Structure metallocene

Figure 8 Stereostructures of polypropylene and its relationship with metallocene structures. (From Ref. 3, with permission from Elsevier Trend Journals, Cambridge, U.K.)...

Many modifications in metallocene structures have been incorporated, as shown in Fig. 9, to synthesize isotactic polypropylene with a range of properties including molecular weight, isotacticity, mechanical properties, etc. 

Figure 9 Metallocene structures for polypropylene synthesis. (From Ref. 2.)...

Fig.5 Examples of simple metallocene structures a parallel sandwich, b multi-decker sandwich, c half-sandwich, d bent/tilted sandwich and e compounds with differently bonded cyclopentadienyl ligands...

On the basis of Cp and its heteroanalogues, a wide series of metallocene structures of types 3, 5, 126, 127 [2-5,7,8,13,132,133,192-205] have been obtained. These complexes are typical for Fe, Co, Ni, Mn, V, Cr, and Ru, and are the standard examples of complex compounds with multicenter delocalized coordination bonds formed as a result of combined grouping orbitals of the cyclopentadienyl rings with rf-metal orbitals. 

The reactions of electrophilic substitution in metallocene structures of the type 3 (Sec. 1.1) are numerous and various [423 133]. The reaction capacity of coordinated cyclopentadienyl ligands in ferrocene 749 is especially studied in detail (3.197) [423-426,433]. It is significant that already in 1968 more than 300 such transformations had been reported [424], for example (3.197) ... 

The organosamarium(II) complex Sm( / 5-CsMc5)2 has the bent-metallocene structure, which can be attributed to polarization effects its THF solvate Sm( / 5-C5Mc5)2(TI If )2 exhibits the expected pseudo-tetrahedral coordination geometry. Sm( j5-C5Me5 )i] is a very powerful reductant, and its notable reactions include the reduction of aromatic hydrocarbons such as anthracene and of dinitrogen, as illustrated in the following scheme ... 

Harding MM, Mokdsi G (2000) Antitumor metallocenes structure-activity studies and interactions with biomolecules. Curr Med Chem 7 1289-1303... 

The two substituted cyclopentadienyl rings in hexakis(trimethylsilyl)-magnesocene and -ferrocene are nonparallel with a Cp-M-Cp angle of 171.5° [average for M = Mg (149)] and 173.9° [M = Fe (99)]. These bent metallocenic structures (Fig. 15a) are in contrast to parallel Cp arrangements observed otherwise for normal of even bulky Cp s [cf. octaphenyl-ferrocene (45)]. The structure of normal magnesocene shows it to be isomorphic with ferrocene (184,185). 

Ab initio calculations on hypothetical organometallic cations Cp2Ln+ (Ln = Sc, La) show bent metallocene structures are favoured due to the covalent a bonding contributions involving the totally symmetric metal d orbitals which favor bent structures. Although the contribution of da is lower than dn, its involvement is sufficient to give bent structures [119]. 

Cationic ansa metallocenes can be utilized as chiral catalysts in Diels-Alder reactions. For example, in the presence of the cationic zirconocene complex [(ebthi)Zr(Ot-Bu) thf]+, the [4 + 2] cycloaddition of acrolein and cyclopentadiene proceeds efficiently to afford endo and exo cycloadducts (equation 71). In reactions in which methyl acrylate is used as the dienophile, cycloadditions occur with lower levels of enan-tioselection (23% ee), but with significantly higher degrees of diastereoselectivity (17 1 endo, exo). In these processes, recent studies demonstrated the great influence of chiral metallocene structure and the dramatic solvent effect. ... 

Metallocene Structure Activity [kg PP/(mol Zr h)] Molecular weight [g/mol] Isotacticity minmm [%] Melting point [°C]... 

A further breakthrough was the synthesis of enantiomeric sterorigid ansa-metallocenes by Brintzinger and co-workers [33] and the discovery by Ewen [34] that such racemic metallocene/methylalumoxane systems generate isotactic polypropylene. It was further found that the metallocene structure determines the polymer structure [35-37]. Again, with these compounds polyolefins such as syndiotactic polypropylene become available on a large scale [38]. Indeed, metallocene/methylalumoxane catalysts offer new prospects for olefin oligomers and polymers [39 2],... 

For example, the cyclocondensation of cyclopentadiene with 2,5-hexanedione in the presence of sodium affords 4,7-dimethylindene in 65 % yield. Subsequent standard transformations yield the desired bridged-metallocene structures. 

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