Chromium Complexes with Me2Si-Bridged Cyclopentadienyl-imidazolin-2-imine Ligands: Synthesis, Structure, and Use in Ethylene Polymerization Catalysis

Abstract

Imidazolin-2-imino-functionalized tetramethylcyclopentadienes 3-H have been prepared in high yields by the reaction of 2 equiv of 1,3,4,5-tetramethylimidazolin-2-imine (1a), 1,3-diisopropyl-4,5-dimethylimidazolin-2-imine (1b), and 1,3-di-tert-butylimidazolin-2-imine (1c) with 5-(chlorodimethylsilyl)-1,2,3,4-tetramethyl-1,3-cyclopentadiene (2), whereas the corresponding indene 6-H has been obtained from the reaction of 1b with 1-(chlordimethylsilyl)indene (5). Deprotonation of 3-H and 6-H with the Schlosser base (KOtBu/nBuLi) affords the highly reactive potassium salts (3)K and (6)K, which are useful precursors for the coordination of these ligands to transition metals. The chromium complexes 4a, 4b, 4c, and 7 have been prepared by salt metathesis reactions of (3)K and (6)K with [CrCl3(THF)3], and the molecular structures of all four paramagnetic complexes have been established by X-ray diffraction analyses. In the case of 4a, 4b, and 7, the formation of constrained-geometry complexes with chelating Me2Si-bridged cyclopentadienyl-imidazolin-2-imine ligands is observed. The presence of short Cr−N bonds is indicative of the strong electron-donating capacity of the imidazoline-2-imino-nitrogen atom, which is based on the ability of the imidazolium ring to effectively stabilize a positive charge. In contrast, the steric bulk of the tert-butyl groups in 4c prevents coordination of the imine moiety. Activation of the chromium complexes 4 and 7 with methylaluminoxane (MAO) affords catalysts for the polymerization of ethylene at ambient temperature and under normal pressure.