Structural and Kinetic Studies of Intermediates of a Biomimetic Diiron Proton-Reduction Catalyst

One-electron reduction and subsequent protonation of a biomimetic proton-reduction catalyst [FeFe(μ-pdt)(CO)₆] (pdt = propanedithiolate), 1, were investigated by UV-vis and IR spectroscopy on a nano- to microsecond time scale. The study aimed to provide further insight into the proton-reduction cycle of this [FeFe]-hydrogenase model complex, which with its prototypical alkyldithiolate-bridged diiron core is widely employed as a molecular, precious metal-free catalyst for sustainable H₂ generation. The one-electron-reduced catalyst was obtained transiently by electron transfer from photogenerated [Ru(dmb)₃]⁺ in the absence of proton sources or in the presence of acids (dichloro- or trichloroacetic acid or tosylic acid). The reduced catalyst and its protonation product were observed in real time by UV-vis and IR spectroscopy, leading to their structural characterization and providing kinetic data on the electron and proton transfer reactions. 1 features an intact (μ²,κ²-pdt)(μ-H)Fe₂ core in the reduced, 1^-, and reduced-protonated states, 1H, in contrast to the Fe-S bond cleavage upon the reduction of [FeFe(bdt)(CO)₆], 2, with a benzenedithiolate bridge. The driving-force dependence of the rate constants for the protonation of 1^- (k_pt = 7.0 × 10⁵, 1.3 × 10⁷, and 7.0 × 10⁷ M^-1 s^-1 for the three acids used in this study) suggests a reorganization energy >1 eV and indicates that hydride complex 1H is formed by direct protonation of the Fe-Fe bond. The protonation of 1^- is sufficiently fast even with the weaker acids, which excludes a rate-limiting role in light-driven H₂ formation under typical conditions.