The electrochemical conversion CO2 presents an intuitive strategy of mitigating greenhouse gas emissions while generating useful organic feedstocks. However, designing efficient molecular catalysts for selective reduction of CO2 remains a challenge. Our previous study has shown that cobalt complexes with pyridyldiimine-based ligands [Co(PDI-R)] can actively reduce CO2 to CO in the MeCN solution with 11.0 M H2O as a proton source. The incorporation of a cationic N-methylpyridinium group into the [Co(PDI)] structure results in the [Co(PDI-PyCH3+I−)] complex which operates with a 0.18 V more positive onset potential and nearly 4 orders of magnitude higher activity for the CO2RR compared to the parent [Co(PDI)] and &gt; 95% FE for CO. We hypothesize that the dramatic improvement in catalytic ability of [Co(PDI-PyCH3+I−)] is due to the intramolecular electrostatic effect introduced by the positively-charged moiety, which can stabilize the reduced CO2-intermediate, increasing catalytic activity and decreasing the effective overpotential. To further analyze the effect of intramolecular electrostatic effects on the activity of molecular catalysts, we prepared a series of analogous complexes each bearing cationic moieties at varying distances from the catalytic active site. We show that increasing the distance between the cationic moiety and active site results in systems that show higher catalytic activity compared to the parent complex [Co(PDI)], but lower catalytic currents and an unfavorable negative shifts in onset potential compared to the [Co(PDI-PyCH3+I−)] complex. We attribute the dependence of catalytic activity on the distance between the cationic moiety and the Co center to the distance-dependent through-space Coloumbic stabilization of reduced CO2-intermediates at the Co center. Overall, our work further highlights the through-space electrostatic effect as a strategy of tuning catalytic activity through ligand modification.

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

ACS Spring Meeting 2021

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