''Constrained geometry'' dialkyl catalysts. Efficient syntheses, C-H bond activation chemistry, monomer-dimer equilibration, and alpha-olefin polymerization catalysis

This contribution reports an efficient synthesis of the ''constrained geometry'' group 4 dibenzyl complexes Me2Si(eta 5-Me4C5)((BuN)-Bu-t)MR2 (CGCMR(2), where R = CH2Ph; M = Ti (1), Zr (2)), as well as the substantially different reaction patterns in the cocatalytic activation of the R = CH2Ph and Me complexes with B(C6F5)(3), PBB (tris(2,2',2 ''-perfluorobiphenyl)borane), and Ph3C+B(C6F5)(4)(-) The resulting cationic complexes are highly but not equivalently active for alpha-olefin polymerization and copolymerization catalysis. The reaction of the neutral free ligand CGCH(2) with Ti(CH2Ph)(4) in aromatic or saturated hydrocarbon solvents at 60 degrees C cleanly affords 1 in 90% yield, while the corresponding reaction with Zr(CH2Ph)(4) produces 2 in lower yield. When activated with Ph3C+B(C6F5)(4)(-) at low temperatures, 2 generates cationic CGCZrCH(2)Ph(+) B(C6F5)(4)(-) (4) However, unlike the corresponding metallocene dibenzyl, the cationic derivative of which (Cp2ZrCH2Ph+B(C6F5)(4)(-) (3)) can be isolated in quantitative yield, the reaction of 1 with B(C6F5)3 and Ph3C+B(C6F5)(4)(-) affords intramolecular C-H metalation products Me2Si(eta 5, eta 1-C5Me3CH2)((BuN)-Bu-t)Ti+[eta(n)-PhCH2B(C6F5)(3)](-) (5) and Me2Si(eta 5,eta(1)-C5Me3CH2)((BuN)-Bu-t)Ti+B(C6F5)(4)(-) (6), respectively. In contrast, the reaction of CGCTiMe(2) with B(C6F5)(3) cleanly generates CGCTiCH(3)(+)CH(3)B(C6F5)(3)(-) (8) without C-H bond activation as well as dinuclear [CGCTiMe(mu-Me)MeTiCGC]+MeB(C6F5)(3)(-) (11), which is in equilibrium with 8 and CGCTiMe(2) (Delta G(298K) = 1.3(2) kcal/mol in favor of 8). The reaction of CGCTiMe(2) with sterically encumbered PBB and Ph3C+B(C6F5)(4)(-) yields predominantly cationic dinuclear species, and analytically pure [CGCTiMe(mu-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 degrees 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 l-hexene copolymerization, the CGCTi(CH2Ph)(2)/MAO system is highly active at 60 degrees 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 a-olefin polymerization activity, catalyst stability, and product polymer microstructure.