Nitro group reduction and Suzuki reaction catalysed by palladium supported on magnetic nanoparticles modified with carbon quantum dots generated from glycerol and urea

Article abstract of DOI:10.1002/aoc.3984

Glycerol and urea were used as green and cheap sources of carbon quantum dots (CQD) for modifying Fe₃O₄ nanoparticles (NPs). The obtained CQD@Fe₃O₄ NPs were used for the stabilization of palladium species and the prepared catalyst, Pd@CQD@Fe₃O₄, was characterized using various techniques. This magnetic supported palladium was applied as an efficient catalyst for the reduction of aromatic nitro compounds to primary amines at room temperature using very low palladium loading (0.008?mol%) and also for the Suzuki–Miyaura cross-coupling reaction of aryl halides as well as challenging heteroaryl bromides and aryl diazonium salts with arylboronic acids and with potassium phenyltrifluoroborate. This magnetically recyclable catalyst was recovered and reused for seven consecutive runs in the reduction of 4-nitrotoluene to p-toluidine and for ten consecutive runs in the reaction of 4-iodoanisole with phenylboronic acid with small decrease of activity. The catalyst reused in the Suzuki reaction was characterized using transmission electron microscopy, vibrating sample magnetometry and X-ray photoelectron spectroscopy. Using experiments such as hot filtration and poisoning tests, it has been shown that the true catalyst works under homogeneous conditions according to the release–return pathway of active palladium species.

Full text of DOI:10.1002/aoc.3984

Received: 16 May 2017
DOI: 10.1002/aoc.3984
Revised: 21 June 2017
Accepted: 23 June 2017
F U L L P A P E R
Nitro group reduction and Suzuki reaction catalysed by
palladium supported on magnetic nanoparticles modified
with carbon quantum dots generated from glycerol and urea
Mohammad Gholinejad1
| Fatemeh Zareh1 | Carmen Nájera2
1 Department of Chemistry, Institute for
Advanced Studies in Basic Sciences
(IASBS), Zanjan, Iran
2 Departamento de Química Orgánica and
Centro de Innovación en Química
Avanzada (ORFEO‐CINQA), Universidad
de Alicante, Alicante, Spain
Glycerol and urea were used as green and cheap sources of carbon quantum
dots (CQD) for modifying Fe3O4 nanoparticles (NPs). The obtained
CQD@Fe3O4 NPs were used for the stabilization of palladium species and the
prepared catalyst, Pd@CQD@Fe3O4, was characterized using various tech-
niques. This magnetic supported palladium was applied as an efficient catalyst
for the reduction of aromatic nitro compounds to primary amines at room
temperature using very low palladium loading (0.008 mol%) and also for the
Suzuki–Miyaura cross‐coupling reaction of aryl halides as well as challenging
heteroaryl bromides and aryl diazonium salts with arylboronic acids and with
potassium phenyltrifluoroborate. This magnetically recyclable catalyst was
recovered and reused for seven consecutive runs in the reduction of 4‐
nitrotoluene to p‐toluidine and for ten consecutive runs in the reaction of 4‐
iodoanisole with phenylboronic acid with small decrease of activity. The
catalyst reused in the Suzuki reaction was characterized using transmission
electron microscopy, vibrating sample magnetometry and X‐ray photoelectron
spectroscopy. Using experiments such as hot filtration and poisoning tests, it
has been shown that the true catalyst works under homogeneous conditions
according to the release–return pathway of active palladium species.
Correspondence
Mohammad Gholinejad, Department of
Chemistry, Institute for Advanced Studies
in Basic Sciences (IASBS), PO Box 45195‐
1159, Gavazang, Zanjan 45137‐6731, Iran.
Email: gholinejad@iasbs.ac.ir
Carmen Nájera, Departamento de Química
Orgánica and Centro de Innovación en
Química Avanzada (ORFEO‐CINQA),
Universidad de Alicante, Apdo. 99,
E‐03080 Alicante, Spain.
Email: cnajera@ua.es
Funding information
Iran National Science Foundation, Grant/
Award Number: 95844587; the Generalitat
Valenciana, Grant/Award Number:
PROMETEOII/2014/017; the Spanish
Ministerio de Economía, Industria y
Competitividad, Agencia Estatal de
Investigación (AEI) and Fondo Europeo
de Desarrollo Regional (FEDER, EU),
Grant/Award Number: CTQ2016‐81797‐
REDC and CTQ2016‐76782‐P; the Spanish
Ministerio de Economía y Competitividad
(MINECO), Grant/Award Number:
CTQ2014‐51912‐REDC and CTQ2013‐
43446‐P
KEYWORDS
carbon quantum dots, glycerol, magnetic, palladium, Suzuki
1 | INTRODUCTION
decreasing or eliminating chemical wastes, particularly
when large‐scale operations or industrial scales are
Nowadays environmental problems encourage chemists
to design new reaction processes complemented with
green chemistry objectives.[1] Along this line, recyclable
heterogeneous catalysts play an important role in
considered.[2] However, despite the extensive applications
of heterogeneous catalysts in organic transformations,
using simple methods such as filtration or centrifugation
are not efficient and precise for the separation of
Appl Organometal Chem. 2017;e3984.
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https://doi.org/10.1002/aoc.3984

