Super Uranophile, Water-Soluble Calixarenes: Their Metal Complexes, Stability Constants and Selective Reactivity to Uranyl Ion

Abstract

The complex-formation reaction of hydroxycalix[n]arene-p-sulfonates, Hj1nn–(n=4,8; j=n/2, n=6; j=2), with various metal ions were studied while focusing on the uranyl ion, UO22+, by the pH-titration method at 25ʰC and m=0.1. From the pH-titration data for Hj1nn–, the acid-dissociation constants were determined to be as follows: pKa1=3.25, pKa2=12.8 for H2144–, pKa1=3.28, pKa2=4.86 for H2166–, and pKa1=3.73, pKa2=4.39, pKa3=8.07, pKa4=10.1 for H4188–. These results indicate that the present ligands are relatively strong multi-protonic acids which release j mol of proton from one mol of ligand, Hj1nn–, having n mol of hydroxyl group in a weakly acidic-to-alkaline aqueous solution. The tetramer ligand, H2144– formed two unstable 1:2 (UO22+:ligand) complexes, [(UO22+)(146–)2(H+)]9– and [(UO22+)(146–)2]10–, and a hexamer ligand, H2166– formed four 1:1 complex species, such as [(UO22+)(168–)]6–, [(UO22+)(168–)(H+)–1]7–, [(UO22+)(168–)(H+)–2]8–, and [(UO22+)(168–)(H+)–3]9– with UO22+. Their selective reactivity to UO22+ over other various metal ions was evaluated based on their pKa values, stability constants and the compositions of the complexes including that for the octamer ligand, H4188–, for which the complex-formation reactions with various metal ions were already studied and found to form an extremely stable 2:1 complex, [(UO22+)2(1812–)(H+)–3]11–.