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Comparative study of o-, m- and p- substituted N-methylpyridinium aldoximes and their reactions with the aquapentacyanoferrate(II) ion

Biological importance of pyridinum aldoximes is closely related to their structure, acidic properties, nuchleophilicity and coordination ability. Compounds of pyridinium aldoxime type have been shown to exhibit a great variety of pharmacological effects. An important pharmacological application is derived from their capability to reactivate organophosphate-inactivated acethylcholinesterase. N-methylpyridinium-2-aldoxime chloride (PAM2-Cl), N-methylpyridinium-3-aldoxime iodide (PAM3-I) and N-methylpyridinium-4-aldoxime iodide (PAM4-I) were used to explore the influence of the position of the aldoxime group in N-methylpyridinium ring on the structure, chemical properties and reactivity toward the pentacyanoferrate(II) moiety. Aquapentacyanoferrate(II) ion, an useful model for biological macromolecules with a labile sixth coordination site, was used as a metal reagent. Crystal structure characterization was done for PAM3-I and compared with the known structures of PAM4-I and PAM2-Cl. Ionization ability for all three ligands was determined and was found to be in good agreement with their structural differences and consequent resonance stabilization of their aldoximato forms. Reactions of the examined ligands with the aquapentacyanoferrate(II) ion were studied in buffered aqueous media by UV/Vis spectroscopy. Comparative kinetic and equilibrium studies on the formation and dissociation of the produced pentacyano(ligand)ferrate(II) complexes were carried out at 25°C and I=0.1 M. Equilibrium constants and stoichiometry of the complexes at pH 6 were determined by mole ratio method. The kinetic properties of the produced complexes were determined in a pH range 5.0-11.5 and correlated with σ-donor and π-acceptor capability of the ligands. Furthermore, the pH-dependency of the complex dissociation rate was analyzed in terms of ionization ability of the coordinated ligand. The obtained values for the equilibrium constants, identified as apparent formation constants, along with the kinetic parameters suggested that all three complexes can be classified as instable and labile.

N-methylpyridinium aldoximes and their pentacyanoferrate(II) complexes

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