Hydrazone to deprotonated azo/azo-enol transformation for isomeric pyrazolone based heterocyclic dyes via metal-ion complexation

Abstract Two pairs of isomeric heterocyclic dyes were synthesized successfully by diazotization reactions between the common 1-phenyl-3-methyl-5-pyrazolone coupling component and 2-aminobenzoic/4-aminobenzoic acid as well as 2-amino-5-nitrophenol/2-amino-3-nitrophenol. The introduction of additional carboxyl and hydroxyl groups in the phenyl ring makes them good multidentate chelating ligands in terms of coordination chemistry, and three Cu(II) and one Fe(II) dye-metal complexes were then produced showing different deprontonated states in ligands (monovalent and divalent), metal-ligand ratios (1:1 and 1:2) and structural diversity (mononuclear, dinuclear and one-dimensional chain). Red shifts of 3–6 nm in the UV–Vis spectra are observed for the ortho -COOH and ortho -NO 2 dyes compared with corresponding para substituted ones, which could be supported by the formation of fused six-membered intramolecular hydrogen-bonding rings found in their single-crystal structures involving the hydrazone protons. More interesting, different from our previously reported solvent and pH induced hydrazone to azo-keto transformation, another kind of hydrazone to azo-enol transformation has been observed for our pyrazolone based dyes in addition to conventional hydrazone to deprotonated azo transformation after metal-ion complexation.