Oxidation of aromatic compounds by hydrogen peroxide catalyzed by mononuclear iron(III) complexes

Article abstract of DOI:10.1016/j.molcata.2016.08.037

In the present work, four mononuclear iron(III) complexes containing BMPA (BMPA?=?bis-(2-pyridylmethyl)amine) and derivative ligands, have been studied as catalyst in toluene oxidation, at 25?°C and 50?°C, using hydrogen peroxide as oxidant and acetonitrile as solvent. All catalysts were able to oxidize toluene with satisfactory yields, producing o-, m-, p-cresols, benzaldehyde and benzyl alcohol, as main products, and traces of 2-methylbenzoquinone and benzoic acid. The catalyst [Fe(BMPA)Cl₃] presented the most promising results, reaching yields up to 30.2% at 50?°C after 24?h. Furthermore, [Fe(BMPA)Cl₃] was applied in the oxidation of other aromatic compounds as benzene, ethylbenzene, cumene, n-propylbenzene, p-xylene and anisole. The reaction with H₂O₂ was monitored by electronic UV–vis spectroscopy in the presence and absence of toluene and its oxidation products, as well as by ESI-(+)-MS/Q-TOF mass spectrometry, in order to provide some information about the reaction mechanism.

Full text of DOI:10.1016/j.molcata.2016.08.037

Accepted Manuscript
Title: Oxidation of aromatic compounds by hydrogen peroxide
catalyzed by mononuclear iron(III) complexes
´
Author: Giselle C. Silva Nakedia M.F. Carvalho Adolfo Horn
Jr. Elizabeth R. Lachter Octavio A.C. Antunes
PII:
S1381-1169(16)30372-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.08.037
MOLCAA 10024
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
29-4-2016
30-8-2016
31-8-2016
´
Please cite this article as: Giselle C.Silva, Nakedia M.F.Carvalho, Adolfo Horn,
Elizabeth R.Lachter, Octavio A.C.Antunes, Oxidation of aromatic compounds by
hydrogen peroxide catalyzed by mononuclear iron(III) complexes, Journal of Molecular
Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.08.037
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Oxidation of aromatic compounds by hydrogen peroxide catalyzed by
mononuclear iron(III) complexes
Giselle C. Silvaa,b, Nakédia M. F. Carvalhoc,*, Adolfo Horn Jr.d, Elizabeth R. Lachtera,*,
Octavio A. C. Antunesa,§
a Universidade Federal do Rio de Janeiro, Instituto de Química, Avenida Athos da Silveira
Ramos, 149, Bloco A, sala 617, Cidade Universitária, Rio de Janeiro, 21941-909, RJ,
Brazil
b Centro Federal de Educação Tecnológica Celso Suckow da Fonseca. Avenida Maracanã,
229, Maracanã, Rio de Janeiro, 20271-110, RJ, Brazil
c Universidade do Estado do Rio de Janeiro, Instituto de Química, Rua São Francisco
Xavier, 524, sala 407, Maracanã, Rio de Janeiro, 20550-013, RJ, Brazil
d Universidade Estadual do Norte Fluminense Darcy Ribeiro, Laboratório de Ciências
Químicas, Avenida Alberto Lamego, 2000, Parque Califórnia, Campos dos Goytacazes,
28013-602, RJ, Brazil
* nakedia@uerj.br, +55-21-2334-0563; lachter@iq.ufrj.br, +55-21-2562-7140
Highlights
- Iron(III) complexes were studied as catalysts in aromatic hydrocarbons oxidation
- Toluene was oxidized in yields up to 30%, forming cresols as major products
- The reactivity against hydrogen peroxide was followed by UV-Vis and ESI-MS
- Iron-hydroperoxide intermediate was proposed as the active catalytic species
- o-cresol and p-cresol interacts with the intermediate forming a yellow chromophore
Graphical abstract
fx1
§ in memoriam
1

Abstract
In the present work, four mononuclear iron(III) complexes containing BMPA (BMPA
= bis-(2-pyridylmethyl)amine) and derivative ligands, have been studied as catalyst in
toluene oxidation, at 25 °C and 50 °C, using hydrogen peroxide as oxidant and acetonitrile
as solvent. All catalysts were able to oxidize toluene with satisfactory yields, producing o-,
m-, p-cresols, benzaldehyde and benzyl alcohol, as main product, and traces of 2-
methylbenzoquinone and benzoic acid. The catalyst [Fe(BMPA)Cl3] presented the most
promising results, reaching yields up to 30.2 % at 50 °C after 24 hours. Furthermore,
[Fe(BMPA)Cl3] was applied in the oxidation of other aromatic compounds as benzene,
ethylbenzene, cumene, n-propylbenzene, p-xylene and anisole. The reaction with H2O2 was
monitored by electronic UV-Vis spectroscopy in the presence and absence of toluene and its
oxidation products, as well as by ESI-(+)-MS/Q-TOF mass spectrometry, in order to provide
some information about the reaction mechanism.
Keywords: toluene oxidation, iron(III) complexes, hydrogen peroxide, iron-peroxo
intermediate
2

