Pillar[5]arene-based phosphine oxides: novel ionophores for solvent extraction separation of f-block elements from acidic media

Abstract

A new class of pillar[5]arene-based phosphine oxides tethered with ten chelating groups on both rims of the pillar were synthesized via two steps from macrocyclization of 1,4-bis(bromoalkoxy)benzenes with paraformaldehyde, followed by Arbusov reaction with iso-propoxydiphenylphosphine. Solvent extraction of these ligands towards selected lanthanides and actinides was investigated under acidic condition. Compared with acyclic monovalent analogs and classical extractant tri-n-octylphosphine oxide (TOPO), the remarkable efficiency and selectivity for thorium(IV) and uranyl(VI) as observed in these novel extractants bearing a varying spacer length revealed the significance of preorganization of chelating groups on pillararene platform. Log–log plot analysis indicated the 1 : 1 stoichiometry (ligand/metal) for the extracted complex formed between the ligand and thorium(IV) or uranyl(VI). The extraction efficiency was considerably improved with increasing acidity in a range of 0.1–1.5 M HNO₃, a result that is distinct from the extraction behaviour of calixarene-based phosphine oxides. The extractability increased with enhancing NaNO₃ concentration and high salinity assisted the preferential extraction of U(VI) over Th(IV). These ligands also showed moderate efficiency in differentiating europium(III) and americium(III) at 1 M HNO₃ in the presence of a synergist (Br₆-COSAN).