Acta Chimica Sinica ›› 2020, Vol. 78 ›› Issue (8): 778-787.DOI: 10.6023/A20030092 Previous Articles     Next Articles



张雷, 马海燕   

  1. 华东理工大学化学与分子工程学院 金属有机化学实验室 上海 200237
  • 投稿日期:2020-03-27 发布日期:2020-06-16
  • 通讯作者: 马海燕
  • 基金资助:

Ethylene-Bridged Multi-Substituted Indenyl-Fluorenyl Zirconocene and Hafnocene Complexes: Synthesis, Structure and Catalytic Behavior for Propylene Selective Oligomerization

Zhang Lei, Ma Haiyan   

  1. Laboratory of Organometallic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237
  • Received:2020-03-27 Published:2020-06-16
  • Supported by:
    Project supported by the National Natural Science Foundation of China (No. 21274041).

In metallocene-mediated propylene polymerization, β-methyl elimination (β-Me elimination) is considered as the key chain-release step for obtaining allyl-terminated products, which are highly preferred as macro(co)monomers or building blocks for preparing novel polymers. However, for most metallocene catalysts the transfer of a β-methyl is instinctively less favored due to its steric and electronic disadvantages. Up to date, very few cases have been found to be efficient for triggering selective β-methyl elimination. In this work, a series of novel ansa-metallocene complexes, ansa-C2H4-{2-Me-3-Bn- 5,6-[1,3-(CH2)3]Ind}(Flu)ZrCl2 (C1), ansa-C2H4-{2-Me-3-Bn-5,6-[1,3-(CH2)3]Ind}(2,7-tBu2-Flu)ZrCl2 (C2), ansa-C2H4-{2-Me-3-Bn-5,6-[1,3-(CH2)3]Ind}(3,6-tBu2-Flu)ZrCl2 (C3) and ansa-C2H4-{2-Me-3-Bn-5,6-[1,3-(CH2)3]Ind}(Flu)HfCl2 (C4), were synthesized via the reaction of the dilithium salts of the corresponding proligand with 1 equiv. of ZrCl4 or HfCl4 in Et2O. All complexes were characterized by 1H NMR, 13C NMR and elemental analysis. The molecular structures of complexes C1, C2, and C3 were further determined via X-ray diffraction method. In the solid state, these complexes adopted an indenyl-backward orientation with rotation angles (RA: the orientation of the indenyl ring with respect to the fluorenyl ring) ranging from -11.30° to -17.07°. Upon activation with modified methylaluminoxane (MMAO) or AliBu3/ [Ph3C][B(C6F5)4] (TIBA/TrB), all these complexes exhibited moderate to high activities for propylene oligomerization at 40~100 ℃, affording propylene oligomers with both allyl and vinylidene chain-ends, which arised from β-Me elimination and β-H eliminations respectively. The methyl group at the 2-position of the indenyl ring turned out to have negative effects on both catalytic activity and β-Me elimination selectivity. Zirconocene complex C1 polymerized propylene to give oligomers with 40%~52% allyl chain-ends. However, further modification of the fluorenyl moiety allowed a great improvement in β-Me elimination selectivity. At 40~100 ℃, zirconocene complexes C2 and C3 bearing a 2,7- or 3,6-di-tert-butyl- substituted fluorenyl moiety showed significantly higher β-Me elimination selectivities (C2, 81%~86%; C3, 68%~77%), affording propylene oligomers (Mn 400~4500 g·mol-1) with allyl-dominant chain-ends. Moreover the substitution pattern of the fluorenyl moiety also substantially influenced the catalytic activities. The incorporation of an electron-donating 2,7-di-tert-butyl groups on the fluorenyl moiety led to notably increased catalytic activities of complex C2 at higher temper- atures above 60 ℃, while complex C3 bearing a 3,6-di-tert-butyl-substituted fluorenyl moiety showed lowest activities among the zirconocene series due to its overcrowded coordination sites. Compared with its zirconocene analogue, the hafnocene complex C4 activated with TIBA/TrB proved to be even more selective toward β-Me elimination, and meanwhile gave products with much lower molecular weights. At 100 ℃, the hafnocene system mainly oligomerized propylene to dimers and trimers. Studies on the dependence of the product molecular weight and the chain-release selectivity on monomer concentration suggested that both β-Me and β-H elimination involved in these systems mainly operate in a bimolecular pathway.

Key words: metallocene catalyst, propylene oligomerization, allyl chain-end, β-methyl elimination