What Are the Oxidizing Intermediates in the Fenton and Fenton-like Reactions? A Perspective ^†

Abstract

The Fenton and Fenton-like reactions are of major importance due to their role as a source of oxidative stress in all living systems and due to their use in advanced oxidation technologies. For many years, there has been a debate whether the reaction of Fe^II(H₂O)₆²⁺ with H₂O₂ yields OH^• radicals or Fe^IV=O_aq. It is now known that this reaction proceeds via the formation of the intermediate complex (H₂O)₅Fe^II(O₂H)⁺/(H₂O)₅Fe^II(O₂H₂)²⁺ that decomposes to form either OH^• radicals or Fe^IV=O_aq, depending on the pH of the medium. The intermediate complex might also directly oxidize a substrate present in the medium. In the presence of Fe^III_aq, the complex Fe^III(OOH)_aq is formed. This complex reacts via Fe^II(H₂O)₆²⁺ + Fe^III(OOH)_aq → Fe^IV=O_aq + Fe^III_aq. In the presence of ligands, the process often observed is L_n(H₂O)_5−nFe^II(O₂H) → L^•+ + L_n−1Fe^III_aq. Thus, in the presence of small concentrations of HCO₃⁻ i.e., in biological systems and in advanced oxidation processes—the oxidizing radical formed is CO₃^•−. It is evident that, in the presence of other transition metal complexes and/or other ligands, other radicals might be formed. In complexes of the type L_n(H₂O)_5−nM^III/II(O₂H⁻), the peroxide might oxidize the ligand L without oxidizing the central cation M. OH^• radicals are evidently not often formed in Fenton or Fenton-like reactions.

1. General Remarks

In 1894, Mr. Fenton reported that Fe^II(H₂O)₆²⁺ catalyzes the oxidation of tartaric acid by H₂O₂ [1]. No mechanism of this process was suggested by Mr. Fenton. Since then, the reaction Fe^II(H₂O)₆²⁺ + H₂O₂ has been called the Fenton reaction and the reactions MⁿL_m + ROOR’—where M is either Fe or another low-valent transition metal, L is either H₂O or another ligand, and R and R’ are either H or another substituent—are called Fenton-like reactions.

The Fenton and Fenton-like reactions are of major importance due to two reasons:

• They are considered to be the major source of oxidative stress in all living systems.
• They are used in the advanced oxidation technologies/processes that are of major importance in the environmental removal of pollutants.

Due to this prominence, a search in SciFinder for Fenton in 2021 results in 3286 references.

The first mechanisms of the Fenton reaction were suggested in 1932 by two groups in parallel. Bray and Gorin [2] suggested that the mechanism is:

Fe^II(H₂O)₆²⁺ + H₂O₂ → Fe^IV=O²⁺_aq

(1)

whereas Haber and Weiss [3,4] suggested that the mechanism of the Fenton reaction is:

Fe^II(H₂O)₆²⁺ + H₂O₂ → Fe^III(H₂O)₆³⁺ + OH^• + OH⁻

(2)

The debate whether the oxidizing intermediate formed in the Fenton reaction is Fe^IV = O²⁺_aq or OH^• has lasted for many decades. Thus, even as recently as this year, it has been suggested that reaction (1) is the correct mechanism, at least in neutral solutions [5], and that (2) is the only process even at pH 5 [6].

The difficulty in differentiating between the two mechanisms stems from the fact that both OH^• radicals and Fe^IV=O²⁺_aq react with organic substrates, usually by abstracting a hydrogen atom, and often form the same, or similar, radicals. Using EPR to quantify the relative yields of the radicals formed in order to decide whether their sources are OH^• radicals often fails due to their different lifetimes [7]. This difficulty was overcome by measuring the final products formed when a mixture of two alcohols is present.⁸ This technique requires that the low-valent metal cation initiating the Fenton-like reaction has a fast ligand exchange rate, i.e., it does not fit Fe^II(H₂O)₆²⁺. Using this technique, it was shown that the reaction Cr^II(H₂O)₆²⁺ + H₂O₂ proceeds via a mechanism analogous to reaction (2), whereas the reaction Cu^I_aq⁺ + H₂O₂ does not yield OH^• radicals or Cu^III_aq [8]. Furthermore, thermodynamic arguments [8] and kinetic arguments using the Marcus theory [9] indicate that the Fenton and Fenton-like reactions do not proceed via the outer sphere mechanism. Therefore, an inner sphere mechanism was proposed [8,9]:

