Mechanistic Promiscuity in Cobalt-Mediated CO2 Reduction Reaction: One- Versus Two-Electron Reduction Process

We compare the carbon dioxide reduction (CO2RR) activity and selectivity of the complexes [(Hbbpya)CoII]2+ and [(Mebbpya)CoII]2+, which contain two 2,2′-bipyridine chelating groups linked by -NH or -NCH3 moieties, respectively. Whereas [(Hbbpya)CoII]2+ forms CO under electrocatalytic conditions in presence of phenol (PhOH) with high selectivity, [(Mebbpya)CoII]2+ shows higher hydrogen evolution reaction activity and low selectivity for CO production. The molecular origin of the difference in product selectivity was analysed based on spectroscopic trapping of reactive intermediates and detailed kinetic and theoretical studies. A difference in mechanism is evident; whereas an efficient proton relay mediated by the -NH group initiates a two-electron reduction of CO2 in the case of [(Hbbpya)CoII]2+, one-electron chemistry prevails for [(Mebbpya)CoII]2+. Under stopped-flow conditions, we trapped the one-electron reduced CO2 radical anion in [(Mebbpya)CoI(CO2-•)], which forms oxalate under aprotic conditions. This study underlines the importance of subtle electronic and protonation changes in controlling the CO2RR product selectivity.