Cu(II) complexes of N-rich aroylhydrazone: Magnetism and catalytic activity towards microwave-assisted oxidation of xylenes

Article abstract of DOI:10.1039/c9dt02196e

The new aroylhydrazone N-(di(pyridin-2-yl)methylene)pyrazine-2-carbohydrazide (HL) species, rich in N-donor sites, has been used to synthesize Cu(ii) compounds with different nuclearities, viz. the binuclear [Cu₂(μ-1κN³,2κN²O-L)(Cl)₃(MeOH)] (1), the octanuclear [Cu₄(μ-1κN³,2κN²O-L)₂(μ-Cl)₃(Cl)₃]₂ (2) and the 1D coordination polymer [Cu₃(μ₃-1κN³,2κN²O,3κN-L)(μ-NO₃)(NO₃)₃(H₂O)₃]_n·nNO₃ (3). They have been characterized by elemental analysis, FT-IR and single crystal X-ray diffraction. The magnetic properties of 2 and 3 have been explored using variable temperature magnetic measurements. The catalytic performances of the compounds were evaluated towards the peroxidative oxidation of o-, p- A nd m-xylenes under microwave irradiation, leading to the formation of the corresponding methyl benzyl alcohol, tolualdehyde and toluic acid as the major products. Complex 3 exhibits the best catalytic activity towards the oxidation of p-xylene with a total yield of 37% (4-methylbenzyl alcohol + p-tolualdehyde + p-toluic acid).

Full text of DOI:10.1039/c9dt02196e

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Trans., 2019, DOI: 10.1039/C9DT02196E.
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Pages 3073-3412
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Cu(II) complexes of N-rich aroylhydrazone: magnetism and
catalytic activity towards microwave-assisted oxidation of xylenes
Manas Sutradhar,*a Tannistha Roy Barman,a Elisabete C.B.A. Alegria,*a,b M. Fátima C.
Guedes da Silva,a Cai-Ming Liu,c Hui-Zhong Kou,d and Armando J. L. Pombeiro*a
aCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av.
Rovisco Pais, 1049-001 Lisboa, Portugal
bChemical Engineering Departament, Instituto Superior de Engenharia de Lisboa, Instituto
Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1, 1959-007 Lisboa, Portugal
cNational Laboratory for Molecular Sciences, Center for Molecular Science, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
dDepartment of Chemistry, Tsinghua University, Beijing 100084, China
*Corresponding authors: manas@tecnico.ulisboa.pt (MS), ebastos@deq.isel.ipl.pt (ECBAA),
pombeiro@tecnico.ulisboa.pt (AJLP)
Abstract
The new aroylhydrazone N'-(di(pyridin-2-yl)methylene)pyrazine-2-carbohydrazide
(HL) species, rich in N-donor sites, has been used to synthesize Cu(II) compounds with
different nuclearities, viz. the binuclear [{Cu2(-1N3,2N2O-L)(Cl)3(MeOH)] (1), the
octanuclear [Cu4(-1N3,2N2O-L)2(-Cl)3(Cl)3]2 (2) and the 1D coordination polymer
[Cu3(3-1N3,2N2O,3N-L)(-NO3)(NO3)3(H2O)3]nnNO3
(3).
They
have
been
characterized by elemental analysis, FT-IR and single crystal X-ray diffraction. The magnetic
properties of 2 and 3 have been explored using variable temperature magnetic measurements.
The catalytic performances of the compounds were evaluated towards the peroxidative
oxidation of o-, p- and m-xylene under microwave irradiation, leading to the formation of the
corresponding methyl benzyl alcohol, tolualdehyde and toluic acid as the major products.
Complex 3 exhibits the best catalytic activity towards the oxidation of p-xylene with a total
yield of 37% (4-methylbenzyl alcohol + p-tolualdehyde + p-toluic acid).
Introduction
Volatile organic compounds (VOCs) are hydrocarbons with a high vapour pressure at
ordinary room temperature, known as major contributors to air pollution, either directly
through their toxic or malodorous nature, or indirectly as ozone and smog precursors.1
1

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Benzene, toluene and xylene are some of the major VOCs with a high toxic potential to
human health.2,3
Catalytic oxidation is considered as one of the most interesting and promising
pathways for the removal of VOCs owing to its easy application with high efficiency and low
costing.4,5 Catalytic oxidation of VOCs over supported noble metals and metal oxides
catalysts has been widely studied.6,7 Besides the elimination of VOCs, the selective
functionalization of aromatic hydrocarbons is also an additional advantage for their catalytic
oxidation. However, the cleavage of C-H bond in large scale is still a challenge, judging by
the high temperature and pressure usually required in the current industrial processes.8–10 The
establishment of clean synthetic routes through a green chemistry approach involves the
design of efficient catalysts from environmental and economic perspectives.11,12 Benzyl
alcohol, benzaldehyde, tolualdehyde, toluic acid, terephthalic acid and benzoic acid constitute
examples of oxygenated products which are relevant intermediates for a diversity of
chemicals, agrochemicals, fragrances, pharmaceuticals and polymers.13-18 Both homogeneous
and heterogeneous catalysts based on transition metal complexes have been reported.19-23
Our group has been involved in the development of catalytic oxidation processes
assisted by microwave (MW) irradiation as a beneficial technique.24 The choice of Cu(II)
catalysts for oxidation reactions is rather attractive on account of their abundant resources,
low cost, non-toxic properties and high catalytic efficiency.25,26 Some of them also exhibit
interesting magnetic properties.26 The oxidation of toluene with Cu(II) complexes was
previously reported by us.27 In the present investigation, o-, m- and p-xylene were chosen as
model substrates for the catalytic oxidation of hydrocarbons in the field of VOCs.
