Efficient and reusable graphene-γ-Fe2O3 magnetic nano-composite for selective oxidation and one-pot synthesis of 1,2,3-triazole using a green solvent

Article abstract of DOI:10.1039/c3ra43184c

A graphene based composite material with γ-Fe₂O ₃ nanoparticles has been synthesized via a simple chemical route. The structure and composition of nanocomposite material have been characterized by TEM, Raman and XRD. The catalytic activity of the composite has been investigated for the oxidation reactions of sulfides and aromatic alcohols using water as the solvent. Sulfides and aromatic alcohols have been selectively oxidized to corresponding sulfoxides and aldehydes, respectively, with excellent yields. In addition this nanocomposite also serves as an efficient catalyst for one-pot synthesis of a series of 1,4-disubstituted-1,2,3-triazoles via click chemistry. The catalyst could be easily isolated from the final product by magnetic decantation (applying an external magnetic field) and can be reused without significant loss of catalytic activity. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3ra43184c

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Page 1 of 28
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DOI: 10.1039/C3RA43184C
GRAPHCAL ABSTRACT
Efficient and reusable grapheneꢀγꢀFe O magnetic nanoꢀ
2
3
composite for selective oxidation and oneꢀpot synthesis of
,2,3ꢀtriazole using green solvent
1
a
b
a
a
b,
Noor Salam, Arjyabaran Sinha, Paramita Mondal, Anupam Singha Roy, Nikhil R. Jana *,
a,
S. M. Islam *
[
a]
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India.
[b]
Centre for Advanced Materials, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata 700 032, India.

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Efficient and reusable grapheneꢀγꢀFe O magnetic nanoꢀ
2
3
composite for selective oxidation and oneꢀpot synthesis of
,2,3ꢀtriazole using green solvent
1
a
b
a
a
b,
Noor Salam, Arjyabaran Sinha, Paramita Mondal, Anupam Singha Roy, Nikhil R. Jana *,
a,
Sk. Manirul Islam *
[
a]
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India.
[b]
Centre for Advanced Materials, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata 700 032, India.
2 3
Abstract: A graphene based composite material with γꢀFe O nanoparticle has been
synthesized via a simple chemical route. The structure and composition of nanocomposite
material have been characterized by TEM, Raman and XRD. The catalytic activity of the
composite has been investigated for the oxidation reactions of sulfides and aromatic alcohols
using water as solvent. Sulfides and aromatic alcohols have been selectively oxidized to
corresponding sulfoxides and aldehydes, respectively, with excellent yields. In addition this
nanocomposite also serves as an efficient catalyst for oneꢀpot synthesis of a series of 1,4ꢀ
disubstitutedꢀ1,2,3ꢀtriazoles via click chemistry. The catalyst could be easily isolated from
the final product by magnetic decantation (applying external magnetic field) and can be
reused without significant loss of catalytic activity.
Keywords: graphene; nanocomposite; selective oxidation; click chemistry; oneꢀpot synthesis;
magnetic separation; green solvent.
*
Authors to whom correspondence should be addressed.
[
a] Dr. Sk. Manirul Islam, Department of Chemistry, University of Kalyani, Kalyani,
Nadia, 741235, W.B., India. Phone: +91ꢀ33ꢀ2582ꢀ8750, Fax: +91ꢀ33ꢀ2582ꢀ8282, eꢀ
mail: manir65@rediffmail.com
[b] Dr. Nikhil R. Jana, Centre for Advanced Materials, Indian Association for the
Cultivation of Science, Kolkata ꢀ 700032, India, Phone: +91ꢀ33ꢀ24734971, Eꢀmail:
camnrj@iacs.res.in.
1

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Introduction
In recent year metal and metal oxide nanoparticles have been attracted great attention
1
ꢀ6
in catalysing different chemical reactions. Although nanoparticles show high catalytic
7
activity their separation from reaction system are difficult. Thus nanoparticle catalysts are
often attached with different solid supports materials such as carbon nanotube, activated
8
ꢀ11
carbon, graphene, silica, zeolites and other materials for easier separation.
Among them
graphene oxide (GO) and graphene are considered as one of the most promising catalyst
1
2,13
support materials due to high surface area and conducting property.
However, uniform
and controlled loading of various nanocatalyst of interest and to stabilize the catalyst is a
12,13
challenging issue.
Graphene oxide (GO) is a lamellar flexible material with a large number of functional
groups such as epoxy (C–O–C), hydroxyl (OH) and carboxyl (COOH) groups on both basal
planes and edges. GO is hydrophilic due to the oxygenꢀcontaining functional groups and it
can be transformed into graphene via chemical reduction. Graphene based materials have
14ꢀ23
15ꢀ18
16ꢀ24
been tested in the removal of ammonia,
hydrogen sulfide,
sulfur dioxide,
or
1
7,20,22,25
nitrogen oxides.
They are advantageous as adsorbents because their physical
properties and surface chemistry can be modified to enhance the interactions with target
molecules. Furthermore, grapheneꢀbased composites with inorganic compounds are expected
to show catalytic properties that may be important for the interactions between adsorbents
26
and target molecules. For example, GO/zinc hydroxide composites showed photoꢀcatalytic
activity significantly enhancing the reactive adsorption of various gases on these
18,24,25
materials.
Recently, much attention has been paid on the synthesis of iron oxideꢀ
graphene based magnetic hybrid material, owing to wideꢀranging applications in catalytic
27
28
29ꢀ30
31
activity, electric doubleꢀlayer capacitance, heavy metal ion removal,
drug delivery,
32a
32b
biosensing and lithium storage. The unique properties of magnetic graphene hybrids
33
such as high conductivity, large surfaceꢀtoꢀvolume ratio, strong magnetism, low cost and
3
4,35
environmentally benign nature
have opened options for multifunctional applications.
However, there are only few reports on the application of magnetic graphene in catalytic
27
oxidation reactions.
Oneꢀpot synthesis is a strategy to improve the efficiency of a chemical reaction
whereby a reactant is subjected to successive chemical reactions in just one reactor. Dipolar
2

