For the electrochemical CO2 reduction (CO2R), molecular catalysts in particular transition metal complexes show promise for the selective conversion of CO2 to single products with catalytic ability tunable through synthetic structure modifications. However, for traditional molecular catalysts, beneficial decreases in effective overpotentials for the CO2R are correlated with detrimental decreases in catalytic activity, which is referred to as “molecular scaling relationships”. Breaking this correlation between effective overpotential and catalytic activity is challenging, because shifting the catalysts' redox potentials more positive, which results in more positive catalytic onset and lower overpotentials, typically leads to a decrease in the metal site nucleophilicity, and in turn decreases the metal site's ability to coordinate and reduce CO2. In the last decade, several strategies have been developed to break “molecular scaling relationships”, primarily focusing on kinetic approaches of stabilizing CO2-adduct intermediates to decrease the activation energy through intramolecular electrostatics, H-bonding effects, or facilitated proton transport. In contrast, there are very few examples of breaking molecular scaling relationships using a thermodynamic approach of modulating the redox potential of metal sites. In our work, our strategy is to design molecular catalysts with redox-active ligands in which catalytic onset is preceded by the formation of a ligand radical in a ligand-based redox process rather than a metal-based redox event. In this scenario, the catalyst's redox potential and the metal site nucleophilicity are no longer necessarily correlated, and both kinetic and thermodynamic approaches can be implemented to break molecular scaling relationships. This talk will show that modulating redox properties and coordination geometry of molecular catalysts with redox-active ligands effectively achieve higher activity at lower overpotentials for the CO2R, presenting an inverse molecular scaling relationship.

Inverse molecular scaling relationships: Achieving high activity at low overpotentials for the electrochemical CO2 reduction by molecular catalysts

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