Alkalide salts

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. 

Alkalide Salt in which one of the alkali metals is the anion. 

We extend the discussion of these alkalide salts and note the existence of numerous species containing a second alkali metal ion but now found as complexed (with a crypt and/or crown ether) cation. These cations may be understood as solvated but with a well-defined solvation shell, both in terms of structure and stoichiometry. The solvation has made it easier to remove an electron from the alkali metal—so much easier that the electron may be put... 

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)... 

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. 

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... 

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 or electrides in Me20 or thf can be used to reduce salts of metals such as Au, Pt, or Cu to give very finely divided metals.12 Solutions of mixed alkali metals in amines can also be made and have been much studied.13 The compounds Li(MeNH2) Na and LiNa(EtNH2) have Li+ and Na in both solid and liquid phases, and have a bronze color. 

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 addition, Y2O3 [100], Gd203 [101], and Dy203 [102] nanoparticles, with size of 10 - 20 nm, have been synthesized by homogeneous reduction using alkalide solutions and subsequent oxidation at 1273 K. Alkalides are crystalline ionic salts... 

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 hydride anion is well known in salts. Much more exotic are the alkalide anions, which, with the exception of Li , have all been synthesized under specialized conditions. 

The next question to be asked concerns the composition of the particles. For the case of monometallic particles this could in principal be a trivial question. If the particle is known to contain a single element, the only question which then arises concerns the oxidation states of the metal, and this can be determined by X-ray photoelectron spectroscopy. For example, the colloidal metals described in Section 6.2.2.1 and prepared by Dye and coworkers by alkalide and electride reduction of salts of gold, copper, platinum, nickel, and molybdenum (as well as several main group metals and metalloids) were analyzed by XPS [73] which showed the presence of only zerovalent metal. Oxidized metal was detected only for nickel and molybdenum (among the transition metals) and this only after exposure to an oxidizing solvent such as methanol. These results show that if a sufficiently powerful reducing agent is used in the colloid synthesis, the surface of the particles can be kept in a reduced metallic state. 

Alkalides and electrides are effective reducing agents comparable to solvated electrons. Alkalides and electrides are crystalline salts consisting of crown ethers complexed with alkali ions or salts with alkali metals as anions, and consist of trapped electrons. They are of the type K+(15 — crown — 6)2Na [197-200]. The reduction is carried out in solvents such as THF under... 

Naphtalides, alkalides, and alkali metals are sufficiently powerful to reduce Ge and Si salts to the elements. Si nanocrystals have been prepared in solution by the reduction of the halides with Na, Li naphthalide, and hydride reagents [216-219]. Similarly, Ge nanocrystals have been made by the reduction of GeCL with Li naphthalide in THF [217]. TEOS (Si(OEt)4) is readily reduced by sodium to yield Si nanocrystals. Si and Ge nanocrystals are frequently covered by an oxide layer. Y2O3 nanocrystals have been made by the alkalide reduction of YCI3 followed by oxidation by exposure to ambient conditions [220]. Yittria nanocrystals could be doped with Eu to render them phosphorescent [221]. ZnO nanoparticles have been prepared from zinc acetate in 2-propanol by the reaction with water [222]. Pure anatase nanocrystals are obtained by the hydrolysis of TiCL with ethanol at 0°C followed by calcination at 87° C for 3 days [223]. The growth kinetics and the surface hydration chemistry in this reaction have been investigated. 

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