Cryptands electride complexes

Alkali metals can dissolve in solvating media such as ethers and amines to form blue solutions of solvated electrons. In the presence of strongly complexing ligands such as crown ethers or cryptands, electrides (complexed alkali cation and electron), or nuclides (complexed alkali cation and alkali metal anion) can be formed as shown in Scheme 7.3 [50]. Nuclides have been shown to react with monomers such as styrene and methyl methacrylate... 

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. 

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)... 

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). ... 

The study of more concentrated alkali metal solutions in a variety of solvents became possible in 1970, when crown ethers were used to enhance the solubility of the metal. The use of crown ethers and cryptands permitted extensive studies of the optical spectra of solvated electrons and alkali metal anions in solution. After the isolation of the first sodide salt in 1974, the optical spectra of polycrystalline films of various alkalides and electrides were determined by rapid evaporation of all solvent from a liquid film on the walls of the optical cell." " Displayed in Fig. 1 are the optical spectra obtained in this way. Clearly, there is a 1 1 correspondence between the spectra in solution and in the solid state. Although the peak positions shift somewhat with temperature and with the complexant used, the optical peaks can be used to verify the presence of particular alkali metal anions or trapped electrons. In addition to rapid solvent evaporation, solvent-free alkalide and electride films can be made by codeposition of the complexant and alkali metal in high vacuum (10 torr)." This permitted the study of optical... 

The species that results from the reductive electrocrystallization of the sodium complex of tris-bipy cryptate, the schematic structure of which is shown in figure 2, has been called a "cryptatium." The name was chosen to express the dual nature of its procedence, since it is part cryptate and part sodium metal. It must be stressed that this cryptatium material is electroneutral, thus forming an expanded atom structure. Figure 2 also shows the schematic structure of an electride and that of a simple sodium atom, to illustrate two additional and extreme situations. In one, the electride, the complexation of the metal ion by the cryptand is so strong that the electron is essentially expelled from its interaction with the cationic center. In the other, the simple sodium atom, the outermost electron resides in an s orbital of the metallic center. Cryptatium thus represents an in-between situation, where the electron is not totally expelled from its interaction with the cation but it does not reside on the cation either, but rather on the ligand. The result is an expanded-metal type structure, where the electron is localized in the ligand structure. 

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