Mechanism of H₂ Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides

The intermediacy of a reduced nickel-iron hydride in hydrogen evolution catalyzed by Ni-Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)Ni(μ-pdt)Fe(CO)(dppv) ([1]⁰ dppv = cis-C₂H₂(PPh₂)₂) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [1]⁰ initially produces unsym-[H1]⁺, which converts by a first-order pathway to sym-[H1]⁺. These species have C₁ (unsym) and C_s (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H1]⁺ protonates at sulfur. The S = 1/2 hydride [H1]⁰ was generated by reduction of [H1]⁺ with Cp∗₂Co. Density functional theory (DFT) calculations indicate that [H1]⁰ is best described as a Ni(I)-Fe(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H1]⁰. Whereas [H1]⁺ does not evolve H₂ upon protonation, treatment of [H1]⁰ with acids gives H₂. The redox state of the "remote" metal (Ni) modulates the hydridic character of the Fe(II)-H center. As supported by DFT calculations, H₂ evolution proceeds either directly from [H1]⁰ and external acid or from protonation of the Fe-H bond in [H1]⁰ to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H1]⁰ initially produces [1]⁺, which is reduced by [H1]⁰. Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit.