Fe3O4@SiO2/EDAC-Pd(0) as a novel and efficient inorganic/organic magnetic composite: Sustainable catalyst for the benzylic C-H bond oxidation and reductive amination under mild conditions

Article abstract of DOI:10.1039/c5nj03413b

A facile and novel methodology has been reported for the synthesis of Pd(0) nanoparticles onto ethylene diamine functionalized inorganic/organic magnetic composite [Fe₃O₄@SiO₂/EDAC-Pd(0)]. The amine functionalized inorganic/organic substrate proved to be an efficient host for the immobilization of palladium nanoparticles giving a highly stable, water dispersible, active and yet magnetically recoverable heterogeneous catalyst. The synthesized catalyst displayed remarkable activity towards the selective C-H bond oxidation using TBHP and reductive amination of aldehydes with nitroarenes using atom efficient molecular hydrogen in green solvents, and could be reused efficiently upto 6 consecutive runs. The novel catalyst is characterized by FT-IR, TGA, XRD, SEM, HRTEM, EDX, ICP-AES, XPS and VSM techniques. Because of the magnetic nature of the catalyst, it can be retrieved using an external magnet, which eliminates the obligation of catalyst filtration after completion of the reaction, thus making it an eco-friendly and economical catalyst to perform the organic transformations.

Full text of DOI:10.1039/c5nj03413b

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Attracting interest for the past several decades, the field of amination of aldehydes with nitroarenes using molecular
catalytic oxidation is continuously and rapidly growing. hydrogen. Further, the catalyst proved its excellent potential
Catalytic oxidation of benzylic C-H bond is one of the key in both reactions and could be reused efficiently with upto 6
transformations in organic synthesis and has
a broad consecutive runs with high efficiency.
application in the manufacture of agrochemicals and
pharmaceuticals.9,10 In spite of the high conversion, benzylic
C-H bond oxidation reactions reported to date largely rely on
homogeneous catalysis and hence recyclability of these often
very expensive catalysts can be problematic.11 Recently, the
use of environment friendly oxidants, such as oxygen,
hydrogen peroxide or tert-butyl hydroperoxide (TBHP)
combined with noble transition metal heterogeneous catalysts
was of particular interest for replacing their homogenous
counterparts.12-14 Hence, the development of a benzylic C-H
bond oxidation reaction with a novel and inexpensive
heterogeneous catalyst of low toxicity would be of
considerable interest and importance.
Results and Discussion
Characterization of palladium nanoparticles onto amine
functionalized inoganic/organic magnetic composite
[Fe3O4@SiO2/EDAC-Pd(0)]
The
general
procedure
for
the
synthesis
of
Fe3O4@SiO2/EDAC-Pd(0) is represented in Scheme 1. It
involves the co-precipitation of Fe3+ and Fe2+ salts to get
Fe3O4 nanoparticles, followed by the formation of silica
spheres26 onto the surface of the Fe3O4 nanoparticles by sol-
gel process. Although, Fe3O4 embedded SiO2 spheres are
quite stable but the aggregation and leaching of metal
nanoparticles from their surface remains the drawback. In this
regard, binding of Fe3O4 embedded SiO2 spheres i.e.,
Fe3O4@SiO2 with natural biopolymers provides more
stability. The chemical modification of cellulose was done by
incorporating the ethylene-1,2-diamine moiety as a pendant
chain covalently bonded to the main polymeric framework
which gives further stability. The inorganic/organic hybrid
magnetite nanoparticles were prepared via formation of
covalent bonds between ethylene diamine functionalized
cellulose (EDAC) and silica coated Fe3O4. Further, EDAC
also acts as a surfactant,27 which restricts the agglomeration
and/or growth of the nanoparticles. Palladium nanoparticles
were then immobilized onto the surface of ethylene diamine
functionalized inoganic/organic substrate by in situ reduction
of palladium chloride using NaBH4. During this process, the
distribution of the size of Pd nanoparticles onto the surface of
Fe3O4@SiO2/EDAC was controlled by the slow addition of
NaBH4 over a time period of 6, 8 and 10 h. It has been
observed that, when NaBH4 was added during 6 h, the size of
Pd (0) nanoparticles obtained was in the range of 10-15 nm
(HRTEM image and particle size histogram, Fig. S1, see
ESI). But, when we increased the time of NaBH4 addition for
8 h, a comparatively smaller Pd(0) nanoparticles in the size
Amines are important building blocks of functional materials,
additives, dyes, agrochemicals and pharmaceuticals.15 Since
controlled mono alkylation of primary amines remains a
challenge,16,17 synthesis of secondary amines rely much on the
reductive amination reactions.18 Domino reductive amination,
using nitroarenes is attractive as it does not require prior
reduction of the nitroarenes.19-24 The traditional methods for
reductive aminations involve the use of stoichiometric
chemical reagents, such as LiAlH4, NaBH4 or the use of
corrosive acetic acid which poses various limitations both
economically and environmentally.25 Thus, methods involving
the use of molecular hydrogen and recyclable catalysts are
often the most preferred pathways to reduce an organic
compound.
