Links between single-site heterogeneous and homogeneous catalysis. DFT analysis of pathways for organozirconium catalyst chemisorptive activation and olefin polymerization on γ-alumina

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(C ₆F₅)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 Al ₂O₃ 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.