Strained Hydrocarbons - Developers

Strained Hydrocarbons 

Edited by Helena Dodziuk

Further Reading 

A. V. Demchenko (Ed.) 

Handbook of Chemical Glycosylation 

2008 

ISBN: 978-3-527-31780-6 

P. G. Andersson, I. J. Munslow (Eds.) 

Modern Reduction Methods 

2008 

ISBN: 978-3-527-31862-9 

L. Kollár (Ed.) 

Modern Carbonylation Methods 

2008 

ISBN: 978-3-527-31896-4 

E. M. Carreira, L. Kvaerno (Eds.) 

Classics in Stereoselective Synthesis 

2008 

ISBN: 978-3-527-29966-9 

M. M. Haley, R. R. Tykwinski (Eds.) 

Carbon-Rich Compounds 

From Molecules to Materials 

2006 

ISBN: 978-3-527-31224-5

Strained Hydrocarbons 

Beyond the van’t Hoff and Le Bel Hypothesis 

Edited by Helena Dodziuk

The Editor 

Prof. Dr. Helena Dodziuk 

Institute of Physical Chemistry 

Polish Academy of Sciences 

Kasprzaka 44 

01-224 Warsaw 

Poland 

Cover illustration 

Cover picture kindly provided by 

Dr. K. S. Nowinski 

All books published by Wiley-VCH are carefully 

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publisher do not warrant the information contained 

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statements, data, illustrations, procedural details or 

other items may inadvertently be inaccurate. 

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� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, 

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All rights reserved (including those of translation 

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ISBN: 978-3-527-31767-7

Contents 

Foreword V 

Preface VII 

List of Contributors XVII 

1 Introduction 1 

1.1 Initial Remarks 1 

Helena Dodziuk 

1.2 Hydrocarbons with Unusual Spatial Structure: the Need to 

Finance Basic Research 5 


1.3 Computations on Strained Hydrocarbons 12 

Andrey A. Fokin and Peter R. Schreiner 

1.4 Gallery of Molecules That Could Have Been Included in 

This Book 18 


1.4.1 Introductory Remarks 18 

1.4.2 Saturated Hydrocarbons 18 

1.4.3 Distorted Double Bonds 21 

1.4.4 Benzene Rings with Nontypical Spatial Structures 22 

1.4.5 Cumulenes 25 

1.4.6 Acetylenes 26 

References 27 

2 Distorted Saturated Hydrocarbons 33 

2.1 Molecules with Inverted Carbon Atoms 33 

Kata Mlinari�-Majerski 

2.1.1 Introduction 33 

2.1.2 Small-ring Propellanes: Computational and Physicochemical 

Studies 35 

2.1.3 Small-ring Propellanes: Experimental Results 38 

2.1.3.1 Preparation and Reactivity of [1.1.1]Propellane 38 

Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk 

Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 

ISBN: 978-3-527-31767-7 

IX

X Contents 

2.1.3.2 Preparation and Reactivity of [2.1.1]Propellane and 

[2.2.1]Propellane 41 

2.1.3.3 [1.1.1]Propellane as the Precursor for the Synthesis of Other 

Unusual Molecules 42 

2.1.4 New Hypothetical Molecules with Inverted Carbon Atoms 43 

2.2 Molecules with Planar and Pyramidal Carbon Atoms 44 


2.3 A Theoretical Approach to the Study and Design of Prismane 

Systems 49 

Tatyana N. Gribanova, Vladimir I. Minkin and Ruslan M. Minyaev 


2.3.2 Prismanes 49 

2.3.3 Expanded Prismanes 52 

2.3.3.1 Asteranes 52 

2.3.3.2 Ethynyl-expanded Prismanes 54 

2.3.4 Dehydroprismanes 55 

2.3.5 Polyprismanes 56 

2.3.5.1 Cubane Oligomers 56 

2.3.5.2 Fused Prismanes 57 

2.3.6 Conclusions 58 

2.4 (CH) 2n Cage Structures, ‘in’-‘out’ Isomerism in Perhydrogenated 

Fullerenes and Planar Cyclohexane Rings 59 


2.4.1 (CH) 2n Cage Structures 59 

2.4.1.1 Tetrahedrane 61 

2.4.1.2 Triprismane 62 

2.4.1.3 Cubane 61, Cuneane 100 and Octabisvalene 101 C 8 H 8 62 

2.4.1.4 C 10 H 10 Saturated Cages 63 

2.4.1.5 C 12 H 12 Saturated Cages 63 

2.4.1.6 Higher [n]Prismanes, Dodecahedrane 64 

2.4.1.7 ‘In’-‘out’ Isomerism in Perhydrogenated Fullerenes C 60 H 60 64 

2.4.1.8 Summary 67 

2.4.2 Planar Cyclohexane Rings 67 

2.5 Ultralong C–C Bonds 70 

Takanori Suzuki, Takashi Takeda, Hidetoshi Kawai and Kenshu Fujiwara 


2.5.2 Ultralong C–C Bonds Confi ned in a Stiff Molecular Frame 72 

2.5.3 Tetraphenylnaphthocyclobutene as a Scaffold to Produce Ultralong 

C–C Bonds 73 

2.5.4 ‘Clumped’ Hexaphenylethane Derivatives with Elongated and 

Ultralong C–C Bonds 74 

2.5.5 HPE Derivatives with a Super-ultralong C–C Bond 78 

2.5.6 ‘Expandability’ of the Ultralong C–C Bond: 

Conformational Isomorphs with Different Bond Lengths 79 

2.5.7 Future Outlook 82

2.6 Ultrashort C–C Bonds 82 

Vladimir Y. Lee and Akira Sekiguchi 


2.6.2 Tricyclo[2.1.0.0 2,5 ]pentanes: Ultrashort Endocyclic Bridging 


2.6.3 Coupled Cage Compounds: Ultrashort Exocyclic Intercage 


2.6.4 Sterically Congested in-Methylcyclophanes: Ultrashort C–C(Me) 

Bonds 91 


References 92 

Contents 

3 Distorted Alkenes 103 

3.1 Nonplanar Alkenes 103 

Dieter Lenoir, Paul J. Smith and Joel F. Liebman 

3.1.1 Introduction and Context 103 

3.1.2 Bridgehead Alkenes 103 

3.1.2.1 t-Butyl-substituted Ethylenes 104 

3.1.2.2 Investigations of t-Butylated Ethylenes and Other Acyclic 

Alkenes 106 

3.1.2.3 Cyclo and Bicycloalkenes … and on to Polycyclic Analogs 107 

3.1.2.4 Adamantylideneadamantane and its Derivatives 108 

3.1.2.5 t-Butyl-substituted and Cyclic Stilbenes 108 

3.1.3 Multiply Unsaturated Bicycloalkenes, Homoaromaticity and 

Cyclophanes 109 

3.1.3.1 The Most Distorted Ethylenes and Seemingly Simple Analogs 111 

3.2 Small Ring and Cage Structures Involving Nonplanar C=C Bonds 

112 

Athanassios Nicolaides 

3.2.1 Pyramidalized Alkenes 112 

3.2.1.1 Tricyclo[3.3.11.0 3,7 ]undec-3(7)-ene 38 115 

3.2.1.2 Tricyclo[3.3.10.0 3,7 ]dec-3(7)-ene 39 and tricyclo[3.3.9.0 3,7 ]non-3(7)-ene 40 

117 

3.2.1.3 Tricyclo[3.3.0.0 3,7 ]oct-1(5)-ene 41 119 

3.2.1.4 (Ph 3 P) 2 Pt Complexes 119 


3.3 Strained Cyclic Allenes and Cumulenes 122 

Richard P. Johnson and Kaleen M. Konrad 


3.3.2 Allene � Bond Deformations and Strain Estimates 123 

3.3.3 Four- and Five-membered Ring Allenes 124 

3.3.4 1,2-Cyclohexadienes 125 

3.3.5 1,2,4-Cyclohexatrienes 127 

3.3.5.1 6-Methylene-1,2,4-Cyclohexatrienes and Related Structures 131 

3.3.6 Seven-membered Ring Allenes 131 

XI

XII Contents 

3.3.6.1 Cycloheptatetraenes 132 

3.3.7 Eight-membered Ring Allenes 134 

3.3.8 Polycyclic Allenes 135 

3.3.9 Cyclic Bisallenes 136 

3.3.10 Cyclic Butatrienes 136 

3.3.10.1 Butatriene � Bond Deformations and Strain Estimates 137 

3.3.10.2 Five- to Nine-membered Ring Cyclic Butatrienes 137 


References 140 

4 Strained Aromatic Molecules 147 

4.1 Nonstandard Benzenes 147 

Paul J. Smith and Joel F. Liebman 

4.1.1 Introduction and Context 147 

4.1.2 Alkylated Aromatics 148 

4.1.3 Helicenes 148 

4.1.4 [n]Circulenes 149 

4.1.5 Cyclophanes 150 

4.2 Distorted Cyclophanes 153 

Henning Hopf 


4.2.2 The [n]Cyclophanes 154 

4.2.2.1 [n]Paracyclophanes 154 

4.2.2.2 [n]Metacyclophanes 160 

4.2.3 The [m.n]Paracyclophanes 161 

4.2.4 Distorted Aromatic Rings and ‘Aromatic Character’ 164 

4.2.5 NMR Characteristics of Cyclophanes 165 

4.3 Helicenes 166 

Ivo Starý and Irena G. Stará 


4.3.2 Synthesis of Helicenes 166 

4.3.3 Nonracemic Helicenes 171 

4.3.4 Intriguing Helicene Structures 172 

4.3.5 Physicochemical Properties and Applications 173 

4.3.6 Theoretical Studies 175 

4.3.7 Outlook 176 

4.4 Cycloproparenes 176 

Brian Halton 


4.4.2 Synthetic Considerations 177 

4.4.3 Chemical Considerations 183 

4.4.4 Heteroatom Derivatives 187 

4.4.5 Physicochemical and Theoretical Considerations 188 

References 193

5 Fullerenes 205 

5.1 Introduction 205 


5.2 Chemistry Infl uenced by the Nontypical Structure: Modifi cation of 

[60]Fullerene 208 

Takuma Hara, Takashi Konno, Yosuke Nakamura and Jun Nishimura 


5.2.2 General Overview 209 

5.2.3 Modifi cation Reactions 215 

5.2.3.1 Reduction and Oxidation 215 

5.2.3.2 Alkylation 217 

5.2.3.3 Cycloadditions 218 


5.3 Physicochemical Properties and the Unusual Structure of 

Fullerenes 225 

5.3.1 Single-crystal X-ray Structures of Fullerenes and Their 

Derivatives 225 

Olga V. Boltalina, Alexey A. Popov and Steven H. Strauss 

5.3.1.1 Introduction 225 

5.3.1.2 Disorder 226 

5.3.1.3 Nonplanar Steric Strain 226 

5.3.1.4 Nonplanar Steric Strain Parameters 229 

5.3.1.5 Are Non-IPR Fullerenes Sterically Unstable? 232 

5.3.1.6 Long and Short C(sp 2 )–C(sp 2 ) Bonds in Fullerene Cages 232 

5.3.1.7 Steric Strain in C 60 (X) n Isomers 236 

5.3.2 Vibrational and Electronic Spectra 238 

Alexey A. Popov 


5.3.2.2 Vibrational Spectra of Fullerenes 239 

5.3.2.3 The Orbital Picture of Fullerenes: High-energy Electronic 

Spectra 243 

5.3.2.4 Electronic Excitations. UPS, UV/Vis/NIR Absorption and 

Fluorescence Spectroscopy 246 

5.3.3 Nuclear Magnetic Resonance 250 

Toni Shiroka 


5.3.3.2 NMR of Fullerenes 251 

5.3.3.3 Concluding Remarks 259 

5.3.4 Electrochemistry 259 

Renata Bilewicz and Kazimierz Chmurski 

5.3.4.1 Electronic Properties of Fullerenes 259 

5.3.4.2 Electrochemical Properties of Soluble Fullerene Derivatives 263 

5.3.4.3 Electrocatalytic Activity of Fullerenes 270 

5.3.4.4 Conclusions and Outlook 272 

5.4 Fullerene Aggregates 273 

Contents 

XIII

XIV 

Contents 

Tommi Vuorinen 

5.4.1 Film Preparation Methods 274 

5.4.2 Fullerene Film Properties 277 


5.5 Endohedral Fullerenes with Neutral Atoms and Molecules 282 

Sho-ichi Iwamatsu 


5.5.2 Preparation 282 

5.5.2.1 Direct Approach Using an Existing Fullerene 282 

5.5.2.2 Molecular Surgery Approach via an Open-cage Fullerene 284 

5.5.2.3 Open-cage Fullerenes, Reversible Molecular Incorporations 

and Ejections 285 

5.5.3 Properties 287 

5.5.3.1 Host Fullerenes 287 

5.5.3.2 Guest Substrates 288 

5.5.4 Binding Energies, Theoretical Investigations 290 

5.5.5 Summary 291 

5.6 Hydrogenated Fullerenes 291 

Mark S. Meier 

5.6.1 Synthesis and Structure 291 

5.6.2 C 70 Chemistry 295 

5.6.3 Higher Fullerenes 297 

5.6.4 Reactivity of Hydrogenated Fullerenes 297 

5.7 Applications of Fullerenes 299 

Rossimiriam Pereira de Freitas and Jean-François Nierengarten 


5.7.2 Applications in Materials Science 299 

5.7.2.1 C 60 Derivatives for Optical Limiting Applications 299 

5.7.2.2 C 60 Derivatives for Photovoltaic Applications 304 

5.7.3 Biological Applications 310 



6 Carbon Nanotubes 335 

6.1 The Structure and Properties of Carbon Nanotubes 335 

Anke Krueger 


6.1.2 The Structure of Single-walled Carbon Nanotubes 335 

6.1.3 The Structure of Multi-walled Carbon Nanotubes 342 

6.1.4 The Aromaticity of Carbon Nanotubes 345 


6.2 The Functionalization of Carbon Nanotubes 347 

Anke Krueger 


6.2.2 Functionalization of the Nanotube Tips 348

6.2.3 Non-covalent Functionalization of Carbon Nanotubes 349 

6.2.4 Covalent Side-wall Functionalization of Carbon Nanotubes 352 

6.2.5 Endohedral Functionalization of Carbon Nanotubes 355 


6.3 Applications of Carbon Nanotubes 356 

Marc Monthioux 


6.3.2 Properties of CNTs 357 

6.3.2.1 Which CNT for Which Application? 357 

6.3.2.2 Why is ‘Nano’ Beautiful? 358 

6.3.2.3 Potential Problems Related to the Use of CNTs 360 

6.3.3 Applications of CNTs 362 

6.3.3.1 Prospective Applications 362 

6.3.3.2 Applications Under Development 364 

6.3.3.3 Applications on the Market 366 



7 Angle-strained Cycloalkynes 375 

Henning Hopf and Jörg Grunenberg 


7.2 Cyclopropyne and Cyclobutyne: Speculations and Calculations 

on Non-isolable Cycloalkynes 376 

7.2.1 Cyclopropyne and Related Systems 376 

7.2.2 Cyclobutyne 378 

7.3 Cyclopentyne, Cyclohexyne, Cycloheptyne: from Reactive 

Intermediates to Isolable Compounds 379 

7.3.1 Cyclopentyne and its Derivatives 379 

7.3.2 Cyclohexyne and its Derivatives 382 

7.3.3 Cycloheptyne and its Derivatives 384 

7.4 The Isolable Angle-strained Cycloalkynes: Cyclooctyne, 

Cyclononyne, and Beyond 385 

7.4.1 Cyclooctyne and its Derivatives 385 

7.4.2 Cyclononyne and Cyclodecyne 386 

7.5 Cyclic Polyacetylenes 387 

7.6 Spectroscopic Properties of Angle-strained Cycloalkynes 392 


8 Molecules with Labile Bonds: 

Selected Annulenes and Bridged Homotropilidenes 399 

Richard V. Williams 


8.2 Annulenes 399 

8.2.1 Cyclobutadiene 399 

8.3 Cyclooctatetraene 403 

Contents 

XV

XVI 

Contents 

8.4 Bond Shifting, Ring Inversion and Antiaromaticity 405 

8.5 Valence Isomerization 409 

8.6 Ions Derived from COT 410 

8.7 The Higher Annulenes 411 

8.8 Bridged Homotropilidenes 413 

8.9 Recent Developments 415 

8.9 Conclusions 419 


9 Molecules with Nonstandard Topological Properties: Centrohexaindane, 

Kuratowski’s Cyclophane and Other Graph-theoretically Nonplanar 

Molecules 425 

Dietmar Kuck 


9.1.1 Is All This Trivial? 425 

9.2 Topologically Nonplanar Graphs and Molecular Motifs 427 

9.2.1 The Centrohexaquinacene Core 427 

9.2.2 The Nonplanar Graphs K 5 and K 3,3 and Some Molecular 

Representatives 428 

9.3 Centrohexaindane 430 

9.3.1 Centrohexaindane and Structural Regularities of the Centropolyindane 

Family 431 

9.3.2 Syntheses of Centrohexaindane 433 

9.3.3 Multiply-functionalized Centrohexaindanes 436 

9.4 K 5 versus K 3,3 Molecules 438 

9.4.1 Topologically Nonplanar Polyethers and Other K 3,3 Compounds 438 

9.5 Kuratowski’s Cyclophane 441 

9.5.1 Synthesis of Kuratowski’s Cyclophane 441 

9.5.2 The Structure of Kuratowski’s Cyclophane 443 

9.6 Conclusions 444 


10 Short-lived Species Stabilized in ‘Molecular’ or ‘Supramolecular Flasks’ 449 



11 Concluding Remarks 459 



Index 463

Foreword 

Chemistry is the truly anthropic science. The molecules we make can heal us, and 

they can hurt us, because they are on the scale of the molecules that make up our 

bodies. And our synthetic creations interact, even react, with the molecules that 

nature – our enzymes, the environment – put into us. 

Molecular science is also anthropic (male and female, of course) because it 

presents a challenge to human intelligence that is just right, commensurate with 

our intellect. The exciting story this book develops bears testimony on every page 

to that anthropic cognitive nature of organic chemistry. 

Let me explain: our remarkable neural system is steered by a complex brain. 

That brain has prejudices for sure; it tends to simplify things, falling at every 

proffered opportunity for beautiful equations, simple mechanisms, Platonic solids 

and the honeyed simplicities of politicians. But when challenged, we can deal with 

substantial complexity. Indeed, the brain relishes being stretched: by rich sensual 

inputs, by patterns, by puzzles. 

Along comes a science, our chemistry. It offers in its molecular structures, a 

game that is at first sight deceptively simple. Take hydrocarbons (most of the 

molecules in this book are in this category) – what could be simpler? Two elements, 

C and H, that by a transparent rule of intercombination form four bonds, and 

one bond, respectively. You are well aware of the manifestation of these rules and 

combinatorics – a chemical universe of incredible diversity. 

These molecules can not only be thought up, they can also be synthesized in a 

human span – roughly the time it takes for a graduate student to get a Ph.D. We 

are not making a ladybug, nor a spiral galaxy; we are making a paracyclophane. 

The complexity of the challenge is on the human scale. And so are the possibilities: 

What can I do to string eight carbons across the para positions of a benzene? Can 

I reduce the bridging carbons to seven? Will I make it easier if the eight carbons 

are partially in a benzene ring themselves? The questions just flow one after the 

other; it takes no talent to ask them, just a normal curious human being, privy to 

the structural codes of chemistry. 

So the game itself, the game of chemical structure, is exciting. Chess pales 

by comparison. Add to that ludic challenge potential utility, and also the natural 

human desire to probe limits (just how far can I distort that double bond out of 

its planar normalcy?), and you have all the makings of intense interaction, part 



ISBN: 978-3-527-31767-7 

V

VI Foreword 

intellectual, part emotional, between a human being and an object of his or her 

creation. 

The object of our intense contemplation – a compound macroscopically, a 

molecule microscopically – is complex enough not to be boring, yet not unpredictably 

chaotic. The strained molecule is just right for some of us to exercise our 

creativity in thinking up these strange beasts, others in coming up with ingenious 

ways of making them (for molecules are real!), all of us admiring the complexity, 

simplicity and function all rolled into one. 

Enjoy reading this book! Roald Hoffmann

Preface 

Strained hydrocarbons represent an amazing domain. About 80 years after 

the formulation of van’t Hoff and Le Bel’s hypothesis new, exciting molecules 

representing in Hoffmann and Hopf formulation (R. Hoffmann, H. Hopf, Angewandte 

Chemie, submitted), what is probably too much of anthropomorphization, 

‘molecular sadism’ were synthesized. Paraphrasing D. J. Cram, one could 

say that such molecules elicit wonder, stimulate the imagination and challenge 

both synthetic talents and interpretive instincts. Up to the early 1990s the field of 

strained hydrocarbons was a kind of elitist area in which only the best synthetic 

and theoretical chemists were active. It was a playground of few, characterized 

by vivid interactions between synthetic and theoretical chemistry allowing one to 

propose plausible synthetic targets on the basis of model calculations. On the other 

hand, it allowed Bader, Wiberg and their followers to refine the definition of the 

chemical bond. The situation in the domain of distorted molecules changed after 

the discovery of fullerenes and nanotubes which attracted numerous researchers. 

These molecules, having nonplanar systems of conjugated bonds, are not 

hydrocarbons but their derivatives are numerous. Thus, they have been included 

into this volume in view of the rapid development of these areas and, still largely 

unfulfilled, prospects of their applications. 

Several researchers helped me in this project. First of all, I would like to thank 

all contributors to this volume. I would like also to acknowledge the support I have 

obtained from Professors T. Marek Krygowski, Jay S. Siegel and Henning Hopf 

in the initial stage. Finding contributors was sometimes a difficult task. The help 

of Professors A. de Meijere, E. Osawa, F. Diederich, C. Thilgen, W. T. Borden, 

J. Cioslowski and H. Kuzmany in the search for coauthors is gratefully acknowledged. 

I am deeply obliged to my colleague, Dr. K. S. Nowinski, for designing the 

cover picture. On the other hand, I owe a deep apology to the authors of many 

interesting papers on strained hydrocarbons which could not be presented in this 

book or were insufficiently covered due to space limitations. 

The question: ‘To what extent can a bond be distorted without being broken?’ 

is fascinating. This book is devoted to the presentation of distorted hydrocarbons. 

It is an effort to counteract, in this limited volume, overspecialization by showing 

not only syntheses, physicochemical studies and theoretical calculations of these 

molecules, but also the prospects of their applications. Strained molecules are 



ISBN: 978-3-527-31767-7 

VII

VIII 

Preface 

exciting objects for studies per se. With several novel hypothetical molecules 

waiting to be synthesized on the one hand, and with the possibility of obtaining 

fascinating supramolecular complexes with distorted hydrocarbons as building 

blocks on the other, this domain will remain enthralling. 

Warsaw, January 2009 Helena Dodziuk

List of Contributors 

Renata Bilewicz 

University of Warsaw 

Department of Chemistry 

Pasteura 1 

02093 Warsaw 

Poland 

Olga V. Boltalina 

Colorado State University 


Fort Collins, CO 80523 

USA 

Kazimierz Chmurski 

University of Warsaw 


Pasteura 1 

02-093 Warsaw 

Poland 


Polish Academy of Sciences 

Institute of Physical Chemistry 

Kasprzaka 44/52 

01-224 Warsaw 

Poland 

Andrey A. Fokin 

Kiev Polytechnic Institute 

Department of Organic Chemistry 

Spr. Pobedy 37 

03056 Kiev 

Ukraine 

Kenshu Fujiwara 

Hokkaido Univeresity 

Faculty of Science 


N10W8, North-ward 

Sapporo 060-0810 

Japan 

Tatyana N. Gribanova 

Southern Federal University 

Institute of Physical and Organic 

Chemistry 

194/2 Stachka Ave 

344090 Rostov on Don 

Russia 

Jörg Grunenberg 

Carolo Wilhelmina Technical 

University of Braunschweig 

Institute of Organic Chemistry 

Hagenring 30 

38106 Braunschweig 

Germany 

Brian Halton 

Victoria University of Wellington 

School of Chemical & Physical 

Sciences 

PO Box 600 

Wellington 6140 

New Zealand 



ISBN: 978-3-527-31767-7 

XVII

XVIII 


Takuma Hara 

Evonik Degussa Japan Co. Ltd. 

Business Line Catalysts 

Tsukuba Minami Daiichi Kogyo 

Danchi 

21 Kasuminosato Ami-machi 

Inashiki-gun Ibaraki-ken 300-0315 

Japan 

Henning Hopf 

Technical University of 

Braunschweig 

Carolo Wilhelmina 

Institute of Organic Chemistry 

Hagenring 30 

38106 Braunschweig 

Germany 


Nagoya University 

Graduate School of Environmental 

Studies 

Chikusa Ku 

Nagoya, Aichi 464-8601 

Japan 

Richard P. Johnson 

University of New Hampshire 


Durham, NH 03824 

USA 

Hidetoshi Kawai 

Hokkaido University 





Japan 

Takashi Konno 

Gunma University 

Graduate School of Engineering 

Department of Chemistry and 

Chemical Biology 

Tenjincho 1-5-1 

Kiryu, Gunma 376-8515 

Japan 

Kaleen M. Konrad 

Merck Research Laboratories 

33 Avenue Louis Pasteur 

Boston, MA 02115 

USA 

Anke Krüger 

University of Kiel 

Otto-Diehls-Institute of Organic 

Chemistry 

Otto-Hahn-Platz 3 

24098 Kiel 

Germany 

Dietmar Kuck 

Universität Bielefeld 

Fakultät für Chemie 

Postfach 100131 

33501 Bielefeld 

Germany 

Vladimir Y. Lee 

University of Tsukuba 

Graduate School of Pure and 

Applied Science 


Tsukuba, Ibaraki 305-8571 

Japan 

Dieter Lenoir 

GSF-Research Center Neuherberg 

Institute of Ecological Chemistry 

Postfach 1129 

85758 Neuherberg 

Germany

Joel F. Liebman 

University of Maryland, 

Baltimore County 

Department of Chemistry & 

Biochemistry 

1000 Hilltop Circle 

Baltimore, MD 21250 

USA 

Mark S. Meier 

University of Kentucky 


Lexington, KY 40506 

USA 

Vladimir I. Minkin 



Chemistry 



Russia 

Ruslan M. Minyaev 



Chemistry 



Russia 

Kata Mlinarić-Majerski 

Rudjer Boškovi� Institute 

Department of Organic Chemistry 

and Biochemistry 

Bijeni�ka 54, P.O. Box 180 

10002 Zagreb 

Croatia 


Université Toulouse III 

CEMES, UPR-8011 CNRS 

BP 94347 

29 rue Jeanne Marvik 

31055 Toulouse Cedex 4 

France 

Yosuke Nakamura 



Department of Chemistry and 

Chemical Biology 



Japan 

Jean-François Nierengarten 

Université de Strasbourg 

Laboratoire de Chemie des Matériaux 

Moléculaires (UMR 7509) 

25 rue Becquerel 

67087 Strasbourg Cedex 2 

France 

Athanassios Nikolaides 

University of Cyprus 


P.O. Box 20537 

Nicosia 1678 

Cyprus 


Jun Nishimura 



Department of Nano-Material 

Systems 



Japan 

XIX

XX 


Rossimiriam Pereira de Freitas 

Universidade Federal de Minas 

Gerais 

Departamento de Química 

Av. Antônio Carlos 6627 

Belo Horizonte 

312-70901, MG 

Brazil 


Leibniz-Institute for Solid State 

and Materials Research IFW 

Group of Electrochemistry and 

Conducting Polymers 

Helmholtzstrasse 20 

01069 Dresden 

Germany 

Peter R. Schreiner 

Justus-Liebig University 

Institut für Organische Chemie 

Heinrich-Buff-Ring 58 

35392 Giessen 

Germany 

Akira Sekiguchi 

University of Tsukuba 


Graduate School of Pure and 

Applied Sciences 

Tsukuba, Ibaraki 305-8571 

Japan 

Toni Shiroka 

Paul Scherrer Institut 

Laboratory for Muon-Spin 

Spectroscopy 

CH-5232 Villigen PSI 

Switzerland 

Paul J. Smith 

University of Maryland Baltimore 

County 

Department of Chemistry & 

Biochemistry 

1000 Hilltop Circle 

Baltimore, MD 21250 

USA 

Irena G. Stará 

Academy of Sciences of the Czech 

Republic 

Institute of Organic Chemistry & 

Biochemistry 

Center for Biomolecules & Complex 

Molecular Systems 

Flemigovo nám. 2 

16610 Prague 6 

Czech Republic 

Ivo Starý 

Academy of Sciences of the Czech 

Republic 

Institute of Organic Chemistry & 

Biochemistry 

Center for Biomolecules & Complex 

Molecular Systems 

Flemigovo nám. 2 

16610 Prague 6 

Czech Republic 

Steven H. Strauss 

Colorado State University 


Fort Collins, CO 80523 

USA 

Takanori Suzuki 






Japan

Takashi Takeda 



Departmeht of Chemistry 



Japan 


Tampere University of Technology 

Institute of Materials Chemistry 

P.O. Box 541 

33101 Tampere 

Finland 


University of Idaho 


P.O. Box 442343 

Moscow, ID 83844-2343 

USA 


XXI

1 

Introduction 

1.1 

Initial Remarks 


Let us start with a bit of history. Today, it is hard to imagine how difficult it was 

to develop basic concepts and ideas of chemistry in the second half of the 19th 

century. The story about Kekulé’s fight for his benzene structure shows that not 

all the arguments he used in its favor are valid today [1]. His idea could not be supported 

by the poor experimental instrumentation of that time. There was no X-ray 

analysis, no modern spectroscopic techniques and no calorimetry. The idea of the 

constitution of molecules, that is building them from a certain number of different 

types of atoms, was established, as well as several experimental findings which 

demanded rationalization. Among them were optical activity and the existence of 

a number of different molecules with the same constitution. Pasteur foresaw that 

the former phenomenon could be related to the positioning of atoms in space but 

only the van’t Hoff [2] and Le Bel [3] hypotheses on the tetrahedral arrangement of 

substituents on the tetravalent carbon atom explained most observations known 

at that time. Interestingly, the independently proposed models differed slightly: 

that of van’t Hoff was based on a regular tetrahedron, while in the second one used 

an irregular tetrahedron to represent the carbon atom. This difference was not 

significant but, remarkably, the more idealized van’t Hoff approach was generally 

accepted. An illustration from the 1908 German edition of van’t Hoff’s book, showing 

two stereoisomers of the tetrasubstituted ethane molecule CR1CR2rCR3CR4r 

shows the way in which molecules were depicted at that time (Figure 1.1). 

The van’t Hoff and Le Bel hypothesis was met with strong criticism, not always 

expressed in impartial scientific language. The renowned chemist and editor of the 

German Journal für praktische Chemie, Prof. Adolf Kolbe wrote: ‘A Dr. H. van ’t Hoff 

of the Veterinary School at Utrecht has no liking, apparently, for exact chemical 

investigation. He has considered it more comfortable to mount Pegasus (apparently 

borrowed from the Veterinary School) and to proclaim in his ‘La chimie 

dans l’éspace’ how the atoms appear to him to be arranged in space, when he is 

on the chemical Mt. Parnassus which he has reached by bold fly’. 



ISBN: 978-3-527-31767-7 

1

2 1 Introduction 

Figure 1.1 The representation of two stereoisomers of a tetrasubstituted ethane molecule 

CR1CR2rCR3CR4r, as published 100 years ago in van’t Hoff’s book [197]. 

In spite of such a strong attack and the difficulties associated with the lack of 

modern physicochemical methods, the idea of the tetrahedral arrangement of 

substituents around a tetravalent carbon atom was generally accepted and van’t 

Hoff became a first recipient of the Nobel prize for Chemistry in 1901, interestingly 

not for his stereochemical ideas but ‘in recognition of the extraordinary 

services he has rendered by the discovery of the laws of chemical dynamics and 

osmotic pressure in solutions’. 

Remarkably, an early idea of Sachse on the cyclohexane conformations (today 

known under the names chair and twist-boat) [4, 5] could not be proved at that 

time and was not accepted. Then, it took almost 80 years to understand all the 

consequences of the van’t Hoff and Le Bel concepts which not always were based 

on justified assumptions. For instance, the van’t Hoff understanding of the C–C 

bond implied free rotation around it. This assumption was only shown to be invalid 

by Pitzer’s work [6, 7] on the hindering of the rotation and preferred orientations 

of substituents on the C–C bond, started in 1936, which marked an important 

step in development of stereochemistry [8]. The combination of the ideas on 

tetrahedral orientation of substituents on a tetravalent carbon atom and of the 

hindered rotation around the C–C bond resulted in rationalization of the cyclohexane 

conformations and the number of isomers of its derivatives summarized 

in the Hassel [9, 10] and Barton [11] studies which were also honored by a Nobel 

Prize ‘for their contributions to the development of the concept of conformation and 

its application in chemistry’. Analogous studies of the spatial structure of alkenes, 

alkynes and aromatic compounds followed. 

With these achievements, the basis of the organic stereochemistry seemed to 

be laid, and models could be built, of spatial structures of molecules from welldefined 

rigid fragments. Eaton’s report on the synthesis of cubane 1 in 1964 [12] 

and especially the Wiberg synthesis of [1.1.1]propellane 2 [13] have shown that, 

in addition to the small-ring cycloalkanes, well-known since the second half of 

the 19th century, that exhibit Bayer strain [14], hydrocarbons having structures 

strongly departing from that suggested by van’t Hoff and Le Bel can exist. This book 

is devoted to such nonstandard structures. Let us first define what the standard 

hydrocarbons are: first, these are saturated hydrocarbons with the arrangement 

of substituents on the carbon atoms close to tetrahedral; then, double bonds and 

aromatic rings lying in a plane with its substituents and, last but not least, linear

1.1 Initial Remarks 

acetylenes. Also of interest are bond lengths that depart far from the standard 

value of 154 pm. Of course, fullerenes and carbon nanotubes in their idealized 

form are not hydrocarbons, but these conjugated aromatic systems are nonplanar 

and they are definitely the most widely studied distorted aromatic systems today. 

They also offer a unique possibility of investigating the effect of nonplanarity on 

structure, physicochemical properties and reactivity. Actually, this book should 

also be understood as part of my private campaign against too deep specialization, 

from which we all suffer. Thus, showing the influence exerted by molecular 

distortions on various physicochemical properties using fullerenes as an example 

seemed to be of importance. 

Highly distorted hydrocarbons are sometimes considered to be of no importance 

in view of their lack of practical applications. The significance of investigating such 

systems is discussed in Section 1.2. They are studied by both experimental and 

theoretical methods that, as discussed in Section 1.3, are of special significance 

in this domain. As shown by several examples (among other 1 [15] and heptacyclo[6.4.0.0 

2,4 .0 3,7 .0 5,12 .0 6,10 .0 9,11 ]dodecane 3 [16, 17] which have first been studied 

theoretically then synthesized [13, 18]), to propose novel plausible synthetic targets 

on the basis of molecular modeling is a reliable aim of calculations. 

We are experiencing such a rapid development of this science that it is not 

possible to discuss all the unusual hydrocarbons. Therefore, a selection, by no 

means considered to be exhaustive, of interesting molecules which did not find 

a place in other chapters is presented in Section 1.4. 

Simple strained saturated hydrocarbons are presented in Chapter 2. Both 

known, such as [1.1.1]propellane 2, and hypothetical molecules having inverted 

carbon atoms are discussed in Section 2.1. The importance of these molecules 

is emphasized by the discussion of the existence of the central bond in 2 which 

permitted precise definition of a bond in quantum chemistry [19]. The fascinating 

Hoffmann idea of planar carbon atom lying in a plane with its four substituents 

[20] was only realized in silico [21] and, as described in Section 2.2, still awaits 

realization. Prismanes (one of which is cubane 1) and asteranes are discussed 

mainly from the theoretical point of view in Section 2.3 but the influence of 

molecular distortions on the properties of the known systems is presented there, 

too. Saturated hydrocarbon cages and planar cyclohexanes (Section 2.4) as well 

as molecules with ultralong (Section 2.5) and ultrashort (Section 2.6) C–C bonds 

are also discussed in Chapter 2. 

In Chapter 3 which is devoted to alkenes, energetic aspects of distorted double 

bonds are presented in Section 3.1, small cage alkenes are discussed in Section 3.2 

while Section 3.3 is devoted to strained cumulenes. 

3


Analogously, in Chapter 4 energetical considerations concerning strained 

aromatic systems are discussed in Section 4.1, while bridged aromatic rings 

(i.e. cyclophanes), helicenes like 4, and cycloproparenes like 5 are presented in 

Sections 4.2 to 4.4, respectively. 

As discussed earlier fullerenes, to which the longest Chapter 5 is devoted, 

are actually not hydrocarbons but their extended closed nonplanar conjugated 

aromatic systems deserve to be discussed in this book. After a short introduction 

in Section 5.1 their chemistry is presented in Section 5.2, and physicochemical 

properties reflecting their distorted structure (X-ray in Section 5.3.1, UV/Vis 

spectra in Section 5.3.2, NMR spectra in Section 5.3.3 and electrochemistry in 

Section 5.3.4) are shown. Next, fullerene films (Section 5.4), endohedral fullerene 

complexes (Section 5.5), an exciting application of NMR to study the structure of 

hydrogenated fullerenes (Section 5.6) and, mostly prospective, fullerene applications 

(Section 5.7). Unfortunately, I did not succeed in finding a specialist willing 

to present theoretical fullerene studies and their limitations due to the size of 

these huge cage molecules. This a serious drawback in spite of the inclusion of 

some fullerene calculations in other chapters. 

The inclusion of nanotubes into this monograph was based on similar arguments 

as those advocating the inclusion of fullerenes. In Chapter 6, structure, chemistry 

and also mostly prospective applications are shown (Sections 6.1–6.3, respectively). 

There has been a fascinating development in the financing of carbon nanotubes: 

people expected to get really big money from investing in CNTs about five years ago 

and then realized that the returns do not come immediately. Some of the applications 

introduced or expected to be introduced soon in large-scale manufacturing 

have not been successful. For instance, using polymer nanocomposites containing 

small amount of CNTs for electropainting of cars has been abandoned by General 

Motors, and Korean plans to build a factory for displays involving CNTs few 

years ago have not been fulfilled. Moreover, a recent observation on the carcenogenicity 

of multiwalled nanotubes may further slow down the development of 

CNT applications [22]. Even if we cannot see their rapid introduction, I am sure 

they will be important in the longer run. 

Cyclic alkynes with nonlinear triple bonds are discussed in Chapter 7, while 

molecules with labile bonds are presented in Chapter 8. They include rigid 

cyclo butadiene 6, highly mobile molecules like cyclooctatetraene 7 and shortlived 

species that could only be trapped at very low temperature in matrices. 

The fascinating discussion of interconversions between some of these species is

1.2 Hydrocarbons with Unusual Spatial Structure: the Need to Finance Basic Research 

presented in some detail from both theoretical and experimental (mainly NMR, 

IR) points of view. 

Hydrocarbons with nonplanar graphs, discussed in Chapter 9, conform to van’t 

Hoff and Le Bel stereochemistry but they are so unusual that they were considered 

to fit into this monograph. Such molecules having distinct topological properties 

represent an exciting border area between chemistry and mathematics. The 

molecules presented in this chapter have been obtained using elegant methods of 

traditional organic chemistry, while other systems exhibiting nontrivial topological 

properties (catenanes, rotaxanes, knots, etc.) could mostly be obtained by taking 

advantage of methods typical of supramolecular chemistry [23]. 

In the last Chapter 10, novel ways of obtaining and stabilizing unstable highly 

strained species in ‘molecular flasks’ are presented. The latter method makes use 

of supramolecular chemistry enabling, for instance, storage of the highly unstable 

cyclobutadiene 6 for a month at room temperature [24]. 

1.2 

Hydrocarbons with Unusual Spatial Structure: the Need to Finance Basic Research 


By reporting the results on hypothetical hexahydrosuperphane 8 [25], a highly 

strained molecule with a planar cyclohexane ring, we have been confronted with 

the question as to whether this molecule had been chosen for study because of 

its future practical applications. The question was posed by a computer scientist 

who had no knowledge of stereochemistry, but it corresponds to a general attitude 

governed by grant funding for scientific research. Namely, any grant application 

has to show its immediate usefulness. However, in reality hardly any grants that 

use such justification will bring marketable results at all. For the applications 

of others we will probably wait 10 or more years and only a few of such grants 

will find their way to industry soon. Let’s inspect some examples in some detail. 

Liquid crystals today are commonly applied in displays but several other uses (as 

surface thermometers showing temperature distribution over a body, in optical 

5


imaging, etc., http://plc.cwru.edu/tutorial/enhanced/files/textbook.htm) are 

foreseen for them. Discovered in 1888 by a botanist, Reinitzer [26], they were for 

almost 100 years considered to be the physicists’ toys. In 1966, an article entitled 

‘Liquid crystals – an area of research of little use?’ appeared in the German journal 

Nachrichten für Chemie, Technik und Labor [27]. Then in 1972 the first liquid crystal 

display was built giving birth to a thriving branch of industry. 

Exciting results reported in a series of works carried out by the Stoddart group 

on so-called molecular machines show that there is a long path from a concept 

to a marketable device [28]. By using rotaxane systems like 9a [29] a family 

of versatile systems has been created which can be used as sensor, switches, 

‘molecular abacus’ or nanomotor. In addition to the spacer 9e, the axle consists 

of the �-electron acceptor groups 9c, 9d which, upon the imposed conditions, 

can selectively bind the �-electron-donating macrocycle 9e shuttling it between 

the position shown in the formula 9a and that around the 3,3�-dimethyl-4,4�bipydidinium 

unit 9c. These studies, combining sophisticated syntheses with 

physicochemical methods, certainly provide an example of an important direction 

for nanotechnology research in the next few years. However, it is questionable 

whether they will bring marketable results within this time. This does not mean 

that such studies are not worth pursuing. Impractical studies elucidating the wave 

or corpuscular character of light and matter have been carried out since the famous 

dispute between Newton and Huygens for almost 300 years and, as the diffraction 

experiments on fullerene C 60 10 (this group of molecules is discussed in detail in 

Chapter 5) from the Zeilinger group show [30], this topic is still vivid. Of course, 

choosing objects for a project out of more than 27 500 000 known molecules is 

a hard task, but its expeditious applicability should not be decisive. Most basic


research is driven by curiosity not practicality but somehow in the long run it pays 

off. No-one thought about chemical applications when the foundations of a new 

branch of mathematics – topology – were formulated in the 1820s, long before 

such abstract concepts as links (catenanes) like olympiadane 11, Möbius strip, 

and knots, like double knot 12, were shown to be of use in chemistry [31] (some 

hydrocarbons with distinct topological properties are presented in Chapter 9). 

7


Moreover, the discovery in the 1990s that circular DNAs in the living organisms 

[32–34] form links, knots and other systems with nontrivial topological properties, 

will have consequences which cannot be anticipated today. We begin to understand 

the mechanism of their formation but the role they play in nature is still unclear. 

Similarly, Einstein’s work on the photoelectric effect published in 1905, for which 

he was awarded the Nobel prize, certainly did not seem to have any practical significance 

at that time. Nor were cosmic studies carried out to develop kevlar and 

teflon! These materials were spin-offs of the space journeys. 

For examples closer to chemistry let us look at the fullerene 10 applications, 

much praised in the 1990s [35]. The Krätschmer method [36] which produces a 

significant amount of the substances spurred numerous proposals for their application. 

They were thought to exhibit superconductivity, serve as a drug carrier, 

its derivative C 60F 60 was anticipated to be an ideal lubricant, etc [35]. 

None of these promises was fulfilled. Superconductivity of fullerene derivatives 

is exhibited only at very low temperatures [37]; C 60F 60 was synthesized but 

it turned out to decompose in air with the HF formation [38]; and, in spite of 

promising reports, to our best knowledge no drug involving fullerene is on 

market. One of few fullerene applications today consists in their use as AFM tips 

(http://www.foresight.org/Updates/Update27/Update27.3.html). Few other are 

presented in Section 5.0 while those discussed in Section 5.7 still await marketable 

applications. This does not mean that fullerenes should not be intensively studied 

and that they will not finally be of practical use. At present these elegant-looking, 

highly symmetrical molecules are exciting and worth studying simply because of 

their unusual properties. They (1) form the nonplanar system of conjugated bonds; 

(2) have a hollow space inside that can accommodate other, smaller molecules, 

ions or even an elementary particle [39]; (3) with cations inside they form unusual 

salts since the fullerene cage assumes the negative charge, thus the salt can be 

dissociated only by its destruction; and, last but not least, (4) their formula merely 

look beautiful or, in other words, they are aesthetically appealing. 

The last point, that is the beauty of molecular formulae as the driving force 

for studying a molecule, was strongly denied by Jansen and Schön in their essay 

provocatively entitled ‘Design in chemical synthesis – an illusion?’ [40]. Their 

argument, which deserves much longer comment, contraposes purely aesthetical 

Gropius’ teapot designs (apparently not constrained by the properties of materials 

from which the objects were to be made) to the design of molecules whose structure 

is unequivocally defined by the energy hypersurface. No objection: molecular 

structure must obey the basic rules of chemistry and physics. However, within 

these limits there is plenty of room for designing molecules with predefined 

desirable properties. In addition to the complicated and not always successful field 

of drug design, hydrocarbons with unusual spatial structure present numerous 

examples of molecular design which was not usually aimed at marketability. 

For instance, let us look at cubane 1 (discussed in Sections 2.3 and 2.4) synthesized 

almost 50 years ago. The molecule was obtained by Eaton [41] not because 

of its immediate applicability. Its synthesis presented a considerable challenge 

and 1 was aesthetically pleasing (the aspect denied by Jansen and Schön [40]). The


molecule turned out to have untypical properties, due to its nonstandard structure, 

e.g. 1 exhibits unusual rearrangement reactions such as the rearrangement of 

cubane to cuneane 13. In addition, NMR spectra of cubane allow one to explore 

the Karplus dependence of the 3 J coupling constants [42]. And as for the cubane 

applications not looked for by Eaton? For more than 30 years secret studies were 

carried out by the American Army on nitro-derivatives of 1 because the high 

energy content of the cubane core magnified by the nitro-substituents suggested 

that these materials might have extraordinary explosive properties. (The synthesis 

of octanitrocubane 14 was eventually reported a few years ago [43].) There were 

also attempts to use cubane derivatives as therapeutic agents [44, 45]. 

However, the emerging applications of some hydrocarbons with unusual spatial 

structure should not deceive us. The main goal of studying them is not their 

marketing but to deepen our understanding of the chemical bond. Interestingly, 

until recently this very fruitful concept, on which all chemistry is based, was not 

anchored in quantum chemistry. We could carry out calculations on the molecule 

as a whole but, without using artificial approximate constructs, were unable to 

analyze properties of specific bonds within it. In particular, studying molecules 

with bonds which are very different from the standard is indispensable to understand 

the limits of the very concept of the chemical bond. The question as to what 

extent a chemical bond can be distorted without breaking is thought provoking. 

Moreover, in certain cases even the mere existence of a bond between two carbon 

atoms has been questioned. This was the case encountered in [1.1.1]propellane 2 

(discussed in some detail in Sections 1.3 and 2.1) [46]. The synthesis of this exciting 

molecule, preceded by the calculations supporting its feasibility and predicting 

the propellane properties, serves as a fascinating example of a mutually fruitful 

interaction of theoretical and experimental studies [47]. 2 represents one of the 

most amazing examples from the point of view of organic stereochemistry since, 

contrary to van’t Hoff [48] and Le Bel [49] hypothesis, all four substituents on its 

bridgehead atoms lie in one hemisphere. Such atoms bearing the name ‘inverted 

carbons’, are also present in other small-ring propellanes (discussed in detail in 

Section 2.1) such as a derivative of [4.1.1]propellane 15 and that of [1.1.1]propellane 

16 [50]. The discussion of the existence of the bridgehead–bridgehead bond 

in small-ring propellanes is remarkable [51]. The distance between the atoms in 

these molecules is about 1.6 Å [52], which is significantly longer than the typical 

C–C bond of 1.54 Å (some authors [50] consider this difference small but the 

energy required for such bond lengthening is considerable). However, 1.6 Å or 

even longer bonds have been encountered in several hydrocarbons [53]. In spite 

9


of the reliable bond length of the former bond in small ring propellanes, the differential 

electron density maps for 15 [54] and 16 [50] measured in X-ray studies 

have not shown any build-up of the differential electron density between the 

bridgehead propellane atoms which should accompany the bond between them. 

The former finding and the possibility of a biradical structure for 14 without the 

central bond triggered a discussion on the reliability of the maps as the criterion of 

the bonding. On the one hand, the formulation of the limitations of this criterion 

resulted, stating that the lack of the build-up of the differential electron density is 

an artifact of the promolecule density distribution not reflecting the relative properties 

of the charge distributions [47]. The quantum calculations for 2 carried by 

Wiberg, Bader and Lau [47] showed that there is the bond-critical point along the 

line connecting bridgehead atoms in this molecule, thus proving the existence of 

the bond between the atoms. These calculations also revealed that the exceptional 

stability of this molecule is not due to the typical two-center integrals describing 

chemical bonds but is the result of the operation of the three-center integrals. In 

addition, other criteria for the existence of the central bond in 2 modeling [1.1.1] 

propellane have been checked [50]. 

Helvetane 17 and israelane 18 appeared as a joke in a 1st April issue of Nouveau 

Journal de Chimie [55]. These highly strained hypothetical molecules belong to 

a very interesting class of (CH) 2n cage compounds to which cubane 1, dodecahedrane 

C 20 H 20 19 and hypothetical perhydrogenated fullerene C 60 H 60 [56] 

(discussed in Sections 2.3 and 2.4) belong. These molecules were shown to be 

of much higher energies than other members of the C 20 H 20 family [57] which 

should be much more easy to synthesize. Nevertheless, they have been calculated 

by several theoreticians who pointed out than removing symmetry constraints 

would significantly lower the energy of 17 and 18. Then such molecules being 

members of a large group of isomers without interesting properties would seem 

to be of no specific interest.


It should be stressed that a molecule dismissed as purely hypothetical today can 

be a plausible synthetic target tomorrow. Herzberg, later awarded Nobel Prize, 

stated in his seminal ‘Infrared and Raman Spectra of Polyatomic Molecules’ in 

1945 that it is not likely that molecules of I h symmetry will ever be found [58]. 

It took several years of hard work for the Paquette group to synthesize the first 

molecule of such a high symmetry, aforementioned dodecahedrane 19, about 

40 years later [59]. (In our opinion this synthesis deserves the name molecular 

design vigorously discredited by Jansen and Schön [40].) Today, the best known 

such molecules are fullerene 10 (discussed in Chapter 10), parent fullerene C 20 

20 [60] and perfluorinated fullerane C 60F 60 (which, as discussed in Chapter 10, 

similar to other short-lived species could be stabilized in molecular flasks) as well 

as the most symmetrical isomers of their higher homologs, like C 240, C 540, C 960, 

etc and some nested fullerenes formed by carbon cage compounds [61] belonging 

to the latter group. 

A kind of laborious play, that seems not to promise serious consequences 

but bears all the attributes of a standard synthetic work, has been reported by 

Chanteau and Tour [62]. They described the syntheses of nanoputanes, like 21, 

the anthropomorphic molecules named after the Jonathan Swift lilliputanes. With 

meticulously described syntheses, the authors showed not only the way to obtain 

the nanokid 21 but also got a ‘dancing’ nanoputanes layer on a surface 22. 

11


To summarize, the choice of a molecule for studies is not simple. Most standard 

systems are trivial and unworthy of serious consideration in basic research. Other 

untypical molecules can also have their pitfalls. Keeping in mind that what is impossible 

to synthesize today can be realizable tomorrow, one should nevertheless 

exhibit caution when choosing an object to study which should be of a serious 

scientific interest. However, it should be stressed once more that immediate applications 

should not be the reason for financing basic research. 

1.3 

Computations on Strained Hydrocarbons 

Andrey A. Fokin and Peter R. Schreiner 

Despite some potential applications as high-energy materials and specialty 

polymers, highly strained compounds mostly play a conceptual and educational 

role. Over 10 000 chemical papers contain the key words ‘strained hydrocarbon’ 

and more than 18 000 ACS papers alone the terms ‘strain energy.’ Despite the 

fact that our rationalistic and thrifty ages leave lesser space for exotic molecules, 

the aesthetic beauty of the cages such as cubane 1, tetrahedrane 23, octahedrane 

24 or dodecahedrane 25 still fascinate organic chemists and represent the artistry 

of organic synthesis. Nature also uses highly strained compounds: The recent 

discovery of ladderanes 26 and 27 as membrane lipids of certain anaerobic bacteria 

[63] and natural antifungal oligocyclopropane antibiotic 28 [64] underline the 

importance of such structures (Scheme 1.1). 

It is generally considered that highly strained compounds are difficult and 

expensive to make and, sometimes, also to store. However, once a challenging 

molecule has been prepared, the development of a simpler way for its synthesis 

is impending. The most recent example is highly strained octahedrane 24, which 

was first prepared in 1993 by an expensive and elaborate procedure [65, 66]. Now 

some octahedrane derivatives such as 30 can be prepared by one-step photochemical 

dimerization of readily available aromatic cyclophane 29 [67]. A simple 

preparation of octacyclopropylcubane 32 by an effective two-step condensation of 

four dicyclopropylacetylenes 31 is another remarkable example (Scheme 1.2) [68]. 

The stability of strained compounds is not necessarily a concern: cubane and its 

derivatives are stable even at high temperatures because the strain is uniformly 

distributed throughout the molecule and orbital symmetry forbids the cleavage 

of two C–C bonds at the same time. Some unstable and highly strained hydro-

1.3 Computations on Strained Hydrocarbons 

Scheme 1.1 Highly strained hydrocarbons and some natural compounds containing strained 

moieties. 

Scheme 1.2 Highly strained derivatives of octahedrane and cubane prepared recently through 

short and simple procedures. 

carbons have been successfully encapsulated and stored at room temperature as 

guest molecules in hemicarceplexes [69]. 

The chemistry of strained organic molecules probably began with the realization 

of cyclopropane derivatives by Perkin [70] that was almost immediately 

followed by the development of strain theory [71]. It was soon recognized that 

Baeyer’s angular strain is the main contributor to the potential energies of organic 

molecules. The quantitative description of strain was first proposed in the mid 

13


1940s by Hill [72] and was developed further by Westheimer into molecular 

mechanics [73]. As with many other useful chemistry concepts like conjugation, 

aromaticity, chemical bonding, etc., strain itself is not defined exactly, but can be 

expressed well quantitatively by ‘strain energy,’ which is, however, not measurable 

experimentally. Its value is calculated as the difference between the experimental 

enthalpy of formation of the molecule of interest and that of a hypothetically strainfree 

structure. The enthalpies of formation of strain-free reference compounds 

are calculated through group additivity schemes [74] based on the ‘averaged’ contributions 

of groups (CH 3, CH 2, CH, etc.) from the straight-chain hydrocarbons. 

The group’s contributions are derived from thermochemical measurements for 

which, however, equilibrium conformer distributions are difficult to take into 

account. Group equivalent schemes also require experimental thermochemical 

data on the molecule of interest but these are equally error-prone. The most recent 

example is the heat of formation of cubane that was reinterpreted based on the 

corrected value of its sublimation enthalpy [75]. Unluckily, cubane has already 

been used for the parameterization of some molecular mechanical methods [76] 

that now require re-parameterization. New additivity schemes [77, 78] demonstrate 

excellent accuracy, but only for moderately strained hydrocarbons. 

The computations of �H f ° through atomization energies and bond/group separation 

reactions are more trustworthy; atomization energies are more useful for 

computation of the enthalpies of formation of small molecules [79–81]. Bond separation 

(isodesmic) equations proposed by Pople [82] for which the bonds between 

the non-hydrogen atoms are separated into strain-free reference molecules, give 

the strain energy directly (cf. Equation 1.1 for the evaluation of the strain energy 

of cubane). Alternatively, homodesmotic [83] equations (such as Equation 1.2), 

that contain an equal number of groups and bonds on both sides of the same 

type, largely cancel systematic computational errors. These two approaches lead 

to different strain energies (Scheme 1.3). 

The choice of strain-free reference compounds is problematic. The generally 

accepted strain energy of cubane (164.8 kcal mol –1 , without the newest correction 

for its enthalpy of sublimation) was computed through homodesmotic Equation 

1.2. However, the strain energy evaluation in Equation 1.2 is not properly 

balanced because the eight isobutane molecules are stabilized by twelve additional 

1,3-interactions (protobranching) [84] relative to cubane. Thus, isoalkanes 

have ‘negative strain’ relative to n-alkanes, and cannot be used as references for 

strain energy evaluations: using branched alkanes artificially increases the strain 

Scheme 1.3 Isodesmic (1.1) and homodesmotic (1.2) equations to determine the strain energy 

(E st ) of cubane.


of the molecules of interest. On the other hand, Equation 1.1 is properly balanced 

because it is based only on strain-free molecules – methane and ethane. The 

strain energy of cubane calculated via Equation 1.1 is 102.9 kcal mol –1 [85] and 

this value should be used. Equally, other strained hydrocarbons appear to be less 

strained than originally assumed. Concerning the validity of isodesmic Equation 

1.1, one can argue that ethane is also not a strain-free molecule because of van 

der Waals’ contacts while methane is destabilized by Pauli repulsions. As these 

effects are present in all organic molecules these two hydrocarbons are still the 

best candidates as strain-free reference hydrocarbons; there is no conformational 

problem either. 

Computations are the only way to evaluate the strain energies of molecules for 

which the experimental thermochemistry is not available. Chemically accurate 

computations for small molecules nowadays are inexpensive, fast, and they can 

be used with ease. However, there are many sources of systematic as well as nonsystematic 

errors in computational chemistry modeling. Some of them are of 

general character, others are typical only for strain energy evaluations. For instance, 

due to the large number of reference molecules used in the above equations, the 

errors in zero-point vibrational energy (ZPVE) corrections, which are usually 

derived from a crude harmonic approximation model, do not effectively cancel. 

The choice of a proper computational method especially for ‘unusual’ strained 

molecules is critical. Computational chemistry estimates the contributions of 

angular strain well, even at the level of molecular mechanics. Another source 

of strain, nonbonding attractions/repulsions, is more challenging to compute 

correctly as only very expensive state-of-the-art computational methods are able 

to describe them accurately. Popular density functional theory (DFT) methods 

offer numerous functionals with different empiric exchange-correlation terms. 

While some of them are especially designed to describe certain types of interactions 

properly, virtually all of them systematically underestimate or completely 

neglect weak interactions [86]. DFT methods, such as the most popular B3LYP 

functional, give rise to various unsystematic errors and, worse, these increase 

dramatically with the size of the molecules [87, 88]. DFT methods may also exhibit 

some artifacts like electron self-exchange, which affects the electron energies considerably. 

Medium-range electron correlation, which contributes to the energies 

of saturated systems significantly, is poorly described both by local and hybrid 

functionals [89]. All of the above problems lead to unacceptable DFT errors for 

unstrained [86, 89] and, especially, strained [88, 90] molecules with more than ten 

heavy (= non-hydrogen) atoms – the ones for which DFT methods currently are used 

most often [91]. The use of new DFT formulations [92, 93] or a posteriori corrections 

(MP2 or coupled cluster) together with accurate thermochemical methods 

(Gn [94], Wn [95], or CBS [96]) significantly improves the quality of strain energy 

evaluations [79]. Nevertheless, most of the computational errors in strain energy 

evaluations are smaller than the discrepancies resulting from the arbitrary choice 

of strain-free reference states. 

Chemists targeting the preparation of a potentially strained compound are faced 

with the problem of predicting its stability and reactivity. The ‘strain energy per 

15


heavy atom’ or ‘per bond’ is a good starting point but it reveals little about the 

kinetic stability of a molecule. If an appreciable reaction pathway for lowering the 

potential energy does not exist, the molecule may be stable despite being highly 

strained. The extraordinary thermal stabilities of prismane and cubane are associated 

with the feature that breaking just one C–C bond causes only minimal 

changes in the remaining part of the molecular structure. Stabilization of tetrahedrane 

with bulky groups, which hinder rearrangements due to the ‘corset effect,’ 

is another example [97, 98]. 

Computational chemistry can help predict the behavior of strained compounds, 

and there are many inspiring examples (for selection see Scheme 1.4). The unusual 

stability of highly strained [1.1.1]propellane 2 [99–101] (discussed in Section 2.1) 

protonated pyramidane derivatives 33 [102, 103], and the T d -1,2-dehydro-5,7-adamantanediyl 

dication 34 [104] first predicted computationally, initiated successful 

attempts to prepare these highly strained systems. 

Scheme 1.4 Some highly strained molecules, whose anomalous stability was predicted 

computationally before preparation. 

The prediction of the thermal stabilities of strained compounds is a routine, 

albeit elaborate, procedure now and involves computations on the barriers 

of the crucial bond breaking pathway (see, for instance, a recent study on the 

kinetic stability of tetrahedrane) [98]. As the kinetic stability is a relative value, 

i.e. it depends on the reaction partner and the conditions, the barriers for the 

attack of radicals on strained compounds not only allow one to analyze their 

potential stability, but also to choose a proper reagent for their derivatization. For 

instance, cubane [105] and octahedrane [66] were found to be highly sensitive to 

the nature of the attacking radicals and they follow either C–C addition or C–H 

substitution paths. Computations on the reactions of strained compounds with 

electrophiles are more difficult, because the carbocationic species that form after 

primary electrophilic attack are largely prone to rearrangements to release the 

strain. Even reproducing experimental proton affinities is difficult, especially for 

a system as strained as cubane [106]. Cyclopropane is an exception because the 

edge-protonated form is a minimum and the downhill ring-opening path has a 

relatively high barrier [107]. 

Highly strained compounds quite often represent intriguing bonding situations, 

which modern computational methods are able to describe well. They offer not 

only accurate energies and geometries of strained compounds, but also provide 

information about electron density distributions, molecular orbitals, bond critical 

points and so forth [108]. One of the examples is the unusual bonding situation 

between inverted carbons of [1.1.1]propellane 2. Twenty years ago theory predicted


[109] that there is a bond critical point between the central carbon atoms of 2 

despite the fact that the electron density does not accumulate in this region. 

Recent synchrotron experiments on derivatives of 2 confirmed that this remarkable 

computational description indeed is correct [110]. The examples of m-benzyne 

35 and m-dehydrocubane 36 demonstrate the borderline between proper C–C 

bonding and open-shell singlet biradical states (Scheme 1.5). The latter is favored 

for m-benzyne if dynamic electron correlation is included exhaustively: the fundamental 

frequencies of the m-benzyne singlet biradical computed at CCSD(T) 

[111] perfectly agree with the experimental IR spectra [112]. m-Dehydrocubane 

forms a singlet state and is predicted to exhibit an extremely long C–C bond 

(1.844 Å from REKS-B3LYP/6-31G* data) [113]. Unusually short bonds were 

found for the dimers of strained compounds. In 1989 the shortest single C–C 

bond (1.438 Å) was computed [114] for bis-tetrahedrane 37; recently this value was 

confirmed experimentally [115]. The properties of highly pyramidalized alkenes 

are difficult to study experimentally as only some matrix IR-spectra are available 

[116] and computational results are difficult to validate. A real breakthrough in 

this area was achieved recently when it was found that the computed proton affinities 

and heats of hydrogenation of 1,5-dehydroquadricyclane 38 agreed well 

with experiment [117]. 

Scheme 1.5 Selected strained molecules that represent unusual C–C bonding situations. 

Most importantly, computational chemistry can not only predict the properties 

of molecules, but also help to discover new classes of strained compounds that 

may challenge experimentalists (Scheme 1.6). Molecules with planar tetracoordinated 

carbon: fenestranes 39 [118], tricyclo[2.1.0.0 1,3 ]pentane 40 [119, 120], and 

tetracyclo[3.1.0.0 1,3 .0 3,5 ]hexane 41 [121]; with inverted geometries around the 

carbon atoms: pyramidane 42 [122] and bowlane 43 [123]; as well as with highly 

twisted double bonds: orthogonene 44 [124] and tetra-t-butyl ethylene 45 [125], 

Scheme 1.6 Highly strained molecules computationally predicted to be isolable. 

17


were computationally predicted to be isolable and thus represent challenging yet 

realistic synthetic targets. 

The development of the chemistry of strained compounds has been closely 

connected to the progress of computational chemistry for the last three decades. 

We have finally reached a situation where geometries, electronic and thermodynamic 

properties as well as the reactivity of highly strained and ‘unusual’ small 

molecules may be estimated computationally with chemical accuracy. 

1.4 

Gallery of Molecules That Could Have Been Included in This Book 


1.4.1 

Introductory Remarks 

The aim of this monograph is to present the richness of the domain of hydrocarbons 

with unusual spatial structure, not only in chemistry but also in their 

physicochemical properties, and not only in experimental studies but also in model 

calculations that play an increasingly important role in this domain. Clearly, such 

a broad scope, to be understood as a protest against the narrow specialization 

from which we all suffer, could not be fully covered in this limited volume. Thus, 

for various reasons, not all molecules deserving incorporation in this book could 

even be mentioned. To counteract this situation, in this chapter several fascinating 

molecules that have not been presented in other chapters will simply be listed 

with short notes showing why they are of interest. This is of particular importance 

since at least some of them merit further, more detailed studies. 

1.4.2 

Saturated Hydrocarbons 

Of the family of bridged spiropentanes 46, the known [4.1.0.0 1,6 ]tricycloheptane 

(n = 2) 46a is stable and exhibits a considerable widening of the C2C1C7 angle up 

to about 160° [126, 127]. There is NMR evidence of [2.1.0.0 1,3 ]tricyclopentane 46b 

for which ab initio calculations yielded a pyramidal configuration on the central 

carbon atom [128, 129]. On the basis of ab initio quantum chemical calculations, 

QC, exciting tricyclo[3.1.0 1,3 ]hexane 46c has been found to exhibit almost linear 

arrangement of the formally Csp 3 –Csp 3 bonds with the

1.4 Gallery of Molecules That Could Have Been Included in This Book 

178° [130]. Wiberg and Snoonian [131] reported the synthesis of the derivative 47 

observed at 10 K having the highly reactive ketene group. Thus, until now the 

highly unusual spatial structure of 46c could not be proven. 

One of the largest member of the trangulane family is branched C 2v-[15]triangulene 

48. A shortening of the central C–C bond in this molecule has been interpreted 

in terms of a considerable change in hybridization of the two central spirocarbon 

atoms due to severe steric strain [132]. Smaller, but also overcrowded, triangulanes 

also studied by the de Meijere group exhibited some unusual reactivity [133]. 

As shown by boat–twist conformation of the central C 6 ring in trispirocyclopropanated 

cyclohexane 49 [134–136] and by the boat conformation of these rings 

in tetraasterane 50 [137, 138] discussed in Section 2.3, the cyclohexane ring does 

not necessarily have to assume the chair conformation. 

Recently synthesized octacyclopropylcubane 51 is not very stable: it has a half-life 

of 3 h at 250 °C and has ‘tremendous overall strain’ of 390 kcal mol –1 [139]. In the 

crystal it exhibits quite rare C 4h symmetry. The average length of C–C bonds in 

the cubane core of 158.3 pm have been found to be slightly but distinctly longer 

than that in cubane (156.5 pm in the gas phase and 155.1 in the crystal). 

Three examples of interesting stereochemical and/or structural phenomena 

will be given at the end of this subchapter. 

19


Figure 1.2 Bicyclic hydrocarbons. 

A rare but interesting observation, i.e. in, out isomerism in bicyclic hydrocarbons 

(Figure 1.2) [140–142], has also been noticed in several natural products [143]. For 

larger values of k, l and m the molecules exhibit dynamic equilibrium. 

Steric strain may also cause ‘squeezing’ a molecule leading to a very close 

distance between the nonbonded atoms. Since X-ray analysis is not the most 

reliable tool for the determination of H atoms’ position, indirect arguments are 

sometimes used for their estimation as was done for 52 [144]. 

As found for cubane 1 [145] and C 60 53, highly symmetrical structures may 

exhibit interesting dynamic behavior in the solid state [146, 147]. In the latter case, 

it also leads to the structural diversity of host–guest and intercalation complexes 

of the fullerene as studied by X-ray technique [148]. 

It took more than 20 years to synthesize dodecahedron 54 [149] which, due to 

its strain, exhibits quite unusual rearrangement reactions. Remarkably, both 53 

and 54 are of I h symmetry, that is every carbon atom (and hydrogen, respectively) 

in these molecules is identical with the other.

1.4.3 

Distorted Double Bonds 


A detailed discussion of several routes that were expected to lead to highly strained 

tetrakis-t-butylethene 55 but were unsuccessful is given in detail in Ref. [150]. 

Hypothetical bicyclo[1.1.0]-1(4)-pentene 56a and bicyclo[1.1.0]-1(3)-butene 56b 

remain unknown but according to ab initio calculations such molecules should 

have considerably pyramidalized formally C sp2 carbon atoms [151, 152]. 

Pyramidalized carbon atoms should be exhibited in known bridged bicyclobutane 

57 (n = 3) [153–156] and 58 which has probably been observed [157]. 

The smallest synthesized [m][n]betweenanene 59 has m = n = 8 [158, 159]. To 

the best of our knowledge, no X-ray structure determination exists but simple 

MM modeling indicates significant distortions from standard geometry [160]. 

The larger (m = 22, n = 10) not highly strained betweenanenes have been expected 

to exhibit interesting dynamic effects involving ‘a jumping of the longer chain’ 

around [161]. 

Tricyclo[4.2.2.2 2,5 ]dodeca-1,5-diene 60 [162] and its tetraaryl-substituted derivative 

61 [163, 164], both with strongly pyramidalized C sp2 carbon atoms, are 

known, while diene 62, also with very close distance between double bonds, is still 

unknown [165]. Noteworthy, the aromatic rings in 61 are planar. 

21


The name Dewar benzene of 63 is thought to have no sound basis [166]. The 

system is stable in the form of its tri-t-butyl [167, 168] or hexamethyl [169, 170] 

derivatives. Although the highly reactive 63 was obtained more than 40 years 

ago and attracted the attention of several theoreticians [171], the reactivity of this 

molecule and/or its derivatives is still the subject of studies today [172]. 

Out of two possible 64a and 64b diastereomers of [2.2]cyclooctatetraenophane, 

which are present as a (d,l) mixture, the former was synthesized and found to 

isomerize to the latter [173]. 

1.4.4 

Benzene Rings with Nontypical Spatial Structures 

Typical structure of aromatic systems consists in the planarity of the aromatic ring 

and its substituents and in close to 120° value of all bond angles. Steric hindrance 

can force another spatial structure. To the best of our knowledge, highly strained 

hexa-t-butylbenzene 65 (X = C) is not known. However, a less strained derivative 

(due to longer C–Si than C–C bonds), hexakis(trimethylsilyl) derivative 65, exhibits 

an unusual distorted chair conformation [174]. Another, less symmetrical type of 

nonplanar distortion of the aromatic ring is provided by 1,2,3-tri-t-butylnaphthalene 

66 [175]. Considerable strain in the latter molecule allowed for the freezing of 

internal rotations of the methyl groups in 1- and 2-positions at 193 K. Interestingly, 

in disagreement with molecular mechanics [176], modeling the barrier for 

the rotation of the groups at the 3-position was the smallest. It is also noteworthy 

that due to its large size, 66 did not form the inclusion complex with �-cyclodextrin 

but rather ‘sat’ on top of the macrocyclic sugar.


Twisted acenes are schematically presented in formula 67 [177]. Additional 

phenyl groups as in 68 stabilize the molecule and have allowed Pascal to achieve 

a formidable twist of 144° between the planes of terminal aromatic rings [178]. 

Interestingly, 68 has been resolved into enantiomers. Such highly twisted aromatic 

systems can find an application as porous solids. They are also expected to have 

chiroptical properties and have been incorporated into light emitting diodes (www. 

cnsi.ucla.edu/arr/paper?paper_id=193298). According to DFT calculations, 68 is 

a disjointed radical exhibiting exciting electronic structure [179]. 

Latos–Grazynski group reported the synthesis of di-p-benzhexaphyrin that is in 

dynamic equilibrium of two forms: ‘standard’ 69a and 69b representing a topologically 

nontrivial Möbius strip [180]. (Other topological nontrivial molecules are 

presented in Chapter 9.) In the solid state, only the latter was found to exist. 

Cyclophanes [181] (covered in Section 4.2) have been studied mainly because 

of their non-standard structure and a strong �–� interaction between close-lying 

aromatic rings [182] manifesting itself in UV/Vis [183] and NMR spectra [184]. 

23


Syntheses of impressive layered para-cyclophanes called cochins having up 

to six aromatic rings as in 70 and 71 were reported by Otsubo and coworkers 

[185–187] while those of three isomers of four-layered [2.2]metacyclophanes 72–74 

were published by Umemoto [188, 189]. In analogy with the smaller [2.2]metacyclophane 

75 discussed in detail in Section 4.2, protons of C–H bonds situated 

between two bridges exhibit very unusual values of chemical shifts since they lie 

above (or below) the plane of the neighboring aromatic ring. 

In spite of its high strain, superphane 76 [190, 191] is relatively stable, even [2 6 ] 

(1,2,3,4,5,6)cyclophane-1-ene 77 with an additional double bond has been reported 

[191]. The benzene C sp2 carbon atoms in 76 all lie in the respective planes but its 

spatial structure is untypical since not all substituents on the aromatic rings lie 

in the plane of the rings [192]. 78 and 79 still await their syntheses [190].


Corannulene 80 has the shape of a bowl because it includes a five-membered 

ring, and is known to invert rapidly [193]. In addition to its nonstandard geometry 

and dynamic behavior, the molecule attracted a lot of interest since it has been considered 

as an important building block that should enable the organic chemistry 

synthesis of C 60 53. Corrannulene derivatives also exhibit interesting packing 

behavior in the solid state [193]. As discussed in detail in Kawase and Kurata 

review [194] not only bowl-shaped but also ball- and belt-shaped aromatic systems 

provide an exciting opportunity to explore the concave–convex �–� interactions 

by studying their complexation. 

Another type of revealing distortion in aromatic rings consists in differentiation 

of their bond lengths achieved by fusing cyclobutane or cyclopentane rings to them, 

resulting in the remarkable differences in the C–C bond lengths for 81 [195] and 

82 [196]. Such systems are indispensable for studying the limits of aromaticity. 

1.4.5 

Cumulenes 

Interestingly, the cumulenes’ structure was predicted by van’t Hoff [197] who stated 

that in cumulenes with an even number of double bonds the four substituents 

must be placed in two perpendicular planes while for the odd-numbered series 

the substituents must lie in one plane with the double bonds. The cumulenes 

discussed in Section 3.3 are distorted from such arrangements. No X-ray structure 

of bicyclic allene 83 [198] and triene 84 [199] have been published but, according 

to MM modeling, the planes of respective bonds are at angles different from 

zero and 180° [160]. Similarly, to the best of our knowledge no structural data for 

hexaene 85 [200] and pentaene 86 [201] have been published but the molecules, 

with t-butyl groups added to increase stability, definitely do not have the standard 

structure. 

25


1.4.6 

Acetylenes 

Permethylated [5]pericyclyne 87 (R = Me) and larger analogs are known [202, 

203]. Interestingly, in analogy with cyclopentane the central ring in 87 adopts the 

envelop conformation even in the solid state while model calculations indicate 

that in permethylated [6]pericyclyne the central ring [204] can adopt either the 

most stable chair conformation or boat or twist-boat ones. Aromaticity and the 

role of conjugation in 88 and other analogous carbocycles have been studied by 

Lepetit [205]. X-ray spectra of an octaphenyl derivative of 89 [206] reveal the planar 

structure of the bicyclic core with considerable bond angle distortions. 

Only a few of many exciting distorted hydrocarbons could be mentioned in this 

chapter. It should be stressed, however, that with a new domain of macrocyclic 

host molecules rapidly developing this area will expand further since not all 

large macrocycles are strain-free. For instance, 90 can host C 60 53 into its cavity 

assuming C 6v symmetry [194, 207].

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2 

Distorted Saturated Hydrocarbons 

2.1 

Molecules with Inverted Carbon Atoms 

Kata Mlinari�-Majerski 

2.1.1 

Introduction 

The tetrahedral geometry at a saturated carbon atom has been known as the 

only possible geometry for almost a century [1]. However, in the late 1960s 

theoretical predictions and some experimental results suggested other possibilities 

such as planar, pyramidal or even inverted geometry at the carbon [2]. The 

interest in nontetrahedral saturated carbon atoms has been growing very rapidly. 

Nontetrahedral geometries at a tetravalent carbon atom are defined by the four 

interatomic vectors emanating from the saturated carbons. Deformation of the 

conventional tetrahedral arrangement (Figure 2.1a) via an umbrella motion leads 

to the inverted geometry in which all of the four substituents lie in the same 

hemisphere (Figure 2.1b) [3a]. 

As established by microwave spectra [4a], the simplest molecule having the 

carbon atoms with inverted geometry is bicyclobutane (Figure 2.1c) but the 

existence of this unusual feature was only recognized later by Paddon-Row and 

coworkers [4b]. The possible existence of this unusual geometry was explored by 

Wiberg who carried out ab initio calculations at the 6–31G* level, first predicted 

Figure 2.1 (a) Tetrahedral geometry at carbon; (b) Inverted geometry at carbon; 

(c) bicyclobutane; (d) [k.l.m]propellanes. 



ISBN: 978-3-527-31767-7 

33

34 2 Distorted Saturated Hydrocarbons 

properties of small-ring propellanes, synthesized the molecules and confirmed 

the predictions [3, 5a]. 

In particular, the ultimate member of the small-ring propellanes, extensively 

studied by Wiberg [3, 5], [1.1.1]propellane 1 was found to be much more stable 

than higher members of the series [k.l.m]propellanes with k = 2, d; l, m = 1, 2. 

This remarkable molecule represents a landmark in the quest for highly strained 

small-ring organic molecules. 

Propellanes are defined as the tricyclic systems (Figure 2.1d) in which three rings 

are fused together by a common central, bridgehead–bridgehead C–C bond [6]. 

Large-ring propellanes (Figure 2.1d; k, l, m � 3) behave chemically like ‘normal’ 

polycyclic hydrocarbons and their bridgehead carbon atoms have tetrahedral configuration 

[7]. However, reduction of the ring size as in 1–7 leads to the small-ring 

propellanes possessing two inverted carbon atoms in the central bond [3]. Such a 

nonstandard structure leads to several peculiarities. For instance, the small-ring 

propellanes 1–7 are remarkably reactive toward electrophiles and free radicals; 

2, 3 and 5 have remarkable thermal lability while 1 is stable [3]. 

Oxa[3.2.1]propellane 8, is the first reported small-ring propellane [8], and the 

synthesis of the parent hydrocarbon 7 soon followed [9, 10]. The X-ray structure 

analysis of dichloro[3.2.1]propellane 7a proved the inverted configuration on the 

bridgehead carbon atoms [11]. Interestingly, in spite of the distance between the 

bridgehead atoms of 1.60 Å in 1 [5a], the mere existence of the central bond has 

been questioned for long time [2a] and the small-ring propellanes have become 

the subject of many theoretical studies and a great challenge to synthetic chemists. 

Until discovery of the molecules having inverted carbon atoms, in quantum 

chemistry there was no definition of such a useful concept as the chemical bond.

2.1 Molecules with Inverted Carbon Atoms 

One could only carry out calculations for a molecule as a whole. As discussed 

below, introduction of bond critical points allowed the researchers to refine the 

definition [34b, 34c]. This topic is discussed in Section 1.3. 

Several other small-ring propellanes 5, 6 and 7 containing a total of six atoms in 

their bridges as in [2.2.2]propellane [12], [3.2.1]propellane [9–11, 13, 14], and [4.1.1] 

propellane [15–17], respectively, have been prepared. Of these, particular interest 

was paid to [2.2.2]propellane 5 and many theoretical studies have been undertaken 

[18–21]. Prior to the synthesis of 5, Stohrer and Hoffmann had predicted a facile 

cleavage of its central C–C bond [20]. Davidson [21] calculated the rearrangement 

of 5 by applying a variety of theoretical methods (CASSCF, PUHF, MUMP2, DFT, 

UDFT, CI) suggested as methods for obtaining a reliable potential energy surface 

for diradicals. The results obtained suggested that [2.2.2]propellane 5 can exist 

and would have a substantial barrier for opening to the bicyclic diradical or rearrangement 

to dimethylenecyclohexane. However, monosubstituted [2.2.2]propellane 

proved highly unstable [12a]. Recently, the extremely stable perfluoro[2.2.2] 

propellane has been prepared [12c]. 

A more strained geometry than that in 5 was expected in [3.1.1]-, [2.2.1]-, [2.1.1]-, 

and particularly in [1.1.1]propellanes (4–1, respectively). As mentioned earlier, 

even the possibility of their existence was questioned [2a, 18]. However, for the 

[1.1.1]propellane 1 the calculations led to the conclusion that 1 should be more 

stable than the corresponding diradical [19, 20]. Wiberg and Walker found that 

high energy of 65 kcal mol –1 is needed to break the central bond of 1 to obtain the 

bicyclo[1.1.1]pentyl diradical [5a]. Amazingly, that calculation result also suggested 

that 1 should be the most easily prepared and the most stable small-ring propellane. 

This prediction proved correct [5]. 

Indeed, the small-ring propellanes, [3.1.1]propellane 4 [15c, 15d, 22, 23] and 

[1.1.1]propellane 1 [5] or their derivatives [24–26] have been synthesized. On the 

other hand, their homologs [2.2.1]propellane 3 [27–29] and [2.1.1]propellane 2 

[30, 31] were observed either at low temperature, captured in the argon matrix, 

or as inter mediates which were subsequently trapped by reagents to give stable 

products. 

2.1.2 

Small-ring Propellanes: Computational and Physicochemical Studies 

In view of the exceptional role that calculations have played for these compounds, 

this Section will begin with a discussion of theoretical results. As mentioned earlier, 

the unusual properties of the inverted carbon atoms and the mere existence and 

nature of the central bond that connects the two inverted bridgehead atoms in the 

small-ring propellanes have been the subject of many theoretical investigations 

[19, 20, 27, 30, 32–41], X-ray [11, 15b, 42] and electron-diffraction analyses [15b, 

43, 44], vibrational [45], photoelectron [46] and electron impact [47] spectroscopic 

and NMR studies [23b, 48–52]. 

The experimental and theoretical results of many different research groups have 

led to the apparently opposite conclusions on the small-ring propellane structure. 

35


Some researchers even questioned the very existence of the central bond assuming 

that the molecules have biradical structure with lone-pairs directed outside the 

cage [19a, 20, 32]. For instance, according to the early low-level ab initio molecular 

orbital studies on [1.1.1]propellane 1 [19a] there is no evidence for a central bond 

in terms of charge distribution since the sp 4 hybrid orbitals forming the bond are 

directed away from each other and with the zero overlap population in this bond. 

Jackson and Allen [32a] pointed out that 1 is electron-deficient because its HOMO 

is nonbonding or slightly antibonding and there is low electronic density between 

the bridgehead carbon atoms C1 and C3. The C1–C3 bond is considered to be 

formed by three-center two-electron molecular orbitals and termed ‘�-bridged � 

bond’. These theoretical results are supported by an experimental investigation 

of the differential electron density of two [1.1.1]propellane derivatives [44]. On 

the basis of ab initio quantum chemical calculations, Feller and Davidson [35] 

came to the conclusion that 1 is just a strained cage with negligible bridgehead to 

bridgehead through-space covalent bonding. Wiberg, Bader and Lau [34b, c] came 

to a different conclusion by analyzing the bond critical points. Their reasoning 

was based on the assumption that the electronic charge density is a physical 

property of the system and as such it should be model-independent. On the basis 

of an analysis of the second derivative of the electron density determined by all 

occupied orbitals, the authors concluded (1) that the exceptional stability of 1 is 

due to specific three-center integrals and (2) that bonding was found between the 

bridgehead atoms since there is an appreciable accumulation of charge between the 

bridgehead atoms. The results of quasi ab initio PRDDO calculation additionally 

support the conclusion that the bridgehead atoms in 1 are weakly bonded in the 

ground state [38]. Recently, the electron density and bonding between the inverted 

carbon atoms have been studied experimentally on the [1.1.1]propellane derivative 

9 by Luger [39] who discussed different criteria for the existence of a bond. 

The authors discussed the quantitative results obtained by a synchrotron 

experiment and model calculations concluding that ‘the bond critical point was 

found between the bridgehead atoms (C1–C3) in 9. This bond is unusual according to 

topological analysis: it has a bond path with a bond critical point of significant density, 

as is characteristic for covalent bond, but no charge accumulation is evident at the bond 

critical point where the Laplacian is positive’. A bond order n was derived from the 

electron density at the critical point and was found to be 0.71 which is close to 

0.73 as determined by Wiberg [34]. A similar low bond order was determined for 

[2.1.1]propellane 2, while in [2.2.1]propellane 3 bond order was 1.0 and in [2.2.2] 

propellane 5 it was 1.3 [34b]. The nature of bonding in 1 is also discussed on the 

basis of localized MOs and bond indices [40]. LMO and bond indices analyses 

indicate that there is a significant interaction between the bridgehead carbon


Figure 2.2 The strain energy (SE, kcal mol –1 ) of small-ring propellanes 1–3, 5 and bicyclo[1.1.1] 

pentane (10) at the 6-31G*/6-31G* level [34a]. 

atoms in 1 and that about 80% of the total contribution of the central bond 

comes from the HOMO. The authors have also found out that in the series of the 

small-ring propellanes the total bond index increases in the order [1.1.1] < [2.1.1] 

< [2.2.1] < [2.2.2]propellane whereas the HOMO contribution remains almost 

constant [40]. Therefore, the difference is in the electron density of the outer 

envelope of the central propellane bond which most probably influences their 

reactivity. However, the strain energies of these propellanes (of 103 kcal mol –1 , 

106 kcal mol –1 , 109 kcal mol –1 , 97 kcal mol –1 and 67 kcal mol –1 for 1, 2, 3, 5, 10, 

respectively) are very similar (Figure 2.2) [34a]. Although they are all sufficiently 

strained to permit facile reaction, some are quite reactive and other quite stable, 

as is [1.1.1]propellane 1. The high stability of 1 is due to the low exothermicity of 

the reactions of breaking the C1–C3 bond to give bicyclo[1.1.1]pentane 10, still 

highly strained structure. The strain release is less than a third of the strain energy 

and breaking the side bond is forbidden by symmetry [34]. 

Several studies of the strain energy of small-ring propellanes have been reported 

[34, 37]. They point to the charge withdrawal from the neighboring groups to the 

bridgehead region with the increasing strain [34c]. 

A photoelectron (PE) spectroscopic study of [1.1.1]propellane 1 revealed that 

there should be only minute change in the geometry between 1 and its radical 

cation [46a]. This was attributed to the nonbonding or slightly antibonding 

character of the HOMO of 1. PE investigation on the less strained [3.1.1]propellane 

4 [46b] was indicative of a slightly bonding HOMO, while the PE spectra of 

several [n.1.1]propellanes [46c] showed that the energy of the first band depends 

very strongly on n. For n = 1 the ionization energy is around 9 eV, while for n = 3 

and 4 it is around 8 eV. 

The central C1–C3 bond in small-ring propellanes are longer (e.g. 159.6 ± 

0.05 pm in 1) than normal C–C bond (154 pm) whereas the length of the bridge– 

bridgehead C–C bonds of 152.5 pm were found to be close to the bond length in 

a cyclopropane ring (151 pm) [43–45b]. The results of natural bond orbital (NBO) 

analysis of the central bond in [1.1.1]propellane and some [1.1.1]heteropropellanes 

37


suggested that in these compounds the C1–C3 bond lengths are closely controlled 

by p character of the hybrid orbitals [53]. While Jarret and Cusumano [49], on the 

basis of NMR measurements, suggested a low p character of the central propellane 

bond other authors assigned a very high p character to this central bond on 

the basis of coupling constants [50]. Recently, the high-level ab initio calculations 

yielded the values of all NMR shielding and spin–spin coupling constants in 1 

[51]. The computed NMR parameters agree with the experimental values, except 

for the C1–C3 coupling constants. The ab initio studies using the equations of 

motion approach have also been reported J(CC) of the bridgehead bond in other 

small-ring propellanes [52]. 

The controversy regarding the central bond and the nature of bonding interactions 

in the small-ring propellanes still revolves around different explanations 

of the similar experimental and theoretical results. Recently, the comprehensive 

work concerning [1.1.1]propellane and the other compounds containing the 

bicyclo[1.1.1]pentane skeleton was reviewed by Michl [54]. The work on the 

propellane addresses the very important issue of the nature of central (C1–C3) 

bond. The results recently reported by Luger [39] represent a culmination of the 

experimental studies of charge density and, as pointed out by Coppens [55], the 

characterization of this chemical bond is not a closed subject. 

2.1.3 

Small-ring Propellanes: Experimental Results 

2.1.3.1 Preparation and Reactivity of [1.1.1]Propellane 

As predicted by Wiberg [5a], the synthesis of [1.1.1]propellane 1 was successfully 

carried out by reduction of dibromide 11 with butyllithium. However, 1 had 

remained rather inaccessible because of the long and tedious synthesis of precursor 

11, until Szeimies [24a] synthesized 1 in a single step (Scheme 2.1), starting from 

the readily available 1,1-dibromo-2,2-bis(chloromethyl)cyclopropane 12. The latter 

procedure made 1 the most easily prepared small-ring propellane. 

Scheme 2.1 

The method of ring closure with 2 equiv. of alkyllithium was also used to prepare 

the bridged [1.1.1]propellanes 13 and the related propellane 14 [24a, 24b, 25b]. 

In addition, the propellane 13 was also prepared by intramolecular addition of 

carbene to a double bond [25a].


Wiberg and McMurdie [56] reported the formation of 1 by nucleophilic attack 

on 1,3-diiodobicyclo[1.1.1]pentane 15, as shown on Scheme 2.2. 

Scheme 2.2 

The availability of 1 enabled the study of properties and reactivity of that remarkable 

molecule. Most of the properties of 1, first predicted by calculation, have been 

confirmed experimentally. For example, the reaction of 1 with acetic acid led to 

ring opening and formation of 3-methylenecyclobutyl acetate (16, Scheme 2.3). 

The enthalpy of that reaction was measured [45a] and the enthalpy of formation 

of 16 was determined to be 85 kcal mol –1 in good agreement with the theoretically 

estimated value of 89 kcal mol –1 [34a, 57]. 

Scheme 2.3 

[1.1.1]Propellane 1 is known to be stable at the room temperature but it rearranges 

to methylenecyclobutene 17 [5a] at 114 °C or to 1,2-dimethylenecyclopropane 

18 at 370 °C (Scheme 2.4) [24b]. At first, the reason for this difference 

Scheme 2.4 

39


was not understood. Recently, Jarosch, Walsh and Szeimies [58] investigated the 

kinetics of thermal rearrangement of 1 by gas-phase pyrolysis in the stationary 

system and found that the unimolecular reaction leads to dimethylenecyclopropane 

18 and its thermal isomerization product ethenylidenecyclopropane 20. The 

ab initio and DFT calculations of the potential energy surface indicated that the 

isomerization follows an asynchronous reaction path in which two side bonds of 

1 are broken with activation barrier of 40.0 kcal mol –1 . Furthermore, the authors 

showed that the minor product methylenecyclobutene 17 and its thermal isomerization 

product 1,2,4-pentatriene 19 resulted from the side reaction catalyzed by 

the reaction vessel surface (Scheme 2.4). 

Wiberg [59] studied a wide variety of the free radical reactions of 1 and compared 

its reactivity to those of bicyclo[1.1.0]butane 21 and bicyclo[2.1.0]pentane 22. 

Having observed that the reactions of 1 with free radicals were faster then that 

of 21, whereas 22 was relatively inert, the authors concluded that the reactivity 

was not determined by the strain energy relief or the HOMO energy of these 

compounds, but rather by the local charge distribution that could be a more 

important factor. 

A free radical addition represents the most effective route of preparing various 

1,3-disubstituted bicyclo[1.1.1]pentane derivatives 23 (Scheme 2.5) [5b, 24a, 

59–61]. 

Scheme 2.5 

In some cases these reactions lead to oligomers [5b]. The free radicals were found 

to react more readily with 1 than with styrene [62]. Furthermore, it was found that 

the electronic energy transfer to 1 occurs with a rate constant significantly below 

the diffusion-controlled limit (for instance, triplet benzophenone was quenched 

by 1 with a bimolecular rate constant of 9.9 � 10 6 M –1 s –1 ) [62b]. 

In addition, the reactions of 1 with electron deficient alkenes and alkynes have 

been studied and compared with the corresponding reactions of 21 and 22 [5b, 

63]. In these reactions relative reactivity of 1 and 21 varied considerably with the 

used reagent while 22 was again unreactive. 

The bridged [1.1.1]propellanes (for example 13 and 14) behave in a similar 

fashion as the parent propellane 1 [24a, 24b, 25].


2.1.3.2 Preparation and Reactivity of [2.1.1]Propellane and [2.2.1]Propellane 

There are few reports on experimental observation of [2.1.1]propellane 2 [30–31] 

and [2.2.1]propellane 3 [27–29]. These two propellanes are the only members of 

the small-ring propellanes that could not be isolated as the stable molecules but 

were observed in the argon matrix at low temperature (~30 K) by IR spectroscopy. 

The propellanes 2 [30] and 3 [27] were obtained by gas-phase dehalogenation of 24 

and 25, respectively, with potassium. When the matrix was warmed up to about 

50 K the compounds polymerized. When bromine was introduced to the matrix 

of 3, 1,4-dibromonorbornane 26 was obtained (Scheme 2.6). 

Scheme 2.6 

It is also suggested that [2.2.1]propellane 3 is an intermediate in the electrolytic 

reduction of the corresponding 1,4-dihalonorbornanes 25 and 26 [28]. 

Experimental results for 2 and 3 and their derivatives [29, 31b, 31c] showed their 

high reactivity toward free radicals and some organometalic reagents. Recently, 

Jarosch and Szeimies [64] studied thermal rearrangement of 2 by using density 

functional and the ab initio molecular orbital calculations and found that the low 

energy isomerization path proceeds via a retro-carbene reaction to give carbene 

27 (Scheme 2.7). 

Scheme 2.7 

The most favorable consecutive reactions were found to be 1,2-hydrogen 

shifts leading to dienes 28 and 29. A few higher propellanes are discussed in 

Section 3.3. 

41


2.1.3.3 [1.1.1]Propellane as the Precursor for the Synthesis of Other Unusual 

Molecules 

The feasibility of radical addition across the central bond of [1.1.1]propellane 

1 enables polymerization of 1 into functionalized [n]staffanes [n]1. Michl [65] 

reported the synthesis of [n]staffanes (Scheme 2.8) as a new family of the endfunctionalized, 

inert, transparent and straight rods with a van der Waals radius 

of 2.3 Å and a length increment of 3.35 Å, which could be used as a construction 

element [54, 65]. Schlüter reported the formation of the same structures by 

spontaneous polymerization of neat 1 [66]. Also, he reported the first anionicallyinduced 

polymerization of the bridged [1.1.1]propellane 13 [67]. The [n]staffane 

skeleton is expected to be of importance for prospective applications as an electrical 

insulator [54, 68]. 

Scheme 2.8 

[1.1.1]Propellane 1 also served as the precursor for the preparation of an 

unnatural amino acid, 3-aminobicyclo[1.1.1]pentane-1-carboxylic acid which has 

been incorporated into linear and cyclic peptides [69]. 

Next, the truly remarkable molecule of high symmetry, [1.1.1.1]paddlane 30 that 

should have two pyramidal carbon atoms, was reported to be one of the products 

(although only in 7% of yield) in the direct photolysis of 1 in the presence of 

diazomethane [70] (Scheme 2.9). 

Scheme 2.9 

The authors reported the NMR spectrum to be compatible with the proposed 

structure. However, in view of high strain in this molecule, which was estimated to 

be 456 kcal mol –1 [71], the claim that they obtained 30 is extremely surprising.

2.1.4 

New Hypothetical Molecules with Inverted Carbon Atoms 


On the basis of simple model arguments an infinite family of [k.1.1]propellanes 

was found to possess inverted carbon atoms independent of the k value [36a]. This 

is not surprising in view of the bicyclobutane structure. Dodziuk and coworkers 

[36, 72], on the basis of molecular mechanics [36a] and ab initio quantum chemical 

calculations [72], proposed small ring geminanes 31–33 as another group of 

molecules that should have inverted carbon atoms. 

The latter molecules have not yet been obtained but they seem to be plausible 

synthetic targets and challenge for the synthetic chemist. 

Acknowledgments 

I am sincerely grateful to my late husband Professor Zdenko Majerski who 

triggered my interest and love for chemistry of the strained ring compounds. 

I gratefully acknowledge financial support of the Ministry of Science, Education 

and Sport of the Republic of Croatia (grant 098-0982933-2911). 

43


2.2 

Molecules with Planar and Pyramidal Carbon Atoms 


About 90 years after its formulation, van’t Hoff and Le Bel’s hypothesis [73, 74] 

on the tetrahedral orientation of substituents on a tetravalent carbon atom has 

proven its validity and predictive power. In particular, it revealed the molecular 

foundations of chirality and the number of isomers of cyclohexane derivatives. The 

idea was commonly accepted in the 1960s. Therefore, it took the great intellectual 

courage of Roald Hoffmann and his collaborators to propose ‘planar methane’, that 

is a tetravalent carbon atom lying in a plane with its four substituents [75, 76] as a 

part of an plausible organic structure. On the basis of very simple model quantum 

chemical calculations, QC, the authors analyzed prospective hydrocarbons that 

could eventually have an atom exhibiting this configuration. 

