284675-05-4Relevant academic research and scientific papers
The mechanism of methane elimination in B(C6F5)3-initiated monocyclopentadienyl-ketimide titanium and related olefin polymerization catalysts
Zhang, Suobo,Piers, Warren E.,Gao, Xiaoliang,Parvez, Masood
, p. 5499 - 5509 (2000)
A new class of monocyclopentadienyl titanium olefin polymerization catalysts and their activation with B(C6F5)3 is reported herein. Dichlorides Cp[(τ)Bu(R)C=N]TiCl2 {Cp = C5H5, R = (τ)Bu (1a); Cp = C5Me5, R = (τ)Bu (2a); Cp = C5Me4SiMe3, R = (τ)Bu (3a); Cp = C5Me5, R = CH2SiMe3 (4a); Cp = C5Me5, R = Me (5a)} were prepared in 50-92% yield from CpTiCl3 and (τ)Bu(R)C=NLi. Analogous dimethyl compounds 1b-5b were prepare via methylation of dichlorides a using MeMgBr in 89-92% yield. Dimethyl compound 6b (L = C5Me5, R = CH(SiMe3)2) was prepared directly from Cp*TiMe3 and (τ)Bu[(Me3Si)2CH]C=NH in 40% yield. Dynamic 1H NMR studies showed that the ketimide ligands in compounds b rotate rapidly about Ti-N on the NMR time scale, with a ΔG(+) of 9.6(6) kcal mol-1 or less. The mixed alkyl compound Cp*-[(τ)Bu(R)C=N]Ti(CH3)CH2SiMe3 {R = (τ)Bu (7)} was prepared via alkylation of the corresponding methyl chloride derivative with BrMgCH2SiMe3. When treated with B(C6F5)3, compounds 1b-6b are rapidly converted into the ion pairs {Cp[(τ)Bu(R)C=N]TiCH3}+[H3C(B(C6F5)3]-, 1c-6c; mixed alkyl compound 7 yields the ion pair [Cp*((τ)Bu2C=N)TiCH2SiMe3]+[H3C(B(C6F5)3]-, 7c, exclusively. Multilinear NMR experiments show that ion pairing is tight in these compounds and that ketimide ligand rotation is occurring with a slightly higher barrier in comparison to the neutral derivatives b. Ion pairs 1c-5c undergo a decomposition processing involving loss of methane and producing the neutral compounds Cp[(τ)Bu(R)C=N]Ti(C6F5)[CH2B(C6F5)2], 1d-5d. The X-ray crystal structure of 1d has been determined. Active chronic compounds are not regenerated from neutral compounds d in the presence of B(C6F5)3 and thus this reaction is a potential deactivation pathway for these particular ion pairs. Detailed kinetic studies on the decomposition of 2c show the reaction to be first order in [2c] with activation parameters of ΔH(+) = 20.6(8) kcal mol-1 and ΔS(+) = -8.5(10) eu, corresponding to ΔG(+)298 of 23.1(8) kcal mol-1. A substantial kinetic isotope effect of k(H)/k(D) = 9.1(6) was measured using d6-2c. Further mechanistic experiments, including crossover and examination of alkane elimination from mixed alkyl ion pair 7c, point to a σ-bond metathesis mechanism for the production of compounds d. The implications of our results for other, related catalyst systems are discussed.
