Sterically encumbered (perfluoroaryl) borane and aluminate cocatalysts for tuning cation - Anion ion pair structure and reactivity in metallocene polymerization processes. A synthetic, structural, and polymerization study

The synthesis and dialkyl abstraction chemistry as well as the unusual cocatalytic characteristics in metallocene-mediated polymerization of two distinctive borane and aluminate cocatalysts tris(2,2',2'nonafluorobiphenyl)borane (PBB) and triphenyl carbenium tris(2,2',2_-nonafluorobiphenyl)fluoroaluminate, (Ph₃C+PBA^-) are reported. Reaction of PBB with Cp'2ThMe2 (Cp' = η⁵-C₅Me₅), CGCZrMe₂ (CGC = Me₂Si(η⁵-Me₄C₅)((l)BuN)), and Cp'MMe₃ (M = Zr, Hf) cleanly affords base-free cationic complexes Cp'₂ThMe⁺MePBB^- (1), CGCZrMe⁺MePBB^- (5), and Cp'MMe₂⁺MePBB^- (M = Zr, 7; Hf, 8). In case of CGCTiMe₂ and dimethyl zirconocenes, μ-methyl dinuclear cationic complexes [(CGCTiMe)₂(μ- Me)]⁺MePBB(6) and [(L₂ZrMe)₂(μ-Me)]⁺MePBB^- (L = η5-C₅H₅ (Cp), 2; η- 1,2-Me₂C₅H₃ (Cp'), 3; Cp', 4; L₂ = Me₂Si(Ind)₂, Ind = η⁵-C₉H₆, 9; L₂ = Me₂C(Flu)(Cp), Flu = η5-C₁₃H₈, 10) are formed. A similar reaction with Ph₃C⁺PBA^- results in the corresponding complexes CGCZrCH₃⁺PBA^- (M = Zr, 19; Ti, 20) and L₂ZrCH₃⁺PBA^- (L = Cp, 15b; Cp_, 16; η⁵-1,3- (SiMe₂)₂C₅H₃, 17; Cp', 18; L₂ = Me₂Si(Ind)₂, 21; L₂ = Me₂C(Flu)(Cp), 22). Two dinuclear complexes 3 and 13 ([Me₂C(Flu)(Cp)Zr(C₆F₅)]₂(μ- F)⁺MeB(C₆F₅)3^-) derived from borane PBB and B(C₆F₅)₃, respectively, and three other PBA^based monomeric complexes 14 (Ph₃C⁺PBA^-), 19, and 21 have been characterized by X-ray diffraction, and these determinations allow detailed analysis of the ion pairing in the solid state. In combination with solution dynamic NMR, all data indicate MePBB^--cation interactions to be considerably weaker than those involving MeB(C₆F₅)₃^-, while the strongly ion-paired chiral PBA^- converts previously enantiomeric cations into pairs of diastereomers. As revealed by dynamic ¹H NMR studies, ion pair reorganization/symmetrization in 5 is significantly more rapid than in the MeB(C₆F₅)3^- analogue, suggesting much looser ion pairing in 5. On the other hand, PBA^- racemization is a rapid process (e.g., ΔG≠(58 °C) = 16.9(2) kcal/mol for 16), while cation-PBA^- ion pairs have higher barriers for ion pair symmetrization than in analogous fluoroaryl borates. Dinuclear complexes 2 and 3 initiate efficient polymerization of methyl methacrylate (MMA) to produce syndiotactic poly(methyl methacrylate) (PMMA), while 9 produces highly isotactic PMMA, and stericalty more accessible complexes 6 and 10 exhibit no activity. For olefin polymerization and copolymerization, PBB-derived cationic complexes, both monomeric and dinuclear, generally exhibit higher catalytic activity and comonomer incorporation levels than the MeB(C₆F₅)3^- analogues, with CGC catalysts exhibiting the greatest activity contrasts. On the other hand, PBA^-derived complexes exhibit a remarkable sensitivity of olefin polymerization characteristics and ion pairing to ancillary ligand bulk, with activity differences of up to 10⁶-fold observed. In regard to stereospecific polymerization, PBA^-derived chiral complex 21 produces highly isotactic polypropylene while B(C₆F₅)A^-derived analogue produces isotactic polypropylene with lower isotacticity under similar conditions. Microstructure analyses of poly(ethylene-co-1-hexene) samples indicate that PBB enhances comonomer incorporation randomness.