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GHOLINEJAD ET AL.
expensive and toxic metals such as palladium which have
less than 5 ppm allowable limits in pharmaceuticals.[3,4]
On the other hand, magnetic materials, due to their
superparamagnetic behaviour and therefore ease of
separation from reaction mixtures, large surface area to
volume ratio and low toxicity, have been considered as
promising supports for the stabilization of metal
catalysts.[5]
Reduction of nitro compounds to the corresponding
amines is a significant chemical reaction from industrial
and academic standpoints. The resulting amines are
valuable intermediates in the synthesis of various
materials such as dyes, polymers, pesticides, antioxidants,
drugs and agrochemicals. Various transition metals show
good activities for the reduction of nitro compounds.
However, palladium‐catalysed nitro reductions offer high
efficiency and selectivity for the formation of primary
amines.[6] Also, among the vast number of reactions
catalysed by palladium, the Suzuki–Miyaura reaction,
which is the cross‐coupling reaction between aryl, vinyl,
or alkyl halides or pseudo‐halides and organoboron
reagents, could be considered one of the most important
methods for the formation of different types of com-
pounds, especially biphenyls.[7]
Concerning solid supports, carbon quantum dots
(CQD) are a new class of carbon‐derived nanomaterials
with unique properties such as high solubility, photo‐
stability and fluorescence emission, being considered
green, non‐toxic, abundant and inexpensive materials.[8]
In the past few years, CQD have been prepared using
various green and cheap sources such as chitosan,[9]
orange peels,[10] coffee grounds,[11] coriander leaves[12]
and bloomed algae.[13] Recently, CQD were synthesized
using glycerol as green solvent via a pyrolysis process
and applied in cell imaging and drug release.[14] New
applications of CQD were reported by Dey and co‐
workers as reducing agents for the synthesis of palladium
nanoparticles (NPs) and their applications as efficient cat-
alysts in Heck and Suzuki reactions.[15] Also, we recently
reported the modification of magnetic NPs and magne-
sium oxide with CQD or graphene quantum dots and
their application for stabilization of Pd and Pd–Cu NPs
in various cross‐coupling reactions.[16] In continuation of
our interest in CQD and magnetic palladium catalysts,
herein, we report palladium supported on magnetic NPs
of Fe3O4 modified with nitrogen‐doped CQD obtained
from glycerol and urea as green and non‐toxic sources.
We envisaged that this material, Pd@CQD@Fe3O4 NPs,
could be used as a recyclable catalyst in the reduction of
nitroarenes and in Suzuki–Miyaura reactions.
2 | RESULTS AND DISCUSSION
Synthetic steps for the preparation of the catalyst are sum-
marized in Scheme 1. The corresponding CQD were
obtained by heating glycerol and urea in a Teflon
autoclave at 180 °C. Photoluminescence emission study
of the prepared CQD showed a broad emission centred
at 430 nm with an excitation wavelength at 350 nm
(Figure 1), which is a characteristic sign for the formation
of CQD.[7] In the case of CQD@Fe3O4 NPs, aqueous
solution of the prepared CQD was mixed with prepared
Fe3O4 NPs[17] at 80 °C for 48 h. In the final step, an
aqueous solution of CQD@Fe3O4 NPs was treated with a
sonication‐assisted solution of PdCl2 under vigorous
stirring, and the final solid Pd@CQD@Fe3O4 NPs were
obtained with a Pd content of 0.08 mmol g−1 determined
using atomic absorption spectroscopy.
Transmission electron microscopy (TEM) images of
Pd@CQD@Fe3O4 at different magnifications show that
the sample has uniform particles of 15–20 nm. Despite
that Pd species are not visible in TEM images, a very thin
layer of CQD shell is detectable around the magnetic NPs
(Figure 2).
Also, scanning electron microscopy (SEM) images
show the presence of highly monodispersed and uniform
magnetic NPs (Figure 3).
The corresponding energy‐dispersive X‐ray spectros-
copy (EDS) mapping images of Pd@CQD@Fe3O4 indi-
cated the presence of C, N, Fe and Pd in the structure
(Figure 4).
SCHEME 1 Steps for the preparation of
Pd@CQD@Fe3O4 NPs

