[FeFe]-Hydrogenase active site models with relatively low reduction potentials: Diiron dithiolate complexes containing rigid bridges

Three diiron dithiolate complexes containing rigid and conjugated bridges, [μ-SC6H4-2-(CO)S-μ]Fe2(CO)6 (1), [2-μ-SC5H3N-3-(CO)S-μ]Fe2(CO)6 (2), and the PPh3-monosubstituted complex [μ-SC6H4-2-(CO)S-μ]Fe2(CO)5(PPh3) (1-P), were prepared as biomimetic models for the [FeFe]-hydrogenase active site. The structures of complexes 1 and 2 were determined by single crystal X-ray analysis, which shows that each complex features a rigid coplanar dithiolate bridge with a 2–3° deviation from the bisect plane of the molecule. The influence of the rigid bridge on the reduction potentials of complexes 1, 2 and 1-P was investigated by electrochemistry. The cyclic voltammograms of complexes 1 and 2 display large positive shifts for the primary reduction potentials, that is, 380–480 mV in comparison to that of the pdt-bridged (pdt = propane-1,3-dithiolato) complex (μ-pdt)Fe2(CO)6 and 160–260 mV to that of the bdt-bridged (bdt = benzene-1,2-dithiolato) analogue (μ-bdt)Fe2(CO)6.