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 Fe 2 (μ-SePh) 2 (CO) 6 (A) and excess PR 3 resulted in the ferrous bis(selenolate) carbonyls Fe(SePh) 2 (CO) 2 (PR 3 ) 2 (PR 3 = PPhMe 2 , 1; PMe 3 , 2) in moderate yields. In striking contrast, the reaction of Fe 2 (μ-SeCH 2 Ph) 2 (CO) 6 (B) with the same PR 3 ligand resulted in the PR 3 -disubstituted models Fe 2 (μ-SeCH 2 Ph) 2 (CO) 4 (PR 3 ) 2 (PR 3 = PPhMe 2 , 3; PMe 3 , 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 PR 3 under UV irradiation afforded the first PR 3 -trisubstituted 2Fe2Se derivatives Fe 2 (μ-SeCH 2 Ph) 2 (CO) 3 (PR 3 ) 3 (PR 3 = PPhMe 2 , 5; PMe 3 , 6). 6 could be further converted into the tetrasubstituted product Fe 2 (μ-SeCH 2 Ph) 2 (CO) 2 (PMe 3 ) 4 (7), while no further substitution was observed with 5 and excess of PPhMe 2 . 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 H 2 evolution reaction under electrochemical conditions.