Quantum mechanics/molecular mechanics investigation of the ethene polymerization mechanism catalyzed by a bulky diimine-Ni(II) complex

Abstract

Neste trabalho descrevemos um estudo teórico sobre a reação de polimerização do eteno catalisada por um complexo catiônico volumoso do tipo Ni(II)-a-diimina. Empregamos a combinação da teoria do funcional de densidade (DFT) e mecânica molecular (MM), contida na aproximação ONIOM, para avaliar as estruturas e energias envolvidas na polimerização do eteno, usando o sistema catalítico de Brookhart. Todas as estruturas intermediárias e de transição ao longo destes passos elementares foram tratadas como representativas do processo de polimerização. Discutimos a conformação do ligante em torno do sítio ativo ao longo das etapas da reação de polimerização, o ângulo de coordenação da olefina, complexo-p, e a formação de ramificações na cadeia polimérica em crescimento durante o processo de polimerização. In this work we describe a theoretical investigation of the ethene polymerization reaction catalyzed by a bulky cationic a-diimine Ni(II) complex. We employed the combination of density functional theory (DFT) and molecular mechanics (MM), within the ONIOM approach, to evaluate the structures and energies involved on the most representative reactions observed on ethene polymerization, using the Brookhart catalytic system. All intermediates and transition state structures along these elementary steps were treated as representatives of the polymerization process. We discuss the conformation of the ligands around the active site along the polymerization reaction steps, the coordination angle of the coordinated olefin, p-complexes, and branch formation of the growing chain during the polymerization process.

Ethylene polymerization catalyzed by a cyclophane-diimine-based Ni(II) complex, a quantum/molecular mechanic study

“…For comparison, the border line adopted between the QM and MM treatments is the same as that used in previews works [18], where, for all practical purposes, the QM region corresponds to the polymerization reaction center. This choice allowed us to consider the minimum number of atoms with QM treatment, without compromising the energies, and the geometry of the molecular structures evaluated [13].…”

“…Also, all energies obtained included the Zero Point Energy (ZPE) correction. It worth to remark that this level of calculation has been adopted by other groups [32,33], as well in a former work of our group [18,34], with good quality of geometries, and energies of the structures obtained. To understand the solvent effect on olefin polymerization by Ni-cyclophane complex was accomplished single point calculations on molecular structures 1, 2, 3 and 4 (optimized by methodology above described) employing the IEFPCM model (Integral-Equation-Formalism Polarizable Continuum Model) containing toluene as solvent.…”

“…On this preview work, we carried out systematic evaluation of the structural arrangement of all structures (intermediates and transition states), giving particular interest in the olefin coordination angles on -complexes, and agostic interactions during the growth of the polymer chain. Here it is important to remark that the -complexes display large olefin coordination distortion angles, with significant participation of the d 2 z orbital in the olefin-metal bond [18].…”

“…Most of these works compare generic substituted and non-substituted ␣-diimine ligands to evaluate the energies involved in the main steps of the polymerization process [17]. Recently, we published a theoretical study in which we treated all of the ground, intermediate and transition state structures of a series of elementary steps as representative structures of the polymerization process for a typical Brookhart bulky ␣-diimine catalyst system [18]. On this preview work, we carried out systematic evaluation of the structural arrangement of all structures (intermediates and transition states), giving particular interest in the olefin coordination angles on -complexes, and agostic interactions during the growth of the polymer chain.…”

“…Furthermore, because of their structural features, cyclophane ligands can improve the thermal stability of the catalytic complex. However, despite their promise, the use and mechanistic implications of cyclophane transition-metal complexes in olefin polymerization remains mostly unexplored [22] Therefore, we report a chemical computational study of the elementary steps of an ethene polymerization reaction performed with an m-terphenyl-based cyclophane Ni(II) catalyst, and compared with the counterpart, bearing the conventional bulky ␣-diimine ligand, bis[N-(2,6-diisopropylphenyl)imino]acenaphthene [18].…”