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, [Fe-2(bdt)(CO)(6)] (bdt = benzenedithiolate), were investigated by time-resolved spectroscopy. The singly reduced catalyst [Fe-2(bdt)(CO)(6)](-), a key intermediate in photocatalytic H-2 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, [Fe-2(bdt)(CO)(6)](-) is readily distinguished from the two-electron reduced catalyst [Fe-2(bdt)(CO)(6)](2-) that is obtained inevitably in the electrochemical reduction of [Fe-2(bdt)(CO)(6)]. For the disproportionation rate constant of [Fe-2(bdt)(CO)(6)](-), an upper limit on the order of 10(7) M-1 s(-1) was estimated, which precludes a major role of [Fe-2(bdt)(CO)(6)](2-) in photoinduced proton reduction cycles. Structurally [Fe-2(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 [Fe-2(bdt)(CO)(6)](-) with bimolecular rate constants of 4 x 10(6), 7 x 10(6), and 2 x 10(8) M-1 s(-1) (trichloroacetic, trifluoroacetic, and toluenesulfonic acids, respectively). The resulting hydride complex [Fe-2(bdt)(CO)(6)H] is therefore likely to be an intermediate in photocatalytic cycles. This intermediate resembles structurally and electronically the parent complex [Fe-2(bdt)(CO)(6)], with very similar carbonyl stretching frequencies.