1. Introduction
The selective functionalization of aromatic hydrocarbons is an important task for the
industry, nevertheless the cleavage of C-H bond in large scale is still a challenge, judging by
the high temperature and pressure required in the actual industrial processes [1-3]. From the
environmental and economic perspective, the development of cleaner synthetic routes to
provide industry feedstocks through a green chemistry approach, involves the design of
efficient catalysts [4,5]. Oxygenated hydrocarbons are important intermediates for chemicals,
agrochemicals, fragrances, pharmaceuticals and polymers manufacture. The industrial
processes usually employ homogeneous catalysts as Cr(VI), Mn(VII) or Os(VII) complexes,
with the inconvenient formation of toxic metallic side products. In this way, cleaner catalytic
systems that can operate at mild conditions are desirable [6].
Non-heme iron(III) complexes inspired in enzymes as methane monooxygenase [7-
13] and toluene 4-monooxygenase [14-16] have been studied as catalysts in the selective
hydrocarbon oxidation, nevertheless, there are still a few reports on the selective oxidation
of aromatic compounds. The iron(III) center in these synthetic models are usually
coordinated to polydentade ligands carrying nitrogen and oxygen donor atoms [17].
Molecular oxygen, hydrogen peroxide and tert-butyl hydroperoxide have been used as
oxidant, however, due to the lower yield usually obtained with the former, peroxides have
been more extensively employed [18-20]. Besides the higher yields obtained with H2O2, its
side product H2O is environmental friendly compared to classical strong oxidants, as
chromium compounds.
In the context of biomimetic models for selective hydrocarbon oxidation, we have
reported the application of iron(III) complexes, coordinated to the ligand BMPA (BMPA =
3

bis-(2-pyridylmethyl)amine) and its derivates, as catalysts in the cyclohexane oxidation using
H2O2, in acetonitrile, and satisfactory yields were achieved [21]. In this work, the catalysts
[Fe(BMPA)Cl3]
(1),
[Fe(MPBMPA)Cl3]
(2),
[Fe(PBMPA)Cl2]
(3)
and
[Fe(PABMPA)Cl2](ClO4) (4) (where: MPBMPA = N-methylpropanoate-N,N-bis-(2-
pyridylmethyl)amine), PBMPA = N-propanoate-N,N-bis-(2-pyridylmethyl)amine and
PABMPA = N-propanamide-N,N-bis-(2-pyridylmethyl)amine)) were investigated in the
oxidation of toluene and other aromatic compounds, such as ethylbenzene, cumene, n-
propylbenzene, p-xylene and anisole. The reaction of the catalysts with H2O2 was monitored
by electronic UV-Vis spectroscopy in presence and absence of toluene and its oxidation
products, as well as by ESI-(+)-MS/Q-TOF mass spectrometry, in order to provide
information about the reaction mechanism.
2. Experimental
2.1. Materials
Toluene, ethylbenzene, n-propylbenzene, p-xylene and anisole (>98% of purity) were
used as received. Cumene was purified by distillation before handling. Commercial 30%
aqueous H2O2 solution was titrated by the iodometric method before use and the determined
concentration was 10.30 mol L-1. Acetonitrile was used as solvent. The catalysts
[Fe(BMPA)Cl3]
(1),
[Fe(MPBMPA)Cl3]
(2),
[Fe(PBMPA)Cl2]
(3)
and
[Fe(PABMPA)Cl2](ClO4) (4) were synthesized according to the literature [22,23].
2.2. Analytic methods
4

The oxidation products were analyzed in a Varian 3800 gas chromatographer with
FID detector and DB-1 column. The analysis conditions were: Tinlet = 220 °C, Tdetector = 250
°C. For toluene reaction analysis the temperature was raised from 50 °C to 55 °C at rate =
1.5 °C/min, then from 55 °C to 82 °C at rate = 10 °C/min, isothermal at 82 °C for 5 min, and
then from 82 to 200 °C at rate = 20 °C/min. For ethylbenzene reaction analysis the
temperature was raised from 50 °C to 70 °C at rate = 1.5 °C/min, then from 70 to 100 °C at
rate = 10 °C/min, and from 100 to 200 °C at rate = 20 °C/min. The analysis of the other
aromatic compundss started at 50 °C to 55 °C at rate = 1.5 °C/min, then from 55 °C to 100
°C at rate = 10 °C/min, and from 100 to 200 °C at rate = 20 °C/min. External calibration
technique was carried out for quantification, where calibration curves for all substrates and
products were used to determine the respective response factor. For a reliable quantification
of the alkyl hydroperoxides, the samples were analyzed before and after addition of an excess
of solid PPh3, which reduces of the hydroperoxide to the corresponding alcohol, since this
product can be partially decomposed in the detector. This method allows the calculation of
the real yield of hydroperoxide [24]. Typical chromatogram is shown in Figures S1-S2. The
products were confirmed by GC/MS analyses, carried out in a Shimadzu GCMS-QP2010S
with electron impact ionization of 70 eV and DB-5 column.
UV-Vis spectra were recorded on a Shimadzu 1601PC UV-Vis spectrophotometer
using a quartz cuvette in acetonitrile solution. ESI-(+)-MS/Q-TOF mass spectrometry
experiments were carried out in a MicrOTOF-Q II equipment from Bruker Daltonics, at
capillary voltage of 4.0 kV in positive ion polarity, in acetonitrile solution.
2.3. Oxidation reactions
5

