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

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Chapter 5 of our results to that listed in Table 5.1 indicates the presence of three-coordinated methoxido ligands under our conditions. Table 5.1: Assignment of 93 Nb NMR signals to [NbCl5-x(OMe)x] species. 4 Solvent x = 0 x = 1 x = 3 x = 4 x = 5 Benzene 2.6 -497 -810 -1010 -1160 Toluene 2 -495 -810 - -1150 Methanol -0.5 -480 -820 -1015 -1155 Ethanol - -470 -830 -1020 -1180 Isopropanol - -470 -830 -1025 -1135 Acetonitrile - -495 / -560 -850 - -1160 x = number of coordinated methoxido species Further experiments by Lee et al. 4 and others 3,5 indicated that there are no bridged (polymeric) niobium species in solution at metal concentrations below 0.185 M. Under our conditions in the kinetic studies, the final metal concentration after mixing is 5 x 10 -5 M, much lower than these conditions. It is therefore postulated here that the starting material in our kinetic studies is in fact a mono-nuclear niobium complex, [NbCl2(OMe)3(MeOH)]. The presence of a coordinated methanol solvento species in the niobium coordination sphere is the most obvious choice since the concentration of methanol under our conditions is in the region of 25 M and since charge balance is maintained in this manner. This bridge splitting by coordinating solvents have also been illustrated for platinum(II) chloride olefin complexes. 6 Another aspect that is important to note here is that there is seemingly only one reaction observed under the conditions of the NMR experiment when excess of acacH is added to [NbCl2(OMe)3(MeOH)]. Later paragraphs would show that there are in fact two reactions in this process which are only observed under conditions where the concentrations of the starting materials are much lower. 6 M. R. Plutino, S. Otto, A. Roodt, L. I. Elding, Inorg. Chem., 38, 1233, 1999. 83
5.3 Results and Discussion 5.3.1 Proposed Reaction Scheme Chapter 5 The following reaction scheme is presented for the reaction between [NbCl2(OMe)3(MeOH)] and acacH. According to Scheme 5.1 the reaction involves a fast initial reaction where acacH substitutes the labile coordinated MeOH to form an intermediate species which undergoes ring-closure in a second, slower, rate- determining step. The validation of this scheme will be done in a stepwise manner in the following paragraphs. [M] [acacH] [M-acacH] [M-acacH] [M-acac] Scheme 5.1: The proposed reaction scheme for the formation of [NbCl(acac)(OMe)3]. Two reactions are observed, the fast first reaction in red and the slower second reaction in blue. 84
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A solution and solid state study of
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Table of contents Abbreviations and
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3.4 Infrared Spectroscopy (IR) ....
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Abbreviations and Symbols Abbreviat
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Abstract 93 Nb NMR was successfully
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Opsomming 93 Nb KMR is met sukses g
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Chapter 1 metals. 3 Due to Wollasto
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Synopsis... 2. Literature Review of
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Chapter 2 Niobium resembles tantalu
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2.1.2 Uses Chapter 2 Niobium has a
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2.2 Separation of Nb and Ta 2.2.1 M
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Chapter 2 Buachuang et al. 16 repor
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Chapter 2 Niobium oxide surfaces ex
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2.4.6 Water absorption Chapter 2 Th
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Chapter 2 been reported in literatu
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Chapter 2 containing niobium as the
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Chapter 2 conclusions, with regard
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Chapter 2 Figure 2.5: Structure of
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2.6.3.3 [NbCl3O(ttbd) - ] Chapter 2
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Chapter 2 (a) (b) Figure 2.10: Stru
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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 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: Chapter 5 corresponds to one specie
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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A solution and solid state study of niobium complexes University of ...

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