Ag-doped nano magnetic γ-Fe2O3@DA core-shell hollow spheres: An efficient and recoverable heterogeneous catalyst for A3 and KA2 coupling reactions and [3 + 2] cycloaddition

Article abstract of DOI:10.1039/c6ra18810a

An effective protocol for the fabrication of Ag-doped nano magnetic γ-Fe₂O₃@DA core-shell hollow spheres (h-Fe₂O₃@DA/Ag) by a simple hydrothermal method is demonstrated without any templates in the reaction system. The synthesized core-shell structure was successfully characterized in terms of the chemical composition, surface morphology, and magnetic properties using FT-IR, VSM, TGA, TEM, FE-SEM, EDS, and XRD patterns. The possible formation mechanism of this magnetic γ-Fe₂O₃@DA/Ag hollow sphere structure is described based on the experimental results. In addition, as a physical property, the apparent shell density was determined using the specific surface area (BET) and shell thickness (BJH). The important, novel and unique catalyst with a hollow structural character and strong magnetic behavior exhibited a high catalytic activity in the A³ and KA² coupling reactions and [3 + 2] cycloaddition for the synthesis of 1H-tetrazoles. Furthermore, the recovered catalyst of the reaction mixture was facilitated by a magnetic field and multiple recycling was carried out without any significant loss in the catalytic activity.

Full text of DOI:10.1039/c6ra18810a

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Ag-doped Nano Magnetic γ-Fe2O3@DA Core–Shell Hollow
Spheres: an efficient and recoverable heterogeneous catalyst for
A3, KA2 Coupling Reactions and [3+2] cycloaddition
Received 00th January 20xx,
Accepted 00th January 20xx
A. Elhampour,a,* M. Malmir,b,* E. Kowsari,b F. Boorboor ajdarib and F. Nematia
DOI: 10.1039/x0xx00000x
An effective protocol for the fabrication of Ag-doped Nano Magnetic γ-Fe2O3@DA Core-Shell Hollow Spheres (h-
Fe2O3@DA/Ag), by a simple hydrothermal method demonstrated without any templates in the reaction system. The
synthesized core-shell structure was successfully characterized in terms of the chemical composition, surface morphology,
and magnet properties by using: FT-IR, VSM, TGA, TEM, FE-SEM, EDS, and XRD patterns. The possible formation
mechanism of this magnetic γ-Fe2O3@DA/Ag hollow sphere structure described based on the experimental results. In
addition, as physical property, apparent shell density was determined using specific surface area (BET) and shell thickness
(BJH). An important, the novel and unique catalyst with hollow structural characters and strong magnetic behavior,
exhibited high performance catalytic activity in the A3, KA2 coupling reactions and [3+2] cycloaddition for the synthesis of
1H-tetrazoles. Furthermore, facilitated recovering by the well manner from the reaction mixture by magnetic field and
multiple recycling without any significant loss in catalytic activity.
www.rsc.org/
Kirkendall effect,23 molten salt corrosion,24 reverse micelle
transport,25 and layer-by-layer assembly26 have been
developed. These fabrication approaches are conventionally
Introduction
Regarding to future technologies and exploring facile, safe
and economic methods for the synthesis of magnetic materials
has spurred intense and rapid development in the field of
material science.1,2 Particularly, iron oxide-based magnetic
materials have been extensively pursued for multidisciplinary
researches, due to their physical and chemical properties and
technical applications.3,4 As one of the most important,
inexpensive, non-toxic, nature-friendly, and the most stable
based on the well-established template methods include:
hard-template, soft-template, and sacrificial template.27-29
Therefore, developing a facile method becomes the main
purpose for the synthesis of magnetic hollow structures. In a
sense, preparing nonspherical hollow structures with one-pot
template-free method is still a perfect method. During the last
decade, different kinds of hollow spheres supported on
various materials including organic,30,31 inorganic32,33 and their
hybrid materials34-37 was reported. Recently, Hollow magnetic
nanoparticles have been also reported as an excellent type of
catalyst support because of their good stability, facile synthesis
and functionalization process, high specific surface area, low
density and facile separation process.38,39
Dopamine (DA) is one of the most significant catecholamine,
plays an important role in the functionalizing of the central
nervous, cardiovascular, renal, and hormonal systems, as well
as in drug addiction and Parkinson’s disease.40,41 Nowadays,
Dopamine derivatives provide a novel and useful alternative
for surface immobilization schemes. Obviously, dopamine and
its derivatives open a new route to the modification of iron
oxide nanoparticles as a novel organic coating material.42-46
Enhancing of the catalytic activity of DA will be another
important property for applications of heterogeneous catalysts
in the future. Hence, noble metals such as Ag, Au, Pt, and Pd
deposited on dopamine surfaces, enhanced the catalytic
activity of dopamine. Also, silver catalyst showed good
catalytic activity towards coupling, cycloaddition, sigmatropic
rearrangement, cycloisomerization, and nitrene transfer
phase of iron oxides, maghemite (γ-Fe2O3), has become an
attractive material. And also, many techniques including
vapor-solid growth,5-7 template aimed synthesis,8 sol-gel
process,9 and hydrothermal synthesis10-12 have been directed
to prepare various types of Fe2O3 crystals. To date, much
attentions have been focused on the size and functionalization
of iron oxide nanopoarticles with various morphologies such as
nanorods, nanowires, nanocubics and etc.10,13-16 lately, hollow
structures with high surface area, low material density as well
as strong magnetic response and good permeation have
attracted many attentions as an important solution in
controlled drug delivery, sensors, lightweight fillers,
catalysis.17-19 Notably, they could be manipulated by an
external magnetic field.20 Very recently, several strategies for
hollow structures synthesis such as the Ostwald ripening,21,22
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reactions; it has been widely employed.47 Among them, A3
coupling reaction (aldehyde, alkyne and amine) has been
widely used for the easy formation of high-value product
propargylamines via C−H bond activation, recently.48
