Why the Reactive Oxygen Species of the Fenton Reaction Switches from Oxoiron(IV) Species to Hydroxyl Radical in Phosphate Buffer Solutions? A Computational Rationale

It has been shown that the major reactive oxygen species (ROS) generated by the aqueous reaction of Fe­(II) and H₂O₂ (i.e., the Fenton reaction) are high-valent oxoiron­(IV) species, whereas the hydroxyl radical plays a role only in very acidic conditions. Nevertheless, when the Fenton reaction is conducted in phosphate buffer solutions, the resulting ROS turns into hydroxyl radical even in neutral pH conditions. The present density functional theory (DFT) study discloses the underlying principle for this phenomenon. Static and dynamic DFT calculations indicate that in phosphate buffer solutions, the iron ion is highly coordinated by phosphoric acid anions. Such a coordination environment substantially raises the pK_a of coordinated water on Fe­(III). As a consequence, the Fe­(III)–OH intermediate, resulting from the reductive decomposition of H₂O₂ by ferrous ion is relatively unstable and will be readily protonated by phosphoric acid ligand or by free proton in solution. These proton-transfer reactions, which become energetically favorable when the number of phosphate coordination goes up to three, prevent the Fe­(III)–OH from hydrogen abstraction by nascent ^•OH to form Fe­(IV)O species. On the basis of this finding, a ligand design strategy toward controlling the nature of ROS produced in the Fenton reaction is put forth. In addition, it is found that while phosphate buffers facilitate ^•OH radical generation in the Fenton reaction, phosphoric acid anions can act as ^•OH radical scavengers through hydrogen atom transfer reactions.