Initially, the catalytic efficiency of the complexes was investigated towards toluene
oxidation at 25 °C or at 50 °C, in acetonitrile, and the best condition was applied to the other
substrates. A typical reaction was carried out in a 25 mL round-bottomed flask, where 7.7
mol of catalyst; 7.7 mmol of substrate; 7.7 mmol of H2O2 and CH3CN to complete 10 mL
of volume, were sequentially mixed and stirred under argon atmosphere, under a
thermostated bath. The reaction was transferred to a separatory funnel and quenched by
adding 20 mL of aqueous 0.4 mol L-1 Na2SO4 solution, followed by extraction with 10 mL
of diethyl ether. The ether layer was dried over anhydrous Na2SO4 and analyzed by GC-FID.
The progress of the reaction over time was followed in a reaction volume of 35 mL, where
aliquots of 5 mL were taken at 1 h intervals during the first 5 h and one last sample after 24
h, and quenched as described above.
The reactions were also followed by UV-Vis spectroscopy employing an acetonitrile
solution containing the catalysts at concentration of 1.010-4 mol L-1 or 1.010-3 mol L-1. For
the reactions with H2O2, 100 equivalents of H2O2 were added. For the reactions in presence
of toluene or the oxidized products (o-cresol, p-cresol, benzaldeyde, benzyl alcohol, benzyl
acid, 2-methylbenzoquinone) 100 equivalents of the respective product were added.
For ESI-(+)-MS/Q-TOF studies, the following amounts of reactants were used:
[Fe(BMPA)Cl3] 1.010-3 mol L-1, H2O2 1.010-2 mol L-1, o-cresol 1.010-2 mol L-1 in
CH3CN/H2O.
3. Results and discussion
3.1 Oxidation reactions
6

The catalysts [Fe(BMPA)Cl3] (1), [Fe(MPBMPA)Cl3] (2), [Fe(PBMPA)Cl2] (3) and
[Fe(PABMPA)Cl2](ClO4) (4), represented in Figure 1, were tested in toluene oxidation,
resulting in the products shown in Scheme 1. Benzaldehyde, benzyl alcohol, 2-
methylbenzoquinone, cresols, benzyl hydroperoxide and benzoic acid, were identified in the
reaction. The catalytic activity of the complexes was investigated at 25 °C and 50 °C, and
higher yields were achieved at 50 °C for all the catalysts. The results for 24 hours are
summarized in Table 1.
Figure 1. Representation of the catalysts 1-4.
Scheme 1. Toluene oxidation reaction promoted by the catalysts 1-4.
7

The reactions at 25 °C were more selective to cresols, in special to o-cresol which
accounts for 30% of all products, followed by benzaldehyde (12%) and 2-
methylbenzoquinone (2%). 2-methylbenzoquinone is formed from oxidation of o-cresol [26],
and its formation is favored at lower temperatures [27]. Traces of benzoic acid were also
detected as product.
Cresols production can be associated with the presence of hydroxyl radicals, which
can be formed via a radicalar mechanism through autoxidation process [25] or to a highly
electrophilic oxo-metal transient reacting with the arene π-system [16]. Interestingly, the
reactions at 50 °C were more selective to benzaldehyde and presented only traces of 2-
methylbenzoquinone and benzoic acid. The decrease in the amount of benzyl hydroperoxide
and benzyl alcohol with concomitant increase of benzaldehyde, in relation to the reaction at
25 °C, suggested that benzaldehyde was formed from the oxidation of these products. The
increase of benzaldehyde selectivity with temperature has been observed by other authors,
nevertheless in higher temperatures benzaldehyde was further oxidized to benzoic acid
[19,28].
8

