Metallocene cations

The structure of the metallocene cation energy minimised with the Car-Parrinello method agrees well with the experimentally obtained crystal structures of related complexes. Typical features of the structure as obtained from X-ray diffraction on crystals of very similar neutral complexes (e.g., the dichlorides), such as small differences in distances between C atoms within a cyclopentadienyl (Cp) ring, as well as differences in distances between the C atoms of the Cp ring and the Zr atom, were revealed from the simulations. 

The cationic character of the active site, proposed as long ago as 1961,32 has recently been confirmed by the synthesis of a wide series of group 4 metallocene cations able to polymerize ethene and propene without any aluminum cocatalyst.33 36 The finding by Ewen that C2H4(l-Ind)2ZrCl2/MAO... 

Homogeneous alkylaluminium (alkylaluminoxane)-free olefin polymerisation catalysts are commonly referred to as catalysts containing group 4 metallocene cationic species. These catalysts resemble the structure and properties of the respective metallocene-based Ziegler-Natta catalysts but, by definition, are not included among the latter catalysts. The development of alkylaluminium-free metallocene-based olefin polymerisation catalysts is connected with discoveries... 

Homogeneous alkylaluminium-free olefin polymerisation catalysts also comprise non-metallocene cationic group 4 metal complexes such as those with benzyl ligands [162]. A distinct group of alkylaluminium-free homogeneous olefin polymerisation catalysts consists of nickel complexes [181-183],... 

The cocatalyst has various functions. The primary role of MAO as a cocatalyst for olefin polymerization with metallocenes is alkylation of the transition metal and the production of cation-like alkyl complexes of the type Cp2MR+ as catalytically active species (91). Indirect evidence that MAO generates metallocene cations has been furnished by the described perfluorophenyl-borates and by model systems (92, 93). Only a few direct spectroscopic studies of the reactions in the system CP2MCI2/MAO have been reported (94). The direct elucidation of the structure and of the function of MAO is hindered by the presence of multiple equilibria such as disproportionation reactions between oligomeric MAO chains. Moreover, some unreacted trimethylaluminum always remains bound to the MAO and markedly influences the catalyst performance (77, 95, 96). The reactions between MAO and zirconocenes are summarized in Fig. 8. 

A few paramagnetic metallocene cations such as chro-mocinium (Cp 2Cr+) have been reported but are only stable when sterically protected. 

The pentamethylcyclopentadiene rings in the arsenic metallocene cation, CpjAs, isolated as the tetrafluoroborate salt, are neither coplanar (angle 36.5°) nor ideally... 

For example, the lattice expansion (A/ = 5.13 A) observed when ferrocene is intercalated into FeOCl suggests that the metallocenes are accommodated with the 7-cyclopentadienyl rings perpendicular to the layers.The stoichiometry FeOCl Fe(Cp)2 o.i6 is close to that calculated for close-packed metallocene cations in this model. In addition, the powder X-ray diffraction patterns from the intercalates indicate a change in lattice symmetry from primitive to body centered showing that the intercalation produces a relative translation of adjacent layers (Figure 63). 

The metallocene cations undergo rapid, stepwise hydrogenolysis to yield mono- and dihydrido complexes (e.g., 35), respectively (Scheme 12). Alter-... 

As discussed above, aluminum trialkyls are often used as in-situ alkylating reagents and as scavengers. In the presence of TMA, group 4 metallocene cations form heterodinuclear complexes (96 eq 55). Alkyl-... 

One method of circumventing the potential limitation is to introduce a degenerate exchange process between protected (meth) acrylate and its counterion where the counterion is itself a metallocene cation (eq 4). 

A cationic metallocene ligand with sterically dissimilar cyclopentadienyl rings joined to a positively charged coordinating transition metal atom, and a stable noncoordinating counter anion for the metallocene cation... 

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]. 

The use of discrete activators was instrumental in elucidating the structure and electronic nature of the active site for Group IVB metallocenes. A number of Lewis base-stabilized metallocene cations such as 41 (132), 42 (133), and 43 (134) were studied, chiefly by the Jordan group at the University of Iowa. Reactivity is not completely shut down by the base, although polymerizations are sluggish. Remarkably, agostic species such as 42 and 43, which are believed to be important in chain transfer and propagation, respectively, can be characterized in such systems. 

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