Density functional theory study of the interaction of CP 2*ZrCH3⁺ (Cp*= C5H 5, C5Me5, and C13H9) with ethylene molecule

Density functional theory was used to study gas-phase reactions between the Cp2*ZrMe+ cations, where Cp* = C5H5 (1), Me5Cp = C5Me5 (2), and Flu = C13H9 (3), and the ethylene molecule, Cp2*ZrMe+ + C2H4 → Cp2*ZrPr+ → Cp2*ZrAllyl+ + H2. The reactivity of the Cp2*ZrMe+ cations with respect to the ethylene molecule decreased in the series 1 > 3 ≈ 2. Substitution in the Cp ring decreased the reactivity of the Cp2*ZrMe+ cations toward ethylene, in agreement with the experimental data on the comparative reactivities of complexes 1 and 3. The two main energy barriers along the reaction path (the formation of the C-C bond leading to the primary product Cp2*ZrPr+ and hydride shift leading to the secondary product Cp2*Zr(H2)Allyl+) vary in opposite directions in the series of the compounds studied. For Flu (3), these barriers are close to each other, and for the other compounds, the formation of the C-C bond requires the overcoming of a higher energy barrier. A comparison of the results obtained with the data on the activity of zirconocene catalysts in real catalytic systems for the polymerization of ethylene led us to conclude that the properties of the catalytic center changed drastically in the passage from the model reaction in the gas phase to real catalytic systems.