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)Co^II]²⁺ and [(Mebbpya)Co^II]²⁺, which contain two 2,2'-bipyridine chelating groups linked by -NH or -NCH₃ moieties, respectively. Whereas [(Hbbpya)Co^II]²⁺ forms CO under electrocatalytic conditions in presence of phenol (PhOH) with high selectivity, [(Mebbpya)Co^II]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)Co^II]²⁺, one-electron chemistry prevails for [(Mebbpya)Co^II]²⁺. Under stopped-flow conditions, we trapped the one-electron reduced CO₂ radical anion in [(Mebbpya)Co^I(CO₂ ^-•)], which forms oxalate under aprotic conditions. This study underlines the importance of subtle electronic and protonation changes in controlling the CO₂RR product selectivity.