ML_mⁿ⁺ + H₂O₂ ⇌ {L_m−1M(H₂O₂)ⁿ⁺ + L}/{L_m−1M(HO₂)⁽ⁿ⁻¹⁾⁺ + L + H⁺}

(3)

For simplicity, it will be assumed in that the complex formed is L_mM(H₂O₂)ⁿ⁺. Reaction (3) might be followed by a variety of routes, e.g., [8,9]:

→ ML_m⁽ⁿ⁺¹⁾⁺ + OH^• + OH⁻

(4a)

L_mM(H₂O₂)ⁿ⁺ → ML_m⁽ⁿ⁺²⁾⁺ + 2OH⁻
RH

(4b)

→ ML_m⁽ⁿ⁺¹⁾⁺ + R^• + OH⁻ + H₂O
R=R

(4c)

→ ML_m⁽ⁿ⁺¹⁾⁺ + HOR-R^• + OH⁻

(4d)

Naturally, L_m−1M(H₂O₂)ⁿ⁺ might also directly oxidize different substrates, e.g., inorganic reducing agents.

It was later discovered that when the central cation M has a too high redox potential, e.g., Co(II) [10], or cannot be oxidized, e.g.: Al^III, Ga^III, In^III, Sc^III, Y^III, La^III, Be^II, Zn^II, and Cd^II [11,12,13], the binding of two or more peroxides to the central cation might lead to the formation of OH^• radicals via disproportionation of the peroxides without involving oxidation of the central cation [10,11,12,13]:

Mⁿ_aq + kH₂O₂ ⇌ Mⁿ(HO₂⁻)_k−1(H₂O₂)_aq + (k−1)H⁺ (k = 2 or 3)

(5)

Mⁿ(HO₂⁻)_k-1(H₂O₂)_aq → Mⁿ(HO₂^•)(HO₂⁻)_k−2(OH⁻)_aq + OH^•

(6)

The observation that ligated H₂O₂ can oxidize a second ligated peroxide suggests that it might also oxidize other ligands. This was tested theoretically, by DFT [14], and experimentally for the oxidation of a carbonate ligated to Co^II [15], thus proving this possibility.

2. The Fenton Reaction Is (Fe(H₂O)₆²⁺ + H₂O₂)

Efforts to determine whether the reaction Fe(H₂O)₆²⁺ + H₂O₂ forms OH^• radicals via following the formation of the DMPO-OH^• adduct by EPR failed, as it was shown that even mild oxidants, e.g., Fe^III_aq, oxidize DMPO via [16]:

DMPO + Ox → DMPO^•+ + Red

(7)

DMPO^•+ + H₂O → DMPOH^• + OH⁻

(8)

The rate constant of the Fenton reaction in acidic media is k(Fe(H₂O)₆²⁺ + H₂O₂)~50 M⁻¹s⁻¹. The measured rate constants depend on the pH and on the ratio [H₂O₂]/[Fe(H₂O)₆²⁺]; the latter dependencies mainly stem from the observation that in the presence of excess H₂O₂ reactions (9) [17] and (10) [17,18] contribute to the observed rate constants [17].

Fe^III_aq + H₂O₂ ⇌ Fe^III(HO₂) + H⁺ (k₉ = 69 M⁻¹s⁻¹ k₋₉ = 0.11 s⁻¹ at pH 2.0)

(9)

Fe(H₂O)₆²⁺ + Fe^III(HO₂) → Fe^III_aq + {Fe^III_aq + OH^•}/{Fe^IV=O_aq}

(10)

K₁₀ = 7.7 · 10⁵ M⁻¹s⁻¹ at pH 1.0

The nature of the products of reaction (10) were later determined [19] to be Fe^III_aq + Fe^IV=O_aq; thus, clearly in acidic solutions when [H₂O₂]/[Fe(H₂O)₆²⁺] > 1, a mixture of OH^• radicals and Fe^IV=O_aq is formed.