Table 1. Results for toluene oxidation promoted by catalysts 1-4 H2O2 in CH3CN after 24 h.
Yield (%)/Selectivity (%)a
Cat T (°C)
Cresols
Benzyl
alcohol
2-Methyl
Benzoic
Acid
Benzyl
TONf
Benzaldehyde
Total
benzoquinone
Hydroperoxide
o-cresol
m-cresol + p-cresol
1b
1b
1c
1d
1e
2b
2b
2c
3b
3b
3c
4b
4b
4c
25
50
50
50
50
25
50
50
25
50
50
25
50
50
3.3 / 14
9.6 / 31
6.9 / 24
3.5 / 13
0.3 / 8
1.1 / 5
3.3 / 11
3.3 / 12
3.1 / 11
0.1 / 3
2.5 / 10
0.1 / 0.3
-
0.5 / 2
1.5 / 6
1.3 / 4
0.1 / 0.3
-
7.9 / 33
7.3 / 30
8.3 / 27
8.6 / 30
9.7 / 36
1.6 / 44
5.3 / 30
6.1 / 21
6.6 / 22
3.8 / 29
5.1 / 21
6.7 / 30
6.3 / 27
4.9 / 21
5.4 / 26
24.1
31.1
28.4
27.2
3.6
240
310
284
273
36
-
8.5 / 27
9.5 / 33
10.9 / 40
1.6 / 44
5.9 / 33
7.7 / 26
8.2 / 28
4.1 / 31
5.2 / 22
7.6 / 34
7.3 / 31
5.5 / 23
6.9 / 33
-
-
-
-
-
-
2.1 / 12
10.5 / 36
9.9 / 33
1.4 / 11
9.1 / 38
4.9 / 22
3.1 / 13
9.3 / 39
6.1 / 29
0.7 / 4
1.4 / 8
0.2 / 0.7
0.2 / 0.7
1.1 / 8
0.3 / 1
-
0.2 / 1
0.4 / 1.5
-
2.3 / 13
0.4 / 1.5
0.6 / 2
2.3 / 17
0.0 / 0.0
0.2 / 0.9
2.8 / 12
0.5 / 2
-
17.9
29.2
29.8
13.2
23.7
22.2
23.5
23.9
20.8
179
291
297
131
237
222
234
239
208
3.9 / 13
4.3 / 14
0.4 / 3
0.1 / 0.8
0.7 / 3
-
3.3 / 14
2.8 / 13
0.9 / 4
2.6 / 11
0.3 / 1
-
0.5 / 2
0.2 / 0.8
-
3.2 / 13
2.4 / 12
a Yields are based on toluene. No product was identified in the blank tests performed in absence of catalyst.
b
Reaction condition: toluene 0.77 mol L-1, catalyst 7.710-3 mol L-1, H2O2 0.77 mol L-1, acetonitrile, argon
athosphere, 24 h.
c Addition of another aliquot of H2O2 at 4 h (catalyst 1, 3 and 4) or 24 h (catalyst 2).
d Addition of 2 equivalents of H2O2 in relation to toluene.
e Addition of 3 equivalents of H2O2 in relation to toluene..
f TON = (nproducts/ncatalyst)100.
Figure 2 shows the reaction progress over time for catalyst 1 at 50 oC (data at 25 oC
and for catalysts 2-4 are shown in Figures S3-S9). Figure 3 compares the total yield progress
o
over time for all catalysts at 50 C. After 5 hours the catalysts 1, 2 and 3 achieved similar
values, around 23 %, while 4 reached 9%, but at 25 oC (Figure S10) the total yield was 20,
10, 6 and 1.5%, for 1, 4, 3 and 2, respectively. The corresponding initial reaction rates W0
9

are presented in Table S1, all catalysts presented higher W0 at 50 °C. The catalytic activity
order in the beginning of the reaction was 1 > 4 > 2 > 3 at 25 °C and 1 > 3 > 4 > 2 at 50 oC.
Similar trend was observed for these catalysts in the oxidation of cyclohexane at 25 °C [21].
In an attempt to increase the total yield, a new batch of H2O2 was added after 4 hours (catalyst
1, 3 and 4) or 24 hours (catalyst 2) of reaction and no significant improvement was observed,
what can indicate the deactivation or decomposition of the catalysts.
For catalyst 1, reactions employing 2 or 3 equivalents of H2O2 in relation to toluene
were carried out, as shown in Table 1. The better condition was obtained with 1 equivalent
of oxidant and a change in color was observed, from yellow to orange. The yield was
considerably reduced with higher amounts of H2O2, in this case the color was changed from
yellow to colorless, probably due to the faster decomposition of the catalyst with peroxide
excess. Besides that, these complexes usually present catalase-like activity [25], which is
accelerated by higher amounts of H2O2, leading to faster peroxide and catalyst
decomposition.
10

25
20
15
10
5
Total yield
o-cresol
m-cresol + p-cresol
benzaldehyde
2-methylbenzoquinone
benzyl hydroperoxide
benzyl alcohol
benzoic acid
0
0
1
2
3
4
5
Time (h)
Figure 2. Reaction progress for toluene oxidation with the catalyst [Fe(BMPA)Cl3] (1),
H2O2, in CH3CN, catalyst : H2O2 : substrate = 1:1000:1000, at 50 °C.
25
20
[Fe(BMPA)Cl3] (1)
15
[Fe(MPBMPA)Cl3] (2)
[Fe(PBMPA)Cl2] (3)
10
[Fe(PABMPA)Cl2](ClO4) (4)
5
0
0
1
2
3
4
5
Time (h)
Figure 3. Total yield progress for toluene oxidation with H2O2, in CH3CN, catalyst : H2O2 :
substrate = 1:1000:1000, at 50 °C.
11

