Macrocyclic complexes, crown ether

The selective binding of ions by synthetic receptors (hosts) attracted significant attention over the past 30 years. Among the synthetic receptors, macrocyclic polyethers (crown ethers and csyptands) were extensively utilized to cooperate with metal ions in forming a stable host-guest complex. In the ion-binding processes of... 

A series of well-defined macrocyclic polyethers (crown ethers) have been synthesized by Pedersen (1, 2), who also demonstrated the capability of com-plexing and lipophilizing alkali metal ions. The KMn04-18-crown-6 complex (1) is soluble in benzene (3). Potassium metal is solubilized by a dinaph-thalenated cyclic polyether to form the interesting K -encapsulated anion radical (2) (4). 

Pedersen s discovery of macrocyclic polyethers (crown ethers) represents one of the most significant innovations in organic chemistry in recent years specific complexation of the cation by the macrocyclic ligand both facilitates the solubility... 

Due to their ability to form complexes selectively with metal cations and thereby solubilize such cations in liquid media of low dielectric constant, macrocyclic multidentate ligands have been widely investigated as carriers in metal ion transport processes for the past two decades (2-P). Such macrocycles include crown ethers, lariat ethers (crown ethers with a side arm which contains potential binding sites) (JO), calixarenes, and some cryptands. Studies of structural variations within a given... 

The strength of this bonding depends on the kind of ether Simple ethers form relatively weak complexes with metal ions but Charles J Pedersen of Du Pont discovered that cer tain polyethers form much more stable complexes with metal ions than do simple ethers Pedersen prepared a series of macrocyclic polyethers cyclic compounds contain mg four or more oxygens m a ring of 12 or more atoms He called these compounds crown ethers, because their molecular models resemble crowns Systematic nomencla ture of crown ethers is somewhat cumbersome and so Pedersen devised a shorthand description whereby the word crown is preceded by the total number of atoms m the ring and is followed by the number of oxygen atoms... 

Although it has been known for many years that ethylene oxide and formaldehyde ean form statistical copolymers, it was not until the interest in crown ethers developed that the potential of macrocyclic formals as complexing agents was recognized. 

Novel electron-transfer reactions mediated by alkali metals complexed with crown ethers as macrocyclic ligands 98ACR55. 

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

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