Calixarenes cation complexation

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... 

Amalgamation of stmctural units typical of crowns and calixarenes has led to the development of calixpodands, calixcrowns, and calixspherands (55). Naturally they behave as cation complexants rather than iaclusion hosts for uncharged molecules. 

The calixarenes have become extremely popular and versatile cation complexing agents and supramolecular scaffolds. 

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

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]... 

Wipff, G. (2001) Molecular dynamics simulations of cation complexation and extraction, in Vicens, J., Asfari, Z., Harrowfield, J., Bohmer, V. (eds.), Calixarenes-2001, Kluwer Acad. Publishers, Dordrecht, pp 312-333 and references cited therein. 

The corresponding dependence of cation complex stability on the anion differs profoundly from that of most other cation receptors such as cyclophanes or calix-arenes [16]. For these cation complex stability decreases on changing the anion from picrate through iodide to tosylate, a dependence that has been attributed to ion-pair aggregation in non-polar solvents. Because the interaction of quaternary ammonium ions with tosylate or iodide in chloroform is considerably stronger than with picrate, cation complexes in the presence of the latter anion are usually more stable. Only when iodide or tosylate cooperatively contributes to cation binding, as in 3 or in some recently described calixarene derivatives [17], is reversal of this order observed. 

P-5 A phosphinic calixarene is complexed to four Ag(I)Q molecules and one additional chloride ion to give an anion (161) that can act as host for alkali metal cations, Hg(II) and Pb(II)235 236. 

Since the calixarenes form complexes with a variety of guests (see Chapter 5) it is not surprising that their conformational mobility is influenced by cations. 

Metal Cation Complexes with Calixarenes Carrying Substituents on the Lower Rim... 

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. 

Biological Models and Their Characteristics, p. 101 Calixarenes and Their Analogues Cation Complexation, p. 137... 

Widening the opposite side of a calixarene upon complexation of small cations to the oxygen atoms of a calixarene lower riin leads to the positive heterotropic allosteric effectin the inclusion o/[60]fullerene in a cage molecule 14 (Fig. 4) derived from two calix[3]aryl esters by complexation of their pyridine groups with Pd(II). " ... 

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... 

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...

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