Alkalides

The alkali metals can be dissolved in liquid ammonia, and also in other solvents such as ethers and organic amines. Solutions of the alkali metals (except Li) contain solvated M- anions as well as solvated M+ cations  

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. 

Elemental sodium dissolves only very slightly ( 10-6 mol L-1) in ethy-lamine, but when C222 is added, the solubility increases dramatically to 0.2 mol L-1, according to the equation 

The cryptated sodium cation surrounded by six natride anions in crystalline [Na+(C222)]-Na.  

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. 

---

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. 

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). 

Although crystalline potassides, rubidides and caesides have all been prepared, sodides are more readily isolated since they tend to be the more kinetically and thermodynamically stable. A number of mixed-metal alkalides has also been isolated but they are all of type M+L Na" (n = 1 or 2). 

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. 

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]. 

Even more exotic than alkalides are electrides. They are formed instead of alkalides when an excess of a strong complexant is present ... 

Interestingly, a considerable number of trapped electrons are present in most alkalides. However, their concentration (up to several percent) can be significantly reduced by applying an excess of metal during the synthesis. Na ... 

NMR is one of the most easy and effective methods in alkalides studies since M4 and M signal positions differ considerably. For instance, Na+ and Na signals of Na+C(222)Na in ethylamine appear at 10.4 0.5 ppm and 62.8 0.2 ppm, respectively, while the signal of uncomplexed Na+ should lie lower than at -10 ppm [30]. Moreover, the signal ofNa+ofthe complex is much broader pointing to a restricted motion of the cation in the cryptand cage. 

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

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)... 

However, the situation is somewhat more complicated than Eq. 12.38 would indicate, because the electrons can react further with the metal to form alkalide ions ... 

On the other hand, a mole ratio of 2 t. mctalrligand. tends to favor the alkalide ... 

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. 

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... 

Dissolution of lithium metal with 12.1.11 in methylamine results in formation of the dark-blue electride (Li 2.1.1 ])+e rather than an alka-lide as deduced from optical transmission and EPR spectroscopy (133). Transparent films containing (M[2.2.2])+M (M = Na, K, Rb) or elec-trides have been prepared by direct vapor codeposition of the metal and the cryptand (134). Optical transmission and infrared spectral data suggest that with the metal in excess only alkalides are formed, whereas electrides are preponderant with the cryptand in excess (138 ). 

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. 

---