Effects of Protonation State on Electrocatalytic CO Reduction by a Cobalt Aminopyridine Macrocyclic Complex.

A critical component in the reduction of CO to CO and HO is the delivery of 2 equiv of protons and electrons to the CO molecule. The timing and sequencing of these proton and electron transfer steps are essential factors in directing the activity and selectivity for catalytic CO reduction. In previous studies, we have reported a series of macrocyclic aminopyridine cobalt complexes capable of reducing CO to CO with high faradaic efficiencies. Kinetic investigations reveal a relationship between the observed rate constant () 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 complexes toward CO reduction. Two deprotonated derivatives, and , of the tetraamino macrocycle were independently synthesized and structurally characterized revealing extensive delocalization of the negative charge upon deprotonation. H nuclear magnetic resonance spectroscopy and ultraviolet-visible titration studies confirm that under catalytic conditions, the active form of the catalyst gradually becomes deprotonated, supporting thus the - 1 relationship with . Electrochemical studies of reveal that this deprotonated analogue is competent for electrocatalysis upon addition of an exogenous weak acid source, such as 2,2,2-trifluoroethanol, resulting in faradaic efficiencies for CO-to-CO conversion identical to those observed with the fully protonated derivative (>98%).