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

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journal contribution
posted on 21.06.2016, 00:00 by Olbelina A. Ulloa, Mioy T. Huynh, Casseday P. Richers, Jeffery A. Bertke, Mark J. Nilges, Sharon Hammes-Schiffer, Thomas B. Rauchfuss
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 Cs (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.