Highly Active Manganese-Based CO<sub>2</sub> Reduction Catalysts with Bulky NHC Ligands: A Mechanistic Study
Yong Yang, Zhenyu Zhang, Xiaoyong Chang, Yaqiong Zhang, Rong‐Zhen Liao, Lele Duan
Abstract
Because of the strong σ-donor and weak π-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-[MnI(bis-MesNHC)(CO)3Br] [1; bis-MesNHC = 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 {[Mn0(bis-MesNHC)(CO)3]0 (2•)} 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-MesNHC)(CO)3]− and CO2; the tetracarbonyl Mn complex [Mn+(bis-MesNHC)(CO)4]+ ([2-CO]+) was captured, and its further reduction proposed as the rate-limiting step.