Herein, we report the syntheses of three new polynuclear Cu(II) complexes (1-3) derived
from the new aroylhydrazone N'-(di(pyridin-2-yl)methylene)pyrazine-2-carbohydrazide
(HL). Variable temperature magnetic measurements have been carried out for 2 and 3. These
complexes have been screened towards the oxidation of the xylene isomers. The influence of
various parameters, such as, reaction time, type of catalyst, temperature and presence of
additives, was also studied towards their optimization.
Results and discussion
The new aroylhydrazone N'-(di(pyridin-2-yl)methylene)pyrazine-2-carbohydrazide
(HL), rich in N-donor sites, has been used to synthesize the polynuclear Cu(II) complexes 1-3
(Scheme 1). The two pyridine moieties in the ligand open the possibility to obtain at least a
binuclear complex, what was not achieved in the previous study (a mononuclear complex was
2

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formed).27 Moreover, the presence of the pyrazine group in the ligand leads to additional
possibilities to generate coordination polymers which was absent in the previous case.27
When copper chloride is treated with HL in methanol the binuclear complex [{Cu2(-
1N3,2N2O-L)(Cl)3(MeOH)] (1) is obtained, but the octanuclear chloro-bridged complex
[Cu4(-1N3,2N2O-L)2(-Cl)3(Cl)3]2 (2) is formed when the reaction is carried out in
acetonitrile. In both 1 and 2 only one of the nitrogen atoms of pyrazine is engaged in
coordination. When copper nitrate reacts with HL in methanol both nitrogen atom of pyrazine
are bonded to copper leading to the formation of the coordination polymer [Cu3(3-
1N3,2N2O,3N-L)(-NO3)(NO3)3(H2O)3]nnNO3 (3). In solution and in the presence of the
metal ion, the aroylhydrazone (HL) deprotonates via enolization and acts as a N5O1 (in 1) or a
N6O1 (in 2 and 3) mono-negative donor towards two metal ions. The molecular structures of
the compounds have been ascertained by single crystal X-ray diffraction. In addition, they
have been characterized by elemental analysis, FT-IR spectroscopy and ESI-MS. Variable
temperature magnetic moment measurements have also been performed. The catalytic
properties of 1-3 have been studied towards the oxidation of o-, m- and p-xylenes with H2O2
under mild conditions. Polymer 3 is the best catalyst towards the oxidation of p-xylene to 4-
methylbenzyl alcohol, p-tolualdehyde and p-toluic acid.
Scheme 1 Syntheses of 1-3.
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X-ray structures
X-ray quality crystals of the compounds were obtained from methanol (for 1 and 3) or
acetonitrile (in the case of 2) upon slow evaporation, at room temperature. The
crystallographic data for 1-3 are summarized in Table 1 and selected dimensions are
presented in Table S1. The molecular structures of 1-3 are presented in Figures 1-3.
Table 1. Crystal data and structure refinement details for complexes 1- 3.
1
C17H15Cl3Cu2N6O2
568.78
2
C64H44Cl12Cu8N24O4
2146.95
3
C16H17Cu3N11O19
858.02
Empirical formula
Formula Weight
Crystal system
Monoclinic
P21/n
Monoclinic
P21/c
Monoclinic
P21/n
Space group
Temperature/K
297(2)
296 (2)
110 (2)
a/Å
7.601 (5)
23.571 (6)
11.823 (5)
90
18.152 (2)
12.223 (2)
18.826 (3)
90
10.6328 (5)
23.5926 (11)
11.5359 (6)
90
b/Å
c/Å

100.410 (5)
90
90.086 (4)
90
99.735 (2)
90
/

V (Å3)
2083.4 (17)
4
4176.9 (11)
2
2852.2 (2)
4
Z
Dcalc (g cm–3)
1.813
1.707
1.998
(Mo K) (mm–1)
2.454
2.439
2.32
Rfls.