Several reviews on the planar and pyramidal carbon atoms have been published 

[77–83]. Most studies in this area dealt with molecules in which heteroatoms took 

part in forcing the planar configuration on a carbon atom [81, 84, 85]. Since this 

monograph is devoted to hydrocarbons, the studies dealing with this mechanism 

of planarization will not be discussed here. 

Interestingly, QC have shown that the planar methane configuration does not 

correspond to a minimum on the potential energy surface [86, 87]. However, the 

latter observation did not hamper the development and the idea of the so called 

planar carbon turned out to be very fruitful resulting, in particular, in vigorous 

research in small ring fenestranes 34 [88, 89] and paddlanes 2 [90, 91] which 

were thought to have carbon atom(s) with the planar configuration. Concerning 

the former group of compounds, QC have shown that [4.4.4.4]fenestrane 34a 

(k = l = m = n = 1), also called windowpane, assumes a nonplanar arrangement 

of substituents on the central atom that is called half-planar (bisphenoidal) configuration 

[92]. This configuration was close to that determined by X-ray for the 

smallest fenestrane synthesized [4.4.4.5]fenestrane 34 (k = 2, l = m = n = 1) [93]. 

As concerns paddlanes 35 [89], using model calculations Wiberg showed that the 

smallest, [1.1.1.1]- and [2.2.2.2]-paddlanes 35 (k = l = m = n = 1 and 2, respectively), 

were too strained to exist [91] (a published synthesis of the former molecule briefly 

discussed in the former section seems unreliable [94]) while the smallest synthesized 

paddlane 36 (k = 10, l = m = n = 2] understandably had a close to tetrahedral 

configuration on the ternary carbon atoms [90].

2.2 Molecules with Planar and Pyramidal Carbon Atoms 

When analyzing the possibility of planar configuration at a tetravalent carbon 

atom Hoffmann group used extended Hückel, EHT, and CNDO/2 calculations 

[75, 76] to estimate the energy difference between the planar and tetrahedral 

configurations of methane 

� �E = E planar – E tetrahedral (2.1) 

equal to 5.5 eV and 10.8 eV, respectively, and discussed two ways in which the value 

can be lowered. The former configuration could be forced either by destabilizing 

the tetrahedral arrangement (for instance by steric hindrance in the system as in 

34 with k, l, m, n smaller than 3) or by stabilizing the planar configuration on the 

carbon atom. With this purpose in mind he discussed the bonding in square planar 

methane in the valence bond framework. Within this model two sp 2 hybrids at the 

carbon are engaged in normal two-electron two-center bonds with two hydrogen 

atoms making use of two out of the four carbon valence electrons. The third of 

the sp 2 hybrids forms a two-electron three-center bond with the remaining two 

hydrogens using only the hydrogen electrons, while the remaining two valence 

electrons of carbon are placed in the 2p orbital perpendicular to the molecular 

plane. Equivalence of all CH bonds results from resonance among equivalent 

structures with different positioning of three-center and two-center CH bonds. 

Such a procedure or analysis of the planar methane orbitals lead to the following 

conclusions: 

1. All CH bonds in planar methane are weaker than in tetrahedral one with the 

average bond order of 3/4 in the former case. In terms of MO, there are only 

six bonding electrons in the planar configuration while there are eight in the 

tetrahedral one leading to a significant � bond weakening. 

2. There is a considerable electron transfer from hydrogens to carbon locating 

considerable electron density on the latter atom. 

3. There is a pure 2p electron lone pair perpendicular to molecular plane. 

4. Two deformations of the tetrahedral methane (T d � D 2 � D 4h and T d � D 2d � 

D 4h ) which represent a symmetry allowed process can transform it to the planar 

one. 

Conclusions 2 and 3 formed the basis of the strategy that should lead to a carbon 

atom lying in a plane with its four substituents. The substitution of hydrogen atoms 

by good electron acceptors, like C�N, should cause delocalization of the lone pair 

reducing �E value (EHT method) to 3.4 eV. An alternative procedure consisted in 

45


incorporation of the lone pair to form (4n + 2) �-electron system as in the �-cation 

of an aromatic anion 37 or benzonium ion 38 resulting in �E (EHT) values of 4.2 

and 2.9 eV, respectively. Substitution of hydrogen atoms by less electronegative 

groups yielded further �E lowering to 1.8 eV in C(BH 2) 4 and 2.9 eV for C(SiH 3) 4 

when 3d orbitals on Si were included. In the latter case Si simultaneously acted 

as a �-donor and �-acceptor. Both factors acting in the direction of favoring the 

planar configuration at the central carbon operate in hypothetical tetrasila[5.5.5.5] 

fenestrane 39. The effect of electronegativity differential should be even stronger 

for Li substitution or replacement of carbon by N + . 

Purely qualitative criteria were invoked to reject unsaturated fenestranes 40–42 

as candidates for an effective �-stabilization while 43–45 were found to be especially 

promising. 

As mentioned before, vigorous activity in the domains of fenestranes and 

paddlanes did not produce hydrocarbons having either pyramidal or planar carbon 

atom. Even if those attempts were unsuccessful, nevertheless they show that a 

incorrect prediction can lead to exciting chemistry. 

By playing with molecular Dreiding models Dodziuk has found that cyclooctane 

in the crown conformation could accommodate a carbon atom in the planar 

configuration. The resulting molecule was dubbed bowlane [95]. Subsequent 

molecular mechanics, MM, [96, 97] calculations [95] yielded a structure 46 of C 4v 

symmetry with the pyramidal carbon atom. A whole family of such molecules

2.2 Molecules with Planar and Pyramidal Carbon Atoms 

46–51 was then studied using MM and semiempirical quantum chemical calculations 

[98] (yielding intermediate configurations between the distorted pyramidal 

and tetrahedral ones) and together with two ‘dimers’ 52 and 53. The former was 

expected to have the planar configuration on the central carbon atom since the 

upper and lower ‘halves’ were thought to force the desired bonds configuration. 

However, these expectations were not fulfilled, resulting in C 4v symmetry and 

both 52 and 53 were found to be highly strained. 

On the other hand, ab initio quantum chemical calculations for the parent 46 

using STO-3G and 6-31G* basis sets carried out by McGrath et al. [99] yielded a 

structure of a lower C 2v symmetry as an energy minimum with the bonds connecting 

the apical carbon atom with its neighbors departing only slightly from the 

planar arrangement. Similarly, HF/6-31G* calculations for 52, named octaplane, 

carried out by the latter group [100] showed that the S 4 structure corresponding 

to a local minimum represents ‘the closest approach to planarity for a tetracoordinated 

carbon atom in a neutral saturated hydrocarbon reported today’. The most 

interesting features of the calculated minimum structure of 52 were considerable 

bond length distortions (up to 161.5 pm), moderate energy cost for achieving 

planarity of ca. 70 kJ mol –1 calculated at MP2/6-31G*//HF/6-31G* level and the 

fact that the highest occupied molecular orbital is essentially a pure lone pair orbital 

localized on the quaternary carbon atom. By capping the central fragment with 

two cyclobutanes 55, cyclohexanes in the boat conformation 56, or cyclooctane 

47


in the crown conformation 52 (that is earlier proposed bowlane ‘dimer’ [98]) the 

authors arrived at the family of alkaplanes [100, 101]. 

The aim of designing theoretically a neutral hydrocarbon with a planar tetracoordinated 

carbon atom was achieved by Radom and Rasmussen [101] when 

dimethanospiro[2.2]octaplane 54 was obtained by linking two opposite pairs of 

adjacent carbon atoms � to the central one and adding methylene bridges connecting 

the top and bottom caps. The calculations at the MP2 level using 6-31G(d) 

basis set resulted in D 2h symmetrical minimum structure having the planar configuration 

on the central carbon atom with the longest C–C bond of 159.1 pm and 

the shortest one in cyclopropane rings of 144.5 pm. By preliminary exploration 

of a number of decomposition pathways corresponding to the lowest vibrational 

modes, the authors checked that, in spite of high degree of strain, the structure 

found for 54 appeared to lie in a relatively deep potential well. Similarly to 46, the 

HOMO in 54 has predominantly p-type lone pair character localized at the central 

quaternary carbon atom and surrounded by the hydrocarbon cage. Interestingly, 

the latter molecule has an extremely low ionization energy of ca. 5 eV comparable 

to those of alkali metals Li and Na. 

Bowlane 46 also formed the basis of a hypothetical hemispiroalkaplanes family 

57–59 consisting, among other of cyclohexane in the boat conformation, norbornane 

or cyclooctane in the crown conformation fragments capped with bent 

spiro[3.3]pentane unit [102]which also contained a pyramidal carbon atom. 

To summarize, up to the present theoretical studies of molecules that should 

have planar carbon atom(s) have been carried out mostly at the HF level using

2.3 A Theoretical Approach to the Study and Design of Prismane Systems 

rather small basis sets. The MP2 calculations have been carried out at best. It 

should be stressed that DFT calculations seem not to be fully reliable, in particular 

when all the rearrangement/dissociation pathways are analyzed. 

Until now no neutral hydrocarbon with a planar carbon atom has been synthesized. 

Hoffmann’s bright idea of molecules having such unusual spatial structure 

still awaits its realization. 

2.3 

A Theoretical Approach to the Study and Design of Prismane Systems 

Tatyana N. Gribanova, Vladimir I. Minkin and Ruslan M. Minyaev 

2.3.1 

Introduction 

The aesthetically attractive highly symmetrical molecular structure and unusual 

properties of prismanes ensure that they continue to draw the attention of both 

experimentalists and theoreticians [103–105]. The starting point for the extensive 

studies of these compounds was the synthesis of cubane [106, 107] which initiated 

the progress of a specific area of these highly strained, albeit kinetically stable 

compounds [107–111]. In contrast with cubane, the experimental study of two 

other currently synthesized members of this class, triprismane and pentaprismane, 

is much scarcer. Theoretical investigations of prismane systems come to 

the fore more and more actively. Whereas in the first years of the development 

of prismane chemistry the theoretical work was mostly limited to the analysis of 

geometries and steric strain of the synthesized compounds [104, 109], the modern 

quantum chemical calculations of prismanes based on more sophisticated and 

reliable methods, and more powerful computers are focused on the design of new 

compounds with interesting properties. This review concentrates on the most 

important and interesting results of the computational chemistry of prismane 

systems. In some cases the calculations performed served as essential additions 

to the experimental data and their explanation. In other cases, they led to the 

formulation of new experiments and opened the way to new structural motifs. 

Special attention is given to the studies related to the theoretical design of new 

hypothetical prismanes. 

2.3.2 

Prismanes 

The molecules of C 2nH 2n prismanes, formed by two parallel regular n-gons 

connected by rectangular faces, are highly strained cage systems in which the bond 

angles differ considerably from the tetrahedral angle of sp 3 -hybridized carbon. 

The first three members of the prismane series have so far been synthesized: 

triprismane 60 [112], cubane 61 [106, 107] and pentaprismane 62 [113]. Attempts 

49


to synthesize hexaprismane 63 have not yet been successful although certain 

progress in this area has been reported [104, 114]. At the same time, according 

to ab initio [115], DFT [116] and molecular mechanics [117] calculations, hexaprismane 

is one of the least stable members of C 12H 12 family with low probability 

of being synthesized [117]. 

Prismane systems provoked the interest of researchers by their aesthetically 

attractive highly symmetrical structure and extraordinary kinetic stability. The 

structural and electronic properties of cubane were investigated using various 

experimental techniques and quantum chemical calculations and the results 

achieved in both directions have been reviewed [103–105, 108–111]. A novel 

focus of the recent work is the insight into structural and dynamic properties of 

solid cubane [118]. The structural parameters of substituted triprismanes and 

substituted pentaprismane were determined by X-ray crystallography and for the 

parent hydrocarbons with help of quantum chemical calculations (see [119–122] 

and references cited therein). There is a good agreement between the calculated 

and experimental results (Figure 2.3). 

The higher prismanes remain a subject of only theoretical study [119, 122, 127, 

128]. The primary challenges are elucidation of the stability limits of the prismatic 

structures with D nh -symmetry and the trends in the geometry and energy characteristics 

with increase in n. According to higher level B3LYP/6-311G(2df,p) 

computational results [122], the highest member of the [n]prismane family is represented 

by decaprismane C 20 H 20 . The principal components of strain energy of 

prismanes are angular strain in the n-membered cycles and in the four-membered 

faces. The calculated SE of cubane (Figure 2.3) is close to the experimental value 

[129] of 157 kcal mol –1 (see, however, discussion in Section 1.3). Penta prismane 

62 is characterized by the lowest strain energy and heat of formation (�H f ) and 

is the most stable molecule within the family [105, 119, 122, 128]. For the other 

members of the series, an increase in n is accompanied by an increase in the 

SE and �H f values and correlates with an increase in the number of strained 

tetragonal faces. Destabilization of the higher prismanes is caused by influence 

of the lateral four-membered cycles as well as by the eclipse of vicinal hydrogens 

[119, 122]. 

A significant contribution to the strain energy of prismanes is their �-antiaromaticity 

[130]. Analysis of �-aromaticity of 60–64 by NICS indices shows that cubane 

61 may be considered to be a ‘super �-antiaromatic’ system [130]. The high strain 

of prismanes determines their low thermodynamic stability: all members of 

the family are strongly destabilized relative to other isomeric forms. The main 

reason for the high kinetic stability of prismanes is that their rearrangement and 

decomposition reactions are forbidden by the rules of conservation of orbital 

symmetry. Stability of the D nh-symmetrical prismane structures is determined 

by the stabilized orbirtal � �–� � interactions between the parallel n-membered 

fragments [131]. 

The CH bonds in prismanes are shortened because the carbon AO has a high 

percent of s-character; this is caused by rehybridization of the sterically strained 

carbon center [113]. The increase in ‘n’ leads to weakening of the CH bonds.


Figure 2.3 Structural characteristics and strain energies (SE) of prismanes calculated [122] 

by the B3LYP/6-311G(2df,p) method. The experimental data obtained by X-ray crystallography 

(italic) for the derivatives of 60 [123] and 62 [124] and by electron diffraction (italic) [125] and 

microwave spectroscopy (bold italic) [126] for 61. Here and in other figures bond lengths are 

given in angström, angles in degrees, strain energies in kcal mol –1 . 

51


Figure 2.4 Calculated [136] and determined experimentally [134, 138] structural characteristics 

of nitrocubane 68 and octanitrocubane 69. 

According to quantum-chemical calculations [132, 133], substitution of hydrogen 

atoms in prismanes by substituents possessing �-donor and �-acceptor properties 

(Li, BeH, BH 2 ) provide, as a rule, for the lowering of the strain energy. The 

�-donating substituents (such as NH 2 ) exert a similar effect. 

As discussed in Section 1.2 nitrocubanes are of great interest as high-energy 

systems suitable for the use as explosives and fuels [108]. Many nitrocubanes 

including hepta- and octanitro derivatives have been synthesized (see [134]). The 

important factor governing the structure and relative stability of nitrocubanes is 

the repulsive interaction of the nitro-groups, which resuls in a decrease in stability 

upon increase in the degree of replacement [135, 136]. According to RHF and 

B3LYP calculations [135], the highest nitrocubanes are characterized by highest 

values of SE and �H f . The calculations of octanitrocubane lead to the conclusion 

on very low rotation barrier (0.006 kcal mol –1 ) of nitro groups, which can be 

described as cooperative disrotatory motion of two subgroups located in the apexes 

of the carbon tetra hedrons [136, 137]. The calculated structural parameters of 

nitrocubanes agree well with those determined by an X-ray study (Figure 2.4). 

2.3.3 

Expanded Prismanes 

2.3.3.1 Asteranes 

The steric strain of prismanes can be lowered considerably by expanding their 

cages by introducing additional atomic groups. Examples of such expanded 

prismanes are asterane molecules formed by inclusion of CH 2-groups in the 

lateral edges of prismanes.


Only [3]-asterane 70 [139] and [3]-asterane 71 [140] have been synthesized. Also 

bi[4]asterane representing the class of fused compounds (see Section 5.2) has been 

preparatively isolated [141]. Like prismanes, asteranes are characterized by high 

kinetic stability: for example, 4-asterane is stable at temperature higher than 300 °C 

[141]. According to the B3LYP/6-311G(2df,p) calculations [122], only asterane with 

n = 3–7 are characterized by stable structure of D nh-symmetry (Figure 2.5). The 

calculated geometric parameters of 70 agree well with the data obtained by the 

electron diffraction study [142]. The calculated SE values of 70–74 (Figure 2.5) 

are notably lower than those for the prismane analogs 60–64. Changes of the 

strain energies in the asterane family are similar to the trends which have been 

found for the prismane systems. In contrast to the prismanes which experience 

the destabilizing influence of the sterically strained lateral four-membered cycles, 

the main factor destabilizing higher asteranes is repulsive interaction between 

hydrogen atoms of the adjacent methylene groups [122,143]. 

Figure 2.5 Structural parameters and strain energies of asteranes 70–74 calculated [122] by the 

B3LYP/6-311G(2df, p) method. Experimental data for 70 (italic) taken from Ref. [142]. 

53


The dehydrogenated asterane frameworks may be used for the formation of 

hypothetical hydrocarbon cages like 75, 76 containing hypercoordinated centers 

[144–146]. The stability of such systems is governed by 8e rule [131]. The regulation 

of charge may occur by variation of basal and bridged atoms and allow the 

design of various non-classical structures [145, 146]. 

2.3.3.2 Ethynyl-expanded Prismanes 

Another interesting type of expanded prismane is ethynyl (77, 78) and diethynylexpanded 

prismanes (79, 80) of which only a derivative of 80 has been synthesized 

[147]. These molecules fall into the category of nonlinear acetylenes discussed in 

detail in Chapter 7. Unlike cubane, synthesized diethynyl-expanded cubane is 

very unstable. Preparative yield of this compound is extremely low, which made 

measurement of its physical properties and calorimetric study difficult or even impracticable. 

However, many important characteristics of the expanded prismanes 

have been obtained by quantum chemical calculations [148–150]. 

According to DFT and MP2 calculations [148, 150] of 77 and 78, the inclusion 

of ethynyl groups into the C–C bonds of triprismane and cubane provides for 

expansion of the valence angles at tetravalent carbon atoms, which values are 

close to that of a tetrahedral angle. The flexibility of the diine fragments leads to 

the formation of the structures with bent bonds. The strain energies of 77 and 78, 

calculated at the MP2/6-31+G* level, are 58.5 and 33.3 kcal mol –1 , respectively, 

and are much lower than those of the parent hydrocarbons. Another interesting 

feature of the expanded systems is the increase in their acidic properties compared 

with parent prismanes; this is caused by the presence of triple bonds enhancing 

the acidity of adjacent protons. The further expansion of the prismane framework 

(systems 79, 80) results in lowering of free energies of the deprotonation and steric 

strain. The structural distortion of the diethynyl fragments leads to a significant 

rise of their reactivity and can be considered as one of the factors of the kinetic 

instability of preparatively accessible diethynyl-expanded cubane [149]. Expanded


prismanes offer a great opportunity to create systems with specific absorption and 

interesting optoelectronic properties [148–150]. In contrast to prismanes, whose 

small size and high steric strain make the formation of endohedral complexes 

difficult [151], the inner cavity of the expanded prismanes allows them to form 

complexes with alkaline and alkaline-earth metals and even with small neutral 

molecules [148–150]. 

2.3.4 

Dehydroprismanes 

An interesting type of extraordinary cage compounds, whose structure is based 

on prismanes is prismenes – the compounds of highly pyramidalized olefin class. 

The possibility of the existence of 1,2–dehydrocubane (cubene) 81 was discussed 

on the basis of ab initio calculations [152, 153] which showed that, in spite of the 

presence of the highly pyramidalized double bond, preparative isolation of this 

compound is possible. This conclusion was corroborated by the experimental 

detection of cubene [154]. 

The calculated [153] characteristics of 81 (olefin strain energy (OSE) = 

58.9 kcal mol –1 ; heat of hydrogenation = 82.5 kcal mol –1 ) are in a good agreement 

with experimental [155] data (OSE = 63±4 kcal mol –1 ; heat of hydrogenation 

= 90±4 kcal mol –1 ). The presence of the strongly distorted double bond in 81 

leads to the increase in the strain energy (227±4 kcal mol –1 ) compared with 

cubane. The calculations indicate that there is sufficient overlap of the p-orbitals 

to consider cubene as an olefin and not as a biradical. According to experimental 

data, exceptionally reactive cubene is capable of reactions typical of olefins [156]. 

In agreement with expectations, ab initio and DFT calculations [157, 158] testify 

for the highest stability of ortho-cubene 81 compared with meta- and para-isomers, 

82 and 83. In all three cases the ground singlet states are well separated from 

the corresponding lowest lying triplet states. Like 81, 83 was generated upon 

treatment of dihalocubanes with organolithium compounds [159, 160] and it has 

been suggested that 82 might be preparable by the same type of reaction [157]. 

According to theoretical and experimental data, 83 does not contain a diagonal 

C–C bond, the most probable state of 83 is a singlet biradical [157, 159, 160]. 

The highly pyramidalized and even more strained dehydrotriprismane (triprismene) 

was studied by ab initio methods [161]. Of two possible triprismene isomers 

the most stable is the structure 84, which can be considered as an olefin. A tentshaped 

isomer 85 is slightly destabilized relative to 84. The calculations indicate 

kinetic stability of triprismene and the possibility of its experimental detection. 

However, attempts [162] at such detection have so far been unsuccessful. 

55


2.3.5 

Polyprismanes 

2.3.5.1 Cubane Oligomers 

An essential property of cubane derivatives is their tendency to form multidimensional 

structures [163–175]. The series of polycubanes have been obtained 

by nucleophilic addition to 1,2-dehydrocubane and 1,4-dehydrocubane [164–166]. 

The simplest system of such type is cubylcubane 86 (Figure 2.6). According to 

experimental and theoretical data [165], a remarkable structural feature of 86 

is the unusually short C–C bond linking two cubane fragments; such systems 

are discussed later in Section 2.6. The high s-character of the exocyclic orbitals 

of cubane leads to the formation of more compact bonds compared with those 

formed by sp 3 -hybridized carbon atoms. 

The p-[n]cubyl oligomers are sufficiently rigid rod-type systems (analogous to 

staffands presented in Section 2.1.3.3) retaining geometrical parameters of the cubane 

fragments since stretching or angle distortions of the intercage bonds require 

significant energy costs [164]. According to PWSCF [168] and B3LYP/6-311G** 

[169] calculations on the polycubane one-dimensional chain and two-dimensional 

network, such properties as ionization potential and electron affinity are only slightly 

dependent on the number of building blocks involved. Cubane chains maintain 

the electronic properties of insulator; at the same time they have very interesting 

elastic qualities and are of interest in the development of electronic nanodevices 

[168]. As shown by DFT/PW91 calculations, inclusion of electron-enriched functional 

groups (NO, NS) in cubane chains favors electronic conduction [170]. 

An example of a three-dimensional polymeric cubane derivative is supercubane 

(C 8) n, proposed as an allotrope of carbon [171–175]. The simplest non-periodic 

supercubane model is percubylcubane 87 (Figure 2.6) which was studied by 

Figure 2.6 Calculated structural parameters of cubylcubane 86 and percubylcubane 87. 

Experimental data for 86 taken from Ref. [165].


HF/STO-3G [174] and PM5 [175] calculations. New structures termed cubanoids 

were computationally designed on the basis of percubylcubane [175]. 

2.3.5.2 Fused Prismanes 

The formation of fused systems joined at n-membered faces serves as another 

way for the formation of polymeric prismane compounds, which may be termed 

poly[n]prismanes [176–179] (such as bi[n]prismanes 88–91). The unusual structural 

feature of poly[n]prismanes is the presence of carbon centers with nonclassic 

bisphenoidal configuration of bonds. 

Bi[n] and tri[n]prismanes (n = 3–6) have been studied using DFT calculations 

[177]. Although poly[n]prismanes are strongly destabilized with respect to their 

‘classic’ isomeric forms, calculations indicate their kinetic stability and possible 

experimental detection. The origin of their relatively high stability is to be found in 

the strong � � –� � orbital interaction between the symmetry adapted frontier MOs 

of the external annulene (CH) n and the inner C n rings. Increase in the number of 

fused cages leads to increase in strain compared with prismanes. Maximal steric 

strain is observed for the first members (n = 3, 4) of each family and for each n the 

strain increases with increase in the size of the structure. The decrease in stability 

of poly[n]prismanes on passing from prismanes to their bi, and tri-congeners 

also manifests itself in progressive decrease of the energy gap between frontier 

orbitals along this set of compounds. The increase in the number of fused cages 

is accompanied by the further lowering of the stability [178]. The interesting 

characteristic of poly[n]prismanes is their auxetic property, i.e. the ability of these 

molecules to become thicker by longitudinally stretching and thinner by compression. 

According to quantum-chemical calculations, poly[n]-prismanes are the first 

systems that manifest auxetic behavior at the molecular level [179]. 

‘Mixed’ polycage prismane derivatives 92–94 containing pentaprismane and 

dodecahedron cages fused by five-membered faces were proposed on the basis 

57


of B3LYP/6-31G* calculations [180–182]. Increase in the number of fused cages 

leads to increase in strain and decrease in stability of these systems compared 

with pentaprismane and dodecahedrane. The possible experimental preparation 

of fused prismanes is supported by successful synthesis of bi[4]asterane 95 and its 

substituted derivatives [141, 183]. In contrast with [4]asterane 71 which is stable at 

temperatures over 300 °C, the transformation of 95 occurs at a lower temperature 

(270 °C) [141] confirming the theoretical conclusions about the decrease of stability 

of fused systems upon increase in the number of building blocks. 

It is also possible to form fused polyprismane structures by fusing fourmembered 

faces of the cage systems. In contrast with cape-fused prismanes 

containing nonclassical bisphenoidal carbon centers, the face-fused prismanes are 

characterized by the presence of carbon atoms with nonclassic inverted (umbrella) 

configuration of bonds. Such molecules called propellanes are discussed in detail 

in Section 2.1. The existence of bipentaprismane 96 was predicted using ab initio 

and DFT calculations [184, 185]. Even more unusual example of face-fused poly[5] 

prismanes 97 is a hypothetical product of multiple pentaprismane splicing, in 

which the [2+2] cycloreversions of cyclobutane rings, leading to the series of 

isomers with pairs of parallel �-bonds interconverting by Cope rearrangement 

may be realized [186]. By analogy with ‘mixed’ cape-fused polyprismanes the 

‘mixed’ face-fused systems can be formed. The stable system 98 investigated [187] 

by B3LYP/6-31G* method is derived from cubane and two pentaprismane cages 

fused by two four-membered faces. Based on the results of the computational 

studies of the fused prismanes, one can conclude that an increase in the number 

of fused cages is accompanied by increase in steric strain, decrease in lowest 

vibration frequencies and HOMO-LUMO energy gap. All these effects lead to 

lowering in the stability of designed complicated systems. 

2.3.6 

Conclusions 

The comparison of theoretical and experimental data indicates that theoretical 

methods can be a reliable tool for comprehensive investigation and design of 

various molecules constructed on the basis of prismanes. Prismane systems, 

which in themselves are unique molecules with nonstandard stereochemistry and 

properties, can form series of novel unusual systems such as dehydroprismanes 

representing a class of pyramidalized olefins; supercubane as an allotrope of 

carbon; fused prismanes, including nonclassical bisphenoidal and inverted carbon

2.4 (CH) 2n Cage Structures, ‘in’-‘out’ Isomerism in Perhydrogenated Fullerenes 

centers, as well as asterane derivatives, containing hypercoordinated centers, etc. 

Various modifications of prismanes lead to the appearance of new intriguing properties. 

Poly[n]prismanes are expected to show auxetic behavior; ethynyl-expanded 

prismanes can reveal useful optoelectronic characteristics; nitro-prismanes are 

good candidates for powerful, high-density explosives; prismanes networks are 

of interest in the development of nanoarchitectures. 

The variety of the systems generated on the basis of prismanes is not exhausted 

(for example, an extensive class of heteroprismanes [128]) and requires further 

studies, which open the way to create new materials with unusual properties. 

Although the majority of reported studies correspond to cubane derivatives, 

available theoretical data reveal similar tendencies in behavior and properties of 

other members of the family. The principles of formation of the lowest prismane 

derivatives can be extrapolated to the area of more complex compounds. Synthesis 

of the molecules considered in this chapter seems to be a great challenge for 

chemists and the success in the field of preparation of numerous cubane derivatives 

(in particular, of octanitrocubane [138]) is an impressive manifestation of 

progress and rich opportunities of modern experimental techniques. The data 

presented show that many hypothetical systems can be suitable objects for the 

synthetic quest and we believe that results of quantum-chemical research help to 

invigorate new experimental efforts in this field. 


This work was supported by Russian Foundation for Basic Research (grant 

07-03-00223), RF Ministry of Industry and Science (grant 4849.2006.3). 

2.4 

(CH) 2n Cage Structures, ‘in’-‘out’ Isomerism in Perhydrogenated Fullerenes and 

Planar Cyclohexane Rings 


2.4.1 

(CH) 2n Cage Structures 

Studies pertaining to saturated cage compounds of a general formula (CH 2 ) n can 

be traced back to the ancient Greeks since the ideal polyhedra: the tetrahedron, the 

cube and the dodecahedron, which represent the carbon skeletons of the molecules 

with n = 2, 4 and 10, were discussed by Plato and Pythagoras [188]. The molecules 

are formed by 2n carbon atoms each of which is connected to three carbon and one 

hydrogen atoms. For n = 2 and 3 there is only one possible isomer: tetrahedrane 

99a and triprismane 60, respectively, while for larger n the number of isomers 

increases. Cubane 61 (discussed in detail in Section 2.3), cuneane 100 and octabis- 

59


valene 101 are the three isomers for n = 4 and the number of possible structures 

increases rapidly with the increase of n; for instance, it is equal to 9 for n = 5, 32 

for n = 6, etc. Of the higher members of the series only pentaprismane 62, and 

diademane 102 with n = 5, heptacyclo[6.4.0.0 2,4 .0 3,7 .0 5,12 .0 6,10 .0 9,11 ]dodecane 103 

(n = 6), dodecahedrane 104 (n = 10) and a derivative of perhydrogenated fullerene 

C 60 X 60 105 (n = 30, X = F) have been synthesized. These molecules are highly 

strained hydrocarbons exhibiting, among other, nontypical rearrangement [189, 

190] (Figure 2.7) and cyclization reactions [191]. The rigid structure of saturated 

cage compounds means that they can serve as ideal models in studies both of the 

angular dependence of the coupling constants, and of dependence of J( 13 C–H) on 

the hybridization (see Section 2.6). 

Figure 2.7 Untypical rearrangement reactions of cubane into cuneane (a) and pagodane into 

dodecahedrane (b).


61 

2.4.1.1 Tetrahedrane 

The elegant molecule tetrahedrane, 99a C 4 H 4 (n = 2), has attracted the attention 

of synthetic and theoretical chemists for almost 100 years [192, 193] and the 

first review on this molecule [194] was published almost at the same time as the 

synthesis of its tetra-t-butyl derivative 99b [195]. Then for a long time, the latter 

molecule was the only tetrahedrane derivative known [196]. A tetrasiladerivative 

99c and other siladerivatives with much smaller substituents 106 (X = Li, CH 3 , 

H, C(SiMe 3 ) 3 ) as well as tetrahedryltetrahedrane 107 (X = SiMe 3 ) [197] followed. 

Syntheses of these molecules have recently been reviewed by de Meijere and 

coworkers [198]. Physicochemical studies of 99b included X-ray [199], spectral [200] 

and NMR [201] measurements. Its high kinetic stability was partly ascribed by 

Minkin [202] to unfavorable steric repulsions in its rearrangement product 108b.


Other theoretical studies ranged from molecular mechanics, MM, [203] prediction 

of T symmetry of 99b [204] to high level quantum chemical, QC, calculations 

indicating that 99a should be isolable [205]. However, unlike cyclobutadiene 108a 

[206], discussed in Chapter 10, unsubstituted tetrahedrane remains unknown. 

The NMR coupling constants in 99a, 60, 61, 62, 102 and hexaprismane 63 were 

calculated by Krivdin [207], while of several calculations analyzing reasons which 

cause shortening of the linking C–C bond in tetrahedranyltetrahedrane 107 

(Y = H), recent work by Mo should be mentioned [208]. 

2.4.1.2 Triprismane 

Triprismane 60 C 6 H 6 (n = 3) [209], also called [3]prismane, is discussed in 

Section 2.3 together with other prismanes. 

2.4.1.3 Cubane 61, Cuneane 100 and Octabisvalene 101 C 8H 8 

Cubane 61 is discussed mainly from theoretical point of view in Section 2.3, so here 

only few aspects of its structure and properties will be covered. This highly strained 

molecule is remarkably stable since it corresponds to a very deep narrow minimum 

on the potential energy surface. Its chemistry, physicochemical properties and 

theoretical studies have been reviewed by Griffin and Marchand [210] and by Hassenrück 

and coworkers [211]. Its high symmetry allowed the Hedberg group [212] 

to compare the accuracy of electron diffraction [213], microwave [214] and X-ray 

[215] determinations of C–C bond length. The unusual skeletal rearrangement of 

61 into 100 (Figure 2.7) was mentioned above. An interesting model of cubane 

hydrogenolysis was discussed by Stober et al. [216]. Several interesting cubane 

derivatives are discussed in Section 2.3. They include known propella[3 4 ]prismane 

109 [217] and cubene 81 [218] (which lies outside the scope of this chapter but was 

presented in the former section). The trimethylene bridges in the former molecule 

are very flexible leading to only three signals in 13 C NMR spectrum [217].


63 

Cuneane 100 and octabisvalene 101 have been much less studied [189, 211, 

219, 220] and the existence of inverted carbon atoms (discussed in Section 2.1) 

in the latter molecule, which can be considered as a bicyclobutane dimer, has 

been mostly overlooked [219]. The X-ray structures of octamethylcubane and 

octamethylcuneane have been determined by Irngartinger [221]. A large group 

recently carried out a calorimetric, crystallographic and computational study of 

61, 100 and their carboxylates [222]. 

2.4.1.4 C 10 H 10 Saturated Cages 

Pentaprismane 62 [223] and diademane 102 [224, 225] are the only known members 

of this family. For the former molecule, only photoelectron spectra [226] and X-ray 

analysis [227] are known. For the latter, several centrally bridgehead-substituted 

derivatives have been obtained recently [228]. According to semiempirical [229] 

and ab initio [230] calculations, the cyclohexane ring in 102 should be planar, as 

are such rings in other molecules having the cis-tris-�-homobenzene fragment 110 

presented later in this chapter. Chemistry of 102 and some of its derivatives and 

heteroanalogs has recently been discussed by de Meijere and coworkers [198]. 

2.4.1.5 C 12 H 12 Saturated Cages 

The highly symmetrical hexaprismane 63 and truncated tetrahedrane 111 have been 

long pursued but, contrary to the common practice not to publish unsuccessful 

attempts, only failed syntheses of them or larger such molecules have been reported 

[231–234]. The synthesis of a derivative of heptacyclo[6.4.0.0 2,4 .0 3,7 .0 5,12 .0 6,10 .0 9,11 ] 

dodecane 103 went unnoticed [235] as there was a considerable puckering of the 

central cyclohexane ring in the parent compound when it was synthesized by de 

Meijere’s group [236]. A more detailed study of the reactivity of 103 was recently 

published by the same group [237] and its chemistry reviewed [198]. Several 

members of this family have been analyzed using MM [203] model calculations 

yielding 103 as the most stable and hexaprismane 63 as the least stable molecules 

among those studied. Both the MM study and QC calculations [238] for 111 

yielded a highly symmetrical T d structure and close values of heat of formation 

(ca 91 kcal mol –1 by QC [239] and ca 87 kcal mol –1 by the MM method [240]).


According to MM modeling [240] (in the QC studies symmetry constraints were 

imposed), 63 and 111 are characterized by planar C 6 rings [240] while the central 

cyclohexane ring in 103 was found to be considerably more puckered than in the 

standard cyclohexane structure, first on the basis of modeling [219, 240] and then 

experimentally [237]. 

Of several theoretical and experimental studies discussing preparation of 

hexaprismanes, papers from the Mehta group aiming at the synthesis of [6]- and 

[7]prismanes [241–243] and the paper by Chou [244] should be mentioned. In 

spite of this massive effort, obtaining hexaprismane 63 remains a great unsolved 

problem in synthetic organic chemistry [245]. Maybe reactions in molecular flasks, 

as presented in Chapter 10, will enable us to obtain 63. 

2.4.1.6 Higher [n]Prismanes, Dodecahedrane 

Hypothetical higher [n]prismanes were discussed in Section 2.3. 

Of still higher prismanes, israelane 112 and helvetane 113 can be mentioned. Following 

a humorous idea of Eschenmoser, these C 24 H 24 molecules were discussed 

by Ginsburg in the 1st April issue of 1982 of the Nouveau Journal de Chimie [246]. 

The family to which they belong consists of numerous molecules and 112 and 113 

would definitely be of very high energy, precluding their existence. Thus, some 

calculations of their structure and energy discussed in [247] seem insignificant. 

It took the Paquette group almost 20 years to synthesize dodecahedrane C 20 H 20 

104 [248]. It was the first molecule of the I h symmetry synthesized in a laboratory, 

contrary to the Herzberg prediction that such molecules would be unlikely to exist 

[249]. It is the only known member of the family under scrutiny. Dodecahedrane 

chemistry has been discussed in detail [250] but its physicochemical properties 

received little attention. 

2.4.1.7 ‘In’-‘out’ Isomerism in Perhydrogenated Fullerenes C 60 H 60 

As fullerenes are discussed at length in Chapter 5, only the exciting ‘in’-‘out’ 

isomerism in hypothetical fully hydrogenated fullerenes C 60 H 60 (named fullerane) 

proposed by Saunders [251] will be considered here. On the basis of MM calculations, 

he found that of all possible ‘in’ and ‘out’ isomers, a nonsymmetrical one 

with 10 C–H bonds pointing inside should be the most stable. ‘In’-‘out’ isomerism 

in hydrogenated fullerenes has also been studied using the MM [252] and/or QC 

[252–254] methods. Two isomers with two C–H bonds pointing inside (other C–H 

bonds are omitted for the sake of clarity) are shown in Figure 2.8. Let us note that,


65 

Figure 2.8 Two geometrical isomers of C 60 H 60 with two CH bonds pointing inside. 

as shown in the latter figure, the carbon cage in the in-isomers undergoes considerable 

distortions and such isomers have a lower symmetry. It should be stressed that 

for higher number n of bonds pointing inside a real challenge in the calculations 

was to generate sufficiently large set of the ‘in’ structures to be sure that the one 

of the lowest energy for the given n represents the lowest energy structure. 

Dodziuk and Nowinski noticed that two isomers with a C–H bond pointing 

inside and outside the cage are topological isomers [255] since in order to come 

from one of them to another, one has to break C–C bonds of the C 60 cage. Dodziuk 

also studied the ‘in’-‘out’isomerism in C 60 H 60 using a different procedure for 

generation of isomers than that applied by Saunders [251] and different parameterization 

and found n = 10 CH bonds pointing inside the C 60 cage for the 

lowest energy structure. The latter result agreed with that of Saunders, however, 

as shown on the right of Figure 2.9, the optimum structure for n = 10 was of high 

symmetry. Comparison of the results by Saunders and those by Dodziuk group 

shows the limitations of the applicability of the simple MM model and only the 

results that do not depend on the force field used are reliable. Dodziuk group 

has also shown that not only several C–H bonds pointing inside the cage lower 

its energy but there is sufficient space in the cage for a methyl, ethyl or n-propyl 

substituent inserted inside and these isomers are considerably more stable than 

their fully ‘out’ counterparts. For iso-propyl and n-butyl substituents the energy of 

‘in’ and ‘out’ isomers assumed comparable values while for more bulky substituents 

the ‘all-out’ isomer was more stable. Moreover, two methyl groups situated 

on the opposite ends of the cage in 1,51-dimethyl fullerane (see Figure 2.10 for 

computer-generated atom numbering) were also calculated to be more stable than 

their isomers with one or both methyl groups pointing out [256]. Interestingly, not 

only was a methyl group ‘in’ more stable than that with the group ‘out’ but also 

its energy, independently of the force field applied, was very close to the energy 

of the C 60H 60 with one hydrogen ‘in’ [256]. 

Saunders [251] and Yoshida [252] groups claimed that one could obtain ‘in’ 

isomers of hydrogenated fullerenes by heating or inversion on a protonated carbon 

atom while Dodziuk group postulated that, in view of high barrier that must be 

overcome to move a hydrogen atom from an outside to an inside position, the 

aimed synthesis must be carried out to obtain the ‘in’ isomers.


Figure 2.9 Two projections of the calculated structures of ‘all-out’ C 60 H 60 (left) and the most 

stable ‘ten-in’ isomer (right). For clarity only carbon and ‘in’ hydrogen atoms are shown. 

The projections along a C 5 symmetry axis are shown in the upper part while the lower ones 

are along one of the C 6 axes. 

Figure 2.10 Atom numbering in C 60 determined by Rose (as cited in Kroto [258]).


67 

As discussed in Chapter 5, in the early days of fullerene chemistry numerous, 

sometimes peculiar, applications of this molecule were proposed [257]. The use 

of C 60F 60 as an ideal lubricant was one of them. The molecule was synthesized 

in all-‘out’ form but proved highly unstable when exposed to air, producing HF 

as a decomposition product [258]. The instability of C 60F 60 can be, at least partly, 

related to the high strain found in its hydrogenated analog [251, 259]. 

2.4.1.8 Summary 

As discussed in Chapter 1, it has become fashionable today to show practical applications 

of the molecules under scrutiny. Let us repeat once more, Eaton tried 

to obtain cubane because synthesis of this attractive highly symmetrical molecule 

was a great challenge. However, in agreement with the above-mentioned trend, 

two interesting applications will be mentioned here. Cage compounds discussed 

in this chapter are too small to host other molecules. However, the possibility that 

they could play the role of host for atomic hydrogen or ions has been explored 

theoretically [260] and the use of a cubane derivative has been proposed in a drug 

delivery system [261]. 

2.4.2 

Planar Cyclohexane Rings 

Let us recognize that the statement ‘a cyclohexane ring can be planar’ can cost a 

student a failure of examination [247]. However, molecules 114–116 and some 

complexes of 117 with cis-fusion of C 3 rings have been proven to exhibit such 

structures [262–265]. As discussed in the former chapter, diademane 102 [224, 

225], hypothetical hexaprismane 63 and truncated tetrahedrane 111 are predicted 

to have planar rings [240]. Cyclophanes 118 and 119 synthesized by the de Meijere 

group [266, 267] should also possess the planar C 6 ring. Similarly, according to 

model calculations a hypothetical hexahydrosuperphane 120 [268] should also 

have such a ring. As will be briefly discussed below, planarization can result 

either from fusion of at least three three-membered rings to the saturated sixmembered 

one [269] or it can be forced by the cage structure in which such a 

ring is built [240, 268]. 

As discussed in detail in Stereochemistry of Organic Compounds [270] the first 

notion of chair and boat conformations of the ring stemming from the tetra hedral 

arrangement of substituents on a tetravalent carbon atom were expressed by 

Sachse as early as in 1890 [271]. For long time physicochemical methods were too 

crude to enable the observation of nonplanarity of these conformations. Although 

several experimental results pointed to the chair shape of the six-membered ring, 

only a pioneering paper by Barton [272] brought a full comprehension of the 

physical and chemical consequences of such a structure. For this and subsequent 

studies he was awarded a Nobel Prize for Chemistry in 1969. 

The ring inversion leading to axial–equatorial equilibrium of substituents in 

monosubstituted cyclohexanes is a rapid process (of ca. 2 � 10 5 s –1 ) the observation 

of which depends on the timescale of experimental technique applied. For


instance, at room temperature two bands corresponding to the stretching C–Br 

vibrations of bromocyclohexane are observed in the IR spectra [273] while only 

one, average signal can be seen in the corresponding spectra of the hydrogen 

attached to the bromine substituted carbon in the NMR spectrum. This signal is 

split into two at about –50 °C when the ring inversion is frozen [274, 275]. Thus, 

in many cases only a combination of different experimental techniques can give 

a real insight into the molecular structure. 

Cyclohexanes with flattened rings have been known for long time [276–278] 

but, as mentioned earlier, the existence of molecules with a planar saturated C 6 

ring is not fully recognized even today. As concerns hydrocarbons, according to 

Engelhardt and Lüttke [279] trans-tris-�-homobenzene 110a should have such a 

ring, although no sound argument in favor of the planarity has been given. On 

the other hand, a close to planar C6 ring in the hexamethyl derivative of 117 [280] 

substantiated this claim. A historical account of the studies of tris-�-homobenzene 

has been published by Rücker [281]. A planar cyclohexane ring was proposed for 

known [224, 225] diademane 102 on the basis of computer modeling [229, 230] 

and, as discussed in Section 2.3, for hypothetical [6]asterane 121 [203]. A planar 

central ring was also found in the X-ray study of the tribenzoderivative of tetracyclo[8.2.0.0 

2,5 0 6,9 ]dodecane 122a [282] but on the basis of model calculations 

[269] discussed below, the planarity cannot be due to the effect of fusion of fourmembered 

rings to a cyclohexane ring. It should rather be ascribed to crystal 

forces, in particular those operating among the stacked aromatic rings. A recent 

determination of the nonplanarity of the central ring in 122b supports the latter 

conclusion. On the other hand, the central ring in 123 and 124 has been proven 

to be planar [283]. 

The influence of a cis fusion of with a smaller ring on the planarization of 

the C 6 ring was systematically studied by Dodziuk [269] using the MM method 

by changing the number from 1 to 6 and size of the fused ring(s) 110, 125–136 

(some of them, like 132, 134 and 136 purely hypothetical). A detailed comparison 

of the calculated geometry with the experimental one proved difficult since 

X-ray analysis is mainly carried out, often for a derivative of the molecule under


69 

scruting, in the solid state while the calculations refer to the isolated molecule. 

Moreover, the crystals sometimes contain solvent molecules influencing the 

results obtained for the molecule under investigation. However, the comparison 

of the calculated and available experimental data showed that the primitive MM 

model used in 1987 reproduced semiquantitatively torsional angles allowing one 

to analyze the planarity of the cyclohexane ring. The calculations led to the conclusion 

that fusion of smaller and/or more rings causes larger distortion of the C 6 

ring toward planarity. In consequence, cis- and trans- tris-�-homobenzenes 110 

should have a planar central ring. A comparison of the results of calculations for 

130 [269, 284], that is the saturated core of 122a and 122b, with the experimental 

data for the latter molecules indicate that, as mentioned earlier, the planarity 

of the central ring in the 122a is forced by intermolecular forces present in the 

crystalline state. Similarly, on the basis of the same type of modeling, planar rings 

should be present in 110a [269], 63, and 111 [240]. For 102, the same conclusion 

was drawn from semiempirical [229] and ab initio [230] QC. The planarity of the 

C 6 ring built into a cyclophane cage in 120 was obtained on the basis of both ab 

initio and DFT calculations [268]. 

Interestingly, both factors forcing planarity are present in cyclophanes 118 and 

119 synthesized by the de Meijere group [228]. Thus, the molecules should have 

a planar cyclohexane ring although no proof of this has been reported.


2.5 

Ultralong C–C Bonds 

Takanori Suzuki, Takashi Takeda, Hidetoshi Kawai and Kenshu Fujiwara 

2.5.1 

Introduction 

The bond length between the two sp 3 carbons is one of the fundamental parameters 

in chemistry. However, there are cases [285] where the bond length d significantly 

exceeds the standard value of 1.54 Å [286], as in sterically congested molecules such 

as t-Bu 3 CH [1.611(5) Å] [287]. Furthermore, it has been experimentally verified 

that a small number of compounds possesses the ultralong C–C bond (d > 1.70 Å) 

[288–290]. This chapter describes how the Csp 3 –Csp 3 bond can be elongated to 

such an extreme length and predicts a limit for the ultralong C–C bond. 

The most popular method to precisely determine the values of the length 

in crystalline materials is single-crystal X-ray analysis, although Raman [289] 

and 13 C NMR spectra [291] were also found effective in some cases. Diffraction 

measurement is often carried out at low temperature to attain high accuracy by 

suppressing thermal motion of atoms in crystal. However, the X-ray methodology 

dose not always give the right answer. It has been pointed out [292] that some of 

the ultralong C–C bonds in the earlier reports [293] were obtained as the result 

of unintentional overestimation by crystallographic artifacts, caused by incorporation 

of the valence isomer with a large separation of atoms in the crystal 

(Scheme 2.10a). In another case, the length was determined to be much smaller

Scheme 2.10 

2.5 Ultralong C–C Bonds 

than the actual d value due to positional disorder [294] or internal motion in the 

crystal [295] (Scheme 2.10b). Despite the opposite outcome, the thermal ellipsoids 

for the atoms in question are mis-shaped to indicate the presence of anomalies 

in both cases. Another common feature is the large estimated standard deviation 

(esd) for the bond length and other structural parameters. To avoid being misled 

by these crystallographic artifacts, only X-ray structures with a suitable accuracy 

(esd for d < 0.01 Å) are considered in this chapter [296]. 

The representative ultralong bonds can be found in Toda’s naphthocyclobutene 

derivatives 137 [288] [1.712(5)–1.734(5) Å] and the benzannulated caged hydrocarbon 

138 [1.713(2) Å] reported by Herges [289]. Before recent results by the authors 

were reported [290], diiodonaphthocyclobutene 137-I [1.734(5) Å] had been considered 

as the world record holder in terms of the length of a conventional C–C 

bond [297]. According to the linear correlation between bond dissociation energy 

(BDE) and d proposed by Zavitsas (d/Å = 1.748–0.002371 � BDE/kcal mol –1 ) [298], 

the experimentally determined values for 137 and 138 are just at the edge of a 

maximum bond length limit of 1.75 Å. In the next section, the pioneering work 

on these compounds is described to show how the syntheses of these elegant 

molecules with such a ultralong C–C bond were successfully realized. 

71


2.5.2 

Ultralong C–C Bonds Confined in a Stiff Molecular Frame 

Tricyclo[4.2.2.2 2,5 ]dodeca-1,3,5,7,9,11-hexaene 139 is the �-bond shift isomer of 

[0.0]paracyclophane 139�. This hydrocarbon is highly reactive because it has a 

strained molecular framework with the bridgehead unsaturated bonds [299]. Its 

tetrabenzannulated derivative 140 is much more stable than 139 [300], yet it still 

can undergo thermal [2+2] cycloaddition with benzyne. In this way, Herges and 

co-workers synthesized the caged hydrocarbon 138 [289], in which the ultralong 

bond in question faces the strongly pyramidalized double bond (Scheme 2.11). 

X-ray analysis at –130 °C revealed that 138 has one of the longest C–C bonds of 

1.713(2) Å among hydrocarbons. The extremely large value of d is not related to 

a contribution from the bond-dissociated isomer 141 because the stiff molecular 

frame prevents symmetry-allowed conrotatory ring opening into the o-quinodimethane 

derivative. The presence of an ultralong C–C bond in 138 was also 

predicted theoretically (B3LYP/6-31G*), where the calculated value of d = 1.721 Å is 

comparable to the experimental value [289]. Even when the calculation started with 

the geometry of ring-opened isomer 141, the high-level optimization converged 

into the structure of ring-closed form 138, since the structure of 141 has been not 

found to be a stationary point at the DFT level. 

Scheme 2.11 

In the Raman spectrum of 138, the C–C stretching vibration was observed at a 

lower wavelength (687 cm –1 ) than the standard value (995 cm –1 for ethane). This 

observation is in accordance with the extraordinary bond expansion that causes 

considerable decrease in strength of the C–C bond. Its force constant (133.1 N m –1 ) 

was calculated as less than one-third of that for ethane (455.7 N m –1 ). Successful 

isolation and characterization of this caged hydrocarbon show that the stiff 

molecular framework effectively maintains the weakened ultralong C–C bond 

in 138.


2.5.3 

Tetraphenylnaphthocyclobutene as a Scaffold to Produce Ultralong C–C Bonds 

Based on their own discovery that thermal cyclization of diallenes proceeds 

smoothly in the solid state [301], Toda, Tanaka, and co-workers prepared a variety 

of cyclobutene-fused aromatic compounds from 1,2-diallenylarenes [288d], 

some of which have an ultralong C–C bonds. Among them, 3,8-dichloro-1,1,2,2tetraphenylcyclobuta[b]naphthalene 

137-Cl [1.720(4) Å at 25 °C] [288a] is the most 

well-studied species, whose structural analyses [1.710(2) Å at –183 °C] [288b] and 

high-level calculations [302] were also carried out by several research groups. All 

the results support the intrinsic nature of the ultralong bond of 137-Cl. 

The origin of the bond elongation is primarily explained by a classical steric 

argument without the need to resort to orbital interaction. The (�-�*)-type 

through-bond orbital interaction had previously been proposed as a plausible 

reason to account for the bond elongation [303], yet several recent experimental 

studies did not support its importance [304]. The through-bond orbital interaction 

[305] is mainly caused by (�-�*)-type filled–filled coupling [306], and perturbation 

through the (�-�*)-type coupling was proven to be negligible according to highlevel 

theoretical calculation [307]. 

Especially indicative that the (�-�*)-type coupling is unimportant are the values 

of d experimentally determined in a series of cyclobutaarenes [288c, 288d]. For 

example, the X-ray structural analyses were carried out on two isomeric compounds 

142, in both of which the two t-butyl groups are substituted for the phenyls in 

137-Cl. The d value in gem-di(t-butyl) derivative gem-142 [1.729(2) Å] is much 

larger than the corresponding value in trans-1,2-di(t-butyl) derivative trans-142 

[1.686(5) Å], although the latter must be quite suitable for the (�-�*)-type coupling. 

The larger d in the former molecule is accounted for mainly by the more severe 

steric repulsion in gem-142 than in trans-142, which also induces outward deviation 

of bonding electron-density on the molecular plane around the carbon with the 

two t-Bu groups attached. 

Compounds 137 and 138 with an ultralong C–C bond have a 1,1,2,2-tetraaryl 

benzocyclobutene skeleton in common, which can be regarded as a sort of 

‘condensed’ derivative [308] of hexaphenylethane (HPE). The parent hydrocarbon 

143 was shown to undergo easy interconversion into 7,7,8,8,-tetraphenylo-quinodimethane 

144, which is too labile to be isolated, as it undergoes further 

73


Scheme 2.12 

pericyclic reaction to dihydroanthracene (Scheme 2.12a) [309]. The successfully 

isolated dibenzo analog of o-quinodimethane 146 [310] exhibits no signs of conversion 

into the cyclobutene-type isomer 145 (Scheme 2.12b) probably because 

of the easy cleavage of the weakened bond in 145, if formed. Thus, the successful 

observation of ultralong C–C bond in 137 and 138 relies on the suppression of their 

ring-opening pericyclic reaction to o-quinodimethanes: its formation is disfavored 

by the delocalization energy loss for the 2,3-naphthoquinodimethane moiety in 

137 and by the stiff molecular framework in 138. In the following sections, other 

members of HPE-type molecules are described which have a longer ‘ethane’ bond 

than the standard. 

2.5.4 

‘Clumped’ Hexaphenylethane Derivatives with Elongated and Ultralong C–C Bonds 

Since the pioneering work by Mislow [291, 311], HPEs with three bulky aryl groups 

at both ends of ethane bond have been used as excellent models to observe the 

elongated C–C bond because the steric repulsion between substituents is the 

most important and powerful factor to expand the C–C bond. One of the major 

obstacles in studying HPEs is the bond-dissociation equilibrium generating trityl 

radicals, which is also related to �,p-dimer formation or high reactivity toward 

oxygen [312].When the two trityls are connected by a suitable bridge, intramolecular 

bond-formation process giving �,�-dimer (HPE) becomes much more facile 

(Scheme 2.13a), thus endowing the ‘cross-clumped’ HPEs such as 147-H with high

Scheme 2.13 


stability despite the elongated C–C bond (calc. 1.64 Å) [311, 313]. Another type of 

clump (‘back-clump’), by which two trityl parts are not connected as in the case of 

9,9�-diphenyl-9,9�-bifluorenyl, does not prevent cleavage of the weakened bond so 

effectively (Scheme 2.13b). Since BDEs decrease with the increase in C–C bond 

length [298], the bridging modification is essential to obtain thermodynamicallystabilized 

HPEs that can allow investigation on the weakened and easily-dissociated 

long C–C bond. 

‘Cross-clumped’ HPE 147-H with the dihydrophenanthrene skeleton was 

prepared from the bis(triphenylmethanol) 148-H via the dianionic species 

(Scheme 2.14a) [313]. When the electron-donating substituents are attached 

to the aryl groups, acid treatment of diols 148-NMe 2 and 148-OMe gave stable 

dications, [biphenyl-2,2�-diyl bis(diarylmethylium)s], which were transformed 

to HPEs 147-NMe 2 and 147-OMe smoothly upon reduction [314]. The reductive 

generation of HPEs via the dicationic species has synthetic merit for generating 

highly congested HPEs 147 from the less-hindered precursors through short-step 

transformation. Dispiro-HPEs (147-spiro-N and 147-spiro-O) were prepared by this 

protocol (Scheme 2.14b), whose ‘ethane’ bond lengths are as long as 1.635(2) Å 

[314c] and 1.656(5) Å [314d], respectively. 

Not only the dihydrophenanthrene-type compounds, but also HPEs with an 

acenaphthene skeleton were available by this method. Thus, naphthalene-1,8diyl 

bis(diarylmethylium)s could be converted into 1,1,2,2-tetraarylacenaphthene 

derivatives 149 (Scheme 2.15) [315], whose ‘ethane’ bond is as long as 1.70 Å 

[1.701(3) Å for 149-H [315b] and 1.707(2), 1.705(2) Å for 149-F (two independent 

75


Scheme 2.14

Scheme 2.15 


molecules in crystal) [315c]. Torsional fixation in the five-membered ring is the 

key feature for producing the larger ‘front strain’ [311a] resulting in elongation of 

the C–C bonds in 149 compared with that in 147, since the latter can relieve the 

strain by adopting a chair-like conformation of the central six-membered ring. 

Although the estimated BDE for the long C–C bond of 1.70 Å is only 20 kcal mol –1 , 

the acenaphthene-type HPEs 149 could be isolated with no signs of decomposition 

under ambient conditions. The remarkable stability of 149 can be rationalized 

by considering that the bond-dissociated species with the non-Kekulé structure 

(naphthalene-1,8-diyl), if formed, only undergo bond-reformation to give 149. This 

is in sharp contrast to the case of tetraarylbenzocyclobutene 143 [309], which is 

transformed into the dihydroanthracene derivative via the bond-dissociated isomer, 

tetraaryl-o-quinodimethanes 144, as discussed previously (Scheme 2.12a). 

It is clear from the above results that the fate of the bond-dissociated species 

is the major determinant factor for successful isolation and characterization 

of a compound with an ultralong C–C bond (d > 1.70 Å). By rational design to 

prevent the bond-dissociated species from intramolecular reaction other than 

bond-reformation, and from intermolecular reactions to give the adduct [316], 

the molecules with a super-ultralong C–C bond (d > 1.75 Å) would be obtained, 

whose bond length comes closer to the shortest nonbonded 1,3 C–C contact 

(1.80–1.90 Å) [317]. 

77


2.5.5 

HPE Derivatives with a Super-ultralong C–C Bond 

The acenaphthene scaffold in HPEs 149 is one of the ideal skeletons for further 

exploration, yet the drawback is that the ‘rigid’ naphthalene-fused five-membered 

ring is not so rigid as expected. The experimentally determined d values for the 

‘ethane’ bond in HPEs 149 exhibit considerable variation [1.633(3) Å for 149-Cl 

[315b] and 1.670(3) Å for 149-MeO [315a]], and the smaller values were observed 

when the acenaphthene skeleton adopted the skewed conformation with the 

larger torsion angle for C 8a -C 1 -C 2 -C 2a as in 149-Cl [27.9(3)°] [315b] and 149-MeO 

[20.6(2)°]. Apparently, the HPEs 149-H [0.0(2)°] and 149-F [3.2(2)°, 3.6(2)°] with 

the eclipsed conformation suffer from the larger front strain, which expands the 

polyarylated C 1 –C 2 bond. Based on our recent results, the electronic effects of the 

substituents on the aryl groups do not determine the preference for the eclipsed/ 

skewed conformation [315c]. Thus, the torsion angle would be much affected by 

crystal packing force. 

When the acenaphthene skeleton is modified to increase its rigidity, skewing 

deformation would become more costly in energy than expanding the long C–C 

bond. Thus, the pyracene-type HPEs 150 with the extra five-membered ring are 

better candidates to exhibit an ultralong C–C bond. Furthermore, the additional 

bridge induces expansion of the C 1–C 2 bond [318] in the parent hydrocarbons 

[calculated by B3LYP/6-31G* [319]: 1.586 Å for pyracene and 1.569 Å for acenaphthene, 

respectively]. Since the bond-elongation effect is strongly enhanced for the 

prestrained bond [303a, 320], the steric repulsion in the pyracene skeleton must 

enlarge the d value for the ‘ethane’ bond more effectively in 150 (Scheme 2.16). 

This is also the heart of the authors’ molecular design to realize the super-ultralong 

C–C bond, whose d is larger than 1.75 Å. 

Various diarylketones were reacted with 5,6-dilithioacenaphthene generated in 

situ from 5,6-dibromide. Upon treatment with acid under dehydrating conditions, 

the resultant diols are smoothly converted to a series of acenaphthene-5,6-diyl 

bis(diarylmethylium)s, from which the desired HPEs 150 were generated upon 

reduction [321]. X-ray analyses have shown that they all adopt the eclipsed conformation 

with the torsion angle of C 8a-C 1-C 2-C 2a less than 3°, and consequently, all of 

Scheme 2.16


Figure 2.11 X-ray structure of 1,1,2,2-tetraphenylpyracene 150-H with a super-ultralong C 1 -C 2 

bond length [1.754(2) Å] (C2/c, Z = 4, R = 5.5%, T = 123 K). The molecule is located on the 

crystallographic 2-fold axis. 

the ‘ethane’ bonds are longer than 1.7 Å. The super-ultralong bond (d > 1.75 Å) is 

found in several of them, such as 150-H [1.754(2) Å] and 150-F [1.761(4) Å] [321]. 

Though largely separated, these carbon atoms must form ordinary covalent bonds. 

The two carbons are hybridized in the sp 3 manner, as judged by the tetrahedral 

coordination determined by X-ray analysis (Figure 2.11). Despite the marginal 

down-field shift, the 13 C signal (78.73 ppm for 150-H in CDCl 3 ) stays within the 

region typical of the sp 3 carbons. Theoretical calculation (B3LYP/6-31G*) gave the 

d values of 1.758 Å for 150-H and 1.762 Å for 150-F, respectively, which correspond 

well with the observed values [321]. Through this achievement, the authors have 

proposed a general scheme to design the HPE derivatives with a super-ultralong 

C–C bond by taking advantage of: (1) increase in front strain by torsional fixation 

and skeletal rigidity; (2) non-Kekulé structure for the bond-dissociated species 

that can only undergo bond-reforming without giving any by-products. The latter 

condition is essential to isolate and characterize the HPE containing a weakened 

ethane bond. 

2.5.6 

‘Expandability’ of the Ultralong C–C Bond: 

Conformational Isomorphs with Different Bond Lengths 

Based on the proposal for a linear relationship between bond lengths and BDEs 

[298], the ‘ethane’ bond in 150-H and 150-F would have ‘negative’ BDEs, which 

is unrealistic. There should be a nonlinear relationship or some special effects in 

the region of the ultralong and the super-ultralong C–C bonds. During further 

examination of the extremely long C–C bond in the pyracene-type HPEs, the 

authors have encountered a special case of a ‘soft’ long C–C bond. That is, the 

79


same C–C bond in a certain HPE exhibits several different values (1.71–1.77 Å) 

depending on the requisites from crystal packing. 

Compared with the conformational polymorphs where different conformers 

crystallize separately in different crystal forms, conformational isomorphs have 

seldom been observed, in which the different conformers are packed orderly in 

the same crystal. The latter is the case for the pyracene-type HPE 151 having two 

spiro(10-methylacridan) units. According to the X-ray analysis at –180 °C, there are 

four crystallographically independent molecules in the crystal of 151. Two adopt 

the eclipsed conformation [torsion angle: 3.4(2), 9.4(2)°] and other two are skewedshaped 

[23.4(1), 24.7(1)°] (Figure 2.12). Adoption of two different conformations 

is similar to the case of acenaphthene-type HPEs (149-H, 149-F: eclipsed; 149-Cl, 

149-MeO: skewed), yet both conformers of 151 do coexist in the same crystal. As 

expected, the eclipsed conformers possess much longer ‘ethane’ bond [1.771(3) 

and 1.758(3) Å] than the skewed molecules [1.712(2) and 1.707(2) Å]. By measuring 

the diffraction data at the elevated temperature, the longest bond length becomes 

as large as 1.781(6) Å at –40 °C. This value of d is one of the largest ever reported 

for the C–C bond length determined with sufficient accuracy [290]. 

The molecular structures in conformational isomorphs often differ significantly 

in torsional angles but not in bond angles; changes in bond lengths are found less 

frequently. The last cases are usually related to the presence of differently charged 

molecules [322] or tautomerism in the crystal [323]. Even when the molecule 

crystallizes in several forms (polymorph), the chemically equivalent bonds in 

general exhibit the same bond length within experimental error. Thus, the crystallographically 

determined value of d has been considered as ‘intrinsic’ or an 

‘eigenvalue’ for a compound of the same charge and the same atom connectivity. 

This provides the basis for the validity of comparison of the bond lengths determined 

by X-ray analyses and to discuss the relationship between bond lengths and 

chemical structures. What happens when the same molecule possesses different 

bond lengths in polymorphs? What happens when the chemically equivalent 

but crystallographically independent molecules exhibit considerably different 

bond lengths in the same single crystal? This is the case for HPE 151 described 

above. So, none of the d values precisely determined [1.771(3), 1.758(3), 1.712(2), 

1.707(2) Å] is the eigenvalue of compound 151, but X-ray analysis just demonstrates 

that the bond in question can be expanded, if it likes, to 1.771(3) Å under 

the given circumstances, and that the bond can also remain much shorter than 

that. This is not a unique and special phenomenon for 151: structurally related 

compound 152, which has the etheno-bridge instead of ethano-bridge in 151, also 

crystallizes as a conformational isomorph [319]. Its crystal packing is quite different 

from that in 151, and there are three crystallographically independent molecules 

of 152: one eclipsed [bond length and torsion angle at –130 °C: 1.749(2) Å and 

5.5(2)°] and two skewed forms [1.726(2) Å and 21.3(2)°; 1.721(2) Å and 22.8(2)°]. 

This dihydropyracylene-type HPE 152 is another example that shows the large 

‘expandability’ of the ultralong C–C bond. In contrast, HPE 153 with a less rigid 

acenaphthene skeleton only adopts the skewed conformation with a much smaller 

d value [1.696(3) Å and 18.1(3)°] [315d] than 151 or 152.


Figure 2.12 X-ray structures of two of four crystallographically independent molecules of 

bis(10-methylspiroacridine)-type HPE 151 with an eclipsed (a) and skewed (b) conformation 

(P1bar, Z = 8, R = 5.2%, T = 93 K). 

81


2.5.7 

Future Outlook 

Ultralong C–C bonds present intriguing facets of organic covalent bonds. For the 

super-ultralong bond with d > 1.75 Å, there is no longer the linear relationship 

between d and BDE. The super-ultralong bond can exhibit a range of d values 

in crystal depending on the packing arrangement since only a small energy is 

necessary to expand the ‘expandable’ covalent bond. Thus, the precisely determined 

d value for the ‘soft’ bond is not necessarily the ‘eigenvalue’ of the bond in a certain 

compound, but it just points out to the length it can exhibit. That value can be the 

higher limit in one case or the lower limit in another case. 

In any case, the experimentally determined large d value will attract attention. It 

surely shows that the bond can be longer than the standard value. Chemists can 

continue the game to find the further elongated C–C bond, and the outstanding 

stability of compounds 150 has prompted the authors to prepare the derivative 

that shows even larger value of d in crystal (1.79 Å) [321], which is nearly equal 

to the shortest nonbonded 1,3 C–C distance (1.80 Å) [317]. The authors appreciate 

Herges’ view [289] that ‘the stable C–C bonds even longer than the smallest 

nonbonding distance are conceivable’. 

2.6 

Ultrashort C–C Bonds 

Vladimir Y. Lee and Akira Sekiguchi 

2.6.1 

Introduction 

The generally accepted normal value for the single C–C bond length between 

two sp 3 -hybridized tetracoordinate carbon atoms, compiled from a large number 

of X-ray structures, is 1.54 Å [324, 325], the value close to that of the C–C bond 

in diamond. In a vast number of crystallographically characterized organic 

compounds, the length of ordinary C–C bonds may deviate slightly from this 

average value, albeit being still very close to it. However, the two limiting cases of 

nonclassical C–C bonds are of particular importance: extremely long (ultralong) 

and extremely short (ultrashort) C–C bonds. The first topic, ultralong C–C bonds, 

is covered by Suzuki (Section 2.5), whereas our goal is to discuss the second 

topic, ultrashort C–C bonds, bringing together the most important experimental 

accomplishments and theoretical considerations in this field.Basically, one can 

distinguish four fundamental classes of compounds featuring ultrashort C–C 

single bonds, which are typically markedly shorter than the border value of 1.50 Å. 

The first class includes those compounds in which the C–C bond is framed on 

both sides by either C=C or C�C multiple bonds: the most common examples are 

1,3-butadiene, H 2 C=CH–CH=CH 2 , and 1,3-butadiyne, HC�C–C�CH, where the 

central C–C bonds are considerably shortened to 1.463(3) Å [326] and 1.384(2) Å

2.6 Ultrashort C–C Bonds 

[327], respectively, being just intermediate between those of the normal C–C 

single (1.54 Å) and C=C double (1.34 Å) bond lengths. This phenomenon is 

typically interpreted in terms of hybridization (increased s-character of the sp 2 - or 

sp-hybrids used to form the C–C �-bonds) and, more importantly, conjugation 

(multiple bonds conjugation leading to the delocalization of �-electrons over the 

entire molecule and shortening of the central C–C bond due to its partial double 

bond character). In contrast to this first class of ultrashort C–C bond compounds, 

well known from standard textbooks of organic chemistry, the other three classes 

are much less familiar to the general chemical community. The second class of 

compounds, exhibiting extraordinarily short C–C bonds between the tetracoordinated 

C atoms, includes the derivatives of tricyclo[2.1.0.0 2,5 ]pentane, featuring 

exceptionally short endocyclic bridging bonds. The third class of compounds, for 

which ultrashort C–C bonds were experimentally found, is represented by the 

dimeric polyhedral compounds (e.g., bi(tetrahedranyl), bi(bicyclo[1.1.0]butyl), 

bi(bicyclo[1.1.1]pentyl), bi(cubyl)), exhibiting remarkably short central intercage 

exocyclic bonds. The fourth and newest class of compounds featuring the ultrashort 

C–C bonds is known more computationally than experimentally at present. 

In such derivatives the C–C bond, force to point inside the inert cage (like cyclophane), 

is compressed by the evident steric congestions. In this section, we will 

deal with the last three classes of compounds featuring ultrashort C–C bonds, 

briefly describing their structural peculiarities and particularly emphasizing the 

origin of the exceptional bond shortening. 

2.6.2 

Tricyclo[2.1.0.0 2,5 ]pentanes: Ultrashort Endocyclic Bridging C–C Bonds 

The derivatives of tricyclo[2.1.0.0 2,5 ]pentane 154a–h, known since 1964 [328, 329], 

particularly those of tricyclo[2.1.0.0 2,5 ]pentan-3-one 154 (R 2 , R 2 = C=O), represent 

the record shortening of the single bond between two tetracoordinate C atoms 

(Table 2.1). 

Indeed, the most remarkable structural feature of all tricyclo[2.1.0.0 2,5 ]pentane 

derivatives 154 is the exceedingly short bond between the bridgehead carbon 

atoms C1–C5, ranging from 1.408 to 1.509 Å (av. 1.45 Å) [330–340]. The only 

exceptional case is represented by the heavy group 14 elements containing 

2,4-disila-1-germatricyclo[2.1.0.0 2,5 ]pentane derivative 155, in which the bridging 

Ge–C bond of 2.242(3) Å was extraordinarily stretched, being 15% longer than 

83


Table 2.1 Endocyclic C 1 –C 5 bridging bonds in tricycle[2.1.0.0 2,5 ]pentane derivatives 154. 

Tricyclo[2.1.0.0 2,5 ]pentane 

derivative 

R 1 , R 2 

154a R 1 = Ph; 

R 2 = OCO(p-Br-C 6 H 4 ) 

154b R 1 = Ph; 

R 2 ,R 2 = C=O 

154c R 1 = Me; 

R 2 ,R 2 = C=O 

154d R 1 = CH 2 OCOCH 3 ; 

R 2 ,R 2 = C=O 

154e R 1 = CH 2 OCOCH 3 ; 

R 2 ,R 2 = –OCH 2 CH 2 O– 

154f R 1 ,R 1 = –CH 2 N(COOMe)N(COOMe)CH 2 –; 

R 2 = OEt 

154g R 1 = COOCH 3 ; 

R 2 ,R 2 = –OCH 2 CH 2 O– 

154h R 1 = COOCH 3 ; 

R 2 ,R 2 = C=O 

r (C 1–C 5), Å Ref. 

1.44(5) [330] 

[331] 

1.444(2) [332] 

1.408(3) at 298 K 

(1.417(1) at 118 K) 

[332] 

[(334)] 

1.416(2) [333] 

[335] 

1.455(3) [336] 

1.509(2) [337] 

1.485(2) [338] 

1.453(2) [338] 

the average value, the fact interpreted in terms of the appreciable singlet biradical 

contribution to the nature of the Ge–C bridging bond [341]. 

The shortest known endocyclic C–C single bonds were measured for 1,5-dimethyltricyclo[2.1.0.0 

2,5 ]pentan-3-one 154c (1.408(3) Å at 298 K [332] and 1.417(1) Å 

at 118 K [334]) and 1,5-bis(acetoxymethyl)tricyclo[2.1.0.0 2,5 ]pentan-3-one 154d 

(1.416(2) Å) [333, 335]. The recent DFT calculations fairly well reproduced the 

structural parameters of 154c: bridging bond length of 1.425 Å (1.417(1) Å by 

X-ray diffraction) and interplanar angle 94.96° (95.0° by X-ray diffraction) [342]. In 

sharp contrast, bicyclo[1.1.0]butane derivatives 156, which represent a part of the 

tricyclo[2.1.0.0 2,5 ]pentane skeleton, exhibit either normal (short bond isomers) or 

very long (long bond isomers) bridging C–C distances (Scheme 2.17). 

Such a striking difference between these two classes of polycyclic derivatives is 

due to the evident geometrical constraints in tricyclo[2.1.0.0 2,5 ]pentanes 154, in 

which the methylene C3-bridge between the C2 and C4 atoms ‘tightens’ the two 

three-membered rings, thus providing a fixed interplanar angle �, much smaller 

than that of the ‘nonfixed’ bicyclo[1.1.0]butanes 156: 94–99° vs. 113–130° [339].

Scheme 2.17 


Consequently, such a small interplanar angle � forces the hybrids at C1 and C5 

atoms in 154 to form an extraordinarily short highly bent C1–C5 bond [339, 340, 

343]. However, this bridgehead C1–C5 bond is not a classical �-bond formed 

by the linear overlap of two sp 3 -hybrids. Thus, the electron density distribution 

study of 1,5-dimethyltricyclo[2.1.0.0 2,5 ]pentan-3-one 154c by X-ray analysis at 

118 K revealed that the electron density maximum of the bridging C1–C5 bond is 

displaced by ~0.40 Å outwards from the bond axis, which amounts to an extreme 

bending by 28° [334]. Consequently, the C1–C5 bridging bond is best described 

as a highly bent �-bond. The early calculations considered the bridging C–C 

bond in 154 to be a result of the �-type overlap of the two p z -orbitals from each 

bridgehead carbon atom, this suggestion being based on the general reactivity 

of this bond [344]; however, later DFT calculations objected such a viewpoint, 

showing that the bridging C–C bond has no � character and is to be described 

as a classical two-center bent �-bond [342]. The hybridization of the bridgehead 

carbon atoms C1 and C5 in 154 sharply deviates from the normal sp 3 -state: the 

hybrid orbitals of the exocyclic bonds are high in s-character (the coupling constant 

1 JCH for exocyclic C1–H and C5–H bonds of 2,4-dimethyltricyclo[2.1.0.0 2,5 ]pentane 

were measured to be as large as 212 Hz) [345], whereas the hybrids used for the 

formation of the endocyclic C1–C5 bond have substantial p-character (the value 

of sp 4.21 was calculated for the unsubstituted tricyclo[2.1.0.0 2,5 ]pentan-3-one 154 

(R 1 = R 2 = H) 342 ) [334, 339]. 

The extent of the shortening of the bridging C1–C5 bond in 154 is greatly affected 

by two principal factors: geometrical (interplanar angle �) and electronic (influence 

of the �-accepting substituents). Accordingly, the bridging bond becomes longer 

when the interplanar angle widens, and vice versa–the bridging bond shortens 

upon decreasing the interplanar angle [332]. Another geometrical factor, greatly influencing 

the length of the bridging bond, is the external bond angle �. In complete 

accordance with the theoretical prediction [343], the relationship between these 

two geometrical parameters is inverse: the larger the bond angle �, the shorter the 

bridging bond, and vice versa [337]. Electronically, �-accepting substituents (like 

carbonyl, phenyl groups) may effectively interact with the bridging C1–C5 bond 

in 154, because it is made of hybrids high in p-character (vide supra); however, the 

direction of such influence depends on the position of the �-accepting group [339]. 

For example, the carbonyl groups at the bridgehead atoms C1 and C5 give rise 

85


to a lengthening of the C1–C5 bridging bond in 154h by ~0.03 Å because of the 

optimal orientation of the carbonyl groups, allowing for the effective interaction 

of their C=O �-bonds with the bridging C1–C5 �-bond [338, 339]. 

In contrast, the carbonyl group at the bridging position C3 (C3=O), tightening 

the bicyclo[1.1.0]butane fragment at the C2 and C4 positions, shows the opposite 

effect, shortening the bridging C1–C5 bond in 154h by the same extent of ~0.03 Å 

[338, 339,346]. 

2.6.3 

Coupled Cage Compounds: Ultrashort Exocyclic Intercage C–C Bonds 

This class of compounds featuring ultrashort C–C bonds best of all fits the original 

definition of such bonds, lacking the highly perturbing factors of either conjugation 

to multiple bonds (as the representatives of the first class of compounds with 

ultrashort C–C bonds, 1,3-butadiene and 1,3-butadiyne) or incorporation into the 

highly strained tricyclic framework (as the representatives of the second class of 

compounds with ultrashort C–C bonds, derivatives of tricyclo[2.1.0.0 2,5 ]pentane). 

Indeed, in striking contrast to the highly bent nature of the endocyclic bridging 

C–C �-bonds of tricyclo[2.1.0.0 2,5 ]pentane derivatives, which makes the definition 

of the bond length somewhat ambiguous, the intercage C–C �-bonds are exocyclic 

and nonbent, thus experiencing minimal geometrical constraints.


The first experimental accomplishments in this field were preceded by the prediction 

that the exocyclic intercage C–C bonds in the hypothetical bi(cubyl) 157 

and 1,1�-bi(bicyclo[1.1.1]pentyl) 159 could be markedly shortened by up to 0.08 Å 

compared with the normal values [347]. 

This hypothesis was based on the general idea that the C–C bonds are expected 

to be appreciably shortened if they were involved in widened bond angles (and vice 

versa: the C–C bonds adjacent to compressed bond angles should be stretched). 

This was the case of compounds 157 and 159, in which the intercage C–C bonds 

participate in six significantly widened C–C–C bond angles [347]. A year later 

this fruitful idea found spectacular experimental confirmation in the synthesis 

and crystal structure determination of bicubyl 157 and (2-t-butylcubyl)cubane 

158 [348]: the intercage C–C bonds in both 157 and 158 were significantly contracted, 

1.458(8) and 1.464(5) Å, respectively, being in very good agreement with 

the predicted value of 1.46 Å [347] and very close to the value of 1.463(3) Å for the 

central C–C bond in 1,3-butadiene [326]. 

Such appreciable shortening was simply explained in hybridization terms: endocyclic 

C–C bonds of bicubyl derivatives are richer in p-character than the normal 

sp 3 -hybrids, whereas the exocyclic C–C bonds are higher in s-character, which 

allows them to form shorter bonds to substituents than their sp 3 -counterparts 

do. The specific geometrical constraints of bicubyl molecules create less steric 

crowding than, for example, in hexasubstituted ethane, the latter factor also 

favoring shortening of the intercage C–C bond. In contrast to 157 and 158, in 

1,1�-biadamantyl 163 the intercage C–C bond is formed by nearly pure sp 3 -hybrids; 

consequently, this bond is not ultrashort but stretched to 1.578(2) Å [349], being 

0.12 Å longer than that in bicubyl 157. 

Other examples of coupled cage compounds featuring ultrashort intercage 

C–C bonds include: bi- and tri(bicyclo[1.1.1]pentyl) derivatives 160–162 and 164, 

87


1,1�-bihomocubyl 165 [350–353]. Thus, in keeping with expectations [347], the 

bi(bicyclo[1.1.1]pentyl) derivative 160 showed a short intercage bond distance of 

1.480(4) Å [350], whereas the tri(bicyclo[1.1.1]pentyl) derivative 164 manifested 

even shorter distances of 1.464(7) and 1.476(7) Å [351]. 

The other two representatives of bi(bicyclo[1.1.1]pentyl) derivatives, 161 and 

162, also displayed diagnostically short intercage C–C bonds of 1.480(3) and 

1.469(6) Å, respectively [352]. An MNDO calculation showed that the nature of 

the substituents at the bridgehead 3,3�-positions (H, NH 2 , or CN) has almost 

no influence on the length of the intercage C–C bond, being nearly invariable 

(1.470–1.472 Å) in all calculated structures. Quite similar to the above cases of 

bicubyl 157 and (2-t-butylcubyl)cubane 158 [348], and in complete accord with the 

prediction [347], 1,1�-bihomocubyl 165 exhibited the short intercage C–C bond 

length of 1.460(1) Å, this value is very close to the corresponding values in 157 

(1.458(8) Å) and 158 (1.464(5) Å) [353]. 

The 1,1�-coupled derivatives of bicyclo[1.1.0]butane were expected to manifest a 

further shortening of the intercage C–C bonds, because their adjacent bond angles 

are very wide, being larger than those in bicubyl 157 and bi(bicyclo[1.1.1]pentyl) 

159. Indeed, in the two structurally characterized examples of such compounds, 

namely, 1,1�-bi(tricyclo[4.1.0.0 2,7 ]heptyl) 166 and 1,1�-bi(tricyclo[3.1.0.0 2,6 ]hexyl) 167 

derivatives the exocyclic intercage C–C bonds were exceptionally short, 1.445(3) 

and 1.440(2) Å, respectively, being ~0.10 Å shorter than the average value of 

1.54 Å [354].


Extrapolating the experimental results on the lengths of the intercage C–C bonds 

in coupled cage compounds, the authors deduced the shortening of ~0.10 Å for 

the central bond in the hypothetical bitetrahedranyl 169 [354]. 

Such impressive experimental accomplishments raised a great deal of interest 

among theoreticians about the problem of extreme shortening of the intercage C–C 

bonds. Thus, the length of this bond in bicubyl 157 was calculated at the HF/DZ+d 

SCF level as 1.484 Å, this value being 0.026 Å longer than the experimental value 

of 1.458(8) Å [355]. Definitely, the Hartree–Fock method markedly overestimated 

the length of the central C–C bond in the coupled cage compounds. Likewise, the 

early calculations on the parent unsubstituted 1,1�-bi(tricyclo[3.1.0.0 2,6 ]hexyl) 168 

produced the overestimated values of 1.477 Å at the STO-3G SCF level and 1.453 Å 

at the DZ SCF level [356] (cf. 1.440(2) Å experimentally observed in 167 [354]). 

On the other hand, the semiempirical AM1 calculations gave underestimated 

values of the intercage C–C bond in the 1,1�-bi(tricyclo[3.1.0.0 2,6 ]hexyl) derivatives: 

1.409 Å for the unsubstituted model 168 and 1.411 Å for the real compound 167 

[357]. The recent DFT B3LYP/6-31G(d,p) calculations on the real molecules 166 

and 167 provided results most closely approaching the experimental X-ray data: 

1.456 and 1.452 Å vs. 1.445(3) and 1.440(2) Å [342]. Consequently, the hybridization 

of the intercage C–C bond in 166 and 167 was calculated as sp 1.31 and sp 1.26 , 

which amounts to an s-contribution to the bonding orbitals of 43.2% and 44.3%, 

respectively. The extreme shortening of the intercage C–C bonds in both 166 

and 167 was further manifested in the extraordinarily large values of the 1 J(C, C) 

coupling constant of 99.8 and 85.9 Hz, respectively, undoubtedly reflecting the 

great s-character of these bonds [342]. 

The most intriguing molecule to study computationally was bi(tetrahedranyl) 

169, whose structure was calculated by several research groups. (Other tetrahedrane 

derivatives were discussed in Section 2.4.1.1.) The earliest calculations 

agreed well with the above prediction of ~1.44 Å [354], giving the estimates of 

the intercage C–C bond in 169 as 1.438 Å at the 6-31G* [358] and 1.444 Å at the 

DZ+P SCF [356] levels of theory. In the latter study, the six equivalent bond angles 

at the central C–C bond were calculated to be 144.6°, which corresponded to a 

bond angle widening of 35.1° compared with a normal tetrahedral angle value of 

109.5° [356]. Consequently, the intercage C–C bond length in 169 was expected to 

be intermediate between the 1.440(2) Å of 1,1�-bi(tricyclo[3.1.0.0 2,6 ]hexyl) derivative 

89


167 (widening angle 130.9° – 109.5° = 21.4°) and the 1.384(2) Å of 1,3-butadiyne, 

H–C�C–C�C–H (widening angle 180.0° – 109.5° = 71.5°). A subsequent study 

by the same authors performed at the MP2 level with the same DZ+d basis set 

showed a slightly shorter intercage C–C bond in 169 of 1.434 Å [355]. A larger 

shortening of the central C–C bond in 169 was found by the semiempirical AM1 

calculations: 1.386 Å Å [357]. It is well-known that the NMR coupling constant 

1 J(C, C) between the two carbon atoms forming the intercage C–C bond is another 

diagnostic measure of the electronic distribution in alkane bonds, effectively indicating 

the alterations in the hybridization of carbon atoms and, consequently, in 

the C–C bond length. Thus, the extreme shortening of the C–C bond linking the 

two tetrahedrane units in 169 (1.444 Å) was manifested in an extraordinarily large 

1 J(C, C) coupling constant of 151 Hz [359]. This 1 J(C, C) value is much greater than 

the values of 35.0 Hz measured for isobutane (CH 3) 2CHCH 3 [360], and 53.7 Hz 

for the central C–C bond of 1,3-butadiene, H 2C=CH–CH=CH 2 [361], closely 

approaching the value of 154.8 Hz measured for 1,3-butadiyne, HC�C–C�CH 

[362]. Consequently, the calculated 1 J(C, C) coupling constant of the intercage 

C–C bond in 169 is very similar to that of a bond between the two sp-hybridized 

carbon atoms, and it possibly represents the highest 1 J(C, C) value ever reported 

for a C–C bond between two saturated tetracoordinated carbon atoms. 

The experimental realization of the theoretical predictions discussed above was 

achieved a couple of years ago, when the structure of the stable hexakis(trimethylsilyl)bi(tetrahedranyl) 

170 was established [363]. The intercage C–C bond in 170 

was exceedingly short, 1.436(3) Å, this value being very close to those predicted 

by calculations (vide supra) of 1.434–1.444 Å, being the shortest acyclic nonbent 

C–C single bond between two saturated tetracoordinated carbon atoms known 

thus far. The principal reason for such a great bond shortening was ascribed to 

the high s-character of this exocyclic C–C bond in 170: the hybridization of this 

bond was calculated (NBO analysis) to be sp 1.53 at the B3LYP/6-31G(d) level [363]. 

However, the latest computations at the HF/6-311+G(d,p) level pointed out that 

apart from this evident cause, there is another reason behind the appreciable 

intercage bond shortening in bi(tetrahedranyl) 169, namely, vicinal hyperconjugative 

orbital interactions between the two tetrahedranyl units over the central 

C–C bond [364]. This hyperconjugation, facilitated by six electron donating silyl 

substituents, was calculated to be even more important than conjugation in 

1,3-butadiene: the hyperconjugation energy in the staggered conformation of 

bi(tetrahedranyl) 169 of –15.2 kcal/mol vs. conjugation energy in 1,3-butadiene 

of –9.9 kcal mol –1 . Such an interaction of the two tetrahedranyl units in 170 

raised the HOMO energy level and lowered the LUMO energy level, resulting 

in the decrease in the HOMO–LUMO energy gap [365]. This was reflected in 

the red shift of the longest wavelength absorption of 170, compared with that of 

tetrakis(trimethylsilyl)tetrahedrane [365–367]. Thus, the two comparable causes 

responsible for the extreme shortening of the intercage C–C bond in 170 were 

suggested: the high s-character of this exocyclic bond in bi(tetrahedranyl) (58% 

contribution) and vicinal hyperconjugative interactions between the tetrahedranyl 

groups (42% contribution) [363, 364].

2.6.4 

Sterically Congested in-Methylcyclophanes: Ultrashort C–C(Me) Bonds 


Recently, other examples of compounds featuring ultrashort C–C bonds were 

computationally studied [368], however, it should be noted that at present such 

chemistry seems to be difficult to realize experimentally. The first class of such 

derivatives includes those compounds in which a particular C–C bond is pressed 

by encapsulation in an inert cage, either inter- (neopentane inside C 60 ) or intramolecularly 

(cyclophanes in which a methyl group is pressed towards the aromatic 

ring similarly to the recently reviewed molecular ‘iron maidens’ [369]. Until now 

only two examples of the compounds of such type were reported by Pascal et al., 

namely, sterically congested in-methylcyclophane derivatives featuring very short 

C–Me bond distances of 1.475(6) and 1.495(6) Å [370]. However, it seems possible 

to shorten the C–C bond without pushing on it, just by covalent means (for 

example, C–C bonds inside the small covalent cage) [368]. Other examples of ultrashort 

C–C bonds are represented by the stiffened and strapped intracage bonds 

or tied-up bicycloalkanes (for example, interior propellanes formed by strapping 

bicyclo[n.n.n]alkane units together) [368]. All of these calculated molecules feature 

very short C–C bonds; however, the reason for such marked bond shortening is 

not as simple as just their increased s-character. Thus, the calculated C–C bond 

shortening at least by 0.1 Å exceeds the value that would be expected from hybridization 

alone. An additional bond shortening originates from the strain caused 

by the threefold symmetric geometry constraints: the computed ultrashort C–C 

bonds in the above systems are simply unable to achieve normal bond lengths 

without causing notable perturbation in the whole molecule. 

2.6.5 

Conclusions 

The general definition of the ‘normal’ C–C single bond length is somewhat arbitrary: 

the value of 1.54 Å is just an average of the vast number of available crystallographic 

data. However, depending on the particular case, the ‘normal’ C–C bond 

can be significantly stretched or shortened; consequently the two extremes, the 

ultralong C–C bonds and the ultrashort C–C bonds, may deviate from the average 

value by ca. 0.10 Å (6.5%) each, thus giving rise to the overall variation of ca. 0.20 Å 

(13%) with respect to the average value of 1.54 Å. In this section we have discussed 

in detail the two most important contributions to the field of ultrashort C–C bonds: 

derivatives of tricyclo[2.1.0.0 2,5 ]pentanes with squeezed endocyclic bridging C–C 

bonds and coupled cage compounds featuring contracted exocyclic intercage C–C 

bonds. The origin of the bond shortening in these two cases is clearly different, 

because in tricyclo[2.1.0.0 2,5 ]pentanes the short C–C bond is endocyclic and highly 

bent, whereas in the coupled cage compounds the short C–C bond is exocyclic 

and nonbent. After the characterization of bi(tetrahedranyl) 170, manifesting the 

shortest acyclic nonbent C–C single bond between two saturated tetracoordinate 

carbon atoms, the next prominent candidates for molecules featuring ultrashort 

91


C–C bonds would be polycyclic compounds, in which the particular C–C bond is 

inter- or intramolecularly involved in the cage fragment. This job has already been 

approached computationally–now it is the experimentalists’ turn. 


We are greatly indebted to all our coworkers, who have made an invaluable experimental 

contribution to this work and whose names are listed in the references. 

This work was supported by a Grant-in-Aid for Scientific Research (Nos. 17550029, 

19105001, 19020012, 19022004, 19029006) from the Ministry of Education, 

Science, Sports, and Culture of Japan. 

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3 

Distorted Alkenes 

3.1 

Nonplanar Alkenes 

Dieter Lenoir, Paul J. Smith and Joel F. Liebman 

3.1.1 

Introduction and Context 

Sterically-strained alkenes are important species in the study of physical organic 

chemistry [1]. Steric strain can result in torsion (twisting) and/or bending of 

the double bond [2]. Bending can occur in a syn- or anti-mode fashion as the 

two pairs of geminal groups on the double bond crinkle towards each other or 

apart. Strain diversely affects alkenes: (a) energy, measured by standard heat 

(enthalpy) of formation (e.g. calorimetric measurement of heat of combustion 

and hydrogenation); (b) kinetics, e.g. as measured by thermal (Z)/(E)-rotational 

barriers; (c) reaction mechanisms and reactive intermediates. While energetics and 

kinetics dominate our interest, examples of such special behavior are sprinkled 

throughout this chapter. 

3.1.2 

Bridgehead Alkenes 

Bridgehead alkenes (also discussed in Section 3.2) have been widely investigated 

[1–3]. For example, consider adamantene 1 [4] shown below. According to Bredt’s 

rule, which is the absolute proscription against bridgehead double bonds, this 

species should not exist. Highly strained bridgehead alkenes such as 1 cannot be 

isolated, since they tend to dimerize spontaneously after their formation. Like some 

other bridgehead alkenes, 1 has been trapped after its formation in solution by 

cycloaddition with butadiene [5]. Wiseman [6] has developed a rule to predict the 

stability of bridgehead alkenes by comparing the stability of these species with that 

of the corresponding E-cycloalkene that serves as the substructure thereof. 1 is a 

derivative of E-cyclohexene, which does not exist under normal conditions, while 

alkene 2 is a derivative of the stable E-cyclooctene and does exist. There is another 



ISBN: 978-3-527-31767-7 

103

104 3 Distorted Alkenes 

general rule developed by Maier and Schleyer [7] to predict whether a cyclic alkene 

can be isolated as a monomer: if the calculated strain energy difference of alkene 

and the corresponding saturated hydrocarbon (OS, the ‘olefin strain’) exceeds a 

value of 21 kcal mol –1 , then the olefin tends to dimerize at room temperature. 

Several highly strained alkenes have been isolated in a cryogenic matrix [8a], 

e.g. 1 and the related aza [8b] and chloro [8c] derivatives. However, 2-(1-adamantyl) 

adamantene cannot be isolated because of fast rearrangement to 3-(1-adamantyl)- 

4-protoadamantylidene [8d]. E-cycloheptene is the smallest E-cycloalkene that can 

be isolated at –40 °C, rapidly transforming to its Z-isomer at room temperature 

[9] through a low barrier biradical Z/E transition state. 

3.1.2.1 t-Butyl-substituted Ethylenes 

t-Butyl is sterically an extremely bulky alkyl group. Sequential addition of this 

group to a C=C double bond leads to a significant increase in strain. The five 

alkenes 3–7, as both the E and Z-isomers, have been prepared. The E-isomers 

can be prepared by McMurry coupling [10] from the corresponding ketones and 

their Z-isomers by photochemical Z/E isomerization and subsequent separation 

using column chromatography. The heats of formation have been determined 

from the measurement of the corresponding heats of hydrogenation [11]. Strain 

energies have been calculated from these values; the highest strain energy value 

of 31.5 kcal mol –1 is found for the Z-isomer of 3,4-diethyl-2,2,5,5-tetramethylhex- 

3-ene 7 [11]. 

The gas phase Z/E barriers of these alkenes have been measured using high 

temperature, low pressure, shock tubes. The values for these alkenes vary greatly 

with a range of ca. 30 kcal mol –1 . The so-called inherent value, the barrier of 

ethylene, has been calculated to be 65.9(0.9) kcal mol –1 [11] (Parentheses convey 

uncertainties in measured values). Some of their isomers, the geminal 1,1-dit-butyl-2,2-dialkyl 

ethylenes, 8 and 9, have been synthesized, and their electrochemical 

oxidation potentials leading to radical cations investigated [12a]. 9 is 

more strained by 9.2 kcal mol –1 than its regioisomer E-7, an effect of geminal vs 

vicinal t-butyl groups in compound 9 [12b]. 

Strain energies of these highly crowded Z-ethylenes can be correlated with rotational 

Z/E barriers [13] using an exponential function, see Figure 3.1 combining 

thermodynamic and kinetic data. Other functional forms relating rotational 

barriers and strain energies are of lower accuracy [13]. 

As recently reviewed [14], tetra-t-butylethylene 10 remains unprepared despite 

numerous attempts by different research groups. However, many tied-back 

derivatives are known [15] such as 11 and its isomer, syn-fenchylidenefenchane.

