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 H-2 as by product, along with a turnover frequency of 1.2 X 10(4) s(-1) for CO production at an overpotential eta 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 X 10(4) s(-1) at eta = 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.