Uniformly dispersed copper nanoparticles onto the modified magnetically recoverable nanocatalyst for aqueous synthesis of primary amides

Article abstract of DOI:10.1002/aoc.3925

Magnetically recoverable and environmentally friendly Cu-based heterogeneous catalyst has been synthesized for the one-pot conversion of aldehydes to their corresponding primary amides. The Fe₃O₄@SiO₂ nanocomposites were prepared by synthesis of Fe₃O₄ magnetic nanoparticles (MNPs) which was then coated with a silica shell via St?ber method. Bi-functional cysteine amino acid was covalently bonded onto the siliceous shell of nanocatalyst. The Cu^II ions were then loaded onto the modified surface of nanocatalyst. Finally, uniformly dispersed copper nanoparticles were achieved by reduction of Cu^II ions with NaBH₄. Amidation reaction of aryl halides with electron-withdrawing or electron-donating groups and hydroxylamine hydrochloride catalyzed with Fe₃O₄@SiO₂@Cysteine-copper (FSC-Cu) MNPs in aqueous condition gave an excellent yield of products. The FSC-Cu MNPs could be easily isolated from the reaction mixture with an external magnet and reused at least 8 times without significant loss in activity.

Full text of DOI:10.1002/aoc.3925

Received: 19 February 2017
DOI: 10.1002/aoc.3925
Revised: 27 April 2017
Accepted: 16 May 2017
F U L L PA P E R
Uniformly dispersed copper nanoparticles onto the modified
magnetically recoverable nanocatalyst for aqueous synthesis of
primary amides
Fariborz Ziaee | Mostafa Gholizadeh
| Seyed Mohammad Seyedi
Department of Chemistry, Faculty of
Science, Ferdowsi University of Mashhad,
Mashhad, 91775‐1436, I.R., Iran
Magnetically recoverable and environmentally friendly Cu‐based heterogeneous
catalyst has been synthesized for the one‐pot conversion of aldehydes to their
corresponding primary amides. The Fe3O4@SiO2 nanocomposites were prepared
by synthesis of Fe3O4 magnetic nanoparticles (MNPs) which was then coated with
a silica shell via Stöber method. Bi‐functional cysteine amino acid was covalently
bonded onto the siliceous shell of nanocatalyst. The CuII ions were then loaded
onto the modified surface of nanocatalyst. Finally, uniformly dispersed copper
nanoparticles were achieved by reduction of CuII ions with NaBH4. Amidation
reaction of aryl halides with electron‐withdrawing or electron‐donating groups
and hydroxylamine hydrochloride catalyzed with Fe3O4@SiO2@Cysteine‐copper
(FSC‐Cu) MNPs in aqueous condition gave an excellent yield of products. The
FSC‐Cu MNPs could be easily isolated from the reaction mixture with an external
magnet and reused at least 8 times without significant loss in activity.
Correspondence
Mostafa Gholizadeh, Department of
Chemistry, Faculty of Science, Ferdowsi
University of Mashhad, Mashhad 91775‐
1436, I.R., Iran.
Email: m_gholizadeh@um.ac.ir
Funding information
Research Council of Ferdowsi University of
Mashhad, Grant/Award Number: 3/41313
KEYWORDS
copper nanoparticles, cysteine, heterogeneous catalyst, magnetically recoverable catalyst
1 | INTRODUCTION
reagents which led to byproducts and chemical wastes.
Boronic acids,[13] N‐heterocyclic carbenes,[14] transition‐
The amide functional group[1] is an important organic unit
in a wide range of chemical structures, including biomole-
cules,[2] natural products, pharmaceuticals,[3] and polymers.
Csp2–N bond as an important organic transformation[1] can
be formed via oxidative amidation of aldehydes to
the oximes from which nitrile intermediates converted
to primary amides via dehydration‐hydration process.
Secondary amides can be generated through an acid‐assisted
Beckman rearrangement.[4] Traditionally amides are synthe-
sized via the reaction of carboxylic acids derivatives such
as acid chlorides,[5] active esters,[6] anhydrides,[7] and acyl
azides[8] in the presence of coupling agents.[9] Classical name
reactions such as Beckmann rearrangement[4] Staudinger
ligation,[10] Ugi,[11] Schmidt‐Abuerearrangement[12] havealso
been developed. Unfortunately, these methods often require
harsh reaction conditions, hazardous and toxic coupling
metals such as ruthenium,[15] rhodium,[16] Au/TiO2,[17]
Au/DNA nanohybrids,[18] Fe,[19] Zn,[20] and Pd as catalyst[21]
have been reported for amidation reactions. From the green
chemistry point of view, the development of efficient
catalytic systems remains a great challenge. Cu(II) salts as
direct catalysts for the one‐pot transformation of aldehydes
to primary amides have also reported.[22] The copper cata-
lyzed oxidative amidation of aldehydes and aliphatic amine
hydrochloride salts was performed by introducing a base.[23]
The conversion of aldehydes to primary amides has been
studied using SBA‐15 grafted ethylenediamine copper com-
plexes.[24] Metal‐free oxidative amidation of aldehydes was
studied by using tert‐butyl hydroperoxide (TBHP) as the
oxidant.[25] The amidation of aldehydes with ammonium
chloride or amine hydrochloride salts has been reported by
using copper sulfate or Cu2O and aqueous TBHP.[26]
Appl Organometal Chem. 2017;e3925.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3925

