Cryptands cation binding

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 . 

In other sections in this chapter, we have referred to a variety of macropolycyclic structures which are more elaborate than the simple three-stranded bicyclic cryptands. This includes bridged double-macrocycles " , in-out bicyclic amines and the macrotricyclic quaternary ammonium salts of Schmidtchen. In addition to these, there are two other types of compounds which deserve special note. The first of these is a stacked twin-ring cryptand, but it is a hybrid molecule rather than a double-cryptand . The species shown below as 20 is a crowned porphyrin, and was designed to provide a pair of metal cation binding sites similar to those which might be available in natural biological systems . 

The cation affinity of aza-crown ethers depends on the type of substituent attached to the nitrogen. Wester and Vogtle (1978) have determined the cation binding constants for substituted [2.2]-cryptands 138] and [89]—191 ] in... 

It should be noted that the quantum yield is affected by cation binding but the absence of shift of the emission or excitation spectra precludes the possibility of intensity-ratio measurements at two wavelengths. An exception is provided by the anthraceno-cryptands 5 which in some cases may lead to disappearance or appearance of exciplexes characterized by an additional band at higher wavelengths. 28 30)... 

J. P. Konopelski, F. Kotzyba-Hibert, J.-M. Lehn, J.-P. Desvergne, F. Fages, A. Castellan, and H. Bouas-Laurent, Synthesis, cation binding, and photophysical properties of macrobicyclic an-thraceno-cryptands,./. Chem. Soc., Chem. Commun. 433 (1985). 

Figure 14, Schematic drawings of conformational changes upon cation binding by glymes, crown ethers, and cryptands (D denotes donor atom) also shown are the slopes (a) and intercepts TASo oi AH-TAS plots as measures of conformational change and desolvation.

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. 

---