In this paper, we reported a facile and novel methodology for
the synthesis of palladium nanoparticles onto amine
functionalized inorganic/organic magnetic composite
[Fe3O4@SiO2/EDAC-Pd(0)] which represent an excellent
example of synergism of the properties of both inorganic and
organic components. The catalytic activity of the novel
Fe3O4@SiO2/EDAC-Pd(0) was tested in the oxidation of
benzylic C-H bond using TBHP and for the one-pot reductive
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range of 6-10 nm were obtained (Fig. 3). While, further groups respectively, both of which were associated with
increase in the time of NaBH4 addition to 10 h gave the ethylene diamine modified Fe3O4@SiO2/EDAC (Fig. 1b and
similar results. The prepared novel magnetic catalyst, 1c). The FT-IR spectra provided evidence of the formation of
Fe3O4@SiO2/EDAC-Pd(0) was characterized by different a silica shell onto the surface of Fe3O4 and also indicated that
techniques such as fourier transform infrared (FT-IR) the ethylene diamine functionalized cellulose was
spectroscopy, thermogravimetric analysis (TGA), X-ray successfully grafted onto Fe3O4@SiO2.
diffraction (XRD), scanning electron microscopy (SEM), high
The thermal stability of Fe3O4@SiO2/EDAC-Pd(0) was
resolution transmission electron microscopy (HRTEM),
examined by thermo gravimetric analysis (Fig. 2). The TGA
energy dispersive X-ray (EDX) analysis, inductively coupled
was recorded by heating the sample at the rate of 10 °C min-1.
plasma atomic emission spectroscopy (ICP-AES), X-ray
photoelectron spectroscopy (XPS) and vibrating sample
magnetometery (VSM).
Fig.
1 FTIR spectra of Fe3O4@SiO2 (a), Fe3O4@SiO2/EDAC (b) and
Fe3O4@SiO2/EDAC-Pd(0) (c).
The TGA curve of Fe3O4@SiO2/EDAC-Pd(0) showed an
initial weight loss up to 150 °C, which was attributed to the
loss of residual solvent and water trapped onto the surface.
The major weight loss beyond 210 °C may be attributed to the
decomposition of cellulose and chemisorbed material i.e.
ethylene diamine groups present in the modified
Fe3O4@SiO2/EDAC substrate. Thus, the catalyst is
reasonably stable up to 210 °C and it is safe to carry out the
reaction under heterogeneous conditions.
Scheme 1. General procedure for the synthesis of Fe3O4@SiO2/EDAC-Pd(0).
The surface modification of the prepared materials was
characterized by FTIR spectroscopy. FT-IR spectra of
Fe3O4@SiO2
(a),
Fe3O4@SiO2/EDAC
(b)
and
Fe3O4@SiO2/EDAC-Pd(0) (c) is shown in Fig. 1. The
characteristic peaks around 1111, 800 and 475 cm-1 were
observed due to SiO2 bonds, while the characteristic peak
around 582 cm-1 was observed due to the presence of Fe-O
bond (Fig. 1a, b and c). Further, the IR spectrum of
Fe3O4@SiO2/EDAC-Pd(0) (Fig. 1b) characterized by the
broad peak centred around 3400 cm-1 was an envelope of
stretching vibrations for O-H of the adsorbed water, silanol
groups, and N-H of the ethylene diamine functionalized
cellulose. Two additional bands around 2895 and 1615 cm-1
were observed due to the stretching vibrations of the C-H of
CH2 groups and the bending vibrations of the -NH of the NH2
Fig. 2 TGA of Fe3O4@SiO2/EDAC-Pd(0).
