The synthesis, structure and reactivity of beta-diketiminato chromium complexes for olefin polymerization.

摘要

This thesis describes research on paramagnetic chromium complexes coordinated by β-diketiminato ligands. Chapter 1 discusses a novel class of homogeneous β-diketiminato chromium complexes that have been prepared and characterized. (Ph)₂nacnacCrCl ₂THF₂ ((Ph)₂nacnac = 2,4-pentane-N,N′ bis(phenyl)ketiminimato) was synthesized and found to polymerize ethylene in the presence of excess methylaluminoxane (MAO). Attempted alkylation of the dichloride resulted in a disproportionation to a chromium(II) species, [(Ph)₂nacnac]₂Cr.; Chapter 2 details the synthesis, characterization and reactivity of β-diketiminato organochromium complexes in various oxidation states. Alkylation of the dinuclear chloride [(2,6-iPr₂Ph)₂nacnacCrCl(μ-Cl)] ₂ resulted in reduction to the chromium (II) species [(2,6-i Pr₂Ph)₂nacnacCr(μ-Cl)] ₂. A series of chromium (II) alkyls of the type [(2,6-iPr ₂Ph)₂nacnacCrR]_n, (R = methyl, ethyl, n = 2; R = trimethylsilylmethyl, benzyl, n = 1) could be isolated by alkylation of the chloride. In the presence of MAO, the chromium (II) alkyls catalyzed the polymerization of ethylene, however, without cocatalyst only oligomers of ethylene were obtained. Oxidation of the methyl derivative resulted in formation of the chromium (III) alkyl cation [(2,6-iPr₂Ph) ₂nacnacCrCH₃(Et₂O)]+BAr₄ −. The chromium (III) alkyl cation catalyzed the polymerization of ethylene and the copolymerization of ethylene with 1hexene. Chromium complexes in the +IV and +V oxidation states have also been isolated and their reactivity is described.; Chapter 3 reports on the synthesis, characterization and reactivity of (2,6-McPh)₂nacnac chromium (III) complexes. The chloride species, (2,6-Me₂Ph)₂nacnacCrCl₂(THF)₂ could be readily alkylated with either LiCH₂SiMe₃ or McMgCl to yield (2,6-Me₂Ph)₂nacnacCr(CH₂Si(CH ₃)₃)₂ or (2,6-Me₂Ph)₂nacnacCrMe ₂(THF), respectively. Although they are coordinatively unsaturated, the dialkyls did not polymerize ethylene without cocatalyst. The addition of cocatalysts (MAO or B(C₆F₅)₃ imparted catalytic activity to the dialkyls and allowed for the isolation of polyethylene. Reaction of the trimethylsilylmethyl derivative with H+(Et ₂O)₂+BARF− resulted in the isolation of a chromium (III) alkyl cation [(2,6-Me₂Ph) ₂nacnacCrCH₂Si(CH₃)₃]+BARF −, which catalyzed the polymerization of ethylene in the absence of cocatalyst.