Multidentate extracting agents based on calix[4]arene scaffold – U^VI/Eu^III separation studies

The development of new ligand systems for solvent extraction demands knowledge of the fundamental chemistry behind these processes. Thus, herein, we report the synthesis and structural characterization of a 6-hydroxymethylpyridine functionalized calix[4]arene (L2). Its complexation behavior towards the divalent uranyl cation as well as the trivalent europium cation was spectroscopically studied. In solution the formation of two uranyl complex species with L2 was proven by spectrophotometric titration experiments, with stability constants of log ß1:1 = 5.82±0.04 and log ß2:1 = 5.57±0.05, and of one europium complex species (log ß1:1 = 6.85±0.03). In addition, the concept of the desired proper planar binding pocket for the linear uranyl cation has been verified in solution. Moreover, comparative UVI and EuIII extraction studies with L2 and a structure related 8-hydroxyquinoline modified calix[4]arene (L1) were performed in which the effects of various conditions such as pH of the aqueous phase, presence of relevant anions (sulfate, phosphate, carbonate) and competitive extraction with ethylenediaminetetraacetic acid (EDTA) were investigated. It has been shown that both macrocyclic ligands offer a strong chelate effect, which enables them to extract even stable uranyl hydrolysis species. With these ligands, the selective separation of UVI from EuIII-containing solutions is possible under a variety of conditions.