Structure, Dynamics, and Polymerization Activity of Zirconocenium Ion Pairs Generated with Boron-C6F5 Compounds and Al2R6

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• 作者：De虂borah Mathis ; Erik P. A. Couzijn ; Peter Chen
• 刊名：Organometallics
• 出版年：2011
• 出版时间：July 25, 2011
• 年：2011
• 卷：30
• 期：14
• 页码：3834-3843
• 全文大小：1077K
• 年卷期：v.30,no.14(July 25, 2011)
• ISSN：1520-6041

文摘

The activation reaction of the olefin polymerization precatalyst Cp*₂ZrMe₂ with a boron-C₆F₅ compound (B(C₆F₅)₃, [Ph₃C][B(C₆F₅)₄]) and an aluminum alkyl species (Al₂Me₆, Al₂p>ip>Bu₆) is studied by NMR spectroscopy in order to determine the nature of the ion pairs that are formed preferentially. We show that a mixture of ion pairs with general formula Cp*₂Zr(Me)鈭捨?Me鈥揈(C₆F₅)_3鈥?i>xR_x (E = Al, B; x = 0, 1; R = Me, p>ip>Bu) (1, 2a/b, 4) is generated due to a rapid transfer of pentafluorophenyl groups from boron to aluminum. Therefore, the molecular ratio of the activators determines the final composition of the ion pairs present in solution. When the pentafluorophenyl group transfer is suppressed, the ion pair Cp*₂Zr鈥?渭-Me)₂鈥揂lMe₂ (5) forms irrespective of the reagent ratio. The high dynamics of these solutions is demonstrated by DNMR studies. Gibbs free energies of activation were determined of 13.6(12) kcal molp>鈥?p> at 298 K for the cocatalyst exchange of ion pair Cp*₂Zr(Me)鈭捨?Me鈥揂l(C₆F₅)₂Me (2a) and 13(2) kcal molp>鈥?p> at 298 K for the methyl exchange in Al₂(C₆F₅)_xMe_6鈥?i>x (x = 0, 1). Due to the stability of the ion pairs generated from the Cp*₂ZrMe₂ precatalyst at temperatures relevant for polymerization, correlations between activities in ethylene polymerization and the nature of the ion pairs can be established. All solutions containing the various ion pairs were found to be catalytically active in ethylene polymerization except that containing the ion pair 2a, which was attributed to the reduced Lewis acidity of the abstractor, as supported by DFT calculations.