[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

Article abstract of DOI:10.1021/ja4092775

The compound [Cp₂Ti(Me)(CD₂Cl₂)][B(C ₆F₅)₄] reacts with trimethylvinylsilane (TMVS) to form the 1,2-insertion product [Cp₂TiCH₂CHMe(SiMe ₃)]⁺ (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(CD ₃)(SiMe₃)]⁺, which converts, via reversible β-elimination, to an equilibrium mixture of specifically [Cp ₂TiCH₂CH(CD₃)(SiMe₃)]⁺ and [Cp₂TiCD₂CD(CH₃)(SiMe₃)] ⁺. Complementing this conventional process, exchange spectroscopy experiments show that the β-H atom of [Cp₂TiCH ₂CHMe(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)(SiMe ₃)]⁺ 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 [Cp ₂TiH{CH₂-CMe(SiMe₃)}]⁺ undergoes reversible exchange with the hydride ligand via the allyl dihydrogen species [Cp₂TiH₂{(η³-CH₂C(SiMe ₃)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.