Redox reactions of [VO(salen)] +/0 couple in acetonitrile: Volume analyses in relation to large chiral recognitions observed for electron self-exchange reactions of [VO(Schiff-Base)] +/0

Abstract

Kinetic measurements were carried out for the outer-sphere electron transfer reactions involving [VO(Schiff-Base)] +/0 couples. Electron self-exchange rate constant for [VO(3-MeOsal-( RR)-chxn)] +/0 couple was determined as k ex = (5.2 ± 0.8) × 10 6 kg mol −1s −1 at 25 °C. It was found that added water in acetonitrile solvent retarded the electron transfer reactions between [VO(salen)] + and [Co( o-phen) 3] 2+: the six-coordinate V(V) species, [VO(salen)(OH 2)] + was found to be ca. 3.5 times less reactive compared with the five-coordinate species, [VO(salen)] +. This small difference between the reactivity of five- and six-coordinate species indicates that the reaction through the direct O V(V)–O V(IV) interaction is outer-sphere in nature, contrary to the previously proposed inner-sphere mechanism. Activation volumes corresponding to the electron self-exchange reactions for [VO(salen)] +/[VO(salen)] 0 and [VO(salen)OH 2] +/[VO(salen)] 0 couples were estimated from the volume profiles of the reduction reactions of [VO(salen)] + by [Co( o-phen) 3] 2+, with and without added water in acetonitrile: Δ V* = −3.4 ± 3.7 and −8.0 ± 4.0 cm 3 mol −1 for [VO(salen)] +/[VO(salen)] 0 and [VO(salen)OH 2] +/[VO(salen)] 0 couples, respectively. It was suggested that previously reported chiral recognitions in the redox reactions involving [VO(Schiff-Base)] +/0 redox couples take place within the encounter complex through the outer-sphere mechanism by maximizing the direct coupling between the dπ orbitals of V(IV) and V(V) species (Scheme 2).