Necklaces, molecular

Mechanically interlocked molecules incorporating cucurbituril and their supramolecular assemblies , 

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Harada et al. were the first to synthesize a polyrotaxane. Using the process shown in Scheme 1, they obtained an inclusion compound in which many a-CDs are threaded by a PEG chain and named it molecular necklace . Wenz et al. [132] reported polyrotaxanes containing polyamines and a-CDs. Because of its significance and interest, the approach used by Harada et al. to obtain the molecular necklace is worth reproducing here in some detail. 

Formation of molecular necklaces [n]MN (n=4-7) has taken on a new 2+2 approach in which pseudorotaxanes containing two molecular beads, e.g. cucurbituril, are treated with the connective metal centers to afford the desired 25<99ACIE638>. 

The molecular necklace 12 of a-cyclodextrin 13 beads threaded on a polyether chain (Figure 1.5) forms spontaneously in solution [17]. This is an example of a so-called one-pot reaction in which complicated structures are... 

Figure 1.5. The one-pot formation of a molecular necklace involving 20-22 a-cyclodextrin beads represented schematically by buckets.

Figure 10.71 Molecular necklace type coordination polymer rotaxanes (a) hexagonal 2D polyrotaxane net and (b) square-grid shaped 2D polyrotaxane net (reproduced by permission of The Royal Society of Chemistry).

Cucurbit [6] uril and increasingly its higher homologues have been used extensively in a variety of host-guest chemistry and nanostructure assemblies. Of particular appeal are rotaxanes and molecular necklaces, mechanically interlocked assemblies of molecules based on CB [6]. Efficient synthesis of ID, 2D and 3D polyrotaxanes and molecular necklaces (cucurbituril beads linked by a macrocyclic molecule string ) has been achieved by a combination of self-assembly and coordination chemistry. We discuss rotaxanes and molecular necklaces in Section 10.7, and cucurbil-based systems are summarised in a recent review.23... 

Catenanes, rotaxanes, molecular necklace, knots, and Borromean link. 

Figure 3 Ball and stick representations for the X-ray crystal structures of molecular necklaces ...

A. Harada, J. Li, M. Kamachi, The Molecular Necklace A Rotaxane Containing Many Threaded a-Cyclodextrin , Natme, 356, 325 (1992)... 

The authors prepared a molecular tube by crossUnking adjacent a-CD units in a polyrotaxane (molecular necklace) [126,127] (Scheme 4). The molecular necklace was dissolved in 10% NaOH, to which epichlorohydrin was then added. Epichlorohydrin was allowed to react with the hydroxyl groups of the threaded a-CD to link up each a-CD. 2,4-Dinitrofluorobenzene moieties, bulky stoppers, at both ends were removed by the treatment with a strong base (25% NaOH). The yield of molecular tube was 92% and the average molecular weight was estimated to be ca. 2 x 10" by GPC. The molecular tube could accommodate Is" ions in an aqueous solution of KI-I2 efficiently, whereas a-CD could not. 

Figure 5.30 A molecular necklace prepared by synkinesis of poly (ethylene glycol) bis-amine and a-cyclodextrin. The molecular assembly was finally locked by capping with 2,4-dinitrofiuorobenzene.

Interlocked Structures, Catenanes, Rotaxanes, and Molecular Necklaces... 

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