Carbene-pyridine chelating 2Fe2S hydrogenase model complexes as highly active catalysts for the electrochemical reduction of protons from weak acid (HOAc).

Two asymmetrically disubstituted diiron complexes (micro-pdt)[Fe(CO)(3)][Fe(CO)(eta(2)-L)] (L = 1-methyl-3-(2-pyridyl)imidazol-2-ylidene (NHC(MePy)), 2; 1,3-bis(2-picolyl)imidazol-2-ylidene (NHC(diPic)), 4) and a mono-substituted diiron complex (mu-pdt)[Fe(CO)(3)][Fe(CO)(2)(NHC(diPic))] (3) were prepared as biomimetic models of the Fe-only hydrogenase active site. X-Ray studies show that the NHC(MePy) and NHC(diPic) ligands in 2 and 4 each coordinate to the single iron atom as NHC-Py chelating ligands in two basal positions and the NHC(diPic) ligand of complex 3 lies in an apical position as a monodentate ligand. The large ranges of the highest and the lowest nu(CO) frequencies of 2 and 4 reflect that the relatively uneven electron density on the two iron atoms of the 2Fe2S model complexes 2 and 4 is as that observed for mono-substituted diiron complexes of good donor ligands. The cyclic voltammograms and the electrochemical proton reduction by 2 and 3 were studied in the presence of HOAc to evaluate the effect of asymmetrical substitution of strong donor ligands on the redox properties of the iron atoms and on the electrocatalytic activity for proton reduction.