A New Bis-Urea Based Cage Receptor for Anions: Synthesis, Solid State Structures and Binding Studies

The synthesis and characterization of a novel bis-urea-based cage receptor for anions (3S,15S)-3,15,20,25-tetramethyl-1,4,6,12,14,17,20,25-octaazatricyclo[15.5.5.17.11]octacosa-7(28),10-diene-2,5,13,16-tetraone (L) is reported. L is a macro-bicyclic ligand built on the 1,7-dimethyl-1,4,7,10-tetraazacyclododecane scaffold to obtain a cage topology in which two ureido moieties have been inserted as binding sites for anions. L can interact with anion guests (G) via H-bonding; in particular, it binds both spherical (Cl^-) and V-shaped anions (AcO^-) as well as more complex carboxylate anions, such as the norfloxacin (Nor^-). NMR experiments highlight that the interaction between L and G mainly occurs at the ureido moieties. L forms L-G adducts of 1 : 1 ([LG]^-) and 1 : 2 ([LG₂]^2-) stoichiometry with Cl^- and AcO^-. Otherwise, in the case of Nor^- only the formation of the [LG]^- complex is observed. L shows higher formation constants values for [LAcO]^- (2.9) and [LNor]^- (3.6) than [LCl]^- suggesting a stronger interaction with the carboxylate anions. In the solid state, three crystal structures of the HL⋅G species were obtained (G=Cl^-, AcO^-, ClO₄ ^-) highlighting the H-bonding interaction between the chloride, acetate or perchlorate anions and the -NH functions of the ureido fragment. The comparison between the two parent open chain receptors (Lb-c) and L has been reported and discussed.