Catalytic oxidation of cyclohexene by aqueous iron(III)/H2O2 in mildly acidic solution: Epoxidation versus allylic oxidation

Article abstract of DOI:10.1016/j.inoche.2013.06.018

Catalytic oxidation of cyclohexene (organic layer) at 25 C in a stirred two phase system with fresh [Fe(OH₂)₆](ClO₄)₃/H₂O₂ in the pH range 2.0-4.0 (I = 0.1 M, NaClO₄/HClO₄) is complete within a 2-minute sampling time before deactivating in contrast to previous reports in the literature. Cyclohexene epoxide is a significant product alongside the more dominant allylic oxidation products; 2-cyclohexen-1-ol and 2-cyclohexen-1-one. Both epoxide and the accompanying allylic products appear over the same time period suggesting that they derive from the same catalytic species. The pH dependence of oxidation product yield (optimizing at ~ pH 3) suggests that the active catalyst derives from reactions involving [Fe(OH₂)₅OH]^{2 +} (pK_a 2.54) and is most likely the hydroperoxo ion [Fe(OH₂)₅OOH]^{2 +}. Yields of cyclohexene epoxide are independent of the presence of dioxygen suggesting that an activated form of H₂O₂ ([Fe(OH₂)₅OOH]^{2 +}) is responsible. Reduction in the relative yield of the two allylic oxidation products in the absence of dioxygen suggests that they derive from 2-cyclohexen-1-peroxy radicals resulting from iron(II)-promoted Fenton chemistry following homolytic FeO or OO cleavage on [Fe(OH₂)₅OOH]^{2 +}. A mechanism for the epoxide formation is proposed involving H₃O⁺-assisted heterolytic OO cleavage on [Fe(OH₂)₅OOH]^{2 +} accompanying O atom transfer to the cyclohexene double bond. Accompanying catalysis of H₂O₂ decomposition persists over several hours. For the first time in a Fenton-like system evidence has been obtained for different timescales for the catalysis of oxidation reactions versus catalysis of H₂O₂ decomposition.

Full text of DOI:10.1016/j.inoche.2013.06.018

Inorganic Chemistry Communications 35 (2013) 284–289
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Catalytic oxidation of cyclohexene by aqueous iron(III)/H2O2 in mildly
acidic solution: Epoxidation versus allylic oxidation
⁎
Ben McAteer, Nicola Beattie, David T. Richens
EaStCHEM School of Chemistry, University of St. Andrews, KY16 9ST Scotland, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalytic oxidation of cyclohexene (organic layer) at 25 °C in a stirred two phase system with fresh
[Fe(OH2)6](ClO4)3/H2O2 in the pH range 2.0–4.0 (I = 0.1 M, NaClO4/HClO4) is complete within a 2-minute
sampling time before deactivating in contrast to previous reports in the literature. Cyclohexene epoxide is
a significant product alongside the more dominant allylic oxidation products; 2-cyclohexen-1-ol and
2-cyclohexen-1-one. Both epoxide and the accompanying allylic products appear over the same time period
suggesting that they derive from the same catalytic species. The pH dependence of oxidation product yield
(optimizing at ~pH 3) suggests that the active catalyst derives from reactions involving [Fe(OH2)5OH]2+
(pKa 2.54) and is most likely the hydroperoxo ion [Fe(OH2)5OOH]2+. Yields of cyclohexene epoxide are inde-
pendent of the presence of dioxygen suggesting that an activated form of H2O2 ([Fe(OH2)5OOH]2+) is re-
sponsible. Reduction in the relative yield of the two allylic oxidation products in the absence of dioxygen
suggests that they derive from 2-cyclohexen-1-peroxy radicals resulting from iron(II)-promoted Fenton
chemistry following homolytic Fe\O or O\O cleavage on [Fe(OH2)5OOH]2+. A mechanism for the epoxide
formation is proposed involving H3O+-assisted heterolytic O\O cleavage on [Fe(OH2)5OOH]2+ accompany-
ing O atom transfer to the cyclohexene double bond. Accompanying catalysis of H2O2 decomposition persists
over several hours. For the first time in a Fenton-like system evidence has been obtained for different time-
scales for the catalysis of oxidation reactions versus catalysis of H2O2 decomposition.
Received 27 February 2013
Accepted 11 June 2013
Available online 27 June 2013
Keywords:
Fenton
Iron(III)
Hydrogen peroxide
Cyclohexene
Epoxidation
Allylic oxidation
© 2013 Elsevier B.V. All rights reserved.
The reactions of hydrogen peroxide with aqueous iron(II)
(Fenton's reagent) Eq. (1) and aqueous iron(III) (the Fenton-like re-
action) Eq. (2) have been known and studied for many years [1–4].