In order to quantify properly the hydroperoxides, GC analyses were carried out before
and after treatment with PPh3 [24]. Comparison of the chromatograms showed the
disappearance of the peak corresponding to the hydroperoxide, with the increase of the peak
of the corresponding alcohol after addition of PPh3, indicating that all the hydroperoxide was
reduced. The presence ofbenzyl hydroperoxide suggested a side-chain oxidation mechanism,
where the hydroxyl and/or hydroperoxyl radicals were responsible to abstract the hydrogen
atoms from alkanes [18].
The catalytic activity of complex 1, which showed the most promising results at 50
°C, was also evaluated toward other aromatic compounds as benzene, ethylbenzene, cumene,
p-xylene, n-propylbenzene and anisole (Table 2).
12

Table 2. Results for the aromatic hydrocarbon oxidation with complex 1, with H2O2, in
CH3CN, at 50 °C, after 24 h.a
Entry
1
Substrate
Products (%)/ Selectivity(%)b
Others
Products
Total
yield (%)
-
29.7
30.0
29.7 / 100
2
0.9
1.5
-
0.4 / 1
1.3 / 4
7.0 / 23
9.3 / 31
1.7 / 6
2.5 / 8
2.8 / 9
4.1 / 14
3
4
5
20.2
16.0
2.0/10
1.7/8
2.5/12
6.6/33
2.3/11
3.6/18
0.4 / 2
6.7 / 42
3.5 / 22
1.3 / 8
4.1 / 26
0.6
31.7
7.6
10.3 / 32
4.1 / 13
2.6 / 34
3.7 / 12
5.4 / 17
2.2 / 7
4.1 / 13
0.5 / 1
0.8/2
6
-
4.5 / 59
0.5 / 7
a Reaction condition: substrate 0.77 mol L-1, catalyst 7.710-3 mol L-1, H2O2 0.77 mol L-1, acetonitrile, argon
atmosphere, 24 h.
b Yields are based on substrate. No product was identified in the blank tests performed in absence of catalyst.
The benzene produced only phenol as product, reaching a yield of 29.7% (entry 1).
Other authors have also described similar selectivity [29].
Acetophenone, 1-phenylethanol, 1-phenylethylhydroperoxide and traces of
benzaldehyde, phenylacetaldehyde and ortho-, metha- and para-ethylphenol, were obtained
from the oxidation of ethylbenzene, with 30% of total yield (entry 2). The same product
profile was already described in the literature [29].
13

n-Propylbenzene oxidation furnished 20.2 % of yield, where o-propylphenol was the
main product, followed by 1-phenylpropan-1-one, 1-phenylpropan-1-ol, p-propylphenol, 1-
phenylpropan-2-one and 1-phenylpropan-2-ol (entry 3). p-Xylene oxidation reached 16.0 %
of total yield (entry 4) and its main product was 2,5-dimethyl-2,5-cyclohexen-1,4-dione,
probably obtained from 2,5-xylenol oxidation. 4-Methylbenzaldehyde was formed from 4-
methylbenzyl alcohol oxidation and p-cresol was formed from ipso-attack in the
hydroxylation of p-xylene. Anisole oxidation furnished the lowest yields, only 7.6 % (entry
6), with phenol being obtained from anisole demethylation and the other products from ring
hydroxylations. In cumene oxidation, cumyl hydroperoxide, phenol and acetophenone were
formed, with traces of 2-phenyl-2-propanol, 2-phenylpropanal, 2-phenyl-1-propanol and
ortho-, metha- and para-cumenol (entry 5).
The formation of hydroperoxides in the reaction of ethylbenzene and cumene
suggests a radicalar mechanism. When n-propylbenzene, p-xylene and anisole were oxidized,
no hydroperoxide was observed. All reactions suggest a competition between ring
hydroxylation (mainly, to ortho and para positions) and benzylic oxidation [29]. In
ethylbenzene, cumene and n-propylbenzene oxidations, the major products were those
obtained from benzylic oxidation, with only 23, 20 and 44 % of selectivity to ring
hydroxylation products, respectively. Nevertheless, in p-xylene and anisole oxidations, the
ring hydroxylation products were obtained preferentially with selectivity of 76 and 66 %,
respectively. In general, the pathway leading to ring hydroxylation was more favored than
that to benzylic oxidation, judging by the high yields obtained for the ortho, meta and para
products.
The high yields obtained in the oxidation of benzene can be associated to its six
positions suitable for hydroxylation. Ethylbenzene has 5 positions for ring hydroxylation
14

