Metalloceniums cations

This review concerns the materials made from the combination as charge transfer salts of the above-mentioned metallocenium cations with transition metal bisdithiolene anions, expanding and updating a previous review [34]. Most of the materials studied so far are decamethylmetallocenium based salts, but other compounds based on different metallocenium derivatives have also been reported and will be also referred. 

In this section the salts based on metallocenium cations and metal bisdichalcogenate anions will be reviewed according to the previously referred structural classification. After referring to the general characteristics of the crystal structures the supramolec-ular features will be correlated with the magnetic properties. 

The study of salts based on metallocenium cations and transition metal bisdichal-cogenide anions was started almost 20 years ago and it experienced a renewed interest in the last few years, allowing a large number of compounds to be characterized. These studies, as summarized in this review, enabled it to be clearly... 

Interestingly, for the series of zirconocene catalysts [Zr]+[MeB(C5F5)3] Marks and coworkers [307] have found no evidence of significant aggregation to ion quadrupoles, as in Eq. (21). These authors have found that the tendency to form aggregates of higher nuclearity than simple ion-pairs is dependent on whether the anion is in the inner or outer coordination sphere of the metallocenium cation [308]. 

Propagation proceeds through an enolate species in the bimetallic mechanism described by Eq. 8-73 in which two metallocene species are involved—one is a neutral enolate, and the other is the corresponding metallocenium cation [Collins et al., 1994 Li et al., 1997]. In LV, the propagating chain is coordinated at the neutral transition metal center (Zr3) and monomer... 

The effect of a counterion on the inversion barrier of the metallocenium cation is of considerable importance the inversion barrier is increased when a counterion is present. To probe this effect, the inversion barrier was calculated without the presence of a counterion and was found [273] for [rac-(IndCH2)2ZrR]+ to be 4kcal/mol, while the observed inversion barrier (with [B(Me)(C6F5)3] ) was found [111] to be 18.3kcal/mol. 

The influence of an anionic counterpartner of the metallocenium cation on its stereospecificity can also be seen when considering polymerisation conditions such as the kind of solvent and temperature. For example, the loss of stereospecificity when propylene polymerisation with Me2C(Cp)(Flu)ZrCl2-based catalysts is performed in polar solvents [142] seems to be caused by inversion of the configuration of the solvent-separated zirconium ion via... 

Substituent Effects So reactive are the coordinatively unsaturated metallocenium cations that one would expect that introduction of substituents into the cyclopenta-dienyl rings would modulate catalytic activity. (11) If substituent effects could be understood, it should be possible to modify the catalyst system to produce polymers having predetermined properties. We suspect that the needed detailed understanding will be hard to achieve for two reasons. First, the polymerization involves a multi-step mechanism and substituents may affect different steps differently. Second, steric and electronic effects of substituents may act in concert or in opposition and they must therefore be disentangled. Table 1 shows the effect of structural changes... 

The intra- and interchain interactions can be schematically summarized in Figure 4. From a simplistic nearest-neighbor model, there is one intrachain interaction, and three interchain interactions arising between two acceptors (A), /aa, two metallocenium cations (D), /dd, or between a cation and acceptor, /da- These interactions can have different magnitudes and either be ferromagnetic (/>0) or antiferromagnetic coupling (/<0). The... 

Preference for neutral metallocene dimethyl coordination over anion coordination to form /(-Me di-nuclear cationic complexes (e.g., 43) is observed for those truly weakly coordinating anions such as B(C6F5)4 and MePBB . Stabilization of highly reactive and unstable metallocenium cations by /(-Me coordination allows isolation and characterization of such species in the pure state, yet affords excellent polymerization activity in solution, presumably via dissociation to a more reactive monomeric form, as indicated by NMR studies.For more coordinating anions such as CH3B(C6F5)3, /(-Me bimetallic cationic complexes are not detected, except when an excess of neutral metallocene dimethyl is employed (Scheme 25) 143,315 equilibrium can be utilized to stabilize... 

Suzuki and Suga reported the use of clays as solid acids to support and activate metallocene catalysts for olefin polymerization. They were able to use much less alkylaluminmn cocatalyst relative to solution polymerization conditions. The clays were slurried with AlMeg in toluene, then treated with a solution containing zirconocene dichloride, II, and AIMeg. The metallocenium cation was presumed formed via abstraction of chloride and/or methyl ligands by acidic sites on the surface of the clay, and the low basicity of the clay smface was proposed to stabilize the coordinatively unsaturated cation. Propylene was copolymerized with 250 psi ethylene at 70°C. For acid-treated KIO montmorillonite, an activity of 3300 X 10 kg polymer/(g Zr h) was obtained. Catalysts based on vermiculite, kaolin, and synthetic hectorite all showed lower but still appreciable activities. In this brief report, the Al/Zr ratio was not specified, and the clay dispersion was not reported. 

Another important family of inorganic hosts include lamellar metal-phosphorus sulfides MPS3 (or better M2P2S6) with M = Mn, Fe, Ni, Zn, Cd. These can incorporate metallocenes or metallocenium cations. Thus cobaltocene and bis(benzene)-chromium have been incorporated into MnPSs [330-334], FePSs and NiPSs [335], ZnPSs [336], and CdPSs [313, 317, 337-341]. The most intensively investigated subject, by far, is the intercalation of cobaltocene into various metal-phosphorus sulfides [330, 338, 339, 342-346]. Unlike metal disulfides, the metal-phosphorus sulfides (e.g. CdPSa) can also incorporate ferrocene [347]. 

Metallocenium cations, such as Co(C5H5)J and Cr(C6Hg)J in MnPSa [348], FePSj [349], NiPSs [350], ZnPSj [330], and CdPSj [349], and [Fe(C5H5)(C6H6)]+ in MnPSa and ZnPSs [330] have also been investigated. 

As proposed in Figure 3.19, ion-exchange reactions might result in the formation of clay anions and metallocene cations (soft ions), and sodium and chloride ions (hard ions). The ion exchange of metallocenium cations with clay surfaces is also discussed by Mariott et al. [83, 84]. 

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