The (Ph)2nacnac ligand in organochromium chemistry

A novel class of beta-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(mu-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 CrR4. 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(eta(1)-C6H4-Ph)nacnacCr (6). However, the chromium(III) aryl complex [(Ph)(2)nacnac](2)CrPh (4) could be synthesized by reaction of CrPh3(THF)(3) with 2 equiv of (Ph)(2)nacnacH. Square-pyramidal 4 featured both a normal planar eta(2)-nacnac and a nonplanar eta(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.