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cycloadditions have captured the attention of organic chemists since their discovery over 50
3
6
years ago by Robert Huisgen. In 2002, the pioneering work on the Huisgen 1,3ꢀdipolar
cycloaddition of terminal alkynes and organic azides catalyzed by homogeneous Cu(I) salts
37
was accomplished by Sharpless and Meldal independently. However, this procedure was
associated with elevated temperature, low yields and lack of selectivity. Thus to overcome
these drawbacks an improved procedure involving Cu(I)ꢀcatalyzed azide–alkyne 1,3ꢀdipolar
38
cycloaddition was reported, which provided high regioselectivity, room temperature
reaction and enlarged scope. More recently, catalytic processes are employing in this field to
produce 1,4ꢀregioisomers exclusively in mild conditions. These reactions are also example of
click reactions as it fulfils the criteria of ‘Click Chemistry’. Its 100 % atom economy and
simple purification have led to various applications for agrochemicals, material science, bioꢀ
chemicals, and functional materials. Triazole compounds can be obtained by Click
Chemistry’ and have tremendous biological importance. The 1,2,3ꢀtriazole may display
biological activities and there are numerous examples in the literature including antiꢀHIV
activity, anti cancer agents, anti mycobacterium tuberculosis strain, antiꢀmicrobial activity
against Gram positive bacteria, antiꢀallergic, antiꢀconvulsant, βꢀlactamase inhibitory,
3
9ꢀ40
selective β3 adrenergic receptor agonism, and so on.
Though these reactions are typically
catalyzed by Pd, Rh, Ir, and Ru complexes, catalytic systems using inexpensive metals such
as Cu, Ni, and Fe are also emerging, indicating another clear direction for the advancement of
4
1
organic reactions. Motivated by the fact in many biologically active natural products,
pharmaceuticals, and optoelectronic materials, we have synthesized such important
composite in green conditions using grapheneꢀγꢀFe O composite. The present work deals
2
3
with economic approach to one pot synthesis of 1,4ꢀdisubstituted 1,2,3ꢀtriazole in mild
conditions.
2 3
Here, we report the synthesis of grapheneꢀγꢀFe O based composite and explored its
potential as heterogeneous catalyst using water as green solvent. Specifically, we have
investigated the oxidation of sulfides and aromatic alcohol and oneꢀpot click chemistry
2 3
reactions. We found that grapheneꢀγꢀFe O catalyst shows high catalytic activity and
selectivity in the above reactions. In addition the composite can be separated and reused for
more than five times without any significant loss in its activities. Due to superparamagnetic
nature of composite the catalyst can be easily removed from the reaction system by using a
laboratory based bar magnet.
3

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Experimental section
Materials and Reagents
All reagents and solvents were purchased from commercial sources and were further
purified by standard procedures. [3ꢀ(2ꢀAminoethylamino)propyl]trimethoxysilane (AEAPS),
tetramethylammonium hydroxide 25 wt% in methanol (TMAH), graphite powder (< 20
micron), hydrazine hydrate (98 %) were purchased from SigmaꢀAldrich and used as received.
Other reagents were purchased from Merck and used as received.
Instrumentation
FEI Technai G2 transmission electron microscope was used for transmission electron
microscopic (TEM) image of sample. Amount of iron present in the composite materials was
estimated by Optima 2100DV (Perkin Elmer) inductively coupled plasma atomic emission
2 3 3
spectroscopy (ICPꢀAES). Solid grapheneꢀγꢀFe O composite was dissolved in HClꢀHNO and
insoluble graphene was separated by centrifuge and supernatant was used for ICP
measurement. Amount of iron was estimate between 3ꢀ7 wt %. XRD measurement of the
0
samples were done by Bruker D8 Advance powder diffractometer, using Cu Kα (λ=1.54A )
as the incident radiation. The products were analyzed by Varian 3400 gas chromatograph
equipped with a 30 m CPꢀSIL8CB capillary column and a Flame Ionization Detector. All
reaction products were identified by using Trace DSQ II GCꢀMS. NMR spectra were
1
monitored on a Bruker AMXꢀ400 NMR spectrophotometer (400 MHz for HꢀNMR and 100
1
3
MHz for CꢀNMR) using CDCl as solvent and tetramethylsilane as internal standard.
3
Synthesis of water soluble silica coated γꢀFe O
2
3
Hydrophobic γꢀFe
synthesis method and then converted to water soluble nanoparticle by silica coating. In brief
0 mL purified toluene solution of γꢀFe (5 mg/mL) was mixed with 15 mL toluene
2 3
O nanoparticle was prepared by standard high temperature
4
3
3
2 3
O
solution of [3ꢀ(2ꢀaminoethylamino) propyl] triethoxysilane (0.01M) and 15 mL methanolic
solution of tetramethylammonium hydroxide (0.01M) under stirring condition and heated to
0
6
0ꢀ70 C. After few minutes precipitation starts and heating was continued for 45 minutes
with the formation of almost compete precipitates. Precipitate was washed with toluene and
ethanol and finally dissolved in 30 mL water with the addition of few drops of dilute acetic
acid solution.
4

