This work is focused on the identification and investigation of the catalytically relevant key iron species in a photocatalytic proton reduction system described by Beller and co-workers. The system is driven by visible light and consists of the low-cost [Fe (CO) ] as catalyst precursor, electron-poor phosphines P(R) as co-catalysts, and a standard iridium-based photosensitizer dissolved in a mixture of THF, water, and the sacrificial reagent triethylamine. The catalytic reaction system was investigated by operando continuous-flow FTIR spectroscopy coupled with H gas volumetry, as well as by X-ray absorption spectroscopy, NMR spectroscopy, DFT calculations, and cyclic voltammetry. Several iron carbonyl species were identified, all of which emerge throughout the catalytic process. Depending on the applied P(R) , the iron carbonyl species were finally converted into [Fe (CO) (μ-CO){μ-P(R) }] . This involves a P-C cleavage reaction. The requirements of P(R) and the necessary reaction conditions are specified. [Fe (CO) (μ-CO){μ-P(R) }] represents a self-assembling, sulfur-free [FeFe]-hydrogenase active-site mimic and shows good catalytic activity if the substituent R is electron poor. Deactivation mechanisms have also been investigated, for example, the decomposition of the photosensitizer or processes observed in the case of excessive amounts of P(R) . [Fe (CO) (μ-CO){μ-P(R) }] has potential for future applications.

Understanding the mechanism of photocatalytic water reduction is essential for the development of effective catalysts. A new, self‐assembling, and sulfur‐free [FeFe]‐hydrogenase mimic, which shows good catalytic activity, is described. The action of the visible‐light‐driven iron catalyst within the cycle of H2 production from water is monitored by a combination of in‐situ spectroscopic methods. More information can be found in the Full Paper by R. Ludwig et al. on page 16052.