Kinetics and mechanism of reduction of oxo-chromium(V) salen by vanadium(IV)

The reduction of oxo-chromium(V) salen with a 40–160-fold excess of oxovanadium(IV) ([H+] = 0.02–0.1 M) at 25 °C has been investigated. The observed absorbance changes fitted a pseudo-first-order process. The nature of the intermediate, final product and reaction mechanism have been proposed on the basis of reaction conditions and observed rate constants. E.s.r. data support 1:1 stoichiometry with VO2+ in a deficiency. With an excess of VO2+ a CrIII product corresponding to a two electron reduction process has been obtained. The spectral and ion exchange properties of the chromium product correspond to that of the N,N′-ethylene-bis(salicylideneimine) derivative of CrIII. The rate of formation of the final product increases with decreasing [H+]. The observed kinetic behavior is consistent with a mechanism involving the formation of a CrIV—VV intermediate in an equilibrium step prior to the electron transfer step. The equilibrium constant for the formation of the intermediate has been estimated to be 11.2 ± 0.8 M−1. The second-order-rate constants for the reduction of CrV species have been estimated to be 0.14 × 102, 0.10 × 102 and 0.05 × 102 M−1 S−1 at [H+] = 0.02, 0.05 and 0.1 M respectively. Like the FeII—CrV redox couple, the VIV—CrV redox reaction also follows an inner-sphere process.