Mechanistic aspects of CO2 reduction catalysis with manganese-based molecular catalysts

One approach for the conversion of CO2 into fuels or fuel precursors is the proton-coupled reduction of CO2 to CO or formic acid, using transition metal complexes as catalysts in either electrocatalytic or photocatalytic processes. While a number of such molecular catalysts have been investigated over the years, many are based on expensive precious metals. However, a growing family of pre-catalysts based on the earth-abundant metal, manganese, originally with the generic formula, [Mn(α-diimine)(CO)3L]+/0, but now expanded to also include non-α-diimine ligands, has recently emerged as a promising, cheaper alternative to the heavily-investigated rhenium-based analogues. In this review, we discuss the current mechanistic understanding of Mn-based CO2 reduction pre-catalysts, from the point of view of both computational modeling and experimental techniques. We also highlight the methods used to accurately determine catalytic figures of merit, such as overpotential and turnover frequency. Finally, we have summarized the major findings in both electrocatalytic and photocatalytic CO2 reduction driven by Mn-based catalysts, including exciting new developments involving immobilization of the molecular catalysts on solid supports or electrodes, and also their use in photoelectrochemical CO2 reduction where solar energy is used to overcome the demanding electrochemical overpotential.