NHC-containing manganese(I) electrocatalysts for the two-electron reduction of CO₂

The synthesis and characterization of the first catalytic manganese N-heterocyclic carbene complexes are reported: MnBr(N-methyl-N'-2- pyridylbenzimidazol-2-ylidine)(CO)₃ and MnBr(N-methyl-N'-2- pyridylimidazol-2-ylidine)(CO)₃. Both new species mediate the reduction of CO₂ to CO following two-electron reduction of the Mn^I center, as observed with preparative scale electrolysis and verified with ¹³CO₂. The two-electron reduction of these species occurs at a single potential, rather than in two sequential steps separated by hundreds of millivolts, as is the case for previously reported MnBr(2,2'-bipyridine)(CO)₃. Catalytic current enhancement is observed at voltages similar to MnBr(2,2'-bipyridine)(CO)₃. Catalytic manganese N-heterocyclic carbene complexes have been synthesized and characterized: [MnBrL(CO)₃] (see scheme; L=N-methyl-N'-2- pyridylbenzimidazol-2-ylidine or N-methyl-N'-2-pyridylimidazol-2-ylidine). Both species mediate the reduction of CO₂ to CO following two-electron reduction of the Mn^I center at a single potential, as observed with preparative scale electrolysis and verified with ¹³CO₂.