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The formation of Pd(0) nanoparticles was confirmed through coupled plasma atomic emission spectroscopy (ICP-AES).
X-ray powder diffraction studies (Fig. S2, see ESI), which The results indicated that Pd content loaded onto
showed three reflection patterns at 2θ = 39.9, 46.0, and 68.2° Fe3O4@SiO2/EDAC was 3.0 wt%.
corresponding to the (111), (200) and (220) crystalline planes
of face-centered cubic (fcc) Pd.28 Further, the diffraction
peaks at 2θ = 30.1, 35.5, 57.1 and 62.3° corresponds to the
(111), (220), (311) and (511) planes of cubic phase of Fe3O4
lattice.29 In addition, a broad diffraction peak at 2θ = 20-25°
was assigned to amorphous silica coated on the surface of
Fe3O4 nanospheres.
The surface morphology of Fe3O4@SiO2/EDAC-Pd(0), was
determined by SEM analysis (Fig. 4a and 4b). The SEM
images showed that the catalyst was a homogeneous powder
and Pd nanoparticles were uniformely distributed onto the
surface of Fe3O4@SiO2/EDAC, and most of the particles have
quasi-spherical shape. The HRTEM images provided a direct
observation of the morphology and distribution of palladium
nanoparticles onto the surface of Fe3O4@SiO2/EDAC (Fig.
3c and 3d). The HRTEM image (Fig. 3c) of
Fe3O4@SiO2/EDAC-Pd(0) exhibited the presence of spherical
particles which were observed in SEM micrographs also.
Further, the inset image in Fig. 3c, reveals the formation of
Fe3O4 core with a diameter of 240 nm and an outer shell
thickness of Silica/EDAC was about 85 nm. Fig. 3d of the
Fe3O4@SiO2/EDAC-Pd(0) catalyst revealed that plenty of Pd
nanoparticles with nearly spherical morphology and good
dispersibility were distributed on the surface of the modified
Fe3O4@SiO2/EDAC. Furthermore, the size distribution of
Fig. 3 SEM (a and b), HRTEM (c and d), particle size histogram (e) and
EDX of Fe3O4@SiO2/EDAC-Pd(0) (f).
X-ray photoelectron spectroscopy (XPS) technique was used
to investigate the electron properties of the species formed on
the surface of Fe3O4@SiO2/EDAC-Pd(0), such as the
oxidation state, and the binding energy of the core electron of
the palladium metal. Fig. 4a shows the overall survey
spectrum of Fe3O4@SiO2/EDAC-Pd(0) in which peaks
corresponding to carbon 1s ( 284.9 eV), oxygen 1s (532.2
eV), nitrogen 1s (401.4 eV), silica 2p (105.3 eV), iron 2p
(712.1 eV) and palladium 3d (335.2 eV) are clearly observed.
Fig. 4b shows the typical Pd(0) absorptions at 335.2 and
340.4 eV for 3d5/2 and 3d3/2 respectively, with a ∆ = 5.2 eV,
which is consistent with the literature for Pd(0).30
palladium
nanoparticles
onto
the
surface
of
Fe3O4@SiO2/EDAC was determined through the histogram
which was proposed according to the data, obtained from the
HRTEM (Fig. 3e). The average size of Pd(0) nanoparticles
was 8.5 nm which showed a close agreement with the values
calculated by XRD data. The elemental composition of
Fe3O4@SiO2/EDAC-Pd(0) was analyzed by energy dispersive
X-ray (EDX) spectroscopy and the outcome was acceptable
with expected elements: iron, silicon, oxygen, carbon and
palladium (Fig. 3f). Moreover, the weight percentage of Pd in
Fe3O4@SiO2/EDAC-Pd(0) was determined by inductively
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Fig. 4 XPS of Fe3O4@SiO2/EDAC-Pd(0); overall survey spectrum (a) Pd 3d core level spectum (b).
acetophenone was choosen as the model reaction
Scheme 2). Then, we set out to evaluate various
The magnetic properties of Fe3O4@SiO2/EDAC-Pd(0) were
evaluated by vibrating sample magnetometery (VSM) at room
temperature (Fig. 5). The VSM magnetization curve
illustrates magnetic saturation value of 38.7 emu/g with a
(
experiments to compare the catalytic efficiency of
Fe3O4@SiO2/EDAC-Pd(0) with other catalysts and to
optimize the appropriate catalytic amount required for
the benzylic C-H bond oxidation. The results are
summarized in Table 1. At first, the model reaction was
performed in water without using any catalyst (Table 1, entry
1). No reaction was observed under this condition. Then, the
model reaction was performed using 0.1 g of different catalyst
systems, while keeping other conditions unaltered.