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ZIAEE ET AL.
Nevertheless, the intricacy of catalysts recovery and
laborious workup procedures are the major drawback of these
catalytic reactions.[27] The development of magnetic nano-
particle‐supported catalysts that can be easily separated from
the reaction mixture has become an important choice. The
easy functionalization of magnetic nanoparticles and
immobilization of various metal complexes make them a
practical catalytic system with highly accessible surface area
that shows enhanced efficiency than their corresponding
homogeneous analogues.[28] Their insolubility and
superparamagnetic nature make it possible to realize various
reactions and reduce the capital and operational costs.[29,30]
The EDTA functionalized magnetic nanoparticles for the
removal of Cu(II) ions were reported as a magnetic nano‐
adsorbent.[31] In the area of magnetic catalysts, few attempts
have been focused on the direct amidation of aldehydes with
Cu nanoparticle‐supported heterogeneous catalysts on the
magnetic nanoparticles.
Herein, we report the design and preparation ofan efficient
heterogeneous magnetically reusable and environmentally
friendly Cu‐based catalyst for the mild aqueous direct conver-
sion of aldehydes into primary amides. The use of less toxic
and inexpensive copper based cysteine functionalized Fe3O4
catalysts has not been reported to date in the synthesis of
amides. The cysteine was connected to magnetic nanoparticles
previously through a sulfur‐metal interaction and was used in a
multi component reactions.[32] In this work, we confined
copper‐cysteine complexes onto the surface of silica coated
magnetic nanoparticles as a highly active catalyst in the
direct synthesis of amides from aldehydes.
for 3 h. After cooling to room temperature, the magnetic
Fe3O4@SiO2@Cysteine nanoparticles (FSC) were collected
with a magnet and washed 6 times with water and ethanol
and dried at 50°C. In the next step, FSC MNPs (0.4 gr) were
dispersed in 25 ml deionized water, and mixed with CuCl2
(0.2 gr in 5 ml EtOH), after sonication for 30 min, the
mixture was refluxed for 4 h. Then, an aqueous solution of
NaBH4(15.0 ml, 0.2 M) was added dropwise to the mixture
and stirred under N2 atmosphere for 2 h. FSC‐Cu
nanocatalyst collected by an external magnet, washed with
EtOH and water 6 times and dried under reduced pressure.
2.3 | General procedure for FSC‐Cu MNPs
catalyzed amide synthesis
Benzaldehyde (1.0 mmol), hydroxylamine hydrochloride
(1 mmol), sodium carbonate (1.1 mmol) and FSC‐Cu MNPs
(1 mol%) were added to 2 mL of water. The reaction mixture
was stirred at 80°C for 24 h. Completion of reaction was
monitored by TLC, and the FSC‐Cu MNPs were separated
with an external magnet. The reaction mixture was quenched
with CH2Cl2, residual purified by column chromatography
using silica gel (n‐hexane/ethyl acetate).1H and 13C NMR
spectra were measured (CDCl3) with a Bruker DRX‐300
AVANCE spectrometer at 300 and 75 MHz, respectively
for all products (see supporting information).
3 | RESULTS AND DISCUSSION
The efficiency of copper loaded Fe3O4@SiO2‐Cysteine
nanocatalyst on the catalytic amidation of aromatic aldehydes
was studied. The Fe3O4@SiO2 MNPs were first prepared
using Stöber method, in the next step cysteine in one step
was covalently bonded to the surface of siliceous shell, and
the Cu(II) ions was then loaded on SH and NH2 functional
groups of FSC MNPs. The FSC‐Cu MNPs were character-
ized by transmission electron microscopy (TEM), Scanning
electron microscopy (SEM), energy dispersive X‐ray
(EDS), ICP‐OES, X‐ray diffractometry (XRD), Thermal
gravimetric analysis (TGA), Vibration Sampling Magnetom-
eter (VSM), and Fourier transform infrared spectroscopy
(FTIR). The effect of several environmental parameters
including catalyst amount, solvents and reaction temperature
have been studied. Scheme 1 show a schematic pathway of
FSC‐Cu MNPs synthesis procedure.
2 | EXPERIMENTAL
2.1 | Synthesis of Fe3O4@SiO2
The Stöber method was utilized for the synthesis of core‐
shell Fe3O4@SiO2MNPs.[33] An aqueous dispersion of
Fe3O4 nanoparticles (6 ml, 0.05 g ml−1) were dissolved into
a mixture of absolute ethanol (400 ml), and deionized water
(100 ml). Ammonia solution (28 wt%, 10 ml) was then
added and the mixture sonicated for 30 min. Then, 8.0 ml
tetraethyl orthosilicate (TEOS) was dropped into the mixture
under vigorous stirring for 12 h. After separation of the
Fe3O4@SiO2MNPs (FS) using an external magnet, catalyst
washed with water and ethanol 4 times and dried at 60°C
in oven.
FTIR spectrum of silica coated Fe3O4@SiO2 MNPs (FS)
showed a broad peak around 475 cm−1 due to the stretching
vibration of Fe‐O bond which can be an evidence for Fe3O4
existence. Asymmetric vibration of Si‐O‐Si bond and the
symmetric stretching of Si‐OH bond were located at
1100 cm−1 and 947 cm−1, respectively (Figure 1a). These
results can be attributed to silica coating of Fe3O4 surface.[34]
2.2 | Synthesis of Fe3O4@SiO2@Cycteine‐Cu
nanoparticles
0.1 g of FS MNPs and 0.4 g cysteine in 20 ml deionized
water was mixed and sonicated for 30 min. The mixture
transferred into Teflon lined autoclave and kept at 150°C

