Cryptands applications

Crown ethers and cryptands in analytical chemistry 82MI1 80ZAK992. Crown ethers and cryptands, application for concentration and separation... 

Cryptands, 7, 731-761 alkali metal complexes NMR, 7, 740 reactivity, 7, 743-744 alkaline earth complexes reactivity, 7, 743-744 anion complexes, 7, 747-748 applications, 7, 753-761 as biological models, 7, 753-754 bis-tren... 

Although the principal application for 9 has been in the synthesis of cryptands (see Chap. 8), this material has also served as precursor to a number of nitrogen based lariat ethers , sometimes referred to as crown complexanes . Binding constants for such compounds have been measured for a few examples in a few cases , but... 

In specific applications to phase transfer catalysis, Knbchel and his coworkers compared crown ethers, aminopolyethers, cryptands, octopus molecules ( krakenmole-kiile , see below) and open-chained polyether compounds. They determined yields per unit time for reactions such as that between potassium acetate and benzyl chloride in acetonitrile solution. As expected, the open-chained polyethers were inferior to their cyclic counterparts, although a surprising finding was that certain aminopolyethers were superior to the corresponding crowns. 

The bulk of the work which has been performed on open-chained crown ether and cryptand equivalents, especially for application to general cation binding studies has been accomplished by Vogtle and his coworkers. Vogtle has reviewed both his own and other work in this field . 

We have noted above (Sect. 8.3) that certain substituted benzoic acids have been used in the synthesis of endolipophilic cryptands but it is the glycerol and penta-erythritol units which have been used most commonly in this application. These are discussed separately, below. 

A particularly imaginative application of this concept has led to the isolation of compounds which contain monatomic alkali metal anions. For example, Na was reacted with cryptand in the presence of EtNHi to give the first example of a sodide salt of... 

Poly (macrocyclic) compounds. The analytical application of compounds such as crown polyethers and cryptands is based on their ability to function as ligands and form stable stoichiometric complexes with certain cations. Special importance is due to their preference for alkali metal ions which do not form complexes with many other ligands. A number of these compounds are commercially available and their properties and analytical applications have been described by Cheng et a/.11... 

Although crown ethers were often found to be as effective as the best onium salts in PTC, they have only found limited commercial application because of their high cost (10 to 100 times that of quats) and perceived toxicity. Cryptands are even more expensive than crown ethers. 

For those applications involving the activation of an inorganic anion (that is, generation of a naked anion), the cryptands, rather than the crowns, tend to be the reagents of choice. Such reagents are thus also ideal for applications involving phase-transfer catalysis of the type discussed previously. 

Cryptands of the type (217)-(220) tend to form stable complexes with a number of heavy metal ions. Of particular interest is the selectivity of (219) for Cd(n) the complex of this metal is approximately 106-107 times more stable than its complexes with either Zn(n) or Ca(n). This reagent may prove useful for removing toxic Cd(n) from biological systems as well as for other applications involving sequestration of this ion (for example, in antipollution systems). The selectivity observed in the above case appears to arise because (i) the nitrogen sites favour coordination to Zn(n) and Cd(n) relative to Ca(n) and (ii) the cavity size favours coordination of Cd(n) relative to Zn(n). 

Another example of a cryptand is PCT-16 (Figure 10.23), which has potential applications as an extracellular probe of potassium. The dissociation constant in water is 1 mmol dm 3, i.e. slightly lower than that of the coumarocryptand PET-5 (see Section 10.3.2). 

It was a result of demand from industry in the mid-1960s for an alternative to be found for the expensive traditional synthetic procedures that led to the evolution of phase-transfer catalysis in which hydrophilic anions could be transferred into an organic medium. Several phase-transfer catalysts are available quaternary ammonium, phosphonium and arsonium salts, crown ethers, cryptands and polyethylene glycols. Of these, the quaternary ammonium salts are the most versatile and, compared with the crown ethers, which have many applications, they have the advantage of being relatively cheap, stable and non-toxic [1, 2]. Additionally, comparisons of the efficiencies of the various catalysts have shown that the ammonium salts are superior to the crown ethers and polyethylene glycols and comparable with the cryptands [e.g. 3, 4], which have fewer proven applications and require higher... 

The first examples of the application of phase-transfer catalysis (PTC) were described by Jarrousse in 1951 (1), but it was not until 1965 that Makosza developed many fundamental aspects of this technology (2,3). Starks characterized the mechanism and coined a name for it (4,5), whilst Brandstrom studied the use of stoichiometric amounts of quaternary ammonium salts in aprotic solvents, "ion-pair extraction" (6). In the meantime Pedersen and Lehn discovered crown-ethers (7-9) and cryptands (10,11), respectively. 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

This issue has been investigated using amalgam cathodes, and the indications are that the cryptand has negligible influence on the rate of incorporation of a sodium ion into the amalgam (Chen et ai, 1991 Doan et al, 1991). One of the most interesting applications of the cryptands is that they increase the mobility of dipositive ions in polymer electrolytes (Chen and Shriver, 1991). 

Cryptands, 42 122-124, 46 175 nomenclature, 27 2-3 topological requirements, 27 3-4 Cryptate, see also Macrobicyclic cryptate 12.2.2], 27 7-10 applications of, 27 19-22 cylindrical dinuclear, 27 18-19 kinetics of formation in water, 27 14, 15 nomenclature, 27 2-3 spherical, 27 18 stability constants, 27 16, 17 Crystal faces, effect, ionic crystals, in water, 39 416... 

As regards other coordination compounds of silver, electrochemical synthesis of metallic (e.g. Ag and Cu) complexes of bidentate thiolates containing nitrogen as an additional donor atom has been described by Garcia-Vasquez etal. [390]. Also Marquez and Anacona [391] have prepared and electrochemically studied sil-ver(I) complex of heptaaza quinquedentate macrocyclic ligand. It has been shown that the reversible one-electron oxidation wave at -1-0.75 V (versus Ag AgBF4) corresponds to the formation of a ligand-radical cation. Other applications of coordination silver compounds in electrochemistry include, for example, a reference electrode for aprotic media based on Ag(I) complex with cryptand 222, proposed by Lewandowski etal. [392]. Potential of this electrode was less sensitive to the impurities and the solvent than the conventional Ag/Ag+ electrode. 

The selective cation binding properties ol crown ethers and cryptands have obvious commercial applications in the separation of metal ions and these have recently been reviewed (B-78MI52103.79MI52102, B-81MI52103). Many liquid-liquid extraction systems have been developed for alkali and alkaline earth metal separations. Since the hardness of the counterion is inversely proportional to the extraction coefficient, large, soft anions, such as picrate, are usually used. 

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