Iron complexes hydrogenase enzymes

More complex assemblies of iron and sulfur, sometimes extended to other metals like nickel, molybdenum, vanadium, or other iron centers are found in some enzymes, that catalyze the transformation of small molecules [1, 14]. Among these centers, we will focus next on the P cluster and the FeMo cofactor of nitrogenase and on the H cluster of the iron-only hydrogenase. 

In view of its application to fuel cell development, research into hydrogen activation remains a forefront area for chemists, physicists, and biologists (7). A rekindling of opportunity and excitement in this field of chemistry has come from the delineation of simple catalytic sites of hydrogenase enzymes as displayed by protein crystal structures published within the last decade (2 10). These active sites hold out promise of using complexes comprised of base metals such as iron or... 

Some iron and nickel cyanide and carbonyl complexes have been reported as models of the [FeNi]-hydrogenase enzymes. The preparation and structures of the trigonal bipyramidal nickel and iron complexes with the tetradentate ligands tris(2-phenylthiol)phosphine (PS3) and tris(3-phenyl-2-thiophenyl)phosphine (PS3 ) have been reported [70, 71]. The nickel carbonyl complex [Ni(PS3 )(CO)] exhibits vco at 2029 cm compared with the value of 1940 em" for the iron earbonyl complex [Fe(PS3 )(CO)]. Both of these complexes lose CO upon oxidation. The use of cyanide in place of carbon monoxide allows for the preparation of both [Fe (PS3)(CN)] and [Fe (PS3 )(CN)] eomplexes. The IR properties of... 

The work of Darensbourg et al. has superseded these early model systems. A series of model compounds with the core unit Fe(CO)2(CN) or Fe(CO)(CN)2 were found to reproduce the unique IR absorption spectra of [FeNi]-hydrogenases very well. The IR spectrum of the [FeNi]-hydrogenase enzyme from D. gigas in the A state exhibits bands at 1947, 2093, and 2083 cm . The IR spectrum of the iron(II) model complex K [CpFe(CO)(CN)2] in acetonitrile exhibits absorption bands at 1949, 2094, and 2088 cm which are assigned to the vco, the symmetric vcn and the asymmetric voj, respectively. The energies and peak-widths at half-maximum of this absorption are sensitive to the oxidation state of the iron center, to the medium and to the counter-ion. Polar media produce broad bands with peak width at half-maximum of the vco band of 17 cm in water. The use of non-polar solvents is required to achieve the narrow (4 cm ) peak width at half-maximum observed for the enzyme. 

In a recent review it has been pointed out that the most probable metal for the role of receptor site for hydrogen in the enzyme hydrogenase is either cobalt or iron ). It is therefore of particular interest to investigate the nature and properties of the hydrogenation catalyst which is formed when KCN is added to an aqueous solution of C0CI2 2). No iron complex is yet known which can react in solution with hydrogen gas. 

Interest in these thiolate-bridged di-iron carbonyl complexes has reemerged recently as protein crystallography revealed that some hydrogenase enzymes possess a similar active site, as discussed next [19-22]. 

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