[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)]+ (III), 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 not 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 III and the observed exchange processes. In all cases where comparisons between experimental and calculated data were possible, agreement was excellent.