The transition to a carbon-free society has driven interest in biomimetic [FeFe]-hydrogenase catalysts for hydrogen evolution. In the active site of the natural enzyme, both the metal center and the nitrogen in the azadithiolate (ADT) bridge are protonated, followed by proton shuttling from the ADT to the terminal hydride, ultimately forming an H–H bond. It has been suggested that under strongly reducing conditions the Fe₂(μ-ADT)(CO)6 (1) turnover mechanism involves a doubly reduced species (1²⁻), and this study investigates its protonation dynamics. Additionally, an analogous catalyst with a benzenedithiolate (BDT) bridge (2) was studied to exclude the possibility of protonating the second basic site. Using stopped-flow Fourier transform infrared (FTIR) spectroscopy, we characterized the metal-protonated doubly reduced species, identified by ~80 cm⁻¹ shift of the three most intense characteristic absorption bands, by introducing 1²⁻ to phenol and 2²⁻ to 3,5-dichlorophenol. Using a stronger acid (pentachlorophenol), a second protonation event occurred, in case of 1, leading to H₂ evolution and catalyst turnover. Upon reductant depletion, only the first reduction is reached, producing 1⁻, which immediately disproportionates, dimerizes, and forms degradation products in the presence of a proton source. When 2²⁻ is mixed with pentachlorophenol, a degradation product also forms, effectively deactivating the catalyst. Understanding the proton reduction mechanism provides valuable insights for future research in optimizing the catalyst design to influence the reactivity, basicity and stability of the key intermediates.
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