3.1 Nonplanar Alkenes 

These species have crystallographically-determined twist angles of 17.0° and 11.8°, 

respectively, and, as their direct equilibration with CH 2 Cl 2 /AlCl 3 shows, their free 

energies differ by only 0.2 kcal mol –1 at 250 °C. However, their reactivity with 

elemental bromine is markedly different, the former rapidly reacting and the other 

very slowly and both yielding ill-defined, but different sets of products. 

Figure 3.1 Correlation of strain energy of Z-ethylenes with Z/E barriers, 

Ea,rot = 64.024exp(-0.0176SE), R 2 = 0.9986. 

105


Some ‘monomeric’ carbenes could be dimerized to the desired alkenes. 

However, di-t-butylcarbene does not dimerize to the hoped for 10 since the 

carbene kinetically prefers intramolecular insertion into the methyl group leading 

to 1,1-dimethyl-2-t-butylcycloprop-ane calculated to be of lower thermodynamic 

stability than the desired olefin [16]. 

3.1.2.2 Investigations of t-Butylated Ethylenes and Other Acyclic Alkenes 

We now turn our attention to other nonplanar alkenes for which thermochemical 

measurements, enthalpies of formation, combustion and/or hydrogenation have 

been reported. The most significant example of this is (Z)-1,2-di-t-butylethylene 

(2,2,5,5-tetramethylhex-3-ene) 3. As cited in a recent monograph on heats of hydrogenation 

[17], there are two reported measurements, both in glacial acetic acid, 

as opposed to the recommended [18] medium of heptane or other nonpolar, inert 

gas mimicking media. The two values are 36.2(0.2) and 37.7(0.1) kcal mol –1 from 

which we derive an average or consensus value of 37.0(0.8) kcal mol –1 . This value 

may be compared with those of a variety of strain-free analogs. Perhaps the most 

direct analogy is with its own (E)-isomer for which the corresponding two primary 

or directly measured values are 26.9(0.1) and 28.1(0.2) kcal mol –1 resulting in a 

consensus value of 27.5(0.6) kcal mol –1 . This suggests some 9.5 kcal mol –1 strain 

energy associated with the vicinal t-butyl groups. Indeed, the difference in hydrogenation 

enthalpies between (Z)- and (E)-alkenes, where no such destabilization 

for the (Z)-isomer is expected beyond about one kcal mol –1 . 

Two other comparisons may naturally be made. The first is with the 1,2-dineopentylethylenes 

12 for which the hydrogenation enthalpies of the (Z) and (E)isomers 

are 26.9(0.1) and 26.0(0.1) kcal mol –1 , respectively, with a difference of 

0.8(0.2) kcal mol –1 . 12 in both the (Z)- and (E)-forms is essentially unstrained. 

The other comparison is with 6, the 1,2-dimethyl derivative of 1,2-di-t-butylethylene. 

The relevant hydrogenation enthalpies for the (Z)- and (E)-isomers are 

43.7(0.2) and 37.4(0.2) kcal mol –1 , respectively, suggesting that the former is only 

6.3(0.3) kcal mol –1 more strained than the latter one. This difference is smaller 

than for the aforementioned difference of the parent di-t-butylethylenes. This 

does not mean that there is less strain in the dimethyl species. Rather, the hydrogenation 

enthalpy is higher for the dimethyl compounds than the corresponding 

parent olefins. This reminds us of the importance of carefully defining reference 

species and strain energies, when making comparisons even for seemingly highly 

related species. 

Reactivity considerations accompany discussions of the large strain energy of 

(Z)-di-t-butylethylene. For example, despite its considerable destabilization, acidcatalyzed 

isomerization to the much more stable (E)-species does not proceed [19]. 

Both isomers of 3 are readily chlorinated: while the (Z)-isomer cleanly yields the 

classical vicinal species, the (E)-isomer results in an isomerized 1,3-dichloride [19, 

20]. Photochemically, the isomers of 3 interconvert, also forming 1,1-dimethyl- 

2-neopentylcyclopropane [21]. Upon irradiation, tri-t-butylethylene, with strain 

presumably interpolating species 3 and 10, results in 1,1-dimethyl-2,3-bis-(t-butyl) 

cyclobutane and 1,2-dimethyl-3-bis(t-butyl)methylcyclopropane.


3.1.2.3 Cyclo and Bicycloalkenes … and on to Polycyclic Analogs 

Let us now consider monocyclic species, and start with cycloalkenes [22]. Solution 

phase hydrogenation enthalpy measurements have also been useful here for 

the understanding of strain and stability of these species. Under comparable 

conditions of an inert media, or otherwise correcting for solvent effects, the difference 

of hydrogenation enthalpies for (Z)- and the distorted (E)-cyclooctene 

13 is about 11.5(0.5) kcal mol –1 ; the latter value is reduced to 9.8(0.4) kcal mol –1 

in acetic acid solution, suggestive of enhanced solvation of the strained � bond 

in the (E)-species over that of its more stable (Z)-isomer. For the corresponding 

cyclononenes, cyclodecenes and cyclododecenes in acetic acid, hydrogenation 

of the (E)-isomers is more exothermic than those of the (Z)-isomers by 2.9(0.1), 

3.3(0.1) and 0.5 kcal mol –1 , respectively, and the (Z)-isomers are somewhat more 

stable than their (E)-counterparts. By contrast, the related acyclic 3-hexene shows 

the (E)-isomer to be more stable by 1.5 kcal mol –1 as determined by hydrogenation 

in hydrocarbon solution. Summarizing, the additional strain in these higher 

cycloalkenes, beyond that of their medium size rings, is minimal. 

Introducing additional unsaturation brings us to the corresponding cycloalkadienes 

[23], in particular the cycloctadienes. Changing one (Z)- linkage to (E)- in 

the 1,3-isomer 14 increases the hydrogenation enthalpy to 15 kcal mol –1 (no experimentally 

determined data for the (E,E)-isomer is available). For the 1,5-isomer 

15, the hydrogenation enthalpies increase in the order (Z,Z) < (Z,E) < (E,E) with 

sequential differences of 12 and 9 kcal mol –1 . The difference between the values for 

the (Z)- and (E)-cyclooctene is about 12 kcal mol –1 , suggesting little thermochemical 

consequence of the additional unsaturation. 

Consider now [22] singly unsaturated bicyclic species. Table 3.1 gives the hydrogenation 

enthalpies of the derivatives of bicyclo[m.n.0] alkenes 16 wherein the 

double bond spans the 0 bridge, always m = n, and some related polycyclic species. 

For comparison, the corresponding data for the acyclic 2,3-dimethyl-2-butene 

Table 3.1 Hydrogenation enthalpies (kcal mol –1 ) of some bicycloalkenes and related species. 

Compound �H H2 Method and comments [Ref.] 

Tetramethylethylene 26.2(0.1) Classical calorimetry 

16, m = n = 1 79(10) Gas phase ion, radical, radical ion cycle [24] 

16, m = n = 2 43(2) Classical hydrogenation calorimetry of 

bicyclo[2.2.0]hexene and -hexane 

17, � 1,5 -dehydroquadricyclane 91(5) Gas phase ion, radical, radical ion cycle [25] 

18, cubene 88(5) Gas phase ion, radical, radical ion cycle [26] 

16, m = n = 3 27(2) Quantum chem calculation for olefin [25, 27] 

19, dodecahedrene 64(4) Gas phase ion comparison [28] 

107


(TME) are also included there. Cubene for which the values are also given in 

Table 3.1 is briefly discussed in Chapter 2.3. 

Enthalpies of formation of many of the aforementioned olefins of interest are 

unavailable because we lack the necessary data for many ancillary species such 

as the saturated cubane [29] (see, however, the discussion of the cubane heat of 

formation in Chapter 1.2) and dodecahedrane [30]. We thus defer making any 

thermochemical comparisons with singly, and a fortiori, the multiply-unsaturated 

derivatives of dodecahedrane [31] and diverse hydrogenated fullerenes (presented 

in Chapter 5.6) and their derivatives [32], which are fascinating, clearly strained 

and worthy of further thermochemical study. For the latter we also need reliable 

heat of formation information [33] for the plausibly strain-free bicyclo[4.4.0]dec- 

1(9)-ene (16, m = n = 4). Other data are absent: for example, the reader might wish 

to use gas phase ion reaction results of the acyclic 2,3-dimethyl-2-butene. This is 

not achievable because the stability of the radical anion of the bicyclobutene arises 

precisely from its nonplanarity [34] as ‘simple’ alkenes fail to bind the defining 

extra electron [35]. 

3.1.2.4 Adamantylideneadamantane and its Derivatives 

While the double bond of adamantylideneadamantane 20 is nearly planar [36] 

when R = H, the three derivatives substituted at the trans allylic bridgehead 

positions show severe twisting and out-of-plane bending of the double bond. 

[37] The calculated strain energy from Allinger’s MM2 method, increases in the 

order; 1,1�-dimethyl, 21.6 kcal mol –1 , 1,1�-diethyl, 28.1 kcal mol –1 compared to the 

parent alkene. The X-ray structure of the ethyl derivative shows a torsion angle of 

12.3° with a pyramidalization angle of 8.9° accompanied by significant elongation 

of the C=C double bond and the vinylic bonds. This highly crowded structure of 

the ethyl derivative has been used to test, and affirm [37] the validity of Allinger’s 

MMP2 method for highly crowded ethylenes. The corresponding 1,1�-diphenyl 

derivative of adamantylideneadamantane (singled out to presage phenylated 

olefins in the following section) has also been prepared recently and likewise 

shows [38] the greatest deformation of the double bond of all species 20. In no 

case, however, is there any measured enthalpy of formation or of hydrogenation 

to test any thermochemical conclusion. 

Adamantylideneadamantanes 20, especially the parent hydrocarbon, have greatly 

contributed to our understanding of diverse reactive intermediates. For example, 

the unsubstituted olefin forms solid bromonium ion salts [39] determined crystallographically 

as the triflate salt [40] while all other derivatives of 20 lead only 

to products of addition/elimination when reacting with bromine [41]. An (asymmetric) 

chloronium salt is known as well [42]. Parent 20 readily exhibits radical 

cation derived chemistry as part of reactions involving excited state uranyl ion 

[43a], strong Brønsted acids [43b], and neutral and cationic NO x species [43c, d]. 

3.1.2.5 t-Butyl-substituted and Cyclic Stilbenes 

When both of the vinylic hydrogens in 1,2-di-t-butylethylene are substituted by 

phenyl groups or equivalently those of stilbene are substituted by t-butyl groups,


to form olefins 21, significant geometric change occurs. The increase of strain is 

calculated to be about 18.0 kcal mol –1 . The central double bonds do not show the 

common distortions like torsion or bending, but the vinylic bonds with the phenyl 

groups are rotated into a perpendicular conformation to avoid steric repulsion of 

the t-butyl groups [44]. The UV spectrum shows two isolated phenyl and olefinic 

chromophores [45]. While the resonance stabilization of the stilbene has presumably 

vanished, calorimetric corroboration remains absent. 

The thermal Z/E barrier for these isomers is significantly reduced from the 

value of 42.8 kcal mol –1 for the parent stilbenes [45]. The Ea of the solid E- and 

Z-21 have been determined in the condensed phase [46] in contrast to the gas 

phase values measured for ethylenes 3–7 [11]. DSC measurements for the temperature 

range of 50–250 °C resulted in a value of Ea = 35.3 (1.0) kcal mol –1 as 

determined using an Ozawa-Flynn-Wall analysis [47], in close agreement with 

32.0(1.3) kcal mol –1 obtained by NMR in solution. Various physicochemical 

properties of stilbenes E- and Z-21 have been studied [48]. Their chemical activity 

includes formation of the first stable � complex found for a bromination reaction 

[49]. 

E-1-((2,2-dimethyl-1-tetralinylidene)-2,2-dimethyltetralin, 22, is the most strained 

of these species with its central double bond twisted by 36.7° [50]. The Z-isomer 

of 22 has evaded isolation. However, its fleeting existence has been documented 

by its NMR and UV spectra, when the E-isomer, dissolved in THF was irradiated 

with UV (235 nm) in a NMR quartz tube at –70° leading to the Z-isomer of 22. 

The Z-isomer rearranges back to the E-isomer at room temperature with a low 

thermal barrier of Ea = 21.0 kcal mol –1 [50]. A torsion angle of 73° has been calculated 

for the Z-isomer by MMP2 [50], but recent DFT and ab initio calculations 

give a smaller value of about 42° [51]. 

3.1.3 

Multiply Unsaturated Bicycloalkenes, Homoaromaticity and Cyclophanes 

We now turn to the parent and hydrogenated derivatives of bicyclo[4.4.1]undecapentaene 

and bicyclo[5.3.1]undecapentaene [52]. These hydrocarbons are 

also known as 1,6-methano[10]annulene 23 and 1,5-methano[10]annulene 24, 

respectively, and may be recognized as strained species with distorted aromatic 

rings. The hydrogenated counterparts of the former species and those of the 

related bicyclo[4.4.2]dodecapentaene 25 series with an ethano bridge, show [53] 

an interesting balance between these compounds and the tricyclic species with a 

0 bridge, a full C–C bond, spanning the bridgeheads to form corresponding propellane 

derivatives (e.g. 26). We recognize the bicyclic species as having severely 

twisted double bonds and the possibility of homoaromaticity [54]; the tricyclic 

species look quite classical – at least when compared with small-ring propellanes 

discussed in Chapter 2.1. To our knowledge, there are no measured enthalpies of 

hydrogenation, nor any of formation, for the mostly saturated bicycloundecenes 

and dodecenes with but one double bond for which we may disentangle the effect 

of twisting, independent of any aromaticity or homoaromaticity. 

109


For the same reason we will ignore here the literature calorimetric discussion 

[55] of the hydrogenation of bicyclo[5.3.1]undeca-1,3,5(11)-triene, also known as 

[5]metacyclophane as well as its [6]metacyclophane (a bicyclo[6.3.1]dodecatriene) 

homolog (species 27, n = 5 and 6, respectively) (cyclophanes are discussed in 

Section 4.2). We merely note that these last species hydrogenate readily, as befits 

their distorted benzene derivatives, to form bridgehead monoenes and then the 

hydrogenation stops, or more properly slows below calorimetric detectability.


Model calculations show these monoenes to be ‘hyperstable’ [56], i.e. their hydrogenation 

enthalpy is less than that of standard olefinic species and so they are not 

really within the purview of the current volume on strained species. Perhaps it 

is ‘sour grapes’ to avoid them here because of the lack of data. Interestingly, and 

perversely, the hydrogenation of [2.2]paracyclophane 28 to a mixture of bridgehead 

dienes has been interpreted in terms of hyperstable olefins [57]. However, while 

the totally saturated polycycle has been calorimetrically investigated [58] but the 

hydrogenation was not, we are thwarted in this case as well. 

3.1.3.1 The Most Distorted Ethylenes and Seemingly Simple Analogs 

Contenders for the most distorted olefins are the fulvalene 29 with a distortion 

angle of 66° [59] and the hydrindanylidenehydrindane 30 with an angle of 49° [60]. 

No heat of formation data is known for either compound. 

We conclude this chapter by acknowledging the diverse fulvalene derivatives as 

examples of distorted olefins (and not just 29 and 30), and note thermochemical 

data are essentially absent for this class of compounds. For the parent fulvalene 

31, we find a measurement [61] for the enthalpy of hydrogenation of its (Cr-Cr) 

bis chromium tricarbonyl derivative to the dihydride, [� 5 -C 5 H 4 Cr(CO) 3 H] 2 32 but 

no enthalpy of formation data to even estimate the enthalpy of formation of the 

parent hydrocarbon (as opposed to its bimetallic complex). For heptafulvalene 33, 

the enthalpy of hydrogenation to form cycloheptylidenecycloheptane 34 has been 

111


known for 50 years [62] but as with the above hydrogenated cyclophanes, the last 

step to form bis(cycloheptyl) has evaded calorimetric determination. So, where are 

the synthetic chemists to make the key compounds and calorimetrists to make the 

key measurements? Quantitative understanding of the so much of the chemistry 

and thermochemistry of strained alkenes is still awaited [63]. 

Acknowledgment 

We thank Dr. C. Wattenbach for his help and technical assistance. 

3.2 

Small Ring and Cage Structures Involving Nonplanar C=C Bonds 

Athanassios Nicolaides 

According to introductory textbooks the olefinic carbons of idealized alkenes are 

sp 2 hybridized and the overall geometry is planar. At this geometry the lateral 

overlap of the two p atomic orbitals of the olefinic carbons is maximal and gives 

rise to a strong � bond (Figure 3.2a). Within this model one can think of two ways 

to distort the system and introduce strain. One is a torsional distortion around 

the C–C � bond, giving rise to torsionally strained olefins, also known as twisted 

or anti-Bredt olefins (Figure 3.2b), which are discussed in the preceding section. 

The second is by moving the olefinic carbons away from the plane defined by 

their four substituents, giving rise to pyramidalized olefins. While twisted olefins 

have received considerable attention [64] rather less is known about pyramidalized 

olefins [65–68]. 

Figure 3.2 Planar and distorted geometries of alkenes. 

In torsionally strained olefins the loss of � bonding due to twisting is partially 

recovered by rehybridization of the olefinic carbons, the consequence of which is 

pyramidalization [64c]. On the other hand it is possible to design purely pyramidalized 

alkenes without torsionally strained � bonds as in 38 and 40. 

3.2.1 

Pyramidalized Alkenes 

In a strict sense pyramidalization of alkenes is very common and is expected 

to take place whenever the two faces of the double bond are not symmetrically


equivalent [66]. However, in the majority of the cases the pyramidalization is quite 

small and trivial. Often the term ‘pyramidalized alkene’ implies that pyramidalization 

is large enough to affect the physical and chemical properties of the system 

in a significant way. 

Several ways to quantify pyramidalization have been proposed [68]. The most 

widely used is that by Borden [69], which defines the pyramidalization angle � 

as the angle between the plane containing one of the doubly-bonded carbons 

and its two substituents and the extension of the double bond (Figure 3.2c). 

In terms of the bond angles � and �, � can be obtained from the formula: 

cos � = –cos �/cos (�/2) [66]. 

9,9�-Didehydroanthracene (DDA, 35) was the first pyramidalized alkene to be 

isolated almost 40 years ago by Weinshenker and Greene [70]. The addition of base 

(t-butoxide) to the pyramidalized C=C double bond provided an early demonstration 

that pyramidalized alkenes are unusually reactive with nucleophiles. The most 

cited pyramidalized polyene is fullerene C 60 [71] and its derivatives, discussed 

in Chapter 5. Alkenes with pyramidalized double bonds include cubene [72], 

bicyclo[1.1.0]but-1(3)-ene [73] and its 2,4-bridged derivatives [74], dodecahedrene 

and related compounds [75] and others which have been reviewed recently in an 

authoritative manner [68]. 

This chapter focuses on tricyclo[3.3.n.0 3,7 ]alk-3(7)-enes 36, a family of pyramidalized 

alkenes that has been studied extensively and systematically both experimentally 

and theoretically [66, 68]. 

In a formal sense, bicyclo[3.3.0]oct-1(5)-ene 37 can be thought of as the generator 

of the homologous series 36 by connecting carbons 3 and 7 with a bridge of n 

methylene groups. The first member of this series is 41 in which carbons 3 and 

7 are directly bonded (n = 0). The five-membered rings of 37 are puckered in the 

same direction [76, 77], making the two faces of the double bond non-equivalent. 

As a result the olefinic carbons are trivially pyramidalized with a computed 

pyramidalization angle of � = 5.8° [77]. The small value of � implies that 36 is a 

normal tetraalkyl-substituted alkene. 

113


Table 3.2 Computational data for alkenes 37–41. 

Alkene n a) 

� b,h) 

c,i) 

ELUMO d,i) 

EHOMO �E e,i) 

C=C f,h) 

41 0 61.9 –0.50 0.61 10.14 1.380 72.8 

40 1 53.7 –2.51 0.65 11.09 1.362 52.3 

39 2 42.2 –1.38 0.33 12.54 1.349 37.4 

38 3 27.9 –0.79 0.24 13.22 1.342 17.7 

37 – 5.8 0 0 14.25 1.337 0 

a) 

Number of methylene groups in the bridge. 

b) 

Pyramidalization angle (°) [68]. 

c) 

LUMO energies relative to that of 37 (5.65 eV) [76]. 

d) 

HOMO energies relative to that of 37 (–8.60 eV) [76]. 

e) 

Energy difference LUMO–HOMO (eV) [76]. 

f) 

Bond-length (Å) [73c]. 

g) –1 

Olefin strain energy (kcal mol ) [68]. 

h) 

Computed with the B3LYP/6-31G(d) method. 

i) 

Computed with the SCF/3-21G method. 

OSE g,h) 

No special properties are expected to arise for olefins 36 as long as the length of 

the methylene bridge is sufficiently long (i.e. for a sufficiently large n). However, 

as the bridge becomes shorter it is expected that at some point it will cause considerable 

puckering of the bicyclic moiety and pyramidalization of the double 

bond. Indeed, the computed data (Table 3.2) show that the pyramidalization 

angle increases from about 28° for 38, having a tri-methylene bridge, to about 

62° for 41, where carbons 3 and 5 are directly bonded. Thus, homologous series 

36 provides a nice way of studying systematically the effect of pyramidalization 

on the properties of the C=C double bond as a function of n. 

A qualitative model based on simple molecular orbital theory has been proposed 

as a guide to understanding the properties of these compounds and of pyramidalized 

alkenes in general [66, 76]. The pyramidalization effect on the electronic 

properties of the C=C double bond can be understood on the basis of two factors 

(Figure 3.3). As pyramidalization (�) increases, the overlap between the p atomic 

orbitals forming the � bond decreases and this raises the energy of the bonding � 

MO (HOMO) but lowers the energy of the anti-bonding �* (LUMO). Pyramidalization 

also causes mixing of the � and � MOs, which in terms of atomic orbitals is a 

form of rehybridization. This increases the s character of the atomic orbitals and 

therefore energetically stabilizes both the HOMO and the LUMO. Overall, both 

factors stabilize the LUMO, but have opposite effects on the HOMO and therefore 

they tend to cancel out. Thus, in the case of the LUMO strong stabilization is 

expected, but the energy of the HOMO is expected to stay roughly the same. 

Indeed computations (Table 3.2) find that the energy of the LUMO in pyramidalized 

alkenes 36 is lower with respect to the reference compound 37 and 

decreases further as pyramidalization of the alkene increases. On the other hand,


Figure 3.3 Effect of pyramidalization on the relative energies of the HOMO (�) and LUMO (�*) 

orbitals of an alkene. 

the energy of the HOMO increases, but this is considerably less in magnitude 

than the corresponding decrease of the HOMO, in agreement with the qualitative 

model. Overall a decrease of the HOMO–LUMO gap is predicted with increased 

pyramidalization. 

Olefin strain energy (OSE) [76, 78] is a quantitative measure of the strain of the 

alkene. With alkene 37 serving as the ‘strain-free’ reference point of homologous 

series 36, the computed OSEs show that there is a considerable increase in the 

strain of the alkene with pyramidalization. Based on empirical rules and the computational 

results (Table 3.2) the most pyramidal alkenes of this series 39–41 are 

not predicted to be isolable under normal conditions, while for 38 it is difficult to 

make a secure prediction. As the strain of the � bond increases, one expects the 

�-bond order to decrease and therefore the C=C bond length to increase. Indeed, 

the C=C bond length of 41 is the longest, but it is still sufficiently short to suggest 

that the residual � bond is still strong despite the large OSE. 

This simple qualitative model rationalizes the computational data shown in 

Table 3.2 in a satisfactory way. Additionally, research has sought to confirm this 

model by generating the pyramidalized alkenes 37–41 and studying their physicochemical 

properties [66, 68]. 

In general, pyramidalized alkenes, like other strained molecules, require the 

development of special synthetic methods for their formation. Apart from the 

synthetic challenge, isolation of the most reactive ones is impossible under 

normal conditions and special techniques are needed. Some of the most reactive 

pyramidalized alkenes have been studied directly by spectroscopy in the gas phase 

or after matrix isolation at low temperatures. More often the intermediacy of pyramidalized 

alkenes is deduced from the product analysis and/or from chemical 

trapping of the alkene [68]. 

3.2.1.1 Tricyclo[3.3.11.0 3,7 ]undec-3(7)-ene 38 

Alkene 38 has been synthesized by the reduction of its dimesylate precursor 

38-OMs. Even though 38 is an isolable solid, X-ray crystallographic data for it have 

115


not been reported. Upon exposure to air it oxidizes readily, forming among other 

things its epoxide [79] a reaction characteristic of pyramidalized alkenes [80, 81]. 

Alkene 38 has been characterized spectroscopically by IR, UV, NMR and electron 

transmission (ET) spectroscopy. 

As expected of symmetrically substituted alkenes, the C=C bond stretching 

vibration of reference compound 37 is forbidden by IR spectroscopy but is observable 

by Raman spectroscopy. In contrast, the C=C bond stretch of 38 appears both 

in the IR and Raman spectra. In the IR spectrum of 38 it appears as a very weak 

band and this can be attributed to the fact that pyramidalization of the double 

bond causes the C=C stretching mode to have a transition dipole, which is oriented 

perpendicular to the C–C bond. It is noteworthy that the C=C stretching frequency 

of 38 is 70 cm –1 lower than that in 37, in qualitative agreement with the weaker 

� bond of the former [82, 83]. 

ET spectroscopy has been employed to measure the vertical electron affinities 

(EA) of 37 and 38. Both are negative indicating that in both cases the radical anion 

is not bound. However, the important result is that 38 has a greater EA than 37, 

compatible with the former having a lower-energy LUMO than the latter [79]. The 

well-known property of C 60 to accept electrons forming anions and polyanions 

can be attributed to the pyramidalization of its C=C double bonds. 

The adiabatic ionization energy (IE) as given by photoelectron spectroscopy is 

found to be lower in 38 than in 37 by 0.31 eV. This difference can be related via 

Koopmans’ theorem [84] to a higher-energy HOMO in 38 than in 37, in agreement 

with the computational data of Table 3.2. 

The UV spectrum of 37 does not show a maximum above 200 nm. According 

to electron energy loss (EEL) spectroscopy, the � � �* singlet excitation requires 

6.54 eV corresponding to photons of 190 nm wavelength. This is very close to the 

� � �* transition of tetramethylethylene (6.61 eV, 188 nm) demonstrating that 37 

is a rather standard alkene. Olefin 38 exhibits a UV absorption with � max = 217 nm 

(5.73 eV). This red shift of 27 nm (or 0.81 eV) is in qualitative agreement with the 

smaller HOMO–LUMO gap of 38 as compared to 37 [79]. 

Protonation of a pyramidalized olefin is expected to relieve olefin strain and 

therefore 38 should have a substantially greater proton affinity, PA, than 37. The 

experiment was carried out in the gas phase using the kinetic method and established 

that the PA of 38 is indeed greater than that of 37. The apparent PA of 38 

was measured at 219 kcal mol –1 or 22.5 kcal mol –1 higher than that of 37. On the 

other hand, the calculations predicted that the difference in PA between 37 and

Table 3.3 Available spectroscopic data for alkenes 37–40. 

Alkene UV a) 


IR b) 

40 1496 [88] 

39 245 [82] 1557 [82] 

38 217 [79] 1615 [79] –2.27 [79] 7.80 [79] 208 [85] f) 

37 190 [68] 1685 [68] –2.44 [79] 8.11 [79] 196.5 [85] 

a) 

nm. 

b) –1 

cm . 

c) 

Vertical electron affinities (eV). 

d) 

Adiabatic ionization potentials (eV). 

e) 

Proton affinities (kcal/mol). 

f) 

Based on the experimental PA of 37 and the computed difference of 11.7 kcal/mol 

between the PAs of 37 and 38. 

38 should be only 11.7 kcal mol –1 . Further experimentation and careful analysis 

of the experimental data with the help of calculations revealed that under the 

experimental conditions, skeletal rearrangements are possible and that what was 

measured was not the PA of 38, but rather of a diene isomer of 38. The skeletal 

rearrangement seems to take place in the protonated form of 38 and involves a 

retrograde vinylcyclopropane rearrangement. This type of rearrangement is endothermic 

for alkene 38, but it is known for the lower homologs of 38 [85]. 

3.2.1.2 Tricyclo[3.3.10.0 3,7 ]dec-3(7)-ene 39 and tricyclo[3.3.9.0 3,7 ]non-3(7)-ene 40 

Alkene 39 was first formed by decarboxylation of lactone 39-lac both in solution 

and in the gas phase (Figure 3.4) [86]. Evidence for the formation of 39 in solution 

was given by trapping the alkene as the Diels–Alder 39-DA. Pyrolysis at 530 °C led 

to the isolation of alkene 43, an isomer of 39, which can be formed via a retrograde 

vinylcyclopropane rearrangement. From experimental data it was estimated that 

the strain in 39 must be at least 21 kcal mol –1 in order for the rearrangement to 

be energetically favorable [87]. 

Figure 3.4 Pyrolysis of lactones 39-lac and 40-lac. 

EA c) 

IP d) 

PA e) 

117


When the pyrolysis was carried out at lower temperature (410 °C) and the pyrolysate 

trapped in an Ar matrix at 10 K, alkene 39 was detected by its IR absorption 

at 1557 cm –1 , 58 cm –1 lower than the C=C stretching frequency of 38 [83]. 

Warming up of the matrix led to the isolation of the dimer of 39, cyclobutane 

39-dim, providing more evidence for the formation of 39. The UV spectrum from 

the matrix isolated 39, exhibited a � max red-shifted to that of alkene 38. 

Lactone 40-lac proved more difficult to pyrolyze than 39-lac. Even at 550 °C only 

50% of the lactone reacted, but not by loss of CO 2 but by isomerizing to ketoketene 

47, which was identified by IR spectroscopy and by chemical trapping with 

methanol. From this remarkable rearrangement it was estimated that the strain in 

40 exceeds 39 kcal mol –1 [88]. At temperatures higher than 550 °C decarboxylation 

did take place, but only traces of the dimer of 40 (40-dim) were isolated, indicating 

that this is not a good route for the formation of 40. The major product of the 

pyrolysis was diene 46, presumably formed via alkene 45, which is the product 

of the retrograde vinylcyclopropane rearrangement of 40. 

Dimesylate 36-OMs (n = 1, 40-OMs) can be easily synthesized from the corresponding 

diol 40-OH. However reduction with Na/Hg does not yield alkene 40. 

A better precursor for alkene 40 is diiodide 40-I (36-I, n = 1). Treatment of 40-I 

in THF at –78 °C with n-butyllithium gave quantitatively the dimer 40-dim [89]. 

When the reaction was repeated in the presence of 1,3-diphenylisobenzofuran 

(1,3-DPIBF, 42), the expected Diels–Alder adduct (40-DA) was isolated. The 

reduction can also be carried out at room temperature with Na/Hg again giving 

products that imply the intermediate formation of 40. Gas-phase dehalogenation 

made possible the matrix isolation of alkene 40 and its IR spectroscopic characterization. 

Like its higher homologs, alkene 40 exhibits a C=C stretch as a weak band 

in the IR spectrum, with a frequency 61 cm –1 lower than that in the spectrum of 

39. Unfortunately, the presence of metal atoms in the matrix prevented the UV 

spectrum from being obtained, although indirect experimental evidence for the 

existence of a long wavelength absorption in 40 was provided.


3.2.1.3 Tricyclo[3.3.0.0 3,7 ]oct-1(5)-ene 41 

Alkene 41 being the consummate member of the homologous series 36 has 

attracted considerable attention. Neither dimesylate 41-OMs, n = 0 nor the bistriflate 

41-OTf, n = 0 (Tf = CF 3SO 2 – ), give the desired alkene by reduction by Na/Hg, 

yielding instead the starting material [68, 90]. In 1996 the first synthesis of 41 

was reported [91]. Reduction of diiodide 36-I, n = 0 (41-I) afforded a mixture of 

the dimer 41-dim and the diene 41-dn, with the latter thermodynamically more 

stable than the former. In the presence of 1,3-DPIBF the Diels–Alder product 

41-DA (36-DA, n = 0) was formed. Unfortunately, no spectroscopic information 

is available for alkene 41. 

Some derivatives of 41 (41� [92], 43 [93] and 44 [94]) have been successfully 

generated, but like 41 they have not been characterized spectroscopically. Evidence 

for their formation is based on product analysis. The least perturbed homolog of 

41 that has been studied is 41�, with two Me groups on the bridge, which can be 

formed from its diiodide precursor (41�-I). Evidence for the generation of 41� is the 

isolation of the dimer 41�-dm, when 41�-I is reduced in solution, and the isolation of 

the expected Diels–Alder products when the reduction is carried out in the presence 

of 1,3-DPIBF (42) or of 11,12-dimethylene-9,10-dihydro-9,10-ethanoanthracene 

(DDE, 48). Some extra experimental evidence for formation of 41� is the detection 

and spectroscopic characterization of its (Ph 3 P) 2 Pt complex 41�-Pt [95]. 

3.2.1.4 (Ph 3 P) 2 Pt Complexes 

Organic reactive intermediates can be stabilized by complexation to organometallic 

fragments. A number of such complexes containing highly reactive � bonds, 

have been isolated as (Ph 3P) 2Pt complexes. For example, the (Ph 3P) 2Pt complexes 

of cycloalkynes that have bent triple bonds: cyclopentyne, cyclohexyne, and cycloheptyne 

are known. (Small-ring cycloalkynes are discussed in Chapter 7.) The 

(Ph 3P) 2Pt complexes of the pyramidalized alkenes 36, n = 1–3 (36-Pt, n = 1–3) have 

been isolated and studied spectroscopically in detail [95]. Recently, the complex 

of the 41� (41�-Pt) has also been reported. 

119


Tetrasubstituted alkenes do not ordinarily form stable complexes with (Ph 3P) 2Pt. 

Indeed, the (Ph 3 P) 2 Pt complex of 37, the unbridged generator of alkenes 36, was 

not formed [96]. This implies that pyramidalization of the double bond is responsible 

for the formation of complexes 36-Pt, n = 1–3 and 41�-Pt. As mentioned 

above, the major electronic effect of double bond pyramidalization is to lower 

the energy of the �* LUMO, making it a better acceptor of electron density from 

the metal fragment [79, 97]. This simple picture is capable of rationalizing most 

of the NMR data of Table 3.4. Thus, with increased pyramidalization, 13 C-NMR 

resonances move to higher field and 195 Pt ones to lower field although the latter 

not in a monotonic way. The trend of the 31 P chemical shifts is not that predicted, 

but it is compatible with transfer of electron density from the (Ph 3 P) 2 Pt fragment 

to the complexed olefin as judged by other experimental data [96, 98, 99]. 

Table 3.4 13 C (of the olefinic carbons), 195 Pt and 31 P chemical shifts (ppm) and 1 J Pt–P and 1 J Pt–C 

coupling constants (Hz). 

41�-Pt a) 

40-Pt a) 

39-Pt a) 

38-Pt a) 

� C � Pt � P 

–4956.6 30.7 b) 

1 JPt–P 

2850 

1 JPt–C 

66.9 –5058.5 30.5 2960 407 

74.9 –5105.5 31.1 3115 343 

78.8,79.2 c) 

–5092.5 32.2 3332 296 d) 

(Ph 3 P) 2 PtC 2 H 4 (39.2) –5146.5 34.1 3740 194 

a) Benzene-d6 , 298 K. 

b) Actual measured value (� 32.4 ppm) adjusted so that the value for the 31 P chemical shift of 

(Ph 3 P) 2 PtC 2 H 4 of Ref. [95] (� 35.8 ppm) becomes the same as the one reported in Ref. [96] 

(� 34.1 ppm). 

c) Toluene-d8 , 229 K. 

d) Toluene-d8 , 338 K.


The 1 J Pt–P and 1 J Pt–C coupling constants follow the expected trend. In particular, the 

former decreases as the latter increases in a linear fashion, implying that electron 

density shifts away from the metal fragment and the Pt–C bonding strengthens as 

pyramidalization of the olefin increases. Thus, complex 41�-Pt is expected to have 

the largest alkene-Pt(PPh 3) 2 binding energy in this series, an expectation that is in 

agreement with computational findings [95]. Despite the stronger C–Pt bonds in 

41�-Pt, this complex was found to react with ethanol to give 45. This is interesting 

because under the same conditions complexes 36-Pt, n = 1–3 are recrystallized 

from ethanol. The alcoholysis of 41�-Pt with ethanol is a rather unusual reaction 

[97] and paves the way for studying the reactivity of such complexes with the aim 

of using them as synthetic equivalents of reactive pyramidalized olefins. 

3.2.2 

Conclusions 

In conclusion, the above synthetic data for alkenes 36, n = 0–3 are summarized 

in Figure 3.5. The general way for the formation of these alkenes is by reduction 

of the corresponding diiodide (for the first two members of the homologous 

series 36) or dimesylate precursor (for the higher homologs 38 and 39). Alkene 

38 is stable enough to be isolated under normal conditions provided oxygen is 

excluded [79]. The more pyramidalized alkenes 39–41, cannot be isolated under 

the same conditions. Instead, in the absence of any trapping reagent their formal 

Figure 3.5 Generation and trapping of tricyclo[3.3.n.0 3,7 ]alk-3(7)-enes (36). 

121


[2+2] dimers (36-dim) are isolated. The strained 36-dim, n = 0 (41-dim) easily 

isomerizes to the diene 41-dn. When the reduction is carried out in the presence 

of 1,3-DPIBF 42, the expected Diels–Alder product is isolated. In the presence 

of (Ph 3P) 2PtC 2H 4 the corresponding organoplatinum complexes 36-Pt of 38–40 

have been isolated. Although, the experimental structure of 38 has not been 

determined yet, this alkene has been characterized extensively spectroscopically. 

Properties of the alkenes 38–40 are also quite well understood, even though the 

UV spectrum of 40 has not been recorded yet. Although, synthetic routes to 41 

have been developed, direct spectroscopic observation of 41 is still lacking, and 

its (Ph 3P) 2Pt complex (41-Pt) is still not known. 

Acknowledgement 

This work has been supported significantly by the U.S. National Science Foundation 

and also by the Research Promotion Foundation of Cyprus. 

3.3 

Strained Cyclic Allenes and Cumulenes 

Richard P. Johnson and Kaleen M. Konrad 

3.3.1 

Introduction 

Cycloalkenes are well known to any student of chemistry but the more exotic 

homologous series of cycloallenes 49–54 and cyclobutatrienes 55–60 rarely find 

their way into textbooks. Decreasing ring size in these two series leads to a rapid 

increase in strain and reactivity caused by deformation of the normally linear 

allene or butatriene. 

Among milestones in the history of cyclic cumulenes, Ball and Landor first 

described the chemistry of 51–53 in 1961 [101], but the synthesis of a derivative 

of 1,2-cyclopentadiene 50 awaited the efforts of Balci and co-workers which were 

reported in 2002 [102]. Evidence for derivatives of 49 was found in studies on enyne 

photochemistry described by Meier and König in 1986 [103]. In the butatriene se-

3.3 Strained Cyclic Allenes and Cumulenes 

ries, a stable 10-membered ring homolog was described by Moore and Ozretich in 

1967 [104]. Synthesis of 57 as a reactive intermediate was reported by Shakespeare 

and Johnson in 1990 [105]. The parent hydrocarbon 56 remains unknown, but 

heterocyclic analogs have been reported by Wong and co-workers [106, 107]. 

Essential features of this structural map are thus known. Experiment and theory 

support the existence of cyclic allenes in all ring sizes 49 through 54 (and larger), 

but structures smaller than an eight-membered ring are not isolable without 

special features. The smallest cyclic butatriene 55 is predicted not to be an energy 

minimum but instead is a transition state on the C 4H 2 energy surface. There is 

evidence for cyclic butatrienes in ring sizes 56 through 60 but only 60 and larger 

ring homologs have been isolated. 

This chapter presents the structure, syntheses and chemical behavior of four- to 

nine-membered ring cyclic allenes and butatrienes, illustrating the status of this 

field with selections from recent chemistry. Readers are referred to earlier reviews 

for more comprehensive treatments [108–111]. 

3.3.2 

Allene � Bond Deformations and Strain Estimates 

In a ring of fewer than ten carbons, the allene group is bent and twisted toward 

planarity. Johnson and co-workers recently estimated strain by applying isodesmic 

and homodesmic relationships [112]. As one example, allene functional group 

strain in 51 may be estimated from the following isodesmic reaction, using 

B3LYP/6-311+G(d,p) energies. This equation exchanges strained and unstrained 

functional groups. 

Figure 3.6 summarizes estimates for allene strain, i.e. strain primarily localized 

in that functional group, as well as total molecular strain, and observed chemical 

behavior for cyclic allenes. Only modest levels of strain result in kinetic instability 

for allenes of eight carbons or fewer; allene 53 and most derivatives dimerize 

quickly at ambient temperature [113, 114]. 

Figure 3.6 Strain estimates and chemical behavior of cyclic allenes. 

123


Ring constraints alter the barrier to allene chiral inversion. For parent allene 63, 

the transition state is best described as planar diradical 64, with one � electron in 

plane and three out of plane [115, 116]. Brudzynski and Hudson [117] measured 

a torsional barrier for allene of 43 kcal mol –1 ; this is easily reproduced by DFT 

calculations [116]. With cyclic allenes, the predicted inversion barrier decreases 

in proportion to strain. For 1,2-cyclohexadiene 51, the predicted inversion barrier 

65 is 14.1 kcal mol –1 [118]. This interconverts the M and P enantiomers (helicity 

nomenclature), consistent with experiments by Balci and Jones that demonstrated 

the chirality of 51 [119]. Smaller homolog 50 is predicted to be chiral but with an 

inversion barrier of < 1 kcal mol –1 [112], while 1,2-cyclobutadiene 49, should be 

a nearly planar diradical [120]. 

3.3.3 

Four- and Five-membered Ring Allenes 

Can a remarkable substance such as 1,2-cyclobutadiene 49 exist? In a 1986 paper, 

Meier and König reported the photorearrangement of 66 to 68 (Scheme 3.1) and 

made the courageous suggestion of 67 as a likely intermediate [103]. Seven years 

later, Johnson and co-workers showed this to be a general singlet photorearrangement 

[120]. Photoreaction of 69 is typical; irradiation results in the interconversion 

of 69 and 71. The most straightforward mechanism is excited state closure to afford 

ground state 70, which can open thermally in either direction. MP2, CASSCF and 

DFT calculations all support the existence of a 1,2-cyclobutadiene intermediate. 

Scheme 3.1 Photorearrangements of enynes. 

Balci’s group reported preparation of the first 1,2-cyclopentadiene derivative in 

2002. Endo fluoro isomer 72 reacts with methyllithium (Scheme 3.2) to give 73, 

which was trapped with furan to give 74 [102]. This achievement followed many 

unsuccessful attempts by the same authors [110]. The same group later reported 

evidence for 1-phenyl-1,2-cyclopentadiene [121].

Scheme 3.2 Synthesis of a 1,2-cyclopentadiene. 

3.3.4 

1,2-Cyclohexadienes 


1,2-Cyclohexadienes are surprisingly common reactive intermediates. The parent 

structure 3 is now securely characterized as a chiral allene [119]. Early synthetic 

routes to 51 included treatment of 79 with strong base (Scheme 3.3) and reaction 

of 77 with methyllithium [101, 108, 122]. This latter reaction presumably proceeds 

through carbene 76 and/or the related carbenoid. This was an early example of the 

Doering-Moore-Skattebøl rearrangement [123–126]. Schleyer and co-workers have 

predicted a barrier of only 0.5 kcal mol –1 for disrotatory ring opening of 76 [127]. 

Shakespeare and Johnson have reported that 51 is easily generated by treatment 

of 78 with fluoride ion [105]. Diels–Alder reaction of enynes is described in a later 

section. Werstiuk and co-workers have reported the photoelectron spectrum of 51 

which was generated from 75 by retro-cycloaddition [128]. Although there have 

been several earlier reports of matrix isolated 51, agreement with the computed 

allene vibrational frequency remains uncertain [129–131]; a definitive route to 

matrix isolated 51 is needed. 

Scheme 3.3 Synthetic routes to 1,2-cyclohexadiene. 

Scheme 3.4 summarizes common reactions of 51 and its derivatives. Dimerization 

and alkene or diene additions proceed rapidly through diradicals 80 or 81 to 

give [2+2] and [2+4] cycloadducts [111]. Caubere reported that nucleophiles such 

as enolates add to 3, affording a wide range of products [132]; of course this is 

facilitated by strain relief. Christl has presented a detailed review of [2+2] and 

[2+4] cycloadditions to 51 and derivatives, summarizing the evidence for stepwise 

reactions [111]. Computational studies by Tolbert, Houk and co-workers support 

the conclusions of a stepwise mechanism [133]. 

125


Scheme 3.4 Reactions of 1,2-cyclohexadiene. 

It has long been recognized that Diels–Alder cycloaddition of enynes with 

alkenes should give 1,2-cyclohexadienes. Computational studies by several groups 

have yielded activation energies only ca. 6 kcal mol –1 above a conventional Diels– 

Alder reaction [134, 135]. Recent examples of this surprising reaction are shown 

in Scheme 3.5. Miller and co-workers showed that dienyne 82 undergoes double 

addition of fumarate to give 84, presumably through allene 83 [136]. Johnson 

and co-workers studied the first intramolecular version of an enyne plus ene 

cyclo addition [134]. Flash vapor pyrolysis (FVP) of enyne 85 cleanly afforded 87; 

the most straightforward mechanism is cycloaddition to give 86, followed by 

cycloreversion. Maas and co-workers have reported preparation of 90 by solution 

thermolysis of pyridinium triflate 88 [137]. 

Many heterocyclic analogs of 1,2-cyclohexadiene 91 have been prepared as 

intermediates and these provide routes to complex ring systems [111, 138–141]. 

Scheme 3.5 Examples of enyne Diels–Alder reactions.


Several different strategies have led to isolable derivatives of 51. Longer carbon– 

heteroatom bonds and steric protection arising from bulky substituents explain 

the isolability of 92 [142] and 93 [143]. Metal complexation as in 94 provides an 

additional approach to kinetic stabilization [144, 145]. 

3.3.5 

1,2,4-Cyclohexatrienes 

1,2,4-Cyclohexatrienes (‘isobenzenes’) and their benzannelated derivatives are 

reactive intermediates that lie at the center of an impressive range of chemistry. For 

61, present results support a chiral structure, and a diradical transition state 61a 

for inversion, much as described for 51. Replacement of one or more ring carbon 

can lead to a large variety of heterocyclic analogs of 1,2,4-cyclohexatrience. As first 

noted by Shevlin, these may exist as either a chiral cyclic allene 95 or planar ylide 

96 [146–148]. Engels has presented a comprehensive analysis of related structures 

containing second and third row elements [118, 149]. Sheridan and co-workers 

have provided the clearest confirmation of allenic structure by generating a variety 

of heterocyclic 1,2,4-cyclohexatriences in cryogenic matrices [150–152]. 

Scheme 3.6 summarizes synthetic routes to 1,2,4-cyclohexatrienes. The two 

pericyclic reactions have been most widely applied. Christl and co-workers 

have reported in great detail on other routes to 1,2,4-cyclohexatriene and its 

benzannelated derivative [153, 154]. The Doering-Moore-Skattebøl reaction of 98 

and dehydrohalogenation of 100 provide excellent routes to 61. 

Common reactions of 1,2,4-cyclohexatrienes are summarized in Scheme 3.7. 

The allene is easily trapped with furan. With styrene, both [2+2] and [2+4] products 

are isolated, presumably through diradical intermediate 101. 

In 1969, Hopf and Musso reported that cis-1,3-hexadiene-5-yne 102 thermally 

isomerizes to benzene [155]. This reaction, now known as the Hopf cyclization, 

provides explanation for diverse high-temperature chemistry [156]. Experimental 

and theoretical results support two general pathways (Scheme 3.8). The 

127


Scheme 3.6 Common routes to 1,2,4-cyclohexatrienes. 

Scheme 3.7 Reactions of 1,2,4-cyclohexatriene. 

Scheme 3.8 Hopf cyclization chemistry.


Type A mechanism proceeds by electrocyclization to 61, followed by sequential 

1,2-hydrogen shifts passing through carbene 104. An alternative Type B route 

begins with Brown rearrangement (1,2-shift) to vinylidene 105, followed by C–H 

insertion. 

Support for the Type A cyclic allene mechanism comes from trapping of 61 

with styrene and inhibition of rearrangement in the presence of O 2 [157]. Hopf 

and co-workers studied the homologous series 106 [158]. In this case, smaller 

rings cyclize readily, while no reaction is observed for the largest homologs. These 

results correlated nicely with computations [156, 158]. 

One chemical process may generate 61 in great abundance. Among the wide 

array of intermediates in alkane combustion, Miller and Melius have proposed that 

61 may be one stop (Scheme 3.9) on the road to aromatic rings and soot formation. 

[159]. Head to tail dimerization of propargyl radical 108, a common species in 

flames, might be followed by Brown rearrangement and C–H insertion to give 

61. This can aromatize by the route described earlier. Alternatively, we note that 

61 should have a low barrier for C–H dissociation to give phenyl radical, another 

flame intermediate. 

Scheme 3.9 1,2,4-Cyclohexatriene in combustion chemistry. 

Photochemical Hopf-type cyclizations are known from the earlier work of Laarhovens 

(Scheme 3.10) [160] who showed that 109 undergoes photocyclization, 

presumably through cyclic allene 110. Uncertainty remains about the aromatization 

steps, since sequential 1,2-H shifts have barriers too high to occur under these 

conditions. Lewis and co-workers have studied several chromophores that should 

photogenerate cyclic allenes [161, 162]. Experiments with 111 lead to the conclusion 

that products arise from intermolecular protonation, rather than hydrogen 

shifts in strained allene 112. 

Scheme 3.10 Photochemical Hopf cyclizations. 

129


The enyne + alkyne cycloaddition has increasingly been applied in the synthesis 

of polycyclic structures. Computational studies on enyne + yne cycloadditions support 

the existence of cyclic allene intermediates [134, 135]. Scheme 3.11 presents 

recent examples. In 1994, Danheiser and co-workers reported that ynone + enyne 

cycloadditions of 113 proceed either by thermal activation or with AlCl 3 catalysis 

[163]. One advantage of this reaction is that it proceeds directly to an aromatic 

ring. Johnson and co-workers applied flash vapor pyrolysis to a simpler structure 

114 [134]. The product distribution was consistent with competitive electrocyclic 

opening and aromatization of the allene intermediate. Ananikov has reported 

computational models for this reaction that agree well with experiment [135]. 

Saa’s group has reported extensive studies on intramolecular enyne + yne cycloadditions.[164]. 

As one example, solution-phase thermolysis of 115 unexpectedly 

afforded a mixture of benzo[b] and benzo[c] fluorenones 116 and 117 [165]. The 

authors proposed that the initially formed allene opens to a 10-membered ring, 

which can then close in two directions. This novel result suggests that at high temperature 

other isonaphthalenes might undergo complex atom scrambling. In the 

final example, Danheiser’s group applied a clever intramolecular benzyne + enyne 

addition to prepare 119 and a variety of related heterocycles [166]. 

Scheme 3.11 Dehydro-Diels-Alder routes to 1,2,4-cyclohexatrienes.


3.3.5.1 6-Methylene-1,2,4-Cyclohexatrienes and Related Structures 

Thermal cyclization of structures 120 affords (Scheme 3.12) an intermediate that 

might be characterized as a chiral allene, diradical or zwitterion. These are not 

resonance structures since each implies a different geometry and electronic state. 

With X = CH 2, this is known as a Myers-Saito cyclization [167, 168]. At present, 

there is neither experimental nor computational support for the allene structure 

121. Computations with a closed-shell configuration optimize to chiral structure 

121 but the wavefunction is unstable, with a lower energy open-shell solution. 

Thus, diradical 122 would appear to be the best description for these structures. 

and not allene 121 [169]. 

Scheme 3.12 Myers-Saito type cyclizations. 

3.3.6 

Seven-membered Ring Allenes 

Strain has been estimated at 14 kcal mol –1 in 1,2-cycloheptadiene 52. The principal 

routes to 52 (Scheme 3.13) have come from dehydrohalogenation [108, 110, 111]. 

The carbenoid route to 52 unexpectedly fails, giving 126 as the major product. This 

longstanding mystery has now been explained by Schleyer and co-workers who 

showed by DFT calculations that the barrier to ring opening of 125 is unusually 

large because of a conformational change leading to the transition state [129]. 

One new route reported to the parent allene is from treatment of �-bromosilane 

124 with fluoride [170]. 

Scheme 3.13 Chemistry of 1,2,-cycloheptadiene. 

Selected examples of other novel 1,2-cycloheptadiene derivatives are summarized 

in Scheme 3.14. Huisgen has reported a [4+3] cycloaddition strategy that 

leads to the isolation of seven-membered ring ketenimines such as 127; this work 

has been reviewed [171]. Agosta and co-workers have reported that the dimer of 

129 is isolated by photolysis of ketone 128 [172]. This type of radical combination 

might offer a more general route to cyclic allenes. A number of sila-substituted 

131


Scheme 3.14 Examples of 1,2-cycloheptadiene chemistry. 

1,2-cycloheptadienes have been reported. In these cases, the longer C–Si and 

Si–Si bonds decrease bending and strain in the allene functional group. In the 

most recent example, Baines and co-workers reacted tetramesityldisylene with an 

alkynylcyclopropane to produce a mixture of allene 133 and other isomers [173]. 

3.3.6.1 Cycloheptatetraenes 

1,2,4,6-Cycloheptatetraene 134 is one of the best studied cyclic allenes, with 

sustained interest in related chemistry over nearly four decades. Interest has 

focused on the structure of 134 and its relationship with carbene 135. Initially 

viewed as highly improbable, the cyclic allene now is securely characterized as 

an important intermediate on the C 7 H 6 potential energy surface. Chemistry of 

benzannelated derivatives is also well known [174]. The most common routes to 

134 are shown in Scheme 3.15. 

Nearly four decades ago, Wentrup showed that 139 could undergo ring expansion 

[175]; without the presence of a trapping agent, dimer 137 was isolated. In 1982, 

Chapman and co-workers generated 134 in an argon matrix by photolysis of 140 

[176]. The IR spectrum showed bands at 1824 and 1816 cm –1 which supported 

the chiral cyclic allene. This result was a revelation at a time when strained cyclic 

allenes were poorly understood; however, the role of carbene 135 remained 

uncertain. In 1996, three theoretical studies converged on the currently accepted 

view of this energy surface [177–179]. Essential features are summarized in 

Figure 3.7. The chiral allene was predicted to be the lowest energy structure on this 

portion of the energy surface, in agreement with experiment and earlier computations. 

More interestingly, it was shown that the closed shell (135, state symmetry 

1 A1) is not an energy minimum. The open shell singlet state 141 ( 1 A 2) was found 

to be the most stable planar structure. This is the transition state for allene

Scheme 3.15 Chemistry of cycloheptatetraene. 


enantiomerization. Matzinger and Bally later subjected 134 and 139 to a complete 

characterization by UV/Vis and IR spectroscopy, where the spectra were assigned 

based on high-level ab initio calculations [180]. The route from singlet carbene 

139 to 134 is a two-step process, passing through 133. This bicyclic intermediate 

has not been observed, probably because it sits in a shallow energy well. Effects 

of aryl substituents on the interconversion of phenyl carbene, bicycloheptatriene 

and cycloheptatetraene were examined by Geise and Hadad [181]. Reaction of 

benzene with atomic carbon offers another entry into the C 7 H 6 surface. Shevlin 

has provided evidence that this reaction proceeds by insertion into a C–H bond 

to generate phenylcarbene [182, 183]. 

Rapid dimerization precludes observation of 134 under ordinary solution-phase 

conditions. However, with the use of Cram’s hemicarcerand [184], Warmuth 

and Marvel reported the generation (Scheme 3.16) of this strained cycloallene 

Figure 3.7 Energetics of 1,2,4,5-cycloheptatrienes. 

133


Scheme 3.16 Generation of “incarcerated” cycloheptatetraene. 

at room temperature inside the carcerand [185, 186]. This and other reactions 

carried out in molecular flasks stabilizing the short-lived guest are discussed in 

Chapter 10. Hemicarceplexed phenylcarbene was found to competitively insert in 

the C–H bonds of the hemicarcerand, resulting in low yields of 134. The yield was 

increased by incarcerating phenyldiazirine in a partially deuterated hemicarcerand, 

resulting in a 67% yield of 134. Similar results were found for the photolysis of 

p-tolyldiazirine in this molecular container [187]. Small molecules can diffuse into 

the hemicarcerand to characterize reactions of the allene. Reaction with HCl gave 

142, while O 2 lead to a novel carbon loss. 

In spite of this intense scrutiny, we can find no strain estimates reported for 134. 

The isodesmic reaction scheme below, calculated at the B3LYP/6-311+G(d,p) + 

ZPVE level of theory, predicts 13.8 kcal mol –1 in allene strain. This is very similar 

to 1,2-cycloheptadiene in the estimate of allene strain. 

Wentrup’s group has reported extensive studies (Scheme 3.17) on the aza 

analogs of 134. In this case, carbene 143 and nitrene 146 interconvert through 

145 [188, 189]. 

Scheme 3.17 Azacycloheptatetraene chemistry. 

3.3.7 

Eight-membered Ring Allenes 

Doering-Moore-Skattebøl chemistry (Scheme 3.18) provides the most common 

route to 1,2-cyclooctadienes 147. Strain in this ring size has diminished to ca. 

5 kcal mol –1 , but the parent structure can only be briefly isolated at ambient temperature 

and most examples should be considered as reactive intermediates [113,


114, 190]. Price and Johnson found that 1-t-butyl-1-2-cyclooctadiene is isolable and 

could even be purified by gas chromatography [191]. 

More complex 1,2-cyclooctadienes have been utilized as intermediates. For 

example, Moore and co-workers designed an alkoxy-Cope rearrangement route to 

eight-membered ring allene 148 which provides a versatile triquinane synthesis 

[192]. 

Scheme 3.18 1,2-Cyclooctadiene chemistry. 

3.3.8 

Polycyclic Allenes 

Only a few bicyclic allenes have been reported. Allene 149 (Scheme 3.19) has 

been of interest for some years; early reports [193] on its preparation now seem 

to be in doubt but Zen and Balci has recently shown that 149 can be successfully 

Scheme 3.19 Polycyclic allenes. 

135


generated by a more reliable carbenoid route [194]. Balci’s group has also reported 

on the allene generated from �-pinene [195]. Okazaki and co-workers recently 

reported evidence for 3,4-homoadamantadiene 151, the first allene built on such 

a complex tricyclic framework [196]. Dehalogenation of 150 led to isolation of 

dimers of allene 151. In the presence of diphenylisobenzofuran (DIBF), adduct 

152 was isolated. DFT calculations on 151 predicted a pure bending deformation 

of the allene, with an angle of 135.9°. Barton and co-workers reported the synthesis 

and isolation of 153. In this structure, the allene is linear but twisted to a dihedral 

angle of 72.4º [197]. 

3.3.9 

Cyclic Bisallenes 

A modest collection of bis-allenes has been reported. Mitchell and Sondheimer 

first described the interesting pericyclic cascade of 154 through bis-allene 155 

(Scheme 3.20) to dimers of 156 [198]. More recently, Wang has developed this as 

a general route to more complex polycyclics [199]. Cyclic bis-allenes can exist as 

diastereomers. Dehmlow and co-workers have shown that dibromides such as 157 

open stereospecifically, in this case giving the dl bisallene [200]. Barton’s group 

reported the synthesis of bis-allenes with silicon in the ring [201]. Kamigata and 

co-workers have reported a detailed ab initio study on meso and dl stereoisomers of 

7–10-membered ring bisallenes [202]. For 7–9-membered rings, the meso isomers 

were predicted to be of slightly lower energy. Strain in this series was estimated 

by comparison to deformation in 2,3-pentadiene. In the 9–10-membered ring 

allenes, strain was predicted to be < 2 kcal mol –1 per allene unit; these structures 

have allenes that are nearly linear. The value increased to 3.6 or 5.9 and 13.7 or 

31.0 kcal mol –1 in dl or meso isomers of the eight- and seven-membered rings, 

respectively. 

Scheme 3.20 Bis-allene chemistry. 

3.3.10 

Cyclic Butatrienes 

Cyclic butatrienes have four contiguous �-bonded carbons. Spectacular examples 

of this functional group include 159 (Scheme 3.21), an intermediate in neo-

Scheme 3.21 Cyclization of cyclic cumulenes to diradicals. 


carzinostatin chemistry [203, 104] and 160, a 10 � aromatic substance first prepared 

by Myers and Finney [205, 206]. These structures have minimal strain but still 

cyclize readily to diradicals with modest thermal activation. 

3.3.10.1 Butatriene � Bond Deformations and Strain Estimates 

Isodesmic and homodesmic estimates for butatriene strain and total strain in cyclic 

butatrienes are summarized in Figure 3. Strain increases rapidly with decreasing 

ring size [112]. In the smallest ring, Mabry and Johnson characterized 1,2,3-cyclobutatriene 

55 as a transition state rather than an energy minimum [207]. DFT 

optimized structures for 56–60 show that the butatriene group remains close to 

planarity. For a comparable ring size, cyclic butatrienes have ca. twice the strain of 

cyclic allenes. Yavari and co-workers have studied the conformational properties 

of 56–60 using HF and MP2 methods [208]. 

Figure 3.8 Strain estimates and chemical behavior of cyclic butatrienes. 

3.3.10.2 Five- to Nine-membered Ring Cyclic Butatrienes 

The parent cumulene 56 remains unreported, in spite of a number of attempts, 

presumably because of its 85 kcal mol –1 of estimated strain. However, Wong and 

137


Scheme 3.22 Generation and trapping of a 1,2,3-cyclopentatriene. 

co-workers have reported routes to thia and azacyclopentatrienes (Scheme 3.22) 

[106, 107]. Treatment of iodonium triflate 161 with fluoride at ambient temperature 

in the presence of various dienes afforded modest yields of substances assigned 

as cycloadducts of 162. Very similar chemistry was utilized to trap the aza analog 

of 162. 

An earlier study by Shakespeare and Johnson had developed this methodology 

(Scheme 3.23) for the first synthesis of 1,2,3-cyclohexatriene 57 [105]. This 

new benzene isomer was easily trapped. Hickey and Paquette later described the 

efficient addition of 57 to cyclopentadiene [209]. 

Scheme 3.23 1,2,3-Cyclohexatriene chemistry. 

Dehydro-Diels–Alder chemistry provides a more novel approach to derivatives 

of 57. Johnson and co-workers found that flash pyrolysis of diyne 164 afforded 

a high yield of dienyne 166 [134]. Intramolecular cycloaddition, followed by 

electrocyclic opening of 117 provide the most logical mechanism. Ring opening 

of 165 is expected to be substantially exothermic and it seems unlikely that this 

process can be run in reverse. Lu and co-workers have predicted that 1,3-diynes 

may add to Si(100) and Ge(100) surfaces to produce layers of reactive 1,2,3-cyclohexatrienes 

[210].


Cyclic medium-ring 1,2,3-butatrienes are highly reactive (Scheme 3.24). 

Szeimies reported the first synthesis of 58 through rearrangement of bicyclobutene 

167 and later described Ni catalyzed dimerization to 168 [211, 212]. The 

eight-membered ring butatriene 59 prepared from 169 has only a brief lifetime 

in solution [213] while the next homolog 60 is isolable [214]. 

Scheme 3.24 Cyclic butatriene chemistry. 

Some of the most spectacular examples of cyclic cumulenes come from the 

domain of organometallic chemistry. In 1993, Buchwald and co-workers reported a 

serendipitous synthesis of zirconacyclohepta-2,4,5,6-tetraene 170 [215]. The crystal 

structure showed a planar cumulene with an unusual zigzag geometry. Rosenthal 

and co-workers have reported in great detail on the chemistry of metallacyclopenta- 

2,3,4-trienes 171 which can be readily prepared by transfer of Cp 2M to 1,3-diynes 

[216, 217]. Internal bond angles in these structures range from 142–150°. The 

molecular geometry and computations indicate strong stabilizing interactions 

between the metal and in-plane cumulene � bond. Steric stabilization of these 

complexes was also suggested. 

3.3.11 

Conclusions 

Strained cyclic allenes and cumulenes present an enormous diversity of structure 

and a complete range of chemical behavior, from shelf-stable substances through 

increasingly fragile reactive intermediates. These simple hydrocarbons and their 

derivatives are now recognized as intermediates in a wide array of chemical 

reactions. Ease of preparation and high chemical reactivity have encouraged a 

growing number of sophisticated applications in synthesis. 

139



Support from the National Science Foundation for our earlier work on strained 

cumulenes is gratefully acknowledged. Thanks to Catherine Johnson for typing 

a semi-legible manuscript. 

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4 

Strained Aromatic Molecules 

4.1 

Nonstandard Benzenes 

Paul J. Smith and Joel F. Liebman 

4.1.1 

Introduction and Context 

It was a tenet of organic chemistry that aromatic species are planar. A lengthy 

section of the now classic book [1] on strained organic molecules was devoted to 

species with distorted rings, and indeed, the current volume elsewhere discusses 

such compounds. Interpolating these past and present monographs on strained 

species, recent topical issues of Chemical Reviews [2–4] document that ‘bent and 

battered’ rings [5] are increasingly part of the chemical landscape. Terms such as 

‘fullerenes’ and ‘single wall nanotubes’ attest to increasing comfort with manifestly 

nonplanar aromatic species. 

We limit our attention to cases where we can compare the energetics of these 

species with essentially planar species. We prefer experimental data over that 

derived from calculations. While it was recently found that many ‘popular theoretical 

methods predict benzene and other arenes to be nonplanar’ [6], it was also 

noted that more sophisticated approaches show this failure while more common, 

cheap and conventional approaches do not. As such, this finding should not 

threaten the increasingly strong coalition of experimentalists and theorists, nor 

prejudice for planar arenes. 

We focus on hydrocarbons, preferably gaseous or in some innocuous solvent 

to minimize the effects of intermolecular interactions. While relative reactivity 

generally runs inverse to stability, we primarily discuss enthalpies of formation 

(�H f) and reaction, and not of activation. 



ISBN: 978-3-527-31767-7 

147

148 4 Strained Aromatic Molecules 

4.1.2 

Alkylated Aromatics 

Starting with derivatives of benzene itself, the three isomeric xylenes have essentially 

indistinguishable gas phase �H f [7], o-, 19.1(1.0); m-, 17.3(0.7); p-, 

18.1(1.0) kJ mol –1 (4.184 kJ � 1 kcal): methyl–methyl repulsions are small. The 

three isomeric t-butyltoluenes show the o-isomer to be less stable by 20 kJ mol –1 

than its m- and p-isomers [8]. For the more strained di-t-butylbenzenes, condensed 

phase studies show that the o-isomer is some 80(10) kJ mol –1 less stable than the 

nearly isoenergetic m- and p-isomers [9]. Encouragingly, this difference is very 

much the same as for 1,2,4- and 1,3,5-tri-t-butylbenzene, 75(15) kJ mol –1 in the 

condensed phase [9a,10]. 

No experimental data allow comparison of 1- and 2-t-butylnaphthalene and 

determination of the destabilization from a t-butyl group in a peri (1,8) position. 

Computational chemistry shows the 2-isomer to be ca. 22 kJ mol –1 more stable 

than its 1-counter part [11]. The 2-isomer behaves as unstrained hydrocarbon: 

the difference of its �H f [12] and that of naphthalene is ca. 110 kJ mol –1 [12], vs. 

106 kJ mol –1 for related benzenes. 

The presence of two t-butyl groups on naphthalene allows for the possibility of 

vicinal (1,2- or 2,3-) and peri- (1,8-) situated groups. Such peri species interconvert 

with their Dewar isomers thermally [13a], photochemically [13b], and via 1-electron 

reduction [13c]. The aromatic isomer is energetically and kinetically disfavored 

by repulsion of the t-butyl groups. Racemization of seriously nonplanar 1,8-dit-butylnaphthalene 

requires ca. 95 kJ mol –1 [14]. 

9-t-butylanthracene 1 is nonplanar [15], readily interconverting with its 9,10- 

Dewar isomer 2 [16]. Equilibration studies show 1- and 9-t-butylanthracene are 

less stable than the 2-isomer [17]: the former are absent under reaction conditions. 

Likewise, the undetected 1- and 4-t-butyl derivatives of phenanthrene 3 must 

be more strained than the 2- or 3-isomers [18]. 4-t-butyl-5-methylphenanthrene 

(studied as its 1,8-dimethylated derivative) is nonplanar [19] and has a strain 

energy (SE) of 142 kJ mol –1 ; by contrast, 4,5-di-t-butylphenanthrene has a SE of 

208 kJ mol –1 . The nonplanar [20] 4,5-dimethyl species has a SE of ca. 50 kJ mol –1 , 

defined as the �H f difference of this species and its 2,7-dimethyl isomer [21]. 

The strain in 4,5-dimethylphenanthrene is also shown by polarography [22a], 

calculation-guided spectroscopy [22b] and mass spectrometry [22c]. Were only 

some of the above simpler alkylated benzenes, naphthalenes and anthracenes so 

thoroughly studied! 

4.1.3 

Helicenes 

As of [4]helicene (benzo[a]phenanthrene, 4), these species are significantly 

nonplanar as shown by X-ray crystallography [23a], NMR [23b], photoelectron 

[23c] and UV spectroscopy [23d]. (A detailed presentation of helicenes is given in 

Section 4.3.) The racemization free energy of the parent [6]helicene 5 and some

4.1 Nonstandard Benzenes 

larger counterparts [24] have been measured. However, to determine their strain 

energies requires �H f measurements which are absent except for [4]helicene 

itself: this species is ca. 17 kJ mol –1 less stable than its tri phenylene isomer 6. 

Thus, we cannot deduce how homologous is this series – when is the �H f difference 

of [n+1] helicene and [n]helicene constant? We recognize these helicenes 

are catacondensed (ortho-fused), and note for the simpler class of homologous 

catacondensed species, the acenes, our thermochemical knowledge tantalizingly 

ends at n = 4, naphthacene. 

We now discuss substituted derivatives. Methyl derivatives of [6]helicene show 

positional effects on racemization rates [25]: unsubstituted ~ 3 ~ 4 ~ 13 ~ 14 

< 2 ~ 15 � 1 ~ 16. The bis-indeno derivatives of 4,5-diphenylphenanthrene[26a], 

1,12-diphenyl[4]helicene and 1,14-diphenyl[5]helicene [26b] show large twist angles 

by crystallography. These species arise from mild base-catalyzed rearrangements 

of unstrained PhC�CC 6H 4C�CCH 2-derivatives of benzene, naphthalene and 

phenanthrene. It is a testimony to the high energy of the C�C triple bond in the 

precursors and the resonance stabilization of newly formed benzene rings in the 

products that such twisted polynuclear aromatic hydrocarbons are readily formed. 

Indeed, the photochemical isomerization of diphenylacetylene to phenanthrene 

[27] is ca. 180 kJ mol –1 exothermic using contemporary �H f [28]. This quantity is 

large enough to accommodate considerable strain in highly distorted aromatic 

products [29]. 

4.1.4 

[n]Circulenes 

Two circulenes with known �H f are corannulene (n = 5, 7) and coronene (n = 6, 

8). The significantly nonplanar corannulene and planar coronene have gas phase 

�H f [30] of 460.6(6.5) kJ mol –1 and 307.5(9.8) kJ mol –1 , respectively. Lacking 4,5 

H,H repulsions we take coronene to be unstrained, neglecting SE as found in 

phenanthrene. Naively, the �H f of corannulene would be 5/6 that of coronene, but 

is more positive by 204 kJ mol –1 , a situation exacerbated by another 40 kJ mol –1 for 

planar corannulene [31]. To put this strain and aromaticity difference into context, 

the isomeric naphthalene and azulene show the former to be more stable by almost 

140 kJ mol –1 : in both cases the totally benzenoid species is more stable by some 

12 kJ mol –1 per carbon. On the other hand, the parent (C 60 ) and C 70 -fullerenes 

(for which corannulene may be understood as en route to) have �H f of ca. 42 and 

39 kJ mol –1 per carbon [32], while ‘gaseous’ benzenoid graphite is much more 

stable, with �H f of 6 kJ mol –1 per carbon [33]. 

While a ‘convenient synthesis … of [7]circulene’ 9 has been reported [34], its 

enthalpy of formation is unknown. Interestingly, bowl-shaped corannulene and 

saddle-shaped [7]circulene have comparable planarization energies [35]: how 

does that relate to their strain energies? We note one synthetic detail [36] for [7] 

circulene: dehydrogenation of 1,16-didehydro-2,15-dimethyl[6]helicene 10 with a 

‘preformed’ 7-membered ring core did not result in 9 or its 1,2-dihydro derivative, 

but instead, the fluorene derivative 11. 

149


4.1.5 

Cyclophanes 

Cyclophanes with small bridges, discussed in detail in Section 4.2, represent 

classic examples [5] of distorted benzene rings, most typically reminiscent of boat 

cyclohexane. Commencing with [n]cyclophanes, the quintessential question is how 

small a polymethylene chain can be used to affix two carbons of a benzene ring. 

Starting with planar orthocyclophanes, perhaps we are jaded but the [1]-derivative 

12 looks normal. More commonly known as cycloproparenes discussed in 

Section 4.4, the SE of this species was indirectly determined by reaction calorimetry 

[37] to be 284 kJ mol –1 . However, orthocyclophanes do not exhibit nonplanar 

distortions and so will not be further discussed. 

What about [1]metacyclophane 13 and/or [1]paracyclophane 14? The �H f of the 

corresponding ‘ring-opened’ species, �,3- and �,4-didehydrotoluene (e.g. 15a/15b), 

are 431(13) kJ mol –1 for both isomers[38]. However, should we preclude the 

existence of [1]-paracyclophane, perchance recognized as norbornadiene-1,7-diyl 

16 the ‘opened’ [2.2.1]propelladiene wherein aromaticity is sacrificed to accommodate 

bonding of the methylene group to ring carbons? We remember the high 

kinetic stability of [1.1.1]propellane presented in Chapter 2.1 and diverse derivatives 

despite exceptional SE relative to ring-opened mono- and bicyclic products 

[39]. Regardless, we know of no evidence for [1]paracyclophane, nor its isomer 

[1]metacyclophane, nor of their presumably less strained [2] and [3] counterparts. 

How small can we go? 

[3]Paracyclophane 17 and its meta isomer remain unknown. [4]Metacyclophane 

18 remains but a reaction intermediate [40] while its Dewar isomer 19 is isolable 

[41], and [4]orthocyclophane is tetralin. [4]Paracyclophanes return us to precariously 

isolable species. Discussed in [42], NMR spectra of [4]paracyclophane and 

even the crystal structure of some derivatives can be obtained. These species 

show both considerable geometric distortion and significant remnants of the 

aromaticity of p-disubstituted benzenes. Interconversion of [4]paracyclophane 

and its more stable Dewar isomer has been observed in both directions. Two 

unsaturated derivatives are particularly interesting: polyene 20, 1,2,3,4-tetradehydro[4]paracyclophane, 

has been observed [43] and shown by reaction chemistry, 

spectroscopy and model calculations to be energetically preferred over its �-bond 

isomer 21, bicyclo[4.2.2]deca-1,3,5,7,9-pentaene, a formally unsaturated analog of 

[2]-1,4-cyclooctatetraenophane. There are seemingly no cyclooctatetraenophanes 

with such a short bridge, although longer, saturated derivatives are known, as are 

longer 1,3- and 1,5-cyclooctatetraenophanes [44]. A less relevant nonhydrocarbon 

example [43] has a (NC) 2C=C-CH=CH-C=C(CN) 2 group para-bridging a benzene 

ring. Powerfully demonstrating the role of substituents in kinetically stabilizing 

small cyclophanes, we have ‘ almost a limiting case of experimentally characterizable 

strained paracyclophanes’ [43]. 

What about [5]cyclophanes? We don’t know �H f and/or reaction enthalpy for [5] 

paracyclophane and its Dewar isomer. [5]Metacyclophane is an isolable species [45], 

but attempts to determine its �H f were thwarted due to formation of a hyperstable

4.1 Nonstandard Benzenes 

olefin during calorimetric hydrogenation [46]. The recently developed alternate 

approach to aromaticity, the so-called ‘isomerization method’ [47], contrasts a 

methylated putative aromatic (e.g., 22) with its ‘isotoluene’ analog (e.g., 23). The 

�H f difference of these isomers (the ISE – isomerization stabilization energy) 

from quantum chemical calculations [47] is 139 kJ mol –1 , reduced to 82 kJ mol –1 

for [5]paracyclophane, suggesting that most of the aromaticity remains for this 

species. We welcome corresponding analysis of other cyclophanes. 

For [6]paracyclophane and its derivatives, Dewar isomers were experimentally 

shown [48] to be less stable than the benzene form by some 130 kJ mol –1 – however 

distorted the benzene ring, aromaticity prevails over strain. [6]metacyclophane is 

isolable, but, as with its smaller [5] counterpart, �H f determination by measurement 

[46] of its enthalpy of hydrogenation was thwarted. 

We now turn to cyclophanes containing ring systems other than benzene. 

Recalling earlier discussion of species containing cyclooctatetraene, we jump to 

those with a 9,10-bridged central anthracene ring, with –CH 2S(CH 2) n-4SCH 2– 

(n = 8, 9, 10, 12). As shown in basic solution by NMR, the methylenedihydroanthracene 

isomer with the tautomerized =CHS(CH 2 ) n-4 SCH 2 - bridge is thermodynamically 

preferred [49]. Similar isomerism is seen for undistorted ‘acyclic’ anthracene 

with two –CH 2 SC 3 H 7 groups, unlike methylanthracene [50]. The all hydrocarbon 

parent and 1,4,5,8-tetrasubstituted [6] (9,10)anthracenophanes [51] undergo acidcatalyzed 

rearrangement to the corresponding methylenedihydroanthracene. Yet 

aromaticity prevails: the Dewar isomers of these anthracenophanes are less stable 

than their benzenoid counterparts. 

Returning to [1]bridged cyclophanes, stable 1,5- and 1,6-methano[10]annulene 

24, 25 were studied by hydrogen calorimetry [52]. The calculated ISE [47] for the 

latter is 106 kJ mol –1 vs. 215 kJ mol –1 for naphthalene. No numbers are available 

for the former species – because different SEs accompany bridgehead double 

bonds in bicyclo[5.3.1] and [4.4.1]undecenes, we hesitate to use the difference for 

�H f of the two methanoannulenes, ca. 80 kJ mol –1 . We are even more hesitant to 

suggest a related difference of 1,5-methano[10]annulene and its unknown tauto mer 

bicyclo[5.3.1]undeca-1,3,5(11),6,9-pentaene 26, a [5]metacyclophane derivative. 

This brings us to [m,n]cyclophanes, which we limit to meta 27, metapara 28 or 

para 29. As reviewed in [42], [1,1]paracyclophanes are characterized in terms of 

structure and activation parameters for isomerization with their Dewar counterparts. 

We also note the authors of [45] are optimistic about the eventual formation 

and characterization of [1,1]metacyclophane. [1,1]metaparacyclophanes are 

likewise unknown – photolysis of diketo derivatives of [2,2]metaparacyclophane do 

not produce the desired species by double elimination of CO. Rather, a different 

fragmentation process [53] results in m-benzyne, another relatively exotic species 

[54]. The relative energies of [1,1]meta, metapara and paracyclophane remain 

unknown. 

For the larger considerably less strained [2,2]cyclophanes (cf. ref. [5]), �H f of 

these species, studied as gases, increase in the same order: meta, 170.5(6.5); 

metapara, 218.4(1.6), para, 244.1(2.6) kJ mol –1 . For the corresponding dienes, the 

enthalpies of formation are ca. 407 kJ mol –1 [32], unknown, and 493.0(5.0) [55]. 

151


We note that the presence of the two double bonds affects changes in �H f : meta, 

237 and para, 249 kJ mol –1 . By contrast, twice the difference of orbitally conjugated 

[56] (E)-stilbene and 1,2-diphenylethane is 186.4(3.6) kJ mol –1 , from which 

it may be deduced that the two double bonds increase the SE of [2,2]meta- and 

para cyclophane. Relative reactivity, rather than calorimetry, has been used to derive 

the same conclusion for [2,2]metaparacyclophane [57].

4.2 Distorted Cyclophanes 

We close this discussion of cyclophanes and this chapter with the conclusion 

that [6,6]paracyclophane has negative SE [58] – while this species is therefore 

outside the scope of the current chapter because of its lack of strain, this observation 

nonetheless intrigues [59]. 

4.2 

Distorted Cyclophanes 

Henning Hopf 

4.2.1 

Introduction 

Since Kekulé proposed the benzene ring structure, the image of this hydrocarbon 

as a rigid, flat, hexagonal molecular object has burned itself into the minds of 

chemists. The ideal of the rigid hexagonal ‘tile’ has not only been reinforced by 

the structures of the polycondensed aromatic systems but also by the Hückel 

theory of aromaticity which proposes that the ‘aromatic character’ of benzene 30 

and its derivatives is most pronounced when the axes of the p-orbitals are parallel 

allowing optimal p,p-overlap. Intuitively, one might expect that any disturbance 

of this overlap, 31, would reduce the aromaticity. This expectation has been the 

main driving force for the attempt to prepare deformed aromatic systems. As 

we will see, the planarity of aromatic compounds can be deformed considerably 

before there is any disturbance in their chemical and spectroscopic behavior 

(Figure 4.1) [60]. 

In principle, there are many ways to deform a benzene ring and a number of 

these distortions are encountered in the different normal modes. To translate this 

dynamic behavior into a stable distorted structure the simplest approach uses the 

incorporation of a molecular bridge, usually a polymethylene chain, causing the 

generation of a cyclophane. For a simple benzene ring as the aromatic moiety to 

be spanned, the bridge may be anchored in 1,4- ([n]paracyclophane 32) or in the 

Figure 4.1 Bridging the benzene ring. 

153


1,3-position ([n]metacyclophane 33). The benzene ring in ortho-arrangement, 34 

is usually planar. Employing the ‘phane concept’ numerous bridged aromatic 

compounds has been prepared during the last decades [61], although the first 

cyclophanes, Lüttringhaus’ ansa compounds [62], were already prepared in the 

early 1940s. A particular attraction is exhibited by cyclophanes possessing a second 

aromatic ‘deck’ as in 35, since in these molecules the distance and orientation of 

the aromatic ‘layers’ may be adjusted by the lengths of the bridges (m, n in 35). 

Because of space limitations this chapter will concentrate on representative 

examples derived from 32, 33, and 35, and the main questions to be answered 

will be the preparation of these compounds and the relationship between bridge 

length, ring distortion, and ‘aromatic character’, the latter being judged mainly be 

the NMR properties of the cyclophanes (see Section 4.2.5). Available calorimetry 

data for cyclophanes are discussed in Section 4.1.5. 

4.2.2 

The [n]Cyclophanes 

4.2.2.1 [n]Paracyclophanes 

Within the class of cyclophanes containing a single aromatic moiety only, this 

group has received the highest attention [61, 63]. Calculations by different methods 

support the expected trend; the shorter the bridge in 32 the more distorted is 

its benzene ring. Table 4.1 summarizes the relationship between bridge length, 

distortion angle �, and heat of formation of the respective [n]paracyclophane 

according to AM1 calculations [64]. 

Table 4.1 Relationship between bridge length, distortion angle �, and heat of formation of 

respective [n]paracyclophane according to AM1 calculations. 

n � [°] �H f (kcal mol –1 ] 

14 0.5 –62.3 

13 1.3 –54.9 

12 1.9 –48.9 

11 2.2 –40.5 

10 3.4 –29.5 

9 9.6 –18.9 

8 13.4 –8.7 

7 16.4 5.1 

6 22.3 26.2 

5 28.6 53.8 

4 35.6 87.8 

3 79.4 117.8

(a) � (23%); (b) Zn/Hg, HOAc, HCl (54%); (c) Br 2 , MeOH, –30 °C, H 2 O (47%); 

(d) LiAlH 4 /AlCl 3 (65%) 

Scheme 4.1 


To prepare [n]paracyclophanes 32 with n down to about 9 should pose no 

particular synthetic problems. The typical methods used to make most of the 

smaller [n]paracyclophanes (n � 8) can all be demonstrated by the preparation of 

[8]paracyclophane 39. By employing a mixture of the two Hofmann bases 36 and 

40 and subjecting it to a 1,6-elimination (Scheme 4.1) [65] the p-xylylene 37 and 

its furanoid analog 41 are generated which cycloadd to produce the furanophane 

38, that subsequently can be reduced directly to 39 or via the diketone 42. 

The gain in aromatic resonance is also made use of when the bis-spiro diene 

43 is pyrolyzed in the presence of trapping agents such as 1,3-butadiene (46, 

Scheme 4.2). The reaction begins with pyrolysis of a three-membered ring to 

provide the diradical 44 � 45 first. Trapping then provides the monoene 47 that is 

hydrogenated to the saturated hydrocarbon 39 [66]. An important general approach 

(Scheme 4.2) to the smaller [n]paracyclophanes employs spirocyclic ketones such 

as 48, which is first converted to a cross-conjugated triene 49 followed by flashvacuum 

pyrolysis (FVP) to provide the diradical 50 that recloses to 39 with the 

remarkable yield of 70% [63]. 

In another approach strain-free precursors, like 51, are first built-up and subsequently 

subjected to ring-contraction processes as shown in Scheme 4.3 for 

4-carboxy[8]paracyclophane 54 [67]. 

51 was prepared by an acyloin condensation/oxidation protocol from the appropriate 

open-chain starting materials, and then ring-contracted by a Wolff rearrangement 

of 52 via ketene 53 to the acid 54. After conversion of 54 into 4-keto[8] 

paracyclophane the sequence could be repeated, and 3-carboxy[7]paracyclophane 

be obtained [68]. The approach beginning with �,��-dichloro-p-xylene 55 and 

1,6-hexandithiol 56 illustrates the principle of the uncoupling of the two critical 

steps in [n]paracyclophane chemistry: the construction of an intermediate that 

155


Scheme 4.2 

Scheme 4.3 

Scheme 4.4

Scheme 4.5 


already has the correct topology of the target molecule (here [10]paracyclo phane 57) 

and the built-up of strain in the last step [69]. Ring contraction is accomplished 

by sulfone pyrolysis [70], using the bis sulfone 58, obtained readily from the bis 

thioether 57. In the acid 54 the experimentally determined deformation angle � 

(see 32) amounts to 9.1° [71], whereas in the lower homolog [68], it has increased 

to 16.5°, in excellent agreement with the calculated value (see above). 

The parent system of the latter was obtained using the approach via reactive 

carbene 60 (Scheme 4.4, Jones route) [72]. 

Here the spirocyclohexadienone tosylhydrazone 59 was metalated with n-butyl 

lithium and the resulting salt thermally decomposed under FVP conditions to 61 

[73]. A conceptually interesting route to [n]paracyclophanes involves the conversion 

of a heterophane into a benzophane (Tochtermann route) [74]. As shown in 

Scheme 4.5 the bridged furan 62 undergoes the expected Diels-Alder addition 

with dimethyl acetylene dicarboxylate (DMDA) to the adduct 63, that is converted 

via the oxaquadricyclane 64 into the bridged oxepin 65 by a sequence of pericyclic 

reactions. When the latter was brominated the dibromide 66 is obtained, which 

on debromination/deoxygenation yielded the diester 67. Various derivatives of 67 

show deformation angles � around 15° [74]. 

Moving down to the parent system [6]paracyclophane 71, both the Jones route 

(Scheme 4.4) [75] as well as the Tochtermann route to the dimethyl diester were 

successful. Apparently, both methods reached their limits here, since they failed in 

the preparation of [5]paracyclophane or its derivatives. Several derivatives of these 

cyclophanes could be prepared, and their solid state structures be determined: for 

[6]paracyclophane-8-carboxylic acid the bending angles � (see 3) amount to 20.3 

and 21.1°, respectively [76], and for the dimethyl 8,9-dicarboxylate this angle is 

19.5° [77]. The 3,3�-disubstutited bicyclopropenyl derivative 68 can be rearranged 

into their Dewar benzene isomers, followed by valence isomerization as shown 

in Scheme 4.6. The metal-catalyzed step afforded a mixture of the two isomeric 

Dewar benzenes 69 and 70, which after separation were thermally opened to 

their aromatic isomers 71 and 72, respectively. On irradiation 71 recloses to 69, 

indicating the high strain of this cyclophane [78]. 

157


Scheme 4.6 

A longer, but superior route was developed to prepare [6]paracyclophane 

(Scheme 4.7) [79]. The bicyclic ketone 73 was first converted into the propellane 

74 by addition of acetylene to the double bond of the substrate 73. Wolff rearrangement 

of the �-diazoketone 75, prepared from 74 by azo group transfer, 

next furnished the ester 78. The Dewar benzene intermediate of 78 was opened 

thermally to the [6]paracyclophane ester 77, which on hydrolysis provided 76 in 

10% yield. Alternatively, the parent system was prepared from 78 by first saponifying 

it and subsequently oxidizing the formed acid. The 1 H NMR spectrum of 

71 does not differ significantly from that of an undistorted aromatic compound 

[75, 78]. 

[5]Paracyclophane 80 could be only generated by irradiation of the 1,4-pentamethylene 

Dewar benzene analogue of 69 in THF-d8 at –80 °C [80]. Hydrocarbon 

80 decomposes rapidly above 0 °C, but it was identified unequivocally by its UV 

Scheme 4.7


and 1 H NMR spectra [81]. Its aromatic protons absorb as an AA�XX�-multiplet at 

7.38 and 7.44 ppm, respectively, whereas the benzylic protons are registered at 

2.77 ppm, both shifted downfield from the respective protons of p-xylene. Esters 

such as 81 [82] and 82 [83], obtained by photoisomerization of their respective 

Dewar benzene isomers have half-lives of several hours at room temperature. 

The smallest [n]paracyclophane, identified by trapping experiments, is the 

tetra methylene bridged system 84 (Scheme 4.8). Irradiation of the butano-bridged 

substrate 83 at –20 °C in THF in the presence of trifluoroacetic acid, provided the 

addition products 86 and 87 [84]. If the experiment is performed in methanol the 

ether 88 is produced. It is rationalized that the precursor 83 indeed photo isomerizes 

to 84, which is protonated to 85, that subsequently yields the trapping products 

86–88. In a second trapping experiment the Dewar benzene ester 89 on photolysis 

in the presence of ethanol yielded the regioisomeric 1,4-trapping products 91 and 

92, for which the [4]paracyclophane ester 90 is the most reasonable precursor. 

When 83 was irradiated in a matrix at 77 K, the UV spectrum of 84 could be 

recorded [85]. Although computational studies concerning [3]paracyclophane have 

been published [86], no preparative efforts have been undertaken. 

Scheme 4.8 

159


4.2.2.2 [n]Metacyclophanes 

A significant ring strain arises in [n]metacyclophanes only when the bridge 

length is seven or smaller [63, 87]. Three basic approaches have been developed 

to synthesize [n]metacyclophanes as summarized in Scheme 4.9. The first route 

begins with cyclic precursors such as dodecanone 93, i.e. substrates that contain 

the future molecular bridge from the very beginning. In case of 93 this is accomplished 

via intermediates such as 94 [88]. In the second route the aromatic unit is 

present from the start, e.g. as the dithiol 97, and the rest of the synthesis involves 

bridging it; a typical example is given by the sulfone pyrolysis of 98 yielding [9] 

metacyclophane 99 [89]. In the last approach the metacyclophane is obtained from 

its para-isomer by acid-catalyzed isomerisation. All three approaches have been 

employed to synthesize e.g. [7]metacyclophane [90–92]. For the synthesis of lower 

homologues such as [5]metacyclophane, methods similar to those employed in 

the para-series have been developed [93]. Again, the [4]cyclophane represents the 

smallest hydrocarbon (generated as an intermediate) in this series also [94]. 

The parent [n]metacyclophanes have low melting points, so only a few derivatives 

have been subjected to X-ray structural analysis. 8,11-dichloro[5]metacyclophane 

possesses a pronounced boat-structure, with the functionalised carbon atoms 

bent out of the plane by 12° (unbridged) and 27° (bridged side). Its 1 H-NMR 

spectrum that displays a ring current equal to that of analogous planar benzene 

derivatives [95]. 

Scheme 4.9

4.2.3 

The [m.n]Paracyclophanes 


Among the [m.n]cyclophanes, those in which the two alkano bridges are anchored 

in the para-positions of both benzene ‘decks’, have received greatest attention. 

We will begin with [0.0]paracyclophane 103 (Scheme 4.10) and end with the [4.4] 

homolog for this discussion. 

Scheme 4.10 

The extremely strained 103 should better be regarded as a valence isomer of 

tricyclo[4.2.2.22,5]dodeca-1,3,5,7,9,11-hexaene 104 of which recently the dicyano 

derivative 76 has been generated as a reaction intermediate [96]. If one of the 

bridges is formally ‘opened’ the para-bridged biphenyl derivatives 106 result [97]. 

The [1.n]paracyclophane series begins with [1.1]paracyclophane 114 (Scheme 4.11); 

its synthesis employs many of the steps that have been discussed above for the 

preparation of the smaller [n]paracyclophanes [98]. 

Tricyclic diketone 107 on photoaddition of acetylene afforded bis-adduct 108, 

which was converted to the bis-diazo diketone 109. Wolff rearrangement in 

methanol then furnished 110, in which the double Dewar benzene structure is 

clearly discernible already. The missing double bonds were introduced by double 

Hofmann elimination of diamine 111, obtained from 110 by routine methods. 

Irradiation of 112 at 77 K in an EPA matrix then furnished 114 via ‘half-opened’ 

113. That 114 had been generated was inferred from UV/Vis and NMR spectra. 

However, on warming to room temperature, 114 was destroyed within 4 h [98]. 

On extended irradiation 114 was converted into the interesting hydrocarbon 115. 

Calculations revealed that its benzene rings are approximately as distorted as the 

one in [5]paracyclophane 80, the bending angle by which the bridgehead carbon 

atoms are ‘removed’ from the plane amounting to 23°, compared with the 12.6° 

of [2.2]paracyclophane (exp., see below) and 23.5° (calc.) for 80. The intraannular 

distance between the atoms of the benzene rings is ca. 236–240 pm, i.e. roughly 

100 pm shorter than the inter-layer distance in graphite. These calculations indicate 

161


Scheme 4.11 

the delocalisation of the electrons despite severe bending of the aromatic moieties. 

The calculated strain energy of 114 is in the range of 93–128 kcal/mol depending 

on the level of theory. 

Although the parent hydrocarbon [1.2]paracyclophane is unknown and presumably 

would not differ very much in strain and benzene ring deformation 

from the [1.1]compound 114 [99], the [1.n]phanes 116 [100] and the analogous 

ketones 117 [101] have been prepared by FVP of the corresponding sulfones. In 

the pentamethylene bridged parent hydrocarbon 118 [102] the bridgehead atoms 

are not only displaced out of the plane by about 10°, but with only 235 pm the 

distance between the substituted carbon atoms at the ‘methylene side’, distinctly 

shorter than the analogous distance in [2.2]paracyclophane (278 pm, see below). 

This observed intraannular distance agrees remarkably well with the calculated 

one for [1.1]paracyclophane 114 (see above) and for different highly functionalized 

derivatives of 114 [103]. 

The by far most thoroughly studied phanes are [2.2]paracyclophane 119 

(Scheme 4.12) and its derivatives. Numerous routes to prepare these compounds 

have been described, and their chemical and structural properties have been 

reviewed [60, 61, 63], that an additional summary is unnecessary here. The basic 

strategies to prepare [2.2]paracylophanes are summarized in highly condensed 

retrosynthetic form in Scheme 4.12.

Scheme 4.12 


Cleavage of one of the ethano bridges in 119 results in the formation of the 

diradical 120 for whose generation either other cyclophanes such as 124 and 125 

or open-chain precursors such as the dibromide 126 may be used. All that remains 

to be done in the last steps is a ring-contraction, for which the photochemical 

or high temperature extrusion of small molecules (e.q. CO, CO 2 or SO 2 ) are the 

methods of choice. In the case of 126 only Wurtz coupling is required; however, 

since in this process intermolecular coupling competes with intramolecular C-C 

bond formation, yields are low [104]. If the other bridge in 120 is also broken, the 

diradical 121 results, which is nothing else than p-xylylene or p-quinodimethane 

37. For this reactive tetraene numerous derivatives of p-xylene 127 can serve as a 

precursor, beginning with p-xylene itself (127, X = Y = H) [105], and ending e.g. with 

the Hofmann base derived from �-bromo-p-xylene 127 (X = H, Y = N(CH 3 ) 3 OH) 

[106]. Continuing our retro-synthetic deconstruction, we next decompose 37 in 

retro-Diels–Alder fashion and arrive at 1,2,4,5-hexatetraene 122 as the diene and 

acetylene 123 as the dienophile component. Bisallene 122 is prepared by dimerization 

of propargyl bromide 128 and since simple alkynes are not reactive enough 

to undergo Diels-Alder additions with it, 123 has to be replaced by an activated 

acetylene 129 with various electron-electron-withdrawing substituents [107]. 

