Sodium cryptates

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). 

Cryptand was obtained as the corresponding sodium cryptate 20 in 27% yield. The cation-free cryptand was isolated by passing an acidic solution of the complex through cation- and anion-exchangers. However, the overall yield was not reported. When sodium carbonate was replaced by potassium carbonate, no detectable amount of cryptate K+ c [2.2.2] was observed this confirms the involvent of a template effect, although this method is rather limited to simple mononucleating cryptands. 

Kinetic parameters for lithium and sodium cryptate [I ] decomplexation (61,64)... 

Figure 9. Two views of the crystal structure of the sodium cryptate Na9. (a) (Left) View into the cavity, showing the coordination of the Na+ ion (solid block circle) to the eight N-sites (dotted circles), (b) (Right) Projection upon a plane perpendicular to the axis connecting the two aliphatic nitrogen atoms of the ligand [75].

ClsHa6CS2N2S4, Caesium di-n-butyldithiocarbamate, 34B, 401 Cl8H36IKN2O6, Potassium cryptate, 39B, 518 Cl8H36lN2Na06, Sodium cryptate, 39B, 519 Cl8H36N2Na20g, Disodium[2.2.2]-cryptate, 40B, 664 Cl8H4aBraCaNaOs, Calcium cryptate, 39B, 519... 

Fig. 3. Arrhenius plots of the propagation rate constants kp of the anionic polymerization of methyl methacrylate in THF for different ion pairs including the propagation rate constant at —98 °C with cryptated sodium and of the free PMMA-anion (H. Jeuck, A. H. E. Muller, Ref. 34 )-...

The dissociation constant of ion-pairs of the cryptated sodium salt is relatively large, 10 5 M at —98 °C, hence the k value of the free polymethyl methacrylate anion is most reliable, when computed from kinetic results of its propagation. 

Both of the above-mentioned catalyst types get the anions into the organic phase, but there is another factor as well. There is evidence that sodium and potassium salts of many anions, even if they could be dissolved in organic solvents, would undergo reactions very slowly (dipolar aprotic solvents are exceptions) because in these solvents the anions exist as ion pairs with Na or and are not free to attack the substrate (p. 443). Fortunately, ion pairing is usually much less with the quaternary ions and with the positive cryptate ions, so the anions in these cases are quite free to attack. Such anions are sometimes referred to as naked anions. 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

Polymer phase-transfer catalysts (also referred to as triphase catalysts) are useful in bringing about reaction between a water-soluble reactant and a water-insoluble reactant [Akelah and Sherrington, 1983 Ford and Tomoi, 1984 Regen, 1979 Tomoi and Ford, 1988], Polymer phase transfer catalysts (usually insoluble) act as the meeting place for two immiscible reactants. For example, the reaction between sodium cyanide (aqueous phase) and 1-bromooctane (organic phase) proceeds at an accelerated rate in the presence of polymeric quaternary ammonium salts such as XXXIX [Regen, 1975, 1976]. Besides the ammonium salts, polymeric phosphonium salts, crown ethers and cryptates, polyethylene oxide), and quaternized polyethylenimine have been studied as phase-transfer catalysts [Hirao et al., 1978 Ishiwatari et al., 1980 Molinari et al., 1977 Tundo, 1978]. 

For incorporation of crown ethers and cryptates into the RTV encapsulant system as sodium and potassium ion scavengers, the total ionic contaminants must first precisely be determined. Atomic absorption is used to measure these ions in commercial silicone RTVs and silicone fluids. Values of "10 ppm for sodium and potassium were obtained in the best samples. Chloride level was determined by potentiometric titration of the silicone with AgN03. A quantity of ion trap (either crown ethers or cryptates) was then added to the RTV silicone encapsulant, and its molar concentration was equal to the combined sodium and potassium contaminant levels. 

Sodium or potassium ions can also participate in the phase-transfer process when they are converted to lipophilic cations by complexation or by strong specific solvation. A variety of neutral organic compounds are able to form reasonably stable complexes with K+ or Na + and can act as catalysts in typical phase-transfer processes. Such compounds include monocyclic polyethers, or crown ethers (1), and bicyclic aminopolyethers (cryptates) (2). They can solubilize inorganic salts in nonpolar solvents and are particularly recommended for reactions of naked anions. Applications of these compounds have been studied.12,21-31... 

Various homo- and heteropolyatomic anions have been stabilized with cryptate counterions. (Na 2.2.21) forms stable complexes with Pb52 (139), Sn94 (140), and Sb73 (141) by inhibiting reversion to the initial sodium-metal alloy phase. In a similar vein, the crystal structures of (K[2.2.2])+(HgTe2) containing a linear anion (142) and (K[2.2.2]+)2Sn42 have been described (143). 

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

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