A Chelating N-Heterocyclic Carbene Ligand in Organochromium Chemistry

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Abstract

Several chromium complexes supported by a chelating N-heterocyclic carbene ligand have been prepared in an attempt to study their ability to catalyze the polymerization of ethylene. LCrCl3(THF) (3) (L = 1,1‘-methylene-3,3‘-di-2,6-diisopropylphenylimidazole-2,2‘-diylidene (2)) was found to polymerize ethylene when activated by either excess methylaluminoxane (MAO) or excess Et2AlCl. Chloride abstraction from 3 with Na+BARF - (BARF = tetrakis(3,5-di-trifluoromethylphenyl)borate) yielded a dinuclear, chloride-bridged product, [(LCrCl)2(μ-Cl)3]+BARF - (4). In a similar fashion, reaction of 3 with 3.0 equiv of AlMe3 gave the dinuclear, chloride-bridged alkyl cation [(LCrMe)2(μ-Cl)3]+AlX4- (X = Me, Cl) (5), which is isostructural to 4. A triaryl complex was also prepared by reaction of 2 with CrPh3(THF)3, giving LCrPh3 (6) in good yield. Similarly, a series of alkyl and aryl chloride complexes of the form LCrRCl2(THF) (R = Me (7a), Ph (7b), tolyl (7c)) have also been made from the appropriate CrRCl2(THF)3 starting material. Further treatement of 7a with Na+BARF- resulted in the formation of the BARF- salt of complex 5. Complexes 3−6 and 7a−c catalyze the polymerization of ethylene when activated by MAO. Preliminary studies with Cr(II) have shown that LCrX2(THF) (X = Cl (8a), Br (8b)) does not polymerize ethylene upon activation by either MAO or alkylaluminum reagents. Complexes 3−6, 7c, and 8b have been structurally characterized by X-ray diffraction methods.

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