Unusual ion-exchange selectivity in crystalline coordination polymers accompanied by structural change of the framework

Ion exchange in crystalline coordination polymers (CPs) has been studied not only as a separation and detection method, but also as a post-synthetic route of new substances. The ion-exchange selectivity, which is one of the most important parameters, sometimes differs from those observed in conventional ion-exchange systems. This is an intriguing phenomena; however, the mechanisms therein have not been elucidated. In the author's opinion, the structural change of the crystalline framework should be one of the most important factors to affect the ion-exchange selectivity. In this review article, two ion-exchange systems using CPs are presented; one is the ion-exchange of lanthanide ion (Ln³⁺) in a CP comprising of Ce³⁺ and bis(nitrophenyl)phosphate (L1), namely CeL1₃, and the other is alkali-metal ion-exchange in a CP based on La³⁺, alkali metal ion (M⁺), and 1,4-phenylenebis(methylidyne)tetrakis(phosphonic acid) (L2). The former offers an exceptional ion-exchange selectivity, i.e., CeL1₃ replaces Ce³⁺ only by Yb³⁺ and Lu³⁺ with a high loading ratio. Powder X-ray diffraction analysis revealed that the ion-exchange reaction concomitants with the crystal-structural transition. According to results obtained, an ion-exchange reaction model was proposed. In the latter system, an exceptional selectivity of alkali-metal ion-exchange has already found by another research group, and was explained by the difference in the ionic size between the outgoing and incoming alkali-metal ions. However, the author demonstrates herein that the proposed model was invalid by examining ion-exchange reactions comprehensively and evaluated with the structural change of the framework. It is deduced for the above two cases that the structural strain generated by the coexistence of two kinds of metal ions with different ionic size and that the resulting structural change would be significant for the ion-exchange reactions. (author)