Crown ethers derived from bicyclocalix[4]arenes as chromoionophores

Abstract

The synthesis of calix[4]arenes in which opposite phenolic units are connected by a poly(oxyethylene) bridge at the narrow rim and a 2,6-dimethylene-4-nitrophenol bridge at the wide rim is reported. For two derivatives with tetra- (4) and penta(oxyethylene) (5) bridges UV-Vis spectrophotometric studies were carried out in buffered solution in the presence of alkali metal ions. Their complexation was associated with changes in their UV-Vis spectra, especially with an increase of the absorption band at 450 nm. For 4 and 5 this was most intense in the presence of potassium and caesium ions, respectively, indicating that the calixarene with the shorter crown ether bridge is selective towards potassium ions and that with the longer bridge towards caesium ions. The X-ray structure analysis of 5 shows a pinched cone conformation for the 1,3-calixcrown part. The p-nitrophenol unit is parallel to one of the unbridged tert-butyl phenol units. Its hydroxyl group points into the cavity, in agreement with comparatively high pK_a values. Molecular mechanics was used to model the geometry of the potassium and caesium complexes.