Zirconium cations

Figure 3.18 Theoretical study of ethylene insertion into the Zr-C bond in the silylene-biscyclopentadienyl(methyl)zirconium cation [H2Si(Cp)2Zr—Me]+. E — energy...

However, such fluorine atoms are hard bases, and have been shown to coordinate to the (presumably) hard zirconium cation. It is worth investigating whether the secondary bonding would be weaker if the fluorine atoms on typical nonbasic anions were replaced with softer chlorine, bromine, or iodine atoms. Perhaps, in this light, it may be significant that zirconocene catalysis is especially efficient when the anion is (CBuHeXe)" (X = Cl, Br, or I). 

The number of articles on zirconium Lewis acids is much smaller than that on titanium, an element in the same group. Basically, the mechanism of reaction using zirconium Lewis acids is the same as that with other Lewis acids. Often the stereoselectivity or enan-tioselectivity of the reaction is emphasized. Hitherto few results have shown zirconium Lewis acids to be superior to other Lewis acids such as TiCU, SnCU, AICI3, etc. Zirconium Lewis acids are often mild, which enables reaction with moderate to good selectivity. Recently, zirconium cation compounds have attracted attention because they are extremely reactive in olefin polymerization. The Kaminsky-type polymerization of olefins with zirconium compounds is interesting, but beyond the scope of this review. 

The most important representatives of tetravalent elements are titanium and zirconium because of their uses as advanced construction materials in different technologies. Systems of titanium and zirconium fluorides and chlorides with alkali metal fluorides and chlorides, respectively, can be found in the Phase Equilibria Diagrams Database (1993). A number of compounds with various stoichiometry are formed in these systems, depending on the size of the titanium and zirconium cation and the polarization ability of the alkali metal. Some of these compounds are very stable and melt congruently even at relatively high temperatures. On the other hand, some compounds melt incongruently and their existence is limited to a certain temperature range. 

Al(Ci2F9)-fluorine interaction consistent with an aluminum anion-zirconium cation pair (Scheme 6.13). Polymerization using this anion-[Pg.205]

Calculated and experimental values, expressed in pmol-CO.g, are collected in Table 1. The ratio of these two values may represent the number of oxygen layer involved in the redox process. In all cases, the calculated values are lower than the experimental ones. This observation shows that 2 to 3 layers are involved in the redox process. The increase of the OSC along with the amount of Pr shows that the presence of this element induces the creation of anionic vacancies. In fact, in substitution to cerium cations, Pr ions may undergo a redox process (Pr /Pr" ) whereas it is not possible for zirconium cations. 

When water salts solutions are used in OPCM synthesis, provided the Me CA ratio is equal to 1 6 in all the composition range studied here (up to Zr relative content equal to 0.9), single phase samples with a cubic fluorite-like structure are formed. Thus, for ceria-zirconia samples with zirconia content up to 50 mol. %, ceria-based cubic solid solution is formed. For those samples, the lattice parameter is decreased as compared with that of a pure ceria phase, due to cerium cations substitution by zirconium cations with a smaller... 

According to SIMS data (not shown here for brevity), zirconium cations and fluorine anions are rather uniformly distributed across the particle depths, while calcium tends to segregate at the surface, where its content is up to several times higher than in the bulk. 

Incorporation of Zr shifts hydroxyl bands to higher frequency and makes them more intense. It implies that a part of the hydroxyls is bound with zirconium cations, since the hydroxyl band positions is rather close to those observed partially stabilized zirconias [15]. The appearance of bands at - 3780 cm implies the emergence of terminal hydroxyls. For... 

To evaluate the usefulness of this model, the barrier to methyl exchange (see eq 6) was computed for MAOg plus bis(l,2-dimethyl cyclopentadienyl)-zirconium cation. The computed barrier of 24 kcal/ mol is in fair agreement with the experimental estimate of 18 kcal/mol. 

A zirconium 1,4-benzenedicarboxylate MOF (UiO-66) is characterized by triply bridged OH groups with v(OH) at 3674—3672 cm (720,721). Full dehydroxylation occurs at 523 K, but the process is reversible and the OH groups are recovered after water adsorption (721). This phenomenon has been related to the ability of zirconium cations to easily change their coordination number (712). The acidity of the OH groups is weak, with a CO-induced shift of the OH modes of —83 cm (721). For an amino-functionahzed UiO-66 sample, v(OH) is observed at sHghdy lower frequencies, 3666 cm (v(OD) at 2708 cm ) (637). 

As reported by Santos and Metzger [50], Ziegler-Natta polymerization can be carried out in a microflow system coupled directly to the ESI source of a Q-TOF mass spectrometer (Figure 14.12). In the first micromixer, catalyst (Cp2ZrCl2-MAO) and monomer solutions are mixed continuously to initiate the polymerization. The polymerization occurs in the microtube reactor. The solution thus obtained is introduced into the second micromixer M2, where the polymerization is quenched by acetonitrile. The quenched solution is fed directly into the ESI source. The transient cationic species can be characterized by mass spectrometry. This is the first case where an alkyl zirconium cation intermediate in the homogeneous Ziegler-Natta polymerization of ethylene is detected directly. 

The next step is the migratory insertion reaction. This leads to the formation of a metal-alkyl complex. Note that we have ended up again with a three-coordinate zirconium cation, which has a metal-alkyl bond. In effect we have lengthened the alkyl chain by the process of olefin coordination followed by migratory insertion. 

Keaton, R. J. Jayaratne, K. C. Fettinger, J. C. Sita, L. R. Structural characterization of zirconium cations derived from a living Ziegler-Natta polymerization system New insights regarding propagation and termination pathways for homogeneous catalysts. J. Am. Chem. Soc. 2000, 122, 12909-12910. 

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