Synthesis and solvent extraction studies of novel calixarene-based uranophiles bearing hydroxamic groups

Abstract

Calix[n]arene-based uranophiles bearing hydroxamic groups on the lower rim (2n; n= 4 and 6) have been synthesized and the extractability (Ex%) and the selectivity towards uranyl ion (UO22+) estimated in a two-phase (water–chloroform) solvent-extraction system. Ex% for 24 and 26 increases from pH 2 and saturation is reached at around pH 5 where 100% extractability occurs. Since the pKa values for hydroxamic acids are 8–9, the apparent pKa shift caused by the UO22+-complexation amounts to 6–7 pK units. Extraction of UO22+ from aqueous carbonate solution established that 26 in the organic phase can compete efficiently with CO32– ions in the aqueous phase for UO22+ whereas in 24 and a calix[6]arene-based uranophile bearing six carboxy groups (16) UO22+ is re-extracted to the aqueous phase. The difference indicates that 26, which has the hexacoordination geometry preorganized for the binding of UO22+, is superior to 24 as a uranophile. We also found that the selectivity of 26 is superior to 16: the Ex% values for 26 is scarcely affected by the addition of competing metal cations (except Fe3+). These results show that 26 serves as an excellent UO22+-selective extraction reagent.