New lower rim calix [4]arene amine derivatives: synthesis, characterisation and acid–base properties. Potential anion and cation binders

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Abstract
The synthesis of new lower rim calix[4]arene derivatives containing aliphatic and alicyclic amines is reported. 1H and 13C NMR characterisation shows that in solution these compounds exist in a cone conformation. UV spectrophotometry was used to investigate the interaction of these amines with the proton. These studies reveal that four protons are taken up for each calix[4]arene unit. The acid–base properties of these macrocyclic amines were investigated potentiometrically using methanol as the reaction medium. From these data, the equilibrium constant for each of the dissociation processes involved was derived. Diagrams showing the percentages of the individual species as a function of the solution pH are given. This information is of fundamental importance for investigating the use of these ligands as cation or anion binders. The higher basic character of aliphatic and alicyclic p-tert-butylcalix[4]arene amines relative to pyridinocalix[4]arene is demonstrated. The implication of these results on their affinity for the proton is discussed. Phase-transfer studies on the water–dichloromethane solvent system provide quantitative evidence that the presence of the calix[4]arene amino derivative in the organic phase leads to an 84% extraction of HAuCl4 from aqueous solutions. The 1H NMR spectrum of the gold(III) adduct is very similar to that of the protonated calix[4]arene amine derivative suggesting that the gold(III) containing anion is transferred by this ligand to the organic phase via an ion-pair mechanism involving the protonated ligand and these anions. The attachment of amino functional groups in the lower rim of p-tert-butylcalix[4]arenes provides a suitable arrangement for interaction with toxic metal cations (Cd2+, Pb2+ and Hg2+) while alkali and alkaline metal cations (Na+, K+ and Ba2+) are discriminated. The potential use of these ligands for the removal of toxic metal cations from contaminated sources is emphasised.

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