NiI/RuII Model for the Ni–L State of the [NiFe]Hydrogenases: Synthesis, Spectroscopy, and Reactivity

This study describes the characterization of a mixed-valence RuII/NiI complex, a structural model for the Ni–L state of the [NiFe]­hydrogenases. One-electron oxidation of (cymene)­Ru­(μ-pdt)­Ni­(diphos) ([1]0, diphos = dppe, C2H4(PPh2)2; [2]0, diphos = dcpe, C2H4(P­(C6H11)2)2] affords the mixed-valence cations [(cymene)­Ru­(pdt)­Ni­(diphos)]+ ([1]+ and [2]+). Crystallographic and spectroscopic measurements indicate that these cations are described as RuII/NiI. Although [1]0 and [1]+ are very similar structurally, the following changes are notable: the Ni–P distances elongate upon oxidation, and the Ru–Ni distance changes insignificantly. The molecular and electronic structures of the Ni center in [1]+ approaches that observed in the [NiFe]­hydrogenases. Density functional theory calculations indicate that [1]0 is best described as RuII/Ni0, consistent with its oxidation to RuII/NiI in [1]+. The fast electron self-exchange rate of 107 M–1 s–1 between [1]0 and [1]+ suggests minor reorganization, more consistent with a Ni0/NiI oxidation state change than a NiI/NiII couple. In solution, [1]+ slowly converts to [H1]+ and [1-H]+, with the latter being a complex of the thioaldehyde SCHCH2CH2S arising from C–H activation of the pdt backbone. Treatment of [1]+ with the H-atom abstracting reagent 2,2,6,6-tetramethylpiperidine-1-oxy also gives [1-H]+.