39522/5172/4569
51711/6324/4777
80728 /8740/7935
collected/unique/observed
Rint
0.0276
0.0892
0.0246
Final R1 a, wR2 b (I  2σ)
Goodness-of-fit on F2
0.0272, 0.0734
0.0990, 0.2433
0.0215, 0.0514
1.077
1.112
1.058
a R = ||Fo|–|Fc||/|Fo|; b wR(F2) = [w(|Fo|2 – |Fc|2)2/w|Fo|4]½
The copper cations in 1 exhibit both a perfect (Cu1,5 of 0.01) and a distorted square
pyramidal geometry and (Cu2,5 of 0.24).28,29 The aroylhydrazone ligand binds the metal
cations in the monoanionic (L) form and acts as a bis-chelate ligand by means of the Oenolate
,
N
imine, Npyridine-atoms for one of the copper cations, and Npyrazine, Namide, Npyridine-atoms for the
other. The coordination spheres of the metals are fulfilled by two chloride anions, or by only
one chloride and a methanol molecule. The asymmetric unit of 2 includes two L ligands
each one coordinating to two copper cations in a way identical to that of complex 1, each
copper cation also binding two chloride anions. Upon symmetry expansion, an octanuclear
compound is revealed with the metals presenting distorted square-pyramidal geometries (5
in the 0.09 – 0.23 range), except for Cu1 which has an octahedral N3Cl3 environment. The
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structure is stabilized by intramolecular ꞏꞏꞏ stacking interactions involving the pyridine
rings (Fig. 4), with centroidꞏꞏꞏcentroid distances of 3.939 and 3.772 Å.30
The asymmetric unit of 3 includes one L ligand coordinated to two copper cations in
a way similar to that of 1 and 2, but the remaining pyrazine nitrogen donates to a third copper
cation. The coordination environments are accomplished with a chelating nitrate anion and a
water molecule (in Cu1), a chelating nitrate as well as a monodentate nitrate (in Cu2) and a
monodentate nitrate anion and two water molecules (in Cu3), rendering the copper cations
Cu1 and Cu2 with a highly distorted octahedral geometry, and Cu3 with a distorted square
planar one which converts to an octahedral arrangement upon symmetry expansion revealing
a bridging chelate nitrate anion linking Cu2 and Cu3.
The different nuclearities in 1 – 3 have an influence in the twisting of L as evidenced
by the angles between the least-square planes of the pyridine rings: 39.49º (1), 39.54 and
44.72º (2) and 44.75º (3). Despite the difference in the bending of L in 2 the intramolecular
CuNNCu distances are equal (4.717 Å) and intermediate between those for 1 and 3 (4.738
and 4.692 Å, in this order). The minimum intermolecular CuCu distances varies in the order
5.544 Å (3) < 6.634 Å (1) < 6.650 Å (2) Hydrogen bond interactions are relevant in 1 (Fig.
1), involving the methanol ligand as donor and the chloride Cl1 as acceptor, and give rise to
an infinite 1D chain. In 3 such a type of interactions is also important, includes the nitrate
anions as acceptors and coordinated water molecules as donors, and extends the structure to
the third dimension.
Fig. 1 Structural representation of 1 with partial atom labelling scheme.
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Fig. 2 Structural representation of 2 with partial atom labelling scheme.
(a)
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(b)
Fig. 3 (a) Asymmetric unit of 3 with partial atom labelling scheme. (b) 1D-polymeric chain
of 3 (hydrogen atoms are omitted for clarity).
Fig. 4 ꞏꞏꞏ stacking interactions in 2.
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Magnetic Properties
The magnetic susceptibilities of 2 and 3 were investigated in the range of 2 – 300 K under
the field of 2000 Oe. As shown in Fig. 5, the room temperature χT value of 2.49 cm3 K mol-1
for 2 is somewhat smaller than 3.00 cm3 K mol-1 expected for eight isolated Cu2+ ions (S =
1/2, g = 2), and the χT value of 1.12 cm3 K mol-1 at 300 K for 3 is in agreement with the
theoretical value for three isolated Cu2+ ions (1.125 cm3 K mol-1, assumed g = 2). Both 2 and
3 show evident antiferromagnetic interactions, which can be easily seen from the gradual
decrease in the χT values of 2 and 3 with the decrease of temperature.
2.5
2
3
2.0
1.5
1.0
0.5
0.0

0
50
100
150
200
250
300
T/K
Fig. 5 Plot of χT vs. T for complexes 2 and 3. The solid lines represent the best theoretical
fitting.
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Cu6
Cu1
J1
J2
J2
J1
Cu7
Cu5
Cu4
Cu2
J3
J2
J2
J1
J1
Cu8
Cu3
(a)
Cu1
Cu5
J2
Cu3
J3
J1
J3
J1
Cu2
Cu4
J2
Cu12
Cu6
J2
J1
J3
Cu10
Cu7
J1
Cu9
J2
J3
Cu8
Cu11
(b)
Fig. 6. Magnetic topology of the Cu8 cluster in 2 (a) and Cu12 cycle in 3 (b) (The copper
numbers do not concern crystallographically independent copper ions, but rather are used to
distinguish them)
As shown in Fig. 6a, the magnetic topology of complex 2 is proposed based on its crystal
structure.
The
Hamiltonian
for
the
Cu8
cluster
is
described
by
H=−2J1(SCu1SCu2+SCu3SCu4+SCu5SCu6+SCu7SCu8)−2J2(SCu1SCu4+SCu2SCu3+SCu6SCu7+SCu5SCu8)−2J3
SCu4SCu5, where J1, J2 and J3 represent the magnetic coupling constants of two copper(II) ions
through the N-N diazine bridge, the monochloro bridge and the dichloro bridge, respectively.
