Dihydrogen phosphate as a hydrogen-bonding donor element: anion receptors based on acylhydrazone

Chromogenic anion receptors based on acylhydrazone are designed and synthesized. UV-vis and (1)H NMR titration showed that receptors 1 and 2 are selective receptors for dihydrogen phosphate (H2PO4(-)). Both showed strong association constants with H2PO4(-) even in polar solvents. Receptor 1 was found to recognize H2PO4(-) through three types of hydrogen-bonding (H-bonding) donors: indole N-H, amide N-H, and imine C-H hydrogens. However, receptor 2 seemed to sense H2PO4(-) through two types of H-bonding donors. Despite this seemingly different number of H-bonding elements, the binding constants of receptors 1 and 2 with H2PO4(-) were almost equal. To understand this puzzling result, we investigated the binding poses of complexes using density functional theory. The proposed 2·H2PO4(-) complex structure revealed another possible H-bonding element involving an aromatic nitrogen acting as a H-bonding acceptor. To confirm this, we synthesized receptor 3, which is devoid of this nitrogen. The binding constant of receptor 3 for H2PO4(-) was 2 orders of magnitude lower than those of receptors 1 and 2. This decreased binding affinity strongly supports the existence of a N(aromatic)···H-O(phosphate) interaction. These results provide a rare opportunity to identify H2PO4(-) acting as a H-bonding donor during an anion-recognition event.