Next, Bakac et al. developed a new procedure to differentiate between OH^• radicals and Fe^IV=O_aq based on the different final products formed in the reactions of OH^• radicals and Fe^IV=O_aq with DMSO, (CH₃)₂SO [20]. This technique can only be used for iron. Using this technique, it was proved that, in acidic solutions, OH^• radicals are formed by the Fenton reaction, whereas in neutral solutions, where pH > 6, the product is Fe^IV=O_aq [20]. This proves that the Fenton reaction under physiological conditions does not form OH^• radicals: However, this statement is not correct for the acidic organelles, e.g., lysosomes [21] and some peroxisomes [22]. This conclusion is correct for reactions of Fe(H₂O)₆²⁺, but not for all Fenton-like reactions of Fe^IIL_m, as seen below.

Recently, it was shown that the Fenton reaction is dramatically accelerated in the presence of low concentrations of bicarbonate well below those present in living cells [19]. The oxidizing transient formed under these conditions is the carbonate anion radical, CO₃^•− [19]. CO₃^•− is a strong oxidizing agent, E⁰(CO₃^•−/CO₃²⁻) =1.57 V vs. NHE [23] and is evidently somewhat stronger in neutral media. CO₃^•− is still a considerably weaker oxidizing agent than OH^• radicals and is, therefore, more selective as a ROS [24,25]. The reactions occurring were proposed to be [19]:

Fe(H₂O)₆²⁺ + H₂O₂ ⇌ (H₂O)₅Fe(O₂H)⁺/(H₂O)₃Fe(O₂H)⁺ + H₃O⁺

(11)

(H₂O)₅Fe(O₂H)⁺/(H₂O)₃Fe(O₂H)⁺ + HCO₃⁻ → Fe^III_aq + CO₃^•−

(12)

Fe(H₂O)₆²⁺ + HCO₃⁻ ⇌ (H₂O)₃Fe(CO₃) + H₃O⁺ + 2H₂O

(11a)

(H₂O)₃Fe(CO₃) + H₂O₂ → Fe^III_aq + CO₃^•−

(12a)

Recent unpublished results [26] suggest that reaction (12) likely proceeds via:

(H₂O)₅Fe(O₂H)⁺/(H₂O)₃Fe(O₂H)⁺ + HCO₃⁻ → (CO₃)Fe^IV_aq

(13)

and reaction (12a) likely proceeds via:

(H₂O)₃Fe(CO₃) + H₂O₂ → (CO₃)Fe^IV_aq

(13a)

The (CO₃)Fe^IV_aq thus formed might decompose via:

(CO₃)Fe^IV_aq→

(14)

→ Fe^III_aq + CO₃^•−

(14a)

Substrate

→ Fe^III_aq + oxidized-substrate + HCO₃⁻

(14b)

The competition between reactions (14a) and (14b) depends on the substrate. Thus, for DMSO k_14a >> k_14b, but for PMSO (phenyl-methyl-sulfoxide) k_14a~k_14b.

3. Fenton-like Reactions Involving Fe^IIL_m

Two types of Fenton-like reactions have to be considered.

When ligands, L, different from H₂O are ligated to The Fe^II central cation, the effect of HCO₃⁻ on the mechanism, discussed above, can be included herein. It should be noted that the technique to distinguish between OH^• radicals and Fe^IV=O_aq, developed by Bakac et al. [20], cannot always be applied here because the mechanism of the reaction LFe^IV=O with DMSO is not known. The mechanism of the reactions of Fe^IIL_m with H₂O₂ for the following ligands was studied.