Propargylamines are frequently used as synthetically
adaptable key intermediates49,50 in a synthesis of varied
natural products and biologically active compounds and
forming of many nitrogen-containing biologically active.51 Up
until now, many methods were carried out by using a wide
range of transition metal catalyst such as copper,52-54 iridium,55
nickel,56 and iron57 which has been well explored for the
synthesis of propargylamines. In addition to the
propargylamine, tetrazole is one of the most important
intermediate and greater synthetic favorites prepared by [3+2]
cycloaddition reaction starting from nitrile. Various synthetic
approaches have been disclosed for this transformation.58-65
Generally, these catalytic systems suffered from some
drawbacks such as: inability of the catalyst for recovering
process. So, the development of environmentally friendly
synthetic methods involved facile separation of catalysts from
the reaction mixture has become increasingly an important
argument. Nowadays, the development of environmentally
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DOI: 10.1039/C6RA18810A
Scheme 1. The possible formation process of Fe2O3@DA/Ag hollow
spheres
Structure characterization of the magnetic
Fe2O3@DA/Ag hollow sphere
FT-IR spectra
friendly catalyzed MCRs enjoying the easy recovery and To clarify the compositions of the synthesized materials, FT-IR
effective recyclability of catalysts as well as improving the was used to characterize the hollow Fe2O3, hollow Fe2O3@DA
atom economy is overgrowing, overshadowing and and hollow Fe2O3@DA doped with Ag nanoparticles in the
compensating the use of catalysts.66-75 Herein, we describe a range of 4000-500 cm-1 are presented in Fig. 1. In all these
novel and unique Ag nanoparticles supported on the magnetic spectra, the strong absorption peaks at 480-580 cm-1 are
Fe2O3 core and DA shell hollow sphere structure is an effective observed. These bands attributed to the Intrinsic stretching
and selective catalyst in the A3 coupling, and KA2 reaction and vibrations of Fe-O in the hematite particle. The significant
[3+2] cycloaddition under mild reaction conditions. The features for h-Fe2O3@DA spectrum (Fig. 1b) are the
magnetic-Fe2O3@DA/Ag hollow sphere was conveniently appearance of the peaks at 3367, 3095, 1629, 1408 and 1276
prepared and its structure was fully characterized by XRD, FT- cm-1 could be assigned to the –NH2 and C-N is stretching
IR, FE-SEM, TEM, EDS, TGA, and VSM analysis.
vibration of dopamine on the surface of magnetic hollow
sphere.77 From the FT-IR spectra, it can be observed that the
magnetic Fe2O3 hollow sphere was functionalized by
dopamine.
Results and discussion
Synthesis of magnetic Fe2O3@DA/Ag hollow
sphere
The process for the preparation of the magnetic Fe2O3@DA/Ag
hollow sphere catalyst is schematically described in Scheme 1.
The nano magnetic Fe2O3@DA/Ag hollow sphere was prepared
from commercially inexpensive available materials and fully
characterized using, the corresponding data, provided by FT-
IR, FE-SEM, TEM, XRD, TGA, and VSM techniques.
As shown in scheme 1, in the first step, Fe2O3 hollow sphere
were synthesized by hydrothermal treatment.76 In the second
step, Fe2O3 hollow sphere were functionalized with dopamine
groups (DA), which the functionalized nanoparticles coated
with dopamine (Fe-Ox dopamine) was performed by
methods63 but without preliminary functionalization of the
terminal amine. Thirdly, Ag(NH3)2NO3 was supported on the
surface of a magnetic Fe2O3@DA hollow sphere through an in
situ wet chemistry,64 and the silver complex was reduced to
Ag(0) particles.
Figure 1. The FT-IR spectra of (a) h-Fe2O3, (b) h-Fe2O3@DA and
(c) h-Fe2O3@DA/Ag.
X-ray diffraction spectra
The phase and composition of the as-obtained products were
examined by X-ray diffraction (XRD) patterns. Fig. 2 (Curve 2a),
shows the XRD pattern of magnetic Fe2O3 hollow spheres, that
the strong diffraction peaks at 30.21, 35.62, 43.34, 53.6, 57.2,
62.83 and 74.5 can be indexed to the {220}, {311}, {400}, {422},
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{511}, {440} and {533} planes of typical cubic structure
hematite (JCPDS card No. 39-1346) without additional peaks
are detected, suggesting high purity of the as-synthesized
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DOI: 10.1039/C6RA18810A
hematite (labeled as F). After coating Fe2O3 by dopamine, the
broad band appearing in the range from 20° to 25° which is
due to the presence of amorphous dopamine shells formed
around the magnetic hollow sphere (labeled as
D).
Additionally, based on the diffraction peaks that correspond to
h-Fe2O3 and DA shell, there are four diffraction peaks else,
labeled as
S in the figure 2c. These diffraction peaks were
indexed to the cubic phase of Ag (JCPDS card No. 04-0783)
with a lattice constant a = 2.41 Å. The mean size of the
ordered γ-Fe2O3 nanoparticles has been estimated from full
width at half maximum (FWHM) and Debye-Sherrer formula
according to equation the following:
Figure 3. TGA analysis of (a) h-Fe2O3 and (b) h-Fe2O3@DA/Ag
Magnetization study
The magnetic properties of the h-Fe2O3, h-Fe2O3@DA and h-
Fe2O3@DA/Ag were measured by vibrating sample
magnetometer (VSM) at room temperature, as shown in Fig. 4.
The maximum saturation magnetization (Ms) values of h-
Fe2O3@DA and h- Fe2O3@DA/Ag were found to be 32.35 and
22.63 emu/g, respectively. It is clear that, the Ms value of the
corresponding magnetic Fe2O3 hollow sphere (45.28 emu/g) is
significantly higher than of the h-Fe2O3@DA and h-
Fe2O3@DA/Ag sphere, which may be due to the h-Fe2O3 were
coated with a layer of dopamine in the Fe2O3@DA and Ag-
doped nano magnetic Fe2O3@DA/Ag hollow spheres. In
addition, it can be seen that there are no obvious changes in
the coercivity from the enlarged view of the central loop of the
samples. The result demonstrates that the as synthesized
Fe2O3@DA/Ag hollow spheres presents excellent magnetic
separation properties and have high potential in applications
as recyclable nano catalysts and facile separation from the
solution with the help of an external magnetic force.
The mean size of as-prepared γ-Fe2O3 nanoparticles was
around 47 nm from this Debye-Sherrer equation. According to
XRD pattern and this investigation, it is clear that maghemite
Fe2O3@DA/Ag hollow spheres have been prepared
successfully.
