Electrochemical properties of vanadium(III,IV,V)-salen complexes in acetonitrile. Four-electron reduction of O2 by V(III)-salen.

The coordination chemistry and electrochemistry of complexes of vanadium(III,IV,V) with salen (H2 salen = N,N'-ethylenebis(salicylideneamine) were reexamined in an attempt to uncover the origin of puzzling results reported in a previous study (Inorg. Chem. 1994, 33, 1056). Microelectrodes were utilized to allow measurements in the absence of supporting electrolyte. The source of the puzzling results was identified and the modifications required in the previous interpretations are specified. Corrected values of formal potentials and diffusion coefficients are also given. The acid-induced disproportionation of V(IV)O(salen), as originally proposed by Bonadies et al. (J. Chem. Soc., Chem. Commun. 1986, 1218; Inorg. Chem. 1987, 26, 1218), was largely supported by the present results. The equilibrium constant for this disproportionation reaction was measured. The stoichiometry and kinetics of the reaction between O2 and the V(III)-salen complex were examined, and a possible mechanism for this four-electron reduction of O2 is suggested.