Four heterocyclic dyes, derived from methyl 4-aminobenzoate and pyridine-2,4-dione components, with different N-substituents in the latter [CH 3 , CH 2 CH 3 , CH 2 CH 2 OH, CH 2 CH 2 CH 2 OCH(CH 3 ) 2 ], were synthesised by the traditional diazotisation-coupling strategy. Furthermore, a Cu(II) dye-metal complex was prepared and characterised by X-ray single-crystal diffraction for comparison, where the configuration transformation from hydrazone to deprotonated azo forms was found before and after Cu(II) ion complexation. In addition, the azo-hydrazone transformation could be observed by acid-base titration experiments. To the best of our knowledge, this work is the first structural study of pyridone dyes based on methyl 4-aminobenzoate and the corresponding dye-metal complex.
A pair of isomeric aromatic heterocyclic dyes, bearing the same N-methyl pyridine-2,6-dione coupling component but a different 2-or 4-aminobenzoic acid diazo component, were characterised structurally and spectroscopically. X-ray single-crystal diffraction analyses revealed that they both adopt the same hydrazone-tautomeric form and planar molecular conformation between the pyridine and phenyl rings. Furthermore, azo-hydrazone transformation was achieved for the 2-aminobenzoic-acid-based dye after Cu (II) ion complexation, as verified by the formation of a neutral Cu(II) dye-metal complex. The coordination geometry of the central Cu(II) ion exhibits a slightly distorted pyramid with a s value of 0.028, where the ligand loses a proton during complexation and serves as the tridentate coordination mode. In addition, thermogravimetric analysis and corresponding differential thermal analysis for the isomeric pair showed that they both have excellent thermal stability, and 4-aminobenzoic-acid-based dye has a higher decomposition temperature (319°C) than the 2-aminobenzoic-acid-based dye (312°C). To the best of our knowledge, this is the first structural study on aromatic heterocyclic dyes having aminobenzoic acid and pyridine-2,6-dione components at the same time.
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