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Synthesis of graphene oxide (GO)
Graphene oxide was prepared via modified Hummer's method. In brief 200 mg
graphite powder and 100 mg NaNO was mixed with concentrated sulfuric acid and then 600
3
0
mg KMnO
4
was added. The temperature of the solution should not rise > 20 C at this
0
addition stage. Temperature of reaction mixture rises to 35 C after some time and mixture
was kept at this condition for another half an hour. After that the solution was diluted with
0
water and the temperature of the solution was maintained 98 C for 15 mints. Next, excess
2 2
permanganate was reduced by adding 3 % H O solution. Then precipitate was washed with
warm water several times and air dried. Finally precipitate was dispersed in water by
sonication and centrifuged at 3000 rpm to remove insoluble graphite. The superannuated was
used as graphene oxide solution.
2 3
Synthesis of grapheneꢀγꢀFe O composite materials
At first 30 mL GO solution (2 mg/mL) was taken in a 100 mL three neck flask and
mixed with 4 mL KOH solution (1M) under stirring condition. The colour of the solution
2 3
turned from brown to light black. Next, 30 mL of silica coated γꢀFe O solution was added to
the solution. No precipitation was observed at this stage. Next, 2 mL of hydrazine solution
0
was added to the reaction mixture and heated at 90 C. Precipitate was observed after one
hour of heating and heating was continued for another 12 hrs. Finally, precipitate was
washed with water, dried and preserved as stocks.
Prussian blue test
Oxidation state of Iron in the composite was determined using prussian blue test.
2 3
About 0.2 mL of concentrated HCl was added in 1 mg of grapheneꢀγꢀFe O composite
materials to dissolve the iron oxide. Next, 1ꢀ2 mL of water was added to dilute the solution.
The solution was centrifuged and supernatant containing dissolved iron was collected and
4 6
divided in two parts. In one part, 10 wt % K [Fe(CN) ] solution was added and in other part
1
0 wt % K [Fe(CN) ] was added.
3
6
2 3
General procedure for oxidation reaction catalyzed by grapheneꢀγꢀFe O composite
The liquid phase oxidation reactions were carried out in a twoꢀnecked round bottom
flask fitted with a water condenser and placed in an oil bath at different temperatures.
Substrates (5 mmol) were taken in water (5 mL) and mixed with catalyst and 30 % H O
2
2
5

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solution (10 mmol). Reaction mixture was under vigorous stirring condition during reaction.
After the reaction, the organic products were separated from the reaction mixture by
extraction with dichloromethane (15 mL x 4). The combined organic portions were dried and
concentrated. Product analysis was performed by Varian 3400 gas chromatograph equipped
with a 30 m CPꢀSIL8CB capillary column and a Flame Ionization Detector using
1
cyclohexanone as internal standard. All reaction products were identified by GCꢀMS and Hꢀ
NMR spectroscopy.
General procedure for click chemistry based the synthesis of 1,4ꢀdisubstituted 1,2,3ꢀ
2 3
triazole catalyzed by grapheneꢀγꢀFe O composite
In a 50 mL round bottom flask 1.0 mmol benzyl halide, 0.065 g sodium azide, 1.0
mmol aryl alkyne and 50 mg grapheneꢀγꢀFe composite were mixed and stirred in 10 mL
2
O
3
distilled water. The reaction was allowed to proceed for 6 h at room temperature under argon
atmosphere. After completion of reaction, the organic products were separated from the
reaction mixture by extraction with ethyl acetate (15 mL x 2). The organic layer was dried
2 4
over Na SO and the solvent was removed under reduced pressure. Reactions were monitored
by Thin Layer Chromatography (TLC) using aluminium backed silica gel 60 (F254) plates,
visualized using UV254 nm excitation and potassium permanganate and ninhydrin dips as
appropriate and purified by silica gel column chromatography. All reaction products were
identified by NMR spectroscopy.
Results and Discussion:
Synthesis and characterization of grapheneꢀγꢀFe O3
2
Synthesis strategy of grapheneꢀγꢀFe O composites shows in the Scheme 1. At first
2
3
iron oxide nanoparticles are prepared by high temperature decomposition of iron stearate.
Then these hydrophobic nanoparticles have been transformed into water soluble nanoparticles
42
by silica coating. Resulting cationic charged amine terminated iron oxide nanoparticles
have hydrodynamic diameter of 10ꢀ12 nm. GrapheneꢀγꢀFe composite materials has been
2
O
3
4
3
synthesized by reported method. At first water soluble GO was synthesized via modified
Hummers method. Next, KOH was added to this solution in such a way that the precipitation
dose not occurs. Next, different amounts of amine terminated iron oxide nanoparticle solution
were added gradually to the GO solution under stirring condition. At this stage iron oxide
nanoparticles were attached to the GO surface via electrostatic interaction between positively
6

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charged silica coated iron oxide and negatively charged GO or via nucleophilic reaction
between amine with epoxide of GO surface. Then GO was reduced by chemical reduction
using hydrazine as reducing agent under the heating condition. Colour of the solution changes
to black with the formation of precipitate. This precipitate was separated from solution using
magnet and washed several times with water and dried in an oven. This dry material was used
for the characterization and catalytic application.
N
N
Br
N
+
+
NaN
3
H
OH
COOH
2
CH OH
OH
HO
O
O
O
OH 1.
HO
O
2
. hydrazine
O
HOOC
CHO
COOH
COOH
HO
+
O
S
S
O
S
O
CH3
CH3
CH3
+
O
NH
2
NH
NH
Si
O
O
NH
2
O
Si
O
Si
O
O
NH
NH
2
2 3
Scheme 1. Schematics of synthesis and catalytic application of grapheneꢀγꢀFe O composite.
The grapheneꢀγꢀFe O composite materials are characterized by XRD and TEM. XRD
2
3
data (Fig. 1) shows that the composite gives the reflection at 30.65, 35.94, 43.55, 53.35,
0
5
7.58 and 63.22 corresponding to the plane of (220), (311), (400), (422), (511) and (440)
0
from γꢀFe
2
O
3
component and the reflection at 23 arises due to graphene component.
7

