Role of Bimetallic Interactions in the Enhancement of Catalytic CO2 Reduction by a Macrocyclic Cobalt Catalyst

The use of two metal centers in a CO2 reduction catalyst that work together synergistically, with similar or complementary functions, can potentially lead to a significant reduction in overpotential, enhance catalytic activity and/or selectivity, and/or enable access to cascade strategies where each metal center catalyzes a different step in the conversion of CO2 to a fuel. Here, the bimetallic reactivity of two metal centers has been identified as the primary route for the reduction of CO2 to CO promoted by the macrocycle, [Co(HMD)](2+) (HMD = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene), based on the experimental characterization of all major steps of the proposed catalytic cycle using pulse radiolysis time-resolved IR (PR-TRIR) spectroscopy, corroborated by density functional theory (DFT) calculations and IR spectroelectrochemistry (IR-SEC). A bimetallic intermediate is formed in situ from two singly reduced [Co(HMD)](+) species bridged by a CO2 molecule, and the presence of a coordinating species, e.g., formate anion, appears to assist in the formation of such an intermediate. It has been demonstrated that this reactivity enables access to elementary steps with lower energy requirements, resulting in overall catalysis being kinetically more facile compared to the mononuclear pathway. A two-step approach that combines chemical reduction followed by PR-TRIR has been successfully used for probing the structure and reactivity of reactive intermediates involved in the advanced stages of a catalytic cycle, which are rarely interrogated using experimental techniques.