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-<i>tert</i>-butylphenyl isocyanate gives rise to ureido-based receptors. These receptors vary in the acidity of their bridging N<i>H</i> 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 <b>5</b>) or lead to anion complexation (macrocycles <b>6</b> and <b>7</b>). Moreover, in the case of tetraureido azacalix[4]arene <b>6</b>, 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.