Multi-electron transfer process of vanadyl complexes for oxidative polymerization of diphenyl disulfide

Oligo(p-phenylene sulfide) is synthesized by oxidative polymerization of diphenyl disulfide with oxygen catalyzed by vanadyl acetylacetonate under strongly acidic conditions. The mechanistic studies reveal that the redox cycles of the vanadyl complexes give rise to catalysis through a two-electron transfer between diphenyl disulfide and molecular oxygen. The VO catalysts act as an excellent electron mediator to bridge a 1.0 V potential gap between the oxidation potential of disulfides and the reduction potential of oxygen. The VO-catalyzed oxygen-oxidative polymerization provides pure oligo(pphenylene sulfide)s containing an S–S bond. The polymeric product is of low molecular weight due to the insolubility under these conditions. (N,N′-ethylenebis(salicylideneaminato))oxovanadium-(IV), VO(salen), was used as an inert model compound to elucidate the redox chemistry of the vanadium complex. VO(salen) reacts with trifluoromethanesulfonic acid (CF₃SO₃H) or triphenylmethyl tetrafluoroborate (?₃C(BF₄)) to form a deoxygenated complex, V^IV(salen)²⁺, and a μ-oxodinuclear complex, [(salen)VOV(salen)]X₂, (X = CF₃SO₃^? or BF₄^?). The dimerization of VO(salen) is initiated by deoxygenation to produce V(salen)²⁺ which enters into an equilibrium with a second VO(salen) complex to produce the μ-oxo dimer. The two-electron transfer of the μ-oxo dinuclear vanadium complex is elucidated.