Most of the studies to date with iron(III) under mildly acidic condi-
tions have focused on its photo [9–10,20] or electrochemical [21] reduc-
tion to generate iron(II) for initiating Eq. (1) etc. or its known catalysis
of H2O2 decomposition [2]. The mechanism of the ferric ion catalyzed
H2O2 decomposition reaction has been the subject of much debate
[22–25] with mono(peroxo)iron(III) and at high [H2O2], bis(peroxo)
iron(III) complexes, invoked to explain the observed kinetics. Studies
by Wynne-Jones et al. [24,25] showed that catalytic decomposition of
H2O2 optimizes around pH 2.4, decreasing sharply at higher pHs. The
rapid formation of yellow–brown colorations upon mixing mildly acidic
solutions of aqueous iron(III) and hydrogen peroxide have been well
documented and attributed to equilibria (3)–(5)[22–25].
Fe2þ þ H2O2→Fe3þ þ HO þ OH−
ð1Þ
aq
aq
Fe2þ þ H2O2→Fe2þ þ Hþ þ HOO·
ð2Þ
aq
aq
As an effective source for the non-discriminating hydroxyl radical
the Fenton reaction has been much studied and indeed widely
employed as a remediation agent for water purification processes,
particularly for the oxidative mineralization of various chlorinated
aromatic hydrocarbon pollutants [5–14]. However speculation con-
tinues as to whether Eq. (1) is truly representative of all the species
generated [4,15–19]. Since aqueous iron(III) eventually precipitates
from neutral or mildly acidic solution as hydrous oxy-hydroxide spe-
cies there have been fewer studies on the corresponding Fenton-like
reaction in regard to the nature of the species generated and their ox-
idizing ability in regard to potential environmental applications.
Â
Â
Â
Ã
Â
Ã
KHOOH
⇌
FeðOH2Þ 3þ þ H2O2
FeðOH2Þ ðHOOHÞ 3þ þ H2O
ð3Þ
ð4Þ
6
5
Ã
Â
Ã
KOOH
⇌
FeðOH2Þ 3þ þ H2O2
FeðOH2Þ ðOOHÞ 2þ þ H3Oþ
6
5
Ã
Â
Ã
K2HOOH
⇌
FeðOH2Þ 3þ þ 2H2O2
FeðOH2Þ ðOOHÞðHOOHÞ 2þ þ H3Oþ þ H2O
6
4
ð5Þ
⁎
Corresponding author at: Department of Chemistry and Biochemistry, New Mexico
Reaction (4) is relevant to the pH range 2–4 with Eq. (3) only rel-
evant below pH 2 and Eq. (5) at high [H2O2]. Values for KOOH have
State University, MSC 3C, P.O. Box 30001, Las Cruces, NM 88003 -8001, USA.
E-mail address: richens@nmsu.edu (D.T. Richens).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.06.018

B. McAteer et al. / Inorganic Chemistry Communications 35 (2013) 284–289
285
been reported as 3.7 × 10−3[26] and 2 × 10−3[23] with values for
HOOH ~3.2 × 10−2 M−1 and for K2HOOH ~6.6 × 10−4 M−1[23].
In 1999, Pignatello et al. reported the reactions of a photo activated
K
Fenton-like reagent (0.2 mM Fe(III)aq/2.0 mM H2O2/I = 0.1 M, NaClO4/
HClO4, pH 1–3) with cyclohexane, cyclohexene, 1,1,2-trichloroethane,
trichloroethene and tetrachloroethene in a stirred two-phase system
at 25 °C [27]. KOOH Eq. (4) was determined spectrophotometrically
at 450 nm to be (9.0
1.5) × 10−3 at pH 2.8 shifting to (1.4
0.5) ×
10−2 at pH 1.45. This latter value is similar to that reported by Lewis et
al. [23] for KHOOH and is presumed here to refer to reaction (3). Interest-
ingly the reactions with the chlorinated hydrocarbons; trichloroethene
and tetrachloroethene studied at pH 2.8 gave a distribution of carboxylic
acid products (particularly high yields of dichloroethanoic acid from
trichloroethene) not consistent with solely radical processes initiated
by H atom abstraction, e.g. by OH radicals, but rather from the decompo-
sition of an intermediate epoxide [27]. Subsequent investigations with
cyclohexene at pH 2.8 led to the detection of significant amounts
of epoxide accompanying formation of the expected allylic oxidation
products; 2-cyclohexen-1-ol and 2-cyclohexen-1-one [27]. However a
surprising observation was the slow appearance of reaction products
over several hours. Substitution of water on [Fe(OH2)5OH]2+, the likely
Fig. 1. GC trace following sampling of the cyclohexene layer after 2 min at 25 °C.
The labeled products are A; cyclohexene epoxide, B; 2-cyclohexen-1-ol/cyclohexanol,
C; 2-cyclohexen-1-one. The peak at Rf 6.91 min is from cyclooctane added as reference
prior to running the GC. Aqueous layer: [H2O2]init = 10 mM; [Fe(III)]init = 0.2 mM, pH 2.7.
aqua iron(III) species present under the stated conditions [28,29], is fast
298
(k
ex
= 1.4 × 105 s−1) [30] suggesting that assembly of the active cata-
lyst from complexes such as [Fe(OH2)4(HOOH)OH]2+ Eq. (6) or more
likely, given the higher acidity of coordinated H2O2[25], its tautomeric
form [Fe(OH2)5OOH]2+ Eq. (7) occurs on the submillisecond timescale.
a slow but steady decrease over the first hour. The addition of further
aliquots of stock H2O2 solution after 10 and 20-minute reaction,
Figure S1, indeed failed to restore the catalytic activity of the solution.
