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, (Ph(3)C(+)PBA(-)) are reported. Reaction of PBB with Cp'2ThMe2 (Cp' = eta(5)-C5Me5), CGCZrMe(2) (CGC = Me2-Si(eta(5)-Me4C5)('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 CGCTiMe(2) and dimethyl zirconocenes, mu-methyl dinuclear cationic complexes [(CGeTiMe)(2)(mu-Me)](+) MePBB- (6) and [(L2ZrMe)(2)(mu-Me)]+MePBB- (L = eta(5)-C5H5 (CP), 2; eta(5)-1,2-Me2C5H3 (CP''), 3; Cp', 4; L-2 = Me2Si(Ind)(2), Ind = eta(5)-C9H6, 9; L-2 = Me2C(Flu)(Cp), Flu = eta(5)-C13H8, 10) are formed. A similar reaction with Ph3C+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; eta(5)-1,3-(SiMe2)(2)C5H3, 17; Cp', 18; L-2 = Me2Si(Ind)(2), 21; L-2 = Me2C(Flu)(Cp), 22). Two dinuclear complexes 3 and 13 ([Me2C(Flu)(Cp)Zr(C6F5)](2)(mu-F)+MeB(C6F5)(3)(-)) derived from borane PBB and B(C6F5)(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(C6F5)(3)(-), while the strongly ion-paired chiral PBA(-) converts previously enantiomeric cations into pairs of diastereomers. As revealed by dynamic H-1 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., Delta G double dagger(58 degrees 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(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 10(6)-fold observed. Ln regard to stereospecific polymerization, PBA--derived chiral complex 21 produces highly isotactic polypropylene while B(C6F5)(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.