Selective precipitation of alkaline earth metal cations with dipicrylamine anion: Structure-selectivity correlation

Abstract

Salts of dipicrylamine anion and alkaline earth metal ions are highly soluble in water, unlike the salts formed with K⁺, Rb⁺, and Cs⁺. We report here that when DPA- is added to a solution containing Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, Ba²⁺ can be precipitated out with high yield and selectivity by optimizing the concentrations. When [DPA^-] = 0.17 M and the molar ratio of each metal ion to DPA^- is 1:1, Sr²⁺ and Ba²⁺ are precipitated out in about 90 % overall yield (with respect to DPA^-) and 85 % selectivity of Ba²⁺. Salts of Mg²⁺ and Ca²⁺ are virtually absent. Single-crystal XRD of the pure Sr²⁺ and Ba²⁺ compounds has been performed. Layered structures are observed with clear evidence that the salts exist in the form of complexes with the composition [M(DPA)₂(H₂O)₄]. This is different to the case of alkali metal ions, where the complexes have no coordinated water molecules. The Ba²⁺ complex has a more compact layered structure than the Sr²⁺ complex, with reduced exposure of the polar entities. As a result it would be expected to have relatively lower solubility in water, which would account for its selective precipitation over the Sr²⁺ complex. That it is legitimate to rationalize the precipitation behavior from the structure deduced by single crystal XRD is verified due to the close similarity between the experimental powder XRD data of the precipitate and the simulated powder XRD pattern obtained from the single-crystal data.