New Chromium(III) Complexes with Imine−Cyclopentadienyl Ligands: Synthesis, Characterization, and Catalytic Properties for Ethylene Polymerization

Abstract

A series of new half-sandwich chromium(III) complexes chelated with (2-((arylimino)methyl)phenyl)tetramethylcyclopentadienyl ligands, 2-(ArN═CH)C6H4Me4CpCrCl2 (Ar = 2,6-Me2C6H3 (1), 2,6-Et2C6H3 (2), 2,6-iPr2C6H3 (3), 4-MeC6H4 (4)), have been synthesized from the reaction of CrCl3 with the lithium salt of the corresponding ligand 2-(ArN═CH)C6H4Me4CpLi (Ar = 2,6-Me2C6H3 (LiL1), 2,6-Et2C6H3 (LiL2), 2,6-iPr2C6H3 (LiL3), 4-MeC6H4 (LiL4)). Free ligands HL1−HL4 were prepared by the condensation reaction of 2-(tetramethylcyclopentadienyl)benzaldehyde with 2,6-dialkylaniline. The free ligands were characterized by 1H NMR spectroscopy, while the chromium(III) complexes were characterized by elemental analyses and single-crystal X-ray crystallography. The X-ray crystallographic analysis indicates that the imine N atom in these complexes coordinates to the central chromium atom. Upon activation with AlR3 and Ph3CB(C6F5)4, these complexes exhibit high catalytic activity for ethylene polymerization and produce polyethylene with moderate to high molecular weights. These chromium(III) complexes can also be activated with AlR3 alone. In the latter case, they show slightly lower catalytic activity for ethylene polymerization in comparison to the AlR3/Ph3CB(C6F5)4 activated catalyst systems. The effects of ligand structure, polymerization temperature, AlR3, and Al/Cr molar ratio on the catalytic behavior of these complexes were examined.