Zirconium and hafnium complexes supported by linked bis(β-diketiminate) ligands: synthesis, characterization and catalytic application in ethylene polymerization

Abstract

Zirconium and hafnium complexes bearing new 1,2-ethanediyl- or 1,3-propanediyl-linked bis(β-diketiminate) ligands, [{C_nH_2n-(BDI^Ar)₂}MCl₂] (Ar = 2,6-Me₂-C₆H₃, 2,6-Cl₂-C₆H₃, 2,6-ⁱPr₂-C₆H₃; M = Zr, n = 2 (4a–c), n = 3 (5a–c); M = Hf, n = 2 (6b)), were synthesized via the reaction of MCl₄·2THF and one equivalent of dilithium salt of the corresponding ligand. Distorted trigonal prismatic and octahedral coordination geometries as well as C₁-symmetric structures are found for zirconium complexes 4c and 5c in the solid state. Variable temperature ¹H NMR spectra indicated the fluxional nature of 4a and 5a in solution. Upon activation with methylaluminoxane (MAO), all these complexes except hafnium complex 6b displayed moderate catalytic activities for ethylene polymerization. 1,2-Ethanediyl-linked complexes 4a and 4b are generally more active than their 1,3-propanediyl-linked analogues. The substituents at the ortho-positions of the phenyl rings have different effect on the catalytic activities of 1,2-ethanediyl-linked series or 1,3-propanediyl-linked series. It is noteworthy that even at a low Al/Zr molar ratio of 500, the catalytic activities of these zirconium complexes could be retained. Polyethylenes with broad molecular weight distributions (MWD = 15.3–20.3) were produced, which might result from the fluxional character of the zirconium complexes. The linear structure of obtained polyethylenes was further determined by ¹³C NMR spectroscopy and DSC analysis.