The structure of 119 has been determined several times [108, 109]. Only some 

important structural data resulting from the latest study (recorded at 19 K) are 

given in Figure 4.2. As expected, extension of the alkano bridges leads to a flattening 

of the benzene rings and an increase in the distance between them. In [3.3] 

paracyclophane 130 the above deformation angle has been halved to 6.4° (intraannular 

distance at the bridgeheads 314 pm and distance between the planes passing 

through the remaining aromatic carbon atoms: 320 pm) [110] and [4.4]paracyclophane 

131 has flat aromatic rings [111]. In the hybrid case of [2.4]paracyclophane 

132 the two benzene rings are inclined at an angle of 21°, the distortion angle 

at the bridgeheads is slightly above 5° (at all positions), and the two non-bonded 

distances at the bridgeheads amount to 279 pm (ethano side) and 386 pm (butano 

163


Figure 4.2 Some structural parameters of [2.2]paracyclophane. 

side) [112]. A fascinating known completely bridged cyclophane called superphane 

is mentioned in Chapter 9, its hypothetical hexahydrogenated analogue with a 

planar cyclohexane ring is briefly discussed in Section 2.4 while known but insufficiently 

studied layered cyclophanes are introduced in Section 1.4. 

4.2.4 

Distorted Aromatic Rings and ‘Aromatic Character’ 

The planarity of classical aromatic systems (usually benzene rings) may be 

distorted effectively by the introduction of molecular bridges. Using this device 

highly bent benzene rings with deformation angles � (see structure 32) up to 

25° may be generated readily. As judged from their X-ray structures and NMRproperties 

(see Section 5) these strongly bent benzene ring systems are delocalized 

to the same extent as benzene and its simple derivatives. On the other hand, 

many cyclophanes with short bridges display a reactivity unprecedented for 

normal aromatics, for example they undergo Diels-Alder additions readily [113] 

or can readily be hydrogenated [114]. For example, it has been calculated that [5] 

metacyclophane has a strain energy of 43 kcal/mol [115]; for [2.2]paracyclophane 

119 a strain energy of 30.1 kcal/mol has been determined [116]. Strain release 

would be effective not only for the products of the reaction, in which the formal 

sp 2 hybridization changed to sp 3 , which makes bridge formation easier. As an additional 

factor the deformation of the �-electron cloud of the benzene ring could 

play a role, since this raises the HOMO and lowers the LUMO energy. Localization 

of �-electrons into a cyclohexatriene moiety would increase the violation of 

Bredt’s rule even further, and is hence avoided. And it may finally play a role that 

it appears to be the �-frame that dominates the delocalization in benzene rather 

than the �-system [117].

4.2.5 

NMR Characteristics of Cyclophanes 


1 H NMR spectroscopy can furnish detailed information with respect to structures, 

conformations and conformational dynamics of this class of compounds 

and a number of NMR reviews on cyclophanes is available [118–122]. The most 

important feature in the 1 H NMR spectra are the unusual chemical shifts of 

protons lying above the plane of the aromatic component of the cyclophane. 

Shortly after Pople suggested [123] that protons connected to aromatic rings 

are deshielded by a ring current induced by the external magnetic field, Waugh 

and Fessenden [124] reported the increased shielding, by 0.7 ppm, of the central 

methylene protons in [10]paracyclophane 32 relative to those in cyclohexane. These 

protons are held in a position over the center of the ring. Shortening the methylene 

bridge decreases the average distance between these protons and the region of 

maximum shielding. The most highly shielded proton in [5]paracyclophane 80 

has � = 0.01 ppm [81] while in [6]paracyclophane 71 � = –0.62 ppm [125]. 

In [2.2]paracyclophane 119 the aromatic rings cause mutual shielding of their 

aryl protons by –0.62 ppm [�(119) = 6.48 ppm [126], �(1,4-diethylbenzene) = 

7.10 ppm)]. This is only a moderate effect because the rings are eclipsed. The 

effect in [2.2.2](1,3,5)cyclophane 133 is somewhat larger because the triple 

bridging decreases the average distance between the rings [�(133) = 5.71 ppm 

[126], �(1,3,5-triethylbenzene) = 6.86 ppm)]. In anti-[2.2]metacyclophane 134, 

however, the protons experience strong shielding [�(134) = –2.75, 4.27 ppm [126], 

�(1,3-diethyl benzene) = 7.02 ppm)] as they are situated much closer to the region of 

maximum shielding of the opposite aromatic ring. Finally, Pascal’s in-cyclophanes 

show extreme shielding effects, for example � = –4.03 ppm for H i in 135 [127], 

because the geometry of the molecular cage forces the internal proton to point 

toward the center of the aromatic ring at a very small distance (172 pm, calc.). 

165


4.3 

Helicenes 

Ivo Starý and Irena G. Stará 

4.3.1 

Introduction 

Helicenes as unique, inherently chiral three-dimensional aromatics have been 

attracting continuous attention for decades. They are usually chemically stable 

and soluble in common organic solvents, which makes a difference from many 

other large �-conjugated systems. The chemistry of helicenes has been reviewed 

several times [128–135]. However, since an enormous amount of progress in their 

synthesis and use took place in the early 1990s, the recent period has not been 

covered by a comprehensive review. Thus, this chapter is mostly focused on the 

important achievements in helicene chemistry within the last two decades. 

Helicenes are polycyclic aromatic systems consisting of all-ortho-fused aromatic 

rings. The prefix before the name expresses the number of fused cycles as exemplified 

by hexahelicene or simply [6]helicene. Provided all these rings are benzenes, 

such compounds are called carbohelicenes 136. If one (or more) benzene unit 

is formally replaced by a heterocycle, such a skeletal modification leads to heterohelicenes 

137. Finally, helicene-like compounds represent the third family of 

helicenes, which can differ significantly from fully aromatic parent helicenes but 

having a similar molecular shape 138. As helicenes can exist in two enantiomeric 

forms regardless of their configurational stability, the handedness of the helix 

is specified by adding the (M) (minus) or (P) (plus) prefix (Scheme 4.13). This 

chapter deals only with carbohelicenes, which are simply called helicenes within 

the following text. 

4.3.2 

Synthesis of Helicenes 

[6]Helicene was the first helicene representative prepared intentionally by Newman 

and Lednicer in 1956 [136], who also pioneered resolution of the racemate 

employing a �-� donor-acceptor interaction between helicene and optically pure (–)- 

or (+)-2-(2,4,5,7-tetranitro-9-fluorenylideneamino-oxy)propionic acid (TAPA).

Scheme 4.13 

4.3 Helicenes 

The real breakthrough in the synthesis of helicenes came in the late sixties when 

the Martin group introduced photodehydrocyclisation of stilbene-type precursors 

[137–139] as the first general method for their preparation [140]. It was based on 

UV-light induced cis/trans isomerisation of 1,2-diarylethylenes followed by conrotatory 

electrocyclisation of the cis isomer to generate a primary dihydroaromatic 

product with trans configuration (Scheme 4.14). 

Scheme 4.14 

Then, in the presence of air and a catalytic amount of iodine, it was immediately 

converted to a fully aromatic system [135]. However, photocyclisation leading to 

an all-ortho-fused system may often be disfavoured for steric or other reasons and, 

therefore, the process may exhibit low regioselectivity. To overcome this problem, 

167


Liu and Katz developed synthetic methodology utilising a bromine substituent 

on a benzene ring, which directs photocyclisation away from its ortho position 

[141]. A remarkable step forward was made in 1991 when Katz and colleagues 

published an updated version of photodehydrocyclisation of stilbene-type precursors 

(Scheme 4.15) [142]. They found that iodine in a stoichiometric amount 

is superior oxidant and propylene oxide is an effective scavenger of hydrogen 

iodide. Since then, this improved methodology has become a standard tool for the 

synthesis of helicenes. Numerous examples of successful use of photodehydrocyclisation 

in its original or innovated version were published to prepare, inter alia, 

[5]helicene [141], [6]helicene [143–145], [7]helicene [143], [8]helicene [145–147], 

[9]helicene [145, 146, 148], [10]helicene [145, 148], [11]helicene [145, 148, 149], 

[12]helicene [145, 149], [13]helicene [145, 148, 150], and [14]helicene [149]. Despite 

the synthetic accessibility of stilbene-type precursors and wide applicability and 

simplicity of photodehydrocyclisation, this methodology suffers from several 

drawbacks. To prevent photodimerisation, the irradiation must be performed 

under high dilution conditions which limits any scale-up [151, 152]. Furthermore, 

acetyl, dimethylamino and nitro groups that can perturb or depopulate the reactive 

excited state should be avoided [152, 153]. 

Scheme 4.15 

Since the early 1990s, various non photochemical approaches have emerged 

to circumvent disadvantages of photodehydrocyclisation. The most important 

method eliminating the irradiation was published by the Katz group [152]. It was 

based on thermal Diels-Alder reaction of aromatic bisvinylethers with p-benzoquinone 

in excess to afford helicenes having terminal quinone rings (Scheme 4.16 

[161]). It evolved into a robust and versatile method, which has also been employed 

Scheme 4.16

4.3 Helicenes 

by other authors [154, 155] to prepare derivatives of [5]helicene [152, 156, 157], 

[6]helicene [152, 157], and [7]helicene [158–160]. The importance of the Katz group 

contribution basically consists of developing the simple methodology, which for 

the first time allows the synthesis of a wide series of functionalised helicenes 

on a multigramme scale and in providing highly attractive compounds for other 

applications [161]. 

The simplicity of Katz’ Diels-Alder methodology represents, on the other hand, 

its drawback. Variations of functionalities on the terminal (hydro)quinone rings 

of a helicene skeleton are rather limited. Therefore, other new methods for the 

preparation of helicenes have recently emerged on the stage. Stará and colleagues 

have developed a new strategy based on intramolecular [2+2+2] cycloisomerisation 

of aromatic triynes catalysed by Ni 0 or Co I complexes (Scheme 4.17 [164]) 

[162–165]. Using this organometallic approach, various helicene derivatives were 

synthesised ranging from penta- to heptacyclic systems. This methodology exhibits 

considerable potential due to its modularity, chemo- and regioselectivity and allows 

for the formation of three new cycles of the helical scaffold all at once. 

Scheme 4.17 

Axially chiral biaryls can be viewed as truncated helicenes. Indeed, there is a 

group of diverse synthetic methods transforming biaryls to helicenes. Bestmann 

and Both [166] and Stará and colleagues [167] described the transformation of 1,1�binaphtyl 

derived bisphosphinealkylene or dihydroazepinium salt, respectively, 

to [5]helicene (Scheme 4.18 [167]). Gingras and Dubois developed syntheses of 

[5]- and [7]helicene based on carbenoid coupling, which started in the case of [7] 

helicene from 4,4�-biphenanthrene-3,3�-diol (Scheme 4.19) [168]. Alternatively, they 

Scheme 4.18 

169


Scheme 4.19 

applied McMurry coupling to the synthesis of [5]helicene from 1,1�-binaphthalene- 

2,2�-dicarbaldehyde [169]. So far the latest contribution to this field was published 

by Collins et al., who employed ring-closing olefin metathesis to construct [5]-, 

[6]- and [7]helicene from divinyl biaryls (Scheme 4.20) [170]. The synthesis of 

helicenes from biaryls has significant potential but such an approach leads to the 

construction of only one new benzene ring in the helicene scaffold. 

Scheme 4.20 

Photocyclodehydrogenation of stilbenes to prepare helicenes has inspired other 

authors to develop its nonphotochemical alternative. Harrowven and colleagues 

used homolytic aromatic substitution in the case of diiodo cis,cis-distilbenes, which 

under treatment with tributyltin hydride and the VAZO radical initiator provided 

functionalised [5]- and [7]helicenes (Scheme 4.21) [171]. 

Scheme 4.21

4.3.3 

Nonracemic Helicenes 

4.3 Helicenes 

Since Newman’s preparation of optically pure [6]helicene [136], different routes 

based on the resolution of racemate have been explored to produce enantiomerically 

enriched or pure helicenes. The resolution of helicene racemates by HPLC 

on a chiral stationary phase is general and can be employed for analytical as well 

as preparative purposes [172–174]. There is only one practical method allowing 

for resolution of racemic helicenes on a multigramme scale so far. Katz and colleagues 

developed a robust procedure using chromatography of diastereomeric 

helicene pairs on silica gel (Scheme 4.22) [175]. This is suitable exclusively for 

helicen-1-ols, which are converted to (1S)-camphanates. 

Scheme 4.22 

Asymmetric synthesis of helicenes and their congeners is envisaged to be 

the most straightforward and efficient route to single enantiomers. Various 

concepts have emerged demonstrating basic principles rather than generally 

useful methodologies. Nevertheless, some of them might be highly promising. 

Classical photodehydrocyclisation of stilbene-type precursors can be carried out 

in an astonishingly stereoselective fashion. This was well demonstrated by the 

pioneering works by Vanest and Martin [176] and Katz and colleagues [177] who 

used stereocentre(s) external or internal to the helix to control stereoselectivity 

of helicene cyclisations. Carreño et al. developed an asymmetric version of the 

Diels–Alder approach providing helicene quinones with excellent optical purities 

(Scheme 4.23) [155, 178–180]. In addition to that, the last decade has witnessed 

other attempts at asymmetric synthesis of helicenes but stereocontrol observed 

has been moderate as published by Stará and colleagues (enantioselective Nicatalysed 

[2+2+2] cycloisomerisation of aromatic triynes) [162, 164, 174]. In spite 

of the above mentioned achievements, practical asymmetric synthesis of helical 

aromatics has so far remained a challenging task. 

171


Scheme 4.23 

4.3.4 

Intriguing Helicene Structures 

Regarding the highest helicene homologues so far synthesised, the world record 

among carbohelicenes belongs to Martin and Baes, who synthesised [14]helicene 

(Scheme 4.24) [149]. In order to prepare much longer helicene structures, the Katz 

group explored the ways of polymerising bifunctional helicene units. Placing a 

salicylaldehyde functionality at each end of optically pure [6]helicene, reaction with 

o-phenylenediamine and nickel(II) salt led to the first polymer with an unbroken 

network of double bonds that winds in one direction along a helix [181, 182]. 

The degree of delocalisation was higher than in optically active [7]helicene-based 

cobaltocenium oligomers [183, 184]. 

Structural diversity of helicene molecules is broad, encompassing helical 

metallo cenes by Katz and Pesti (the Fe or Co atom spans the ends of a helicene 

backbone, Scheme 4.25) [185], helical metal phthalocyanine derivatives by the 

Katz group [186], conjugated helical acetylene-bridged cyclophanes by Fox et al. 

[187], 2,2�-bis[6]helicyl by Laarhoven and Veldhuis [188], helicenes containing a 

cyclophane unit by Nakazaki et al. [189], Martin and colleagues [190], and S-shaped 

[191] or 3-shaped [192] double helicenes by the Laarhoven group, to mention but 

a few. 

Scheme 4.24 Scheme 4.25

4.3.5 

Physicochemical Properties and Applications 

4.3 Helicenes 

As helicenes have robust, inherently chiral scaffolds, it is not surprising that they 

were soon applied by the Martin group to asymmetric synthesis as chiral auxiliaries 

(in diastereoselective reduction of �-keto esters [193], ene reaction [194] and 

atrolactic synthesis [195]) or chiral reagents (in hydroxyamination [196] or epoxidation 

[197] of olefins, Scheme 4.26) with remarkable success. After certain delays, 

attention has turned to enantioselective catalysis and so pioneering exploitations of 

helicene ligands have recently emerged. Highly stimulating results were obtained 

in asymmetric hydrogenation (Reetz et al. [198], Nakano and Yamaguchi [199]), 

allylic substitution (Reetz and Sostmann [200], Scheme 4.27) and diorganozinc 

addition to aldehydes (Katz et al. [156], Soai et al. [201]). 

Scheme 4.26 

Scheme 4.27 

Regarding self-assembly, one of the most astonishing attributes of helicene 

assemblies was described by the Katz group [204] (Scheme 4.28) [130]. Properly 

substituted nonracemic helicenes, possessing both electron-rich inside and 

electron-deficient outside regions, can aggregate spontaneously to create columnar 

structures exhibiting enormous optical rotation values and NLO properties. 

173


Scheme 4.28 

In Langmuir-Blodgett films, an array of parallel columns can be observed 

directly by AFM [202]. Moreover, these columns are further organised into long 

micrometre-wide lamellar fibres visible under an optical microscope [203]. The 

chiroptical properties of such assemblies are so remarkable that CD spectra could 

be measured for a monolayer [204]. The Moore group showed that a [6]helicenecontaining 

foldamer provided highly solvent dependent CD spectra [205]. 

There are remarkable examples of helicene use in molecular recognition. The 

[7]helicene-based helicopodand was used by Diederich and colleagues in molecular 

recognition of dicarboxylic acids with high diastereoselectivity [206]. Nonracemic 

helicene diol was successfully used by Reetz and Sostmann reporting on enantioselective 

fluorescence quenching by chiral amines [173]. A helicene derivatising 

reagent developed by the Katz group can serve as a remote chirality sensor for chiral 

alcohols, amines and phenols when detected by NMR measurements [207]. 

Helicenes were deposited on metal surfaces and studied by various techniques. 

Ernst et al. used near-edge X-ray absorption spectroscopy with linearly polarised 

synchrotron radiation (NEXAFS) to study the orientation of (P)-[7]helicene on 

a Ni(100) surface under ultrahigh vacuum (UHV) conditions [208]. This group 

also studied chirality transfer from (M)-[7]helicene into handed supramolecular 

structures on a Cu(111) surface by STM [209] and the orientation and the intramolecular 

relaxation due to adsorption of nonracemic [7]helicene on Cu(111) 

and Cu(332) surfaces by means of angle-scanned full-hemispherical X-ray photoelectron 

diffraction [210]. 

Various spectral and physicochemical properties of helicenes have been 

investigated. The fluorescence spectra, emission lifetimes, quantum yields of 

fluorescence and triplet state formation of a series of helicenes were studied by 

Vander Donckt and colleagues [211, 212]. They found the photophysical properties 

of the helicenes evolved steadily as a function of the number of ortho-fused 

benzene rings. Experimental photoelectron spectra of helicenes were analysed 

by Obenland and Schmidt to conclude that interaction between the �-orbitals of 

overlapping benzene rings is much smaller than in cyclophanes [213]. The comparison 

between experimental and calculated VCD spectra allowed Bürgi et al. 

to make the unequivocal assignment of the absolute configuration of [7]helicene 

[172]. An electrochemical study on helicenes was performed by Laarhoven and Brus 

who measured values of polarographic half-wave potentials [214]. Experimental

Scheme 4.29 

4.3 Helicenes 

estimation of the reduction and oxidation potential of [7]helicene was done by 

Rulíšek et al. by measuring current-voltage curves on inert electrodes [215]. 

Focussing on molecular machinery, there is a fascinating application of a 

helicene structure by the Kelly group. In an artificial molecular motor, a helicene 

ratchet ensured unidirectional rotary motion fuelled by the chemical energy of 

periodic bond making/bond breaking processes (Scheme 4.29) [216]. 

4.3.6 

Theoretical Studies 

The unique helicene structure has steadily attracted the attention of theoretical 

chemists. Grimme and Peyerimhoff studied the relationship between the structure 

and racemisation barrier in the series of helicenes by means of semiempirical 

AM-1 and ab initio SCF methods [217]. Similarly, Haufe and colleagues recalculated 

barriers to racemisation for helicenes finding an excellent agreement with the 

experimental results [218]. They confirmed the fact that the barriers for carbohelicenes 

converge to the value of about 45 kcal/mol. Ab initio calculations were 

carried out by Schulman and Disch in the series of helicenes and their closest 

topological isomers, planar phenacenes [219]. Their comparison revealed only 

slight loss of aromatic character in the former molecules. 

Most interestingly, the current-voltage characteristics of helicenes were calculated 

and their intriguing conductance was foreseen by Treboux et al. [220]. Helicenes 

can also be viewed as tiny mechanical objects since their structure resembles a 

molecular spring. Hartree–Fock calculations by Lipkowitz and colleagues using 

PM3 Hamiltonian revealed that the nanospring stiffness could be modulated by 

increasing/decreasing the electron density and length of helicenes [221]. 

Optical and chiroptical properties of helicenes were studied in detail by computational 

methods. The electronic CD spectra were calculated by the Ahlrichs 

group for helicenes, exploiting the adiabatic time-dependent DFT method as a 

prime tool for chiroptical property investigations. Thus agreement between the 

most important spectral features and theory was found [222]. There are also other 

theoretical approaches to chiroptical spectroscopy published by Hansen and Bak 

[223]. The hyper-Rayleigh scattering second-order NLO responses of helicenes 

were investigated by the Botek group employing the time-dependent Hartree-Fock 

175


approach and AM1 semi-empirical Hamiltonian [224]. The results of a series of 

DFT and DFT-D (the empirical dispersion energy terms included) calculations 

were reported by Rulíšek et al. with the aim to predict the physicochemical properties 

(equilibrium structures, stabilisation energies, redox potentials, excitation and 

CD spectra, electronic conductivity and elasticity) of elongating helicene structures 

[215]. It was shown that many of them are converged on [14]helicene. 

4.3.7 

Outlook 

After five decades of helicene chemistry, interest in these unique aromatic 

compounds will certainly continue in the forthcoming years and there are many 

good reasons for it. As the synthesis of helicenes has recently witnessed significant 

progress, various helical aromatics have become more easily available than 

before. Important achievements might be expected, for instance in developing 

general asymmetric synthesis of helicenes, which remains still unsolved. However, 

applications of helicenes in various branches of chemistry, material science and 

nanoscience will be central to further efforts. 

4.4 

Cycloproparenes 

Brian Halton 

4.4.1 

Introduction 

The most highly strained ring-fused aromatics, the cyclopropa- and cyclobutabenzenes, 

have provided distinct fields of study for more than 50 years [225, 226]. 

Cyclopropabenzene 139 made its debut in an 1888 paper by Perkin [227] where 

its synthesis was noted as yet to be done; cyclobutabenzene 140 appeared some 

20 years later in a 1909 thesis footnote [228]. Each parent has provided classes of 

compounds that continue to be the subjects of detailed scrutiny with numerous 

reviews covering various aspects of cycloproparene [226, 229–232] and cyclobutarene 

[225, 233] chemistry. The present cycloproparene synopsis summarizes 

developments in cycloproparene chemistry over the past five years [229]. 

The first cycloproparene claim [234] made in 1930, was inconclusive [235] and 

the 1953 addition [236] of Ph 2 CN 2 to imide 141 did not give 143 as thought but 

the sulfonamides 142 [237] as now confirmed from X-ray data [238].

4.4.2 

Synthetic Considerations 

4.4 Cycloproparenes 

That 3H-indazoles, analogs of 143, are precursors to cycloproparenes was 

confirmed by dinitrogen loss from 144 that gave the first stable derivative [239], 

but the route is not general. Wege found [240] that debromosilylation of 145 led 

to quinone 146 as a transient trapped by furan as exo/endo adducts. Attempts to 

obtain a kinetically more stable quinone targeted [241] C1 dialkylation via diazopropane 

addition to 1,4-naphthoquinone and subsequent deazetation. 

The addition gave 147 that was easily oxidized to 148 (37%) (Scheme 4.30) 

but irradiation (350 nm) then gave a complex mixture [241]. Ultimately, primary 

adduct 147 was diverted to 149 (30%) and this lost nitrogen affording 150 (50%) 

and 151 (38%) as expected [232, 239, 242]. Attempts to dideacetylate 149 to hydroquinone 

and/or quinone were unsuccessful. Modification of the acetate groups 

of 149 gave further six derivatives of which only the 2-acetoxy-7-methoxy affords 

a cycloproparene on photolysis (Table 4.2, see p. 179). 

The Collis–Wege results [241] indicate limitations in the 3H-indazole route. 

Sylvania 350 nm lamps are ideal for excitation of C9 acetoxy compounds absorbing 

close to 350 nm (Table 4.2). Substrates that absorb at longer wavelength do not 

react because of insufficient energy absorption or enhanced mesomerism to N1 

that strengthens the C9a–N1 bond; impact of the C9 substituent on the excited 

state also needs consideration [241]. 

The planarization of annulenes by annelating small rings across strategic sites 

has been recognized for some time. Dürr [243] synthesized 152 in 1983 and 

demonstrated close planarity by X-ray analysis. Schleyer’s group subsequently 

predicted highly aromatic planar all-cis-[10]annulenes from cyclopropa fusions 

[244], and Sastry’s group now have extended this to the rim of corannulene 153 

(B3LYP/6-31G+G* calculations) where the inversion barrier is reduced by > 50% 

if all rim bonds are cyclopropannelated [245]. Now, the first parent non-benzenoid 

177


Scheme 4.30 

cyclopropannulene has been isolated by Stevenson [246]. Reaction of CH 2 Cl 2 and 

cycloC 8 H 7 Br with t-BuOK in HMPA generates anion radical 154, but in THF 155 

is formed from I 2 oxidation of the dianion. Unlike the foul smell of 139, 155 has a 

sweetly olefinic odor. NMR spectra have the CH 2 protons at � 4.61 (139: � 3.11) and 

those of the eight-membered ring at � 3.6–3.7; C1 (� 90.4) is deshielded 72 ppm 

relative to 139 and 13 C labeling gives J C1–H 162 Hz (139: 170 Hz [247]; cycloC 3H 4: 

167 Hz [248]). Clearly 155 has C1a–C2 and C7–C7a � bonded and a paratropic 

ring current. Calculations (B3LPY/6-31G*) confirm this with C1a–C7a 141.7 and 

C1a–C2 132.9 pm [246]. 

The formation of benzdiynes by loss of CO and CO 2 from designed precursors 

continues with Sato confirming [249] 157 as a primary product from 156, 

and that loss of CO then gives diyne 158. Coupled with low temperature IR 

studies, energy diagrams (B3LPY/6-31G*) place the derivatives in the order of 

stability 157 > 159 > 160 > 158 with energy differences as shown in Scheme 4.31. 

Likewise, benzocyclopropenone 161 is 138 kJ mol –1 more stable than 162, but only 

42 kJ mol –1 below ring contracted 163.


Table 4.2 Photochemical behavior of 4,9-disubstituted 3,3-dimethyl-3H-benz[f]indazoles. a) 

R 1 

R 2 

� max 

log � 

Outcome b) 

Ac Ac 354 (3.49) CPN (50%) + STY (38%) 

Me Ac 357 (3.57) CPN (32%) + STY (43%) 

H TBDMS c) 

390 (3.68) decomposition 

Ac TBDMS 368 (3.65) decomposition 

Me TBDMS 375 (3.73) decomposition 

Ac H 372 (3.67) no reaction 

Ac Me 366 (2.99) no reaction 

Me H 379 (3.74) no reaction 

Me Me 372 (3.71) no reaction 

a) 

Data reproduced with permission from the Australian Journal of Chemistry: 

htpp://www.publish.csiro.au/journals/ajc; see Ref. [17]. 

b) 

CPN = cyclopropanaphthalene; STY = styrene. 

c) t 

TBDMS = BuMe2Si- 179


Scheme 4.31 

Scheme 4.32 

The most general route to the cycloproparenes is from use of 1-bromo-2-chlorocyclopropene 

in Diels–Alder cycloaddition and subsequent didehydrohalogenation 

as illustrated for 164 [229, 250, 251]. Addition of :CCl 2 to cycloC 6H 8 and

Scheme 4.33 


subsequent dehydrochlorination (the Billups synthesis; Scheme 4.32) remains 

the method of choice for parents 139 and 165 [229, 252]. Detailed procedures for 

preparation of alkylidenes 167 from 165 via anion 166 and silyl-Wittig olefination 

(Scheme 4.33) [229, 253] are now formalized into four protocols that gave 

~30 new 3,6-dimethoxycyclopropa[b]naphthalenes [254]. Applications are largely 

to �-extended, conjugated and cross-conjugated derivatives with ‘push–pull’ 

character that include dithioles 168/169 [255], trienes 170–172 [256], and ‘spaced’ 

[257] 173–175; p-HCO-C 6 H 4 -CHO and 166 give 174 with 173 as a trace product. 

Not all reactions go to plan [258]. 

181


Scheme 4.34 

Reactive bicyclopropenylidenes–novel C 6 H 4 valence bond isomers–are formed 

from anion 166 with bulkily substituted cyclopropenones 176 (Scheme 4.34) [259]. 

While complex mixtures ensue, those from 176b/176c afford diones 178b/178c 

in low yield (~ 8%) via triafulalvenes 177, as established from the mass spectrum 

of isolated 177b and its subsequent transformation to 178b in air. 

An inverse electron demand route to alkylidenecycloproparenes has yet to receive 

the detailed assessment it deserves [260]. Condensation of cation 179 [261] with 

anion 180 results in coupling and in situ HCl loss to 181 that co-crystallizes with 

water as a hemihydrate.

4.4.3 

Chemical Considerations 


Cycloproparene functional group modification can provide access to difficultly 

accessible derivatives. Dilithiation of 182, reaction with PhCONMe 2, and condensation 

of the ensuing dione with LiC 5H 5 gives � extended 183 that packs in sheets 

in the solid state (Scheme 4.35) [255]; by analogy 184 gives 185 that provides 186 

and 187. Horner–Wittig reaction of cyclopropaquinone 188 provides 189 and 190 

but the reaction fails with lower homolog 191, likely due to its enhanced enedione 

character [255]. Bis(dithiole) 190, sensitive to acid and light, decomposes on purification. 

In fact, many � extended alkylidenes are only available in poor yields 

with their ‘push–pull’ ability for the new materials arena untested. 

The HOMO of 139 is located at the bridge and the C3–C4 bond [262] so that 

inverse electron demand [2+4] cycloadditions are to the bridge as illustrated by 

Scheme 4.35 

183


192/193 [263]. Strain is manifested by these reactions and those that use the 

lateral � bond as a two-electron component [229]. Such [3+2] cycloadditions, 

usually catalyzed by AgBF 4 or Eu(fod) 3 , involve ionic intermediates and proceed 

best in polar media to a wide range of heterocycles as summarized for 165 in 

Scheme 4.36 [264]. Notable is the cyclization to 195 via C–S bond formation in 

194 in a yield that almost doubles under Eu(fod) 3 catalysis; the alternative C–N 

closure is not seen [265]. As for formation of 200, thiotropone adds to 165 but 

the initial spirocycle rearranges via [1.7] C shift as shown to give 201 in analogy 

to tropone; the yield is but 5% [266]. 

Exocyclic alkene 181, explicitly prepared for flash vacuum thermolysis (FVT) 

study, ejects Me 2 CO and CO 2 as expected and gives 202 (Ar matrix, 20 K) characterized 

by its IR spectrum matching that calculated (B3LPY/6-31G*) [260]. The 

existence of such strained and bent ethylideneone derivatives of cyclopropabenzene, 

first proposed some 17 years ago from bis(diazoketone) photolyses [229, 

267] (cf. 163, Scheme 4.31) is now settled. 

FVT details for methylidenes 203 and 204 alluded to [268] in 1990 have appeared 

[269]; they do not parallel parents 139 and 165 in their behavior [270]. Diphenyl 

203 cyclodehydrogenates to 204 and both proceed to a range of C 24H 14 polycyclic 

aromatics (Schemes 4.37 and 4.38). FVT of 203 gives [e,k]acephen-206 [271] (7%) 

and [a,e]ace-anthrylene 209 (12%). With 204, [e,l]acephenanthrylene 212 (47%) is 

obtained via 210; 206 is the minor polycyclic aromatic hydrocarbon (< 4%). Traces 

of dimer 215 were also detected. Automerization of the products in analogy to 

acephenanthrylene [272] could lead from 206 to 208, and from 212 to 214 [269].

Scheme 4.36 


185


Scheme 4.37 

Scheme 4.38 

The well used [250, 273] Ag(I) catalyzed dimerization of cycloproparenes fails 

[274] with alkylidenes 167 (Scheme 4.39). Conditions effective for 139 and 165 

do not apply to 216, the simplest compound tested; ethyne 217 and ethanone 

218 (3 : 1) are obtained without 219. While metal ion is involved, none of the aryl 

derivatives studied dimerizes due to steric constraints at C1; simpler alkylidene 

derivatives such as C1 ethylidene (=CHMe) remain unknown.

Scheme 4.39 

4.4.4 

Heteroatom Derivatives 


Cycloproparenes carrying a heteroatom other than in the 6–3 fusion are unexceptional 

[229], those with the atom in the fused six-membered ring date to 1987 [275], 

and those involving N [276], S or Se [277] at C1 have been covered [229]. Recent 

efforts are aimed at incorporating group III/IV atoms at C1. Benzoborirenes 220 

have been characterized [278]. Computed structures of 220 (R 1 = R 2 = H) and the 

cyclopropabenzenyl cation [279] show marked reverse Mills–Nixon deformation 

within the aromatic unit due to strong � delocalization over the three-membered 

ring and rehybridization of the fusion sites. 

Kinetically stabilized metallacyclopropabenzenes are obtained as stable compounds 

when the very bulky Dip and Tbt substituents (Scheme 4.40) are present 

at C1 [280]. Dibromides 221 transform to 222 that react with o-dibromobenzene 

to give not only 223 (34%) [281, 282] and 224 (40%) [283], but also the bisheterocycles 

225 [281, 284] and 226 [285], as separable cis/trans-isomers. Structures are 

confirmed from X-ray and spectroscopic data (Section 4.4.5) and delineate the 

impact of strain in the cycloproparenes. 

187


Scheme 4.40 

4.4.5 

Physicochemical and Theoretical Considerations 

The cycloproparenes are ‘push–pull’ � systems, the direction of which depends on 

the nature of the attached substituents [231, 286] as confirmed by 3,6-di methoxynaphthalene 

derivatives [254, 287]. Cycloheptatrienylidenes 227–231 with electron 

donation from the cycloproparene to the seven-membered ring best illustrate this 

[287]. The dipole moments of 227–231 range from 1.4(5) D to 1.8(5) D with those 

of parents, 233 and 234 directed away from the cycloproparene core and that of 

232 directed towards it (HF/6-31G(d,p) calculations) [288]. That diethers 229 and 

231 are each more polar than parents 228 and 230 verify this for 227, 228 and 234. 

Furthermore, X-ray analyses of 227 and 229 show the seven-membered rings 

markedly distorted from planarity. The cycloheptatrienylidene –CH=CH– moiety 

of 227 is bent out of the cycloproparene plane by ~28° and that of more polar 229 

by ~45° (Figure 4.3). This represents physical resistance of the seven-membered 

ring to developing 8� antiaromatic character enforced by the more powerful 

electron donating cycloproparene. In contrast, the 6� 5C cyclopentadienylidenes 

235 and 204 are essentially planar [288]. 

Figure 4.3 Superimposed side perspectives of cycloheptatrienylidenes 227 (dashed) and 229 

(solid). (Reproduced from [63] with permission from Elsevier).


The linearly dependence of cycloproparene 13 C NMR shifts on p-aryl- and p,p�diarylmethylidene 

substituents [289] extends to the range of 3,6-dimethoxy derivatives 

[254] as shown in Figure 4.4. The correlations apply also to the �-extended 

derivatives 168–175. 

Cycloproparene charge transfer (CT) complexation is demonstrated by admixture 

of dithiole 190 and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in 

MeCN with an instantaneous green that has a new broad CT absorption in the 

540–620 nm range; the complex was too unstable for isolation [255]. A more systematic 

study [290] with p-substituted mono- and diaryl derivatives of 167 showed 

only the bis(dimethylanilino) of the diaryls to complex, and it did so with, DDQ, 

7,7,8,8-tetracyano-1,4-benzoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8tetracyano-1,4-benzoquinodimethane 

(TCNQF 4 ), tetracyanoethene (TCNE), and 

o-chloranil. In contrast, the mono anilino complexes with DDQ only and in 

MeCN, while the p-OMe, p-SMe, and p-Ph homologs complex with TCNQF 4 , but 

only in CH 2 Cl 2 . HF/6-31G**-derived HOMO-LUMO interactions account for 

the complexation as illustrated by 236 and 237. That of the diaryl is via a pendant 

aromatic while the essentially planar mono-aryls facilitate face-to-face interaction. 

A more detailed study is needed. 

189


+ 

Figure 4.4 Plots of �p vs �C for (a) 1-(arylmethylidene)-1H-cyclopropa[b]naphthalenes and 

(b) 3,6-dimethoxy analogs. (Reproduced from [254[ with permission from the Royal Society of 

Chemistry). 

Much impetus for cycloproparene study comes from the juxtaposition of aromaticity 

and strain with the concepts of aromatic bond localization (Mills–Nixon 

effect) and � delocalization central. Noted earlier [245] was the flattening of 

cyclopropa-fused conjugated polyenes (B3LPY/6-31G*), and now we know that 

the bond localization imposed on benzene by tris-fusion as in 238 [291] and 239 

[292] extends to the pyridazines 240 and 241 [293]. Crystallographic analyses of 

the essential cycloproparenes have been discussed [229] and current interest lies


with heteroatom derivatives. The cycloproparenes are fully delocalized aromatics 

whose � frames are distorted primarily about the fusion sites [139: C1a–C5a, 

133.4(4) pm, C1a–C2, 136.3(3) pm] [294] as expected from strain induced bond 

localization (SIBL) [295]. Optimized structures (B3LYP/aug-cc-pVDZ) give negative 

nucleus independent chemical shift (NICS) values fully commensurate with aromaticity 

[296]. Maksi� identifies positive and negative Mills–Nixon effects with 

differing substituents [279, 297] (cf. 220 and cyclopropabenzenyl cations [279]). 

Importantly, clear distinction between crystallographic � frame measurements 

and strain effects that impact on the � orbitals is vital; cycloproparenes have no 

meaningful bond alternation. The diatropic ring current can now be visualized 

using the distributed-origin, coupled Hartree–Fock method [298]. 

Structures of heteroatom derivatives 223–226 have appeared [280, 285]. Table 4.3 

shows that a large atom at the 1-position alleviates steric constraints and reduces 

bond length deviations; comparative data for 139 and (unknown) biscycloproparene 

242 are included. Bridge bonds are lengthened from 133.4 pm in 139 to 

between 139.0 pm (223) and 141.5 pm (cis-225), and there is essentially no aro matic 

bond length variation in bis-225 and 226. This is in stark contrast to unknown 242 

where marked variations are predicted. Despite larger atom sizes, the hetero atom 

derivatives retain internal six-membered ring angles � less (111.9–117.3°) and �/� 

greater (121.2–124.7°) than the 120° of the ideal sp 2 -hybrid with deviations greatest 

in the bis-derivatives. Three-membered ring fusion continues to manifest itself 

even with heteroatoms present, but it is the parent hydrocarbons that are forced 

to maximize the distortions and provide unique structures. 

Cyclopropabenzenyl anion (139A), generated by hydroxide ion deprotonation 

in the gas phase and computed at MP2/6-31+G(d)//HF/6-31+G(d), is more 

stable [299] than its C2 and C3 isomers by 28 and 42 kJ mol –1 . According to 

MP2/6-31+G(d) computation, 4� interaction in the three-membered ring is minimized 

when bonds about the bridge sites (C1a–C5a, 139 pm; C1a–C2, 137.9 pm) 

are lengthened and C1 pyramidalized (C1–H is bent 50.6° from the cycloproparene 

plane) [300]. Homolog 165A comes from fluoride ion deprotonation (Fourier 

transform mass spectrometry) [300, 301]. The proton affinities (PA) are 139A: 

1614.2; 165A: 1535.5; 243A (unknown) 1607.5 kJ mol –1 , respectively [300]. While 

the calculated energies of 165 and 243 differ by only 4 kJ mol –1 , the PAs show the 

former to be 72 kJ mol –1 more acidic consistent with resonance structures and 

avoidance of 8� antiaromaticity. Notable is the lower acidity of 139 than that of 

toluene by ~12.5 ± 13 kJ mol –1 while 165A is more acidic than 2-methylnaphthalene 

by 32 ± 13 kJ mol –1 due to increased ring size and reduced importance of the 4� interaction; 

165A is an aromatic anion [301]. Because 243A is ~40 kJ mol –1 less stable 

191


Table 4.3 Observed and calculated structure parameters for cyclopropabenzene and its 

heteroatom derivatives. a) 

139 b) 

223 c) 

224 c) 

cis-225 c) 

trans- 

225 c,d) 

Mol A 

trans- 

225 c,d) 

Mol B 

cis-226 c) 

trans- 

226 c) 

242 e) 

a 133.4(4) 139.0(4) 139.1(3) 141.5(6) 140.5(11) 139.8(11) 139.4(6) 139.4(6) 134.6 

b 136.3(3) 138.8(4) 

139.4(4) 

c 138.7(4) 138.1(4) 

138.3(4) 

138.5(3) 

138.8(3) 

139.2(3) 

139.1(2) 

140.9(5) 

141.4(6) 

139.7(6) 

139.4(6) 

138.8(11) 138.9(11) 138.1(8) 

141.4(11) 137.6(11) 140.3(9) 

139.4(6) 

140.3(6) 

139.6(9) 140.5(11) 139.6(9) 139.7(6) 

138.9(11) 140.1(11) 139.1(6) 

139.2 

139.2 

d 139.0(5) 140.3(3) 139.7(3) 141.2(6) 1.420(10) 138.0(11) 136.4(9) 138.9(6) 134.6 

e 149.8(3) 182.6(2) 

182.8(3) 

� 124.5(2) 121.3(3) 

121.5(2) 

� 113.2(2) 117.3(3) 

117.3(3) 

� 122.4(2) 121.2(3) 

121.4(3) 

198.2(2) 

194.0(2) 

121.9(2) 

122.0(2) 

116.5(2) 

116.3(2) 

121.6(2) 

121.7(2) 

– – – – – – 

123.2(4) 

122.9(3) 

113.7(7) 

114.3(4) 

122.9(4) 

123.1(4) 

124.7(7) 

122.7(4) 

113.7(7) 

114.8(7) 

122.9(4) 

122.7(4) 

123.0(7) 

123.2(8) 

113.0(7) 

114.6(7) 

124.2(7) 

121.8(7) 

123.1(6) 

123.5(6) 

113.3(6) 

111.9(6) 

124.2(6) 

122.7(4) 

123.8(4) 

123.6(4) 

112.5(4) 

112.3(4) 

123.7(4) 

124.2(4) 

� 52.8(2) 44.7(1) 42.11(8) – – – – – – 

a) Data taken from Refs. [280] and [285]; bond lengths in picometres (pm), angles in degrees (°). 

b) 

X-ray data at –150 °C taken from Ref. [294]. 

c) 

X-ray at –170 °C. 

d) 

Independent molecules A and B are in the unit cell. 

e) 

Optimized at B3LYP/6-311G(2d,p). 

126.1 

107.8 

126.1

References 

than its acyclic counterparts, it is less able to alleviate the unfavorable 4� interaction 

in the three-membered ring. It is best regarded as antiaromatic. Calculated 

proton acidities of homologs 244A–246A are 1483.6, 1611.7, and 1531.3 kJ mol –1 

respectively [300]. Computational study of mono-anionic dicycloproparenes 247A 

and 248A (PA: 1579.0/1605.4 kJ mol –1 ) shows that the second three-membered 

ring enhances acidity. The proton affinity of 139A is 35.2 kJ mol –1 above that of 

247A but only 8.8 kJ mol –1 above that of 248A [300]. 

The impact of mesomeric CN groups and inductive F substituents on the 

stability of 139A has been assessed [301]. The CN group, able to conjugate to 

the C1 anion, increases acidity more at C2(5) than C3(4). Difluorination inductively 

raises acidity by 43.1 kJ mol –1 at C2(5) but reduces it by 43.9 kJ mol –1 at 

C3(4); replacement of all aromatic ring protons in 139A by F decreases acidity 

by 83.7 kJ mol –1 [300]. A range of C1 substituents enhance acidity; Me has least 

(< 1 kJ mol –1 ) and SO 2 H most (141 kJ mol –1 ) impact [302], but their impact is less 

than on the cyclopropenyl anion. 

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and roughly entitled “What 

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in a listener’s question “What do you 

do when you find a client boring?”, also 

admitting concern because it seemed 

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Attempting to answer this, now that I 

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203

5 

Fullerenes 

5.1 

Introduction 


At the beginning of this chapter the inclusion of fullerenes and nanotubes in 

this book has to be explained once more. Of course, speaking formally these 

compounds are not hydrocarbons, since in the pure state they consist only of 

the carbon atoms. However, their extended systems of nonplanar aromatic rings 

provide a model for the analysis of the effect of distortions on physicochemical 

properties. 

The serendipitous discovery of C 60, 1, resembles a detective story [1]. The 

molecule is remarkable for its high symmetry that determines the equivalence 

of all carbon atoms. However, the possibility of the mere existence of molecules 

of this point group was denied by Herzberg [2], later awarded Nobel Prize in 

chemistry for his spectroscopic studies. In his seminal monograph he stated: 

‘The regular icosahedron and the regular dodecahedron belong to point group 

I h. It is not likely that molecules of such a symmetry will ever be found.’ Then C 60 

was independently proposed by two groups of theoreticians [3, 4]. However, the 

suggestion was totally forgotten. Interestingly, Rohlfing and coworkers observed 

a strong signal corresponding to C 60 in their study of small carbon clusters in 

1984 [5]. They even reported the whole mass spectrum with the remarkable C 60 

signal but, focused on small structures, they limited their discussion exclusively 

to ions smaller than 40. The following year, an observation by Curl, Kroto and 

Smalley of a strong C 60 peak (accompanied by a smaller C 70 one) in a carbon soot, 

produced by discharges in vacuum that should mimic conditions in the cosmos, 

marked the beginning of the fullerene era. Intriguingly, the authors, not knowing 

the earlier proposals, had trouble assigning the structure and did not know what 

shape it represented. Kroto’s hobby interest in graphics and his fascination for 

the geodesic domes of the American architect Buckminster Fuller provided clues, 

helped clear up the problem and brought the molecule its name. Then, a phone 

call to the Mathe matical Department brought the answer: soccer ball. 



ISBN: 978-3-527-31767-7 

205

206 5 Fullerenes 

To begin with, the available amount of fullerene was very small, preventing proof 

of the structure. A purification procedure developed by the Krätschmer group 

solved the problem of obtaining bulk amounts of C 60 [6], thus enabling further 

studies. Four sharp bands in the IR spectrum of the enriched soot were compatible 

with the I h symmetry of the molecule. Kroto learned about the purification 

procedure by refereeing the paper. After writing a positive review, he purified a 

sample and let it be measured by 13 C NMR [1]. The spectrum consisted of only 

one line providing the first proof of the equivalence of all 60 carbon atoms, thus 

proving the spatial fullerene structure. Interestingly, the first X-ray measurements 

yielded the C 60 cage diameter and the distance between the molecules in the crystal 

only since the near-spherical shape allowed practically undisturbed rotation of the 

molecules at room temperature. As a result, two different C–C bond lengths were 

first, although quite inaccurately, determined by NMR technique [7]. Structural 

aspects of fullerene chemistry and chirality were recently summarized by Thilgen 

and Diederich [8]. 

The highly symmetrical structure of 1 seems quite artificial but the molecule 

has been found in some minerals (http://webmineral.com/data/Fullerite.shtml) 

and in the cosmos [9, 10] and its presence in Nature precluded its patenting. A few 

C 60 molecules are even produced by burning dinner candles [11]. 

All fullerenes are composed of 12 cyclopentane rings and a certain number of 

six-membered ones. Most stable fullerenes are known to conform to so called 

Isolated Pentagon Rule, IPR, stating that their cyclopentane rings do not have 

common atoms or bonds. The smallest carbon cage satisfying this condition is 

that of 1 and it represents the most frequently encountered fullerene structure. 

The next one, discovered together with C 60 , is 2. In addition, numerous smaller 

(not complying with IPR) and higher fullerenes have been reported. Contrary to 

1 and 2, for which only one isomer is possible, the higher members of the family 

can assume several structures of, sometimes different, symmetry. An Atlas of 

Fullerenes [12] collects all possible IPR structures and their symmetries from C 20 

to C 100 . The Fullerene Gallery at the address: http://cochem2.tutkie.tut.ac.jp:8000/ 

Fuller/fsl/fsl.html presents many of them including non-IPR ones. Seven possible 

IPR isomers of C 80 are shown in Figure 5.1. 

C 20 can be considered as the archetypal fullerene, built of fully unsaturated 

five-membered rings, since it is a highly unstable fulleroid structure without any 

six-membered rings. Its synthesis was accomplished by the Prinzbach group 

[13–15].

Figure 5.1 Seven possible isomers of C 80 with their symmetry according to 

An Atlas of Fullerenes [12]. 

Scheme 5.1 

5.1 Introduction 

As noticed by J.-M. Lehn [16], the synthesis of C 60 represents a unique example 

of the covalent self-assembly in which 90 bonds are formed simultaneously. Efforts 

of several groups trying to synthesize the molecule in a series of ‘true chemical 

reactions’ resulted in rich corannulene chemistry (Scheme 5.1) [17–19]. 

The interest in fullerenes was triggered by the observation that other molecules 

or ions can be hosted inside their cages. Such supramolecular complexes, which 

bear the name endohedral fullerene complexes, were thought to allow one to 

propose exciting applications. The first suggestions seemed to be too good to 

be true. For instance, Stoddart wrote about a fullerene cage with ‘a door’ which 

could serve as a drug carrier releasing the content at a desired place [20]. He also 

predicted that extending the superconductivity of 1 to room temperature or using 

perfluorinated C 60 F 60 as a lubricant would revolutionize our all lives since we 

would avoid losses due to electrical resistance or friction. Later we have learned 

207


that these hopes (discussed in some details in Chapter 2.4) cannot be fulfilled 

[21, 22]. Model calculations have shown that production and purification of 

much larger fullerenes must be mastered to allow much larger guests of practical 

interest to be hosted [23]. 

Fullerenes are relatively large objects, difficult to study both experimentally 

and computationally. Highly symmetrical C 60 is a kind of exception although it 

also poses several problems. Other fullerenes are available in small quantities 

and their lower symmetry complicates their study. Only the most important 

experimental methods, often combined with discussions of theoretical results, 

will be presented in this chapter and several exciting aspects of fullerene research 

will not be covered. For instance, endohedral fullerene complexes with metal 

ions inside forming untypical salts that can be dissociated only by destroying the 

cage, will practically not be discussed here. It should be stressed, however, that 

the formation of endohedral complexes with these or other guests can stabilize 

otherwise unstable non-IPR fullerenes [24]. 

Fullerene studies are often triggered by practical considerations. However, 

these highly symmetrical systems are of critical importance for basic research. As 

shown by thought-inspiring experiments in the Zeilinger group on the obtaining 

of diffraction patterns of C 60 and C 70 [25, 26], such purely scientific studies could 

eventually lead to marketable devices. 

Applications of fullerene will be in detail discussed in Section 5.7. It suffices 

to mention here that at present only few have reached the market. Interestingly, 

the hopes for the applications were high in the late 1980s and early 1990s. Then 

carbon nanotubes took the lead and electronic devices built from one nanotube 

were thought to revolutionize our whole life. For instance, nanotube transistor 

[27] or flat displays (http://news.com.com/Carbon+nanotube+TV+trials+ 

on+horizon/2100-1041_3-6051476.htm) were expected to be sold soon. However, 

they seem to pose numerous technical problems, resulting in new interest in 

fullerene applications. In any case, in spite of very few practical applications of 

fullerenes and nanotubes at present, these molecules have given a strong boost to 

the development of nanotechnology which sooner or later is destined to change 

our everyday life. 

5.2 

Chemistry Influenced by the Nontypical Structure: Modification of [60]Fullerene 

Takuma Hara, Takashi Konno, Yosuke Nakamura and Jun Nishimura 

5.2.1 

Introduction 

As is well known, Kroto, Smally, and Curl worked together with their associates 

and during their study on interstellar carbon clusters, serendipitously discovered 

a soccer ball compound or C 60 which originally was named buckminsterfullerene

5.2 Chemistry Infl uenced by the Nontypical Structure: Modifi cation of [60]Fullerene 

Figure 5.2 Number of papers reported on fullerenes and CNTs. 

dedicated to genius architect Richard Buckminster Fuller [28]. Since its large-scale 

preparation was developed by Krätschmer and Huffman [29], the chemical world 

has been caught by the fullerene fever for many years [30]. Only recently, because 

of the prospects of their applications, the fever has passed most of its heat to carbon 

nanotubes (CNTs), as shown in Figure 5.2. However, the unusual properties of C 60 

and other fullerenes continue to make them fascinating objects for study. 

During the hot years, many useful C 60 derivatives were obtained. Most modifications 

are of three general types: i.e. those with nucleophilic, radical, and 

electrophilic reagents. Since C 60 is an electron acceptor itself, most of the modifications 

have been carried out by using nucleophilic, electron-donating species 

or radicals. 

In this chapter we first overview the fullerene reactions and then summarize the 

modifications, dividing them into six subclasses according to the addend structures. 

Since, at least theoretically, almost all reactions are applicable to all types 

of fullerenes, the reactions of C 60 which is the best studied, most easily available 

and also interesting for applications, will mainly be discussed here. 

Moreover it should be stressed that within the last 15 years the circumstances 

around the fullerene chemistry have been well established except for the safety 

issues [31]; i.e. the supply of various fullerenes [32], the IUPAC nomenclature 

[33], and many useful HPLC columns for their separations [34], etc. A variety 

of findings in C 60 chemistry have surprised a number of audiences not only in 

organic, theoretical and general chemistry but even in natural philosophy. 

5.2.2 

General Overview 

Fullerenes have a system of conjugated double bonds and receive various additions 

at the sites, although these are not regular double bonds. The bonds are classified 

into (5,6) junction (single bond) and (6,6) junction (double bond), which mean 

the edges connecting a five-membered ring and a six-membered ring, and two 

five-membered rings, respectively [35]. These bonds have different properties, as 

shown in Figure 5.3. Many reactions do not usually occur at (5,6) junctions, but 

at (6,6) junctions to give 1,2-adducts [36]. 

209


Figure 5.3 Two different bonds: (5,6) junction with 1.45 Å of bond length and (6,6) one 

with 1.38 Å [9]. 

A variety of reaction mechanisms have been suggested for the novel electronaccepting, 

large �-conjugating fullerenes, which can be roughly classified as 

depicted in Figure 5.4. 

Figure 5.4 Reactions classified.


Species 3 are generated in many ways by the reactions of organolithium [37] 

including acetylides [37d], Grignard reagents with [30] or without copper salt 

[37b,c, 38a–c], and typical nucleophiles [39] such as CN – [40]. It was reported 

that the negative charge of a typical t-BuC 60 – anion 3 is not fully delocalized but 

rather localized at the adjacent carbon (2-position) of the t-Bu addend (1-position) 

[37b]. Grignard reagents react with C 60 more slowly than organolithium, and the 

products are 1,4-adducts in contrast with the organolithium cases providing 1,2adducts 

[37b,c]. Grignard reagents like the phenyl one together in the presence 

of CuBr-Me 2S complex react with the fullerene to afford interesting adducts 

having five addends at the 2,5,10,21,24-positions, which have been extensively 

applied to various fields, not only organic chemistry but also materials science 

[38d–f]. Sodium cyanide and fullerene dissolved in DMF-o-dichlorobenzene mixed 

solvent generated carbanion 3, which was trapped by various electrophiles like 

p-t-butyl(bromomethyl)benzene to give the appropriate 1,2-adducts [40]. Diethyl 

bromomalonate gives 3 through the addition of its ester enolate, which rapidly 

eliminates bromide anion to end up with the formation of the corresponding 

methanofullerene. This reaction is called the Bingel reaction, dedicated to the 

inventor [39]. Dianion C 60 2– formed by the two-electron reduction of C60 can 

act as a nucleophile, which reacts with, for example, benzyl bromide to afford 

1,4-dibenzylfullerene [41]. 

Since fullerene reacts with radicals so rapidly, it is often called a radical 

sponge [42]. Generally, a radical species adds at the fullerene double bonds and 

generates a fullerene monoadduct radical species, which accepts another radical 

at the 4-position toward the 1-addend position. Such reactivity is explained by 

acknowledging that fullerene does not have a fully conjugated �-system but rather 

[5]-radialene substructure which becomes the stable conjugated cyclopentadienyl 

radical 10 after the addition of five radicals as shown in Scheme 5.2. 

Scheme 5.2 Radical addition to the fullerene with [5]radialene substructure. 

211


The fullerene does not usually react well with Brønsted acids, but does somehow 

with sulfuric acid–nitric acid mixture to afford polyhydroxyfullerenes [43]. Lewisacid 

catalyzed transformation was reported to proceed through arenium cation 

[44]. 

The double bond of fullerene generally behaves as an electron-accepting site 

(7) such as dienophile or dipolarophile. Therefore, C 60 exhibits many pericyclic 

reactions; for example, Diels–Alder reaction, 1,3-dipolar cycloaddition (Huisgen 

reaction) such as Prato reaction, and so forth. 

There are many synthetic data accumulated so far on several reactions. These 

reactions proceed with the interaction of fullerene (acceptor) LUMO and reagent 

(donor) HOMO (7) or the reverse combination (8). The bisadduct distribution 

seems to provide a method that can qualitatively define whether the reagents 

behave as acceptors or donors. Several popular reactions like Bingel [45], Prato 

[46], and Diels–Alder [47], which give 3-membered, 5-membered, and 6-membered 

rings as the junctions between the fullerene and addends, were reported to have 

nucleophilic character, while benzyne [48] and nitrene [49] cycloadditions seem 

to be electrophilic. 

The multi-addition, especially bisaddition, is one of the interesting fields in 

fullerene chemistry. Yet even the bisaddition of a symmetric reagent to C 60 can 

give at least 8 isomers: cis-1, cis-2, cis-3, e, trans-4, trans-3, trans-2, and trans-1 (see 

Figure 5.5 and Table 5.1). The regioselectivities of bisaddition were thoroughly 

examined on the three major fullerene modification reactions mentioned above. 

Moreover, several C 2 -symmetric regioisomers, cis-3, trans-3, and trans-2, are chiral, 

and their absolute configurations have been determined by CD spectroscopy after 

optical resolution by using some elaborate chiral HPLC columns [34]. 

Generally speaking, the regioisomer distribution of bisadducts was qualitatively 

correlated with the frontier orbital coefficients of the corresponding monoadducts; 

e and trans-3 bisadducts were formed preferentially among eight isomers [45–49]. 

The isomer ratios in Bingel, Prato, and Diels–Alder bisadditions are depicted in 

Figure 5.6a. The reactive species in these reactions, malonate anions, Huisgen 

ylides, and dienes, are nucleophiles in their nature, so that similar regioselectivities 

are obtained. 

The isomer ratios in benzyne and nitrene bisadditions are shown in Figure 5.6b. 

The most remarkable difference between these bis-additions and those shown 

in Figure 5.6a is the high ratio of cis-1 in the former. Generally, the cis-1 position 

is most stressed by the first addition so that this double bond is apt to be more 

reactive and to accept the addition of species more than the other sites. However, 

the reactions except for benzyne and nitrene additions suffer from steric hindrance 

between the first addend moiety and incoming species, and usually result in a 

Table 5.1 Number of isomers with symmetrical achiral addends [50]. 

Number of additions Mono Bis Tris Tetrakis 

Number of isomers 1 8 46 262


Figure 5.5 Eight possible isomers of bisadducts. 

negligible amount of cis-1 product. Actually benzyne and nitrene can give flat 

addends, while other reactions afford more bulky addends. 

The second apparent difference of benzyne addition from others is that it has 

less total content of the southern hemisphere products. The same tendency is 

observed for the nitrene bisaddition, since the methoxycarbonylnitrene is regarded 

as an electrophilic species, although there is some subtle difference between the 

two electrophilic additions. 

For selective modification, especially for multiadditions, the use of tethers 

has been proposed. For example, Diederich’s and Hirsch’s groups developed the 

selective Bingel addition toward bisadducts and trisadducts [45c, 51]. Nakamura’s 

group successfully obtained a chiral bisadduct having a cyclopentane ring at the 

junction [52]. Recently a selective Prato reaction was also achieved using the tether 

manipulation [53]. Diels–Alder reaction was made selective at the bisaddition 

using the same procedure [54]. 

213


Figure 5.6 Distribution of regioisomeric bisadducts. 

Another method to make multi-additions selective was developed by Hirsch’s 

group, using a reversible template [55]. Using the reversible Diels–Alder reaction of 

9,10-dimethylanthracene as a template, they obtained O h -symmetrically modified 

hexakisadducts by Bingel addition, although a recent paper reported that similar 

selectivities were obtained without using the anthracene derivative [56]. 

Returning to Figure 5.4, finally the thermal or photochemical electron transfer 

reactions from a donor like alkylamine to the fullerene must be mentioned. At 

the very beginning of the fullerene chemistry, the addition of alkylamines was 

discovered [57]. It is suggested that this reaction involves the formation of fullerene 

radical anion and amine radical cation through single electron transfer (SET) and


the radical coupling of both species to afford the corresponding amine adduct 

through the proton transfer [57a]. On the other hand, under photoirradiation 

an electron-rich ketene silylacetal 11 reacts with C 60 through SET to give an ene 

reaction product as shown in Scheme 5.3 [58]. In this reaction, first the fullerene 

absorbs light to excite itself to 1 C 60*, which rapidly and efficiently renders intersystem 

crossing to 3 C 60*. Then 3 C 60* gets an electron from the acetal to generate 

its anion radical and acetal cation radical, which combine with each other finally 

giving product 12. 

Scheme 5.3 SET under photoirradiation. 

Many reactions follow one of the above reaction types. The stability of adducts 

is sometimes an important issue, and depends largely on the reversibility of 

modification reactions. Since pristine fullerene itself is thermodynamically stable 

enough, most fullerene derivatives reproduce fullerene under harsh conditions, 

even though the reactions are not reversible. Bingel adducts are decomposed by 

electrolysis [59]. Prato adducts give pristine fullerene when they are treated with a 

strong dipolarophile such as maleic anhydride in the presence of a catalyst such as 

Wilkinson’s complex or copper triflate in o-dichlorobenzene at reflux for 8 to 18 h 

[60]. Diels–Alder adducts easily decompose or usually undergo retro-Diels–Alder 

reaction at high temperatures or sometimes even at r.t. (see below) [61]. 

5.2.3 

Modification Reactions 

Showing typical products with their structures, a variety of modifications are 

introduced in this section, focusing on the joint structures between the fullerene 

and addends. 

5.2.3.1 Reduction and Oxidation 

Many investigations related to the reduction have been reported [62], using direct 

hydrogenation at high temperatures, electrochemical reduction, BH 3 -THF, 

NH=NH, LiBEt 3 H then treated with HBF 4 , Birch reduction, Zn-HCl, Zn(Cu), 

hydrozirconation, H 2 Rh(0)-Al 2 O 3 , Pd/C-catalyzed reduction, and so forth. 

Theoretically all 30 double bonds ((6,6) junctions) can accept hydrogen atoms 

leading to C 60 H 60 . After receiving a certain number of hydrogen atoms, however, 

the remaining double bonds are buried inside the C 60 surface too deeply to react or 

the products are too much deformed to stay stable. The theoretical studies indicate 

that C 60 H 60 with all CH bonds pointing outwards is much less stable than the 

215


Figure 5.7 Hydrogenation and oxidation of C 60 . 

isomers with the bonds pointing inside the C 60 cage [63] (see Section 2.4.1.7). Until 

now the structures of C 60 H 2 , C 60 H 4 , C 60 H 6 , and C 60 H 18 have been well examined 

and reported (Figure 5.7). Among the series of C 60 H 24 to C 60 H 36 various reducing 

agents under various conditions selectively gave C 60 H 36 , which is revealed as a 

mixture of isomers. The structure of hydrogenated fullerenes mainly studied by 

NMR spectra is discussed in Section 5.6. 

The fullerene is easily oxidized, and even its commercial samples usually contain 

the oxide. The reactivity to oxidation was anticipated at the very beginning of 

C 60 studies. It is oxidized to form fullerene oxide or epoxide 13. The oxidation is 

carried out by electrochemical reaction [64a], irradiation with oxygen [64b], direct 

oxidation at high temperatures [64c], oxygen with a radical initiator like AIBN 

[64d], ozone [64e], dimethyldioxolane [64f], MCPBA [64g], P450 model system 

[64h], and so forth. 

Recently Tajima and his co-workers reported an intriguing transformation 

of the oxide shown in Scheme 5.4 [65], which may be useful in the materials 

science. 

Scheme 5.4 Transformation of fullerene oxide.


The nitration of C 60 with NO 2 gave mixtures of tetra- and hexanitro derivatives 

in the ratio depending on the reaction time [66]. Then a selective synthetic method 

toward hexanitro[60]fullerenes was developed [43c]. The nitroallylic moieties in 

hexanitro[60]fullerenes were found to be excellent leaving groups for the nucleophilic 

substitution by amines, such as anilines, leading to the formation of 

hexaanilino[60]fullerenes. Moreover, hexanitro[60]fullerene was reported to be a 

better electron acceptor and form nitrofullerene complexes with aniline, o-, m- and 

p-phenylenediamine, pyridine, tacrine, triethylamine, 1-aminoanthraquinone, 

N,N-dimethylaniline, TTF, and BEDT-TTF (ET) [67]. 

The nitrated products partly isomerize to the nitrite esters, which, with subsequent 

hydrolysis by atmospheric moisture, give nitrofullerols containing 6–8 nitro 

and 7–12 hydroxy groups per fullerene [68]. Hydrolysis of polynitrofullerenes in 

aqueous NaOH gave the corresponding polyhydroxylated fullerene derivatives or 

fullerols in moderate overall yields. Halogenated fullerenes were used as reagents 

to prepare fullerols. Halogens were selectively substituted when fullerene bromides 

and chlorides were treated with silver trifluoroacetate [69]. 

5.2.3.2 Alkylation 

Here the alkylations of fullerene are summarized. These addends are formed via 

intermediates 3, 4, 5, and 9. 

Some alkyl- and ethynyl-fullerenes were produced by the reaction with t-butyl-, 

9-fluorenyl-, and phenylethynyl-lithium reagents. Typical reaction products are 

listed in Figure 5.8 [37c, 38c, 70]. 

Figure 5.8 Some derivatives obtained from intermediate 3; 15a [37c], b [70a,b], c [70a], d [70c], 

e [70e], f [70d], g [70f], h [70f], i [70f], j [37c], k [37c], l [37c], m [37c], n [37c], o [37c], p [38c]. 

217


Grignard reagents react with fullerene to afford some intriguing products as 

shown in Figure 5.8 [38c]. It was reported that the reaction is influenced by solvents 

and presence of oxygen in the reaction media. Nakamura and his co-workers 

discovered so-called Nakamura reaction that, using Grignard reagent combined 

with copper reagents and dimethyl sulfide, affords several penta-adducts around 

the corannulene subunit in excellent yields [71]. Recently they were successful in 

the first synthesis of beltane derivative or 40�-[10]cyclophenacene 16, which can 

be regarded as the smallest unit of (5,5)CNT [72]. 

5.2.3.3 Cycloadditions 

Wudl once suggested that some cycloadditions are the most useful in the modifications 

of fullerenes [73]. The following four subsections summarize these useful 

cycloaddition reactions. 

Three-membered Ring-fused C 60 . A three-membered ring as fullerene-addend linkage 

is formed through intermediates, 3, 7, and 8. Among the reactions through 3, 

Bingel reaction is the most popular [39], which becomes now one of the most 

useful fullerene modification reactions (Scheme 5.5) [73, 74]. Using this reaction, 

many fascinating and intriguing derivatives have been synthesized as shown in 

Figure 5.9. For example, Nakamura, Nishimura, and their co-workers carried out 

Bingel reaction at –78 °C to r.t. and for the first time successfully obtained some 

catenanes possessing C 60 surface as the ring component [75]. Other interesting 

Bingel products are also depicted in Figure 5.9 [36, 76]. 

Scheme 5.5 Bingel reaction [39].


Figure 5.9 Fascinating Bingel products 18 [75], 19 [36], and 20 [76]. 

The Bingel products are apt to render electrochemically induced isomerization 

by the migration of cyclopropane rings on the C 60 surface, if the controlled 

potential electrolysis is not exhaustive [77]. The isomerization was expressed as 

‘electrochemical walk on the sphere’. 

Many related reactions have been reported. Monoalkyl malonates react with 

iodine in the presence of DBU to afford iodomethanofullerenes 21 [78]. BrCH 2CN 

and bromoform also give corresponding methanofullerenes by the treatment of 

strong base LDA [79]. The reaction with a special enolate ion gives a derivative 22 

[80]. Phosphonium [81] and sulfonium [82] ylides afford a variety of methanofullerenes. 

Recently Saigo and his co-workers developed fullerene-acid chloride 

23 as a building block of the synthetic chemistry via the sulfonium ylide transformation 

[83]. 

219


Scheme 5.6 Stepwise and direct formation of fulleroid and/or methanofullerene. 

Diazomethane and related diazo compounds react with fullerene double bonds 

and form corresponding pyrazolines, which decompose to give fulleroids with 

the loss of nitrogen (see Scheme 5.6) [84]. Often fulleroids readily rearrange into 

methanofullerenes under heating or photoirradiation [85]. 

The reactions of diazo compounds with the fullerene do not always provide 

pyrazolines. In some cases, the corresponding carbenes are first generated and 

then add to fullerene to give methanofullerenes. Recently Akasaka and his coworkers 

examined thoroughly whether the reactions with diazoadamantane and 

alkyldiazirines pass through carbenes or pyrazolines. They successfully partitioned 

reactions after the analysis of methanofullerene and fulleroid contents [86]. 

As other three-membered ring formations, aziridine framework is used as the 

junction between the fullerene and a functional group. RO-CO-N 3 forms aziridine 

ring between fullerene and alkoxy- or aryloxy-carbonyl group (see 27) [87]. On the 

other hand, RCH 2 N 3 adds to the fullerene to afford azafulleroid 28 [88], one of 

which is derived to aza[60]fullerene as its dimer form 29, only accessible hetero-


fullerene. Recently the chemistry of heterofullerene has been developed by Hirsch’s 

group [89]. Silylenes generated in situ add to the fullerene to afford corresponding 

silacylopropane product 30 [90]. 

Four-membered Ring-fused C 60 . The most striking four-membered ring between 

fullerene and a partner was disclosed as the form of fullerene-dimer 31 by Komatsu 

and his co-workers [91]. This dumbbell type structure is often used as a symbol 

of chemical achievement. The dumbbell is formed through 3. Naturally a kind of 

[2+2] cycloaddition was expected to occur under photoirradiation. Using supramolecular 

and photochemical technique, McClenaghan and his co-workers first 

successfully obtained corresponding fullerene dimer [92]. Some 1,3-butadienes 

[93a], styrenes [93b], and enones [93c] also gave cyclobutane-annulated fullerenes 

under photoirradiation through radical species 5. Under thermal conditions 

some reactive acetylenes [94a], dimethyleneketene acetals [94b], ketenes [94c], 

and allenamides [94d] made cyclobutene-ring-joints for the fullerene. Besides 

the examples mentioned above, there are a few ones, mostly from the reaction of 

benzyne [47, 95] as shown in Scheme 5.7. 

Scheme 5.7 Four membered-ring formation; 31 [91], 32 [94a], and 33 [47]. 

221


Five-membered Ring-fused C 60 . The five-membered rings can be formed by [3+2] 

cycloaddition, where three sp 2 atoms come from reagents and two sp 2 atoms derive 

from the fullerene double bond at 1,2-positions. Reagents possessing three-atom 

active site are 1,3-dipoles or Huisgen ylides, trimethylene methane diradical, 

disilylane, and so forth (Scheme 5.8). The reaction path usually is considered 

to be through 5 for photoreactions or 7 for pericyclic reactions of electron-rich 

ylides. Here in this section only stable products are dealt; unstable pyrazolines 

mentioned above are not included. Moreover, some cases treated above are not 

touched below such as dioxacyclopentane system 14. 

Scheme 5.8 Prato reaction. 

Figure 5.10 Fascinating Prato products; 35 [98], 36 [99], and 37 [100].


Figure 5.11 Some five-membered ring-fused fullerenes with reagents or reactive species; 

38 [101], 39 [102], 40 [103], 41 [104], 42 [105], 43 [106], 44 [107], 45 [107], and 46 [107]. 

The most useful convenient method among the kind of modifications is Prato 

reaction, one of pericyclic reactions [96]. It is one of convergent syntheses in which 

two high molecular weight substrates can be joined by one step. Actually fullerene, 

N-alkylglycine, and an aldehyde, such as formaldehyde or a high molecular weight 

aldehyde prepared, are dissolved in toluene and the mixture is heated to reflux. 

The procedure is just simple and the yield is usually high to excellent. Therefore it 

is so popular in materials science field as well as the fullerene chemistry. Recently 

this reaction was applied to make CNTs soluble in conventional organic solvents 

such as acetone and chloroform [97]. 

Fascinating Prato products 35–37 [98–100] are depicted in Figure 5.10. Other 

five-membered-ring formations are summarized in Figure 5.11 with reagents or 

reactive species [101–107]. 

Six-membered Ring-fused C 60 . There are some stepwise and concerted [4+2] cyclization 

reactions toward six-membered ring-fused fullerenes. The most popular one 

of the latter category is Diels–Alder reaction. As discussed above, the Diels–Alder 

adducts of fullerene are not always stable enough [61], so that unstable dienes such 

223


Figure 5.12 Some Diels–Alder examples together with a stepwise addition of amine; 47 [108e], 

48 [111], 49 [112], and 50 [113]. 

as o-quinodimethanes are used to prevent the retro-Diels–Alder reaction [108] or 

the adducts are further transformed into more stable products [109]. Moreover the 

instability of Diels–Alder adducts is applied to the regioselective Bingel reaction 

mentioned above [54, 55], as well as the isolation of a higher fullerene such as 

C 84 from the mixture with C 76 and C 78 [110]. Nevertheless making precursors for 

quinodimethanes, for example, is time consuming so that the Diels–Alder modification 

is not applied frequently to the materials science fields. Some Diels–Alder 

examples together with a stepwise addition of amine are depicted in Figure 5.12 

[108e, 111–113]. 

5.2.4 

Conclusions 

In the last one-and-a-half decades, a variety of modification reactions have 

been developed using the most easily available C 60 in the chemistry. Yet still 

some methods are being searched especially in the transition metal catalyzed 

reactions. 

Since the application to materials science is now booming, the fullerene properties 

have attracted much attention, such as the high coagulation tendency, the 

poor solubility in usual organic solvents, the nanometer-scale volume, as well as 

the electron-accepting nature. When its electronic properties are important at the 

applications, however, the tendency must be kept in mind that C 60 gradually loses 

its properties by the modification. For example, its quantum yield of the singlet 

oxygen generation under photoirradiation decreases from 1.0 for the pristine 

one as a sensitizer to 0.9 for monoadduct, and then to 0.5 for some bisadduct 

isomers [114]. 

And now the isolation of unstable fullerenes seems to be focused on, which 

is mentioned briefly above [110]. In the next decade we will see a lot of exotic

5.3 Physicochemical Properties and the Unusual Structure of Fullerenes 

fullerenes isolated from the soot obtained by a variety of methods. Those will not 

only be limited to the higher fullerenes but also found in lower fullerenes, which 

will give some impacts in organic chemistry itself at the same time. 

5.3 

Physicochemical Properties and the Unusual Structure of Fullerenes 

5.3.1 

Single-crystal X-ray Structures of Fullerenes and Their Derivatives 

Olga V. Boltalina, Alexey A. Popov and Steven H. Strauss 

5.3.1.1 Introduction 

Fullerenes are closed-carbon-cage molecules containing 12 pentagons and (except 

for C 20) one or more hexagons of three-connected C atoms that are nominally sp 2 

hybridized [115]. All isolated-pentagon-rule (IPR) [116] C 60+x fullerenes have 60 

C atoms that are at the junction of two hexagons and one pentagon (HHPJs); the 

other x C atoms are at the junctions of three hexagons (triple-hexagon junctions, 

or THJs). The curvature of fullerene cages requires that fullerene C atoms cannot 

be coplanar with their three nearest neighbors, as shown in Figure 5.13. This is 

a significant departure from the usual planar geometry of C(sp 2 ) atoms, and the 

nonplanarity introduces steric strain in all fullerenes relative to planar graphite. 

The exohedral derivatization of fullerenes is driven, in large measure, by the relief 

of this strain as substituents [117] are added to some cage C atoms, changing 

the hybridization of those C atoms from (approximately) sp 2 to sp 3 [118]. The 

drawings in Figure 5.13, made with the coordinates of DFT-optimized C 60 and 

the known D 2 -symmetry isomer of C 76 (which is denoted C 76 -D 2 (1)) [119], show 

that HHPJs are more pyramidal with respect to their neighbors than THJs, and 

consequently no hollow fullerene derivative with fewer than 38 substituents has 

been found to have a substituent on a THJ [120–122]. In other words, HHPJs are 

Figure 5.13 One of the 60 hexagon–hexagon–pentagon junctions (HHPJs) in the 

DFT-optimized structure of C 60 (top) and one of the 16 triple-hexagon junctions (THJs) in the 

DFT-optimized structure of C 76 -D 2 (1) (bottom). Both views are parallel to the plane of the 

three darkly-shaded C atoms to which the junction carbon atom is attached. The darkly-shaded 

C atoms are connected with dashed lines. 

225


more reactive with respect to addition reactions than THJs because of the difference 

in pyramidalization [117]. 

There are two excellent reviews of fullerene X-ray structures, one by Neretin 

and Slovokhotov [123], which lists nearly 400 fullerene structures, and the more 

focused review by Balch [124]. The focus of this chapter is the inherent steric 

strain in fullerenes, as measured by �-orbital axis vector (POAV) analysis [125]. 

Only leading references will be cited. The examples of fullerene derivative X-ray 

structures we have chosen for analysis have F and CF 3 substituents because these 

structures are more numerous [126, 127] than structures with other substituents. 

This will allow a comparison to be made of structures with identical compositions. 

X-ray structures of endohedral metallofullerenes (EMFs) will not be discussed, 

and space limitations further prevent us from reviewing the many structures of 

fullerene cycloadducts [128] and the growing class of norfullerene derivatives [128, 

129]. Finally, the DFT-predicted results (PBE functional) in this chapter that are 

not explicitly attributed to previous publications were determined for this chapter 

using methods previously described [126]. 

5.3.1.2 Disorder 

The first attempts to determine precise C–C distances in crystalline C 60 by X-ray 

diffraction revealed that the icosahedral molecule exhibits rotational disorder, 

and only the cage radius and intercage distance could be determined [123]. 

Consequently, as discussed in Section 5.3.3, the first determination of C 60 bond 

distances was carried out using solid-state NMR spectroscopy [130], and only later 

by X-ray [131] and neutron diffraction [132] (in the latter two cases the fullerene 

was disordered even at low temperature). Although the solid-state disorder is 

severe, it is not completely isotropic [133]. To varying extents, higher fullerenes and 

endohedral fullerenes without exohedral substituents also exhibit disorder of cage 

C atoms and hence generally afford only imprecise diffraction-derived positional 

parameters [123, 124]. This problem has been overcome in favorable cases by cocrystallization 

with other molecules or by lowering the temperature of the crystal 

[123, 124, 134, 135]. The most precise structure of C 60 is in C 60 ·Pt(OEP)·2 C 6 H 6 ; 

the fullerene is on a general position and the estimated standard deviations (esd) 

for the 90 cage C–C distances are 0.003 Å [135]. In most endohedral fullerene 

structures, some or all of the endohedral atoms exhibit positional disorder, even 

when exohedral substituents or cocrystallization with metalloporphyrins has 

minimized or eliminated cage disorder. However, the first completely ordered 

EMF derivative was recently reported [136]. 

5.3.1.3 Nonplanar Steric Strain 

The structures, Schlegel diagrams, and IUPAC numbering schemes for C 60 and 

C 70 are shown in Figure 5.14. Every fullerene larger than C 60 has more than 20 

hexagons (C 70 has 25 hexagons and ten THJs). The nonplanar strain energy is 

considerable. It is believed to be ca. 2 000 kJ mol –1 in C 60 (33 kJ mol –1 C-atom –1 ), 

which is 80% of the excess energy of the C atoms relative to graphite [125]. The 

POAV angle � �� , defined in Figure 5.15 [125], is 101.64° for DFT-optimized C 60


Figure 5.14 DFT-optimized structures and Schlegel diagrams of C 60 -I h (1) and C 70 -D 5h (1). 

The 10 triple hexagon junctions around the equator of C 70 are C2, C5, C12, C20, C30, C40, C50, 

C59, C65, and C70. 

Figure 5.15 In this version of the �-orbital axis vector approximation, the POAV is defined as 

the vector that makes equal angles to the three � bonds at a conjugated C atom. The common 

angle is denoted � �� . In C 60 , the � p angle (i.e. � �� – 90°) is 11.64°. 

227


(the reduced POAV angles, defined as (� �� – 90°) and hereinafter denoted � p, 

range from 11.4° to 11.9° for the C 60 molecule in C 60·Pt(OEP)·2 C 6H 6 [135], and 

are 8.6°, 10.2°, 11.5°, 11.8°, and 12.0° for C2, C1, C7, C24, and C8, respectively, 

in the DFT-optimized structure of C 70-D 5h(1). DFT average values of � p are 10.9° 

for C 70, 10.3° for the known D 5d(1) isomer of C 80, and 9.9° for the known D 2(22) 

isomer of C 84. It is sensible that the average value of � p decreases as the number 

of C atoms in the cage increases because, in a C 60+x fullerene, x C atoms are THJs 

and are therefore in a more planar environment than the 60 C atoms that form 

the 12 pentagons. 

The strain energy in fullerenes varies from isomer to isomer in higher fullerenes, 

being lowest for isomers in which the pentagon-induced curvature is distributed 

as uniformly as possible over the fullerene surface (i.e. for isomers with pentagons 

as far apart as possible) [115]. This is an extension of the isolated-pentagon rule 

(IPR) [116]. The lower stability of fullerenes with adjacent pentagons is generally 

understood to be due to two factors, (1) resonance destablization and (2) an 

increase in steric strain [118]. The larger steric strain in some of the cage C atoms 

of a fullerene with adjacent pentagons is clearly seen in Figure 5.16, which shows 

the DFT-optimized structure of the hypothetical non-IPR fullerene C 84 -C s (51365) 

(an actual derivative of it, the EMF Tb 3 N@C 84 -C s (51365), has been studied by X-ray 

diffraction [137]). The two pent-pent junction C atoms have � p angles of 16.1° 

(note that the corresponding angle for a C(sp 3 ) atom would be 19.5°). However, the 

evidence discussed below, based on DFT-optimized structures [138], indicates that 

the overall nonplanar steric strain in C 84 -C s (51365) is not unusually high relative 

to many stable IPR fullerenes. 

Figure 5.16 The DFT-optimized structure of the hypothetical non-IPR fullerene C 84 -C s (51365). 

Note the unusually prominent non-planarity of the cage C atoms that form the pentagon– 

pentagon junction at the top of the drawing. The � p angle for these symmetry-related C atoms 

is 16.1° (cf. 11.6° for C 60 and 10.5° for the two C atoms the form the pentagon–hexagon junction 

at the bottom of the structure.


5.3.1.4 Nonplanar Steric Strain Parameters 

A measure of steric strain in IPR fullerenes known as � h was described in the 

Atlas of Fullerenes [115]. It is ‘the standard deviation … of the hexagon neighbor 

index distribution’ (each hexagon in a fullerene has an index, 3, 4, 5, or 6, which 

is the number of hexagons that surround it) [115]. For IPR fullerenes up to the 46 

isomers of C 90, � h varies from 0.000 (for the ‘least strained’ fullerenes according 

to this concept, which include C 60, C 80-I h, and C 80-D 5h) to ca. 2.400, but nearly all 

are between 0.000 and 1.000. This follows Raghavachari’s suggestion [139] that 

nonplanar steric strain in IPR fullerenes is minimized when ‘the pentagon-induced 

curvature is distributed as uniformly as possible over the fullerene surface’ 

[115]. Some relevant � h values are listed in Table 5.2 [119, 126, 135, 138, 140–151]. 

For example, � h for C 70 is 0.490. The I h and D 5d isomers of C 80, which differ substantially 

in shape and therefore in the distribution of strain energy, are shown in 

Figure 5.17. Their � h values are 0.000 and 0.817, respectively. It has proven difficult 

to apportion the energy difference between fullerene isomers to nonplanar steric 

strain and to � electronic effects that are not due to nonplanarity [115]. What is 

known is that, except for C 84 isomers, IPR fullerenes with maximum electronic 

stability generally have high steric strain and vice versa [115]. For example, despite 

the greater steric strain implied by its high � h value, the D 5d isomer of C 80 has a 

DFT energy that is 72 kJ mol –1 lower than for the I h isomer [142]. 

Several authors have proposed using either (1) the parameter � p (earlier defined 

as � �� – 90°) [116] or (2) the fractional s character for a fullerene C atom � atomic 

orbital as a measure of the nonplanar strain energy for that C atom, and the sum 

over all C atoms as an index of the nonplanar strain energy for the entire fullerene 

[115, 152]. Since � h is only defined for IPR fullerenes with no substituents, we 

propose that a similar parameter be used to compare overall nonplanar steric 

strain in all fullerenes and their derivatives. That dimensionless parameter, �(� p 2 ), 

is the standard deviation of the � p 2 distribution for all C(sp 2 ) atoms in the cage. 

Figure 5.17 The DFT-optimized structures of two of the seven IPR isomers of C 80 that have 

D 5d and I h symmetry. The steric parameter � h is 0.817 for C 80 -D 5d (1) and 0.000 for C 80 -I h (7). 

The dimensionless steric parameters �(� p 2 ) is 36.0 for C80 -D 5d (1) and 10.1 for C 80 -I h (7). 

229


2 

Figure 5.18 The correlation of the steric parameters �h and �(�p) for the 24 DFT-optimized IPR 

isomers of C84 (the parameters are defined in the text). The line is a linear least-squares fit to 

the data. The DFT structure of C84-D2 (1) is shown with its 84 POAV vectors. It is the least stable 

of the IPR C84 cages as well as the least uniform in surface curvature. The �p angles for the 

vectors labelled a and b are 12.3° and 4.6°, respectively. 

To demonstrate that � h and �(� p 2 ) can be used interchangeably for IPR fullerenes, 

and therefore that �(� p 2 ) reliably measures nonplanar steric strain, we determined 

�(� p 2 ) values of the 24 IPR isomers of C84 and compared them with their � h values 

(Figure 5.18). A similar plot for IPR isomers of C 80 (not shown) is also essentially 

linear with similar slope and intercept. This suggests that it may be meaningful 

to compare �(� p 2 ) values for cages with different numbers of C atoms such as 

those listed in Table 5.2. 

Table 5.2 Structural parameters and DFT-predicted relative �H f values for fullerenes and 

fullerene(X) n derivatives. a) 

Compound �H f for isomers, 

kJ mol –1 

b) 

�h [115] 

2 c) 

�(�p) Average � p , 

deg g) 

C 60 0.000 0.0 11.6 DFT 

DFT or X-ray 

C 60 0.000 2.5 11.6 X-ray [135] 

C 70 0.490 25.0 10.9 DFT 

C 70 (CF 3 ) 10 -1 49.3 9.0 X-ray [140] 

C 74 -D 3h (1) 0.415 23.2 10.5 DFT [119] 

(C 74 -D 3h (1))(CF 3 ) 12 35.7 8.4 X-ray [141]

Table 5.2 (continued) 

Compound �H f for isomers, 

kJ mol –1 

C 80 -I h (7) 72 d) 

C 80 -C 2v (5) 18 d) 

C 80 -D 5d (1) 0 d) 

C 84 -D 2 (22) 0.0 e) 

C 84 -D 2 (21) 54.0 e) 

C 84 -C 2v (7) 88.3 e) 

C 84 -D 2 (1) 200.8 e) 

C 84 -C s (51365) 258.5 e) 

C 84 -D 2 (22) 6– 

C 84 -D 2 (21) 6– 

C 84 -C 2v (7) 6– 

C 84 -C s (51365) 6– 

38.5 e) 

0.0 e) 

48.6 e) 

1.8 e) 


b) 

�h [115] 

2 c) 

�(�p) Average � p , 

deg g) 

DFT or X-ray 

0.000 10.1 10.1 DFT [138] 

0.365 23.4 10.1 DFT [138] 

0.817 36.0 10.3 DFT [138] 

0.331 22.8 9.9 DFT [138] 

0.331 23.2 9.9 DFT [138] 

0.485 28.7 9.9 DFT [138] 

0.927 46.0 10.0 DFT [138] 

36.1 9.9 DFT [138] 

0.331 28.7 9.9 DFT [138] 

0.331 27.9 9.9 DFT [138] 

0.485 33.1 9.9 DFT [138] 

37.5 9.9 DFT [138] 

C 60 (CF 3 ) 10 -2 0.1 31.5 9.5 X-ray [143] 

C 60 (CF 3 ) 10 -3 4.7 35.9 9.4 X-ray [144] 

C 60 (CF 3 ) 10 -4 7.6 36.4 9.5 X-ray [142] 

C 60 (CF 3 ) 10 -5 1.2 34.3 9.5 X-ray [126] 

C 60 (CF 3 ) 10 -6 0.0 28.7 9.4 X-ray [146] 

C 60 (CF 3 ) 12 -1 0.0 f) 

C 60 (CF 3 ) 12 -2 39.8 f) 

29.7 9.0 X-ray [147] 

33.3 8.9 X-ray [148] 

C 1 -C 60 (CF 3 ) 18 28.8 7.0 X-ray [150] 

C 3v -C 60 F 18 53.9 8.6 X-ray [149] 

T-C 60 F 36 3.3 1.7 X-ray [151] 

a) All data from this work unless otherwise noted. Reference numbers are shown in square brackets. 

b) The �h values are standard deviations of the hexagon neighbor index distributions. 

The � h parameter is only defined for IPR fullerenes, not for non-IPR cages (e.g. C 84 -C s (51365)) 

or for exohedral fullerene derivatives. 

c) These dimensionless steric parameters, defined in the text, were calculated for this chapter 

using the DFT-optimized coordinates for all of the three-connected carbon atoms in a fullerene 

or fullerene derivative. 

d) Ref. [142]; these relative energies were calculated for singlet ground states. 

e) 

Ref. [138]. 

f) 

Ref. [148]. 

g) 2 

This average is over all cage C atoms that are nominally sp hybridized and does not include 

sp 3 hybridized C atoms bearing substituents. 

231


5.3.1.5 Are Non-IPR Fullerenes Sterically Unstable? 

2 

The relative �Hf and �(�p) values in Table 5.2 reveal an interesting aspect about 

the structure of non-IPR C84-Cs(51365). It is predicted to be 58 kJ mol –1 less stable 

than the least stable IPR isomer, C84-D2(1), and 258 kJ mol –1 less stable than the 

2 

most stable IPR isomer, C84-D2(22). However, its �(�p) value is significantly lower 

2 

than that for C84-D2(1) (it is also lower than �(�p) for two other IPR C84 cages). In 

2 

addition, its �(�p) value is the same as for IPR C80-D5d(1), the most stable of the 

IPR C80 cages [142]. The average values of �p for C84-D2(22) and C84-Cs(51365) are also the same. Based on these results, it does not appear that C84-Cs(51365) has an unusual amount of nonplanar strain energy relative to some of the most 

stable IPR fullerenes that are known to exist.The DFT relative energies of several 

6– 

C84 cages were recently reported [128]. They were calculated to predict the most 

stable cages that should be present in isolable iM3N@C84 compounds. The data 

2 – 

in Table 5.2 show that �(�p) values for the neutral cages and the 6 ions are only 

marginally different. Significantly, the C84-Cs (51365) 6– cage is energetically as stable 

as the most stable hexaanionic IPR cage, C84-D2 (21) 6– . With the right electron 

count, the non-IPR isomer can be as stable as any IPR isomer. This can only be 

true if the non-IPR cage is not sterically very unstable. Therefore, it appears that 

the instability of neutral C84-Cs(51365) is predominantly electronic in origin. This 

may also be true for other non-IPR fullerenes, and the idea that non-IPR fullerenes 

are sterically very unstable should be carefully reexamined. 

5.3.1.6 Long and Short C(sp 2 )–C(sp 2 ) Bonds in Fullerene Cages 

With the exception of three non-IPR EMF X-ray structures, Tb 3N@C 84-C s(51365) 

[137]. La@C 72(C 6H 3Cl 2) [153], and Sc 3N@C 68-D 3(6140) [151], all other fullerene 

X-ray structures reported to date are of IPR fullerenes. The first X-ray structures 

of non-IPR hollow fullerenes were reported in late 2008 [591]. The smallest IPR 

fullerene is C 60, and its X-ray structure [135] revealed that it has 30 short bonds 

(1.379(3)–1.396(3) Å), which are hexagon-hexagon junctions, and 60 long bonds 

(1.440(3)–1.461(3) Å), which are hexagon-pentagon junctions, as shown in 

Figure 5.19 (the DFT-optimized distances are 1.399 and 1.453 Å, respectively, and 

the NMR-determined distances are 1.40 and 1.45 Å, respectively [130]). The short 

hex-hex junctions and long hex-pent junctions are usually referred to as ‘double’ 

bonds and ‘single’ bonds, respectively, although this is clearly an oversimplification. 

These and other results [155, 156] have led to the oft-quoted ‘rule’ that double 

bonds in pentagons (DBIPs) are destabilizing in fullerenes and exohedral fullerene 

derivatives, although the underlying steric and/or electronic basis for this rule 

has not been convincingly explained. 

Higher fullerenes have more than two types of cage C–C bonds. The structures 

of C 70 ·6 S 8 [157, 158] and C 76 ·6 S 8 [159] were reported in the 1990s, but they did 

not have a precision that allowed the C–C bond distances of the various bond 

types to be distinguished from one another. For C 70 , this was still true at the end 

of 2006 [160]. In order to get a solution to the structure in some cases, noncrystallographic 

symmetry had to be imposed (e.g. D 5h symmetry for C 70 [158]) or a 

rigid-body DFT-optimized cage has been assumed and used in the refinement (e.g.


Figure 5.19 The 90 fullerene cage C–C bond distances in the structure of C 60 · Pt(OEP) · 2 C 6 H 6 [135]. 

The error bars shown are ±3�. The 30 shorter distances (‘double bonds’) are hexagon-hexagon 

junctions and the 60 longer distances (‘single bonds’) are hexagon-pentagon junctions. 

Figure 5.20 The shortest and longest distance for the eight types of cage C–C bonds in two of 

the three C 70 molecules in the X-ray structure of C 9 H 3 Cl 6 N · 3 (C 70 ) · 3 (C 6 H 5 Cl) (±3� error bars) 

[163]. Each of the eight diamond-shaped points is the DFT-optimized distance for that C–C 

bond type (this work). 

233


for BaC 74-D 3h [161] and Sc 3N@C 78-D 5h(5) [162]). A precise structure of C 70 was 

finally determined in 2007 using crystals of C 9H 3Cl 6N·3(C 70)·3 (C 6H 5Cl), with esd 

for cage C–C distances of 0.004 Å for two of the three independent C 70 molecules 

[163]. The ranges for the eight types of C–C distances are shown in Figure 5.20 

and are compared with the corresponding DFT predicted distances. The addition 

of substituents to the fullerene cage can also lead to ordered and precise structures, 

albeit at the expense of altering the fullerene cage from its original state as 

a pure carbon allotrope. The esd for cage C–C distances can be as low as 0.003 

[140], 0.002 [141], or 0.001 Å [147]. For example, the X-ray structure of C 70(CF 3) 10 

[140] is shown in Figure 5.21a along with a plot of its 75 C(sp 2 )–C(sp 2 ) distances. 

The esd for these distances are 0.003 Å. A plot of the 60 C(sp 2 )–C(sp 2 ) distances 

in an isomer of C 60 (CF 3 ) 10 is also shown in Figure 5.21b. These plots show that 

the sharp distinction between cage C–C ‘single’ and ‘double’ bonds, which was 

meaningful when applied to C 60, is much less distinct for higher fullerenes and 

fullerene(X) n derivatives (including C 60X n derivatives). 

Figure 5.21 Plots of the 75 (a) and 60 (b) cage C(sp 2 )–C(sp 2 ) distances in the X-ray structures 

of one of the isomers of C 70 (CF 3 ) 10 [140] and C 60 (CF 3 ) 10 [143]. The insets are (upper left in 

both cases) a 50% probability thermal ellipsoid plot of the molecule and (lower right in both 

cases) a Schlegel diagram showing the ribbon of edge-sharing C 6 (CF 3 ) 2 hexagons (each black 

circle represents a cage C atom bearing a CF 3 group; the letter ‘m’ denotes the meta-C 6 (CF 3 ) 2 

hexagon).


There are now nearly 30 X-ray structures of fullerene(CF 3) n compounds (and a 

growing number of fullerene(C 2F 5) n structures) [126, 127], and many of these are 

among the most precise X-ray structures of any fullerene or fullerene derivative. 

For the CF 3 compounds, the value of n varies from 2 to 18. The CF 3 substituent 

is sterically bulky (it is larger than an iodine atom) [144]. In most of the structures 

with n � 12, the CF 3 groups are distributed one per pentagon (there are only two 

exceptions), and CF 3 groups are rarely found on adjacent cage C atoms (two 

different exceptions). In nearly every case, the CF 3 groups are found on isolated 

para-C 6(CF 3) 2 hexagons and/or on one or more ribbons or loops of edge-sharing 

meta- and para-C 6(CF 3) 2 hexagons [126, 127]. In two cases, C 2-p 11 -(C 74-D 3h) 

(CF 3) 12 and C 2-p 11 -(C 78-D 3h(5))(CF 3) 12 [141] the X-ray structures were the first 

unambiguous proof of the existence of the hollow C 74 -D 3h and C 78 -D 3h (5) cages 

(‘p 11 ’ in the formula indicates that the 12 CF 3 groups are arranged on a ribbon 

of 11 edge-sharing para-C 6(CF 3) 2 hexagons). The structure of the C 74 compound 

was sufficiently precise (esd = 0.002 Å) that the cage C–C distances could be 

compared with the DFT-predicted distances (Figure 5.22). For all but three of the 

C–C bonds, the X-ray distance matched the DFT distance to within ±3�, validating 

the PBE functional used in that study (and in our other publications, including 

this chapter) [125]. 