ZIAEE ET AL.
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SCHEME 1 Schematic representation of the FSC‐Cu MNPs preparation
FIGURE 2 Thermogravimetric (TG) analysis curve of FSC MNPs
FIGURE 1 IR spectra of (a) Fe3O4@SiO2, (b) Cysteine, (c)
Fe3O4@SiO2‐cysteine
card no. 19–0629 (Figure 3a). As shown in Figure 3b, no
remarkable changes occurs in the number of peaks, but due
to the heavy atom effect of Cu the characteristic peaks of
Fe3O4 becomes weak in curve 3b. These results indicate that
crystal structure of Fe3O4 core after immobilization of Cu
nanoparticles retain unchanged. Also, new peaks which were
located at 2θ =43.65°, 50.84° and 74.26° attributed to the
(111), (200), and (220) planes of the fcc structure of Cu nano-
particles in the catalysis matrix according to the JCPDS card
no. 04–0836.[36] The ICP and EDS analyzes confirm the pres-
ence of Cu units in the FSC‐Cu matrix.
Cysteine amino acid show characteristic peaks at 1610 cm−1
and 1397 cm−1 which attributed to the asymmetric and
symmetric stretching of carboxylic group. A very broad peak
of NH3+ stretch is observed between 3000 and 3500 cm−1. A
weak peak at 2541 cm−1 is due to the presence of S‐H groups
of cysteine (Figure 1b). Cysteine functionalized FS MNPs
exhibit several changes in characteristic peaks in comparison
with Figure 1a due to high concentration of grafted cysteine
units. The peak located at 2541 cm−1 is related to the presence
of S‐H groups onto the FS MNPs. Also, two peaks due to
asymmetric and symmetric N‐H stretching were located at
3400 cm−1 and 3220 cm−1 respectively. (Figure 1c), which
confirms the functionalization of surface with cysteine
molecules.[35]
The morphology of the synthesized products was charac-
terized by SEM and TEM. The Fe3O4 MNPs are multifaceted
cubic particles with average diameters of about 40 nm
To examine the thermal stability of FSC‐Cu MNPs the
thermogravimetric (TG) analysis was used. As shown in
Figure 2 the initial weight loss up to 200°C was probably
related to the loss of absorbed water molecules[35] The weight
loss in the temperature range of 200‐600°C was attributed to
the evaporation and subsequent decomposition of surface
grafted cysteine groups of FSC MNPs.[35] Therefore, these
results confirm that the ligand grafting had been successfully
achieved.
The crystal structure of Fe3O4 and FSC‐Cu MNPs were
characterized by XRD. Six diffraction peaks at 2θ = 30.31°,
35.64°, 43.31°, 53.86°, 57.24°, 62.83° attributed to the
(220), (311), (400), (422), (511), (440) crystal planes of the
Fe3O4 crystal[34] with cubic phase according to the JCPDS
FIGURE 3 XRD patterns of (a) Fe3O4 and (b) FSC‐Cu MNPs

