Pyridylamido Bi-Hafnium Olefin Polymerization Catalysis: Conformationally Supported Hf...Hf Enchainment Cooperativity

作者：Gao, Yanshan; Mouat, Aidan R.; Motta, Alessandro; Macchioni, Alceo^*; Zuccaccia, Cristiano; Delferro, Massimiliano; Marks, Tobin J.

来源：ACS Catalysis, 2015, 5(9): 5272-5282.

DOI：10.1021/acscatal.5b00788

• hafnium
• bimetallic
• olefin polymerization
• NMR spectroscopy
• cooperativity effects

Homobimetallic Hf(IV) complexes, L-2-Hf2Me5 (3) and L-2-Hf2Me4 (4) (L-2 = N,N'-{[naphthalene-1,4-diylbis (pyridine- 6,2-diyl)] bis [(2-isopropylphenyl)-methylene)]bis(2,6-diisopropylaniline}), were synthesized by reaction of the free ligand L-2 with the appropriate Hf precursor and were characterized in solution (NMR) and in the solid state (X-ray diffraction). In 3, L-2 acts as a dianionic tridentate ligand for one Hf metal center and as a monoanionic bidentate ligand for the other, whereas in 4, both Hf units are tricoordinated to opposite sides of L-2. In the solid state, the Hf...Hf distance is significantly different in 3 vs 4 (6.16 vs 8.06 angstrom, respectively), but in solution, the structural dynamics of the two linked metallic units in bis-activated complex 3 accesses conformers with far closer Hf...Hf distances (similar to 3.2 angstrom). Once activated with Ph3C+B(C6F5)(4)(-) (B-1) or PhNMe2H+B(C6F5)(4)(-)(NB), 3 exhibits pronounced bimetallic cooperative effects in ethylene homopolymerization and ethylene +1-octene copolymerization vs the monometallic analogue L-1-HfMe2 (1, L-1 = 2,6-diisopropyl-N-{(2-isopropylphenyl)[6-(naphthalen-1-yl)pyridin-2-yl]methyl}aniline) and bimetallic 4, producing polyethylene with 5.7 times higher M-w, and poly(ethylene-co-1-octene) with 2.4 times higher M-w and 1.9 times greater 1-octene enchainment densities than 1. The activation chemistry of 3 and 4 with 1 or 2 equiv of B-1 and NB is characterized in detail by NMR spectroscopy. In sharp contrast to 1, which undergoes Hf-C-naph protonolysis followed by naphthyl remetalation with NB as the cocatalyst, activation of 3 with B-1 or NB proceeds by consecutive-CH3 protonolysis/abstractions at each Hf center, explaining the higher polymerization activity of 3/NB versus 1/NB. All product polymers have narrow (2-3) PDIs, and this is explained by NMR evidence for very fast exchange of alkyl moieties between the two active Hf metal centers. Key experimental findings are supported by DFT analysis.

• 出版日期2015-9
• 单位Perugia; 西北大学