Anion-induced conformational changes in 2,7-disubstituted indole-based receptors

The conformational preorganization and anion-induced conformational changes of indole-based receptors functionalized with an amide group at the 2-position and a variety of amide, urea and thiourea moieties at the 7-position have been studied by the means of NMR spectroscopy. NOE experiments showed that anti–anti orientation across C2–C2 and C7–N7 bonds is preferred for receptors 1–4 in acetone solution in the absence of anions. Anion–receptor interactions have been evaluated through 1H and 15N chemical shift changes. In 2,7-bis-carboxamido functionalized indoles the interaction with chloride and bromide anions primarily occurs at the indole H1 proton. The introduction of urea and thiourea moieties increases the number of hydrogen bond donor sites which manifests itself in a distribution of halide–receptor interactions among the H1, H7 and H7 protons. Acetate anions also interact strongly with indole and urea NH donor groups, whereas nitrate anions interact solely with H7 and H7 urea/thiourea protons. NOE enhancements in the presence of anions revealed that anion–receptor complexes favour the syn–syn conformation of the C2 and C7 substituents