Chromium alkyl compounds

Homoleptic carbonyl ligands, in palladium complexes, 8, 197 Homoleptic chromium alkyl compounds, preparation,... 

The tetravalent chromium alkyl compounds were found to give catalysts that are somewhat more active than the catalyst made from the divalent chromium counterpart, under commercial reaction conditions (90-110 °C, 0.5-1.5 mol ethylene L ). Indeed, they were among the most active organochromium catalysts tested in our laboratory. Their overall 1-h yield was usually also superior to that observed with some of the best chromium oxide on silica-titania catalysts. Even when compared with chromium oxide systems used with a cocatalyst, the catalysts made with tetravalent chromium alkyls were equal or better in activity. Unfortunately, for commercial applications, these catalysts also tend to make some oligomers and wax as well. 

Chromium alkyls [(>j -(3-t-Bu-5-Mepz)3B)CrR] (R = Et, CH2SiMc3, Ph) follow from the corresponding homoleptic complexes and alkyllithium compounds (97CEJ1668). 

A further dramatic comparison of the comparative reactivities of chromium alkyls in diverse oxidation states was furnished by another set of benzyl complexes. [7] Shown below, these three compound are isomers, yet they range in oxidation state from Cr to Cr °. Of the three, only the mixed-valent complex Cp G0i-ti n3-Bz)Cr(Bz)Cp, containing a Bivalent chromium bound to an T)3-benzyl and a ni-benzyl ligand, catalyzed the polymerization of ethylene. 

The obvious next step was oxidation of the tris(pyrazolyl)borate chromium alkyls to the catalytically active -t-III oxidation state. However, cyclic voltammetry experiments did not show a reversible oxidation in any case, and all attempts to prepare complexes of the type [Tp Bu,Meci-R]+X by chemical oxidation failed, yielding [Tp Cr(THF)n] X instekl. TTie reasons for the apparent instability of TpCr alkyls are not clear, and we arc continuing our efforts to isolate related compounds,... 

B. Chromium(III) Alkyl Compounds Polynuclear Chromium(III) Complexes Polyaminocarboxylic Ligands... 

One is left to ponder initiation by other organochromium catalysts. Chromium allyls or 2,4-dimethylpentadienylchromium(II) could conceivably rearrange into p-l coordination upon addition of ethylene. However, chromocene must initiate the first chain in some other way, because the site must retain the ring. Thus, for chromocene catalysts, the initiation problem is similar to that described for chromium oxide. The diarene-chromium(O) and Cr(0)(CO)6 catalysts may also have this problem. Perhaps this is why these catalysts sometimes initiate polymerization more sluggishly than the chromium alkyls. However, there is also some evidence that the Cr(0) compounds can be oxidized by surface OH groups to leave a Cr-H group, which could also be considered an alkylated species. 

Indeed, one need not necessarily use a chromium alkyl for this purpose, as other organochromium compounds can also be used successfully. The open-ring chromocene, bis(2,4-dimethylpentadienyl) chromium(II), called Cr(DMPD)2, was tested and performed similarly in many respects. Figure 200 presents an example in which this organochromium compound, called Cr(DMPD)2, was added to the reactor along with Cr(VI)/silica (or at the right in the figure, just silica) calcined at 800 °C. 

Other chromium-based catalysts have been explored which, when supported, afford polyethylene with relatively narrow molecular weight distribution. These are based on mono(cyclopentadienyl) chromium alkyl complexes first explored by Theopold. These may be Cr(II) compounds such as [Cp -CrMe]2, Cr(III) oxo compounds such as Cp Gr(0)-Me2, neutral and cationic Cr(III) compounds such as Cp CrMe2(THF) and [Cp CrMe(THF)2][BPh4],227 mixed valence dimers such as Gp Gr( -GH2Ph)(M- 7 7 -CH2Ph)CrCp, or even anionic complexes such as [Li] [Cp Cr(CH2Ph)3] (Table... 

No ji-coordination compounds with a coordinate bond from an aryl group to the metal of the type shown in compound 4.19 that are as stable as those in which the ligand atom is nitrogen, phosphorus, arsine, oxygen, or sulfur have yet been reported [19, 20] the synthesis of these 4.19 compounds is difficult, because p-elimination readily occurs in these alkyl compounds and the Group 6 metals chromium, molybdenum, and tungsten as shown in Fig. 4.5. 

Enantiomeric separation of nonpharmaceutical compounds include IV-alkyl-Af-methylaniline W-oxides (ethyl to butyl plus isomers) on a Chiralcel OD column. (A = 210 nm) using 1% to 3% ethanol in hexane [143]. Carrea et al. [144] separated the enantiomers of various substituted chromium and magnesium tricarbonyl metallocenes ( / -benzene and -cyclopentadiene) on a Chiralcel OD column (A = 315 run) with an isocratic mobile phase that varied from 1% to 10% ethanol in hexane depending on the enantiomeric pair involved. Chromium tricarbonyl compounds complexed with a variety of ij -arenes were separated on a Whelk-O column (A = 315 nm) using a 20/80 IPA/hexane as the mobile phase [145]. 

The Nobel Prize in Chemistry was awarded to Karl Ziegler and Giulio Natta in 1963 for their research in developing olefin polymerization catalysts primarily based on titanium compounds and aluminum alkyl compounds required for the catalyst initiation process. However, by 1963 the details on the discovery of the Cr-based catalyst system in which the chromium compound was supported on amorphous silica were widely published and commercially important for the manufacture of HDPE. In the view of this author, the 1963 Nobel Prize awarded in chemistry should also have included two additional scientists, John P. Hogan and Robert L. Banks from Phillips Petroleum. 

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

Organochromium Catalysts. Several commercially important catalysts utilize organ ochromium compounds. Some of them are prepared by supporting bis(triphenylsilyl)chromate on siUca or siUca-alumina in a hydrocarbon slurry followed by a treatment with alkyl aluminum compounds (41). Other catalysts are based on bis(cyclopentadienyl)chromium deposited on siUca (42). The reactions between the hydroxyl groups in siUca and the chromium compounds leave various chromium species chemically linked to the siUca surface. The productivity of supported organochromium catalysts is also high, around 8—10 kg PE/g catalyst (800—1000 kg PE/g Cr). 

Chromium carbene complexes like 13, which are called Fischer carbene complexes, can conveniently be prepared from chromium hexacarbonyl 11 and an organolithium compound 12, followed by an O-alkylation step ... 

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