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 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.
Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Olbelina A
OA
Ulloa
Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Mioy T
MT
Huynh
Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Casseday P
CP
Richers
Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Jeffery A
JA
Bertke
Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Mark J
MJ
Nilges
Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Sharon
S
Hammes-Schiffer
Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Thomas B
TB
Rauchfuss
D020134:Catalytic Domain; D006859:Hydrogen / Q000378:metabolism*; D006864:Hydrogenase / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011522:Protons; D011789:Quantum Theory
D011522:Protons; D006859:Hydrogen; C043296:nickel-iron hydrogenase; D006864:Hydrogenase