Cation Binding by Cryptands

In contrast to the crown ethers, the three-dimensional cryptands display peak selectivity. The macrobicyclic cavities are more rigid and restricted so that they are unable to constrict sufficiently 

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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.

M. Albrecht, H. Rottele, P. Burger, Alkali-Metal Cation Binding by Self-Assembled Cryptand-Type Supermolecules , Chem. Em. J., 2, 1264 (1996)... 

Fig. 8.5. Slope a as a measure of conformational changes and intercept 7AS°o as a measure of desolvation upon complexation, initially proposed for cation binding by acyclic glyme, cyclic crown ether, and bicyclic cryptand, where D denotes a donor atom such as oxygen and nitrogen.

In the early years of supramolecular chemistry, a number of studies showed that in comparison to the cyclic crown ethers and bicyclic cryptands, alkali metal cation binding by podands of the oligoethylene glycol type is relatively weak in polar solvents such as water and methanol. Thus [18]crown-6 binds K+ some four orders of magnitude more strongly in methanol solution compared to its open chain podand analog, 2. Representative binding constants for cyclic and acyclic species are shown... 

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 . 

The preferential stabilization of conformation [96] by the cation is annulled upon binding of the cation with a cryptand. Similar effects have been observed with crown ethers. The sodium salt of acac in methanol at —58°C consists of 35% [104] and 65% [105]. The addition of 18-crown-6 [3] shifts the... 

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)... 

The electrochemistry associated with these redox-active cryptands is quite intriguing. As pointed out earlier, anodic shifts of the ferrocene redox potential may be used via the Nernst equation to estimate the decrease in binding capacity (Xj/Ki) on coordination with a cation. Beyond this, however, if Kj is determined independently as is the case for 38 (m, n = 2) [63] then Xj may be calculated and correlated with the ratio of cationic radius/charge (Fig. 6-7) — data that reveal that increasing charge density on the cation destabilizes the complex between the oxidised cryptand and the cation, presumably by charge repulsion [68]. Alkali metal cations gave only small (< 20 mV) anodic shifts with this cryptand. 

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