“Constrained Geometry” Dialkyl Catalysts. Efficient Syntheses, C−H Bond Activation Chemistry, Monomer−Dimer Equilibration, and α-Olefin Polymerization Catalysis

This contribution reports an efficient synthesis of the “constrained geometry” group 4 dibenzyl complexes Me 2 Si(η 5 -Me 4 C 5 )( t BuN)MR 2 (CGCMR 2, where R = CH 2 Ph; M = Ti ( 1 ), Zr ( 2 )), as well as the substantially different reaction patterns in the cocatalytic activation of the R = CH 2 Ph and Me complexes with B(C 6 F 5 ) 3, PBB (tris(2,2‘,2‘‘-perfluorobiphenyl)borane), and Ph 3 C + B(C 6 F 5 ) 4 - . The resulting cationic complexes are highly but not equivalently active for α-olefin polymerization and copolymerization catalysis. The reaction of the neutral free ligand CGCH 2 with Ti(CH 2 Ph) 4 in aromatic or saturated hydrocarbon solvents at 60 °C cleanly affords 1 in 90% yield, while the corresponding reaction with Zr(CH 2 Ph) 4 produces 2 in lower yield. When activated with Ph 3 C + B(C 6 F 5 ) 4 - at low temperatures, 2 generates cationic CGCZrCH 2 Ph + B(C 6 F 5 ) 4 - ( 4 ). However, unlike the corresponding metallocene dibenzyl, the cationic derivative of which (Cp 2 ZrCH 2 Ph + B(C 6 F 5 ) 4 - ( 3 )) can be isolated in quantitative yield, the reaction of 1 with B(C 6 F 5 ) 3 and Ph 3 C + B(C 6 F 5 ) 4 - affords intramolecular C−H metalation products Me 2 Si(η 5,η 1 -C 5 Me 3 CH 2 )( t BuN)Ti + [η n -PhCH 2 B(C 6 F 5 ) 3 ] - ( 5 ) and Me 2 Si(η 5,η 1 -C 5 Me 3 CH 2 )( t BuN)Ti + B(C 6 F 5 ) 4 - ( 6 ), respectively. In contrast, the reaction of CGCTiMe 2 with B(C 6 F 5 ) 3 cleanly generates CGCTiCH 3 + CH 3 B(C 6 F 5 ) 3 - ( 8 ) without C−H bond activation as well as dinuclear [CGCTiMe(μ-Me)MeTiCGC] + MeB(C 6 F 5 ) 3 - ( 11 ), which is in equilibrium with 8 and CGCTiMe 2 (Δ G 298K = 1.3(2) kcal/mol in favor of 8 ). The reaction of CGCTiMe 2 with sterically encumbered PBB and Ph 3 C + B(C 6 F 5 ) 4 - yields predominantly cationic dinuclear species, and analytically pure [CGCTiMe(μ-Me)MeTiCGC] + [MePBB] - ( 9 ) can be isolated in quantitative yield. Complexes 5 and 6 are highly active homogeneous catalysts for ethylene and propylene polymerization, producing ultra-high molecular weight ( M w > 10 6 ) polyethylenes with high melting transition temperatures ( T m = 142 °C), as well as syndiotactic-enriched atactic polypropylenes having appreciable molecular weights. Although C−H bond-activated complexes 5 and 6 are ineffective for ethylene and 1-hexene copolymerization, the CGCTi(CH 2 Ph) 2 /MAO system is highly active at 60 °C to incorporate 1-hexene in large quantities (69.9%). Finally, comparisons of polymerization catalysts bearing different counteranions at various temperatures demonstrate the substantial influence of anion identity on α-olefin polymerization activity, catalyst stability, and product polymer microstructure.