X-ray crystallographic, electrochemical and spectroscopic properties of 2-pyridinio 2-pyridyl ketone phenyl hydrazone chloride hydrate

In contrast to the reaction between di-2-pyridyl ketone with a variety of hydrazines or hydrazides in refluxing acidified alcoholic solution to form unprotonated di-2-pyridyl ketone hydrazones (dpkhydrazones), the reaction between di-2-pyridyl ketone and phenyl hydrazine hydrochloric acid under the same conditions gave unprecedented pyridyl protonated 2-pyridinio 2-pyridyl ketone phenyl hydrazone chloride hydrate, dpkphh·HCl·3H2O. Crystals of dpkphh·HCl·3H2O obtained from an ethanolic solution of dpkphh·HCl·3H2O that contains a few drops of HCl are in the centric triclinic space group P-1: Structure analysis reveals non-coplanar dpkphh·Hþ along with a chloride anion and three water molecules. The molecules pack show infinite stacks of anti-parallel dpkphh·Hþ locked to the chloride anion and water molecules via novel fused hydrogen bonded oxygen–chloride four and six-membered cyclic rings propagating between the stacks. Electrochemical measurements on dpkphh·HCl·3H2O in non-aqueous solvents show solvent dependence, single and multi-electronic transfers in DMF and in CH3CN single electronic redox transfers. The reversibility of the second electronic reduction following the first irreversible electronic transfer hints to the stability of electrochemically generated intermediate following the sequential electronic transfers. In contrast to the optical behavior of a variety unprotonated and metal coordinated dpkhydrazones, protonation of one pyridine ring in dpkphh·HCl·3H2O decreases the electron density on the pyridine ring ceases the intraligand charge transfer electronic transition and renders the protonated systems (dpkphh·HCl·3H2O plus surrounding solvent molecules) insensitivity to their surroundings although the spectra show strong solvent–solute interactions. 1H NMR measurements on dpkphh·HCl·3H2O in non-aqueous media reveal sensitivity to solvent and temperature variations that point to strong solvent–solute interactions. The amide and water protons show sensitivity to solvent and temperature variations higher than the aromatic protons consistent with their participation in non-covalent hydrogen bonds. q 2004 Elsevier B.V. All rights reserved