Fenton-Like Reactions for Pyrazolone Ester Based Heterocyclic Dyes with Reversible Hydrazone-Azo Tautomerism

In this work, two new pyrazolone ester based heterocyclic hydrazone dyes have been prepared and characterized by NMR, FT-IR and MS spectrometry. The acid/base-activated hydrazone-azo tautomerism has been monitored by UV-Vis and 1 H NMR spectra. What is worth mentioning, three Fenton catalysts: Cu II , Ni II and Co II are introduced to form reactive hydroxyl radicals from hydrogen peroxide to attack the ortho -position of phenyl unit of one pyrazolone ester based heterocyclic dye and resulted in another ortho -OH substituted pyrazolone ester based heterocyclic dye via Cu II /Ni II /Co II -H 2 O 2 catalytic oxidation, metal-ion complexation, and ending demetallization. Furthermore, three Cu II , Ni II and Co II centered complexes have been isolated and structurally characterized including the resultant heterocyclic dye as the anionic ligand. UV-Vis spectral comparisons show that the absorption maximum ( λ max ) values of Cu II , Ni II and Co II centred complexes are significantly shifted to higher wavelengths compared to their precursor, originated from the increased electron density by forming the divalent L 2- ligand in the deprotonated azo form, which is comparable to the pyrazolone ester based heterocyclic dye in deprotonation azo form under alkaline condition. Furthermore, the hydrazone-azo tautomerism has been verified by the significant changes of hydrazone proton signals observed in the 1 H NMR spectra. In addition, two possible pathways for Fenton catalysts corresponding to three types of metal ions were proposed. We believe that the acquisition of these intermediate metal complexes is conducive to understanding on the applications of Fenton-like reactions in the syntheses of heterocyclic dyes