Selective isolation of Keggin ions using self-assembled superanion capsules†

Abstract

Crystals of composition [Al₁₃O₄(OH)₂₄(H₂O)₁₂][Na(18-crown-6)(H₂O)₂L₂(H⁺)_w]_0.5^x–[{L(H⁺)_y}]^z–·29H₂O 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)₂L₂(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 π–π stacking.