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 delta+-O delta- 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.