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 (2n; n = 4 and 6) have been synthesized and the extractability (Ex%) and the selectivity towards uranyl ion (UO2(2+)) estimated in a two-phase (water-chloroform) solvent-extraction system. Ex% for 2(4) and 2(6) 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 UO2(2+)-complexation amounts to 6-7 pK units. Extraction of UO2(2+) from aqueous carbonate solution established that 2(6) in the organic phase can compete efficiently with CO3(2-) ions in the aqueous phase for UO2(2+) whereas in 2(4) and a calix[6]arene-based uranophile bearing six carboxy groups (1(6)) UO2(2+) is re-extracted to the aqueous phase. The difference indicates that 2(6), which has the hexacoordination geometry preorganized for the binding of UO2(2+), is superior to 2(4) as a uranophile. We also found that the selectivity of 2(6) is superior to 1(6): the Ex% values for 2(6) is scarcely affected by the addition of competing metal cations (except Fe3+). These results show that 2(6) serves as an excellent UO2(2+)-selective extraction reagent.