Adamantane bisurea derivatives: anion binding in the solution and in the solid state

1,3-Bis-(3-arylureido)adamantane receptors, bearing phenyl (5,6), 1-naphthyl (7,8) and 9-anthryl (9) fluorophore, were synthesized. Their ability for complexation with F−, Cl−, Br−, OAc−, image in solution was investigated by UV–vis and fluorescence spectrophotometry. The binding was compared to that of 2-naphthyl bisurea derivatives with flexible spacers (bearing propylene or pentalene, 2 and 4) and rigid adamantane analogues (1 and 3). In solution, the receptors form stable complexes with all anions except with image. The complexation ability in CH3CN correlates with the basicity of anion and the acidity of the urea N–H, whereas in DMSO the complexes stability variations are less pronounced. The X-ray structure of receptor 1 indicates that incorporation of the adamantane moiety preorganizes the receptor in a tweezer-like conformation for the optimal formation of hydrogen bonding network and high selectivity for image anion. Incorporation of the methylene spacers between the adamantane and the urea additionally increases stability of the complexes with anions. X-ray structural analysis was performed on the following complexes: 1·Bu4NH2PO4, 3·Bu4NH2PO4, 5·Bu4NH2PO4·4H2O, and 5·Bu4NOAc·3H2O. All image complexes include extensive image hydrogen bonds, essential for the anion recognition, as well as image hydrogen bonds.