Ionizable 1,2 crown calix arenes

The first member of a new class of crowned calix[4]arenes, 16a [44] (Structure 9), possesses two cavities. A lipophilic one enclosed by four aromatic nuclei and a hydrophilic one formed by the oligo(oxyethylene) ring and two ionizable phenolic hydroxyl groups. This ionophore binds Kd", NH4, and Ba " ions. 

Electrospray Ionization/Mass Spectrometry (ES/MS), a soft-ionization desorption technique using polar solvents such as water, methanol, or acetonitrile, was used for direct measurement of cations in solution. The first measurements carried out with mono or bis(crown-6) calix[4]arenes from an equimolar cation-extractant solution confirm that the calixarenes mono(crown-6) extract only one cesium cation. On the contrary, in the same conditions, bis(crown-6) calix[4]arenes can extract two cesium cations for a ratio Cs/BC6 equal to 2.5. The binuclear complex (composed of two cesium cations) is the major species. Cesium/sodium selectivity measurements implementing various mono or bis(crown-6) calix[4]arenes were in agreement with liquid/liquid results.42... 

Zhou et al. studied the extraction of alkaline earth metal cations with p-tert-calix[4]arene-l,2-crown-5 under different conformations. The pH for half loading, pH0 5, is a qualitative measure of ligand acidity. The pH0 5 values for a given conformation decrease as the electron-withdrawing ability of X increases. Moreover, the ligand acidity increases uniformly as the conformation is varied in the order 1,3-alternate < partial-cone < cone. This order is related to the proximity of the ionized groups to the complexed cations.140... 

Lamouroux, C., Aychet, N., Lelievre, A., Jankowski, C.K., Moulin, C. 2004. High-performance liquid chromatography with electrospray ionization mass spectrometry and diode array detection in the identification and quantification of the degradation products of calix[4]arene crown-6 under radiolysis. Rapid Commun. Mass Spectrom. 18 1493-1503. 

The introduction at the lower rim of calix[4]arenes of acetic acid moieties, easily ionizable to carboxylates upon metal ion binding, also resulted in receptors being quite selective for alkaline-earth and, particularly, calcium ions. One of the first receptors synthesized, which shows a high selectivity for alkaline-earth ion, was the tetracid 39, but later on it was found that the diametrically 1,3-diamide-diacid derivatives 40 are even more efficient and selective (Ca Sr alkali ions). The introduction of ionizable carboxylic acid groups close to the crown-ether moieties affords a receptor 41 showing high efficiency and selectivity in Ca + extraction, also as a consequence of the crown-ether size. 

---