Metal-Dependent\nMechanism of the Electrocatalytic\nReduction of CO<sub>2</sub> by Bipyridine Complexes Bearing Pendant\nAmines: A DFT Study

In this study, the electrocatalytic reduction of carbon\ndioxide\nby Mn<sup>I</sup>, Re<sup>I</sup>, and Ru<sup>II</sup> bipyridine\ncomplexes bearing pendant amines is evaluated by DFT methods. Prior\nexperimental studies showed that introducing pendant amines in the\nsecondary coordination sphere of the catalyst shifts product selectivity\nfrom CO to HCOO<sup>–</sup> (in the presence of a proton source)\nin the case of Mn. In contrast, CO is the major product with Re and\nRu. This work includes a comprehensive study of the pathways leading\nto CO, HCOO<sup>–</sup>, and H<sub>2</sub> to elucidate the\nenergetic preferences that underlie product selectivity. Our results\nshow that switching the metal center leads to changes in the preferred\nmechanism. While with Mn, the reaction is preferred in an <i>endo</i> configuration, allowing the participation of amines\nin the hydride formation, reactivity on the <i>exo</i> configuration\nis preferred with Re. In addition, the distinct redox properties of\nRe allow for the formation of Re OCOCO<sub>2</sub>-bridged adducts\nthat lead to CO without a proton source. Further, the ability of Ru\nto exchange the two Cl<sup>–</sup> anions changes the preferred\ncoordination number of Ru compared to Mn and Re and, consequently,\nits reaction mechanism. Overall, this study provides the structure\nand reactivity insight needed for further catalyst design.