Metallic alkyl compounds

D. G. H. Ballard Polymerisation by homogeneous transition metal alkyl compounds, pp. 213-264 (25). 

II. Soluble Transition Metal Alkyl Compounds as Polymerization Catalysts. 266... 

III. Ligand Replacement in Transition Metal Alkyl Compounds and... 

Attempts to synthesize transition metal alkyl compounds have been continuous since 1952 when Herman and Nelson (1) reported the preparation of the compound C H6>Ti(OPri)3 in which the phenyl group was sigma bonded to the metal. This led to the synthesis by Piper and Wilkinson (2) of (jr-Cpd)2 Ti (CH3)2 in 1956 and a large number of compounds of titanium with a wide variety of ligands such as ir-Cpd, CO, pyridine, halogen, etc., all of which were inactive for polymerization. An important development was the synthesis of methyl titanium halides by Beerman and Bestian (3) and Ti(CH3)4 by Berthold and Groh (4). These compounds show weak activity for ethylene polymerization but are unstable at temperatures above — 70°C. At these temperatures polymerizations are difficult and irreproduceable and consequently the polymerization behavior of these compounds has been studied very little. In 1963 Wilke (5) described a new class of transition metal alkyl compounds—x-allyl complexes,... 

The results of polymerizing ethylene using varying sigma-bonded transition metal alkyl compounds are summarized in Table VII. It is evident that none of the catalysts are very active and are comparable with the simple allyl compounds listed in Table I. 

The above substitution effects appear to be independent of the nature of the ligand (16) since the benzyl compounds behave similarly, Table XI. It would appear from these observations that the introduction of anionic ligand would be sufficient to increase activity of transition metal alkyl compounds for polymerization. This, however, is probably an oversimplifica-... 

IV. Heterogeneous Polymerization Catalysts Derived from Transition Metal Alkyl Compounds... 

Transition metal alkyl compounds react with the -OH groups on the surface of silica in a manner similar to that described for the silanol [reaction (13)] and as with the latter more than one type of bonding is possible. Silica dried at 200°C reacts with Zr(allyl)4 to give two molecules of propene per metal atom and utilizing in the course of this process two -OH groups per metal atom. The chemistry of the process is accurately described by the equation... 

Polymerization of Ethylene with Transition Metal Alkyl Compounds in Toluene at 80°C... 

Polymerization of Propylene by Transition Metal Alkyl Compounds Toluene as Solvent, Temperature 65°C. Ethylene Pressure 10 atm (IS, 16)... 

Ballard et al. (15) have found that transition metal alkyl compounds of... 

It has been shown (p. 266) that transition metal alkyl compounds containing Cpd and C6H6 groups, ir-bonded to the metal inactivate the metal center for polymerization. It has also been shown by Nyholm and Aresta (45), in the platinum series, that five- or six-membered rings containing only sigma and ir-carbon-to-metal bonds are very stable compounds. These observations add chemical plausibility to reaction (29). 

Comparison of the homogeneous polymerizations of transition metal alkyl compounds with their heterogeneous equivalents shows that the higher activity of the latter is due to ... 

Certain metal alkyl compounds from p- and d-block elements react under very mild conditions with 1 under insertion into the element-carbon bond. Some examples are shown in Scheme 9. 

Finally, let us briefly consider the analogous behavior of MR metal alkyl compounds. Tables 4.8 and 4.9 summarize geometrical and NBO data for monomethy-lated complexes of the form MHnMe. In agreement with simple Lewis-like pictures... 

The transition group compound (catalyst) and the metal alkyl compound (activator) form an organometallic complex through alkylation of the transition metal by the activator which is the active center of polymerization (Cat). With these catalysts not only can ethylene be polymerized but also a-olefins (propylene, 1-butylene, styrene) and dienes. In these cases the polymerization can be regio- and stereoselective so that tactic polymers are obtained. The possibilities of combination between catalyst and activator are limited because the catalytic systems are specific to a certain substrate. This means that a given combination is mostly useful only for a certain monomer. Thus conjugated dienes can be polymerized by catalyst systems containing cobalt or nickel, whereas those systems... 

Metal of the metal alkyl compound Ionic radii of the metal, A. Polypropylene not extractable in boiling n-heptane %... 

Simple fractional distillation processes for purification of metalorganics can be employed to remove some of these impurities, but this is a very inefficient approach. A dramatic improvement in the yield of many high-purity metal alkyl compounds resulted from the development of the adduct-purification scheme for the purification of metal alkyls, which was commercially developed by A. C. Jones and coworkers. This process uses the strong tendency of many metal alkyls to form stable adduct compounds with other reactants, thus making a difficult problem that is encountered in the epitaxial growth arena into an useful advantage in the synthetic arena. Actual synthetic and purification routes employed in the manufacture of metal alkyls are proprietary. It is a challenge to develop an optimized synthetic process that has the required purity, efficiency, volume, reproducibility, and yield. 

Finally, the apparent thermal stabilities of alkyl-cobalamins, as well as of some of the other transition-metal-alkyl compounds that have been examined in the course of these studies, generally are higher than would correspond to their metal-C bond-dissociation energies. The most probable explanation for this is that, in the absence of effective radical scavengers, homolytic dissociation of metal-alkyl bonds occurs reversibly because of selective recombination of the initially produced radicals and metal complexes. 

The position of the methylene group in both spectra is significantly different from that in the spectra of the corresponding metal alkyl compounds and could be considered generally as a measure of the isomer ratio. For rapid exchange between two tautomeric forms, whose spectra are known, the ratio of the tautomeric forms could be calculated by using the additivity of CH2 group chemical shift. 

Two notable points from the aforegoing discussion are as follows. (1) By far the majority of the known monohalogenoalkyl compounds are of the group VTII transition elements there are very few early transition metal halogenoalkyl compounds known. (2) Very few monofluoroalkyl metal complexes have been prepared. The lack of early transition metal halogenoalkyl compounds may be in part due to the high electropositivity of these metals, which facilitates a- and / -elimination reactions. Related hydride elimination reactions almost certainly occur more easily for early transition metal alkyl compounds than for later transition metal compounds. In this regard it is particularly noteworthy that one of the only early transition metal haloalkyl compounds mentioned is the fluoroethyl scandium com-... 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

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