A Density Functional Study of Ethylene Insertion into the M‐methyl (M = Ti, Zr) Bond for Different Catalysts, with a QM/MM Model for the Counterion, B(C6F5)3CH3−

Abstract

AbstractSingle site homogeneous catalysts have been studied extensively in recent years as alternatives to traditional heterogeneous catalysts. The current theoretical study uses density functional theory to study the insertion process of the ethylene monomer into the titanium‐carbon chain for contact ion‐pair systems of the type [L1L2TiCH3‐μ‐CH3‐B(C6F5)3], where L1, L2, are Cp, NPH3, and other ligands. Different modes of approach cis and trans to the μ‐CH3 bridge were considered. The counterion, B(C6F5)3CH3−, was modeled by QM/MM methods. The value of ΔHtot—the total barrier to insertion—was found to be positive (in the range of 4–15 kcal/mol). The ability of the ancillary ligands, L1 and L2, to stabilize the ion‐pair was found to be an important factor in determining the value of δHtot. On replacing the titanium metal center with zirconium, the ΔHtot values were found to be lowered (in the range of 2–9 kcal/mol), indicating that they would be better catalysts than their titanium analogues. The size of the ligands L1 and L2 was increased by replacing hydrogens in the ligands with tertiary butyl groups. The value of ΔHtot was found to increase (in the range of 10–28 kcal/mol) in contrast to the simple systems, for both the cis and trans cases of approach, with the cis mode of approach giving lower values of ΔHtot. Solvent effects were incorporated with cyclohexane (ε = 2.023) as the solvent, and were found to have a minor influence, ±(0.5–1.5) kcal/mol) on the insertion barrier for all the cases studied.