The MagPack software31 was used to fit the magnetic data, but, J2 and J3 always went in the
opposite direction, and no reasonable fitting resulted. Since both are magnetically exchanged
through the chlorine atom, J3 can be approximately equal to J2. In this case, the best fitting
gave J1 = 104.4 cm1, J2 = 40.6 cm1, g = 2.29, R = 9.6ⅹ105. The J1 value is comparable to
that
of
[Cu2(POAP)Br3(H2O)]
(POAP
=
N-(amino(pyridin-2-
yl)methylene)picolinohydrazonate) (J1 = 123 cm1),32 suggesting the antiferromagnetic
interaction mediated by the N-N diazine bridge in complex 2 is strong.
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As described above, complex 3 is a polymeric chain composed of trinuclear units, and thus
three magnetic coupling constants J1, J2 and J3 are necessary, which correspond to the
magnetic exchange through the N-N diazine bridge, the nitrate bridge and the pyrazine bridge,
respectively. Unfortunately, there is no suitable formula in the literature, but it can be
approximated by MagPack software31 using a cluster loop of Cu12 that is connected by the
Cu3 units end to end (notably, the maximum paramagnetic centers are 14 for this software).
As shown in Fig. 6b, the magnetic topology of the Cu12 cycle in complex 3 is proposed. The
Hamiltonian is described by
H
=
−2J1(SCu1SCu2+SCu4SCu5+SCu7SCu8+SCu10SCu11
)
−
2J2(SCu2SCu3+SCu4SCu6+SCu7SCu9+SCu10SCu12) − 2J3(SCu3SCu4+SCu6SCu7+SCu9SCu10+SCu2SCu12). The
best fitting gave J1 = 112.9 cm1, J2 = 38.8 cm1, J3 = 0.2 cm1, g = 2.32, R = 3.9ⅹ105.
The distance of the two copper(II) ions bridged by the N-N diazine group in complex 3 is
4.692 Å, a little smaller than that in complex 2 (4.717 Å). Hence, a corresponding stronger
magnetic coupling mediated by the N-N diazine bridge is expected for 3 (J1 = 112.9 cm1)
with respect to 2 (J1 = 104.4 cm1).
Catalytic studies
o-, m- and p-xylenes were chosen as model substrates to investigate the catalytic properties of
complexes 1-3 for selective oxidation of aromatic hydrocarbons. The oxidation reactions
were carried out in liquid-phase (acetonitrile as solvent) under microwave irradiation and, for
comparative purpose, under conventional heating, using hydrogen peroxide (H2O2, 30% aq.
sol.) as oxidant (Scheme 2 and Table 2). The oxidation of the xylene isomers follows a
sequence of partially oxidized intermediates, e.g., for p-xylene (p-xyl) being p-methylbenzyl
alcohol (p-tolylmethanol) (p-BA) → p-methylbenzaldehyde (p-tolualdehyde) (p-TOL) → p-
methylbenzoic acid (p-toluic acid) (p-TA) (Scheme 2). No subsequent oxygenated products,
such as p-carboxybenzaldehyde or terephthalic acid, were detected (by GC-MS and HPLC).
Moreover, no oxidation of the aromatic ring was observed under the reaction conditions, in
accord with the rather inert character of the aromatic ring, which is not activated by the
Cu(II) catalysts under study. Blank experiments have been conducted and no reaction is
observed in the absence of 1-3.
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Scheme 2
The results of the oxidation of xylenes by 1-3 are listed in Table 2 and depicted in Figs. 7 and
8. For all catalytic systems, the trend of the overall yield of oxygenated products of xylene
isomers follows the order of p-xylene > m-xylene > o-xylene, for each copper(II) complex (1-
3) (Fig. 7, Table 2). In all cases, methylbenzyl alcohol (BA), tolualdehyde (TOL) and toluic
acids (TA) are detected as the major products. Fig. 8 shows that the different xylene isomers
yield distinct amounts of oxygenated derivatives, with the maximum total yield occurring for
p-xylene oxidation. Such results indicate that the positions of the two weakly electron-
donating methyl groups of xylenes have influence on the reactivity of the substrate. Lower
yields are observed for the xylenes with the methyls in the ortho and meta positions, what can
be accounted for by the higher steric constrains associated to such positions.33,34 For example,
the microwave assisted oxidation of p-xylene at 50 ºC for 3 h using complex 3 as catalyst
leads to the formation of 4-methyl benzyl alcohol (20.2%), p-tolualdehyde (9.2%) and p-
toluic acid (7.6%) (entry 13, Table 2), whereas 2-methyl benzyl alcohol (7.2%), o-
tolualdehyde (6.2%) and o-toluic acid (1.3%) (entry 7, Table 2) are obtained from the
oxidation of o-xylene under the same conditions. It is worth to note that, for all xylene
isomers, oxidation of methyl groups results also in the formation of methylbenzyl alcohol
(BA). This product can be an intermediate for the aldehyde formation. The methyl benzyl
alcohol and tolualdehyde are usually the preferred products, being complex 3 favourable to
the formation of the alcohol, whereas complexes 1 and 2 tend to exhibit slight increases of
the aldehyde over the alcohol (Table 2). In the oxidation of p-xylene the formation of a
significant amount (12%) of toluic acid was observed in the presence of 1. For all complexes,
the highest produced amount of toluic acid occurs for the para isomer.