GHOLINEJAD ET AL.
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FIGURE 1 Fluorescence spectrum of CQD prepared from
glycerol and urea
Further insight into the surface composition of the
Pd@CQD@Fe3O4 NPs was obtained from X‐ray photo-
electron spectroscopy (XPS). High‐resolution XPS of the
obtained materials in the C 1 s region confirmed the pres-
ence of carbon from the appearance of four peaks at
284.58, 285.3, 286.02 and 288.80 eV corresponding to
C―C/C═C, C―N, C―O and C═O forms of carbon,
respectively (Figure 5).[18]
FIGURE 3 SEM image of Pd@CQD@Fe3O4 NPs
showed the presence of two peaks at 335.6 and 340.8 eV
related to Pd(0) and peaks at 338 and 343 eV related to
Pd(II) species.[19,20] As can be seen in Figure 6, Pd is
mainly in metallic state proving the noteworthy ability
of CQD for reducing Pd(II) to Pd(0). The mechanism of
the metal reduction is not clear for us. However, it is
believed that the various hydroxyl and carboxyl
Also, the presence of Pd in the structure was proved
using XPS by studying the binding energy range of
335–342 eV related to Pd 3d5/2 and Pd 3d3/2. These results
FIGURE 2 TEM images of Pd@CQD@Fe3O4 NPs at different magnifications

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GHOLINEJAD ET AL.
FIGURE 5 XPS spectrum of Pd@CQD@Fe3O4 NPs in C 1 s
region
FIGURE 6 XPS spectrum of Pd@CQD@Fe3O4 NPs in Pd 3d
region
functionalized groups on the CQD surface can act as
reducing agents and produce metal NPs.[15,21]
Powder X‐ray diffraction (XRD) study of
Pd@CQD@Fe3O4 showed peaks at 2θ values of 40.2°,
46.7° and 68.2° related to (111), (200) and (220) planes
of Pd(0) and 2θ values of 30.1°, 35.5°, 43.1°, 53.4°, 57.0°
and 62.6° corresponding to (210), (311), (400), (422),
(511) and (440) planes of Fe3O4 (Figure 7).[22]
The superparamagnetic property of the catalyst was
confirmed from the magnetization curve (74 emu g−1),
showing zero coercivity and remanence on the magneti-
zation loop and absence of hysteresis loop (Figure 8).
The nitrogen adsorption–desorption isotherms for
Pd@CQD@Fe3O4 NPs were of type IV with H3 hysteresis
loop according to the IUPAC classification, and the
Brunauer–Emmett–Teller surface areas of 71 m2 g−1
(Figure S1, supporting information). The pore size distri-
bution of the catalyst was calculated from adsorption
branch of the isotherm using the Barrett–Joyner–Halenda
method. A relatively narrow pore size distribution at a
mean value of rp = 1.64 nm and pore volume of
vp = 0.45 cm3 g−1 for the catalyst indicate appropriate size
distribution (Figure S2, supporting information).
FIGURE 4 EDS mapping images of (a) C, (b) N, (c) Fe and (d) Pd.