However, the addition of aliquots of fresh iron(III) solution did re-
store some activity over the first 30 min, Figure S2, when amounts
of H2O2 were still present, Fig. 2. A gradual darkening of the aqueous
phase followed by a brown turbidity after 1 h suggested that the
loss of catalytic oxygenation activity is due to gradual depletion of
soluble mononuclear aqua iron(III) via hydrolytic polymerization to
inactive colloidal forms and finally precipitation of insoluble iron(III)
oxy-hydroxides [28]. A closer inspection of the H2O2 decay profile,
Fig. 2, reveals an induction period within the first 2 min correlating
with the maximum rate of H2O2 activation towards oxygenation.
Thereafter, efficient catalysis of H2O2 decomposition dominates.
A further insight into the nature of the putative iron(III) oxygenation
catalyst came from monitoring the pH dependence of the yields of oxi-
dation products following the initial sampling after 2-minute reaction,
Fig. 3. The yields of all products increase sharply in the pH range 2.7–
3.2 before decreasing at higher pHs. Interestingly there is a small but
noticeable increase in the relative yield of epoxide compared to the
allylic oxidation products; the epoxide yield increasing between pH
2.7 and 3.2 to reach ~13% of the total oxidation products seen. Above
pH 3.2 the total product yield decreases but the relative yield of epoxide
remains steady at ~11%. The speciation profiles of aqua iron(III) at
below mM concentrations (data from [29a] shown in Fig. 3 for com-
parison) indicate a correlation of oxygenation catalytic activity with
amounts of the hydroxopentaaqua ion; [Fe(OH2)5OH]2+ (pK11 = 2.54
at I = 0.1 M, NaClO4) [29a]. With oxo-aquairon(IV) not proven as an
effective epoxidation catalyst [31c], we tentatively propose the follow-
ing mechanism for alkene epoxidation, Fig. 4. Firstly, an activated com-
plex forms between [Fe(OH2)5OOH]2+ and cyclohexene which then
undergoes intramolecular iron-bound oxygen atom transfer (most
electrophilic) to the cyclohexene double bond accompanying (H3O+)-
assisted O\O heterolysis. Formation of the allylic oxidation products;
2-cyclohexen-1-ol and 2-cyclohexen-1-one will arise via the generally
accepted radical autoxidation mechanism (8)
½FeðOH2Þ OHꢀ2þ þ H2O2→½FeðOH2Þ ðHOOHÞOHꢀ2þ þ H2O
ð6Þ
5
4
ð7Þ
Furthermore, the formation of epoxide, which is only observed in
the presence of iron(III), is not consistent with reactions involving OH
radicals e.g. via Eq. (1) and suggests another source for the oxidation.
It was speculated [27] that [Fe(OH2)5OOH]2+ could homolytically
cleave at the O\O group to generate the oxo-aquairon(IV) complex;
[Fe(_O)(OH2)5]2+ which actually transfers oxygen to the cyclohexene.
However extensive studies by Bakac and co-workers on the chemistry
of oxo-aquairon(IV), generated via reaction of ozone with [Fe(OH2)6]2+
,
show that it tends to rather promote H atom or hydride abstraction and
only transfers oxygen to highly oxophilic substrates such as sulfoxides
or phosphines [31,32].
We have re-investigated Pignatello's reaction of aqueous iron(III)
(added as a freshly made up acidified solution of [Fe(OH2)6](ClO4)3
adjusted to pH 2–4) and H2O2 with neat cyclohexene in a stirred
two-phase system [33]. A typical GC trace for the reaction products
isolated from the cyclohexene layer at pH 2.7, Fig. 1, confirms the
presence of cyclohexene epoxide A (Rf = 5.78 min) accompanying
the formation of 2-cyclohexen-1-ol/cyclohexanol B (not resolved)
(Rf = 6.23) and 2-cyclohexen-1-one C (Rf = 6.68). The peak at
Rf = 6.91 is from cyclooctane added as an internal reference. Fig. 2
shows a typical plot (25 °C) at pH 3.0 representing integration of
the GC peaks A–C as a function of sampling time after the initial injec-
tion of H2O2. The product yields were calculated relative to the
amount of the internal reference added to each sample. Clearly appar-
ent from Fig. 2 is the rapid formation of products within the first
2-minute sampling time with little or no change thereafter over a
1-hour period. It was initially thought that the loss of activity after
the initial 2-minute sampling was due to rapid depletion of H2O2
from the reaction solution. However subsequent monitoring of the
[H2O2] concentration via permanganate titration of aliquots of the
aqueous phase in a separate run (shown also in Fig. 2) showed only
ð8Þ

286
B. McAteer et al. / Inorganic Chemistry Communications 35 (2013) 284–289
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
10
20
30
40
50
60
Fig. 2. Time dependence of formation of oxidation products (black symbols) at 25 °C following sampling of the cyclohexene layer respectively at various time intervals [32]: A
cyclohexene epoxide; B 2-cyclohexen-1-ol/cyclohexanol; C 2-cyclohexen-1-one. Aqueous layer: [H2O2]init = 10 mM; [Fe(III)]init = 0.2 mM, pH 3.0. The open circles show the
time dependence of the H2O2 concentration in the aqueous layer in a separate blank run under the same conditions. Here 5 cm3 aliquots were removed from a 100 cm3 volume
of the aqueous layer after the set time interval and quenched by injection into a 25 cm3 solution of 0.1 M HClO4 for titration with standard permanganate.
