Synthesis and Characterization of Novel Half-Metallocene-Type Group IV Complexes Containing Phosphine Oxide–Phenolate Chelating Ligands and Their Application to Ethylene Polymerization

A series of novel half-metallocene-type group IV metal complexes containing phosphine oxide–phenolate chelating ligands of type CpMCl2[O-2R1-4R2-6(Ph2PO)C6H2] (Cp = C5H5, M = Ti, 2a: R1 = R2 = H; 2b: R1 = Ph, R2 = H; 2c: R1 = tBu, R2 = H; 2d: R1 = R2 = tBu; M = Zr, 3b: R1 = Ph, R2 = H; 3c: R1 = tBu, R2 = H; 3d, R1 = R2 = tBu) have been synthesized in high yields (60–76%) from CpMCl3 (M = Ti, Zr) with 1.0 equiv of 2R1-4R2-6(Ph2PO)C6H2OH in THF and in the presence of triethylamine, and the complexes were identified by NMR and mass spectra as well as elemental analysis. Structures for 2a–d and 3d were further confirmed by X-ray crystallography. Complexes 2a–d adopt a five-coordinate, distorted square-pyramidal geometry around the titanium center. Complex 3d has a six-coordinate, distorted octahedral geometry around the zirconium center, in which the equatorial positions are occupied by oxygen atoms of the chelating phosphine oxide–phenolate ligand and two chlorine atoms. The cyclopentadienyl ring and the oxygen atom of the THF molecule are coordinated on the axial position. When activated by modified methylaluminoxane, all complexes exhibited moderate to high activities toward ethylene polymerization, giving high molecular weight polymer with a unimodal molecular weight distribution. The substituents on ligands and the metal center as well as the reaction conditions have a profound effect on the polymerization. It should be noted that zirconium complexes 3b–d displayed notable ethylene polymerization activity even at high temperature (75 °C). Moreover, using Ph3CB(C6F5)4/i-Bu3Al in place of MMAO as a cocatalyst, 3b–d also generate high molecular weight polymer with high efficiency under the same conditions.