Models of intermediates in metallocene-catalyzed alkene polymerizations: observation of a d 0 cationic titanium–alkyl-alkene complex and decomposition by β-allyl elimination

Abstract

Low temperature activation of Cp * 2Ti[ η 1, η 1- CH 2CH(CH 2CHCH 2)CH 2] (3) with [HN(CH 3)(C 6H 5) 2] [B(C 6F 5) 4] led to the formation of Cp * 2Ti[ η 1, η 2-CH 2CH(CH 3)CH 2CHCH 2][B(C 6F 5) 4] ( 6) as determined by 1H NMR spectroscopy. Cp * 2Ti[ η 1, η 2-CH 2CH(CH 3)CH 2CHCH 2][B(C 6F 5) 4] undergoes rapid quantitative β-allyl elimination at temperatures as low as −140 °C. The resulting cationic titanium allyl complex [Cp * 2Ti( η 3-CH 2CHCH 2)][B(C 6F 5) 4] ( 4) exhibits a static structure at low temperatures, but interconversion of η 3– η 1 binding modes can be observed at higher temperatures. Lineshape analysis of this process yielded Δ G ‡(−10 °C)= 13.7 ± 0.6 kcal mol −1, Δ H ‡=9.8 ± 0.6 kcal mol −1, and Δ S ‡=−15 ± 3 eu. The use of neutral borane B(C 6F 5) 3 also resulted in β-allyl elimination with the formation of [Cp * 2Ti( η 3-CH 2CHCH 2)][CH 2=CHCH 2B(C 6F 5) 3] ( 8).