A dihydrogen phosphate selective anion receptor based on acylhydrazone and pyrazole

A novel chromogenic anion receptor 1 based on acylhydrazone and pyrazole has been designed and synthesized. Chromogenic anion receptor 1 forms stable 1:1 complexes with dihydrogen phosphate (H₂PO₄^-) and other halide (Cl^-, Br^-) anions in DMSO solution, as shown by UV-vis, fluorescence and ¹H NMR spectroscopic experiments. The pyrazole containing host 1 in the optimized geometry of the complex has been noted as planar. The planarity of the host with the H₂PO₄^- anion resulted from four strong N-H⋯O and two weak C-H⋯O types of H-bonding. In total six H-bonding and the planarity of the host in the complex are responsible for the high binding energy. The addition of an excess of more basic anions (F^- and CH₃COO^-) induces stepwise deprotonation, an event signalled by the appearance of a bright yellow color. The intensity of the fluorescence spectrum increases when hydrogen bonding occurs and decreases when deprotonation occurs, which is evidenced by fluorescence titrations.