Direct Spectroscopic Detection of Key Intermediates and the Turnover Process in Catalytic H₂ Formation by a Biomimetic Diiron Catalyst

[FeFe(Cl₂-bdt)(CO)₆] (1; Cl₂-bdt=3,6-dichlorobenzene-1,2-dithiolate), inspired by the active site of FeFe-hydrogenase, shows a chemically reversible 2 e⁻ reduction at −1.20 V versus the ferrocene/ferrocenium couple. The rigid and aromatic bdt bridging ligand lowers the reduction potential and stabilizes the reduced forms, compared with analogous complexes with aliphatic dithiolates; thus allowing details of the catalytic process to be characterized. Herein, time-resolved IR spectroscopy is used to provide kinetic and structural information on key catalytic intermediates. This includes the doubly reduced, protonated complex 1H⁻, which has not been previously identified experimentally. In addition, the first direct spectroscopic observation of the turnover process for a molecular H₂ evolving catalyst is reported, allowing for straightforward determination of the turnover frequency.