Electrochemical Properties and CO2-Reduction Ability of m-Terphenyl Isocyanide Supported Manganese Tricarbonyl Complexes

To circumvent complications with redox-active ligands commonly encountered in the study of manganese electrocatalysts for CO2 reduction, we have studied the electro-chemistry of the manganese mixed carbonyl/isocyanide complexes XMn(CO)(3)(CNArDipp2)(2) (X = counteranion), to evaluate the pairing effects of the counteranion and their influence over the potential necessary for metal-based reduction. The complexes described herein have been shown to act as functional analogues to the known homoleptic carbonyl manganese complexes[Mn-(CO)(5)] (n = 1-, 0, 1+). The m-terphenyl isocyanide ligand CNArDipp2 improves the kinetic stability of the resulting mixed carbonyl/isocyanide systems, such that conversion among all three oxidation states is easily effected by chemical reagents. Here, we have utilized an electrochemical study to fully understand the redox chemistry of this system and its ability to facilitate CO2 reduction and to provide comparison to known manganese-based CO2 electrocatalysts. Two complexes, BrMn(CO)(3)(CNArDipp2)(2) and [Mn(THF)(CO)(3)(CNArDiPP2)(2)]OTf, have been studied using infrared spectroelectrochemistry (IR-SEC) to spectroscopically characterize the redox states of these complexes during the course of electrochemical reactions. A striking difference in the necessary potential leading to the first one-electron reduction has been found for the halide and triflate species, respectively. Complete selectivity for the formation of CO and CO32- is observed in the reactivity of [Mn(CO)(3)(CNArDiPP2)(2)](-)with CO2, which is deduced via the trapping and incorporation of liberated CO into the zerovalent species Mn(CO)(3)(CNArDiPP2)(2) to form the dimers Mn-2(CO)(7)(CNArDiPP2)(3) and Mn-2(CO)(8)(CNArDiPP2)(2).