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

Abstract

Novel Zr(IV) and Hf(IV) complexes bearing two constrained bulky β-enaminoketonato ligands {[ArNCH–C₈H₃(CH₂)_n(R)O]₂MBn₂, M = Zr or Hf; n = 1, 2 or 3; R = H or C₆H₅; Ar = C₆H₅ or C₆F₅} 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.