beyond the α-carbon suitable for oxidation. Similarly, cumene possesses positions available
for ring and benzylic oxidation, and a variety of products is obtained because the ramified
chain stabilizes the radicals formed, especially in the α-position [29]. The side chain radical
stabilization is less favored in n-propylbenzene because the linear chain decreases its
reactivity. The ring hydroxylation in p-xylene oxidation is less favorable because it has only
four C-H bonds available for oxidation. The anisole oxidation furnished the lowest yields,
which may be due to the electron withdrawing effect of the methoxy group. The order of
substrate reactivity was: benzene ≥ cumene ≥ ethylbenzene > n-propylbenzene > p-xylene >
anisole.
3.2. Reactivity of the complexes toward H2O2 followed by electronic UV-Vis spectroscopy
It was possible to observe a color change during the oxidation reactions from yellow
to orange, probably due to the formation of an oxygenated intermediate species, such as
FeIII(OOH) or FeIV(O), or from coordination of an oxidized product to the iron center, either
to the free complex or to the oxygenated intermediate. To address this question, the reaction
of the complexes with H2O2 in CH3CN, at 25 °C, was followed bt UV-Vis spectroscopy, in
absence as well as in presence of toluene and some representative oxidation products.
Catalyst 1 spectrum (Figure 4-solid line) presents a band at 253 nm attributed to
pyridine intraligand CT transition π→π*, and two other bands at 290 and 379 nm attributed
to LMCT [23]. After addition of 100 equivalents of H2O2 to 1, the band at 379 nm was blue-
shifted to 359 nm and bands at 312 and 269 were formed (Figure 4-dashed blue line). The
spectra presented two isosbestic points at ~430 and ~370 nm, indicating the presence of two
coexisting species. When the experiments were carried out employing a higher comcentration
of the catalyst (Figure 4-inlet) it was possible to observe a new band at max = 570 nm. It has
15

been described that intermediates possessing the FeIII(OOH) core present a signature band in
the region of 530-590 nm and were proposed as the active species in the oxidation of
hydrocarbons catalyzed by iron complexes, in some cases competing with a radical
mechanism [33-38]. The FeIV(O) signature band usually appears in the region of 650-800
nm, which is absence in our case, supporting the formation of a FeIII(OOH) intermediate [37].
The time trace at 570 nm shows a growth region with maximum around 34 minutes, which
can be fit to a single exponential function, with kgrowth = 3.410-3 s-1 (Figure S11), indicating
a first order process. A decay is observed after 34 minutes and can also be fit to a single
exponential function, with kdecay = 2.910-3 s-1 (Figure S12). Que and coworkers have shown
that (TPA)FeIII(OOH) formed at 540 nm from [FeII(TPA)(NCCH3)2]2+ and H2O2 in CH3CN
at -40 oC and decays, kdecay = 2.4(3)10-3 s-1, in presence of water to form FeV(O)(OH), which
2
is responsible for 1-octene oxidation [39]. Isolated [(L5 )Fe(OOH)](PF6)2 complex with a
o
amine/pyridine ligand also degraded in acetonitrile solution at 7 C in a first-order decay,
kdecay = 5.310-3 s-1 at 519 nm, due to the oxidation of CH3CN to CH2CN radical, which by
its turn consume the FeIII(OOH) species [40].
16

1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.15
0.10
0.05
269
570 nm
0
10
20
30
40
50
60
70
80
90
Time (min)
253
t=0min
t=30 min
t=60 min
312
500
550
600
650
359
 (nm)
379
290
250
300
350
400
450
500
550
600
 (nm)
Figure 4. Reaction between [Fe(BMPA)Cl3] (1) and H2O2 followed by UV-Vis spectra in
CH3CN, at [complex] = 1.010-4 mol L-1 and [H2O2] = 1.010-2 mol L-1. Inlet: spectra at
complex and H2O2 concentration 10 higher and time trace of the absorption spectra at λmax
= 570 nm.
A titration experiment (Figure S13) was carried out in an attempt to determine the
molar ratio of H2O2 per complex molecule, however, spectral changes were only observed
after addition of 10 equivalents of H2O2. With 20 equivalents of H2O2 the complete
disappearance of the band at 379 nm and the formation of the band at 359 nm were observed.
The reaction of 2 and 3 with H2O2 (Figures S17-18) showed no spectral change in
the region of 500-700 nm, while the appearance of a band at 530 nm was observed for 4
(Figure S19-20). The time trace (Figure S21) presented first-order growth and decay, with
kgrowth = 7.410-4 s-1 and kdecay = 9.610-4 s-1 (Figures S22), one order of magnitude smaller
17

