A comparative dynamic nuclear magnetic resonance study of degenerate enantiomerization and dionate site exchange in octahedral diolatobis(pentane-2,4-dionato)titanium(IV) complexes

Abstract

Dynamic ¹H n.m.r. spectroscopy is used to investigate the mechanism of metal-centred rearrangement in the title complexes containing 2,4-dimethylpentane-2,4-diolate, 2,3-dimethylbutane-2,3-diolate, and isopropoxide ligands. The rate of degenerate enantiomerization (k₁), determined from the coalescing diolate gem-dimethyl signals, is compared with the rate (k₂) of dionate methyl-site exchange. The value of k₁/k₂ is relatively insensitive to solvent but varies between 2.0 and 1.0 according to the nature of the diolate ligands. The free-energy barriers lie in the range 11.9–16.2 kcal mol^–1. The balance of evidence favours a common non-rupture trigonal twisting mechanism for both processes. A higher-energy process (ΔG^‡ 25 kcal mol^–1) bringing about coalescence of the pentanedionate and acetyl methyl signals in the spectrum of bis(3-acetylpentane-2,4-dionato)(2,4-dimethylpentane-2,4-diolato)-titanium(IV) at 196 °C involves rupture of a Ti–O bond.