engleski

Kinetic and thermodynamic properties of pentacyano[bis(pyridinium-4-aldoxime)]ferrate(II) complex

1, 1'-bis(pyridinium-4-aldoxime)trimethylene dibromide (TMB-4) is a potent reactivator of the acetylcholinesterase inhibited by organophosphorus poisons and seems to have various other pharmacological actions. Trying to give more insight into the correlation between the chemical structure and the biological function of that aldoxime we undertook a kinetic study of its reactions with the aquapentacyanoferrate(II) ion, [Fe(CN)5(H2O)]3-, as an often used model for systems with labile sixth coordination site. The kinetics has been studied spectrophotometrically in an aqueous media, pH 5-11, 25.0 °C and ionic strength 0.50 M. Formation rate constants were found to be pH-dependent due to pH-related reactive ionic forms of the TMB-4. Dissociation rates of [Fe(CN)5(TMB-4)](3-n)- (n=1-3) were also found to be pH-dependent, and have been measured in the presence of an excess of dimethyl sulfoxide (DMSO). An increase in the dissociation rate of [Fe(CN)5(TMB-4)]-/2- complex containing protonated TMB-4 forms with respect to the dissociation rate of its conjugated base ([Fe(CN)5(TMB-4)]3-) is attributed to the relative basicities of the leaving ligand and solvatation effects. The rate constants of formation and dissociation along with activation parameters will be presented and discussed in terms of, nowadays widely accepted, dissociative mechanism of the ligand substitution reactions in various pentacyanoferrates(II). The equilibrium constant for the complex formation was deduced from kinetic measurements.

kinetics; oximes; equilibrium constant

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