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

Article abstract of DOI:10.1021/ic0610278

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}Fe ₂(CO)₆] (1) and [{μ-(SCH₂) ₂N(CH₂CH₂CH₃)}Fe₂(CO) ₆] (2) with dppe gave double butterfly complexes [{μ-(SCH ₂)₂CH₂}Fe₂(CO)₅(Ph ₂-PCH₂)]₂ (3) and [{μ-(SCH₂) ₂N(CH₂CH₂CH₃)}Fe₂(CO) ₅(Ph₂PCH₂ (4), where two Fe₂S ₂ units are linked by the bisphosphine. In addition, an unexpected byproduct, [{μ-(SCH₂)₂N(CH₂CH ₂CH₃)}Fe₂(CO)₅{Ph ₂PCH₂CH₂(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 P=S, presumably by nucleophilic attack on bridging sulfur. By contrast, the reaction of 1 and 2 with dppm under mild conditions gave only complexes [{μ-(SCH ₂)₂CH₂}Fe₂(CO)₅(Ph ₂PCH₂-PPh₂)] (6) and [{μ-(SCH ₂)₂N(CH₂CH₂CH₃)}Fe ₂(CO)₅(Ph₂PCH₂PPh₂)] (8), where one ligand coordinated in a monodentate fashion to one Fe ₂S₂ unit. Furthermore, under forcing conditions, the complexes [{μ-(SCH₂)₂CH₂}-Fe ₂(CO)₄{μ-(Ph₂P)₂CH₂}] (7) and [{μ-(SCH₂)₂N(CH₂CH ₂CH₃)}Fe₂(CO)₄{μ-(PH ₂P)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).