The Oxidation of Iron(II) by Molecular Oxygen Catalyzed by Chloride Ion, Thiocyanate Ion, and Ethylenediamine in Dimethyl Sulfoxide

Abstract

Abstract The rates of the oxidation reactions of iron(II) to iron(III) by molecular oxygen in DMSO were measured under the catalyses of Cl−, SCN−, and en. The rate was proportional to the partial pressure of oxygen and was of the second order to [Fe(II)] in catalyses of Cl− and SCN− while it was of the first order to [Fe(II)] in catalysis of en. The reaction rate was also linear to [X]2 and [H+], where X represents Cl− or SCN−, indicating that the reaction proceeds via both the electron-transfer and hydrogen-atom-transfer mechanisms, side by side. The reaction order of [Fe(II)] depends upon which step is rate-determining among those of the oxygenation of Fe(II), its spontaneous decomposition, and its bimolecular reaction with another Fe(II) and, therefore, may be variable according to the concentrations of the reacting species. The autoxidation rate can be affected by the kinds of the catalyzing ligands. The autoxidation may be slow when an Fe(II) complex with low spin is oxidized to an Fe(III) complex with high spin and may be fast when from high spin type to low spin one. In the present cases of Cl−, SCN−, and en in DMSO, it seems to go from high spin type to high spin one with moderately large rates.