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-Me0sal-(RR)-chxn)(+/0) couple was determined as k(ex)= (5.2 +/- 0.8) x 10(6) kg mol(-1)s(-1) at 25 degrees 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 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)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: Delta V* = -3.4 +/- 3.7 and -8.0 +/- 4.0 cm(3) 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 d pi orbitals of V(IV) and V(V) species (Scheme 2). (c) 2005 Elsevier B.V. All rights reserved.