"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 Me₂Si(η⁵-Me₄C₅)( ^tBuN)MR₂ (CGCMR₂, 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 α-olefin polymerization and copolymerization catalysis. The reaction of the neutral free ligand CGCH₂ with Ti(CH₂Ph)₄ in aromatic or saturated hydrocarbon solvents at 60°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 CGCZrCH₂Ph⁺ 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(η⁵,η¹-C₅Me ₃CH₂)(^tBuN)Ti⁺[η ⁿ-PhCH₂B(C₆F₅)₃] ^- (5) and Me₂Si(η⁵,η¹-C₅Me ₃CH₂)(^tBuN)Ti⁺B(C₆F ₅)₄- (6), respectively. In contrast, the reaction of CGCTiMe₂ with B(C₆F₅)₃ cleanly generates CGCTiCH₃⁺CH₃B(C₆F₅) ₃^- (8) without C-H bond activation as well as dinuclear [CGCTiMe(μ-Me)MeTiCGC]⁺MeB(C₆F₅) ₃^- (11), which is in equilibrium with 8 and CGCTiMe₂ (ΔG_298K = 1.3(2) kcal/mol in favor of 8). The reaction of CGCTiMe₂ with sterically encumbered PBB and Ph₃C⁺B(C₆F₅)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 > 10⁶) 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(CH₂Ph)₂TMAO 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 Dolvmer microstructure.