A Mesoionic Carbene-Pyridine Bidentate Ligand That Improves Stability in Electrocatalytic CO 2 Reduction by a Molecular Manganese Catalyst.

Tricarbonyl Group 7 complexes have a longstanding history as efficacious CO 2 electroreduction catalysts. Typically, these complexes feature an auxiliary 2,2'-bipyridine ligand that assists in redox steps by delocalizing the electron density into the ligand orbitals. While this feature lends to an accessible redox potential for CO 2 electroreduction, it also presents challenges for electrocatalysis with Mn because the electron density is removed from metal-ligand bonding orbitals. The results presented here thus introduce a mesoionic carbene (MIC) as a potent ligand platform to promote Mn-based electrocatalysis. The strong σ donation of the N,C-bidentate MIC is shown to help centralize the electron density on the Mn center while also maintaining relevant redox potentials for CO 2 electroreduction. Mechanistic investigation supports catalytic turnover at two operative potentials separated by 400 mV. In the low operating potential regime at -1.54 V, Mn(0) species catalyze CO 2 to CO and CO 3 2- , which has a maximum rate of 7 ± 5 s -1 and is stable for up to 30.7 h. At higher operating potential at -1.94 V, "Mn(-1)" catalyzes CO 2 to CO and H 2 O with faster turnovers of 200 ± 100 s -1 , with the trade-off being less stability at 6.7 h. The relative stabilities of Mn complexes bearing MIC and 4,4'-di- tert -butyl-2,2'-bipyridine were compared by evaluation under the same electrolysis conditions and therefore elucidated that the MIC promotes longevity for CO evolution throughout a 5 h period.