Figure 5.22 X-ray vs DFT-predicted cage C–C distances for the structure of C 2 -(C 74 -D 3h )(CF 3 ) 12 

(the molecule has crystallographic C 2 symmetry) [141]. The error bars shown are ±3�. The 

insets are (upper left) a 50% probability thermal ellipsoid plot of the molecule and (lower right) 

a Schlegel diagram showing the p 11 [125] ribbon of edge-sharing C 6 (CF 3 ) 2 hexagons (each black 

circle represents a cage C atom bearing a CF 3 group). 

235


5.3.1.7 Steric Strain in C 60(X) n Isomers 

One question that has not been addressed very often in the literature is whether 

or not different C 60(X) n isomers might have different amounts of nonplanar steric 

strain that can affect their relative energies [164–166]. (In this part of our analysis, 

we do not consider steric strain induced by proximate X substituents [165] or 

potential angle strain in cage C(sp 3 ) atoms). Listed in Table 5.2 are the DFT 

relative energies and X-ray derived �(� p 2 ) values for five isomers of C60(CF 3) 10. 

Schlegel diagrams of the isomers are shown in Figure 5.23 (the X-ray structure 

of isomer 1 has not yet been determined). Not listed in Table 5.2 are the range, 

average, and standard deviation for the bond angles for C(sp 2 ) atoms within 

the pentagons for each isomer. All three of these angle parameters are virtually 

the same for the five isomers as well as for C 60, indicating negligible differences 

in angle strain due to C(sp 2 ) atoms having C–C–C angles of ca. 108° [167]. For 

example, the sets of {range, average, standard deviation of the average} for these 

angles are {107.4–108.6°, 108.0°, and 0.2°} for C 60 , and {106.9–110.8°, 109.0°, 

and 0.9°} for C 60 (CF 3 ) 10 -2, and {106.9–110.7°, 109.0°, and 0.9°} for C 60 (CF 3 ) 10 -6 

(all three sets from X-ray structures). The relative energies for the five isomers of 

C 60(CF 3) 10 range from 0.0 to 7.6 kJ mol –1 and the �(� p 2 ) values range from 28.7 

to 36.4. Since these are both small ranges, we conclude that this series of nearly 

equienergetic isomers have comparable values of total steric strain (since differences 

in angle strain are also negligible) and therefore have similar � electron 

energies. 

Two isomers of C 60(CF 3) 12 are also listed in Table 5.2 (see Figure 5.23 for Schlegel 

diagrams). In this case the relative energies differ by 40 kJ mol –1 , a significant 

amount. However, the �(� p 2 ) values are similar, so it would seem that the energy 

difference can be attributed to differences in � electron energy. This tentative 

conclusion should be tested in the future by a computational study. 

Next, compare the two X-ray structures with very different addition patterns, 

C 3v -C 60 F 18 and C 1 -C 60 (CF 3 ) 18 (see Table 5.2 and Figure 5.23). In this case, the 

�(� p 2 ) values are quite different, 53.9 for C3v -C 60 F 18 but only 28.8 for C 1 -C 60 (CF 3 ) 18 . 

This shows that different addition patterns for a given number of substituents 

can lead to significantly different amounts of nonplanar steric strain, although 

this was not true for the five structurally similar isomers of C 60 (CF 3 ) 10 . The DFT 

energies of C 3v -C 60 F 18 and the isomer of C 60 F 18 with the same addition pattern of 

C 1 -C 60 (CF 3 ) 18 revealed that C 3v -C 60 F 18 was 290 kJ mol –1 more stable. How much 

of that difference is due to differences in steric strain and how much is due to 

differences in � electron energy remains to be seen. 

Finally, consider the concept that relative planarity for one part of a C 2n fullerene 

is sterically destabilizing because, since the surface must be closed, it introduces 

‘high curvature in another part of the fullerene surface’ [115, 152, 168]. This is a 

valid concept for fullerenes without exohedral substituents, but is it also valid for 

fullerene(X) n derivatives? The answer may be ‘No.’ In C 2n fullerenes, all C atoms 

are nominally sp 2 hybridized, but in fullerene derivatives there are cage C(sp 3 ) 

atoms, and these atoms, which do not experience the nonplanar steric strain that 

the C(sp 2 ) atoms experience, can contribute to closing the surface of the fullerene.


Figure 5.23 Schlegel diagrams of some of the C 60 X n derivatives listed in Table 5.2 (X = F, CF 3 ). 

Each black circle is a cage carbon atom to which a substituent is attached. For the C 60 (CF 3 ) n 

compounds with n = 10 and 12, the ribbons or loops of edge-sharing meta- and para-C 6 (CF 3 ) 2 

hexagons are highlighted (the letter ‘m’ denotes the meta-C 6 (CF 3 ) 2 hexagon) and the IUPAC 

lowest locants of the cage C(sp 3 ) atoms are indicated. 

237


In fact, the average � p for the cage C(sp 2 ) atoms in the X-ray structures of C 60, 

C 60(CF 3) 10-6, C 60(CF 3) 12-2, C 3v-C 60F 18, C 1-C 60(CF 3) 18, and T-C 60F 36 (Figure 5.23) 

are 11.6°, 9.5°, 8.9°, 8.6°, 7.0°, and 1.7°, respectively. The introduction of exohedral 

substituents clearly reduces the average nonplanar steric strain in cage C(sp 2 ) 

atoms, to the point where the 24 remaining C(sp 2 ) atoms in T-C 60F 36 are in nearly 

planar environments. 

In reviewing the X-ray structures (and some relevant DFT-optimized structures) 

of fullerenes and their derivatives, we have raised several questions about how 

to determine steric strain in these compounds, especially in exohedral derivatives. 

We do not believe that we have answered these questions fully, because our 

analysis has been simple. Instead, we have highlighted some aspects of steric 

strain in closed-curved-surface carbon compounds that should be studied with 

more sophisticated computational methods in the future, especially as more and 

more precise X-ray structures become available. 

Acknowledgment 

We thank Prof. Marilyn Olmstead for her assistance in preparing this chapter, and 

we acknowledge the financial support of the Civilian Research and Development 

Foundation (RUC2-2830-MO-06 to AAP and SHS) and the U.S. National Science 

Foundation (CHE-0707223 to OVB and SHS). 

5.3.2 

Vibrational and Electronic Spectra 



Spectroscopic studies of the fullerenes played an important role from the very 

beginning of the fullerene era. Historically, when Wolfgang Krätschmer and 

coauthors discovered that the soot prepared by the arc-discharge method under 

certain conditions was enriched with fullerenes, a characteristic IR spectrum with 

four sharp lines emerging over the broad band background was the first proof that 

C 60 was present in the sample, because exactly four IR active modes are expected 

for the icosahedral C 60 molecule from group theory analysis [169]. Subsequent 

discovery of the method of extracting the fullerenes from carbon soot [170] as well 

as progress in HPLC separation of fullerene mixtures, boosted numerous studies 

of spectroscopic and photophysical properties of these molecules. 

In this chapter, a survey of vibrational and electronic spectroscopic studies of 

fullerenes is presented. We should note that the number of published papers 

dedicated to the given fullerene molecule scales in accordance with the fraction 

of this fullerene in the arc-discharge soot. Hence, the ‘archetypical fullerene’ 

C 60, which constitutes ca 85% of the crude fullerene extract, is by far the best 

studied member of the fullerene family, and it is inevitable that any review like 

this is to a large extent a review of the numerous studies on C 60. The properties


revealed for C 60 are to some extent inherited by all other fullerenes; however, the 

high symmetry of the C 60 molecule makes this fullerene very special in terms 

of spectroscopic properties, since the majority of its energy levels are degenerated, 

and many vibrational and electronic transitions are forbidden by symmetry 

selection rules. 

5.3.2.2 Vibrational Spectra of Fullerenes 

It is natural to expect the vibrational spectra of fullerenes to be quite complex: 

even for C 60, the smallest classical fullerene, the number of modes is as large 

as 174. However, vibrational representation of the free C 60 molecule spans 

2 A g + 3 T 1g + 4 T 2g+ 6 G g + 8 H g + A u + 4 T 1u + 5 T 2u + 6 G u + 7 H u symmetry types 

of I h group, and due to the high symmetry, 174 vibrational degrees of freedom 

are substantially degenerated and grouped into 46 normal modes, of which only 

14 are optically active (A g and H g symmetry types in Raman and T 1u type in IR 

spectra). In accordance with the symmetry considerations, four primary IR and 

ten Raman lines are observed in the experimental spectra of crystalline C 60 . As the 

highest site symmetry of C 60 in the room-temperature lattice is T h (and is further 

reduced at lower temperature), the fact that the spectra of solid C 60 samples can 

be described in terms of the isolated fullerene molecule with unperturbed I h 

symmetry show that crystalline fullerenes can be treated as molecular crystals 

with weak intermolecular interactions. The same conclusion can be drawn from 

the X-ray data discussed in the former section. 

Contrary to the optically-active modes, assignment of the many of 32 inactive 

fundamentals of C 60 is still ambiguous (see, for instance, detailed analysis given 

in Refs. [171] and [172]). The most detailed, but also the most complicated, information 

can be obtained from the second-order IR and Raman spectra [173, 174], 

i.e. the spectra of thick films or single crystals, which exhibit multiple bands with 

intensities at least an order of magnitude lower than those of the symmetry-allowed 

modes (Figure 5.24). The spectra are very rich and can be attributed to the silent 

modes as well as overtones and combination modes. The vibrational spectrum 

of C 60 was also studied by fluorescence spectroscopy (the measurements of the 

matrix-isolated C 60 at helium temperatures are particularly informative) [175, 176], 

inelastic neutron scattering (INS) spectroscopy [177, 178], photoluminescence 

spectroscopy of the singlet oxygen entrapped in C 60 lattice (SOPL) [179], and high 

resolution electron energy loss spectroscopy (HREELS) [180] (see Figure 5.24). 

While fluorescence spectroscopy provides information on some optically-forbidden 

ungerade modes, INS, SOPL and HREELS in principle provide complete vibrational 

density of states (VDOS) irrespective of the symmetry types (however, 

selection rules for SOPL spectrum are not yet clear). Unfortunately, the spectral 

resolution of these methods is rather low, especially in the high frequency range. In 

1998 Heid [178] reported assignment of all vibrational modes in the 250–600 cm –1 

range based on the INS study of a large single crystal of C 60. 

Besides C 60, detailed vibrational studies by means of IR, Raman, fluorescence, 

HREELS, INS and SOPL spectroscopies were reported only for D 5h–C 70. As can be 

expected from the lower symmetry of the molecule, the spectra of C 70 are consider- 

239


Figure 5.24 (a) IR and Raman spectra of C 60 . Raman spectra are shown for two excitation 

wavelengths. Note strong resonance enhancement of A g (2) modes in the spectrum excited in 

the visible range. (b) ‘Second-order’ IR and Raman spectra of C 60 . (c) Vibrational density of 

states in C 60 measured by INS [177], HREELS [181], and SOPL [179] and calculated with DFT 

(PBE/TZ2P) [182]. Stretching O-O modes in SOPL spectra of 18 O 2 and 16 O 2 are denoted by 

asterisks.


ably richer than those of C 60. Due to the much higher number of fundamentals 

(46 in C 60 vs 122 in C 70, including 53 Raman active and 31 IR active vibrations), 

assignments of C 70 normal modes are less reliable than those for C 60, and some 

ambiguities exist even in the assignment of several IR and Raman active modes. 

The use of INS or SOPL for C 70 appears less advantageous than for C 60 because 

the lower degeneracy of C 70 normal modes and their much higher total number 

result in the smeared VDOS in the whole frequency range [183, 184]. 

The difficulties with the isolation of the isomerically pure samples of higher 

fullerenes result in the lack of vibrational spectroscopic data for many of them. 

To our knowledge, IR and Raman spectra have been reported for D 2(1)-C 76 [185], 

C 2v(2)-C 78 [186], C 2v(3)-C 78 [186], D 2(22)-C 84 [187, 188], and D 2d(23)-C 84 [187, 188]; 

for D 3 (1)-C 78 and D 2 (2)-C 80 only Raman spectra are available [189]. As anticipated, 

complexity of the spectra increases with the number of atoms in the fullerene 

molecule and with the decrease of its symmetry. At the same time, vibrational 

spectra appear to be very structure sensitive (for instance, IR and Raman spectra 

of two C 2v isomers of C 78 are substantially different, Figure 5.25), a property that 

can be used for structure determination of the new fullerene isomers, especially 

if experimental studies are supported by theoretical calculations. 

A systematic study of the Raman spectra of the group of fullerenes were 

performed by Eisler et al. [189, 190] in 2000. The authors studied isomerically 

pure fullerenes C 60 , C 70 , C 76 (D 2 ), C 78 (D 3 , C 2v , C� 2v ), and C 80 (D 2 ) and isomeric 

mixtures of C 82 (two C 2 isomers in 1 : 1 ratio) and C 84 (D 2 and D 2d isomers in 2 : 1 

ratio). Room-temperature spectra of all fullerenes were measured with 514, 693, 

794, 1064 nm excitation wavelengths (see Figure 5.25 for the spectra obtained 

at 693 nm wavelength). To establish the common vibrational phenomena for all 

fullerenes, the authors employed Lamb theory of oscillations of isotropic spherical 

and spheroidal shells. Two types of vibration, referred to as monopolar and quadrupolar, 

were found to be especially characteristic in the analysis of the Raman 

spectra of fullerenes. Monopolar modes can be easily recognized as they sustain 

medium to strong Raman intensity over all excitation wavelength (the example 

is ‘breathing’ A g (1) vibration of C 60 ). Their frequencies scale inversely with the 

square root of mass and are shape-insensitive, that is, different isomers with the 

same mass (e.g. C 78 ) exhibit nearly the same frequency. Quadrupolar mode (the 

example is ‘squashing’ H g (1) vibration of C 60 ) is five-fold degenerate for a sphere 

and C 60, but is split into several components for all other fullerenes. Importantly, 

the magnitude of splitting correlates with the deviation of the fullerene shape 

from the sphere (viz. 15 cm –1 for nearly spherical D 2 –C 84 versus 45 cm –1 for 

elongated D 2-C 80), and thus can be used to determine the isomeric structures of 

newly isolated fullerenes. 

241


Figure 5.25 (a) IR spectra of higher fullerenes [185–188]; (b) Raman spectra of higher fullerenes 

[185–188] (� ex = 1064 nm for C 70 –C 78 , 514 nm for C 84 ); (c) Raman spectra of some fullerenes, 

excitation 693 nm [189]. Asterisks in (c) denote breathing mode.


5.3.2.3 The Orbital Picture of Fullerenes: High-energy Electronic Spectra 

Though non-planarity of the fullerene molecules results in the partial mixing of � 

and � states (see Section 5.3.3.3), high-lying occupied and low-lying unoccupied 

molecular orbitals (MOs) of fullerenes are still of the essentially �-nature and may 

be adequately described by the theoretical methods developed for the �-systems. 

Due to the high symmetry of C 60, it is straightforward to treat this molecule as 

a spherical shell whose electronic states can be classified in terms of the orbital 

momentum l. In such a ‘superatomic’ model, all levels up to l = 4 are occupied, 

while the level with l = 5 is partially filled with 10 electrons (Figure 5.26) [180]. 

Though very simple, the model gives a reasonable description of the electronic 

structure of C 60 which can be correlated with the orbital picture obtained by using 

more sophisticated theoretical approaches (Figure 5.26), especially for the occupied 

states. Under icosahedral perturbation, the electronic levels are split but still retain 

a substantial extent of degeneracy. The majority of C 60 MOs have either t (u,g) , g (u,g) 

or h (u,g) symmetry, including five-fold degenerated HOMO (h u ) and three-fold 

degenerated LUMO (t 1u ). LDA calculations of the band structure in crystalline 

C 60 have shown that the widths of the bands derived from the frontier orbitals do 

not exceed 0.5 eV [191], and hence discrete density of states (DOS) characteristic 

for C 60 in the gas should be preserved in the solid phase. 

Experimentally electronic states of C 60 were probed by a variety of high-energy 

spectroscopic techniques (see Refs. [180, 192, 193] for reviews). Valence occupied 

states were extensively studied by ultraviolet photoemission spectroscopy (UPS) 

[180, 194] providing a direct measure of orbital ionization energies and DOS. 

A complementary technique, providing information about density of unoccupied 

states (DUOS) is inversed photoemission spectroscopy (IPES) [195], in which 

the surplus electron is immersed into unoccupied orbitals. Other methods extensively 

used to analyze DUOS of fullerenes are core-level electron energy loss 

spectroscopy (EELS) [180, 192, 193] and X-ray absorption spectroscopy (XAS) 

[196, 197]. In core-level EELS and XAS, electrons from C1s levels are excited 

to un occupied C2p-derived states. Formally this provides a measure of DUOS, 

however the energy levels are affected by the interactions with the core hole, 

resulting in the possible difference from the IPES spectra (which are free from 

the excitonic effect). 

Figure 5.27 shows representative UPS, IPES, XAS and EELS spectra of C 60 

compared with the DOS and DOUS of the isolated molecule computed at the PBE/ 

TZ2P level. The spectra in the valence region are characterized by sharp distinct 

peaks, which reflect the extensively degenerated MO levels of C 60 . UPS and IPES 

spectra show good agreement with theoretically computed DOS and DUOS in the 

energy range of � and �* states, respectively, and the features derived from the 

highest-energy occupied orbitals can be assigned straightforwardly (Figure 5.26). 

The patterns of EELS and XAS spectra are virtually identical and, though they also 

resemble IPES spectra, the shift of the peak positions is obvious, pointing to the 

importance of the excitonic effect in the fullerene. 

UPS spectra measured in the gas and the solid state are very similar proving 

weak intermolecular interaction in the latter, however the fine vibronic structure 

243


Figure 5.26 Upper panel: Electronic energy levels in C 60 as obtained with the model: electron 

on a sphere [180] (left), Hückel approximation (middle) and DFT PBE/TZ2P [182] (right). 

Note that the energy scales for the three models are different. Middle and lower panels: DOS 

and DUOS in C 60 as measured by different methods [180, 194–197] and calculated by DFT [182]. 

The asterisks in the gas-phase spectra denote CO bands.


resolved in the spectrum of the gaseous C 60 is absent in the solid state spectra 

[194]. Ionization potential (IP) and electron affinity (EA) of C 60, estimated from 

the gas-phase UPS spectra of neutral C 60 and its anion [198], are 7.3 and 2.67 eV, 

respectively, which formally gives the energy gap between the HOMO and LUMO 

levels of 4.7 eV, while in the solid state the energy difference of the HOMO and 

LUMO-derived peaks of UPS and IPEs spectra is reduced to 3.75 eV due to the 

screening effect of the surrounding lattice molecules. Significantly, both gas 

and solid state values are considerably larger than the band gap � = 1.8–2.2 eV 

measured by optical absorption spectroscopy and low-energy EELS. The reason 

for such a discrepancy is the different nature of the final states. While the � value 

corresponds to the formation of the exciton (hole-electron pair) localized on one 

molecule, in the UPS and IPES techniques the final states are ionic. That is, (IP-EA) 

difference or the peak-to-peak distance in UPS/IPES spectra is the formation 

energy of the hole and the electron localized on two different molecules. The difference 

(IP–EA)–� defines the energy of on-site Coulomb interaction (so called 

charge correlation energy), Hubbard U, which therefore has the order of 3.0 eV 

for the free molecule and 1.6 eV for C 60 in the solid state. 

As can be expected on the basis of their common molecular architecture and 

all-carbon � systems, DOS and DUOS of C 70 and the higher fullerenes resemble 

those of C 60 (Figures 5.26 and 5.27). However, the lower symmetry and larger 

number of atoms result in an increase of the number of states with lower degeneracy, 

and the spectral patterns are less resolved than those in C 60 exhibiting many 

overlapping bands. Meanwhile, the spectra still exhibit individual characteristic 

Figure 5.27 UPS and C1s EELS spectra of selected fullerenes [193] (middle panels) compared 

to the DOS (left) and DUOS (right) computed at the PBE/TZ2P level [182]. C 84 is a mixture of 

D 2 (22) and D 2d (23) isomers in 2 : 1 ratio. 

245


features, resembling peculiarities of the electronic structure of each fullerene, 

which can be best demonstrated by significant difference observed in the DOS 

and UPS spectra of two C 2v isomers of C 78 [180, 192, 193]. 

5.3.2.4 Electronic Excitations. UPS, UV/Vis/NIR Absorption and Fluorescence 

Spectroscopy 

Excitation spectra of the fullerenes in the near-infrared, visible and near-UV 

range are attributed to the �–�* transitions and are very structure sensitive. 

The lowest energy transition, which can be ascribed to the excitation through 

the HOMO-LUMO gap, is an important fundamental property determining the 

kinetic stability of the given fullerene. Only molecules with a considerably large 

HOMO-LUMO gap can be isolated from the soot by conventional organic solvents, 

while structures with a small gap, albeit they may be formed in considerable 

amount and be thermodynamically stable, cannot be extracted from the soot, 

presumably because they have polymerized. C74, D3h(5)-C78 or Td-C76 are examples 

of such ‘insoluble’ fullerenes, whose formation in the arc-discharge synthesis was 

proven by electrochemical or chemical transformation to the soluble forms [199]. 

Excitation spectra of fullerenes have been studied by many techniques, including, 

but not limited to, low-energy EELS, core-level XPS (satellite structure of C1s peak), 

UPS of the anionic species, and optical absorption and fluorescence spectroscopy. 

Description of the results obtained by EELS and XPS are beyond the scope of this 

review (for a review, see Refs. [180, 192, 193]), and here we mainly focus on UPS 

and optical spectroscopy of fullerenes. 

The UPS of the gas-phase anions is an important information source for two 

kinds of fundamental properties of fullerenes. First, the lowest energy band in 

the UPS spectrum corresponds to the detachment of the electron from the singleoccupied 

orbital of the anion and therefore corresponds exactly to the electron 

affinity. Secondly, photodetachment of the electron from the doubly-occupied 

orbital results in singlet or triplet excited state of the neutral molecule. Thus, UPS 

spectrum of the anion provides information on the excited states of the fullerene 

and is especially useful in the determination of the lowest energy excitations. As 

UPS measurements of the gas-phase anions require very small amounts of the 

sample, while the mass-selection technique allows the mixtures of fullerenes to 

be studied without their preliminary separation, it is not surprising that the first 

study of the anions of C60 and C70 fullerenes by UPS was done in 1987 [200], 

before the method of their bulk-production was discovered. Likewise, UPS is the 

only method of choice for the studies of the electronic structure of exotic small 

fullerenes C20 –C50 (Figure 5.28) [201, 202]. Thus, C32 has been shown to have a 

large HOMO-LUMO gap and is therefore expected to be stable [201]. Vibrationally 

resolved UPS spectra of C – 60 [198], and C– 70 [203], cooled in the ion trap are shown 

in Figure 5.28. The second and the third lowest energy bands in the spectrum of 

C – 

60 are attributed to the first triplet and singlet excited states of C60 . The energy 

of the former is estimated as 1.62 ± 0.01 eV, while the multiplet splitting (i.e. the 

energy gap between triplet and singlet states derived from the same excitation) 

is 0.2–0.3 eV.


Figure 5.28 UPS spectra of anions of some fullerenes; vibrationally resolved UPS spectra of C – 

60 

[198], and C – 

70 

[203]. ‘A. D.’ in the spectrum of C– 

60 

marks an auto-detachment band. 

Though UPS remains an invaluable source of information on the structures 

whose isolation in bulk amounts is not possible, the method requires rather 

complex equipment, while the information it gives is actually limited to the lowest-energy 

excitation, because at the higher energies the density of excited states 

of fullerenes is usually very high. For this reason, the UV/Vis/NIR absorption 

spectroscopy is much more advantageous now, when at least > 1 mg amounts 

of the fullerenes have become available. Simple relatively inexpensive instrumentation 

and sampling techniques make this method accessible to virtually 

all researchers worldwide, and the richness and uniqueness of �–�* excitation 

spectrum for each fullerene has made it almost a standard now that the characterization 

of any new fullerene cannot be complete without the UV/Vis/NIR 

absorption spectrum. 

Room-temperature UV/Vis absorption spectrum of C 60 in n-hexane is shown in 

Figure 5.29. Three regions with the intensities covering three orders of magnitude 

can be roughly distinguished in the spectrum: low-intensity highly structural 

bands in the visible range (450–700 nm), several more intense sharp peaks 

around 380–420 nm, and finally three very strong broad absorptions at 328 nm 

(log(� max ) = 5.20), 256 nm (5.24) and 211 nm (5.20) in the UV range [204]. As only 

the excitation to the singlet states of T 1u symmetry are optically allowed for C 60 , 

its HOMO � LUMO (h u � t 1u ) transition is optically forbidden, giving rise to 

the excited states of T 1g , T 2g , G g , and H g symmetry. The lowest energy 1 T 1u state 

can be attributed to the sharp feature at 408 nm (3.04 eV) in the experimental 

247


Figure 5.29 Left panel: Room-temperature UV/Vis absorption spectrum of C 60 in n-hexane. 

Insets show low temperature ‘absorption’ spectra: resonant two-photon ionization spectrum 

in ultrasonic beam [210] (R2PI); fluorescence-excitation spectrum in Ne matrix at 4 K [175], 

absorption spectrum in helium droplets at T~0.4 K [207]. Vertical bars show excitation energies 

and oscillator strengths predicted by TD-DFT [205]. Right panel: Low temperature fluorescence 

spectra of C 60 in different matrices [175, 208]. 

spectrum. Interpretation of the absorption spectrum of C 60 in the UV range was 

given in 1992 by Leach [204]. A new assignment for some bands was proposed by 

Bauernschmitt [205] in 1998 with the use of more refined time-dependent (TD) 

DFT calculations (see Figure 5.29 for the comparison of TD-DFT computed and 

experimental spectra of C 60 ). 

Though HOMO � LUMO transitions in C 60 are symmetry forbidden, they 

may be activated through a Herzberg–Teller (HT) coupling to the appropriate 

molecular vibrations. In due turn, each HT-activated transition is an origin for 

the Franck–Condon (FC) and Jahn–Teller (JT) progressions via coupling to the A g 

and H g modes, respectively. Altogether, these multiple vibronic excitations form 

a complex pattern observed in the experimental spectrum of C 60 in the visible 

range. There have been numerous efforts, both theoretical and experimental, to 

interpret the low-energy excitation spectrum of C 60 (see Ref. [176] for a detailed 

review). Vibrationally resolved data on cold C 60 molecules were obtained by twophoton 

ionization spectroscopy in supersonic beam [206], absorption spectra 

of C 60 in He droplets (T � 0.4 K) [207], or by measuring the fluorescence and 

fluorescence excitation spectra in various matrices or single-crystal C 60 at low 

temperatures (T = 1.2–5 K) [175, 176, 208]. The survey of experimental spectra 

is presented in Figure 5.29. Semiempirical (CNDO/S, QCFF/PI) and TD-DFT 

calculations predict that three lowest energy excited states, 1 T 1g , 1 T 2g and 1 G g at 

ca 1.7–2.2 eV are quasi-degenerated within 0.05 eV. Excitations to 1 T 1g state are 

activated by A u , H u , and T 1u vibrational modes, to 1 T 2g state – by G u and H u modes, 

and to 1 G g state – by T 2u , G u , and H u modes, and therefore vibrationally resolved


excitation and especially fluorescence spectra provide additional information on 

the silent fundamentals of C 60. Detailed analysis of the vibronic structure based 

on the CNDO/S calculations was reported by Negri [175, 209]. It was shown that 

the lowest excited state is 1 T 1g with the energy of 1.93–1.94 eV depending on the 

matrix and temperature, while 1 T 2g and 1 G g states are ca 10 and 50 cm –1 higher 

in energy. 

The survey of available absorption spectra for the major isomers of the higher 

fullerenes C 70-C 84 is given in Figure 5.30. It can be seen that the spectra are highly 

structural and specific for each fullerene. Significant variation of the spectra with 

the isomeric structures of the same molecular size observed in C 78 [186, 211], C 80 

[212, 213] and C 84 [212, 213] is especially remarkable as it shows that UV/Vis/ 

NIR absorption spectroscopy may be used to distinguish the fullerene isomers. 

In 1998 Bauernschmitt [205] reported that TD-DFT calculations gave very good 

agreement with the experimental absorption spectra and thus can be used to 

determine the isomeric structures of newly isolated fullerenes when assignment 

based on 13 C NMR is ambiguous. 

IR and Raman spectra of isomerically pure C 2 (3)-C 82 fullerene have been recently 

reported [590]. 

Figure 5.30 Room-temperature UV/Vis/NIR absorption spectra of isomerically pure fullerenes 

[186, 205, 211–213] (for the isomers of a given C 2n , the spectra are given in the order of 

their yield; the ratio of C 80 -D 2 (2) to C 80 -D 2d (1) is ca 30 : 1 [213]; for C 84 isomers, the ratio of 

D 2(22) : D 2d(23) : C 2(11) : C s(16) : C s(14) : D 2d(4) : D 2(5) is 1 : 0.5 : 0.2 : 0.1 : 0.1 : 0.05 : 0.03 [214]). 

Enhanced spectra in the low-energy range are given as additional curves. 

249


5.3.3 

Nuclear Magnetic Resonance 

Toni Shiroka 


Nuclear magnetic resonance (NMR) represents the selective absorption of electromagnetic 

radiation by nuclei with nonzero spin placed in an external magnetic 

field. The dependence of its main parameters (such as the resonance frequency 

� and width ��, the spin–lattice T 1 and spin–spin T 2 relaxation times) from the 

nucleus surroundings, makes it a highly sensitive probe to the immediate environment 

and, therefore, an extremely versatile tool of spectroscopic investigation. 

The magnetic resonance spectra can broadly be classified into two categories: 

high-resolution solution NMR and solid-state NMR [215, 216]. In the first case, 

most appropriate for structural analysis of single molecules, the material is 

dissolved in suitable solvents, where the interactions that cause the broadening of 

spectral lines are generally averaged to zero as a result of the random molecular 

motion. The relative differences in resonance frequency with respect to a reference 

nucleus, known as chemical (orbital) shifts, reflect the different electronic screening 

of the external field in various chemical environments. 

The same principles are also applicable to solid-state NMR. However, the restricted 

molecular motion in solids can result in very broad spectra. For metallic 

samples moreover, there is an additional contribution to the frequency shift, 

arising from the spin polarization of the conduction electrons, known as Knight 

(spin) shift [217]. The latter implicitly includes a contact hyperfine interaction 

term, due to electrons in s orbitals, and a dipolar contribution arising from non-s 

electron spins, respectively related with the isotropic and anisotropic part of the 

shift. Advanced techniques, including magic-angle spinning, cross polarization, 

spectral editing, etc., allow nevertheless to recover a wealth of information even 

from the overcrowded solid-state spectra, thus successfully complementing or 

supporting other data. 

Two other techniques, closely related to NMR, are the nuclear quadrupole 

resonance (NQR) [218] and the muon spin rotation (µSR) [219, 220], both of 

which have also contributed to our knowledge of doped fullerenes. NQR relies 

on the fact that in many solids, the spectra of nonspherical nuclei (spin I > 1/2) 

are dominated by quadrupole effects, arising from the interaction of the nuclear 

electric quadrupole moment with the nonspherically symmetrical field gradient 

generated by the surrounding electrons. The NQR spectra can be collected even 

in zero magnetic field and often provide decisive information about structural 

properties and dynamics. µSR on the other hand makes use of 100% polarized 

muons, elementary particles similar to electrons, but positively charged, hence 

chemically behaving as light isotopes of hydrogen. Once implanted in matter, 

muons precess in the local magnetic field and information on both precession 

and relaxation is delivered by the decay positrons, emitted preferentially along 

the muon spin direction. The main µSR advantages with respect to conventional


NMR consist in the higher sensitivity, the complementary (interstitial) ‘point of 

view’, and the possibility to perform measurements even in the absence of an 

applied magnetic field. 

5.3.3.2 NMR of Fullerenes 

To illustrate the NMR contribution in the study of fullerene properties we resort 

to some selected representative examples which, however, are not intended to be 

either definitive, or exhaustive. For a detailed account of the field the interested 

reader should refer to the many excellent reviews [221–224]. 

Depending on the unit being investigated: individual molecules, or ensembles 

of them, the reported examples are conveniently grouped accordingly, roughly 

corresponding to solution NMR and solid-state NMR experiments, respectively. 

Individual Fullerene Property Studies. The first data to support the highly symmetric 

structure of C 60 were due to NMR [225]. The exact equivalence of all the carbon 

atoms, arising from the icosahedral I h symmetry, implies a single narrow line for 

the 13 C solution NMR spectrum. This is indeed the case, as shown in Figure 5.31a, 

where a remarkable number of atoms gives rise to a single line resonance, an 

Figure 5.31 (a) 13 C NMR spectra of pure C 60 , (b) of a mixture of C 60 with C 70 (b), and 

(c) of pure C 70 . Notice the presence of five different lines in the symmetry reduced C 70 , 

whose intensity ratio 10 : 10 : 20 : 20 : 10 reflects the five sets of nonequivalent carbons. 

(Reprinted with permission from [225]). 

251


unprecedented feature for such a large molecule. Despite its simplicity, this early 

spectrum is even more revealing. Indeed, the observed chemical shift, 142.7 ppm 

from a tetramethyl silane (TMS) reference, is significantly lower than the peaks 

for the corresponding positions in aromatic compounds such as benzene 

(120 ppm) or naphthalene (133.7 ppm). On the other hand, it is well known that 

the strain-induced hybridization produces a downfield shift, as seen for example 

in the 13 C peaks for the bridgehead carbons in indane C 9H 10 (143.9 ppm) and 

benzocyclobutene C 8H 6 (146.3 ppm). The observation of a similar shift also in 

C 60 represented a beautiful demonstration of its curved geometry. This first, 

simple NMR experiment contributed decisively to the elucidation of the fullerene 

structure, consisting of pentagonal and hexagonal carbon rings. 

Even more interesting is the case of C 70, where the icosahedral symmetry is reduced 

to a lower D 5h symmetry. Accordingly, its solution NMR spectrum consists 

of five different lines [225], reflecting the five nonequivalent carbon atom sites, as 

shown in Figure 5.31c. More complex, two-dimensional NMR experiments, making 

use of double-quantum coherence in 13 C enriched samples, were able to examine 

also the nature of the bonding and to determine the bonding topology [226]. 

The observation of an isotropic Knight shift, K iso , in the NMR spectra of conducting 

fullerenes provides another, independent confirmation of the curved geometry 

of the single fullerene units [227–230]. Indeed, since the probability density of 

p-electrons at the nucleus site vanishes, they cannot contribute to the isotropic 

Knight shift. In the case of a perfect sp 2 hybridization, where neither the sp 2 orbitals, 

nor the residual (non hybridized) p z orbitals overlap with the nucleus, K iso should 

be exactly zero. In the presence of curvature, though, this simple picture will 

change. On curved carbon surfaces, the normally planar sp 2 orbitals (orthogonal 

to the axis defined by the p z orbital) must tilt somewhat to match the neighboring 

carbons (see Figure 5.32a). This distortion is achieved by a small admixture of 

the 2s wave function into the 2p z orbital, producing a state whose hybridization is 

intermediate between the planar sp 2 and the tetrahedral sp 3 . The same process can 

be described also as a small �–� hybridization, if the overlap of neighboring carbon 

atom orbitals is considered. The rehybridization process on curved surfaces will 

therefore always retain a small 2s character, resulting in a nonzero direct Fermi 

contact interaction and hence in a small isotropic Knight shift, whose magnitude 

depends on the degree of curvature. NMR measurements in a metallic compound, 

such as Rb 3 C 60 [228], fully confirm this picture by providing an isotropic Knight 

shift value of ~44 ppm, less than half the anisotropic shift value. Much smaller 

shifts, ~10 ppm, inversely proportional to the tube diameter, are foreseen for the 

conducting nanotubes, where the curvature is present only along one direction 

[231] (carbon nanotubes will be discussed in Chapter 6). 

Quantitative predictions about the degree of orbital mixing, based on the 

so-called ‘orbital following’ assumption, initially assigned a ~9% 2s character to 

the fullerene molecule, with a pyramidalization angle of ~12% [232, 233]. The 

resulting hyperfine coupling, together with the measured susceptibility values, 

predicted nevertheless an isotropic Knight shift value (K = A · �) 10 times higher 

than that actually measured! The solution to this puzzle came again from NMR


Figure 5.32 Two possible carbon–carbon bonding schemes in C 60 , depicting one of the three 

2sp 2 orbitals, together with the non hybridized 2p z orbital. (a) Diagram showing the ‘orbital 

following’ bonding scheme, with the hybridized orbitals inclined with respect to the radially 

oriented 2p z orbitals. (b) In the more realistic ‘bent’ � bonding, the hybridized orbitals remain 

nearly perpendicular to the respective 2p z orbitals, resulting in a smaller 2s admixture, thus 

accounting for the anomalously small isotropic Knight shift observed. 

experiments. From the 13 C coupling constants in a 2D experiment, Hawkins and 

co-workers [234] could estimate a fractional rehybridization of 3% in neutral C 60 

and, more importantly, established that the correct carbon bonding model is that of 

‘bent’ �-bonding and not the stiff orbital following (see Figure 5.32b). This model, 

which is the currently accepted one [224], resulted also in the correct K iso values. 

Because it has a nearly spherical shape, at room temperature C 60 maintains a 

high reorientation rate, even in the solid state (vide infra discussed also in the 

preceding sections). The resulting static and dynamic disorder makes diffraction 

investigations problematic, giving access only to the average charge distribution 

and precluding the details of the internal molecular structure. Carr-Purcell solidstate 

NMR measurements of 13 C– 13 C magnetic dipolar coupling, on the other 

hand, provided the first experimental determination of bond lengths in C 60 [235]. 

This particular pulse sequence can selectively remove the broadening effects due 

to chemical shift anisotropy (differences in local magnetic field shielding in a 

powder sample), but retains the dipole–dipole coupling. 

Since the dipolar interaction strength depends on the inverse cube of the C–C 

distance, the method not only can distinguish between the 1.40 and 1.45 Å bond 

lengths, but can assess them with better than 1% precision! Only at a later time, 

as described in Section 5.3.1, could low-temperature X-ray studies determine the 

bond lengths in C 60 even more accurately. 

Figure 5.33 shows both the measured and the simulated powder spectra at 

a temperature sufficiently low, so that molecular rotation does not average the 

~4 kHz dipolar coupling between directly bonded carbons. The very large central 

peak (cropped for clarity) arises from 13 C nuclei with no 13 C nearest neighbors, 

and is flanked by two weak doublets with different intensity and frequency, arising 

from the directly bonded 13 C atoms. As expected, the inner doublet, due to 60 

pentagon bonds, is twice as intense as the external one, due to 30 bonds between 

pentagons. This fully confirms the truncated icosahedron geometry, since an early 

proposed structure, consisting in a truncated dodecahedron, yields a rather poor 

agreement with the experimental data (Figure 5.33, panel c). 

253


Figure 5.33 (a) Fourier transform of the 13 C signal obtained in a Carr–Purcell sequence on 

13 C enriched fullerene sample at 77 K. The intense center line has been cropped for clarity 

(see text). (b) Simulation of two Pake doublets with carbon–carbon bond lengths of 1.45 and 

1.40 Å; (c) as in (b), but with bond lengths of 1.451 and 1.345 Å. (Reprinted with permission 

from [235]). 

To conclude this section on individual fullerene properties, we present an 

ingenious way to follow the fullerene reactions by monitoring the NMR signal 

of their endohedrally labeled versions [236]. Although 13 C NMR spectra of C 60 

and C 70 are simple (see above), the spectra of the reaction products are rather 

complex, since the attachment of groups to the fullerene skeleton reduces the 

initially high symmetry. The study of these spectra is not only time consuming 

but, especially when many products are involved, it presents serious difficulties 

in assigning the carbon peaks. The 1 H NMR signal of the ubiquitous protons can 

in principle help. However, the tendency of solvents to be trapped in the fullerene 

lattice, and the hydrogen presence also in the reagents and byproducts makes it 

unpractical. On the other hand, (as discussed also in Section 5.6 on hydrogenated 

fullerenes) the use of endohedral fullerene complexes of noble gases, as e.g. 3 He, 

does not have any of the previous drawbacks. Each helium-labeled fullerene gives 

a single sharp peak with a typical line width below 1 Hz and a relatively large 

negative chemical shift (–6.3 ppm and –28.8 ppm from a 3 He reference in C 60 

and C 70 respectively). The diamagnetic chemical shifts of the centrally located 

helium nucleus, not only prove the substantial aromatic ring currents present 

in all the fullerenes, but their modifications can be used as a reaction indicator. 

Indeed, the conjecture that alterations of the �-bonding structure of the fullerene 

through reaction would produce substantial shifts in the 3 He peak, has been 

successfully verified experimentally. A simple addition reaction, with just one of 

the 30 double bonds being modified, produced a strikingly large –3 ppm shift of 

the helium signal. Considering also that in this case the peak areas are accurate 

indicators of the relative amounts, and that to date no two different products have 

been found to have the same helium shift, the potentialities of the method as a 

reaction monitoring tool are clear.


Collective Fullerene Property Studies. While solution NMR measurements can 

reveal properties of single molecules, solid-state NMR is an excellent probe of the 

collective behavior of fullerenes. This, together with the fact that both pure and 

doped fullerenes are available mainly as powders, explains the extremely vast and 

diversified range of experiments belonging to this class. 

The investigation of C 60 rotational dynamics in the solid state, represents 

perhaps one of the best known examples of this kind of studies. Figure 5.34 shows 

the evolution of 13 C NMR line shape of fullerene, as the temperature is lowered 

[237, 238]. Surprisingly, at room temperature, instead of a broad and featureless 

spectrum, typical of a powder sample, C 60 shows an extremely narrow line, very 

similar to those observed in a solution. This extreme ‘motional narrowing’ effect 

implies a rapid and isotropic molecular reorientation which, successive relaxation 

rate studies have shown to be even more effective than in the liquid phase. At 

the lowest temperature the molecules are stationary but randomly oriented, and 

Figure 5.34 13 C NMR spectra of solid fullerene taken at different temperatures. The surprisingly 

narrow line at room temperature gradually broadens and changes its shape, as the isotropic 

rotational motion slows down. (Reprinted with permission from [237]). 

255


therefore subject to different magnetic field shieldings (technically, to a chemical 

shift anisotropy). A fit of the powder pattern yields an asymmetric chemical shift 

tensor (220, 186, 25) [237], quite similar to those of aromatic carbon compounds. 

The coexistence, at intermediate temperatures, of sharp features superimposed 

on a broad spectrum indicates the simultaneous presence of two different phases: 

a fast rotating phase with another one where the rotations are blocked. To find 

more information about them one can resort to the spin–lattice NMR relaxation 

rates. The motional dynamics affects the intensity of magnetic field fluctuations 

which, in turn, will be reflected in the measured relaxation rates (1/T 1). 

The numerical analysis of room temperature data provides an astonishingly fast 

rotational correlation time, � = 9.2 ps, only three times longer than that measured 

in the gas phase! Some fascinating, recent attempts towards applications of this 

finding, although somewhat speculative, include the use of C 60 as a molecular 

ball bearing for nanoscale machine design [239]. 

Temperature dependent X-ray and neutron diffraction studies [240] hinted 

at a structural fcc–sc phase transition at 249 K, but little was known about the 

details. An NMR examination of the transition [241, 242], could find a sharp drop 

in the T 1 relaxation times, despite no appreciable change in the spectrum shape. 

These measurements suggested a high-temperature phase, characterized by free 

rotation (‘rotator’ phase), and a low-temperature (‘ratchet’) phase, characterized 

by rotational jump diffusion among symmetry-equivalent orientations. Due to 

the hampered rotations, the ‘ratchet’ phase shows also much higher relaxation 

rates (i.e. shorter relaxation times), and its slower dynamics seems to arise from 

an orientational ordering, such that electron-rich interpentagon bonds are facing 

electron-poor pentagon centers of neighboring C 60 molecules. This also explains 

the lowered structural symmetry (sc instead of fcc) of this phase. 

The strong tendency toward polymerization of fullerene molecules into one-, 

two-, and even three-dimensionally connected polymers, either by photoexcitation, 

or by high-temperature and high-pressure treatments, has attracted a continuous 

scientific interest. From the early studies of relatively simple monodimensional 

chains in AC 60 (A = K, Rb, Cs) [243], to RbC 60 dimers [244], up to the most recent 

elucidation of a particularly complex polymerization in Li 4 C 60 [245], NMR has 

always played a key role. This class of experiments has in common the successful 

use of magic angle spinning (MAS) [246] to simplify the extremely broad powder 

pattern arising from carbon polymerization [247]. The sample spinning produces 

an effect very similar to the motional narrowing, by suppressing all the anisotropic 

contributions from the relevant interactions. 

Figure 5.35 shows the 13 C NMR MAS spectrum of Li 4 C 60 , together with its 

structure, consisting in a 2D planar polymerization pattern, characterized by the 

coexistence of single and ‘double’ C–C bonds propagating along two orthogonal 

directions. The spectrum is broadly divided into two regions, whose frequency 

domains are commonly attributed to sp 2 (~150 ppm) and to sp 3 (~60 ppm) hybridized 

carbon atoms. The fullerene distortion induced by the polymerization 

removes the equivalence of the sp 2 atoms, giving rise to a multitude of isotropic 

chemical shifts. The two distinct peaks in the sp 3 region, instead, are reminiscent


Figure 5.35 Room temperature 13 C MAS (8 kHz) spectrum of Li 4 C 60 showing the different 

intensities for sp 2 and sp 3 carbon atoms. The two peaks in the sp 3 region arise from two 

different types of C 60 polymerization along a and b directions (see structure in inset). 

(Reprinted with permission from [245]). 

of the two different kinds of covalent bonds, as confirmed both by the dissimilar 

shifts and by the 4 : 2 integrated intensity ratio. Despite the abundance of bonds, 

conventional NMR measurements (not shown) did not find any trace of Knight 

shifts (either isotropic, or anisotropic), thus demonstrating the insulating character 

of the polymer. 

We dedicate the last example to the A 3 C 60 (A = K, Rb, Cs) fulleride superconductors, 

because their NMR investigation represents also a remarkable example 

of electronic property studies [224, 248, 249]. 

Contrary to endohedral fullerene complexes involving He discussed above, 

the solid state fullerides consist of empty fullerenes, with the alkali ions residing 

in the voids of the fullerene lattice. Since the superconductivity in fullerides is 

widely accepted to be of phononic nature, the most important NMR method in 

their studies has naturally been the measurement of the spin-lattice relaxation 

times T 1 . Differently from the insulating undoped C 60 , where the main relaxation 

mechanisms are the molecular reorientation and the chemical shift anisotropy 

(see above), in the metallic A 3 C 60 family the most important mechanism is the 

so-called Korringa relaxation [250]. It stems from the significant electron-nucleus 

hyperfine interaction in metals which, therefore, provides also the dominant relaxation 

channel, with the nuclei being restored to their ground state level through 

spin-flip exchanges with electrons. Since the only electronic states available for the 

spin-flip transitions are those within kT of the Fermi energy level, the resulting 

relaxation rate is proportional to temperature, i.e. [216]: 

257


1 

T 

1 

� kT 

= ⋅ A N( EF) 

� 

2 2 

, (5.1) 

with A the hyperfine coupling constant, and N(E F) the electronic density of states at 

Fermi level. Taking into account the expression for the (temperature independent) 

Knight shift, K = A �, it can easily be shown that (Korringa relation): 

1 2 

n 

2 

2 

� �e 

1 

T T = ⋅ ⋅ , (5.2) 

4 � k � K 

with � e and � n the gyromagnetic ratios of the electron and the nucleus. Since the 

spin-lattice relaxation in alkali fullerides is dominated by the dipolar, rather than 

the contact hyperfine interaction [251], the Korringa relation should include a 

scaling factor of ~3 [229]. However, provided powder averages are considered [252], 

the above relation is still valid. 

In the normal metallic state, we would therefore expect a temperature independent 

1/(T 1 T) factor and, to a very good extent, this is what one really observes in 

many alkali-metal fullerides [228]. More interesting, but also more challenging, 

were the experiments performed in the superconducting state. Here, the gradual 

formation of the Cooper pairs implies an exponential increase of the relaxation 

times. This behavior not only was observed, but it could be also used to evaluate 

the low-temperature superconducting gap � 0 [228]. The final, and for a long time 

puzzling, issue was the absence of a Hebel–Slichter peak [253] in the transition 

to the superconducting state. This peak, which appears as a slight upsurge in the 

1/(T 1 T) data vs. temperature, is due both to the electronic state ‘pile up’ during 

the opening of the superconducting gap and to an electronic coherence factor. 

Since it is universally regarded as a strong indicator of BCS superconductivity, its 

first observation in Rb 3 C 60 [254] (see Figure 5.36) using the muon-spin relaxation 

technique (µSR), and, successively, also in other compounds [255], definitely 

Figure 5.36 Muon -spin relaxation (µSR) measurement of 1/(T 1 T) in Rb 3 C 60 at 1.5 T. 

The solid line is a fit to the Hebel–Slichter theory, with a broadened density of states. 

The almost constant 1/(T 1 T) value above the critical temperature corresponds to 

normal metallic behavior. (Reprinted with permission from [254]).


confirmed the BCS nature of alkali-fulleride superconductors. Subsequent NMR 

measurements [256], performed at relatively low fields, could reproduce these 

results and explain the absence of a peak in early data by its suppression by the 

high field. 

5.3.3.3 Concluding Remarks 

Magnetic resonance spectroscopy, both in its solution- and solid-state NMR 

forms, has proved an extremely useful tool for the investigation of the structural, 

dynamical and electronic properties of fullerenes, which rivals in importance and 

successfully complements techniques such as X-ray and neutron diffraction. Even 

simple, one-dimensional NMR experiments, carried out on static samples, can 

provide useful and often decisive information, otherwise difficult or impossible 

to obtain by other methods. 

The elucidation of fullerene structure, the study of its rotational dynamics in the 

solid state, the investigation of the different electronic properties of doped fullerenes, 

and the study of carbon polymerization are just a few of the most important 

contributions of NMR to the fullerene field. Even though current research interests 

have gradually shifted to nanotubes and similar classes of materials, the investigative 

role of NMR remains central in our continuously advancing understanding 

of these unusual carbon forms. 

5.3.4 

Electrochemistry 

Renata Bilewicz and Kazimierz Chmurski 

5.3.4.1 Electronic Properties of Fullerenes 

Fullerenes and their derivatives are unique in the richness of their redox behavior 

and electrochemical reactions, and every year new publications including reviews 

and monographs appear highlighting various electrochemical aspects of these 

fascinating molecules [257–262]. Since early research on the electrochemistry of 

C60 to C84 , their isomers and their derivatives has been summarized in excellent 

chapters of Echegoyen et al. [261, 263], in the following chapter we focus mainly 

on more recent work devoted to the relationship between the fullerene structure 

and their electrode behavior. 

Fullerenes exhibit a triply degenerated LUMO (t1u ) of low energy and, therefore, 

behave as an electronegative molecule reversibly accepting up to 6 e – [264–265]. 

Electron affinities, EA, and ionization potentials, IP, calculated for C60 to C84 [263] 

are shown in Table 5.3. 

In early reports, only two or three reduction steps were recorded for C60 using 

cyclic voltammetry [264], however, in mixed solvents and lowered temperatures 

six reversible 1 e – reductions can be easily resolved (Figure 5.37) [266]. For C70 , 

3– 

all six waves are seen at room temperature and starting from C70 the reduction is 

easier than that of corresponding C60 anions. This was explained by Cox in terms 

of larger size of C70 and charge separation delocalization model [267]. 

259


Table 5.3 Estimated electron affinities and ionization potentials of selected fullerenes [263]. 

(isomer) EA (eV) IP (eV) 

C 60 2.7 7.8 

C 70 2.8 7.3 

C 76 (D 2 ) 3.2 6.7 

C 78 (C 2v ) 3.4 6.8 

C 82 (C 2 ) 3.5 6.6 

C 84 (D 2 ) 3.5 7.0 

C 84 (D 2d ) 3.3 7.0 

Figure 5.37 Reduction of C 60 in CH 3 CN/toluene at –10 °C using cyclic voltammetry at a 

0.1 V s –1 scan rate and differential pulse voltammetry (0.050 V pulse width, 0.3 s period, 

0.025 V s –1 ) upper and lower curves, respectively. (Adapted from [266]). 

The interactions of solvent molecules with fullerenes and fullerides are significant 

mainly due to the size of the fullerene molecule compared to those of the 

solvent, hence, large solvent–fullerene interaction surface [268–274]. The surface 

exposes a strained �-orbital system consisting of sp 2 orbitals with enhanced s 

n–/(n + 1)– 

character. The formal redox potentials of the C60 couples become more 

solvent–dependent as the charge increases due to stronger electrostatic interactions 

with solvent dipoles and the hydrogen bonding interactions with the fullerides of 

increasing basicity (Table 5.4).


n–/(n + 1)– 

Table 5.4 Formal reduction potentials of C60 couples referenced to the reduction 

potential of the Me10Fc +/0 couple in 0.1 M tetra-n-butyl ammoniumperchlorate (TBAClO4 ) [273]. 

0/1– 

Solvent C60 1–/2– 

C60 2–/3– 

C60 3–/4– 

C60 acetonitrile –735 –1225 –1685 

aniline b) 

–396 c) 

–693 c) 

–1158 c) 

–1626 d) 

benzonitrile –397 –817 –1297 –1807 

benzyl alcohol b) 

bromobenzene b) 

–443 c) 

–587 d) 

–817 c) 

chlorobenzene –573 –953 –1438 

chloroform b) 

1,2-dichlorobenzene b) 

–554 d) 

–535 c) 

–908 d) 

–907 c) 

–1360 c) 

1,2-dichloroethane –448 –848 –1298 

–1841 d) 

dichloromethane –468 –858 –1308 –1758 

N,N-dimethylaniline b) 

–547 c) 

dimethylformamide –312 –772 –1362 –1902 

N-methylaniline b) 

–442 d) 

nitrobenzene –406 

–782 d) 

pyridine –343 –763 –1283 –1813 

tetrahydrofuran –473 –1063 –1633 –2133 

a) 

From Ref. [268]. Data recorrected to the Me10Fc +/0 couple. 

b) 

Determined in the present study. 

c) 

CV data. 

d) 

DPV data. 

Large interactions of the charged fullerides with the solvent are explained in 

terms of solvent dependent Jahn–Teller distortions [273, 275] and large quadrupole 

moments [276]. �-stacking interactions between C 60 and aromatic solvents 

result in a substantial contribution of the polarizability correction term to the first 

reduction. Formation of ion pairs between the more electron-rich fullerides and 

the cations of the supporting electrolyte add to the complex solvent/solute interactions 

influencing also the values of reduction potentials. Interactions with cations 

are especially important in halogenated and nitrile solvents for which slowing 

diffusion rates, slower kinetics and changes of reduction potentials were reported 

261


[274, 277–279]. Dimerization of C – 60 may occur in more concentrated solutions, 

+ + 

and in the presence of NBu4 cations, the salts of [NBu4 ][C60][C – 60] stoichiometry 

can be formed [277, 280]. Fairly large solvent/solute interactions observed were 

shown to arise from contributions of different, relatively weak interactions [273 

and refs therein]. Attempts to compensate for the effects of counterions included 

the use of microelectrodes instead of conventional size electrodes which allowed 

one to work without added supporting electrolyte [281]. This useful alternative 

approach should be used with caution since unknown residual impurities in the 

solution may play the role of counter ions, and affect the experimentally obtained 

values of potentials. In the analysis of solvent and counter ions effects on the 

electroreduction potential of fullerenes, the quality of the reference electrode or 

of the internal redox standard are of major importance. Decamethylferrocenium/ 

decamethylferrocene couple was proposed as favorable internal standard for the 

electrochemical measurements compared to the ferrocene (Fc/Fc + ) system due to 

its negligible solvent dependence and interactions with counterions [273]. 

Low value of HOMO of C60 and high value of IP (Table 5.3) indicate that fullerene 

should not be prone to oxidation [263]. When a low-nucleophilicity electrolyte 

(Bu4NAsF6 ) was used, three 1 e – oxidation peaks were reported [282]. Contrary to 

stable multianions formed upon reduction [266], the cationic species produced 

0/+1 

upon electrooxidation of C60 or C70 are rather unstable [283]. Reversible C60 oxidation at 1.26 V vs. Fc/Fc + was found in 1,1,2,2-tetrachloroethane (TCE) as 

0/+1 

solvent [284]. For the analogous C70 process the potential is 1.21 V. The addition 

of CF3SO3H to the TCE solutions increases longevity of the cation radicals. C + 60 

radicals formed by constant potential electrolysis in a thin layer electrochemical 

cell are stable for 3–4 h at 233 K. The characteristic IR bands appear at 10 170 cm –1 , 

11 820 and less intense at 8950 cm –1 . Upon transfer to the EPR tubes, the X-band 

EPR spectra of frozen solutions of C + 60 contain one rhombic shaped signal at 133 K 

consistent with the cation radical having rhombic symmetry (hence distorted from 

the Ih symmetry of the neutral species) [285]. The electrochemical data obtained 

upon long term oxidation of C70 are more complicated and indicated instability 

of the cation radical even at low temperatures and in the TCE/CF3SO3H medium 

in comparison to those in C60 . 

With increasing fullerene size and the number of carbon atoms in the cage, 

fullerenes become more readily reduced. Half-wave reduction and oxidation potentials 

of higher fullerenes (C70 , C76 , C78 , C84 ) and of their isomeric forms were 

discussed by Echegoyen et al. [263] (see Chapter 1 and Tables 5.3 to 5.6). The higher 

fullerenes are easier to oxidize, e.g. C + 76 is formed in a reversible process at 0.8 V 

and isomers of C84 are oxidized at ca. 0.9 V [284, 286, 287]. 

The low HOMO to LUMO gap (2.34 eV) resulting from unusual hybridization 

and the extended conjugated �-electronic structure leads to special electronic 

and electrochemical properties of C60 . Both oxidation and reduction peaks can be 

recorded for it at 1.27 V and –1.06 V vs. Fc/Fc + reference potential, respectively 

[282]. The difference between these values agrees well with the optical gap. The 

energy gap decreases as already mentioned with the increasing number of carbon 

atoms in the cage.


Figure 5.38 Potassium ion induced switching of intra- to intermolecular electron transfer 

(forward reaction) and 18-crown-6 induced reversible switching of the inter- to intramolecular 

process (backward reaction) [289]. 

Fullerenes act as moderate electron acceptors and form weak charge transfer 

complexes with common electron donors both inorganic and organic e.g. TTF 

[259, 288]. In the donor–acceptor complexes fullerenes accelerate forward electron 

transfer and slow backward electron transfer resulting in the formation of longlived 

charge separated states. Porphyrin and fullerene entities show weak �–� 

type interactions which can be strengthened by covalent linking the porphyrin 

to a crown ether in such a way that both functionalities contribute to the stability 

of the complex [288]. Due to two modes of binding depending on the location of 

the crown ether either intra or intermolecular electron transfer can be realized. 

Interestingly, shifting of the electron-transfer pathway from intra to intermolecular 

route was achieved by complexing potassium ions in the crown ether cavity 

(Figure 5.38), thus potassium ion induced a reversible switching phenomenon 

in this system. With strong electron donors such as alkali metals, radical anion 

salts are generated in the solution and often reveal interesting superconducting 

properties [260, 289]. 

5.3.4.2 Electrochemical Properties of Soluble Fullerene Derivatives 

In order to tune the electronic properties of fullerenes for specific electronic and 

photophysical devices additional units are introduced into the fullerene structure. 

Schematic representation of such building blocks linked to the C 60 moiety with 

spacer units is shown in Figure 5.39. 

Functionalization of fullerene in most cases reduces its electron affinity due 

to disruption of the conjugated system. The reduction potentials of fullerene 

derivatives classified conveniently as: singly bonded functionalized derivatives, 

cyclopropanated and cycloaddition products are collected in [263] while those 

of several C 60 -acceptor dyads are listed in Table 5.4 of Ref. [259]. Saturation of a 

double bond of the fullerene structure leads to a higher LUMO energy [292, 293]. 

The loss of conjugation may be, however, compensated by the introduction of 

electron withdrawing groups directly connected to the C 60 cage. Such functionalization 

may result in even stronger electron accepting molecule than the parent 

263


Figure 5.39 Schematic representation of dyads and triads involving the C 60 electrophore: 

(a) C 60 donor, (b) C 60 acceptor, (c) C 60 donor 1 -donor 2 (monoadduct with respect to C 60) and 

(d) C 60 donor-acceptor (bisadduct with respect to C 60 ) [291]. 

one. (Fullerene functionalization is discussed in Section 5.2.) The reduction 

potentials of [1,2]dicyanofullerenes [294] or dicyano-[6,6]methano fullerene [295] 

are ca. 0.1 V more positive than that of the parent fullerene. The electron affinity 

of fluorofullerenes increases by ca. 0.05 eV per each fluorine atom added, thus, 

highly fluorinated fullerenes become exceptionally strong electron acceptors 

[296, 297]. Other strategies to improve electron acceptor properties of a fullerene 

derivative include insertion of electron-withdrawing substituents connected to 

the fullerene cage through a methano bridge, and formation of pyrro lidinium 

salts [288, 298]. 

The electroreduction potential depends on whether the first reduction is 

fullerene or addend based. Efficient electron acceptor organofullerenes are those 

exhibiting the periconjugative effect. In case of quinone type methanofullerenes, 

the intramolecular electronic interaction can occur between p z –� orbitals of the 

olefinic carbons of the quinone moiety and the adjacent carbon atoms of C 60 

separated by a spiro carbon atom. The molecule with more extended conjugation 

has better acceptor properties [288]. Five reversible electroreduction waves were 

observed for fullerene linked to the malonate group containing flexible long alkyl 

chains to two biphenyl-phenyl units [299]. For some of the derivatives, following 

first electron transfer a hemolytic cleavage of one of the bonds connecting the 

addend to C 60 may take place leading to irreversible electrochemical behavior 

[300]. The voltammetric experiment may even show reversible behavior of the 

compound, but the constant potential electrolysis (CPE) performed at a longer 

time-scale, often reveals the presence of several side products of chemical reactions 

in the reaction mixture which can be identified by HPLC and MALDI-TOF 

(Figure 5.40) [288]. In the case of CPE of methanofullerenes with nitrophenyl 

groups shown in Figure 5.40 after reoxidation, small amounts of C 60 , bisadducts 

and ca. 40% of starting material were found. 

Voltammetry performed during CPE may lead to valuable information concerning 

the mechanism of reactions and is often very helpful for understanding of the


Figure 5.40 Cyclic voltammetry of methanofullerenes terminated with an ESR active nitrophenyl 

group [288] in THF vs. Fc. 

pathways involved in the process. For example, in case of bis(ethoxycarbonyl) derivative 

of methano C 70 fullerene, Echegoyen and coworkers [301] obtained evidence 

of a new stable intermediate exhibiting reversible electrochemical behavior. The 

malonate group with its electron withdrawing affinity allowed stabilization of the 

negative charge in the adduct and then formation of an intercage bond between 

two fullerene core radicals. Dimerization was found here to inhibit the expected 

electrochemical retro-cylopropanation reaction. The proposed mechanism of the 

retro-Bingel reaction taking place during reductive electrochemistry is presented 

in Figure 5.41. 

Addition of K + ion to the solution can cause a positive shift of reduction potential 

by ca. 90 mV as shown for a C 60 -dibenzo-18-crown6-conjugate (Figure 5.42) [302, 

303]. This shift can be explained by the electrostatic effect of K + bound in the 

cavities of the crown ether which decreases the influence of electronic interaction 

of the C 60 moiety with the substituent. However, the reduction potential is still 

more negative than that of pristine C 60 , showing that raising the LUMO level by 

saturation of few double bonds of the fullerene cage is the main factor determining 

the value of the reduction potential in this case. 

Finding new and improved C 60 -based acceptors, that would show less negative 

reduction potentials but at the same time, high stabilities of the anionic radicals 

formed, is a challenging goal important for designing fullerene based photovoltaic 

devices and artificial photosynthetic systems. 

265


Figure 5.41 Proposed mechanism for the formation of dimeric C 70 fullerene derivatives from 

the electroreduced form (radical position and connecting bond between the C 70 units are 

chosen arbitrarily with respect to possible regioisomers) [301]. 

Figure 5.42 Representative examples of fullerene intramolecular complexes with better 

reduction potentials than pristine C 60 fullerene. (Adapted from [288]).


Effect of Addition Pattern on the Electrochemical Properties of Fullerenes. The addition 

pattern is an important factor influencing the value of E 1/2 potential of the C 60/C – 60 

step of the exohedral fullerene derivatives. For 18 C 60(CF 3) n derivatives (which 

had been discussed in Section 5.3) this potential has been shown to be a linear 

function of the DFT-predicted E(LUMO) values [304]. 

The reduction potential can be changed to a great extent by altering the addition 

pattern of C 60(X) n and C 70(X) n as shown for C 60(CF 3) n, C 60(C(CO 2Et) 2) 3 [305] and 

C 70(Ph) i [306]. The effect is especially strong when X is large enough so that 1,4 

additions are favored over 1,2 ones. Moving e.g. one CF 3 group by only two cage 

carbon atoms on its own pentagon, would change the reduction potential by 0.45 V. 

The effect of particular substituent on shifting E(LUMO) values is a function of 

the substituent ability to withdraw electrons from the fullerene cage [307, 308]. 

As shown, it depends on the addition patterns of the compounds in question. 

Different addition patterns result in different numbers of non-terminal double 

bonds in pentagons (defined as short pent-hex junctions that have two C(sp 2 ) 

nearest neighbors). When the LUMO fragments associated with them overlap, 

E(LUMO) becomes very low and the E 1/2 values are relatively high. This shows, 

that the addition pattern of a fullerene derivative may be even a more important 

factor than the number of subtituents for the electron acceptor efficiency of the 

molecule (Figure 5.43) [309]. 

Figure 5.43 E 1/2 values of C 70 (CF 3 ) n derivatives from electrochemical and DFT study [309]. 

Addition patterns leading to non-terminal double bonds in pentagons and 

extensive LUMO delocalization allow one to obtain controlled electron–acceptor 

properties. 

Organofullerenes with Electron Donor Moieties. Due to the degree of electron delocalization 

within the �-system and large size, the molecules under discussion 

are convenient building blocks for the construction of covalent donor–acceptor 

267


dyads, triads and more complicated systems in molecular devices for electronics, 

molecular switches, and photoconductors. Absorption of fullerenes in the visible 

spectrum region and their ability of rapid photoinduced charge separation make 

fullerene derivatives fascinating materials for photovoltaic applications [310]. 

The intramolecular transfer dynamics in molecules with fullerenes linked to an 

electroactive or photoactive species is function of excited state of the antenna 

molecule, donor–acceptor distance and the solvent used [311]. 

The energy levels of the charge-separated states (� GRIP) can be evaluated using 

the Weller type approach utilizing the redox potentials, center-to-center distance, 

and dielectric permittivity of the solvent [312]: 

� = E − E + �G 

, 

GRIP ox red s 

where �G s = –e 2 /4 � � 0 � R R Ct–Ct and � 0 and � R refer to vacuum and solvent permittivity. 

By comparing these energy levels with the energy levels of the excited states, 

the driving forces are evaluated. Electrochemical gap as small as 1 eV has been 

reported for some fullerene–ferrocene dyads [259]. Ferrocene (Fc, E 1/2 = 0.50 V 

vs. SCE), N,N�-dimethylaniline (E 1/2 = 0.81 V vs. SCE), N-methylphenothiazine 

(E 1/2 = 0.70 V vs. SCE) and tetrathiafulvalene (TTF, E 1/2 = 0.37 V vs. SCE) are 

common donor components of these dyads [313]. This type of hybrids were 

often classified as electroactive but, due to the insignificant VIS absorption of 

the added group, photoinactive. The electrochemical data for the TTF–C 60 dyads 

are reviewed by Bendikov et al. [310]. Due to very different structures such dyads 

exhibit different solvation effects and dependence of the electrochemical gap on 

the solvent used [314]. A thermodynamically stable supramolecular donor–acceptor 

system with C 60 and TTF assembled through guanidinium–carboxylate ion pair 

stabilized by two hydrogen bonds has been proposed [315]. Multiple substitution 

of the fullerene core leading from dyads (Figure 5.44) to triads [315] tetrads and 

quintads [316] enlarged the HOMO–LUMO gap and lead to a decrease of the C 60 

reduction potential. 

Because of the moderate fullerene absorption in the VIS region, functionalization 

of C 60 with a chromophoric addend improves the light harvesting efficiency 

of the fullerene dyad. In such a system fullerenes operate as electron or energy 

acceptor moieties. Donors such as metalloporphyrins, zinc phthalocyanines, 

ruthenium (II) polypyridyl complexes with strong metal-to-ligand charge transfer 

(MLCT) are powerful units in the excited state. The radical pair for the latter case 

involves oxidized complex and anionic fullerene radical with a lifetime of 304 ns in 

deoxygenated CH 2 Cl 2 . However, in the presence of traces of oxygen the fullerene 

triplet excited site is quenched to produce cytotoxic singlet oxygen species [311]. 

In the C 60 -ferrocene conjugates the intimate contacts between the donor and 

acceptor result in large ground-state interactions suggesting a substantial charge 

density shift from donor to acceptor. In the excited states, the processes in these 

conjugates are dominated by rapid charge separation reactions (0.8 ps) to yield 

metastable ion pairs with the radical pair lifetime of ca. 30 ps. As discussed by 

Guldi and coworkers [317], no prominent charge-transfer features were observed


Figure 5.44 Cyclic voltammogram of TTF-C 60 diad [R=S(CH 2CH 2O) 4Me] recorded in 

1,2-dichlorobenzene. (Adapted from [310]). 

for the bucky ruthenocene conjugates and only an intrinsically faster excitedstate 

deactivation (ca. 200 ps) evolved. The authors attribute this difference to the 

unfavorably shifted oxidation potential of about 0.61 V in ruthenocene. For the 

fullerene derivative –�G ET is 0.35 eV, while for ruthenocene conjugate it is –0.26 eV 

rendering the charge separation thermodynamically unfeasible. Modulation of the 

electronic coupling between C 60 acceptor and various donors for the construction of 

new optoelectronic devices requires also searching for new type of connecting units 

between the donor and acceptor. Longer-distance charge separation was achieved 

for oligothiophene–fullerene dyads (nT–C 60 ) and triads 8T–4T–C 60 [318]. 

Making use of coordination of fullerene derivatives to transition metal ions is 

in general a promising approach. Fullerene coordination ligands in which one 

bipirydine or terpyridine unit was directly attached to the nitrogen atom of a fulleropyrrolidine 

ensure a linear communication pathway between fullerene and 

coordinated ruthenium (II). This formed a linear rod-like donor–acceptor system 

(Figure 5.45) [319]. 

The reduction of the fullerene unit in the complex is easier than in the free 

ligand while Ru-based reductions are shifted by 50 mV more negative relative to 

the simple [Ru(bpy) 3 ]Cl 2 complex. These electrochemical results indicate electronic 

coupling existing between the fullerene core and the ruthenium ion center and are 

in line with the MLCT transition band, which is subjected to a red shift for the dyad. 

Ferrocene–porphyrin–fullerene constructs with crown ether appended porphyrins 

were self-assembled through host–guest interactions with alkylammonium cation 

functionalized fullerenes. Interactions between the porphyrin and fullerene entities 

were followed by voltammetry. The crown ether–alkylammonium cation complexation 

binding strategy lead to well-defined stable triads with charge-recombination 

in two steps and direct charge recombination from the porphyrin cation radical. 

This system fulfilled the condition of long-lived charge-separated state. 

269


Figure 5.45 Cyclic voltammograms of [Ru(bpy) 3 ]Cl 2 (black), bpy-C 60 (red) and complex (blue) in 

CH 2 Cl 2 with 0.1 M TBA[PF 6 ] (tetrabutylammonium hexafluorophosphate) [319]. 

An interesting design strategy is based on dendritic molecules appended with 

multiple zinc porphyrin units, to trap pyridine compounds carrying multiple 

(n = 1–3) fullerene units (Figure 5.46) [320]. The complexes presented in the 

scheme have a photoactive layer of spatially segregated donor and acceptor arrays. 

Voltammetry experiments revealed electron donation from ligating pyridine to 

the zinc porphyrin units. Photoinduced electron transfer was confirmed by flash 

photolysis measurements. High ratio of charge-separation rate constant to charge– 

recombination rate constant (3400) was obtained. 

5.3.4.3 Electrocatalytic Activity of Fullerenes 

Early attempts to use fullerenes as catalysts involved reduction of organic halides 

[321]. Regeneration of the oxidized form of C60 catalyst at the electrode surface 

was responsible for the electrocatalytic current. Electrocatalytic reduction of 

dihalogenated alkanes was investigated using C60 and C70 in solution and using 

2–/3– 1–/2– 

coated electrodes [322–324]. C60 and C60 were the catalytically active couples. 

In Figure 5.47, the cyclic voltammograms reveal the catalytic effects observed for 

2–/3– 

dihaloethanes in the range of potentials corresponding to the C60 electrode 

process. The peaks appearing at positive potentials are due to the oxidation of 

halogenide released in the catalytic reduction process. 

Anions of higher fullerenes C76 , C78 , C84 are relatively more stable with respect 

to derivatization and can serve as better electrocatalysts than anions of C60 [323]. 

The rate constants for the pseudo-first-order conditions can be calculated from 

the rotating disk electrode measurements [325]. Alkanes and monoiodoalkanes 

were products of the C60 reaction with �,�-diiodoalkanes except for reaction 

3– 

with 1,3-diiodopropane and 1,5-diiodopentane where C60 adduct formation was 

reported [322]. For each fullerene anion kcat increased in the order Cl 

representation of the complexation of DP 24 with fullerene appended bipyridine ligand Py 2 F 3 . 

(Adapted from [320]). 

271


Figure 5.47 Cyclic voltammograms for 0.0001 M C 70 (curve 1), 0.0001 M C 70 and 0.4 M 

1,2-dichloroetane (curve 2), 0.0001 mM C 70 and 0.4 M 1,2-dibromoethane (curve 3), as well as 

0.0001 M C 70 and 0.0014 M 1,2-diiodoethane (curve 4) in 0.00001 M (TBA)PF 6 in benzonitrile, 

at Pt electrode. Potential scan rate is 0.1 V s –1 . 

agreement with a dissociative mechanism of reduction [326] and increased in the 

order C84 < C78 < C76 < C70 < C60 for each 1,2-dihaloethane [327]. Although, 

lower rate constants were obtained for the higher fullerenes, less negative potentials 

and the lack of alkyl adduct formation were of advantage. 

Electrochemically generated anions of C60 were used also in deprotonation of 

the organic acids. A weekly basic monoanion of fullerene deprotonates a relatively 

strong acid e.g. ethyl nitroacetate. C – 60 anion catalyzed nitroacetate reaction with 

ethyl acrylate and acetonitrile to form double addition products [328]. 

5.3.4.4 Conclusions and Outlook 

Fullerene based films, discussed in the next section, attract considerable interest as 

materials possessing the unique properties of fullerenes, but at the same time more 

suitable for practical applications in nanotechnological, electrochemical sensing 

and photophysical devices. Since C 60 molecules exhibit electron-mediating properties, 

upon immobilization in mono- and multilayer films on electrodes they can efficiently 

promote reduction and oxidation processes. In these exciting applications, 

albeit outside the scope of this chapter, their poor solubility in many solvents and 

water becomes an advantage, contrary to the case of diffusion charge mediation. 

In general, more uniform coverage of the electrode substrates and higher reversibility 

of the electrode processes were reported for C 60 embedded in different type 

of matrices than for the polycrystalline C 60 films deposited on electrodes [262]. C 60 

modified electrodes can be easily prepared by placing a C 60 drop onto the electrode 

which is then overlaid with nafion protecting coat, as shown first by Compton et al. 

[280]. Two to four quasireversible 1 e – reductions were usually observed in aqueous 

or mixed solvent and electrocatalytic application of surface immobilized C 60 

molecules were demonstrated e.g. for oxygen and nitrobenzene reductions [262].

5.4 Fullerene Aggregates 

Figure 5.48 Proposed mechanism of cytochrome c immobilization, and electrochemical 

reduction by C 60 -Pd polymer film. (Adapted from [331]). 

Several attempts concern biocatalytic systems important for the field of bioelectronics 

[329], e.g. C 60 molecules assembled in monolayers were used to mediate 

electrons between enzymes and the electrodes [330]. Two different fullerenemodified 

electrodes were used: electrical contacting an enzyme and cytochrome 

c with the electrode (Figure 5.48) [331]. 

Fast development of applications of the C 60 films as mediating units in sensing 

and catalytic devices may be expected in the near future. 

5.4 

Fullerene Aggregates 


Many extraordinary properties of fullerenes have alerted researchers to find and 

study the properties of aggregated fullerene structures. The fullerenes have also 

drawn much interest as construction materials for thin films and other ordered 

solid nanostructures. One can prepare photovoltaic devices which use photogenerated 

charge separation as a result of the excellent ability of fullerenes to 

accept electrons from electron-donating compounds. A well-known example is 

the plastic solar cell with bulk hetero junction structure, where an electronegative 

fullerene derivative is incorporated into conjugated electron-rich polymer [332]. 

Many interesting features of fullerene and fullerene derivative films have been 

discovered, for example, superconductivity [333, 334], unique redox properties 

[335], photo-electrochemical response [336, 337] among others. The most common 

fullerenes studied in all respects are C 60 and C 70 , so this chapter will concentrate 

on the research based on these two carbon balls. 

273


5.4.1 

Film Preparation Methods 

Numerous supramolecular assemblies containing fullerene as a functional moiety 

have been synthesized [338–340]. In order to use the functions of the supramolecular 

structures created, the assemblies should in most cases be bound as solid 

well-ordered films [340]. With ordered nanostructured films one may be able to 

scale the functions taking place at the molecular level up to macroscopic level. 

This may happen when billions of supramolecular assemblies work coherently 

in an ordered film and the sum of all actions at the nanoscale is seen as the 

resulting macroscopic function. For this reason thin fullerene films are also in 

focus when, for example, molecular electronics are developed. A variety of film 

preparation methods have been used in fullerene thin film fabrication and will 

be briefly reviewed here. 

Drop casting and spin-coating are based on solvent evaporation and precipitation 

of the solute. In general, the film morphology can be varied by changing 

the solvent evaporation rate. The type of the solvent can also influence how the 

precipitate will crystallize. The drop cast fullerene films are used, for example, 

as starting material for electrochemical polymerization of fullerene copolymers 

[341]. Compared with drop casting, spin-coating can provide better control over 

the film formation process. In spin-coating the film is formed by precipitation 

onto a substrate which is rotated while evaporation takes place. Thus in addition 

to the solvent evaporation the film morphology is affected by the centrifugal 

forces caused by the substrate rotation. The film morphology and thickness can 

be varied, for example, by changing the rotation speed, the solvent, and the concentration 

of active material in the solution. Spin-coating is usually used when 

a fullerene derivative is mixed with a conjugated polymer. This way one can get 

a bulk hetero junction between the electron-donating conjugated polymer and 

electron-accepting fullerene. The bulk hetero junction structure has been proved 

to act as an efficient organic photodiode when placed between two electrodes with 

different work functions [332]. 

The Langmuir–Blodgett (LB) and Langmuir–Schäffer (LS) methods are sophisticated 

techniques for the preparation of well-ordered one molecule thick films from 

surface active molecules [342, 343]. The first step in both techniques is formation 

of the Langmuir film that is a stable molecular monolayer at the gas–liquid 

interface. Water is definitely the most commonly used liquid as the subphase for 

monolayer formation. Water can easily provide very strong anisotropic interaction 

between the molecules and the interface. In practice, the subphase is either pure 

water or water including a small amount of some salt. Naturally, it is easiest to use 

ambient air as the gas phase. The need of anisotropic interaction sets a prerequisite 

for the molecules which are used for the monolayer formation. The molecules 

should be amphiphilic, having one part of the molecule hydrophilic, i.e. soluble 

in water, and the other part hydrophobic. The strong attraction to water forces 

molecules to be as close to the water surface as possible. On the other hand the 

water insoluble part stops the molecules from dissolving in the subphase. This


kind of anisotropic interaction makes it possible to form stable monomolecular 

films at the air–water interface. The attraction to water prevents the molecules at 

the interface from forming three-dimensional crystals. 

The Langmuir film preparation starts by dissolving the surfactants in a waterimmiscible 

volatile solvent, such as chloroform or hexane. The solution is then 

carefully spread onto the clean water surface so that the total area of all the 

molecules at the air–water interface is much smaller than the available area of the 

water surface. One can get a simplified, but descriptive, picture of the phase transitions 

of the molecular film at the interface when one considers two-dimensional 

phases analogous to the three-dimensional ones. Thus in the beginning, when 

the molecules have space to move freely in two dimensions they behave like a 

two-dimensional gas (a in Figure 5.49). In this phase the surfactants have very 

small, if any, effect on the surface tension of water. 

As three-dimensional gas, the two-dimensional gas can also be condensed by 

increasing the pressure. In order to increase the two-dimensional pressure of the 

molecules at the interface the surface area should be decreased. Usually, the LB 

system consists of a trough with computer controlled barriers. By moving these 

barriers the area of the water surface can be controlled. When the surface area is 

decreased, the free space around the molecules is reduced and molecules start 

to influence each other. At some point of the compression, when the surfactants 

are close enough to each other, they start to decrease the surface tension of water. 

The observed decrease in the surface tension is referred to as the surface pressure 

(�) which is the difference in the surface tensions of the pure water surface (�*) 

and the water surface with the surfactants (�): � = �* – �. The surface pressure is 

plotted as a function of available area per molecule, i.e. mean molecular area. The 

resulted graph is referred to as surface pressure–area isotherm. For an example, 

an isotherm of an imaginary surfactant is presented in Figure 5.49. The phase 

transitions cause changes in the gradient of the area–surface pressure isotherm. 

The most common method of obtaining information about the phase transitions in 

the monolayer is to measure the surface pressure online with the Wilhelmy method 

while the molecules are compressed; the more densely the molecules are packed 

Figure 5.49 Schematic illustration of a surface pressure–area isotherm of a surfactant. 

The monolayer phases are indicated and illustrated: (a) two-dimensional gas, 

(b) two-dimensional liquid, (c) two-dimensional solid, and (d) collapse of the monolayer. 

275


in the monolayer, the more they lower the surface tension of water. With decreasing 

surface tension the surface pressure increases. When the gaseous molecules 

are compressed enough a two-dimensional liquid, or expanded condensed phase 

(b in Figure 5.49), is formed. Further compression results in a faster rising in the 

surface pressure when compared with that observed for the gas phase. 

In order to transfer the monolayer from the interface onto a solid substrate, the 

monolayer should be stable. An essential requirement to get a stable monolayer 

is that the surfactants are able to form a solid phase at the ambient temperature. 

When the monolayer has reached the two-dimensional solid phase, or condensed 

phase (c in Figure 5.49), the available surface area per molecule corresponds the 

molecular cross section which is approximately 0.2 nm 2 for saturated fatty acids. 

Usually, the first Langmuir–Blodgett deposition takes place by passing the hydrophilic 

substrate through the monolayer at the interface from water to air, so 

that the water soluble ends are attached to the substrate. For the layers following 

the first deposition, depending on the dipping direction, either from air to water 

or from water to air, the molecules in the monolayer may have different orientation 

in respect to the substrate. Concurrently with passing the substrate through 

the monolayer the surface pressure is kept constant by moving the barriers with 

proper speed. The transfer ratio is the area the barriers moved during deposition 

divided by the substrate area passed though the film. The transfer ratio describes 

the quality of the film deposition, and for the ideal case it is unity. The Langmuir– 

Schäffer method is sometimes referred to as horizontal lifting. In the LS method 

the substrate is kept parallel to the water surface when the substrate is brought 

in contact with the Langmuir film. The Langmuir monolayer adheres onto the 

substrate and is lifted with the substrate from the interface. 

It might seem that the LB technique is strictly limited by the requirement of 

amphiphilicity of the molecules but actually it is possible to prepare, for example, 

films where totally hydrophobic compounds are mixed in a surfactant matrix. 

Formation of the Langmuir and Langmuir–Blodgett films of pristine fullerenes 

is somewhat controversial, but this will be discussed later in the text. 

Physical vapor deposition in high vacuum is a very well known technique to 

prepare thin metal, semiconductor, and organic films. Among other organic 

materials, also fullerene thin films have been prepared by thermal vapor deposition 

in vacuum [344–347]. The fullerene vaporized is produced in the high vacuum 

by heating electrically the resistive source at temperature of 300 to 550 �C. With 

organic materials the temperature control of the source has high importance in 

order to avoid destruction of the material. 

Nanostructured fullerene films can be fabricated by the method referred to as 

electrodeposition [348, 349]. In this method one uses the ability of C 60 and C 70 to 

form clusters in solution of mixed solvents at room temperature. A usual diameter 

for such a fullerene cluster is 100–300 nm. The driving force for cluster formation 

is the three-dimensional hydrophobic interaction between fullerene units in the 

solution of polar and non-polar solvents. One method of preparing clusters is to 

inject a toluene solution of fullerene into acetonitrile. In the final cluster suspension 

the ratio of mixed solvents is between 3 : 1 and 9 : 1 of acetonitrile–toluene. The


fullerene cluster formation process and the stability of the clusters are affected by 

the ratio between the solvents and by the fullerene concentration in the original 

toluene solution. Especially, C 60 can precipitate, for example, when the fullerene 

concentration exceeds 50 µM and when volume fraction of acetonitrile is more 

than half [350]. The cluster formation is confirmed by solvatochromic changes 

when acetonitrile is added to the toluene solution of fullerene. The color changes 

from the purple of C 60 dissolved in toluene to yellow-green of the C 60 cluster 

solution. By electrodeposition one can transfer these fullerene aggregates onto a 

conductive solid substrate, for example indium-tin-oxide (ITO) coated glass. The 

electrodeposition takes place in a two-electrode cell where one of the electrodes 

is the substrate. The spacing between the electrodes is few millimeters and a DC 

voltage from 20 to 400 V is applied between the substrate and the other electrode. 

Under the influence of the DC electric field, the fullerene clusters can become 

charged. The charged clusters are transferred by the electric field onto the substrate 

with positive potential. Pristine fullerenes in solution could not be charged by 

the electric field in a similar way as the clusters, thus fullerene aggregation must 

first take place before electrodeposition. By this method relatively thick fullerene 

films can be deposited easily and fast. For example, when the cell has been filled 

with approximately 0.1 mM fullerene cluster solution and electric field is applied 

for one minute, the solution becomes colorless and a brown film forms onto the 

substrate. With these conditions the resulted film has a thickness of approximately 

1 µm which corresponds to surface concentration of 0.2 µmol cm –2 of fullerene 

molecules [348]. The film thickness can be controlled by varying the fullerene 

cluster concentration, the deposition time, and applied DC voltage. In order to 

increase the film thickness one should use higher cluster concentration, longer 

deposition time, and/or apply higher DC voltage. 

5.4.2 

Fullerene Film Properties 

Pristine C 60 and C 70 consist solely of carbon atoms and thus are purely hydrophobic. 

The formation of a stable fullerene Langmuir monolayer at the air–water 

interface is somewhat controversial. There are reports about successfully formed 

monolayers which have been transferred onto the solid substrate from the interface 

[351, 352]. When fullerene forms a real monolayer on the water surface the limiting 

area, i.e. the molecular area obtained by extrapolating the isotherm to zero pressure 

from its steepest part, is approximately 1 nm 2 . Most of the results indicate that 

fullerenes tend to form an aggregated multilayer structure rather than a stable 

monolayer [353]. The mean molecular area obtained decreases dramatically when 

the fullerenes do not form a monolayer at the air–water interface. Usually, the 

molecular area is less than half of that of the perfect monolayer. When the surface 

pressure–area isotherm results in a molecular area lower than one could expect 

from the physical size of the molecule, the conclusion is that the molecules do 

not form a perfect monolayer but they are placed on top of each other as a double 

layer or a multilayer. The Langmuir monolayer measurements have shown that a 

277


stable fullerene monolayer can be formed on the water surface in very narrow range 

of experimental conditions [351, 352]. For example, with concentrations higher 

than 10 –4 M, a perfect monolayer cannot be obtained [351]. The simulations of the 

fullerene monolayers on the water surface help to understand the reason for the 

observed irregular monolayer formation [354]. The pristine fullerene does not have 

anisotropic interaction with the water to keep a stable monolayer structure. 

For Langmuir film preparation a dilute fullerene solution is spread onto the 

water surface. After solvent evaporation fullerenes form a two-dimensional gas 

phase. The single molecule can move away from this phase in two ways: it may 

attach itself to other fullerenes and form a monolayer raft at the interface, or it may 

escape from the water surface onto the formed raft. A stable monolayer is formed 

if every fullerene in the raft has six neighboring fullerenes. Six fullerene–fullerene 

bonds keep the surrounded fullerene tightly in place on the water surface [354]. 

Naturally, the raft is not infinite but it has boundaries where there are also fullerenes 

which have less than three neighboring fullerenes. The fullerenes on the edge 

can easily be promoted on top of the fullerene raft. For example, if a fullerene has 

only two neighboring fullerenes no fullerene–fullerene bonds have to be broken 

when this fullerene turns onto the fullerene raft. When conditions are such that 

this promotion takes place it is impossible to obtain a fullerene monolayer at the 

air–water interface. On the water surface the monolayers of pristine fullerene are 

extremely rigid and incompressible having high collapse pressure [351]. 

In order to overcome the problematic monolayer formation of pristine fullerenes, 

caused by the fullerene–fullerene and fullerene–water interactions, one can use 

two different strategies. The first is to use some matrix compound which prevents 

direct contact, and thus interaction, between fullerenes. The pristine fullerene 

can be mixed with long chain alcohols or fatty acids [351]. The surfactants and 

fullerenes are supposed to spread homogenously onto the water surface and 

thus fullerenes tend to aggregate less than when only pristine fullerene is spread 

onto the water surface. In addition, some special matrices with cavities where 

fullerene can fit in, such as azocrowns [355], calixarenes [356], and resorcarenes 

[357] have been used to enclose fullerenes in the LB films. These cavitants offer a 

hydrophobic cavity where fullerene can be incorporated and so isolated from the 

water surface when a one-to-one mixture of the cavitant and fullerene is spread 

on the water. The cavitant has a hydrophilic part which enables formation of 

stable Langmuir monolayer at the air–water interface. When any matrix is used 

the resulting fullerene film does not solely consist of fullerene. A rather exciting 

way to make a uniform fullerene LB film is to use specially selected matrix 

molecules which can be selectively dissolved away from the film deposited onto 

the solid substrate [358]. One can enhance miscibility of fullerene in the matrix 

by replacing fatty acids with surfactant with, for example, a fulvalene moiety. The 

electron-accepting fullerene and the electron-donating matrix molecule may have 

ground state interactions which can increase mixing compared with that of fatty 

acids and fullerene [358]. 

The second option to decrease the tendency of fullerenes to form multilayer 

stacks at the air–water interface is to use fullerenes substituted with some hydro-


philic group in order to increase surface–molecule interaction. The amphiphilic 

fullerene derivatives are capable of forming the Langmuir monolayers themselves 

[337, 359–361]. An amphiphilic fullerene can be also mixed with fatty acid matrix 

to ease film formation and deposition [336]. The hydrophilic adducts on fullerenes 

will keep the molecules as a monolayer at the air–water interface even during compression. 

In other words, the interaction between the water surface and the adducts 

will defeat the fullerene–fullerene attraction, which usually causes formation of 

fullerene multilayers at the interface during compression. Addition of hydrophilic 

side group to the fullerene core does not cancel attraction between the fullerenes. 

The attraction between the fullerene cores causes irreversible clustering, which 

is seen as unequal isotherms for the first compression and for the subsequent 

expansion of the molecular film [362]. This is observed when the monolayer of 

the hydrophilic fullerene derivative on the water surface is compressed until it 

collapses and after that the surface area is increased. The expansion isotherm 

resembles the compression isotherm observed for pristine fullerene forming a 

multilayer on the water surface. This means that after the monolayer collapse the 

fullerene derivatives are no longer spread uniformly onto the water surface but 

the fullerene aggregates are held together by the attraction between the fullerene 

cores. In order to improve the spreading behavior one can combine the shielding 

effect of matrix and the properties of hydrophilic adducts by chemical engineering 

[362]. Fullerene derivatives with several long hydrocarbon chains and hydrophilic 

groups as adducts have been designed and the LB properties of these molecules 

have been tested [363]. Already four alkyl chains added onto the fullerene core 

reduced the attraction between the fullerene cores at the air–water interface. This 

is seen as better reversibility in isotherms measured with successive compression/expansion 

cycles. Still the alkyl chains do not provide proper shielding of 

fullerene cores but, for example, the film absorption spectra show broadening 

as a result of fullerene aggregation. By placing the fullerene core in the middle 

of an amphiphilic dendritic structure, one can achieve a fullerene derivative 

with excellent spreading properties on the water surface. The dendritic adducts 

make it possible to prepare films where fullerenes are isolated from each other 

[364, 365]. The LB monolayers of such fullerene derivatives have shown similar 

absorption spectra to those measured for dilute dichloromethane solutions of the 

same compounds. Similar absorption spectra for film and solution indicate that 

the fullerene cores are isolated even in the LB film. Films consisting of isolated 

fullerenes are interesting in point of view of nonlinear optics, especially, optical 

limiting applications [362]. 

Electronegative fullerene can be also modified by linking it covalently with an 

electron donating chromophore, such as, porphyrin [338, 366], phthalocyanine 

[367], or phytochlorin [368]. The resulting compound is capable of the photoinduced 

intramolecular electron transfer from the donor moiety to the fullerene 

moiety [366]. The donor–fullerene dyads can be synthesized to have amphiphilic 

character and thus they are suitable for the LB film preparation [369–372]. By the 

LB technique the electron donor–acceptor dyads can be deposited as monolayers 

with specified orientation of all the molecules in the film. In other words, in the 

279


dyad monolayer the electron transfer takes place perpendicular to the substrate 

and in the same direction in all dyad molecules coherently [369, 371]. When the 

vectorial photoinduced intramolecular electron transfer takes place from donor 

to fullerene in billions of dyads simultaneously, a potential of several millivolts 

is produced [371]. 

Fullerenes are electrochemically interesting. In solutions, pristine C60 and C70 6– 6– 

are able to reversibly accept up to six electrons resulting in C60 and C70 anions, 

respectively [373]. Fullerenes electrochemistry is discussed in more detail in the 

former section. In contrast, the oxidation of fullerene is more difficult and is 

usually limited to the formation of dication as the highest oxidation state [374]. 

Similar step-wise reduction is also seen in fullerene films, but the behavior has 

been found to be more complicated than for dissolved molecules [335]. The voltammograms 

for drop-cast fullerene films showed larger peak splitting between 

the measured reduction and re-oxidation peaks than for dissolved fullerenes. The 

observed hysteresis for the films suggests large structural changes in the film [335, 

375]. Upon reduction, most of fullerenes are reduced to mono-anions. The charge 

in the film is balanced by diffusion of cations from the supporting electrolyte. 

The bigger the diffused cations are in size, the larger are the structural changes 

seen in the film. The changes are reversible when fullerenes are re-oxidized back 

to the neutral form. 

With the electrodeposition one can easily prepare micrometer thick fullerene 

films on optically transparent electrodes. Absorption spectra for electrodeposited 

C60 films on three different substrates are presented in Figure 5.50. The inset 

Figure 5.50 Absorption spectra of C 60 clusters deposited on different substrates (a) OTE/(C 60 ) n , 

(b)OTE/TiO 2 /(C 60 ) n , and (c) OTE/SnO 2 /(C 60 ) n film. The absorption spectra of native electrodes, 

(d) OTE/TiO 2 and (e) OTE/SnO 2 , recorded prior to electrodeposition are also shown. The inset 

shows solution absorption spectra of (f) 19 µM C 60 in toluene and (g) same amount of C 60 in 

the form of clusters in 1:3 (v/v) toluene–acetonitrile. (Reproduced with permission from J. Phys. 

Chem. B 2000, 104, 4014–4017; Copyright 2000 America Chemical Society).


in Figure 5.50 shows the difference in the absorption spectra of the C 60 toluene 

solution and the cluster suspension. The characteristic absorption band of C 60 at 

approximately 350 nm is red-shifted and broadened when clusters are formed. 

In addition, the very weak absorption band of C 60 monomer in the 500–600 nm 

region gets stronger and shifts slightly to shorter wavelengths when aggregation 

takes place [350]. Similar spectral shifts and broadening of the absorption bands, 

compared with the fullerene solution spectrum, can be seen as an indication of 

fullerene–fullerene interaction in films prepared by various methods. 

The fullerene films electrodeposited onto the transparent electrode coated with 

tin oxide nanoparticles show photocurrent generation in electrochemical photocurrent 

measurements. In the experiments, the fullerene film is immersed in an 

acetonitrile electrolyte with iodine/iodide redox-pair [348]. The current–voltage 

characteristics for this system established a photodiode behavior. The photocurrent 

generation in this system is most likely due to photogalvanic type of behavior. This 

involves electron injection from the iodine/iodide pair to photoexcited fullerene. 

The generated fullerene anion transfers electron to SnO 2 nanoparticle. 

For nonlinear optics, fullerenes and fullerene derivatives have shown good 

responses, for example, as optical limiting materials. Usually, for this purpose 

fullerenes are dispersed into a solid polymer [376] or phosphate glass [377] matrix. 

