Coordination of BF₄^- to oxovanadium(V) complexes, evidenced by the redox potential of oxovanadium(IV/V) couples in CH₂Cl₂

The oxidation of oxovanadium(IV) complexes [LV^IVO] (L = tetradentate Schiff-base ligands such as N,N′-ethylenebis- (salicylideneaminate)(2-) (salen) and N,N′ -2,2-dimethylpropylenebis(salicylideneaminate)(2-) (salpn)) to [LV^VO]⁺, believed to be responsible for the voltammetric response near 0.6 V vs Ag/AgC in CH₂Cl₂ in the presence of tetrabutylammonium tetrafluoroborate as a supporting electrolyte, is in fact coupled to a homogeneous process where [LVO]⁺ coordinates BF₄^- to form a neutral complex formulated as [LVOBF₄]. The formation constants for [VO(salen)BF₄] and [VO(salpn)BF₄] are evaluated to be K_salen^-1 = 1.1 × 10² M^-1 and K_salpn^-1 = 1.4 × 10 M^-1, respectively. Crystal structure of [VO(salen)BF₄] reveals that one of the fluorine atoms in BF₄^- is so close to the vanadium(V) atom as to be practically bound in the solid state.