Uptake of Molecular Oxygen by Ferrous Complexes

Abstract

DURING the course of a deliberate study of models for iron porphyrin complexes of biological importance we have shown that ferrous dimethylglyoxime, Fe(DMG)₂, is capable of combining reversibly with molecular oxygen. The reaction occurs in a mixed solvent, 50 per cent aqueous dioxan, in which the complex is appreciably soluble in the presence of various added bases such as pyridine, ammonia, histidine and imidazole. The pH of the solution must be in the range 7–11 if the reaction is to be observed. We consider the reversible reaction to be Fe(DMG)₂(base)₂ + O₂ ⇌ Fe(DMG)₂(base) O₂ + base The forward reaction can be followed by the changes in absorption spectrum as oxygen is bubbled through the solution of the Fe(DMG)₂(base)₂ complex and the back reaction can be followed by observing similar changes bubbling pure nitrogen through a solution of the oxygenated complex. The spectrophotometric measurements show that the reaction is completely reversible for some of the complexes but that some autoxidation occurs with others. The changes in absorption spectrum with uptake of oxygen are very similar to those which occur on the uptake of carbon monoxide or cyanide by the complex. Both the liberation of oxygen and the liberation of carbon monoxide from the complexes have been studied manometrically. There would appear to be two complexes in one of which two molecules of oxygen or carbon monoxide are bound to the cation and, in the other, one molecule is bound as in the above equation. Comparison of the manometric and spectrophotometric measurements indicates that the first molecule of oxygen or carbon monoxide is picked up slowly but the second molecule is picked up so very slowly that the two stages of the reaction can be followed.