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

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 activate the complex (CN and CF₃), destabilized the catalyst. An added proton source both increased the activity and decreased the overpotential by 200 mV for all catalyst derivatives, shifting the catalytic mechanism from an electron-first pathway to a proton-first pathway. We used binding energies calculated via density functional theory to help understand the substituent effect on the catalyst affinity for CO₂ and other intermediates relevant to the catalytic mechanism. Catalyst activity was quantified using intrinsic rate constants determined through the utilization of catalytic plateau currents, as well as the application of a foot of the wave analysis, which yielded incongruent values. Of those complexes tested, Re(4,4′-tBu-bpy)(CO)₃Cl with an added 1 M phenol yielded the most active catalytic system (k_cat = 6206 s^-1) at an overpotential of 0.67 V.