The kinetics of trisbidentate chelate complexes revisited. A two-dimensional NMR study of the isomerization and racemization processes of the complexes [M(TTA) 3] (M = Al III, Ga III; TTA = 2-thenoyltrifluoroacetone)

Abstract

The octahedral tris-chelate complexes [M(tta) 3] (M = Al III , Ga III; tta = 2-thenoyltrifluoroacetone or 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione) were isolated as mixtures of cis- and trans-isomers. Their fluxionality in solution was measured by 19F NMR two-dimensional exchange spectroscopy and one-dimensional bandshape analysis. Rates of exchange between all four fluorine signals for a given temperature imply a common transition state for both the cis-trans isomerization and the Δ-Λ racemization processes. A bond rupture mechanism involving a trigonal bipyramidal or, more probably, a square planar transition state structure with a pendant axial ligand is proposed for the fluxion. The indium(III) complex [In(tta) 3] was also prepared but its fluxionality was too rapid to be followed by 19F NMR spectroscopy. Activation energies [ ΔG † (298.15 K ] for the fluxions were 89.8 ± 0.3 (Al III) and 75.4 ± 0.2 KJ mol −1 (Ga III). 1H and 19F NMR date for the octahedral complex [Co(tta) 3], and the tetrahedral complexes [Be(tta) 2] and [Zn(tta) 2] are also reported.