COMPLEXATION BEHAVIOR OF HETEROCYCLIC HYDRAZONES .1. STRUCTURE AND ACID-BASE EQUILIBRIA FOR HETEROCYCLIC HYDRAZONES

The acid-base equilibria of twenty-one hydrazones substituted by pheny l, pyridyl and/or quinolyl groups have been investigated in aqueous so lutions over the region of H- to H-0 acidity function by a spectrophot ometric method at 25-degrees-C. For 2-pyridinecarbaldehyde 2-(5-substi tuted)pyridylhydrazones, although the proton dissociation of the neutr al species (HL) satisfied a Hammett correlation, those of H2L+ and H3L 2+ 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-degrees-C and an ionic strength of 0.1 (KCl). The enthalpy and entropy changes for the proton dissocia tions of H3L2+ and H2L+ were influenced by the steric effects of the m ethyl group and/or the introduced quinolyl ring. An analysis of the pH dependence of the H-1 NMR signals for three hydrazones in an acetone- d6-D2O solution gave more profitable information concerning the fine s tructures of HL, H2L+, and H3L2+ . Each H-1 and C-13 NMR chemical shif t of some ring-substituted methyl derivatives (HL form) in a dioxane-D 2O solution has been briefly assigned. On the other hand, a single-cry stal X-ray analysis, H-1 NMR data in chloroform-d and thermodynamic da ta for di-2-pyridyl ketone 2-pyridylhydrazone (DPPH) suggested that th e intramolecular hydrogen bond in the DPPH molecule is broken by the a ddition of a proton to the H2L+ species in an aqueous solution.