The (Ph)₂nacnac Ligand in Organochromium Chemistry

A novel class of β-diketiminato chromium complexes was prepared to serve as a homogeneous model system for the heterogeneous Phillips ethylene polymerization catalyst. (Ph) 2 nacnacCrCl 2 (THF) 2 ( 1, (Ph) 2 nacnac = N, N ‘-diphenyl-2,4-pentanediimine anion) was found to polymerize ethylene in the presence of excess methylaluminoxane (MAO). Reaction of 1 with 1.0 equiv of an alkylzinc compound yielded dimeric, chloride-bridged monoalkyl complexes of the type [(Ph) 2 nacnacCrR(μ-Cl)THF] 2 (R = Me ( 3a ), Et ( 3b )). Reaction of 1 with 2.0 equiv of alkyllithium reagents resulted in a disproportionation to [(Ph) 2 nacnac] 2 Cr ( 2 ) and CrR 4 . 2 could be oxidized by treatment with tetrachloroethane to give trigonal-bipyramidal [(Ph) 2 nacnac] 2 CrCl ( 5 ). Attempted alkylation of 5 with aluminum alkyls yielded the ortho-metalated product (Ph) 2 nacnac(η 1 -C 6 H 4 -Ph)nacnacCr ( 6 ). However, the chromium(III) aryl complex [(Ph) 2 nacnac] 2 CrPh ( 4 ) could be synthesized by reaction of CrPh 3 (THF) 3 with 2 equiv of (Ph) 2 nacnacH. Square-pyramidal 4 featured both a normal planar η 2 -nacnac and a nonplanar η 3 -nanac ligand. Thermolysis of 4 resulted in the loss of benzene along with the formation of 6 . The crystal structures of 2, 3a, and 4 − 6 were determined by X-ray diffraction. Complexes 1 and 3 − 5 catalyzed the polymerization of ethylene when activated with MAO, but they did not catalyze the polymerization by themselves.