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.
A structurally characterized cationic chromium(III) alkyl featuring a bulky nacnac ligand catalyzes the polymerization of ethylene as well as the copolymerization of ethylene with alpha-olefins. This well-characterized homogeneous catalyst constitutes a structural as well as functional model of the widely used heterogeneous Phillips olefin polymerization catalyst.
Hydrogenolysis of a Cr(III) dialkyl precursor produced a binuclear chromium complex with a bridging hydride and a bridging alkyl; this structurally characterized organometallic compound is thermally very stable and does not undergo the expected reductive elimination of alkane.
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