The incorporation of a second coordination sphere into the ligand scaffold of an organometallic catalyst plays an important role in improving catalytic efficiency and enhancing product selectivity. Herein, we report a zinc porphyrin complex with multiple flexible triazole units comprising the second coordination sphere, as an electrocatalyst for the highly selective electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO). This electrocatalyst converted CO2 to CO with a Faradaic efficiency of 99% and a current density of –6.2 mA/cm2 at –1.9 V vs Ag/AgCl in N,N-dimethylformamide using water as the proton source. Long-term bulk electrolysis at –1.6 V vs Ag/AgCl produces carbon monoxide at a current density of –2.0 mA/cm2 and Faradaic efficiency of 93% for 10 h. 1H NMR and UV-visible absorption spectroscopy of the solution after electrolysis confirmed the stability of the catalyst and the absence of liquid product. Control compounds lacking triazole groups or a zinc center were not electrocatalytic, establishing the importance of the second coordination sphere in catalysis and the redox-innocent metal center as the binding site. Structure-function relationship studies were carried out on the catalyst’s analogs containing different numbers of triazole units and distinct triazole geometries; these unveiled that the triazole units function cooperatively to stabilize the CO2-catalyst adduct in order to facilitate intramolecular proton transfer. A catalytic mechanism was proposed based on our experimental observations. Our findings and mechanistic interpretations demonstrate that incorporating a flexible second coordination sphere in the ligand is a versatile strategy to enhance the activity of electrocatalytic CO2 conversion.

Enhanced electrocatalytic activity of a zinc porphyrin for CO2 reduction: Effects of a flexible second coordination sphere

ACS Spring 2022

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