Catalytic oxidation of a model volatile organic compound (toluene) with tetranuclear Cu(II) complexes

Article abstract of DOI:10.1016/j.ica.2021.120314

A tetranuclear Cu(II) cubane [Cu₂(μ-1κONO’:2κOO’:3κO-HL)(μ-1κONO’:2κOO’-HL)]₂?4dmf (1) derived form (2,3-dihydroxybenzylidene)-2-hydroxybenzohydrazide (H₃L) was synthesized at room temperature and characterized by elemental analysis, IR spectroscopy, ESI-MS and single crystal X-ray diffraction. The known tetranuclear Cu(II) open-cubane [Cu(HL)]₄?4EtOH (2) was synthesized from the same pro-ligand following a similar method or the reported one. The different tautomeric forms (keto and enol) of the organic ligands in 1 and 2 explain their different structural features. Both complexes were screened as catalysts for the peroxidative oxidation of toluene with tert-butyl hydroperoxide, achieving benzaldehyde and o-cresol as the main products. Complex 1 exhibits the highest activity (maximum product yield of 11%).

Full text of DOI:10.1016/j.ica.2021.120314

Inorganica Chimica Acta 520 (2021) 120314
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Research paper
Catalytic oxidation of a model volatile organic compound (toluene) with
tetranuclear Cu(II) complexes
a
,
*
a
,
b
,
*
a
a,b
Manas Sutradhar , Elisabete C.B.A. Alegria
, Tannistha Roy Barman , Hugo M. Lapa
,
a
a,c,*
M. F ´a tima C. Guedes da Silva , Armando J.L. Pombeiro
a
Centro de Química Estrutural, Instituto Superior T ´e cnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Departamento de Engenharia Química, Instituto Superior de Engenharia de Lisboa, Instituto Polit ´e cnico de Lisboa, R. Conselheiro Emídio Navarro, 1, 1959-007 Lisboa,
b
Portugal
c
Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya Street, Moscow 117198, Russian Federation
A R T I C L E I N F O
A B S T R A C T
A tetranuclear Cu(II) cubane [Cu
2
(
μ
-1κONO’:2κOO’:3κO-HL)(
L) was synthesized at room temperature and charac-
terized by elemental analysis, IR spectroscopy, ESI-MS and single crystal X-ray diffraction. The known tetra-
nuclear Cu(II) open-cubane [Cu(HL)] ‧4EtOH (2) was synthesized from the same pro-ligand following a similar
μ
-1κONO’:2κOO’-HL)]
2
‧4dmf (1) derived form
Dedicated to Prof. M. Peruzzini on occasion of
his 65th birthday as a recognition of his
contribution to science.
(
2,3-dihydroxybenzylidene)-2-hydroxybenzohydrazide (H
3
4
method or the reported one. The different tautomeric forms (keto and enol) of the organic ligands in 1 and 2
explain their different structural features. Both complexes were screened as catalysts for the peroxidative
oxidation of toluene with tert-butyl hydroperoxide, achieving benzaldehyde and o-cresol as the main products.
Complex 1 exhibits the highest activity (maximum product yield of 11%).
Keywords:
Cu(II) complexes
Aroylhydrazone
X-ray structure
Oxidation of toluene
Peroxidative oxidation
1
. Introduction
bond in alkanes, remains a huge challenge for the scientific community.
At the industrial level, activation involving the C H cleavage usually
–
The hydrocarbons with a high vapour pressure at ordinary room
requires drastic conditions such as high temperatures [24–30]. There
has been a considerable effort focusing on the establishment of clean
synthetic routes, involving the design and development of new and
efficient catalysts for the selective oxidation of hydrocarbons, from
environmental and economic points of view [27,31,32]. Possible
oxygenated products, such as benzyl alcohol, benzaldehyde, tolualde-
hyde, cresols, benzoic acid, toluic acid etc., can be useful intermediates
for chemicals, agrochemicals, fragrances, pharmaceuticals and polymers
manufacture [33–36].
Our group has been involved in developing and exploring the cata-
lytic activity of a variety of transition metal complexes towards catalytic
oxidation processes of industrial interest [37–43]. In particular Cu(II)
complexes have been chosen for oxidation reactions due to their abun-
dant resources, low cost, non-toxic properties and high catalytic effi-
ciency [44,45]. In fact, Cu(II) species can act as attractive catalysts in the
oxidation of aromatic hydrocarbons (BTX) [19–21]. In the present
investigation, we have synthesized and characterized a new tetranuclear
temperature, commonly entitled as volatile organic compounds (VOCs),
are major contributors for air pollution, either directly through their
toxicity or pungent smelling, or indirectly by generating other pollutants
[
1]. Benzene, toluene and xylene (BTX) are among the VOCs that are
mainly used as common solvents as well as raw materials in the pro-
duction of other chemicals [2–4]. All these three substances are known
to be toxic for human health [5–8]. There are several technologies of
VOC elimination such as flame combustion, catalytic combustion, cat-
alytic destruction using ozone and plasma, photocatalytic decomposi-
tion, adsorption processes and biological treatments. [9,10,11–16].
Catalytic oxidation is acknowledged as one of the Best Available Tech-
niques (BAT) normally applied in the Large Volume Organic Chemical
Industry (LVOC) for the abatement of VOCs owing to its easy application
with high efficiency and economically feasibility [17–23]. Apart from
the elimination of VOCs, an important motive of catalytic oxidation of
aromatic hydrocarbons (BTX) concerns their selective functionalization.
The activation of bonds considered practically inert, such as the C
–
H
cubane complex [Cu
2
(μ-1κONO’:2κOO’:3κO-HL)(μ-1κONO’:2κOO’-
*
Corresponding authors at: Centro de Química Estrutural, Instituto Superior T ´e cnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.
