Comparative CO2 Hydrogenation Catalysis with MACHO-type Manganese Complexes

A pair of manganese complexes containing MACHO-type pincer ligands bearing a secondary amine, [HN­{CH2CH2(PiPr2)}2]­MnH­(CO)2, which can participate in pathways involving metal–ligand cooperation (MLC), and a tertiary amine, [MeN­{CH2CH2(PiPr2)}2]­MnH­(CO)2, which cannot participate in pathways involving MLC, are compared for the hydrogenation of CO2 to formate in the presence of a base. Lewis acid cocatalysts are crucial for increasing the activity of both catalysts, with [MeN­{CH2CH2(PiPr2)}2]­MnH­(CO)2 reaching TONs of up to 18,300 and yields of up to 73% in the presence of lithium triflate. This productivity is far greater than for the MLC capable secondary amine MACHO-supported manganese catalyst. Preliminary mechanistic experiments indicate that CO2 insertion into the Mn–H of each catalyst affords a stable manganese formate complex. In situ NMR spectroscopy and comparative catalytic experiments are consistent with the intermediacy of these manganese formate complexes in the catalytic cycle, likely representing the catalyst resting states. Our findings suggest that the tertiary amine ligated system gives greater productivity due to a combination of longer catalyst lifetime and greater enhancement from Lewis acid additives.