Solvent Extraction Behavior of Calixarene-Type Cyclophanes Towards Trivalent La, Nd, Eu, Er and Yb

Abstract

In this paper we describe the solvent extraction behaviour of three calixarene-type cyclophanes towards trivalent lanthanides Ln³⁺ (Ln = La, Nd, Eu, Er and Yb). By using p-tert-butylcalix/6/arene hexacarboxylic acid (LI), the lanthanides were extracted from the aqueous phase at pH 2 to 3.5 into chloroform according to a cation exchange mechanism as 1:2 metal:ligand complexes. The extractability is: Nd, Eu > La > Er > Yb. In the presence of excess Na⁺ in the aqueous phase, this order was not changed but the lanthanides were extracted as 1 : 1 complexes at low extractant concentration and their extractability decreased. The use of p-tert-butylcalix/4/arene.tetracarboxylic. acid (L2) with a smaller cavity size dissolved in chloroform or toluene results in lower distribution coefficients, D_Ln, compared with LI. The extraction constants of the 1 : 2 metaLextractant complexes for the water-toluene system follow the order Eu > Nd > Yb > Er > La. Upon addition of excess Na⁺, both extractability and selectivity increased.The extraction behaviour of a third cyclophane (L3) with a cavity size similar to that of LI containing four carboxylic acid groups was different from that of LI with respect to exlractability and order of extraction of the lanthanides.