A Combined Density Functional Theory and Molecular Mechanics (QM/MM) Study of Single-Site Ethylene Polymerization Catalyzed by [(C₆H₅NCH)C₄H₃N]₂-RM⁺ {M = Ti, Zr} in the Presence of the Counterion CH₃B(C₆F₅)₃^-

Calculations have been carried out to investigate the insertion of the ethylene monomer into the M−methyl bond for the systems [(C 6 H 5 N CH)C 4 H 3 N] 2 -CH 3 M-μ-CH 3 -B(C 6 F 5 ) 3 (M = Ti and Zr), using density functional theory. Second insertion studies have also been conducted for the [(C 6 H 5 N CH)C 4 H 3 N] 2 -C 3 H 7 Ti-μ-CH 3 -B(C 6 F 5 ) 3 system. A validated QM/MM model was used to represent the counterion. The C 6 H 5 groups in the ligands were modeled with QM/MM, with hydrogens being used as the capping atoms. With R = Me (the initiation step), approach of the ethylene cis to the μ-Me bridge (cis approach) and from between the pyrrolide imine ligands (trans approach) were considered, along a path leading first to uptake of ethylene to form a π-complex followed by insertion of ethylene into the M−R bond. For the titanium-based system, the uptake was found to be rate determining for the cis approach, the total barrier being 12.9 kcal/mol, and the insertion barrier was found to be rate determining for the trans approach, the total barrier being 13.8 kcal/mol. For the zirconium-based system, the insertion barriers was rate determining for both cis and trans approaches, the total barriers being 9.4 and 10.6 kcal/mol, respectively. The propagation step was then studied for the contact ion-pair [(C 6 H 5 N CH)C 4 H 3 N] 2 -C 3 H 7 Ti-μ-CH 3 -B(C 6 F 5 ) 3 ( 4 ). Different conformations of the propyl chain in 4 were considered by altering θ, the dihedral angle formed between the C β −C α −Ti and the C α −Ti−μC planes. The resting states were found to be at θ = −80° ( 4a ), −180° ( 4b ), and 40° ( 4c ). A maxima was found near θ = 0° ( 4d ). Cis and trans approaches of the ethylene monomer were considered for each of the four cases. All approaches of ethylene toward 4a and 4c conformers led to insertion, as did cis approach toward 4b and trans approach toward 4d . Uptake barriers were found to be rate determining for all the cis cases. The insertion barriers for all the trans cases of approach were uniformly higher than the corresponding cis barriers, by 5−10 kcal/mol. The lowest rate-determining insertion barrier was found for the uptake step for the cis approach in 4c: 11.1 kcal/mol. Termination by hydrogen transfer from chain to monomer was found to occur for the cis approach of monomer toward 4d and trans approach toward 4b . The lowest termination barrier was higher than the lowest rate-determining insertion barrier by 6.3 kcal/mol.

Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.