Hydrogenation of CO2 to Methanol with Mn‐PNP‐Pincer Complexes in the Presence of Lewis Acids: the Formate Resting State Unleashed

The hydrogenation of CO2 to methanol was achieved using a catalytic system comprising metal complexes of the form [Mn(CO)2[N(C2H4PR2)] (R=iPr/Ph, [HN(C2H4PPh2)2]=MACHO−Ph) together with Lewis acid additives. Mechanistic studies suggest initial CO2 insertion into a Mn−H bond leads to a formate complex as resting state. By systematically balancing the interaction between the acidic additive and the catalyst, the formate ligand could be removed through esterification to unleash the full catalytic potential. The reaction conditions were optimized on basis of the partial reaction order of relevant compounds. The combination of MACHO−Ph and Ti(OiPr)4 was identified as the most active system with a TON of 160 (p(CO2)=5 bar, p(H2)=160 bar, T=150 °C). Using methanol as solvent and co‐reagent allows the catalytic conversion of CO2/H2 in a liquid phase process comprising only the substrates and products. Breeding methanol: Hydrogenation of CO2 to methanol with a manganese‐based catalytic system was studied using alcohols as co‐reagents. Experimentally derived mechanistic insight led to the choice of suitable Lewis acid co‐catalysts to unlock the limiting resting state by transferring the formate ligand into a formate ester. This allowed to develop the conditions where the product methanol acts as the co‐reagent and solvent (ROH=MeOH). The resulting reaction system corresponds to a base‐metal catalyzed liquid phase process for “breeding” methanol via CO2 hydrogenation.