GHOLINEJAD ET AL.
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FIGURE 7 XRD pattern of Pd@CQD@Fe3O4 NPs
FIGURE 9 Thermogravimetric analysis of Pd@CQD@Fe3O4 NPs
different reaction conditions gave lower yields for the
reaction (Table 1, entries 6–12). Thus, we selected NaBH4
as a source of hydrogen and 0.008 mol% of Pd loading at
room temperature as the optimized reaction conditions.
It is worth mentioning that NaBH4 is a mild and efficient
reducing agent for the reduction of nitro group[23] and the
model reaction in the absence of Pd catalyst failed to
proceed (Table 1, entry 13).
Next, the scope and limitations of this reductive meth-
odology using a series of nitroarenes were examined
(Table 2). The results indicated that the reduction of aro-
matic nitro compounds containing electron‐donating
groups such as Me, ―OMe and ―NH2 (Table 2, entries
2–5) as well as nitroarenes having electron‐withdrawing
groups such as ―NO2, F, ―COMe and Br proceeded well
and afforded the corresponding amines in high to excel-
lent yields (Table 2, entries 6–10). Also, reactions of
nitroarenes containing o‐substituents proceed effectively
and gave amines in high to excellent yields (Table 2,
entries 11–14). Finally the reduction of 2,6‐
dimethylnitrobenzene, as a highly sterically hindered
nitroarene, took place giving the corresponding amine in
high 91% yield after 24 h (Table 2, entry 15).
In an attempt to extend the application of this catalyst,
Pd@CQD@Fe3O4 was used in the Suzuki–Miyaura cross‐
coupling reaction of aryl halides. In order to study the opti-
mized reaction conditions, 4‐bromoanisole was allowed to
react with phenylboronic acid and the effects of various
factors such as solvent and base were considered. Results
of this study indicated that using equal mixture of ethanol
and water as a solvent, t‐BuOK as a base and 0.3 mol%
of Pd catalyst gave the desired product in 92% yield
(Table 3, entry 4).
FIGURE 8 Magnetization curves of Pd@CQD@Fe3O4 NPs and of
reused catalyst after third run.
Thermogravimetric analysis of the Pd@CQD@Fe3O4
NPs showed one weight loss in the range 25–100 °C which
is related to physisorbed water and solvents and a second
main weight loss between 250 and 340 °C, which is
related to the decomposition of CQD. This result con-
firmed the thermal stability and negligible leaching of
the structure up to 250 °C (Figure 9).
The catalytic properties of the Pd@CQD@Fe3O4 NPs
were first assessed in the reduction of aromatic nitro com-
pounds. We focused our initial investigation on the effect
of various types of reducing agents and palladium amount
on the reduction of nitrobenzene as a model reaction in
aqueous ethanol (Table 1). Results indicated that using
NaBH4 as source of hydrogen and 0.1 mol% Pd at room
temperature, the reaction proceed well and 100% conver-
sion of nitrobenzene to aniline was obtained (Table 1,
entry 1). Therefore, we used a smaller amount of Pd
(0.002–0.008 mol%) at room temperature (Table 1, entries
2–5). Results of this study showed excellent conversion to
desired product using 0.008 mol% of Pd catalyst during 2 h
(Table 1, entry 5). Using other hydrogen sources under
Using the optimized reaction conditions, the Suzuki
reaction of structurally different aryl halides with various
arylboron compounds was studied. Reactions of aryl
bromides containing electron‐withdrawing groups such
as 4‐NO2, 4‐CN, 4‐Cl, 4‐COCH3, 4‐CHO and 3‐CHO with

