Indole-based macrocycles as a class of receptors for anions.

The development of synthetic receptors and chemosensors for anions has attracted much attention in recent years as the fundamental role of anions in biological and chemical processes has become increasingly understood. Biomacromolecules such as sulfate-binding proteins and a CIC chloride channel make use of hydrogen bonds as a major driving force to bind or transport anions through cell membranes. Similarly, hydrogen-bonding interactions are widely implemented for the construction of artificial receptors, and amido and ureido groups have frequently been incorporated for this purpose. 4] Furthermore, the pyrrolic NH group has also been used as a hydrogen-bond donor to an anion, as demonstrated in calixpyrroles, expanded porphyrinoids, and amidopyrroles. Herein, we describe the synthesis and binding properties of indole-based macrocycles 1 and 2, which can function as a new class of synthetic receptors for anions. The macrocycles contain a well-defined internal cavity surrounded by four indole NH hydrogen atoms, thus are capable of binding anions strongly through hydrogen-bonding interactions. More interestingly, 1 and 2 showed separate sets of H NMR signals for the free macrocycles and their complexes at room temperature in all cases and thus can serve as chemosensors for anions based on H NMR chemical shifts. The synthesis of 1 and 2 is outlined in Scheme 1. The key molecular building block 7,7’-diiodo-2,2’-biindole (4) was prepared by oxidative dimerization (Cu(OAc)2, 95%) [9]