Sulfane Decreases the Nucleophilic Reactivity of Zinc Thiolates: Implications for Biological Reactive Sulfur Species

A zinc dithiolate complex supported by a [N3S2] ligand was studied as a model for zinc-mediated thiolate-disulfide exchange, enabling isolation of a zinc-bound mixed-disulfide intermediate. Solution-phase characterization of this zinc-disulfide complex indicates an interaction between the zinc center and the disulfide moiety that results in activation of the S-S bond for subsequent reactions. Comparison of this reaction with disulfide exchange by a previously prepared zinc tetrasulfanido complex demonstrates that sulfane sulfur (S-0) acts as an efficient thiolate trapping agent, that is, polysulfanide anions are much less basic than thiolates. The resulting polysulfanide anions also exhibit decreased nucleophilicity compared to the parent thiolate anions. Alkylation kinetics comparisons between the zinc dithiolate and zinc tetrasulfanido complexes indicate attenuation of zinc-bound thiolate nucleophilicity by sulfane. These results suggest a general interplay between zinc, sulfane, and thiol/thiolate reactivity that can significantly impact biological redox processes.