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GHOLINEJAD ET AL.
TABLE 1 Optimization of reaction conditions for reduction of nitrobenzenea
Entry
H donor
Temp. (°C)
Time (h)
Catalyst (mol%)
Yield (%)b
1
NaBH4
r.t.
2
0.1
100
2
NaBH4
NaBH4
NaBH4
NaBH4
NH4HCO2
NH4HCO2
N2H4
r.t.
r.t.
r.t.
r.t.
90
2
0.002
0.002
0.004
0.008
0.008
0.1
10
64
91
97
7
3
24
24
2
4
5
6
24
24
24
24
24
24
24
24
7
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
3
8
0.008
0.1
2
9
N2H4
2
10
11
12
13
HCO2H
Glycerol
i‐PrOH
NaBH4
0.008
0.008
0.008
—
1
1
0
0
aReaction conditions: nitrobenzene (0.5 mmol), H donor (2 eq.), catalyst and solvent (2 ml).
bGC yields using normalization method.
phenylboronic acid proceeded efficiently and the desired
biphenyls were obtained in high to excellent yields
(Table 4, entries 1–6). Also, reactions of less reactive aryl
bromides containing electron‐donating groups such as
―OH and ―OMe as well as bromobenzene and
1‐bromonaphthalene proceeded efficiently and afforded
biphenyls in high to excellent yields (Table 4, entries
7–10). Reactions of challenging 2‐bromopyridine and
5‐bromopyrimidine as heterocyclic compounds proceeded
efficiently without poisoning and deactivation of the
catalyst by heteroatoms and afforded products in 82–90%
isolated yields (Table 4, entries 11 and 12). Furthermore,
reactions of aryl bromides with other arylboronic acids
and potassium phenyltrifluoroborate gave products in
high to excellent yields (Table 4, entries 13–21). Reactions
of aryl chlorides under the optimized reaction conditions
were sluggish; therefore different reaction conditions
were studied. It has been found that using PEG200 and
NaOH at 120 °C are the most effective conditions for the
Suzuki reaction of aryl chlorides (Table 4, entries
22–25). In addition, reactions of aryl iodides with
arylboronic acids under optimized reaction conditions
proceeded very efficiently and afforded coupling products
in excellent yields in less than 4 h (Table 4, entries 26–28).
Arenediazonium salts are alternative substrates to aryl
halides in various Pd‐catalysed coupling reactions[24] and
can be easily prepared from commercially available
anilines. In order to show the potential activity of the
catalyst under investigation, we studied the Suzuki
reaction of different aryldiazonium tetrafluoroborates
with phenylboronic acid under the optimized reaction
conditions. Results of this study showed formation of
desired products in high to excellent yields using either
electron‐donating or electron‐withdrawing substituents
in the aryldiazonium salt (Table 4, entries 29–33).
In order to study the recycling properties of the cata-
lyst, reduction of 4‐nitrotoluene under optimized reaction
conditions was studied. After completion of the reaction,
the catalyst was separated using an external magnet and,
after washing with diethyl ether and drying, it was used
in another reaction run. Results showed that this catalyst
can be recovered and reused for seven consecutive runs
without significant decrease in activity (Figure 10). Also,
the recyclability of the catalyst was studied in the Suzuki
reaction of 4‐iodoanisole with phenylboronic acid under
the optimized reaction conditions. Also in this case, the
catalyst can be easily recycled, using an external magnet
(Figure S3, supporting information), during ten consecu-
tive runs with very small decrease in catalytic activity
(Figure 10).
TEM images of the reused catalyst after ten runs
showed that the structure was preserved and also the
presence of the CQD shell (Figure 11). Leaching of Pd
and Fe from the catalyst after ten runs was found to be
16 and 0.2%, respectively. These results confirm the strong
ability of CQD for protection of Fe3O4 NPs and preventing
their leaching and aggregation during the recycling
processes. However, XPS study of reused catalyst after
the third run showed that all palladium species are
excited in reduced Pd(0) form (Figure 12). Study of the

GHOLINEJAD ET AL.
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TABLE 2 Pd@CQD@Fe3O4 NPs‐catalysed reduction of nitroarenes using NaBH4 as a hydrogen source in aqueous ethanol at room
temperaturea
Entry
Substrate
Product
Yield (%)b
Time (h)
1
97
2
2
3
87 (100)
86 (100)
2 (4)
2 (4)
4
5
93
4
90 (100)
4 (12)
6
7
99
98
2
4
8
9
97
97
8
5
10
11
96
95
4
4
12
13
14
91
98
96
4
6
2
(Continues)

