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

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 kex = (5.2 ± 0.8) × 106 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)(OH2)]+ 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 Odouble bond; length as m-dashV(V)–Odouble bond; length as m-dashV(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)OH2]+/[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: ΔV* = −3.4 ± 3.7 and −8.0 ± 4.0 cm3 mol−1 for [VO(salen)]+/[VO(salen)]0 and [VO(salen)OH2]+/[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).