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 Cr^III 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 Cr^III. 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 Cr^IV—V^V 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 Cr^V 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 Fe^II—Cr^V redox couple, the V^IV—Cr^V redox reaction also follows an inner-sphere process.