Unfortunately, the yields obtained with other catalysts were
not satisfactory (Table 1, entries 2-5). Further, we employed
homogenous catalysts like PdCl2 and Pd(OAc)2 to catalyse
the benzylic C-H bond oxidation (Table 1, entries 6 and 7).
With homogenous salts, the yields of the desired product were
good but the reaction was not selective and many side
products were formed. Further, we carried out the reaction
using Pd(0) based heterogeneous systems (Table 1, entries 8
and 9) which resulted in satisfactory increase in yield of the
desired product. Then, we examined the use of heterogeneous
magnetic catalyst, Fe3O4@SiO2/EDAC-Pd(0) for the model
reaction. As can be seen from the results, the present catalyst,
Fe3O4@SiO2/EDAC-Pd(0) gave the desired product with
characteristic hysterisis loop which exhibited
a
soft
ferromagnetic behaviour. Fig. 5, also showed that the catalyst
allow a facile separation from the reaction mixture using an
external magnet.
Fig. 5 Room temperature magnetization curve of Fe3O4@SiO2/EDAC-
Pd(0).
Catalyst testing for benzylic C-H bond oxidation
Initially, to obtain the most appropriate conditions for the
desired protocol, the oxidation of ethylbenzene to
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O
Fe3O4-SiO2/EDAcel@Pd(0)
R
R
TBHP, H2O, 60 o
C
Scheme 2 Fe3O4@SiO2/EDAC-Pd(0) catalyzed C-H bond oxidation
Table 1. Optimization for the C-H bond oxidation and reductive amination
Entry
Catalyst
Amount
(g)
C-H bond oxidationa
Time (h) Yield (%)c
Reductive aminationb
Time (h) Yield (%)c
1.
2.
-
-
0.1
10
10
traces
traces
35
15
NRd
NRd
SiO2
15
3.
Fe3O4
0.1
10
15
20
20
25
65
70
80
85
4.
Fe3O4@SiO2
0.1
10
10
42
15
5.
Fe3O4@SiO2/NH2cel
Pd(OAc)2
0.1
45
15
6.
0.1
4.0
4.0
3.5
2.45
2.0
2.0
3.0
60
7.0
6.0
4.5
3.0
2.5
2.5
3.5
7.
PdCl2
0.1
65
8.
Cellulose-Pd(0)
Silica/Cellulose-Pd(0)
Fe3O4@SiO2/EDAC-Pd(0)
Fe3O4@SiO2/EDAC-Pd(0)
Fe3O4@SiO2/EDAC-Pd(0)
0.1
72
9.
0.1
79
10.
11.
12.
0.1
90
93
0.05
0.025
90
93
81
82
aReaction conditions: ethyl benzene (1 mmol), 70% t-BuOOH (1.5 mmol), water (5 mL) at 60 oC.
bReaction conditions: benzaldehyde (1 mmol), nitrobenzene (1.2 mmol), ethanol:water (3:1, 5 mL) under H2 atmosphere at room
temperature.
cIsolated Yield
dNR: No reaction.
maximum yield and selectivity in 2h (Table 1, entry 10). significant role for the excellent behaviour of
Fascinated by these results, we carried out the test Fe3O4@SiO2/EDAC-Pd(0) in catalyzing the desired
reaction using 0.05
g
(1.5 mol% Pd) of reaction.
