Indenyl Functionalized N-Heterocyclic Carbene Complexes of Chromium: Syntheses, Structures, and Reactivity Studies Relevant to Ethylene Oligomerization and Polymerization

The chromium(III) complexes of the type (LCrXY)-Cr-1 (L-1 = 1-(2,6-diisopropylphenyl)-3-[beta-(4,7-dimethylindenyl)ethyl]-imidazol-2-ylidene, X, Y = Cl, 2a-Cl; X = Cl, Y = Me, 2a-Me; X = Cl, Y = Ph, 2a-Ph; X, Y = benzyl, 2a-Bz) and (LCrCl2)-Cr-2 (L-2 = 1-(2,6-diisopropylphenyl)-3-[gamma-(4,7-dimethylindenyl)propyl]-imidazol-2-ylidene, 2b-CI) have been prepared and characterized by analytical and diffraction methods. The salts [(LCrCl)-Cr-1(THF)](+)[BAr4F] and [(LCrCl)-Cr-2(THF)]+[BAr4F] (Ar-F = 3,5-bis(trifluoromethyl)phenyl) were also characterized, having been obtained by abstraction of chloride, or indirectly a methyl, from 2a-Cl or 2a-Me, and 2b-Cl, by NaBAr4F, respectively. Reaction of 2a-Bz with 1 equiv of [H(Et2O)(2)][BAr4F] in THF gave the thermally unstable salt [(LCr)-Cr-1(Bz)(THF)][BAr4F], [2a-Bz](+)[BAr4F], which was also characterized crystallographically. The chromium(II) analogue (LCrCl)-Cr-1, 3a, was prepared from CrCl2 and (LK)-K-1, but on prolonged standing in THF, it converted to the dimeric complex 4a. The complexes 2a-Cl, 2b-Cl, 2b-Me, and 2a-Bz in the presence of an Al activator were tested as ethylene polymerization and oligomerization catalysts, in general, showing activity toward polymer formation.