Ni^I/Ru^II model for the Ni-L state of the [NiFe]hydrogenases: Synthesis, spectroscopy, and reactivity

This study describes the characterization of a mixed-valence Ru ^II/Ni^I 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(PPh ₂)₂; [2]⁰, diphos = dcpe, C₂H ₄(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 Ru^II/ Ni^I. 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 Ru^II/Ni ⁰, consistent with its oxidation to Ru^II/Ni^I 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/Ni ^I oxidation state change than a Ni^I/Ni^II 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]⁺.