Kinetic and Mechanistic Effects of Bipyridine (bpy) Substituent, Labile Ligand, and Brønsted Acid on Electrocatalytic CO2 Reduction by Re(bpy) Complexes

Abstract

In order to help develop robust and deployable molecular electrocatalysts for the reduction of CO2 to CO, we must understand the effects of tuning their structure and catalytic conditions. To this end, we quantify how modifications to the catalyst fac-Re(4,4′-R-bpy)(CO)3X (bpy = 2,2′-bipyridine, R = OCH3, CH3, tBu, H, CN, CF3; X = Cl, Br, py(OTf), or CH3CN(OTf)) with and without an added proton source (phenol, acetic acid, 2,2,2-trifluoroethanol) affect the catalyst stability, activity, and overpotential. Through cyclic voltammetry experiments, we found that the substituents and proton source had a large effect on both overpotential and activity. Substituents with moderate electron-donating ability (tBu and CH3) increased activity and overpotential in comparison to the unsubstituted complex Re(bpy)(CO)3Cl. In contrast, substituents resulting in too much electron density distributed over the bpy ligand, either from too-strong electron-donating ability (OCH3) or from the requirement of a third reduction to ...