3-(para-Substituted phenylhydrazo)pentane-2,4-diones: Physicochemical and solvatochromic properties

New azoderivatives of pentane-2,4-dione, 3-(4-bromophenylhydrazo)pentane-2,4-dione (HL1) and 3-(4-acetylphenylhydrazo)pentane-2,4-dione (HL2), have been synthesized by reaction between pentane-2,4-dione and the corresponding aryldiazonium salts, and characterized by IR, UV–vis, 1H and 13C NMR spectroscopies, potentiometry, X-ray diffraction analysis (for HL2) and thermogravimetry. The collected data confirm that HL1–2 exist in DMSO solution and in solid phase exclusively in the hydrazo form, being stabilized by an intramolecular hydrogen bonding. The thermodynamics of proton dissociation of HL1–2 and of the related known 3-(4-nitrophenylhydrazo)pentane-2,4-dione (HL3), in water–ethanol media, were studied quantitatively and the corresponding functions were calculated, showing a dependence on the inductive electron-acceptor character of the substituent in para-position of the aromatic part of the molecule. The thermal decompositions of HL1–3 were also investigated showing their high thermal stability with well-defined peaks of phase transition at 413–418 K. The wavelengths (or wavenumbers) of maxima of absorption bands in the UV–vis spectra of HL1–3 and the corresponding absorption coefficients are dependent on the para substituent, but no correlation was found with any particular property of the solvent, such as polarity, H-bond donating or accepting ability.