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

Abstract

One-electron reduction and subsequent protonation of a biomimetic proton-reduction catalyst [FeFe­(μ-pdt)­(CO)₆] (pdt = propanedithiolate), <b>1</b>, 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. <b>1</b> features an intact (μ²,κ²-pdt)­(μ-H)­Fe₂ core in the reduced, <b>1^–</b>, and reduced-protonated states, <b>1H</b>, in contrast to the Fe–S bond cleavage upon the reduction of [FeFe­(bdt)­(CO)₆], <b>2</b>, with a benzenedithiolate bridge. The driving-force dependence of the rate constants for the protonation of <b>1^–</b> (<i>k</i>_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 <b>1H</b> is formed by direct protonation of the Fe–Fe bond. The protonation of <b>1^–</b> is sufficiently fast even with the weaker acids, which excludes a rate-limiting role in light-driven H₂ formation under typical conditions