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Fig. 7 Total yield (BA + TOL + TA) in the presence of 1-3 for o-, m- and p-xylene oxidation.
Reaction conditions: o-, m- and p-xylene (5 mmol), 1, 2 or 3 (10 μmol), H2O2 30% aq. sol.
(10 mmol), NCMe, 50 ºC, 3 h under MW-irradiation (5 W).
Fig. 8 o-, m- and p-xylene oxidation in the presence of complexes 1-3. Reaction conditions:
o-, m- and p-xylene (5 mmol), 1-3 (10 μmol), H2O2 30% aq. sol. (10 mmol), NCMe (3 mL),
50 ºC, 3 h under MW-irradiation (5 W). xyl = xylene. BA = methylbenzyl alcohol, TOL =
tolualdehyde, TA = toluic acid (TA).
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Complex 3, besides being the most active catalyst for all xylene isomers (Fig. 8), is
also the most selective for any of the isomers, with the selectivity for alcohol ranging from 49
to 62% (entries 7, 10 and 13, Table 2). However, although complex 3 gives greater yields of
products relative to 1 and 2, if we consider the number of metal centres, the activities of 1 and
3 are comparable. Hence, for example, in the oxidation of p-xylene (entries 3, 6 and 13, Table
2) we have 11.3% total yield/Cu atom for 1 (binuclear complex), 2.7% total yield/Cu atom
for 2 (octanuclear complex) and 12.3% total yield/Cu atom for 3 (on the basis of the
repeating trinuclear unit).
The oxidation reaction of the p-xylene isomer was also carried out under conventional
heating and it was found that in some cases it requires twice the reaction time to achieve
similar yields to those obtained under microwave radiation (entries 16-18, Table 2). The
oxidation reaction of p-xylene was attempted in the absence of solvent but without success.
For comparative purposes, the starting salts used in the synthesis of the complexes 1-3 were
also tested, with total yields of only up to 3%, thus showing the relevance of the N-donor
ligands in the Cu coordination sphere. The ligand itself did not exhibit any activity.
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Table 2 - Oxidation of o-, m- and p-xylene with H2O2 under microwave irradiation (MW) and
under conventional heating at 50 °C, using 1-3 as catalyst precursors.
Yield (%)
Entry Catalyst Substrate
BA
4.1
TOL
6.1
TA
1.8
Totalb
1
2
o-xyl
12.0
1
2
m-xyl
7.1
5
6.6
9
2.3
8.6
1.4
2.6
7.5
1.3
1.0
0.3
3.4
3.0
1.7
7.6
6.5
1.1
12.4
8
16.0
22.6
7.1
3
p-xyl
o-xyl
4
1.3
4.4
6.6
8.5
6.2
5.8
0.4
5.9
5.4
1.9
9.2
9.3
1.7
10.3
7.5
6.6
0.1
0.1
5
m-xyl
8.2
17.4
21.6
14.7
15.7
2.0
6
5.6
p-xyl
o-xyl
7
7.2
8c
o-xyl
o-xyl
m-xyl
m-xyl
m-xyl
8.9
9d
1.3
10
11c
12c
13
14c
15d
16e
17e
18e
19f
20g
21h
15.3
16.3
4.9
24.6
24.7
8.5
3
20.2
23.9
5.6
37.0
39.7
8.4
p-xyl
p-xyl
p-xyl
1
10.1
7.3
32.8
22.8
26.3
0.4
2
p-xyl
3
1
16.4
0.2
3.3
0.1
0.1
p-xyl
1
0.3
0.5
CuCl2
0.4
1.6
1.5
0.7
0.2
0.6
2.1
2.9
p-xyl
p-xyl
22h
Cu(NO3)2
aReaction conditions: o-, m- and p-xylene (5 mmol), catalyst precursor 1-3 (10 μmol), H2O2
b
(30% aq.) (10 mmol), NCMe (3 mL), 50 ºC, 3 h, MW (5W). Moles of products [alcohol +
c
aldehyde + acid]/100 moles of o-, m- or p-xylene isomer, determined by GC. An excess of
d
triphenylphosphine (PPh3) was added prior to the GC analysis. Reaction performed in the
14

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e
f
presence of TEMPO radical (0.125 mmol). Under conventional heating, 6 h. MW, solvent-
g
h
free conditions, 50ºC, 3 h. Under conventional heating, solvent-free conditions, 3 h. For
comparative purposes. o-, m-, and p-xyl = o-, m-, and p-xylene; BA = methylbenzyl alcohol,
TOL = tolualdehyde, TA = toluic acid.
The effect of acid additive (HNO3) on the peroxidative oxidation of xylenes was also
explored for 1-3. Following the same experimental conditions, we observed a drop of the
total yield [BA + TOL + TA] of the reaction (Fig. 9 for p-xylene oxidation in the presence of
3 and Figs. S1, S2 and Table S2, ESI, for o- and m-xylene oxidation in the presence of 1-3).
The decrease in yield was approximately the same for the various n(HNO3)/n(cat) ratios used
(10, 25 or 50). It is believed that the presence of an acid may deactivate the catalyst.
