A solution and solid state study of niobium complexes University of ...

More documents

Recommendations

Info

Chapter 2 A slower rate was observed in benzene than in ethanol. The reason for this is that alcohol favours the conversion of the dimer to the monomer. 87 The absorption curves illustrate the fast absorption of CO2 during the first few minutes, followed by a slow continuous absorption over several hours. This is displayed by both the ethanol and benzene solutions. This supports the hypothesis that the carboxylation of the monomer is faster than the dissociation of the dimer. Vcarb = k2[Nb(OEt)5][CO2] (2.15) Second order kinetics was determined for the carboxylation reaction of [Nb(OEt)5]2 (Eq. 2.15). k2 was calculated as 1.19 x 10 -1 l.mol -1 .s -1 for the first part of the absorption curve, that is the carboxylation step. The same value was determined for both the ethanol and benzene solutions. The second part of the curve was used to calculate the dissociation process. The k1 value in ethanol was determined as 1.15 x 10 -5 s -1 and that for benzene was determined as 0.97 x 10 -5 s -1 . The insertion of CO2 into the Nb-OEt bond was found to be reversible. The CO2 was released by the hemicarbonate under vacuum. This was confirmed by the fact that the IR signal due to the carbonate group disappeared and the starting complex ([Nb(OEt)5]2) was quantitatively regenerated. 2.9 Conclusion Most of the work done on niobium focuses on the +5 oxidation state. The syntheses of these complexes are complicated due to the metal’s affinity to readily form oxides. A few niobium complexes have also been characterized to contain oxo bridges. Relatively few papers have been published on the kinetics of niobium complexes. From literature, it is clear that there is a significant need for research in the separation of niobium and tantalum without the use of HF. 87 D. C. Bradley, C. E. Holloway, J. Chem. Soc., 219, 1968. 41
Synopsis... 3. Synthesis and Characterisation of Niobium(V) Complexes In this chapter the major characterisation techniques employed in this study as well as the detailed synthetic procedures of several metal complexes are described. 3.1 Introduction ooooooooooooooo All of the niobium complexes synthesized was characterised using various techniques, including nuclear magnetic resonance (NMR), ultraviolet-visible (UV-Vis) and infrared (IR) spectroscopy. Single Crystal X-ray crystallography (XRD) was also used for characterisation when crystals, suitable for this technique, could be isolated. A detailed discussion of the complexes studied by this technique is given separately in Chapter 4. Brief discussions on these basic techniques are included, as well as some basic principles of chemical kinetics which serves as an introduction to the kinetics addressed in Chapter 5. 3.2 Nuclear Magnetic Resonance Spectroscopy (NMR) NMR is a spectroscopic technique that relies on the magnetic properties of the atomic nucleus. When placed in a strong magnetic field, certain nuclei resonate at a particular frequency in the radio frequency range. From the small variations in the resonant frequency, detailed information about the molecular structure in which the atom resides can be obtained. 1 1 N. E. Jacobsen, NMR Spectroscopy Explained, Wiley and Sons, New Jersey, 1, 2007, 42
