Synthesis and characterization of chromium complexes 2-Me4CpC6H4CH2(R)NHCrCl2 and their catalytic properties in ethylene homo- and co-polymerization

Abstract

A series of new half-sandwich secondary amine-coordinated dichlorochromium complexes chelated by 2-(tetramethylcyclopentadienyl)benzylamine ligands, 2-Me₄CpC₆H₄CH₂(R)NHCrCl₂ [R = ⁱPr (1), Cy (2), Ph (3), 4-MePh (4), 2,6-Me₂Ph (5), 2,6-Et₂Ph (6)], have been synthesized from the reactions of CrCl₃(THF)₃ with the dilithium salts of the corresponding ligands in THF, followed by the addition of 1/2 eq. of H₂O to the reaction mixtures. The isolated yields of the chromium complexes were found to increase with the increase in the amount of H₂O introduced and reach the highest values (66–76%) when 1/2 eq. of H₂O is added. Attempts to isolate the 2-(tetramethylcyclopentadienyl)benzylamidochromium complexes, 2-Me₄CpC₆H₄CH₂(R)NCrCl, were not successful. The new dichlorochromium complexes were characterized by IR, ¹H NMR, EPR, and UV/Vis spectroscopy and elemental analyses, and the molecular structures of complexes 1, 5 and 6 were determined by X-ray crystallography. The X-ray crystallographic analysis reveals that these chromium complexes possess a three-legged piano-stool geometry with the amine N atom in a mitered six-membered chelating ring and the two chloride atoms as the legs. Upon activation with AlR₃ and Ph₃CB(C₆F₅)₄, complexes 1–6 exhibit reasonable catalytic activity for ethylene polymerization and copolymerization with 1-hexene, producing polyethylenes with moderate to high molecular weights and poly(ethylene-co-1-hexene)s with moderate comonomer incorporation which are typical linear low-density polyethylenes (LLDPE). Complex 4 was found to show higher catalytic activity for ethylene homo- and co-polymerization than other complexes under similar conditions, while complex 3 produced poly(ethylene-co-1-hexene)s with the highest comonomer incorporation.