Highly Thermally Stable Eight-Coordinate Dichloride Zirconium Complexes Supported by Tridentate [ONN] Ligands: Syntheses, Characterization, and Ethylene Polymerization Behavior

A series of novel eight-coordinate
dichloride zirconium complexes
with the general formula L2ZrCl2 supported by
two tridentate [ONN] ligands have been synthesized by the reaction
of ZrCl4·2THF with 2 equiv of the corresponding ligands
(tridentate phenoxy-imine ligands (L1H–L4H) or tridentate phenoxy-amine ligands (L5H and L6H)). These complexes were characterized
by NMR and elemental analyses. The molecular structures of representative
zirconium complexes C2 and C6 were confirmed
by single-crystal X-ray diffraction and revealed that the metal center
is eight-coordinated by two tridentate [ONN] ligands and two chlorides
in a distorted-square-antiprismatic geometry. When C1–C6 were activated by modified methylaluminoxane
(MMAO), the resultant catalysts displayed notable thermal stability
and high activities toward ethylene polymerization. The ligand substituents,
the metal coordination environment, and the reaction conditions had
a profound effect on the polymerization. The catalytic activity increases
consistently with increasing polymerization temperature, and the highest
activity of 1.04 × 106 g of PE (mol of Zr)−1 h–1 was achieved in o-xylene
at 140 °C. A catalytic lifetime of nearly 5 h was observed for
the C5/MMAO catalyst system at 140 °C.