Cation-binding selectivities crown ethers

The ditopic receptor 119 has been shown to bind both inorganic anions and alkali metal cations simultaneously (233). Selectivity was observed for HS04-over CP and the binding of alkali metal cations in the crown ethers was proven to substantially enhance the magnitude of anion binding (through electrostatic interaction). 

It was initially proposed that there is an optimal spatial fit between crown ethers and particular cations. It is true that [18]crown-6 is selective for K, whereas the larger [21]crown-7 has a higher affinity for Rb and Cs than K. However, modem understanding of these systems has somewhat modified this simple size-fit idea, particularly because of the flexibility of the crown ethers - their size is not a constant. Table 2.1 shows the binding constants obtained for a selection of cations with various crown ethers. 

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. 

A good deal of work has been done on polymeric crown ethers during the last decade. Hogen Esch and Smid have been major contributors from the point of view of cation binding properties, and Blasius and coworkers have been especially interested in the cation selectivity of such species. Montanari and coworkers have developed a number of polymer-anchored crowns for use as phase transfer catalysts. Manecke and Storck have recently published a review titled Polymeric Catalysts , which may be useful to the reader in gaining additional perspective. 

Alcohols can be selectively bound to the same host type if they are combined with an amine and vice versa, considering that a cation and an anion will be formed through a proton transfer. The so-formed alkoxide anion will bind to the boron atom, while the ammonium ion will be complexed by the crown ether (147, Fig. 39). Competition experiments involving benzyl-amine have shown enhanced selectivity for the complexation of alcohols with... 

Sr2+ so well that it was selectively extracted from a bulk sample of a barium salt (Helgeson et al., 1973a). Binding constants for metal-cation complexes of 1,3-xylyl-crown ethers [66]—[69] carrying an additional carboxylate binding... 

On the size of the cavity in the crown ether which can be tailored to allow for the selective binding of only certain cations, i.e., whose ionic radius is best accommodated by the poly ether. Thus 18-crown-6 shows a high affinity for K (ionic diameter 2.66 A) and 15-crown-5 for Na+ (ionic diameter 1.80A). 

Glasses exist that fnnction as selective electrodes for many different monovalent and some divalent cations. Alternatively, a hydrophobic membrane can be made semiper-meable if a hydrophobic molecnle called an ionophore that selectively binds an ion is dissolved in it. The selectivity of the membrane is determined by the structnre of the ionophore. Some ionophores are natnral products, such as gramicidin, which is highly specific for K+, whereas others such as crown ethers and cryptands are synthetic. Ions such as, 1, Br, and N03 can be detected using quaternary ammonium cationic surfactants as a lipid-soluble counterion. ISEs are generally sensitive in the 10 to 10 M range, but are not perfectly selective. The most typical membrane material used in ISEs is polyvinyl chloride plasticized with dialkylsebacate or other hydrophobic chemicals. 

The selective cation binding properties ol crown ethers and cryptands have obvious commercial applications in the separation of metal ions and these have recently been reviewed (B-78MI52103.79MI52102, B-81MI52103). Many liquid-liquid extraction systems have been developed for alkali and alkaline earth metal separations. Since the hardness of the counterion is inversely proportional to the extraction coefficient, large, soft anions, such as picrate, are usually used. 

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