Towards understanding of the selective precipitation of alkali metal cations in presence of dipicrylamine anion

Abstract

Dipicrylamine anion (DPA-) precipitates out [K(DPA)] with high selectivity from salt bitterns containing Na⁺, K⁺, and Mg²⁺, whereas the same ligand shows poor selectivity towards K⁺ - and much higher selectivity towards Cs⁺ - in studies conducted with a mixture of K⁺, Rb⁺, and Cs⁺. Their single-crystal structures reveal that the K⁺ and Rb⁺ salts have similar layered structures, with 8 oxygen atoms from seven DPA^- anions encapsulating the metal cation, whereas the Cs⁺ salt possesses a channel-like structure with the metal ion encapsulated by ten oxygen atoms from six DPA^-. The conformation of DPA^- in the [Cs(DPA)] single crystal matches closely that of DPA in crystalline state. M...O and intermolecular C-H...O interactions together stabilize the structures. The ¹³³Cs NMR spectrum of the poorly soluble [Cs(DPA)] shows an upfield shift of the peak with respect to CsCl as a result of the interaction with the oxygen atoms of DPA^-, whereas ²³Na NMR spectrum of the highly soluble [Na(DPA)] shows no such upfield shift compared to NaCl. Powder XRD patterns of bulk [M(DPA)] (M = K⁺, Rb⁺, and Cs⁺) precipitates show that these are similar to the patterns obtained by simulation of the single-crystal X-ray data. The selectivity of precipitation correlates qualitatively with the size and hydration enthalpies of the ions.