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

详细信息    查看全文

• 作者：Yanshan Gao ; Aidan R. Mouat ; Alessandro Motta ; Alceo Macchioni ; Cristiano Zuccaccia ; Massimiliano Delferro ; Tobin J. Marks
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：September 4, 2015
• 年：2015
• 卷：5
• 期：9
• 页码：5272-5282
• 全文大小：751K
• ISSN：2155-5435

文摘

Homobimetallic Hf(IV) complexes, L²-Hf₂Me₅ (3) and L²-Hf₂Me₄ (4) (L² = N,N鈥?{[naphthalene-1,4-diylbis(pyridine-6,2-diyl)]bis[(2-isopropylphenyl)methylene)]bis(2,6-diisopropylaniline}), were synthesized by reaction of the free ligand L² with the appropriate Hf precursor and were characterized in solution (NMR) and in the solid state (X-ray diffraction). In 3, L² 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 L². In the solid state, the Hf路路路Hf distance is significantly different in 3 vs 4 (6.16 vs 8.06 脜, respectively), but in solution, the structural dynamics of the two linked metallic units in bis-activated complex 3 accesses conformers with far closer Hf路路路Hf distances (鈭?.2 脜). Once activated with Ph₃C⁺B(C₆F₅)₄^{鈥?/sup> (B₁) or PhNMe₂H+B(C₆F₅)₄鈥?/sup> (NB), 3 exhibits pronounced bimetallic cooperative effects in ethylene homopolymerization and ethylene +1-octene copolymerization vs the monometallic analogue L1-HfMe₂ (1, L1 = 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 B₁ and NB is characterized in detail by NMR spectroscopy. In sharp contrast to 1, which undergoes Hf鈥揅_naph protonolysis followed by naphthyl remetalation with NB as the cocatalyst, activation of 3 with B₁ or NB proceeds by consecutive 鈭扖H₃ protonolysis/abstractions at each Hf center, explaining the higher polymerization activity of 3/NB versus 1/NB. All product polymers have narrow (2鈥?) 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.}