N-heterocyclic carbenes as alternative platforms for electrocatalytic CO2 reduction by tricarbonyl Mn complexes

The electrochemical conversion of carbon dioxide is nowadays one of the most promising approaches for a sustainable production of renewable fuels and value-added chemicals. Among the different families of reported molecular catalysts for CO2 reduction, the fac-[Mn(CO)3(bpy–R)Br] complexes (bpy-R=2,2’-bipyridyl derivatives) are well known to efficiently convert CO2 to CO with excellent faradaic efficiencies, although further improvement in the field is mainly limited to structural modifications of the bipyridyl unit.[1-2] For instance, analogous mixed pyridyl/N-heterocyclic carbene (NHC) Mn counterparts recently showed considerably lower selectivity to CO and catalyst stability. Herein, we explore the electrocatalytic CO2 reduction properties of a series of novel tricarbonyl Mn complexes bearing the bidentate methylene-bis(N-methyl-imidazolium) ligand, [Mn(CO)3(bis-MeNHC)X]n (X=Br, n=0; X=CH3CN, n=1), comparing them to the reference [Mn(CO)3(py-MeNHC)I] catalyst (py-MeNHC=N-methyl-N’-2-pyridyl-imidazolium) under the same conditions. The replacement of a pyridine ring with a NHC unit is found to significantly affect the catalyst performances, improving substantially the kinetics and selectivity for catalytic CO production of the well-established C^N ligand-based Mn system. Complementary spectroelectrochemical (UV-Vis, FTIR) measurements and computational data give some hints about the role of the redox-innocent bis-NHC moiety on catalysis.