Stable electrides

Removal of enclathrated oxygen ions from the cavities in a single crystal of 12Ca07Al203 leads to the formation of the thermally and chemically stable electride [Ca24AI28064l4+(c )4. 

It is likely that the study of electrides would be more widespread if thermal stability at room temperature could be achieved. The recent success in synthesizing a sodide and potasside that are stable at room temperature provided the incentive to find aza-based complexants that will permit the crystallization of stable electrides. The cation of choice would be Li", because there would be no competition from Li. an alkalide that has never been made. A number of candidates were tried, from methylated cyclens to fully methylated azacryptands to ada-manzane-like complexants. Thus far none produced a crystalline electride of known structure. When the cation is kinetically able to leave the complexant. dissociation without decomposition occurs. The aza complexants used to date apparently do not form strong enough complexes to yield thermodynamically stable electrides. although aikalides can be made in some cases. The search for the ideal aza complexant continues. 

It is also possible to prepare crystalline electrides in which a trapped electron acts in effect as the anion. The bnUc of the excess electron density in electrides resides in the X-ray empty cavities and in the intercoimecting chaimels. Stmctures of electri-dides [Li(2,l,l-crypt)]+ e [K(2,2,2-crypt)]+ e , [Rb(2,2,2-crypt)]+ e, [Cs(18-crown-6)2]+ e, [Cs(15-crown-5)2]" e and mixed-sandwich electride [Cs(18-crown-6)(15-crown-5)+e ]6 18-crown-6 are known. Silica-zeolites with pore diameters of vA have been used to prepare silica-based electrides. The potassium species contains weakly bound electron pairs which appear to be delocalized, whereas the cesium species have optical and magnetic properties indicative of electron locahzation in cavities with little interaction between the electrons or between them and the cation. The structural model of the stable cesium electride synthesized by intercalating cesium in zeohte ITQ-4 has been coirfirmed by the atomic pair distribution function (PDF) analysis. The synthetic methods, structures, spectroscopic properties, and magnetic behavior of some electrides have been reviewed. Theoretical study on structural and electronic properties of inorganic electrides has also been addressed recently. ... 

In electrides, electrons behave as anions. In a typical organic electride Cs+ (15-crown-5)2 e, the electrons occupy cavities connected by narrow channels. The first electride stable at room temperature was made by ionizing Cs into Cs+ and e within the channels of an aU-sUica zeohte. Petkov et confirmed that Cs+ and a low-density electron gas are present in the one-dimensional nanopores of the sihceous zeolite ITQ-4 (Si32064). The PDF technique provided direct structural evidence that Cs intercalated in the form of Cs+ ions arranged in short zigzag chains (see Figure 10). This structural model... 

The sodide, stable to —25°C, has been dubbed an inverse sodium hydride (see Figure 22). The strategy of using kinetically trapped cations in polyaza cages may lead to new classes of stabilized alkalides and electrides. 

There is evidence that the definition of chemical elements must be broadened to include the electron. Several compounds known as electrides have recently been made of alkaline metal elements and electrons. A relatively stable combination of a positron and electron, known as positronium, has also been studied. 

The per-aza cryptand TriPip222 (Figure 8.8c) is able to complex Na ions and is extremely robust towards reduction. In 2005 this cryptand was reported to form Na(TriPip222)]+e, the first electride to be stable at room temperature. Cavities within crystalline [Na(TriPip222)] e accommodate the electrons. The sodide [Na(TriPip222)] Na, with Na trapped in the cavities, was also prepared (Figure 8.8d). ... 

We recently added alkali metals from the vapor phase to two pure silica zealots to yield adducts that have a 1 1 stoichiometry of cations to electrons.The properties strongly point to ionization to yield alkali cations that interact with the Si—O—Si units that line the zeolite channels. Apparently, the released electrons occupy the open spaces of the channels, which have diameters of — 7 A. The electrons are strongly coupled to form nearly diamagnetic materials. These inorganic electrides are stable to at least 100°C. Potentially, zeolite-based systems... 

Although we studied alkalides and electrides for nearly 30 years, there is much that remains to be done to make them useful members of the chemistry-physics communities We hope that the existence of stable, easy-to-prepare alkalides and electrides will prompt a number of other researcheis to entei this intriguing field... 

No stable, well-defined complexes with group 1 metal - metal single bonds are known to date, though related alkalide [41,42] and electride [41,43-45] compound classes with formal low oxidation state group 1 metals have been studied, and some alkali metal suboxide and subnitride materials are... 

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