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

摘要

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-[H>1]⁺, which converts by a first-order pathway to sym-[H>1]⁺. These species have C1 (unsym) and Cs (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H>1]⁺ protonates at sulfur. The S = 1/2 hydride [H>1]⁰ was generated by reduction of [H>1]⁺ with Cp*2Co. Density functional theory (DFT) calculations indicate that [H>1]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 [H>1]⁰. Whereas [H>1]⁺ does not evolve H2 upon protonation, treatment of [H>1]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 [H>1]⁰ and external acid or from protonation of the Fe–H bond in [H>1]⁰ to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H>1]⁰ initially produces [>1]⁺, which is reduced by [H>1]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.