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ZIAEE ET AL.
(Figure 4a). Silica coated surface of Fe3O4 MNPs were
showed in Figure 4b, which in contrast to the Figure 4a show
a Fe3O4 MNPs within a siliceous shell. Figure 4c show a high
dispersity of Cu NPs onto the surface of FSC MNPs, which
indicates core‐shell structure has not influenced after immo-
bilization of Cu nanoparticle on the surface of FS MNPs.
As shown in Figure 4d the SEM image of FSC‐Cu
nanocatalyst showed a spherical morphology. The content
of Cu in the sample was confirmed by inductively coupled
plasma/optical electron microscopy (ICP‐OES). The amount
of loaded Cu onto the FSC was 14.981 wt. %. In addition,
the attendance of Fe, Si and Cu elements are shown in
Figure 4e. These results confirmed the existence of Fe3O4,
SiO2and Cu elements in this catalyst.
As shown in Figure 5 the magnetic hysteresis loops of the
Fe3O4 and FSC‐Cu MNPs exhibit a ferromagnetic behavior
at the room temperature. The magnetic saturation value
(Ms) of the Fe3O4 and FSC‐Cu MNPs is 65 and 35 emu/g,
respectively. This decrease in the Ms value of FSC‐Cu MNPs
is due to disordered structure of silica shell at interfaces
which provide less magnetic moment per unit mass than that
of ferromagnetic core regions. These results demonstrated
that the core‐shell particles possess magnetic properties.
The catalytic performance of the FSC‐Cu as a magneti-
cally recoverable nanocatalyst was investigated through the
synthesis of primary amides from aromatic aldehydes. The
reaction of benzaldehyde and hydroxylamine hydrochloride
carried out under aqueous condition as a model reaction.
Different reaction conditions have been studied which is
summarized in Table 1. For comparative purposes, SiO2‐Cu
was prepared and its effect was compared with FSC‐Cu.
The SiO2‐Cu catalyst showed lower yield than FSC‐Cu. Also
hot filtration test for SiO2‐Cu revealed the leaching of Cu
units after 4 recycle. The FSC‐Cu could be reused 8 run with-
out any significant decrease in activity (Figure 6). Hot
filtration tests for FSC‐Cu showed negligible Cu leaching
up to 6 run. Various solvents and bases were tested in the
model reaction which based on Table 1 results water was
found to be the optimum solvent (Table 1, entries 10–15).
No amide product was obtained without any base. Cs2CO3
was effective base and up to 99% of benzamide was formed
(Table 1, entry 15), which was in good agreement with
previously reported results[37] Because of low price and
availability, Na2CO3 was selected as a base for optimized
condition. The amount of the Cu units had a critical effect
on the yields and time of reaction. Model reaction conducted
with 0.5 mol% catalyst could provide 70% yield of
benzamide after 30 h at 100°C (Table 1, entry 13). Based
on these results, 1 mmol of aldehyde, 1 mmol of hydroxyl-
amine hydrochloride, 1 mol% FSC‐Cu, 1.1 mmol of Na2CO3
and 2 ml of water were chosen as optimized conditions.
To assay the scope of the optimized conditions, a wide
range of aromatic aldehydes were tested and the results are
FIGURE 4 TEM images of (a) Fe3O4 MNPs, (b) Fe3O4@SiO2
MNPs, (c) FSC‐Cu MNPs. SEM image of (d) FSC‐Cu MNPs. EDS
analyzes of (e) FSC‐Cu MNPs