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Grapheneꢀ
γ
ꢀFe O composite
2
3
20
30
40
50
60
70
80
2θ (degree)
2 3
Figure 1. Xꢀray diffraction pattern of grapheneꢀ γꢀFe O composites materials.
In Prussian blue test (Fig. 2), it has been observed that composite materials solution
produces blue precipitate after addition of K [Fe(CN) ] but such precipitation is not observed
by K [Fe(CN) ] addition. This result confirms that iron present in the grapheneꢀγꢀFe O
4
6
3
6
2
3
composite is of (+3) oxidation state.
K
4
[Fe(CN)
6
]
K
3
[Fe(CN)
+
6
]
+
Composite
Composite
Figure 2. Prussian blue test of grapheneꢀγꢀFe
0 nm
2 3
O composites materials
5
2
00 nm
8

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2 3
Figure 3. High and low resolution TEM image of grapheneꢀ γꢀFe O composites materials.
Figure 3 shows the high and low resolution TEM image of composite materials,
indicating uniformly dispersed iron oxide nanoparticles at the surface of graphene. Raman
2 3
spectra of graphene and grapheneꢀγꢀFe O composite shows two well documented D and G
ꢀ1
ꢀ1
bands at 1310 cm and 1608 cm , respectively (Figure 4). It is well known that higher
disorder in graphite leads to higher relative intensity of D band compared to that of G band.
Therefore the disorder is measure by the intensity ratio of D and G band (I
D G
/I ). In graphene
this intensity ratio is 1.92 but in grapheneꢀγꢀFe O this increases to 2.24. This result indicates
2 3
2
the decrease in the size of sp domain structure upon the composite formation.
grapheneꢀγꢀFe O
2
3
graphene
8
00 1000 1200 1400 1600 1800 2000
ꢀ1
Raman shift (cm)
Figure 4. Raman spectra of graphene and grapheneꢀγꢀFe O composites materials having D
2
3
ꢀ1
ꢀ1
and G band at 1310 cm and 1608 cm , respectively.
The magnetic property of the grapheneꢀγꢀFe O composite materials was investigated
2
3
at 5K and 300K (Figure 5). Magnetization curve shows that the grapheneꢀγꢀFe
materials are superparamagnetic in nature with coercivity value 215 Oe and 100 Oe at 5K and
00K, respectively. The saturation magnetization values of the grapheneꢀγꢀFe composite
are 20 and 16 emu/g at 5K and 300K, respectively, indicating that composite has higher
magnetism. The digital image shows the magnetic response of the grapheneꢀγꢀFe
composite towards magnetic field. It can be seen that the grapheneꢀγꢀFe composite
materials easily separated under the external magnetic field but in absence of magnetic field
2 3
O composite
3
2 3
O
2
O
3
2
O
3
the grapheneꢀ γꢀFe
2
O
3
composites materials disperse in solution.
9

