Anions alkalides

Cryptands were found to react with metal solutions in basic solvents to generate the alkali metal cryptate and an alkali anion (alkalide), for example (Na[2.2.2])+Na (62, 63). 23Na-NMR measurements of this salt in methylamine, tetrahydrofuran, and ethylamine solutions showed that the Na resonance is shifted strongly upheld from the Na resonance (free or complexed) as shown in Fig. 7. The anion resonates at approximately the same frequency as that calculated for the free... 

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. 

Although the alkali metals are known primarily for their formation of unipositive ions, numerous examples of alkali metal anions (alkalides) have been reported since 1974. 

Alkalides and electrides are stoichiometric salts containing alkali metal cations complexed by crown ethers. Charge balance is provided by the alkali metal anions (alkalides) or trapped electrons (electrides). Rb and Rb NMR has been used to study a number of mbidium alkalides, electrides and related compounds (Kim et al. 

Electron-donating dopants may reduce polyacetylene or other conjugated polymers giving rise to -type conductivity. Doping occurs when the polymer is immersed in a tetrahydrofuran solution of radical anion/alkalide where the alkalide components can be Li, Na, K, Cs, or Rb, and the radical anion can be naphthalene, anthracene, or benzo-phenone. A doping reaction such as the following occurs (Nph = naphthalene) ... 

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. 

The alkalides. The first crystalline alkalide to be prepared in this manner was [Na+(2.2.2)].Na. This salt is obtained as shiny, gold-coloured crystals (Dye etal., 1974). The 23Na nmr spectrum yields a narrow upfield signal for the Na- ion (Dye, Andrews Ceraso, 1975) the X-ray structure indicates close-packed sodium cryptate cations with Na" anions occupying octahedral holes between the cryptate layers (Tehan, Barnett Dye, 1974). 

The equilibrium (1) is very sensitive to several factors. The complexing ability of L, the electron affinity of M and the lattice energy of the resulting salt drive the alkalide formation while the lattice energies ofthe solid metal M(s) and the complexant L, the ionization energy of M, and the unfavourable entropy of formation ofthe well ordered crystalline product oppose it. Not only alkalides with the cation and anion of the same element but also mixed ones such as K+C(222)Na have also been obtained [24]. 

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]. 

The high affinity of crown ethers and cryptands for alkaii metai cations will cause the metals themselves to disproportionate into cations and alkalide anions or electride salts. 

Alkalides are crystalline compounds that contain the alkali metal anions, M-(Na-, K-, Rb, or Cs ). The first alkalide compound Na+(C222)-Na was synthesized and characterized by Dye in 1974. 

Table 12.4.1. Radii of alkali metal anions from structure of alkalides and alkali metals ...

More than 40 alkalide compounds that contain the anions Na , K , Rb , or Cs have been synthesized, and their crystal structures have been determined. Table 12.4.1 lists the calculated radii of alkali metal anions from structures of alkalides and alkali metals. The values of rM (av) derived from alkalides and rM- from the alkali metals are in good agreement. 

While the trapped electron could be viewed as the simplest possible anion, there is a significant difference between alkalides and electrides. Whereas the large alkali metal anions are confined to the cavities, only the probability density of a trapped electron can be defined. The electronic wavefunction can extend into all regions of space, and electron density tends to seek out the void spaces provided by the cavities and by intercavity channels. 

The combination of the reducing power of alkali metal-ammonia solutions with the strong complexing power of macrocyclic ligands allows compounds to be made containing unusual anions, such as [Sn9]4-. Among the unexpected products of such reactions are alkalide and electride salts. An example of an alkalide is [Na(2.2.2.crypt)]+ Na-, where crypt is the crypt and... 

Alkalides and electrides allow homogenous reduction reactions for various transition metals, even for main group metals in aprotic solvents. These reducers consist of alkali metal anions or electrons trapped in crown ethers. 

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.- ... 

Solutions of the alkali elements, in a range of polar solvents, have been shown to contain M , M ", e, and the unusual anion, M , in equilibrium. Early studies were hindered by solubility and decomposition problems. The addition of a macrocylic chelate to these solutions provided a means of increasing the solubility of the metal as well as controlling what species predominate in solution. A metal chelate mole ratio of 2 1 leads to formation of the alkalides [M -i- L] M while a ratio of 1 1 leads to the electrides (see 10.2.2.5), [M -t- L] e (where M and M = Li, Na, K, Rb, Cs (not always the same) and L = two crown ethers (combinations of 12-C-4, 15-C-5, 18-C-6) or one cryptand [2.2.2]). Furthermore, these unique compounds could be isolated as solids and fully characterized. A wider range of alkalides than electrides is known including those with other chelates such as ethylenediamne , and aza crown ethers . Some representative examples are [Na -t- crypt]Na"(the first to be structurally characterized) , and [Cs + 18-C-6)(15-C-5)]Na- . 

A more iuterestiug electride, [K([2.2.2]cryptand)] e isformedby thereactiouof [2.2.2]cryptand with potassium metal in diethyl ether at -50 °C. In this material, the X-ray crystal structure shows that the [K([2.2.2]cryptand)] cations are separated by large dumbbell-shaped cavities in which pairs of electrons reside, some 5.3 A apart. Each pair of cavities is connected with adjacent cavities via channels 7.8 and 8.4 A long (Figure 3.77). The structures of the alkalide anion analogues [K([2.2.2]cryptand)] have also been reported,and show that the alkali metal anions also... 

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