7,7'-Diamino-2,2'-diindolylmethane: a building block for highly efficient and selective anion receptors-studies in solution and in the solid state.

The easy-to-make 7,7'-diamino-2,2'-diindolylmethane was used as a building block for the construction of anion receptors operating by hydrogen bonds. Its various bisamide and bisurea derivatives were designed and synthesised as acyclic as well as macrocyclic molecules, then their structural and anion binding properties were studied in solution and in the solid state. The bisamide receptors demonstrate high affinity towards oxoanions in highly polar and partially aqueous solutions (DMSO with up to 25 % H(2)O) with significant selectivity for dihydrogen phosphate. Remarkably, the bisurea-based molecules are able to bind anionic guests even in pure methanol and show selectivity toward tetrahedral oxoanions, that is, hydrogen sulphate and dihydrogen phosphate. X-ray analysis revealed that both classes of molecules adopt a similar conformation in the solid state: a bent sheet shape with a binding pocket equipped with hydrogen-bond donors (four for the amides and six for the bisureas), whose orientation is particularly tailored for oxoanions. The results of ROESY NMR experiments are in agreement with the findings for the solid state and confirmed that both bisamides and bisureas can easily adapt the conformation with convergent hydrogen-bond donors, which is highly suitable for anion binding.