Computational Understanding of Catalyst Design: Palladium Pincer Complexes for Carbon Dioxide Hydrogenation

Experimental catalyst design to promote the hydrogenation of CO₂ has led to the synthesis of an efficient tripodal palladium pincer complex. This work presents a computational reading key of the ligand design for efficient transition metal catalysts using palladium pincer complexes as precise representatives. By investigating the electronic structure/reactivity relationship, we demonstrate that all the progressive adjustments of the pincer architecture do not significantly affect the Pd-L (L = PCy₃, PPh₃) bond and the electron density rearrangement in the first coordination sphere of the complex. We find a strong Pd-L bond, where the dispersion energy contribution is substantial, that rules out the previously proposed mechanism involving Pd-L bond breaking. Mechanisms of H₂ activation and CO₂ reduction are revisited, which showcase how the secondary coordination sphere plays a critical role in reactivity, strongly affecting the Pd-O (carboxylic arm) bond polarization due to hydrogen bond formation with the O atom. The catalytically active complex is demonstrated to perform through a stepwise mechanism involving polar addition of H₂ across the Pd^δ+-O^δ- bond and subsequent OH deprotonation by an external base (DBU), forming a Pd-H hydride complex, which easily reacts with CO₂ through an outer-sphere hydride transfer path.