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=Pr-i/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((OPr)-Pr-i)(4) was identified as the most active system with a TON of 160 (p(CO2)=5 bar, p(H-2)=160 bar, T=150 degrees C). Using methanol as solvent and co-reagent allows the catalytic conversion of CO2/H-2 in a liquid phase process comprising only the substrates and products.