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The (Ph)2nacnac Ligand in Organochromium Chemistry

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journal contribution
posted on 2002-02-06, 00:00 authored by Leonard A. MacAdams, Woo-Kyu Kim, Louise M. Liable-Sands, Ilia A. Guzei, Arnold L. Rheingold, Klaus H. Theopold
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 46 were determined by X-ray diffraction. Complexes 1 and 35 catalyzed the polymerization of ethylene when activated with MAO, but they did not catalyze the polymerization by themselves.