Diiron dithiolate complexes containing intra-ligand NH ... S hydrogen bonds: [FeFe] hydrogenase active site models for the electrochemical proton reduction of HOAc with low overpotential

Four diiron dithiolate complexes containing ortho-acylamino-functionalized arenethiolato ligands, [(micro-S-2-RCONHC6H4)2Fe2(CO)6] (R = CH3, 1; CF3, 2; C6H5, 3; 4-FC6H4, 4), were synthesized and well characterized as biomimetic models of the Fe-Fe hydrogenase active site. The molecular structures of and 4 were determined by X-ray crystallography. The intra-ligand NHS hydrogen bonds were studied by the X-ray analysis and by the (1)H NMR spectroscopy. The contribution of the NHS hydrogen bonds to the reduction potentials of complexes was investigated by electrochemistry. The first reduction potentials of complexes exhibit large positive shifts, that is, 220-320 mV in comparison to that of the analogous complex [(micro-SPh)2Fe2(CO)6] and 370-470 mV to that of [(micro-pdt)2Fe2(CO)6] (pdt = propane-1,3-dithiolato). Complex is capable of electrocatalysing proton reduction of acetic acid at relatively low overpotential (ca. 0.2 V) in acetonitrile.