Polynuclear structures, chirality

The Chirality of Polynuclear Transition Metal Complexes 4 CHIRALITY IN POLYNUCLEAR STRUCTURES... 

Polynuclear complexes based on octahedral building blocks may be structurally not well defined because of stereogenic problems [8]. However, clever synthetic strategies have recently been devised to obtain chirally pure species [61-65]. Synthesis and, of course, photophysical and photochemical studies of stereochemically pure metal-based dendrimers are still in their infancy. 

Chiral bis-(binaphthophosphole) (bis(BNP)) ligands have been used in the asymmetric hydroformylation of styrene. In solution, the free diphospholes display fluxional behavior. Consistent with their structure, the reaction of the bis(BNP) compounds with platinum(II) derivatives gives either cis chelate mononuclear complexes or trans phosphorus-bridged polynuclear derivatives. Coordination to platinum enhances the conformational stability of bis(BNP)s and diastereomeric complexes can be detected in solution. In the presence of SnCl2, the platinum complexes give rise to catalysts that exhibit remarkable activity in the hydroformylation of styrene. Under optimum conditions, reaction takes place with high branched selectivity (80-85%) and moderate enantio-selectivity (up to 45% ee). 

Capillary electrophoresis has been used for the analysis of chiral pollutants, e.g., pesticides, polynuclear-aromatic hydrocarbons, amines, carbonyl compounds, surfactants, dyes, and other toxic compounds. Moreover, CE has also been utilized to separate the structural isomers of various... 

If the neglect of chirality may be justified in mononuclear systems, this is not generally the case in polynuclear systems where several chiral centres may be present. This leads to a large number of possible diastereomers, with different symmetries, structures and properties. In general the complexes discussed in this... 

Apart from the structural aspects and the mechanism of self-assembly, there is another reason to study the chirality of these systems. There is an increasing body of evidence that suggests that polynuclear complexes generated by self-assembly reactions may show much greater kinetic inertness than simple mononuclear... 

Our intention in this chapter is to draw attention to the chiral aspects of self-assembled polynuclear complexes, and to show how the study of properties related to chirality can afford useful information. After a brief review of the nature of chiral centres in these complexes and the experimental methods used for studying them, we will discuss a number of structure types which are currently attracting interest, such as helicates, dendrimers, molecular boxes, and topologically complex molecules. 

In the first part of this chapter we have tried to present the general features of chirality in transition metal complexes. In the remainder, we shall discuss a number of polynuclear complexes where chirality is important, either as a factor in analysing the structure or as a means of studying the properties. 

Coordination isomerism is another relatively rare, though well-known, phenomenon, exemplified in the compounds [Cr(NH3)6][Co(CN)6]/[Cr(CN)6][Co(NH3)6] and [Cr(en)20x]-[Cr(ox)2en]/[Cr(en)3][Cr(ox)3] (ox = 204 ). It is related to the structural isomerism that might be expected to be encountered in polynuclear systems (Figure 3). Although examples of such isomer types are known, e.g. the chiral cobalt(III) hexol , [Co Co(OH)2(NH3) 4]3] , and the chromium(III) rhodoso ion with the alternative achiral structure shown in Figure 3, no isomeric pairs have been thoroughly characterized. 

CE has been used for the analysis of chiral pollutants, e.g., pesticides, polynuclear aromatic hydrocarbons, amines, carbonyl compounds, surfactants, dyes, and other toxic compounds. Moreover, CE has also been utilized to separate the structural isomers of various toxic pollutants such as phenols, polyaromatic hydrocarbons, and so on. Sarac, Chankvetadze, and Blaschke " resolved the enantiomers of 2-hydrazino-2-methyl-3-(3,4-dihydroxyphenyl)propanoic acid using CD as the BGE additive. The CDs used were native, neutral, and ionic in nature with phosphate buffer as BGE. Welseloh, Wolf, and Konig investigated the CE method for the separation of biphenyls, using a phosphate buffer as BGE with CD as the chiral additive. Miura et al., used CE for the chiral resolution of seven phenoxy acid herbicides using methylated CDs as the BGE additives. Furthermore, the same group resolved 2-(4-chlorophenoxy) propionic acid (MCPP), 2-(2,4-dichlorophenoxy) propionic acid (DCPP), (2,4-dichlorophenoxy) acetic acid (2,4-D), 2-(4-chlorophenoxy) propionic acid (2,4-CPPA), [(2,4,5-... 

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