Selective isolation of Keggin ions using self-assembled superanion capsules

Crystals of composition [Al13O4(OH)(24)(H2O)(12)][Na(18-crown-6)(H2O)(2)L-2(H+)(w)](0.5)(x-)[{L(H+)(y)}](z-). 29H(2)O 1, where x + z = 7 and L = p-sulfonatocalix[4]arene, were obtained from an aqueous solution containing monomeric and oligomeric aluminium species, Na4L and 18-crown-6. The crystal structure features three distinct elements: the cationic Keggin ion [Al13O4(OH)(24)(H2O)(12)](7+), an anionic capsule [Na(18-crown-6)(H2O)(2)L-2(H+)(w)](x-) and a water bearing calixarene. The anionic capsule comprises a central sodium ion bound by 18-crown-6 and two trans water molecules, which is in turn encapsulated by two p-sulfonatocalix[4]arene molecules in a head-to-head arrangement. The different components are held together in the solid state by an intricate pattern of hydrogen bonds, complemented by other supramolecular interactions such as pi-pi stacking.