Figure 2. XRD pattern of (a) h-Fe2O3, (b) h-Fe2O3@DA and (c) h-
Fe2O3@DA/Ag
TG analyzer
The temperature-dependent changes can be verified by the
determination of mass loss employing thermo gravimetric
analysis (TGA). The TGA of the synthesis h-Fe2O3 and h-
Fe2O3@DA/Ag are shown in Fig. 3. As shown in Fig 3a, smaller
o
weight losses below around 200 C are perhaps due to the
removal of surface hydroxyls and/or surface adsorbed water,
o
as well as the endothermic weight loss at (300-600 C) is
attributed to water loss from organic groups on the surface.
Based on the results displayed in fig 3b, by increasing the
temperature a vigorous weight loss for H-Fe2O3@DA/Ag was
observed, owing to decomposition of adsorbed organic groups
on their surfaces. The removal of organic components starts at
around 200 °C and is virtually completed at 500 °C.
Figure 4. The magnetization curves of (a) h-Fe2O3, (b) h-
Fe2O3@DA and (c) h-Fe2O3@DA/Ag
Thus, the successful doping of dopamine with Ag nanoparticles
on the surface of the hollow-Fe2O3 is verified and exhibits a
considerable thermal stability at high temperatures.
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FE-SEM-EDS and TEM analysis
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DOI: 10.1039/C6RA18810A
Figure 5. The FEG-SEM-EDS analysis of (a,b) h-Fe2O3, (c,d) h-Fe2O3@DA and (e,f) h-Fe2O3@DA/Ag
The morphology and structure of the obtained h-Fe2O3, h- are covered with medial quantities of small particles. As
Fe2O3@DA and h-Fe2O3@DA/Ag were investigated by FE-SEM, expected, the corresponding EDS pattern revealed the
EDS and TEM analysis to characterize the resultant products at presence of Fe, O, N, C and Ag in the structure. Furthermore,
different stages during the course of synthesis (Fig. 5). From based on TEM image (6) and EDS, products were covered by
Fig. 5a, b and c, it can be clearly observed that the nano Ag nanoparticles. In addition, EDS results indicated that the
magnetic Fe2O3 are spherical in shape with an average loading capacity of the Ag nanoparticles on the support is 4.42
diameter of 400 nm. From the FE-SEM image shown in Figure wt%.
5a with the significant magnification (Fig. 4a), the surface To confirm further successfully functioned fertilizer, the final
morphology was clearly revealed. After that, the surface of sample was determined by TEM observations. Generally, From
Fe2O3 hollow spheres was subsequently modified with a layer the TEM image (Figure 6), the magnetic cores are black
of dopamine group. Clearly, the DA layer was formed via tiny spheres, and a thin gray color shell with an average thickness
shelled on the surface of Fe2O3 hollow sphere and it is of about 10 nm appears around the core. It should be noted
noteworthy; that the presence of the DA shell was efficient in that the Ag nanoparticles could be observed with an average
working as a capping agent to stabilize the nanoparticles. On diameter of about 10-20 nm and homogeneously immobilized
the other hand, the EDS pattern of Fe2O3@DA hollow spheres on the surface of Fe2O3@DA hollow spheres. Hence, the
confirms the existence of a DA layer (Fig. 5c and d). Finally, in successful functionalization of Fe2O3@DA hollow sphere by DA
order to prepare h-Fe2O3@DA/Ag core shell structure, Ag ions and Ag nanoparticles is obviously evidenced based on the
adsorbed on Fe2O3@DA hollow sphere through an in situ wet images examined by TEM observation.
chemistry. Based on the Figure 5c, the surfaces of the DA shell
Figure 6. The TEM image of h-Fe2O3@DA/Ag
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DOI: 10.1039/C6RA18810A
Determination of the physical property of shell in
hollow-sphere
Determination of the physical property of shell in hollow-sphere
particles is important since it is a factor which directly affects the
functionality of the desired catalyst. Thus we determined the shell
density using specific surface area and shell thickness in
accordance with the method developed and reported
previously.78
Table 1 Synthesis of three-component reaction of derivatives aldehyde,
secondary- amines and terminal alkynes catalyzed by h-Fe2O3@DA/Aga
Yieldb
(%)
98
93
90
90
91
87
94
90
93
88
80
96
90
89
88
90
80
70
Prod
uct
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
5q
5r
Time
(min)
60
60
60
60
65
65
60
75
50
75
85
60
70
70
65
R1
R2
R3
R4
1a: C6H5
1b: p-Cl-C6H4
1c: o-Cl-C6H4
1d: p-Br-C6H4
1e: p-Me-C6H4
1f:p-MeO-C6H4
1g:o-Thiophen
1h: p-OHCC6H4
3a: -(CH2)5-
4a: C6H5
4a
4a
4a
4a
4a
4a
4a
The ρshell was calculated with a shell thickness of t (nm)
(obtained from XRD), the inner radius of r (nm) (obtained from
BET)76 and specific surface area SA (m2.g-1) (obtained from
BJH).76 Therefore the ρshell value was calculated to be 2.9 g.cm-3
The magnetite’s density is established at 3.25 g.cm-3, therefore
the as-synthesized hollow spheres have lower density than
solid sphere-like Fe2O3 which show they have a good potential
being applied at industrial levels.
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
1i: o-Naphthyl
4a
1a
1a
1a
1b
1e
1d
1a
1a
1a
4b:p-CH3-C6H4
4c:p-F-C6H4
Catalytic activity of nano magnetic
Fe2O3@DA/Ag hollow sphere
3b:-(CH2)2-O-(CH2)2-
4a
4a
4a
4a
4b
4a
4a
3b
3b
3b
3b
Catalytic performance of nano magnetic
Fe2O3@DA/Ag hollow sphere in A3 coupling and
70
65
60
3c: -(CH2)4-
3d:C2H5
KA2 reaction
C2H5
2a: H
2a
2a
2a
2a
2b: C6H11
3a
3a
3b
3c
3d
3a
3b
3b
3c
3d
3a
4a
4b
4a
4a
4a
4a
4b
4a
4a
4a
4c
4a
4a
4a
4a
6a
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
6l
6m
6n
6o
40
40
40
40
45
45
45
45
45
45
45
45
45
45
45
90
81
95
85
78
90
85
88
85
65
81
90
85
80
75
Achieving successfully synthesized and characterization of
catalyst, its role as a catalyst was evaluated in A3 and KA2
coupling reactions. At first, the proper reaction was chosen
based on the treatment of benzaldehydes (1.0 mmol),
piperidine (1.2 mmol) and phenyl acetylene (1.5 mmol)
(scheme 2). Figure 7 summarizes the catalytic performances of
the catalyst with various parameters such as solvent
temperature and the amount of catalyst in this reaction. Then,
the model reaction was carried out in the presence of different
loading of catalyst. The catalytic activity first increased with
the increase of the h-Fe2O3@DA/Ag loading up to 10 mg.
