Accessing and Photo-Accelerating Low-Overpotential Pathways for CO2 Reduction: A Bis-Carbene Ruthenium Terpyridine Catalyst

A ruthenium catalyst bearing a bidentate bis(carbene) ligand is prepared and studied as a catalyst for CO2 electroreduction. The catalyst [Ru(tpy)(bis-mim)(MeCN)][PF6]2 (tpy) is 2,2 ',:6 ',2 ''-terpyridine; bis-mim is (methylenebis(N- methylimidazol-2-ylidene)) mediates reduction of CO2 into CO with a turnover frequency of 630 s-1 and Faradaic efficiency (FE) of 30% at an overpotential of 730 mV. The strongly donating bis(carbene) ligand also enables access to a pathway operating at a lower overpotential of ca. 310 mV. While low-overpotential catalysis is slow in the dark (TOF = 0.01 s-1), visible light illumination increases the rate 10-fold (TOF = 0.11 s-1). A full mechanistic picture is developed using kinetic analysis from cyclic voltammetry, spectroelectrochemistry, and computational methods, with the bis-mim ligand facilitating rapid CO2 activation at low overpotentials. Comparisons with other ruthenium catalysts yield insight into the ability to tune the rate of chemical steps (e.g., ligand dissociation and CO2 nucleophilic attack) and the overpotential by tailoring the primary coordination sphere while retaining the "redox-active" tpy ligand.