Also, sol-gels of fullerene derivatives can be prepared [378]. By this manner one 

may have films with high concentration of isolated fullerenes. The optical power 

limiting is a nonlinear optical phenomenon where the optical absorption increases 

while incident light intensity increases [379]. As seen in the inset in Figure 5.50, 

isolated C 60 molecules in solution have low steady-state absorption in the visible 

range. C 60 at its excited singlet or triplet state has considerably higher molar absorption 

coefficient in the 300–700 nm range than at the ground state [380]. The 

absorption properties result in reverse saturable absorption characteristics for 

fullerene, and thus make it a suitable material for optical limiting applications. 

Since reversible changes in optical transparency are based on fullerene excited 

state absorption, a requirement for an efficient operation is the long lifetime of the 

excited state. Actually, fullerene aggregation causes self-quenching of the excited 

states and thus restricts the optical limiting time to sub-nanosecond level. The 

desire for longer working times directs the interest towards solid films where 

fullerenes are not aggregated but are isolated from each other. 

High temperature superconductivity has been observed for alkali metal fullerides 

[334]. The critical temperature of 33 K can be observed for alkali fulleride 

salts with stoichiometries A 3 C 60 , discussed in Section 5.3.3.2. The salt is formed 

by refluxing fullerene and alkali metal under an oxygen free atmosphere, and 

electronegative fullerenes oxidize alkali metal atoms. The superconductive 

phase of the resulted salt adopts, in general, face-centered-cubic or primitive 

cubic structure. Essentially complete electron transfer from electropositive alkali 

metal to electronegative fullerene fills the fullerene conduction band, the lowest 

unoccupied molecular orbital, halfway. Fullerene films with superconductive 

character have been prepared also by doping fullerene LB films with potassium 

[335]. After a fullerene multilayer LB film has been deposited it has been doped 

281


with potassium in high vacuum at elevated temperature. The potassium doped 

film had superconducting transition at approximately 8 K. 

5.4.3 

Conclusions 

A vast amount of research has been reported in scientific journals, and review 

articles in journals and books, about fullerene films, their functions, and future 

prospects. The fullerenes have not yet been – and probably never will be – used as 

tiny bearings for nanosized machines, although it was seen as a future application 

a long time ago. But the fullerenes have shown to be excellent material for organic 

solar cells and other applications related to producing and storing electrical energy. 

In addition, some applications can be found from the area of optical devices. Thus 

far no commercial application of fullerene has been introduced. There have been, 

and there are, many potential trials to find a fullerene application useful for the 

human race. The enthusiastic research on fullerenes has opened already markets 

for mass production of these tiny and fascinating carbon balls. 

5.5 

Endohedral Fullerenes with Neutral Atoms and Molecules 


5.5.1 

Introduction 

The inside cavity of a fullerene (3.5 Å in diameter for C 60 ) is large enough to 

enclose an atom or a very small molecule. This section describes the endohedral 

fullerenes enclosing nonmetal substrates inside the cages. Unlike metallofullerenes, 

in which an electron transfer takes place from the metal to the fullerene 

cage, these guests are bound to the cage only by weak van der Waals’ forces and 

by the structural impossibility of escaping. Consequently, there are few changes 

in properties of the host fullerenes. Such inert guests have served as valuable 

probes for exploring both the interior and exterior of fullerenes. Also, fullerene 

containers can keep highly reactive chemical species that are unable to exist in 

an ordinary system. Synthetic procedures and properties of the products will be 

presented along with theoretical studies. 

5.5.2 

Preparation 

5.5.2.1 Direct Approach Using an Existing Fullerene 

Since He@C 60 (C 60 with a helium atom inside) was discovered in a fullerene soot 

[381], continuous efforts have been paid to the syntheses of such gas complexes

5.5 Endohedral Fullerenes with Neutral Atoms and Molecules 

Scheme 5.9 Direct approach to the endohedral nonmetal complexes of fullerenes. 

with high yields. The first successful methodology was the forced penetration of 

a guest into an existing fullerene. This is different from the case of metallofullerene 

in which a metal fragment is captured in a cage during the formation of a 

fullerene framework. (a) High-pressure and high-temperature encapsulations; 

(b) ion beam implantations; and (c) nuclear reactions have been developed to 

achieve such incorporations (Scheme 5.9). 

The high-pressure and high-temperature incorporations are generally carried 

out by heating a pristine fullerene at 650 °C under 300 MPa of a desired gas using 

a hydrostatic high-pressure vessel [382, 383]. A series of noble gas complexes, from 

He@C 60 to Xe@C 60 , have been synthesized by this method [383–389]. Fractions of 

the endohedral complexes in recovered fullerenes were 0.03–0.1%. Trace amounts 

of complexes containing two helium or neon atoms inside (He 2 @C n and Ne 2 @C n , 

n = 60, 70) were also detected by 3 He NMR and/or mass spectrometry [382, 389]. 

Chemical bonding between two atoms confined in a fullerene cage have been 

investigated in theory [390]. Similarly, formation of some diatomic molecule 

complexes with diatomic molecule quests, such as 13 CO@C 60 , N 2 @C 60 , and N 2 @ 

C 70 , was confirmed by mass spectrometry [391]. Fractions of these products were 

estimated to be in the range of 0.02–0.05%. The reaction is applicable to higher 

or mixed fullerenes. 

Ion beam implantations have been mainly employed for the syntheses of group 

V atomic complexes, such as N@C 60 [382, 392]. Commonly, low energetic ions 

are provided to a continuously growing fullerene film. The amounts of fullerenes 

recovered were 10–20%, because the fullerenes employed decomposed. Fractions 

of the desired endohedral complexes in these portions were in the order of 0.01%. 

At present, successful examples are limited to N@C 60 , N@C 70 , P@C 60 , and 

N 2 @C 60 [393–396]. It has been suggested that an atomic complex with a higher 

fullerene cage would be unstable because decreased curvature of the cage might 

increase its reactivity toward a substrate inside [392, 394]. Also, the incorporation 

of arsenic has been attempted without success [392, 394]. 

Though the smallest 1 H@C 60 has not been synthesized so far by the above 

mentioned methods, its isotopic and radioactive homolog, 3 H@C 60 ( 3 H = T, 

tritium), has been synthesized by the nuclear reaction of 6 Li with C 60 or neutron 

irradiation on the 3 He atom in 3 He@C 60 [17, 382, 383]. Counting a soluble portion 

in a scintillation counter showed the presence of tritium. Other radioactive 

complexes, such as 41 Ar, 85 Kr, and 133 Xe@C n (n = 60 or 70), were synthesized by 

neutron irradiations or ion implantation [398–400]. 

283


Table 5.5 Separation factors in HPLC and 13 C NMR chemical shifts (in ppm) of X@C 60 . 

Separation 

factor 

in HPLC a) 

X@C 60 

13 C NMR, � �� b) 

Refs. 

In all cases, fractions of the desired complexes in recovered fullerenes are less 

than 0.5%. Despite such extremely low content, Ar@C 60 [384, 385], Kr@C 60 [386], 

Xe@C 60 [388], N@C 60 [393, 395], and N 2 @C 60 [396] were enriched to the analytical 

level by high performance liquid chromatography (HPLC). Multistage purification 

is essential because of poor separation factors (see retention times relative to that 

of C 60 collected in Table 5.5) in addition to low fractions. 

5.5.2.2 Molecular Surgery Approach via an Open-cage Fullerene 

Since the carbon–carbon bond of C 60 was cleaved to give a hole-opened, so-called 

an open-cage fullerene 51 (Scheme 5.10) [401], an alternative strategy has been 

proposed as a rational approach to obtain the desired endohedral complex at a 

high incorporation level [402]. It consists of the following three stages: (1) chemical 

cage scission to make an opening, (2) insertion of a guest through the opening, 

and then (3) closing the cage with the guest inside. He@51 prepared from He@ 

C 60 revealed that an opening formed by one bond scission is too small to permit 

passage even of the smallest helium atom. Then, larger openings in 52–56 were 

created by regioselective and multiple bond scissions (Scheme 5.10) [403–413]. 

Among successful molecular incorporations into these derivatives (described in 

Section 5.5.2.3), a pure and macroscopic quantity of H 2 @C 60 was recently synthesized 

from C 60 through 53 (Scheme 5.11) [406–408]. First, the 100% encapsulation 

of a hydrogen molecule into 53 was achieved at 80 MPa of H 2 and at 200 °C [406]. 

Then, closure of the opening in H 2 @53 was carried out with a trapped H 2 in 

four steps to afford H 2 @C 60 [407, 408]. Although slight escape of the trapped H 2 

had taken place during the closure, a rich fraction of H 2 @C 60 , higher than 90%, 

permitted its complete separation by using the repeated HPLC system, despite 

an extremely low separation factor (Table 5.5). 

C 60 

Ar@C 60 1.04–1.05 143.4 143.2 c) 

Kr@C 60 1.09 143.6 143.2 d) 

Xe@C 60 1.08 144.5 143.5 c) 

+0.2 [384, 385] 

+0.4 [386] 

+1.0 [388] 

N@C 60 1.06–1.08 – – – [393, 395] 

N 2 @C 60 1.10–1.13 – – – [396] 

H 2 @C 60 1.01 142.84 142.76 e) 

a) 

Retention time relative to that of C60 . 

b) 

��(X@C60 )–�(C60 ). 

c) 

in C6D6. d) 

in C6D6/C6H6. e) 

in o-dichlorobenzene-d4. 

+0.08 [407, 408]


Scheme 5.10 Molecular structures of open-cage C 60 derivatives 51–56. 

Scheme 5.11 Synthesis of H 2 @C 60 following the molecular surgery approach. 

5.5.2.3 Open-cage Fullerenes, Reversible Molecular Incorporations and Ejections 

Though successful restoration of the opening in open-cage fullerenes has been 

limited to H 2 @53, several molecular encapsulations have been performed using 

52–56. The first successful examples were incorporations of helium and hydrogen 

gas (H 2 ) into 52 [404]. Encapsulation of helium was achieved by heating solid 52 

at 305 °C under 48 MPa of helium (Table 5.6). The fraction of He@52 was 1.5%. 

Compound 53 has a larger opening than that of 52, and the same encapsulation 

could be done below 2 MPa [405]. Despite easy encapsulation, however, the fraction 

of He@53 was nearly the same as that of He@52. This is because molecular incorporation 

is a reversible and equilibrium process, and a larger opening allows 

285


Table 5.6 Endohedral He and H 2 complexes of open-cage C 60 derivatives: reaction conditions, 

fractions, energy barriers to escape (kcal mol –1 ), and NMR chemical shifts of the guests 

(in ppm). 

Condition Fraction 

(%) 

Barrier to escape 

exp. (theory) 

3 He/ 1 H 

NMR a) 

Refs. 

3 He@C60 – – – – –6.40 [38] 

3 He@2 48 MPa, ~305 °C 1.5 24.6 (24.3) –10.10 [404] 

3 He@3 2 MPa, 80 °C 1.5 22.8 (18.6) –11.86 [405] 

H 2 @C 60 – – – – –1.44 [408] 

H 2 @2 10 MPa, 400 °C 5 – (40.0) –5.43 [404] 

H 2 @3 81 MPa, 200 °C 100 34.3 (28.7) –7.25 [406] 

H 2 @4 13.5 MPa, 100 °C 83 21.7 (19.8) –7.34 [410] 

a) 3 He signals NMR relative to dissolved 3 He gas, 1 H signals NMR relative to TMS. 

not only for encapsulation but also rapid escape. Actually, the trapped helium 

atom can be released from the cage by heating. The activation energies for the 

helium escape from He@52 and He@53 were estimated to be 24.6 kcal mol –1 

and 22.8 kcal mol –1 , respectively, in accordance with the size of the openings in 

52 and 53 (Table 5.6) [404, 405]. Easy escape also suggests that there is negligible 

binding energy between the trapped helium and the fullerene cage. 

Encapsulation of molecular hydrogen has been achieved using 52–54 [404, 

406, 409, 410]. H 2 @52 was obtained in 5% fraction by heating solid 52 at 400 °C 

under 10 MPa of H 2 (Table 5.5) [404]. Compound 53 allowed for 100% H 2 encapsulation 

at 80 MPa of H 2 and at 200 °C as mentioned earlier [406]. Compound 

54 has a larger opening than those of 52 and 53, and allowed entry of H 2 under 

mild conditions [410]. The trapped H 2 can be released by heating, as in the case 

of helium complexes. Energy barriers of the H 2 escape from H 2 @53 and H 2 @54 

were estimated to be 34.3 kcal mol –1 and 21.7 kcal mol –1 , respectively (Table 5.6) 

[406, 410]. Recently, an useful index was reported for the evaluation of effective 

areas of these openings bearing different shapes and functional groups [409]. 

Among the open-cage derivatives of fullerenes, compound 55 has the largest 

opening at present. Although endohedral fullerene complexes bearing a di- or 

tri-atomic molecule inside are still limited [391, 396, 407, 408], compound 55 

can hold one water or a carbon monoxide molecule inside the cage [411–413]. 

Unlike other entries, a water molecule is incorporated into 55 spontaneously 

[411]. H 2 O@55 was obtained as a mixture with the empty 55 in the product. In 

solution, the trapped water molecule in H 2 O@55 is in rapid equilibrium with 

residual water in the solvent. The fraction of H 2 O@55 was about 75% in commercial 

CDCl 3 , but decreased with decline of the water content in the solvent.


Also, in the 1 H NMR spectrum the signal of H 2O in H 2O@55 disappeared rapidly 

after treatment with D 2O. The inclusion property can be refined by the size and 

structure of the opening. Compound 56 has a similar but smaller opening than 

that in 55. The fraction of H 2O@56 was less than 10% under conditions identical 

to those in which H 2O@55 was obtained, but could be improved to 85% by 

refluxing in a mixture of toluene and water, indicating higher activation energies 

of both incorporation and escape [412]. Functional groups around the opening 

also affected the inclusion property of a water molecule. 

Carbon monoxide was incorporated into 55 by heating a solution of a mixture 

of H 2O@55 and 55 in the presence of CO [413]. The fraction of CO@55 reached 

84% under 9.0 MPa of CO and at 100 °C. CO@55 gradually released the trapped 

CO and reverted to a mixture of 55 and H 2O@55 under ambient conditions. This 

is in contrast to the spontaneous H 2O encapsulation into 55, indicating that H 2O 

binds more tightly than does CO. 

5.5.3 

Properties 

5.5.3.1 Host Fullerenes 

Basically, complete endohedral fullerenes are stable under ambient conditions. No 

decomposition was observed on H 2 @C 60 at 500 °C [407, 408]. Thermal escape of 

helium and neon from He@C 60 or Ne@C 60 only took place above 600 °C [381–383, 

414]. Since the energy required to escape from the pristine cage is too high to 

produce it thermally, it has been proposed that one or two fullerene bonds are 

reversibly cleaved to form a temporary window to allow the guest release. On the 

other hand, escape of the nitrogen atom in N@C 60 took place at a relatively low 

temperature (260 °C) [392, 394]. A different pathway through the insertion of the 

trapped nitrogen atom into a fullerene bond has been proposed to account for 

the difference. Endohedral open-cage complexes release the guests by heating, 

as described earlier. 

There are only weak van der Waals’ interactions between the guests and the 

fullerene cages. The 13 C NMR chemical shifts of the X@C 60 cages (X = guest) 

are representatives as well as poor separation factors in HPLC (Table 5.5). In all 

cases, the signals appeared downfield relative to that of the empty C 60 cage. The 

shift increased with the size of the guest but was less than 1 ppm even for the 

maximum case of Xe@C 60 . 

Slight changes were observed in the IR and UV/Vis spectra of X@C 60 as 

shown below, but further study is necessary for systematic considerations. In 

the IR spectrum of Kr@C 60 , three of the four well-known C 60 bands (528, 1183, 

and 1429 cm –1 ) shifted by 8–16 cm –1 to higher frequencies [386]. The remaining 

absorption band at 577 cm –1 became significantly weak, but the reason for this 

observation is not clear. The IR spectrum of Ar@C 60 also showed small (2–4 cm –1 ) 

shifts below 600 cm –1 in the same direction, but opposite shifts were observed on 

other higher frequency bands [385]. For H 2 @C 60 , only a small shift was observed 

on the absorption band at 577 cm –1 [408]. The UV/Vis spectrum of Kr@C 60 

287


showed red shifts by 1–3 nm in the range of 580–640 nm [386], but no detectable 

change was observed on either Ar@C 60 or H 2@C 60 [385, 408]. N@C 60 displays 

a yellowish-brown color in solution, unlike the magenta color of C 60 [395]. Accordingly, 

broad absorption bands of C 60 in the range of 440–640 nm are notably 

suppressed in the case of N@C 60. 

The chemical reactivity of the fullerene cage is not affected by such a guest, as 

expected. Organic reactions developed for fullerenes are reproducible under the 

same conditions [383, 415, 416]. The availability of the products will be shown in 

the next paragraph. 

5.5.3.2 Guest Substrates 

In contrast to the insensitivity of host fullerenes, guests are quite sensitive to the 

cage size, electronic state, and organic addends of the hosts. This property has 

enabled their valuable applications as chemical probes and markers. The smallest 

helium atom has played a major role in NMR studies [382, 338]. The abundant 4 He 

is not an NMR active nucleus, but isotopic 3 He has a spin of I = 1/2 and exhibits 

a good NMR sensitivity. Generally, signals of the trapped atoms and molecules 

shift upfield due to magnetic shielding by the fullerene cage. Helium-3 in 3 He@ 

C60 and 3 He@C70 appeared at � = –6.4 and –28.8 ppm, respectively, relative to the 

dissolved 3 He gas [383, 417]. Chemical shifts of the 3 He atoms trapped in higher 

fullerenes vary in the range between those of 3 He@C60 and 3 He@C70 . Higher 

fullerenes often have several isomeric structures with different symmetries, and 

each shows multiple signals in the 13 C NMR spectra. Also, multiple addition 

reactions on C60 generally produce an inseparable mixture of up to 8 regioisomers, 

and each adduct shows multiple signals in 13 C NMR due to a lowering of 

molecular symmetry. In contrast, a trapped 3 He atom in any structure shows only 

one single line in the 3 He NMR spectrum. Thus, 3 He complexes can be used as 

markers in the analyses of mixtures of higher fullerenes and multiple adducts 

of fullerenes [383, 417, 418]. Their use for studying structures of hydrogenated 

fullerenes will be discussed in Section 5.6. 

Organic addends on 3 He@C60 generally produce additional upfield shifts [383]. 

A [5,6]-opened homofullerene structure retains the original 60 � elelectronic structure, 

and the internal 3 He atom shows only a small shift (�� = –0.2 ppm) (Table 5.7). 

On the other hand, a [6,6]-closed structure accompanies a loss of �-electrons, and 

the trapped He atoms show much larger shifts (�� = –1.8 – –3.2 ppm) than those in 

the former [5,6]-opened structure. Adding electrons to 3 H@C60 produces a drastic 

6– 

upfield shift [417]. The hexa-anion, He@C60 showed the signal at � = –48.7 ppm 

corresponding to �� = –42.4 ppm relative to that of He@C60 . It should be noted 

that He@C70 shows opposite downfield shifts by both organic addends and electronic 

reductions. For example, 3 6– 

He@C70 showed the signal at � = +8.3 ppm, 

which corresponded to �� = +37.1 ppm relative to He@C70 . 

Recent success in the macroscopic synthesis of H2@C60 has allowed measurement 

of the most familiar 1 H NMR spectra. The trends observed were the same 

as in the case of 3 He@C60 . The trapped H2 molecule in H2@C60 appeared at 

� = –1.44 ppm, which corresponded to �� = –5.97 ppm relative to the dissolved H2


Table 5.7 3 He and 1 H NMR chemical shifts (in ppm) of the functionalized endohedral 

fullerenes with 3 He atom or H 2 molecule inside [383, 408]. 

3 He NMR –6.40 –6.63 –8.11 –8.11 –9.45 

1 H NMR a) 

a) Relative to dissolved H2 gas (not TMS). 

–5.97 – – –7.80 –9.17 

gas (Table 5.7) [407, 408]. Organic addends induced additional upfield shifts [407, 

408, 416]. The relaxation time (T 1 ) of H 2 in H 2 @C 60 was in the range of 0.04–0.12 s, 

10–20 times shorter than that of the dissolved H 2 (0.84–1.44 s) [419]. 

With respect to the guests in open-cage C 60 derivatives, creating an opening 

on the C 60 surface induces an upfield shift similar to those observed in organic 

addends. Shifts become larger with increase in the size of the openings in both 3 He 

and 1 H NMR. 3 He@52 and 3 He@53 showed upfield shifts of �� = –3.70 ppm and 

�� = –5.46 ppm, respectively, relative to 3 He@C 60 [404, 405] (Table 5.6). Similarly, 

H 2 @52–54 showed upfield shifts of �� = –3.99 ppm (H 2 @52), �� = –5.81 ppm 

(H 2 @53), and �� = –5.90 ppm (H 2 @54) relative to H 2 @C 60 [404, 406, 408, 410]. 

The proton signals of water molecule in H 2 O@55 appeared at � = –11.4 ppm in 

the 1 H NMR spectrum [411]. Although H 2 O@C 60 has not still been synthesized, it 

corresponds to the upfield shift by �� = –13 ppm relative to the residual water in 

the solvent (� = 1.6 ppm). The signal was a sharp singlet indicating free rotation of 

the trapped H 2 O in the C 60 cage. The carbon signal of the trapped CO in CO@55 

appeared at � = 174.6 ppm shifted by �� = –10 ppm relative to the dissolved free 

CO gas (� = 184.6 ppm) [413]. The signal appeared as a singlet line even at –80 °C, 

indicating that CO rotates rapidly on the NMR time scale. In the IR spectrum, 

in contrast, two CO absorption bands were observed at � = 2125 and 2112 cm –1 . 

These are shifted by –18 and –31 cm –1 to lower frequencies, respectively, from 

the CO gas frequency (� = 2143 cm –1 ). The observed difference between NMR 

and IR can be interpreted in terms of the difference in the time scale of these 

measurements. 

As for N@C 60 , atomic nitrogen has unpaired electrons and is paramagnetic. The 

excellent sensitivity of EPR (electron paramagnetic resonance) spectroscopy has 

allowed the detection and analysis of N@C 60 even at an extremely low fraction. 

289


The ESR spectrum of N@C60 shows three lines originating from the hyperfine 

inter action of the electron spin with the nuclear spin of 14 N (I = 1) [392–395], 

indicating that the trapped nitrogen atom keeps its quartet spin ground state 

configuration without an electron transfer to the cage. Regarding cage size, 

electronic state, and organic addends, trends observed were the same as in the 

cases of 3 He and 1 H NMR. Compared with N@C60, the central line of the triplet 

of N@C70 shifted upfield by 0.0065 mT in the magnetic field corresponding to 

6– 

–19 ppm [392]. The hexa-anion N@C60 showed an upfield shift by –35 ppm 

6– 

relative to N@C60, but N@C70 did a downfield shift by +41 ppm relative to N@ 

C70. Organic addends also caused an upfield shift for N@C60 but a downfield 

shift for N@C70 [392]. 

5.5.4 

Binding Energies, Theoretical Investigations 

To maximize yield of the endohedral fullerenes, and to construct an efficient host 

system using open-cage fullerene derivatives, it is important to understand the 

interaction between the guest and the fullerene cage. Generally, trapped guests 

have been believed to have attractive interactions with the fullerene cage, because 

molecular incorporation is an entropically disfavored process. Various theoretical 

studies have been carried out along with synthetic experiments [420–427]. 

The binding energies of noble gas complexes of C 60 were estimated by ab 

initio and DFT (density functional theory) calculations. At the MP2 (secondorder 

Møller–Plesset perturbation) theory, a series of noble gas complexes, from 

He@C 60 to Xe@C 60 , were computed to have stabilization energies of –0.3 to 

–7.5 kcal mol –1 , indicating attractive interactions in all cases [421, 422]. In contrast, 

the frequently used B3LYP (Becke three-parameter exchange functional coupled 

with the Lee-Yang-Parr correction) functional in DFT studies generally underestimates 

weak molecular interaction and predicts repulsive destabilization [421, 

423]. For example, H 2 @C 60 was estimated to be unstable by +1.6 kcal mol –1 at 

the B3LYP/6-31G(d,p) level, whereas it was stabilized by –4.0 kcal mol –1 at the 

MP2/6-31G(d,p) [423, 424]. Recently, the MPWB1K functional (modified Perdew- 

Wang and Becke functionals) of DFT was reported to give results consistent with 

the MP2 calculations (–7.9 kcal mol –1 for H 2 @C 60 ) [424]. In a real experiment, 

however, the DSC (differential scanning calorimetry) measurement for the H 2 

escape from the open-cage H 2 @54 suggested that the trapped H 2 destabilizes 

the cage [410]. Also, simple molecular mechanics and semiempirical quantum 

calculations have been discussed over a variety of endohedral complexes including 

multiple guest systems [425–427]. The possibility of housing more than one H 2 

molecule in the C 60 and higher fullerene cages has been examined by Dodziuk 

[426]. Further experiments with a variety of guests are needed to reach reliable 

and systematic conclusions.

5.5.5 

Summary 

5.6 Hydrogenated Fullerenes 

Endohedral nonmetal complexes of fullerenes allow us to study the inside of the 

fullerene sphere. The properties of host fullerenes scarcely change in response 

to the neutral guest substrates. In contrast, guests are quite sensitive to the host 

size, structure, electronic state, and organic addends, including chemical openings. 

Along with first generations produced by forced gas penetrations, new endohedral 

complexes are beginning to be synthesized by the molecular surgery method, 

which will continue to expand the library of endohedral fullerenes in macroscopic 

quantities and at high incorporation levels, leading to practical applications. 

5.6 

Hydrogenated Fullerenes 

Mark S. Meier 

Fullerenes are among the most highly strained aromatic compounds known, approaching 

500 kcal mol –1 in strain energy. Pyramidalization of sp 2 carbon atoms, 

by ~8° – 12° in fullerenes, is required to form the closed shells, but also results 

in a degree of rehybridization [428] that diminishes overlap in the �-system. The 

pyramidalization induces strain, and in turn that strain introduces reactivity 

[429] not normally seen in other aromatic systems. The interplay between strain, 

aromaticity [430], thermodynamics, and kinetics leads to a very rich chemistry, 

producing some beautiful structures in situations where one might expect intractable 

mixtures of products. Fullerene reactions are also discussed in more 

general terms in Section 5.2. 

The hydrogenation of fullerenes [431] provides a clear context for the study of 

fullerene reactivity. The continuous series of C 60H 2n, with n ranging (at least in 

principle) from 1 to 30, covers a molecular formula range from C 60 to C 60H 60. 

Hydrogen (followed closely by fluorine) is the least bulky covalent addend that 

may be bonded to a fullerene, and hence provides an excellent probe for revealing 

the reactivity of the fullerene core without interference from steric effects. 

In practice, only a handful of the 30 different C 60H 2n molecular formulae are 

formed in the course of most hydrogenations of C 60. This result is striking in its 

own right, but the selectivity for a limited number of isomers, when vast numbers 

are possible in most cases, is truly amazing. 

5.6.1 

Synthesis and Structure 

The simplest of the hydrogenated fullerenes is C 60 H 2 . This compound has been 

prepared by a long list of methods, including methods as diverse as hydroboration 

[432], electrochemical reduction/protonation [433–435], dissolving metal 

291


Figure 5.51 C 60 H 2 (57), shown in a 3-dimensional view and in a planar projection 

(Schlegel diagram). 

reduction/protonation [436], borohydride reduction [437], hydrozirconation [438], 

thermal [437] and sonochemical [439] transfer hydrogenation, direct hydrogenation 

with metal catalysts [440–444], and with diimide [440]. This compound has 

been made indirectly through decomposition of a Si-bridged C 60 dimer [445], as 

well as by hydrolysis of acylfullerenes [446], and it has been detected in fullereneproducing 

flames [447]. 

While a considerable number of isomers of C 60 H 2 are possible [448, 449], 

only one has been prepared, purified, and fully characterized. The 1,2-isomer 

(57, Figure 5.51) appears to be the most thermodynamically stable isomer, and 

given the high acidity of fullerene C–H bonds [450, 451], it seems likely that any 

unstable isomers that may be formed would be transient and easily undergo 

isomerization. 

C 60 H 2 forms a brown solution in toluene, strikingly different from the magenta 

color of C 60 solutions. The 1 H NMR spectrum is quite simple – a singlet at 

5.12 ppm (CS 2 solution). The significant downfield shift of this resonance is likely 

caused by large ring currents in the remaining �-electron system [452]. The 13 C 

NMR spectrum is consistent with the C s -symmetry, with a single sp 3 resonance 

at 54.2 ppm [453]. Notably, the 1 H-coupled 13 C spectrum reveals the 2-bond H-C 

coupling constant (6.7 Hz), which is a useful indicator of a 1,2 arrangement of 

hydrogen atoms on the fullerene surface. 

Essentially any reagent that is capable of hydrogenating C 60 is likely to be capable 

of hydrogenating C 60 H 2 to C 60 H 4 or farther. An elegant study by Cahill produced 

and characterized several isomers of C 60 H 4 [454]. Hydroboration–protonolysis of 

C 60 produces a mixture of six isomers (Figure 5.52) of which two could be isolated 

and definitively identified, with 58a being the major product. A mixture of C 60 H 4 

compounds could be isomerized on platinum to the 1,2,3,4-isomer (cis-1) 58a, 

indicating that this not only the kinetic isomer but is likely the thermodynamically 

most stable isomer as well. This crucial experiment validates the computational 

prediction that the cis-1 isomer (58a) is one of the lowest energy structures studied. 

Dissolving metal reduction also produces C 60 H 4 , with 58b and 58c being identified 

in the Zn(Cu) reduction [455]. 

Dissolving metal reduction of C 60 using Zn(Cu) results in the formation of a 

major isomer 59 (trans-3, trans-3) and 2 minor isomers [455]. The remarkably 

simple 1 H (singlet) and 13 C NMR spectra (10 resonances) indicates a structure

Figure 5.52 Isomers of C 60 H 4 . 

Figure 5.53 C 60 H 6 (59). 


with D 3 -symmetry, and the measured 6.8 Hz 2-bond H–C–C coupling constant 

(indicating a 1,2-arrangement of hydrogens in 3 equivalent sets) permits assignment 

of the structure (Figure 5.53). 

Chromatography suggests that a second, minor isomer is formed along with 

59, and investigation of the 3 He NMR spectra of 3 HeC 60 H 4 proves that there are at 

least two minor isomers present in the Zn(Cu) reduction [456], although the structures 

have not been determined. Clearly the major isomer 59 does not result from 

hydrogenation of the most stable isomer of C 60 H 4 (58a) but from one of the less 

stable ones (58b). Computational work [457] has suggested that most stable C 60 H 6 

structures include one related to the thermodynamic isomer of C 60 H 4 (58a) and 

one that is analogous to the C 60 Cl 6 structure [458]. This result highlights the fact 

that fullerene chemistry is a kinetic world not always ruled by thermodynamics. 

Beyond C 60 H 6 lies a homologous series of compounds (C 60 H 8 , C 60 H 10 , C 60 H 12 , 

etc.) in which no one species emerges as significantly more stable than others 

in the immediate neighborhood. Accordingly, to date none of these species has 

been isolated in good yield in pure form. The remarkable exceptions to this rule 

are C 60 H 18 and C 60 H 36 . The fact that specific isomers of these two molecular 

formulae appear as islands in a vast sea of possible formulae and isomers is one 

of the most startling results in fullerene chemistry. 

293


Figure 5.54 C 3v -C 60 H 18 60. Left and middle: bottom and side views; right: planar projection. 

C 60H 18, like C 60H 36 (below), is more stable than the nearby homologs (C 60H 16, 

C 60H 20, etc), appearing as a significant product in the Birch reduction [459], 

transfer hydrogenation [460], and in the Zn/HCl reduction [461] of C 60. These 

reactions could produce every adjacent oxidation state, but C 60H 18 is formed to 

the near exclusion of the adjacent formulae. This compound is formed as a C 3v 

‘turtle shell’ isomer 60 [462]. This structure contains a nearly planar benzene ring 

at the base (Figure 5.54). 

A wide array of reaction conditions lead to this same C 3v isomer of C 60 H 18 . 

These methods include reduction of C 60 with amines [463], hydrogenation [464, 

465], and dehydrogenation of C 60 H 36 [466]. 

Reduction of C 60 with lithium in ammonia containing t-BuOH (Birch reduction) 

results in the formation of C 60 H 36 [459]. This result is striking, as there are 

numerous adjacent oxidation states (… C 60 H 34 , C 60 H 38 , etc) that might be formed, 

but C 60 H 36 is formed as a major product to the near exclusion of other products. 

Other reagents also lead to C 60 H 36 , including transfer hydrogenation [467, 468], 

reduction by Zn metal in acidic solution [469], and catalytic hydrogenation 

[441]. 

The number of possible isomers for C 60 H 36 is truly staggering, exceeding 10 14 

by one report [448]. A number of methods for preparation of this compound have 

been reported [441, 459, 460, 467, 469, 470], but most result in complex mixtures 

of species from which C 60 H 36 must be isolated. Transfer hydrogenation [460, 467, 

468, 471] using 9,10-dihydroanthracene produces a mixture of three isomers, 

with a C 1 isomer 61a being strongly dominant, although a smaller amount of a 

C 3 isomer 61b and a trace of T isomer 61c are observed [466]. Detailed NMR work 

on this mixture has led to the structural assignments shown (Figure 5.55). 

It is believed that the formation of this limited array of structures, out of the 

huge number possible, is the result of rapid isomerization under the conditions 

of transfer hydrogenation. Further heating of this mixture results in conversion 

of the T and C 1 isomers to the C 3 isomer, supporting both the notion of thermal 

isomerization as well as supporting calculations that suggest that the C 3 isomer 

is the most stable. 

Reduction of C 60 H 36 with lithium in diethylamine leads to a mixture of highly hydrogenated 

products, composed of C 60 H 38 though C 60 H 44 [472]. These compounds 

were detected by MS, but despite extensive HPLC work no one compound could 

be isolated. 3 He NMR spectra of the Li/diethylamine reduction of He@C 60 H 36

Figure 5.55 C 60 H 36 structures. 


confirms that numerous different compounds are present. A small amount of 

C 60 H 48 was observed in the Zn/HCl reduction of C 60 , but again no structural assignment 

has been made [469]. Isolation and definitive structural assignment of 

any member of this class of compounds is a major challenge, as they are relatively 

insoluble (even among fullerenes) as well as fragile, decomposing in solution in 

air and light. 

The endpoint of hydrogenation of C 60 , C 60 H 60 , has not been isolated or characterized. 

This is not surprising, as the large number of eclipsing interactions 

would make an all-exo structure impossibly high in energy. Structures with some 

of the hydrogens on the interior would be significantly more stable [473, 474], but 

synthesis is a challenge. These structures are discussed in Section 2.4.7.1. 

5.6.2 

C 70 Chemistry 

The reactivity of C 70 is similar to that of C 60 and as expected, the less pyramidalized 

[429] carbons along the equator are typically less reactive than the more pyramidalized 

carbon at the poles. As is the case with the reduction of C 60 to C 60 H 2 , 

reduction of C 70 to C 70 H 2 can be accomplished by a variety of methods and has 

been demonstrated with many of the same reagents used with C 60 [440]. 

The first step in the hydrogenation of C 70 results in the formation of 1,2-C 70 H 2 

(62a, Figure 5.56), resulting from hydrogenation of the most highly strained part 

of the molecule [475, 476]. This regiochemistry is the common result for addition 

to one C-C bond in C 70 , being observed as the major product in Diels–Alder, 

Hirsch-Bingel, Prato, and other additions. The 2-bond H-C-C coupling constants 

(4.7–5.2 Hz) in 62a are somewhat smaller than the analogous coupling in C 60 H 2 . 

A smaller amount of the 5,6-isomer 62b is formed along with 62a, resulting from 

reduction of the second most strained site in C 70 . 

295


Figure 5.56 1,2-C 70 H 2 (6a), 5,6-C 70 H 2 (62b), and C 70 H 4 (63a). 

The second hydrogenation step is an interesting one, because if strain determines 

the regiochemistry of addition then at least five different isomers of C 70 H 4 

could result from the reduction of C 70 H 2 (62a), resulting from addition to any one 

of the remaining 6–6 ring fusions at the poles. In practice, the Zn(Cu) reduction 

of C 70 results in the formation of C 70 H 4 (63a) from C 70 H 2 (62a) [476]. 

The Zn(Cu) reduction of C 70 produces C 70 H 8 (64) as a major product (Figure 

5.57) [476]. This compound does not form by reduction of C 70 H 4 (63a), but 

instead from an ‘unseen’ pathway starting at C 70 and proceeding through a series 

of intermediates (presumably C 70 H 2-6 species) that are reactive and rapidly reduce 

again, until this relatively stable isomer is formed. At that point, 64 is relatively 

slow to reduce again, and hence it builds in concentration. 

Structure 64 is analogous to the structure of C 70 Cl 10 [477], but is unusual for the 

1,4-pattern of hydrogenation. A 1,2-pattern is found is virtually all other examples. 

The crowded 1 H NMR resonances are more upfield than seen in 62a and 63a, 

consistent with hydrogenation away from the poles. The 1,4-pattern is suggested 

from the absence of the familiar 2-bond C–H coupling constant, replaced instead 

by smaller coupling constants. 

Figure 5.57 C 70 H 8 (64).

Figure 5.58 Polar views of C 70 H 8 (64) and C 70 H 10 (65). 


Further reduction of 64 leads primarily to C 70H 10 (65), although two minor 

and unidentified isomers are also formed (Figure 5.58). The 13 C spectrum of 65 

indicates a plane of symmetry, and a typical 2-bond C–H coupling constant (5.6 Hz) 

appears again. This structure results from reduction of 64 in such a manner as to 

cause minimal disruption of the remaining aromatic system. 

As with C 60, powerful hydrogenating conditions result in highly hydrogenated 

compounds. With extended reaction times, the Zn(Cu) reduction produces 

material beyond C 70 H 10 , but numerous compounds result. The Zn/HCl reduction 

can also produce highly hydrogenated materials, 

Strongly reducing conditions (catalytic hydrogenation [441], Zn/HCl reduction, 

transfer hydrogenation [460, 468]) leads to the formation of C 70 H 36 , in a close 

parallel to the fate of C 60 under the same conditions. 

5.6.3 

Higher Fullerenes 

Reduction of C 76 (as a mixture of isomers) with Zn in HCl leads to a mixture 

of products, ranging from C 76 H 46 to C 76 H 50 . Likewise, treatment of C 78 and C 84 

(again, both as mixtures of isomers) under these conditions produces complex 

mixtures of products. The reduction of C 78 produces a spectrum of products, with 

C 78 H 36 being a major product accompanied by C 78 H 48 . 

Reduction of the higher fullerenes has been reported to be accompanied by a 

measure of breakdown of the fullerene cages, as evidenced by the observation 

C 60 H 36 and/or C 70 H 36 species [478]. 

5.6.4 

Reactivity of Hydrogenated Fullerenes 

Hydrogenation of fullerenes is an easily reversible process, with dehydrogenation 

back to the parent fullerene being brought about by reagents such as DDQ 

[459] and metal hydrogenation catalysts [479]. The most remarkable aspect of the 

chemistry of the higher fullerenes is the pronounced acidity of the C-H bonds. 

It is possible to deprotonate t-BuC 60 H with weak bases such as acetate, and the 

pK a of the fullerene C-H bond was estimated to be 5.7 [450]. In DMSO, the pK a 

of C 60 H 2 was determined to be 4.6, quite acidic for a C n H n compound [451]. 

297


The acidity of other hydrogenated fullerenes is pronounced [480, 481], and a 

prediction of 8.0 has been made for C 60H 6 [480]. 

In practice, deprotonation of hydrogenated fullerenes is a facile method for 

formation of fullerene anions. These anions react readily with electron deficient 

alkenes such as acrylonitriles to form aminocyclopentenes [482]. Treatment of 

C 60H 2 with weak bases and alkyl halides produces 1,2-monoalkyl- or 1,4-dialkyl 

fullerenes [483] although yields are mediocre, presumably because the anions 

undergo rapid oxidation by adventitious oxidants. Alkylation with alkyl tosylates 

is not effective, because fullerene anions typically react by electron transfer [484] 

rather than by a standard S n2 mechanism. 

Dialkylation of C 60H 2 with CH 3I results in a mixture of 1,2- and 1,4-dialkylation, 

which is consistent with 1,2-addition being nominally the most stable arrangement, 

but the crowding of addends destabilizes this arrangement, leading to the 

second alkylation step proceeding at an allylic position. 

Treatment of C 70H 2 (63a) with one equivalent of base and ethyl bromoacetate 

results in a 37 : 1 ratio of C-1 alkylation 66a to C-2 alkylation 66b (Figure 5.59) 

[485]. The selectivity here probably results from more rapid deprotonation at C-1, 

a reaction that probably produces a more stable, delocalized anion than would be 

produced by deprotonation at C-2. 

Figure 5.59 Monobenzyl C 70 from alkylation at C-1 (66a) and from alkylation at C-2 (66b). 

Treatment of C 70 H 2 (62a) with two equivalents of base and alkyl halide produces 

a mixture of products (Figure 5.60). While a trace amount of the 1,2-bibenzyl 

product 67a is obtained the dominant product, resulting from alkylation near the 

equator 67b, is obtained in only 10% yield [483]. 

2– 

Figure 5.60 Products of dibenzylation of C70.

5.7 

Applications of Fullerenes 

Rossimiriam Pereira de Freitas and Jean-François Nierengarten 

5.7.1 

Introduction 

5.7 Applications of Fullerenes 

When fullerenes were first discovered more than two decades ago [486], there was 

much excitement about possible applications for this new molecular material. In 

spite of initial speculation, the fullerenes could not be extensively studied until 

1990 when Krätschmer et al. [29] made C 60 available in macroscopic quantities. 

Since then, the unusual properties of this fascinating allotropic carbon form have 

been intensively investigated. Among the most spectacular findings, C 60 was 

found to behave like an electronegative molecule able to reversibly accept up to six 

electrons [487], to become a supraconductor in M 3 C 60 species (M = alkali metals) 

[488, 489] or to be an interesting material with nonlinear optical properties [490]. 

Although possessing exceptional properties, C 60 is difficult to handle because 

it forms aggregates and is insoluble or only sparingly soluble in most solvents 

[491]. This serious obstacle for practical applications can be overcome, at least in 

part, with the help of organic modification. Effectively, the recent developments 

in the functionalization of fullerenes allow the preparation of highly soluble C 60 

derivatives which are easier to handle, and whose electronic properties, such as 

facile multiple reducibility, optical nonlinearity or efficient photosensitization 

that are characteristic of the parent fullerene, are maintained for most of the C 60 

derivatives [492–497]. 

After several years of research, fullerene chemistry is now a well-established field 

and the knowledge acquired has revealed both potentials and limitations of this 

class of compounds. The present chapter illustrates some of the most promising 

applications for chemically modified fullerenes. The objective is not to present 

an exhaustive review but to describe some of the most illustrative examples in 

materials science and biology. 

5.7.2 

Applications in Materials Science 

5.7.2.1 C 60 Derivatives for Optical Limiting Applications 

Optical Limiting (OL) is a nonlinear phenomenon in which the absorption of a 

material increases when the incident radiation intensity increases. Materials or 

devices with transmission that decreases with light level are called optical limiters 

and can potentially be used to protect optical sensors, including the human eye, 

from dangerous laser beams. Recently much effort has been invested in the 

research of organic materials that can behave as nonlinear absorbers because 

they are, in general, easily integrated into optical devices. 

C 60 itself has been widely investigated for potential application in the field of 

optical limiting [490, 498, 499]. Effectively, the transmission of fullerene solutions 

299


decreases by increasing the light intensity. For short pulses (ps), the limiting action 

is ascribed to pure reverse saturable absorption (RSA), whereas for longer pulses 

(ns-µs) thermal effects are also invoked [500–505]. Even if fullerene solutions are 

efficient optical limiters, the use of solid devices is largely preferred for practical 

applications because they are easier to handle. Therefore, crystalline films of C60 have been studied, but found to be inefficient against pulses longer than tens of 

ps. This result is ascribed to a fast de-excitation of the laser-created excited state 

due to the interactions of neighboring C60 molecules in the solid phase [501]. In 

contrast, it has been shown that C60 keeps its limiting properties after inclusion in 

solid matrices such as sol–gel glasses [506–508], polymethylmethacrylate (PMMA) 

matrices [509] and glass–polymer composite samples [510]. As far as sol–gel glasses 

are concerned, special procedures have to be employed since good solvents for 

fullerenes are incompatible with the sol–gel process [500, 511–513]. Actually, incorporation 

of fullerenes is typically achieved by soaking mesoporous silica glasses 

with a solution of C60 [506–508]. Another efficient approach is to incorporate 

water-soluble C – 60 derivatives, compatible with the sol–gel process, directly into 

Figure 5.61 Transmission versus incident fluence at 532 nm of a sol-gel sample containing 

compound 68.


the sol [514]. For example, the water soluble methanofullerene 68 (Figure 5.61) 

has been synthesized [514] and successfully included in a sol–gel during the 

gelation process. The transmission of the sample at 532 nm as a function of the 

incoming laser fluence is shown in Figure 5.61. With increasing pulse energy, the 

transmission of the sample clearly decreases. The threshold for the onset of the 

limiting action is located at about 3 mJ cm –2 , a value comparable or even slightly 

lower than that obtained with inclusions of C 60 in sol–gel matrices [501]. The 

damage threshold of samples is about 200 mJ cm –2 . Up to this fluence, the effect 

is fully reversible. For higher values, cumulative damage of the C 60 molecules was 

observed in their glass environment. With the same pulse lengths, the threshold 

value is considerably smaller than that of materials showing simultaneous twophoton-absorption. 

Even among materials showing RSA, this value is quite good 

[501] and similar to that found in C 60 solutions, confirming the fact that C 60 keeps 

its favorable limiting properties even after chemical modification. 

However, for sol–gel glasses doped with plain C 60 or methanofullerene 68, faster 

de-excitation dynamics and reduced triplet yields were observed when compared 

with the solutions. The latter observations have been mainly explained by two 

factors: (1) perturbation of the molecular energy levels by the interactions with the 

sol–gel matrix and (2) interactions between neighboring fullerene spheres caused 

by aggregation [500, 514]. To prevent such undesirable effects, fullerodendrimers 

69–70 (Figure 5.62) in which the C 60 core is buried in the middle of a dendritic 

structure [514–519] were prepared and incorporated in sol–gel glasses by soaking 

mesoporous silica glasses with a solution of 69 and 70 [516]. 

Figure 5.62 Fullerodendrimers 69 and 70. 

301


Figure 5.63 Transmission versus incident fluence at 532 nm of a sol-gel sample containing 

compound 70. 

The resulting samples contain only well-dispersed fullerodendrimer molecules. 

Measurements on the resulting doped samples have revealed efficient optical 

limiting properties [516]. The transmission as a function of the fluence of the laser 

pulses is shown in Figure 5.63. It remains nearly constant for fluences lower than 

5 mJ cm –2 . When the intensity increases above this threshold, the effect of induced 

absorption appears, and the transmission diminishes rapidly, thus showing the 

potential of these materials for optical limiting applications. 

Stable sol–gel glasses have been also prepared from several fullerene derivatives 

containing both solubilizing chains and siloxane groups (Figure 5.64) [520, 521]. 

The optical-limiting properties of these derivatives have been investigated both 

in solution and in sol–gel glasses. By irradiation at 652 nm compounds 71–75 

gave better results than nonfunctionalized fullerene, while at 532 nm C 60 gave 

the best result. 

The photoinduced optical second-harmonic generation (PISHG) and the transmission 

versus the intensity of the laser beam have been studied in solution and 

in the solid state for a series of C 60 -tetrathiafulvalene (TTF) dyads (Figure 5.65) 

[522–524]. Compounds 76a–b and 77 revealed improved nonlinear optical (NLO) 

properties when compared with C 60 . Using molecular dynamics simulations and 

quantum chemical calculations, the authors have shown that an asymmetry in the 

excited states influences the NLO properties of these systems as a result of a photoinduced 

charge redistribution both on the intra- and intermolecular levels. 

The synthesis and electronic properties of a series of multiple C 60 terminated 

oligo(p-phenylene ethynylene) (OPE) hybrid compounds obtained by an in situ 

ethynylation method [525] was reported by Tour and coworkers (Figure 5.66). There 

was evidence of a synergistic interaction between the conjugated OPE backbone 

and fullerene. C 60 -OPE hybrid 77 presented an enhanced NLO performance 

relative to its OPE precursor; this behavior is presumably due to the occurrence 

of periconjugation and/or charge transfer effects in the excited state.

Figure 5.64 Fullerene derivatives containing siloxane groups. 

Figure 5.65 Fullerene-TTF dyads studied as optical limiters. 

Figure 5.66 Fullerene-OPE hybrid 78. 


303


Figure 5.67 Combination of phthalocyanine and fullerene moieties for optical limiting. 

In the optical limiting field, combination of phtalocyanine and fullerene moieties 

has been made by different researchers [526, 527] in order to provide improved 

performances through a synergistic effect. The optical limiting behavior of copper 

phthalocyanine-fullerene dyad 78 [526] (Figure 5.67) has been investigated in 

solution and as a nanoparticle dispersion using nanosecond laser pulses at 

532 nm. An enhanced optical limiting performance of the nanoparticle sample 

compared with that of the solution sample has been observed. The formation of 

ordered aggregates with a well-defined ‘face-to-face’ packing fashion is proposed 

to be responsible for the enhancement of the optical limiting performance of the 

nanoparticle sample. 

5.7.2.2 C 60 Derivatives for Photovoltaic Applications 

The interaction of C 60 with light has attracted considerable interest in the exploration 

of applications related to photophysical, photochemical and photoinduced 

charge transfer properties of [60]fullerene derivatives. Following the observation 

of ultrafast photoinduced electron transfer from �-conjugated polymers to C 60 

core [528] giving rise to long-lived charge-separated states [529], intensive research 

programs are focused on the use of fullerene derivatives acting as electron 

acceptors in organic solar cells. The development of these devices was stimulated 

by the inherent advantages of organic materials such as their low weight and cost, 

and by the possibility of fabricating large active surfaces thanks to their processability. 

After only ten years of studies, it is now clearly established that [60]fullerenebased 

materials are among the most important candidates for the expansion of 

plastic solar cells and for renewable sources of electrical energy [530]. 

Similarly to the photosynthesis process in plants, organic solar cells are based 

on absorption of light followed by cascades of energy and electron transfer events. 

Typically, organic solar cells are constituted of at least four distinct layers [531], 

not counting the substrate, which may be glass or some flexible, transparent 

polymer (Figure 5.68). On top of the substrate the cathode is laid. Indium tin oxide 

(ITO), is a popular cathodic material because it is transparent, and glass substrate 

coated with ITO is commercially available. The cathode is very often aluminum 

(calcium, magnesium, gold are also used). Inserted (or sandwiched) between the


Figure 5.68 Typical structure of an organic solar cell (a) and production of photocurrent (b). 

two electrodes, the photoactive layer is responsible for light absorption, exciton 

generation/dissociation and charge carrier diffusion. These heterojunctions are 

typically fabricated using p-type donor (D) and n-type acceptor (A) semiconductors. 

A layer of the conductive polymer mixture poly(3,4-ethylenedioxythiophene)/ 

poly(styrenesulfonate) (PEDOT-PSS) may be applied between the cathode and the 

active layer. The PEDOT-PSS layer serves several functions. Not only does it serve 

as a hole transporter and exciton blocker, but it also smooths out the ITO surface, 

seals the active layer from oxygen, and keeps cathode material from diffusing into 

the active layer, which can lead to unwanted trap sites. Under illumination, electron 

transfer from the donor to the acceptor and generation of excitons followed by 

charge separation and transport of carriers to the electrodes induces a photocurrent 

(see Figure 5.69). At present, thanks to the invention of N.S. Sariciftci and 

A.J. Heeger [532], one of the most used acceptors in heterojunction photovoltaic 

cells is plain C 60 or fullerene derivatives. 

From the theoretical point of view, the principal feature of the p–n heterojunction 

is the built-in potential at the interface between both materials presenting a 

difference of electronegativities [532]. In fact, the absorption of light induces the 

promotion of an electron from the highest occupied molecular orbital (HOMO) 

[or the valence band (VB)] of the donor to the lowest unoccupied molecular orbital 

(LUMO) [or the conducting band (CB)] of the acceptor, generating an exciton at 

the interface of the junction. Then the built-in potential and the associated difference 

in electronegativities of materials allow the exciton dissociation [533]. The 

charge separation occurs at D/A interfaces and free charge carriers are transported 

through semiconducting materials with the electron reaching the cathode (Al) 

and the hole reaching the anode (ITO). 

The first heterojunction with a conjugated polymer and C 60 was reported in 

1993 [534]. In this work, the device ITO/MEH-PPV C 60 /Au (or Al) was fabricated 

by sublimation of fullerene onto a MEH-PPV (poly[2-methoxy-5-(2’-ethylhexyloxy)- 

1,4-phenylenevinylene]) layer spin-coated on ITO-covered glass. This solar cell 

showed a relative high FF of 0.48 and a power conversion efficiency (PCE) of 

0.04% under monochromatic illumination. 

305


In such devices, the interaction between the electron donor and the electron 

acceptor materials is only effective at the interface, and is limited by the diffusion 

length of the exciton (near 20 nm maximum). As a consequence, low short-circuit 

photocurrent (Isc) values and conversion efficiency were obtained. In order to 

overcome these deficiencies, interpenetrating networks were developed as ideal 

photovoltaic materials for a high-efficiency photovoltaic conversion. A crucial and 

major breakthrough towards efficient organic devices was realized by Heeger, Wudl 

and coworkers with the development of the ‘bulk-heterojunction’ concept [535]; 

that is an interpenetrating network of a (p-type) donor conjugated polymer and 

C 60 or another fullerene derivative as (n-type) acceptor material. Consequently, the 

photoactive layer of these solar cells consists of blending the conjugated polymer 

and the fullerene derivative. 

The effective interaction between the donor and the acceptor compounds within 

these so-called ‘bulk-heterojunction’ solar cells can take place in the volume of the 

entire device. Subsequently, the separated charge carriers are transported to the 

electrodes via an interpenetrating network. A major shortcoming of this kind of 

device is the tendency, especially for pristine C 60 , to phase separate and then to 

crystallize. This aggregation phenomenon imposes important consequences on 

the solubility of C 60 within a conjugated polymer matrix. For that reason, intensive 

efforts on organic solar cells rapidly focused on interpenetrated networks of conjugated 

polymers with the 1-(3-methoxycarbonyl)propyl-1-1-phenyl-[6,6]methanofullerene 

80 ([60]PCBM) (Figure 5.69). This compound, initially synthesized 

by Wudl and coworkers [536], is more soluble in organic solvents than pristine 

C 60 . The first example of blend between MEH-PPV and [60]PCBM 80 exhibited a 

PCE of 2.9% [530], but under monochromatic low intensity light [535]. 

Figure 5.69 Structure of [60]PCBM 80 and some conjugated polymers commonly used for the 

preparation of organic solar cells.


Remarkably, the efficiency of the bulk heterojunction devices consisting of 

poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-p-phenylene vinylene (MDMO-PPV) and 

[60]PCBM was increased to 3% by the group of Sariciftci [537]. In the recent years, 

regioregular polyalkylthiophenes (PAT) which combine the potential advantages of 

a better photostability and a smaller bandgap than PPV derivatives, were the most 

widely used �-donor polymers associated with [60]PCBM. Bulk-heterojunction 

solar cells consisting of poly(3-hexylthiophene), P3HT and [60]PCBM have reached 

a PCE around 5% [538, 539]. 

However, for commercial use the efficiency and the stability of the organic 

photodiodes have to be improved dramatically. For these purposes, �-conjugated 

polymers with a strong absorption in all visible range and a good stability towards 

light are needed. Additionally, the initially formed phases between the donor 

and acceptor have to be fixed by either crosslinking the two compounds or using 

polymer/polymer mixtures with high T g since the two phases tend to separate as 

a result of the the operational heat through illumination, thus reducing progressively 

the performances of the device. Hence, in parallel to the development of 

the polymer/fullerene derivative bulk-heterojunctions, C 60 functionalized macromolecules 

[540–544] have been investigated for the preparation of all-polymer 

solar cells (Figure 5.70) [540, 541]. The controlled incorporation of fullerenes into 

well-defined linear polymers was achieved by polycondensation of a bifunctional 

fullerene with diols affording the C 60 -based polymer 81 with an average polymerization 

degree of 25 [540]. Photovoltaic cells were prepared by blending this 

soluble C 60 -polymer 81 with MDMO-PPV. The device ITO/PEDOT-PSS/MDMO- 

PPV:C 60 -polymer(1:5)/LiF/Al showed clear photovoltaic behavior (V oc = 0.36 V, 

Figure 5.70 C 60 -functionalized macromolecules for the preparation of all-polymer solar cells. 

307


J sc = 0.7 µA cm –2 , FF = 0.36) but these weak values of short- and open-circuit 

currents might be ascribed to the low conductivity of the fullerene polymer. The 

presence of large solubilizing groups may inhibit interactions between fullerenes, 

thus decreasing the charge transport [545]. 

The performances of bulk heterojunction devices obtained from �-conjugated 

polymers and C 60 derivatives are very sensitive to the morphology of the blend 

[546]. Ideally (to ensure efficient exciton dissociation), an acceptor species should 

be within the exciton diffusion range from any donor species and vice versa. 

Moreover, both the donor and the acceptor phases should form a bi-continuous 

microphase separated network to allow bipolar charge transport. However, the 

donor and acceptor molecules are usually incompatible and tend to undergo uncontrolled 

macrophase separation. In particular, phase separation and clustering 

of the fullerene can occur caused by the operational heat through illumination, 

thus reducing the effective donor/acceptor interfacial area and the efficiency of 

the devices [546]. In order to prevent such undesirable effects, it was proposed 

that the bicontinuous network could be simply obtained by chemically linking a 

hole-conducting moiety to the electron-conducting fullerene subunit [547]. Based 

on these considerations, compound 82 in which an oligophenylenevinylene (OPV) 

moiety is covalently linked to the fullerene sphere (Figure 5.71) was prepared [547]. 

The use of a fulleropyrrolidine derivative attached to an oligophenylenevinylene 

can be considered as the first example of a molecular heterojunction specifically 

designed for photovoltaic conversion. The fulleropyrrolidine OPV-C 60 82 was 

incorporated in a photovoltaic device by spin-casting between aluminum and 

ITO electrodes. The device ITO/OPV-C 60 82/Al delivered a low PCE of 0.01% 

(V oc = 0.46 V, J sc = 10 µA cm –2 , FF = 0.3) under monochromatic irradiation 

(400 nm, 12 mW cm –2 ). This study showed that plastic solar cells can be obtained 

by chemically linking the hole-conducting and the electron-conducting units. The 

length of the OPV was increased from three to four units and the performances of 

the ITO//OPV-C 60 83/Al were significantly improved with a monochromatic PCE 

of 0.03%. The limited efficiency of these devices was attributed to the competition 

between energy transfer and electron transfer [547]. 

The OPV-C 60 hybrid 84 was tested as an active material in photovoltaic cells 

and the device ITO/PEDOT-PSS/84/Al presented enhanced I/V characteristics 

(V oc = 0.65 V, J sc = 235 µA cm –2 under white light illumination at 65 mW cm –2 ) 

Figure 5.71 OPV-C 60 hybrids as molecular heterojunctions for photovoltaic conversion.


(Figure 5.72). Even if these solar cells are not prepared under similar conditions 

used for OPV-C 60 derivatives 82 and 83, the increased donating ability of the 

OPV moiety is an important argument for the improvement as that of the device 

performance [548]. Photovoltaic devices were also prepared from oligophenyleneethynylene 

– C 60 (OPE-C 60) oligomers 85 and 86. Under light, both devices show 

clear photovoltaic behavior. Interestingly, the performances of the devices prepared 

from the N,N-dialkylaniline terminated derivative 86 are significantly improved 

when compared with those obtained with 85. The latter observation can be related 

to the differences in their first oxidation potentials. Effectively, due to the increased 

donating ability of the OPE moiety in 86 when compared with 85, the energy level 

of the charge separated states resulting from a photoinduced electron transfer is 

significantly lower in energy. Therefore, the thermodynamic driving force is more 

favorable, thus electron transfer which is one of the key step for the photocurrent 

production must be more efficient for 86. As a result, the power conversion efficiency 

of the devices is increased by one order of magnitude [549]. This clearly 

demonstrated the interest of the molecular approach, which allows to establish 

structure/activity relationships. 

Figure 5.72 OPV- and OPE-C 60 hybrids tested as active material in photovoltaic cells. 

309


5.7.3 

Biological Applications 

[60]fullerene derivatives have attracted attention regarding their pharmacological 

properties since their discovery, but the low solubility of this material in aqueous 

media was initially an obvious problem for biological studies. Over the past few 

years, several strategies have been developed to overcome the natural repulsion 

of fullerenes for water and render them biocompatible. Generally, these strategies 

including chemical covalent modification of their surface with polar groups 

as terminal amines, alcohols, carboxylic acids, amino acids [550, 551] and sugars 

[552], or preparation of water soluble supramolecular complexes with macrocyclic 

host systems such as cyclodextrin, cyclotriveratrylene or calixerene derivatives 

[553–556]. Once in solution, fullerenes and derivatives exhibit a wide range of 

biological activity [557–564]. 

A potential application of fullerene derivatives is related to the easy photoexcitation 

of C 60 by visible light. The resulting singlet excited-state 1 C 60 is readily 

converted to the long-lived triplet 3 C 60 via intersystem crossing. In the presence 

of molecular oxygen, the fullerene can decay from its triplet to the ground state, 

transferring its energy to O 2, generating singlet oxygen 1 O 2, known to be a highly 

cytotoxic species. Therefore fullerenes constitute an excellent photosensitizer to 

be used in photodynamic therapy (PDT) [565]. There are two different pathways 

of DNA photocleavage acting mainly at guanine sites. The generation of singlet 

oxygen (type II photosensitization) as well as the energy transfer from the triplet 

excited state of fullerene to bases (type I photosensitization) can be responsible of 

the oxidation of guanosines and these modifications increase the instability of the 

phosphodiesteric bond that becomes easily susceptible of alkaline hydrolysis. 

The first report on the DNA photocleaving activity of fullerenes was made in 

1993 by Nakamura and coworkers [566]. The brief light exposure of a mixture of 

compound 87 (Figure 5.73) and a plasmid DNA resulted in single-strand nicking, 

and longer exposure led to double-strand cleavage, largely at guanine sites. No 

cleavage was observed in the dark. 

Since then, numerous approaches [557–564] have been developed to obtain 

fullerenes derivatives capable of cleaving the DNA under photoirradiation. Among 

recent reports, there is the preparation of derivatives 88–91 a–e containing mono- 

and disaccharides (Figure 5.74) [552]. The introduction of sugar moieties improves 

the biological properties of fullerene because they increase its solubility and they 

play an important role in cell–cell interaction. The production of singlet oxygen 

was analyzed by measuring near IR emission at 1270 nm. The bisadducts were 

Figure 5.73 Example of a fullerene derivative used for DNA photocleavage.


Figure 5.74 Water soluble derivatives containing mono- and disaccharides substituents. 

less effective in generating singlet oxygen. The treatment of HeLa cells with these 

derivatives was almost not effective in dark condition, while phototoxicity was 

more efficient upon incubation with monoadducts. 

Fullerene derivative 92 presenting a water-soluble �-cyclodextrin as appendage 

was able to act as DNA-cleavage agent after photoirradiation (Figure 5.75) [567]. 

The authors studied the mechanism of action and demonstrated the necessity of 

oxygen in the system. Moreover, EPR studies evidenced the presence of 1 O 2 as 

active species in the cleavage process. 

Figure 5.75 Fullerene derivative substituted with a �-cyclodextrin subunit. 

311


Figure 5.76 Fullerene-phorphyrin hybrids. 

Fullerene-phorphyrin hybrids possess attractive photoabsorption properties and 

application of this material in PDT has been realized by some researchers. Comparison 

between porphyrin, porphyrin-fullerene dyad (P-C 60 ) 93 and metalated 

porphyrin-fullerene dyad (ZnP-C 60 ) 94 (Figure 5.76) showed that the phototoxic 

activity decreased from P-C 60 to ZnP-C 60 and to porphyrin alone. This behavior 

was found also in anaerobic conditions, demonstrating that in this case both Type 

I and Type II mechanisms were involved [568]. 

Fullerene is an excellent acceptor in the ground state and can accept, reversibly, 

up to six electrons in solution. This property renders C 60 an excellent radical 

scavenger and provides a possible therapeutic approach for some neurodegenerative 

disorders such as Parkinson’s, Alzheimer’s and Lou Gehrig’s diseases. There 

is evidence to suggest that these diseases are due to hyper-production of reactive 

oxygen species (ROS). 

An important class of fullerene derivatives studied mainly as neuroprotective 

agents and radical scavengers comprises carboxyfullerenes such as the tris-malonic 

acid derivatives [569] with C 3 or D 3 symmetry 95 and 96 (Figure 5.77). Many researchers 

have investigated the antioxidant mechanism of these two regioisomers 

Figure 5.77 Regioisomers of tris-malonic acid derivatives with C3 and D3 symmetry.


[570, 571]. For the compound C 3 the neuroprotective effect was not only related to 

its ability of scavenging free radicals but also to the nitric oxide synthase inhibition 

and the possible suppression of toxic cytokines. 

Polyhydroxylated fullerenes named fullerenols [C 60(OH) n] have been shown 

to be excellent antioxidants. Djordjevic and coworkers analyzed the mechanism 

of action of C 60(OH) 2–26 by ESR in presence of 2,2-diphenyl-1-picryhydrazyl 

(DPPH) free radical and OH radicals generated by Fenton reaction [572]. The 

addition of fullerenol to the DPPH solution decreased the radical concentration 

as demonstrated by reduction of EPR signal of DPPH. The same behavior, with 

better results, was found in the case of OH radicals produced by Fenton reaction. 

Electron or hydrogen atom donation from fullerenol to free radicals was the 

proposed mechanism. Fullerenols C 60(OH) 22, C 60(OH) 7±2 and C 60(OH) 24 have 

been also tested as scavenger of ROS and nitric oxide [573–575]. 

The ability of �-alanine fullerene derivatives 97–99 in scavenging OH radicals 

has been studied by chemiluminescence (Figure 5.78) [576]. The radical sponge 

action is dose-dependent and the three adducts presented a variable scavenger 

activity. Compound 98 was the best, followed by 99, while 97 was the less effective 

in the series. 

Figure 5.78 �-alanine fullerene derivatives with scavenger activity. 

The antibacterial activity of fullerene derivatives was first reported in 1996 for 

fulleropyrrolidinium salts 100a–c (Figure 5.79) [577]. The hypothesized mechanism 

of action was attributed to the cell membrane disruption by the bulky carbon 

cage, which seemed not really adaptable to planar cellular surface. Since this first 

report, numerous papers have related the potential antimicrobial effects of C 60 and 

derivatives. More recently, Mashino and coworkers [578] studied the antibacterial 

activity of C 60 -bis(N,N-dimethylpyrrolidinium salts) regioisomers 101–103 against 

Escherichia coli and Gram-positive bacteria such as Enterococcus faecalis. The three 

compounds demonstrated excellent and not significantly different antibacterial 

activity, which was comparable with that of vancomycin. The mechanism of action 

seemed to be inhibition of the respiratory chain. 

In 1993 Wuld and coworkers [579] reported for the first time the inhibition of 

HIV-protease probably by interaction of fullerene with the hydrophobic active site 

of the enzyme. Since then, many studies on different fullerene derivatives have 

been performed [580–584]. Recently Marchesan and coworkers obtained good 

results against HIV on CEM cells infected by HIV-1(III B ) or HIV-2 (ROD) using 

a series of N,N-dimethyl bis fulleropyrrolidinium salts. However, the mechanism 

involved in the antiviral activity was not elucidated. Analyses in this sense have 

been performed by Mashino and coworkers. They identified efficient inhibitors of 

313


Figure 5.79 Fulleropyrrolidinium salts with antibacterial activity. 

the HIV reverse transcriptase, more active than nevirapine. The same compounds 

were found to be active also against hepatitis C virus RNA polymerases [585]. 

Recent studies have shown that chemically modified fullerenes can still be 

considered potentially interesting systems for drug delivery [586] and gene 

therapy [587]. Finally, radiolabeled fullerenes [588, 589] can became nano-vehicles 

for imaging, diagnosis, therapy and microsurgery because they can be used as 

contrast agents or radiotracers. 

5.7.4 

Conclusions 

Recent progress in the chemistry of C 60 allowed the synthesis of a large variety 

of fullerene derivatives for various applications in materials science and biology. 

In the first part of this chapter we have shown that fullerenes are efficient optical 

limiters and interesting acceptors for photovoltaic applications. In the second part, 

the biological properties of fullerene derivatives have been summarized. Despite 

some remarkable recent achievements, it is clear that the examples discussed 

herein represent only the first steps towards the design of commercial drugs or 

fullerene-based materials which can display functionality at the macroscopic level. 

More research in these areas is clearly needed to fully explore the possibilities 

offered by these compounds.

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333

6 

Carbon Nanotubes 

6.1 

The Structure and Properties of Carbon Nanotubes 

Anke Krueger 

6.1.1 

Introduction 

Another class of strained (hydro)carbon structures consists of only carbon in 

its pristine state. Tubular objects with a graphitic structure can be considered 

an extremely unsaturated form of an aromatic hydrocarbon. These so-called 

carbon nanotubes (CNTs) had been discovered long before they became a major 

research object for chemists and physicists alike. M. Endo described tubular 

carbon structures with several walls and coined the name ‘carbon nanotubes’ 

for these interesting objects (Figure 6.1a) and S. Iijima recognized the concentrically 

rolled shape of these objects [1]. Originally, they were thought to be just 

another type of carbon fiber before their unique properties were discovered. It 

was only in the 1990s that another type of CNT, the single-walled parent systems 

of the more complex MWNT (multi-walled nanotubes), were described by Iijima 

(Figure 6.1b) [2]. Their structure was unambiguously determined by electron 

microscopy (HRTEM, STEM; STM, AFM) and their spectroscopic properties 

investigated [3]. To understand these materials the knowledge of their structure 

is of utmost importance. This chapter will discuss the structure of single-walled 

nanotubes (SWNT) as well as their multi-walled counterparts. Additionally, the 

aspect of aromaticity will be reviewed. 

6.1.2 

The Structure of Single-walled Carbon Nanotubes 

The basic structural element of SWNT is the graphene plane. This is the basic 

unit of graphite, which can be formally rolled up to yield a tubular structure in 

the case of CNTs (Figure 6.1). Of course, the real production of CNTs is carried 

out in different ways. Various methods including CVD techniques, laser ablation, 



ISBN: 978-3-527-31767-7 

335

336 6 Carbon Nanotubes 

Figure 6.1 HRTEM images of (a) MWNT (from Nature 1991, 354, 56), 

(b) SWNT (courtesy of F. Banhart) and (c) the formal roll-up of graphene to form tubes. 

arc discharge, the HiPCo process etc. have been developed for different demands 

of quality, size and purity of CNT samples [3]. 

The seamless tubular objects obtained are nevertheless directly related to 

their planar counterpart (graphene or graphite) concerning their symmetry, 

spectroscopic and chemical properties, but their unique structure causes various 

peculiarities as well. In order to understand these features the structure of the 

different types of CNTs has to be known in detail. 

Dresselhaus defined a logical system for all types of CNT in order to approach the 

structural description systematically [4]. The basis of this method is the graphene 

layer. Depending on the way of seamless roll-up, one obtains different structure 

types of CNT. There are two privileged directions on the graphene sheet. They are 

represented by the unit vectors of its unit cell and form an angle of 60° (Figure 6.2). 

When a nanotube is formed from a graphene sheet, the roll-up is defined by a 

vector C that corresponds to its circumference after the tube formation (Figure 6.2). 

The direction of this vector defines the roll-up direction of the graphene sheet 

with the tube axis being perpendicular to C. 

The vector C can be described as the vector sum of multiples of the unit vectors 

a 1 and a 2 of the graphene lattice with 

C = n ⋅ a + m ⋅ a with n ≥ m and m ≥ 0 

(6.1) 

1 2 

A CNT is then unambiguously described by the set of descriptors n and m. Only 

its length needs to be defined, as all tubes with the same diameter and orientation 

but different lengths possess the same pair (n, m). It turns out that the length 

does not have a major importance for most of the real CNTs as they are very 

long compared to their diameter (up to several microns compared to a few nano-

6.1 The Structure and Properties of Carbon Nanotubes 

Figure 6.2 Construction of CNTs from graphene (top: the vector for a (6,3) tube is inscribed), 

and chiral angle � and length of the translational unit cell of a (6,3) nanotube (bottom). 

meters) [5]. Only in the case of very short tubes do the effects of the ends, such as 

wall bending, have to be taken into account [6]. The diameter and the length of 

CNTs are quantified according to the quantification of the underlying graphene 

lattice. The diameter needs to yield a seamless tube; the length is determined by 

the numbers of hexagons along the tube axis. 

Some rules exist for the unambiguous description of CNT by this system. 

Firstly, m and n are real numbers; secondly, m must be greater than or equal to n 

in order to avoid redundancy in the description, e.g. a (6,0) nanotube is identical 

to a (0,6) nanotube. In this case only the orientation of the rolled-up graphene 

sheet is changed by 60°. 

Three different types of SWNT can be distinguished (Figure 6.3): When n = 0, 

the orientation of the vector C is parallel to a 1 . This type is called zigzag nanotube 

because it has a zigzag structure at either end. The other archetype nanotube with 

m = n is called armchair nanotube. Its ends show a pattern that resembles the 

armchair conformation of cyclic hydrocarbons. 

337


Figure 6.3 Zigzag, armchair and chiral carbon nanotubes. 

In between these two extremes, all sets of m and n shape chiral nanotubes. The 

chirality originates from a helical roll-up of the graphene sheet and the chiral 

angle � is defined as 

⎛ 3 m ⎞ 

� = sin ⎜ ⎟ 

−1 2 2 

⎝n + n m + m ⎠ (6.2) 

These chiral tubes exist in two enantiomeric forms (left-handed and right-handed 

roll-up), which have been separated after noncovalent functionalization [7]. 

The diameter of the nanotube is also defined by the structure parameter set: 

1 

2 2 

dCNT = ⋅ a ⋅ n + n m + m 

(6.3) 

� 

According to Equation (6.3), a (5,5) nanotube, a (7,3) nanotube and a (9,0) tube 

have a very similar diameter of ~0.7 nm, close to the diameter of the archetypical 

fullerene C60 . Although indistinguishable by their size (e.g. in a HRTEM image), 

these tubes exhibit completely different properties. Whereas the (5,5) armchair 

tube is electrically conducting, the zigzag (9,0) tube is a semiconductor at absolute 

zero but conducting at room temperature, whereas the (7,3) nanotube is a semiconductor. 

The different symmetry of chiral, armchair and zigzag tubes is also 

responsible for their different Raman spectra [8]. 

In general, all properties depend on the respective nanotube symmetry. The 

density of states (see Figure 6.4 for examples) and consequently the electrical 

conductivity depend on the following conditions. The conductivity is governed 

by the existence of a gap at the Fermi level. All armchair nanotubes are metallic. 

In the case of semiconducting tubes a gap is opening between the valence and 

the conduction band [9]. Those nanotubes with n – m = 3 q (q � 0) possess a small 

gap at the absolute zero; at room temperature they behave like metallic tubes 

(Table 6.1) [9].


Figure 6.4 Density of states (DOS) of a (9,0) and a (10,0) carbon nanotube [12]. 

In the case of the (9,0) tube the density of states is not zero at the Fermi level. 

Table 6.1 Electrical conductivity as a function of structure parameters m and n [9]. 

Nanotube Conditions 

Electrically conducting metallic nanotubes n = m 

Semiconducting nanotubes with a small band gap 

(conducting at room temperature) 

n – m = 3 q (q � 0) 

Semiconducting nanotubes n – m � 3 q 

In real samples of CNTs these types coexist, and so far it has not been possible 

to produce a certain type or even tubes with a defined set (m,n) selectively [10]. 

Only the enrichment was achieved by different methods including electrophoresis 

[11] and selective functionalization (see next section). 

Another important aspect of the CNT structure is the construction of the tips. 

The formal roll-up of a graphene sheet would lead to unsaturated bonds at the 

339


Figure 6.5 Short carbon nanotubes with functionalized tips; (a) hydrogenated, (b) oxidized tip. 

In the case of the short hydrogenated tube the widening of the ends is highlighted. 

terminal carbon atoms at both rims and therefore to an elevated and unfavorable 

energetic state. In reality, the ends of CNTs show many different ways to overcome 

the dangling bonds and to produce a valence-saturated structure. The simplest 

would be the saturation of all dangling bonds with hydrogen (Figure 6.5a). Calculations 

by Schleyer et al. showed that short fragments of CNTs with hydrogenated 

ends are widened at their tips and no longer exhibit a perfect tubular shape [6]. 

Other functional groups can be also present at the tips of CNTs (Figure 6.5b). They 

are usually introduced by oxidative reagents, e.g. mineral acids, and are often used 

to introduce more complex moieties at the CNT surface (see Section 6.2). 

After production, the tips of SWNTs are usually closed by caps of different shape 

(see Figure 6.6). In principle, it is possible to postulate partial fullerene structures 

that fit exactly to the rim to be covered with a cap. Fujita and Dresselhaus have developed 

a formalism for the construction of CNT tips by arranging five-membered 

rings on the graphene sheet at the ends of the tube shell (Figure 6.6b) [13]. The 

five-membered rings are responsible for the curvature of the cap and the formation 

of a partial fullerene structure. The opening angle of the cap depends linearly on 

the number of five-membered rings in the fullerenic cap (Table 6.2) [14]. 

In the case of the archetypical zigzag (9,0) and armchair (5,5) carbon nanotubes 

it is theoretically possible to cover their ends with cut C 60 molecules. The fullerene 

hemisphere has to be selected according to the rim structure of the nanotube 

(Figure 6.7). Although highly symmetrical, those structures are rarely formed. 

Most often somewhat irregular nanotube caps are observed, often having sharp 

ends [15]. 

Real CNTs are usually not defect free (Figure 6.8). In addition to the imperfect 

structures at their ends they have also defects in the sidewalls. Several types of 

these defects are known. Missing carbon atoms create holes in the graphene 

network of the wall, disturbing the electronic properties as well as the �-conjugation 

Table 6.2 Opening angle of the nanotube cap as a function of five-membered rings in the cap. 

Number of five-membered ring 1 2 3 4 5 

Opening angle (deg.) 112.9 83.6 60.0 38.9 19.2


Figure 6.6 (a) HRTEM image of open (arrows) and closed SWNTs (from Chem. Phys. Lett. 

2000, 316, 349). (b) An example for the construction of a theoretically possible cap according 

to the method of Fujita and Dresselhaus. Highlighted hexagons represent the positions of 

pentagons in the cap (following the scheme in the lower right corner a segment from these 

hexagons is removed). Superimposing the fields with equal numbers results in the formation of 

the curved cap. 

Figure 6.7 Capping of a (5,5) and a (9,0) carbon nanotube by appropriate half-spheres of C 60 . 

The rims of the tubes are marked grey. 

341


Figure 6.8 Defects in the side wall of CNTs. (a) Holes; (b) Stone–Wales defect (side and 

top view) with the highest reactivity at the borders of the defect (highlighted in grey); 

(c) the migration of Stone–Wales defects (top) and Stone–Wales rearrangement (bottom). 

(see below) [16]. Another defect consists in the substitution of six-membered rings 

by pairs of five-membered and seven-membered rings. This so-called Stone–Wales 

defect is usually built by two sets of these rings (Figure 6.8) [17]. It can be produced 

either by a rearrangement where a bond between two six-membered rings is turned 

by 90° or by the addition of a C 2 unit to the graphene network [18]. 

In summary, the effect of a Stone–Wales defect on the curvature is zero as the 

convex contribution of the pentagons is compensated by the concave contribution 

of the heptagons. These rings do not necessarily occupy neighboring positions. 

In a rearrangement reaction including a four-membered Hückel transition state 

they migrate away from each other leading to bent or even coiled nanotubes, 

if a certain number of these defects is positioned in a regular manner [19]. For 

example, a pentagon on one (convex) side of the nanotube and a heptagon on the 

other (concave) side leads to a kink in the respective nanotube. 

In the case of a complete Stone–Wales defect the curvature is not evenly distributed 

over the whole defect structure. Thus the reactivity of the different bonds 

depends on the prehybridization and strain exerted by the additional curvature 

(see Figure 6.8). 

6.1.3 

The Structure of Multi-walled Carbon Nanotubes 

The structure of MWNTs is closely related to that of SWNTs. Each wall of the 

MWNT represents a SWNT, only their diameters differ. The whole object is a


combination of these walls and the main question is whether there are interactions 

between the single layers of a MWNT. The inter-wall distance in MWNT is mostly 

slightly larger than the distance between the graphene sheets in graphite [20]. 

This indicates that the interactions between the walls should be somewhat weaker 

than in graphite. Inspection of the possible interactions between the walls in 

different types of MWNT reveals a fundamental difficulty: The interlayer distance 

of 0.34 nm defines a certain increase of � u = 2.14 nm in circumference (because 

of the increase of � d = 0.68 nm in diameter). Depending on the type of CNT this 

can be expressed in multiples of the basic unit of the circumferential vector C. 

In the case of armchair nanotubes this corresponds well to � u = 5 · 0.426 nm. On 

the other hand, it is difficult to produce a good fit for zigzag tubes as the nearest 

multiple is � u = 9 · 0.246 nm = 2.214 nm. With this increase in diameter the interwall 

distance increases to 0.352 nm. 

A real MWNT will rarely be formed of just one type of CNT, making the situation 

even more complex. Chiral tubes are very unlikely to fit with their neighboring 

tubes and a mixture of zigzag, armchair and chiral tubes is usually present in real 

MWNTs [21]. As the interaction between the walls is rather weak, the mobility of 

the inner walls is only slightly hindered and rotation is possible [22]. This is also 

responsible for the ‘sword in a sheath’ phenomenon which is observed when a 

MWNT filled composite is mechanically strained in the direction of the MWNT 

[23]. Multi-walled carbon nanotubes show mixed electronic behavior [24]. Some 

authors state that the outermost wall determines whether a tube is metallic or 

semiconducting [25]. 

In similarity to single-walled CNTs, MWNTs exhibit even more complex 

structures at the ends of the cylindrical body. Especially, the presence of sevenmembered 

rings in addition to the usual five-membered rings, opens many 

possibilities for tip structures. Mostly, slightly unsymmetrical tips are observed, 

beak-like structures as well as so-called ‘acute’ tips with sharp ends can also be 

found (Figure 6.9) [26]. 

Another feature of real MWNTs is the occurrence of internal structures. 

These include inner caps as well as bamboo-like structures (Figure 6.10). These 

features can be formed by one or more internal walls of the MWNT [27]. In 

some cases one observes toroidal structures at the tips of opened MWNT. 

The neighboring walls are joined by C–C bonds, diminishing the number of 

dangling bonds [28]. 

Figure 6.9 Cap structures of real MWNTs. 

343


Figure 6.10 Internal structures of multi-walled carbon nanotubes. Internal caps at a bend (left) and 

HRTEM image of bamboo-like carbon structures (right). (From Chem. Phys. Lett. 2000, 323, 560). 

Figure 6.11 Electron-microscopic images of coiled multi-walled carbon nanotubes 

(from J. Mater. Res. 2000, 15, 808 (top) and Appl. Phys. Lett. 2002, 81, 3567 (bottom)). 

Coiled and bent MWNTs have been also found in nanotube samples. These 

defects are formed by the existence of five- and seven-membered rings in the 

graphene network (see above) [19]. With appropriate conditions, strongly coiled 

MWNTs can be produced in macroscopic amounts (Figure 6.11) [29]. 

In comparison to SWNTs the defectiveness of the outer walls of a MWNT does 

not affect the mechanical and chemical properties in the same dramatic way. 

Even if the outermost walls have holes, the tube shape remains still closed as 

the inner walls are still intact. On the other hand, the electronic properties are


affected more seriously because of the major contribution of the outer shell to 

the electrical transport [30]. 

6.1.4 

The Aromaticity of Carbon Nanotubes 

When we regard carbon nanotubes as a type of highly unsaturated form of hydrocarbon 

the issue of aromaticity needs to be discussed. 

The concept of aromaticity is difficult to apply to carbon nanotubes as the conjugated 

system shows a significant curvature along its circumference. On the other 

hand there is no curvature in the direction of the tube axis. Is a nanotube aromatic 

or not? The question has to be answered whether a sufficient delocalization of the 

�-electrons is realized or not. According to the bond lengths, conjugation reaches a 

high level as there is no significant alternation of single and double bonds [31]. 

For the discussion it is useful to neglect the effects of the tips in the first instance 

as they represent defects in the perfect structure of a straight, defect-free CNT. The 

endless graphene layer, from which a hypothetical perfect nanotube is formally 

derived, possesses a planar, fully delocalized �-electron cloud and therefore can 

be considered to be aromatic [32]. In the case of the rolled-up nanotube the participating 

electrons and their orbitals are still the same. Only the orientation and 

to some extent the shape of the �-orbitals is different. A 3D structure is formed 

and the conjugation is weakened by the radial distribution of the �-orbitals 

(Figure 6.12). Along the tube axis the extent of conjugation remains the same but 

in the circumferential direction it changes the more, the smaller the diameter of 

the nanotube [31, 33]. 

Figure 6.12 Orientation and conjugation of �-orbitals in graphene and CNTs. 

It has been shown that the form of possible resonance structures depends on 

the set of structure parameters (m, n). Metallic tubes with m – n mod 3 = R and 

R = 0 (n mod x yields the remainder of the division of n by x) can be described 

with a Clar resonance structure consisting only of complete benzene rings. These 

are fully conjugated [34]. All other tubes with R = 1 or R = 2 (m – n not divisible 

by 3) have at least one seam of isolated double bonds in their resonance structure 

(Figure 6.13) [34]. These tubes are semiconducting. A special case are the tubes 

with (m, 0) and R = 1. Here it would be possible to construct a chiral Clar resonance 

structure with the double bond seam as the chiral element. To avoid this, a structure 

is formulated that shows an achiral quinoidal seam (Figure 6.13, right). 

345


Figure 6.13 Resonance structures for a (12,9), a (12,8), a (12,7) and a (19,0) nanotube 

(from J. Org. Chem. 2004, 69, 4287). 

The calculation of NICS (nucleus independent chemical shift) values by Ormsby 

and King has shown the validity of the model and the dependence of the electronic 

properties on the tube diameter and the graphene sheet orientation [35]. 

In reality, the nanotubes are not endless and therefore some restrictions apply 

to the number of their �-electrons and the options for fully delocalized systems. 

To assess the �-electron delocalization, the resonance structures should be 

inspected for finite pieces of CNTs. Nakamura showed that the length of the tube 

fragment plays an important role in the full delocalization of the �-electrons in the 

case of armchair CNTs [36]. Only in the case of a length that accommodates full 

aromatic sextets can a complete conjugation be achieved. Otherwise, complete 

and incomplete Clar structures best represent the situation in finite CNTs. Interestingly, 

the HOMO-LUMO gap and the frontier orbital energy oscillate as well 

(Figure 6.14). 

Figure 6.14 Resonance structures in finite armchair CNTs. 

Functionalization has an important influence on the conjugation of the 

�-electrons [37]. Depending on the addition mode of reactants, the conjugation can 

be diminished or even completely destroyed. In all cases this has an influence on 

the electronic properties of the tube as well. Metallic CNTs become semiconducting 

when they are functionalized on the side walls. The conjugation is interrupted 

at the position of an addition reaction.

6.2 The Functionalization of Carbon Nanotubes 

Similar effects are observed for defects in the side wall. The conjugation is 

broken at places where carbon atoms are missing or where the graphene network 

is interrupted by the existence of ring defects or functional groups [38]. 

6.1.5 

Conclusions 

In summary, the tubular structure of CNTs is unique in the field of highly unsaturated 

hydrocarbons. Depending on the structure parameters quite different 

properties are observed. Fully conjugated �-systems show metallic conductivity 

whereas semiconductivity occurs with isolated double bonds in the resonance 

structure of the respective tube. The curvature induces an increased reactivity of 

the carbon atoms. 

In the next section the reactivity of these highly inspiring molecules will be 

discussed. The three-dimensional �-system allows for a variety of useful reactions 

and the application of CNTs in various field, e.g electronics, composites, and 

biomedical applications. 

In order to achieve these high expectations, it will be necessary to produce 

homogeneous samples of CNTs with defined electronic properties, i.e. defined 

structure parameters. So far only mixtures are available, but significant progress 

has been made towards the separation of different types of nanotubes. 

6.2 

The Functionalization of Carbon Nanotubes 

Anke Krueger 

6.2.1 

Introduction 

Functionalization of materials is one of the most important research areas. It 

is only after appropriate modification of the pristine material’s properties that 

many applications open up for the new compounds or composites. Especially for 

biological applications such as drug delivery or sensing applications, a suitable 

and stable functionalization is of importance. It is only natural that for carbon 

nanotubes (CNT) the research on their surface modification began just shortly 

after their production in sufficient amounts. The work has continued ever since 

then, as a vast range of reactions has been tested and various new materials have 

been developed. 

In the following sections we will discuss the different types of surface modification 

for CNTs. In this, MWNTs will only be separately mentioned if a significant 

difference is observed compared with SWNTs. Normally, the behavior of these two 

classes of CNTs is rather similar. Only large MWNTs with decreased curvature show 

lower reactivity because there is a smaller prehybridization of the carbon atoms, 

i.e. a smaller deviation from sp 2 towards sp 3 hybridization caused by curvature. 

347


Figure 6.15 Functionalization types for carbon nanotubes. 

The topology of CNTs permits different kinds of modification: reaction at the 

tips, at the outer or inner wall or the inclusion of objects in the internal space 

(Figure 6.15). Functionalization can be realized by covalent bonds or by noncovalent 

inter actions, e.g. electrostatic forces [39]. 

Compared with the fullerenes, there are some structural differences. In fullerenes 

the six-membered rings are partially connected to five-membered rings 

inducing three-dimensional curvature and strain, whereas CNTs are not curved 

along their tube axis. Additionally, the diameter of the tubes is often (especially in 

the case of MWNTs) larger than for the archetypical fullerene C 60 . Therefore, the 

reactivity of CNTs should be reduced compared with C 60 , which is actually the case 

[40]. Many of the reactions described for fullerenes can be carried out with CNTs 

as well but need harsher conditions and/or longer reaction times [41]. Only the 

tips of carbon nanotubes exhibit a comparable reactivity because of the existence 

of pentagons [42]. A similar effect is observed for the so-called Stone–Wales defect. 

They consist of two pentagons and two heptagons (see former Section) and some 

of their bonds are much more strained than the usual bonds in CNT [43]. 

6.2.2 

Functionalization of the Nanotube Tips 

The first procedures for the functionalization of CNTs were closely related to their 

purification. It was observed that reaction with oxidizing acids or piranha water 

(mixture of sulfuric acid and hydrogen peroxide) not only opened the nanotube tips 

but introduced carboxylic groups at the rims of the cylindrical structures [44]. 

It is obvious why this reaction preferably takes place at the ends of the tubes. 

The higher strain and prehybridization induced by the five-membered rings in 

the caps cause a higher reactivity. The carbon is oxidized and the caps removed. 

The resulting dangling bonds are saturated with carboxylic groups, which can 

further react with, for example, amines or alcohols to form amides or esters [45]. 

Using long-chain alkyl alcohols or amines for this purpose results in solubility of 

the modified tubes in organic solvents (Figure 6.16) [46]. 

Solubility of the CNTs in reaction or physiological media (important for future 

bioapplications) is one of the most challenging and important issues of CNT 

research [47]. Certain reactions only take place in solution; homogeneous distribution 

is possible and applications in blood or serum depend on stable solutions 

of the nanotubes. For nanotubes there are several approaches to this problem: 

Addition of surface active compounds [48] or coating of the tube with alkyl chains 

[49] or hydrophilic groups [50] result in improved dispersibility.

Figure 6.16 Functionalization of the tips of a CNT. 


Under the reaction conditions that are necessary to remove the caps of CNTs, 

a large number of defects is created in the side walls of the tubes [51]. Especially 

at the position of already existing small holes, larger defects are produced by 

oxidative removal of carbon atoms. The rims of these holes are also covered with 

carboxylic groups (Figure 6.17). 

Figure 6.17 Defects in the side wall carry carboxylic groups at the rims after oxidative treatment. 

Further grafting of e.g. alkyl chains via amidation or esterification leads to improved 

dispersibility. 

6.2.3 

Non-covalent Functionalization of Carbon Nanotubes 

As fully unsaturated hydrocarbons, carbon nanotubes are strongly hydrophobic. 

They are therefore not dispersible in any polar solvent or water. But they can 

undergo strong interactions with other hydrophobic compounds, especially when 

these are able to form �–� interactions [52]. Benzene and other small aromatic 

compounds do not establish very strong interactions with the nanotube surface. 

349


Figure 6.18 Arrangement of pyrene molecules along the nanotube axis (left); 

a larger planar aromatic system does not improve the interaction with the nanotube (right). 

But condensed aromatic compounds such as pyrene are highly suitable for this 

kind of surface modification [53]. Pyrene with its elongated shape is able to 

establish �–� interactions throughout its �-system. It can arrange itself with its long 

axis parallel to the tube axis in order to maximize the binding force (Figure 6.18). 

Bigger planar aromatics are not necessarily more strongly bound because of the 

increasing distance between the �-system of the nanotube and the ends of the 

aromatic compound (Figure 6.18) [54]. A perfect fit would be achieved with a 

curved structure that imitates the curvature of the respective nanotube. In this 

case it would be possible to observe selective interactions with the nanotubes of 

the best-fitting diameter. 

Depending on the functionalization of the pyrenes that are interacting with 

the nanotubes it is possible to tune the hydrophilicity of the resulting composite 

resulting ultimately in water solubility (Figure 6.19) [55], debundling [56] or solubility 

in organic solvents if the pyrenes carry long side chains [57]. At the same 

time they influence the band structure of the functionalized nanotube as can be 

seen from the resulting absorption spectra (Figure 6.20) [58]. Polymers bearing 

pyrenes in their side chains can be mixed with nanotubes to yield a non-covalently 

bound composite material that unites the properties of the nanotubes with those 

of the polymer [59]. 

Figure 6.19 Functionalization of a CNT with pyrene derivatives. The pyrene moieties are 

arranged along the tube axis. Depending on the side chain the conjugates are dispersible in 

different media.


Figure 6.20 Absorption spectra of CNTs before and after functionalization with DomP 

(from J. Am. Chem. Soc. 2004, 126, 10234). 

Similar to pyrenes, phthalocyanins and porphyrins interact with the CNT surface 

via van der Waals interactions and �-stacking [60]. They can be used to study the 

energy transfer from the porphyrin skeleton to the nanotube. In 2004 Sun et al. 

reported on the selective interaction of semiconducting nanotubes while the 

metallic ones remained unchanged and insoluble [61]. 

Not only organic molecules can be deposited on the nanotube surface but also 

metallic clusters. Usually, this requires the addition of a reducing agent (except 

for electrodeposition) and a thorough surface cleaning of the nanotube surface 

[62]. Deposited clusters include gold, platinum and palladium nanoparticles, 

zinc and magnesium oxide [63]. These composites are interesting for catalytic 

applications. 

Another topologically completely different approach is taken with long, chainlike 

molecules, in general polymeric materials. These can be wrapped around 

single nanotubes or small bundles [64]. Usually the interaction is based on electrostatic 

forces or �-stacking. Examples (Figure 6.21) for this functionalization method 

include the wrapping of CNTs with amylose, peptides, polyaniline and polymers 

Figure 6.21 Non-covalent wrapping of a CNT with hydrophilic amylose (lower left) or 

hydrophobic PmPV (lower right). 

351


such as PVP (polyvinylpyrrolidone) or PmPV (poly-m-phenylenevinylidene) [65]. 

The latter produces interesting composites with CNTs where the electrical conductivity 

of the composite is significantly increased whereas the luminescence 

properties of PmPV remain basically unchanged [66]. The electrical conductivity 

is also increased in polyaniline (PANI) composites [67]. The circular and sizeselective 

complexation of CNTs is achieved with cyclodextrins [68, 69]. 

6.2.4 

Covalent Side-wall Functionalization of Carbon Nanotubes 

Instead of using electrostatic interactions and �-stacking, CNTs can also be functionalized 

covalently at their side-walls. The conceptually easiest way of modifying 

the surface structure of a CNT consists in reducing the number of unsaturated 

bonds, and therefore its hydrogenation (Figure 6.22). So far, a completely hydrogenated 

CNT has not been reported. Only partial reduction was observed 

after reaction in a hydrogen plasma or with lithium in liquid ammonia (Birch 

type reaction), after high temperature treatment in a hydrogen atmosphere or 

electrochemical hydrogenation [70, 71]. According to calculations a completely 

hydrogenated CNT should be stable up to a diameter of 1.25 nm [72]. 

Figure 6.22 Hydrogenation of CNTs. 

In close resemblance to the hydrogenation, nanotubes can also be halogenated. 

