Highly Active Manganese-Based CO2 Reduction Catalysts with Bulky NHC Ligands: A Mechanistic Study

Because of the strong sigma-donor and weak pi-acceptor of the N-heterocyclic carbene (NHC), Mn-NHC complexes were found to be active for the reduction of CO2 to CO with high activity. However, some NHC-based manganese complexes showed low catalytic activity and required very negative potentials. We report herein that complex fac-[Mn-I(bis-(NHC)-N-mes)(CO)(3)B]r [1; bis-(NHC)-N-Mes = 3,3-bis(2,4,6-trimethylphenyl)-(1,1'-diimidazolin-2,2'-diylidene)methane] could catalyze the electrochemical reduction of CO2 to CO with high activity (TOFmax = 3180 +/- 6 s(-1)) at a less negative potential. Due to the introduction of the bulky Mes groups, a one-electron-reduced intermediate {[Mn-0(bis-(NHC)-N-Mes)(CO)(3)](0) (2(center dot))} was isolated as a packed dimer and crystallographically characterized. Stopped-flow Fourier-transform infrared spectroscopy was used to prove the direct reaction between doubly reduced intermediate fac-[Mn(bis-(NHC)-N-Mes)(CO)(3)](- )and CO2 ; the tetracarbonyl Mn complex [Mn+(bis-(NHC)-N-Mes)(CO)(4)](+) ([2-CO](+)) was captured, and its further reduction proposed as the rate-limiting step.