Tuning Reactivity of Platinum(II) Complexes

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Chapter 8 Tuning Reactivity of platinum(II) complexes: A Kinetic and Mechanistic Investigation into Substitution Behaviour of Mono- and Dinuclear Platinum(II) Complexes Summary Inspired by the trend to generate new types of Pt(II) compounds with anti-cancer activity, the main aim of this study was to investigate the thermodynamic and kinetic properties of square-planar Pt(II) complexes with N-donor ligands that exhibit antitumor activity. A further aim was to determine whether the linker remains attached to the metal centre when the geometry and environment of the Pt(II) centre is cis to the leaving ligand in multinuclear platinum(II) compounds. A series of complexes introduced in Chapter 3 consists of mononuclear Pt(II) centres coordinated by tridentate N-donor ligands. The main feature of these complexes is that they are all characterised by the presence of a delocalised π-system that permits facile transfer of electrons from the metal to the ligand, while they differ by the way the fused ring system around the terpy moiety is structured. The objective was to investigate thermodynamic and kinetic properties of the complexes, mainly focusing on their dependence on the strength of π-backbonding of the spectator ligands around the Pt(II) centre. Therefore, substitution reactions with thiourea and its derivatives and anions (SCN¯, I¯ Br¯) were performed with the corresponding chloro complexes at an ionic strength of 0.1 M NaClO4 in presence of 10 mM LiCl to prevent solvolysis. In comparing the reactivity of [Pt(terpy)Cl] + (PtCl) and pyPhenPtCl, (phen = phenanthroline), the extensive π-conjugation of the fused-ring system of the phen ligand, coupled with enhanced electronic communication in the aromatic system around the platinum centre, led to higher reactivity for pyPhenPtCl than PtCl. The most important feature of this work however, was elucidation of the effect of introducing the isoquinoline moiety instead of pyridine on the lability of the chloride ligand in CH3PhisoqPtCl 1
compared to CH3PhPtCl. The “unusual” decrease in reactivity of CH3PhisoqPtCl is ascribed to the decrease in delocalisation of π-electron density as a result of the poor π- acceptor property of the isoquinoline ligand. The isoquinoline ligand is a net σ-donor, which slows down substitution reactions. This is supported by the Electronic absorption spectra and DFT-calculations which show that this system has the largest HOMO-LUMO energy gap. This behaviour is contrary to the cis σ-effect reported in other studies. The activation parameters obtained in the study supported an associative mode of substitution mechanism in all cases. In Chapter 4, a series of bridged dinuclear platinum(II) complexes of the type, [{cis– Pt(H2O)(NH3)2}2–µ–pzn] 4+ {where pzn = pyrazine (pzn), 2,3-dimthylpyrazine (2,3pzn), 2,5-dimethylpyrazine (2,5pzn) and 2,6-dimethylpyrazine (2,6pzn) were synthesised to investigate the influence of the rigid bridging ligand on the reactivity of the platinum(II) centres in dinuclear complexes. The higher pKa values for first and second deprotonation of 2,3pzn, 2,5pzn and 2,6pzn compared to pzn are indicative of the σ-inductive effect of the methyl substituents on the pyrazine moiety that increases the electron density at the Pt(II) centre. The effect of increasing or decreasing electron density on the Pt(II) centre is also visible in the reactivity of the complexes. The reactivity of pzn is higher in comparison to 2,3pzn, 2,5pzn and 2,6pzn. This is due to the moderate electrophilicity of the Pt(II) centre, which also experiences less steric hindrance. In addition, The DFT calculated bond angles show that the bridging pyrazine ligand lies almost perpendicular to the square- planar Pt(II) centre enabling the methyl groups on the linker ligand to effectively block the entry of the incoming nucleophile from above or below the metal centre ,slowing down the substitution process. The introduction of the methyl groups to bridging ligand also results in an increase in the separation energy of the frontier molecular orbitals (ΔE), leading to a less reactive metal centre in the ground-state. 1 H and 195 Pt NMR spectroscopy was applied to demonstrate stepwise substitution of the chloro ligands from the pyrazine bridged dichloro Pt(II) complex (pzn) by thiourea. A third step was observed in all the complexes and was attributed to degradation of the 2
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Tuning Reactivity of Platinum(II) C
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Declaration This thesis report is b
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Abstract Systematic kinetic and the
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Table of contents Acknowledgements
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2.5.5 Effect of Non-participating G
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5.2.1 Chemical and Solutions ......
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7.2 Experimental Section ..........
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procurements, Messers P. Forder and
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Figure 2.2 Potential energy profile
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Figure 4.6 Concentration dependence
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Figure 6.1 Spectrophotometric titra
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List of Tables Table 2.1 A selectio
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Table 6.4 Summary of rate constants
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TU thiourea DMTU 1,3-dimethyl-2-thi
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Table of Contents-1 Chapter 1 .....
