Crown ethers molecules

In Pedersen s early experiments, the relative binding of cations by crown ethers was assessed by extraction of alkali metal picrates into an organic phase. In these experiments, the crown ether served to draw into the organic phase a colored molecule which was ordinarily insoluble in this medium. An extension and elaboration of this notion has been developed by Dix and Vdgtle and Nakamura, Takagi, and Ueno In efforts by both of these groups, crown ether molecules were appended to chromophoric or colored residues. Ion-selective extraction and interaction with the crown and/or chromophore could produce changes in the absorption spectrum. Examples of molecules so constructed are illustrated below as 7 7 and 18 from refs. 32 and 131, respectively. 

Polyphosphazenes bearing crown ethers (12-crown-4,15-crown-5 and 18-crown-6) as single or as mixed substituents with trifluoroethoxy or methoxy-ethoxyethoxy groups were synthesized by Cowie [601,602] and Allcock [484] and their conductivity studied because it was shown that the incorporation of crown ether molecules into a polymer electrolyte could increase their ionic conductivity. In these macromolecules, the crown ether units were linked to the backbone through oxymethylene spacer groups. 

In the cation [Pb(15-crown-5)2]2+ the lead atom is sandwich-like coordinated by the ten oxygen atoms of the two crown ether molecules. (Figure 13). 

The electrochemical properties of ferrocene have been utilized by many workers in the field of electrochemical molecular recognition. Saji (1986) showed that the previously synthesized (Biernat and Wilczewski, 1980) ferrocene crown ether molecule (Fig. 3 [1]), whose binding properties had previously been studied only by nmr and UV/Vis techniques (Akabori et al., 1983), could be used as an electrochemical sensor for alkali metal cations involving a combination of through-space and through-bond interactions. 

In 1990 we reported the synthesis of new redox-responsive crown ether molecules that contain a conjugated link between the crown ether unit and a ferrocene redox-active centre (Beer et al., 1990a). Examples of some of the species synthesized are shown in Fig. 5. The electrochemical behaviour of these species was investigated and also the electrochemical behaviour of their analogues with a saturated link between the ferrocene unit and the crown ether. The changes in the CVs of [2a] upon addition of magnesium cations are shown in Fig. 6. The metal cation-induced anodic shifts of [2a], [2b] and also their saturated analogue [3] and vinyl derivatives [4a], [4b] are shown in Table 1. 

On the other hand, since the increase of ion conductivity is realized through the coordination of lithium ions by crown ether molecules, the lithium... 

The crystalline solid contains a 4 1 stoichiometric guest/host ratio and is composed of 2 1 guest/host complexes with two additional thiourea molecules hydrogen bonded to the guest molecules of the complex. There are three independent crown ether molecules in the solid. Only one of these is shown in Fig. 19, and it lies on a center of symmetry. 

Addition of a second crown produces the loose ion pair A, Cr,K, Cr. However, the complexation constant for adding the second crown is 1800 M 1 for the fluorenyl carbanion and only 200 M 1 for the picrate salt. The lower value for picrate may in part be due to less charge delocalization, e.g., the free ion dissociation constant for potassium fluorenyl in TEF is 1.6 x 10 7M (18) as compared to 9.2 x 10 M for potassium picrate (17). The two N02 substituents close to the 0 bond in picrate may also hinder the enlargement of this ionic bond and the insertion of a crown ether molecule because of electronic or sterlc effects. 

Basak, A. Dugas, H., (1986) Design and synthesis of DNA intercalating crown ether molecules Tetrahedron Letters. 27, 3-6. 

The separation selectivity often can be modified by adding to the mobile phase reagents that form complexes with the separated solutes and affect the retention and the selectivity of separation as a result of competing complexing equilibria [68]. Addition of crown ethers to the mobile phase can be used to form selective complexes with molecules or ions whose dimensions correspond to the inner cavity in the crown ether molecule [69]. Similarly, formation of inclusion complexes with p- or y-cyclodextrin added to the mobile phase can be utilised to improve the separation of both geometric and optical isomers [70,71 ]. 

The centrosymmetric dimer [Li(12-crown-4)]2, in which each lithium ion forms an intermole-cular Li—O bond with a neighboring crown ether molecule (Li—0 = 2.01 A) in a rectangular four-membered Li202 ring has been described. ... 

Crown ethers Due to their inclusion capability, crown ether molecules are interesting hosts. Some detailed insights into the adsorption of crown ethers on surfaces as well as its inclusion of hosts have been already obtained in STM studies. 

Since the discovery of crown ethers in 1967 by Pedersen, thousands of crown ethers have been synthesized and their properties studied. In recent years, there has been a great deal of interest in using crown ethers for designing metal ion sensors (vide supra). In this section the construction of novel crown ether molecules with interesting properties is highlighted. 

In contrast to the CnE06 molecule which contains a linear oxyethylene chain, the crown ether molecule possesses a closed crown ring as shown schematically here ... 

Recrystallization from toluene afforded crystals suitable for X-ray analysis which contained one molecule of the solvent per crown ether molecule. The free ligand adopts a conformation in the crystal which fills the cavity space almost completely. The toluene molecules are not strongly associated with those of the tricycle. This can be perceived by the easy loss of toluene from the solvate. 

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