Lanthanide Ion Exchange Modulated via Crystalline Phase Transition of a Crystalline Coordination Polymer

Abstract

Metal-ion exchange in crystalline coordination polymers (CCPs) and MOFs, which are porous CCPs, are strongly affected by the flexibility of the crystalline framework. However, the effect of the structural flexibility on the ion exchange reactivity (and selectivity) have not been discussed. In this study, a CCP containing Ce³⁺ and bis(4-nitrophenyl) phosphate (L), CeL₃, was synthesized and its unique ion-exchange selectivities toward Yb³⁺ and Lu³⁺ (and Tm³⁺ depending on the condition) in the lanthanide series were found. The relatively large difference in ion-exchange selectivity between the neighboring Ln³⁺ ions (e.g., Tm³⁺-and-Yb³⁺ and Er³⁺-and-Yb³⁺) is noteworthy because the reactivities of heavy lanthanides are generally considerably similar. Large differences in the ionic sizes of the heavier Ln³⁺ ion compared to that of Ce³⁺ in the parent framework may induce a structural strain and the structural strain of the polymeric framework is likely responsible for this unusual trend. Powder X-ray diffractometry measurements of the de novo-synthesized mixed-metal CCPs (MM-CCPs) indicated that the Ln³⁺-to-Ce³⁺ mixing limit, x_lim, in the crystal is an important factor that determines the feasibility of the structural transition. The ion exchange mechanism in CeL₃ was suggested; ion exchange occurs on the surface of CeL₃, and the structural transition-and-ion exchange proceeds in a chain reaction like manner only when ion exchange triggers the structural transition. The findings obtained in this study strongly indicate that the structural flexibility including structural phase transition of CCPs and MOFs enables us to modulate an ion-exchange reactivity in addition to selectivity.