However, the activity decreased with further increases in
catalyst loading up to 15 mg. The optimal catalyst loading was
determined to be 10 mg. Besides the h-Fe2O3@DA/Ag loading,
and also the effects of reaction solvents, temperatures on the
catalytic efficiency were investigated. The above experiment
was also performed in various solvents including water,
ethanol and dimethylsulfoxide (DMSO), that the yields were
lower. Whereas, the best result with highest yield was
achieved by carrying out the reaction under solvent free condition.
2b
2b
2b
2b
2b
2c: -(CH2)2-Me
3a
2c
2c
2c
3b
3c
3d
a Reaction conditions: aliphatic aldehyde (1.1 mmol) or arylaldehydes (1 mmol),
amines (1.1 mmol), alkynes (1.2 mmol), h-Fe2O3@DA/Ag (10 mg) under solvent-
free conditions at 90oC.
b Isolated yield.
c Aqueous formaldehyde (37%, 0.4 mL).
carried out and the results were summarized in figure 7. When the
reaction was performed at room temperature, only a trace of the
desired product was formed during 5h (green column). On the
o
other hand, at 50 and 90 C, (yellow and pink column), the product
yield decreased. As results, the reaction temperature 90 oC was
chosen for subsequent experiments.
Scheme 2. Model reaction
Finally, in order to study the effect of temperature on the synthesis
of propargylamine, the synthesis of 5a at various temperatures was
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As demonstrated in Table 1, the cyclic amines such as morpholine
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and pyrrolidine gave similar results to DtOhIo: s1e0.10o3b9t/aCi6nReAd188w1i0tAh
piperidine, whereas diethylamine rendered the corresponding
product in modest yield. In order to obtain better results, we
carried out a synthesis of 5a using nano γ-Fe2O3 and hollow γ-Fe2O3
catalyst to compare the catalytic activity of prepared nano magnetic
γ-Fe2O3@DA/Ag hollow sphere as a catalyst under solvent-free
conditions from the above observations in figure 8a.
It is clear that nano magnetic Fe2O3@DA/Ag hollow sphere plays a
main role compared to other catalysts, due to its nano active
surface and high performance of Ag nanoparticles. Also, to show
the superiority of the present method over previous ones, we
compared our results with some other results reported in the
literatures79-83 (Fig. 8b). Therefore, the h-Fe2O3@DA/Ag was used as
catalyst for the synthesis of propargylamine. As shown, magnetic γ-
Fe2O3@DA/Ag hollow sphere as a simple catalyst has several
advantages such as short reaction times, excellent yields, simple
recovery of magnetic catalyst and avoiding the use of solvent in the
reaction system.
Figure 7. Effects of loading of catalyst, duration of reaction and solvent for
the synthesis of propargylamine (Scheme 2)
Thus, the optimized reaction conditions include 1.0 mmol of
benzaldehyde, 1.2 mmol of piperidine, 1.5 mmol of phenyl
acetylene and 10 mg nano magnetic Fe2O3@DA/Ag hollow sphere
of catalyst, at 90 oC under solvent free condition. Encouraged by
these results, with the optimized reaction conditions in hand, we
initiated our investigation into the scope of aldehydes, amines, and
terminal alkynes applicable to the present A3 coupling reaction and
the results are summarized in Table 1. According to these results it
was found that, the aromatic aldehydes and aryl acetylenes bearing
electron-donating substituents as well as electron-withdrawing
groups and aliphatic aldehydes provided the desired products in
moderate to high yields within short reaction time.
A possible mechanism was proposed based on reported literature.81
According to figure 6, the role of Ag nanoparticles as a Lewis acid
catalyst in the synthesis of propargylamine is shown which the
immobilized Ag nanoparticles on the surface of γ-Fe2O3@DA hollow
sphere insert in terminal alkyne to form the Ag acetylide
intermediate (I) with the activation of C-H bond. Meanwhile, the
nucleophilic addition of amine to aldehyde leads to generate
iminium ion (II). Thus, the silver acetylide intermediate attacks the
iminium ion in situ, to obtain the corresponding propargylamine
(III).
Scheme 3. Plausible mechanism of the A3-coupling reaction by h-
Fe2O3@DA/Ag
In order to show the practical applicability of the nano magnetic γ-
Fe2O3@DA/Ag hollow sphere, the synthesis of propargylamines of
three components coupling reaction of Secondary amines, aliphatic
ketones and aryl acetylene (KA2 reaction) was examined.
Importantly, Ketoimines are considered to be less reactive towards
nucleophilic additions than aldimines, due to the steric hindrance
and electronic effects. Therefore, the utilization of in situ
generation of ketoimines some reactions, including in the direct
alkyne addition reaction still remains a significant challenge for
Figure 8. Comparison of h-Fe2O3@DA/Ag with various γ-Fe2O3 catalysts (a)
and other recently reported silver catalyst (b) in the preparation of
propargylamine (scheme 2)
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chemists which should be circumvented.84 So, the catalytic activity
of the catalyst was tested by the model reaction of cyclohexanone
(1.5 mmol), phenyl acetylene (1.5 mmol) and piperidine (1 mmol) to
obtain 5a using 10 mg nano magnetic Fe2O3@DA/Ag hollow sphere
under similar optimized reaction conditions. The scope of the KA2
coupling reaction was evaluated using cyclic ketones, secondary
amines and alkynes, applying the optimal condition, and exhibited
good reactivity (Table 2). However, the reaction proceeds were
utilized cyclic ketones, giving the corresponding propargylamines in
high yields. Notably, aromatic ketones gave none of the desired
product in long reaction time processes.
