Direct observation of key catalytic intermediates in a photoinduced proton reduction cycle with a diiron carbonyl complex

The structure and reactivity of intermediates in the photocatalytic cycle of a proton reduction catalyst, [Fe₂(bdt)(CO)₆] (bdt = benzenedithiolate), were investigated by time-resolved spectroscopy. The singly reduced catalyst [Fe₂(bdt)(CO)₆]^-, a key intermediate in photocatalytic H₂ formation, was generated by reaction with one-electron reductants in laser flash-quench experiments and could be observed spectroscopically on the nanoseconds to microseconds time scale. From UV/vis and IR spectroscopy, [Fe₂(bdt)(CO)₆]^- is readily distinguished from the two-electron reduced catalyst [Fe₂(bdt)(CO)₆]^2- that is obtained inevitably in the electrochemical reduction of [Fe₂(bdt)(CO)₆]. For the disproportionation rate constant of [Fe₂(bdt)(CO)₆]^-, an upper limit on the order of 10⁷ M^-1 s^-1 was estimated, which precludes a major role of [Fe₂(bdt)(CO)₆]^2- in photoinduced proton reduction cycles. Structurally [Fe₂(bdt)(CO)₆]^- is characterized by a rather asymmetrically distorted geometry with one broken Fe-S bond and six terminal CO ligands. Acids with pK_a ≥ 12.7 protonate [Fe₂(bdt)(CO)₆]^- with bimolecular rate constants of 4 × 10⁶, 7 × 10⁶, and 2 × 10⁸ M^-1 s^-1 (trichloroacetic, trifluoroacetic, and toluenesulfonic acids, respectively). The resulting hydride complex [Fe₂(bdt)(CO)₆H] is therefore likely to be an intermediate in photocatalytic cycles. This intermediate resembles structurally and electronically the parent complex [Fe₂(bdt)(CO)₆], with very similar carbonyl stretching frequencies.