Binuclear iron-sulfur complexes with bidentate phosphine ligands as active site models of Fe-hydrogenase and their catalytic proton reduction

The displacement of CO in a few simple Fe(I)-Fe(I) hydrogenase model complexes by bisphosphine ligands Ph(2)P-(CH(2))(n)-PPh(2) [with n = 1 (dppm) or n = 2 (dppe)] is described. The reaction of [{mu-(SCH(2))(2)CH(2)}Fe(2)(CO)(6)] (1) and [{mu-(SCH(2))(2)N(CH(2)CH(2)CH(3))}Fe(2)(CO)(6)] (2) with dppe gave double butterfly complexes [{mu-(SCH(2))(2)CH(2)}Fe(2)(CO)(5)(Ph(2)PCH(2))](2) (3) and [{mu-(SCH(2))(2)N(CH(2)CH(2)CH(3))}Fe(2)(CO)(5)(Ph(2)PCH(2))](2) (4), where two Fe(2)S(2) units are linked by the bisphosphine. In addition, an unexpected byproduct, [{mu-(SCH(2))(2)N(CH(2)CH(2)CH(3))}Fe(2)(CO)(5){Ph(2)PCH(2)CH(2)(Ph(2)PS)}] (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 [{mu-(SCH(2))(2)CH(2)}Fe(2)(CO)(5)(Ph(2)PCH(2)PPh(2))] (6) and [{mu-(SCH(2))(2)N(CH(2)CH(2)CH(3))}Fe(2)(CO)(5)(Ph(2)PCH(2)PPh(2))] (8), where one ligand coordinated in a monodentate fashion to one Fe(2)S(2) unit. Furthermore, under forcing conditions, the complexes [{mu-(SCH(2))(2)CH(2)}Fe(2)(CO)(4){mu-(Ph(2)P)(2)CH(2)}] (7) and [{mu-(SCH(2))(2)N(CH(2)CH(2)CH(3))}Fe(2)(CO)(4){mu-(Ph(2)P)(2)CH(2)}] (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/AgNO(3) (-1.0 V vs NHE).