Influence of the Bis-Carbene Ligand on Manganese Catalysts for CO Electroreduction.

First row transition metal complexes have attracted attention as abundant and affordable electrocatalysts for CO reduction. Manganese complexes bearing bis-N-heterocyclic carbene ligands defining 6-membered ring metallacycles have proven to reduce CO to CO selectively at very high rates. Herein, we report the synthesis of manganese carbonyl complexes supported by a rigid ortho-phenylene bridged bis-N-heterocyclic carbene ligand (ortho-phenylene-bis(N-methylimidazol-2-ylidene), Ph-bis-mim), which defines a 7-membered ring metallacycle. We performed a comparative study with the analogues complexes bearing an ethylene-bis(N-methylimidazol-2-ylidene) ligand (CH-bis-mim) and a methylene-bis(N-methylimidazol-2-ylidene) ligand (CH-bis-mim), and found that catalysts comprising a seven-membered metallacycle retain similar selectivity and activity as those with six-membered metallacycles, while reducing the overpotential by 120-190 mV. Our findings reveal general design principles for manganese bis-N-heterocyclic carbene electrocatalysts, which can guide further designs of affordable, fast and low overpotential catalysts for CO electroreduction.