Session: Electrochemistry and Electrocatalysis:

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 > 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.

Charles McCrory

University of Michigan

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