while keeping other Further, in order to optimize the reaction conditions with
Fe3O4@SiO2/EDAC-Pd(0),
conditions unaltered and the results indicated no change respect to solvents, oxidants and temperatures, the model
in the yield of product (Table 1, entry 11), although, reaction was examined under different set of conditions
further reduction in the amount of Fe3O4@SiO2/EDAC- to obtain the best possible combination and the results
Pd(0) led to the decrease in the yield of product (Table 1, are presented in Table 2. Initially, the effect of various
entry 12). These results clearly indicated that the type of solvents such as CH2Cl2, DMF, CH3CN and H2O at
catalyst as well as its appropriate amount is essential for temperatures ranging from R.T. to 100 °C was tested in
the oxidation to occur. Further, the high surface area to case of the model reaction. As could be seen in Table 2
,
volume ratio of palladium nanoparticles together with the reaction in CH2Cl2 and DMF gave the poorest yields
their distribution on the magnetic support, played a
(
Table 2, entries 1-7), while the yields have been much
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improved in case of CH3CN (Table 2, entries 8-10), but from
ARTICLE
the
oxidation
of
ethyl
benzene
using
the best results were obtained in H2O at 60 °C (Table 2, Fe3O4@SiO2/EDAC-Pd(0), the major product formed was
entry 12). However, when we carried out the model acetophenone with high selectivity and other by-products
reaction in water at elevated temperatures of 80 and 100 were not observed. Thus, the mild reaction conditions,
°C, it has led to decrease of selectivity of the product excellent stability and high yield for the transformation of
formation. Thus, 60 °C was the appropriate temperature ethylebenzene to acetophenone encouraged us to extend the
selected for carrying out the desired reaction in water. scope of Fe3O4@SiO2/EDAC-Pd(0) as heterogeneous catalyst
Next, the effect of various oxidants i.e., O2, H2O2 and to other benzylic hydrocarbons. As shown in Table 3, the
TBHP was examined. The results indicated that the oxidation of several benzylic C-H substrates to the
oxidation with TBHP gave the best results in terms of corresponding products using Fe3O4@SiO2/EDAC-Pd(0)
time, yield and selectivity.
(Table 2, entry 12). proceeded smoothly with moderate to high yields. Ethyl
However, the oxidation using O2 and H2O2 gave the poor benzene, fluorene, diphenylmethane and anthrone were
yields.
efficiently converted to the corresponding ketone derivatives
(Table 3, entries 1-4). Cyclohexane was also converted to
cyclohexanone under the same conditions (Table 3, entry 5).
Extending the scope of the optimized conditions to other
alkyl benzenes, it has been observed that toluene derivatives
with electron-donating groups (Table 3, entries 7-8) undergo
oxidation faster than those with electron-withdrawing
counterparts (Table 3, entries 9-10). Moreover, when p-
xylene was used, both methyl groups were oxidized to
aldehydic groups, giving p-terephthaldehyde in good yield
(Table 3, entry 7). The oxidation of toluenes to the
corresponding benzoic acids, which is an important
transformation in organic chemistry, could be efficiently
achieved by increasing the reaction time. Thus, various
toluene derivatives were tested and their corresponding acid
derivatives were obtained in excellent yields (Table 3,
Table 2. Optimization of different solvents, oxidants and reaction
temperatures for benzylic C-H bond oxidationa
Entry
Solvent
Oxidant
Temp
(°C)
Time
(h)
Yield
(%)b
1.
2.
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DMF
TBHP
TBHP
O2
RT
40
3
3
15
45
3.
40
3
Traces
Traces
32
4.
H2O2
TBHP
O2
40
3
5.
90
3
6.
DMF
90
3
20
7.
DMF
H2O2
TBHP
O2
90
3
25
8.
CH3CN
CH3CN
CH3CN
H2O
80
2.5
3
80
9.
80
35
10.
11.
12.
13.
14.
15.
H2O2
TBHP
TBHP
TBHP
O2
80
3
40
RT
60
3
25
H2O
2
90
entries
11-13).
These
results
showed
that
H2O
100
100
100
2
83
H2O
3
Traces
15
Fe3O4@SiO2/EDAC-Pd(0) can facilitate the oxidation of
benzylic C-H bonds and serve as a highly efficient catalyst. A
thorough literature survey showed no reference to the use of
palladium nanoparticles onto inorganic/organic magnetic
composite as catalysts for selective C-H bond oxidation.
H2O
H2O2
3
aReaction conditions: ethyl benzene (1 mmol), oxidant (1.5 mmol), catalyst
(0.05 g, 1.5 mol% Pd) and solvent (5 mL).
bIsolated Yield.
A thorough literature survey shows that the selective
synthesis of acetophenone from ethylbenzene is a challenge to
heterogeneous catalysts because selectivity towards
acetophenone is mainly hampered due to the formation of
other oxygenates depending upon the type of catalyst used.
