Di-, tri- and tetraphosphine-substituted Fe/Se carbonyls: synthesis, characterization and electrochemical properties

The active sites of [FeFe]-hydrogenase promoted by Fe/E (E = S, Se) clusters have attracted considerable interest due to their significance in understanding the interconversion of hydrogen with protons and electrons. As an extension of the study on Fe/Se clusters related to H-cluster model compounds of [FeFe]-hydrogenase, a series of tertiary phosphine substituted Fe/Se carbonyls were successfully prepared. The treatment of Fe₂(μ-SePh)₂(CO)₆ (A) and excess PR₃ resulted in the ferrous bis(selenolate) carbonyls Fe(SePh)₂(CO)₂(PR₃)₂ (PR₃ = PPhMe₂, 1; PMe₃, 2) in moderate yields. In striking contrast, the reaction of Fe₂(μ-SeCH₂Ph)₂(CO)₆ (B) with the same PR₃ ligand resulted in the PR₃-disubstituted models Fe₂(μ-SeCH₂Ph)₂(CO)₄(PR₃)₂ (PR₃ = PPhMe₂, 3; PMe₃, 4) as the principal products. The more interesting finding is that two independent isomers (anti- and syn-) can be isolated according to different reaction temperatures. Further reactions of 3 or 4 with PR₃ under UV irradiation afforded the first PR₃-trisubstituted 2Fe2Se derivatives Fe₂(μ-SeCH₂Ph)₂(CO)₃(PR₃)₃ (PR₃ = PPhMe₂, 5; PMe₃, 6). 6 could be further converted into the tetrasubstituted product Fe₂(μ-SeCH₂Ph)₂(CO)₂(PMe₃)₄ (7), while no further substitution was observed with 5 and excess of PPhMe₂. All the prepared compounds were fully characterized by elemental analysis, various spectroscopic techniques and X-ray crystallography. In addition, some electrochemical properties of these models were studied by cyclic voltammetry (CV) in MeCN. Compounds 4, 6 and 7 were found to be catalysts for the H₂ evolution reaction under electrochemical conditions.