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|Title:||Nuclear Magnetic Resonance Studies of Reactions of β-diketonate Complexes|
|Authors:||Nixon, John Chamberlain Alan|
|Abstract:||<p>Metal derivatives of β-diketones form a very extensive class of coordination compounds. This thesis deals with three related aspects of the chemistry of these compounds; the effect of acid on a number of acetylacetonate complexes; the kinetics of ligand exchange reactions of V(acac)3 with Hacac and Hhfac and the rearragement reaction of some vanadium β-diketonate complexes. These topics have been investigated primarily by means of nuclear magnetic resonance technique.</p> <p>Kinetic evidence of protonation of a number of acetylacetonate complexes on the γ-carbon has been obtained. No evidence was found to support the hypothesis of species containing a 'dangling' monodentate ligand protonated on oxygen.</p> <p>The ligand exchange reaction of V(acac)3 with Hacac and Hhfac has been found to occur through two independent pathways, one of which is acid catalysed and another by the reaction of the neutral ligand with a species which probably contains the "dangling" monodentate ligand. The 'dangling ligand' hypothesis is also supported by the observation that the ligand exchange reaction of V(acac)3 with Hacac is also catalysed by water.</p> <p>The rearrangement of tris β-diketonato V(III) complexes has been found to proceed via an intramolecular mechanism. Total lineshpae analysis of the temperature dependent NMR spectra of V(tfac)3 indicate that the trigonal twist (RG3(p)) and mechanisms involving only trigonal bipyramidal states can be ruled out as the sole reaction pathways. V(dibm)3 has been synthesized and confirmation of enantiomerization of this complex has been inferred from the lineshape analysis of the NMR spectra of the diastereotopically non-equivalent methyl groups.</p> <p>In an attempt to establish the existence of a bond rupture process, an attempt was made to synthesize V(triac)3. Because of partial decomposition of the ligand, the product was a mixture of V(triac)3 and V(triac)2(acac). Although coalescence of the resonances of the methyl groups bonded to the ring carbons of the triac‾ ligand of V(triac)2 (acac) was observed at about 76ºC, no evidence of site exchange of the acetyl methyl group and the ring methyl groups (linkage isomerization) was obtained up to 200ºC. for either compound. This evidence does not support a bond rupture mechanism.</p> <p>The effect of solvent on the rates of rearrangement of V(tfac)3 and V(hfac)2 (acac) has been determined. For V(tfac)3, the effects are small. However, for V(hfac)2 (acac), the solvent effects are pronounced. The large solvent effect is most readily interpreted in terms of a bond rupture mechanism.</p> <p>The enthalpies and entropies of the rearrangement processes of a number of β-diketonato V(III) complexes have been estimated and found to exhibit a linear relationship suggesting that the rearrangement processes may occur via a common mechanism.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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