New organo-lewis acids. Tris(β-perfluoronaphthyl)borane (PNB) as a highly active cocatalyst for metallocene-mediated ziegler-natta α-olefin polymerization

Tris(β-perfluoronaphthyl)borane (B(C₁₀F₇)₃, PNB) is synthesized from β-perfluoronaphthyllithium and BCl₃ to serve as a new strong organo-Lewis acid cocatalyst. PNB efficiently activates a variety of group 4 dimethyl complexes to form highly active homogeneous Ziegler-Natta olefin polymerization catalysts. Reaction of PNB with rac-Me₂Si(Ind)₂ZrMe₂ and CGCMMe₂ (M = Zr, Ti; CGC = Me₂Si(η⁵-Me₄C₅)( ^tBuN)) (1:1 molar ratio) rapidly produces the base-free cationic complexes rac-Me₂Si(Ind)₂ZrMe⁺MePNB^- (1) and CGCMMe⁺MePNB^- (M = Zr, 2; Ti, 3), respectively. The μ-methyl dinuclear cationic complex [(CGCTiMe)₂(μ-Me)]⁺MePNB^- (4) is formed when a 2:1 CGCTiMe₂:PNB stoichiometry is employed: In the case of group 4 dimethyl zirconocenes, L₂ZrMe₂ (L = η⁵-C₅H₅, Cp; η⁵-1,2-Me₂C₅H₃, Cp″), reaction in a 1:1 metallocene:PNB ratio affords cationic complexes L₂ZrMe⁺MePNB^- (L = Cp, 5; Cp″, 6), while the reaction with a 1:2 molar ratio affords dinuclear μ-methyl cationic complexes [(L₂ZrMe)₂(μ-Me)]⁺MePNB^- (L = Cp, 7; Cp″, 8). In both reactions, μ-F dinuclear cationic complexes [(L₂ZrMe)₂(M-F)]⁺MePNB^- (L = Cp, 9; Cp″, 10) are formed as byproducts. (C₆F₅)₃BNCCH₃ and PNBNCCH₃ were synthesized and characterized. Analysis of the PNBNCCH₃ + B(C₆F₅)₃ ⇄ (C₆F₅)₃BNCCH₃ + PNB equilibrium yields ΔH° = +0.7(2) kcal/ mol and ΔS° = +4.3(5) eu, suggesting PNB has somewhat higher Lewis acidity and is sterically more encumbered than B(C₆F₅)₃. Solution ν(CN) values for PNBNCCH₃ and (C₆F₅)₃-BNCCH₃ are 2365.3 and 2366.5 cm^-1, respectively, which indicate strong Lewis acidity. PBBNCCH₃ cannot be detected in the reaction of (C₆F₅)₃BNCCH₃ with PBB [PBB = tris-(2,2′,2″-perfluorobipheny)lborane] over prolonged periods at 60°C. In ethylene polymerization, PNB-derived cationic complexes 5, 6, 7, and 8 have catalytic activities similar to the B(C₆F₅)₃-derived analogues, while 2 and 3 have substantially higher activities. In propylene polymerization, catalyst 1 has higher activity than the B(C₆F₅)₃ analogue. In the case of ethylene and 1-hexene copolymerization, PNB-derived cationic complex 3 exhibits higher polymerization activity with similar 1-hexene incorporation versus the B(C₆F₅)₃-derived cationic complex. In large-scale batch copolymerizations of ethylene and 1-octene mediated by CGCTiMe₂, the PNB-based catalytic systems exhibit approximately twice the activity of the B(C₆F₅)₃-based systems.