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(IV)O(salen) (H(2)salen = N,N '-ethylenebis(salicylideneamine)) in dichloromethane produces the -oxo dinuclear vanadium(IV) complex [(salen)V(IV)OV(IV)(salen)](2+) ([V(IV)OV(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(IV)OV(IV)](2+) complex undergoes disproportionation and dissociation reactions to produce a solution containing an equimolar mixture of V(IV)O(salen), [V(III)(salen)](+), and [V(IV)OV(V)](3+). The oxophilic [V(III)(salen)](+) complex reacts with O(2) to accomplish a four-electron reduction of O(2): 2[V(III)(salen)](+) + O(2) --> 2[V(V)O(salen)](+). These reactions can be exploited to carry out catalytic electroreductions of O(2) to H(2)O in acidified dichloromethane solutions of V(IV)O(salen) at unusually positive potentials.