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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry