Liquid phase oxidation of xylenes catalyzed by the tripodal C-scorpionate iron(II) complex [FeCl2{κ3-HC(pz)3}]

Article abstract of DOI:10.1016/j.poly.2016.10.037

A simple and mild (room temperature) process for oxidation of o-, m- or p-xylene to the corresponding methylbenzyl alcohols, tolualdehydes and toluic acids, using H₂O₂(30% aqueous solution) and the iron(II) C-scorpionate [FeCl₂{κ₃-HC(pz)₃}] (pz = pyrazol-1-yl) catalyst is presented. Remarkably, after 5 min of reaction an overall oxygenates yield of 22% (TOF = 1.3 × 10²h^?1) was obtained. The effects of reaction parameters, such as reaction time, temperature, amount of catalyst, type and amount of oxidant are reported and discussed.

Full text of DOI:10.1016/j.poly.2016.10.037

Accepted Manuscript
Liquid phase oxidation of xylenes catalyzed by the tripodal C-scorpionate
iron(II) complex [FeCl2{κ3-HC(pz)3}]
Marta Mendes, Ana Paula C. Ribeiro, Elisabete C.B.A. Alegria, Luísa M.D.R.S.
Martins, J.L. Pombeiro
PII:
S0277-5387(16)30534-4
DOI:
http://dx.doi.org/10.1016/j.poly.2016.10.037
POLY 12290
Reference:
Polyhedron
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Revised Date:
Accepted Date:
19 August 2016
23 October 2016
24 October 2016
Please cite this article as: M. Mendes, A.P.C. Ribeiro, E.C.B. Alegria, L.M.D. Martins, J.L. Pombeiro, Liquid phase
oxidation of xylenes catalyzed by the tripodal C-scorpionate iron(II) complex [FeCl2{κ3-HC(pz)3}], Polyhedron
(2016), doi: http://dx.doi.org/10.1016/j.poly.2016.10.037
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Liquid phase oxidation of xylenes catalyzed by the tripodal C-
scorpionate iron(II) complex [FeCl2{κ3-HC(pz)3}]
Marta Mendes,1 Ana Paula C. Ribeiro,1,* Elisabete C.B.A. Alegria,1,2,*
Luísa M.D.R.S. Martins,1,2,* Armando J.L. Pombeiro1
1 Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av.
Rovisco Pais, 1049-001 Lisboa, Portugal.
2 Chemical Engineering Department, Instituto Superior de Engenharia da Lisboa,
Instituto Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1959−007 Lisboa,
Portugal. E-mail: lmartins@deq.isel.ipl.pt
Abstract
A simple and mild (room temperature) process for oxidation of o-, m- or p-xylene to the
corresponding methylbenzyl alcohols, tolualdehydes and toluic acids, using H2O2 (30%
aqueous solution) and the iron(II) C-scorpionate [FeCl2{κ3-HC(pz)3}] (pz = pyrazol-1-
yl) catalyst is presented. Remarkably, after 5 minutes of reaction an overall oxygenates
yield of 22% (TOF = 1.3 x 102 h-1) was obtained. The effects of reaction parameters,
such as reaction time, temperature, amount of catalyst, type and amount of oxidant are
reported and discussed.
Keywords
Xylene, oxidation, scorpionate, iron, hydrogen peroxide
Introduction
Terephthalic acid production from the oxidation of para-xylene is a very important
process in polyester industry [1,2]. This large scale produced acid is manly used in
polyethylene terephthalate (PET) manufacturing, a commodity polymer that finds use in
fibers, bottles, and films. Currently, oxidation of para-xylene is conducted in a
homogeneous catalytic system: cobalt, manganese and the highly corrosive bromide
dissolved in aqueous acetic acid (Amoco process) [1]. Although very high conversions
of the reactant are attained, and greener processes (e.g., a spray process [3], the use of
1

supercritical CO2 [4] or water [5] as solvents, or N-hydroxyimides and guanidines as
catalyst additives [6] or other promoters [7]) have been recently reported, the
development of a halogen free, low cost, sustainable and still highly efficient catalytic
system for the production of this commodity is needed.
Halogen free heterogeneous initiator and catalyst systems have also been reported [1],
potentially combining the benefits of easy separation with the advantage of non-
corrosive reaction mixtures. However, all systems described so far lack activity
compared to homogeneous initiator system and the active metal compounds are leached
in the reaction environment. In addition, for obvious environmental constraints,
classical stoichiometric oxidants, such as dichromate or permanganate, should be
replaced by new environmental friendly catalytic processes using clean oxidants like
molecular oxygen or hydrogen peroxide. However, in spite of the advantages of
aqueous H2O2 as a cheap and benign oxidant, its catalytic application for the oxidation
of para-xylene remains underexplored [1,8]. In fact, besides the use of iron(III) ions
supported on neutral γ-alumina as catalyst for the oxidation of p-xylene with hydrogen
peroxide in acetonitrile at 60 ºC [9], the application of this abundant, cheap and non-
toxic metal as catalyst for the said reaction is still quite understudied [8]. Although
active, the Fe3+-Al2O3 catalyzed conversion of p-xylene (24%) involves both the side
chain oxidation (to tolualdehyde, 11%) and the hydroxylation of the aromatic ring (to
2,5-dimethylphenol, 13%), being unable to afford toluic acid.
On the other hand, the use of metallic complexes with C-scorpionate poly(pyrazol-1-
yl)methane ligands as homogeneous catalysts have experienced significant advances in
the last years [10].
In this work, we aim to use the tripodal C-scorpionate iron(II) complex [FeCl2{κ3-
HC(pz)3}] (pz = pyrazol-1-yl) [11] that has proven its efficiency as catalyst for the
partial oxidation of cyclohexane to cyclohexanol and cyclohexanone [11,12] (both
under homo- and heterogeneous [13,14] conditions), as well as for the oxidation of
secondary alcohols to the corresponding ketones, [15] but was not applied for other
substrates.
Complex [FeCl2{κ 3-HC(pz)3}] is easily synthesized [11] at room temperature, directly
from the corresponding iron(II) chloride and hydrotris(1-pyrazolyl)methane HC(pz)3,
and exhibits a high solubility in water (in spite of the water insolubility of the
uncoordinated hydrotris(1-pyrazolyl)methane), [12] favoring the use of an aqueous
reaction medium, an interesting possibility for a greener xylene oxidation process.
2

Moreover, the coordination polyhedron of complex [FeCl2{κ 3-HC(pz)3}] is a tetragonal
pyramid. Thus, the iron atom has one empty coordination site that can be easily
occupied by a substrate species. On the other hand, the tripodal C-scorpionate ligand,
bearing three pyrazolyl moieties (via their N atoms) can assist proton-transfer steps that
are believed to be involved in key catalytic processes [16]. These features were
particularly significant for its choice to attempt developing green catalytic processes for
oxidation of p-xylene.
To our knowledge, this is the first time that the successful use of an Fe(II) catalyst for
the oxidation of xylenes is reported.
Materials and Methods
All the reagents and solvents were purchased from commercial sources and used as
received. The water used for all preparations and analyses was double-distilled and de-
ionized. The scorpionate iron(II) complex [FeCl2{κ3-HC(pz)3}] was synthesized
according to a reported procedure and characterized accordingly [11].
1H and 13C NMR spectra were recorded at ambient temperature on a Bruker Avance II +
300 (UltraShieldTM Magnet) spectrometer operating at 300.130 and 75.468 MHz for
proton and carbon-13, respectively, at ambient temperature. The chemical shifts are
reported in ppm using tetramethylsilane as an internal reference.
Infrared spectra (4000–400 cm−1) were recorded on a Vertex 70 (Bruker) instrument in
KBr pellets. Far infrared spectra FIR (400-200 cm-1) were recorded on a Vertex 70
spectrophotometer in CsI pellets.
Elemental analyses were carried out by the Microanalytical Service of the Instituto
Superior Técnico.
SEM and EDX analyses were carried out on a scanning electron microscope JEOL
7001F with Oxford light elements EDS detector and EBSD detector.
XPS were performed at CEMUP on a Kratos AXIS Ultra HAS equipment, with
VISION software for data acquisition and CASAXPS software for data analysis.
Chromatographic analyses were undertaken by using a Perkin-Elmer Clarus 600 gas
chromatograph, equipped with two capillary columns (SGE BPX5; 30 m x 0.32 mm x
25 mm), one having an EI-MS (electron impact) detector and the other one with a FID
detector, were used for analyzing the reaction mixtures. Helium was used as the carrier
gas.
The xylenes oxidations were carried out in air in round bottom flasks with vigorous
3

stirring and using CH3CN as solvent (up to 3 mL total volume). Typically, [FeCl2{κ3-
HC(pz)3}] (10 µmol) was added to the solvent as a solid or in the form of a stock
solution. After the addition of xylene (o-, m- or p-; 5 mmol) the reaction started when
hydrogen peroxide (50% aq. sol., 10 mmol) was added in one portion. In the
experiments with HNO3 (65%, 0.1 – 0.5 mmol) this acid was added immediately before
the addition of the substrate. The reactions were stopped by cooling the reactional
solution. Extraction was performed with 10 mL of diethyl ether and 90 µL of
cyclopentanone were inserted in the extracted mixture. All products formed were
identified by GC.
Results and Discussion
In this work, for the first time, the morphological aspects and microstructure
distribution of the tripodal C-scorpionate iron(II) complex [FeCl2{κ3-HC(pz)3}] (pz =
pyrazol-1-yl) were characterized by scanning electron microscopy (SEM) and energy-
dispersive X-ray spectroscopy (EDX). The iron is detected (EDX, Figure S1b and Table
S1) at the surface, an important feature for its application as catalyst. This and the know
(see Introduction) features of [FeCl2{κ3-HC(pz)3}] prompted us to test its catalytic
activity for the oxidation of xylenes.
p-Xylene can be successfully activated at a methyl group using the above complex as
catalyst and aqueous hydrogen peroxide at 35 °C in acetonitrile, leading to the
formation of 4-methylbenzyl alcohol, p-tolualdehyde and p-toluic acid (Figure 1).
Figure 1 – Oxidation of p-xylene to 4-methylbenzyl alcohol, p-tolualdehyde and p-toluic
acid.
Remarkably, after only 5 minutes of reaction an overall oxygenates yield of 22% (TOF
= 1.3 x 102 h-1) was obtained in the presence of nitric acid as co-catalyst (entry 8, Table
1). Even in this short reaction time p-toluic acid is already formed (4% yield). The
4

presence of HNO3 in the reaction medium is needed to afford p-toluic acid (compare
entries 1 and 5, Table 1).
Table 1 - Oxidation of p-xylene catalyzed by the scorpionate complex [FeCl2{κ3-
HC(pz)3}].a
Yield (%)b
Reaction
time (h)
Acid
Total TOFd
Entry
[n(acid)/n(catalyst)]
TONc
(h-1)
Alcohol
2.2
-
Aldehyde
2.2
Acid
-
Total
4.4
1
2 e
3
1
1
-
22
6
22
6
-
1.1
-
1.1
0.75
0.25
1
-
3.6
0.9
4.3
0.8
1.0
2.4
3.5
3.7
4.3
4.0
6.0
7.0
4.8
8.2
8.5
0.6
0.8
2.2
1.9
2.9
3.4
3.0
2.9
-
6.5
33
43
4
-
0.6
-
1.5
8
30
5
10
25
50
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
50
17.6
5.5
4.5
1.6
3.8
4.0
4.4
4.1
4.5
4.8
5.1
5.8
5.2
3.9
4.4
0.4
0.3
1.9
3.2
4.1
4.2
-
26.3
7.9
132
40
132
40
6
1
7
1
9.0
13.8
21.8
23.1
23.8
26.3
29.0
31.3
33.9
31.7
29.4
28.6
2.0
69
69
8
0.08
0.25
0.75
1
15.5
15.3
16.0
17.6
20.2
20.2
21.1
21.7
17.2
15.7
1.0
109
116
119
132
145
157
170
159
147
143
10
1363
462
159
132
48
9
10
11
12
13
14
15
16 f
17 g
18 h
19 i
20 j
21k
22 l
23m
24 n
3
6
26
9
19
12
1
13
147
143
10
1
1
1
3.8
4.9
25
25
1
8.9
13.0
15.5
22.8
23.9
65
65
0.08
0.08
0.08
1
10.4
15.8
16.3
1.2
155
76
1938
950
747
21
60
4.2
21
3
a Typical (unless stated otherwise) reaction conditions: p-xylene (5 mmol), [FeCl2{κ -HC(pz)3}] (10 µmol),
H2O2 (10 mmol), HNO3 (65%, 0.1 – 0.5 mmol), 35 ºC, MeCN (3 mL). b Product yield = moles of product/100
d
e
mol of p-xylene. c Total turnover number = moles of product per mol of catalyst. TOF = TON per hour.
f
TBHP (70 % aq. solution, 10 mmol) as oxidant. Reaction performed at 50 ºC. Reaction performed at 70
g
h
i
3
l
ºC. Reaction performed at 100 ºC. H2O2 (5 mmol). j H2O2 (25 mmol). k [FeCl2{κ -HC(pz)3}] (5 µmol).
3
3
[FeCl2{κ -HC(pz)3}] (15 µmol). m [FeCl2{κ -HC(pz)3}] (20 µmol). n Using FeSO4 .7H2O as catalyst.
5

The aldehyde is always the main product (Table 1) and the highest total yield is
obtained after 9 h reaction time (33.9 %, entry 14, Table 1 and Figure 2). The achieved
yields (under mild conditions), along with the selectivity found for the activation of the
methyl group of p-xylene (no hydroxylation of the aromatic ring was observed) and the
ability to afford toluic acid indicate an exceptional activity of the catalytic system based
on the complex [FeCl2{κ3-HC(pz)3}], in acetonitrile medium. Moreover, compared with
the use of iron(III) ions supported on neutral γ-alumina as catalyst [8] (see above) our
system is much more advantageous (higher conversion and selectivity obtained with
lower loading of catalyst, temperature and reaction time) for the oxidation of p-xylene.
The use of a common iron(II) salt did not afford toluic acid (entry 24, Table 1)
suggesting a significant role for the C-scorpionate ligand at our catalyst.
40
35
30
25
20
15
10
5
0
0
2
4
6
8
10
12
Time / h
Figure 2 - Effect of the reaction time on the total yield for the oxidation of p-xylene
catalyzed by [FeCl2{κ3-HC(pz)3}].
The use of hydrogen peroxide as oxidant revealed to be preferable (besides
environmental issues) to, e.g., tert-butylhydroperoxide, TBHP (compare entries 1 and 2,
Table 1) since the latter, under identical conditions, affords only the aldehyde in low
yield (1.1%). The amount of H2O2 used affects the overall yield of products: the yield is
quite small (4.9%, entry 19, Table 1) for 5 mmol of H2O2 and increases to 26% when
the oxidant amount is 10 mmol (entry 11, Table 1). The use of a higher amount (up to
25 mmol) leads to a yield decrease of more than 50% (entry 20, Table 1).
6

In the temperature range of 35 to 70 ºC, the alcohol yield increases with increasing
temperature, whereas the aldehyde and the acid yields do not show a marked variation
(Figure 3). However, at 100 ºC, a drastic yield decrease is observed, conceivably on
account of the decomposition of H2O2.
Figure 3 - Effect of the temperature on the products yield from the oxidation of p-xylene
catalyzed by [FeCl2{κ3-HC(pz)3}].
The effect of the amount of catalyst was also investigated (Table 1, entries 8 and 21-23
and Figure 4). An increase in the partial and total yields was observed by increasing the
load of catalyst, although rather more pronounced when the catalyst amount is increased
from 5 to 10 µmol. In fact, 10 µmol appears to be the best catalyst load, since further
increasing up to 20 µmol leads to a ca. 2 % increase in the products yield, mostly due to
the alcohol and aldehyde formation.
25
20
15
10
5
0
0
5
10
15
20
Amount of catalyst / micromol
Figure 4 - Effect of the catalyst loading on the products yield from the oxidation of p-
xylene catalyzed by [FeCl2{κ3-HC(pz)3}].
7

In contrast to previous studies [1,9], a marked catalytic activity of the C-scorpionate
iron(II) complex [FeCl2{κ3-HC(pz)3}] is observed under milder conditions (lower
temperatures and a milder oxidant, in the absence of any bromine source), what is
significant towards the development of a greener and more sustainable process for p-
xylene oxidation. The process with bromide uses acetic acid as solvent [1], while in
another work with p-xylene [9] using MeCN no acid is observed after 6 h.
The catalytic applicability of [FeCl2{κ3-HC(pz)3}] was extended to o- and m-xylenes in
order to observe the influence of the methyl group position (if any) in the oxidation
process (Table 2 and Figure 5).
Table 2 - Oxidation of o-, m- and p-xylene catalyzed by [FeCl2{κ3-HC(pz)3}].a
Yield (%)b
Acid
Total
TOFd
(h-1)
Entry Substrate [n(acid)/n
TONc
Alcohol Aldehyde Acid Total
(catalyst)]
1
2
-
0.9
5.0
1.6
9.8
0.6
2.7
3.4
17
93
17
93
10
18.6
3
4
5
-
10
-
0.2
3.7
2.2
1.3
11.9
2.2
-
2.7
-
2.0
18.3
4.4
10
92
22
10
92
22
6
10
4.3
17.6
4.5
26.3
132
132
3
a Typical (unless stated otherwise) reaction conditions: xylene (5 mmol), [FeCl2{κ -HC(pz)3}] (10 µmol),
H2O2 (10 mmol), HNO3 (65%, 0.1 mmol), 35 ºC, MeCN (3.0 mL), 1 h reaction time. b Product yield =
d
moles of product/100 mol of xylene. c Total turnover number = moles of product per mol of catalyst.
TOF = TON per hour.
8

Figure 5 - Overall yield for the oxidation of xylenes, at 1 h reaction time, in the absence
() or in the presence () of nitric acid.
The methyl in the para position leads to the highest total yield as well as to the highest
yield of aldehyde and acid. This possibly is accounted for by the higher steric constrains
associated to the orto and meta positons [17]. For all substrates, the presence of nitric
acid highly promotes the formation of the oxidation products (Figure 5). The selectivity
to the above monofunctional products is an indication that no ring hydroxylation
occurred. This suggests that the oxidation of the xylenes in our system is governed by a
free-radical mechanism [18] assisted by the iron(II) catalyst [FeCl2{κ3-HC(pz)3}]
(Scheme 1). In fact, a unit change of the metal oxidation state (+2/+3) at the iron centre
was detected by XPS: a mixture of both Fe3+ and Fe2+ oxidation states, with a
predominance of Fe3 is observed at XPS spectrum (Figure S3) and appears to be a
consequence of the oxidation process.
The catalytic process may involve HO˙ and HOO˙ radicals produced in the iron
promoted decomposition of hydrogen peroxide, [10] according to equations (1) to (7):
Fe2+ + H2O2 → HO• + Fe3+ + HO−
Fe3+ + H2O2 → HOO• + H+ + Fe2+
(1)
(2)
(3)
(4)
(5)
(6)
(7)
•
HO• + RCH3 → H2O + RCH2
•
RCH2 + O2 → RCH2OO•
RCH2OO• + RCH3 → RCH3OOH + RCH2
RCH2OO• + Fe2+ → Fe3+ + RCHO + HO−
2 RCH2OO• → RCH2OH + RCHO + O2
•
Scheme 1
9

From Table 2 we can observe the influence of the alkyl group position in the final
result. The orto- position produces more alcohols, while the meta- and para- produces
more aldehyde. The higher amount of acid produced occurs in the para- position, which
is the more stable one.
Our catalytic system, in comparison with the Amoco process [1,8] has the advantage of
not requiring the utilization of hydrobromic acid (HBr) or sodium bromide (NaBr) as
the bromide ion source that leads to a hazardous reaction due to the corrosive nature of
HBr [18] and requires a reactor very costly to build and maintain.
Conclusions
A new process for the selective oxidation of a methyl group of xylenes catalyzed by the
C-scorpionate iron(II) complex [FeCl2{κ3-HC(pz)3}] is presented. Remarkably fast
conversion of xylenes to the corresponding alcohols, aldehydes and acids (no
hydroxylation of the aromatic ring) is achieved under very mild conditions (low
temperature, green oxidant, low catalyst loading) and in the presence of the co-catalyst
nitric acid. Thus, our catalytic system is a significant finding towards the development
of a greener process for p-xylene oxidation and seems to be worth of further
development, a topic that will be addressed in future studies. The generality of the
above advantages should also be tested by extending the investigation to other
important industrial substrates.
Acknowledgements
Financial support from the Fundação para a Ciência e a Tecnologia (FCT), Portugal, for
fellowships to A. P. C. R. (SFRH/BPD/90883/2012), and for the UID/QUI /00100/2013,
PTDC/QEQ-ERQ/1648/2014 and PTDC/QEQ-QIN/3967/2014 projects are gratefully
acknowledged. The authors also acknowledge Dr. Pierre Dournel (Solvay) for
stimulating discussion and the Portuguese NMR Network (IST-UL Centre) for access to
the NMR facility.
References
[1] R. A. F. Tomás, J. C. M. Bordado, J. F. P. Gomes, Chem. Rev. 113 (2013) 7421–
7469.
[2] R. J. Sheehan, in: Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; Wiley-
10

−651.
VCH: Weinheim, Germany, 2003, Vol. 35, pp. 639
[3] a) M. Li, T. Ruddy, D. Fahey, D. H. Busch, B. Subramaniam, ACS Sustainable
Chem. Eng. 2 (2014) 823−835. b) M. Li, F. Niu, X. Zuo, P. D. Metelski, D. H. Busch,
B. Subramaniam, Chem. Eng. Sci. 104 (2013) 93–102.
[4] D. S. Kim, Y. H. Shin, Y.-W. Lee, Chem. Eng. Comm. 202 (2014) 78–84.
[5] a) M. Osada, P. E. Savage, AIChE J. 55 (2009) 1530-1537. b) J. B. Dunn, P. E.
Savage, Ind. Eng. Chem. Res. 41(2002) 4460-4465.
[6] a) Y. Cheng, X. Li, Q. Wang, L. Wang, Ind. Eng. Chem. Res. 44 (2005) 7756-7760.
b) Q. Wang, X. Li, L. Wang, Y. Cheng, G. Xie, Ind. Eng. Chem. Res. 44(13) (2005)
4518-4522. c) M. N. Kopylovich, K. T. Mahmudov, M. F. C. G. Silva, L. M. D. R. S.
Martins, M. L. Kuznetsov, T. F. S. Silva, J. J. R. Fraústo da Silva, A. J. L. Pombeiro, J.
Phys. Org. Chem. 24 (2011) 764-773. d) H. Falcon, J. M. Campos-Martin, S. M. Al-
Zahrani, J. L. G. Fierro, Cat. Com. 12 (2010) 5-8. e) B. Saha, N. Koshino, J. H.
Espenson, J. Phys. Chem. A, 108 (2004) 425-431.
[7] a) B. Saha, J. H. Espenson, J. Mol. Catal. A: Chem., 207 (2004) 123-129. b) K.
Deori, D. Gupta, B. Saha, S. K. Awasthi, S. Deka, J. Mater. Chem. A, 1 (2013) 7091-
7099.
[8] N. A. M. Fadzil, M. H. A. Rahim, G. P. Maniam, Chinese J. Catal. 35 (2014) 1641–
1652.
[9] H. H. Monfared, Z. Amouei, J. Mol. Catal. A: Chem. 217 (2004) 161-164.
[10] a) C. Pettinari, C. Santini, in: J.A. McCleverty, T.J. Meyer (Eds.), Comprehensive
Coordination Chemistry II: From Biology to Nanotechnology, vol. 1, Elsevier
Pergamon (APS), Amsterdam, 2003, p. 159-172. b) C. Pettinari, R. Pettinari, Coord.
Chem. Rev. 249 (2005) 525-543. c) H. R. Bigmore, C. S. Lawrence, P. Mountford, C.
S. Tredget, Dalton Trans. (2005) 635-651. d) A. Otero, J. Fernández-Baeza, A. Lara-
Sánchez, L. F. Sánchez-Barba, Coord. Chem. Rev. 257 (2013) 1806-1868. e) L. M. D.
R. S. Martins, A. J. L. Pombeiro, Eur. J. Inorg. Chem. (2016) 2236-2252.
[11] T. F. S. Silva, E. C. B. A. Alegria, L. M. D. R. S. Martins, A. J. L. Pombeiro, Adv.,
Synth. Cat. 350 (2008) 706-716.
[12] L. M. D. R. S. Martins, A. J. L. Pombeiro, Coord. Chem. Rev. 265 (2014) 74-88.
[13] L. M. D. R. S. Martins, A. Martins, E. C. B. A. Alegria, A. P. Carvalho, A. J. L.
Pombeiro, Appl. Cat. A: Gen. 464-465 (2013) 43-50.
[14] L. M. D. R. S. Martins, M. Peixoto de Almeida, S. A. C. Carabineiro, J. L.
Figueiredo, A. J. L. Pombeiro, ChemCatChem 5 (2013) 3847-3856.
11

[15] L. M. D. R. S. Martins, A. P. C. Ribeiro, S. A. C. Carabineiro, J. L. Figueiredo, A.
J. L. Pombeiro, Dalton Trans. 45 (2016) 6816-6819.
[16] a) B. Zhao, Z. Han, K. Ding, Angew. Chem. Int. Ed. 52 (2013) 4744-4788. b) H.
Grutsmacher, Angew. Chem. Int. Ed. 47 (2008) 1814-1818. c) J. I. Van der Vlugt, J. N.
H. Reek, Angew. Chem. Int. Ed. 48 (2009) 8832-8846. d) C. Gunanathan, D. Milstein,
Acc. Chem. Res., 44 (2011) 588-602.
[17] a) Y. Zhao, M. Zhang, D. Ren, W. Tan, X. Li, X. Wang, X. Guo, Reac. Kinet.
Mech. Cat. 114 (2015) 519–527. b) P.R.H. P. Rao, K.R. Reddy, A.V. Ramaswamy,
P. Ratnasamy in: J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier, M.
Guisnet, G. Pérot (Eds.), Heterogeneous Catalysis and Fine Chemicals III:
Procedings to the 3rd international symposium, Pointiers, April 5-8 1993, Elsevier
Science Publishers B.V., Amsterdam, 1993, pp. 385-392.
[18] R. A. Sheldon, J. K. Kochi, Metal Catalyzed Oxidation of Organic Compounds,
Academic Press, New York, 1981.
12

Figures captions
Figure 1 – Oxidation of p-xylene to 4-methylbenzyl alcohol, p-tolualdehyde and p-toluic
acid.
Figure 2 – Effect of the reaction time on the total yield for the oxidation of p-xylene
catalyzed by [FeCl2{κ3-HC(pz)3}].
Figure 4 - Effect of the catalyst loading on the products yield from the oxidation of p-
xylene catalyzed by [FeCl2{κ3-HC(pz)3}].
Figure 5 - Overall yield for the oxidation of xylenes, at 1 h reaction time, in the absence
() or in the presence () of nitric acid.
13

Graphical Abstract
+ +
2%
4%
16%
H2O2
r.t., 5 min
Tripodal iron(II) C-scorpionate complex [FeCl2{κ3-HC(pz)3}] (pz = pyrazol-1-yl) acts
as homogeneous catalyst for the fast oxidation of xylenes with hydrogen peroxide,
under mild conditions, affording the corresponding methylbenzyl alcohol, tolualdehyde
and toluic acid.
14

Fe2+ + H2O2 → HO• + Fe3+ + HO−
Fe3+ + H2O2 → HOO• + H+ + Fe2+
(1)
(2)
(3)
(4)
(5)
(6)
(7)
•
HO• + RCH3 → H2O + RCH2
•
RCH2 + O2 → RCH2OO•
RCH2OO• + RCH3 → RCH3OOH + RCH2
RCH2OO• + Fe2+ → Fe3+ + RCHO + HO−
2 RCH2OO• → RCH2OH + RCHO + O2
•
Scheme 1 - Proposed mechanistic pathway for the oxidation of xylenes catalysed by [FeCl2{κ3-
HC(pz)3}].