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

Abstract

Four diiron dithiolate complexes containing ortho-acylamino-functionalized arenethiolato ligands, [(μ-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 3 and 4 were determined by X-ray crystallography. The intra-ligand NH⋯S hydrogen bonds were studied by the X-ray analysis and by the ¹H NMR spectroscopy. The contribution of the NH⋯S hydrogen bonds to the reduction potentials of complexes 1–4 was investigated by electrochemistry. The first reduction potentials of complexes 1–4 exhibit large positive shifts, that is, 220–320 mV in comparison to that of the analogous complex [(μ-SPh)₂Fe₂(CO)₆] and 370–470 mV to that of [(μ-pdt)₂Fe₂(CO)₆] (pdt = propane-1,3-dithiolato). Complex 4 is capable of electrocatalysing proton reduction of acetic acid at relatively low overpotential (ca. 0.2 V) in acetonitrile.