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

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 25°C and μ=0.1. From the pHtitration 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-.