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2
2
1
1
5
0
5
0
5
0
5
b
a
M
A
G
N
E
-
3
5K
00K
-
-
-
-
10
15
20
25
T
-
2000 -1000
0
1000
2000
H(Oe)
Figure 5. (a) Field dependent magnetization curve of grapheneꢀγꢀFe
2 3
O composites at 5K
and 300K (b) digital image of grapheneꢀ γꢀFe composites materials in water dispersion
2
O
3
and after separation by laboratory based bar magnet.
Catalytic activity
Since grapheneꢀmetal systems exhibit catalytic activity in a wide range of the
industrially important processes and have been extensively studied, we decided to investigate
the catalytic activity of grapheneꢀγꢀFe
2 3
O composite in oxidation of sulfides, aromatic
alcohols with H O as oxygen source and oneꢀpot synthesis of 1,2,3ꢀtriazole.
2
2
2 2 2 3
Oxidation of sulfides with H O catalyzed by grapheneꢀγꢀFe O composite
The efficacy of grapheneꢀγꢀFe
2 3
O composite in the oxidation of organic sulfides,
using H O as terminal oxidant, has been explored. Thus when methyl phenyl sulfide was
2
2
2 3 2 2
treated with grapheneꢀγꢀFe O composite and 30 % H O in water, methyl phenyl sulfoxide
was obtained. The general reaction is shown in Scheme 2. Considering methyl phenyl sulfide
as a model substrate, it was subjected to different reaction conditions as shown in Table 1.
The effect of temperature was investigated for the oxidation of methyl phenyl sulfide and
results are shown in Table 1. A maximum of 98 % conversion was achieved on carrying out
0
0
0
the reaction at 60 C. At 50 C, the conversion was low. At 70 C, the initial conversion of
0
methyl phenyl sulfide was higher than that at 60 C but when reaction continues the
0
conversion of that sulfide was almost same with conversion at 60 C at 8 h of reaction time.
0
Therefore, 60 C was the optimum temperature and all catalytic oxidation reactions of
sulfides were carried out at this temperature. Amount of H
conversion and selectivity of methyl phenyl sulfide oxidation reaction. Conversion increased
when amount of H increased from 5 mmol (run 4) to 10 mmol (run 2) but there was no
2 2
O has a significant effect on the
2
O
2
1
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significant change in conversion when amount changed to 15 mmol (run 5) at the same
reaction conditions. The selectivity of sulfoxide decreased with the increase of the amount of
2 2
H O . This is due to further oxidation of sulfoxide. The reaction was also monitored with
different catalyst amount. At a lower catalyst amount (40 mg, run 6) the reaction took longer
time and was not complete. While with 50 mg of catalyst (run 2) the reaction was complete in
8
h, yielding 90 % sulfoxide with a small amount of sulfone (10 %). It was also observed that
by increasing the amount of catalyst to 60 mg (run 7), the reaction led to more sulfone along
with sulfoxide. Therefore, an optimum amount of catalyst and oxidant with a suitable solvent
are necessary for the above oxidation and based on the above observation we come to a
conclusion that 50 mg of catalyst, 10 mmol 30 % H
best condition for the oxidation of sulfides to sulfoxides with good selectivity (run 2). It was
also found that without H , grapheneꢀγꢀFe composite alone cannot oxidize sulfides (run
).
2 2
O and water as a solvent make for the
2
O
2
2 3
O
8
O
S
O
S
S
CH3
2
GꢀγꢀFe O3
CH3
CH3
O
H O , Water
2
2
o
6
0 C
Methyl Phenyl Sulf oxide
Methyl Phenyl Sulfide
Methyl Phenyl Sulfone
Scheme 2. Oxidation of methyl phenyl sulfide.
To examine the activity of the catalyst further, oxidation of other sulfides also has
been investigated (Table 2). Substrate scope is extended to diphenyl sulfide, ethyl phenyl
sulfide and dibutyl sulfide etc. The sulfoxides were selectively obtained in all cases. A series
of substrates, aryl alkyl, aryl allyl and dialkyl sulfides, can be oxidized to the corresponding
sulfoxides. The reactivity and conversion were dependent on the nature of the substituent. In
the case of allyl sulfides, small oxidation was observed at the carbon–carbon double bond.
Similarly, benzylic sulfides can be oxidized to the corresponding sulfoxides with small
oxidation products of the benzylic C–H bond.
Oxidation of aromatic alcohols with H
2 2 2 3
O catalyzed by grapheneꢀγꢀFe O composite
Although several excellent catalysts for the oxidation of alcohols to the corresponding
44
aldehydes or ketones have been reported , most of them required organic solvents and
addition of base, which violate the requirements of the “green chemistry”. Water is the most
ideal solvent because it is not only a green solvent but also can avoid the hazards associated
1
1

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with the use of oxidisable organic solvents. We examined the catalytic activity of the
grapheneꢀγꢀFe O composite for the oxidation of benzyl alcohol by varying the various
2
3
reaction conditions. The general reaction is shown in Scheme 3. The effect of the amount of
on the oxidation of benzyl alcohol into benzaldehyde was studied and the results are
shown in Table 3. The percent conversion of benzyl alcohol into benzaldehyde is dependent
upon H up to a considerable amount. The amount of H was varied in terms of benzyl
alcohol to H
2 2
H O
2
O
2
2 2
O
2
O
2
molar ratio. Benzyl alcohol to H
2
O
2
molar ratio of 1:1 resulted in 60 %
0
conversion when the amount of catalyst was 50 mg, temperature was 50 C and reaction time
was 6 h. When benzyl alcohol to H
to 94 % under all other similar conditions. However, the conversion was found almost
constant when benzyl alcohol to H molar ratio was further increased up to 1:3. Therefore
:2 molar ratio of benzyl alcohol and H was found to be optimum. The results for the
2 2
O molar ratio was raised to 1:2, the conversion increased
2
O
2
1
2 2
O
oxidation of benzyl alcohol in the presence of different amounts of catalyst were presented in
0
Fig. S1. All the tests were conducted with 5 mmol substrate, 10 mmol H
2
O
2
and at 50 C in 5
mL water under base free condition. 20 mg grapheneꢀγꢀFe
2
O
3
composite afforded a
conversion of 25 % within 6 h. No byꢀproducts such as benzoic acid were detected under this
reaction condition. When the amount of the catalyst increased up to 35 mg, the conversion
had an apparent increase and a 75 % conversion was achieved. But in this condition, a little
amount of benzoic acid was detected. A further increase in catalyst amount up to 50 mg led to
an increase in the conversion (93 %). However, when the catalyst amount continuously
increased up to 65 mg and 75 mg, the conversion remains almost constant but the selectivity
of benzaldehyde reduces. These findings indicate that the oxidation of benzyl alcohol is
highly dependent on the amount of the solid catalyst. The influence of temperature on the
oxidation of benzyl alcohol was investigated by performing the reaction at a temperature
0
range from 40ꢀ 60 C with all other parameters fixed. The results are given in Table 3 which
reveals that the conversion is dependent on temperature. 55 %, 94 % and 94.4 % conversion
0
0
was recorded corresponding to 40, 50 and 60 C. Therefore, 50 C was selected as the
optimum temperature. At lower temperature, the conversion of benzyl alcohol into
benzaldehyde was considerably decreased.
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2