Fig. 9 Effect of HNO3 additive on the peroxidative oxidation of p-xylene in the presence of 3.
Reaction conditions: p-xylene (5 mmol), 3 (10 μmol), H2O2 30% aq. sol. (10 mmol), NCMe
(3 mL), additive HNO3 [n(HNO3)/n(cat) = 10, 25 or 50], 50 ºC, 3 h under MW-irradiation (5
W).
When radical trapping experiments were performed with the addition of 2,2,6,6-
tetramethylpiperidin-l-oxyl (TEMPO), a stable free radical, to the reaction medium, a marked
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reduction of total yield occurs. For instance, for the o-, m- and p-xylene oxidation performed
in the presence of 3, when the reaction is carried out in the presence of TEMPO (0.125
mmol), only 14, 34 or 23% of total yield is obtained respectively, relative to the total yield
obtained when the reaction is carried out without any additive (entries 9, 12 and 15 relatively
to entries 7, 10 and 13, Table 2). This suggests that the reaction mechanism may involve
radicals, conceivably produced in the catalyst promoted decomposition of H2O2 with the
formation of a reduced form of the catalyst and possibly an hydroxo-Cu species,23,35-37
according to equations 1-7. The HO/HOO radicals can abstract an H atom from the

substrate (RCH3) to form the RCH2 benzyl radical which then reacts with dioxygen, leading
to the RCH2OO peroxyl radical. RCH2OOH can be formed, e.g., upon hydrogen abstraction
from RCH3 by RCH2OO, whereas RCHO and RCH2OH can be produced by reduction of
RCH2OO• (for RCHO) or through RCH2OO• (for RCHO and RCH2OH).
The involvement of the xylylhydroperoxide (RCH2OOH) as a primary oxidation
product is accounted for by a method developed by Shul'pin [35] for alkane oxidation.
Reaction samples of the microwave-assisted oxidation of o-, p- and m-xylenes in the presence
of 3 were treated with an excess of triphenylphosphine (PPh3) prior to the GC analysis and
the results compared with those obtained without PPh3 (entries 8, 11 and 14 versus entries 7,
10 and 13, respectively, Table 2). There occurs an increase in the amount of the alcohol
RCH2OH due to the reduction of the peroxide RCH2OOH by PPh3 (with formation of
O=PPh3), with a concomitant decrease in the amounts of the other oxidized products.
In order to try to detect possible species involved in the reaction mechanism, we have
analyzed by ESI-MS the reaction solution of p-xylene+H2O2 in CH3CN in the presence of
catalyst 1 (after 3 h under microwave radiation at 50 ºC) and observed (in the negative mode)
a peak at m/z = 597 which possibly can be accounted for by the peroxido/xylylhydroperoxido
[1-(MeOH+3Cl)+(OO)+(OOCH2C6H4CH3)]-. In accord, the corresponding peroxido [1-
(MeOH+3Cl)+2(OO)]- (m/z = 493) was observed when the experiment was undertaken in the
absence of the substrate (p-xylene). Moreover, the presence of the p-xylylhydroperoxide
RCH2OOH (see above) was also confirmed by ESI-MS(+) by the detection of a peak at m/z =
139 in the p-xylene+H2O2+CH2CN+1 system.
Cu(II)+ H2O2 → HOO + H+ + Cu(I)
Cu(I) + H2O2 → HO + Cu(II) + HO−
(1)
(2)
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
HO + RCH3 → H2O + RCH2
(3)
(4)
(5)
(6)
(7)
RCH2 + O2 → RCH2OO

RCH2OO + RCH3 → RCH2OOH + RCH2
RCH2OO + Cu(I) → Cu(II) + RCHO + HO−
2 RCH2OO → RCH2OH + RCHO + O2

Conclusions
Xylenes, major volatile organic compounds (VOCs), have a high toxic potential to
human health, and in this study we show a way of their catalytic conversion to valuable
organic compounds. The aroylhydrazone N'-(di(pyridin-2-yl)methylene)pyrazine-2-
carbohydrazide (HL), rich in N-donor sites, is a convenient pro-ligand to form multinuclear
copper(II) complexes. In fact, its reactions with copper(II) chloride dihydrate in different
solvents (MeOH and NCMe) lead to the formation of Cu(II) complexes with different
nuclearities, [{Cu2(-1N3,2N2O-L)(Cl)3(MeOH)] (1) in MeOH and [Cu4(-1N3,2N2O-
L)2(-Cl)3(Cl)3]2 (2) in NCMe, whereas the use of copper(II) nitrate results in the 1D
coordination polymer [Cu3(3-1N3,2N2O,3N-L)(-NO3)(NO3)3(H2O)3]nnNO3 (3). These
compounds catalyze the oxygenation of a methyl group of xylene isomers (o-, m-, p-xylene)
yielding methyl benzyl alcohol, tolualdehyde and toluic acid as major products (no
hydroxylation of the aromatic ring), under mild reaction conditions (low temperature, H2O2,
low catalyst loading) and in the absence of any additive. The methyl benzyl alcohol and
tolualdehyde are usually the main products, with polymer 3 being favourable to the formation
of the alcohol and complexes 1 and 2 to the aldehyde. The ability to afford toluic acid (yields
up to 12%) indicate an exceptional activity of the catalytic systems. Complex 3 exhibits the
best catalytic activity towards the oxidation of p-xylene with a total yield of 37% (4-
methylbenzyl alcohol + p-tolualdehyde + p-toluic acid). The reactivity of xylene isomers
follow the order: p-xylene > m-xylene > o-xylene for each copper(II) complex. Microwave
irradiation reduced to half the reaction time to achieve similar yields to those obtained under
conventional heating. These catalytic systems are of significance towards the development of
sustainable processes for p-xylene oxidation and deserve further development.
