Mechanism of H<sub>2</sub> 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) ([<b>1</b>]<sup>0</sup>; dppv = <i>cis</i>-C<sub>2</sub>H<sub>2</sub>­(PPh<sub>2</sub>)<sub>2</sub>) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [<b>1</b>]<sup>0</sup> initially produces <i>unsym</i>-[H<b>1</b>]<sup>+</sup>, which converts by a first-order pathway to <i>sym</i>-[H<b>1</b>]<sup>+</sup>. These species have <i>C</i><sub>1</sub> (unsym) and <i>C</i><sub><i>s</i></sub> (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H<b>1</b>]<sup>+</sup> protonates at sulfur. The <i>S</i> = 1/2 hydride [H<b>1</b>]<sup>0</sup> was generated by reduction of [H<b>1</b>]<sup>+</sup> with Cp*<sub>2</sub>Co. Density functional theory (DFT) calculations indicate that [H<b>1</b>]<sup>0</sup> 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<b>1</b>]<sup>0</sup>. Whereas [H<b>1</b>]<sup>+</sup> does not evolve H<sub>2</sub> upon protonation, treatment of [H<b>1</b>]<sup>0</sup> with acids gives H<sub>2</sub>. The redox state of the “remote” metal (Ni) modulates the hydridic character of the Fe­(II)–H center. As supported by DFT calculations, H<sub>2</sub> evolution proceeds either directly from [H<b>1</b>]<sup>0</sup> and external acid or from protonation of the Fe–H bond in [H<b>1</b>]<sup>0</sup> to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H<b>1</b>]<sup>0</sup> initially produces [<b>1</b>]<sup>+</sup>, which is reduced by [H<b>1</b>]<sup>0</sup>. 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.