Anion recognition through hydrogen bonding by adamantane-dipyrromethane receptors

Adamantane-dipyrromethane (AdD) receptors [di(pyrrole-2-yl)methyladamantane (1), 2,2-di(pyrrole-2-yl)adamantane (2), 1,3-bis[di(pyrrole-2-yl)methyl]adamantane (3), 2,2,6,6-tetra(pyrrole-2-yl)adamantane (4)] form complexes with F−, Cl−, Br−, AcO−, NO3−, HSO4−, and H2PO4−. The association constants of the complexes were determined by 1H NMR titrations, whereas the geometries of complexes 1·F− (2:1), 2·F− (2:1), 2·Cl− (2:1), 2·AcO− (2:1), and 4·F− (1:1) were determined by X-ray structural analysis. The most stable complexes are of 2:1 stoichiometry with F− and AcO−. The stability constants are in accordance with the anion basicity and the ability of AdD receptors to place the hydrogen bonding donor groups in a tetrahedral fashion around anions. The binding energies of the complexes between receptors 1–4 and F− anion are calculated using quantum chemical methods. The calculated results show that the solvent polarity is important for the complexation of fluoride ion with AdD receptors 1–4.