As can be seen from the results (Table 1, entry 12) obtained
To gather insights into benzylic C-H bond oxidation and to
ascertain if the catalytic reaction involves a radical process, a
radical‐trapping experiment employing TEMPO (a radical
scavenger) was performed (Scheme 3a). The first reaction
(Table 3, entry 1) has been carried out until the conversion
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Table 3. C-H bond oxidation of arylalkanes catalyzed by
was 45% (0.6 h) after which 1 mmole of TEMPO was added
to it and the reaction was allowed to proceed further up to the
completion (2 h). In this case, the reaction stopped, which
could indicate that the oxidative transformation most likely
occurred via radical intermediates. Based on the previous
literature protocols, an alcohol derivative might be a possible
intermediate in the formation of ketones. Thus, we also used
1-phenylethanol as the substrate under standard reaction
conditions and almost quantitative yield was obtained
(Scheme 3b), which indicates that the benzyl alcohol
derivative may be an intermediate in the benzylic C-H bond
Fe3O4@SiO2/EDAC-Pd(0) using TBHP under aqueous media at 60
°Ca
Entry
Substrate
Product
Time
(h)
Yield
(%)b
O
1.
2
90
O
2.
1.5
94
O
3.
4.
5.
6.
7.
1.5
1.5
1.5
2
95
94
88
89
88
oxidation.
O
O
O
Fe3O4-SiO2/EDAcel@Pd(0)
TEMPO
No conversion
R
R
TBHP, H2O, 60 oC, 0.6h
2h
O
O
Scheme 3a Radical trapping experiment
O
O
OH
H
Fe3O4-SiO2/EDAcel@Pd(0)
TBHP, H2O, 60 oC, 2h
R
CH3
O
H
99% conversion
O
3.25
Scheme 3b Oxidation of 1-Phenylethanol
H
O
Catalyst testing for reductive amination of aldehydes with
nitroarenes
H
8.
2.5
90
H2
N
H2
N
Encouraged by the impressive performace of the
Fe3O4@SiO2/EDAC-Pd(0) for C-H bond oxidation, we
further explored the catalytic activity of the catalyst for the
reductive amination reaction (Scheme 4). The reaction of
benzaldehyde with nitrobenzene using molecular hydrogen at
room temperature was chosen as a model reaction. Initial
experiments were done for the comparison of its activity with
other catalysts and for the optimization with respect to the
amount of catalyst required for catalysing the desired
reaction. The results are summarised in Table 1, which shows
that 0.05 g (1.5 mol% Pd) of Fe3O4@SiO2/EDAC-Pd(0)
showed best performance with respect to yield and time
(Table 1, entry 11).
O
H
9.
3.75
90
89
O2N
O2N
O
10.
H
3.5
4.5
Br
Br
O
OH
OH
11
88
87
O
12.
5.5
4.5
O2N
O2N
O
13.
86
OH
H2N
H2N
aReaction conditions: arylalkane (1 mmol), 70% t-BuOOH (1.5 mmol),
Fe3O4@SiO2/EDAC-Pd(0) (0.05g, 1.5 mol% Pd) and water (5 mL) at 60 oC.
bIsolated yield.
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Next, we studied the effect of the solvent on reductive donating as well as electron-withdrawing groups ran well and
amination and observed that the nature of the solvent affected the corresponding products were obtained in good yields.
the conversion of the reaction. Solvents such as CH2Cl2,
Comparison with other catalyst systems
EtOH, CH3CN and H2O were studied, but these solvents gave
low yields for the reaction (Table 4, entries 1-5). However,
In order to evaluate the uniqueness and efficiency of
when we chose organic/aqueous co-solvent system, high
Fe3O4@SiO2/EDAC-Pd(0) catalyst among the different
yields of 82% to 93% were obtained (Table 4, entries 6-8).
catalysts reported in literature, we have compiled the data for
This may be attributed to the beneficial effect of the co-
benzylic C-H bond oxidation and for the reductive amination,
solvent which resulted in good solubility of the organic
the results are collected in Table 6 and 7. The results show
substrates.
the supremacy of the present catalytic system in terms of
R2
time, yield and selectivity compared to others reported in the
literature. Furthermore, the present protocol affords a truly
green process using benign reaction media as well as
achieving higher product yields in shorter reaction time.
CHO
NO2
Fe3O4-SiO2/EDAcel@Pd(0)
EtOH:H2O, H2, R.T.
N
H
+
R2
R1
R1
Scheme 4 Reductive amination of aldehydes with nitroarenes
Recyclability and heterogeneity
Table 4. Effect of solvents for the reductive amination in the
presence of Fe3O4@SiO2/EDAC-Pd(0) using molecular hydrogen at
room temperaturea.