involving the trapping of 2-cyclohexene-1 radicals by dioxygen
present in the air or from the accompanying H2O2 decomposition. Con-
firmation of this was provided by runs conducted air-free wherein the
yields of 2-cyclohexen-1-ol and 2-cyclohexen-1-one are reduced sig-
nificantly, Table 1, compared to the epoxide; which now assumed
N57% of the total products observed at the optimum pH. The flux of
2-cyclohexen-1 radicals most likely derives from iron(II)-promoted
Fenton chemistry involving H atom abstraction through OH radicals
and/or oxo-aquairon(IV). Both species can be readily generated
from [Fe(OH2)5OOH]2+ either via Fe\O homolysis (e.g. Eq. (9) then
Eq. (1)) or O\O homolysis Eq. (10)[34,35]. The most likely pathway is
via Eq. (9) given the presence of high spin iron(III) in [Fe(OH2)5OOH]2+
.
FeðOOHÞ2aqþ→Fe2þ þ HOO·
ð9Þ
ð10Þ
aq
FeðOOHÞ2þ→FeIV
O
þ HO·
2þ
aq
aq
The insensitivity of epoxide formation to the presence of molecu-
lar oxygen suggests that an activated form of H2O2 is responsible
for its formation rather than organoperoxy radicals deriving from
Eq. (8) which have been reported to generate epoxides from alkenes
but in very small yields.
An additional mechanistic possibility is for the H3O+-assisted
O\O heterolysis on [Fe(OH2)5OOH]2+ to take place prior to addition
of the ‘O’ atom across the double bond [36]. Such a reaction would
generate
a ‘perferryl’ oxo-aquairon(V) species, [17,22] the oxo
group of which would be expected to be very much more electro-
philic than oxo-aquairon(IV) or the iron bound peroxooxygen of
[Fe(OH2)5OOH]2+ leading to rapid ‘O’ addition across the cyclohexene
double bond. In polar aprotic solvents in the presence of a number of
2-pyridylmethylamine chelate ligands, evidence has been recently
claimed [37] for the formation of short lived highly reactive perferryl
oxoiron(V) species, generated via H2O2 or peroxycarboxylate oxidation
of their iron(II) counterparts. These iron species have been suggested as
responsible for the catalysis of alkene epoxidation by H2O2 in the
presence of ethanoic acid [38] and the insertion of ‘O’ into unreactive
C\H bonds such as in the formation of salicylate from peroxybenzoate
[39] or H2O2/benzoate [40]. Aqueous iron(V) however has an ex-
tremely short lifetime away from strongly alkaline media (lifetime
b10 μs below pH 4) [41]. So while we cannot directly rule out an oxo
aquairon(V) species as responsible for alkene epoxidation in the
present system, its trapping (reduction) by cyclohexene would require
an extremely fast and efficient process at the solvent interface capable
of competing successfully with its rapid reduction by excess H2O2
Eq. (11).
FeVOa3qþ þ H2O2→Fe3þaq þ O2 þ H2O
ð11Þ
Fig. 3. pH dependence of the yields of oxidation products at 25 °C following sampling
of the cyclohexene organic layer after 2 min: A cyclohexene epoxide; B 2-cyclohexen-
To date the only example of
a
stable spectroscopically-
1-ol/cyclohexanol;
C 2-cyclohexen-1-one. Aqueous layer: [H2O2]init = 10 mM;
characterized oxoiron(V) complex is [Fe(O)(TAML)]− (TAML4−
=
[Fe(III)]init = 0.2 mM. The iron(III) speciation data is from ref. 30a (with permission)
for I = 0.1 M, NaClO4, [Fe(III)] = 0.01 mM.
various deprotonated tetraamido macrocycle ligands) reported in

B. McAteer et al. / Inorganic Chemistry Communications 35 (2013) 284–289
287
deactivation
hydrolytic polymerization
2+
H
H2O2
H2O
H2O + O2
H2O2
O
III
(H2O)4 Fe
[Fe(OH2)5OH]2+
H3O+
O
O
+
H
H
H2O
2+
H
[Fe(OH2)6]3+
O
III
(H2O)4 Fe
H
O
O
O
H
H
O
+
H
+
H2O
H
H
O
III
Fe
(H2O)4
H
O
O
H
H
O
+
H
H
H
H3O+
O
III
O
(H2O)
4Fe
+
H
[Fe(OH2)5OOH]2+
O
+ H2O
O
H
[Fe(OH2)6]3+
H2O2
Fig. 4. Mechanism for cyclohexene epoxidation catalyzed by [Fe(OH2)5OOH]2+
.