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DOI: 10.1039/C3RA43184C
CH OH
CHO
COOH
2
GꢀγꢀFe2O3
H O , Water
2
2
o
5
0 C
Benzyl Alcohol
Benzaldehyde
Benzoic Acid
Scheme 3. Oxidation of benzyl alcohol.
Catalytic activity of the catalyst has been extended for the oxidation of aromatic
alcohols, a series of benzylic alcohols (Table 3). Substituted benzyl alcohols are also oxidized
to corresponding aldehydes. In all above cases trace amount of acids were observed. On the
other hand, 2,2ꢀdimethylꢀ1ꢀphenylꢀ1ꢀpropanol,2ꢀhydroxyꢀ1,2ꢀdiphenylethanone, 1ꢀphenyl
ethanol and diphenyl methanol are selectively converted to ketones on oxidation with this
catalyst.
Click chemistry based oneꢀpot synthesis of 1,2,3ꢀtriazole, catalyzed by grapheneꢀγꢀFe
composite
2 3
O
Azoles and their derivatives are important class of organic compounds which have
numberless applications in organic synthesis and these compounds are highly bioꢀactive. We
initially carried out the standard three component click reaction of benzyl bromide and NaN
3
with phenylacetylene in water to investigate the catalyst activity of grapheneꢀγꢀFe O
2
3
composite (Scheme 4). After stabilizing the optimum conditions for the reaction we turn our
attention to investigate the effect of changing the solvent on the efficiency of the coupling
reaction. We have studied this reaction in various polar and nonꢀpolar solvents. It is found
that water is the most suitable solvent and it gives best result for oneꢀpot three component
click reaction in presence of this composite (Table 4). To verify the amount of catalyst and
temperature effect in representative reaction of benzyl bromide, sodium azide, a series of
reactions were investigated by taking the model reaction in different amount catalyst and
0
temperature. It was found that the best yield was obtained using 50 mg of catalyst at 60 C
(run 8). When the amount of the catalyst loading was increased to 60 mg, there was no
obvious increase in the yield of the reaction (run 10), and the reaction did not occur under
catalystꢀfree conditions (run 11). At room temperature the yield is not good enough and it can
be assumed that higher temperature is needed to activate this type of coupling (run 13). Thus,
the optimized conditions for the oneꢀpot Click reaction are 50 mg catalyst and water as
0
solvent at 60 C temperature. Using these optimized conditions, various substituted 1,2,3ꢀ
1
3

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triazoles are synthesized. Results are summarized in Table 5. Substrates have been selected
by varying both the aromatic halides (run 1ꢀ5) and the alkynes (run 6ꢀ10). It is found that all
the reactions proceed smoothly to give the corresponding 1,4ꢀdisubstitutedꢀ1,2,3ꢀtriazoles in
high yield, which clearly indicates the generality and scope of the reaction with respect to the
aromatic halides and the alkynes. It is noticeable that increase of electronic effect in the
benzyl moiety increases the yield whereas it decreases with substrates containing electron
deficient aromatic halides. The same observation is found while different alkynes are studied.
N
N
GꢀγꢀFe O3
2
Br
+ NaN3
N
+
0
water, 60 C, Argon atmosphere
H
benzyl bromide phenyl acetylene
sodium azide
1ꢀbenzylꢀ4ꢀphenylꢀ1H ꢀ1,2,3ꢀtriazole
Scheme 4. Synthesis of 1,4ꢀsubstituted 1,2,3ꢀ triazole.
Among pure γꢀFe
Table 6). GrapheneꢀγꢀFe
which indicates that the catalytic activity of γꢀFe O can be remarkably improved by
2
O3, graphene and grapheneꢀγꢀFe
2
O3, graphene has less catalytic activity
(
2
O
3
hybrids exhibited better catalytic activity than pure γꢀFe
2 3
O
2
3
combining them with graphene sheets. This implies the crucial role of interaction between
γꢀFe and graphene. The enhancement in catalytic activity can be attributed to three
factors: (i) Graphene has conducting property and provides high migration efficiency of
2
O
3
45ꢀ
electrons, which plays an important role in enhancing the catalytic activity for this reaction.
4
6
(ii) As compared with γꢀFe
2 3 2 3
O and graphene, the grapheneꢀγꢀFe O can offer an
environment to prevent aggregation of γꢀFe O and graphene and obstruct facile loss of
2
3
2 3
activity. GrapheneꢀγꢀFe O hybrids have a higher surface area which can provide more active
sites for adsorption of reactants and catalytic oxidation and coupling reaction. (iii) Graphene
is not only a support, but also as catalyst for the reaction. Nevertheless, the novel properties
2 3 2 3
of grapheneꢀγꢀFe O as a catalyst lead us to believe that grapheneꢀγꢀFe O hybrids will find
application in various catalytic fields. The roles of the active sites of graphene and the
reaction mechanism itself are still unclear and need further investigation. But based on the
4
7ꢀ49
studies,
proposed. First, the oxygen containing groups (ꢀOH, ꢀCOOH etc) on the GO provided the
anchoring sites for the deposition of γꢀFe , which enabled γꢀFe to distribute on the
surface of GO thus avoiding their aggregation. Second, reactants can be adsorbed onto
a plausible explanation regarding active sites and reaction mechanism can be
2
O
3
2 3
O
1
4

