Alkalides stability

Novel anions stabilized by alkali-polyether cations The ability of a crown (such as 18-crown-6) or a cryptand (such as 2.2.2) to shield an alkali cation by complex formation has enabled the synthesis of a range of novel compounds containing an alkali metal cation coordinated to a crown or cryptand for which the anion is either a negatively charged alkali metal ion or a single electron (Dye Ellaboudy, 1984 Dye, 1984). Such unusual compounds are called alkalides and electrides , respectively. 

Polyether complexation. The solution of the above problem is to add a suitable crown ether or cryptand to the alkali metal solution. This results in complexation of the alkali cation and apparently engenders sufficient stabilization of the M+ cation for alkalide salts of type M+L.M" (L = crown or cryptand) to form as solids. Thus the existence of such compounds appears to reflect, in part, the ability of the polyether ligands to isolate the positively charged cation from the remainder of the ion pair. 

This general trait of crown ethers and cryptands (to be discussed later) to stabilize alkali metal salts has been extended to even more improbable compounds, the al-kalides and electrides, which exist as complexed alkali metal cations and alkalide or electride anions. For example, we saw jn Chapter 10 that alkali metals dissolve in liquid ammonia (and some amines and ethers) to give solutions of alkali electrides 10 M M+ f e" (12.38)... 

Aside from the effects of ligand stoichiometry and the nature of the solvent, there are also differences in stability of the alkalide ions. The sodide anion is the most stable, and the ceside ion the leash Because of differential stabilities of the alkalide ions and... 

Of particular significance in this respect has been the ability to prepare, characterize and study most intriguing species, the alkalides [2.79, 2.80] and the electrides [2.80, 2.81] containing an alkali metal anion and an electron, respectively, as counterion of the complexed cation. Thus, cryptates are able to stabilize species such as the sodide [Na+ c 9]Na- and the electride [K+ c 9]e-. They have also allowed the isolation of anionic clusters of the heavy post-transition metals, as in ([K+ c cryp-tand]2 Pb52-) [2.82]. 

When cryptands or crown ethers are added to stabilize the cations, crystalline solids may be isolated, some containing the M anions, alkalides, and others containing trapped electrons, electrides. 

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. 

Alkalide anions, M", discussed in Chapter 12, may be stabilized by various macrocylic ligands.- The dissolution of sodium and heavier alkali metals in ethers gives not only solvated and e, but also solvated M , which results from disproportionation of the metal atom.- ... 

Many properties of alkalide crystals, powders, and films were measured. The original alkalide, Na (C222)Na , has been most thoroughly studied, in part because of the high stability of pure samples. In contrast to most alkalides, single crystals and vapor-deposited films of this sodide are stable in vacuo for many hours, even at room temperature. In addition to the crystal structure, we measured the thermodynamics of formation by an EMF method, optical... 

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. 

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