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Scheme 4. Synthesis of 5-Substituted 1H-Tetrazoles by h-Fe2O3@DA/Ag
To investigate the effect catalyst loading, the model reaction was
carried out in the presence of different amount of catalyst. Hence
30 mg of h-Fe2O3@DA/Ag can be catalyzed in [3+2] cycloaddition
reactions for synthesis of tetrazoles. More amount of the catalyst
has no significant change for the yield of tetrazoles, whereas
decreasing the amount of h-Fe2O3@DA/Ag lead to decrease the
yield a lot. Experiments with different aprotic and protic solvents
and solvent free condition revealed the best results in DMF as
solvent (blue column) as indicated in figure 9. Water was not a
suitable solvent for this reaction, but the experimental results
show that DMF, DMSO and Toluene is good solvent with 94%,
73%, and 44% yields, respectively, under the optimum conditions.
Among them, DMF is more preferable. To investigate the effect of
temperature on the reaction rate, [3+2] cycloaddition was
performed at different temperatures. When the reaction was
performed at room temperature, the reaction was found to be
inefficient during 12 h (red column). Generally, whilst the reaction
temperature was increased to 110 ℃, the yield was sustainable.
Table 2 Synthesis of the three-component reaction of aliphatic ketones,
secondary- amines and terminal alkynes catalysed by h-Fe2O3@DA/Aga
Prod
uct
Time
(min)
Yieldb
(%)
R1 , R2
R3 , R4
R5
7a: -(CH2)4-
3a: -(CH2)5-
4a: C6H5
8a
8b
120
120
90
85
7a
3b: -(CH2)2-O-(CH2)2-
4a
7a
3c: -(CH2)4-
4a
4a
8c
140
150
82
87
7b: -(CH2)5-
3a
8d
7b
3a
4b: p-CH3-C6H4
8e
175
81
7b
7b
7b
3b
3b
3c
4a
4b
4a
8f
8g
8h
130
170
140
92
89
90
7b
3b
4c: p-F-C6H4
8i
145
82
a Reaction conditions: ketones (1.5 mmol), amines (1 mmol), alkynes (1.5
mmol), h-Fe2O3@DA/Ag (10 mg) under solvent-free conditions at 90oC.
b Isolated yield
Catalytic testing for synthesis of 1H-Tetrazole in [3+2]
cycloaddition
Figure 9. Effects of loading of catalyst, duration of reaction and solvent for
the synthesis of 5-Substituted 1H-Tetrazoles (Scheme 4)
To achieve better results catalytic activity of nano magnetic
Fe2O3@DA/Ag hollow sphere, its role as a catalyst was evaluated for
the synthesis of tetrazoles (scheme 4).
And also in order to show the efficiency of this catalytic system
further, the synthesis of 5-Substituted 1H-Tetrazoles was
investigated in the presence of magnetic Fe2O3@DA/Ag hollow
sphere. As shown, the optimized reaction conditions (1:1.5 molar
ratio of benzonitrile: sodium azide, 30 mg catalyst, in DMF at 110
oC) were used for all organic benzonitrile and the corresponding 5-
substituted-1H tetrazoles were prepared within 5-13 h with good to
excellent yields.
At the beginning, the standard reaction was chosen for the
treatment of benzonitrile and sodium azide. Then, this reaction was
carried out with various amounts of catalysts, solvents and at
different reaction temperature (Fig. 9). To investigate the effect
catalyst loading, the model reaction was carried out in the presence
of different amount of this.
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DOI: 10.1039a/C6pRrAo1p8o8s10eAd
By considering results in the literature report,85
mechanism for the formation of the 1-substituted tetrazole is
shown in scheme 5. It is clear that, silver nanoparticles act as a
Lewis acid activating the nitrile groups via complexation (I).
Therefore, it enhances the electrophilic property of cyanide group,
which then reacts with sodium azide to form of intermediate (II). At
the end, magnetic h-Fe2O3@DA/Ag catalyst-assisted elimination of
ethanol from (III) leads to the final product (IV).
Table 3. Synthesis of 5-substituted-1H-tetrazole from derivatives of aryl
cyanide and sodium azide catalyzed by magnetic Fe2O3@DA/Ag hollow
spherea
Time
(h)
Yield
R
Product
(%)b
Recycling and leaching of the catalyst
C6H5
9a
9b
9c
9d
9e
9f
9g
9h
9i
9J
9k
9l
9m
9n
9o
7
5
8
9
7
8
7
8
7
11
5
6
5
13
9
94
90
89
86
85
79
85
85
87
87
75
85
88
70
82
As the possibilities of recycling the catalyst are one of the key
advantages of organic procedures concerning green chemistry
aspects, the catalyst was recycled and reused. In order to study
these important features and constancy of the activity of this novel
used catalyst system, the catalyzed three-component reaction
involving benzaldehyde, piperidine and phenylacetylene was
selected as a model reaction and conducted under already secured
optimal reaction conditions including the use of the exact amount
of optimized quantity of the catalyst. Figure 10 indicates that the
heterogeneous catalyst was used for 5 successive times in the new
experiments without dramatic yield loss and generates a product
with purity similar to that obtained in the first run.
p-O2N-C6H4
p-CH3-C6H4
p-OCH3-C6H4
β-Naphtol
Α-Naphtol
m-Cl-C6H4
p-Cl-C6H4
p-Br-C6H4
p-OH-C6H4
p-CN-C6H4
2-Pyridine
4-Pyridine
CH2-C6H5
o-NH2-m-NO2-C6H4
a
Reaction condition: nitriles (1.0 mmol), Sodium azide (1.5 mmol),
magnetic h-Fe2O3@DA/Ag (30 mg), DMF (2 mL), 110 oC
b Yields are after work-up.
Benzonitrile containing both electron withdrawing and
electron donating groups underwent the conversion smoothly.
The benzonitrile having electron donating substituents require
longer reaction time as donating substituent on aromatic ring
decreases the electrophilic character of benzonitrile.
Generally, this catalyst has some advantages such as giving the
selected three-component reaction a short reaction time,
providing excellent yields and offering simple and almost
quantitative recovery of the magnetized catalyst in pure form
to be re-used in several further runs without appreciable loss
in its catalytic activity.
