Cation extraction, using cryptands

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. 

Crown ethers and cryptands, either alone or fixed on a polymer support [2.89], have been used in many processes, including selective extraction of metal ions, solubilization, isotope separation [2.90], decorporation of radioactive or toxic metals [2.17, 2.49], and cation-selective analytical methods [2.89, 2.91, 2.92] (see also Sect. 8.2.2 and 8.4.5). A number of patents have been granted for such applications. 

In a cryptate complex, the cation is enclosed wholly or partially in a hydrophobic sheath, so that not only are salts of this complexed cation soluble in nonpolar organic solvents but also extractable from aqueous solutions into organic solvents immiscible with water (144). Specific cryptands may be used to selectively complex metals from crude materials or wastes, particularly if they are immobilized on a polymer support (101, 114, 145). 

Macrocycllc compounds (some crown ethers and cryptands) are selective reagents for extractive separation of alkali metals [22-27]. These ligands form cationic complexes with alkali metal ions, and these can be extracted as ion-pairs with suitable counter-ions e.g., picrate) [28], most often into chloroform. For potassium, p-nitrophenoxide was used as counter-ion [29]. In cases, where a coloured anionic complex is a counter-ion [30], the extract may serve as a basis for determining the alkali metal. The effect of the structure of the dibenzo-crown ether rings upon the selectivity and effectiveness of isolation of alkali metals has been studied in detail [31]. Chromogenic macrocyclic reagents applied for the isolation and separation of alkali metals have been discussed [32]. 

This mechanism also applies when lipophilic crown ethers or cryptands are used as catalysts. The extent of anion extraction into the organic phase and its reactivity depend on the combination of many parameters, including the natme of the anion (its charge, size, polarizabihty, etc.), the concentration of the inorganic salt in the aqueous phase, the dielectric constant of the organic solvent the separation between cation and anion in the ion pair, and the number of water molecules associated with the anion in the organic phase. The influence of these parameters on the reaction rate was exhaustively discussed. ""... 

The Crowns have been used as models for the transport of anions across membranes, against a concentration gradient (cf Type 2 transport, p. 68). The anions transported were A -benzoylated amino acids and small peptides the membrane was a stirred layer of chloroform bounded on each face by water. Because the Crowns had little solubility in water, they remained mainly in the chloroform. At the first water/chloroform interface, potassium ions were introduced. This led to extraction of the ions into the chloroform layer to form a ternary complex (anion, K , ionophore) which released both ions into the post-membrane aqueous phase. The depleted ionophore continued the cycle by extracting more anion and cation from the first aqueous phase, thus starting another round of the cycle (Tsukube, 1982). For anion-complexing Cryptands, see Dietrich et al. (1978). 

The main use of crown ethers in PTC is in solid-liquid phase-transfer. In particular, it has been emphasized that they should be the catalysts of choice under such conditions. Due to its particular structure, the crown ether can approach the crystalline lattice so that the extraction and subsequent complexation of the cation require very little cation displacement, while the anion is contemporaneously associated to the complex. In the case of quaternary salts, on the other hand, the steric hindrance around the cationic center makes its interaction with the surface of the crystal difficult and the solution mechanism more complicated in this case only the anion must be extracted to displace the one originally associated with the lipophilic cation. These conclusions met with some scepticism Indeed, onium salts, cryptands, polypodes, polyamines, etc. have been successfully employed as solid-liquid PTC catalysts (see Sects. 4, 5.2, 5.3, 5.4). Moreover, when the catalytic activity of quaternary salts, crown ethers and polyamines was compared with respect to the extraction of anions from the crystalline state into an organic solvent, crown ethers were found to be the best system for the transport of CN . The catalytic effectiveness is completely reversed in the case of other anions, such as F and CHjCOO , the quaternary salt being the most efficient in these cases... 

---