• L = PO₄³⁻/HPO₄²⁻ [20]. The results suggest that the Fenton reaction in the presence of phosphate in neutral solutions yields OH^• radicals and not (PO₄³⁻)_mFe^IV=O_aq [20].
• L = edta [22]. The reaction Fe^II(edta)²⁻ + H₂O₂ was studied at pH > 5.5 using the technique developed by Masarwa et al. [8]. The results indicate that OH^• radicals are the product of this reaction [27].
• L = nta, nta = N(CH₂CO₂⁻)₃³⁻ [28]. The reaction Fe^II(nta)⁻ + H₂O₂ was studied. Surprisingly, though edta and nta are very similar ligands, the results differ considerably. The results suggest that the major product of the Fe^II(nta)⁻ + H₂O₂ is a (nta)Fe^IV=O_aq complex [28]. The yields of the final products are pH dependent [28].
• L = citrate [29]. The reaction of Fe^II(citrate)⁻ with H₂O₂ was studied. This reaction is of importance because Fe^III(citrate) is a major component of the non-transferrin iron mobile pool [30]. The results indicate that the reaction Fe^II(citrate)⁻ + H₂O₂ in neutral solutions does not yield OH^• radicals. The results do not answer the question whether a Fe^IV(citrate)_aq species is a transient formed by this reaction. When low concentration of HCO₃⁻ are added to this system, the kinetics and final products are changed dramatically, indicating that the CO₃^•− radical anion is a major product of the reaction under these conditions [29].

The results presented in this section indicate that the mechanism of the Fenton-like reactions of Fe^IIL_m complex dramatically depend on the nature of the ligand. Therefore, one cannot assume that Fe^II complexes with analogous ligands react via the same mechanism.

When different peroxides are used as oxidants in the Fenton-like reaction, such as in biological systems, the most important peroxides are the ROOH compounds, where R is an alkyl. The ROOH peroxides are formed in biological systems, mainly in lipids, via the chain reaction [30,31]:

RH + Ox → R^• + Ox-H/(Ox⁻ + H⁺) (Ox = OH^•, R’^•, Fe^IV=O_aq etc.)

(15)

R^• + O₂^• → RO₂

(16)

RH + RO₂^• → RO₂H + R^•

(17)

Therefore, the mechanism of the reaction (CH₃)₃COOH + Fe(H₂O)₆²⁺ was studied. The results indicate that in this system Fe^IV=O_aq is also formed in neutral solutions in the absence of bicarbonate. In the presence of low concentrations of bicarbonate, CO₃^•− radical anions are the product of this Fenton-like reaction [32].

The S₂O₈²⁻ and HSO₅⁻ peroxides are of major importance in advanced oxidation technologies [33,34,35,36]. Therefore, the mechanisms of the reactions Fe(H₂O)₆²⁺ + HSO₅⁻/S₂O₈²⁻ were studied. The results indicate that in acidic media, SO₄^•− radical anions are the active oxidizing species formed, in neutral solutions, Fe^IV=O_aq is formed, and in the presence of low concentrations of bicarbonate, CO₃^•− is the oxidizing intermediate formed [26].

4. Other Fenton-like Reactions

Fenton-like reactions are reported for most low-valent transition metals and even for cations that are not involved in redox processes [11,12,13]. Herein, only Fenton-like reactions involving Cu^I [37] and Zn^II [38,39,40,41] that are of biological importance and Co^II, due to its role in advanced oxidation technologies [15], are discussed.

The reaction of Cu^I with H₂O₂ was long thought to yield OH^• radicals [42], but it was later shown that the active oxidizing agent is Cu^I(H₂O₂) [8] or Cu^III_aq [43]. It was also proposed that the reaction of Cu^I with S₂O₈²⁻ yields Cu^III_aq [44]. Conversely, it was proposed that the reactions of Cu(II) with HSO₅⁻and S₂O₈²⁻ yield Cu^III_aq and SO₄^•− [45].

