Monomer and dimer pathways of earth-abundant manganese tricarbonyl pre-catalysts for CO ₂ reduction studied by time-resolved IR spectroscopy

The activation mechanism of Mn-based molecular catalysts is reported in a three-component system (photosensitizer, electron donor, catalyst), investigated by time-resolved infrared spectroscopy. In total four complexes were studied that are derived from Mn(2,2'-bipyridine)(CO)₃Br by varying substituents on the ligand, which impose steric constraints or modulate electronic properties. Thereby, ligand effects on catalyst activation pathways are systematically assessed. A unified feature across all systems is that the intermediate after one-electron reduction and subsequent Br^- dissociation, <i>i.e.</i>, Mn⁰(L)(CO)₃, possesses a more positive reduction potential than the parent complex, leading to its rapid second reduction. This step outcompetes dimerization of Mn⁰ radicals, which instead proceeds through a symproportionation between the two-electron reduced species and the parent complex. However, when dimer formation is sterically hindered, two Mn⁰ species arise instead. The final process is the re-oxidation of the reduced intermediates by either hydrogen evolution or regeneration of the electron donor. Although CO₂ conversion was not the focus of this work, the elucidated pathways clarify how competing re-oxidation channels can limit reduction efficiency or alter product selectivity. These mechanistic insights provide a foundation for rational strategies to control the selectivity and amplify the desired catalytic reactions.