Zirconocene-ethene complex

Examine the sequence of structures corresponding to Ziegler-Natta polymerization of ethene, or more specifically, one addition step starting from a zirconocene-ethene complex where R=CH3. Plot energy (vertical axis) vs. frame number (horizontal axis). Sketch Lewis structures for the initial complex, the final adduct and the transition state. Indicate weak or partial bonding by using dotted lines. 

For unsymmetrical zirconacyclopentadienes, Cp2ZrEt2, which we developed as an equivalent to the zirconocene—ethene complex (3), is a very useful reagent [13]. Two different alkynes couple selectively via zirconacyclopentenes (4) (Eq. 2.3). 

An easier route to the same unsymmetric zirconacyclopentadiene 20 is the use of Cp2ZrEt2 as reagent (easily prepared from Cp2ZrCl2 with two equivalents of EtMgBr). This reagent (Scheme 9), equivalent to a zirconocene-ethene complex 24, reacts similarly to the Negishi-type reagent, which was prepared in the... 

In a simplified picture, the mechanism of the Zr-catalyzed ethylmagnesation can be rationalized as shown in Scheme 1 [8]. At first, the zirconocene-ethene complex 12 is generated from the catalyst precursor Cp2ZrCl2. Complex 12 can also be regarded as a metallacyclopropane 16. After coordination and insertion of the alkene 10, a metalla-cyclopentane 13 is formed, which subsequently reacts with the Grignard reagent regioselectively to the open-chain intermediate... 

Reductive alkylations. Reaction o philes followed by protonation accomplis zirconocene-ethene complex effects c>d... 

Reductive alkylations. Reaction of the complexed species with electrophiles followed by protonation accomplishes reductive alkylations. However, the zirconocene-ethene complex effects cyclopropylmethylation of alkynes with 4-bromobutene. ... 

Zirconocene alkene complexes 29 are readily prepared by the p-hydride activation route from dialkyl zirconocenes 28 or methyl alkyl zirconocenes 27 (Scheme 5.10). It is also possible to prepare the zirconocene complexes of ethene, styrene, and, to a lesser extent, vinyltrimethylsilane by displacement of the weakly bound 1-butene ligand from 4. ... 

Complex 115 reacts analogously with ethene. At ambient temperatures, trimethylgallium is rapidly liberated. At the same time, the remaining (r 2-aryne)zirconocene fragment takes up 1 equivalent of ethene to form the metallaindane system 122. Compound 122 was identified by comparison with an authentic sample [194], Prolonged exposure of the reaction mixture to ethene eventually led to the formation of 123. 

In the zirconocene-catalyzed polymerization of alkenes, Landis and coworkers [20] have reported in situ observation of a Zr-polymeryl species, 15, at 233 K (Figure 1.5). Complex 15 is formed by partial reaction of 14 with excess 1-hexene. Derivatives 16 and 17 are generated quantitatively from 15 by addition of ca. 10 equiv. of propene and ethene, respectively. No other intermediates, such as alkene complexes, secondary alkyls, diasteromers of 15 or 16, or termination products, accumulate to detectable levels. These NMR studies permit direct monitoring of the initiation, propagation and termination processes, and provide a definitive distinction between intermittent and continuous propagation behavior. 

Bis[iV,iV -di(2-pyridyl)-imidazol-2-ylidene]aurate(I) tetrafluoroborates, preparation, 2, 292-293 Bis[iV,iV -di(2-pyridyl-methyl)-imidazol-2-ylidene]aurate(I) tetrafluoroborates, preparation, 2, 292-293 Bis(diselenolate) complexes, dinuclear iron compounds, 6, 242 Bis(dithiolene) compounds, in tungsten carbonyl and isocyanide complexes, 5, 644 Bis(enolato) complexes, with bis-Cp Ti(IV), 4, 589 Bis(enones), in reductive cyclizations, 10, 502 Bis(ethanethiolato) complexes, with bis-Cp Ti(IV), 4, 601 Bis(ethene)iridium complexes, preparation, 7, 328-329 -Bis(fluorenyl)zirconocene dichlorides, preparation,... 

Polyethylenes synthesized by metallocene/MAO catalysts have a molecular weight distribution of M /M = 2. The molecular weight can easily be lowered by increasing the temperature, increasing the metallocene concentration, or decreasing the ethene concentration. The narrow molecular weight distribution is characteristic for a single site catalyst. Nearly every zirconocene forms an active site of a cationic metallocene - and an anionic MAO compound or a complex of both. The nature of the active site would be clearer if more details were known about the structure of the alumoxane. 

Early transition metal catalysts such as vanadium complexes and zirconocenes effectively copolymerize ethene with norbornene [81]. This capabihty eventually led to the commercial development of the APEL and TOPAS line of cyclic olefin copolymers by Mitsui and Ticona (formerly Hoechst), respectively [82]. Interest in this class of polymers is due to its high glass transition temperatures and transparency that is imparted by the norbornene component. 

Metallocenes, especially zirconocenes but also titanocenes, hafnocenes, and other transition metal complexes treated with MAO are highly active for the polymerization of olefins, diolefins, and styrene. The polymerization activity, which is up to 100 times higher than for classical Ziegler catalysts, as well as the possibility to easily tailor the microstructure of the polymer chain and to obtain polymers with special properties have motivated research groups worldwide to produce thousands of patents and publications in the last 20 years. An overview can be found in selected review articles and books [55-68]. A metallocene/MAO catalyst containing 1 g zirconium produced 40 x 10 g polyethylene in 1 h at 95°C and 8 bar ethene pressure (Table 1). 

Naga and Imanishi [30] studied the effects of the ligand structure of zirconocene catalysts on the copolymerization of cyclopentene (Cp) and ethene. They found that non-bridged zirconium complexes together with MAO as cocatalyst were not able to incorporate cycloolefin units into the polymer chain. The copolymers obtained with rac-[Et(lnd)2]ZrCl2 (I-l in Fig. 3) contained not only c T-1,2-units but also 20-30% cis-l,3-units of cyclopentene. DSC measurements showed multiple melting... 

Zirconocene ii -alkyne complexes generated by the C-H activation method may be trapped intermolecularly with alkenes, but excepting ethene and nor-bornene the yields are generally low. ... 

Zirconocene ri -alkyne complexes generated by elimination of ethene (P-P C-C bond activation) or elimination of methane (C-H activation) as illustrated in Scheme 5.5 may be trapped with nitriles and aldehydes to provide enones and allylic alcohols 25 and 26 on hydrolysis (Scheme 5.9). ... 

---