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)( t BuN)) (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(μ-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)3BNCCH3 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.