Figure 10. Reusability of the h-Fe2O3@DA/Ag catalyst in A3 Coupling
Conclusions
The present work demonstrates a unique and facile approach
to the fabrication of Ag-doped nano magnetic-Fe2O3@DA
Core–Shell Hollow Spheres. The formation mechanism of the
core shell nanoparticles was directly related to DA molecules,
attached to silver nanoparticles seeds, dissociate from h-Fe2O3
surfaces. The phase and structure of the as-obtained were
identified by XRD, FT-IR, FE-SEM, EDS, TGA and VSM
measurements and shells was clearly revealed by TEM
characterizations. Additionally, apparent shell density as
physical property was determined using the specific surface
area and shell thickness. The nano magnetic γ-Fe2O3@DA/Ag
hollow sphere product exhibited an excellent catalytic activity
for the synthesis of propargylamine by A3, KA2 coupling
reaction and 1H-tetrazoles via [3+2] cycloaddition. Moreover,
this catalyst could be easily separated and reused up to five
times with no significant loss of activity and selectivity. The
easy reusability of the catalyst without any further purification
proved as a high potential candidate for organic reactions. It is
Scheme 5. Plausible mechanism for the synthesis of 5-Substituted 1H-
Tetrazoles by h-Fe2O3@DA/Ag
8 | J. Name., 2012, 00, 1-3
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believed that this facile synthesis method reported here can refluxed for 12 h at 100°C. The final product (h-Fe2O3@DA)
View Article Online
DOI: 10.1039/C6RA18810A
be applied to the preparation of effective catalyst with hollow was collected from the reaction medium by employing of a
sphere structure.
magnet, and then washed with acetone and dried at 70°C.
Experimental
Loading Ag nanoparticles to obtain h-
Fe2O3@DA/Ag
Materials and instruments
The doping of Ag nanoparticles onto h-Fe2O3@DA shell was
performed as follows.64 Initially, 0.06 g of h-Fe2O3@DA was
dispersed in 15 mL of 5 × 10−3 M Ag(NH3)2NO3 solution under
stirring at room temperature for 30 minutes. After that, the
[Ag(NH3)2]+ ions were adsorbed onto the surfaces of h-
Fe2O3@DA via the electrostatic attraction. The dispersion was
added in 15 mL of ethanol contained PVP (0.1 g), followed by
All chemicals, including FeCl3∙6H2O, trisodium citrate
dihydrate, sodium acetate trihydrate, ethanol, ethylene glycol
(EG), PVP, urea, dopamine, AgNO3 and NH3∙H2O, were
analytical grade reagents, purchased from Sigma-Aldrich, and
used without further purification. The progress of the reaction
was monitored by TLC on commercial aluminum-backed plates
of silica gel 60 F254, visualized, using ultraviolet light. Melting
points were determined in open capillaries using an
Electrothermal 9100 without further corrections.1H NMR and
13C NMR spectra were recorded using a Bruker DRX-400
spectrometer at 400 and 100 MHz respectively. Magnetic-
Fe2O3@DA/Ag hollow sphere was characterized by; FT-IR
spectra were obtained with potassium bromide pellets in the
range of 400–4000 cm-1 using a Shimadzu 8400s spectrometer;
X-ray diffraction (XRD) was detected by Philips using Cu-Ka
radiation of wavelength 1.54Å; Scanning electron Microscopy,
FE-SEM-EDX, analysis was performed using Tescanvega II XMU
Digital Scanning Microscope. Samples were coated with gold at
10 mA for 2 min prior to analysis; the magnetic properties
were characterized using a vibrating sample magnetometer
(VSM, Lakeshore7407) at room temperature. Thermo-
gravimetric analyses (TGA) were analyzed with a LINSEIS model
STS PT 16000 thermal analyzer under air atmosphere at a
heating rate of 5 °C min−1.
o
heating the solution by reflux at 70 C for 4 h. The final
precipitate was collected by magnetic separation, washed
three times with a recycle of ethanol and deionized water, and
dried at 50 oC for 12 h.
Investigating of magnetic Fe2O3@DA/Ag
hollow sphere as an efficient nano catalyst in
some organic reaction
General procedure for synthesis of 1-(1,3-
diphenylprop-2-ynyl)piperidine: (5a)
To a mixture of benzaldehyde (1.0 mmol), piperidine (1.2
mmol) and phenyl acetylene (1.5 mmol) without any solvent,
magnetic h-Fe2O3@DA/Ag (10 mg), was added. The mixture
was heated at 90 °C for 1h (monitored by TLC). And then, the
reaction mixture was cooled to room temperature and diluted
with hot ethanol (10 ml). Then, the catalyst was separated by
an external magnet from the cooled mixture, washed with
acetone, dried in the oven and re-used for a consecutive run
under the same reaction conditions. The filter was
concentrated and the resulting residue was purified by short
column chromatography on silica gel to obtain the excellent
yield of the desired product.
Preparation of Nano magnetic Fe2O3 hollow
sphere
Hollow magnetic particles were prepared through
solvothermal method.76 At first, ferric chloride hexahydrate (5
mmol) was dissolved in 70 mL of ethylene glycol (EG) in a flask,
followed by the addition of 1.5 mmol of trisodium citrate
dehydrate, 30 mmol of sodium acetate (NaAc) and urea (17
mmol). The resulting mixture was stirred vigorously for 1 h,
and was transferred to a Teflon-lined stainless-steel autoclave
(150 mL capacity), at 220 °C for 24 h. And after the reaction,
the autoclave was cooled to room temperature. The resulting
brown product was washed several times with ethanol and
deionized water and was finally dried at 80 °C in the oven for
overnight.
General procedure for synthesis of 1-phenyl-1H-
tetrazole: (9a)
To a mixture of nitrile (1.0 mmol) and NaN3 (1.5 mmol) in DMF
(2 mL), magnetic h-Fe2O3@DA/Ag (30 mg), was added. The
mixture was stirred at 110 °C for appropriate time until the
reaction was completed (monitored by TLC), and then, that
was cooled to room temperature and diluted with ethyl
acetate (10 ml) and 5 N HCl (20 mL). The catalyst was
separated by an external magnet from the cooled mixture,
washed with acetone, dried in oven and re-used for a
consecutive run under the same reaction conditions. Then, the
organic layer was separated, and the aqueous layer was
extracted with ethyl acetate (20 mL). The combined ethyl
acetate layer was washed with water, dried over anhydrous
Na2SO4, The residue was purified by recrystallization or short
Preparation of h-Fe2O3@DA Core shell
The h-Fe2O3@DA shells were prepared according to a
previously reported method.63 Briefly, (1 g) of the as-prepared
Fe2O3 hollow spheres were dispersed in 50 mL of deionized
water, and later a certain amount of dopamine (1 g) was
added to the suspension. Then, the above suspension was
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4H), 2.67−2.70 (m, 2H), 2.79−2.83 (m, 2H), 3.82-4.13V(imew, A1rtHic)le, O4.n1lin5e-
column chromatography on silica gel to afford the target
products in excellent yield.
