Enhancing a Molecular Electrocatalyst’s Activity for CO2 Reduction by Simultaneously Modulating Three Substituent Effects

The electrocatalytic activity for CO₂ reduction is greatly enhanced for Co complexes with pyridyldiimine-based ligands through the stepwise integration of three synergistic substituent effects: extended conjugation, electron-withdrawing ability, and intramolecular electrostatic effects. The stepwise incorporation of these effects into the catalyst structures results in a series of complexes that show an atypical inverse scaling relationship for CO₂ reduction—the maximum activity of the resulting catalysts increases as the onset potentials are driven positive due to the ligand electronic substituent effects. Incorporating all three effects simultaneously into the catalyst structure results in a Co complex [Co(PDI-PyCH₃⁺I^–)] with dramatically enhanced activity for CO₂ reduction, operating with over an order of magnitude higher activity (TOF_cat = 4.1 × 10⁴ s^–1) and ～0.2 V more positive catalytic onset (E_onset = ?1.52 V vs Fc^+/0) compared to the parent complex, an intrinsic activity parameter TOF₀ = 6.3 × 10^–3 s^–1, and >95% Faradaic efficiency for CO production in acetonitrile with 11 M water. This makes [Co(PDI-PyCH₃⁺I^–)] among the most active molecular catalysts reported for the CO₂ reduction reaction. Our work highlights a promising catalyst design strategy for molecular CO₂RR catalysts in which catalytic ability is enhanced by tuning three synergistic substituent effects simultaneously in a single catalyst structure.