E-mail addresses: manas@tecnico.ulisboa.pt (M. Sutradhar), elisabete.alegria@isel.pt (E.C.B.A. Alegria), pombeiro@tecnico.ulisboa.pt (A.J.L. Pombeiro).
https://doi.org/10.1016/j.ica.2021.120314
Received 30 December 2020; Received in revised form 15 February 2021; Accepted 21 February 2021
Available online 26 February 2021
0
020-1693/© 2021 Elsevier B.V. All rights reserved.

M. Sutradhar et al.
Inorganica Chimica Acta 520 (2021) 120314
Scheme 1. Synthetic routes for 1 and 2.
2 4
HL)] ‧4dmf (1) and the known [Cu(HL)] ‧4EtOH (2) [46]. Both cubanes
were tested as catalysts for the peroxidative oxidation of VOCs and their
activities were compared. We have chosen toluene as a model substrate
for the study and tert-butyl hydroperoxide as oxidant achieving benz-
aldehyde and o-cresol as the main products.
2
. Experimental
2
.1. Materials and methods
The synthetic part of this study was performed in air. Commercially
available reagents and solvents were used as received, without further
purification or drying. Cu(NO
source.
3
)
2
2
⋅2.5H O was used as Cu(II) metal
C, H, and N elemental analyses were carried out by the Microana-
lytical Service of the Instituto Superior T ´e cnico. Infrared spectra
ꢀ
1
(
4000–400 cm ) were recorded on a Bruker Vertex 70 instrument in
1
KBr pellets; wavenumbers are in cm . The 1H NMR spectra were
ꢀ
Scheme 2. Conversion of hazardous toluene in added-value products.
recorded at room temperature on a Bruker Avance II + 400.13 MHz
(
UltraShieldTM Magnet) spectrometer. Tetramethylsilane was used as
the internal reference and the chemical shifts are reported in ppm. Mass
spectra were run in a Varian 500-MS LC Ion Trap Mass Spectrometer
equipped with an electrospray (ESI) ion source. For electrospray ioni-
zation, the drying gas and flow rate were optimized according to the
particular sample with 35p.s.i. nebulizer pressure. Scanning was per-
formed from m/z 100 to 1200 in methanol solution. The compounds
were observed in the positive mode (capillary voltage = 80–105 V). Gas
chromatographic (GC) measurements were carried in a FISONS In-
struments GC 8000 series gas chromatograph with a capillary DB-WAX
column (30 m × 0.32 mm) and a FID detector under the following
Mass Spectrometry (GC–MS) analyses were performed using a Perkin
Elmer Clarus 600C (Shelton, CT, USA) instrument (He as the carrier gas).
The ionization voltage was 70 eV. Gas chromatography was conducted
in the temperature-programming mode, using a SGE BPX5 column (30
m × 0.25 mm × 0.25 m).
3
2.2. Syntheses of the pro-ligand H L
The aroylhydrazone Schiff base pro-ligand (2,3-dihydrox-
ybenzylidene)-2-hydroxybenzohydrazide (H L) (Scheme 1) was pre-
3
◦
◦
◦
conditions: program 120 C for 1 min, 10 C/min, 200 C for 1 min,
pared following a literature method [46] upon condensation of the 2-
hydroxy benzohydrazide with 2,3-dihydroxybenzaldehyde.Scheme 2.
◦
injector at 240 C and helium as the carrier gas. Gas Chromatography-
2

M. Sutradhar et al.
Inorganica Chimica Acta 520 (2021) 120314
2
.3. Synthesis of [Cu
2
(
μ
-1κONO’:2κOO’:3κO-HL)(
μ
-1κONO’:2κOO’-
Table 1
Crystal data and structure refinement details for complex 1.
HL)] ‧4dmf (1)
2
1
⋅4dmf
To a 10 mL DMF solution of H
3
L (0.272 g, 1.00 mmol), 0.280 g (1.20
Empirical formula
Formula Weight
Crystal system
Space group
Temperature/K
a/Å
C
68 4 12 20
H68Cu N O
mmol) of Cu(NO O in 20 mL methanol was added and the re-
3
)
2
⋅2.5H
2
1627.50
action mixture was stirred for 30 min at room temperature. The resulting
dark green solution was then filtered, and the filtrate was kept in a 100
mL beaker in open air at room temperature for slow evaporation. After 3
days, green crystals suitable for X-ray diffraction analysis were isolated,
washed 2 times with cold methanol and dried in open air. The batch sent
for microanalysis fits well for two crystallization water molecules per
tetramer.
Monoclinic
C2/c
296(2)
26.58(4)
21.76(3)
14.060(19)
102.89(3)
7926(18)
4
b/Å
c/Å
◦
β/
3
V (Å )
Z
Yield: 72%. Anal. Calcd for (1‧4dmf) C68
4 12
H72Cu N O22: C, 49.10; H,
ꢀ
3
D
calc (g cm
F000
(Mo K
Rfls. collected/unique/observed
)
1.364
ꢀ 1
4
.36; N, 10.10. Found: C, 49.04; H, 4.31; N, 10.02. IR (KBr; cm ): 3446
3344
–
(C–O)enolic and 1158
(N–N). ESI-MS(+):
ꢀ 1
) (mm )
ν
(OH), 1609
ν
(C
–
N), 1252
ν
ν
μ
α
1.132
+
m/z 1336 [1+H] (100%).
66144/ 7618/ 4323
0.1611
R
int
a,
b
Final R1 wR2 (I ≥ 2
σ
)
0.0741, 0.1716
1.022
2
.4. Synthesis of [Cu(HL)]
4
‧4EtOH (2)
2
Goodness-of-fit on F
a
◦
R = Σ||F
o
|–|F
c
||/Σ|F
o
o
|.
Compound 2 was synthesized as reported earlier at 50 C [46], but it
b
2
2
2
2
4 ½
wR(F ) = [Σw(|F
|
– |F
c
| ) /Σw|F
o
| ] .
can be obtained by following the method described for 1 at room tem-
perature using ethanol instead of methanol as solvent and an excess of
pyridine. In this case, ethanol as solvent yielded good quality crystals.
The presence of excess of pyridine (base) helps enolization and depro-
tonation of all coordinated ligands during the complex formation.
◦
reaction mixture was maintained under stirring for 3 h at 80 C. When
3
HNO was used as an additive, the additive/catalyst molar ratio of 25 or
5
1
0 was used. Finally, 90 µL of cycloheptanone (internal standard) and
0 mL of diethyl ether (for substrate and organic products extraction)
To a 30 mL ethanol solution of H
3
L (0.272 g, 1.00 mmol), 0.280 g
were added. The obtained mixture was stirred for 10 min and then an
aliquot (1 µL) from the organic phase was analyzed by GC using the
internal standard method. For longer reaction periods, the reaction
mixtures were also analyzed by GC–MS to detect possible formation of
benzyl benzoate and benzoic acid, among other oxidation products.
(
1.20 mmol) of Cu(NO O in 10 mL ethanol was added and the
3
)
2
⋅2.5H
2
reaction mixture was stirred for 20 min at room temperature. To this
reaction mixture, 0.10 g (1.2 mmol) of pyridine was added dropwise and
the stirring was continued for 10 min. The resulting dark green solution
was then filtered, and the filtrate was kept in a 100 mL beaker in open air
at room temperature for slow evaporation. Green crystals suitable for X-
ray diffraction analysis were isolated in ca.3 days, washed 2 times with
cold ethanol and dried in open air.
3. Results and discussion
Anal. Calcd for C56
H
40
N
8
O
16Cu
4
(2): C, 50.38; H, 3.02; N, 8.39.
The aroylhydrazone Schiff base 2,3-dihydroxybenzylidene-2-
ꢀ 1
Found: C, 50.30; H, 3.05; N, 8.31. IR (KBr; cm ): 3426
ν
(OH). 1606
hydroxybenzohydrazide (H
different tetranuclear Cu(II) compounds, the cubane [Cu
1κONO’:2κOO’:3κO-HL)( -1κONO’:2κOO’-HL)] ‧4dmf (1) and the
known [46] open-cubane [Cu(HL)] ‧4EtOH (2). The aroylhydrazone
3
L) [46] was used to synthesize two
–
(C–O)enolic and 1071 (N–
ν
(C
–
N), 1252
ν
ν
N). ESI-MS(+): m/z 1336
2
(μ-
+
[
1+H] (100%).
μ
2
4
2
.5. X-ray measurements
exhibits keto-enol tautomerism in solution (Scheme 1). Complex forma-
tion via deprotonation of the enol form in solution is more common and
that can be accelerated by increasing the reaction temperature or by
addition of a base [43,46,53]. However, in the absence of such a type of
accelerating effect, the Cu(II) salt of a base of a strong acid can form an
aroylhydrazone-Cu(II) complex in the keto form at room temperature
[38,54]. In this study, we have synthesized a tetranuclear Cu(II) cubane
where the aroylhydrazone is in both keto and enol forms.
A poor-quality single crystal of 1 suitable for X-ray diffraction was
immersed in cryo-oil, mounted in a Nylon loop and measured at room
temperature. Intensity data were collected using a Bruker AXS PHOTON
1
00 diffractometer with graphite monochromated Mo-K
α (λ 0.71073)
radiation. Cell parameters were retrieved using Bruker SMART [47]
software and refined using Bruker SAINT [47] on all the observed re-
flections. Absorption corrections were applied using SADABS [48]. The
structure was solved by direct methods by using SHELXT2014/5 [49]
and refined with SHELXL-2018/3 [50]. Calculations were performed
using WinGX version 2018/3 [51]. All non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were included at geometrically
calculated positions and refined using a riding model. A void of 742 Å3
with 220 electrons was found in the structure by using the SQUEEZE
routine of PLATON [52] but the identification of the disordered mole-
cules could not be achieved; such electrons were removed from the
model. Least square refinements with anisotropic thermal motion pa-
rameters for all the non-hydrogen atoms and isotropic for the remaining
atoms were employed.
Reaction of H
3
L with Cu(NO
3
)
2
⋅2.5H
2
O in the methanol-dmf medium
and at room temperature results the formation of cubane 1 (Scheme 1) in
Table 2
◦
Selected bond distances (Å) and angles ( ) in complex 1.
N1–N2
1.419(7)
N3–N4
1.423(8)
1.909(7)
2.056(5)
2.301(6)
1.994(5)
1.998(4)
90.6(2)90.6(2)
82.5(2)
N1–Cu1
1.953(6)
N3–Cu2
O2–Cu2
O4–Cu2
O5–Cu2
O6–Cu2
O1–Cu1
1.987(5)
O2–Cu1
2.007(4)
O6–Cu1
2.023(5)
i
O7–Cu1
2.340(5)
i
i
N1–Cu1–O1
N1–Cu1–O2
O1–Cu1–O2
N1–Cu1–O6
O1–Cu1–O6
O2–Cu1–O6
N1–Cu1–O7
O1–Cu1–O7
O2–Cu1–O7
O6–Cu1–O7
81.8(2)
N3–Cu2–O6
N3–Cu2–O5
92.16(19)
167.52(17)
178.01(18)
96.64(19)
89.64(17)
101.72(19)
91.97(18)
99.96(18)
77.13(16)
O5–Cu2–O6
N3–Cu2–O2
O6–Cu2–O2
O5–Cu2–O2
N3–Cu2–O4
O6–Cu2–O4
O2–Cu2–O4
170.5(2)
172.5(2)
88.87(17)
97.16(19)
110.8(2)
97.59(19)
76.71(17)
2
.6. Catalytic studies
i
i
Under typical conditions, the peroxidative oxidation reactions were
carried out in a glass tube under atmospheric pressure as follows: to 3
mL of acetonitrile (NCMe), 5 mmol of toluene, 10 mol of catalyst 1 or 2
and 10 mmol of H (30% aq.) or TBHP (70% aq.) were added and the
μ
2 2
O
3

M. Sutradhar et al.
Inorganica Chimica Acta 520 (2021) 120314
Fig. 1. (a) Thermal ellipsoid plot (drawn at 20% probability level) of the asymmetric unit of complex 1 with partial atom labelling scheme (dmf molecules are
omitted for clarity, but their O101 and O201 oxygen atom are indicated). (b) The central cubane structure of 1 showing only the O- and N-coordinating atoms.
◦
Symmetry code: (i) 1-x,y,1/2-z. Hydrogen bonds, represented in dashed cyan colour, d(D⋯A) and ∠(D–H⋯A): N2–H2N⋯O3: 2.639(8) Å, 133 ; O4–H4A⋯O201:
◦
◦
◦
2
.728(15) Å, 144 ; N4–H4N⋯O8: 2.610(10) Å, 134 ; O4–H/A⋯O101: 2.669(10) Å, 130 .
◦
good yield. The open-cubane complex 2 can be synthesized at 50
C
cubane Cu
Cu1 cations can be considered to be in a Nimine(Oketo
phenol environment defining a highly distorted octahedron, despite
4 4
O moiety built from the four phenoxo atoms (Fig. 1b). The
according to the literature method [46] or alternatively by following a
similar procedure to that of 1 at room temperature but using ethanol and
adding an excess of pyridine (Scheme 1). Complex 1 was characterized
by elemental analysis, IR spectroscopy, ESI-Mass spectrometry, and
single crystal X-ray crystallography. The m/z value at 1336 corresponds
to [1+H+]+ and reveals the tetranuclear core in 1.
)
1
( -Ophenox-
μ
o 3
) O
3
the long Cu1-O2 bond distance (2.768(5) Å ). The Cu2-O6 dimension is
3
even longer (2.849(5) Å ), though shorter than the sum of the van der
Waals radii of copper and oxygen (1.40 and 1.52 Å, respectively), and
the geometry around this metal cation is perhaps better described as a
square pyramid drawn by a Nimine(Oenol
The remaining Cu–O bond distances (Table 1) range from 1.987(5) to
.340(5) Å, and the Cu–N distances (1.953(6) and 1.969(6) Å) are
)
1
(μ
2
-Ophenoxo) Ophenol setting.
3
.1. X-ray structures
2
slightly shorter than those found in related compounds [46]. The
intramolecular Cu⋅⋅⋅Cu distances range from 3.359(5) to 3.505(5) Å,
while the shortest intermolecular one assumes the value of 9.926(1) Å.
The structure is stabilized by intramolecular bond interactions involving
the NH group and the deprotonated and uncoordinated phenolate O3
and O8 atoms; the most relevant intermolecular H-contacts include the
bridging phenolic groups as donors and the Odmf-atoms as acceptors
Crystals of the compound 1 were obtained from methanol upon slow
evaporation, at room temperature. The molecular structure of 1, crys-
tallographic data and processing parameters are summarized in Table 1
and selected dimensions in Table 2.
Complex 1 crystallizes in the monoclinic system with space group
C2/c and the asymmetric unit contains two copper cations, two HL2
ꢀ
organic ligands, and two dmf molecules. Both the Schiff base anions
coordinate in the keto tautomeric form (Scheme 1) using the O1 and O5
keto atoms, the deprotonated phenoxo atoms O2 and O6 (Fig. 1a), and
the phenolic groups O4 and O7. Symmetry expansion revealed the
(
Fig. 1a).
Fig. 2. Effect of catalyst amount on the yield of the major products of oxidation of toluene (benzaldehyde + benzyl alcohol + o-cresol), catalyzed by 1 (a) and 2 (b).
◦
Reaction conditions: toluene (5.0 mmol), catalyst precursors 1 or 2 (2.5–20
μ
mol), H
2
O
2
(30% aq), NCMe (3 mL), 80 C, nacid/ncat = 25, 3 h reaction time.
4

M. Sutradhar et al.
Inorganica Chimica Acta 520 (2021) 120314
Table 3
a
2 2
Yields (benzaldehyde, benzyl alcohol and ortho-cresol) for the oxidation of toluene by TBHP or H O with 1 or 2 as catalyst precursors.
b
c
c
c
d
e
Entry
Cat.
Ox.
(nacid/ncat
)
Benzyl alcohol (%)
Benzaldehyde (%)
o-cresol (%)
Total Yield
TON
1
1
H
H
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
2
2
–
0.3
0.4
–
1.7
1.9
2.2
4.3
2.8
6.1
5.2
3.3
0.3
2.1
3.6
3.8
3.1
2.8
3.0
1.7
2.7
3.0
3.0
3.6
–
3.9
4.9
5.2
7.3
7.3
6.5
5.9
10.7
0.6
5.3
4.1
4.3
6.2
6.6
5.4
19
24
26
36
37
130
59
27
3
f
2
–
g
3
–
4
5
25
50
25
25
25
25
25
25
25
–
–
–
h
6
0.4
0.7
–
i
7
j
–
8
7.4
0.3
3.2
–
–
3.1
2.2
1.5
h,k
9
–
–
h,l
1
1
1
1
1
1
0
1
2
3
4
5
26
82
87
31
33
27
h,m
h,n
0.5
0.5
–
TBHP
TBHP
TBHP
25
50
1.6
0.9
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
H
H
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
2
2
–
–
0.4
0.4
–
2.7
2.8
2.8
5.0
4.9
5.5
4.8
3.6
0.13
0.9
3.4
3.8
1.8
3.1
3.2
–
2.9
5.7
6.5
3.0
5.2
1.8
4.3
5.2
–
0.4
0.04
0.4
–
5.6
8.9
9.7
8.0
10.1
7.3
9.1
8.8
0.13
1.4
3.5
4.3
1.8
4.1
4.1
–
28
48
44
40
51
146
91
22
0.7
7
f
–
g
–
25
50
25
25
25
25
25
25
25
–
–
h
–
i
–
j
–
h,k
h,l
h,m
h,n
–
–
–
70
85
9
–
TBHP
TBHP
TBHP
–
25
50
–
–
1.0
0.9
–
21
21
–
–
–
H
2
O
2
–
a
◦
Reaction conditions: toluene (5.0 mmol), 10
μ
mol of catalyst precursor 1 or 2, H
2
O
2
(10 mmol, 30% aq.) or TBHP (10 mmol, 70% aq.), NCMe (3 mL), 80 C, 3 h
reaction time.
b
3
HNO /catalyst molar ratio.
c
d
e
f
Molar yield (%) based on the substrate i.e. moles of product (benzaldehyde, benzyl alcohol or ortho-cresol) per mole of substrate, determined by GC.
Total Yield (%) = (moles of benzaldehyde + benzyl alcohol + ortho-cresol)/100 mol of substrate.
Turnover number = (moles of benzaldehyde + benzyl alcohol + ortho-cresol)/mol of Cu catalyst.
6
hours reaction.
g
h
i
2
4 hours reaction.
2
.5 µmol of catalyst.
µmol of catalyst.
5
2
j
0 µmol of catalyst.
k
l
◦
5
0
C.
Room temperature.
m
n
5
mmol of oxidant.
2
0 mmol of oxidant.
3
.2. Catalytic studies
benzaldehyde for lower amounts of catalyst (2.5–10
and 7, Table 3) and o-cresol appears as the main product when an
increased amount of catalyst (20 mol) (entry 8, Table 3) is used, for 2 a
μ
mol) (entries 1, 6
The catalytic performance of complexes 1 and 2 was evaluated for
μ
the homogeneous oxidation of toluene using H
2
O
2
(aq. 30%) or t-
considerable amount of o-cresol is already detected even for low Cu
concentrations (entries 21 and 22, Table 3). Benzoic acid, a more
oxidized product, is not observed even for the applied highest amount of
BuOOH (aq. 70%) as oxidants, in acetonitrile, under typical conditions
◦
of 80 C and 3 h reaction. The Cu(II) complexes 1 and 2 show compa-
rable catalytic performances, achieving an overall yield up to ca. 11%.
Under the abovementioned reaction conditions, toluene is oxidized
to yield predominantly benzaldehyde and o-cresol (the m and p-cresol
isomers were not observed), along with some amount of benzyl alcohol
1 or 2 (20 mol) (entries 8 and 23, Table 3, for 1 and 2, respectively)
μ
excluding the possibility of oxidation of the aldehyde to acid under these
reaction conditions [56].
The effect of the amount and type of oxidant on the oxidation reac-
tion was also investigated considering the reaction conditions of entries
(
Scheme 1), indicating the occurrence of both side-chain and ring
oxidation. No traces of benzyl benzoate (PhCOOCH Ph) were observed,
even for longer reaction periods (24 h) thus indicating that hemiacetal
formation (PhCH(OH)OCH Ph) is not a preferred pathway under the
2
6 and 21 from Table 3 (2.5
respectively, for comparison. In both cases, the molar ratio 1:2 toluene/
seems to be the most favourable to produce o-cresol (entries 1 and
μmol and nacid/ncat = 25) for 1 and 2,
2
2 2
H O
applied conditions [55]. The oxidation of toluene was also investigated
in the absence of the Cu(II) metal complexes and under the typical re-
action conditions no products were detected significantly.
16, Table 3, for 1 and 2, respectively). For both cases it appears that the
production of o-cresol relates to the amount of catalyst, since for the
lower amount of catalyst (2.5
seem to affect its production. For the same amount of catalyst (2.5
the 1:1 toluene/H molar ratio favours mostly the formation of ring
μ
mol), the amount of oxidant does not
Fig. 2 shows the effect of the amount of catalyst on the oxidation of
toluene. In general, the total yield increases with the catalyst amount.
The results also indicate a relationship between the selectivity and the
amount of catalyst used. Whereas 1 was found to be selective for
μmol)
2 2
O
oxidation products such as benzyl alcohol and benzaldehyde (entries 11
and 26, Table 3, for 1 and 2, respectively) and for a 1:2 M ratio the
5

M. Sutradhar et al.
Inorganica Chimica Acta 520 (2021) 120314
Fig. 3. Effect of reaction time on the yield of the major products of oxidation of toluene (benzaldehyde + benzyl alcohol + o-cresol), catalysed by 1 (a) and 2 (b).
◦
Reaction conditions: toluene (5.0 mmol), catalyst precursors 1 or 2 (10
μ
mol), H
2
O
2
(30% aq), NCMe (3 mL), 80 C, nacid/ncat = 25, reaction time = 3, 6 or 24 h.
amount of benzaldehyde almost doubles (entries 6 and 21, Table 3, for 1
and 2, respectively), whereas a high excess of oxidant (1:4 toluene/H
with high selectivity towards benzaldehyde after 10 h over a CuCr
spinel nanoparticles catalyst (0.1 g) and using H (50% aq. solution)
as oxidant [66]. Djinovi ´c et al. reported the production of CO
2 4
O
2
O
2
2 2
O
molar ratio) (entries 12 and 27, Table 3, for 1 and 2, respectively)
reduced the formation of the less oxidized products (benzyl alcohol and
benzaldehyde) with the formation of benzoic acid (detected by GC–MS),
probably due to the overoxidation of the benzaldehyde formed from
benzyl alcohol oxidation [55,57].
2
and water
◦
from the oxidation of toluene at high temperature (270–480 C) in the
presence of Cu-Fe functionalized disordered mesoporous silica catalysts.
Under the applied reaction conditions, products of partial oxidation
(benzaldehyde and benzoic acid) were not detected [67]. The predom-
inance of ring-oxidation products (cresols) as well as the high selectivity
for o-cresol isomer was also observed in the toluene oxidation with 30%
Fig. 3 shows the influence of reaction time on the product yields and
selectivities for the periods of 3, 6 and 24 h (10 mol of 1 or 2). When the
μ
reaction time is extended from 3 to 24 h, the yield of benzaldehyde in-
creases slightly in the presence of 1 and keeps almost unchanged in the
presence of 2, whereas the amount of ortho-cresol increases continuously
in both cases (e.g., in the presence of 2 it increases ca. 97% (from 2.9 to
2 2
aqueous H O catalyzed by vanadium(V)-substituted polyoxometalates,
at 80◦C for 3 h [68].
4. Conclusions
5
.7%) from 3 to 6 h reaction. In the case of 2 and for longer reaction
periods, residual amounts of benzoic acid were detected probably due to
the overoxidation of benzaldehyde since by using benzaldehyde as
starting material and under the same reaction conditions just benzoic
acid was formed [56].
A
novel tetranuclear Cu(II) cubane complex [Cu2( -
μ
1κONO’:2κOO’:3κO-HL)(
(2,3-dihydroxybenzylidene)-2-hydroxybenzohydrazide (H L) is re-
μ
-1κONO’:2κOO’-HL)] ‧4dmf (1) derived from
2
3
ported. Compound 1 is neutral as confirmed by X-ray crystallography, as
Considering the previously recognized promoting effect of an acidic
medium on the peroxidative oxidation of toluene [58], the presence of
an acid additive was explored. Among the tested acids, such as pyridine-
the previously reported open-cubane [Cu(HL)] ‧4EtOH (2). Both copper
4
(II) complexes were tested successfully as catalysts for the oxidation of
toluene, used as a model substrate for the oxidative conversion of toxic
VOCs to added value chemical products. Under the explored experi-
mental conditions, both side-chain and ring oxidations occur, with for-
mation of benzaldehyde and ortho-cresol as the main products. This
study emphasizes the easy conversion of VOCs to valuable compounds
by easy single-pot catalytic oxidations.
4
-carboxylic acid (HPCA), trifluoroacetic acid (TFA) and nitric acid
(
HNO ), the last two were equivalent and their effect was more
3
expressive. The presence of the acid additive resulted in an improvement
in the total yield and amount of produced o-cresol (entries 1, 4 and 5 for
1
, and 16, 19 and 20 for 2, Table 3).
The mechanism of this reaction is believed [59–62] to start with the
•
•
formation of oxygen-based radicals HOO and HO (upon oxidation or
involving the Cu2 /Cu redox pair),
+ +
Declaration of Competing Interest
reduction of the oxidant H O
2 2
which can abstract an H atom from toluene (PhCH
3
) to form the benzyl
•
•
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
radical PhCH
2
that by reaction with O
2
leads to the PhCH
2
OO peroxyl
radical. The peroxyl radical may abstract hydrogen from PhCH
3
and give
rise to PhCH
2
OOH, whereas PhCHO and PhCH
2
OH can be produced by
•
metal-assisted reduction of PhCH
2
OO (for PhCHO) or through
•
Acknowledgements
PhCH
2
OO dismutation leading to PhCHO and PhCH
2
OH (plus O
2
). On
the other hand, the H atom abstraction may occur at the aromatic ring to
form an aromatic radical which may react with oxygen-based radicals
producing cresoxy radicals. The resonance stabilized cresoxy radicals
upon combination with H-atoms give cresols [63,64].
This work has been supported by the Fundaç ˜a o para a Ci ˆe ncia e
Tecnologia (FCT) 2020–2023 multiannual funding to Centro de Química
Estrutural (project UIDB/00100/2020) and by the RUDN University
Strategic Academic Leadership Program. Authors are also grateful to the
FCT project PTDC/QUI-QIN/29778/2017 for financial support. M.S.
acknowledges the FCT and IST for a working contract “DL/57/2017”
Our catalytic system can be compared to other Cu-based catalytic
systems reported previously, and we can essentially highlight its selec-
tivity for the toluene oxidation products, benzaldehyde and ortho-cresol
isomer. Wang et al. conducted the liquid phase oxidation of toluene with
molecular oxygen (1.0 MPa) and in the presence of pyridine (to avoid
the overoxidation of benzaldehyde to benzoic acid) over heterogeneous
catalysts of copper-based binary metal oxides achieving 7% conversion
of toluene (86% selectivity for benzaldehyde) at 463 K after 2 h [65].
More recently, Acharyya et al. reported a toluene conversion of 57.5%
(
Contract no. IST-ID/102/2018).
Appendix A. Supplementary data
CCDC 2053025 for 1.4dmf contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Center via www.ccdc.cam.
6

M. Sutradhar et al.
Inorganica Chimica Acta 520 (2021) 120314
ac.uk/data_request/cif. Supplementary data to this article can be found
online at https://doi.org/10.1016/j.ica.2021.120314.
[34] K. Gauli, R.N. Ram, H.P. Soni, J. Mol. Catal. A: Chem. 242 (2005) 161–167.
[
[
[
35] H. Gao, G.W. Chen, Q. Yuan, H.Q. Li, Catal. Today. 110 (2005) 171–178.
36] M.A. Gonzalez, S.G. Howell, S.K. Sikdar, J. Catal. 183 (1999) 159–162.
37] M. Sutradhar, L.M.D.R.S. Martins, M.F.C. Guedes, da, Silva, A.J.L. Pombeiro, Appl.,
Catal. A: Gen. 493 (2015) 50–57.
References
[
[
38] M. Sutradhar, L.M.D.R.S. Martins, M.F.C. Guedes da Silva, E.C.B.A. Alegria, C.-
M. Liu, A.J.L. Pombeiro, Dalton Trans. 43 (2014) 3966–3977.
[
[
[
1] J.A. Horsley Catalytica Environmental Report No. E4, Catalytica Studies Division,
Mountain View. 1993, CA, USA.
2] T. Cai, H. Huang, W. Deng, Q.G. Dai, W. Liu, X.Y. Wang, Appl. Catal. B. 166 (2015)
39] M. Alexandru, M. Cazacu, A. Arvinte, S. Shova, C. Turta, B.C. Simionescu,
A. Dobrov, E.C.B.A. Alegria, L.M.D.R.S. Martins, A.J.L. Pombeiro, V.B. Arion, Eur.
J. Inorg. Chem. (2014) 120–131.
3
93–405.
[
40] M.-F. Zaltariov, M. Alexandru, M. Cazacu, S. Shova, G. Novitchi, C. Train,
A. Dobrov, M.V. Kirillova, E.C.B.A. Alegria, A.J.L. Pombeiro, V.B. Arion, Eur. J.
Inorg. Chem. 29 (2014) 4946–4956.
3] S.A.C. Carabineiro, X. Chen, O. Martynyuk, N. Bogdanchikova, M. Avalos-Borja,
A. Pestryakov, B. Tavares, J.J.M. Orfao, M.F.R. Pereira, J.L. Figueiredo, Catal.
Today 244 (2015) 103–114.
[
[
[
[
[
41] M. Sutradhar, E.C.B.A. Alegria, K.T. Mahmudov, M.F.C. Guedes da Silva, A.J.
L. Pombeiro, RSC Adv. 6 (2016) 8079–8088.
[
4] J.L. Zeng, X. Liu, L.J. Wan, H.L. Lv, T.Y. Zhu, J. Mol. Catal. A: Chem. 408 (2015)
2
21–227.
42] M. Sutradhar, L.M.D.R.S. Martins, S.A.C. Carabineiro, M.F.C. Guedes da Silva, J.G.
Buijnsters, J.L. Figueiredo, A.J.L. Pombeiro, ChemCatChem. 8 (2016) 2254-2266.
43] M. Sutradhar, E.C.B.A. Alegria, M.F.C. Guedes da Silva, L.M.D.R.S. Martins, A.J.
L. Pombeiro, Molecules 21 (2016) 425.
[
[
[
5] A. Jones, Atmos. Environ. 33 (1999) 4535–4564.
6] F.I. Khan, A.K. Ghoshal, J. Loss Prevent. Proc. 13 (2000) 527–545.
7] R. Balzer, V. Drago, W.H. Schreiner, L.F.D. Probst, J. Braz. Chem. Soc. 25 (2014)
2
026–2031.
44] S.E. Allen, R.R. Walvoord, R.P. Salinas, M.C. Kozlowski, Chem. Rev. 113 (2013)
[
[
8] R. Balzer, L.F.D. Probst, V. Drago, W.H. Schreiner, H.V. Fajardo, Braz. J. Chem.
Eng. 31 (2014) 757.
9] A.G.M. da Silva, H.V. Fajardo, R. Balzer, L.F.D. Probst, A.S.P. Lovon, J.J. Lovon-
Quintana, G.P. Valenca, W.H. Schreine, P.A. Robles-Dutenhefner, J. Power Sources
6
234–6458.
45] M. Sutradhar, L.M.D.R.S. Martins, M.F.C. Guedes da Silva, A.J.L. Pombeiro in: A.J.
L. Pombeiro, M.F.C Guedes da Silva, (Eds.), Alkane Functionalization, Wiley,
Hoboken, NJ, 2019, ch 16, pp 319-336.
2
85 (2015) 460.
[
46] M. Sutradhar, M.V. Kirillova, M.F.C. Guedes da Silva, C.-M. Liu, A.J.L. Pombeiro,
Dalton Trans. 42 (2013) 16578–16587.
[
10] A.G.M. da Silva, T.S. Rodrigues, T.J.A.; Slater, E.A. Lewis, R.S. Alves, H.V. Fajardo,
R. Balzer, A.H.M. da Silva, I.C. de Freitas, D.C. Oliveira, J.M. Assaf, L.F.D. Probst, S.
J. Haigh, P.H.C. Camargo, ACS Appl. Mater. Interfaces 7 (2015) 25624-25632.
11] W.X. Tang, G. Liu, D.Y. Li, H.D. Liu, X.F. Wu, N. Han, Y.F. Chen, Sci. China Chem.
[
[
47] Bruker, APEX2, Bruker AXS Inc., Madison, Wisconsin, USA, 2012.
48] G.M. Sheldrick, SADABS. Program for Empirical Absorption Correction. University
of Gottingen, Germany, 2000.
[
[
5
8 (2015) 1359–1366.
[
[
49] M. George, Sheldrick, SHELXT – Integrated space-group and crystal structure
Determination, Acta Cryst. A71 (2015) 3–8.
12] Z. Jiang, M.X. Chen, J.W. Shi, J. Yuan, W.F. Shangguan, Catal. Surv. Asia 19 (2015)
1
–16.
50] G.M. Sheldrick (2015) Crystal structure refinement with SHELXL, Acta Cryst., C71,
[
[
[
13] Q.H. Trinh, S.B. Lee, Y.S. Mok, J. Hazard. Mater. 285 (2015) 525–534.
14] C. Zhang, H. Cao, C. Wang, M. He, W. Zhan, Y. Guo, J. Hazard. Mater. 402 (2021).
15] C. Zhang, Y. Wang, G. Li, L. Chen, Q. Zhang, D. Wang, X. Li, Z. Wang, Appl. Surf.
Sci. 532 (2020).
3
-8 (Open Access). https://doi.org/10.1107/S2053229614024218.
[
[
[
51] L.J. Farrugia, J. Appl. Crystallogr. 45 (2012) 849–854.
52] A.L. Spek, Acta Crystallogr., Sect. C 71 (2015) 9–18.
53] M. Sutradhar, L.M.D.R.S. Martins, M.F.C. Guedes da Silva, K.T. Mahmudov, C.-
M. Liu, A.J.L. Pombeiro, Eur. J. Inorg. Chem. (2015) 3959–3969.
54] M. Sutradhar, E.C.B.A. Alegria, M.F.C. Guedes da Silva, C.-M. Liu, A.J.L. Pombeiro,
Molecules 23 (2018) 2699.
[
16] K. Zeng, Z. Wang, D. Wang, C. Wang, J. Yu, G. Wu, Q. Zhang, X. Li, C. Zhang, X.
S. Zhao, Catal. Today (2020), https://doi.org/10.1016/j.cattod.2020.05.043.
17] P. G ´e lin, M. Primet, Appl. Catal. B: Environ. 39 (2002) 1–37.