ZIAEE ET AL.
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FIGURE 4 (Continued)
A hot filtration test[38] was performed to determine the
activity of catalyst from the Cu leaching point of view under
optimized reaction conditions. The catalyst was removed by
an external magnet after 10 h reaction (~35% conversion).
No further increase in product yield was observed even after
50 h upon catalyst removal. These results indicated that Cu
ions are strongly coordinated with cysteine and showed the
heterogeneous nature of the FSC‐Cu MNPs.
Reusability of the catalyst was evaluated in the model
reaction (Figure 6). In terms of copper leaching, our study
showed that apart from being highly stable under the
optimized conditions. After completing the reaction the
FSC‐Cu MNPs were separated from reaction mixture easily
by an external magnet, washed with water and ethanol 3
times, then dried at 60°C for 2 h. The FSC‐Cu catalyst could
be reused at least 8 times without a significant loss of its
activity Figure 6. (Reaction conditions: benzaldehyde
(1 mmol), hydroxylamine hydrochloride (1 mmol), 1.1 mmol
of Na2CO3 (1.1 mmol), FSC‐Cu MNPs (1 mol %), water
(2 ml). As shown in Figure 6b the powder XRD data of
FIGURE 5 Magnetization hysteresis loops measured at room
temperatures: (a) Fe3O4, (b) FSC‐Cu MNPs
summarized in Table 2. The reaction gave an excellent result
for aryl aldehydes either with electron‐withdrawing or elec-
tron‐donating groups. Ortho substituted aryl aldehydes
showed lower yield and required extended reaction time which
can be attributed to steric effects (Table 2, entries 3, 5, 8, 12).

