Crown ether like associations with

It is well known that small cations form crown-ether-like associations with PEG polymers (77). The cations interact with pseudocrown ether structures of the polymers formed by coiling of the macromolecules. Furthermore, it has been shown that AuCU" ions become coordinated to the pseudocrown ether structures via ion-pairs (76). The Au(IU) complexes are attracted through electrostatic interactions to cations that are bound to the oxyethylene groups of the polymers. In our systems Na+ ions can bind to the pseudocrown ether structures, and form ion-pairs with the gold complexes. The bound complexes are reduced via oxidation of the oxyethylene groups by the metal center ... 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

These macrocyclic ethers assume a crown-like shape in solution with a central cavity capable of containing a small solute. They bind to small cationic species through association with the electron-rich oxygens of the ether linkage. Chiral crown ethers (Fig. 31) serve as selectors for enantiomeric amines in the protonated state. They have been used as mobile-phase... 

Figure 5. It is likely that the intercalated alkali cations are surrounded by ethereal oxygen atoms like the cations in crown ethers, the iodine and thiocyanate anions being associated with the cations. It is interesting to note that the organic derivatives of y-zirconium phosphate synthesized in this study can form the complexes only with the alkali salts having soft base anions such as I and SCN , but not with the salts having hard base anions such as Br and N03 .

A series of diamide-based cryptands derived from bis(m-phenylene)-[32]crown-10 have been synthesized to complex divalent salts such as paraquat (N,N-dimethyl-4,4 -bipyridinium) dichloride. The synthetic project started from the observation that readily prepared bicyclic crown ether 23 containing two 1,3,5-phenylene units linked by three tetra(ethyleneoxy) units forms a pseudorotaxane-like inclusion complex with Af,Af-dimethyl-4,4-bipyridinium bis(hexafluorophosphate), with an association constant /fa = 6.1 X 1() M that is 100-fold greater than that of the analogous simple crown ether. On this basis, additional anion-binding moieties, such as isophthalamide or 2,6-pyridinedicarboxamide unit, were intfoduced in the third chain of the cryptand receptor. The anion-assisted complexation of bypyridinium dications was analyzed by a combination of different techniques ( H NMR,... 

Many organic reactions which involve ionic species involve the cations associated with the reactive anions, even though most of the attention is focused on the latter. Some reactions, like the displacement of bromide by cyanide in the synthesis of a cyano-alkane [5] (see Eq. 7.2), appear to involve the cation in only the most peripheral way. In the above example, replacing sodium cyanide by tetrabutylammonium cyanide [6,7] or the 18-crown-6 complex of potassium cyanide does not alter the course of the reaction [8, 9], only its rate. There are, however, quite a few examples now available of alterations in the course of the reaction in the presence of crown ether, cryptate, or ammonium salts. 

In this volume we have collected 10 review chapters from distinguished scientists who have contributed extensively to the study and development of supramolecular assemblies that contain metals and metal-like elements with unusual structures and morphologies and possess potentially useful (and applicable) physical and biological properties. The first chapter by K. Ariga et al. is a general discussion of supramolecular structures that contain inorganic building blocks for hybrid lipid thin films, layer-by-layer assemblies, structure transcription, and functional mesoporous hybrids. This is followed by two chapters, the first by M. L. Kistler et al., who describe the self-assembly of hydrophilic polyoxometalate (POM) macro-anions and examine the structure and behavior of POM macro-ions in solution. This is followed by a chapter by S. K. Das, who provides an overview of the supramolecular features of POM-supported transition metal complexes, POM-crown ether complexes with supramolecular cations, and supramolecular water clusters associated with POMs. 

---