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, diphos = dppe, C2H4(PPh2)2; 2, 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 1 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 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 10(7) M(-1) s(-1) between 1 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.