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1.0 Introduction 1.1 Cancer Disease
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toxic potential. The most well-know
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1.3.2.2 Cellular Uptake Cisplatin i
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H 3N OH 2 Pt H 3N OH 2 Active Pt(II
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transformational pathways that comp
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the hydrolysis of the complex, wher
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1.3.4 Terpyridine Platinum(II) Comp
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H 3 N Cl Pt NH 3 H 3 N NH 2 (CH 2 )
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1.4 Kinetic Interest The platinum-b
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3. The effect of varying the positi
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17 R. A. Henderson, The Mechanism o
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Altona J. H. van Boom, G. A. van de
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76 (a)J. Kašpárková, J. Zehnulov
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Table of Contents-2 Chapter Two ...
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List of Tables Table 2.1: A selecti
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The mononuclear Pt(II) complexes 1-
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Potential Energy R + X RX 1 transit
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For the associative mechanism (A),
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the concentration of one of the rea
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k obs , s -1 0.00030 0.00025 0.0002
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k = Ae -Ea/RT 2.14 lnk = lnA - E a
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= 23.76 + R Hence, a plot of ln ⎛
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iii. # Δ V ≈ 10 cm3 mol-1 featur
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conventional methods are classical
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Figure 2.6: Schematic diagram of a
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The light transmitted from the samp
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c. Oxidizability: Ligands that are
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Table 2.1: A selection of n o pt va
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eaction with different nucleophiles
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eaction site from direct attack by
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PEt 3 PEt 3 R PEt 3 Pt Pt Y Y Cl R
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direct displacement of the leaving
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therefore, weaken the bond of the l
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σ-Donation According to classical
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References 1 (a) J. Reedijk, Chem.
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34 R. B. Jordan, Reaction Mechanism
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Table of Contents-3 Chapter 3.The
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Chapter 3 The π-Acceptor Effect in
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In order to extend our understandin
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after which water was added to quen
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O CH 3 + I + N O oH - N O O 7 CH 3
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84% (34.7 mg, 0.0618 mmol). 1 H NMR
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PhCN PhCN Pt Cl Cl + N N CH 3 N CH
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Complex Structure HOMO LUMO PtCl CH
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The geometry-optimised structures i
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against the concentration of the in
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Table 3.2: Summary of the second-or
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constants of CH3PhisoqPtCl decrease
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with π*-orbitals of the ligand. Th
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3.6 References 1 D. Rosenberg, L. V
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30 Microcal TM Origin TM Version 5.
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Figure S3.1: Kinetic trace at 448 n
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ln(k 2 /T) -6.0 -7.5 -9.0 -10.5 -12
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Table S3.3b: Average observed rate
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Table S3.5b: Temperature dependence
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Table S3.8: DFT calculated electros
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List of Figures Figure 4.1: Structu
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Table 4.2: Summary of pKa values fo
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4.1 Introduction Platinum compounds
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cis geometry, leading to dramatic c
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ligand was added to the [{cis-PtCl(
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spectra were measured in and refere
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4.3.1 DFT calculated Optimized Stru
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Table 4.1: A summary of the DFT cal
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H2O-Pt-L-Pt-OH2 H2O-Pt-L-Pt-OH2 H2O
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electrophilicity and acidity of the
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(A) 18 Absorbance 0.08 0.07 0.06 0.
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k obs(3 rd ) , s -1 -5 6.00x10 TMTU
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4.3.4 Kinetics with NMR The substit
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ln([ML] t ) 4.0 3.5 3.0 2.5 2.0 1.5
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ln(k 2(1 st ) /T) -3.5 -4.0 -4.5 -5
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Comple x Table 4.4: Summary of Acti
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The decrease in reactivity of 2,6pz
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Table 4.5: DFT calculated (NBO) cha
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eaction proceeds via bimolecular pa
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References 1 T. Storr, K. H.Thomson
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36 D. Jaganyi, D. Reddy, J.A. Gerte
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Appendix 4 THE INFLUENCE OF THE PYR
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Absorbance at 368. 0 nm 0. 0 8 0. 0
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Table S4.3: Average observed rate c
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k nd obs(2 ) , s-1 0.003 TU DMTU TM
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k obs2 , s -1 2.40x10 -4 2.20x10 -4
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Table S4.13: Average observed rate
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k obs(1 st ) , s -1 0.06 0.04 0.02
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ln(k 2(3 rd ) /T) -10.0 -10.5 -11.0
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SpinWorks 2.5: 2,6 pznClO4 in D2O N
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Table of Contents-5 Chapter 5 .....