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Experimental
General materials and procedures
The synthetic part of this study was performed in air. Commercially available
reagents and solvents were used as received, without further purification or drying. Two
different metal sources, CuCl2.2H2O and Cu(NO3)2.2.5H2O, were used for the synthesis of
complexes.
C, H, and N elemental analyses were carried out by the Microanalytical Service of the
Instituto Superior Técnico. Infrared spectra (4000–400 cm–1) were recorded on a Bruker
1
Vertex 70 instrument in KBr pellets; wavenumbers are in cm–1. The H NMR spectra were
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 ionization, the drying gas
and flow rate were optimized according to the particular sample with 35 p.s.i. nebulizer
pressure. Scanning was performed from m/z 100 to 1200 in methanol solution. The
compounds were observed in the positive mode (capillary voltage = 80–105 V). The magnetic
susceptibility measurements were carried out on polycrystalline samples with a Quantum
Design MPMS-XL5 SQUID magnetometer in the temperature range of 2–300 K and at
applied field of 2000 Oe. Diamagnetic corrections were estimated from Pascal’s constants for
all constituent atoms.38 The microwave-assisted xylene oxidation reactions were performed
using a 10 mL capacity reaction tube with a 13 mm internal diameter and carried out in a
focused Anton Paar Monowave 300 microwave fitted with a rotational system and an IR
temperature detector. Gas 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. Helium was used as the carrier gas. HPLC was conducted on a Waters
Alliance 2695 HPLC system equipped with a sample cooler, a column heater and a 2998
photodiode array detector (Massachussets, USA). Analysis were performed with a Luna C18
(2) column (150 mm ×4.6 mm; 5 μm; Phenomenex, Torrance, CA), at a flow rate of 1 mL
min−1, following elution gradient program: isocratic conditions with 95% water and 5%
acetonitrile for 5 min; gradient elution from 5 to 12 min until a mobile phase composition of
55% water, 35% acetonitrile and 10% methanol was reached; a second linear gradient from
12 to 19 min, thus reaching the final composition of 15% water, 75% acetonitrile and 10%
methanol; isocratic elution from 19 to 29 min followed by a 6 min re-equilibration step. The
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volume of the samples injected by the autosampler was 10 μL. The UV absorbance was
monitored at 254 nm.
Syntheses of the pro-ligand HL
The new pro-ligand N'-(di(pyridin-2-yl)methylene)pyrazine-2-carbohydrazide (HL)
was prepared in a way that is similar to that described in the literature for other
carbohydrazides39 upon condensation of di(2-pyridyl) ketone with pyrazine-2-
carbohydrazide.
Yield: 82.0%. Anal. calc. for C16H12N6O: C, 63.15; H, 3.97; N, 27.62%. Found: C,
1
63.11; H, 3.95; N, 27.58%. IR (KBr pellet, cm1): 2323 (NH), 1693 (C=O), H NMR
(DMSO-d6, ): 9.73-7.51 (s, 11H, C4H3N2 and C5H4N).
Synthesis of Cu(II) complexes (1-3)
Synthesis of [{Cu2(-1N3,2N2O-L)(Cl)3(MeOH)] (1)
To a 20 mL methanol solution of HL (0.304 g, 1.00 mmol), a 10 mL methanol
solution of CuCl2ꞏ2H2O (0.340 g, 2.00 mmol) was added and the reaction mixture was stirred
for 20 min at room temperature. The resultant dark green solution was filtered and the filtrate
was kept in open air. Dark green single crystals suitable for X-ray diffraction analysis were
isolated after 3 days. Crystals were washed 3 times with cold methanol and dried in open air.
Yield: 0.390 g (69%, with respect to Cu(II)). Anal. Calcd for (1) C17H15Cl3Cu2N6O2:
C, 35.90; H, 2.66; N, 14.78. Found: C, 35.86; H, 2.64; N, 14.74. IR (KBr; cm1): 3237
(OH), 1586 and 1559 (C=N), 1249 (C–O)enolic and 1160 (N–N). ESI-MS(+): m/z 569
[CuL+H]+ (100%).
Synthesis of [Cu4(-1N3,2N2O-L)2(-Cl)3(Cl)3]2 (2)
A similar synthetic procedure as mentioned for 1 was employed, but acetonitrile was
used as the solvent instead of methanol. On slow evaporation in air, green crystals were
obtained after 2 days.
Yield: 0.400 g (74%, with respect to Cu(II)). Anal. Calcd for C66H50Cl12Cu8N24O4 (2):
C, 36.41; H, 2.31; N, 15.44. Found: C, 36.38; H, 2.29; N, 15.40. IR (KBr; cm1): 1559
(C=N), 1252 (C–O)enolic and 1161 ( N–N). ESI-MS(+): m/z 1083 [½(2)+H]+ (100%).
