Alkylated metallocenes

Cationic alkyl metallocene complexes are now considered the catalytically active species in metallocene/MAO systems. Spectroscopic observation has confirmed the presence of cationic catalytic centers. X-ray photoelectron spectroscopy (XPS) on the binding energy of Zr(3d5/2) has suggested the presence of cationic species, and cationic hydride species such as ZrHCp2 that are generated by /1-hydride elimination of the propagating chain end... 

The active species of the metallocene/MAO catalyst system have now been established as being three-coordinated cationic alkyl complexes [Cp2MR] + (14-electron species). A number of cationic alkyl metallocene complexes have been synthesized with various anionic components. Some structurally characterized complexes are presented in Table 4 [75,76], These cationic Group 4 complexes are coordinatively unsaturated and often stabilized by weak interactions, such as agostic interactions, as well as by cation-anion interactions. Under polymerization conditions such weak interactions smoothly provide the metal sites for monomers. 

As far as organometallics are concerned, and important exception is represented by the (per)alkylated metallocenes and the related bis(arene) complexes of various transition metals. These compounds have been intensively studied in connection with the magnetic properties of their CT complexes, mainly by Miller and co-workers [11]. These studies led to the discovery of the first organometallic compound displaying bulk ferromagnetic properties (vide infra). 

Using deuterium-labeling experiments, about 100% of the metal was shown to be active in 1-hexene polymerizations with the [rar -C2H4( 1 -lnd)2ZrMc [McB((4,f5)3] catalyst,922 and the reactivity of M-(secondary alkyl) bonds at —80 °C was comparable to that of primary alkyl metallocenes.302 These relative monomer insertion rates appear strongly ligand specific. However, when these comparisons are made, it must be borne in mind that different authors use very different catalysts, as well as different definitions of the term active center Landis defined the active... 

The alkylated metallocene, Cp2Zr(CH3)2, further reacts with MAO, forming a compound (A) that features the structural element Zr-O-Al ... 

New Ziegler-Natta catalysts are homogeneous systems. These are based on a combination of metallocenes and MAO. By appropriate use of these catalysts linear polyethylene, and all the three types of polypropylene (atactic, isotactic and syndiotactic) can be produced in a specific manner. A cationic alkyl-metallocene complex is the active species in the polymerization. MAO helps in alkylating the metallocene (if the metallocene is a metallocene dihalide like Cp2ZrCl2) and also in assisting the removal of a CHa. Further it stabilizes the cationic center and also possibly scavenges impurities. 

CyclopentadienylthaHium and its alkylated derivatives are used in the synthesis of metallocenes (qv) and other transition-metal cyclopentadienyl complexes (29). 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Alkyl, aryl and allyl complexes of 5 some metallocenes (43)... 

Further studies quickly revealed that the rapid dehydrogenative coupling of primary organosilanes to oligomers and the slower coupling of secondary silanes to dimers can be effected under ambient conditions with compounds of the type CP2MR2 (M = Ti, R = alkyl M = Zr, R = alkyl or H)(11,12,13). None of the other metallocenes, metallocene alkyls, or metallocene hydrides of groups 4, 5 or 6 have shown any measurable activity for polymerization... 

Like many homogeneously catalyzed reactions, the overall cycle (or cycles) in these polymerization reactions probably contains too many steps to be easily analyzed by any single approach. Both kinetics and model compound studies have thrown light on some of the steps. However, as indicated above, many of the model compounds isolated from the reactions of primary silanes with metallocene alkyls and hydrides are too unreactive to explain the polymerization results. 

Since group 3 metallocene alkyls are isoelectronic with the cationic alkyls of group 4 catalysts they may be used as olefin polymerization initiators without the need for cocatalysts. The neutral metal center typically results in much lower activities, and detailed mechanistic studies on the insertion process have therefore proved possible.216-220 Among the first group 3 catalysts reported to show moderate activities (42 gmmol-1 h-1bar-1) was the yttrocene complex (77).221... 

The use of weakly coordinating and fluorinated anions such as B(C6H4F-4)4, B(C6F5)4, and MeB(C6F5)3 further enhanced the activities of Group 4 cationic complexes for the polymerization of olefins and thereby their activity reached a level comparable to those of MAO-activated metallocene catalysts. Base-free cationic metal alkyl complexes and catalytic studies on them had mainly been concerned with cationic methyl complexes, [Cp2M-Me] +. However, their thermal instability restricts the use of such systems at technically useful temperatures. The corresponding thermally more stable benzyl complexes,... 

Marks et al. reported the co-polymerization of ethylene and 1-hexene by using ansa-type complexes of lanthanide metals [127]. Recently, bulky alkyl substituted ansa-type metallocene complexes of yttrium have been reported to exhibit high activity for the polymerization of 1-hexane. [114, 119, 128]... 

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