2007 by Collins et al. [42] and even here it is stable only in polar apro-
tic solvents at −60 °C.
The rapid appearance of oxidation products observed in this study,
including cyclohexene epoxide, before catalyst deactivation contrasts
with their reported ‘slow’ (hours) appearance in the same pH range in
the earlier published work [27]. However the rapid processes observed
here are entirely consistent with the expected rapid formation and
subsequent reactions of [Fe(OH2)5OOH]2+ with the cyclohexene organ-
ic layer in the stirred two phase system. A slow reaction over several
hours is hard to rationalize given the lability of aqueous iron(III) species
and known rapidity of reactions (1)–(11)[4] and subsequent aging pro-
cesses. The rapid stirring of the two phases performed here (rate
~1000 rpm) is in keeping with the experimental conditions previously
reported [27] so different rates of mass transfer between the two phases
Table 1
Yields of oxidation products in cyclohexene layer following sampling after 2 min as a function of aqueous phase pH. The values in italics are from runs conducted in the absence of
air. [H2O2]init = 10 mM; [Fe(III)]init = 0.2 mM.
pH Moles 2-cyclohexan-1-ol/ Moles 2-cyclohexan-2-one Moles cyclohexene Moles total
% Epoxide in total % Efficiency total products TON total products
cyclohexanol
epoxide
oxidation products products
based on H2O2
based on Fe(III)
2.0 1.12 × 10−4
2.7 3.87 × 10−4
4.48 × 10−5
1.75 × 10−4
5.03 × 10−4
4.71 × 10−5
1.47 × 10−3
5.04 × 10−5
1.88 × 10−3
5.23 × 10−5
1.43 × 10−3
6.25 × 10−4
8.44 × 10−5
1.05 × 10−5
4.66 × 10−5
3.82 × 10−5
1.34 × 10−4
1.05 × 10−4
4.94 × 10−4
1.33 × 10−4
3.45 × 10−4
1.15 × 10−4
1.52 × 10−5
2.98 × 10−4
9.37 × 10−4
1.3 × 10−4
3.5
5.0
29.3
5.9
51.2
13.1
57.1
12.7
11.9
10.7
119
375
52
916
82
1500
93
584
386
57
60
187
26
458
41
752
47
292
193
28
3.0 6.91 × 10−4
4.99 × 10−5
2.29 × 10−3
2.05 × 10−4
3.76 × 10−3
2.33 × 10−4
2.71 × 10−3
9.66 × 10−4
1.42 × 10−4
3.2 1.39 × 10−3
4.67 × 10−5
3.4 9.38 × 10−4
3.6 3.13 × 10−4
4.0 4.27 × 10−5

288
B. McAteer et al. / Inorganic Chemistry Communications 35 (2013) 284–289
(b) F. Chen, Y. Xie, J. He, J. Zhao, Photo-Fenton degradation of a dye in methanolic
solution under both UV and visible irradiation, J. Photochem. Photobiol. A
138 (2001) 139–146;
(c) F. Herrera, J. Kiwi, A. Lopez, V. Nadtochenko, Photochemical decoloration
of Remazol Brilliant Blue and Uniblue A in the presence of Fe3+ and H2O2,
Environ. Sci. Technol. 33 (1999) 3145–3151;
(d) K. Wu, Y. Xie, J. Zhao, H. Hidaka, Photo-Fenton degradation of a dye under
visible light irradiation, J. Mol. Catal. A 144 (1999) 77–84;
(e) K. Matsunaga, S. Yamabe, T. Mori, Photodegradation of polychlorobiphenyls
by titanium dioxide/ferric ion/hydrogen peroxide/UV light system, Jpn. J.
Toxicol. Environ. Health 43 (1997) 174–181;
would seem unlikely as an explanation. Also in the previous work, the
photochemical nature of the Fenton-like catalysis was emphasized. Fol-
lowing runs conducted in daylight, in the dark and under mild UV light
it appears that the profile of product formation in the present system is
not influenced by photo-activation.
For the first time in a Fenton-like system evidence has been
obtained for different timescales for the catalysis of oxidation reac-
tions versus catalysis of H2O2 decomposition. The active oxidation
catalyst is only short lived (≤2 min) following the addition of
fresh [Fe(OH2)6]3+ and thereafter quickly deactivates presumed due
to depletion of [Fe(OH2)5OH]2+/[Fe(OH2)5OOH]2+ from the aqueous
phase. The accompanying catalysis of H2O2 decomposition however
persists over 1–2 h as the iron(III) solution ‘ages’. The development of
(f) J.J. Pignatello, G. Chapa, Degradation in PCBs by ferric ion, hydrogen peroxide
and UV light, Environ. Toxicol. Chem. 13 (1994) 423–427.