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GHOLINEJAD ET AL.
TABLE 2 (Continued)
Entry
Substrate
Product
Yield (%)b
Time (h)
15
91
24
aReaction conditions: nitroarene (0.5 mmol), catalyst (0.008 mol% Pd), NaBH4 (2 eq.), H2O–EtOH (5:1, 2 ml) at room temperature under argon atmosphere.
bGC yield.
TABLE 3 Optimization of reaction conditions of 4‐bromoanisol and phenylboronic acid catalysed by Pd@CQD@Fe3O4 NPsa
Entry
Catalyst (mol% Pd)
Base
Solvent
Yield (%)b
1
0.3
K2CO3
H2O–EtOH
53
2
0.3
0.3
0.3
0.2
0.1
0.01
0.3
0.3
0.3
0.3
0.3
0.3
K2CO3
NaOH
H2O–CH3CN
H2O–EtOH
H2O–EtOH
H2O–EtOH
H2O–EtOH
H2O–EtOH
H2O–EtOH
EtOH
10
49
92
71
53
7
3
4
t‐BuOK
t‐BuOK
t‐BuOK
t‐BuOK
t‐BuOK
t‐BuOK
t‐BuOK
NaOH
5
6
7
8
15c
63
72
53
70
41
9
10
11
12
11
H2O
H2O
NaOH
EtOH
NaOH
H2O–EtOH
aReaction conditions: 4‐bromoanisole (0.5 mmol), phenylboronic acid (0.75 mmol), base (0.75 mmol), catalyst and solvent (2 ml).
bGC yields using normalization method.
cReaction performed at 50 °C.
magnetization curve of reused catalyst showed that the
catalyst preserved its superparamagnetic properties with
very small decrease of magnetization (Figure 8). Also,
the nitrogen adsorption–desorption isotherms for
Pd@CQD@Fe3O4 NPs showed preservation of the struc-
ture with isotherms similar to that of the fresh catalyst
(Figure S4, supporting information). In addition, these
results indicated vp = 0.30 cm3 g−1and rp = 4.61 nm for
the recycled catalyst which may be due to damage of walls
of pores during the washing and recycling of the catalyst
(Figure S5, supporting information).
bromobenzaldehyde with phenylboronic acid under the
optimized reaction conditions. During this test, the cata-
lyst was stirred in the reaction solvent (H2O–EtOH) for
30 min at 60 °C. Then, the catalyst was removed by filtra-
tion and 4‐bromobenzaldehyde, phenylboronic acid and t‐
BuOK were added and the resulting mixture was allowed
to react. After 2 h, GC analysis of reaction showed the for-
mation of 25% of the desired coupling product and 75% of
4‐bromobenzaldehyde. It should be noted that a positive
hot filtration test is interpreted as proof for homogeneous
catalysis, whereas a negative hot filtration test does not
necessarily indicate the emergence of heterogeneous
catalysis due to the possibility of fast deactivation or
redeposition of soluble active species. Furthermore, using
To understand whether the catalyst works via a het-
erogeneous pathway, or proceeds using leached Pd, we
used the hot filtration test for the reaction of 4‐

GHOLINEJAD ET AL.
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TABLE 4 Reactions of structurally different aryl halides and aryldiazonium tetrafluoroborates with arylboronic acids and potassium phenyl
trifluoroborate in the presence of Pd@CQD@Fe3O4 NPs catalysta
Entry
Ar1
Ar2BR
Time (h)
Product
Yield (%)b
1
PhB(OH)2
2
94
2
3
PhB(OH)2
PhB(OH)2
2
100c
80
12
4
5
PhB(OH)2
PhB(OH)2
2
2
99c
94
6
PhB(OH)2
12
85
7
8
PhB(OH)2
PhB(OH)2
12
12
88
99c
9
PhB(OH)2
PhB(OH)2
24
24
96
77
10
11
12
PhB(OH)2
PhB(OH)2
24
24
90
82
13
14
24
3
95
100c
(Continues)