4.17 (m, 4H), 7.46−7.50 (m, 3H), 7.62−7.64 (m, 2H).
DOI: 10.1039/C6RA18810A
1-(1-(2-p-tolylethynyl)cyclohexyl)piperidine (Table 2, 8e):
Yellow oil; 1H NMR (400 MHz, CDCl3, ppm) δ 1.39-1.93 (m, 16H),
2.17-2.20 (m, 2H), 2.53 (s, 3H), 2.73-2.83 (m, 2H), 7.26-7.27 (m, 3H),
7.46-7.48 (m, 2H); 13C NMR (100 MHz, CDCl3, ppm) d 21.32, 23.4,
24.4, 25, 26.7, 37.6, 47.9, 58.8, 85.4, 92.1, 123, 127.6, 128.3, 133.
General procedure for recycling of magnetic
Fe2O3@DA/Ag hollow sphere
Recyclability of Fe2O3@DA/Ag (10 mg) was examined for the
synthesis of propargylamine between benzaldehyde (1.0
mmol), piperidine (1.2 mmol) and phenyl acetylene (1.5 mmol)
and under solvent free conditions for 60 minutes at 90 °C.
After that, the mixture was washed five times with ethanol,
filtered by using external magnet, and dried in air, and then
reused for several runes of reactions under identical
conditions.
4-(1-(2-phenylethynyl)cyclohexyl)morpholine (Table 2, 8f):
1
Pale yellow oily liquid; H NMR (400 MHz, CDCl3, ppm) δ 1.28-1.30
(m, 1H), 1.52 (m, 2H), 1.63-1.67 (m, 3H), 1.73 (br.s, 2H), 2.03-2.05
(m, 2H), 2.74 (br.s, 4H), 3.78 (br.s, 4H), 7.27 (m, 3H), 7.44-7.45 (m,
2H), 13C NMR (100 MHz, CDCl3, ppm) δ 22.7, 25.7, 35.4, 46.6, 58.8,
67.4, 86.4, 89.8, 123.4, 127.7, 128.1, 131.7.
4-(1-( (4-fluorophenyl)ethynyl)cyclohexyl)morpholine (Table 2,
1
8i): Yellow oil; H NMR (400 MHz, DMSO-d6, ppm) δ 1.26-1.34 (m,
1H), 1.57-1.62 (m, 2H), 1.69-1.78 (m, 3H), 1.80-1.86 (m, 2H), 2.00-
2.02 (m, 2H), 2.78 (s, 4H), 3.70 (br.t, J = 4.2 Hz, 4H), 6.97-7.00 (t, J =
8.6 Hz, 2H), 7.32-7.40 (m, 2H);
Spectral data of some selected compound
1-(1,3-diphenylprop-2-ynyl)piperidine (table 1, 5a): Pale yellow
oily liquid; 1H NMR (400 MHz, CDCl3, ppm) δ 1.45-1.49 (m, 2H),
1.58-1.65 (m, 4H), 2.59 (t, 4H), 4.81 (s, 1H), 7.31-7.40 (m, 6H), 7.53-
7.55 (m, 2H), 7.65-67 (d, J=7.6 Hz, 2H).
5-Phenyl-1H-tetrazole (Table 3, 9a): White solid; mp 213–215
o
1
oC (Lit.88 214–215 C); H NMR (400 MHz, DMSO-d6, ppm) δ 7.68 (s,
3H, Ph), 7.92 (s, 2H, Ph); 13C NMR (100 MHz, DMSO-d6, ppm) δ
126.6, 128.6, 130.3, 134.6, 155.
1-(3-phenyl-1-(thiophen-2-yl)prop-2-ynyl)piperidine (table 1,
5-(4-Nitrophenyl)-1H-tetrazole (Table 3, 9b): Yellow solid; mp
218–219 oC (Lit.88 220-222 oC); 1H NMR (400 MHz, DMSO-d6, ppm) δ
8.30 (d, 2H, J 8.4, Ph), 8.39 (d, 2H, J 8.8, Ar-H); 13C NMR (100 MHz,
DMSO-d6, ppm) δ 127.6, 129.1, 131, 149.5.
o
o
1
5g): Yellow solid; mp 50-51 C (Lit.86 52-53 C); H NMR (400 MHz,
CDCl3, ppm) δ 1.48-1.52 (m, 2H), 1.63-1.70 (m, 4H), 2.62-2.71 (m,
4H), 5.03 (s, 1H), 7.00 (dd, J1=J2=3.6 Hz, 1H), 7.25-7.30 (m, 1H), 7.31
(d, J=4.4 Hz, 2H). 7.36-7.38 (m, 3H), 7.54-7.57 (m, 2H); 13C NMR (100
MHz, CDCl3, ppm) δ 24.4, 26.1, 50.6, 58.2, 85.3, 86.9, 123, 125.3,
125.8, 126.2, 128.2, 128.3, 131.8, 144.
5-(4-Methylphenyl)-1H-tetrazole (Table 3, 9c): White solid; mp
249-251 oC (Lit.88 247-249 oC); 1H NMR (250 MHz, DMSO-d6, ppm) δ
2.35 (s, 3H, CH3), 7.37 (d, 2H, J 7.6 Hz, Ph), 7.90 (d, 2H, J 7.5 Hz, Ph).
