Redox Interconversion of Non-Oxido Vanadium Complexes Accompanied by Acid-Base Chemistry of Thiol and Thiolate

The redox nature of the non-oxido vanadium sulfur center is associated with several biological systems such as vanadium nitrogenase, the reduction of vanadium ion in ascidians, and the function of amavadin, which is a vanadium(IV) natural product contained in Amanita mushrooms. But the related chemistry is less explored and understood compared to oxido vanadium species due to the oxophilic character of high valent vanadium ions. Herein, we present a class of non-oxido vanadium thiolate complexes, [V^III(PS2″S^H)₂]^- (1) (PS2″S^H = [P(C₆H₃-3-Me₃Si-2-S)₂(C₆H₃-3-Me₃Si-2-SH)]^2-), [V^IV(PS3″)(PS2″S^H)]^- (2) (PS3″ = [P(C₆H₃-3-Me₃Si-2-S)₃]^3-), [V(PS3″)₂]^- (3), [V(PS3″)(PS2″S^H)] (4), and [V^IV(PS3*)₂]^2- (5a) (PS3* = [P(C₆H₃-3-Ph-2-S)₃]^3-), and study their interconversion through the redox and acid-base reactions. Complex 1 consists of a six-coordinate octahedral vanadium center; complexes 2 and 4 are seven-coordinate with distorted capped trigonal prismatic geometry. Vanadium centers of 3 and 5a are both eight-coordinate; the former adopts ideal dodecahedral geometry, but the latter is better viewed as a distorted square antiprism. Complex 1 is oxidized to complex 2 and then to complex 3 with dioxygen. Each one-electron oxidation process is accompanied by the deprotonation of unbound thiol to bound thiolate. Complex 3 is also produced from complex 2 through stepwise addition of Fe(Cp)₂⁺/n-BuLi, or in the reverse order. The formation of 2 from 3 is achieved in the order of adding Co(Cp)₂ and acid or, as with the previous complex, inversely. Notably, the reduction of complex 2 to complex 1 accompanying the protonation of bound thiolate to unbound thiol only occurs with the presence of both Co(Cp)₂ and acid, indicating a cooperative effect between the metal-centered reduction and bound thiolate protonation. The conversions among these complexes are observed with ESI-MS and UV-vis-NIR spectroscopies. The work demonstrates two-electron redox interconversion in these complexes mediated by transformations between unbound thiol and bound thiolate.