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GHOLINEJAD ET AL.
TABLE 4 (Continued)
Entry
Ar1
Ar2BR
Time (h)
Product
Yield (%)b
15
4
93
16
17
24
24
93
92
18
PhBF3K
4
93
19
20
PhBF3K
PhBF3K
4
94
80
12
21
22
PhBF3K
9
83
PhB(OH)2
24
86d
23
PhB(OH)2
24
95d
24
25
26
27
28
PhB(OH)2
PhB(OH)2
PhB(OH)2
PhB(OH)2
PhB(OH)2
24
24
1
75d
91c,d
95
4
100c
95
4
(Continues)

GHOLINEJAD ET AL.
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TABLE 4 (Continued)
Entry
Ar1
Ar2BR
Time (h)
Product
Yield (%)b
29
PhB(OH)2
3
91
30
31
3
3
97
90
PhB(OH)2
32
33
PhB(OH)2
PhB(OH)2
3
1
88
96
aReaction conditions: Ar1X (0.5 mmol), Ar2BR (0.75 mmol), t‐BuOK (0.75 mmol), EtOH–H2O (2 ml, 1:1), catalyst Pd@CQD@Fe3O4 NPs (0.3 mol% Pd) at 60 °C.
bIsolated yield.
cGC yield.
dReaction conditions: ArCl (0.5 mmol), PhB(OH)2 (0.75 mmol), NaOH (0.75 mmol), PEG200 (2 ml), catalyst Pd@CQD@Fe3O4 NPs (0.3 mol% Pd) at 120 °C.
showed that the reaction was quenched without forma-
tion of product and starting material was intact. Also,
addition of SH@Fe3O4, obtained from modification of
Fe3O4 NPs with 3‐mercaptopropyltrimethoxysilane,
showed complete suppression of activity.[25] Another
method for detection of homogeneous systems versus het-
erogeneous ones is an inverse relationship between the
active Pd concentration and the conversion in homoge-
neous mechanisms. To investigate the effect of Pd concen-
tration, reaction of 4‐bromobenzaldehyde with
phenylboronic acid using 0.3, 1 and 2 mol% was studied.
Results showed the formation of products in 92, 90 and
78% yields, respectively. This reduction of reactivity with
higher percentage of Pd catalyst is attributed to the forma-
tion of inactive Pd clusters from leached soluble Pd
species.[26]
FIGURE 10 Recycling of Pd@CQD@Fe3O4 NPs catalyst in the
reduction of 4‐nitrotoluene (red) and Suzuki reaction of 4‐
iodoanisole with phenylboronic acid (green)
From the results of experiments discussed above, we
conclude that when using this catalyst, reactions proceed
via a release–return Pd(0) system in which leached active
Pd(0) species catalysed the reaction and transfer of Pd
between solvent and support is responsible for its
recycling.[27] These results are similar to those obtained
by Arai and co‐workers in which when a homogeneous
Pd catalyst was used in the presence of additional support,
the Pd species deposited on the support after the comple-
tion of the reaction.[28]
solely hot filtration test for detection of heterogeneous
versus homogeneous catalysis is not sufficient and further
experiments are needed for finding the nature of the cata-
lyst.[25] In order to gain more insight into the behaviour of
the catalyst in the reaction, we added polyvinylpyridine
(PVPy; 2% cross‐linked) as an efficient poisoning agent
for soluble palladium species to the reaction mixture of
4‐bromobenzaldehyde with phenylboronic acid in a molar
ratio of 100:1 (PVPy:Pd). Very interestingly, results

