Alkyls, transition metal

Tp )FeMe] [Tp = phenyltris(3-tert-butylpyrazol-l-yl)borate] (98JA10561) and [(> -Tp )FeR] [Tp = hydrotris(3,5-diisopropylpyrazol-l-yl)borate R = Ft, CH2CHCH2, CH2T0I-P] [98JCS(CC)973, 99JCS(CC)417] are prepared by methods similar to those described for the similar non-transition and late-transition metal alkyls. 

R = CH3, CH3, CD3), which were characterized by single-crystal X-ray diffraction and NMR and IR spectroscopy These complexes are rare examples of first-row transition metal alkyl-hydrido species. ... 

Two additional areas are under study at this time. The reaction of fluorine with transition-metal alkyls appears to be quite promising. 

Among others, alkyls of Be, Cd, Hg, Al, Sn, Pb, Co, Pt, and Au have been prepared by treatment of Grignard reagents with the appropriate halide. The reaction has been used to prepare alkyls of almost all non-transition metals and even of some transition metals. Alkyls of metalloids and of nonmetals, including Si,... 

Transition metal alkyls may be divided into two categories with respect to solvent dependence of their CO insertions ... 

Transition metal alkyls are often relatively unstable earlier views had attributed this either to an inherently weak M—C bond and/or to the ready homolysis of this bond to produce free radicals. Furthermore, the presence of stabilizing ir-acceptor ligands such as Cp , CO, or RjP was regarded as almost obligatory. However, (1) the M—C bond is not particularly weak compared say to the M—N bond, and (2) the presence of the new type of ligand on the metal could make the complex kinetically stable thus, even isoleptic complexes, i.e., compounds of the form MR , might be accessible 78, 239). These predictions have largely been borne out (see Table VII). 

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

Polymerisation by transition metal alkyl and hydride complexes... 

Reactions of carbon subsulphide and of elementary phosphorus, sulphur and selenium with complexes of the platinum metals Sulphur dioxide insertion reactions of transition metal alkyls and related complexes... 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

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

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

V. Stereoregular polymerization with Transition Metal Alkyls. 298... 

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 way in which aluminum alkyls and titanium halides combine together to form propagating centers have been discussed in depth for the last 15 years without any one mechanism taking precedence over another (6). The simplest explanation is that aluminum alkyl alkylates the transition metal in the crystal lattice to give a transition metal alkyl center. Polymerization takes place by... 

Wilke s allyl compounds were found to be very poor catalysts indeed, e.g., Ti(2 Me-allyl)4, was found only to have an activity equal to 0.5 gm/m.M Ti/atm/C2H4/hr. For this reason there has been considerable dispute that transition metal alkyls can be the intermediates in Ziegler polymerization. 

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... 

The surfaces of some types of silica and alumina freed from adsorbed water contain acidic -OH groups. Ballard et al. (15) showed that these -OH groups react readily with transition metal alkyls giving stable compounds that are highly active polymerization catalysts for olefins. These systems are best described with reference to silica. 

In this way the number of -OH groups per gram of support that are available for reaction with the transition metal alkyl, can be measured. If this information is combined with surface-area measurements, the number of... 

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 suggested that polymerization initiated by transition metal alkyls is preceded by coordination of the monomer to the metal atom. 

These initiators cannot, however, produce high molecular weight poly-ethylenes except at high pressures ( 1000 atm). The difference is presumably that (XXVI) cannot coordinate ethylene, whereas the propagating centers derived from transition metal alkyls have this characteristic. 

It was shown (12, 41) with transition metal alkyls containing 14C in the alkyl group that the polymers produced contain alkyl groups covalently... 

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). 

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