[21] (a) N. Masomboon, M.-J. Chao, J. Anotai, M.-C. Lu, Treatment of 2,6-
dimethylaniline by electrochemical-assisted Fenton-Like process, J. Hazard.
Toxicol. Radioactive Waste 16 (2012) 63–67;
(b) A. Sobkowiak, The influence of pH on the catalytic current of the
iron(III)/hydrogen peroxide system, Electrochim. Acta 32 (1987) 1251–1252;
(c) A. Sobkowiak, B. Fleszar, The equation of catalytic current of the ferric
ion-hydrogen peroxide system with regard to hydroxylation, Electrochim.
Acta 26 (1981) 847–850.
methods for maintaining the flux of mononuclear [Fe(OH2)5OH]2+
/
[Fe(OH2)5OOH]2+ in solution over longer periods is the subject of ongo-
ing research.
[22] M.L. Kremer, G. Stein, Catalytic decomposition of hydrogen peroxide by ferric per-
chlorate, Trans. Faraday Soc. 55 (1959) 959–973.
[23] T.J. Lewis, D.H. Richards, D.A. Salter, Peroxy-complexes of inorganic ions in hydrogen
peroxide-water mixtures. I. Decomposition by ferric ions, J. Chem. Soc. (1963)
2434–2446.
[24] M.L. Haggett, P. Jones, W.F.K. Wynne-Jones, Peroxycomplexes as intermediates in
the catalytic decomposition of hydrogen peroxide, Discuss. Faraday Soc. 39
(1960) 153–162.
Acknowledgment
B.N. and N. B. thank the University of St. Andrews, Scotland, UK for
financial support.
[25] P. Jones, R. Kitching, M.L. Tobe, W.F.K. Wynne-Jones, Behavior of mixtures of hy-
drogen peroxide + water. IV. Catalytic decomposition of hydrogen peroxide,
Trans. Faraday Soc. 55 (1959) 79–90.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.inoche.2013.06.018.
[26] M.G. Evans, P. George, N. Uri, [Fe(OH)]+2 and [Fe(O2H)]+2 complexes, Trans. Faraday
Soc. 45 (1949) 230–239.
[27] J.J. Pignatello, D. Liu, P. Huston, Evidence for an additional oxidant in the
photo-assisted Fenton reaction, Environ. Sci. Technol. 33 (1999) 1832–1839.
[28] C.M. Flynn Jr., Hydrolysis of inorganic iron(III) salts, Chem. Rev. 84 (1984)
31–41.
References
[1] H.J.H. Fenton, Oxidation of tartaric acid in presence of iron, J. Chem. Soc. 65
(1894) 889–910.
[29] (a) A. Stefansson, Iron(III) hydrolysis and solubility at 25 °C, Environ. Sci.
Technol. 41 (2007) 6117–6123;
[2] F. Haber, J. Weiss, The catalytic decomposition of hydrogen peroxide by iron salts,
Proc. Roy. Soc. London A147 (1934) 332–351.
[3] W.G. Barb, J.H. Baxendale, P. George, K.R. Hargrave, Reactions of ferrous and ferric
ions with hydrogen peroxide. II. Ferric-ion reaction, Trans. Faraday Soc. 47 (1951)
591–616.
(b) R.H. Byrne, Y.-R. Luo, R.W. Young, Iron hydrolysis and solubility revisited:
observations and comments on iron hydrolysis characterizations, Mar.
Chem. 70 (2000) 23–35.
[30] (a) M.W. Grant, R.B. Jordan, Kinetics of solvent water exchange on iron(III),
Inorg. Chem. 20 (1981) 55–60;
[4] C. Walling, Fenton's reagent revisited, Acc. Chem. Res. 8 (1975) 125–131.
[5] J.W. Moffett, R.G. Zika, Reaction kinetics of hydrogen peroxide with copper and
iron in seawater, Environ. Sci. Technol. 21 (1987) 804–810.
[6] B.C. Faust, Photochemistry of clouds, fogs, and aerosols, Environ. Sci. Technol. 28
(1994) 216A–222A.
[7] J.J. Pignatello, Dark and photoassisted iron(3+)-catalyzed degradation of
chlorophenoxy herbicides by hydrogen peroxide, Environ. Sci. Technol. 26
(1992) 944–951.
[8] Y. Sun, J.J. Pignatello, Photochemical reactions involved in the total mineralization
of 2,4-D by iron(3+)/hydrogen peroxide/UV, Environ. Sci. Technol. 27 (1993)
304–310.
(b) T.W. Swaddle, A.E. Merbach, High-pressure oxygen-17 Fourier transform
nuclear magnetic resonance spectroscopy. Mechanism of water exchange
on iron(III) in acidic aqueous solution, Inorg. Chem. 20 (1981) 4212–4216.
[31] (a) O. Pestovsky, A. Bakac, Aqueous ferryl(IV) ion: kinetics of oxygen atom trans-
fer to substrates and oxo exchange with solvent water, Inorg. Chem. 45
(2006) 814–820;
(b) O. Pestovsky, S. Stoian, E.L. Bominaar, X. Shan, E. Munck, L. Que Jr., A. Bakac,
Aqueous FeIV = O: spectroscopic identification and oxo-group exchange,
Angew. Chem. Int. Ed. 44 (2005) 6871–6874.
