Reversible Conversion of Disulfide/Dithiolate Occurring at a Vanadium(IV) Center: A Biomimetic System for Redox Exchange in Vanabin.

Ascidians use a class of cysteine-rich proteins generally referred to as vanabins to reduce vanadium ions, one of the many biological processes that involve the redox conversion between disulfide and dithiolate mediated by transition-metal ions. To further understand the nature of disulfide/dithiolate exchange facilitated by a vanadium center, we report herein a six-coordinate non-oxido V IV complex containing an unbound disulfide moiety, [V IV (PS3″)(PS1″ S-S )] ( 1 ) (PS3″ = [P(C 6 H 3 -3-Me 3 Si-2-S) 3 ] 3- , where PS1″ S-S is a disulfide form of PS3″). Complex 1 is obtained from a reaction of previously reported [V V (PS3″)(PS2″S H )] ( 2 ) (PS2″S H = [P(C 6 H 3 -3-Me 3 Si-2-SH)(C 6 H 3 -3-Me 3 Si-2-S) 2 ] with TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yl)oxyl) via hydrogen atom transfer. Importantly, complex 1 can be reduced by two electrons to form an eight-coordinate V IV complex, [V IV (PS3″) 2 ] 2- ( 4 ). The reaction can be reversed through a two-electron oxidation process to regenerate complex 1 . The redox pathways both proceed through a common intermediate, [V(PS3″) 2 ] - ( 3 ), that has been previously reported as a resonance form of V V -dithiolate and a V IV -(thiolate)(thiyl-radical) species. This work demonstrates an unprecedented example of reversible disulfide/dithiolate interconversion mediated by a V IV center, as well as provides insights into understanding the function of V V reductases in vanabins.