than those obtained for 1 and H2O2. This result is in accordance with the initial reaction rate
W0 obtained in toluene oxidation (Table S1), where W0 for catalyst 1 was one order of
o
magnitude higher than 4 at 25 C. Complexes 2 and 3 presented the lowest W0 and no
intermediate was observed from their reaction.
The reaction in presence of toluene (Figure S25) showed similar spectral changes as
those observed for 1 and H2O2, indicating no influence on the Fe(OOH) intermediate
formation. The same behavior was observed for the oxidation products benzoic acid (Figure
S36) and 2-methylbenzoquinone (Figure S48) due to their low coordination ability. Similar
results were described for the system [(bpmen)2Fe2O(-O)(-OH)](ClO4)3, benzene and
H2O2 (bpmen = N,N’-dimethyl-N,N’-bis(2-pyridylmethyl)-1,2-diaminoethane), where the
formation of the intermediate iron(III)-phenolate was independent of the initial substrate
concentration [16]. However, when the reaction of 1 and H2O2 was carried out in presence
of benzyl alcohol or benzaldehyde (Figure 5) the bands of the catalyst showed a blue shift
compared to the reaction of 1 and H2O2 right after the reaction started, indicating the
coordination of these products to the Fe(OOH) intermediate. In the course of the reaction,
new bands appeared as the solution color changed from yellow to bright yellow, due to the
displacement of the bands from visible to the ultraviolet range, but the signature band of
Fe(OOH) was not observed.
Control experiments without H2O2 showed that the coordination of the oxidized
products to the pure complex can be neglected, judging by the lack of changes in the
electronic spectrum after 60 min (Figures S28, 34, 38, 41, 46) except for sodium benzoate
(Figure S50) and benzaldehyde (Figure S31) which showed subtle spectral changes, the
latter after easy oxidation to benzoate under air. No further oxidation of the products were
18

observed in absence of the complex (Figures S24, 29, 35, 39, 42, 47, 51), except for
benzaldehyde as mentioned (Figure S32).
1.2
complex
benzyl alcohol, t = 0 min
benzyl alcohol, t = 60 min
benzaldehyde, t = 0 min
1.0
275
benzaldehyde, t = 60 min
0.8
0.6
316
340
0.4
379
0.2
0.0
200
250
300
350
400
450
500
 (nm)
Figure 5. Reaction between [Fe(BMPA)Cl3] (1) (1.010-4 mol L-1) and H2O2 (1.010-1 mol
L-1), in presence of benzyl alcohol or benzaldehyde, followed by UV-Vis spectroscopy, in
CH3CN, during 60 minutes,.
In the reaction of 1, H2O2 and o-cresol or p-cresol, immediate formation of a red-
orange chromophore was observed at λmax = 388 nm and 360 nm, respectively, which are
responsible for the color change during the toluene oxidation reaction. In presence of o-cresol
(Figure 6(a)) the complex absorption at 379 nm was red-shifted to 388 nm, as the intensity
has also increased. A new band at λmax = 570 nm attributed to the Fe(OOH) species emerged,
and the time trace was fitted to a first-order growth (Figure S40), kgrowth = 3.910-4 s-1. The
reaction in presence of p-cresol (Figure 6(b)) presented a similar behavior, however a blue
shift to 360 nm was observed, with a less intense absorption. The band at λmax = 570 nm
19

increased in the beginning of the reaction and then decreased after 40 min (Figure 6(b)-
inlet), the time trace was fit with a first-order growth, kgrowth = 2.010-4 s-1, and decay, kdecay
= 1.510-3 s-1 (Figures S44-45). Both kgrowth values are one order of magnitude smaller than
that for 1 and H2O2, indicating that the buildup of the Fe(OOH) intermediate is slower when
ortho or para cresol is bound. However, the kdecay observed for p-cresol is higher and
indicates that the intermediate consumption is accelerated in presence of p-cresol. Moreover,
o-cresol inhibited the intermediate decay.
1.0
(a)
0.35
388
0.30
0.9
317
0.25
0.20
0.8
0.15
0.10
0.7
0.05
0.6
0.00
0
20
40
60
80
100
120
Time (min)
0.5
0.4
0.3
0.2
0.1
0.0
482
Complex
t=0min
t=20min
t=60min
t=100min
570
250
300
350
400
450
500
550
600
650
 (nm)
20

(b)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
317
360
0
20
40
60
80
100
Time (min)
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
570
complex
t=20min
t=60min
t=100min
450
500
550
600
650
700
 (min)
300
350
400
450
500
550
600
650
 (min)
Figure 6. Reaction between [Fe(BMPA)Cl3] (1) (1.010-4 mol L-1) and H2O2 (1.010-2 mol
L-1) followed by UV-Vis spectroscopy, in CH3CN, during 100 minutes. (a) in presence of o-
cresol 1.010-2 mol L-1 or (b) p-cresol 1.010-2 mol L-1. Inlet: time trace at λmax = 570 nm.
3.3. Reactivity of [Fe(BMPA)Cl3] toward H2O2 followed by ESI-(+)-MS/Q-TOF mass
spectrometry
To confirm the formation of the intermediates proposed by UV-Vis, we followed the
reaction of 1, H2O2, and o-cresol by ESI-(+)-MS/Q-TOF mass spectrometry, as shown in
Figure 7. The spectrum of the pure complex in CH3CN (Figure 7a) presented signals from
mononuclear species at m/z 325.0 and m/z 289.0, assigned to [FeIII(BMPA)Cl2]+ and
[FeIII(BMPA – H+)Cl]+, respectively. Besides, binuclear species such as [FeIII (-
2
Cl)(BMPA)(BMPA – H+)Cl3]+ and [FeIII (-Cl)(BMPA)2Cl4]+ were also observed at m/z
2
21