(c) A. Bakac, private communication.
[32] O. Pestovsky, A. Bakac, Reactivity of aqueous Fe(IV) in hydride and hydrogen
atom transfer reactions, J. Am. Chem. Soc. 126 (2004) 13757–13764.
[33] The catalytic runs were conducted in stirred (1000 rpm) thermostated 100 cm3
[9] R. Chen, J.J. Pignatello, Role of quinone intermediates as electron shuttles in
Fenton and photo-assisted Fenton oxidation of aromatic compounds, Environ.
Sci. Technol. 31 (1997) 2399–2406.
conical flasks at 25.0
0.1 °C open to the air. The two phase system comprised
[10] R. Bauer, H. Fallmann, The photo-Fenton oxidation — a cheap and efficient waste-
water treatment method, Res. Chem. Intermed. 23 (1997) 341–354.
[11] A. Safarzadeh-Amiri, J.R. Bolton, S.R. Cater, Ferrioxalate-mediated solar degrada-
tion of organic contaminants in water, Sol. Energy 56 (1996) 439–443.
[12] R. Watts, S. Kong, M. Dippre, W.T.J. Barnes, Oxidation of sorbed hexachlorobenzene
in soils using catalyzed hydrogen peroxide, Hazard. Mater. 39 (1994) 33–47.
[13] B.N. Aronstein, R.A.L. Lawal, A. Maka, Chemical degradation of cyanides by
Fenton's reagent in aqueous and soil-containing systems, Environ. Toxicol.
Chem. 13 (1994) 1719–1726.
[14] J.J. Pignatello, M. Day, Mineralization of methyl parathion insecticide in soil by hy-
drogen peroxide activated with iron(III)-NTA or -HEIDA complexes, Hazard.
Waste Hazard. Mater. 13 (1996) 237–244.
[15] D.T. Sawyer, A. Sobkowiak, T. Matsushita, Metal [MLx; M = Fe, Cu, Co,
Mn]/hydroperoxide-induced activation of dioxygen for the oxygenation of hy-
drocarbons: oxygenated Fenton chemistry, Acc. Chem. Res. 29 (1996) 409–416.
[16] T.J. Conocchioli, E.J. Hamilton Jr., N. Sutin, Formation of iron(IV) in the oxidation
of iron(II), J. Am. Chem. Soc. 87 (1965) 926–927.
[17] M.L. Kremer, “Complex” versus “free radical” mechanism for the catalytic decompo-
sition of hydrogen peroxide by ferric ions, Int. J. Chem. Kinet. 17 (1985) 1299–1314.
[18] D.A. Wink, R.W. Nims, M.F. Desresiers, P.C. Ford, L.K. Keefer, A kinetic investiga-
tion of intermediates formed during the Fenton reagent mediated degradation
of N-nitrosodimethylamine: evidence for an oxidative pathway not involving hy-
droxyl radical, Chem. Res. Toxicol. 4 (1991) 510–512.
[19] C. Walling, K. Amarnath, Oxidation of mandelic acid by Fenton's reagent, J. Am.
Chem. Soc. 104 (1982) 1185–1189.
[20] (a) S. Yang, B. Tie, T. Zhao, Z. Qian, J. Sun, UV/Fe3+/H2O2 oxidation-coagulation
process for treatment of landfill leachate, Gongye Cuihua. 15 (2007) 48–52;
a 25 cm3 aqueous layer containing a 0.2 mM solution of iron(III) adjusted to
the appropriate pH with diluted aqueous HClO4 and maintained at a constant
ionic strength of 0.1 M with NaClO4. The pH was measured using a standard
glass electrode calibrated with accurately diluted solutions of standardized aque-
ous HClO4 (I = 0.1 M, NaClO4). The runs were conducted in unbuffered solutions
to avoid complexation of Fe(III) by buffer anions. Only a small pH drift of ~0.1
units lower was observed over the first 30 minutes. The iron(III) was always di-
luted from freshly made up stock solutions of [Fe(OH2)6](ClO4)3 (Aldrich) in
1.0 M HClO4. The use of fresh iron(III) solutions for each run was vital to avoid
rapid loss of activity through hydrolytic polymerization and catalysis of H2O2 de-
composition. The organic layer consisted of 25 cm3 of cyclohexene (Aldrich). The
purity of the cyclohexene was checked by GC. The reactions were initiated by the
injection of 0.5 cm3 of a 0.5 M aqueous H2O2 solution supplying a 10 mM initial
concentration. The H2O2 concentration was determined by standard permanga-
nate titration. After the set time interval a 2 cm3 sample from the catalytic run
was removed from the organic layer and quenched by adding to 0.5 g anhydrous
sodium carbonate (Aldrich) in a small sample vial. Adjustments were made for
the amounts of sample removed in computing the relative yields of products in
the bulk organic layer at each subsequent sampling time. A 0.5 M solution of
cyclooctane (0.5 cm3) in cyclohexene was added to each 2 cm3 sample to act as
an internal reference before
a 1 μl sample was removed for GC analysis. A
Hewlett-Packard 6890 gas chromatograph fitted with a 5973-MSD (mass selective
detector) was used for the GC analysis. A 30 m column of dimethylpolysiloxane
served as the stationary phase. The temperature program adopted was 50 °C for
2 minutes then 130 °C for a further 2 minutes and finally a ramp to 260 °C for a fur-
ther 6 minutes. Helium flow gas was used at a pressure of 55 kPa and a flow rate of
1.0 cm3 min−
.