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DOI: 10.1039/C3RA43184C
graphene through π–π electronic interactions, which increased the effective contact between
the reactant molecules and the γꢀFe O . Oxygen can adsorb on the γꢀFe O sites and then spill
2
3
2
3
over to the adjacent bridge sites of graphene. In this way, the adsorption of oxygen on
grapheneꢀγꢀFe composite is greatly promoted. The existence of mobile oxygen on
2
O
3
graphene support can promote the oxidation reactions. In 1,4ꢀdisubstitutedꢀ1,2,3ꢀtriazoles
synthesis the GO acts as a base and removes the acetylenic proton to form a Fe–acetylide
50,51
complex.
triazole. Considering its magnetic separation behaviour and relatively high activity, grapheneꢀ
γꢀFe hybrids are promising catalytic materials for environmentally friendly oxidation and
This complex undergoes cycloaddition with an in situ formed azide to form a
2
O
3
also coupling processes.
Recycling of catalyst
Another point of great concern for most of anchored catalysts is the ease of separation
as well as the deactivation and reusability of the catalyst. The immobilized grapheneꢀγꢀFe O
3
2
composite was therefore investigated for recoverability and reusability. Magnetic separation
is a more attractive and simple technique than the filtration or centrifugation techniques as it
prevents the loss of the catalyst and increases the reusability of the catalyst. Separation of the
grapheneꢀγꢀFe O composite from the reaction mixture becomes very easy owing to the
2
3
superparamagnetic nature of γꢀFe
2
O
3
nanoparticles at room temperature. We have first
composite for the oxidation of
carried out recycling uses of the grapheneꢀγꢀFe
2
O
3
methyl phenyl sulfide and 98 % conversion was obtained in the first cycle. After completion
of the reaction, an external magnetic field was applied on the outer surface of the glass
2 3
reaction vessel containing grapheneꢀγꢀFe O composite using a small permanent magnet. The
reaction solution was then easily removed from the reaction vessel by decantation while the
external magnet held the catalyst inside the vessel. The magnetic catalyst was washed with
deionized water, acetone and nꢀhexane to remove any physisorbed reagents, dried under
vacuum at room temperature overnight, and reused in further reactions under identical
conditions to the first run. It is seen that the catalyst remained excellent catalytic activity even
after five times of recycling. There is no loss of magnetic responsiveness or morphological
change after recycle. TEM image and Raman spectra (Figure S3 and S4 in the supporting
information) of the reused catalyst did not show any substantial change compared with the
fresh catalyst. Similar recycling of the catalyst was also carried out for the oxidation reaction
3
of benzyl alcohol and click reaction of benzyl bromide and NaN with phenylacetylene. The
1
5

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results confirm that the catalyst has good stability and recyclable applicability for both of
these reactions. The obtained results are presented in Fig. 6. As seen from Fig. 6, the recycled
catalyst did not show any appreciable change in the activity which indicates that the catalyst
is stable and can be regenerated for repeated use.
Figure 6. Recycling activity of the grapheneꢀγꢀFe O composite.
2
3
Conclusions
We have developed a new graphene based composite with γꢀFe
2
3
O nanoparticle as a
robust, safe and magnetically recoverable heterogeneous catalyst. The resulting catalyst
showed enhanced catalytic activity with efficient recycling for the oxidation reactions of
sulfides, aromatic alcohols and synthesis of 1,2,3ꢀtriazole using water as solvents. The
magnetic nature of this heterogeneous nanocatalyst allows for its easy separation from the
reaction mixture by using a simple bar magnet. The inexpensive nature of the catalyst, use of
environmentally benign oxidant, excellent product yields and wide substrate scope makes this
catalyst an improved and superior than the existing one. Leaching test indicates that the
catalytic reaction is mainly heterogeneous in nature. The reusability of this catalyst is high
and can be reused five times without significant decrease in its initial activity. We hope that
the present catalytic systems would be useful to synthesize industrially important organic
compounds, synthetic starting materials in future.
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6

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Acknowledgments
We gratefully acknowledge the DST, New Delhi, for award of grant under FIST and
PURSE program to the Department of Chemistry, University of Kalyani. SMI acknowledges
DST, CSIR, New Delhi, India for funding and NRJ acknowledges DST, India for funding.
PM and NS are thankful to the UGC, New Delhi for research fellowship and ASR is thankful
to the CSIR, New Delhi, India for senior research fellowship.
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Table 1. The effect of different reaction parameters on oxidation of methyl phenyl sulfide
a
with grapheneꢀγꢀFe O composite (7 % Fe).
2
3
Run
Temperature
Amount of H
2
O
2
Conversion
Selectivity of
sulfoxide/Sulfone (%)
b
b
(
mmol)
(%)
0
(
C)
o
1
2
3
4
5
50 C
10
10
10
5
85
98
93
60
97
75
98
0
79/21
90/10
80/20
95/5
0
60 C
0
70 C
0
60 C
0
60 C
15
10
10
ꢀ
70/30
81/19
60/40
0/0
c
0
6
60 C
d
0
7
60 C
0
8
60 C
a
Conditions: methyl phenyl sulfide (5 mmol), water (5 mL); time (8 h), 50 mg catalyst.
b
c
d
Conversion and selectivity were determined by GC. Catalyst amount = 40 mg. Catalyst
amount = 60 mg.
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Table 2. Oxidation of sulfides using 30 % H
2
O
2
catalyzed by grapheneꢀγꢀFe
2
O
3
composite (7
a
%
Fe).
b
Entry
Substrates
Time (h)
Conversion (%)
Selectivity of product
b,c
(
%)
70
91
85
83
81
87
71
84
76
80
74
81
76
1
2
3
4
5
6
7
8
9
C
4
H
9
SC
4
H
9
5
4
4
5
4
5
5
4
4
5
5
4
5
80
89
90
81
80
61
70
75
82
86
78
84
79
PhSPh
PhSC
2
H
5
PhSC H
6
11
PhSCH
2
Ph
PhSCH CH=CH
2
2
pꢀCNC
6
H
4
SCH
3
pꢀCH
3
C
6
H
4
SCH
3
2
Me S
1
0
1
2
3
pꢀOMeꢀPhSMe
1
1
1
C H SC H
3
7
3
7
pꢀNO
2
C
6 4 3
H SCH
PhSC H OH
2
4
a
Conditions: Sulfide (5 mmol); 30% aq H
2
O
2
(10 mmol); water (5 mL); 50 mg catalyst at
o
b
c
6
0 C temperature. Conversion and selectivity were determined by GC. Products were
1
characterized by GC–MS and HNMR.
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Table 3. Oxidation of aromatic alcohols with 30 % H
2
O
2
2 3
catalyzed by grapheneꢀγꢀFe O
a
composite (7 % Fe).
b,h
Entry
Aromatic alcohol
Conversion
Product Selectivity (%)
b
(
%)
60
94
c
1
Benzyl alcohol
Benzaldehyde (90)
d
2
Benzyl alcohol
Benzaldehyde (96)
e
3
Benzyl alcohol
93
55
Benzaldehyde (71)
f
4
Benzyl alcohol
Benzyl alcohol
Benzaldehyde (85)
Benzaldehyde (73)
g
5
94.5
82
6
7
8
9
4ꢀmethoxybenzyl alcohol
4ꢀmethylbenzyl alcohol
4ꢀbromobenzyl alcohol
2ꢀ methoxybenzyl alcohol
2,2ꢀdimethylꢀ1ꢀphenylꢀ1ꢀpropanol
4ꢀmethoxy benzaldehyde (82)
4ꢀmethyl benzaldehyde (87)
4ꢀbromo benzaldehyde (90)
2ꢀmethoxy benzaldehyde (83)
83
70
80
1
0
68
2,2ꢀdimethylꢀ1ꢀphenylꢀ1ꢀ
propanone(100)
1
1
4ꢀnitrobenzyl alcohol
75
4ꢀnitro benzaldehyde (90)
4ꢀchloro benzaldehyde (89)
1
2
3
4ꢀchlorobenzyl alcohol
72
62
1
2ꢀhydroxyꢀ1,2ꢀdiphenylethanone
1,2ꢀdiphenylethaneꢀ1,2ꢀ
dione(100)
1
4
5
1ꢀphenyl ethanol
diphenylmethanol
76
73
Acetophenone (100)
benzophenone(100)
1
a
2 2
Reaction condition: aromatic alcohol (5 mmol), 30% H O (10 mmol), water (5 mL),
0
b
c
2 2
catalyst (50 mg), temperature (50 C), time (6 h). Determined by GC. benzyl alcohol:H O
d
0
e
=
2 2 2 2
1:1. benzyl alcohol:H O = 1:2, temperature (50 C). benzyl alcohol:H O = 1:3.
f
0
g
0
h
temperature (40 C). temperature (60 C). Products were characterized by GC–MS and
1
HNMR.
2
3