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Synthesis of [Cu3(3-1N3,2N2O,3N-L)(-NO3)(NO3)3(H2O)3]nnNO3 (3)
To a 20 mL methanol solution of HL (0.304 g, 1.00 mmol), an excess of
Cu(NO3)2ꞏ2.5H2O (0.930 g, 4.00 mmol) was added and the reaction mixture was stirred for
20 min at room temperature. The resultant greenish brown solution was filtered and the
filtrate was kept in open air. Dark brown single crystals suitable for X-ray diffraction analysis
were isolated in ca. 3-4 days. Crystals were washed 3 times with cold methanol and dried in
open air.
Yield: 0.648 g (71%, with respect to Cu(II)). Anal. Calcd for C20H28Cu3N11O19 (2): C, 26.19;
H, 3.08; N, 16.80. Found: C, 26.16; H, 3.05; N, 16.76. IR (KBr; cm1): 3092 (OH), 1270
(C–O)enolic, 1363 (NO3) and 1160  (N–N). ESI-MS (+): m/z = 916 [M+H]+ (100%).
X-ray crystallography
Good quality single crystals suitable for X-ray diffraction of 1 - 3 were immersed in cryo-oil,
mounted in Nylon loops and analyzed at the temperature of 296 K (for 1 and 2) and 110 K for
3. Intensity data were collected using a Bruker AXS PHOTON 100 diffractometer with
graphite monochromated Mo-Kα ( 0.71073) radiation. Data were collected using omega
scans of 0.5º per frame and full sphere of data were obtained. Cell parameters were retrieved
using Bruker SMART40 software and refined using Bruker SAINT40 on all the observed
reflections. Absorption corrections were applied using SADABS.40 Structures were solved by
direct methods by using SIR9741 and refined with SHELXL-2014 or -2018.42 Calculations
were performed using WinGX v2014.1.43 All non-hydrogen atoms were refined
anisotropically. Those H-atoms bonded to carbon were included in the model at
geometrically calculated positions and refined using a riding model. Uiso(H) were defined as
1.2Ueq of the parent carbon atoms for aromatic residues and 1.5Ueq for the methyl groups.
The hydrogen atoms attached to oxygen were located in the difference Fourier map and
refined using Uiso(H) as 1.5 of the parent atom. Least square refinements with anisotropic
thermal motion parameters for all the non-hydrogen atoms and isotropic for the remaining
atoms were employed.
Catalytic studies
The oxidations of o-, m- and p-xylene were carried out under microwave irradiation and
conventional heating conditions. For the microwave-assisted reactions, o-, m- or p-xylene (5
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mmol, 615 μL), 30% aqueous H2O2 (10 mmol, 1.02 mL), acetonitrile (3 mL) and catalyst 1-3
(10 μmol) were added to an appropriate Pyrex tube (10 mL). The system was placed in the
microwave reactor and left with stirring and under microwave irradiation (5-10 W), at 50 °C
for 1, 3 or 6 h. Finally, 90 µL of cycloheptanone (internal standard) and 10 mL of diethyl
ether (for substrate and organic products extraction) were added. On the bench, the oxidation
reactions were conducted in a round-bottomed flask equipped with a conventional heating
system. In a typical procedure, the same reaction mixture of reactants with the same ratios as
those used in MW experiments were placed in the reactor and stirred for 3 or 6 h. A magnetic
stirrer with a thermostat and an oil bath were used to maintain the temperature at 50°C. The
analysis of the products was done by GC as indicated above. The composition of the reaction
mixture was confirmed by GC-MS and, for the oxidation of p-xylene in the presence of 3, by
HPLC.
Appl. Organometal. Chem. 2010, 24, 464. Catalysts, 2016, 6, No. 4, paper No. 50 (40 pages).
doi:10.3390/catal6040050).
Acknowledgements
The authors gratefully acknowledge the Fundação para a Ciência e a Tecnologia (FCT),
Portugal, and its projects PTDC/QEQ-ERQ/1648/2014, PTDC/QEQ-QIN/3967/2014, and
UID/QUI/00100/2019. M.S. acknowledges the FCT and IST for a working contract
“DL/57/2017” (Contract no. IST-ID/102/2018). C.-M. L. acknowledges the funding from the
National Natural Science Foundation of China (21871274).
The authors acknowledge the Portuguese NMR Network (IST-UTL Centre) for access to the
NMR facility, and the IST Node of the Portuguese Network of mass-spectrometry (Dr
Conceição Oliveira) for the ESI-MS measurements.
Supporting information
CCDC 1918246-1918248 for 1-3 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data to this article can be
found online at .....
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Page 25 of 25
Dalton Transactions
View Article Online
DOI: 10.1039/C9DT02196E
Graphical Abstract
VOC
VOC
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Three new polynuclear Cu(II) complexes derived from the new aroylhydrazone N'-
(di(pyridin-2-yl)methylene)pyrazine-2-carbohydrazide (HL) have been screened towards the
oxidation of the xylene isomers.
25