Cation Complexation by Calixarenes

The calixarenes are extremely versatile host frameworks and, depending on their degree of fnnction-alisation, may act as hosts for cations, anions and neutral molecules, as we will see in Chapters 4 and 5. 

The parallel ring geometry observed in both X-ray crystal structures may also be achieved in the 

3-alternate conformation, and it has been shown by NMR spectroscopy that K+ and Ag+ binding also 

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Atwood, J. L., Cation complexation by calixarenes . in Cation Complexation by Macrocycles, Inoue, Y., Gokel, G., eds. Dekker New York, 1991. 

Fig. 1 Molecular structures of the calixarenes, where, commonly, n = 4. 6. and 8 and the calixresorcinarene (A). The strategy for cation complexation by the introduction of ligand groups at either the lower or upper rims of calixarene derivatives (B). Metal complex with...

Figure 13 illustrates all the known modes of cation binding by native and functionalized calixarenes exploiting cation-TT, induced dipole, or electrostatic interactions. Strictly speaking, only for situations (c)-(f), we can talk of cation recognition by calixarene receptors, whereas in metallation with transition and f-element (situations (a) and (b)), which are often exploited in anion complexation or catalysis, little control is experienced on selectivity. Usually, the most important calixarene-based cation receptors are obtained by the introduction of chelating units at the lower rather than at the upper rim. This ensures a more... 

Results obtained by ES/MS confirm that the stability of calixarene/cation complexes depends upon the medium. The calixarene in solution presents a strong affinity for cesium, whereas in the gas phase, it displays a stronger affinity for sodium. Moreover, the stability of calixarene/Na+ complexation in a solvent phase is increased by the presence of water in the dilution system (up to 40% in acetonitrile), whereas other alkali complexes are destabilized by the presence of water. Finally, affinity for sodium, which is weak in the solution for calixarenes bearing benzo moieties, considerably increases in the gas phase. These results confirm the interpretation of the MD simulations in an aqueous phase, which lead the authors to conclude that cesium-over-sodium selectivity is governed by the hydration of the sodium cation in the complex, and by the higher hydrophobicity of the complexation site leading to an enhancement of selectivity for cesium over sodium 49... 

Meanwhile, cation - crown complexes were fully optimized by ab initio QM techniques in the gas phase, allowing to investigate the effect of electronic reorganization (mainly charge transfer + polarization effects) on the recognition properties/15,161 QM/MM hybrid methods which combine a QM representation of the ion coordination sphere with a MM representation of the more remote species, have been tested on 18C6/ cation complexes/17, 18] Typical applications of "computational approaches in supramolecular chemistry" up to 1994 can be found in ref. [191 There are also reviews on more specialized fields like calixarenes/20,21]... 

Another example of how PET can be modulated by complexation involves the use of bulky carboxylates as molecular curtains . Intramolecular PET occurs in the Zn complex of the poly amine (166) between the dimethylaniline donors and the anthracene acceptor, however when triphenylacetic acid is added a strong enhancement of fluorescence occurs. This effect is ascribed to complexation of the triphenylacetate at the Zn cation in such a way as to block PET between the other two units in the complex. The host-guest chemistry afforded by calixarenes has also been adapted to the construction of efficient ET systems. 

Capsule-shaped Ir(l) and Rh(l) cationic complexes with a triphosphinocaltx[6]arene as a multidentate ligand were recently synthesized (Figure 33). These organometallic bis-caltxarene complexes showed dynamic behavior with size-selective molecular encapsulation, which was confirmed by variable-temperature P H NMR measurements in the presence of various molecules. X-ray crystal analysis showed that the calix[6]arene moiety adopted the same pinched-cone conformation as the non-coordinated caltxarene. Molecules such as CFlj,Cly or CICH2CH2CI are too small to fit the cavity of the iridium and rhodium bis-calixarene complexes and cannot restrict the dynamic behavior at 25 °C. On the contrary, molecules such as X2CHCHX2 (X = C1 or Br), benzene, toluene, o- or w-xylene just fit in the cavity and show the dynamic behavior. Finally, large molecules (p-xylene, cumene, mesitylene, etc.) could not enter the cavity. 

The main classes of calixarene derivatives for the complexation and extraction of alkali and alkali earth cations are based on the total subsitution at the lower rim by alkoxyester. alkoxyamide. or alkoxyketone functions. Fig. 2. In this way. suitable donor ligand functions for cation complexation are introduced. Such derivatives were used for the complexation and solvent-to-solvent... 

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