Fenton Like Reactions Proceed Via a Variety of Mechanisms

The Fenton and Fenton-like reactions are of key importance in biology and in advanced oxidation processes, AOPs. Their mechanism was in debate for many years: the question being whether the active intermediates formed are hydroxyl radicals or L_m-1Mⁿ⁺²=O complexes. It is now accepted that usually the mechanism involves two steps:

(1) MⁿL_m + H₂O₂ ⇔ L_m-1Mⁿ(O₂H)/L_m-1Mⁿ(H₂O₂) + L + H⁺

+ L → Mⁿ⁺¹L_m + ^.OH + OH^-

(2) L_m-1Mⁿ(H₂O₂) ————— → L_m-1Mⁿ⁺²=O + H₂O

+ RH → Mⁿ⁺¹L_m + R^. + H₂O + OH^‑

It was also shown that at least for M = Fe, in the presence of excess H₂O₂, the following reaction is of major importance:

(3) (H₂O)₅Fe(OOH)²⁺+Fe(H₂O)₆²⁺+2H+→((H₂O)₅Fe^IV=O + Fe(H₂O)₆³⁺+OH^-)/(2Fe(H₂O)₆³⁺+ ^.OH)

Recent results point out that the reactions:

(4) (H₂O)_6-l-1Mⁿ(OOH)_l(H₂O₂) → (H₂O)_6-l-1Mⁿ(^.OOH)(OOH)_l-1(OH) + ^.OH

(5) (H₂O)_6-l-1Mⁿ(OOH)_l(H₂O₂) → (H₂O)_6-l-1Mⁿ⁺¹(^.OOH)(OOH)_l-1(OH) + OH^-

are the source of the ROS when reaction (2) is endothermic. Furthermore, in the presence of HCO₃^-/CO₃^2-, which is present in AOPs and in biology, the reactions:

(6) LMⁿ(CO₃)(H₂O)_n + H₂O₂ → LMⁿ(CO₃)(H₂O)_n-1(OOH) + H₃O⁺

(7) LMⁿ(CO₃)(H₂O)_n-1(OOH) → LMⁿ(CO₄)(H₂O)_n-1 + OH^-

(8) LMⁿ(CO₄)(H₂O)_n-1 + H₃O⁺ → LMⁿ⁺¹(OH)(H₂O)_n + CO₃^.-

(9) LMⁿ(CO₄)(H₂O)_n-1 + H₂O₂ → LMⁿ(CO₄)(H₂O)_n-2(OOH) + H₃O⁺

(10) LMⁿ(CO₄)(H₂O)_n-2(OOH) + H₂O → LMⁿ(OH^-)₂(^.OOH)(H₂O)_n-2 + CO₃^.-

have to be considered. This means that in these systems carbonate anion radicals are the major reactive species.

Furthermore, this means that a ligated peroxide may oxidize another ligand without oxidizing the central cation. DFT examples for these possibilities will be presented.