Spectroscopic and structural insights into N-substituted pyridinium-4-aldoximes and their pentacyanoferrate(II) complexes

Abstract

Comparative kinetic and equilibrium studies on the formation and dissociation of three mono- and one bis-pyridinium-4-aldoxime pentacyanoferrate(II) complexes have been carried out in aqueous solutions at 25°C and I=0.1M. The synthesis, spectroscopic and thermal characterization of a new N-methylpyridinium-4-aldoxime pentacyanoferrate(II) complex is presented. The obtained values for the equilibrium constants, identified as apparent formation constants (βf/M−1) along with kinetic parameters, the formation (kf/M−1s−1) and dissociation (kd/s−1) rate constants indicated the behaving of all protonated pyridinium-4-aldoximes as weak π-acceptors. The pH-dependence of the dissociation rates has been analyzed in terms of ionization abilities of the coordinated ligands. The magnitude of the dissociation rates suggested that both protonated and deprotonated ligand forms are effective σ-donors that bind to the [Fe(CN)5]3− moiety through the nitrogen atom. The deprotonation of the coordinated aldoxime group leads to the reduced lability of the complexes due to increased σ-donor capability of aldoximato nitrogen causing the strengthened of the iron(II)–nitrogen bond. The values of dissociation activation parameters, ΔH‡ and ΔS‡, are found to be consistent with the SN1 dissociative type of mechanism. The spectroscopic data (FT-IR, NMR and UV–Vis) of the isolated coordinated pentacyanoferrates(II) conforms with the weak π-acceptor properties of the pyridinium-4-aldoxime ligands. A detailed structural characterization of the iodide and chloride salt of N-methylpyridinium-4-aldoxime was also presented using NMR, FT-IR and UV–Vis spectroscopy, as well as X-ray diffraction.