Manganese(I) Hydridocarbonyl Catalysts for Homogeneous CO2 Reduction to Formate and Methanol Under Mild Conditions

Carbon dioxide is considered as a useful C1 feedstock for chemical synthesis, providing that its inertness is overcome using appropriate catalysts. In the last few years, an increasing number of research groups worldwide have reported on the use of earth-abundant metal catalysts able to bring about CO2 activation and conversion to useful chemicals, by 2 to 6-electron reduction processes. Whereas Fe, Co and Ni have been extensively explored, the use of Mn(I) organometallic complexes has so far been in its infancy, albeit rapidly growing in the past two years. We demonstrated for the first time that well-defined Mn(I) complexes, i.e. bearing PNP pincer ligands based on bis(aminophosphine)pyridine scaffold, are active catalysts for homogeneous CO2 efficient hydrogenation to formate (TONs > 30,000 in the presence of DBU and Lewis acid co-catalysts) and selective hydrosilylation to methoxysilanes, i.e. methanol precursors. Mechanistic studies showed that both ligand-assisted and metal-centred mechanisms can be active, depending on the nature of the ligand and the type of reaction, and their implications in catalysis will be discussed.