In sustainable organic synthesis, the recovery of the catalyst is
of prime importance, which is generally accomplished via
laborious filtration or centrifugation techniques. Due to the
magnetic character of Fe3O4@SiO2/EDAC-Pd(0), the catalyst
could be recovered easily using an external magnet. To
confirm the reusability and stability of the catalyst, we carried
out a set of experiments in case of Table 3, entry 1 and
Table 5, entry 1 using Fe3O4@SiO2/EDAC-Pd(0). After
completion of reaction, the catalyst was recovered using an
external magnet, washed successively with EtOAc, double
distilled water, and then dried. A fresh reaction was then
setup using recovered Fe3O4@SiO2/EDAC-Pd(0) and new
substrates were added under similar reaction conditions.
Fe3O4@SiO2/EDAC-Pd(0) could be used at least six times
without any change in activity. The results are summarized in
Table 8. Further, to rule out the contribution of
homogeneous catalysis, a reaction (Table 3, entry 1) was
carried out until the conversion was 40% (0.5 h) and at that
point the solid was filtered off at the reaction temperature.
The liquid phase was then transferred to another flask and
again allowed to react, but no further significant
Entry
Solvent
Time (h) Yield (%)b
1.
2.
3.
4.
5.
6.
7.
8.
CH2Cl2
H2O
6
6
45
60
68
72
80
82
85
93
CH3CN
5
MeOH
4.5
3
EtOH
EtOH/H2O (1:3)
EtOH/ H2O (1:1)
EtOH/ H2O (3:1)
3
3
2.5
aReaction conditions: benzaldehyde (1 mmol), nitrobenzene (1.2 mmol),
Fe3O4@SiO2/EDAC-Pd(0) (0.05g, 1.5 mol% Pd), solvent (5 mL) under H2
atmosphere at room temperature.
bIsolated yield.
With a reliable set of conditions in hand, the scope and
generality of the developed protocol with respect to various
aldehydes and nitroarenes were investigated using
Fe3O4@SiO2/EDAC-Pd(0). As shown in Table 5, various
aldehydes were reductively aminated with nitroarenes in
EtOH/H2O (3:1) at room temperature under hydrogen
atmosphere. Furthermore, reaction of most of the substrates
with aromatic aldehydes and nitroarenes bearing electron-
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Table 5. Reductive amination of aldehydes with nitroarenes catalyzed by Fe3O4@SiO2/EDAC-Pd(0) under molecular H2 at room
temperaturea
Entry
Aldehyde
Nitroarene
Product
Time (h)
Yield (%)b
CHO
N
H
1
2
O2N
2.5
3
93
91
CHO
N
H
O2N
MeO
MeO
Me
CHO
CHO
N
H
3
4
5
6
7
8
2.5
3
90
89
91
90
88
90
O2N
Me
Br
N
H
O2N
O2N
Br
Cl
CHO
N
H
2
Cl
CHO
CHO
CHO
N
H
3.5
3
O2N
HO
Me
HO
Cl
Cl
N
H
O2N
Me
Cl
Cl
2.5
O2N
N
H
Cl
CHO
Cl
9
3
3
90
89
N
H
MeO
O2N
MeO
OMe
OMe
CHO
N
H
10
O2N
aReaction conditions: benzaldehyde (1 mmol), nitrobenzene (1.2 mmol), Fe3O4@SiO2/EDAC-Pd(0) (0.05g, 1.5 mol% Pd), ethanol:water (3:1, 5
mL) under H2 atmosphere at room temperature.
bIsolated yield.
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Table 6 Comparison of the catalytic activity of Fe3O4@SiO2/EDAC Pd(0) with some reported catalytic systems for the C-
H bond oxidation of ethylbenzene to acetophenone
Catalyst
Reaction conditions
Time /yield
(h) (%)
Reference
CuI, L1
SDS, T-Hydro, H2O, r.t.
