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 MnI, ReI, and RuII 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 endo configuration, allowing the participation of amines in the hydride formation, reactivity on the exo 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.本研究采用密度泛函理论（Density Functional Theory, DFT）方法，对带有悬挂胺基的Mn(I)、Re(I)及Ru(II)联吡啶配合物催化二氧化碳电还原的性能进行了评估。此前的实验研究表明，在催化剂的二级配位球中引入悬挂胺基，可使Mn基催化剂的产物选择性从CO转向HCOO⁻（在质子源存在的条件下）。与之相对，Re和Ru基催化剂的主要产物仍为CO。本研究对生成CO、HCOO⁻及H₂的反应路径开展了全面探究，以阐明支撑产物选择性的能量偏好因素。研究结果显示，更换金属中心会改变反应的优选路径。对于Mn基催化剂，反应更倾向于采取内型（endo）构型，使得胺基能够参与氢化物的形成过程；而Re基催化剂则更偏好外型（exo）构型的反应路径。此外，Re独特的氧化还原性质使其能够形成Re-OCOCO₂桥联加合物，该加合物可在无质子源的条件下生成CO。进一步而言，Ru能够交换两个Cl⁻阴离子，这一特性改变了Ru相较于Mn和Re的优选配位数，进而改变了其反应机制。总体而言，本研究为后续的催化剂设计提供了结构与反应活性方面的关键认知。