Electrochemical Synthesis of Mono‐ and Disubstituted Diiron Dithiolate Complexes as Models for the Active Site of Iron‐Only Hydrogenases

[Fe 2 (S 2 C 3 H 6 )(CO) 5 {P(OMe) 3 }] ( 2 ) and [Fe 2 (S 2 C 3 H 6 )(CO) 4 {P(OMe) 3 } 2 ] ( 3 ) were selectively prepared by the electrochemical reduction of [Fe 2 (S 2 C 3 H 6 )(CO) 6 ] ( 1 ) in the presence of trimethyl phosphite ligand. Electrochemical data indicate a CO‐displacement reaction catalyzed by electron transfer. Complexes 2 and 3 were characterized by X‐ray crystallography, which shows that in the solid state, the trimethyl phosphite ligands lie in the apical configuration. NMR suggests that multiple isomers exist in solution. Both 2 and 3 exhibit a primary reduction step that involves two electrons. No stable hydride derivatives could be isolated by the reaction of 2 or 3 with strong acid. However, for both 2 and 3 , proton reduction catalysis was detected at a potential less negative than that of the reduction of the diiron complex working as catalyst. The proposed mechanism involves a protonation of the diiron complex in the vicinity of the electrode, which is triggered by the more facile reduction of the protonated form (CE mechanism). The protonation and reduction steps are followed by a bimolecular reaction that regenerates the starting complex. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)