Mechanism of H2 Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides
Journal of the American Chemical Society
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) ( 0 ; dppv = cis-C 2 H 2 (PPh 2 ) 2 ) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of  0 initially produces unsym-[H1] + , which converts by a
... der pathway to sym-[H1] + . These species have C 1 (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* 2 Co. 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 H 2 upon protonation, treatment of [H1] 0 with acids gives H 2 . The redox state of the "remote" metal (Ni) modulates the hydridic character of the Fe(II)-H center. As supported by DFT calculations, H 2 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  + , 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. should reflect on the mechanism since oxidation of [H1] 0 predominantly gives significant sym, and protonation of  0 generates exclusively unsym (see above). Indeed, treatment of Ulloa et al. Comparisons of these NiFe models with the [NiFe]-hydrogenases are instructive. Overall, the biological system operates at one oxidation state higher than these models (Table 7) . This difference reflects the effects of charge, since the model complexes are almost always 7 Ulloa et al.