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, H_j1_nⁿ–(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 H_j1_nⁿ–, the acid-dissociation constants were determined to be as follows: pK_a1=3.25, pK_a2=12.8 for H₂1₄^4–, pK_a1=3.28, pK_a2=4.86 for H₂1₆^6–, and pK_a1=3.73, pK_a2=4.39, pK_a3=8.07, pK_a4=10.1 for H₄1₈^8–. These results indicate that the present ligands are relatively strong multi-protonic acids which release j mol of proton from one mol of ligand, H_j1_nn–, having n mol of hydroxyl group in a weakly acidic-to-alkaline aqueous solution. The tetramer ligand, H₂1₄^4– formed two unstable 1:2 (UO₂²⁺:ligand) complexes, [(UO₂²⁺)(1₄^6–)₂(H⁺)]^9– and [(UO₂²⁺)(1₄^6–)₂]^10–, and a hexamer ligand, H₂1₆^6– formed four 1:1 complex species, such as [(UO₂²⁺)(1₆^8–)]^6–, [(UO₂²⁺)(1₆^8–)(H⁺)_–1]^7–, [(UO₂²⁺)(1₆^8–)(H⁺)_–2]^8–, and [(UO₂²⁺)(1₆^8–)(H⁺)_–3]^9– with UO₂²⁺. Their selective reactivity to UO₂²⁺ over other various metal ions was evaluated based on their pK_a values, stability constants and the compositions of the complexes including that for the octamer ligand, H₄1₈^8–, 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(1₈^12–)(H⁺)_–3]^11–.