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

Abstract

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, (Ph 3 C + PBA - ) are reported. Reaction of PBB with Cp‘ 2 ThMe 2 (Cp‘ = η 5 -C 5 Me 5 ), CGCZrMe 2 (CGC = Me 2 Si(η 5 -Me 4 C 5 )( t BuN)), and Cp‘MMe 3 (M = Zr, Hf) cleanly affords base-free cationic complexes Cp‘ 2 ThMe + MePBB - ( 1 ), CGCZrMe + MePBB - ( 5 ), and Cp‘MMe 2 + MePBB - (M = Zr, 7; Hf, 8 ). In case of CGCTiMe 2 and dimethyl zirconocenes, μ-methyl dinuclear cationic complexes [(CGCTiMe) 2 (μ-Me)] + MePBB - ( 6 ) and [(L 2 ZrMe) 2 (μ-Me)] + MePBB - (L = η 5 -C 5 H 5 (Cp), 2; η 5 -1,2-Me 2 C 5 H 3 (Cp‘‘), 3; Cp‘, 4; L 2 = Me 2 Si(Ind) 2, Ind = η 5 -C 9 H 6, 9; L 2 = Me 2 C(Flu)(Cp), Flu = η 5 -C 13 H 8, 10 ) are formed. A similar reaction with Ph 3 C + PBA - results in the corresponding complexes CGCZrCH 3 + PBA - (M = Zr, 19; Ti, 20 ) and L 2 ZrCH 3 + PBA - (L = Cp, 15b; Cp‘‘, 16; η 5 -1,3-(SiMe 2 ) 2 C 5 H 3, 17; Cp‘, 18; L 2 = Me 2 Si(Ind) 2, 21; L 2 = Me 2 C(Flu)(Cp), 22 ). Two dinuclear complexes 3 and 13 ([Me 2 C(Flu)(Cp)Zr(C 6 F 5 )] 2 (μ-F) + MeB(C 6 F 5 ) 3 - ) derived from borane PBB and B(C 6 F 5 ) 3, respectively, and three other PBA - -based monomeric complexes 14 (Ph 3 C + 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(C 6 F 5 ) 3 -, while the strongly ion-paired chiral PBA - converts previously enantiomeric cations into pairs of diastereomers. As revealed by dynamic 1 H NMR studies, ion pair reorganization/symmetrization in 5 is significantly more rapid than in the MeB(C 6 F 5 ) 3 - analogue, suggesting much looser ion pairing in 5 . On the other hand, PBA - racemization is a rapid process (e.g., Δ G ‡ (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 sterically 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(C 6 F 5 ) 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 10 6 -fold observed. In regard to stereospecific polymerization, PBA - -derived chiral complex 21 produces highly isotactic polypropylene while B(C 6 F 5 ) 4 - -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.