A critical component in the reduction of CO2 to CO and H2O is the delivery of two equivalents of protons and electrons to the CO2 molecule. The timing and sequencing of these proton and electrons transfer steps are essential factors in directing the activity and selectivity for catalytic CO2 reduction. In previous studies we have reported a series of macrocyclic aminopyridine cobalt complexes capable of reducing CO2-to-CO with high faradaic efficiencies. Kinetic investigations reveal a relationship between the observed rate constant (kobs) and the number of pendant amine hydrogen bond donors minus one, suggesting the presence of a deprotonated active catalytic state. Herein we investigate the feasibility of these proposed deprotonated states towards CO2 reduction. Two deprotonated derivatives, Co(L4–) and Co(L2–), of the parent tetraamino macrocycle Co(L) were independently synthesized and structurally characterized revealing extensive delocalization of negative charge across the macrocycle upon deprotonation. Controlled potential electrolysis studies of Co(L4–) reveal that the complex is competent for catalytic CO2 to CO conversion upon addition of an exogenous weak acid source (2,2,2-trifluoroethanol), with Faradaic efficiencies identical to those observed for Co(L) (98%). Spectroscopic analysis of the post-catalysis solution reveals an identical deprotonated state for Co(L) and Co(L4–), confirming the role of a deprotonated active catalyst.

Effects of protonation state on electrocatalytic CO2 reduction by a cobalt aminopyridine macrocyclic complex

ACS Spring Meeting 2021

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