COMPLEXATION OF PR3+, EU3+, YB3+ AND TH4+ IONS BY CALIXARENE CARBOXYLATES

The binding abilities of a series of ionizable calixarenes towards thr ee lanthanides (Pr3+, Eu3+ and Yb3+) and one actinide (Th4+) have been established in methanol by potentiometric measurements. The calixaren es result from progressive substitution of the phenolic hydrogens of p -tert-butylcalix[4]arene by carboxylic acid functions. Calixarene deri vatives with mixed carboxylic and ester functional groups, as well as octa-O-carboxymethyl-p-tert-butylcalix[8]arene and the two oxa-derivat ives, boxymethyl-p-tert-butyltetrahomodioxacalix[4]arene and boxymethy l-p-tert-butylhexahomotrioxacalix[3]arene have also been studied. The formation of 1:1 species partially protonated [M(H(z)L)] and totally d eprotonated (ML) as well as methoxo species [{ML(OMe)(z)}] has been es tablished with lanthanides. Additional binuclear comlexes and their me thoxo forms have been found in some cases. With thorium the mononuclea r ML and MHL complexes are mainly formed in addition to the correspond ing methoxo species at high pH. For a given ligand, the stability of t he complexes increases with the cationic charge: complexes of thorium are more stable than those of lanthanides, which are themselves genera lly more stable than the corresponding alkaline-earth- and alkali-meta l complexes. The predominance of electrostatic interactions in the bin ding is further confirmed by a linear relationship between the stabili ty of mononuclear complexes (log beta(110)) and the total basicity of the ligands (Sigma pK(ai)). The p-tert-butylcalix[8]arene octaacid and the p-tert-butylcalix[4]arene monoacid are respectively the best comp lexing agents for the lanthanides and thorium. The monoacid derivative shows significant Yb3+/Eu3+ and Th4+/Eu3+ selectivities in appropriat e pH ranges.