B. McAteer et al. / Inorganic Chemistry Communications 35 (2013) 284–289
289
[34] (a) W.C. Bray, M.H. Gorin, Ferryl ion, a compound of quadrivalent iron, J. Am.
Chem. Soc. 54 (1932) 2124–2125;
(d) A. Bassan, M.R.A. Blomberg, P.E.M. Siegbahn, L. Que Jr., A density functional
study on a biomimetic non-heme iron catalyst: insights into alkane hydrox-
(b) B. Ensing, F. Buda, P.E. Blöchl, E.J. Baerends, A Car-Parrinello study of the for-
mation of oxidizing intermediates from Fenton's reagent in aqueous solu-
tion, Phys. Chem. Chem. Phys. 4 (2002) 3619–3627;
(c) B. Ensing, F. Buda, P.E. Blöchl, E.J. Baerends, Chemical involvement of solvent
water molecules in elementary steps of the Fenton oxidation reaction,
Angew. Chem. Int. Ed. 40 (2001) 2893–2895.
ylation by a
formally HO–FeV = O oxidant, Chem. Eur. J. 11 (2005)
692–705;
(e) K. Chen, L. Que Jr., Stereospecific alkane hydroxylation by non-heme iron
catalysts: mechanistic evidence for an FeVO active species, J. Am. Chem. Soc.
123 (2001) 6327–6337.
[38] R. Mas-Balleste, L. Que Jr., Iron-catalyzed olefin epoxidation in the presence of
acetic acid: insights into the nature of the metal-based oxidant, J. Am. Chem.
Soc. 129 (2007) 15964–15972.
[39] N.Y. Oh, M.S. Seo, M.L. Lim, M.B. Consugar, M.J. Park, J.-U. Rohde, J. Han, K.M. Kim,
J. Kim, L. Que Jr., W. Nam, Self-hydroxylation of perbenzoic acids at a nonheme
iron(II) center, Chem. Commun. (2005) 5644–5646.
[40] (a) O.V. Makhlynets, P. Das, S. Taktak, M. Flook, R. Mas-Balleste, E.V.
Rybak-Akimova, L. Que Jr., Iron-promoted ortho- and/or ipso-hydroxylation
of benzoic acids with H2O2, Chem. Eur. J. 15 (2009) 13171–13180;
(b) S. Taktak, M. Flook, B.M. Foxman, L. Que Jr., E.V. Rybak-Akimova,
Ortho-hydroxylation of benzoic acidswith hydrogen peroxide at a nonheme
iron center, Chem. Commun. (2005) 5301–5303.
[41] J.D. Rush, B.H.J. Bielski, Decay of Ferrate(V) in neutral and acidic solutions. A pre-
mix pulse radiolysis study, Inorg. Chem. 33 (1994) 5499–5502.
[42] F. Tiago de Oliveira, A. Chanda, D. Banerjee, X. Shan, S. Mondal, L. Que Jr., E.L.
Bominaar, E. Münck, T.J. Collins, Chemical and spectroscopic evidence for an
FeV-Oxo complex, Science 315 (2007) 835–838.
[35] B. Ensing, F. Buda, E.J. Baerends, Fenton-like chemistry in water: oxidation catal-
ysis by Fe(III) and H2O2, J. Phys. Chem. A 107 (2003) 5722–5731.
[36] D. Quinonero, K. Morokuma, D.G. Musaev, R. Mas-Balleste, L. Que Jr., Metal-
peroxo versus metal-oxo oxidants in non-heme iron-catalyzed olefin oxidations:
computational and experimental studies on the effect of water, J. Am. Chem. Soc.
127 (2005) 6548–6549.
[37] (a) O.Y. Lyakin, K.P. Bryliakov, E.P. Talsi, EPR, 1H and 2H NMR, and reactivity
studies of the iron oxygen intermediates in bioinspired catalyst systems,
Inorg. Chem. 50 (2011) 5526–5538;
(b) O.Y. Lyakin, K.P. Bryliakov, G.J.P. Britovsek, E.P. Talsi, EPR spectroscopic trap-
ping of the active species of nonheme iron-catalyzed oxidation, J. Am. Chem.
Soc. 131 (2009) 10795–10799;
(c) E.A. Duban, K.P. Brylyakov, E.P. Talsi, The nature of active species in cata-
lytic systems based on non-heme iron complexes, hydrogen peroxide, and
acetic acid for selective olefin epoxidation, Kinet. Catal. 49 (2008)
375–385;