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

Abstract

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(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 [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 10(7) M(-1) s(-1) between [1](0) 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](+).