CATION COMPLEXATION BY CHEMICALLY-MODIFIED CALIXARENES .5. PROTONATION CONSTANTS FOR CALIXARENE CARBOXYLATES AND STABILITY-CONSTANTS OF THEIR ALKALI AND ALKALINE-EARTH COMPLEXES

The complexing properties of some calix[4]arenes in which carboxylic acid ligating groups have been progressively attached to the lower rim through the phenolic hydrogen atoms have been measured with alkali and alkaline-earth metal cations in methanol by means of potentiometric titrations. The acid-base characteristics of these compounds were first determined. The weak acidities generally observed may be explained by the formation of intramolecular hydrogen bonding. The formation of mononuclear 1:1 complexes and their corresponding protonated forms was established in most cases. The latter species were predominent in the acidic range of pH. Additionally, dinuclear complexes were also found with the diacid derivatives. The results show that, in appropriate acidity conditions, calixarene acids, which are very efficient complexing agents for alkalis and especially alkaline-earth metals, are much stronger binders than calixarene esters, ketones, or amides. In the tetraphenol or the tetraester, the stepwise substitution of the phenolic hydrogens or ester functions by a carboxylic acid leads to an enhancement of the stability of the resulting complexes but a decrease of the Na+/K+ selectivity. With respect to the alkaline-earth metals, a huge Ca2+/Mg2+ selectivity is exhibited by all the compounds.