Manganese Catalysts with Bulky Bipyridine Ligands for the Electrocatalytic Reduction of Carbon Dioxide: Eliminating Dimerization and Altering Catalysis

With the goal of improving previously reported Mn bipyridine electrocatalysts in terms of increased activity and reduced overpotential, a bulky bipyridine ligand, 6,6′-dimesityl-2,2′-bipyridine (mesbpy), was utilized to eliminate dimerization in the catalytic cycle. Synthesis, electrocatalytic properties, X-ray diffraction (XRD) studies, and infrared spectroelectrochemistry (IR-SEC) of Mn­(mesbpy)­(CO)₃Br and [Mn­(mesbpy)­(CO)₃(MeCN)]­(OTf) are reported. Unlike previously reported Mn bipyridine catalysts, these Mn complexes exhibit a single, two-electron reduction wave under nitrogen, with no evidence of dimerization. The anionic complex, [Mn­(mesbpy)­(CO)₃]⁻, is formed at 300 mV more positive potential than the corresponding state is formed in typical Mn bipyridine catalysts. IR-SEC experiments and chemical reductions with KC₈ provide insights into the species leading up to the anionic state, specifically that both the singly reduced and doubly reduced Mn complexes form at the same potential. When formed, the anionic complex binds CO₂ with H⁺, but catalytic activity does not occur until a ∼400 mV more negative potential is present. The Mn complexes show high activity and Faradaic efficiency for CO₂ reduction to CO with the addition of weak Brønsted acids. IR-SEC experiments under CO₂/H⁺ indicate that reduction of a Mn­(I)–CO₂H catalytic intermediate may be the cause of this unusual “over-reduction” required to initiate catalysis