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Synthesis, Structures, and Reactivity of Weakly Coordinating Anions with Delocalized Borate Structure:  The Assessment of Anion Effects in Metallocene Polymerization Catalysts

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journal contribution
posted on 15.12.2000, 00:00 authored by Jiamin Zhou, Simon J. Lancaster, Dennis A. Walker, Stefan Beck, Mark Thornton-Pett, Manfred Bochmann
The formation of adducts of tris(pentafluorophenyl)borane with strongly coordinating anions such as CN- and [M(CN)4]2- (M = Ni, Pd) is a synthetically facile route to the bulky, very weakly coordinating anions [CN{B(C6F5)3}2]- and [M{CNB(C6F5)3}4]2- which are isolated as stable NHMe2Ph+ and CPh3+ salts. The crystal structures of [CPh3][CN{B(C6F5)3}2] (1), [CPh3][ClB(C6F5)3] (2), [NHMe2Ph]2[Ni{CNB(C6F5)3}4]·2Me2CO (4b·2Me2CO), [CPh3]2[Ni{CNB(C6F5)3}4]·2CH2Cl2 (4c·2CH2Cl2), and [CPh3]2[Pd{CNB(C6F5)3}4]·2CH2Cl2 (5c·2CH2Cl2) are reported. The CN stretching frequencies in 4 and 5 are shifted by ∼110 cm-1 to higher wavenumbers compared to the parent tetracyano complexes in aqueous solution, although the M−C and C−N distances show no significant change on B(C6F5)3 coordination. Zirconocene dimethyl complexes L2ZrMe2 [L2 = Cp2, SBI = rac-Me2Si(Ind)2] react with 1, 4c or 5c in benzene solution at 20 °C to give the salts of binuclear methyl-bridged cations, [(L2ZrMe)2(μ-Me)][CN{B(C6F5)3}2] and [(L2ZrMe)2(μ-Me)]2[M{CNB(C6F5)3}4]. The reactivity of these species in solution was studied in comparison with the known [{(SBI)ZrMe}2(μ-Me)][B(C6F5)4]. While the latter reacts with excess [CPh3][B(C6F5)4] in benzene to give the mononuclear ion pair [(SBI)ZrMe+···B(C6F5)4-] in a pseudo-first-order reaction, k = 3 × 10-4 s-1, [(L2ZrMe)2(μ-Me)][CN{B(C6F5)3}2] reacts to give a mixture of L2ZrMe(μ-Me)B(C6F5)3 and L2ZrMe(μ-NC)B(C6F5)3. Recrystallization of [Cp‘ ‘2Zr(μ-Me)2AlMe2][CN{B(C6F5)3}2] affords Cp‘ ‘2ZrMe(μ-NC)B(C6F5)3 6, the X-ray structure of which is reported. The stability of [(L2ZrMe)2(μ-Me)]+X- decreases in the order X = [B(C6F5)4] > [M{CNB(C6F5)3}4] > [CN{B(C6F5)3}2] and increases strongly with the steric bulk of L2 = Cp2 ≪ SBI. Activation of (SBI)ZrMe2 by 1 in the presence of AlBui3 gives extremely active ethene polymerization catalysts. Polymerization studies at 1−7 bar monomer pressure suggest that these, and by implication most other highly active ethene polymerization catalysts, are strongly mass-transport limited. By contrast, monitoring propene polymerization activities with the systems (SBI)ZrMe2/1/AlBui3 and CGCTiMe2/1/AlBui3 at 20 °C as a function of catalyst concentration demonstrates that in these cases mass-transport limitation is absent up to [metal] ≈ 2 × 10-5 mol L-1. Propene polymerization activities decrease in the order [CN{B(C6F5)3}2]- > [B(C6F5)4]- > [M{CNB(C6F5)3}4]2- ≫ [MeB(C6F5)3]-, with differences in activation barriers relative to [CN{B(C6F5)3}2]- of ΔΔG = 1.1 (B(C6F5)4-), 4.1 (Ni{CNB(C6F5)3}42-) and 10.7−12.8 kJ mol-1 (MeB(C6F5)3-). The data suggest that even in the case of very bulky anions with delocalized negative charge the displacement of the anion by the monomer must be involved in the rate-limiting step.