ja6b04579_si_001.pdf (14.84 MB)
Mechanism of H2 Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides
journal contribution
posted on 2016-06-21, 00:00 authored by Olbelina
A. Ulloa, Mioy T. Huynh, Casseday P. Richers, Jeffery A. Bertke, Mark J. Nilges, Sharon Hammes-Schiffer, Thomas B. RauchfussThe
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.