Catalysis of the electroreduction of O₂ to H₂O by vanadium-salen complexes in acidified dichloromethane

Addition of stoichiometric quantities of trifluoromethanesulfonic (triflic) acid to solutions of V^IVO(salen) (H₂-salen = N,N′-ethylenebis(salicylideneamine)) in dichloromethane produces the μ-oxo dinuclear vanadium(IV) complex [(salen)V^IVOV^IV(salen)]²⁺ ([V^IVOV^IV]²⁺). Addition of significant excesses of triflic acid converts the dimer into [V^IV(salen)]²⁺. In dichloromethane solutions containing 0.1 M tetrabutylammonium tetrafluoroborate, the [V^IVOV^IV]²⁺ complex undergoes disproportionation and dissociation reactions to produce a solution containing an equimolar mixture of V^IVO(salen), [V^III(salen)]⁺, and [V^IVOV^V]³⁺. The oxophilic [V^III(salen)]⁺ complex reacts with O₂ to accomplish a four-electron reduction of O₂: 2[V^III(salen)]⁺ + O₂ → 2[V^VO(salen)]⁺. These reactions can be exploited to carry out catalytic electroreductions of O₂ to H₂O in acidified dichloromethane solutions of V^IVO(salen) at unusually positive potentials.