18] J.R.; Gonz ´a lez-Velasco, A. Aranzabal, J.I. Guti ´e rrez-Ortiz, R. L o´ pez-Fonseca, M.A.
Guti ´e rrez-Ortiz, Appl. Catal. B: Environ. 19 (1998) 189-197.
[
[
[
[
55] M.I.B. Saiman, G.L. Brett, R. Tiruvalam, M.M. Forde, K. Sharples, A. Thetford, R.
L. Jenkins, N. Dimitratos, J.A. Lopez-Sanchez, D.M. Murphy, D. Bethell, D.
J. Willock, S.H. Taylor, D.W. Knight, C.J. Kiely, G.J. Hutchings, Angew. Chem. Int.
Ed. 51 (2012) 1–6.
[
[
[
[
[
19] M. Sutradhar, E.C.B.A. Alegria, T. Roy Barmana, F. Scorcelletti, M.F.C. Guedes da
Silva, A.J.L. Pombeiro, Mol. Catal. 439 (2017) 224-232.
20] M. Sutradhar, T. Roy Barman, E.C.B.A. Alegria, M.F.C. Guedes da Silva, C-M. Liu,
H-Z. Kou, A.J.L. Pombeiro, Dalton Trans. 48 (2019) 12839-12849.
21] M. Sutradhar, T. Roy Barman, E.C.B.A. Alegria, H.M. Lapa, M.F.C. Guedes da Silva,
A.J.L. Pombeiro, New J. Chem. 44 (2020) 9163-9171.
[
56] X. Wang, J. Wu, M. Zhao, G. Yufen Lv, C.Hu Li, J. Phys. Chem. C 113 (2009)
1
4270–14278.
[
[
57] M. Nawab, S. Barot, R. Bandyopadhyay, New J. Chem. 43 (2019) 4406–4412.
58] H.M. Lapa, M.F.C. Guedes da Silva, A.J.L. Pombeiro, E.C.B.A. Alegria, L.M.D.R.
S. Martins, Inorg. Chim. Acta 512 (2020).
22] M. Sutradhar, T. Roy Barman, A.J.L. Pombeiro, L.M.D.R.S. Martins, Molecules 24
(
2019) 47 (1-12).
[
59] M.V. Kirillova, M.L. Kuznetsov, Y.N. Kozlov, L.S. Shul’pina, A. Kitaygorodskiy, A.J.
L. Pombeiro, G.B. Shul’pin, ACS Catal. 1 (2011) 1511–1520.
23] H. Falcke, S. Holbrook, I. Clenahan, A. L. Carretero, T. Sanalan, T. Brinkmann, J.
Roth, B. Zerger, S. Roudier, L. D. Sancho, Best Available Techniques (BAT)
Reference Document for the Production of Large Volume Organic Chemicals; EUR
[
[
[
[
[
60] G.B., Shul’pin,, J. Mol. Catal. A: Chem. 189 (2002) 39–66.
61] G.B. Shul’pin, C. R. Chim. 6 (2003) 163–178.
2
9
8882 EN; Publications Office of the European Union, Luxembourg, 2017, ISBN
78-92-79-76589-6, doi:10.2760/77304, JRC109279.
62] Y.N. Kozlov, G.V. Nizova, G.B. Shul’pin, J. Phys. Org. Chem. 21 (2008) 119–126.
63] F. Battin-Leclerc, Prog. Energy Combust. Sci. 34 (2008) 440–498.
64] M.D. Boot, M. Tian, E.J.M. Hensen, S.M. Sarathy, Prog. Energy Combust. Sci. 60
[
[
[
[
24] S.-J. Li, Y.-G. Wang, Tetrahedron Lett. 46 (2005) 8013–8015.
25] A. Gunay, K.H. Theopold, Chem. Rev. 110 (2010) 1060–1081.
26] J.-B. Feng, X.-F. Wu, Appl. Organometal. Chem. 29 (2015) 63–86.
27] A.J.L. Pombeiro, M.F.C. Guedes da Silva (Eds.), Alkane Functionalization, Wiley,
Hoboken, NJ, 2019.
(
2017) 1–25.
[
[
[
65] F. Wang, J. Xu, X. Li, J. Gao, L. Zhou, R. Ohnishi, Adv. Synth. Catal. 347 (2005)
1
987–1992.
66] S.S. Acharyya, S. Ghosh, R. Tiwari, B. Sarkar, R.K. Singha, C. Pendem, T. Sasakib,
[
28] A.J.L. Pombeiro, Alkane Functionalization: Introduction and Overview, in: Alkane
Functionalization, A.J.L. Pombeiro, M.F.C Guedes da Silva, (Eds.), Wiley & Sons,
Hoboken, NJ, 2019, Chapter 1, pp. 1-15.
R. Bal, Green Chem. 16 (2014) 2500–2508.
67] P. Djinovi ´c a, A. Risti ´c a, T. Zumbara, V.D.B.C. Dasireddya, M. Rangusa, G. Drazica,
ˇ
ˇ ´
M. Popovab, B. Likozara, N.Z. Logara, N.N. Tu ˇs ar, Appl. Catal. B: Environ. 268
[
[
[
[
[
29] A.E. Shilov, G.B. Shul’pin, Chem. Rev. 97 (1997) 2879–2932.
30] G.B. Shul’pin, Org. Biomol. Chem. 8 (2010) 4217–4228.
(
2020).
68] K. Nomiya, K. Hashino, Y. Nemoto, M. Watanabe, J. Mol. Catal. A: Chem. 176
2001) 79–86.
[
31] H.R. Mardani, H. Golchoubian, J. Mol. Catal. A Chem. 259 (2006) 197–200.
32] L.I. Matienko, L.A. Mosolova, G.E. Zaikov, Russ. Chem. Rev. 78 (2009) 211–230.
33] A. Bottino, G. Capannelli, A. Comite, R. Di Felice, Catal. Today 99 (2005) 171–177.
(
7