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Links Between Single-Site Heterogeneous and Homogeneous Catalysis. DFT Analysis of Pathways for Organozirconium Catalyst Chemisorptive Activation and Olefin Polymerization on γ-Alumina

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journal contribution
posted on 10.12.2008, 00:00 authored by Alessandro Motta, Ignazio L. Fragalà, Tobin J. Marks
This contribution focuses on the catalytic properties of the organozirconium precatalyst Cp2Zr(CH3)2 chemisorbed on dehydroxylated γ-alumina (Al2O3), as analyzed via density functional theory. The interactions of the catalytically active cationic Cp2ZrCH3+ adsorbate species are scrutinized at two possible model Al2O3 (110) surface sites, namely μ2-O and μ3-O, representing the principal reactive species on the alumina surface. It is found that zirconocenium coordination occurs via two different geometries (dioxo-bridged and oxo-bridged) at both the μ3-O and μ2-O surface sites. This process is compared to that for forming the related homogeneous phase Cp2ZrCH3+H3CB(C6F5)3 ion pair structure. It is found that the interaction of the Cp2ZrCH3+ adsorbate species with the μ2-O sites is far stronger than that with the μ3-O sites due to the greater unsaturation of the former. Furthermore, the interaction with the μ3-O sites is weaker than that in the parent homogeneous ion pair. The catalytic activity of the chemisorbed Cp2ZrCH3+ systems for ethylene polymerization is investigated at both μ2-O and μ3-O sites and compared with the analogous Cp2ZrCH3+H3CB(C6F5)3 -mediated process in solution. A Cossee enchainment mechanism proceeds via ethylene π-complex formation and an α-agostic assisted transition state to yield γ- and β-agostic insertion products. The overall kinetics of enchainment are closely correlated with the energetics of π-complex formation, and it is suggested that the differing kinetic behaviors of the surface-bound Cp2ZrR+ species on the various Al2O3 coordination sites and the analogous homogeneous species reflect differences in the olefin π-complex stabilization energies. These computational results agree well with the experimental data which indicate that only fractions of the surface bound species are catalytically significant but that these are far more catalytically active than the homogeneous analogues.