Electrocatalytic Reduction of Carbon Dioxide by Mn(CN)(2,2'-bipyridine)(CO)3: CN Coordination Alters Mechanism.

MnBr(2,2'-bipyridine)(CO)3 is an efficient and selective electrocatalyst for the conversion of CO2 to CO. Herein, substitution of the axial bromide for a pseudohalogen (CN) is investigated, yielding Mn(CN)(2,2'-bipyridine)(CO)3. This replacement shifts the first and second reductions to more negative potentials (-1.94 and -2.51 V vs Fc/Fc(+), respectively), but imparts quasi-reversibility at the first feature. The two-electron, two-proton reduction of CO2 to CO and H2O is observed at the potential of the first reduction. Data from IR spectroelectrochemistry, cyclic voltammetry, and controlled potential electrolysis indicate that this behavior arises from the disproportionation of two one-electron-reduced species to generate the catalytically active species. Computations using density functional theory are also presented in support of this new mechanism.