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Synthesis and characterization of selected N, N'‐ substituted bis(pyridinium aldoximes) and their pentacyano(pyridinium aldoxime)ferrate(II) complexes

Aldoxime derivatives of pyridinium cation are biologically active compounds having potential pharmacological application. As strong nucleophilic agents in aldoximato zwitterion form, R-py+–C(H)=N–O–, they are used as effective reactivators of the human acetylcholinesterase inhibited by organophosphorus compounds (e.g. pesticides or chemical warfare nerve agents). The biological function and pharmacological effects of pyridinium aldoximes are closely related to their structural features, acidity of aldoxime functional group and their capability to coordinate to metal ions. Aquapentacyanoferrate(II) ion, [Fe(CN)5(OH2)]3–, represents a structural model of important octahedrally coordinated iron(II) centres found in protein macromolecules. The labile sixth coordination site and great stability of pentacyanoferrate(II) moiety in aquapentacyanoferrate(II) ion make it a good selective probe for the investigation of ambidentate ligand reactivity and the coordination mode. N, N'-bis(pyridinium-2- aldoxime)tetramethylene dibromide (QMB2-2Br) and its 3-isomer (QMB3-2Br) and 4-isomer (QMB4-2Br) were used to explore the influence of the position of aldoxime group on pyridinium ring on their properties and reactivity towards [Fe(CN)5(OH2)]3–. Ionization ability in the order QMB2 > QMB4 > QMB3, determined in aqueous media, was found to be in good agreement with their structural differences and consequent resonance stabilization of their aldoximato zwitterions. The kinetic study on the formation and dissociation of the corresponding [Fe(CN)5(QMB)](3–n)– complexes was performed in buffered aqueous media in pH range 4–11 at 25 °C and the deduced kinetic parameters have been correlated with - and - bonding properties of the examined bis(pyridinium aldoxime) ligands. The notable influence of the ligands' isomerism on their reactivity towards aquapentacyanoferrate(II) ion was obtained. The established decrease of the formation rate constants in the order QMB3 > QMB4 > QMB2 was in accordance with the increasing steric hindrance. The slightly pH-dependent dissociation rates were found, which along with the distinctive position of the MLCT band in the electronic absorption spectra, strongly supported the N-coordination of the aldoxime group. Furthermore, all complexes can be classified as labile while the ligands have more pronounced σ-donor than π-acceptor ability. Spectroscopic, elemental and thermogravimetric analysis of the isolated solids support the fact that all complexes are mononuclear pentacyano(QMB)ferrates(II).

bis(pyridinium aldoximes) ; pentacyano(pyridinium aldoxime)ferrate(II) complexes

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