Role of Iron(III)-salen Chloride as Oxidizing Agent with Thiodiglycolic Acid: The Effect of Axial Ligands

The sulfoxidation of thiodiglycolic acid with iron(III)-salen chloride, which acts as an oxidizing agent without any terminal oxidant, in 50% aqueous acetonitrile medium has been studied. A substantial red shift in the λ_maxvalue of FeIII-salen was observed in aqueous medium. The spectrophotometric kinetic study indicates that [Fe^III(salen)]+ is the active oxidizing species and the reaction follows Michaelis-Menten kinetics with respect to the substrate. The rate of the reaction is highly sensitive to the medium. A reaction mechanism involving electron transfer from sulfur atom of thiodiglycolic acid to the central iron atom of [Fe^III(salen)]⁺ is proposed. The presence of nitrogenous bases like pyridine, imidazole and 1-methylimidazole shows a retarding effect on the reaction rate. This can be explained on the basis of binding of these ligands to the coordination sphere of [Fe^III(salen)]⁺ prior to the reaction with the substrate. The observed order of reactivity, pyridine &gt; 1-methylimidazole &gt; imidazole, is in accordance with the inverse of <em>π</em>-donating ability of nitrogen bases.