Amplified Recognition of Basic Anions Induced by Cooperative Interaction of Ureido-Binding Sites Preorganized by Azacalix[4]arene Skeleton.

The reaction of tetranitropolyamino carriers with 4-tert-butylphenyl isocyanate gives rise to ureido-based receptors. These receptors vary in the acidity of their bridging NH groups on supporting skeletons, significantly affecting their complexation ability. While introducing nonbasic anions leads to the independent action of ureido-binding sites, accomplished with low binding efficiency for all studied compounds, the results for basic anions depend on the system used. Here, due to a different electron density distribution on supporting skeletons, the interaction with basic anions may cause unwanted deprotonation (acyclic systems 5) or lead to anion complexation (macrocycles 6 and 7). Moreover, in the case of tetraureido azacalix[4]arene 6, the addition of carboxylates and phosphate induces system cooperativity, which changes the complex stoichiometry from 1:4 to a 1:2 ratio (receptor: anion) and positively improves binding efficiency.