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

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Chapter 2 Figure 2.14: Structure of [Nb(OEt)5]2. The sensitivity of metal chlorides towards alcohols decreases with the progressing electropositive character of the metal. An exothermic reaction occurs when NbCl5 is added to an alcohol, producing a niobium chloride alkoxide as displayed below 76 : NbCl5 + 3 ROH → NbCl2(OR)3 + 3 HCl (2.13) The replacement of the chlorides by the methoxide groups does not proceed to completion without the addition of an activating agent like ammonia. Comprehensive studies have been carried out for NbCl5 complexes with ROH (R = Me, Et, Pr) in the solid state to determine influencing factors and products formed. 2.7.3 93 Nb-NMR Studies Figure 2.15: Structure of [NbCl2(OMe)3]2. Karaliota et al. 77 found that the molecular constitution of the methanol solutions is dependent on the NbCl5 concentration, the temperature and storage time. 1 H-, 13 C- and 93 Nb-NMR spectroscopy was used to identify the substitution products formed in the solvolysis of NbCl5 by MeOD. The chloro-methoxo complexes contain bridging methoxo groups and act as precursors for the formation of organic products like MeCl and Me2O. Results confirmed previous reports 78,79 that only [NbCl2(OMe)3] is formed in non-polar solvents. Their investigation also indicated that the formation of 76 C. Sanchez, J. Livage, M. Henry, F. Babonneau, J. Non-Cryst. Solids, 100, p65, 1988. 77 A. Karaliota, M, Kamariotaki, D, Hatzipanayioti, Transition Met. Chem., 22, 411, 1997. 78 L. Hubert-Pfalzgraf, A. Pinkerton, J. Riess, Inorg. Chem., 17, 663, 1978. 79 C. Alquier, M. T. Vanderborre, M. Henry, J. Non-Cryst. Solids, 79, 383, 1986. 33
Chapter 2 other NbCl5-x(OMe)x products is heavily dependent on the storage time of the sample as indicated in Table 2.5. The number of coordinated MeO groups per metal atom was calculated from the integrals of peaks corresponding to terminal-MeO (MeO-ter) and briging-MeO (MeO-br) using 1 H NMR spectroscopy. In the NMR experiments the ratio MeO-coordinated/Nb was calculated as follows, such that α is a percentage and the quantities in the square brackets are the NMR integrals: MeOcoord / Nb = α x (initial moles of MeOD) / (moles of Nb) α = [MeOcoord] = 34 x 100 (2.14) From the 1 H NMR spectrum, the number of coordinated MeO groups (terminal and bridging) was calculated per metal atom. As seen in Table 2.5, the ratio MeOcoord/Nb decreases with an increase in NbCl5 concentration. The decrease in MeOcoord/Nb is due to the conversion of methoxo groups into Me2O and MeCl, but the percentage of free methanol remains the same, indicating that the MeCl was derived from the coordinated methoxide groups. The 93 Nb NMR spectra of NbCl5 in MeOD (1.18 M) over time, is displayed in Figure 2.16 (at 4 °C). The two bands at -500 and -850 ppm are assigned to [NbCl4(OMe)] and [NbCl2(OMe)3] respectively. 80,81,82 After considerable time a broad band appeared in the -600 to -800 ppm region with a shoulder on the side at -850 ppm (Figure 2.16). These resonances were attributed to species with a higher number of oxygens around the Nb centre. After 37 days the solution was heated to 50 °C, then cooled to 25 °C and a band at -1100 ppm was observed. This could be due to the coordination of 5 methoxo groups per Nb centre or the coordination of four methoxo groups and a Nb=O group. 80 G. R. Lee, J. Crayston, Dalton Trans., 3073, 1991. 81 R. Idd, H. Spinney, Inorg. Chem., 12, 1967, 1973. 82 C. Jameson, D. Pehder, M. Hoch, Inorg. Chem., 27, 3490, 1988.
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: Chapter 2 Reactions of dialkylamid
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 and 52: Chapter 2 The hemicarbonate formed
Page 53 and 54: Synopsis... 3. Synthesis and Charac
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
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