The mechanism of methane elimination in B(C6F5)3-Initiated monocyclopentadienyl-ketimide titanium smd related olefin polymerization catalysts

A new class of monocyclopentadienyl titanium olefin polymerization catalysts and their activation with B(C6F5)(3) is reported herein. Dichlorides Cp[Bu-t(R)C=N]TiCl2 {Cp = C5H5, R = Bu-t (1a); Cp = C5Me5, R = Bu-t (2a); Cp = C5Me4SiMe3, R = Bu-t (3a); Cp = C5Me5, R = CH2SiMe3 (4a); Cp = C5Me5, R = Me (5a)} were prepared in 50-92% yield from CpTiCl3 and Bu-t(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 Bu-t[(Me3Si)(2)CH]C=NH in 40% yield. Dynamic H-1 NMR studies showed that the ketimide ligands in compounds b rotate rapidly about Ti-N on the NMR time scale, with a a Delta G(double dagger) of 9.6(6) kcal mol(-1) or less. The mixed alkyl compound Cp*-[Bu-t(R)C=N]Ti(CH3)CH2SiMe3 {R = (t)3u (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[Bu-t(R)C=N]TiCH3}(+)[H3C(B(C6F5)(3)](-), 1c-6c; mixed alkyl compound 7 yields the ion pair [Cp*((Bu2C)-Bu-t=N)TiCH2SiMe3](+)[H3C(B(C6F5)(3)](-), 7c, exclusively. Multinuclear 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-5e undergo a decomposition process involving loss of methane and producing the neutral compounds Cp[Bu-t(R)C=N]Ti(C6F5)[CH2B(C6F5)(2)], 1d-5d. The X-ray crystal structure of Id has been determined. Active cationic 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 Delta H-double dagger = 20.6(8) kcal mol(-1) and Delta S-double dagger = -8.5(10) eu, corresponding to Delta G(298)(double dagger) of 23.1(8) kcal mol(-1). A substantial kinetic isotope effect of k(H)/k(D) = 9.1(6) was measured using d(6)-2c. Further mechanistic experiments, including crossover and examination of alkane elimination from mixed alkyl ion pair 7c, point to a sigma-bond metathesis mechanism for the production of compounds d. The implications of our results for other, related catalyst systems are discussed.