Ligand-Controlled Product Selectivity in Electrochemical Carbon Dioxide Reduction Using Manganese Bipyridine Catalysts

Magnus H Rønne, Dasol Cho, Monica R Madsen, Joakim B Jakobsen, Seunghwan Eom, Émile Escoudé, Hans Christian D Hammershøj, Dennis U Nielsen, Steen U Pedersen, Mu-Hyun Baik, Troels Skrydstrup, Kim Daasbjerg

Electrocatalysis is a promising tool for utilizing carbon dioxide as a feedstock in the chemical industry. However, controlling the selectivity for different CO2 reduction products remains a major challenge. We report a series of manganese carbonyl complexes with elaborated bipyridine or phenanthroline ligands that can reduce CO2 to either formic acid, if the ligand structure contains strategically positioned tertiary amines, or CO, if the amine groups are absent in the ligand or are placed far from the metal center. The amine-modified complexes are benchmarked to be among the most active catalysts for reducing CO2 to formic acid, with a maximum turnover frequency of up to 5500 s–1 at an overpotential of 630 mV. The conversion even works at overpotentials as low as 300 mV, although through an alternative mechanism. Mechanistically, the formation of a Mn–hydride species aided by in situ protonated amine groups was determined to be a key intermediate by cyclic voltammetry, 1H NMR, DFT calculations, and infrared spectroelectrochemistry.