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 Ru-II/Ni-I complex, a structural model for the Ni-L state of the [NiFe]hydrogenases. One-electron oxidation of (cymene)Ru(mu-pdt)Ni(diphos) ([1], 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 Ru-II/Ni-I. Although [110 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 [11 is best described as Ru-II/Ni-I, consistent with its oxidation to Ru-II/Ni-I in [1](+). The fast electron self-exchange rate of 10(7) M-1 s(-1) between [110 and [1]+ suggests minor reorganization, more consistent with a Ni-0/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](+).