Computational Study of an Iron(II) Polypyridine Electrocatalyst for CO₂ Reduction: Key Roles for Intramolecular Interactions in CO₂ Binding and Proton Transfer

A solar-driven conversion of CO₂ into fuels by artificial photosynthesis would not only mitigate the greenhouse effect but also provide an alternative to obtain fuels in a renewable fashion. To this end, the new iron polypyridine catalyst [Fe(bpy^NHEtPY2Me)L₂]²⁺ (L = H₂O, CH₃CN) was recently developed for the electrochemical reduction of CO₂ to CO. In this study, we performed density functional theory (DFT) electronic structure calculations to shed light on a possible pathway for CO₂ reduction and the origin of the selectivity between CO₂ reduction versus the hydrogen evolution reaction. The metal center remains Lewis acidic throughout the reduction process due to ligand loss and mainly ligand-based reduction stabilized by antiferromagnetic coupling to a high-spin Fe(II) center. This results in a high barrier for hydride formation but a facile addition and activation of CO₂ via an η² coordination and stabilizing hydrogen bonding by the amine group. The second unoccupied equatorial coordination site opens up the possibility for an intramolecular protonation with a coordinated water ligand. This facilitates protonation because not only CO₂ but also the proton source H₂O is activated and properly aligned for a proton transfer due to the Fe-OH₂ bond; consequently, both protonation steps are facile. The moderate ligand field allows a rapid ligand exchange for a second intramolecular protonation step and facilitates an exergonic CO release. The lower selectivity of the related [Fe(bpy^OHPY2Me)L₂]²⁺ complex can be related to its more acidic second coordination sphere, which enables an intramolecular proton transfer that is kinetically competitive with CO₂ addition.