Polyethers solvate metal ions

Polyethers solvate metal ions crown ethers and ionophores... 

Usually, the ion exchange rate between a solvated metal ion and the ion fixed in the complex compoimd is very fast. This is the reason why most of the cation complexes with cyclic polyethers have high exchange rates which are in the order of 10 s >20) Although the exchange rate for metal ions with a bicyclic cryptand is significantly lower than with monocyclic crown ethers the kinetic isotopic... 

Cyclic polyethers, or crown ethers, are capable of solvating metal ions in organic (nonpolar) solvents. 

As polymer matrices, various polyethers are frequently used because the mobile metal ions existing within are not only solvated but also coordinated by oxygen atoms originating from polyethers, e.g., poly(ethylene oxide)/PEO. The respective materials are often cross-linked, and they are bound together to form single three-dimensional networks. 

The importance of crown ethers derives from their extraordinary ability to solvate metal cations by sequestering the metal in the center of the polyether cavity. For example, 18-crown-6 complexes strongly with potassium ion. 

Crown ethers, a class of macrocyclic polyethers,22,23) have the remarkable ability to solubilize alkali metal salts in solvents of low polarity. The crown ether solvates a metal cation by binding the ion to its oxygen atoms by means of ion-dipole interactions. The metal ion becomes located near or inside the cavity of the ether illus-... 

Crown ethers or cryptates 493) are cyclic polyethers that selectively solvate alkali-metal ions, although depending on the size of the hole, other ions can be extracted by these reagents. Some recent work on extraction using these crown ethers has been reported 494-496) and more results can soon be expected from the use of crown ketones 408, 409). 

A significant advance in overcoming this problem is the use of macrocyclic polyethers in catalytic quantities in such media. Macrocyclic polyethers, such as 18-crown-6, the structure of which is shown in Figi 5.3, have a pronounced ability to coordinate metal ions. This ability allows for more salt to be dissolved in the organic solvent equally important, coordination of the metal ion leaves a relatively poorly solvated anion behind. The anions behave under these conditions as highly reactive species, sometimes termed naked anions. A study of the relative rates of... 

Dioxane and Water. Grunwald and co-workers (GBK) (7) used a vapor pressure method to obtain the differential of the free energy of transfer of a solute with respect to solvent mole fraction at 50 wt% dioxane. On the basis of what has now become known as the large-ion assumption (8), they separated cation and anion effects by equating the free energies of transfer for tetraphenylborate and tetraphenylphosphonium ions. They concluded that Na+ was preferentially solvated by dioxane, a surprising result then, but less unexpected now that complexes of the alkali metals with polyethers have been discovered (dioxane... 

The transformation of alkyl halides into alkanenitriles with cyanide ions has frequently been carried out in protic solvents such as methanol or ethanol, sometimes with the addition of water or acetone, and often at elevated temperatures. Under these conditions reaction rates decrease in the order iodides, bromides, chlorides, as would be expected. Accordingly iodide ions have a catalytic effect and increase reaction rates. The use of anhydrous ethylene glycol or di- and poly-ethylene glycols and their corresponding ethers allows the use of higher temperatures, which means better solubility of the alkali metal cyanides. There is probably additional help from the extensive solvation of the countercations by some of these hydroxy polyethers. While for primary halides yields for nitriles range up to 90% (Table 1), they drop sharply with secondary and tertiary halides. ... 

There are now several structural studies on cobalt(II) crown ether and cryptand complexes (Table 78), which show the coordination mode to be markedly sensitive to the macrocycle cavity diameter. Both 12-crown-4 and 15-crown-5 (cavity diameters 120-150 pm and 170-220 pm respectively) can include ions to form structures in which every ether oxygen atom is bound to the metal. In contrast Co—O (ether) bonding is destabilized in the larger 18-crown-6 and dicyclohexyl-18-crown 6 polyethers. In the blue complexes obtained from the reaction of these compounds with C0CI2 the role of the cyclic polyether is to solvate discrete [Co(H20)6] cations and [CoC ] " anions and there are no direct Co—O (ether) bonds.A similar effect is seen in the Co" complex of a 27-membered-ring macrocycle where the pentacoordinate structure (267) features only one long Co—O (ether) bond. " ... 

A study of i.r. vibrations of alkali-metal cations encased in dibenzo-18-crown-6 reveals that the ions Na+ and K+ have about equal complexation forces, and cation selectivity therefore stems entirely from the difference in stability of the solvated cations. I.r. methods have also been used in characterizing stable H30+-polyether complexes formed in aqueous perchloric acid, and X-ray diffraction studies of barium thiocyanate complex with isomer A of dicyclohexyl-18-crown-6 (obtained with isomer B on hydrogenation of dibenzo-18-crown-6) show that this isomer has the cis-syn-cis configuration. ... 

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