Abstract
Formate dehydrogenase (FDH) enzymes catalyze redox interconversion of CO(2) and HCO(2)(-), with a key mechanistic step being the transfer of H(-) from HCO(2)(-) to an oxidized active site featuring a [M(VI)≡S] group in a sulfur-rich environment (M = Mo or W). Here, we report reactivity studies with HCO(2)(-) and other reducing agents of a synthetic [W(VI)≡S] model complex ligated by dithiocarbamate (dtc) ligands. Reactions of [W(VI)S(dtc)(3)][BF(4)] (1) conducted in MeOH solvent generated [W(VI)S(S(2))(dtc)(2)] (2) and [W(V)S(μ-S)(dtc)](2) (3) products by a solvolysis pathway that was accelerated by the presence of [Me(4)N][HCO(2)] but did not require it. Under MeOH-free conditions, the reaction of 1 with [Et(4)N][HCO(2)] produced some [W(IV)(μ-S)(μ-dtc)(dtc)](2) (4), but predominantly [W(V)(dtc)(4)](+) (5), along with stoichiometric CO(2) detected by headspace gas chromatography (GC) analysis. Stronger hydride sources such as K-selectride generated the more reduced analogue, 4, exclusively. The reaction of 1 with the electron donor, CoCp(2), also produced 4 and 5 in varying amounts depending on reaction conditions. These results indicate that formates and borohydrides act as electron donors rather than hydride donors toward 1, an outcome that diverges from the behavior of FDHs. The difference is ascribed to the more oxidizing potential of [W(VI)≡S] complex 1 when supported by monoanionic dtc ligands that allows electron transfer to outcompete hydride transfer, as compared to the more reduced [M(VI)≡S] active sites supported by dianionic pyranopterindithiolate ligands in FDHs.