Complexation Behavior of Heterocyclic Hydrazones. I. Structure and Acid-Base Equilibria for Heterocyclic Hydrazones

Abstract

Abstract The acid-base equilibria of twenty-one hydrazones substituted by phenyl, pyridyl and/or quinolyl groups have been investigated in aqueous solutions over the region of H− to H0 acidity function by a spectrophotometric method at 25 °C. For 2-pyridinecarbaldehyde 2-(5-substituted)pyridylhydrazones, although the proton dissociation of the neutral species (HL) satisfied a Hammett correlation, those of H2L+ and H3L2+ did not. The thermodynamic parameters for the proton dissociations of H2L+ and H3L2+ of seven representative liqands were determined by a temperature-coefficient method at 25 °C and an ionic strength of 0.1 (KCl). The enthalpy and entropy changes for the proton dissociations of H3L2+ and H2L+ were influenced by the steric effects of the methyl group and/or the introduced quinolyl ring. An analysis of the pH dependence of the 1H NMR signals for three hydrazones in an acetone-d6–D2O solution gave more profitable information concerning the fine structures of HL, H2L+, and H3L2+. Each 1H and 13C NMR chemical shift of some ring-substituted methyl derivatives (HL form) in a dioxane–D2O solution has been briefly assigned. On the other hand, a single-crystal X-ray analysis, 1H NMR data in chloroform-d and thermodynamic data for di-2-pyridyl ketone 2-pyridylhydrazone (DPPH) suggested that the intramolecular hydrogen bond in the DPPH molecule is broken by the addition of a proton to the H2L+ species in an aqueous solution.