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The oxidation of oxovanadium(IV) complexes [LV(IV)O] (L = tetradentate Schiff-base ligands such as N,N'-ethylenebis(salicylideneaminate)(2-) (salen) and N,N'-2,2-dimethylpropylenebis(salicylideneaminate)(2-) (salpn)) to [LV(V)O](+), believed to be responsible for the voltammetric response near 0.6 V vs Ag/AgCl in CH(2)Cl(2) in the presence of tetrabutylammonium tetrafluoroborate as a supporting electrolyte, is in fact coupled to a homogeneous process where [LVO](+) coordinates BF(4)(-) to form a neutral complex formulated as [LVOBF(4)]. The formation constants for [VO(salen)BF(4)] and [VO(salpn)BF(4)] are evaluated to be K(salen)(-)(1) = 1.1 x 10(2) M(-)(1) and K(salpn)(-)(1) = 1.4 x 10 M(-)(1), respectively. Crystal structure of [VO(salen)BF(4)] reveals that one of the fluorine atoms in BF(4)(-) is so close to the vanadium(V) atom as to be practically bound in the solid state.

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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 &mgr;-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.