Binuclear Iron−Sulfur Complexes with Bidentate Phosphine Ligands as Active Site Models of Fe-Hydrogenase and Their Catalytic Proton Reduction

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Abstract
The displacement of CO in a few simple Fe(I)−Fe(I) hydrogenase model complexes by bisphosphine ligands Ph2P−(CH2)n−PPh2 [with n = 1 (dppm) or n = 2 (dppe)] is described. The reaction of [{μ-(SCH2)2CH2}Fe2(CO)6] (1) and [{μ-(SCH2)2N(CH2CH2CH3)}Fe2(CO)6] (2) with dppe gave double butterfly complexes [{μ-(SCH2)2CH2}Fe2(CO)5(Ph2PCH2)]2 (3) and [{μ-(SCH2)2N(CH2CH2CH3)}Fe2(CO)5(Ph2PCH2)]2 (4), where two Fe2S2 units are linked by the bisphosphine. In addition, an unexpected byproduct, [{μ-(SCH2)2N(CH2CH2CH3)}Fe2(CO)5{Ph2PCH2CH2(Ph2PS)}] (5), was isolated when 2 was used as a substrate, where only one phosphorus atom of dppe is coordinated, while the other has been converted to PS, presumably by nucleophilic attack on bridging sulfur. By contrast, the reaction of 1 and 2 with dppm under mild conditions gave only complexes [{μ-(SCH2)2CH2}Fe2(CO)5(Ph2PCH2PPh2)] (6) and [{μ-(SCH2)2N(CH2CH2CH3)}Fe2(CO)5(Ph2PCH2PPh2)] (8), where one ligand coordinated in a monodentate fashion to one Fe2S2 unit. Furthermore, under forcing conditions, the complexes [{μ-(SCH2)2CH2}Fe2(CO)4{μ-(Ph2P)2CH2}] (7) and [{μ-(SCH2)2N(CH2CH2CH3)}Fe2(CO)4{μ-(Ph2P)2CH2}] (9) were formed, where the phosphine acts as a bidentate ligand, binding to both the iron atoms in the same molecular unit. Electrochemical studies show that the complexes 3, 4, and 9 catalyze the reduction of protons to molecular hydrogen, with 4 electrolyzed already at −1.40 V versus Ag/AgNO3 (−1.0 V vs NHE).