Molecular Electrochemical Catalysis of the CO₂-to-CO Conversion with a Co Complex: A Cyclic Voltammetry Mechanistic Investigation

The electrochemical catalytic reduction of CO₂ into CO could be achieved with excellent selectivity and rate in acetonitrile in the presence of phenol with cobalt 2,2′:6′,2″:6″,2′″-quaterpyridine complex [Co^II(qpy)(H₂O)₂]²⁺ (Co) acting as a molecular catalyst. Upon using cyclic voltammetry at low and high scan rate (up to 500 V/s) two catalytic pathways have been identified. At a low concentration of phenol (<1 M), catalysis mainly occurs after the reduction of Co with three electrons. In that case, the selectivity for CO production is ca. 80% with 20% of H₂ as by product, along with a turnover frequency of 1.2 × 10⁴ s^-1 for CO production at an overpotential η of ca. 0.6 V. The triply reduced active species binds to CO₂ and the C-O bond is cleaved thanks to the acid. At very large concentration of phenol (3 M), another pathway becomes predominant: the doubly reduced species binds to CO₂, while its reductive protonation leads to CO formation. As already shown, this later process is endowed with fast rate at low overpotential (turnover frequency of 3 × 10⁴ s^-1 at η = 0.3 V) and 95% selectivity for CO production. By varying the phenol concentration and the scan rate in voltammetry experiments, it was thus possible to identify, activate, and characterize several pathways for the CO₂-to-CO conversion and to decipher Co electrochemical reactivity toward CO₂.