Selectivity Switching by Ligand Coordination Sites─The Key to Promote the CO2/C2H4 Coupling Reaction over the Ru-Based Catalyst

The mechanism of the CO2/C2H4-coupling reaction catalyzed by Ru/dmpe (dmpe = PMe2CH2CH2PMe2) and Ru/PP3 (PP3 = P(CH2CH2PMe2)3) catalysts has been revealed using density functional theory method. Three possible pathways for the catalytic conversion of CO2/C2H4 were proposed, including the formation of acrylic acid, the insertion reaction of ruthenalactone, and the base-assisted formation of acrylate. The resting state was studied by considering the two possible spin states (singlet and triplet) through a principal interacting orbital analysis to anticipate potential competition between low-lying spin states. The higher energy of the triplet compared with the singlet state is due to the interplay between orbital interactions and the coordination mode. Then, the differences in the catalytic mechanism between diphosphine ligands and tetradentate phosphine ligands have been revealed. In the Ru/dmpe system, the size of ruthenalactone can be influenced by the addition of ethylene, resulting in the formation of a homologous series of unsaturated Ru carboxylate products. However, additional ligands cannot bind to the transition metal because the tetra-coordinated PP3-ligated ruthenalactone is saturated with 18 electrons. Meanwhile, the release of the ligand site by dissociation of the Ru–P bond turned out to be infeasible, because it was a high-energy step. As another possible pathway for catalytic synthesis in the Ru/PP3 system, base-promoted β-H abstraction to produce acrylate salts is found to occur readily. In contrast, the electron-deficient Ru/dmpe system is unlikely to produce acrylate salts due to the methoxide coordinated complexes being too stable. Subsequently, potential enhancements to the Ru-catalyzed acrylate salt formation reaction were identified through an extensive screening of ligands and methoxides. Overall, the coordination sites of the phosphine ligand switch the selectivity of the reaction by influencing the electronic arrangement of the transition metal valence orbitals. The coordination sites and electronic properties of the ligand are important descriptors in determining the fate of the CO2/C2H4 coupling, which provides a valuable perspective for future catalyst design.