SBIC ECR Symposium & Poster Presentation 21st International Conference on Biological Inorganic Chemistry 2025

The transformation of CO₂ into valuable carbon-based fuels through reduction presents a sustainable approach to addressing global energy demands and reducing greenhouse gas emissions.^[1] Efficient catalysts are necessary that can selectively perform CO₂ reduction reactions (CO₂RR) to various C₁ and C₂ products over the kinetically and thermodynamically competitive hydrogen evolution reaction (HER).[^2] Despite advances in the development of molecular catalysts with highly specialized ligand scaffolds[^3], spectroscopic trapping of the key reactive intermediates are lacking in many cases,  which makes the mechanism ambiguous.

In this study, 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 and low selectivity for CO production. Furthermore, [(Hbbpya)Co]²⁺ maintains its exceptional reactivity and selectivity for CO production in aqueous solutions achieving a record high turnover number (TON) of ~6 × 10⁴. The molecular origin of the difference in product selectivity has been analysed based on spectroscopic trapping of reactive intermediates and detailed kinetic and theoretical studies. A change in mechanism is evident; whereas, an efficient proton relay mediated by the -NH group initiates a two-electron reduction of CO₂ in case of [(Hbbpya)Co^II]²⁺, a one-electron chemistry prevails for [(Mebbpya)Co^II]²⁺. We show in this study the concurrent effect of both the ligand non-innocence and proton shuttles in controlling two- vs one-electron reduction of CO₂. Most importantly, in the case of [(Mebbpya)Co^II]²⁺, we have trapped the one-electron reduced CO₂ radical anion in [(Mebbpya)Co^I(CO₂^-•)] under stopped-flow conditions, which forms oxalate under aprotic conditions. This study underlines the importance of subtle electronic and protonation changes in controlling the CO₂RR product selectivity.