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

摘要

Low temperature activation of Cp_2~8Ti[eta~1,eta~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_2~*Ti[eta~1,eta~2-CH_2CH(CH_3)CH_2CH=CH_2][B(C_6F_5)_4] (6) as determined by ~1H NMR spectroscopy.Cp_2~*Ti[eta~1,eta~2-CH_2CH(CH_3)CH_2CH=CH_2][B(C_6F_5)_4] undergoes rapid quantitative beta-allyl elimination at temepratures as low as -140 deg C.The resulting cationic titanim allyl complex [Cp~*_2Ti(eta~3-CH_2CHCH_2)][B(C_6F_5)_4] (4) exhibits a static structure at low temperatures,but interconversion of eta~3-eta~1 binding modes can be observed at higher temperatures.Lineshape analysis of this process yielded DELTAG(-10 deg C)=13.7(+-)0.6 kcal mol~(-1),DELTAH=9.8(+-)0.6 kcal mol~(-1),and DELTAS=-15(+-)3 eu.The use of neutral borane B(C_6F_5)_3 also resulted in beta-allyl elimination with the formation of [Cp_2~*tI(ETA~3-ch_2chch_2)][ch_2=chch_2b(c_6f_5)_3] (8).