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ZIAEE ET AL.
TABLE 1 Optimization of reaction conditionsa
Entry
1
Catalyst
FSC‐Cu
FSC‐Cu
FSC‐Cu
SiO2‐Cu
FSC‐Cu
FSC‐Cu
FSC‐Cu
FSC‐Cu
FSC‐Cu
FSC‐Cu
FSC‐Cu
FSC‐Cu
FSC‐Cu
FSC‐Cu
FSC‐Cu
Base
Solvent
EtOH
EtOH
EtOH
MeOH
MeOH
MeOH
Toluene
DMF
DMSO
H2O
Cu (mol%)
T (°C)
80
Time (h)
36
Yieldb (%)
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K3PO4
4
2
52
52
50
33
55
49
60
55
58
65
80
75
70
97
>99
2
80
36
3
1
80
48
4
5
80
48
5
2
80
48
6
1
80
48
7
1
80
55
8
1
100
100
80
36
9
1
36
10
11
12
13
14
15
1
40
K2CO3
H2O
1
80
30
NaOAc
Na2CO3
Na2CO3
Cs2CO3
H2O
1
80
48
H2O
0.5
1
100
80
30
H2O
24
H2O
1
80
24
aReaction conditions: benzaldehyde (1 mmol), NH2OH·HCl (1 mmol), base (1.1 mmol) in 2 ml of solvent.
bIsolated yield.
FIGURE 6 (a) Reusability of FSC‐Cu
MNPs in the conversion of benzaldehyde to
benzamine, (b) XRD analyze of recycled
FSC‐Cu MNPs
recycled FSC‐Cu MNPs after 8 runs clearly illustrate that the
catalyst still remains as crystalline. The content of Cu in the
sample was confirmed by ICP‐OES. The ICP‐OES
analysis of catalyst resulted in a value of about 14.896 wt.
% of Cu. In comparison with as‐synthesized FSC‐Cu MNPs
with 14.981 wt. % of Cu, only 0.085 wt. % of Cu was leached

ZIAEE ET AL.
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TABLE 2 Aqueous conversion of aldehydes to primary amides using FSC‐Cu
Entry
Product
Time (h)
Yield (%)
1
24
97
2
3
22
26
95
90
4
22
92
5
23
23
24
25
24
24
89
90
87
89
91
95
6
7
8
9
10
11
24
94
(Continues)

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ZIAEE ET AL.
TABLE 2 (Continued)
Entry
Product
Time (h)
Yield (%)
12
25
90
13
23
95
SCHEME 2 Proposed mechanisms for the catalytic conversion of aldehydes to amides
after 8 runs which is negligible and confirm that catalyst is
really heterogeneous.
The catalytic activity of FSC‐Cu MNPs was comparable
to those of reported homogeneous Cu systems (Table 3).
The proposed mechanism for amidation of aldehydes is
shown in Scheme 2. Two different metal catalyzed pathways
have been previously reported.[37] The pathway A involves
five‐membered cyclic intermediate, which decomposes into
the amide product. The pathway B involves dehydration/
hydration process with a rearrangement to the amide
product.[37] We believe that pathway B seems to be the most
plausible on the basis of obtained results and previously
reported Cu activities in chemical processes.[39]
4 | CONCLUSION
In summary, we have developed the magnetically recover-
able Cu‐based catalyst for direct amidation of aromatic
aldehydes with NH2OH.HCl under aqueous conditions.
The mild reaction conditions, broad substrate scope make
FSC‐Cu MNPs efficient and environmentally friendly for
the synthesis of primary amides. Cysteine was used as a
ligand for modification of silica coated magnetic nanoparti-
cles which used for generation of Cu NPs. This catalyst can
be reused for at least 8 consecutive runs without significant
loss of catalytic activity.
TABLE 3 Comparison of catalytic performance of different systems
Cat.
T
Yield
Catalyst
(mol%)
Solvent
(°C) (%) Ref
[37b]
Pd(OAc)2
5
2
2
1
1
2
1
DMSO‐H2O 100 98
H2O 110 99
Solvent free 110 95
[22a]
[22b]
[37e]
[40]
Cu(OAc)2
CuSO4.5H2O
TerpyRu(PPh3)Cl2
[RuCl(CO)(PPh3)(TAC)]
SBA‐15/En‐Cu
FSC‐Cu
Toluene
110 88
110 95
80 95
ACKNOWLEDGMENTS
Toluene
[24]
H2O/48 h
H2O/24 h
The authors gratefully acknowledged for partially financial
support of this study (Grant No: 3/41313) by Research Coun-
cil of Ferdowsi University of Mashhad.
80 97 This
work

ZIAEE ET AL.
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[26] S. C. Ghosh, J. S. Ngiam, A. M. Seayad, D. T. Tuan, C. L. Chai,
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