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Flexible supramolecular ring-like host for various tetrahedral anion guests

Anion binding by synthetic receptors is a rapidly growing area of interest in supramolecular chemistry. Such research enables better understanding of the role of anion binding in biological systems. The design of anion receptors is challenging. Anions can be pH sensitive, complex anions can have different geometries. Therefore, anion receptors must function in active pH range of anions. Also, it is optimal for the receptors to have flexible geometry and so become complementary to various anion guests. In our work, we prepared a supramolecular host for two tetrahedral anions: SO42- and ClO4-. The crystals are unstable upon exposure to air so the single crystal diffraction data were collected from the crystal mounted in a loop in a mixture of methanol and glycerol, and cooled in nitrogen vapour stream at 100 K. The structures revealed that the ring-like host is formed by three molecules which have flexible parts. When sulphate anion is the guest it occupies the center of the ring since the hydrophilic parts of the molecules are at the inside of the formed host ring. The disordered sulphate anion forms 12 hydrogen bonds with the host molecules. On contrary, when perchlorate anion is the guest it occupies voids between the rings formed by three molecules which are now folded in such a way that the methyl groups point inward leaving no space within the ring for the guest.

supramolecular receptor; anion binding; hydrogen bond

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