Dichlorides metallocene, structural

Using bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2 Structure 6) and MAO, up to 40 000 000 g polyethylene/g Zr per h are obtained. Every zirconium atom forms an active complex and produces about 46 000 polymer chains per h. The time of insertion of one ethylene unit is only 3 X 10 s. Table 2 shows the polymerization behavior of different metallocene/alumoxane catalysts. 

Mortazavi SMM, Arabi H, Zohuri G, et al Copolymerization of ethylene/a-olefins using bis(2-phenylindenyl)zirconium dichloride metallocene catalyst structural study of comonomer distribution, Polym Int 59(9) 1258—1265, 2010. 

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 metallocene dichloride of zirconium and hafnium 20b and 20c were also prepared and underwent reduction with potassium to give monomeric metallocene monochloride complexes 21b and 21c (Eq. 8) [39b]. The structure of the zirconocene complex 21 b in the crystal showed a conformation which suggests a less steric strain as compared to 21a due to zirconium s larger atomic size. As a consequence of the coordinative unsaturation an unusually short Zr —Cl bond length was found. 

Based on Chien s research results, Collins et al. modified the basic structure of the catalysts and also achieved elastic material [8,18,19]. In both cases the elastic properties of the polymers are justified in a block structure with isotactic and atactic sequences. In 1999 Rieger et al. presented a couple of asymmetric, highly active metallocene catalysts, e.g., the dual-side catalyst rac-[l-(9-r 5-fluorenyl)-2-(5,6-cyclo-penta-2-methyl-l-q5-indenyl)ethane]zirconium dichloride (Fig. 3). These catalysts allowed building of isolated stereoerrors in the polymer chain to control the tacticity and therefore the material properties of the polymers [9],... 

A special case of the chain back skip polymerization mechanism and therefore an entirely different polymerization behavior was observed for differently substituted asymmetric complexes (for example catalyst 3). Although asymmetric in structure, these catalysts follow the trend observed for C2-symmetric metallocenes [20], Chien et al. [23] reported a similar behavior for rac-[l-(9-r 5-fluorenyl)-2-(2,4,7-trimethyl-l-ri5-indenyl)ethane]zirconium dichloride and attributed this difference in the stereoerror formation to the fact that both sides of the catalyst are stereoselective thus isotactic polypropylene is obtained in the same manner as in the case of C2-symmetric metallocene catalysts. 

Recent advances in the development of well-defined homogeneous metallocene-type catalysts have facilitated mechanistic studies of the processes involved in initiation, propagation, and chain transfer reactions occurring in olefins coordi-native polyaddition. As a result, end-functional polyolefin chains have been made available [103].For instance, Waymouth et al.have reported about the formation of hydroxy-terminated poly(methylene-l,3-cyclopentane) (PMCP-OH) via selective chain transfer to the aluminum atoms of methylaluminoxane (MAO) in the cyclopolymerization of 1,5-hexadiene catalyzed by di(pentameth-ylcyclopentadienyl) zirconium dichloride (Scheme 37). Subsequent equimolar reaction of the hydroxyl extremity with AlEt3 afforded an aluminum alkoxide macroinitiator for the coordinative ROP of sCL and consecutively a novel po-ly(MCP-b-CL) block copolymer [104]. The diblock structure of the copolymer... 

Titanocene dichloride was the first metallocene derivative to be converted53 into dithiolato complexes, for example compound (38). Some other examples of this structural type exist, and their structural features are described in Section 16.5.3.1. 

A variety of new ligand designs and ligand combinations were used in attempts to mimic some properties of the ubiquitous bent metallocene environment at the early metal centers consequently, some of these systems were used in the further development of butadiene zirconium chemistry. The pyridine based chelate zirconium dichloride complex 43 cleanly formed the butadiene complex 44 upon treatment with butadiene-magnesium. Its structure shows that the C4H6 is arranged perpendicular to the chelate ligand plane. Complex 44 inserts one equivalent of an alkene or alkyne to form the metallacyclic 7i-allyl system 4545 (Scheme 13). 

Cyclopentadiene, whose deprotonation in the presence of a base gives relatively stable cyclopentadienyl anion, has also been found to be a suitable substrate for palladium catalyzed arylation [79]. Metallocenes, typically zirconocene dichloride, are also arylated [79,80]. By using excess aryl bromides, they are completely arylated to produce structurally interesting pentaarylcyclopentadienes (Eqs. 38 and39). 

This type of selectivity originates solely from steric interactions between the auxiliary ligands, polymer chain, and the incoming propene. It was first explained qualitatively by means of visualization of the structure of the catalyst precursors. A more quantitative approach led naturally to molecular mechanics models in order to explain and even predict the stereospecificity of catalysts with different ligand environments. Due to the limitations of MM models to describe metallocene complexes as well as bond breaking and bond formation processes (see Section 3.1.2.1), the models were initially based on some rigid core structures derived from the measured structures, e. g., of the dichloride precursors [25, 26]. In order to achieve more accurate results, core structures, calculated by ab initio methods, were employed later. A further step in this direction is the joint description of the... 

Phosphonium-bridged cationic //// //-metallocenes 1161 have been synthesized by salt metathesis, as illustrated in Scheme 274.903 Methylation of the zirconium complex yields the methyl derivative 1162, which reacts rapidly with CO at room temperature to give the acyl derivative 1163. The molecular structures of the dichloride complexes have been determined by X-ray diffraction. 

Mono-diene complexes of zirconocene and hafnocene have been prepared by two methods [129-131 ], viz the photochemical reaction of diphenylzirconocene in the presence of diene and the reaction of metallocene dichlorides with diene magnesium adduct. The structures and reactivity of s-cis-dicm complexes indicate that the metal-lacyclopentene (B) is the preferred canonical form. Complexes of the type Cp2Zr(j-/ra/25-1,3-diene), have been prepared they were the first examples of this mode of coordination (C). Insertion of unsaturated compounds into a diene coordinated to zirconocene results in regioselective C—C bond formation [132-136]. 

Chiral C2-symmetric ansa-metallocenes, also referred to as bridged metallocenes, have found extensive use as catalysts that effect C-C bond-forming processes in an enantioselective manner [3]. In general, bridged ethylene(bis-tetrahydroin-denyl)-metallocene dichlorides (1-3, Scheme 1) put forth attractive options for the design of asymmetric reactions because of their geometrically-constrained structure and relative ease of preparation. 

Thus, the size of M influences the Cl-M-Cl bonding angle, the M-Cl bonding distance and the non-bonding Cl...Cl distance (bite) within the dichlorometal moiety of the complexes. These structural parameters are given in Table II for metallocene dichlorides (8 - 11) and for the inorganic cytostatic drug DDP (12). 

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