Preparation of ortho hydroxyl azobenzene dyes by Cu(II)/H2O2 catalytic oxidation and complexation and the following demetallization

Abstract In this work, an ortho hydroxyl azobenzene heterocyclic dye has been prepared by using the post-modification synthetic strategy via the Fenton-type reaction, namely in situ Cu(II) / H 2 O 2 catalytic oxidation and complexation, and the following demetallization. Special attention is given to the structural and spectral differences between dyes before and after implanting the ortho phenolic hydroxyl group and the copper(II) ion complexation. Solvatochromic experiments for two dyes with and without the additional ortho phenolic hydroxyl group in four organic solvents with different solvent polarity index reveal distinguishable UV–Vis spectral behavior. The existence of hydrazone, mixture of hydrazone and azo, and azo tautomers is observed in different organic solvents in the ortho hydroxyl azobenzene heterocyclic dye, and only the hydrazone tautomer can be found in the dye without the hydroxyl group by contrast. Further acid-base titration verifies the equilibrium between hydrazone and deprotonated azo forms in both dyes. In addition, X–ray single-crystal diffraction studies have proved the configurational transformation from the hydrazone to deprotonated azo forms and returning to the hydrazone form before and after the copper(II) ion complexation and the following demetallization.