The tautomerization between keto- to phenol-hydrazone induced by anions in the solution

Two simple anion receptors, 2-[(2-hydroxy-5-nitrophenyl)methylene]hydrazone (1) and 2-[(3,5-dibromo-2-hydroxyphenyl)methylene]hydrazone (2) with –OH binding sites, were synthesized and characterized. The anion binding ability of receptors 1 and 2 with halide anions (F−, Cl−, Br− and I−), AcO− and H2PO4- was investigated using visual (naked-eye), UV–vis titration experiments in dry DMSO together with DFT theoretical calculation. The addition of F−, AcO− and H2PO4- to the host solution resulted in a red shift of the charge-transfer absorbance band accompanied by a color change from yellow to orange in the naked-eye experiments. Receptor 1 containing a nitro group at the para position and receptor 2 containing two bromine groups at the ortho and para positions both showed strong binding ability for H2PO4- ion in the form of phenol-hydrazone. Moreover, receptor 1, induced by anion species in the solution, converted to the form of phenol-hydrazone from keto-hydrazone.

► Two receptors with –OH binding sites show strong binding ability for H2PO4- ion.
► Compound 1 converts to phenol-hydrazone from keto-hydrazone induced by anions.
► Compound 2 recognizes anion in the form of phenol-hydrazone.
► The anion binding ability of compound 1 is weaker than that of compound 2.