A Mesoionic Carbene-Pyridine Bidentate Ligand Improves Stability in Electrocatalytic CO2 Reduction by a Molecular Manganese Catalyst

Abstract

Tricarbonyl group 7 complexes have a longstanding history as efficacious CO2 electroreduction catalysts. Typically, these complexes feature an auxiliary 2,2-bipyridine ligand which assists in redox steps by delocalizing electron density into the ligand-orbitals. While this feature lends to accessible redox potential for CO2 electroreduction, it also presents challenges for electrocatalysis with manganese, since the electron density is removed from metal-ligand bonding orbitals. The results presented here thus introduce mesoionic carbene (MIC) as a new potent ligand platform to promote Mn-based electrocatalysis. The strong σ-donation of the N,C-bidentate MIC is shown to help centralize electron density on the Mn-center while also maintaining relevant redox potentials for CO2 electroreduction. Mechanistic investigation supports catalytic turnover at two operative potentials separated by 400 mV. In the low overpotential regime, Mn(0) species catalyze CO2 to CO and CO32- with a maximum rate of 7 ± 5 s-1 and is stable for up to 30.7 h. At higher overpotential, “Mn(-1)” catalyzes CO2 to CO and H2O with faster turnovers of 200 ± 100 s-1 with the trade-off of being less stable at 6.7 h. The relative stability of Mn-complexes bearing MIC and 4-4’-diterbutyl-2,2’-bipyridine was compared by evaluating under the same electrolysis conditions, and therefore elucidated that the MIC promotes longevity for CO evolution through-out a 5 h period.