Especially the reaction with fluorine yields samples with a high halogen content 

of up to 100% [73]. This does not mean that each carbon is carrying exactly one 

fluorine atom, but there are positions like the tips and defects where the fluorination 

grade is elevated. The reaction is reversible; treatment with hydrazine yields 

the clean nanotubes after annealing [74]. The fluorination reaction was used to 

study the addition mechanism of the addition to CNTs. Usually, the fluorine atoms 

are positioned close to each other and further fluorination continues along the 

circumference, not the axis of the tube [75]. This results in a nonregular distribution 

of highly fluorinated areas and some with almost no fluorine. One explanation 

for this behavior is the preferred 1,4-addition (over 1,2-addition), which is also 

corroborated by computational results (Figure 6.23) [76]. The electronic properties 

of fluorinated CNT as very good insulators strengthen the argumentation, 

too [77]. The 1,4-addition destroys the conjugation of the �-bonds over the whole 

diameter. Fluorinated CNTs can be used in further reactions such as nucleophilic 

substitutions with amines or carbanions [78]. Other halogens are much less 

reactive towards CNTs. Only an electrochemical method for the halogenation 

has been described [79].

Figure 6.23 Fluorination of CNTs (1,4- vs. 1,2 reaction). 


Figure 6.24 Bingel reaction on CNTs and further modification of the ester groups. 

Another important reaction of CNTs is the nucleophilic attack by bromomalonates 

in the presence of a base (Figure 6.24). This so-called Bingel reaction is 

one of the most versatile reactions in fullerene chemistry and can be applied 

to CNTs, although only under harsher conditions [80]. The ester groups of the 

malonate can be modified in various ways, enabling a vast range of further CNT 

derivatives and improved solubility [81, 82]. 

In general, the reaction with nucleophiles as described for fullerenes can be 

transferred to CNTs (e.g. addition of carbenes and nitrenes [83]), but the lower 

reactivity in some cases remains an obstacle to efficient functionalization. 

Radical reactions are another option for the surface grafting of organic moieties. 

Perfluoroalkyl radicals as well as acyloyls radical have been used for the modification 

of the nanotubes surface [84]. The reaction with diazonium salts is a very 

important reaction of CNTs. The intermediate phenyl radicals directly bind to the 

tube by C–C bonds [85]. Strano et al. reported on the selective functionalization 

of metallic tubes with this reaction [86]. 

CNTs are good candidates for cycloaddition reactions because they have a system 

of conjugated double bonds (Figure 6.25). They always react as the -ene component, 

even in [4+2] cycloadditions [87]. It is rather unlikely that a reaction as the diene 

component would be observed, because the nanotube has unfavorable geometry 

and also is electron deficient [88]. The most common cycloaddition on CNTs is 

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Figure 6.25 Cycloaddition reactions of CNTs. 

the Prato reaction, that has also been extensively studied in the case of fullerenes 

[89]. This 1,3-dipolar cyclodaddition of azomethineylides yields functionalized 

pyrrolidine rings on the nanotube surface. This reaction accepts a broad range 

of functional groups in the sidechains. That is why this reaction is often used to 

immobilize biological structures like DNA, antibodies or viruses on CNTs for applications 

such as drug or antibody delivery [90]. Solubility in physiological media 

is achieved with bridging oligoethylenglycol moieties [91]. 

Additionally, the reactions with nitrile imines and ozone are [3+2] cycloadditions. 

The latter can be used to establish carboxylic, keto or hydroxylic groups at 

the nanotube surface depending on the selected work-up [92]. 

Compared to [3+2] cycloadditions, the [4+2] addition has rarely been reported. 

Only a few examples using o-chinodimethanes have been described [93]. Fluorinated 

CNTs are better dienophiles because of the strain induced by the sp 3 

carbon atoms that exist in the neighborhood of the remaining double bonds and 

the electron-withdrawing properties of the fluorine substituents [94]. [2+2] cycloadditions 

to CNTs have not been reported so far.


Figure 6.26 Transition metal complexes with CNTs have so far been reported only via functional 

groups (right). Dihapto and hexahapto complexes are difficult to achieve due to the weak donor 

character of the tubes and the unfavorable geometry. 

Altogether the cycloaddition chemistry of CNTs is less pronounced than for 

fullerenes. The lower curvature and the lower electrophilicity play the key roles 

for the lower reactivity. 

An interesting question is whether CNTs are able to form coordination 

complexes with transition metals. Their double bonds could act as �-donors in 

these compounds. So far, only complexes via functional groups of the nanotube 

have been described, such as the CNT–Wilkinson complex conjugate (Figure 6.26) 

[95]. These derivatives are interesting objects for catalyst research. The reasons for 

the inability to complex the metal centers directly at the nanotube are the better 

conjugation of the double bonds, the absence of five-membered rings and the 

larger HOMO-LUMO gap compared with C 60 [96]. 

6.2.5 

Endohedral Functionalization of Carbon Nanotubes 

After the removal of the caps from the nanotube tips the inner space becomes 

accessible. Many reports on the endohedral functionalization of carbon nanotubes 

have been published [97]. Most of these results have been obtained for SWNTs. 

Small molecules as well as metals [98] or metal oxides [99] can be included in 

the tube. 

Besides, organic molecules can be filled into CNTs. These include large 

molecules such as polypyrrole or carotene [100] and small structures like metallocenes 

[101] and aromatic compounds [102, 103]. A very special example for the 

endohedral functionalization of SWNTs is the inclusion of fullerenes inside CNTs 

[104]. The resulting material is a ‘carbon-only’ compound with a tube-like shell 

and dot-shaped inclusions. 

For a long time it was believed that covalent functionalization of the nanotube 

wall is prevented by the small orbital lobes inside the tube. However, Gao et al. 

reported on the grafting of amino groups inside CNTs [105]. 

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6.2.6 

Conclusions 

In summary, the chemistry of carbon nanotubes is a vast research area. There are 

many options for the modification of both SWNTs and MWNTs. These include the 

removal of the caps with subsequent formation of carboxylic groups, the endohedral 

inclusion of small clusters and molecules, and the side-wall functionalization 

at defects or regular double bonds. This can be done with surface active agents 

leading to non-covalent composites of nanotubes with organic molecules. Metal 

clusters can be deposited on the CNT surface and wrapping with macromolecular 

compounds results in better dispersibility of the tubes. In the case of SWNTs this 

can even break up the bundles of tubes. Covalent surface modification can make 

use of existing functional groups or directly use the conjugated double bonds of the 

graphene lattice. Cycloadditions, radical additions of diazonium salt etc. and the 

Bingel reaction are examples for the efficient modification of the carbon nanotube 

surface. All these reactions lead to new materials with interesting properties, e.g. 

the selective functionalization of metallic or semiconducting tubes. For new applications 

in composites or electronics these features are very useful. 

6.3 

Applications of Carbon Nanotubes 


6.3.1 

Introduction 

Carbon nanofilaments – or nanotubes – were actually very probably the core of 

the much larger vapor-grown carbon filaments used by Edison to operate the 

early version of his light-bulb. However, although carbon nanotubes (CNTs) were 

unambiguously revealed as early as 1952 [1], and shown to exhibit diameters as 

low as 3–4 nm as early as 1976 [1], they have only been thought to be useful for a 

wide range of applications from the 1990s, i.e. from the time of the well-known 

paper by Iijima [1]. In this regard, CNTs are a good example of availability being 

not the only condition for a material to be considered of some interest from the 

technological point of view. Both technology and the minds (of scientists and 

engineers) have to be prepared, specifically when downsizing is involved [106]. 

Fifty years ago, vapor-grown carbon (nano)filaments were mostly undesirable 

by-products of industrial processing that were studied to be avoided. Nanotubes 

are nowadays the most popular nanomaterials, as measured from the number of 

scientific articles published yearly, and from the amount of investment dedicated 

to related R&D. Consequently, applications involving CNTs are now coming to the 

market although they are probably still too few with respect to the expectations. 

This section will briefly address these aspects, along with providing examples of 

applications.

6.3.2 

Properties of CNTs 

6.3 Applications of Carbon Nanotubes 

6.3.2.1 Which CNT for Which Application? 

CNTs were considered very promising because they exhibit extreme properties 

in almost every aspect. Table 6.3 provides the most important ones, as gathered 

from literature. 

As with other kinds of graphene-based carbon materials (otherwise called polyaromatic 

carbons, including graphite), the properties of CNTs are determined 

by the level of anisotropy, i.e. the extent to which the graphenes are aligned with 

respect to the direction at which the property is measured, because graphene and 

graphene stacks exhibit high intrinsic anisotropy (Table 6.4). 

Table 6.3 Some extreme properties of CNTs. 

Property Values Comments 

Aspect ratio ~1000 Diameter > 0.4 nm, length up to cm 

Specific surface area ~2700 m 2 g –1 

The highest! 

Tensile strength > 45 GPa The highest ever! 

Tensile modulus 1–1.3 TPa The highest! 

Tensile strain > 40% Provides the highest toughness ever! 

Thermal stability > 3000 °C In oxygen-free atmosphere 

Electrical conductivity 10 4 –10 7 S cm –1 

Better than that of copper 

Transport regime Ballistic Induced superconductivity with T c < 1 K 

Thermal conductivity ~6000 W mK –1 

Electron emission 10 6 –10 12 A cm –2 

Higher than those of diamond or graphite 

Highest current density! 

Table 6.4 Properties for a stack of perfect graphenes (except the tensile strength-to-failure, 

calculated for a single graphene) as measured parallel or perpendicular to the graphene plane 

direction. 

Direction in graphene (stack) Parallel Perpendicular 

Bond energy kJ mole –1 

Thermal expansion °C –1 

Thermal conductivity W (m · K) –1 

524 7 

1.5 · 10 –6 

Electrical resistivity � cm 3.8 · 10 –5 

3000 8 

Young modulus GPa > 1000 ~50 

Strength-to-failure GPa ~100 ? 

27 · 10 –6 

10 –2 

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Hence, properties are maximized along the axis of single-wall CNTs (SWNTs) 

because anisotropy is at maximum (or ‘assumes maximum value’) on the one 

hand, and SWNT structure is close to perfection on the other hand, with only 

pentagons or heptagons (see the preceding section) as possible structural defects 

(vacancies are minor in pristine SWNTs). The fact that SWNTs can be as perfect 

as a (macro)molecule makes a major difference with regular materials. 

The situation is quite different for multi-wall CNTs (MWNTs), in which all 

kinds of orientation of graphene stacks can be found with respect to the nanofilament 

axis, from fully parallel (as in MWNTs with concentric texture) to fully 

perpendicular (as in nanofibers with platelet texture), including combinations of 

both (as in MWNTs with bamboo texture). In addition, they may be affected by 

a much larger variety of possible structural defects, such as sp 3 carbons, heteroatoms, 

disclinations, in addition to other than six-membered rings as in SWNTs, 

making graphenes in graphene stacks and the mutual orientation of the latter 

more or less perfect. Hence, a much wider range of property values can be found 

for MWNTs. 

An interesting property of CNTs is their intrinsic biocompatibility, which is 

of importance not only for medical applications but also for safety issues. Such 

biocompatibility has long been known for pyrolytic carbon or graphite, because 

they contain a limited amount of accessible graphene edges, which actually are 

the reactive sites, as opposed to the graphene plane. In this regard, although the 

reactivity of SWNTs or concentric MWNTs is low (for being free of graphene 

edges), using high purity materials is compulsory, otherwise carbon or noncarbon 

impurities will affect the reactivity, and hence the biocompatibility, of the 

whole. 

These brief comments explain why research efforts regarding the synthesis 

of nanotubes ultimately aim at high-yield production of SWNTs, preferably to 

MWNTs, although production capacity is still much larger for the latter so far. It 

also illustrates that, considering carbon nanotube properties (physical as well as 

chemical, including reactivity), it is of utmost importance to define which kind 

of nanotubes is involved. 

6.3.2.2 Why is ‘Nano’ Beautiful? 

As mentioned before, extreme properties of CNTs come from their high anisotropy. 

But other highly anisotropic polyaromatic carbon materials, carbon fibers, known 

for long time, also exhibit extreme properties (sometimes close to those of CNTs, 

e.g. Young’s modulus, thermal conductivity, etc.). They have already found many 

applications, from daily-life ones, such as sports goods, to high-tech uses in space 

shuttles. One might then wonder whether it is worth investing millions of dollars 

worldwide in the R&D of CNTs. Reasons for this are multiple: 

� CNTs exhibit a much higher specific surface area, thus a much higher surface 

of interaction, than carbon fibers. This is especially useful not only for composites 

but also for applications where the specific reactivity of CNTs is exploited 

(e.g. sensors).


� CNTs exhibit a much higher aspect ratio than carbon fibers. This feature is very 

favorable for applications where they are used as a network, e.g. as fillers in electrically 

or thermally conductive composites built with an insulating matrix (e.g. 

most of polymers). It is actually well-known that the percolation threshold (i.e. 

the proportion of filler from which the conductivity suddenly jumps) decreases 

as the aspect ratio increases. 

� CNTs have a much better propensity to structural perfection than carbon 

fibers, especially when considering SWNTs or possibly concentric MWNTs. 

This provides some of the as-prepared CNTs with ultimate properties, as 

opposed to carbon fibers which contain numerous and various kinds of defects 

(Figure 6.27). 

� Rather simple, mostly single step, fabrication processes can be used to prepare 

CNTs whereas carbon fibers require complex, multi-step processes. This makes 

the synthesis of CNTs a much more affordable technology, regarding both the 

needed equipment and the level of manpower qualification. 

� Contrary to the reagents used in production of carbon fibers, feedstocks for 

carbon nanotube synthesis are basic compounds with perfectly defined chemical 

composition, easy availability, and constant quality (CH4 , C2H2 , CO, graphite, 

pure metal catalysts, …). 

Whereas the first three reasons relate to the intrinsic superiority of CNTs over 

carbon fibers, the last two relate to the superiority of their synthesis processes, 

which are likely to ultimately result in much cheaper materials. 

Figure 6.27 Tensile mechanical properties of the fibers available on the market, among the 

most performing ones. Circles and triangles are carbon fiber, polyacrilonitrile-based and 

pitch-based, respectively. Only carbon nanotubes (SWNT) are able to exhibit both high modulus 

(1–1.2 TPa) and high strength (45 GPa or more). (Modified from [107]). 

359


6.3.2.3 Potential Problems Related to the Use of CNTs 

However, there are obviously still unsolved problems that prevent CNTs from 

overwhelming the market: 

� The most important one is quality. Many marketed CNTs exhibit low purity, 

since they come along with other carbon nanophases (amorphous carbon, 

fullerenoids, polyaromatic carbon shells, nano-horns …) and catalyst and/or 

solvent remnants (Figure 6.28). Enriching in CNT content is a technological 

challenge because purification processes based on chemicals are obviously more 

or less harmful to the CNT structure, because the CNTs are chemically similar 

to some of the various phases to be discarded. 

Likewise, all marketed nanotube grades exhibit poor selectivity, e.g. regarding 

the helicity of SWNTs (which controls the metallic vs semi-conductor behavior), 

CNT diameters, number of walls in MWNTs … (Figure 6.29). 

� Processing is another major issue because the aspect ratio of nanotubes makes 

them entangle (except for the carpet-type growth mode, where CNTs are aligned 

and perpendicular to the substrate). Meanwhile, CNT nanosize makes them 

sensitive to weak surface forces and clump upon any attempt to mix and disperse 

them into liquids (e.g. solvents, matrix precursors) using regular mixing procedures 

(e.g. see Figure 6.30, left). This is highly detrimental to the properties of 

the resulting composite materials. Various solutions are however being studied 

(e.g. see Figure 6.30, right). 

� The low surface reactivity of CNTs causes a problem in some cases, e.g. when 

strong nanotube/matrix interactions are needed as in composites for structural 

applications (in which stress transfer from the matrix to the nanotubes would 

then be minimized), or when there will be a need to interconnect them, (e.g. 

when used as electronic components). CNT functionalization discussed in the 

preceding section is a way to overcome this problem. 

� Cost is still a major issue, as for any recently developed material. Marketed prices 

are in the range 80–2000 $/g for SWNTs, and higher than 0.05 $/g for MWNTs, 

depending on the synthesis process, number of post-treatments, and resulting 

purity, structural quality, etc. Obviously, this is by far too high for incorporating 

most of nanotube grades in any mass application. But prices tend to get lower 

every year, and were already divided by 2 to 4 within the past decade. 

� Health and safety issues are nowadays a major concern for any nanosized 

material. The toxicity of CNTs is supposed to increase as the CNT aspect ratio, 

specific surface area, and surface reactivity (when any, e.g. for functionalized 

nanotubes) increase. But results in literature are still contradictory [110], and 

there is no certainty yet regarding the actual cyto- and eco-toxicity of CNTs. 

A significant reason is the very large variety of CNTs on the one hand, and 

the relatively poor characterization of the nanotubes investigated on the other 

hand, which make results barely comparable and understandable. Another 

significant reason is the absence of standard procedures for the evaluation of 

toxicity. Investigations are in progress worldwide in this regard.


Figure 6.28 Example of the typical 

aspect of an as-prepared nanotube 

material from the arc discharge 

process (from [108]). Round morphologies 

with dark contrast are metal 

catalyst remnants. Every other phase 

is carbon-based. 

Figure 6.29 Example from purified 

CNTs, formerly prepared via catalystenhanced 

chemical vapour deposition 

(CCVD). Even rather clean, nanotubes 

exhibit discrepancies in diameter, 

and number of walls (one or two). 

Figure 6.30 In spite of the addition of a surfactant such as sodium dodecyl sulfate (SDS), the 

dispersion of as-prepared SWNTs (from arc discharge) is poor and forms agglomerates visible 

with an optical microscope (left). If the material is pre-treated with freeze-drying, agglomeration 

occurs only at a much lower scale, invisible to the optical microscope (right) (from [109]). 

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6.3.3 

Applications of CNTs 

Applications of nanotubes are currently at various levels of development. Considering 

the variety and excellence of the physical properties of nanotubes, areas 

in which they can be applied are too numerous to be listed here (Figure 6.31). 

Hence, only examples will be provided, including (1) prospective applications; 

(2) applications under development; and (3) applications already on the market. 

Figure 6.31 The ‘application-tree’ for CNTs. Not exhaustive (by courtesy of M. Endo, modified 

from original). 

6.3.3.1 Prospective Applications 

Applications from this category still have to face one or several drawbacks from 

those listed in Section 6.3.2.3 (beside cost) to such an extent which severely – yet not 

definitively – prevents their development. They are therefore still in the research 

laboratories, although examples of prototypes can be found, mostly fabricated by 

companies for high-tech advertising purposes rather than for real advances in 

performance with respect to the existing technology (e.g. sport goods).


Structural composites. While the benefit of CNTs for tribological applications is 

still being investigated (for which both the mechanical and thermal performances 

are likely to be useful), the benefit expected from incorporating nanotubes into 

various matrices to make composites for structural applications appears obvious 

from considering Figure 6.27. Difficulty regarding processing (dispersion) is the 

main problem still to be solved, as well as the low reactivity that prevents good 

bonding to the matrix. Consequently, even if CNTs (e.g. SWNT, and concentric- 

MWNT types) are actually known to reinforce matrices efficiently [111, 112], the 

resulting performances still hardly beat those of composites reinforced with regular 

carbon fibers available on market. Cost and safety are also issues that contribute to 

delaying the development. Meanwhile, various ways to make nanotube-based fibers 

are being investigated: (1) as a filler to fiber precursors at soften state (polymer 

[e.g. 113] or pitch [114]), (2) as a dry-spun [115] or wet-spun [116] nanotube fiber 

from as-prepared nanotubes. The interest of making micrometer-sized fibers out 

of nanotubes is to allow weaving fabrics, that can be put in composites or to be 

used as such. 

Once the pending problems are solved, nanotube-containing composites will 

lead to mass applications, starting with sport goods (e.g. golf club shafts, tennis 

racket frame, fishing rods) then structural parts for transportation vehicles of all 

kinds, from bicycle to space shuttle, and many other areas. Fabrics prepared with 

nanotube-based fibers (i.e. with no matrix involved) will be also used for functional 

clothes (e.g. bullet-proof vest), cables, stain-resistant textiles, etc. 

Electronic parts. Thanks to the amazing versatility of electronic behavior of SWNTs, 

they could be components of miniaturized computers, or, in a more distant future, 

in ultra-fast quantum computers, e.g. as [117]: (1) connectors, using metallic-type 

SWNTs, with a possibility of building 3D circuitry for better compactness; (2) transistors, 

using semi-conducting SWNTs (ten times faster than regular metal-oxidesemiconductor 

transistor); (3) diodes, using combined metallic/semi-conductor 

SWNT junctions via pentagons and heptagons; (4) superconducting transistors 

[118]. A common advantage of such carbon-based nano-components for electronics 

is low energy consumption and their ability to withstand high temperatures, thus 

not needing cooling systems. Most current obstacles to development are the lack 

of sources for SWNTs with specific transport behavior, the quality and feasibility 

of contacts, and more generally the technical difficulty to build electronic circuits 

with nanosized components. 

Actuators. Based on the finding that the lattice constants of graphene expand differently 

when doped with anions and when doped with cations, the idea of using 

doped nanotube-based fibers as current-powered actuators was proposed in 1999 

[119]. Making artificial muscles is expected, thanks to both the biocompatibility 

and the unprecedented mechanical properties of CNTs. Such artificial muscles 

could be used for repairing surgery (and robotics), if internally implanted, but 

they also could be used for military purpose and hard structures, if supported by 

an external frame providing extra strength to anyone wearing it as some kind of 

363


suit. Problems are many and studies are in progress, in particular at the University 

of Texas (R. Baughman’s group) and in France (P. Poulin’s group). Voltagepowered 

nano- or micro-scaled tools, although not necessarily requiring previous 

ion-doping, such as micro-tweezers [120], have also been proposed. Related applications 

are mechanical memory elements (as developed by NANTERO), and 

nanoscale electric motors. 

Membranes and low-friction surfaces. Filtering membranes can be made which 

are able to let water molecules go through the inner channels while large ions 

are blocked. Thanks to the perfect smoothness of the graphene surface, high 

speed transfer of fluids throughout the inner cavity of nanotubes, five orders of 

magnitude higher than predicted by theory, was recently demonstrated [121]. 

Likewise, again thanks to the low friction, but also to the low reactivity and thermal 

resistance, slick surfaces (slicker than Teflon) can be envisaged. 

6.3.3.2 Applications Under Development 

Applications from this category are being developed by profit-based companies, 

which have invested enough to prepare prototypes that exhibit one or several 

benefits with respect to existing materials and devices. Reaching the market 

is however still prevented by one or several drawbacks from those listed in 

Section 6.3.2.3, generally including cost issues. 

Conductive composites (part 1). They mostly correspond to any application in 

which static electricity is a problem [112]. Thanks to the huge aspect ratio of 

CNTs (in the range of 1000), loading an insulating polymer matrix with them 

allows reaching the percolation threshold with a CNT content as low as 1 wt% 

or less (depending on the type of CNTs used), as opposed to ~15 wt% for regular 

nanosized carbon fillers such as carbon blacks. One of the benefits is that the 

color of the resulting composite is not affected, as opposed to becoming black, and 

even photo-transparent polymers remain so. The development of nanotube-loaded 

conductive composites is less problematic than that of structural composites 

because there is no need for a specific bonding with the matrix. Consequently, 

transparent films are prepared as an alternative to regular indium tin oxide films 

to make flexible polymer-based transistors, touch-screens, and displays (Eiko, 

Unidym). Good thermal conductivity brought by incorporating nanotubes into 

materials is also of use. Nanotubes loaded polymer composites are also planned 

to be applied in tanks dedicated to the storage of spark-sensitive, explosive or inflammable 

compounds (Arkema). Other related applications are electro-magnetic 

shielding, radar-absorbing materials, electric motor brushes, heat exchangers and 

dissipaters, etc. 

Bio-related materials. Other nanotube-loaded polymer-based manufactured materials 

are also under development, yet not with the goal to make them electrically 

conductive but biocompatible. Polymer-based surgery wires (catheters) were successfully 

tested as an alternative to currently used animal-derived wires (‘catguts’).


Prosthetic coatings are also considered because cells were shown not to adhere 

to (some of) nanotubes, on the one hand, while graphene surface has a very low 

friction coefficient on the other hand. The same properties made them applicable 

for anti-fouling coatings for boat hulls. Likewise, water and air filtration devices 

made from nanotubes are supposed to be more resistant to fungus and bacteria 

colonization. 

Batteries and supercapacitors. Appropriate features for energy storage device electrodes 

are both a high amount of mesoporosity (allowing for an easy circulation 

of the electrolyte) and a high amount of microporosity (presenting a high surface 

area of charge exchange to the electrolyte which is required for fast current delivery 

and high speed charging), as well as good electrical conductivity. Appropriate pore 

size distribution was actually previously achieved using other kinds of carbon 

materials. However, the preparation of the latter involved chemical treatments 

(activation) detrimental to the material conductivity. Based on the early work by 

Niu et al. [122], CNTs were found to be able to intrinsically exhibit all the required 

features while requiring limited pre-treatments. Development is in progress [123], 

but nanotubes-based supercapacitors (with capacitance > 300 F/g, exhibiting faster 

speed charging with respect to other carbon-based supercapacitors) are now built, 

e.g. by Montena) as well as fuel cell with nanotube electrodes (Nec, Motorola, 

Intematix). Recently, a foldable, postage-stamp-sized supercapacitor with a voltage 

of ~2.5 V was developed from a nanotube reinforced cellulose paper [124]. 

Support for catalysts. Based on a pioneering demonstration by Planeix and 

coworkers [125], nanotubes were also found to be good catalyst supports for catalyst 

particles for heavy chemical industrial process, with promises for needs of mass 

production (Arkema). High surface area, low chemical reactivity, large mesopore 

network, and tailorable surface energetics (for retaining catalyst nanoparticles 

and preventing them from coalescence) are among the suitable properties of 

nanotubes for this application. With respect to regular ceramic-based catalyst 

supports (alumina, typically), CNTs also exhibit much higher mechanical strength, 

which goes with higher durability. 

Chemical sensors. Based on pioneering works published in 2000 [126, 127] CNTs 

can be used as chemical sensors, either under the form of a single SWNT (semiconductor 

type), or a paper-like SWNT network. Because electron transport in 

CNTs is a surface phenomenon, the conductance response is affected by the occurrence 

of molecule(s) adsorbed onto the nanotube surface. More interestingly, the 

response can be specific since it differs depending on the nature of the molecule 

adsorbed. Highly sensitive (sometimes in the range of ppm) chemical sensors are 

therefore currently developed (Nanomix, Motorola) for use in atmosphere control 

for instance. Among other companies involved are Applied Nanotech, Carbon 

Nanoprobes, Honeywell International, Nanosensors, etc. 

365


Electron emitters (part 1). Thanks to their ability to withstand enormous current 

density (see Table 6.3), it was proposed as early as 1995 [128] that CNTs could be 

used as powerful field emitters [128, 129]. Alternatively, they are able to carry the 

same current density as regular emitters (e.g. in Mo or Si) but with a much lower 

extraction voltage threshold, typically of several orders of magnitude. Depending 

on the target onto which the emitted electrons are projected, various applications 

are proposed. If projected onto a phosphor layer, nanotube-based emitter arrays can 

serve for low consumption displays for TV sets or computers, or high autonomy 

portable devices (e.g. cell-phones). Samsung was first to develop a prototype of TV, 

then Motorola, Futaba Corp., Copytele Inc., etc. However, it seems that the fact 

that no nanotube-based flat screen TV has yet been marketed, while prototypes 

were shown several years ago, could be because its life-time is still insufficient. 

On the other hand, instead of considering a periodic array of nanotube emitters, 

a single nanotube-based tip can be used as a field emission electron source 

with high coherence and brightness for electron microscopes [129], as currently 

developed by FEI). 

6.3.3.3 Applications on the Market 

Applications from this category are those available on market or used by companies 

to process manufactured products. There are still few, and considering arguments 

given in Section 6.3.2.3, no wonder if they correspond to applications with low 

requirements regarding purity and selectivity, and/or with low requirement 

regarding quantity. 

Electron emitters (part 2). If the target is a metal cathode (Cu, Mo), the very 

high current density carried by a single nanotube tip allows building portable 

X-ray generators. As opposed to electron sources for electron microscopes (see 

Section 6.3.3.2), ultimate, expensive technology (e.g. ultra-high vacuum) is not 

required in that case, which has allowed e.g. Oxford Instruments to put such an 

application on the market. 

Near-field microscopy probes. In 1996 Dai et al. [130] were first to propose to use 

a single carbon nanotube as a tip for atomic force microscopy, with several advantages 

with respect to regular ceramic tips (Si, Si 3 N 4 ), typically a much higher 

mechanical resistance, providing a longer life-time, a much higher aspect ratio, 

and a better lateral resolution, and minimizing tip artefacts. Benefits are high 

enough to compensate still prohibitive prices, in the range of several hundred 

dollars per tip (Nanoscience Instruments). 

Conductive composites (part 2). An application of nanotube-loaded conductive polymers 

that can be considered on the market is electro-painting (in which applying an 

electric potential difference between the painting nozzle and the piece to be painted 

prevents electrostatic repulsion effects and allows for a good adhesion of the paint), 

since General Motors has already applied it to some car parts. Electro-painting of 

polymer-based parts is also proposed by Hyperion for a good surface finish.

6.3.4 

Conclusions 


CNTs dominate R&D in the field of nanotechnology. This is fully justified by their 

large panel of amazing properties. On the other hand, only a few applications are 

on the market so far, even though about 15 years have elapsed from the moment 

when the scientists realized the full potential of nanotubes. This has been enough 

to inspire recent pessimistic comments regarding the ultimate usefulness of 

nanotubes that would finally not fulfill the expectations regarding their ubiquitous 

applicability. Such doubts are also inspired by the previous disappointment related 

to fullerenes, whose discovery in 1985 was acknowledged with a Nobel Prize in 

1996, which have generated millions of dollars investments in research programs 

worldwide, but have finally resulted in only a few examples of applications. 

Such an analysis is misleading because it ignores the major difference between 

fullerenes and nanotubes, which is the high morphological, textural, nanotextural, 

and structural versatility of the latter with respect to the former. It also ignores 

the time usually needed to bring the idea to the market in the field of advanced 

materials. In this regard, the example of carbon fibers teaches us that CNTs are 

just following the regular path (Figure 6.32). 

Carbon fibers were actually invented in the 1960s, were first used in sports goods 

about 12 years later, then started being used in aircraft and space industries after 

10 more years. Finally, it took about 40 years in total for the market to become 

mature and demanding for large quantities, synonymous of large profits. CNTs 

do not behave differently, and they still have the potential to ultimately dramatically 

influence our daily life. 

Figure 6.32 Evolution of the world market for carbon fibers (by courtesy of M. Endo). 

367


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373

7 

Angle-strained Cycloalkynes 

Henning Hopf and Jörg Grunenberg 

7.1 

Introduction 

According to the sp-hybridization model, the carbon–carbon triple bond possesses 

a linear structure, and indeed, many alkynes fulfill this prediction. High level ab 

initio calculations [1] of isolated alkyne molecules, as well as gas phase electron 

diffraction experiments [2] reveal their linear minimum structure. On the other 

hand, given that the C�C–R bending force constant becomes intrinsically smaller 

for substituted alkynes and polyynes, crystal packing effects may lead to deviations 

from strictly linear geometries [3]. From a dynamic point of view even in the case 

of acetylene one has to include the – albeit reduced – flexibility of the carbon– 

carbon triple bond in order to describe, for example, the vinylidene acetylene 

rearrangement (see below) [4]. Further, the linear sp-hybrid picture only holds true 

for the electronic ground state. Cis- and trans-bent configurations are known in 

the case of low-lying electronic states [5]. Nevertheless, many alkynes correspond 

to the prediction of linearity due to the sp-hybrid model, and we have collected 

a selection of typical examples, reaching from the parent compound acetylene 1 

via a few alkyl 2–4 and aryl acetylenes 5–7 to a functionalized acetylene, dimethyl 

acetylenedicarboxylate 8, in Scheme 7.1, together with the appropriate references 

[1, 2, 6–11], all dating from the recent literature. 

The easiest way to distort a triple bond is to incorporate it into a sufficiently small 

ring structure. The question, of course, is what this ring size is, and from what 

ring size on the respective cycloalkynes can we isolate compounds, that can be 

worked with under normal laboratory conditions [12]. Rather than using the nonspecific 

term ‘distorted cycloalkyne’ Krebs [12] prefers to speak of ‘angle-strained 

cycloalkynes’. Since it is easier to deform a C�C–C arrangement than its more 

hydrogenated olefinic and saturated analogs, relatively large angle deformations 

are possible in cycloalkynes without significant changes in energy. Krebs arbitrarily 

considers all cycloalkynes in which the C�C–C angle is deformed by more than 

10° as angle-strained [12]. Accepting this definition, the stable cyclononyne (see 

below) is an angle-strained cycloalkyne, whereas cyclodecyne is not. 



ISBN: 978-3-527-31767-7 

375

376 7 Angle-strained Cycloalkynes 

Scheme 7.1 

7.2 

Cyclopropyne and Cyclobutyne: Speculations and Calculations 

on Non-isolable Cycloalkynes 

7.2.1 

Cyclopropyne and Related Systems 

The smallest conceivable cyclic acetylene is cyclopropynylidene (9, Scheme 7.2), 

the lowest member of the cyclocarbons (see below). 

Scheme 7.2 

This structure has been discussed in the chemical literature [13], but only in the 

context of theoretical investigations. On the other hand, cyclopropyne (general 

structure 10) has been invoked both in spectroscopic [14] and theoretical studies 

[15]. According to computational evidence, it appears likely that the tetragonal 

form of this hydrocarbon, 14, is a transition state in the automerization of propadienylidene 

13; whereas planar cyclopropyne 15 could be involved as a transition 

state in the rearrangement of 13 into cylopropenylidene (11, Scheme 7.3) [15], a 

C 3H 2 isomer that has already been prepared in matrix in 1984 [16] and has been 

discussed in connection with interstellar chemistry [17]. 

The olefinic system, corresponding to 10, cyclopropene 12, known for a long 

time, has often been used as an addition partner in preparative chemistry [18].

7.2 Cyclopropyne and Cyclobutyne: Speculations and Calculations on Non-isolable Cycloalkynes 

Scheme 7.3 

Replacement of the methylene group in 10 by a SiH 2 moiety results in silacyclopropyne 

19, a cycloalkyne that has been generated as summarized in 

Scheme 7.4 [19]. 

Pulsed flash pyrolysis of the precursor 16 results in the generation of the 

silylene 17 that undergoes rearrangement to the silacyclopropene 18. Photolysis 

of either 17 or 18 furnished 19, the first formal cyclopropyne ever to be prepared 

and characterized experimentally by matrix IR spectroscopy and theoretically by 

ab initio calculations. 

The even simpler SiC 2 20 has been prepared by laser vaporization of a silicon 

carbide rod within a pulsed supersonic nozzle [20]. By spectroscopic analysis it was 

shown that the molecule is triangular in both the ground and excited electronic 

states. The carbon–carbon length in the ground state is 125 pm and the C–Si–C 

angle amounts to 40°; i.e. the cycloalkynes possesses C 2v symmetry. 

Didehydrooxirene (21, oxacyclopropyne) has been discussed in connection with 

mass spectrometric studies of the hydroxypropynal molecular ion [21]. 

Scheme 7.4 

377


7.2.2 

Cyclobutyne 

Experiments to generate and trap cyclobutyne have been scarce until the present 

day although there were several early attempts to generate this next higher cycloalkyne 

homolog [22]. 

According to theoretical studies, cyclobutyne may be the limiting ring size for 

simple cycloalkynes: in ab initio calculations both singlet and triplet cyclobutyne 

correspond to energy minima [23]; the structural parameters of a calculated 

structure, 22, are shown in Scheme 7.5. 

Scheme 7.5 

Two isomerization processes have been considered likely for the highly reactive 

22: thermal isomerization to butatriene 23, and ring contraction to cyclopropylidene 

methylene 24 [24]. Whereas the (calculated) activation energy for the 

unimolecular rearrangement of 22 to 23 should be substantial [25], cyclobutyne 

22 should isomerize to 24 with little or no barrier making the direct observation 

of this cycloalkyne difficult or impossible [26]. 

These calculations are supported by the results summarized in Scheme 7.6 

[27]. 

Scheme 7.6

7.3 Cyclopentyne, Cyclohexyne, Cycloheptyne: from Reactive Intermediates to Isolable Compounds 

When the halides 25 are treated with lithium diethylamide (LDA) in THF in 

the presence of lithium thiophenolate, the two thio ethers 27 and 29 are obtained 

after work-up. To rationalize these findings it has been proposed that initially a 

cyclo butyne derivative, bicyclo[3.2.0]-hept-6-yne 26 is generated, that subsequently 

either adds the trapping agent to form 29 or undergoes the ring-contraction to 28 

mentioned, that then reacts with the thiophenolate to give 27. 

Annelation with an aromatic system has often been employed to stabilize 

reactive organic molecules; however, in the case of synthesizing benzannulated 

cyclo butynes this approach failed [28]. Another method of influencing the stability 

and reactivity of hydrocarbons consists in exchange of all hydrogen substituents 

for fluorine. However, in the case of cyclobutyne, ab initio calculations suggest that 

fluorine substitution leads to destabilization with respect to cyclobutene [29]. 

The ability of metal atoms to complex and stabilize highly reactive molecules is 

well known [30], and metal complexes containing the ‘cyclobutyne’ ligand such as 

Os 3(CO) 9(µ-� 2 -cyclo-butyne)(µ-SPh)(µ-H) have been described [31]. 

7.3 

Cyclopentyne, Cyclohexyne, Cycloheptyne: from Reactive Intermediates 

to Isolable Compounds 

7.3.1 

Cyclopentyne and its Derivatives 

Reliable experimental ground is reached with cyclopentyne 34, which has been 

generated by various routes [32]; three conceptually different ones are summarized 

in Scheme 7.7. 

For example, on phenyl lithium treatment, 30 is debrominated to carbene 33, 

that stabilizes itself by the already discussed vinylidenecarbene � acetylene rearrangement 

to 34 [26, 33]. 

That 34 has actually been produced was shown by different trapping experiments: 

for example, in the presence of trans-2-butene the cyclobutene 36 was 

obtained [34]. In another route cyclopropenone 31 was prepared by photodecomposition 

of 2,6-diazocyclohexanone in an argon matrix and was decarbonylated on 

further irradiation at 8 K to 34 [35]. Originally, only the allene 37 as a secondary 

product of cyclopentyne was observed; in later experiments the IR spectrum of 

34 could be recorded in matrix [36]. Finally, cyclobutanone 32 was converted to 

34 via 35 and (presumably) 33. In this case trapping with trans-1-methoxypropene 

yielded the trans-cycloadduct 38 [37]. 

Cyclopentyne 34 can be stabilized in various ways (Scheme 7.8). 

4-Thia-3,3,5,5-tetramethylcyclopentyne 39 has been obtained by oxidation of the 

corresponding bis-hydrazone and has been trapped by various addition reagents 

[38]; the intermediate is presumably less strained because of the longer C–S bonds 

and consequently larger C�C–C bond angles, and the neighboring gem-dimethyl 

groups provide further protection (see below). 

379


Scheme 7.7 

Scheme 7.8�


Metal complexation provides another means of stabilization. Thus the platinum 

complex 40 is reduced by sodium amalgam to give a colorless, very reactive solid 

for which structure 41 has been proposed on the basis of NMR and IR data [39]. 

On methanol treatment 41 gives instantaneously the dinuclear complex 42, the 

structure of which was established by X-ray crystallography [39]. 

Stabilization by condensed aromatic rings is realized in the case of the aromatic 

hydrocarbon acenaphthyne 45, which has been prepared by several routes as 

shown in Scheme 7.9. 

Thus photoextrusion of nitrogen from 43 [35] or nitrogen and carbon monoxide 

from the precursor 44 in a matrix led to 45 – allowing its UV and IR spectra to be 

recorded at 15 K – as did the Ramberg–Bäcklund rearrangement of the dibromo 

sulfone 48 [40]. Chemical proof that 45 has been generated in these transformations 

was established by its interception with oxygen leading to the quinone 46, 

and self-trapping furnished the trimer decacyclene (47, see below) [35]. 

The bicyclic acetylene norbornyne 51 was first prepared [41] by metalating 49 

with n-butyllithium; the resulting organolithio intermediate 50 on mild warming 

lost lithium chloride to provide 51 that trimerizes in poor yield (ca. 10%) to the 

polycyclic aromatic stereoisomers 52 and 53 (Scheme 7.10) [42]. 

Scheme 7.9 (a) h�, Ar, 15 K; (b) h�, Ar, 15 K; (c) t-BuOK, t-BuOH, room temp.; (d) room temp. 

(5% from 44); (e) O 2 . 

381


Scheme 7.10 

The still higher unsaturated cyclopentyne derivative bicyclo[2.2.1]hept-2-en- 

5-yne [43] has also been generated by an elimination reaction [43, 44]. With a 

calculated strain energy of ca. 97 kcal mol –1 it is approximately 30 kcal mol –1 more 

strained than cyclopentyne itself [44]. Lower bicyclo[2.1.1]hex-2-yne [45] and higher 

homologs of 51 have also been produced (see below) [46]; like norbornyne 51 they 

trimerize to aromatic compounds of type 52/53. 

7.3.2 

Cyclohexyne and its Derivatives 

Among all angle-strained cycloalkynes cyclohexyne 56 has received the greatest 

attention. Since this classical work has been reviewed several times [47], only the 

most important approaches to 56 will be discussed here (Scheme 7.11). 

In the debromination of 1,2-dibromocyclohexene 54 [48] and the dehydrochlorination 

of 1-chlorocyclohexene 55 [49] the relationship between precursor 

and the desired hydrocarbon is obvious. The oxidation of the bis-hydrazone 57 

to 56 can be rationalized by postulating a bis-carbene intermediate [50]. And the 

last two approaches [51] could also proceed via a vinylidene carbene, 59. Since 

cyclohexyne 56 is still too reactive to be isolated as such, the chemical proofs of its 

intermediate existence again rest on trapping and matrix isolation experiments. For 

example, it has been reacted with tris(triphenylphosphine)platinum(0) to provide a 

stable platinum complex [52], and trimerization leads to dodeca hydrotriphenylene 

(see below) [40, 48]. 

Just as in the case of the lower homolog, matrix isolation has allowed recording 

of the IR spectrum of 56 (Scheme 7.12) [53, 54]. 

Flash vacuum pyrolysis of 4-cyclopentylidene-3-methyl-isoxazol-5(4H)-one 61 

at 700–800 °C and 10 –4 torr causes fragmentation to the carbene 59 that isomerizes 

to 56. The IR spectrum recorded at 77 K shows absorption bands at 2090 and 

2105 cm –1 that were assigned to the stretching vibrations of the deformed triple 

bond. These signals disappeared on warm-up between –110 and –100 °C, and 

when room temperature had been reached, 62 had been formed.


Scheme 7.11 (a) Li/Hg, THF; (b) PhLi, ether; (c) HgO, benzene; (d) 480–640 °C; (e) tBuOK. 

Scheme 7.12 

The tetramethyl flanked cyclohexyne 64 is produced on heating 63 at 370 °C and 

then quenching the pyrolysate at 12 K in an Ar matrix [55]. Again, the vibrational 

spectrum could be recorded at low temperature, but, disappointingly, the C�C 

stretching vibration of 64 could not be identified; very likely because it is hidden 

under one of the (intense) CO-bands at 2139 and 2149 cm –1 . On warming the 

reaction mixture to 45 K dimerization of 64 to the enyne 65 took place. 

383


Scheme 7.13 

As in the case of the bicyclic cyclopentyne 51 a relatively large number of 

cyclo hexyne derivatives is known [12, 56] in which this strained moiety has been 

incorporated into a polycyclic framework. 

The surprisingly stable tetrasila cyclohexyne 68 is obtained in good yield when 

the tetrasilane 66 is reacted with acetylene bis-Grignard (67, Scheme 7.13). 

This cyclohexyne is stable at room temperature and has a half life of 8 h at 

174 °C in decane; clearly, it is stabilized both by steric protection through the 

methyl substituents and the long carbon–silicon and silicon–silicon bonds and 

large C�C–C bond angles, respectively [57]. Still, the acetylene–vinylidenecarbene 

equilibrium can evidently take place here as well, since in the presence of diphenyldiazomethane, 

68 is converted into the allene 70 in quantitative yield, presumably 

by forming the carbene intermediate 69 first. 

7.3.3 

Cycloheptyne and its Derivatives 

The methods used to prepare cycloheptyne do not differ from those employed to 

prepare the lower homologs (see above). However, with this hydrocarbon we are 

beginning to reach the shores of stability. In dilute dichloromethane solution at 

–25 °C this cycloalkyne (72, Scheme 7.14) has a half-life of less than a minute, but 

at –78 °C this has already increased to one hour [58]. 

Irradiation of the cyclopropenone 71 in an Ar matrix at 17 K provided 72 as 

expected and allowed the measurement of the vibrational spectrum: the triple 

bond stretching frequency is registered at 2121 cm –1 [59]. 

The blockage of the immediate vicinity of the triple bond by methyl groups leads 

to a drastic increase in stability. 3,3,7,7-Tetramethylcycloheptyne 73 is a stable hydrocarbon 

at room temperature with a half-life for dimer formation of one day at 

25 °C [60]. Compared to the parent hydrocarbon 72 the methyl protected derivative

Scheme 7.14 

7.4 The Isolable Angle-strained Cycloalkynes: Cyclooctyne, Cyclononyne, and Beyond 

dimerizes 10 7 to 10 8 times more slowly. Krebs and co-workers have synthesized 

numerous cycloheptyne derivatives of the general structure 74 [12]; most of them 

are more stable than 72. For example the thio ether (74, X = S) shows no tendency 

to di- or oligomerization even at 140 °C [61]. Because of the stability of the sulfur 

compound it was possible to determine its molecular structure by electron diffraction; 

with 145.8° for the C�C–C angle the deformation is about 12.7° higher 

than that observed for cyclooctyne (see below) [62]. 

As described for cyclohexyne (see above), the platinum complex 75 was 

obtained from intermediately generated 72 [52]. Its X-ray structural analysis 

shows that the triple bond deviates from linearity by ca. 40°. The dimeric metal 

complex 76 is produced if 73 is reacted with CuCl·Me 2 S. X-ray crystallography 

reveals that the ‘deformation angle’ at the triple bond amounts to 34.4° in this 

case [63]. 

7.4 

The Isolable Angle-strained Cycloalkynes: Cyclooctyne, Cyclononyne, and Beyond 

7.4.1 

Cyclooctyne and its Derivatives 

With cyclooctyne (79, Scheme 7.15) we have definitely reached the region of stable 

acetylenes, and, in fact, the hydrocarbon is so readily available [64], that it has 

become a versatile building block in organic synthesis [65]. The presently most 

convenient methods to prepare 79 are summarized in Scheme 7.15. 

385


Scheme 7.15 (a) Br 2 ,CH 2 Cl 2 , –40°C; (b) t-BuOK, ether, THF, 0–15 °C; (c) LDA, hexane, 

THF, –25 – +15 °C (80–84%); (d) n-BuLi, –70 °C (X = S; 70%. X = Se; 85%). 

Starting from cyclooctene 77 bromination first provides the expected dibromide 

which is dehydrobrominated as shown in the scheme to the vinylbromide 78. After 

a base switch to LDA a second elimination step furnishes 79 that can be obtained by 

this route in 30-g lots [66]. Alternatively, the also readily available 1,2,3-thiadiazole 

(80, X = S) or 1,2,3-selenodiazole (80, X = Se) on treatment with n-butyl lithium 

at low temperature furnish the cycloalkyne in good yield as well [67]. 

Yet cyclooctyne still possesses a quite strongly deformed C–C�C–C-unit. 

Structure determination by electron diffraction shows a reduced C�C–C angle of 

154.5° [68]. Whereas many medium-ring cycloalkynes have an overall flat structure 

(see below), cyclooctyne is twisted. 

Because of the strained nature of their triple bonds, cyclooctynes form metal 

complexes readily [69]. 

A sizeable number of derivatives of cyclooctyne is known, and many of these 

are summarized in compilations by Krebs and Meier and their coworkers [12, 

70–72]. 

7.4.2 

Cyclononyne and Cyclodecyne 

With cyclononyne and cyclodecyne we are ending our journey through the 

homologous series of cycloalkynes. Both hydrocarbons had been obtained by 

various elimination reactions from the appropriate precursors but were often 

contaminated with their allenic and dienic isomers. They were finally obtained in 

pure form by employing the Curtius route (oxidation of 1,2-bis-hydrazones, see 

above) to cyclonona- and cyclodeca-1,2-dione [73]. Still, cyclononyne is not free 

from angle-strain. With a C�C–C angle of 160.2° – determined by electron diffraction 

[74] – this is only slightly larger than in cycloctyne (see above), while the 

triple bond has a normal length in cyclononyne. No experimental structural data 

of cyclodecyne seem to be known. According to molecular mechanics calculations 

the above angle is 171.6°, i.e. still not yet 180° [75]. It has been pointed out, though, 

that because of non-bonded interactions, additional deformation and hence additional 

strain is produced, making it unlikely that cyclodecyne will possess a linear 

arrangement of the triple bond and the two carbon atoms flanking it [76].

7.5 

Cyclic Polyacetylenes 

7.5 Cyclic Polyacetylenes 

As cyclic polyacetylenes we want to define all cycloalkynes possessing at least two 

triple bonds, whether these are conjugated or not. Many of these systems possess 

nonlinear acetylene or diacetylene moieties, and the number of know representatives 

of this class is growing rapidly. The following compilation does not intend to 

be comprehensive; it only aims at demonstrating the large variation in structures 

one is encountering in this sub-group. 

One the most famous hydrocarbons in this class is 1,5-cyclooctadiyne 81, 

originally prepared in small yield by the dimerization of butatriene 23 [77], and 

later with far better success from the dibromide 82 under the conditions shown 

in Scheme 7.16 [78]. The hydrocarbon is essentially planar, the deviation from the 

180° degree geometry of the acyclic model compounds summarized in Scheme 7.1 

amounts to 20.7°, and the distance between the parallel double bonds is 259.7 pm. 

Compared to cyclooctyne 79 (see above) the angle deformation at the triple bond 

is less pronounced. The two benzannelated hydrocarbons 83 and 84 have also 

been investigated by X-ray crystallography. In 83 the C�C–C angle is 155.8° and 

the distance between the triple bonds amounts to 261 pm. The molecule is nearly 

planar, but the two benzene rings are slightly folded out of the plane of the eightmembered 

ring by 2°, a distortion that has been related to packing effects [79]. 

For 84 the corresponding structural data are: acetylene angle distortion 154°, 

(average) distance between the triple bonds 285 pm [80]. 

Scheme 7.16 

387


The interesting tetrasila analog of 81, the octamethyl derivative 86, is obtained, 

when 85 is subjected to flash vacuum pyrolysis at 680 °C or to irradiation [81]. 

The compound is practically planar, its deformation from linearity at the triple 

bonds is smaller than that for 81 (average angle 166°), and the intra-annular 

distance between the acetylene moieties has increased to 324 pm, a consequence 

of the longer bonds to silicon. When 86 is pyrolyzed a second time, another 

die methylsilylene unit is split off and the seven-membered diacetylene 87 is 

produced. 

Gleiter and co-workers have prepared the symmetrical bis homolog of 81, the 

decadiyne 88 [82], and determined its structural parameters. As indicated in 

Scheme 7.17 the molecule possesses a chair conformation and the molecular arrangement 

at the triple bonds (intraannular distance 299 pm) is clearly nonlinear 

(C�C–C angle 171.7°). In the higher homolog, the dodecadiyne 89, the triple bonds 

are crossed (crossing angle 24°) and still not quite linear (C�C–C angle 173.8°) 

[82]. The introduction of heteroatoms into the carbon framework of 89 has no 

significant influence on the structure. In all systems 90 chair-like conformations 

prevail, the triple bonds are always slightly bent, the deviation from 180° varying 

between 6 and 10°, and the distance between the triple bonds lies between 290 

and 310 pm [83]. 

As expected, ring enlargement causes stronger linearization of the C–C�C–C 

fragment. For example, in the 12-membered hydrocarbon 91 the deviation from 

180° amounts to 5.9° only [84], and in the 14-membered bis-ether 94, obtained 

from the dibromide 92 via the bis-acetylene 93 (Scheme 7.17), the four carbon 

atoms lie practically on one single line (C�C–C angle 178.7°) [85]. 

Numerous cyclic and polycyclic systems have been obtained in the meantime 

that contain more than two triple bonds [86, 87]. A classical example is shown in 

Scheme 7.18 in which the cyclic polyacetylene 95 is isomerized by base treatment 

to a fully conjugated dehydroannulene 96. Although many of these intermediates 

and products are highly instable and could not be studied by X-ray crystallography, 

in some cases single crystals for X-ray investigations could be obtained, and they 

showed that these compounds did indeed contain distorted acetylene or diacetylene 

moieties as subunits. For example, in the tetrayne 97, a hydrocarbon possessing a 

Scheme 7.17

Scheme 7.18 


chair conformation, the angle �C–C�C amounts to 176.7°, whereas the C�C–CH 2 

angle is 172.9°; the C1–C11 distance is 309.8 pm, whereas the C2–C10 distance is 

slightly extended to 339.0 pm [88]. In the even more strained tetrayne 98 the two 

mentioned distortion angles are 166.6° and 166.3°, respectively, and the C1–C10 

distance is 269.6 pm, whereas the C2–C9 separation amounts to 323.6 pm [89]. 

Overall these molecules adopt an ellipsoidal shape. 

Under the influence of the discovery of new allotropes of carbon (fullerenes) 

there has been a veritable renaissance of annulene chemistry and a huge number 

of new carbon-rich compounds has been prepared. Since there are very recent 

reviews available in this area, these studies will not be discussed here [90, 91]. 

A small selection of stable and intermediately produced distorted alkynes are 

summarized in Scheme 7.19 to illustrate where the development is going. 

The octaacetylene 99 (Scheme 7.19) has, as expected, a planar structure [92]. That 

there must be considerable strain in the molecule is indicated by the butadiyne 

moieties which have bond angles as low as 164.5°. 

More and more examples are reported in the chemical literature in which 

distorted alkyne units are incorporated in three-dimensional structures. For 

example, the vinyl bromide 100 on treatment with potassium tert-butoxide 

evidently yields the (simplest) cyclophyne, 101. This has not only been inferred 

from the isolation of its trimer, trifoliaphane 102 [93], but also by a metal complexation 

experiment similar to the one discussed above for cyclopentyne (cyclophanes 

are presented in Section 4.2). Thus treatment of dibromide 103 with 

sodium amalgam in the presence of platinum tris(triphenylphosphine) afforded 

the complex 104 in good yield. As shown by X-ray structural analysis, the C–Pt–C 

angle amounts to 38.9°, whereas the C1–C2–C3 angle is only 121°. Very likely the 

corresponding angle in the precursor acetylene 101 has a similar value [94]. 

Another phane system with distorted triple bonds is the [2.2]paracyclophane/ 

dehydroannulene hybrid 105 (Scheme 7.20) [95]. A prominent feature of the [14] 

annulene core of this hydrocarbon is the inward bending of the monoyne units 

389


Scheme 7.19 

by 4.5–11.8°, whereas the diyne moiety bends outwards by 8.1–11.9°. A similar 

structure was observed for the dehydroannulene 106, that corresponds to one deck 

of 105. Here the monoyne units are bent inwards by 3.9–11.5° and the diyne unit 

is bent outward by 8.6–11.2° [96]. 

Finally, the [6.6]naphthalenophane 107 shows interesting structural features 

in its unsaturated bridges: the angle between the triple bond and the aromatic 

carbon atom to which it is bound is 173.8°, and that between it and the neighboring 

sp 2 -hybridized carbon atom amounts to 175.5°, their four atoms being 

arranged in a transoid fashion, not linear as for the simple model compounds in 

Scheme 7.1 [97]. 

Pericyclynes [98], cyclic compounds in which acetylene units are separated from 

each other by a methylene group or a heteroatom, are another interesting class 

of polyacetylenes, since they could possess homoaromatic character. Several of 

Scheme 7.20

Scheme 7.21 


these contain distorted acetylene units. For example (Scheme 7.21) in the thioether 

108 the C�C–C angles lie between 172.5° and 173.3° [99]; and in cyclopentayne 

109 the corresponding values for the isolated triple bonds range from 170.8° to 

178.8°, whereas in the butadiyne moiety the ‘outer’ angle (to the saturated carbon 

atom) amounts to 168.2° and the ‘inner’ enclosing three atoms of the triple bonds 

is 168.5° [99]. 

Although cyclic polyacetylenes possessing three, 110 (Scheme 7.21) or more 

consecutive triple bonds apparently have not been prepared, the ultimate molecules 

in this series are the completely dehydrogenated cyclic polyacetylenes, the cyclo[n] 

carbons 111. 

There have been several reports on the preparation – or at least the detection – of 

this novel allotrope of carbon [100], and the topic has been reviewed several times 

[101]. Certainly, the lower members must possess (strongly) distorted acetylene 

units. However, because of the instability of the cyclocarbons no experimental 

evidence concerning their molecular structures is available. Mass spectrometric 

techniques are usually employed to detect them after their generation, for example 

by laser evaporation of graphite or from stable organic precursors [102]. To study 

this unique form of carbon by electronic and/or vibrational spectroscopy in the 

gas phase or under matrix isolation conditions would be extremely interesting 

[103]. It has been estimated that the deformation of the bond angle at each sphybridized 

carbon atom of cyclo-C 18 112 amounts to ca. 160° [104]. 

With compound 112 we are ending our journey through the interesting world 

of deformed acetylenes that began with another cyclocarbon, the cyclocarbene 9 

(Scheme 7.2). Theoretical studies on ethynyl expanded prismanes are discussed 

in Section 2.3.3.2. 

391


7.6 

Spectroscopic Properties of Angle-strained Cycloalkynes 

Although spectroscopic data of selected strained cycloalkynes have been discussed 

several times above it seems reasonable to present these in a separate chapter 

in comparison to possibly discover any general trends. Needless to say, that this 

discussion suffers from the fact that the available spectroscopic information is 

not only incomplete but in the cases of the very reactive cycloalkynes impossible 

to obtain experimentally. 

IR and Raman spectra are available for a sizeable number of cyclohexynes to 

cyclononynes, most of them derivatives [12]; they indicate that a reduction in the 

C�C–C angle causes a shift to lower wave numbers. A linear correlation between 

absorption maxima and experimental and/or calculated C�C–C bond angles has 

been proposed; however, strong deviations from this correlation has also been 

noted [105]. To rationalize this observation it has been proposed that the hybridization 

is changed by the deformation from sp towards sp 2 . This would lead to 

a lower force constant for the C�C bond stretching vibration and hence a lower 

frequency of absorption [12]. In several cases two or three bands are registered 

in the region between 2100 and 2300 cm –1 , and this has been attributed to Fermi 

resonance with overtones or combination bands. 

Since the triple bond does not carry any hydrogen atom substituents, 1 H NMR 

data are of limited importance to probe the angle strain/deformation in small-ring 

cycloalkynes. However, 13 C NMR data indicate that increasing the ring strain by 

going to smaller ring sizes or introducing additional double bonds into the rings, 

shifts the 13 C signals to lower fields. A case in point is 3,3,7,7-tetramethylcycloheptyne 

73 which with � = 109.8 ppm for the acetylenic carbon atoms has not only 

the highest value for a simple cyclooctyne but is shifted downfield by 22.8 ppm in 

comparison to the unstrained reference compound 2,2,5,5-tetramethyl-3-hexyne 

[12]. Again, the observed shifts due to ring strain are rationalized as resulting 

from a changed hybridization of the acetylenic carbon atoms in the angle-strained 

cycloalkynes. In many of the NMR studies reported on these hydrocarbons the 

emphasis is on the dynamic behavior of these compounds (flexibility of the 

saturated polymethylene bridge). 

Hints concerning the orbital structure of angle-strained cycloalkynes may be 

derived from photoelectron (PE) and electron transmission (ET) spectra of these 

hydrocarbons. According to ab initio [106] and extended Hückel calculations [107] 

the energy of the HOMO is hardly effected by cis-bending of a triple bond up to 

ca. 30°. In contrast, the LUMO energy is decreased considerably in the cis-bent 

models. Experimental studies on various angle-strained cycloalkynes confirm these 

predictions. For seven-membered cycloalkynes such as 73 is has been observed 

that the otherwise degenerate �-orbitals are split into two PE bands [108, 109].

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Chem. Ber. 1994, 127, 1529. 

79 R. Destro, T. Pilati, M. Simonetta, J. Am. 

Chem. Soc. 1975, 97, 658. 

80 R. A. G. de Graaff, S. Gorter, C. Romers, 

H. N. C. Wong, F. Sondheimer, J. Chem. 

Soc. Perkin Trans. 2, 1981, 478. 

81 H. Sakurai, Y. Nakadeira, A. Hosomi, 

Y. Eriyama, C. Kabuto, J. Am. Chem. Soc. 

1983, 105, 3359. 

82 R. Gleiter, M. Karcher, R. Jahn, H. Irngartinger, 

Chem. Ber. 1988, 121, 735. 

395


83 R. Gleiter, St. Rittinger, H. Irngartinger, 

Chem. Ber. 1991, 124, 365; for 1-thia-6azacyclodeca-3,8-diynes 

see R. Gleiter, 

J. Ritter, H. Irngartinger, J. Lichtenthaler, 

Tetrahedron Lett. 1991, 32, 2883; for 

1,6-diazacyclodeca-3,8-diynes see 

J. Ritter, R. Gleiter, H. Irngartinger, 

Th. Oeser, J. Am. Chem. Soc. 1997, 119, 

10599; review: R. Gleiter, R. Merger in 

Modern Acetylene Chemistry, P. J. Stang, 

F. Diederich (Eds.), VCH, Weinheim 

1995, pp. 285; cf. R. Gleiter, Angew. 

Chem. 1992, 104, 29; Angew. Chem. Int. 

Ed. Engl. 1992, 31, 27. 

84 R. Gleiter, R. Merger, H. Irngartinger, 

J. Am. Chem. Soc. 1992, 114, 8927. 

85 R. Gleiter, M. Ramming, H. Weigl, 

V. Wolfart, H. Irngartinger, Th. Oeser, 

Liebigs Ann./Recueil, 1997, 1545. 

86 Review: R. Gleiter, D. B. Werz in 

Carbon Rich Compounds, M. M. Haley, 

R. R. Tykwinski (Eds.), Wiley-VCH, 

Weinheim 2006, pp. 295. 

87 Review: F. Sondheimer, Pure Appl. 

Chem. 1963, 7, 363; cf. M. Nakagawa in 

The Chemistry of Triple-bonded Functional 

Groups, S. Patai (ed.), Wiley-Interscience, 

New York, 1978, Vol. I, pp. 635. 

88 R. Gleiter, R. Merger, J. Chavez, T. Oeser, 

H. Irngartinger, H. Pritzkow, B. Nuber, 

Eur. J. Org. Chem. 1999, 2841; X-ray of 

1-aza-cyclotetradeca-3,5,10,12-tetrayne 

derivatives: E. M. Schmidt, R. Gleiter, 

F. Rominger, Chem. Eur. J. 2003, 9, 

1814 and E. M. Schmidt, R. Gleiter, 

F. Rominger, Eur. J. Org. Chem. 2004, 

2818. 

89 M. Kaftory, I. Agmaon, M. Ladika, 

P. J. Stang, J. Am. Chem. Soc. 1987, 

109, 782; K. N. Houk, L. T. Scott, 

N. G. Rondan, D. C. Spellmeyer, 

G. Reinhardt, J. L. Hyun, G. J. DeCicco, 

R. Weiss, M. H. M. Chen, L. S. Bass, 

J. Clardy, F. S. Jørgensen, T. A. Eaton, 

V. Sarkozi, C. M. Petit, L. Ng, 

K. D. Jordan, J. Am. Chem. Soc. 1985, 

107, 6556; A. G. Myers, S. D. Goldberg, 

Angew. Chem. 2000, 112, 2844; Angew. 


90 M. M. Haley, R. R. Tykwinski (Eds.), 

Carbon-Rich Compounds – from Molecules 

to Materials, Wiley-VCH, Weinheim, 

2006; V. Maraval, R. Chauvin, Chem. 

Rev. 2006, 106, 5317; E. L. Spitler, 

C. A. Johnson, II, M. M. Haley, Chem. 

Rev. 2006, 106, 5344. 

91 M. E. Maier, Synlett, 1995, 13. 

92 F. Mitzel, C. Boudon, J.-P. Gisselbrecht, 

J.-P. Seiler, M. Gross, F. Diederich, Helv. 

Chim. Acta, 2004, 87, 1130. 

93 M. Psiorz, H. Hopf, Angew. Chem. 1982, 

94, 639; Angew. Chem. Int. Ed. Engl. 1982, 

21, 640; cf. C. W. Chan, H. N. C. Wong, 

J. Am. Chem. Soc. 1988, 110, 462. 

94 K. Albrecht, D. C. R. Hockless, 

B. König, H. Neumann, M. A. Bennett, 

A. de Meijere, J. Chem. Soc. Chem. 

Commun., 1996, 543; cf. B. König, 

M. A. Bennett, A. de Meijere, Synlett, 

1994, 653. 

95 H. Hinrichs, A. K. Fischer, P. G. Jones. 

H. Hopf, M. M. Haley, Org. Lett. 2005, 

7, 3793; cf. H. Hinrichs, A. J. Boydston, 

P. G. Jones, K. Hess, R. Herges, 

M. M. Haley, H. Hopf, Chem. Eur. J. 

2006, 12, 7103. 

96 K. P. Baldwin, A. J. Matzger, 

D. A. Scheiman, C. A. Tessier, 

K. P. C. Vollhardt, W. J. Youngs, Synlett, 

1995, 125. 

97 H. Hopf, Chr. Werner, unpublished 

results. 

98 Reviews: L. T. Scott, M. J. Cooney 

in Modern Acetylene Chemistry 

(P. J. Stang, F. Diederich, Eds), VCH, 

Weinheim, 1995, pp. 321; A. de Meijere, 

S. I. Kozhushkov, Topics in Current 

Chemistry, Vol. 201, Springer, Berlin, 

1999, p. 1. 

99 L. T. Scott, private communication. 

100 Y. Rubin, M. Kahr, C. B. Knobler, 

F. Diederich, C. L. Wilkins, J. Am. Chem. 

Soc. 1991, 113, 495. 

101 F. Diederich in Modern Acetylene 

Chemistry (P. J. Stang, F. Diederich, 

Eds), VCH, Weinheim 1995, pp. 443; 

F. Diederich, L. Gobbi, Topics in Current 

Chemistry, Springer Verlag, Berlin, 

Vol. 201, 1999, pp. 43; K. Tahara, Y. Tobe, 

Chem. Rev. 2006, 106, 5274. 

102 F. Diederich, Y. Rubin, O. L. Chapman, 

N. S. Goroff, Helv. Chim. Acta, 1994, 77, 

1441; Y. Tobe, T. Fujii, H. Matsumoto, 

K. Naemura, Y. Achiba, T. Wakabayashi, 

J. Am. Chem. Soc. 1996, 118, 2758; 

Y. Tobe, T. Fujii, H. Matsumoto, 

K. Tsumuraya, D. Naguchi, 

N. Nakagawa, M. Sonoda, K. Naemura,

Y. Achiba, T. Wakabayashi, J. Am. Chem. 

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103 M. Grutter, M. Wyss, E. Riaplov, J. P. 

Maier, S. D. Peyerimhoff, M. Hanrath, 

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104 F. Diederich, Y. Rubin, C. B. Knobler, 

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1088. Apparently cyclocarbons were 

first discussed in a publication in 

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a (fictional) contribution by H. Musso 

and H. Hopf under alias names 

(S. Housmans, H. P. Honnef). 

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105 H. Meier, H. Petersen, H. Kolshorn, 

Chem. Ber. 1980, 113, 2398. 

106 N. G. Rondan, L. N. Domelsmith, 

K. N. Houk, A. T. Bowne, R. H. Levin, 

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107 D. M. Hoffmann, R. Hoffmann; 

C. R. Fiesel, J. Am. Chem. Soc., 1982, 

104, 3858. 

108 H. Schmidt, A. Schweig, A. Krebs, 

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109 G. Bieri, E. Heilbronner, 

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J. Wirz, Helv. Chim. Acta, 1974, 57, 

1265. 

397

8 

Molecules with Labile Bonds: 

Selected Annulenes and Bridged Homotropilidenes 


8.1 

Introduction 

Arguably, molecules with labile bonds are involved in any and every chemical 

reaction. Therefore, to be tractable, the scope of this chapter must be limited to 

a highly select range of molecules with especially labile bonds – some annulenes 

and the bridged homotropilidenes. The emphasis of this review is further refined 

to concentrate on the properties and consequences of the possession of labile 

bonds. In-keeping with the title ‘Strained Hydrocarbons’, heteroatoms are only 

included as modifiers of the parent hydrocarbon and systems in which they are the 

prime motivators are excluded. In general, metal containing compounds are not 

considered. The coverage throughout this chapter is limited to material published 

subsequent to the last major review in that particular area, with a brief overview of 

prior material sufficient to set the newer work in an appropriate context. Access 

to the older primary literature is provided, wherever possible, through citations 

to appropriate reviews. 

8.2 

Annulenes 

8.2.1 

Cyclobutadiene 

Interest in the simplest annulene, cyclobutadiene or [4]annulene (1), extends back 

for more than 130 years. Sondheimer introduced the term annulenes to describe 

‘the completely conjugated monocarbocyclic polyenes, the ring size being indicated 

by a number in brackets’ [1]. The salient areas of fascination with cyclobutadiene 

have been its structure, whether it has a square planar or rectangular, triplet or 

singlet ground state, and its (anti)aromaticity or lack thereof. Its preparation, 

chemistry and physical properties are summarized in several excellent reviews 



ISBN: 978-3-527-31767-7 

399

400 8 Molecules with Labile Bonds: Selected Annulenes and Bridged Homotropilidenes 

[2–8]. In a very recent overview Bally presents the current state-of-the-art in the 

cyclobutadiene story as it relates to antiaromaticity [9]. Despite this long history, 

the first synthesis of a cyclobutadiene was only achieved in 1965. Petit et al. 

prepared 1 by ceric ion oxidation of the corresponding iron tricarbonyl complex 

2 [10]. Subsequently, many alternative syntheses of 1 involving the photolysis or 

thermolysis of strained precursors such as 3–7 have appeared [11]. 1 is an exceptionally 

reactive transient intermediate that can only be isolated in low-temperature 

matrices or even in a hemicarcerand at room temperatures [3–8] (see Chapter 10 

for discussion on reactions in “molecular flasks”). It dimerizes by an extremely 

rapid Diels–Alder self condensation even at cryogenic temperatures, but it can 

be trapped in situ to form derivatives with suitable reagents. Highly sterically 

encumbered derivatives, e.g. 8–13, are stable at ambient temperature and several 

of these compounds have had their X-ray structures determined [3–8]. 

Cyclobutadienes provide prime examples of the synergy between experiment 

and theory. Much of the experimental work was and continues to be driven by 

theory, and theory has played a key role in the interpretation and rationalization of 

experimental results [3–9]. Coulson recognized that the Hückel molecular orbital 

theory predicted square planar (D 4h) triplet ground state of 1 would be subject 

to second order Jahn–Teller distortion which should result in a rectangular (D 2h) 

singlet ground state [12]. More advanced calculations incorporating electron 

correlation favor a rectangular singlet ground state for cyclobutadiene [13]. IR 

spectra from matrix isolated cyclobutadiene were initially interpreted as indicating 

a square geometry for 1 [17–19]. However, theory and experiment came into 

concert when Masamune et al. clearly demonstrated, using a Fourier transform 

IR spectrometer, that the IR spectrum of 1 does indeed correspond with a rectangular 

geometry. The earlier investigators were hampered by the low intensity 

of the signals and were misled by bands due to carbon dioxide (which complexes 

with the cyclobutadiene) produced in the photolytic preparation of 1 [4, 20]. In 

addition to the extensive discussion of the IR spectra of cyclobutadienes in the 

general reviews [4, 6, 7], several detailed analyses of the IR absorptions of cyclobutadienes 

have appeared [21–23]. Following the first IR investigations on 1, X-ray 

crystal structures confirming rectangular geometries for the stable substituted 

cyclobutadienes 8 and 10 became available [24, 25]. Subsequent X-ray structures 

determinations of 9, 11 [26] and 13 [27] all reveal a rectangular cyclobutadiene

8.2 Annulenes 

Figure 8.1 Several isolated cyclobutadiene derivatives with X-ray determined bond lengths. 

core. Examination of Figure 8.1, in which the X-ray determined bond lengths 

for 8–11 and 13 are shown, reveals that in 11 the cyclobutadiene core is almost 

square. Borden and Davidson rationalized this seeming anomaly by considering 

the reduction in steric strain (between the tertiary-butyl groups) upon lengthening 

the double bonds, and the reduction in energy upon concomitant shortening of 

the formal single bonds [28]. 

Although the familiar term aromaticity defies a universally acceptable definition 

[29, 30, 31], it is generally used to describe completely conjugated cyclic systems 

in which (4n + 2) (where n is an integer) electrons are delocalized in a cyclic 

array. Such systems enjoy a ‘special stability’ (the aromatic molecule is of lower 

energy than a suitable non-aromatic model) due to this aromatic delocalization. 

Conversely, conjugated cyclic systems of 4n electrons do not enjoy significant 

special stability (non-aromatic) and may even be less stable (antiaromatic) than 

the appropriate model compound. It should be noted that geometric and magnetic 

criteria are also used in awarding the designation aromatic or anti aromatic to 

candidate compounds. It would be preferable if the terms, introduced by Garratt 

[1], diatropic (for systems having a diamagnetic ring current, equated with 4n + 2 

aromatics) and paratropic (for systems having a paramagnetic ring current, 

equated with 4n species which may or may not be destabilized by cyclic delocalization) 

were to be universally adopted. Just as benzene is the archetype aromatic 

molecule, cyclobutadiene was considered to be the prime example of an antiaromatic 

[5, 8, 32]. Recent work has increasingly tended to call this conclusion into 

question [9]. It should be noted, Bally and Masamune pointed out in their 1980 

review that the high energy of cyclobutadiene could result from several factors, 

not just its antiaromaticity, and that the reported degree of ‘negative resonance 

energies’ varied between 0 and more than 30 kcal mol –1 [4]. There is no doubt that 

1 exhibits properties associated with antiaromaticity and that it should be classed 

as antiaromatic [3–9]. However, Mo and Schleyer, in a very detailed analysis of 

the thermodynamics of (anti)aromaticity, show that the destabilization of cyclobutadiene 

by �-antiaromaticity is rather small. They attribute the majority of the 

classical thermodynamic instability of cyclobutadiene to result from strain and 

�–� and �–� repulsions [33]. 

401


As the ground state of 1 is rectangular, then the two degenerate D 2h forms must 

lie on (at least) a double-minimum potential energy surface (PES). These forms 

could readily interconvert if the barrier(s) between them is sufficiently low. Balaban 

and F�rca�iu introduced the term automerization that they defined as being a particular 

case of isomerization in which both the molecular and structural formulae 

are conserved in the ‘reactant’ and ‘product’ [34]. Valence bond isomerization 

or valence tautomerization equally well describe such processes; however, the 

interconversions of 1a � 1b and the rectangular forms of other cyclobutadienes 

are most commonly referred to as automerizations. The first indication that such 

an automerization is facile came from the NMR spectra of the tri-tertiary-butyl 

derivatives 10 and 14. In each case the tertiary-butyl substituents at C2 and C4 

gave only one 1 H resonance. For 14 the C1 1 H resonance showed no line broadening 

down to 133 K and the C2/C4 13 C single resonance remained sharp to 88 K 

giving an estimated �G ‡ of less than 2.5 kcal mol –1 for automerization [35–37]. 

Interestingly, the ring 1 H resonance in 14 occurs at 1.04 ppm higher field than for 

the C2 1 H resonance of cyclopentadiene. This significant upfield shift is taken as 

indicative of a paramagnetic ring current in 14 and of supporting its designation 

as antiaromatic [36]. The PES for the automerization of 1 (and some derivatives) 

has been extensively explored by calculation and there is general agreement that 

classically (vide infra) 1a automerizes to 1b through the square planar D 4h 1c [11]. 

The calculated barrier heights have varied widely [11]. The current best level of 

theory (optimized multireference average quadratic coupled cluster, MR-AQCC, 

employing various extended correlation consistent basis sets) predicts a barrier 

height of 6.3 kcal mol –1 (electronic and zero-point energies) for the 1a � 1b automerization 

[16]. 

Trapping experiments of specifically 1,2-dideuterated cyclobutadiene generated 

in solution convincingly demonstrate that cyclobutadiene has a rectangular D 2h 

ground state in solution as well as in low temperature matrices [38]. Repeating 

these trapping experiments at various temperatures and concentrations allowed 

the estimation of the activation barrier (�H ‡ ~1.6–10 kcal mol –1 ) for the automer-

8.3 Cyclooctatetraene 

ization [39]. This latter study also revealed a surprising large negative entropy of 

activation. Carpenter proposed that his negative entropy of activation could be 

rationalized by assuming the automerization of 1 proceeds mostly by heavy-atom 

tunneling [40]! This controversial proposal stimulated much research resulting 

in a consensus supporting tunneling in the automerization of 1 [41–43]. In an 

important experiment, Maier et al. found for the first time in a cyclobutadiene 

that the 13 C NMR spectra of the cyclobutadienes 15 are temperature dependent 

[44]. Coalescence temperatures were determined for the C2/C4 signals of 15b 

and 15c and for the quarternary carbons attached to C2/C4 of 15c. From these 

results 15a was estimated to have �G ‡ ~3.5 kcal mol –1 and for 15b and 15c, �G ‡ 

was determined to be 4.5 and 5.8 kcal mol –1 respectively. These data demonstrate 

that the automerization of these substituted cyclobutadienes proceeds by a classical 

mechanism and does not involve tunneling. 

A further reflection of the lability of cyclobutadienes is their photochemical 

valence isomerization to the most strained cage compounds, tetrahedranes [6, 

45–48]. The first tetrahedrane, tetra-tertiary-butyltetrahedrane 16a, to be isolated 

and fully characterized (including X-ray structure determination) can be prepared 

by low-temperature irradiation of 11. Similarly, low-temperature irradiation of 12 

and 15a yield tetrahedranes 16b and 16c respectively. 

8.3 

Cyclooctatetraene 

Cyclooctatetraene ([8]annulene, COT) (17) was the next annulene to be isolated 

after benzene [49]. Contrary to the expectations of the time that it should exhibit 

similar properties (aromaticity) to benzene, it behaves as a simple polyolefin. 

There are several earlier and extensive reviews [5, 8, 50–54] of cyclooctatetraenes 

(COTs) and one recent general review of the annulenes which makes brief mention 

of COTs [55]. In 2001, Klärner reviewed the antiaromaticity of planar COT [56]. 

Although 17 was originally prepared in 1911 by a low yielding multistep synthesis 

[49], it remained relatively unavailable until the advent of the Reppe et al. synthesis 

in 1948 [57]. With practical amounts of material then accessible, progress in the 

chemistry of COTs was rapid. The infrared and Raman spectra [58–60], and X-ray 

403


[61, 62] and electron diffraction [63, 64] determinations of COT revealed that it 

adopts a nonplanar equilibrium conformation. There was initially some dispute 

as to whether the structure is of D 2d (tub-shaped), D 4 (crown-shaped with alternating 

single and double bonds) or D 4d (crown-shaped with all equivalent bond 

lengths). Subsequent electron diffraction studies clearly established a D 2d tub 

geometry 17a for COT [65, 66]. 

There are 21 isomeric (CH) 8 hydrocarbons possible, some of which can be 

accessed from COT and many of which lead to COT upon thermolysis or photolysis 

[67, 68]. In the context of this review the low temperature processes of valence 

isomerization via bond shifting (BS, 17a � 17c), ring inversion (RI, 17a � 17d) 

and valence isomerizations (VI, 17a � 18) to bicyclo[4.2.0]octa-2,4,7-triene 18 and 

to semibullvalene 19 via high temperature equilibration are of most importance. 

It was early recognized that as a consequence of COT’s nonplanar equilibrium 

geometry, mono- and appropriately polysubstituted derivatives would be chiral 

and that racemization may ensue should the nonplanar ring undergo inversion 

with a sufficiently low activation barrier, e.g. 20a � 20d [69, 70]. Similarly, racemization 

can also result from the higher energy BS [52–54]. In 1962 Mislow and 

Perlmutter reported the first isolation of an optically active COT, the diacid 21 [71]. 

From the rate of racemization of 21, they estimated the RI activation energy to 

be 27 kcal mol –1 . The substituents hinder planarization which doubtless results 

in a higher RI activation energy than that for simpler COTs and permitted the 

isolation of each enantiomer. 

Also in 1962 Anet reported an elegant NMR experiment using the splittings 

in the 13 C satellites of the single 1 H resonance of 17 to estimate the barrier for 

BS (�G ‡ ~13.7 kcal mol –1 ) in COT, 17a � 17c [72]. The BS activation energy was 

later refined by Luz and Meiboom (10.9 kcal mol –1 ) from their dynamic NMR 

studies on partially perdeutero (all hydrogens of some molecules replaced with

8.4 Bond Shifting, Ring Inversion and Antiaromaticity 

deuterium but not for every molecule in the macroscopic sample) COT in nematic 

solvents [73], and again by Naor and Luz in a new analysis of Luz and Meiboom’s 

[73] data obtained for C 8H 8 COT in nematic solvents (�H ‡ = 10.0 kcal mol –1 ) [74]. 

Shortly after studying BS in COT, Anet group in another ingenious NMR experiment 

determined the activation parameters for both BS (�G ‡ = 17.1 kcal mol –1 at 

–2 °C) and RI (�G ‡ = 14.7 kcal mol –1 at –2 °C) in the alcohol 22 [75]. Of perhaps 

greater significance, they postulated that BS involves a planar fully delocalized 

bond equalized transition state (TS) 23 while RI proceeds through a planar bond 

alternating TS 24. This notion of planar bond equalized or alternating transition 

states (TSs) persists today, at least for the parent and simple substituted COTs (vide 

infra). Oth surveyed his own results and those of others studying the dynamics 

of COTs [76]. 

Subsequent to these pioneering studies, many syntheses for specifically functionalized 

COTs were developed and the BS and RI for a multitude of these COTs 

were examined [5, 8, 50–54, 68]. In addition to NMR determinations of the BS and 

RI activation parameters, Paquette, in particular, extracted the RI data from detailed 

analyses of the complex kinetics for the loss of optical activity in appropriate COTs 

and their NMR determined BS values [52–54]. It was found that increasing substitution 

of the COT nucleus tended to slow the rates of both BS and RI. 

8.4 

Bond Shifting, Ring Inversion and Antiaromaticity 

The proposal of Anet group that the TS for COT BS is the planar D 8h and for COT 

RI the planar D 4h species 17e and 17f respectively [75], appeared to offer the perfect 

method of evaluating antiaromaticity in (planar) COT. Assuming BS and RI go 

through 17e/f, then the difference in activation barriers for BS and RI (��G ‡ , 

��H ‡ , etc.) directly yields the (classical) resonance energy for the antiaromatic 

17e [52]. However, the current view is that both D 8h and D 4h COT are paratropic 

and antiaromatic [56, 77]. While 17f appears to be universally accepted as the RI 

TS, the veracity of 17e as the BS TS has been questioned. Dewar et al. considered 

the experimental ��G ‡ s for a range of COTs (typically 2–4 kcal mol –1 ) to be far 

too small compared with their calculated differences in the heats of formation 

for 17e/f (13.9, MINDO/2, and 15.4, � approximation, kcal mol –1 ) for 17e to be 

the BS TS [78]. They suggested a crown-shaped species similar to 17b may be the 

BS TS. Based on symmetry and qualitative energy considerations, Ermer et al. 

concluded that neither 17e nor a crown TS were likely and that a flattened saddle 

405


of D 2d symmetry 17g was the best choice for the TS [79]. Paquette group initially 

supported 17e as the BS TS [80], but, following extensive additional studies, later 

concluded for certain highly substituted or annelated (cyclooctatetraenophanes) 

COTs that a planar TS is untenable for the observed rapid BSs [54]. He suggested 

that a lower energy pathway would result from pseudorotation of the COT nucleus 

to give a flattened saddle TS (analogous to 17g). He left unanswered the question 

of the BS TS in the parent COT 17. 

Hrovat and Borden, using CASSCF calculations, provided an answer to this 

question [81]. They clearly demonstrated that 17e and 17f are the TSs for the 

parent COT and that the CASSCF/6-31G* activation BS and RI barriers (including 

zero-point correction) calculated at the optimized CASSCF/3-21G* geometry 

(CASSCF/6-31G*//CASSCF/3-21G*) are 14.7 and 10.6 kcal mol –1 , respectively. 

These barriers (and the difference between them, 4.1 kcal mol –1 ) are in excellent 

agreement with experiments. The results from several more recent calculations 

at higher levels of theory all support Borden and Hrovat’s earlier results [82–84]. 

Garavelli et al. optimized 17e and 17f using CASSCF/6-31G* and found activation 

barriers of 14.5 and 10.9 kcal mol –1 (including zero-point correction) [84]. Incorporation 

of dynamic electron correlation (CASPT2) results in 17e and 17f becoming 

essentially degenerate and after zero-point correction 17e is 1.9 kcal mol –1 lower 

in energy than 17f results in the authors claim to be within the error bars for 

CASPT2. It is of interest to note that the singlet D 8h TS 17e, just as the singlet D 4h 

TS 1c for cyclobutadiene automerization, violates Hund’s rule [81]. 

In concurrence with an earlier CASSCF/6-31G* study of the RI and BS processes 

by Castaño et al. [83], Garavelli et al. also propose that there is a bifurcation point 

(BP) of D 4h symmetry between the two TSs, 17e and 17f and similarly 17e and 17f �, 

and that BS avoids passage through the lower lying 17f/17f �, instead proceeding directly 

from the BP to the D 2d COT ground state (Figure 8.2). The path followed from 

17e toward 17f to yield D 2d COT is illustrated by the dashed lines in Figure 8.2.

8.4 Bond Shifting, Ring Inversion and Antiaromaticity 

Figure 8.2 Bond shifting (BS), ring inversion (RI) showing the proposed bifurcation point (BP) 

between 17e and 17f. 

The COT PES has also been probed by means of transition state spectroscopy 

[85, 86]. Linberger, Borden and coworkers used photodetachment of an electron 

from the COT radical anion to generate and study the planar TSs 17e/f by means 

of photo electron spectroscopy [87]. They confirmed that 17e and 17f are separated 

in energy by 3–5 kcal mol –1 and that, in violation of Hund’s rule, the D 8h singlet 

is 12.1 kcal mol –1 lower in energy than the D 8h triplet [87, 88]. Zewail et al. also 

generated the D 4h 17f by photodetachment of the COT radical anion and examined 

the TS dynamics for RI using pump/probe femtosecond laser pulses and timedependent 

mass spectrometric analysis [89]. 

It has been suggested that heavy atom tunneling is involved in the COT BS, just 

as in cyclobutadiene automerization [82, 90]. The small atomic displacements and 

activation barrier and the negative entropy of activation for BS are supportive of 

the participation of tunneling in BS. 

Through suitable substitution the activation barrier for BS can be raised to such 

a level as to allow the isolation of ‘shelf-stable’ bond shift isomers [52, 54]. For 

example, the annelated derivatives 25a and 25b were the first bond shift isomers 

to be isolated and the tetramethyl COTs 26a/26b followed shortly after. 

Similarly Ermer et al. predicted, from geometric considerations, and Trindle later 

confirmed, from DFT and ab initio calculations, that annelation of COT with small 

rings or bicycles would favor a planar conformation of the COT nucleus [79, 91]. 

407


The X-ray structure of perfluorotetracyclobuteno-COT (27a) has a planar COT 

nucleus with ~D 4h symmetry resulting from the small ring annelation increasing 

the internal COT angle from 120° to 135° [92]. The endocyclic (to the cyclobutene) 

bonds (en) are on average 0.072 Å shorter than the exocyclic bonds (ex). Baldridge 

and Siegel calculated (B3PW91/DZ(2d,p)) that 27a is 17.2 kcal mol –1 more stable 

than 27b, while for the corresponding hydrocarbons 28b is 2.3 kcal mol –1 more 

stable than 28a [93]. They attribute this reversal in tautomeric stability to the 

electronic perturbations induced by perfluorination. They also predicted that 

tetrakisbicyclohexeno-COT should be planar with 29b favored by 32.7 kcal mol –1 

over 29a. Matsuura and Komatsu confirmed these predictions by the synthesis 

of 29b [94]. Considering the 1 H chemical shift of the bridgehead bicyclohexeno 

protons, the nucleus independent chemical shifts (NICS) value and the magnetic 

susceptibility exaltation of 29, they concluded that 29 is less antiaromatic than the 

parent D 4h COT. They attributed this decreased antiaromaticity to the electronic 

interaction between the bicyclohexeno-annelating groups and the COT nucleus. 

Planar COTs are antiaromatic [56], with the ‘D 4h’ conformer 2–4 kcal mol –1 

lower in energy than the ‘D 8h’ in the parent COT and simple derivatives. However, 

just as for cyclobutadiene the antiaromatic destabilization is only small and is 

certainly not responsible for the nonplanar D 2d tub ground state [77]. Departure 

from planarity can be understood in terms of Jahn–Teller distortion from the D 8h 

to the D 4h geometry which puckers to the D 2d tub principally to relieve angle strain. 

Pseudo-Jahn–Teller coupling has also been invoked to account for the puckering of 

the D 4h COT [95]. The Hückel rule (4n + 2 = aromatic, 4n = ‘antiaromatic’) strictly 

only applies to planar monocyclic fully conjugated systems. However, Haddon 

showed using his �-orbital axis vector (POAV) analysis that conjugation is hardly 

interrupted even when the �-system is significantly distorted from planarity, as 

in, for example, the various bridged [10]annulenes which are strongly aromatic 

despite their considerable nonplanarity [96–98]. Jenneskens et al. determined from 

the analysis of current density maps that D 4h COT has a large paramagnetic ring 

current and that even as the ring puckers toward the D 2d tub a paramagnetic ring 

current is maintained for ~80% of the geometry change [99, 100].

8.5 

Valence Isomerization 

8.5 Valence Isomerization 

At ambient temperature COT is in rapid equilibrium with bicyclo[4.2.0]octa- 

2,4,7-triene 18 formed by thermally allowed 6-� disrotatory electrocyclization. At 

100 °C ~0.01% of 18 is present in the equilibrium mixture [50, 51, 67, 68, 101, 

102]. In agreement with Huisgen and Mietzsch’s experimental determination 

of the 17 � 18 activation barrier (28.1 kcal mol –1 ) [101], Gravelli et al. calculated 

(CASPT2, zero point corrected) this barrier to be 26.9 kcal mol –1 (Figure 8.3) [84]. 

Due to its ground state tub conformation, no diene unit is available in COT to 

participate in Diels–Alder cycloadditions. However, 18 is an active participant in 

Diels–Alder chemistry [50, 51]. Polymethylation and bridging of the COT nucleus 

can increase the thermodynamic stability of the bicyclic tautomers. Such stabilization 

is apparent for the bridged 30a/b, octamethyl-COT and for tetramethyl 26a 

and pentamethyl 31a, but not their BS isomers 26b and 31b [50]. Bicyclo[4.2.0] 

octa-2,4,7-triene 18 and its derivatives are doubtless involved in the rich high 

temperature chemistry of the COTs [67]. 

Paquette and coworkers showed that thermolysis of semibullvalene 19 and 

some of its derivatives yielded COTs [103]. A later kinetic study of the 19 � 17 

equi librium found an activation energy of 39.8 kcal mol –1 with 17 more stable 

than 19 (�H 0 = 2.4 kcal mol –1 ) and ~2.7% of 19 at 300 °C [104]. Recent CASPT2 

calculations are in remarkably good agreement with these experimental data [84], 

and in accord with earlier CASSCF calculations [105], predict a single step reaction 

passing through a TS of C 2 symmetry and a bifurcation point from which the two 

semibullvalene tautomers result (Figure 8.3). 

The position and ease of attaining the semibullvalene � COT thermal equilibrium 

is highly dependent on substitution [106, 107]. In general, it is more 

common for semibullvalenes to function as synthetic precursors of specifically 

substituted COTs than for them to be prepared by this thermal isomerization. 

The complex photochemistry of COTs is summarized in these referenced 

reviews and articles [51, 67, 84, 108, 109]. A practical synthesis of semibullvalene 

19 results from the gas phase (vibrationally hot) [84] photolysis of COT [109]. 

409


Figure 8.3 Equilibrium between 17, 18 and 19. 

Another (CH) 8 isomer important in the synthesis of semibullvalene is barrelene 

32 [110]. Initial interest in barrelene centered on Hine’s proposal that inter action 

of its 6-� electrons may lead to aromatic character [111]. Zimmerman, Paufler 

et al. were the first to prepare 32 and the discovery of semibullvalene resulted 

from the Zimmerman group’s later investigations of its properties [110, 112, 

113]. Zimmerman concluded that barrelene is a delocalized system, but, due to 

the enforced Möbius �-orbital overlap, it is not stabilized by this delocalization 

having the same �-energy as three isolated ethylenes and hence is non-aromatic 

[113, 114]. Photoelectron spectroscopy studies confirmed the delocalization and 

also suggested a degree of through-bond (hyperconjugative) interaction [115]. 

The acetone sensitized photolysis of barrelene not only marked the discovery of 

semibullvalene but also the discovery of the di-�-methane rearrangement [116]. 

8.6 

Ions Derived from COT 

Based on the known ease of reduction of COT, Katz carried out a detailed study of 

the COT dianion 33 [117, 118]. He concluded that 33 is planar and aromatic. The 

reduction of COTs by both dissolving metals and electrochemically affirms these 

conclusions [50, 51, 55]. Despite intensive efforts and its anticipated aromaticity, 

the COT dication 34 remains unknown. A few substituted COT dications, e.g., 35a 

and 35b, have been prepared at low temperature and deemed to be planar aromatics 

[119]. Komatsu and coworkers prepared the bicyclo[2.2.2]octeno annelated COT

8.7 The Higher Annulenes 

dication 36 which is stable at room temperature, has a tub conformation and 

undergoes RI with �G ‡ ~10.8 kcal mol –1 [120, 121]. 

Protonation of COT or reaction with some other electrophiles yields the homotropylium 

cation 37 or one of its derivatives [50, 51]. Homotropylium cations are the 

most extensively studied and best established homoaromatics [122, 123]. The idea 

of homoaromaticity, first proposed by Roberts [124] and Doering and coworkers 

[125] in 1956, was generalized 3 years later by Winstein [126] who actually introduced 

the term ‘homoaromaticity’. If the cyclic conjugation of an aromatic 

system is interrupted by one or more saturated units (often a CH 2 group) and the 

resulting molecule still enjoys ‘special stability’, then it is termed homoaromatic 

[122, 123, 127–134]. Thus a molecule will be considered homoaromatic if one 

(or more) through space interactions complete the cyclic delocalization of 4n + 2 

electrons and that this through space interaction is energy lowering. Winstein 

termed homoaromatic systems with one interruption to the cyclic conjugation 

monohomoaromatic and those with multiple interruptions, bis-, tris-, and tetrahomoaromatic 

etc. [127, 128]. 

8.7 

The Higher Annulenes 

Two recent reviews [55, 135] and earlier articles [1, 5] provide extensive coverage 

of the higher annulenes. The focus of interest in these annulenes continues to 

be the study of their aromaticity, their structures and their complex conformational 

and configurational isomerism. All annulenes are known between [4]- and 

[30]annulene except for [26]- and [28]annulenes. Most of the higher ([10]- and larger 

for the purposes of this review) annulenes are examples of systems with labile 

bonds and exist in a wide variety of configurations and conformations [55, 135]. 

In addition to reviewing the dynamics of COTs, Oth also reviewed the dynamics 

of [12]-, [14]-, [16]- and [18]annulenes as determined by variable-temperature NMR 

studies [76]. It has long been held that: (1) the 4n + 2 annulenes are diatropic with 

bond-equalized structures while the singlet planar 4n annulenes are paratropic 

and bond alternating [30]; (2) as the annulene ring size increases, bond length 

alternation will set in. The position of onset of bond alternation was debated 

with a mild consensus in favor of about [20]annulene [55, 135]. Schleyer et al. 

have challenged this uneasy truce by asserting both [14]- and [18]annulene have 

significantly bond alternating structures [136]. X-ray structures for [18]annulene 

determined at 80 K [137] and 111 K [138] indicate a bond equalized D 6h geometry. 

411


Schleyer et al. suggest that these structures are incorrect as a consequence of 

insoluble disorder problems [136]. In a commentary on the Schleyer et al. paper, 

Ermer concludes that if there is indeed disorder, it may be possible to solve these 

problems experimentally [139]. He also pointed out that the Schleyer et al. data 

apply to the gas and solution phases and postulated that crystal packing forces 

may favor the D 6h geometry. 

Modeling [10]annulene computationally has proved to be difficult. Castro, 

Karney et al. summarize previous work in this area and examine the interconversion 

of various [10]annulene isomers [140]. They support, from their BH&HYLP 

and coupled cluster calculations, Masamune’s [141] NMR based structural assignments 

for the two crystalline compounds mono-trans 38 and all-cis 39 isolated by 

his group and his proposed equilibration of 38 with heart-shaped 40. 

As already discussed for cyclobutadiene and COT, the higher annulenes can 

also undergo bond shift isomerization. This bond shifting along with the conformational 

and other configurational equilibria [1, 55, 76] of [12]-, [14]- and 

[16]annulenes were re-examined computationally by Castro and Karney et al. 

