Chromium alkyls

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. 

Constrained geometry chromium alkyls catalyzed the polymerization of ethylene however, the reaction was relatively slow, and elevated pressures (PC2H4 = 500 psi) were required to generate significant amounts of polymer. Not surprisingly then, no homopolymoization or copolymerization of a-olefins was observed. Instead, catalytic isomerization and dimerization of the alkyl-substituted olefins was found. 

Based on our observation in these two systems, it would appear that Cp Cr -alkyls, if rendered electrophilic and/or sufficiently coordinatively unsaturated, will both bind and insert a-olefins. However, the more heavily substituted alkyl ligands thus formed (i.e. CrBl-CH2-CH(R)-P vs. Crni-CH2-CH2-P resulting from ethylene insertion) seem to be very susceptible to facile 3-hydrogen elimination. Rapid chain transfer and very low molecular weights are the results of this tendency. Whether the latter is an innate property of all chromium alkyls or reflects the particular chemical nature of the Cp Cr-moiety remains to be established. To this end, investigation of chromium alkyls with a variety of other ancillary ligands are needed. 

CHROMIUM ALKYLS WITH NITROGEN- AND OXYGEN-LIGANDS 3.1. Tris(pyrazolyl)borate complexes... 

The alkyls Tp Cr-R are the best test case yet of the catalytic activity of CrU alkyls (see Section 1). However, they did not react with ethylene, even at elevated temperature. On the contrary, Tp - Cr-Et eventually decomposed by an apparent P-hydrogen elimination yielding Tp - Cr-H and ethylene. Thus our notion that divalent chromium alkyls are not the chain propagating species in polymerization catalysis receives further support... 

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,... 

Paramagnetic chromium alkyls in the +11 and +111 oxidation states have been reacted with O2. For example, treatment of Cp Cr(py)Me2 [21] or... 

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

Pulse radiolysis has been used to study the transient formation and decomposition of cobalt-alkyl bonds in aqueous solution in the same manner as it has been used for chromium alkyls. And as for chromium alkyls, bond homolysis is a major decomposition pathway (28). For bond formation reactions, pulse radiolysis shows that they are assisted by increases in pressure. This feature results from the homolysis having a larger activation volume than the bond formation reaction, resulting in a significantly negative overall reaction volume for the process (29). In general for all of these metal-alkyl bond homolysis reactions of the aquo complexes, steric hindrance facilitates the reaction. Ligand effects also play a role, but the factors involved are more subtle. 

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. 

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. 

For example, the trimethylsilylmethyl derivative of chromium(II) is well suited to this purpose. Although it produces a highly active catalyst on aluminophosphate or fluoride-treated alumina supports, it is barely active on silica by itself. Nevertheless, when added to silica-supported Cr(II) oxide, the result is a highly active catalyst that produces branched polymer. In addition to reacting with silanol groups, the chromium alkyl may also react with chromium oxide to again produce mono-attached species, such as is shown in Scheme 44. Coordination between one Cr atom and its chromium or oxide neighbor also seems likely. 

One of the most interesting results of this approach of using "two-valent" chromium species is the effect on catalyst activity. Yields as high as 16 kg g 1 h 1 were obtained, which was more than twice that of the chromium oxide parent, and many times more than that of the chromium alkyl when deposited on silica. The induction time of chromium oxide was eliminated, and also the declining kinetics profile of the chromium alkyl catalyst. That is, the hybrid catalyst seemed to have incorporated the best aspects of both parents to yield unusually high polymerization activity. 

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. 

Figure 11 Well-defined cationic chromium alkyl catalysts for ethylene polymerisation...

It may be noted also that there are a number of reasonably stable lithium alkyl anions, which are made by interaction of lithium alkyls with metal halides. Some of these, notably the copper(i) alkyls, Li[CuR2], are important reagents in organic synthesis (see Section 25-H-2). Some chromium alkyl salts have anions with multiple metal—metal bonds,49 e.g., Li4[Cr2Me8] 4C4HsO. 

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... 

To contrast our findings for late TMs with data from a metal situated earlier in the d-block, we return to the Cp-donor-based chromium-alkyl complexes already addressed in Section 1.2. 

A chromium alkyl bond that may be formed from L CrX , (X = halogen) by addition of an alkylating agent (usually MAO) ... 

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