Lanthanide ion exchange modulated via crystalline phase transition of a mixed-metal coordination polymer based on bis(4-nitrophenyl) phosphate

Abstract

We previously reported that the CeL3 [L = bis(4-nitrophenyl) phosphate] coordination polymer (CP) exhibits exceptionally large differences in ion exchange selectivity toward neighboring ions in the lanthanide series (Ln3+). Herein we aimed to elucidate the mechanism of the unique ion-exchange behavior of CeL3 by focusing on the formation reaction of a mixed-metal CP (MM-CP) and the phase transition between two crystalline types, which occurs during the course of the ion-exchange reaction. The results of powder X-ray diffractometry of the de novo-synthesized MM-CPs indicated that the Ln3+-to-Ce3+ mixing limit, xlim, in the crystal is an important factor that determines the feasibility of the structural transition. Smaller Lu3+, Yb3+, and Tm3+ electrostatically interact with ligand molecules very strongly, which causes structural strain in MM-CP. Thus, the crystal-to-crystal structural transition tends to occur when the Ln3+ mixing ratio (x) is above xlim (∼0.2). The exchange of Ce3+ for Ln3+ (=Lu3+, Yb3+, and Tm3+) in the MM-CPs is further facilitated following structural transition because the crystalline structure formed after phase transition is known to prefer the heavy Ln3+ ions to Ce3+. In addition, the crystal lattice contracts during structural transition, and the intact CeL3 inside the particles is exposed to the solution phase, which may result in ion-exchange proceeding as a chain reaction. Hence, Ce3+-for-Ln3+ exchange is accelerated when x > xlim and disfavored when x < xlim. Consequently, large Ln3+-dependent differences in final reactivity are observed.