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

Abstract

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−, NO3−,HSO4−,andH2PO4− 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 NO3−. 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 H2PO4− 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 H2PO4− complexes include extensive receptor⋯H2PO4− hydrogen bonds, essential for the anion recognition, as well as H2PO4−⋯H2PO4− hydrogen bonds.