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 (ΔG298K = 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 (Mw > 106) polyethylenes with high melting transition temperatures (Tm = 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.
|Number of pages||9|
|Publication status||Published - Aug 5 1997|
ASJC Scopus subject areas
- Inorganic Chemistry
- Organic Chemistry