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GHOLINEJAD ET AL.
reduce Pd(II) to Pd(0) and the obtained Pd@CQD@Fe3O4
NPs catalyst showed high activity in the selective reduc-
tion of nitroarenes and the Suzuki cross‐coupling reac-
tion. This magnetically recyclable catalyst was reused for
several times in both reactions without significant loss of
activity, and the fresh and recovered catalysts were char-
acterized using various techniques. The true nature of
the catalyst in Suzuki reaction was also determined using
different tests to be a homogeneous system.
4 | EXPERIMENTAL
4.1 | Synthesis of Fe3O4 NPs
FeCl3⋅6H2O (16 mmol, 4.32 g) and FeCl2⋅4H2O (8 mmol,
1.59 g) were dissolved in deionized water (150 ml) under
argon atmosphere and vigorous mechanical stirring.
Then, aqueous ammonia (25%, 13 ml) was added slowly
under argon atmosphere and the resulting mixture was
stirred at 80 °C for 4 h. The black magnetic precipitates
were separated using an external magnet and washed sev-
eral times with deionized water and ethanol and dried in
a vacuum oven at 60 °C for 24 h to afford black magnetic
NPs.
4.2 | Synthesis of CQD@Fe3O4
The CQD used in our experiments were synthesized by
carbonization of glycerol and urea. A solution of glycerol
(12.5 ml) and urea (1 g) in distilled water (10 ml) was
stirred at room temperature for 10 min and the resulting
mixture was heated in a Teflon autoclave at 180 °C for
4 h. Then, the mixture was cooled to room temperature
and the obtained material was centrifuged at 10 000 rpm
for 10 min to remove larger particles. After centrifugation
the viscous oil was transferred to a 25 ml flask containing
Fe3O4 NPs (500 mg in 10 ml of water) and the mixture
sonicated for 15 min. Then, this mixture was stirred using
a mechanical stirrer at 80 °C for 48 h. Resulting
CQD@Fe3O4 was subjected to magnetic separation and
the obtained material was washed with deionized water
(3 × 20 ml) and ethanol (3 × 20 ml) and dried under
vacuum.
FIGURE 11 TEM images of reused catalyst after tenth run for the
reaction of 4‐iodoanisole with phenylboronic acid
4.3 | Synthesis of Pd@CQD@Fe3O4
FIGURE 12 XPS spectrum of reused Pd catalyst in 3d region for
the reaction of 4‐iodoanisole with phenylboronic acid after third run
In a 50 ml flask, CQD@Fe3O4 (1 g) was sonicated in water
(7 ml) for 10 min. In another batch, PdCl2 (20 mg,
0.11 mmol) was sonicated and dissolved in water (3 ml)
and the resulting solution was added slowly to the flask
containing CQD@Fe3O4 under argon atmosphere and
the mixture stirred at 60 °C for 24 h. Then, the resultant
solids were collected using a magnet, followed by washing
3 | CONCLUSIONS
In this work we prepared Fe3O4 NPs modified with CQD
obtained from glycerol and urea as cheap and green
starting materials. The new magnetic compound can

GHOLINEJAD ET AL.
13 of 14
with water (2 × 10 ml) and EtOH (2 × 10 ml) and dried
under vacuum at 70 °C.
Europeo de Desarrollo Regional (FEDER, EU) (projects
CTQ2016‐76782‐P and CTQ2016‐81797‐REDC), the
Generalitat Valenciana (PROMETEOII/2014/017) and
the University of Alicante.
4.4 | General procedure for catalytic
reduction of nitro compounds
ORCID
Nitroarene (0.5 mmol), NaBH4 (2 eq.), catalyst
(0.008 mol% Pd) and water–ethanol (5:1, 2 ml) were added
to a 5 ml flask and stirred for appropriate reaction time at
room temperature. Progress of the reactions was moni-
tored by GC analysis. After completion of the reaction,
crude product was extracted with ethyl acetate
(3 × 5 ml) and purified using column or plate
chromatography.
Mohammad Gholinejad
4509
http://orcid.org/0000-0003-0209-
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The authors are grateful to the Institute for Advanced
Studies in Basic Sciences (IASBS) Research Council and
Iran National Science Foundation (INSF grant no.
95844587) for support of this work. C.N. is also grateful
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How to cite this article: Gholinejad M, Zareh F,
Nájera C. Nitro group reduction and Suzuki
reaction catalysed by palladium supported on
magnetic nanoparticles modified with carbon
quantum dots generated from glycerol and urea.
Appl Organometal Chem. 2017;e3984. https://doi.
org/10.1002/aoc.3984
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