Unusual Lanthanide Ion Exchange Selectivity in Phosphoester Coordination Polymers

Abstract

Ion exchange properties in coordination polymers based on phosphoesters were investigated. The ion-exchange selectivity of [Ce(dehp)₃], formed by Ce^3＋ and di(2-ethylhexyl phosphoric acid), is in the order Nd^3＋＜Gd^3＋＜Dy^3＋＜Lu^3＋ when a small amount of Ln^3＋ is loaded, but Lu^3＋ ≈ Nd^3＋＜Gd^3＋ ≈ Dy^3＋ for a high loading ratio. It is experimentally suggested that a steric effect is involved in the reactions, which becomes stronger for the smaller Ln^3＋, in addition to a simple electrostatic interaction. This trend is attributed to the differences in the ionic sizes between an original Ce^3＋ and an incoming Ln^3＋. Coordination polymers are candidates for a new type of ion-exchange material of which properties are different both from inorganic and organic ion-exchange materials.