Lewis bases external

A hither facet of research has involved the structural characterisation of aluminium complexes which incorporate polydentate salen-type ligands. These have been noted in both neutral and monocationic (ion-separated) contexts (the latter requiring that the metal centre be stabilised by an external Lewis base) [35]. While such charged systems are invariably mononuclear the same is only usually true of their neutral analogues by virtue of the sterically demanding bis(aryloxide), chelating ligand. In the context of these latter complexes, dimerisation has been noted [251] while, more recently, the employment of flexible alkyl chains between two salen-coordinated aluminium ions has enabled the observation of dinuclear compounds [160, 161]. 

Subsequently, Landis studied the coordination of the ferrocene-based diphosphine-benzoxaborolidine ligands 53 with the aim of supporting secondary interactions between the pendant Lewis acid and an external Lewis base.63 Stable platinum complexes 63a-c were formed by reacting... 

One has to realise the extreme chemical complexity of catalysts such as MgCl2/internal Lewis base/TiCU AlEt3/external Lewis base. Moreover, the... 

To date, the ability of external Lewis bases to bind to the pendant Lewis acid (anchoring effect) has not been structurally substantiated, but H/"B NMR evidence has been reported in a few instances using A -based donors. ... 

The positively charged carbon atom in a carbocation is an extremely electron-dehcieni (electrophilic) carbon. As such, its behavior is dominated by a need to obtain an electron pair from any available source. The Sn I reaction illustrates the most obvir>"s fate of a < nr .- -t on c a.bination with an external Lewis base, forming a new bond to carbon. However, the electron deficiency of cationic carbon is so great that even under typical SnI solvolysis conditions, surrounded by nucleophilic solvent molecules, some of the cations won t wait to combine with external electron-pair sources. Instead, they will seek available electron pairs within their own molecular structures. The most available of the.se are electrons in carbon-hydrogen bonds one carbon removed from the cationic center (at llic so-called carbon) ... 

There are two distinct paths for cleavage of H-H (and X-H) bonds homolysis to a dihydride (as above) and heterolysis, that is, breaking the H-H bond into H and H fragment. Both paths have been identified in catalytic hydrogenation and are available for other cr-bond activations. A proton can split off from the H2 and migrate to either an external Lewis base (intermolecular) or a ligand/anion (intramolecular) as in Scheme 3. 

Lewis base is called the internal Lewis base. This is followed by addition of Group I-in metal-alkyl component with an additional Lewis base. The base that is added the second time may be the same or a different one from that used in the original milling. In either case it is called the external Lewis base. It is not unconunon to use an ester as the internal base and an oiganosilane compound, like phenyltriethoxysilane, as the external one. 

From the obtained polymerization data we have evaluate the activity of the catalysts employed. Table 1 lists the activity of the catalyst by us prepared (FOT cat.) compared with that exhibit by the commercial catalyst and the molecular weights of the obtained polymers. It is to be outlined that both these polymerizations were carried out in absence of hydrogen and external Lewis bases. Furthermore the polymerization promoted by the FOT catalyst was carried out also in absence of the internal base. As the presence of these components influences both the activity and stereospecificity of the catalysts, a comparison with the features shown by the commercially produced polypropylene it is not possible. 

Ciardelli, F., Menconi, F., Altomare, A., and Carlini, C. (1988) Polymerization of olefins by highly active Ziegler-Natta catalysts in presence of vinyl esters as "external" Lewis bases, XVI Congress of Italian Chenoical Soc., Bononiachem 88, Bologna, October 9-14, Abstracts, p.278... 

Both internal and external Lewis bases react with aluminum alkyls forming a 1 1 complex in the first step (57) [352,353]. The second step is an alkylation reaction affording a new alkoxyaluminum species (58). 

On the other hand, the traditional catalytic system designed by Natta, i.e. TlCl,-AlRjCl, is now to be replaced by the more active catalytic system supported on NgClj, including a solid precatalyst with the support, TiCl and a so-called internal Lewis base (ILB), and a cocatalytic solution with AIR,and an external Lewis base (ELB). Two main families of these supported catalytic systems are to be considered, according to the nature of the Lewis bases. In the most studied family, the two Lewis bases are aromatic esters in a more recent, but more efficient family, the ILB is an aromatic diester. 

Internal Lewis bases (ILB) and external Lewis bases (ELB) are both necessary with the supported Ziegler-Natta catalysts to achieve the requirements of high activity and stereospecificity. The ILB is not directly involved in the active center. During the catalyst preparation the ILB masks the magnesium chloride surface, blocking the acidic defects. The catalyst is then formed by a selective replacement of the weakest bonded ILB by the titanium tetrachloride, giving thus access to low acidic active centers. The scheme can be considered as an active center selection. High activity is observed when a dynamic selection process results in an accumulation of trapped defects. 

The effects of external Lewis bases containing oxygen is given in Table 2. 

Fully aware of the paramount importance of his discovery, Ziegler sought to optimize the catalytic composition, and it quickly appeared that the best system is the one comprised of a mixture of TiCls/AlEts dispersed in a hydrocarbon solvent. These systems have been immensely improved since their discovery and, currently, the most efficient ones are supported catalysts utilized in association with AlEts as cocatalyst and an external Lewis base as stereoregulating agent (for the polymerization of propene). The two essential components are TiCU and AlEts the role played by other components will be discussed after the presentation of the basic mechanisms. Active centers are generated through the reaction pathway shown below ... 

The mode of action of external Lewis base—so called because it is added to the catalytic system together with the organoaluminum cocatalyst—is more difficult to account for. This base generally consists of a dialkoxysilane molecule, and its role would be to inhibit certain nonspecific sites in addition to the internal Lewis base or transform some of them into isospecific ones by the steric effect. It is admitted that it coordinates simultaneously to a titanium atom and an aluminum atom via its two oxygen atoms, thus corroborating the Rodriguez and Van Looy mechanism. 

The key role of polar solvents and/or external Lewis bases for the stabilization of the propagating carbocationic chain, so that to prevent undesirable chain-transfer and termination reactions, is well-documented [8]. Using related EtAlCl2-incorporating initiators but in the presence of SnCLj, allowed for the highly... 

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