Surprisingly, Zn²⁺_aq and Zn^II-complexes were shown to be involved in the formation of reactive oxygen species (see references [38,39,40,41] for example.). However, no chemical mechanism initiating this process was forwarded. One possible mechanism is that suggested by Shul’pin et al. [13]. According to this mechanism, the reactions involved are:

Zn²⁺_aq + H₂O₂ ⇌ Zn^II(O₂H⁻)⁺_aq + H⁺

(18)

Zn^II(O₂H⁻)⁺_aq + H₂O₂ ⇌ Zn^II(O₂H⁻)(H₂O₂) ⁺_aq

(19)

Zn^II(O₂H⁻)(H₂O₂)⁺_aq → Zn²⁺_aq + OH^• + HO₂^• + OH⁻

(20)

As the steady state concentration of H₂O₂ in biological media is very low, the probability that two H₂O₂ will bind to the same Zn²⁺_aq is low. Therefore, it is tempting to propose that the process leading to the formation of reactive oxygen species catalyzed by Zn²⁺_aq is:

Zn²⁺_aq + HCO₃⁻ ⇌ Zn^II(HCO₃⁻)⁺_aq

(21)

Zn^II(HCO₃⁻)⁺_aq + H₂O₂ ⇌ Zn^II(HCO₃⁻)(H₂O₂)⁺_aq

(22)

Zn^II(HCO₃⁻)(H₂O₂)⁺_aq → Zn²⁺_aq + OH^• + CO₃^•− + H₂O

(23)

These two plausible mechanisms must be studied experimentally to prove one or both of them.

The reaction Co(H₂O)₆²⁺ + H₂O₂ to yield OH^• radicals is endothermic due to the high redox potential of the Co^III/II couple [10]. However, it was shown that the following reactions replace the simple Fenton-like reaction [14]:

Co(H₂O)₆²⁺ + 3H₂O₂ ⇌ (H₂O)Co^II(HO₂⁻)₂(H₂O₂)

(24)

(H₂O)Co^II(HO₂⁻)₂(H₂O₂) → (H₂O)Co^II(HO₂⁻)(HO₂^•)(OH⁻) + OH^•

(25)

In the presence of bicarbonate, the complex cyclic-(CO₄)Co^II(HO₂⁻)₂(H₂O) is formed. This complex decomposes via [15]:

cyclic-(CO₄)Co^II(HO₂⁻)₂(H₂O) → (H₂O)Co^II(HO₂^•)(OH⁻)₂ + CO₃^•−

(26)

The reaction of HSO₅⁻ with Co(H₂O)₆²⁺ and with Co^II(P₂O₇)(H₂O)₂²⁻ require more than one peroxymonosulfate to form radicals [46].

Finally, it should be pointed out that it is likely that ligands other than carbonate, with the proper redox potential, might also be oxidized directly by peroxides [14].

5. Heterogeneous Fenton-like Processes

A variety of heterogeneous catalysts react with H₂O₂ in Fenton-like processes. Thus, ZnO-nanoparticles induce the formation of reactive oxygen species in biological systems. However, this is attributed to the dissolved Zn²⁺_aq ions [39] and is, therefore, not truly heterogeneous.

The most important heterogeneous catalysts of Fenton-like processes have iron atoms/cations as the active participants, e.g., zero-valent iron [47], MFe₂O₄ (e.g., Fe₃O₄ [48] and MgFe₂O₄ [49]), and LaFeO₃ [50]. These systems are used in advanced oxidation processes and not in biological ones. Therefore, their mechanisms are not discussed herein.

6. Concluding Remarks

The major conclusions of this perspective are:

• The reaction Fe^II(H₂O)₆²⁺ + H₂O₂ yields OH^• radicals as the active oxidizing agent in acidic solutions when [Fe^II(H₂O)₆²⁺] > [H₂O₂], a mixture of OH^• radicals and Fe^IV=O_aq in acidic solutions when [Fe^II(H₂O)₆²⁺] < [H₂O₂], Fe^IV=O_aq in neutral solutions, and CO₃^•− in solutions containing even low concentration of HCO₃⁻, i.e., under physiological conditions.
• It is important to note that mechanisms of the reactions H₂O₂ + Fe^IIL_m(H₂O)_k, where L are ligands different than water, depend dramatically on the properties of L. Thus, one must study the mechanism for each ligand separately.
• The study of the mechanisms of Fenton-like reactions with other peroxides requires separate studies.
• The mechanisms of Fenton-like reactions of other low-valent metal cations differ from each other and thus require separate studies.

Therefore, it must be concluded that the mechanism of each Fenton-like reaction should be studied before concluding which oxidizing transient is formed in that reaction.