Monochloro non-bridged half-metallocene-type zirconium complexes containing phosphine oxide-(thio)phenolate chelating ligands as efficient ethylene polymerization catalysts

Abstract

A series of novel monochloro half-zirconocene complexes containing phosphine oxide-(thio)phenolate chelating ligands of the type, ClCp′Zr[X-2-R¹-4-R²-6-(Ph₂PO)C₆H₂]₂ (Cp′ = C₅H₅, 2a: X = O, R¹ = Ph, R² = H; 2b: X = O, R¹ = F, R² = H; 2c: X = O, R¹ = ^tBu, R² = H; 2d: X = O, R¹ = R² = ^tBu; 2e: X = O, R¹ = SiMe₃, R² = H; 2f: X = S, R¹ = SiMe₃, R² = H; Cp′ = C₅Me₅, 2g: X = O, R¹ = SiMe₃, R² = H), have been synthesized in high yields. These complexes were identified by ¹H {¹³C} NMR and elemental analyses. Structures for 2b, 2c and 2f were further confirmed by X-ray crystallography. Structural characterization of these complexes reveals crowded environments around the zirconium. Complexes 2b and 2c adopt six-coordinate, distorted octahedral geometry around the zirconium center, in which the equatorial positions are occupied by three oxygen atoms of two chelating phosphine oxide-bridged phenolate ligands and a chlorine atom. The cyclopentadienyl ring and one oxygen atom of the ligand are coordinated on the axial position. Complex 2f also folds a six-coordinate, distorted octahedral geometry around the Zr center, consisting of a Cp–Zr–O (in PO) axis [177.16°] and a distorted plane of two sulfur atoms and one oxygen atom of two chelating phosphine oxide-bridged thiophenolate ligands as well as a chlorine atom. When activated by modified methylaluminoxane (MMAO), all the complexes exhibited high activities towards ethylene polymerization at high temperature (75 °C), giving high molecular weight polymers with unimodal molecular weight distribution. The formation of 14-electron, cationic metal alkyl species might come from the Zr–O (in phenol ring) bond cleavage based on the DFT calculations study.