Central metal ion exchange in a coordination polymer based on lanthanide ions and di(2-ethylhexyl)phosphoric acid: Exchange rate and tunable affinity

Abstract

In this paper the exchange of lanthanide(III) ions (Ln3+) between a solution and a coordination polymer (CP) of di(2-ethylhexyl)phosphoric acid (Hdehp), [Ln(dehp)3], is studied. Kinetic and selectivity studies suggest that a polymeric network of [Ln(dehp)3] has different characteristics than the corresponding monomeric complex. The reaction rate is remarkably slow and requires over 600h to reach in nearly equilibrium, and this can be explained by the polymeric crystalline structure and high valency of Ln3+. The affinity of the exchange reaction reaches a maximum with the Ln3+ possessing an ionic radius 7% smaller than that of the central Ln3+, therefore, the affinity of the [Ln(dehp)3] is tunable based on the choice of the central metal ion. Such unique affinity, which differs from the monomeric complex, can be explained by two factors: the coordination preference and steric strain caused by the polymeric structure. The latter likely becomes predominant for Ln3+ exchange when the ionic radius of the ion in solution is smaller than the original Ln3+ by more than 7%. Structural studies suggest that the incoming Ln3+ forms a new phase though an exchange reaction, and this could plausibly cause the structural strain.