Page 1 and 2: A solution and solid state study of
Page 3 and 4: Table of contents Abbreviations and
Page 5 and 6: 3.4 Infrared Spectroscopy (IR) ....
Page 7 and 8: Abbreviations and Symbols Abbreviat
Page 9 and 10: Abstract 93 Nb NMR was successfully
Page 11 and 12: Opsomming 93 Nb KMR is met sukses g
Page 13 and 14: Chapter 1 metals. 3 Due to Wollasto
Page 15 and 16: Synopsis... 2. Literature Review of
Page 17 and 18: Chapter 2 Niobium resembles tantalu
Page 19 and 20: 2.1.2 Uses Chapter 2 Niobium has a
Page 21 and 22: 2.2 Separation of Nb and Ta 2.2.1 M
Page 23 and 24: Chapter 2 Buachuang et al. 16 repor
Page 25 and 26: Chapter 2 Niobium oxide surfaces ex
Page 27 and 28: 2.4.6 Water absorption Chapter 2 Th
Page 29 and 30: Chapter 2 been reported in literatu
Page 31 and 32: Chapter 2 containing niobium as the
Page 33 and 34: Chapter 2 conclusions, with regard
Page 35 and 36: Chapter 2 Figure 2.5: Structure of
Page 37 and 38: 2.6.3.3 [NbCl3O(ttbd) - ] Chapter 2
Page 39 and 40: Chapter 2 (a) (b) Figure 2.10: Stru
Page 41 and 42: 2.7 Alkoxides Chapter 2 Specific kn
Page 43 and 44: Chapter 2 Reactions of dialkylamid
Page 45 and 46: Chapter 2 other NbCl5-x(OMe)x produ
Page 47 and 48: Chapter 2 In 1991 Lee et al. 83 pub
Page 49 and 50: EtO EtO EtO EtO Cl Nb Cl Cl Nb Cl R
Page 51: Chapter 2 The hemicarbonate formed
Page 55 and 56: Chapter 3 with γ = magnetogyric ra
Page 57 and 58: Chapter 3 electromagnetic spectrum
Page 59 and 60: 3.5.1 Bragg’s law Chapter 3 Bragg
Page 61 and 62: 3.5.3 ‘Phase problem’ Chapter 3
Page 63 and 64: Chapter 3 A = ∑ ε cl (3.14) In
Page 65 and 66: Chapter 3 t1/2 = 54 = . 3.7 Sy
Page 67 and 68: 3.7.2.4 Synthesis of [NbCl4(acac)]:
Page 69 and 70: 4. Crystallographic Synopsis... Cha
Page 71 and 72: Chapter 4 packages 2 respectively.
Page 73 and 74: Chapter 4 4.3 Crystal Structure of
Page 75 and 76: Chapter 4 longer bonds (C1-C2, C2-C
Page 77 and 78: Chapter 4 Figure 4.5: Packing of [N
Page 79 and 80: Chapter 4 Figure 4.7: Molecular str
Page 81 and 82: Chapter 4 Table 4.5: Selected bond
Page 83 and 84: Chapter 4 Figure 4.11: Molecular st
Page 85 and 86: Chapter 4 Figure 4.12: The phacac p
Page 87 and 88: Chapter 4 Table 4.9: Hydrogen bonds
Page 89 and 90: Chapter 4 According to our knowledg
Page 91 and 92: 5.2 Experimental procedures 5.2.1 K
Page 93 and 94: Chapter 5 corresponds to one specie
Page 95 and 96: 5.3 Results and Discussion 5.3.1 Pr
Page 97 and 98: Chapter 5 Figure 5.5: Typical UV/Vi
Page 99 and 100: 5.3.3 Derivation of the rate law Ch
Page 101 and 102: Chapter 5 The Stopped-flow data (fa
Page 103 and 104:
Chapter 5 Figure 5.9: Eyring plot,
Page 105 and 106:
Chapter 5 However, more information
Page 107 and 108:
Chapter 6 second reaction entailed
Page 109 and 110:
Appendix A Table 1.3: Bond angles (
Page 111 and 112:
Appendix A A 2 Supplementary Data f
Page 113 and 114:
Appendix A C(16)-H(16B) 0.96 C(16)-
Page 115 and 116:
Appendix A O(7)-C(10)-C(11) 123.6 (
Page 117 and 118:
Appendix A H(18E)-C(18B)-H(18F) 109
Page 119 and 120:
Appendix A H(18F) 6243 6605 575 75
Page 121 and 122:
Appendix A Table 3.3: Bond angles (
Page 123 and 124:
Appendix A Table 3.4: Anisotropic d
Page 125:
Appendix B Appendix B B 1 Supplemen
show all

A solution and solid state study of niobium complexes University of ...

Create successful ePaper yourself

Delete template?

Save as template?