1-(3-phenyl-1-(4-(3-phenyl-1-(piperidin-1-yl)prop-2-
5-(3-Chlorophenyl)-1H-tetrazole (Table 3, 9g): White solid; mp
ynyl)phenyl)prop-2-ynyl)piperidine (table 1, 5h): White solid; mp
o
o
1
138-140 C (Lit.83137-139 C); H NMR (250 MHz, DMSO-d6, ppm) δ
7.55 (m, 2H, Ph), 7.96 (d, 1H, J 7.6, Ph), 7.99, (s, 1H); 13C NMR (62.9
MHz, DMSO-d6, ppm) δ 125.4, 126.2, 126.4, 130.7, 131.1, 133.9,
154.6.
o
157-159 C (Lit.86 158-160 oC); 1H NMR (400 MHz, CDCl3, ppm) δ
1.47 (m, 2H), 1.59-1.63 (m, 4H), 2.59 (m, 4H), 4.81 (s, 1H), 7.33-7.35
(m, 3H), 7.52-7.55 (m, 2H), 7.63 (s, 2H).
1-(1-(naphthalen-3-yl)-3-phenylprop-2-ynyl)piperidine (table
5-(4-Chlorophenyl)-1H-tetrazole (Table 3, 9h): White solid; mp
251-253 oC (Lit.88 251-252 oC); 1H NMR (400 MHz, DMSO-d6, ppm) δ
7.61 (d, 2H, J 8.4, Ph), 8.09 (d, 2H, J 8.8, Ph).
1
1, 5i): Yellow oil; H NMR (400 MHz, CDCl3, ppm) δ 1.47-1.51 (m,
2H), 1.60-1.67 (m, 4H), 2.64 (t, 4H), 4.97 (s, 1H), 7.36-7.40 (m, 3H),
7.48-7.52 (m, 2H), 7.58-7.61 (m, 2H), 7.79 (dd, J1=J2=8.4 Hz, 1H),
7.85-7.91 (m, 3H), 8.11 (s, 1H); 13C NMR (100 MHz, CDCl3, ppm) δ
24.4, 26.2, 50.8, 62.5, 86, 88.1, 123.3, 125.8, 125.9, 126.7, 127.2,
127.5, 127.7, 128.1, 128.12, 131.8, 132.9, 133.1, 136.3.
5-(4-Hydroxyphenyl)-1H-tetrazole (Table 3, 9j): White solid; mp
235 oC (Lit.88 233-234 oC); 1H NMR (400 MHz, DMSO-d6, ppm) δ 6.91
(d, 2H, J 8.4, Ph), 7.58 (d, 2H, J 8.4, Ph), 10.11 (s broad, OH); 13C
NMR (100 MHz, DMSO-d6, ppm) δ 116.1, 117.4, 128.8, 153.2, 159.8.
N,N-diethyl-1,3-diphenylprop-2-yn-1-amine (table 1, 5r): Pale
1
4-(1H-tetrazol-5-yl)-benzonitrile (Table 3, 9k): White solid; mp
257-259 oC (Lit.89 258-260 oC); 1H NMR (250 MHz, DMSO-d6, ppm) δ
8.06 (d, 2H, J 7.1, Ph), 8.19 (d, 2H, J 8.6, Ar-H); 13C NMR (62.9 MHz,
DMSO-d6, ppm) δ 113.3, 118.1, 127.6, 128.7, 133.1, 155.2, 162.2.
yellow oily liquid; H NMR (400 MHz, CDCl3, ppm) δ 1.04 (m, 6H),
2.36-2.62 (m, 4H), 5.19 (s, 1H), 7.15-7.27 (m, 4H), 7.29-7.38 (m, 3H),
7.39-7.41 (m, 2H).
1
4-(3-phenylprop-2-ynyl)morpholine (table 1, 6c): yellow oil; H
2-(1H-tetrazol-5-yl)pyridine (Table 3, 9l): White solid; mp 210-
213 oC (Lit.90 211-212 oC); 1H NMR (400 MHz, DMSO-d6, ppm) δ 7.75
(s, 1H, Ph), 8.07 (s, 1H, Ph), 8.20 (d, 1H, J 8.4 Ph), 8.63 (s, 1H).
NMR (400 MHz, CDCl3, ppm) δ 2.64-2,67 (m, 6H), 3.52 (s, 3H), 3.69-
3.71 (m, 1H), 3.77-3.79 (m, 6H), 7.28-7.31 (m, 4H), 7.43-7.46 (m,
2H).
4-(1H-tetrazol-5-yl)pyridine (Table 3, 9m): White solid; mp 256-
258 oC (Lit.91 256-258 oC); 1H NMR (250 MHz, DMSO-d6, ppm) δ 8.10
(d, 2H, J 6.0, Ph), 8.77 (d, 2H, J 6.5, Ph); 13C NMR (62.9 MHz, DMSO-
d6, ppm) δ 120.9, 121.3, 133.8, 149.9, 165.7.
1-(1-cyclohexyl-3-phenylprop-2-ynyl)pyrrolidine (table 1, 6i):
Colorless liquid; H NMR (400 MHz, CDCl3, ppm) δ 1.05-1.36 (m,
1
5H), 1.56-1.63 (m, 2H), 1.75-1.79 (m, 6H), 1.82-2.10 (m, 4H), 2.5-
2.98 (m, 4H), 3.36-3.38 (d, J =7.6 Hz, 1H), 7.14-7.33 (m, 3H), 7.50-
7.63 (m, 2H). 13C NMR (100 MHz, CDCl3, ppm) δ 24.9, 26.9, 27.1,
28.3, 32.7, 33, 42.9, 51.1, 61.1, 86.1, 88.9, 125.9, 128.9, 129.8,
132.6.
Acknowledgements
4-(1-phenylhex-1-yn-3-yl)morpholine (Table 1, 6m): Yellow oil;
1H NMR (400 MHz, DMSO-d6, ppm) δ 0.97 (m, 3H), 1.45-1.75 (m,
10 | J. Name., 2012, 00, 1-3
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32 Y. Yang, W. Zhang, Y. Zhang, A. Zheng, H. Sun, Xi. Li, S. Liu, P.
The authors wish to gratefully acknowledge the partial support
of this study by Semnan University and Amirkabir University of
Technology (AUT) Research Council.
View Article Online
Zhang and X. Zhang, Nano Research, 2015, 10.1007/s12274-
DOI: 10.1039/C6RA18810A
015-0840-9.
33 C. Zhou, K. Huang, L. Yuan, W. Feng, X. Chu, Zh. Geng, X. Wu,
L. Wang and Sh. Feng, New J. Chem., 2015, 39, 2413.
34 W. Yan, Zh. Yang, W. Bian and R. Yang, Carbon, 2015, 92, 74.
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