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)₂nacnacCrCl₂(THF)₂ (1, (Ph)₂nacnac = 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)₂nacnacCrR(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)₂nacnac]₂Cr (2) and CrR₄. 2 could be oxidized by treatment with tetrachloroethane to give trigonal-bipyramidal [(Ph)₂nacnac]₂CrCl (5). Attempted alkylation of 5 with aluminum alkyls yielded the ortho-metalated product (Ph)₂nacnac(η¹-C₆H₄-Ph)nacnacCr (6). However, the chromium(III) aryl complex [(Ph)₂nacnac]₂CrPh (4) could be synthesized by reaction of CrPh₃(THF)₃ with 2 equiv of (Ph)₂nacnacH. Square-pyramidal 4 featured both a normal planar η²-nacnac and a nonplanar η³-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.