A series of twelve [fac-Mn(R₂bpy)(CO)₃(CH₃CN)]⁺ pre-catalysts with systematically varied second coordination sphere functionality for the proton-coupled electrocatalytic reduction of CO₂ are reported, whereby a structure-determined shift in catalytic pathway is demonstrated and product selectivity is tuned from CO to competing HCO₂H and H₂ production.

Abstract

A series of twelve second coordination sphere (SCS) functionalized manganese tricarbonyl bipyridyl complexes are investigated for their electrocatalytic CO₂ reduction properties in acetonitrile. A qualitative and quantitative assessment of the SCS functional groups is discussed with respect to the catalysts’ thermodynamic and kinetic efficiencies, and their product selectivities. In probing a broad scope of functional groups, it is clear that only the aprotic ortho-arylester SCS is capable of promoting the highly desired low-overpotential proton-transfer electron-transfer (PT-ET) pathway for selective CO production. The ortho-phenolic analogues cause an increase in overpotential with a product selectivity favoring H₂ evolution, consistent with a high-overpotential pathway via the anionic [Mn−H]⁻ intermediate. Alternative aprotic Lewis base functional groups such as trifluoromethyl, morpholine and acetamide are shown to also be capable of intermediate manganese hydride generation. The tertiary amine substituent, 2-morpholinophenyl, exhibits a desirable product distribution characteristic of syn-gas (CO : H₂=30 : 48) with an impressive turnover frequency, while the secondary amine group, 2-acetamidophenyl, induces a notable shift in selectivity with a faradaic yield of 55 % for the formate (HCO₂ ⁻) product. In addition to their catalytic properties, cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC) studies are presented to probe pre-catalyst electronic properties and the two-electron reduction activation pathway.