651.0 and m/z 686.9, respectively. The assignments were confirmed by concordance between
the experimental and simulated isotopic pattern (Figures 53-56).
The spectrum after reaction of 1 and H2O2 (Figure 7b) showed new signals attributed
to Fe(OOH) species, at m/z 288.0 and m/z 323.0. The first peak is a combination of three
different species, as confirmed by the isotopic pattern simulation (Figure 7d): [FeIII(BMPA
– H+)(OOH)]+, [FeII(BMPA)(OOH)]+ and [FeIII(BMPA – H+)Cl]+. The second peak is a
combination of two species: [FeIII(BMPA)Cl(OOH)]+ and [FeIII(BMPA)Cl2]+ (Figure 7e). A
third peak at m/z 510.1 is attributed to a combination of two binuclear species: [FeIII,II2(-
O)(BMPA)(BMPA – PyCH2)(CH3CN)(OOH)]+ and [FeIII,II2(-O)(BMPA)(BMPA –
PyCH)(OH)2(CH3CN)]+ (Figure S57). Similar Fe(OOH) intermediate was detected by ESI-
MS from the reaction of [FeII(bpmen)(CH3CN)2][ClO4]2 (bpmen = N,N’-dimethyl-N,N’-
bis(2-pyridylmethyl)ethane-1,2-diamine) and H2O2: [Fe(bpmen)(OOH)+ Na+ + ClO4] [41].
The reaction of 1, H2O2 and o-cresol (Figure 7c) presented signals corresponding to
the species formed from 1 and H2O2. Besides that, new peaks were observed at m/z 318.1,
389.1 and 660.2, which were assigned to a species containing o-cresol coordinated to the iron
center. The first peak is due to the species [FeIV(BMPA – PyCH)(OOH)(3-(O-)-o-
cresolate)]+, which presents a iron-peroxo units and an anionic form of the 3-hydroxy-o-
cresol (Figure S58). The peak at m/z 389.1 corresponds to [FeIV(BMPA – Py – H+)(o-cresol
– H+)2]+, a species containing two molecules of o-cresol coordinated to the iron center
(Figure 7f). Finally, a binuclear species at m/z 600.2 is assigned to [FeIII,II2(-
O)(BMPA)(BMPA – PyCH)(OH)(CH3CN)(o-cresol – H+)]+ (Figure S59). Similar
intermediate with phenolate-bound iron(III) species was observed for the reaction of the
22

[(bpmen)2Fe2O(-O)(-OH)](ClO4)3, benzene and H2O2, which exhibited a peak assigned to
[(bpmen)FeIII(OPh)(MeCN) + H+] [16].
Figure 7. ESI-(+)-MS/Q-TOF mass spectra of: (a) [Fe(BMPA)Cl3] (1.010-3 mol L-1 in
CH3CN); (b) after 30 minutes of H2O2 (1.010-2 mol L-1) addition; (c) after 60 minutes of p-
cresol (1.010-2 mol L-1) addition. Experimental and isotopic pattern simulation for the ions:
(d) m/z 288.0 of spectrum (b); (e) m/z 323.0 of spectrum (b); (f) m/z 389.1 of spectrum (c).
4. Conclusions
The iron(III) complexes studied were able of oxidize toluene in presence of H2O2,
furnishing satisfactory results. The higher yields were obtained at 50 °C, with the complex
[Fe(BMPA)Cl3] (1), reaching up to 31%. The reactions at 25 °C were more selective to o-
23

cresol while the reactions at 50 °C were more selective to benzaldehyde. In the oxidation of
benzene, phenol was obtained with 100% of selectivity and 30 % of yield, representing a
very promising result. In the ethylbenzene and cumene oxidation, a higher selectivity was
obtained to the benzylic chain products in relation to the ring hydroxylation products. In n-
propylbenzene, p-xylene and anisole oxidation, the ring hydroxylation was more favoured.
The reaction of [Fe(BMPA)Cl3] and H2O2 was followed by electronic UV-Vis spectroscopy
and ESI-(+)-MS/Q-TOF mass spectrometry. The data supported the formation of a
intermediate Fe(OOH). In presence of o-cresol and p-cresol a chromophore was observed
due to coordination to Fe(OOH) as phenolate.
Acknowledgements
The CNPq and CAPES for the financial support to the present work.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version.
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