Hydrogenase

The hydrogenase enzyme catalyses the redox equilibrium, 2H⁺ + 2e^- <--> H₂. Despite the simplicity of the reaction, hydrogenase enzymes are complex multi-metal domain proteins of high molecular weight. The Fe-only hydrogenase is primarily responsible for the reduction of protons, this process is thought to occur at the H centre of the enzyme. Crystal structures of the enzymes have shown that the H centre consists of an Fe₄S₄ unit linked via sulfur to a remarkable bimetallic complex, an Fe₂S₂ unit with mixed CO/CN ligands. Recent work on abiological model compounds based on Fe₂(m₂-S{CH₂}₃S)(CO)₆, 1, and Fe₂(m₃-RSCH₂C{Me}{CH₂S}₂(CO)₅, R = Me (2a), CH₂Ph (2b) has provided an important starting point for the understanding of this chemistry in general and the effect of the additional sulfur atom in the coordination sphere of one of the iron atoms in particular.
The above complexes are known to react with KCN to give dicyano dianionic complexes, which are very similar in structure to the hydrogenase active site (below). All complexes have an Fe₂S₂ fragment ligated by CO/CN ligands, where the two bridging S ligands are connected by three small molecules. The complexes 2a and 2b also bring a third sulfur coordination, a thioether, to one of the iron atoms, making these complexes even closer structural models of the hydrogenase active site.

	The hydrogenase active site, the H-centre
Since hydrogenase functions as a redox catalyst it is critical that the changes in reactivity and structure accompanying oxidation/reduction be better understood. It is this aspect of the chemistry that is the focus of our research. High-pressure IR spectroelectrochemical (IR-SEC) studies of the reduction of 1 reveal a complex series of reactions involving both CO dissociation and CO rearrangement. For 2a and 2b facile dissociation of the pendant S atom provides an alternative reaction channel.
	Much of this chemistry can be unravelled by a series of IR-SEC measurements under pressures of either an inert gas (N₂, Ar) or CO. On the left are shown a variety of IR spectra obtained at various points along the redox pathways of the Fe₂ complexes. The spectra were obtained by subtracting band sets which were attributed to a single species from differential spectra which were due to a mixture of species. The identification of the band profiles of the different species is obtained from the time and concentration dependence of the different SEC experiments.
M. Razavet, S. C. Davies, D. L. Hughes, C. J. Pickett, Chem. Commun., 2001, 847. A. Le Cloirec, S. P. Best, S. Borg, S. C. Davies, D. J. Evans, D. L. Hughes and C. J. Pickett, Chem. Commun., 1999, 2285-2286.