Cryptands useful properties

Until very recently, studies of the use of luminescent lanthanide complexes as biological probes concentrated on the use of terbium and europium complexes. These have emission lines in the visible region of the spectrum, and have long-lived (millisecond timescale) metal-centered emission. The first examples to be studied in detail were complexes of the Lehn cryptand (complexes (20) and (26) in Figure 7),48,50,88 whose luminescence properties have also been applied to bioassay (vide infra). In this case, the europium and terbium ions both have two water molecules... 

The strongly basic properties of potassium hydroxide are apparent from the work of Dietrich and Lehn (1973) who reported that the liquid-solid system KOH/THF/[2.2.2]-cryptand was capable of generating the anions of weak carbon acids such as triphenylmethane [142], diphenylmethane [143], and fluorene. The same anions could be generated using NaNH2 instead of KOH. 

Lobacz et al. [52] have described partial adsorption ofTl+-cryptand (2,2,2) complex on mercury electrode. From voltocoulom-etry, cyclic voltammetry, and chrono-coulometry, it has been deduced that electroreduction of this complex proceeds via two parallel pathways from the solution and from the adsorbed states, which are energetically close. Also, Damaskin and coworkers [53] have studied adsorption of the complexes of alkali metal cations with cryptand (2,2,2) using differential capacity measurements and a stationary drop electrode. It has been found that these complexes exhibit strong adsorption properties. Novotny etal. [54] have studied interfacial activity and adsorptive accumulation of U02 " "-cupferron and UO2 - chloranilic acid complexes on mercury electrodes at various potentials in 0.1 M acetate buffer of pH 4.6 and 0.1 M NaCl04, respectively. 

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. 

The versatile solubility properties of the crown ethers and cryptands are important in two of their major applications, phase transfer catalysis and anion activation. Phase transfer catalysis involves the transport of guest species from one phase to another. The two phases in question are usually two immiscible liquids (liquid-liquid phase transport). In practice, this usually means the use of a... 

A number of ferrocene cryptand molecules (66-74) ((29)—(31)) have been reported in the literature and it is only relatively recently that their electrochemical coordination properties have been disclosed. We have synthesized potassium-selective metallocene cryptands (72) (30) and (31) the electrochemistry of the former in the presence of K+ guest cations proved disappointingly irreversible (75). Hall and co-workers (76) have used cyclic voltammetry to investigate the coordination of... 

Ion transport by macrobicyclic ligands (cryptands) has been investigated using a bulk liquid membrane149. It was found that the relative transport rates are not proportional to complex stability. E.g. cryptand (Fig. 67 m = n = 1) which forms a very stable K+ complex, is an inefficient K+ carrier, because of the slow exchange rate of the complex167. On the other hand, cryptand (Fig. 68 3) which forms a more lipophilic and less stable complex, is quite an efficient K+ carrier. It appears that the cryptands display efficient carrier properties for those cations which form complexes with stability constants of about 10s in methanol. 

The synthesis of cu-amino-substituted 18-crown-6 and [2.2.2]cryptand readily bonded with chloromethylated polystyrene cross-linked by different amounts of DVB, is described [86]. Such bonded polyesters are used as interfacial transfer catalysts promoted by anions. As in the case of analogous soluble systems, the catalytic activity of cryptands is higher than that of crown ethers and quaternary onium salts. Because of their high chemical stability, such catalysts can be regenerated without chemical decomposition. However, the impairment of mechanical properties caused by comminution of the polymer matrix remains to be solved. 

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