Models of intermediates in metallocene-catalyzed alkene polymerizations: observation of a d0 cationic titanium–alkyl-alkene complex and decomposition by β-allyl elimination

Low temperature activation of Cp*2Ti[η1,η1- CH2CH(CH2CHCH2)CH2] (3) with [HN(CH3)(C6H5)2] [B(C6F5)4] led to the formation of Cp*2Ti[η1,η2-CH2CH(CH3)CH2CHCH2][B(C6F5)4] (6) as determined by 1H NMR spectroscopy. Cp*2Ti[η1,η2-CH2CH(CH3)CH2CHCH2][B(C6F5)4] undergoes rapid quantitative β-allyl elimination at temperatures as low as −140 °C. The resulting cationic titanium allyl complex [Cp*2Ti(η3-CH2CHCH2)][B(C6F5)4] (4) exhibits a static structure at low temperatures, but interconversion of η3–η1 binding modes can be observed at higher temperatures. Lineshape analysis of this process yielded ΔG‡(−10 °C)= 13.7 ± 0.6 kcal mol−1, ΔH‡=9.8 ± 0.6 kcal mol−1, and ΔS‡=−15 ± 3 eu. The use of neutral borane B(C6F5)3 also resulted in β-allyl elimination with the formation of [Cp*2Ti(η3-CH2CHCH2)][CH2=CHCH2B(C6F5)3] (8).