Selective isolation of Keggin ions using self-assembled superanion capsules

摘要

Crystals of composition [Al_(13)O_4(OH)_(24)(H_2O)_(12)][Na(18-crown-6)(H_2O)_2L_2(H~+)_w]_(0.5)~(x-)[{L(H~+)_y}]~(z-) centre dot 29H_2O 1, where x + z = 7 and L = p-sulfonatocalix[4]arene, were obtained from an aqueous solution containing monomeric and oligomeric aluminium species, Na_4L and 18-crown-6. The crystal structure features three distinct elements: the cationic Keggin ion [Al_(13)O_4(OH)_(24)(H_2O)_(12)]~(7+), an anionic capsule [Na(18-crown-6)(H_2O)_2L_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.