Complexation of Pr³⁺, Eu³⁺, Yb³⁺ and Th⁴⁺ ions by calixarene carboxylates

Abstract

The binding abilities of a series of ionizable calixarenes towards three lanthanides (Pr³⁺, Eu³⁺ and Yb³⁺) and one actinide (Th⁴⁺) have been established in methanol by potentiometric measurements. The calixarenes result from progressive substitution of the phenolic hydrogens of p-tert-butylcalix[4]arene by carboxylic acid functions. Calixarene derivatives with mixed carboxylic and ester functional groups, as well as octa-O-carboxymethyl-p-tert-butylcalix[8]arene and the two oxa-derivatives, tetra-O-carboxymethyl-p-tert- butyltetrahomodioxacalix[4]arene and tri-O-carboxymethyl-p-tert- butylhexahomotrioxacalix[3]arene have also been studied. The formation of 1 : 1 species partially protonated [M(H_zL)] and totally deprotonated (ML) as well as methoxo species [{ML(OMe)_z}] has been established with lanthanides. Additional binuclear complexes and their methoxo forms have been found in some cases. With thorium the mononuclear ML and MHL complexes are mainly formed in addition to the corresponding methoxo species at high pH. For a given ligand, the stability of the complexes increases with the cationic charge: complexes of thorium are more stable than those of lanthanides, which are themselves generally more stable than the corresponding alkaline-earth- and alkali-metal complexes. The predominance of electrostatic interactions in the binding is further confirmed by a linear relationship between the stability of mononuclear complexes (log β₁₁₀) and the total basicity of the ligands (ΣpK_ai). The p-tert-butylcalix[8]arene octaacid and the p-tert-butylcalix[4]arene monoacid are respectively the best complexing agents for the lanthanides and thorium. The monoacid derivative shows significant Yb³⁺/Eu³⁺ and Th⁴⁺/Eu³⁺ selectivities in appropriate pH ranges.

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