Dual roles of the electronic effect on selectivity: pincer nickel-electrocatalyzed CO2 reduction

Abstract

The electronic effect is crucial for the electrocatalytic reduction of CO₂. In contrast to the previous understanding of the monotonic influence of the electronic effect on the selectivity of CO₂ reduction, the dual roles of the electronic effect on the selectivity are revealed in the present study, i.e., (1) the electronic effect on redox originating from σ-donation and (2) the electronic effect on π-back-donation, via comprehensive DFT studies on four representative classes of pincer NHC Ni^II catalysts. On the one hand, the electron-rich C, B-coordinating (CCC and CBC) ligands guarantee that the catalysts possess the driving force to reduce CO₂ in a lower-electron reduction state (Ni^I), leading to lower free energy barriers for the formation of HCOOH, which results from the lower ligand-field deformation energies for the hydride transfer and the stronger p–σ* interactions in the metal–hydride intermediates. In contrast, the less electron-rich N-coordinating (CNC and C(B)NC) ligands require an Ni⁰ electron reduction state to reduce CO₂, preferring kinetic-controlled CO formation due to the higher free energy barriers for the generation of HCOOH. This redox effect well explains the unprecedented experimentally observed selectivity of HCOOH in the stronger electron donor CCC-Ni system, which is different from the traditional understanding of the electronic effect. On the other hand, at an identical reduction state, the electronic effect plays an important role in tuning the back-donation ability of the metal center, benefiting the π-back-donation in the metal–carbonyl intermediates, and thus favors the formation of CO. This back-donation effect is consistent with the traditional understanding of selectivity. This work provides comprehensive insights into the dual role of the electronic effect on the selectivity for CO₂ reduction, which can be instructive for the future design and development of catalysts.