Novel zirconium complexes with constrained cyclic β-enaminoketonato ligands: improved catalytic capability toward ethylene polymerization

• Published in: Dalton transactions : an international journal of inorganic chemistry 2016-06, Vol.45 (25), p.138-1318
• Main Authors: Wang, Kai-Ti, Wang, Yong-Xia, Wang, Bin, Li, Yan-Guo, Li, Yue-Sheng
• Format: Article
• Language: English
• Subjects: Bearing; Catalysis; Catalysts; Constraints; Ethylene; Molecular weight; Polymerization; Zirconium
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c6dt01391k

Novel Zr( iv ) and Hf( iv ) complexes bearing two constrained bulky β-enaminoketonato ligands {[ArN&z.dbd;CH-C 8 H 3 (CH 2 ) n (R)O] 2 MBn 2 , M = Zr or Hf; n = 1, 2 or 3; R = H or C 6 H 5 ; Ar = C 6 H 5 or C 6 F 5 } were synthesized and clearly characterized. X-ray crystal structure analysis reveals that these complexes adopt a distorted octahedral geometry. Compared with non-constrained analogues, the Zr( iv ) complexes with a cyclic skeleton exhibited high catalytic activities (up to 16.4 kg PE mmol Zr −1 h −1 ) toward ethylene polymerization at ambient pressure and elevated temperatures. Moreover, the catalytic properties of these complexes could be governed exquisitely by appropriate variation of the N -aryl substituents and the size of the benzocyclane. The Zr( iv ) complexes bearing a non-fluorinated N -aryl group yielded oligomers, while the fluorinated analogues bearing a five-membered or six-membered cyclane group produced high molecular weight polyethylenes (33.4-306 kg mol −1 ) under similar conditions on account of the suppression effects on β-H elimination. The Zr( iv ) complexes are more active toward ethylene polymerization than the Hf( iv ) analogues, and the resulting polymers exhibited higher molecular weight and narrower molecular weight distribution. The synthesized bis(β-enaminoketonato) zirconium complexes bearing a cyclic skeleton exhibited improved capabilities toward ethylene polymerization even at high temperatures.