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

Calix[n]arene-based uranophiles bearing hydroxamic groups on the lower rim (2_n; 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 2 ₄ and 2₆ increases from pH 2 and saturation is reached at around pH 5 where 100% extractability occurs. Since the pK_a values for hydroxamic acids are 8-9, the apparent pK_a 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 CO ₃^2- ions in the aqueous phase for UO₂ ²⁺ whereas in 2₄ and a calix[6]arene-based uranophile bearing six carboxy groups (1₆) UO₂²⁺ is reextracted to the aqueous phase. The difference indicates that 2₆, which has the hexacoordination geometry preorganized for the binding of UO ₂²⁺, 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.