[Cp₂TiCH₂CHMe(SiMe₃)]⁺, an Alkyl–Titanium Complex Which (a) Exists in Equilibrium between a β‑Agostic and a Lower Energy γ‑Agostic Isomer and (b) Undergoes Hydrogen Atom Exchange between α‑, β‑, and γ‑Sites via a Combination of Conventional β‑Hydrogen Elimination–Reinsertion and a Nonconventional CH Bond Activation Process Which Involves Proton Tunnelling

The compound [Cp₂Ti­(Me)­(CD₂Cl₂)]­[B­(C₆F₅)₄] reacts with trimethylvinylsilane (TMVS) to form the 1,2-insertion product [Cp₂TiCH₂CHMe­(SiMe₃)]⁺ (<b>III</b>), which exists in solution as equilibrating β- and γ-agostic isomers. In addition, while free rotation of the β-methyl group results in a single, averaged γ-H atom resonance at higher temperatures, decoalescence occurs below ∼200 K, and the resonance of the γ-agostic hydrogen atom at δ ∼ −7.4 is observed. Reaction of [Cp₂Ti­(CD₃)­(CD₂Cl₂)]⁺ with TMVS results in the formation of [Cp₂TiCH₂CH­(CD₃)­(SiMe₃)]⁺, which converts, via reversible β-elimination, to an equilibrium mixture of specifically [Cp₂TiCH₂CH­(CD₃)­(SiMe₃)]⁺ and [Cp₂TiCD₂CD­(CH₃)­(SiMe₃)]⁺. Complementing this conventional process, exchange spectroscopy experiments show that the β-H atom of [Cp₂TiCH₂CHMe­(SiMe₃)]⁺ undergoes exchange with the three hydrogen atoms of the β-methyl group (β-H/γ-H exchange) but <i>not</i> with the two α-H atoms. This exchange process is completely shut down when [Cp₂TiCH₂CH­(CD₃)­(SiMe₃)]⁺ is used, suggesting an H/D kinetic isotope effect much larger (apparently >16 000) than the maximum possible for an over-the-barrier process. It is proposed that β-H/γ-H exchange is facilitated by quantum mechanical proton tunnelling in which a hydrogen atom of the 2-methyl group of the alkene–hydride deinsertion product [Cp₂TiH­{CH₂CMe­(SiMe₃)}]⁺ undergoes reversible exchange with the hydride ligand via the allyl dihydrogen species [Cp₂TiH₂{(η³-CH₂C­(SiMe₃)­CH₂}]⁺. Complementing these findings, DFT calculations were carried out to obtain energies and NMR parameters for all relevant species and thence to obtain better insight into the agostic preference(s) of complex <b>III</b> and the observed exchange processes. In all cases where comparisons between experimental and calculated data were possible, agreement was excellent