Abstract
Dipicrylamine anion (DPA^–^) precipitates out [K(DPA)] with high selectivity from salt bitterns containing Na^+^, K^+^, and Mg^2+^, 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 ^133^Cs 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 ^23^Na 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)