A molecular receptor that selectively binds dihydrogen phosphate

A dihydrogen phosphate-binding receptor (4) containing both hydrogen bond donors and acceptors has been prepared by incorporating two pyridyl units to a preorganized biindole scaffold. Receptor 4 strongly and selectively binds dihydrogen phosphate via multiple hydrogen bonds with an association constant (K_a) of 1.1 × 10⁵ M⁻¹ in CH₃CN at 22 ± 1 °C. The high selectivity toward the target anion over other anions is proven to be due to two additional hydrogen bonds between the phosphate hydroxyl groups and the pyridyl nitrogens, each of which increases the complex stability by the free energy of 1.6 kcal/mol. This result clearly demonstrates that a selective receptor for a polyprotic anion can be developed by combining both hydrogen bond donors and acceptors.