[142–144]. They concluded that the bond shifting, 41 � 42, 43 � 44 and 45 � 46, 

proceeds through a Möbius transition state (antiaromatic for the [14]- and aromatic 

for the [12]- and [16]annulenes). The recently revived interest in and study of 

Möbius systems is reviewed by Herges [145]. 

Bridged derivatives are known for most of the higher annulenes and these 

derivatives are discussed in the two most recent reviews on annulenes [55, 135]. 

In the majority of these annulenes the bridging group is a methano group, e.g., 

the methano-bridged [10]annulene (47 and 48) and [14]annulene (49 and 50). 

At ambient temperature, 1,6-methano[10]annulene 47 is in equilibrium with a 

very small amount of the isomeric norcaradiene 51 [146–152]. Substitution on

8.8 Bridged Homotropilidenes 

C11 tends to increase the stability of the norcaradienic form and, reaffirming the 

homoaromatic transannular (C1,C6) interaction [123], it has even been suggested 

that the dimethyl compound 52 is nonclassical (a barrierless flat PES linking 

52a � 52b) [153]. 

Dimethyldihydropyrene 53, another bridged [14]annulene, has long served as 

an excellent NMR probe for aromaticity [154, 155] and is now being studied for its 

potential applications as a photoswitch [156]. The dark green 53 is photo-bleached 

by visible light to give the colorless cyclophanediene 54 which undergoes ready 

thermal, although forbidden, electrocyclization to return 53. A recent computational 

investigation (UB3LYP/6-31G*) revealed the transition state for this 6-� 

conrotatory electrocyclization is biradical-like accounting for the low thermal 

activation barrier [157]. 

8.8 

Bridged Homotropilidenes 

The trivial name for 1,3,5-cycloheptatriene 55 is tropilidene and hence homotropilidene 

is bicyclo[5.1.0]octa-2,5-diene 56. Doering reasoned that the Cope 

rearrangement in homotropilidene, via the cis conformer 56b, would be greatly 

accelerated compared with the parent 1,5-hexadiene due to the ring strain induced 

upon cyclopropanation and to the restricted conformational mobility of the vinyl 

groups [158]. The homotropilidene degenerate Cope rearrangement proved to 

be exceptionally rapid (�H ‡ 21.4 kcal mol –1 less than for 1,5-hexadiene), which 

lead Doering to introduce a new term ‘fluctional’ now commonly fluxional [159, 

160]. He defined fluxional (structure), which refers to a dynamic system and is 

413


not to be confused with resonance hybrid, as ‘an organic molecule which must be 

described as the mean between two identical structures’. The next logical development 

in designing molecules capable of ever more rapid Cope rearrangement 

was to restrict conformational mobility even further and to more closely mimic 

the required boat conformation for the Cope TS by imposing a bridge between 

C4,C8 as in 57 [158–160]. 

Again expectations were met; the bridged homotropilidenes, bullvalene 58 

[161], barbaralane 59 [162], barbaralone 60 [162] and semibullvalene 19 [110], all 

undergo degenerate Cope rearrangement much more rapidly than 56. Remarkably, 

by a series of Cope rearrangements every hydrogen and every carbon atom 

in bullvalene become equivalent. The order of reactivity is 19 > 59 > 60 > 58. 

Thus began the quest for a neutral bishomoaromatic bridged homotropilidine 

[123, 132, 163, 164]. 

It was suggested long ago that the semibullvalene nucleus is the system which 

most closely approaches the elusive goal of neutral homoaromaticity [165]. Semibullvalene 

19 was first prepared by Zimmerman and Grunewald in 1966 [110]. 

With the low field NMR spectrometer available to them at that time the degenerate 

Cope rearrangement 19a � 19b could not be frozen out. Later, Anet group using a 

251 MHz ( 1 H) NMR spectrometer were able to achieve temperatures lower than 

the coalescence temperature and consequently determined the activation barrier 

for this Cope rearrangement (�G ‡ 5.5 kcal mol –1 , �H ‡ 4.8 kcal mol –1 ) [166]. The 

value for the activation barrier was later refined using 13 C dynamic NMR data 

(�G ‡ 6.2 kcal mol –1 , �H ‡ 5.2 kcal mol –1 ) [167]. It is generally accepted that the 

C 2v symmetric homoaromatic species 19c is the transition state for this process 

[163, 164]. 

Calculations by Dewar and Lo [168] and Hoffmann and Stohrer [169] suggested 

that substitution of the semibullvalene nucleus with electron withdrawing groups 

at the 2, 4, 6, and 8 positions and electron donating groups at the 1 and 5 positions 

would lead to a decrease in the activation energy for the Cope rearrangement 

through stabilization of the TS. Many syntheses of substituted semibullvalenes 

and barbaralanes were developed, and in agreement with Dewar’s and Hoffmann’s 

predictions the activation barrier was lowered [163, 164]. However, no example has 

yet been prepared in which this ‘barrier’ becomes negative resulting in the delocalized 

species, analogous to 19c, becoming the ground state [123, 163, 164].

8.9 Recent Developments 

In an alternative approach to eliminating the barrier to Cope rearrangement in 

semibullvalenes, small ring annelation was calculated to destabilize the localized 

forms and to favor the delocalized form, e.g., 61, 62, 63 and 64 [123, 132, 163, 

164]. Again no neutral homoaromatic ground state bridged homotropilidene has 

yet been characterized experimentally. 

8.9 

Recent Developments 

Bullvallene. This was used as a starting material in the synthesis of triaza- and tri thia- 

[3]-peristylanes 65 [170] and 66 [171] and also the bullvalene trisepoxide 67 [172]. 

Barbaralane. Reiher and Kirchner and Kirchner and Sebastiani computationally 

examined the tetraphosphabarbaralane 68, and briefly the corresponding tetraazaanalog 

and the parent 59 [173, 174]. They conclude that 68 is homoaromatic with 

a C 2v ground state which is not biradicaloid. Using molecular dynamics simulations 

they demonstrate that at finite temperatures (as opposed to the usual 0 K) 

68 still prefers a C 2v ground state and that distortion (in a Cope rearrangement 

sense) to degenerate C s forms is a vibrational mode (no energy barrier) and not a 

reaction. Using molecular dynamics modeling they present 3-dimensional NICS 

analysis confirming the homoaromatic nature of 68. By (computationally) linking 

together two or more barbaralanes, e.g. 69 and 70, Tantillo et al. arrived at a new 

class of compounds – �-polyacenes [175]. These extended barbaralanes were 

computed to have completely delocalized bishomoaromatic singlet ground states 

and again are predicted not to be biradicaloid. The authors originally envisaged 

these extended barbaralanes as potential sigmatropic shiftamers in the sense of 

69a � 69b [175, 176]. 

415


Semibullvalene. In addition to semibullvalenes, many of the studies considered here 

also discuss barbaralanes. Some semibullvalenes and barbaralanes, with extremely 

low activation barriers to the Cope rearrangement, are thermochromic and solvatochromic 

[177]. The observed solvatochromism is attributed to the changing concentration 

of the delocalized homoaromatic species. Time-dependent density functional 

theory (TD-DFT) calculations confirm that the long-wavelength absorptions, 

responsible for the color of these compounds, arise solely from the delocalized 

species. For the semibullvalene 71 and barbaralanes 72 and 73, increasing solvent 

dipolarity (dipolar solvents possess a permanent dipole moment whereas polar 

solvents are characterized by a significant dielectric constant [178]) increases the 

concentration and correspondingly the thermodynamic stability of the delocalized 

forms. The stabilization of the delocalized form by dipolar solvents is so successful 

with 72 that it goes from a localized (in most solvents) to a delocalized homoaromatic 

solvate ground state in the highly dipolar solvent N,N�-dimethylpropylene 

urea [177]. In complete contrast, the bisanhydride 64 appears to follow the opposite 

trend with the delocalized form increasing in concentration with decreasing solvent 

polarity, perhaps supporting the notion that 64 is homoaromatic in the gas phase 

[164, 179]. Various self-consistent reaction field (SCRF) methods were used to 

model the effect of solvent on these bridged homotropilidenes [177]. While the 

results of these calculations indicated small selective solvent stabilizations of the 

delocalized over localized forms for 71–73, the calculated solvent effects on 64 were 

negligible. A gas phase electron diffraction structure determination of 64 resulted 

in three models, a delocalized homoaromatic structure, a localized structure or a 

2 : 1 mix of localized:delocalized forms, none of these possibilities could be ruled 

out as they all fit the diffraction data [179].

8.9 Recent Developments 

Confirming suggestions in an earlier review [163], calculations on 19, 58, 59 

and dihydrobullvalene 74 reveal that semibullvalene rearranges most rapidly of 

the group as it enjoys the greatest relief of strain in going from the localized form 

to the homoaromatic TS and that the degree of aromaticity in the TS is greatest 

for barbaralane (and least for semibullvalene!) [180]. 

Recently Brown, Henze and Borden used unrestricted DFT, CASSCF and 

CASPT2 to reinvestigate parent semibullvalene 19 and some derivatives (61 and 

75) along with the new system, epoxide 76, with a view to determining if the C 2v 

symmetric species were necessarily homoaromatic [181]. They concluded that 

the C 2v geometries of 19, 75 and 76 enjoy homoaromatic stabilization while that 

of 61 does not and that these C 2v species are the ground states for 61, 75 and 76. 

Both 61 and 76 were calculated to possess significant triplet character and 61 was 

predicted to have a triplet ground state. 

Calculations on a series of substituted semibullvalenes 71, 77–82 revealed that 

the B3LYP/6-31G* method tends to overestimate the stability of the delocalized 

homoaromatic species by up to about 3 kcal mol –1 compared with experimental 

values determined in solution [182]. In agreement with experiment, two phenyl 

and two cyano groups 80 were found to be more effective in stabilizing the delocalized 

form than four phenyl 82 or four cyano 81 substituents. Derivatives 80, 

81 and 82 were all predicted to have homoaromatic ground states. Subsequent 

(TD) B3LYP/6-31G* calculations on semibullvalenes 19, 71, 78, 79, 80 and 83 

and barbaralanes 59, 72, 73, 84, 85 and 86 reaffirmed that the delocalized species 

are responsible for the long wavelength electronic absorptions observed in the 

visible region for some of these compounds [183]. The position of the calculated 

and observed IR bands are in good agreement. 80 was predicted to be blue and 

to be the best target for synthesis of a neutral bishomoaromatic. 

Sauer et al. published full experimental details for their ingenious one-pot 

syntheses of a wide variety of 1,5-dimethyl-3,7-disubstitued semibullvalenes 

417


Scheme 8.1 

(Scheme 8.1) [163, 184], which prompted Zhou and Birney to computationally 

examine the fragmentation of the proposed intermediate diazo compounds 87 to 

semibullvalenes 88 [185]. Zhou and Birney concluded that the fragmentation led 

to a bifurcation point and from there to the localized semibullvalenes and thus 

avoiding the Cope TS for these semibullvalenes. In another one-pot synthesis, Xi 

group developed a copper (I) mediated approach to octasubstituted semibullvalenes 

from 1,4-diiodobutadienes (Scheme 8.2) [186]. Thermolysis of the resulting 

octasubstituted semibullvalenes provides octasubstituted COTs. 

Scheme 8.2 

In an extension of Gompper’s modification of the Saunders’ isotopic perturbation 

method [187, 188], Quast, Williams et al. studied, by variable-temperature 

13 C NMR spectroscopy in a variety of solvents, the nondegenerate Cope rearrangement 

of a series of unsymmetrically substituted semibullvalenes (89–96) 

[189]. They developed a new treatment in which the effects of the substituents on 

chemical shift were specifically accounted for and determined the thermodynamic 

quantities for these skewed equilibria. The Cope equilibrium could only be frozen 

out for 89 and it was determined that in 89 the preferred valence tautomer is the 

one with the ethyl group on the cyclopropane unit.

8.9 Conclusions 

Jiao and coworkers used the isolobal relationship of a boron carbonyl (BCO) 

moiety to a CH group in their design of potentially homoaromatic semibullvalenes, 

barbaralanes and bullvalenes [190]. Their calculations (B3P86/6-311+G**) 

indicate that 97 and 98 should be homoaromatic. The other BCO ‘replaced’ bridged 

homotropilidines under investigations showed reduced barriers to their Cope 

rearrangement compared to their respective hydrocarbons, but still possessed a 

localized ground state. 

8.9 

Conclusions 

The rapid development and current affordability of computer hardware and 

software has resulted in an unprecedented ability to carry out ever more sophisticated 

calculations on increasingly bigger molecules. These developments have 

had a major impact in the area of molecules with labile bonds. In all the systems 

discussed in this chapter, theory has played a leading role in identifying synthetic 

targets and important experimental studies and the rationalization and interpretation 

of experimental results. Through this synergy between experiment and 

theory, the complex properties of cyclobutadiene and cyclooctatetraene are now 

well understood. Enticing new results resurrect old questions about the annulenes: 

Where does natural bond alternation begin and just how (anti)aromatic are 

they? The location of Möbius transition states for annulene isomerization nicely 

addresses the long-standing and vexing question of why these formal cis � trans 

isomerizations are so facile. The cyclophanediene � dihydropyrene equilibrium 

is now better understood and predictions regarding substituent effects on the 

barrier to this electrocyclization are being tested experimentally. Many barbaralanes 

and semibullvalenes with miniscule experimental barriers to their Cope 

rearrangement are now known. The thermochromism and solvatochromism of 

these bridged homotropilidenes has been thoroughly investigated and even a 

semibullvalene solvate with a homoaromatic ground state found. Again, through 

calculations, several ground state bishomoaromatic semibullvalenes have been 

identified. Now all that remains is the synthesis and experimental verification of 

the first neutral bishomoaromatic! 

419


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9 

Molecules with Nonstandard Topological Properties: 

Centrohexaindane, Kuratowski’s Cyclophane and 

Other Graph-theoretically Nonplanar Molecules 

Dietmar Kuck 

9.1 

Introduction 

It may be suspected that students of organic chemistry believe, for probably quite 

varying periods of time, in the perfect tetrahedral coordination of the saturated 

carbon atom. Colloquial mentioning that an organic molecule, say, methanol, 

contains a ‘tetrahedral carbon’ incorrectly implies a sphere of a regular tetrahedron. 

Van’t Hoff and Le Bel’s model [1] has just been too appealing. The propensity to 

oversimplify seems to be a major driving force but also a means for our understanding 

chemical structures. Simplicity comes and goes with symmetry and our 

individual views of chemical beauty change over the years. What is more beautiful: 

ideal shape or a (slightly) pertubated appearance [2]? 

In fact, there are only a few perfectly tetrahedral molecules in organic chemistry. 

Methane, the tetrahalomethanes and also neopentane belong to this group, albeit 

only the central carbon atom has the perfectly tetrahedral coordination, that is, 

its valence orbitals are subject to perfect sp 3 -hybridization. The central carbons 

in tetraethylmethane [3] and tetrabenzylmethane [4] do not, nor do many more 

‘organic’ carbon atoms [5]. 

9.1.1 

Is All This Trivial? 

Another insight that comes late, or later, in the process of learning about organic 

chemistry concerns structural topology, rather than three-dimensional (3-D) topography, 

of organic molecules. It frequently comes as a challenge to students to 

present the 3-D structural formula of a molecule in the plane of a sheet of paper. 

Strychnine is a clear example of a complex polycyclic organic structure which can 

be projected so that we ‘see’ its seven rings put one alongside the other in apparently 

two dimensions. However, there are naturally occurring and also non-natural 

organic compounds which cannot be drawn in the plane with one ring beside the 

other. Strychnine is too simple a structure! This is documented by several reports 

on topologically nonplanar organic molecules [6–14]. 



ISBN: 978-3-527-31767-7 

425

426 9 Molecules with Nonstandard Topological Properties 

Ask students for an organic compound the structure of which cannot be drawn 

in the plane without crossing at least two bonds. It is very likely that they won’t 

find any but they would spontaneously suggest either C 60, or dodecahedrane (or 

one of the other two Platonic hydrocarbons) – or diamond, which, notwithstanding 

the carats involved, may be considered a set of ultimate organic molecules given 

its hydrogen-terminated surface [8, 15]. 

In fact, all of the known or otherwise conceivable globular molecules, such 

as the fullerenes [16] (discussed in Chapter 5), the dodecahedranes [17] (briefly 

presented in Section 2.4.1.6), superphane [18, 19] (mentioned in Section 4.2), 

and their synthesis precursors, or the various carcerands [20], are ‘topologically 

planar’. Even ‘oligomeric’ fullerenes [21] are � since their structures can be represented 

as ‘planar graphs’, that is, as polycyclic arrangements of rings that are 

multiply fused side-by-side in the plane. It is not possible to draw topologically, 

or graph-theoretically nonplanar (‘gt-nonplanar’), molecules in such a simple way 

(Figure 9.1). At least one crossing of a pair of bonds cannot be avoided. 

The smallest conceivable gt-nonplanar’carbon-based structure bearing, at the 

same time, a carbon atom with perfect tetrahedral coordination would be the 

carbon cluster C 5 1. However, C 5 isomers are known to be linear and, of course, 

extremely difficult to generate experimentally [22]. A somewhat more realistic 

analog of 1 is the next-higher homolog bearing a perfectly ‘tetrahedral’ carbon 

atom, viz. ‘centrohexaquadrane’ (C 11 H 12 , 2). This hypothetical hydrocarbon was 

discussed in 1981 in an inspiring conceptual article concerning the aggregation 

of rings about a common carbon center [23]. The first experimentally realistic 

gt-nonplanar hydrocarbon bearing a central carbon atom with ideal tetrahedral 

symmetry is centrohexaquinane (C 17 H 24 , 3). This molecule has been calculated to 

have overall T molecular symmetry [23, 24] and represents, owing to the almost 

perfect geometrical fit of the cyclopentane units with the central neopentane core, 

a low-strain ‘polycyclane’ [23]. More specifically, it may be considered the prototypical 

member of the unusual family of ‘centropolycyclanes’, that is, polycyclic 

hydrocarbons bearing several (up to six) rings mutually fused about the four C–C 

bonds of neopentane [23]. Notwithstanding its geometrically familiar constituents, 

including the unique, perfectly sp 3 -hybridized central carbon atom (and 16 

nonperfect cases), centrohexaquinane (3) has remained a hypothetical molecule 

to date [25–29]. The same holds true for its highest unsaturated congener, centrohexaquinacene 

(4, Figure 9.2), which also has low internal strain but has been 

calculated to have a perfect T d -symmetrical molecular structure [24]. 

Figure 9.1 The three simplest topologically nonplanar carbon-based K 5 molecules are 

all hypothetical: tetrahedral cluster C 5 1; centrohexaquadrane 2 [23]; centrohexaquinane 3.

9.2 

Topologically Nonplanar Graphs and Molecular Motifs 

9.2.1 

The Centrohexaquinacene Core 

9.2 Topologically Nonplanar Graphs and Molecular Motifs 

What is the beauty of the centrohexaquinane motif, and of centrohexaquinacene 

4, in particular? Most of their properties become evident when regarded with the 

eyes of an organic chemist and analyzed with a touch of mathematics. 

1. It represents a relatively large organic structure centered about a truly sp 3 - 

hybridized carbon atom. 

2. The tetrahedral symmetry of the central carbon atom is extended into the 3-D 

space. 

3. Centrohexaquinacene 4 contains six perfectly planar cyclopentene rings. 

4. Each of these six rings is bisected by one of the Cartesian axes and the central 

carbon atom shared by all rings is placed in the origin of this Cartesian coordinate 

system. 

5. Thus, placed at one of the six tips of an octahedron, each of the six double 

bonds of centrohexaquinacene 4 points out into one of the six directions of 

the 3-D space (�x, �y, �z) [28, 29]. 

6. The organic substructures of 3 and 4 are manifold: They contain three mutually 

fused spiro[4.4]nonane units (and thus represent ‘superspiro’ arrangements), 

four intermingled [3.3.3]propellanes as well as four intermingled triquinacenetype 

entities, and finally, three partially intercrossing [5.5.5.5]fenestrane units 

[23, 28]. 

7. In these various substructures, the geometrical abnormalities and the corresponding 

symmetries are largely preserved. In turn, all deviations from 

perfect geometries and minor symmetries in the substructures are completely 

cancelled in the complete centrohexacyclic frameworks of 3 and 4, in particular. 

Figure 9.2 The hypothetical centrohexaquinacene 4 and an ‘exploded’ view illustrating the 

ortho gonal orientation of the six cyclopentene rings of 4 in the Cartesian coordinate system 

[28, 29]. 

427

428 

9 Molecules with Nonstandard Topological Properties 

8. Stepwise dismantling of 3 and 4, representing the largest congeners, by 

removing one C 2 (ethano or etheno) bridge after the other leads to all of the 

lower centropolyquinanes and centropolyquinacenes, respectively. 

9. The centrohexacyclic structures of 3 and 4 give rise to another special molecular 

topography: All surfaces of these C 17 molecular cores are concave when 

regarding the cyclopentene rings of 4 as strictly two-dimensional units. 

10. To complete this survey of abnormalities, it is repeated here that the molecular 

topology of 3 and 4 is nonplanar: The constitution of the 17 carbon atoms corresponds 

to the complete graph K 5, a nonplanar graph comprising five centers 

all of which are pairwise interconnected. This special abnormality will be 

discussed in detail below. 

9.2.2 

The Nonplanar Graphs K 5 and K 3,3 and Some Molecular Representatives 

A key publication on graph theory appeared in 1930 in Fundamenta Mathematicae, 

written by the Polish mathematician C. Kuratowski [30]. In this article entitled ‘Sur 

le problème des courbes gauches en topologie’, Kuratowski showed that all topologically 

nonplanar graphs can be reduced to two graphs, K 5 and K 3,3 , both of which 

can be sketched by pencil-and-paper drawings of a simple tetrahedron. These two 

sketches are depicted by a modern computer-aided ‘drawing’ in Figure 9.3. 

Although the two graphs K 5 and K 3,3 have a common origin, they are fundamentally 

different. In the case of the graph K 5 , the central point of the tetrahedron is 

added and connected to the four corner points. The whole set of ten lines (edges 

of the graph) connecting five tetravalent points (the vertices of the graph) comprise 

the ‘complete graph’ K 5 . Each of the five vertices is connected to all of the other 

ones. The graph K 3,3 , also nonplanar, is somewhat less simple and elegant. In 

this case, two opposite edges of the tetrahedron are divided by two additional 

points, which are mutually connected. Coloring these additional points and their 

neighboring points at the tetrahedron’s tips in opposite ways, as depicted in 

Figure 9.3, yields two sets of three vertices of the ‘bipartite’ K 3,3 graph. Notably, 

the three trivalent vertices of each set are not interconnected, but each of them is 

associated with each vertex of the other set. Thus, the graph K 3,3 comprises two 

‘isolated’ sets of three vertices and a total of nine edges. 

Figure 9.3 The sketches of the K 5 graph (a) and the K 3,3 graph (b), as reproduced from 

Kuratowski’s publication of 1930 [30] using modern computer-aided graphics.

9.2 Topologically Nonplanar Graphs and Molecular Motifs 

Figure 9.4 The complete graph K 5 in four different representations (see text). 

The graph K 5 is often represented as a regular pentagon with ‘exhaustive’ 

interconnection of the tips (Figure 9.4a). The impossibility of projecting it into 

the plane without any mutual crossing of edges is most simply illustrated in 

Figure 9.4b. Slight rearrangement in the plane gives a filled square (Figure 9.4c) 

with the same topological properties, and distortion into the third dimension 

generates the tetrahedron with its central vertex being topographically different but 

topologically equivalent to the other four (Figure 9.4d). This recalls Kuratowski’s 

original representation of the K 5 graph, as depicted in Figure 9.3a. 

The graph K 3,3 is often represented in a rectangular form, as depicted in 

Figure 9.5a. Maximum efforts to separate the edges again leave one unavoidable 

crossing, as most simply shown in Figure 9.5b. Rearrangement of points, as 

discussed for the graph K 5, yields the (unusual) rectangular representation of 

Figure 9.5c, which can easily be converted into the regular hexagonal form of 

Figure 9.5d. Of course, this representation is still topologically nonplanar, as shown 

in Figure 9.5e. Finally, out-of-plane distortion generates a spatial impression of 

the interconnection of the six vertices of the K 3,3 graph. Kuratowski’s tetrahedronbased 

definition (Figure 9.3b) can be most easily recognized from the illustrations 

in Figure 9.5d and 9.5e, in which the extra diametrical connectivity is represented 

by the two middle vertices and their common edge. 

Figure 9.5 The complete graph K 3,3 in six different representations (see text). 

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Figure 9.6 Centrohexaindane 5, a T d -symmetrical K 5 -hydrocarbon and the hexabenzo analog of 

3 and 4, and Kuratowski’s cyclophane 6, a formally D 2d -symmetrical K 3,3 -polyether. Note that the 

front and the back naphthalene-2,7-diyl bridges have been simplified by a two extra long bonds 

(O–O). 

The number and variety of gt-nonplanar organic compounds has increased 

recently, and the examples are collected and discussed in several reports, including 

quite recent ones [8–14, 28, 29, 31–34]. Instead of commenting on the various sorts 

of such nevertheless unusual structures, two of the most representative cases are 

presented in more detail below. To date, the most versatile among these is centrohexaindane 

(5, Figure 9.6) [35–37], the hexabenzo analog of hypothetical parent 

K 5 hydrocarbons, centrohexaquinane 3 and centrohexaquinacene 4. This highest 

member of the centropolyindane family can be made in gram amounts [37] and 

a variety of derivatives has become accessible [9, 28, 29]. The most representative 

K 3,3 counterpart is considered to be Kuratowski’s cyclophane 6 [38], a macrocyclic 

polyether, although a number of other topologically nonplanar organic compounds 

of the K 3,3 topology were known previously [8, 10, 11, 39–41]. The synthesis of 

Kuratowski’s cyclophane was designed as a directed construction of a nonplanar 

organic compound of the K 3,3 type and, as such, it appears to represent a unique 

example of a non-natural polycyclic structure. 

9.3 

Centrohexaindane 

Centrohexaindane 5 is a colorless, high melting solid (m.p. > 400 °C). Its solubility 

in various solvents is surprisingly high and single crystals can be grown from 

some of them, including para-xylene and triethylamine [29]. The fact that this 

‘heavy’ C 41 H 24 hydrocarbon does not behave as a stone-like solid is attributed to 

the peculiarity that it is one of the very organic molecules that, disregarding the 

rims of the benzene rings, have exclusively concave faces. Thus, solvent molecules 

like triethylamine tend to fill the four ‘hollow’ tribenzotriquinacene cavities. In 

fact, a strongly negative electrostatic potential has been calculated for the concave 

side of the tribenzotriquinacene skeleton [42].

9.3 Centrohexaindane 

Figure 9.7 The structure of centrohexaindane 5, as deduced from X-ray structure analysis 

of a single crystal containing one molecule of triethylamine per 5 (C 41 H 24 �NEt 3 ) [28, 29, 43]. 

Views of the tribenzotriquinacene cavity (stick model, left) and of the triptindane (propellane) 

axis (space-filling model, right). 

The molecular structure of centrohexaindane as determined by X-ray diffraction 

of single crystals grown from triethylamine solutions is shown in Figure 9.7 [43]. 

All three intermingled [2,2�]spirobiindane units are strictly linear and the two 

benzene rings of each are mutually orientated at right angles. The four tribenzotriquinacene 

entities are strictly C 3v -symmetrical; not only are the planes of their 

three indane wings orientated at 120° but the long axes of the indane wings are 

strictly orthogonal to each other. The three fenestrindane units comprised in 5 are 

held in a perfect D 2d -symmetrical conformation. The six C–C–C bond angles at 

the central carbon atom of 5 are identical, within the limits of experimental error, 

with the ideal tetrahedral angle (109° 28�) expected for the four C–C bonds at an 

undisturbed sp 3 -hybridized atom. From all this follows the overall T d -symmetry 

of the ground state of centrohexaindane, which is only very slightly affected by 

the crystal packing. 

9.3.1 

Centrohexaindane and Structural Regularities of the Centropolyindane Family 

As the highest member of the regular centropolyindanes [28, 29, 44], centrohexaindane 

(5) contains all of the lower members of this family as substructures. Thus, 

removal of just one indane unit leads to centropentaindane 7 [45], another very rigid 

polycyclic system bearing five benzene rings stretching practically ortho gonally 

into five of the six directions of the 3-D space. Removal of another indane unit gives 

rise to either fenestrindane 8 [46], an interesting congener of the widely studied 

all-cis-[5.5.5.5]fenestranes [5, 28], or to a trifuso-centrotetraindane 9 [47] which, like 

5 and 7, represents a rigid molecular framework. Among the higher centropolyindanes, 

fenestrindane 8 is particular since it is conformationally flexible, as are 

two of the three regular centrotriindanes comprised in the framework of centrohexaindane, 

viz. 11 [46b, 48] and 12 [49, 50]. The only centrotriindanes having a 

conformationally rigid framework are the tribenzotriquinacenes, e.g. 10 [48, 51]. 

In fact, the three indane wings of the latter centrotriindane have been shown to 

stretch into the 3-D space almost perfectly at right angles [28, 29]. 

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Figure 9.8 Centrohexaindane 5 and the lower centropolyindanes 7�12. The highly systematic 

structural and geometrical relationship between the centropolyindanes is reflected by the nearly 

constant � max value of their UV/Vis absorption and the systematic incremental 13 C chemical 

shifts of the central carbon atoms. a The � max value of 12 was taken from [49b]. 

Centrohexaindane has such a very close structural and geometrical relationship 

with the various lower centropolyindanes, that a comparison of some spectroscopic 

properties is informative. The UV/Vis spectrum of centrohexaindane reveals that 

the six �-electron systems of the mutually fused indane units do not interact, in 

agreement with the diphenylmethane-type junction of the aromatic rings and the 

orthogonal arrangement in space [28, 29]. Thus, the absorption at � max = 276.5 nm 

(� = 5800) in n-heptane is roughly four times as strong as the corresponding lowenergy 

band in indane, and there is only a very minor bathochromic shift compared 

with the latter hydrocarbon (� max = 273.0 nm, � = 1500) [48]. With the decreasing 

number of indane units attached to the neopentane core, the extinction coefficient 

decreases steadily. By contrast, the � max values of the centropolyindanes were found 

to reflect a characteristic feature of the indane units annealed to the neopentane


core, namely their conformational geometry. In centrohexaindane 5 and in all other 

congeners containing at least one tribenzotriquinacene unit (cf. 10), the indane 

wings are forced into a planar conformation and their � max values are identical, 

viz. 276.0�276.5 nm. Of course, this also holds true for the tribenzotriquinacenes 

themselves, e.g. 10. Fenestrindane 8 and the other two centrotriindanes, 11 and 

12, and indane itself have conformationally flexible frameworks [28]. Their cyclopentane 

rings are allowed to partially move out-of-plane giving an envelope form 

even in fenestrindane, and the UV/Vis spectra of all of these indane hydrocarbons 

exhibit � max values in the range of 273.0�274.0 nm (Figure 9.8). 

In view of the fact that the central carbon atom of centrohexaindane (5) is a 

perfectly ‘aliphatic’, sp 3 -hybridzed atom, the resonance of the 13 C nucleus at this 

particular position is also noteworthy. It appears at �(C centro ) = 95.0 with particularly 

low relative intensity, as expected for such a nucleus embedded in scaffold 

of sixteen (!) other quaternary carbon atoms. As measured under standard conditions 

at 600 MHz, the intensity ratio falls considerably short of the expected 

value, i.e. [C centro ] : [C � ] � 0.25. The chemical shift of the central carbon nucleus 

in centrohexaindane decreases systematically when one or more indane wings are 

removed stepwise from the neopentane core. Thus, the value of �(C centro ) = 83.2 

was found for centropentaindane 7, whereas fenestrindane 8 has �(C centro ) = 71.9 

and the isomeric trifusocentrotetraindane 9 has �(C centro ) = 70.9. Thus, as illustrated 

in Figure 9.8, stepwise removal of the ortho-phenylene bridges from 

centrohexa indane gives rise to a constant upfield shift of 10�11.5 ppm. Similarly 

to the UV/Vis data, this finding reflects the highly systematic aufbau principle 

of the centropolyindane family, based on largely non-strained indane units and 

culminating in the T d -symmetrical centrohexaindane. 

9.3.2 

Syntheses of Centrohexaindane 

Centrohexaindane 5 can be prepared along three independent synthesis routes and 

in gram amounts. The first of these involves 11 steps starting from 1,3-indanedione, 

a commercially available building block. It involves the synthesis of several 

benzoannelated [5.5.5.6]- and [5.5.5.5]fenestranes, including fenestrindane 8, and 

affords centrohexaindane in an overall yield of ca. 12%. One of the variants of the 

‘fenestrane route’ to centrohexaindane is shown in Scheme 9.1 [35, 37, 46]. 

Two-fold Michael addition of 1,3-indanedione 13 to dibenzylideneacetone 14 

yields the trans-diphenylspirotriketone 15 [52, 53]. Protection of the sterically accessible 

cyclohexanone functionality followed by reduction of the other two carbonyl 

groups gives the dispirane 16. Originally, the trans orientation in 15 and 16 of the 

phenyl groups appeared to be particularly favorable to construct the all-cis-[5.5.5.6] 

fenestrane framework of ketone 17 by bicyclodehydration [44b] with concomitant 

hydrolysis of the acetal group and, in fact, the yield of this conversion is excellent. 

However, the cis-diphenyl stereoisomers of 15 and 16 were found to undergo the 

two-fold cyclodehydration as well, giving rise to the strained cis,cis,cis,trans-[5.5.5.6] 

fenestrane skeleton [54]. Subsequent ring contraction of fenestranone 17 via 

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Scheme 9.1 The synthesis of centrohexaindane 5 along the fenestrane route. 

the corresponding �,��-dibromoketone, followed by Favorskii rearrangement, 

yields the all-cis-tribenzo[5.5.5.5]fenestrane carboxylic acid 18. Decarboxylation 

of the latter compound suffers from mediocre efficiency but cleanly affords the 

tribenzo[5.5.5.5]fenestrene 19, which undergoes benzoannelation at its double 

bond by use of Raasch’s reagent (tetrachlorothiophene-S,S-dioxide) [55] and 

subsequent dehalogenation/aromatization. Noteworthily, fenestrindane 8, the 

all-cis-tetrabenzo[5.5.5.5]fenestrane, is a highly versatile parent centropolyindane 

which has been functionalized in various ways at both its four bridgehead positions 

and at the eight outer positions of the arene periphery [56, 57]. In the two final 

steps, fenestrindane is converted into its tetrabromo derivative 20, a remarkably 

distorted fenestrane bearing significantly flattened geometry at the central carbon 

atom [�(C–C–C) = 121.5°] [28, 56, 58] and subsequent condensation with two 

molecules of benzene under Lewis acid catalysis yields centrohexaindane [35]. 

Although the majority of the steps of this multistep synthesis have good or even 

very good yields, the fenestrane route to centrohexaindane is rather lengthy and 

its efficiency is limited. On the other hand, the eventual incorporation of two 

molecules of benzene across the open angles of the fenestrane framework offers 

valuable advantages in view of the construction of substituted centrohexaindanes 

(see below).


Scheme 9.2 The synthesis of centrohexaindane 5 along the propellane route (via 22 and 23) 

and along the broken fenestrane route (via 11 and 7). 

Two much shorter and more efficient syntheses of centrohexaindane have 

been developed later and are illustrated in Scheme 9.2. The shortest access to 5 

starts again from 1,3-indanedione 13 which, in the first step, is converted into its 

2,2-dibenzyl derivative 21 and then to the [3.3.3]propellane ketone 22. Two-step 

oxidation of the latter compound affords the highly versatile 1,3,3�-triketone 23, 

called ‘triptindanetrione’ because of its relation to the parent centrotriindane 

12, dubbed ‘triptindane’ (Figure 9.8) [49]. Two further steps including three-fold 

addition of the elements of benzene (from phenyllithium and by hydrolysis) and 

subsequent three-fold cyclodehydration of the intermediate, highly crowded propellanetriol 

completes the six-step ‘propellane route’ to centrohexaindane 5 [36, 37]. It 

is remarkable that a total of ten new C–C bonds are formed by this sequence with 

an overall yield of 25%. Since three adjacent indane wings are attached in the last 

two steps, the propellane route represents a valuable alternative to the fenestrane 

route in case of the synthesis of substituted centrohexaindanes (see below). 

The third synthesis of centrohexaindane also involves 2,2-dibenzyl-1,3-indanedione 

21. This time, however, the diketone is converted into a ‘broken fenestrane’, 

viz. the C 2 -symmetrical centrotriindane 11, by reduction to the corresponding 

trans-1,3-indanediol and subsequent acid-catalyzed two-fold cyclo dehydration 

(Scheme 9.2) [44b]. Careful bromination of the benzhydrylic and benzylic positions 

of 11 and subsequent incorporation of two further benzene units furnish centropentaindane 

7, the second-highest centropolyindane, in strikingly high yield 

[45]. In the final two steps, the bromination/condensation sequence is repeated, 

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involving the highly reactive bridgehead dibromo derivative of 7 and leading to 

centrohexaindane 5 in an overall yield of 40%. To date, this seven-step ‘broken 

fenestrane route’ has not been used for the synthesis of derivatives of centrohexaindane 

but it offers the possibility of incorporating one single arene unit in 

a directed way [37]. 

9.3.3 

Multiply-functionalized Centrohexaindanes 

In fact, the directed synthesis of centrohexaindanes bearing various patterns of 

substitution at the six poles of the polycyclic framework appears to be a promising 

goal. The spatial orientation of the six arene units is well defined, so unusual 

and interesting supramolecular aggregation, in particular in the solid state, may 

be envisioned. However, only a few centrohexaindanes bearing a pinpointed set 

of functional groups have been made to date, and they are shown in Figure 9.9. 

Nitration of the parent centrohexaindane 5 under optimized conditions gives 

a mixture of four hexanitro derivatives 24a�d, all of which bearing each nitro 

group at one of the six peripheral C–C edges [29, 59]. Two of these constitutional 

isomers have C 1 molecular symmetry and the other two are C 3 symmetrical. 

Thus, four racemates are formed in ratios (ca. 3 : 3 : 1 : 1) that correspond to the 

ratios expected for the case of statistical attack of the nitrating reagent. The four 

racemates can be separated from each other and have been fully characterized 

[59]. Conversion of the original mixture to the as yet hypothetical twelve-fold 

nitrated centrohexaindane and use of the latter to generate the corresponding 

T d -symmetrical dodeca aminocentrohexaindane represents a similarly challenging 

and promising goal. 

At variance from the a posteriori functionalization of the parent centrohexaindane, 

the a priori incorporation of substituents turned out to be useful in several 

cases. Following the propellane route, the C s -symmetrical dimethyl and tetramethyl 

derivatives 25 and 26 were synthesized [60]. Remarkably, these centrohexaindanes 

contain the substituents at the strongly sterically-shielded ortho positions; in the 

case of the tetramethyl derivative 26, each of the methyl groups points into one of 

the four sterically-shielded propellane cavities of the centrohexaindane framework. 

The tetramethoxycentrohexaindanes 27 and 28 represent two examples of several 

electron-rich derivatives of 5 that have become accessible along the fenestrane 

route [61]. The four-fold bridgehead bromination of fenestrindane (8 � 20, 

cf. Scheme 9.1) can easily be used to prepare the corresponding bridgehead 

tetraalcohol, which in turn can be condensed under Brønsted acid catalysis with 

various methoxybenzenes, e.g. with veratrole and hydroquinone dimethyl ether in 

the cases of 27 and 28, respectively. Again, the facile incorporation of four methoxy 

groups into the sterically-shielded inner arene positions of 28 is remarkable. Thus, 

the 1 H NMR spectrum of 28 clearly reflects the close neighborhood of the four 

methoxy groups to the eight ortho protons at the adjacent benzene rings, which 

resonate at particularly low field (� = 8.22 ppm, compared with � = 7.74 ppm in 

the case of 27) [59, 61].


Figure 9.9 Selected derivatives of centrohexaindane synthesized either by a posteriori functionalization 

of 5 (24a�d) or by a priori incorporation of the functional groups (25�29, see text). 

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The only twelve-fold functionalized derivative of 5 reported so far is the T dsymmetrical 

dodecamethoxycentrohexaindane 29 [62]. In this case, the propellane 

route proved to be viable, albeit with some inevitable restrictions and losses of 

yields in the last steps. As mentioned above, the construction of centrohexaindanes 

bearing functional groups at geometrically well-definable points is considered a 

great challenge. 

9.4 

K 5 versus K 3,3 Molecules 

Compared with the K 3,3 hydrocarbons and the heterocyclic organic molecules with 

K 3,3 topology presented in the next section, most of the gt-nonplanar counterparts 

of the K 5-type are remarkably simple. However, as mentioned above, the synthesis 

of the smallest prototypical hydrocarbon that conceivably should be accessible, 

centrohexaquinane 3, has never been achieved. To our knowledge, the singly benzoannelated 

derivative of 3, benzocentrohexaquinane 30, represents the smallest 

K 5 hydrocarbon synthesized so far, albeit obtained only in minute amounts [27]. 

The C s -symmetrical dibenzo and the C 3v -symmetrical tribenzo analogs 31 and 32, 

and other higher benzoannelated congeners have been reported since the first 

synthesis of centrohexaindane 5 [27, 36]. 

Figure 9.10 The lowest congener of the K 5 -hydrocarbons synthesized to date, benzocentrohexaquinane 

30 and its next higher C s -symmetrical and C 3v -symmetrical benzo analogs, 31 and 

32, respectively. Hydrocarbons 31 and 30 have been synthesized from 32 by partial oxidative 

degradation of the benzene rings [27]. 

9.4.1 

Topologically Nonplanar Polyethers and Other K 3,3 Compounds 

The K 3,3-cyclophane 6, termed ‘Kuratowski’s cyclophane’, represents a macrocyclic 

polyether [38]. Remarkably, other representative topologically nonplanar organic 

compounds are also polyethers. One of them is the trioxacentrohexa quinane 33 

(Figure 9.11), a polycyclic polyether of K 5 topology published back-to-back in 1981 

by Simmons [25], Paquette [26] and coworkers and sometimes just named after 

both these authors. The other is the first molecular Möbius strip ever known, the 

twisted and thus gt-nonplanar belt-type structure 34, reported by Walba in 1982 

[39], representing a K 3,3 -type molecule [14, 63, 64]. In contrast to a sizable number 

of other heterocyclic K 3,3 molecules, including several peptides [32–34] and

9.4 K5 versus K3,3 Molecules 

Figure 9.11 Two complementary topologically nonplanar polyethers: The ‘Simmons–Paquette’ 

molecule 33, a molecular K 5 graph [25, 26], and Walba’s Möbius ladder 34, a molecular K 3,3 

graph [39]. Note that two diglycol ether strips of 34 are drawn extra long and that they cross 

each other in front of the major part of the ladder. 

heterobridged pagodanes [41], K 3,3-type hydrocarbons that have been studied experimentally 

and discussed in the literature, are extremely scarce. Thus, the triplelayered 

naphthalenophane 35 (Figure 9.12) has been synthesized and its electron 

spectra studied in detail [40], and this compound may be regarded as the first K 3,3hydrocarbon 

ever known. More recently, a hexamantane, viz. ‘cyclohexamantane’ 

36, was isolated from petroleum and its structural and spectroscopic properties 

were reported [65]. It has been discussed previously that at least five adamantane 

units are required to generate a topologically nonplanar diamondoid framework 

[8]. However, none of the ten possible members of the pentamantane branch of 

the polymantane family [66] is known by experiment; rather, anti-tetramantane 

is the highest congener ever synthesized [67]. Cyclohexamantane 36 undoubtedly 

represents a gt-nonplanar hydrocarbon, the framework of which contains several 

K 3,3 -graphs. While the chemistry of diamondoids beyond adamantane has started 

to grow impressively [68, 69], the existence and synthesis of other gt-nonplanar 

diamondiod hydrocarbons in petroleum can be envisioned, just as numerous 

gt-nonplanar peptides and proteins may exist in biological systems. 

Figure 9.12 The first K 3,3 hydrocarbon, Otsubo’s cyclophane 35 [40], representing a 

gt-nonplanar triple-layered [2.2:2.2]naphthalenophane (top view showing one naphthalene- 

2,6-diyl unit on top and another one below the central naphthalene-2,3,6,7-tetrayl unit), and 

Dahl’s gt-nonplanar K 3,3 diamondoid hydrocarbon, cyclohexamantane 36 (side view) [65]. 

The central naphthalene 35 or decalin 36 units are highlighted in blue. 

It is interesting to locate the structural features that render compounds 35 and 36 

topologically nonplanar. This is even amusing since, counter-intuitively, these hydrocarbons 

may be considered closely interrelated, both being naphthalene-based 

cyclophanes. The K 3,3 graph underlying in the triple-layered naphthalenophane 

35 is highlighted in Figure 9.13a. Comparison with the simpler benzene-based 

analog (Figure 9.13b) and the Schlegel diagram of the latter (Figure 9.13c) clearly 

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Figure 9.13 (a) The gt-nonplanarity of Otsubo’s cyclophane 35 as evidenced by its K 3,3 

topology. (b) Simplification to the corresponding benzene-based triple-layered cyclophane. 

(c) Demonstration of the gt-planarity of the latter structure, implying that all even higher multilayered 

cyclophanes of this type are gt-planar as well. 

Figure 9.14 (a) The gt-nonplanarity of cyclohexamantane 36 as evidenced by its K 3,3 topology. 

(b–d) Gt-planar diamond cuttings derived from 36 by hypothetical removal of the central C–C bond 

or one or two central CH units to give spherical, bowl-type and belt-type structures, respectively. 

demonstrates that multilayered cyclophanes containing only monocyclic units, 

e.g. benzene rings, are all gt-planar. This also holds true for ‘[6]chochin’, a chiral, 

six-layered [2.2:2.2:2.2:2.2:2.2]paracyclophane [70–72]. 

The gt-nonplanarity of cyclohexamantane 36 is illustrated in Figure 9.14a. 

Similar to the presentation of Otsubo’s cyclophane 35 in Figure 9.13a, it is evident 

that it is the central C–C bond which renders 36 topologically nonplanar. Thus, 

the hypothetical removal of that central bond would produce a cage hydrocarbon 

with a topologically planar structure. (Note that due to the strong transannular 

H in …H in repulsion, the conformation would deviate markedly from that shown 

in Figure 9.14b). Further removal of the top and bottom methane units would 

lead to another interesting, belt-like cutting of the diamond lattice (Figure 9.14d). 

The diagrams clearly show the change from gt-nonplanarity of 36 to gt-planarity 

in spherical, bowl- and belt-like structures.

9.5 

Kuratowski’s Cyclophane 

9.5 Kuratowski’s Cyclophane 

Kuratowski’s cyclophane 6, described by Siegel et al. in 1995 [38], represents the 

most recent and certainly also the ‘heaviest’ K 3,3 -type non-natural topologically 

nonplanar organic molecule that has been assembled by a designed chemical 

synthesis. However, it is worth noting that, similarly to the synthesis of Walba’s 

Möbius-type polyether mentioned above, the construction of 6 notoriously involves 

the concomitant formation of a topologically planar isomer. Thus, although being 

a designed synthesis, as is that of centrohexaindane 5, the preparation of 6 is based 

on a compromise from the very start, as will be shown in the next section. 

9.5.1 

Synthesis of Kuratowski’s Cyclophane 

The synthesis strategy for the construction of 6 can be envisioned easily from the 

representation of the K 3,3 graph shown in Figure 9.5f. One of the ether components 

is a doubly branched octiphenyl 46 comprising all the six vertices of the K 3,3 graph 

and five of the nine edges required. The remaining four edges are provided by 

incorporating four naphthalene-2,7-diyl units as the complementary ether components. 

Hence, the challenge of this synthesis was two-fold: (1) The construction of 

a suitable octiphenyl derivative, viz. the octabromide 47, and (2) the achievement 

of the four etherification processes in a one-pot synthesis affording, at least in 

part, the desired topologically nonplanar octaether 6 (Schemes 9.3�9.5). 

The assembly of the octiphenyl 46 is based on the synthesis of several 2�-functionalized 

meta-terphenyls (Scheme 9.3). Coupling of two equivalents of 3,5-dimethylphenylmagnesium 

bromide 39 with 1,3-dichloro-2-iodobenzene 38 in the 

course of a Hart reaction [73] and subsequent quenching of the product mixture 

with trimethyl borate leads to the terphenylboronate 37a. The corresponding 

boronic acid 37b can be subjected to a two-fold Suzuki coupling with 4,4�-diiodobiphenyl 

45 to furnish the desired octiphenyl (46) in a remarkably short sequence 

(Scheme 9.4, yields have remained unpublished). Another but relatively lengthy 

route to 46 involves 2�-iodoterphenyl 40 as the product of the Hart reaction between 

38 and 39. Subsequent Ullmann reaction of 40 with 4-iodonitrobenzene (41) to 

quaterphenyl 42 followed by reduction of the nitro group and Sandmeyer reaction 

affords the corresponding iodoquaterphenyl 43, and subsequent exchange of 

the halogen for the boronic acid group gives the quaterphenylboronic acid 44 

(Scheme 9.3). The latter two quaterphenyl derivatives both represent suitable 

moieties of the desired octiphenyl 46, which is accessible (Scheme 9.4, yields 

have remained unpublished). 

The remaining section of the synthesis consists of an apparently routine 

sequence of cyclophane chemistry. Nevertheless, it comprises an eight-fold refunctionalization 

of the methyl groups of octiphenyl 46 in four subsequent preparative 

steps to give, in a stated 50% yield, the octiphenyl 47 containing eight benzylic 

bromomethyl functionalities (Scheme 9.4). In the very last step (Scheme 9.5), 

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Scheme 9.3 Synthesis of the building blocks 37b, 43 and 44 for the construction of Kuratowski’s 

cyclophane 6 [38]. 

Scheme 9.4 Syntheses of octiphenyl (46) and its octabromo derivative (47) [38].

9.5 Kuratowski’s Cyclophane 

Scheme 9.5 Assembly of Kuratowski’s cyclophane 6 and its topologically planar isomer 49 [38]. 

this intermediate, illustrated in an axial representation corresponding to the 

‘inner’ edge of the graph shown in Figure 9.5f, is subjected to an eight-fold 

Williamson ether synthesis with four equivalents of 2,7-dihydroxynaphthalene 

48. Two macrocyclophanes result from this condensation: (1) the desired topologically 

nonplanar isomer 6 of formal D 2d symmetry and (2) the topo logically 

planar isomer 49, of formal D 2h symmetry, which can be separated from each 

other and isolated by flash chromatography and isolated in yields of 15% and 

10%, respectively (Scheme 9.5) [38]. 

9.5.2 

The Structure of Kuratowski’s Cyclophane 

X-ray crystal structure analysis [38] revealed that neither of the macrocyclic polyethers 

6 and 49 adopts the ideal molecular symmetries mentioned above. This 

is not surprising in view of the numerous rotational degrees of freedom and the 

presence of solvent molecules in the crystal. Instead, the topologically nonplanar 

isomer 6 was found to prefer an approximately S 4-symmetrical conformation. 

Moreover, in both cases, two molecules of benzene per macrocyclophane were 

443

444 


incorporated in the crystals. The use of Kuratowski’s cyclophane 6 for the design 

of larger molecules in the 10 kDa range has been emphasized by the authors [38] 

and, in fact, extension of the molecular poles at the quaterphenyl axis of 6 and 

at the four lateral naphthalene wings appears tempting. On the other hand, the 

presence of several benzylic ether linkages may cause limitations in the chemical 

stability under various reactions conditions. 

9.6 

Conclusions 

It has been demonstrated in this chapter that the use of regular and essentially 

non-strained organic building blocks, such as the sp 3 -hybridized carbon centers 

in the neopentane core and in cyclopentane and cyclopentene rings, as well as 

the sp 2 -hybridized carbon atoms in benzene and naphthalene rings, can be used 

to construct novel polycyclic molecular structures of highly unusual topological 

properties. Centrohexaindane 5, a large but conceptually simple, possibly ‘elegant’ 

or even ‘beautiful’, and experimentally easily accessible hydrocarbon, and Kuratowski’s 

cyclophane 6, an even larger and likewise elegant macrocyclic polyether, 

represent outstanding examples of such cases. Whereas the former compound is 

a unique hydrocarbon containing a C 17 core that corresponds to the complete and 

topologically nonplanar graph K 5 , the latter is a cyclophane designed to contain a 

bipartite and also topologically nonplanar graph K 3,3 . However, there are notable 

differences. Centrohexaindane has a strictly T d -symmetrical and rigid molecular 

framework consisting of normal (five-membered) rings; it lacks any conformationally 

dynamic behavior. By contrast, Kuratowski’s cyclophane is a conformationally 

flexible macrocyclic polyether and exists in a dynamic equilibrium that 

comprises conformers of lower symmetry than the ideal shape would suggest. 

Hence, the spatial orientation of the six indane wings of centrohexaindane is 

perfectly orthogonal to each other and may be accessible to extension into the six 

directions of the Cartesian coordinate system. The geometrical orthogonality of 

the centrohexaindane framework is of particular importance in view of the fact 

that the benzene units can be modified by various functional groups. Kuratowski’s 

cyclophane, with its central quaterphenyl axis and its lateral naphthalene units may 

also be extendable by functionalization of the aromatic peripheries, however, the 

conformational flexibility renders its scaffold less geometrically defined. Whether 

the conformationally extreme rigidity and exact geometrical shape of 5 and/or 

the limited flexibility of 6 will turn out to be useful for future development, may 

depend on the requirements and feasibility of the nanoscale molecular interaction 

of such unusual organic compounds. 

In any case, inspiration to create novel molecular architectures based on 

building blocks that are in conformity with van’t Hoff’s and Le Bel’s hypothesis 

has brought about untypical organic systems with mathematically interesting, 

topological properties and aesthetic construction principles [74]. There is no doubt 

that further such fruitful creativity is needed.

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38 C. T. Chao, P. Gantzel, J. S. Siegel, 

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43 D. Kuck, J. Tellenbröker, H. Bögge, 

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47 D. Kuck, M. Seifert, Chem. Ber. 1992, 

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48 D. Kuck, Angew. Chem. Int. Ed. Engl. 

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50 B. Paisdor, D. Kuck, J. Org. Chem. 1991, 

56, 4753–4759. 

51 (a) D. Kuck, T. Lindenthal, A. Schuster, 

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(b) D. Kuck, E. Neumann, A. Schuster, 

Chem. Ber. 1994, 127, 151–164. 

52 I. Ya. Shternberga, Ya. F. Freimanis, 

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55 M. S. Raasch, J. Org. Chem. 1980, 45, 

856–867. 

56 D. Kuck, A. Schuster, R. A. Krause, 

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unpublished work. 

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63 (a) For the first syntheses of molecules 

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systems, see D. Ajami, O. Oeckler, 

A. Simon, R. Herges, Nature 2003, 426, 

819–821. (b) Note that, in contrast to 

their �-electron system, the constitution 

of these hydrocarbons is topologically 

planar. 

64 For related studies on Möbius 

�-electron systems, see (a) C. Castro, 

Z. Chen, C. S. Wannere, H. Jiao, 

W. L. Karney, M. Mauksch, R. Puchta, 

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65 J. E. P. Dahl, J. M. Moldowan, 

T. M. Peakman, J. C. Clardy, 

E. Lobkovsky, M. M. Olmstead, 

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66 A. T. Balaban, P. von Ragué Schleyer, 

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67 (a) W. Burns, M. A. McKervey, 

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68 (a) P. R. Schreiner, A. A. Fokin, 

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References 

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11, 7091–7101; (c) P. R. Schreiner, 

N. A. Fokina, B. A. Tkachenko, 

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(d) A. A. Fokin, P. R. Schreiner, 

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2006, 71, 8532–8540. 

69 Note that, in contrast to cyclohexamantane 

(36), the globular framework 

of [1(2,3)4]pentamantane (‘T d -pentamantane’, 

cf. ref. [68d]) is topologically 

planar. 

70 (a) T. Otsubo, S. Mizogami, I. Otsubo, 

Z. Tozuka, A. Sakagami, Y. Sakata, 

S. Misumi, Bull. Chem. Soc. Japan 1973, 

46, 3519–3530; (b) T. Otsubo, H. Horita, 

S. Misumi, Synth. Commun. 1976, 6, 

591–596. 

71 M. Nakazaki, K. Yamamoto, S. Tanaka, 

H. Kametani, J. Org. Chem. 1977, 42, 

287–291. 

72 H. Dodziuk, K. S. Nowi�ski, Tetrahedron 

1998, 54, 2917–2930. 

73 C. J. F. Du, H. Hart, K. K. D. Ng, 

J. Org. Chem. 1986, 51, 3162–3165. 

74 The structure of centrohexaindane 

(5) was used as ‘The Graph of the 

Conference’, 19th LL-Seminar on Graph 

Theory, Vienna, April 25–28, 2002. 

The author thanks Professor Dr. Peter 

F. Stadler (Leipzig, Germany) for kindly 

having communicated this to us. 

447

10 

Short-lived Species Stabilized in ‘Molecular’ or 

‘Supramolecular Flasks’ 


Highly strained hydrocarbons with unusual spatial structure are often short-lived 

species observed only in cryogenic matrices [1]. Several exciting systems discussed 

in this volume are of such character. Recently, it has been shown that encaging a 

highly reactive species in a larger molecule, i.e. forming an inclusion complex, can 

in certain cases stabilize it. As will be in detail discussed below, such an approach 

rooted in supramolecular chemistry – a new interdisciplinary border area situated 

among chemistry, physics, biology and technology – allows one to observe such 

species even at room temperature. 

Although there is no precise definition of this field, it flourishes basing on new 

concepts of molecular and chiral recognition, self-assembly, self-organization 

and preorganization as well as cooperativity [2]. Formation of a supramolecular 

complex is known to change the properties of its constituent parts. The most impressive 

examples of this phenomenon are represented, probably, by the existence 

of a single nitrogen atom [3, 4] or noble gas molecule [5, 6] in a fullerene cage 

and by alkali metal anions [7] created due to the high affinity of crown ethers to 

alkali metal cations resulting in the formation of alkalides like 1. As exemplified 

by cyclodextrin complexes, the host molecules in such complexes can either 

stabilize the guest (as in drugs marketed in form of cyclodextrin complexes) or 

act as catalyst [8]. The former effect has been exploited in the recent syntheses of 

short-lived species stabilized inside ‘molecular flasks’. 



ISBN: 978-3-527-31767-7 

449

450 10 Short-lived Species Stabilized in ‘Molecular’ or ‘Supramolecular Flasks’ 

The beginning of this exciting development was marked by the synthesis of 

cyclobutadiene 2 in hemicarcerand 3 (Scheme 10.1) by the Cram group [9] in 1990 

few years after he had obtained the Nobel Prize for his participation in the formulation 

of ideas which formed the basis of supramolecular chemistry. As formulated 

by these authors, ‘Cyclobutadiene, (CH) 4 , is the Mona Lisa of organic chemistry in 

its ability to elicit wonder, stimulate the imagination, and challenge interpretive 

instincts. No other organic compound combines such a fleeting existence and so 

many different syntheses, with such a propensity for different chemical reactions, 

and with the variety of calculations of its structure.’ [9]. Prior to the work by the 

Cram group the existence of cyclobutadiene was proved but its rapid decomposition 

precluded detailed experimental studies of its structure and properties 

[10–13]. Against all expectations, 2 synthetized in the hemicarcerand molecular 

flask 3 (easily obtained by the guest templated synthesis depicted in Scheme 10.2) 

was found to be extraordinary stable for months at room temperature. The guest 

molecule was obtained by the photoisomerization and fragmentation �-pyrone

10 Short-lived Species Stabilized in ‘Molecular’ or ‘Supramolecular Flasks’ 

Scheme 10.1 Photochemical and thermal reactions of �-pyrone: 

at 8 K in an argon matrix (---------); in solution and gas phase (–––––); 

in the inner phase of the hemi carcerand 3 (––––– –––––). 

Scheme 10.2 Synthesis of the cage compound templated by guest. 

451


according to Corey procedure [14, 15]. Cyclobutadiene obtained in this way was 

proved to be in a singlet ground state and to rotate rapidly in the flask 3. However, 

in the oxygen-free atmosphere, 2 could be kept at room temperature for months. 

The ‘inner phase’ of carcerands and hemicarcerands was dubbed in [9] ‘a new 

phase of matter’ essentially different from the inside phases of clathrates and 

zeolites since ‘one host molecule provides one discrete molecular inner phase not 

depending on the bulk phase’. Cram predicted that the chemical reactions carried 

out in such phases could enable one to obtain and study many highly reactive species. 

The achievement stimulated vigorous research of cyclobutadiene centered 

mainly on its antiaromaticity [16], the mechanism of its automerization [17], the 

Jahn–Teller effect [18] and its exotic analogs [19]. However, only recently the few 

syntheses of new short-lived species in molecular flasks and systematic studies 

of such reactions have been reported. 

In 1997 Warmuth accomplished an exciting synthesis of a compound 4 in the 

carcerand cage 5 [20]. 4 within 5 was prepared in a solution in deuterated THF 

by photochemical reaction at 77 K. The highly unstable 4 undergoes [4+2] Diels– 

Alder reaction with the host during the warming according to Scheme 10.3 [21, 22] 

yielding cycloaddition product 6. Thought-provokingly, on the basis of NMR spectra 

Warmuth could not decide whether the compound obtained had the o-benzyne 

4a or allene 4b structure [20]. The alternative was solved on the basis of high-level 

ab initio calculations showing that the calculated structure was closer to that of 

o-benzyne 4a [23, 24]. 

Another recent accomplishment in this field is the synthesis of highly unstable 

1,3,5,6-cycloheptatetraene 7 [25, 26] in the carcerand flask by Warmuth and Marvel 

[27, 28] (Scheme 10.4). Obtaining 7 by the latter authors was made possible by 

taking advantage of a small isotope effect taking place in cage 8 which slowed 

down a competitive reaction leading to the insertion product 9 analogous to 6.


Scheme 10.3 [4+2] Diels–Alder reaction of the highly unstable 4 with the host during the warming. 

Scheme 10.4 Obtaining 1,3,5,6-cycloheptatetraene 7 in photochemical reactions and its 

decomposition. 

In this case the authors were able to assign the singlet 7 not diradical (with a 

triplet ground state) structure to the product. These studies were reviewed by 

Warmuth [29]. 

The observation that C=C bonds in some terpenes have been found to avoid the 

bridgehead positions was summarized as Bredt’s rule [30]. So-called anti-Bredt 

molecules violating this rule have been studied extensively [31, 32]. Roach and 

Warmuth [33] used the cage of 5 as molecular flask enabling them to obtain two 

such hydrocarbons 10–12. The latter highly strained olefins containing a transcyclohexane 

or trans-cycloheptane rings are unexpectedly stable when kept in 

oxygen-free atmosphere. 

453


Carcerands 3 and 5 played the role of a molecular flask in the syntheses and 

stabilization of the short-lived hydrocarbon molecules 2, 4, 7, 10–12. The syntheses 

of the chiral and somewhat hydrophilic hemicarcerand 13 [34] and of the watersoluble 

hemicarcerand 14 [35] pave the way for the synthesis of chiral and polar 

short-lived species. 

The syntheses of 2, 4a, 7, 10–12 were carried out in ‘molecular flasks’. Another 

milestone in the domain of the synthesis of unstable species were the syntheses in 

self-assembled cages playing the ‘flask’ role. Several such syntheses were carried 

out in Fujita [36–39], Raymond [40] and Warmuth [41–43] groups leading to the 

stabilization of various types of short-lived species that are not hydrocarbons. They 

represent supramolecular reactions of a higher complexity than those discussed 

above since they were carried out in self-assembled, not covalently bound, cages. 

Some of the most important results of this kind have been reviewed by Schmuck 

[44]. An exciting recognition by a self-assembled cage of a nine-residue peptide, 

combined with its �-helical folding described by Fujita group deserves mentioning 

although it falls outside the scope of this book [45]. 

Fujita [38, 46, 50], Raymond [51–55] and Warmuth [56–62] groups started 

systematic studies of chemical reactions carried out inside molecular or supramolecular 

cages allowing us to hope that several other short-lived species will be 

possible to obtain.


Only few reactions in ‘molecular flasks’ have been described in literature but 

they clearly mark the beginning of an exciting development in the border area 

among organic, supramolecular and theoretical chemistry. Numerous unusual 

molecules that have been recently proposed as plausible synthetic targets [63] 

on the basis of quantum calculations may be only short-lived species. Noble gas 

compounds like HHeF [64], tetrahedrane 15 (of which only two derivatives are 

known) [65] and its truncated analog 16 [66], bowlane with a pyramidal carbon 

atom 17 [67], dimethylspiro[2.2]octaplane 18 with a planar carbon atom developed 

on its basis [68], tricyclohexane 19 having a bond angle, formed by three tetracoordinated 

carbon atoms, of ca. 180° [69], [1.1.1]geminane 20 with inverted carbon 

atoms presented in Chapter 2.1 [70], fused bicubane 21 [71], to name but a few, 

455


may prove to be not only feasible but stable inside molecular capsules which force 

close contacts of reagents and may even catalyse the synthesis, on one hand, and 

protect the product from decomposition, on the other. Such prospects will undoubtedly 

trigger further computational studies of nonstandard species. Moreover, the 

enabling of the capture of short-lived molecules like silacyclopropyne 22 known 

as matrix-isolated species with C�C–Si angle close to 60° [72], fullerene C 20 23 

[73, 74], or transient intermediates like carbocations [75, 76] and carbenes [77] in 

(super)molecular cages will beyond question give an impetus to the studies of 

reaction mechanisms. 

The molecules 2, 4a, 7 and 23 are symmetrical, aesthetically pleasing systems. 

So are the hypothetical light noble gas compounds and most of 15–22 awaiting 

their syntheses. At the advent of a fruitful mutual interaction among organic 

synthesis and supramolecular and quantum chemistry one can say repeating, 

with some additions, the words by Goethe’s Faust: You highly strained molecule 

‘Stay indeed! You are so beautiful!’ 

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12, 3211. 

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Soc. 2007, 129, 7000. 

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Kawano, M.; Fujita, M. Dalton Trans. 

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Science 2006, 312, 251. 

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Warmuth, R. Science 2007, 316, 85. 

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129, 2746. 

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Bergman, R. G.; Warmuth, R. 

J. Am. Chem. Soc. 2006, 128, art. no. 

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11 

Concluding Remarks 


Our journey through the domain of strained hydrocarbons with unusual spatial 

structure is coming to its end. It is time to attempt to sketch its future development 

as it seems today. As described in detail in preceding chapters, several fascinating 

hydrocarbons with unusual spatial structure still await their syntheses. Geminanes, 

like 1, with inverted carbon atoms discussed in Chapter 2.1; bowlane 2 and dimethanospiro[2.2]octaplane 

3 (Chapter 2.2) with pyramidal and planar carbon 

atoms, respectively; parent diethynyl expanded prismanes, like 4 (Chapter 2.3); 

truncated tetrahedrane 5 and hexaprismane 6 (Chapter 2.4); and centrohexaquinane 

7, which exhibits little strain but is still unknown (Chapter 9) as well as 

numerous unusual unsaturated and/or conjugated hydrocarbons that have been 

predicted to be stable but whose syntheses have so far been unsuccessful. Taking 

into account the great achievements of the Cram group, that succeeded in obtaining 

cyclobutadiene 8 [1], and of the Warmuth group involving o-benzyne 9 [2, 3] taking 

advantage of supramolecular approach (Chapter 10), even the synthesis of the 

elusive highly strained parent tetrahedrane 10 cannot be excluded. These reactions 

in ‘molecular flasks’ allow one to obtain and stabilize short-lived molecules, and 

will certainly enable studies of exciting molecules with the structure strongly 

departing from the standard one. 



ISBN: 978-3-527-31767-7 

459

460 11 Concluding Remarks 

The synthetic chemistry of fullerenes will undoubtedly flourish. One can expect 

that it will develop in four directions: 

1. In addition to a total synthesis of the parent fullerene, those of more specific 

C 60 and higher fullerene derivatives will be vigorously pursued, driven not only 

by pure interest but also by the prospects of marketable applications. 

2. Syntheses of larger fullerene cages will be indispensable for applications of 

endohedral fullerene complexes. 

3. With the same purpose in mind, the syntheses of ‘opened’ fullerenes with a 

hole and their chemical closing, is discussed in Chapter 5.5. In this respect, 

the author is very interested to discover whether Murata group [4] will succeed 

in closing C 70 with two hydrogen molecules inside (discussed in Section 5.5), 

as this had been calculated to be less stable than the complex involving only 

one H 2 molecule inside the cage [5]. 

4. Today ‘in’ isomers of hydrogenated fullerenes are hypothetical moieties but, 

taking into account their unusual structure and predicted interesting properties 

(for example, the exciting possibility of reactions between substituents inside 

the cage discussed in Chapter 2.4), they could become a hot topic when the 

appropriate synthetic methods will be developed. 

One can also expect numerous synthetic investigations in the field of carbon 

nanotubes although in this area aggregate formation, called synkinesis by Fuhrhop 

and Köning [6], will be more actively carried on than the standard synthesis. 

Physicochemical studies, interesting per se and forming the basis of applications 

of strained hydrocarbons as well as model calculations of these systems 

will thrive. 

As discussed in detail in Chapter 1.2, the applications are not (and should not 

be) the major driving force for studying strained hydrocarbons. However, eventually 

many strained hydrocarbons will be used as drug carriers, parts of molecular 

(or supramolecular) devices, etc. 

And last, but not least, unusual hydrocarbons or reagents from which they are 

made will be the subject of rapidly developing supramolecular chemistry. Endohedral 

fullerene complexes as well as nested fullerenes and multiwalled carbon 

nanotubes on the one hand and gorgeous supramolecular cube (crystallized from 

water solution of 11) obtained by the Kuck group in their quest for centropolyindanes 

[7] illustrate this point.

References 

I hope that this book has shown that the domain of strained hydrocarbons is 

an interdisciplinary field in which synthetic, physical and theoretical chemists are 

actively involved. Driven by the interest of numerous specialists from diversified 

areas the field will undoubtedly flourish. 

References 

1 Tanner, M. E.; Knobler, C. B.; Cram, D. J. 


2 Warmuth, R. Angew. Chem. Int. Ed. 1997, 

36, 1347. 

3 Jiao, H. J.; Schleyer, P. V.; Beno, B. R.; 

Houk, K. N.; Warmuth, R. Angew. Chem. 

Int. Ed. 1998, 36, 2761. 

4 Koichi, K.; Murata, M. M. S.; Murata, Y. 

In: 230th ACS National Meeting 2005 

2005, p ORGN-338. 

5 Dodziuk, H. Chem. Phys. Lett. 2005, 410, 

39. 

6 Fuhrhop, J.-H.; Koening, J. Membranes 

and Molecular Assemblies. The Synkinetic 

Approach; The Royal Society: Cambridge, 

1994. 

7 Kuck, D. Chem. Rev. 2006, 106, 4885. 

461

Index 

a 

acenaphthene skeleton 78 

acenaphthene-5,6-diyl bis(diarylmethylium) 

78 

acene 

– twisted 23 

[e,l]acephenanthrylene 184 

acetylene 26 

actuator 363 

adamantene 103 

adamantylideneadamantane 108 

– derivative 108 

alkaplane 48 

alkenes 

– acyclic 106 

– bridgehead 103 f. 

– computational data 114 

– distorted 103 ff., 112 

– nonplanar 103 ff. 

– planar 112 ff. 

– pyramidalized 112 ff. 

alkylidenecycloproparene 182 

alkynes 75 f. 

all-cis-tribenzo[5.5.5.5]fenestrane 434 

allene 

– bicyclic 25 

– � �-bond 

deformation 123 

– cyclic 123 

– eight-membered ring 134 

– four- and fi ve-membered ring 124 

– polycyclic 134 f. 

– seven-membered ring 131 

– strain estimate 123 

– strained cyclic 122 ff. 

amylose 351 

annulene 399 ff. 

– higher 411 ff. 

[4]annulene 399 ff. 

[8]annulene 403 ff. 

[10]annulene 411 f. 

methano-bridged 412 




[18]annulene 411 

antiaromaticity 399 ff. 

�-antiaromaticity 401 

�-antiaromaticity 50 

antibacterial activity 313 

armchair nanotube 337 f. 

aromatic character 164 

aromatic ring 

– aromatic character 153 f. 

– distorted 164 

aromatics 

– alkylated 148 

– strained 147 ff. 

aromaticity 401 

�-aromaticity 50 

asterane 52 f. 

– 3-asterane 53 

– 4-asterane 53 

automerization 402 f. 

azacycloheptatetraene 135 

aza[60]fullerene 220 

b 

B3LYP (Becke three-parameter exchange 

functional coupled with Lee-Yang-Parr 

correction) 290 

back-clump 75 

barbaralane 414 f. 

barbaralone 414 

barrelene 410 

battery 365 

benzdiyne 178 

benzene 

– non-standard 147 ff. 

benzene rings 

– nontypical spatial structure 22 



ISBN: 978-3-527-31767-7 

463

464 

Index 

benzocentrohexaquinane 438 

benzophane 157 

benzo[a]phenanthrene 148 

m-benzyne 17 

[m][n]betweenanene 21 

1,1�-bi(adamantyl) 87 

bi[4]asterane 58 

bi(bicyclo[1.1.0]butyl) 83 

bi(bicyclo[1.1.1]pentyl) 83 


1,1�-bi(bicyclo[1.1.1]pentyl) 87 

bi(cubyl) 83 

bicycloalkane 91 

bicyclo[n.n.n]alkane 91 

bicycloalkene 107 

– hydrogenation enthalpy 107 

– multiply unsaturated 109 

bicyclobutane 

– inverted geometry 33 

bicyclo[2.2.1]hept-2-en-5-yne 382 

bicyclo[3.3.0]oct-1(5)-ene 113 

bicyclo[4.4.1]undecapentaene 109 

bicyclo[4.4.2]dodecapentaene 109 

bicyclo[5.1.0]octa-2,5-diene 413 

bicyclo[5.3.1]undecapentaene 109 

bicyclo[5.3.1]undeca-1,3,5(11)-triene 110 

bifurcation point 410 

1,1�-bi(homocubyl) 88 

Bingel reaction 214 ff. 

bio-related material 365 

bipentaprismane 58 

bi[n]prismane 57 

bis(acetoxymethyl)tricyclo[2.1.0.0 2,5 ]pentan- 

3-one 84 

bisaddition 

– fullerene 213 

bisallene 


bishomoaromatic 419 

bis(triphenylmethanol) 75 

bi(tetrahedranyl) 83 

1,1�-bi(tricyclo[3.1.0.0 2,6 ]hexyl) 88 f. 

1,1�-bi(tricyclo[4.1.0.0 2,7 ]heptyl) 88 

bond shifting 405 

bond 

– nonplanar C=C 112, 144 

– nonplanar C–C 44, 68 f. 

– shifting 405 

– ultralong C=C 70 

– ultrashort C=C 82 

bowlane 17, 46, 455 

– dimer 48 

bulk-heterojunction 305 

bullvalene 414 f. 

butatriene 

– � �-bond 

deformation 137 

– cyclic 137 ff. 

– fi ve- to nine-membered ring 137 

– strain estimate 137 

9-t-butylanthracene 148 

(2-t-butylcubyl)cubane 88 

c 

C60 , see also fullerene 

– derivative for optical limiting 299 

– derivative for photovoltaic application 

304 

– fi ve-membered ring-fused 222 

– four-membered ring-fused 221 

– six-membered ring-fused 223 

– terminated oligo( p-phenylene ethynylene) 

(OPE) hybrid compound 302 

– three-membered ring-fused 218 

C60-OPE hybrid 302 

C 60 (X) n isomer 

– steric strain 236 ff. 

C70 295 

C–C bond 

– elongated 74 

– expandability of ultralong bond 79 

– stiff molecular frame 72 

– super-ultralong 78 

– ultralong 70 ff. 

– ultrashort 82 ff. 

– ultrashort endocyclic bridging 83 ff. 

– ultrashort exocyclic intercage 86 

C(sp 2 )-C(sp 2 ) bond 

– fullerene cage 232 

C=C bond 


– stretch 116 

C–C(Me) bond 

– ultrashort 91 

C2v-[15]triangulene 19 

C10H10 saturated cage 63 

C12H12 saturated cage 63 

C60H6 293 

C60H18 294 

C60H36 294 f. 

C60H60 – all-out isomer 66 

– atom numbering 66 

– in isomers 66 

– ten-in isomer 66 

C 60 (OH) n 313 

C 60 -tetrathiafulvalene (TTF) dyad 302 

C70H2 298 

2– 

C70 298

C70H8 296 

(CH) 2n cage structure 59 ff. 

cage structure 

– nonplanar C=C bond 112 ff. 

carbene 456 

carbocation 456 

carbohelicene 166 

carbon atom 

– inverted 43, 58 

– planar 44 ff. 

– pyramidal 44 ff. 

carbon nanotube (CNT) 335 ff. 

– applications 356 ff. 

– armchair 337 

– aromaticity 345 

– bamboo-like structure 344 

– cap structure 343 

– chiral 338 

– cycloaddition 353 

– electrical conductivity 339 

– fl uorination 353 

– functionalization 347 ff. 

– HRTEM 341 

– hydrogenation 352 

– multi-walled (MWNT) 342 ff. 

– properties 357 

– resonance structure 346 

– safety 360 

– semiconductor-type 365 

– single-walled (SWNT) 335 ff. 

– structure 342 f. 

– symmetry 338 

– toxicity 360 

– zigzag 337 

carboxyfullerene 312 

carcerand cage 452 ff. 

catalysis 

– electrocatalytic activity of fullerenes 

270 

catenane 7 

centrohexaindane 430 ff. 

– multiply functionalized 436 

centrohexaquadrane 426 

centrohexaquinane 426 

centrohexaquinacene 427 

centropentaindane 432 ff. 

centropolycyclane 426 

centropolyindane 431 

trifuso-centrotetraindane 431 f. 

centrotriindane 432 

chemical sensor 365 

chiral nanotube 338 

[6]chochin 440 

[n]circulene 149 

[7]circulene 149 

Clar resonance structure 345 f. 

clathrate 451 

clump 75 

chochin 24 

computational chemistry 15 

conductive composite 364 ff. 

corannulene 25, 149 

coronene 149 

coupled cage compound 86 

– ultrashort exocyclic intercage C–C 

bond 86 

cubane 8 f., 50, 59 ff. 

– highly strained derivative 13 

– oligomer 56 

cubene 55, 62 

cubylcubane 56 

cumulene 25, 122 


cuneane 59 ff. 

cyclic system 

– conjugated 401 

cyclization 

– Hopf 127 

– Myers-Saito type 131 

cycloaddition 353 f. 

– carbon nanotube 353 f. 

cycloalkene 107 

cycloalkyne 

– angle-strained 375 ff. 


– spectroscopic property 392 

cyclobutadiene 399 ff., 450 

– bond length 401 

1,2-cyclobutadiene 124 

cyclobutyne 378 

cyclodecyne 386 

cyclodextrin 449 

1,2-cycloheptadiene 131 

trans-cycloheptane 453 

cycloheptatetraene 132 

– 1,2,4,6-cycloheptatetraene 132 

– 1,3,5,6-cycloheptatetraene 453 

– incarterated 134 

1,2,4,5- cycloheptatriene 133 

1,3,5-cycloheptatriene 413 

cycloheptatrienylidene 188 

cycloheptyne 384 

1,2-cyclohexadiene 125 

cyclohexamantane 439 f. 

cyclohexane 

– planar ring 67 ff. 

– trans 454 

– 

trispirocyclopropanated 19 

Index 

465

466 

Index 

1,2,3-cyclohexatriene 138 

1,2,4-cyclohexatriene 127 f. 

E-cyclohexene 103 

cyclohexyne 382 

cyclononyne 386 

1,2-cyclooctadiene 135 

cyclooctatetraene (COT) 403 ff. 

– annelated 406 

– bond shifting 407 

– conformation 404 

– ion 410 f. 

– ring inversion 407 

[2.2]cyclooctatetraenophane 22 

E-cyclooctene 103 ff. 

Z-cyclooctene 107 

cyclooctyne 385 f. 

1,2-cyclopentadiene 124 f. 

cyclopentyne 379 

cyclophane 12, 23, 67, 109, 150 ff. 


– K3,3 

438 

– Kuratowski 430 ff., 441 ff. 

– naphthalene-based 439 

– NMR characteristics 165 

[n]cyclophane 154 

[m,n]cyclophane 151 

[2,2]cyclophane 151 

[2.2.2](1,3,5)cyclophane 165 

[26 ](1,2,3,4,5,6)cyclophane-1-ene 24 

cyclopropabenzene 192 

cyclopropabenzenyl anion 191 

cyclopropannulene 178 

cycloproparene 176 ff. 

– charge transfer (CT) complexation 189 

– heteroatom 187 

cyclopropyne 376 

cyclopropynylidene 376 

d 

decaprismane C20H20 50 

Td-1,2-dehydro-5,7-adamantanediyl 

dication 16 

1,2-dehydrocubane 55 

m-dehydrocubane 17 

dehydroprismane 55 

1,5-dehydroquadricyclane 17 

dehydrotriprismane 55 

density functional theory (DFT) method 15 

density of state (DOS) 243 f. 

density of unoccupied state (DUOS) 243 f. 

diademane 60 ff. 

di-p-benzhexaphyrin 23 

dibenzylation 

C 

2– 298 

– 70 

di-t-butylbenzene 148 

1,2-di-t-butylethylene 108 

(Z)-1,2-di-t-butylethylene 

(2,2,5,5-tetramethylhex-3-ene) 106 

dichloro[3.2.1]propellane 34 

9,9�-didehydroanthracene (DDA) 113 

Diels-Alder reaction 


diiodonaphthocyclobutene 71 

5,6-dilithioacenaphthene 78 

dimethanospiro[2.2]octaplane 48 

N,N�-dimethylaniline 268 

1,1-dimethyl-2,3-bis-(t-butyl)cyclobutane 

106 

1,2-dimethyl-2,3-bis-(t-butyl) 

methylcyclopropane 106 

2,3-dimethyl-2-butene 108 

1,1-dimethyl-2-neopentylcyclopropane 

106 

dimethylspiro[2.2]octaplane 455 

E-1-(2,2-dimethyl-1-tetralinylidene)- 

2,2-dimethyltetralin 109 

1,5-dimethyltricyclo[2.1.0.0 2,5 ]pentan- 

3-one 84 

2,4-dimethyltricyclo[2.1.0.0 2,5 ]pentane 85 

3,6-dimethoxycyclopropa[b]naphthalene 

181 

9,9�-diphenyl-9,9�-bifl uorenyl 75 

trans-diphenylspirotriketone 433 

DNA photocleavage 310 

dodecadiyne 388 

dodecahedrane 12, 60 

– C20H20 

64 

dodecahedron 20 

Doering-Moore-Skattebøl rearrangement 

125 ff. 

double bond 


drop casting method 274 

e 

electrochemistry 259 f. 

electron emitter 366 

electron energy loss spectroscopy 

(EELS) 243 ff. 

electron-mediating property 272 

electronic part 363 

enyne 

– Diels-Alder reaction 126 

– photorearrangement 124 

ethylene 

– most distorted 111 

– t-butyl-substituted 

104 ff. 

Z-ethylene 104

f 

syn-fenchilydenefenchane 104 

fenestrane 17, 44 ff. 

[4.4.4.4]fenestrane 44 

all-cis-[5.5.5.5]fenestrane 431 

all-cis-[5.5.5.6]fenestrane 433 

fenestranone 434 

fenestrindane 431 

ferrocene 268 

ferrocene (Fc/Fc + ) system 262 

ferrocene-porphyrin-fullerene 

construct 269 

fi lm preparation method 274 

fullerene 6 ff., 205 ff. 

– absorption spectrum 280 

– � �-alanine 

derivative 313 

– alkylation 217 

– application 299 ff. 

– binding energy 290 

– biological application 310 ff. 

– bond 209, 232 

– C20 

11, 206 

– C60 

, see also C60 6 f., 205 ff. 

– C80 

207 

– cage 232 

– cyclic voltammetry 269 f. 

– cycloaddition 218 

– cyclobutane-annulated 221 

– � �-cyclodextrine 

subunit 311 

– density of state (DOS) 243 f. 

– density of unoccupied state (DUOS) 

243 f. 

– Diels-Alder adduct 223 

– DNA photocleavage 310 

– electrocatalytic activity 270 

– electron affi nity 260 

– electronic property 259 

– electronic structure 246 

– endohedral 282 ff. 

– fi lm 274 ff. 

– fi lm property 277 

– fullerene interaction 278 

– functionalization 263 

– guest substrate 288 

– higher 297 

– host 287 

– hydrogenated 291 

– hydrogenation 216 

– intra-/intermolecular association 263 

– in-out isomerism 64 

– ionization potential 260 

– isomer 212, 292 ff. 

– nanostructured fi lm 276 

– non-IPR stability 232 

Index 

– nonplanar steric strain 226 ff. 

– nonplanar steric strain parameter 229 

– nuclear magnetic resonance (NMR) 

250 ff. 

– open cage 284 f. 

– orbital picture 243 ff. 

– oxidation 215 

– perhydrogenated 60 

– physicochemical property 225 

– POAV ( �-orbital axis vector) analysis 

226 ff. 

– polyhydroxylated 313 

– porphyrin hybrid 312 

– property 255 

– radical addition 211 

– reaction 210 

– reactivity of hydrogenated fullerene 

297 

– reduction 215 

– reduction potential 261 

– reversible molecular incorporation 

and ejection 285 

– Schlegel diagram 226 f. 

– siloxane group 303 

– single crystal X-ray structure 225 

– sol-gel glasses 300 ff. 

– structure 291 

– superconductivity 281 

– synthesis 291 

– tris-malonic acid 312 

– UV/Vis absorption spectrum 247 f. 

– vibrational spectra 239 ff. 

– water interaction 278 

[60]fullerene 208 

– photovoltaic application 304 

fullerene aggregate 273 

fullerene-ferrocene dyad 268 

fullerene-OPE hybrid 303 

fullerene-porphyrin hybrid 312 

fullerene-TTF dyad 303 

fullerenol (C60 (OH) n ) 313 

fullerodendrimer 301 

fulleroid 220 

fulleropyrrolidinium salt 313 

fulvalene 111 

functionalization 347 ff. 

– covalent side-wall 352 

– endohedral 355 

– non-covalent 349 f. 

g 

graph 

– nonplanar 428 

graphene 337 

467

468 

Index 

h 

H2@C60 284 ff. 

He@C60 282 ff. 

helicene 166 

– asymmetric synthesis 171 

– nonracemic 171 

– physicochemical property 173 

– structure 172 

[4]helicene 148 

[5]helicene 168 ff. 

[6]helicene 166 ff. 

[7]helicene 168 f. 

[14]helicene 172 

helvetane 64 

hemicarcerand 450 ff. 

heptacyclo[6.4.0.0 2,4 .0 3,7 .0 5,12 .0 6,10 .0 9,11 ] 

dodecane 63 

heterophane 157 

heterohelicene 166 

heterojunction 305 ff. 

hexaanilino[60]fullerene 217 

hexa-t-butylbenzene 22 

hexaene 25 

hexagon-hexagon-pentagon junction 

(HHP) 225 

hexahelicene 166 

hexahydrosuperphane 67 

hexakis(trimethylsilyl) derivative 22 

hexanitro[60]fullerene 217 

hexaphenylethane (HPE) 73 

– acenaphthalene-type 80 

– bis(10-methylspiroacridine)-type 81 

– clumped derivative 74 

– cross-clumped 75 

– derivative with super-ultralong C–C 

bond 78 

– dihydropyracylene-type 80 

– dispiro 75 

hexaprismane 67 

homoaromaticity 109, 411 

homotropilidene 413 

bridged 413 ff. 

homotropylium cation 411 

Hopf cyclization chemistry 128 

Hückel rule 408 

hydrocarbon 

– bicyclic 20 

– distorted saturated 33 ff. 

– highly strained natural compound 13 

– K3,3 

438 

– K5 

438 

– saturated 18, 33 ff. 

– short-lived 449 ff. 

– strained, see strained hydrocarbon 

– unusual spatial structure 5 

hydrogenation enthalpy 

bicycloalkene 107 

i 

in-out isomerism 59 

– perhydrogenated fullerene C60H60 

64 

indium tin oxide (ITO) 304 

inversed photoemission spectroscopy 

(IPES) 243 f. 

inverted geometry 33 

isobenzene 127 

isodesmic equation 15 

isolated-pentagon rule (IPR) 228 

isomorph 

– conformational 79 

israelane 64 

k 

K3,3 molecule 438 

K5 molecule 438 

Knight shift 

– isotropic 252 

Korringa relation 258 

Kuratowski 

– cyclophane 430 ff., 441 ff. 

– graph 428 

l 

ladderane 12 

Langmuir-Blodgett (LB) method 274 ff. 

Langmuir-Schäffer (LS) method 274 ff. 

low-friction surface 364 

m 

magic angle spinning (MAS) 256 

membrane 364 

[n]metacyclophane 154 ff. 

[1]metacyclophane 150 

[2.2]metacyclophane 24 

[2,2]metaparacyclophane 152 

[5]metacyclophane 110, 150 f. 

[6]metacyclophane 110, 151 

metallacyclopenta-2,3,4-triene 139 

metallacyclopropabenzene 187 

methane 

– planar 45 

– tetrahedral 45 

1,5-methano[10]annulene 109 

1,6-methano[10]annulene 109, 412 

methanofullerene 220 

methano[70]fullerene 265 

1-(3-methoxycarbonyl)propyl-1-1-phenyl-[6,6] 

methanofullerene ([60]PCBM) 306

in-methylcyclophane 91 

6-methylene-1,2,4-cyclohexatriene 131 

N-methylphenothiazine 268 

Mills-Nixon effect 190 

Möbius strip 23 

molecular fl ask 449 ff. 

molecule 


monobenzyl C70H2 298 

multiporphyrin dendrimer 271 

n 

N@C 60 289 

nanokid 11 

nanoputane 11 

nanotechnology 6 

nanotube, see also carbon nanotube 

335 ff. 

nanotube cap 340 

naphthalene 148 

naphthalene-1,8-diyl bis(diarylmethylium) 

75 

naphthalenophane 439 

[6.6]naphthalenophane 390 

naphthocyclobutene derivative 71 

Ne@C 60 287 

near-fi eld microscopy probe 366 

nitrocubane 52 

NMR (Nuclear Magnetic Resonanz) 

38 f., 109, 151, 250 ff. 

norcaradiene 412 

o 

octabisvalene 59 ff. 

octacyclopropylcubane 12 ff. 

octahedrane 12 

– highly strained derivative 13 

octaplane 47 

octiphenyl 442 

– octabromo derivative 442 

olefi n strain energy (OSE) 55 

olympiadane 7 

OPE-C60 hybrid (oligophenyleneethynylene-C60 

) 309 

optical limiting (OL) 299 

OPV-C60 hybrid (oligophenylenevinylene-C60 

) 308 f. 

organofullerene 267 

– electron donor moieties 267 

orthogonene 17 

oxa[3.2.1]propellane 34 

p 

�–� interaction 

– carbon nanotube 345 ff. 

– concave–convex 25 

P3HT (poly(3-hexylthiophene)) 307 

paddlane 44 ff. 

[1.1.1.1]paddlane 42 

padogane 60 

– heterobridged 439 

para-cyclophane 24 

[n]paracyclophane 154 ff. 

[m.n]paracyclophane 161 

[0.0]paracyclophane 161 

[1.n]paracyclophane 161 

[1.1]paracyclophane 161 f. 

[2.2]paracyclophane 110, 161 f. 

[2.2]paracyclophane/dehydroannulene 

hybrid 389 

[2.2:2.2:2.2:2.2:2.2]paracyclophane 440 

[4]paracyclophane 159 

[4.4]paracyclophane 163 



[60]PCBM 306 

pentaene 25 

pentamantane 439 

pentaprismane 50 ff., 60 ff. 

percubylcubane 56 

perfl uorotetracyclobutenocyclooctatetraene 

408 

[5]pericyclyne 

– permethylated 26 

[6]pericyclyne 

– permethylated 26 

(Ph3P) 2Pt complex 119 f. 

phenylcarbene 

hemicarceplexed 134 

photocyclodehydrogenation 169 

photodehydrocyclisation 168 

photodynamic therapy (PDT) 310 

photorearrangement 

– enyne 124 

photovoltaic cell 309 

photovoltaic conversion 308 

phthalocyanine 304, 351 

POAV (�-orbital axis vector) analysis 

226 ff., 408 

polyacetylene 

– cyclic 387 f. 

polyaniline (PANI) 351 

polycubane 56 

polycyclane 426 

Index 

469

470 

Index 

polyether 

– topologically nonplanar 438 

polyhydroxyfullerene 212 

polymantane 439 

polyprismane 56 

poly[n]prismane 57 

porphyrin 351 

porphyrin-fullerene-dyad (P-C60 ) 312 

– metalated 312 

Prato reaction 213 ff. 

prismane 16, 49 

– C2nH2n 

prismane 49 

– expanded 52 ff. 

– ethynyl-expanded 54 

– fused 57 

[n]-prismane 64 

[6]prismane 64 

[7]prismane 64 

pristine fullerene 278 

propellane 

– small-ring 35 f. 

[k.l.m]propellane 33 f. 

– inverted geometry 33 

[k.1.1]propellane 43 

[1.1.1]propellane 9 ff., 34 ff. 

– precursor 42 

– preparation and reactivity 38 ff. 

[2.1.1]propellane 

– preparation and reactivity 40 f. 

[2.2.1]propellane 

– preparation and reactivity 40 f. 

[2.2.2]propellane 35 

[4.1.1]propellane 35 

propella[34 ]prismane 62 

pyramidalization 114 ff. 

pyramidane 17 


pyrene derivative 350 

r 

ratchet phase 256 

reverse saturable absorption (RSA) 300 

ring 

– inversion 405 

– nonplanar C=C bond 112 ff. 

rotator phase 256 

rotaxane system 6 

s 

scavenger activity 313 

semibullvalene 404 ff., 414 ff. 

– octasubstituted 418 

short-lived species 

– stabilization in molecular fl ask 449 ff. 

silacyclopropyne 377, 456 

sol-gel glasses 300 f. 

solar cell 305 ff. 

– all-polymer 307 

spin coating method 274 

spiropentane 18 

spiro[3.3]pentane 48 

[n]staffane 42 

stilbene 

– cyclic 108 f. 

– t-butyl-substituted 

108 f. 

strain energy 14, 104 f. 

strain estimate 123, 137 

strained hydrocarbon 12 

– computation 12 

structural composite 363 

supercapacity 365 

superconductivity 8 


supercubane 56 

supramolecular fl ask 449 ff. 

surfactant 275 

t 

1,1,2,2-tetraarylacenaphthene derivative 

75 

tetraasterane 19 

tetracyclo[3.1.0.0 1,3 .0 3,5 ]hexane 17 

tetra-tertiary-butyltetrahedrane 403 

tetrahedral coordination 425 f. 

tetrahedrane 12, 61 ff., 403, 455 

– bis-tetrahedrane 

17 

tetrakisbicyclohexenocyclooctatetraene 

408 

tetrakis-t-butylethene 21 

tetralin 150 

anti-tetramantane 439 

tetramethyl cyclooctatetraene 407 

tetraphenylnaphthocyclobutene 73 

7,7,8,8-tetraphenyl-o-quinodimethane 73 

tetraphosphabarbaralane 415 

tetrathiafulvalene (TTF) 268 

3,3,7,7-tetramethylcycloheptyne 384 

4-thia-3,3,5,5-tetramethylcyclopentyne 

379 

topography 425 ff. 

topology 425 ff. 

triangulane 19 

all-cis-tribenzo[5.5.5.5]fenestrane 434 

tribenzo[5.5.5.5]fenestrene 434 

tri(bicyclo[1.1.1]pentyl) derivative 88 

tri-t-butylethylene 106 

1,2,3-tri-t-butylnaphthalene 22 

tricyclo[2.1.0.0 1,3 ]pentane 17

tricyclo[2.1.0.0 2,5 ]pentan-3-one 83 ff. 

tricyclo[2.1.0.0 2,5 ]pentane 83 f. 

– ultrashort endocyclic bridging C–C 

bond 83 ff. 

tricyclo[3.1.0 1,3 .0 3,5 ]hexane 18 

tricyclo[3.3.n.0 3,7 ]alk-3(7)-ene 113 ff. 

tricyclo[3.3.0.0 3,7 ]oct-1(7)-ene 119 

tricyclo[3.3.9.0 3,7 ]non-3(7)-ene 117 

tricyclo[3.3.10.0 3,7 ]dec-3(7)-ene 117 

tricyclo[3.3.11.0 3,7 ]undec-3(7)-ene 115 

[4.1.0.0 1,6 ]tricycloheptane 18 

tricyclo[4.2.2.2 2,5 ]dodeca-1,5-diene 21 

triene 25 

trifuso-centrotetraindane 431 f. 

trioxacentrohexaquinane 438 

triprismane 50, 62 

tri[n]prismane 55 

triprismene 55 

tris-�-homobenzene 69 

– cis and trans 69 

tropolidene 413 

u 

ultraviolet photoemission spectroscopy 

(UPS) 243 ff. 

umbrella confi guration 58 

UV/Vis spectra 432 

UV/Vis spectra of fullerenes 246 ff. 

v 

valence isomerization 404 ff. 

w 

windowpane, see fenestrane 

Index 

x 

X-ray of fullerenes 225 ff. 

X-ray absorption spectroscopy (XAS) 243 

xylene 148 

z 

zeolite 452 

zero-point vibrational energy (ZPVE) 15 

zigzag nanotube 337 

zirconacyclohepta-2,4,5,6-tetraene 139 

471

Strained Hydrocarbons - Developers

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