Solubilization of alkali metals in tetrahydrofuran and diethyl ether by use of a cyclic polyether

Modern methods for the radical deoxygenation of alcohols

Die wichtigste und nahezu einzigartige Eigenschaft der makrocyclischen Polyather („Kronenverbindungen”) ist ihre Tendenz zur Komplexbildung mit Alkalimetallsalzen und Salzen mit ahnlichen Kationen. Derartige Komplexe werden durch nichtgerichtete Coulomb-Krafte zwischen dem Kation und den negativen Enden der C—O-Dipole zusammengehalten. Die Stabilitat der Polyather-Komplexe hangt vor allem davon ab, wie gut das Kation in den Polyatherring hineinpast, aber auch von der Ladungsdichte des Kations und — in Losung — von der Solvatationsstarke des Mediums. Die cyclischen Polyather sind mit Erfolg u. a. bei Arbeiten mit Ionenverbindungen in organischen Losungsmitteln sowie zur Untersuchung des Ionentransports in biologischen Systemen verwendet worden.

Makrocyclische Polyäther und ihre Komplexe

Electrides are ionic solids with cavity-trapped electrons, which serve as the anions. Localization of electrons in well-defined trapping sites and their mutual interactions provide early examples of quantum confinement, a subject of intense current interest. We synthesized the first crystalline electride, Cs(+)(18-crown-6)(2)e(-), in 1983 and determined its structure in 1986; seven others have been made since. This Account describes progress in the synthesis of both organic and inorganic electrides and points to their promise as new electronic materials. Combined studies of solvated electrons in alkali metal solutions and the complexation of alkali cations by crown ethers and cryptands made electride synthesis possible. After our synthesis of crystalline alkalides, in which alkali metal anions and encapsulated alkali cations are present, we managed to grow crystalline electrides from solutions that contained complexed alkali cations and solvated electrons. Electride research is complicated by thermal instability. Above approximately -30 degrees C, trapped electrons react with the ether groups of crown ethers and cryptands. Aza-cryptands replace ether oxygens with less reactive tertiary amine groups, and using those compounds, we recently synthesized the first room-temperature-stable organic electride. The magnetic and electronic properties of electrides depend on the geometry of the trapping sites and the size of the open channels that connect them. Two extremes are Cs(+)(15-crown-5)(2)e(-) with nearly isolated trapped electrons and K(+)(cryptand 2.2.2)e(-), in which spin-pairing of electrons in adjacent cavities predominates below 400 K. These two electrides also differ in their electrical conductivity by nearly 10 orders of magnitude. The pronounced effect of defects on conductivity and on thermonic electron emission suggests that holes as well as electrons play important roles. Now that thermally stable organic electrides can be made, it should be possible to control the electron-hole ratio by incorporation of neutral complexant molecules. We expect that in further syntheses researchers will elaborate the parent aza-cryptands to produce new organic electrides. The promise of electrides as new electronic materials with low work functions led us and others to search for inorganic electrides. The body of extensive research studies of alkali metal inclusion in the pores of alumino-silicate zeolites provided the background for our studies of pure silica zeolites as hosts for M(+) and e(-) and our later use of nanoporous silica gel as a carrier of high concentrations of alkali metals. Both systems have some of the characteristics of inorganic electrides, but the electrons and cations share the same space. In 2003, researchers at the Tokyo Institute of Technology synthesized an inorganic electride that has separated electrons and countercations. This thermally stable electride has a number of potentially useful properties, such as air-stability, low work function, and metallic conductivity. Now that both organic and inorganic electrides have been synthesized, we expect that experimental and theoretical research on this interesting class of materials will accelerate.

Electrides: early examples of quantum confinement.

Theory and applications of ion-selective electrodes

Anions of sodium, potassium, rubidium, and cesium are stable both in suitable solvents and in crystalline solids. The latter can be prepared either by cooling a saturated solution or by rapid solvent evaporation. Thermodynamic arguments show that alkali metal anions can probably exist in saturated solutions of the alkali metals in any compatible solvent, but that below saturation, dissociation into the cation and solvated electrons is favored in highly polar solvents such as ammonia. The key to solvent-free salts of the alkali metal anions is stabilization of the cation by incorporation into a suitable crown or cryptand complex. By using such complexes it also appears possible to produce “electride” salts in which the charge of the complexed cation is balanced by a trapped electron. The chemical, electrical, and optical properties of salts of the alkali metal anions and “electrides” could provide useful applications.

Solubilization of alkali metals in tetrahydrofuran and diethyl ether by use of a cyclic polyether

Citations

Modern methods for the radical deoxygenation of alcohols

Makrocyclische Polyäther und ihre Komplexe

Electrides: early examples of quantum confinement.

Theory and applications of ion-selective electrodes

Compounds of Alkali Metal Anions

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Crystalline Salt Complexes of Macrocyclic Polyethers

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Stability constants of cyclic polyether complexes with univalent cations

Crystalline salt of the sodium anion (Na

New Macrocyclic Polyethers