Theoretical Studies On Metal-Ligand Cooperation In Chemical Bond Activation And Activity Differences In Styrene Polymerization Catalyzed By Rare-Earth Metal Complexes

The research on chemical bond activation by using metal-ligand cooperation of organometallic complexes has drawn more and more attention during these years. The fundamental mechanism of metal-ligand interactions plays a significant role in developing new cooperative catalysis system. However, with the existing experimental method of measurement and analysis, the clear image of microscopic reaction mechanism and metal-ligand interactions can not be described. On the other hand, it is also difficult for experimental measurement to clarify the activity difference caused by different metal centers and ligands in olefin polymerization. Quantum chemistry theory was proven to be an effective tool for exploring the reaction mechanism of organometallic complexes at the molecular level. In this paper, the metal-ligand cooperation of N-H bond activation catalyzed by bis(imino)pyridine complex of aluminum and cycloisomerization of4-pentynoic acid catalyzed by SCS pincer palladium complex have been theoretically studied. At the same time, the activity difference in styrene polymerization of different rare-earth metal catalysts has also been well illustrated. The main results in this paper are described as follows:1. Bis(imino)pyridine complex of aluminum exhibits good activity in activation of the N-H bonds in aromatic amines. The mechanism of N-H bond activation was studied using quantum chemical theory, and the results indicated that the metal-ligand cooperation played a vital role in this catalytic process. Bis(imino)pyridine ligands indicated strong abilities to serve as electron reservoirs, which was called the redox noninnocent ligands. The second substrates as a good hydrogen transfer agent could facilitate the catalytic process and decreased the energy barrier of the rate-determining step to25kcal/mol.2. Tridentate SCS pincer palladium complex performs spectacular activities in catalyzing the cycloisomerization of4-pentynoic acid to liberate the alkylidene lactone, which makes it to be very few examples of cooperative catalysis in Pd complexes. The catalytic cycle has been well investigated by DFT calculation. SCS pincer palladium complex, one of the typical hemilabile ligands, expressed the metal-ligand cooperation by switching the coordination state with the metal center. The second substrates made up the electron loss of palladium in the rate-determining step by coordinating to the metal center and hydrogen bonding interactions, effectively reducing the energy barrier. When the ligand backbone is methylated, the frontier orbital of the ligand changed dramatically. Thus, it led to different catalytic activities of two catalysts. 3. Yttrium, scandium, and lutetium allyl complexes bearing a pyridyl-functionalized cyclopentadienyl ligand exhibited apparently different activities in syndiotactic polymerization of styrene. By investigating the mechanism and structures of insertion product, the computational results suggested that the kinetic factor was not the fundamental reason for activity differences but the coordination modes. The η7coordination mode will make the system into the resting state, reducing the coordinating possibility of subsequent monomer to the metal center. The chemical hardness of the metal atom determined the coordination mode. With lower value of chemical hardness, the easier the product will enter into the dormancy.