Metal and ancillary ligand structural effects on ethylene insertion processes at cationic group 4 centers. A systematic, comparative quantum chemical investigation at various ab initio levels

Ethylene insertion into the metal-methyl bonds of group 4 (Ti, Zr) (C₅H₅)₂MCH₃⁺ and H₂Si(C₅H₄)(^tBuN)MCH₃ ⁺ catalyst cations has been investigated at the ab initio level employing DZV- and DZP-quality basis sets together with Moller-Plesset perturbative and coupled-cluster single-double excitation wave function expansions. All reactions are found to proceed from reactants to products via intermediate π-complexes and subsequent Cossee-Arlman four-center transition state structures. Enthalpic barriers for the insertion step strongly depend on the nature of the ancillary ligand and metal, with ΔH‡ increasing in the order (C₅H₅)₂TiCH₃⁺ <(C₅H₅)₂ZrCH₃ ⁺ ≈ H₂Si(C₅H₄)(^tBuN) TiCH₃⁺ <H₂Si(C₅H₄)(^tBuN) ZrCH₃⁺. Furthermore, metallocene H₂Si <bridging has the effect of increasing the electrophilicity toward ethylene. The observed ethylene activation/insertion structural and energetic trends may be rationalized using qualitative electronic structure arguments, ancillary ligand steric hindrance, and metal ionic radius. Electron correlation effects are found in all cases to play a crucial role in predicting reaction energetics. Reasonable, incremental convergence in computed energies is obtained for Zr systems and for the H₂Si(C₅H₄)(^tBuN) TiCH₃⁺ cation upon increasing the calculational level (MP2 → MP3 → MP4-SDQ → CCSD). In contrast, fluctuations in results are found for the (C₅H₅)₂ TiCH₃⁺ cations, indicating the desirabilit of high-level calculations.