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DOI: 10.1039/C3RA43184C
2 3
Table 4. Influence of different reaction parameters on grapheneꢀγꢀFe O composite catalyzed oneꢀpot
a
Click reaction .
0
b
Entry
Solvent
Toluene
THF
Temperature ( C)
Yield (%)
1
2
3
4
5
6
7
8
60
60
60
60
60
60
60
60
60
60
60
40
RT
46
27
Acetone
48
CH
2
Cl
2
43
MeCN
EtOH
DMSO
Water
Water
Water
Water
Water
Water
50
60
66
92
c
9
67
d
1
0
1
93
e
1
Trace
60
1
2
1
3
35
a
Reaction condition: [benzyl bromide] = 1 mmol, [phenyl acetylene] = 1 mmol, NaN
3
(0.065
o
b
g), 60 C temperature, catalyst 50 mg, Isolated yields after silica gel column chromatography,
c
d
e
Catalyst amount = 40 mg. Catalyst amount = 60 mg, Absence of catalyst. Products were
characterized by comparison of their NMR spectra with those reported in the literature.
2
4

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Table 5
GꢀFe O Composite
2
3
hal
H
+ NaN3
N
N
+
0
water, 60 C, Argon atmosphere
N
hal = Br
Cl
Z
=
b
c
Sl.
No.
Aliphatic halide
Aryl alkyne
Z
Product
Yield (%)
N
1
92
93
Br
N
N
N
2
Z
Z
N
Br
N
HO
HO
N
3
4
91
80
N
Br
N
H CO
3
O
N
Z
Z
Br
N
N
O N
2
O N
2
Br
N
5
N
84
N
2
5

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N
6
7
72
84
Br
N
N
nꢀHexyl
H
nꢀHexyl
N
OEt
O
Br
N
N
O
EtO
N
8
9
(4ꢀOCH )ꢀZ
N
91
85
Br
3
N
O
N
4ꢀMeꢀZ
Br
N
N
N
1
0
1
2
4ꢀBrꢀZ
N
78
70
88
Br
N
Br
Br
N
1
1
Z
Z
N
N
N
Cl
N
N
2
6

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N
13
80
N
Cl
N
H CO
3
O
a
Reaction condition: [Aliphatic halide] = 1 mmol, [alkyne] = 1 mmol, NaN (0.065 g), water
3
o
b
(
10 ml), 60 C temperature, catalyst 50 mg. Products were characterized by comparison of
c
their NMR spectra with those reported in the literature. Isolated yields after silica gel column
chromatography
Table 6. Effect of different catalysts on the oxidation of the methyl phenyl sulfide, benzyl
alcohol and 1,2,3ꢀtriazole synthesis.
Entry
Catalyst
Oxidation of
Oxidation of
1,2,3ꢀ
Triazole
Synthesis
Methyl Phenyl
Sulfide
Benzyl Alcohol
Conversion (%)
Conversion (%) Yield (%)
1
2
3
γꢀFe
2
O
3
46
13
74
51
10
71
39
trace
68
Graphene
2 3
GrapheneꢀγꢀFe O
(3 % Fe)
4
GrapheneꢀγꢀFe O
98
94
92
2
3
(7 % Fe)
2
7