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

Yong Yang; Zhenyu Zhang; Xiaoyong Chang; Ya-Qiong Zhang; Rong-Zhen Liao; Lele Duan

doi:10.1021/acs.inorgchem.0c01364

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

Inorg Chem. 2020 Jul 20;59(14):10234-10242. doi: 10.1021/acs.inorgchem.0c01364. Epub 2020 Jun 25.

Authors

Yong Yang¹, Zhenyu Zhang¹, Xiaoyong Chang¹, Ya-Qiong Zhang², Rong-Zhen Liao², Lele Duan^{1

3}

Affiliations

¹ Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055, P. R. China.
² Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China.
³ Shenzhen Grubbs Institute and Guangdong Provincial Key Laboratory of Energy Materials for Electric Power, Southern University of Science and Technology (SUSTech), Shenzhen 518055, P. R. China.

PMID: 32585094
DOI: 10.1021/acs.inorgchem.0c01364

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-[Mn^I(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 (TOF_max = 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.