The Mechanism of Methane Elimination in B(C6F5)3-Initiated Monocyclopentadienyl-Ketimide Titanium and Related Olefin Polymerization Catalysts

A new class of monocyclopentadienyl titanium olefin polymerization catalysts and their activation with B(C 6 F 5 ) 3 is reported herein. Dichlorides Cp[ t Bu(R)C N]TiCl 2 {Cp = C 5 H 5, R = t Bu ( 1a ); Cp = C 5 Me 5, R = t Bu ( 2a ); Cp = C 5 Me 4 SiMe 3, R = t Bu ( 3a ); Cp = C 5 Me 5, R = CH 2 SiMe 3 ( 4a ); Cp = C 5 Me 5, R = Me ( 5a )} were prepared in 50−92% yield from CpTiCl 3 and t 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 = C 5 Me 5, R = CH(SiMe 3 ) 2 ) was prepared directly from Cp*TiMe 3 and t Bu[(Me 3 Si) 2 CH]C NH in 40% yield. Dynamic 1 H NMR studies showed that the ketimide ligands in compounds b rotate rapidly about Ti−N on the NMR time scale, with a Δ G ‡ of 9.6(6) kcal mol - 1 or less. The mixed alkyl compound Cp*[ t Bu(R)C N]Ti(CH 3 )CH 2 SiMe 3 {R = t Bu ( 7 )} was prepared via alkylation of the corresponding methyl chloride derivative with BrMgCH 2 SiMe 3 . When treated with B(C 6 F 5 ) 3, compounds 1b − 6b are rapidly converted into the ion pairs {Cp[ t Bu(R)C N]TiCH 3 } + [H 3 C(B(C 6 F 5 ) 3 ] -, 1c − 6c; mixed alkyl compound 7 yields the ion pair [Cp*( t Bu 2 C N)TiCH 2 SiMe 3 ] + [H 3 C(B(C 6 F 5 ) 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 − 5c undergo a decomposition process involving loss of methane and producing the neutral compounds Cp[ t Bu(R)C N]Ti(C 6 F 5 )[CH 2 B(C 6 F 5 ) 2 ], 1d − 5d . The X-ray crystal structure of 1d has been determined. Active cationic compounds are not regenerated from neutral compounds d in the presence of B(C 6 F 5 ) 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 Δ H ‡ = 20.6(8) kcal mol - 1 and Δ S ‡ = −8.5(10) eu, corresponding to Δ G ‡ 298 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 σ-bond metathesis mechanism for the production of compounds d . The implications of our results for other, related catalyst systems are discussed.