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List of Tables Table 5.1: A summary
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5.1 Introduction Multinuclear plati
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onding. For this reason, pKa titrat
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400-300 cm -1): 3308, 3117, 3071 (N
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5.2.6 Spectrophotometric pKa Titrat
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Table 5.1: A summary of DFT-calcula
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However, because the highest occupi
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Table 5.2: Acid dissociation consta
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Table 5.3: A summary of DFT calcula
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H3N 6 eq TU 0 eq TU Ha NH3 Ha Cl TU
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third step due to the trans-effect
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[H 2 O-Pt-(NN)-Pt-OH 2 ] +4 [NU-Pt-
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k obs(1st) / s -1 0.20 TU DMTU TMTU
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thiourea nucleophile is large enoug
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ln(k st 2(1 ) /T) -3 -4 -5 -6 -7 -8
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is the same as the electron-withdra
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associative mode of substitution me
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16 H. Ertürk, J. Maigut, R. Puchta
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43 (a) D. Jaganyi, A. Hofmann and R
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276 nm Absorbance 0 . 6 5 0 . 6 4 0
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k obs(1 st ) , s -1 0.4 0.3 0.2 0.1
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ln(k 2(2 nd ) /T) -8.0 TU -8.5 -9.0
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pzn PPM -1750.0 -1850.0 -1950.0 -20
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Figure S5.13: UV/Visible spectra fo
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k obs(1 st ) in s -1 0.030 0.025 0.
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ln(k 2(2 nd ) /T) -10 -11 -12 -13 -
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9.61 ppm Ha PPM 9.8 9.6 9.4 9.2 9.0
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k obs(3rd) / s -1 -5 8 .00 x 10 T U
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ln(k st 2(1 ) /T) -1.5 TU DMTU TMTU
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ln(k rd 2(3 ) /T) -8.5 -9.0 -9.5 -1
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SpinWorks 2.5: znPt(II)-OP4 in D2O
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Figure S5.31: Mass spectrum for com
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ln(k st 2(1 ) /T) -4 -5 -6 -7 -8 -9
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SpinWorks 2.5: phtPt(II)-OP2 in D2O
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Figure S5.41: Mass spectrum for com
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List of Figures Figure 6.1: Spectro
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Chapter 6 Tuning Reactivity of Plat
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Against this background, several re
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6.2.2 Instruments Microanalyses wer
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Metal Complex Pt3 Yield: 52.5 mg (0
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6.3 Results 6.3.1 Synthesis and Cha
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The pKa values obtained are summari
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Table 6.2: DFT-calculated parameter
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that of dinuclear Pt(II) complexes
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It can be concluded that substituti
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ate constants, kobs(1 st /2 nd ), w
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Table 6.3: Summary of rate constant
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6.3.6 Activation Parameters The act
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pKa1 values become smaller. In addi
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of steric influence is felt by the
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6.5 Conclusion The present study ha
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17 O. F. Wendt and L. I. Elding, 19
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51 Y. Iwadata, K. Kawamura, K. Igar
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Table S6.4(b): Average observed rat
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ln(k 2(2 nd ) /T) -4 -6 -8 -10 -12
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45.0 40 35 30 25 20 %T 15 10 5 0 -5
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k st obs(1 ) in s-1 0.30 Br TU 0.25
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-2304.16 ppm H 3N PPM -2200.0 -2220
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Absorbance 1.6 1.4 1.2 1.0 0.8 0.6
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SH N SH + Mechanism Br N + CO 3 2-
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Figure 7.5: 195Pt NMR spectra of mi
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linker remained coordinated to the
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complexes, have a lower charge and
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ange 326-400 cm -1 (weak) for Pt-Cl
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ButPt, HexPt, OctPt and DecPt, were
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7.3 Results 7.3.1 DFT Calculations
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Structure HOMO LUMO EnPt (C2h) Prop
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Absorbance Table 7.2: Summary of pK
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Page 380 and 381: 7.3.4 Activation Parameters The tem
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Page 388 and 389: 32 N. Summa, J. Maigut, R. Puchta a
Page 390 and 391: Appendix 7 Table S7.1: Summary of s
Page 392 and 393: k obs(2 nd ) , in s -1 0.00020 0.00
Page 394 and 395: ln(k 2(2 nd ) /T) -8.5 -9.0 -9.5 -1
Page 396 and 397: k obs(1 st ) , in s -1 0.06 TU DMTU
Page 398 and 399: ln(k 2(1 st ) /T) -3.2 TU DMTU TMTU
Page 400 and 401: Table S7.11: Summary of kobs(2 nd )
Page 402 and 403: Table S7.13: Average observed rate
Page 404 and 405: %T 22.0 20 18 16 14 12 10 8 6 4 2 0
Page 406 and 407: k st obs(1 ) , in s-1 0.10 0.08 0.0
Page 408 and 409: ln(k st 2(1 ) /T) -4.0 TU DMTU TMTU
Page 410 and 411: Figure S7.23: Mass spectra for HexP
Page 412 and 413: Table S21: Average observed rate co
Page 414 and 415: Table S7.23: Summary of kobs(2 nd )
Page 416 and 417: Table S7.25: Average observed rate
Page 418 and 419: 90.0 80 70 60 50 40 30 20 %T 10 0 -
Page 420 and 421: Figure S7.37: Mass spectrum for Hex
Page 424 and 425: dinuclear Pt(II) complexes to relea
Page 426 and 427: finally Pt2. The order of reactivit
Page 428: • prolonged survival in the cell
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Tuning Reactivity of Platinum(II) Complexes

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