Formation, Preservation, and Cleavage of the Disulfide Bond by Vanadium

Reaction of the disulfide {HpicanS}2 (HpicanS is the carboxamide based on picolinate (pic) and o‐mercaptoaniline (anS); the {} brackets are used to denote disulfides) with [VOCl2(thf)2] leads to reductive scission of the disulfide bond and formation of the mixed‐valence (VIV/VV) complex anion [(OVpicanS)2μ‐O]− (1), with the dianionic ligand coordinating through the pyridine‐N atom, the deprotonated amide‐N atom, and thiophenolate‐S atom. Reductive cleavage of the SS bond is also observed as [VCl2(tmeda)2] (tmeda=tetramethylethylenediamine) is treated with the disulfides {HsalanS}2 or {HvananS}2 (HsalanS and HvananS are the Schiff bases formed between o‐mercaptoaniline and salicylaldehyde (Hsal) or vanillin (Hvan), respectively), yielding the VIII complexes [VCl(tmeda)(salanS)] (2 a), or [VCl(tmeda)(vananS)] (2 b). The disulfide bond remains intact in the aerial reaction between {HsalanS}2 and [VCl3(thf)3] to yield the VV complex [VOCl{salanS}2] (3), where (salanS)2− coordinates through the two phenolate and one of the imine functions. The SS bond is also preserved as [VO(van)2] or [VO(nap)2] (Hnap=2‐hydroxynaphthalene‐1‐carbaldehyde) is treated with bis(2‐aminophenyl)disulfide, {anS}2, a reaction which is accompanied by condensation of the aldehyde and the diamine, and complexation of the resulting bis(Schiff bases) {HvananS}2 or {HnapanS}2 to form the complexes [VO{vananS}2] (4 a) or [VO{napanS}2] (4 b). In 4 a and 4 b, the phenolate and imine functions, and presumably also one of the disulfide‐S atoms, coordinate to VIV. 2Mercaptophenyl‐2′‐pyridinecarboxamide (H2picanS) retains its identity in the presence of VIII; reaction between [VCl3(thf)3] and H2picanS yields [V{picanS}2]− (5). The dithiophenolate 2,6‐bis(mercaptophenylthio)dimethylpyridine (6 a) is oxidized, mediated by VO2+, to the bis(disulfide) octathiadiaza‐cyclo‐hexaeicosane 6 b. The relevance of these reactions for the speciation of vanadium under physiological conditions is addressed. [HNEt3]‐1⋅0.5 NEt3, 3⋅3 CH2Cl2, {HsalanS}2, [HNEt3]‐5, and 6 b⋅4 THF have been characterized by Xray diffraction analysis. Model reactions for the interaction of vanadium compounds with cysteine‐ and cystine‐containing peptides, under physiological conditions, show that vanadyl (VO2+) may mediate the oxidation of thiolate to disulfides, the reduction of disulfide to thiolate (see scheme), or the stabilization of disulfide by coordination. VIII stabilizes thiolate by coordination.