Highly substituted zirconium and hafnium cyclopentadienyl bifunctional β-diketiminate complexes – Synthesis, structure, and catalytic activity towards ethylene polymerization

Abstract

A series of tetramethyl-(Cp′) and pentamethylcyclopentadienyl (Cp*) metal complexes (η5-C5Me4R)ZrCl2LCO (R = H(1), Me(2)) and (η5-C5Me4R)HfCl2LCO (R = H(3), Me(4)) where LCO is a bifunctional β-diketiminate ligand [2-(MeO)–C6H4)]NC(Me)CHC(Me)N[2-(MeO)–C6H4)] (LCO) were prepared from corresponding tetramethyl- and (pentamethylcyclopentadienyl)metal trichlorides and the lithium precursor LiLCO. All complexes were characterized by their melting point, elemental analysis, 1H and 13C NMR spectroscopy, IR spectroscopy. In addition their solid-state structures were determined by X-ray diffraction techniques. In all cases, a pseudo-tetragonal pyramidal coordination environment of the metal centre with cyclopentadienyl ligand on the top and nearly isobidentately bonded nitrogen atoms of the chelating LCO ligand in an open η5-fashion was observed. The oxygen atoms positioned at the diketiminate periphery remained uncoordinated to transition metal in the solid phase as well as in solution.Besides the desired compounds, two hafnium complexes bearing either 2-methoxyaniline or 2-methoxyanilide ligands were isolated and structurally characterized as minor decomposition and dismutation products of 3 and 4, respectively.Complexes containing the β-diketiminate ligand as well as its precursors were tested for catalytic activity in the polymerization of ethylene using different activators (MAO, MMAO and AliBu3/(Ph3C)+ [B(C6F5)4]−). Among the new complexes, the best activity as high as 340 kgPE (molMhbar)−1 was achieved with 1/AliBu3/(Ph3C)+ [B(C6F5)4]− system.In attempts to clarify the activation process of group 4 metal-based precatalysts, the synthesis and NMR studies of the aluminium β-diketiminate complex Al(iBu)2LCO were also carried out.