Electrides and alkalides

Recent work has helped to refine the understanding of the physical and magnetic properties of these systems. The synthesis, structure, polymorphism, and electronic and magnetic properties of the electride Rb(cryptand[2.2.2])e have been described. Depending on the manner of preparation and the temperature, the antiferromagnetic electride can display a range of elecrical conductivity, from poor ( 10 Scm )—consistent with localized electrons— to near-metallic electrical conductivity. Studies of the phase transitions in Cs+(18-crown-6)2e with NMR, EPR, and variable-temperature powder X-ray diffraction indicates that it undergoes a slow irreversible 

Parallels have been proposed between the dissolution of the alkali metals in nonaqueous solvents and the interactions of alkali metals with zeolites.The sorption of sodium or potassium vapor into dehydrated zeolites produces intensely colored compounds, ranging from burgundy red to deep blue, depending upon the metal concentration. A combination of EPR, 

As early as 1969, Pedersen was intrigued by the intense blue colour observed upon dissolution of small quantities of sodium or potassium metal in coordinating organic solvents in the presence of crown ethers. Indeed, the history of alkali metal (as opposed to metal cation) solution chemistry may be traced back to an 1808 entry in the notebook of Sir Humphry Davy, concerning the blue or bronze colour of potassium-liquid ammonia solutions. This blue colour is attributed to the presence of a solvated form of free electrons. It is also observed upon dissolution of sodium metal in liquid ammonia, and is a useful reagent for dissolving metal reductions , such as the selective reduction of arenes to 1,4-dienes (Birch reduction). Alkali metal solutions in the presence of crown ethers and cryptands in etheric solvents are now used extensively in this context. The full characterisation of these intriguing materials had to wait until 1983, however, when the first X-ray crystal structure of an electride salt (a cation with an electron as the counter anion) was obtained by James L. Dye and 

Since X-ray crystallography cannot observe the lone electron directly (Box 2.1), it is questionable whether it is really situated at such a distance from the Cs cation. If true, this would represent a very extreme example of the naked anion effect (Section 3.8.2). An alternative explanation localises the electron on the Cs cation, which would also account for the observed low conductivity. However, convincing evidence for the separation of cation and electron comes from the nearly isostructural sodide (Na ) and kalide (K ) analogues of [Cs([18]crown-6)2] e. In these, species the alkali metal anions are sitnated in the same localised cavities as their electride analogues. 

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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. 

Role of Cation Complexants in the Synthesis of Alkalides and Electrides... 

Tsai, K.-L. and Dye, J.L., Synthesis, properties, and characterization of nanometer-size metal particles by homogeneous reduction with alkalides and electrides in aprotic solvents, Chem. Mater., 5, 540,1993. 

Alkalides and electrides are the strongest known reducing agents in a given... 

The chemistry of the alkali metals has in the past attracted little attention as the metals have a fairly restricted coordination chemistry. However, interesting and systematic study has blossomed over the past 25 years, largely prompted by two major developments the growing importance of lithium in organic synthesis and materials science, and the exploitation of macrocyclic ligands in the formation of complexed cations. Section 12.4 deals with the use of complexed cations in the generation of alkalides and electrides. 

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... 

Solutions of Alkali Metals Solvated Electrons, Alkalides, and Electrides... 

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. 

Synthesis of Alkalides and Electrides James L. Dye, Mikhail Y. Redko,... 

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. 

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. 

In this overview of alkalides and electrides, the methodologies and special techniques used will be briefly described, and the structural features and electronic, optical and magnetic properties wiU be summarized. The reference lisl is far from complete, but the information given can provide many additional references to the original literature in this field. 

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