Pseudo-crown ether

Figure 5. Pseudo-crown ether phase transfer mechanism...

Ishida Y, Sasaki D, Miyauchi H et al (2(X)4) Design and synthesis of novel imidazolium-based ionic liquids with a pseudo crown-ether moiety diastereomeric interaction of a racemic ionic liquid with enantiopure europium complexes. Tetrahedron Lett 45 9455-9459... 

The main obstacle to wide usage of crown-ethers in composition formulations is their comparatively high cost. One way of reducing costs is replacement of traditional crown-ethers with open-chain pseudo-crown-ethers. Furthermore, the use of crown-ethers allows the utilization for impregnation of very cheap monomers that may even be wastes from chemical production. 

Roncali, J., R. Garreau, and M. Lemaire. 1990. Electrosynthesis of conducting poly-pseudo-crown ethers from substituted thiophenes. J Electroanal Chem 278 373-378. 

Binding of hard anions occurs strongly at the hard Lewis acidic uranyl center, whereas cation- tt interactions are established between the aromatic side arms and the cation counterpart of the ion pair. Thus, complexation of alkali metal salts (MX) such as CsCl and RbCl with 15 resulted in the formation of isomorphous supramolecular assemblies in the solid state. In the dimeric [15-CsCl], each cation is coordinated to six oxygens, three from each receptor, thus creating a pseudo-crown-ether-Uke environment for the cation. Additionally, each metal ion in the dimeric unit is coordinated to both halide ions and, most importantly, to two aromatic side arms, one from each of the receptors giving decacoordination for the cation. The closest metal ion-aromatic carbon distances of 3.44(1) A for CsCl, 3.34-3.38(1) A for RbCl, and 3.58(1) A for CsF are observed in the respective alkali halide complexes [15 MX] indicating the conformational flexibility of the side arms and adaptability of the receptors 15 and 16 to form multiple cation- rt interactions with the hosted cations. 

Fig. 10. Electrofoimation of pseudo-crown ether cavities containing materials according to a two-step anodic route.

A novel strategy for forming controlled-size pseudo-crown ethers from acyclic polyethers has recently been proposed by Fabre et al. [258]. As conceptualized in Fig. 10, it consisted of the electrosynthesis of electroactive copolymers from compounds possessing two different electropolymerizable aromatic groups (pyr-role/thiophene (10), pyrrole/dimethoxybenzene (11), and thio-phene/dimethoxybenzene (12)) linked together by a polyether chain. 

Because of possible ion-selective effects, pseudo-crown ethers were prepared by electropolymerization from suitably substituted thiophenes by Roncali et al. [1093]. The polymer formed from l,14-(3-thienyl)-3,6,9,12-tetraoxatetradecane showed an absorption maximum at A = 430 nm in the undoped state. Compared to the respective value for poly(3-(3,6,9-trioxade( l))thiophene, a blue shift of about SO nm, indicating a shorter mean conjugation length, was found. In the oxidized form, this band was considerably reduced in intensity, and a new band at A = 750 nm, attributed to a transition into the upper bipolaron band, was seen. The results were found to be inferior to those of polymers prepared from 3-polyether-substituted monomers. A general review of this field was provided by Fabre and Simonet [1094] for further details, see also [1095]. 

This approach has been extended to the synthesis of macrocyclic ethers (pseudo-crown ethers) incorporated as part of a macromolecular network (styrene-divinylbenzene copolymer) and results in polymers of high coordinating power for various ions (139). The combination of macrocyclic structures with polymer will allow, in the near future, the development of new catalysts containing specific binding properties together with effective catalytic behavior (348). 

Polymeric pseudo-crown ether (139). Reprinted with permission. Copyright 1979 by the American Chemical Society. 

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