Highly Selective Molecular Catalysts for the CO₂-to-CO Electrochemical Conversion at Very Low Overpotential. Contrasting Fe vs Co Quaterpyridine Complexes upon Mechanistic Studies

[M^II(qpy)(H₂O)₂]²⁺ (M = Fe, Co; qpy: 2,2′:6′,2″:6″,‴-quaterpyridine) complexes efficiently catalyze the electrochemical CO₂-to-CO conversion in acetonitrile solution in the presence of weak Brönsted acids. Upon performing cyclic voltammetry studies, controlled-potential electrolysis, and spectroelectrochemistry (UV-visible and infrared) experiments together with DFT calculations, catalytic mechanisms were deciphered. Catalysis is characterized by high selectivity for CO production (selectivity >95%) in the presence of phenol as proton source. Overpotentials as low as 240 and 140 mV for the Fe and Co complexes, respectively, led to large CO production for several hours. In the former case, the one-electron-reduced species binds to CO₂, and CO evolution is observed after further reduction of the intermediate adduct. A deactivation pathway has been identified, which is due to the formation of a Fe⁰qpyCO species. With the Co catalyst, no such deactivation occurs, and the doubly reduced complex activates CO₂. High scan rate cyclic voltammetry allows reaching kinetic conditions, leading to scan-rate-independent plateau-shaped voltammograms from which catalytic rate constant was obtained. The molecular catalyst is very active for CO production (turnover a frequency of 3.3 × 10⁴ s^-1 at 0.3 V overpotential), as confirmed by catalytic a Tafel plot showing a comparison with previous catalysts.