The electrochemical CO2 reduction reaction (CO2RR) is a promising strategy for the mitigation of greenhouse gas emissions in which CO2 is converted to useful organic feedstocks using renewable energy sources. In recent years, significant effort has been made in designing efficient molecular catalysts for the selectively reducing CO2 to single products. Previous studies have shown that cobalt complexes with pyridyldiimine-based ligands [Co(PDI-R)] can actively reduce CO2 to CO in MeCN solution with 11.0 M H2O as a proton source. In our recently published work, we demonstrated how the incorporation of a cationic N-methylpyridinium group into the [Co(PDI)] framework results in the [Co(PDI-PyCH3+I−)] complex which shows ~ 4 orders of magnitude higher activity at 0.18 V more positive onset potential for the CO2RR compared to the parent complex [Co(PDI)], with &gt; 95% FE for CO. We believe the dramatic improvement in catalytic ability of [Co(PDI-PyCH3+I−)] may be attributed to the intramolecular electrostatic effect introduced by the positively-charged moiety, which can increase catalytic activity and decrease the effective overpotential by stabilizing reduced CO2-intermediates. To better ascertain the influence of the intramolecular electrostatic effect on catalytic activity, we prepared a series of analogous complexes bearing cationic moieties at varying distances from the catalytic active site. Complexes with increasing distance between the cationic moiety and active site still show higher catalytic activity compared to the parent complex [Co(PDI)], yet lower catalytic current and unfavorable negative shifts in onset potential compared to [Co(PDI-PyCH3+I−)]. We attribute the dependence of catalytic activity on the distance between the cationic moiety and the Co center to the distance-dependent, through-space Coulombic stabilization of reduced CO2-intermediates at the Co center. Overall, our work further highlights the through-space electrostatic effect as one method of tuning catalytic activity through ligand modification.

Electrocatalytic CO2 reduction by cobalt pyridyldiimine complexes featuring cationic substituents: Effect of distance of the charged feature from the metal center on the electrocatalytic activity for CO2 reduction

ACS Fall 2021

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