Metal-Dependent Mechanism of the Electrocatalytic Reduction of CO₂ by Bipyridine Complexes Bearing Pendant Amines: A DFT Study

In this study, the electrocatalytic reduction of carbon dioxide by Mn^I, Re^I, and Ru^II bipyridine complexes bearing pendant amines is evaluated by DFT methods. Prior experimental studies showed that introducing pendant amines in the secondary coordination sphere of the catalyst shifts product selectivity from CO to HCOO^- (in the presence of a proton source) in the case of Mn. In contrast, CO is the major product with Re and Ru. This work includes a comprehensive study of the pathways leading to CO, HCOO^-, and H₂ to elucidate the energetic preferences that underlie product selectivity. Our results show that switching the metal center leads to changes in the preferred mechanism. While with Mn, the reaction is preferred in an <i>endo</i> configuration, allowing the participation of amines in the hydride formation, reactivity on the <i>exo</i> configuration is preferred with Re. In addition, the distinct redox properties of Re allow for the formation of Re OCOCO₂-bridged adducts that lead to CO without a proton source. Further, the ability of Ru to exchange the two Cl^- anions changes the preferred coordination number of Ru compared to Mn and Re and, consequently, its reaction mechanism. Overall, this study provides the structure and reactivity insight needed for further catalyst design.