1/68
31
(Homogenous)
Sc(OTf)3
(Homogenous)
RFT, CH3CN, air, blue
light (440 nm)
2.5/71
24/75
32
14
SUT-6-Zn
(Heterogeneous)
TBHP, CH3CN/
AcOH, 65 oC
Ag/ZnO
(Heterogeneous)
solvent-free, H2O2,
80 oC
7/90
48/38
2/90
13
33
Cu-MOF-SiF6
(Heterogeneous)
TBHP, CH3CN, 60 oC
Fe3O4@SiO2/EDAC-Pd(0)
TBHP, H2O, 60 oC
Present
work
(Heterogeneous)
Table 7 Comparison of the catalytic activity of Fe3O4@SiO2/EDAC-Pd(0) with some reported catalytic systems
for the reductive amination of benzaldehyde with nitroarene
Catalyst
Reaction conditions
Time/yield
(h) (%)
Reference
HMMS-salpr-Pd
Ethanol, H2, R.T.
6/93
19
Pd NPs
Pd1Ag1.70
Ethanol, H2, R.T.
Ethanol, H2, R.T.
Methanol, H2, R.T.
Ethanol, H2, R.T.
Water, 60 oC, H2
6/92
4/93
20
21
GA-Pd Nanoparticles
Pd/Fe3O4
6/88
22
6/92
23
24
Pd/Fe3O4@C
8/92
Fe3O4@SiO2/EDAC-Pd(0)
EtOH/H2O, H2, R.T.
2.5/93
Present work
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conversion was observed. This strongly suggests that the
catalyst is heterogeneous in nature. Furthermore, the ICP-
AES analysis of the used Fe3O4@SiO2/EDAC-Pd(0) after
the 6th run was performed to calculate the Pd concentration.
The initial concentration (3.0 wt% of Pd) before the reaction
was reduced to 2.93 wt % after the 6th run. The small amount
of Pd leaching may be due to microscopic changes that
occurred on the surface of the magnetic catalyst. FTIR and
TGA studies showed that the spent catalyst had no
observable structural change relative to the fresh catalyst.
Further, to examine the stability of Fe3O4 nanoparticles and
Fe3O4@SiO2/EDAC under acidic conditions, 20 mg of each
substrate was dispersed in 10 ml of HCl solution with
concentrations of 0.05, 0.1, 0.1, and 0.15 mol/L, respectively.
After continuous contact with acid for 2, 4 and 6 h, the
suspension was magnetically separated and the leached iron
concentration in the supernatant was determined by ICP-AES
(Table 9). These results indicated that the coating of Fe3O4
with SiO2/EDAC prevented the leaching of Fe to a major
extent. However, we have not used acidic conditions in the
reactions under study.
Table 9. Leaching test of Fe3O4 and Fe3O4@SiO2/EDAC under
acidic environment.
Entry HCl
Sol.
Leached Fe content (%)
Fe3O4
Fe3O4@SiO2/EDAC
2 h 4 h 6 h
Conc.
(M)
2 h
4 h 6 h
1
2
3
0.05
0.1
0.14
0.46
2.01
0.35
1.99
0.06
0.28
0.56
0.15 0.48
0.61 1.05
1.02 1.83
1.15 3.02
0.15
3.90
4.96
Conclusion
In conclusion, we have developed Fe3O4@SiO2/EDAC-Pd(0)
as a novel, magnetic, and highly active heterogeneous catalyst
for the benzylic C-H bond oxidation and reductive amination.
The catalyst can be easily recovered using an external magnet
and can be reused upto six times without significant
deterioration in activity. The salient features of the proposed
methodology include generality, operational simplicity, easy
recyclability, a cleaner reaction profile which lead to short
reaction time and high yields, thus adding a credit towards the
development of greener methodologies for organic synthesis.
Further investigations on the use of Fe3O4@SiO2/EDAC-
Pd(0) catalyst for other potential transformations are being
carried out in our laboratory.
Table 8. Recyclability of Fe3O4@SiO2/EDAC-Pd(0) for the C-
H bond oxidation and reductive amination reactionsa
Catalytic
runs
C-H bond oxidation
Yield (%)b
Reductive amination
Yield (%)b
Experimental
1.
2.
3.
4.
5.
6.
90
90
89
88
88
87
93
93
92
92
91
91
General
All the chemicals and solvents used were purchased
1
from Aldrich Chemical Company or Merck. The H and
13C NMR data were recorded in CDCl3 on Bruker
Avance III (400 MHz and 100 MHz) and mass spectral
data on Bruker Esquires 3000 (ESI). The FTIR spectra
were recorded on Perkin-Elmer FTIR spectrophotometer.
TGA was recorded on Linsesis STA PT-1000 make
thermal analyzer. X-ray diffratogram (XRD) was
recorded on a Bruker AXSDB X-ray diffractometer using
Cu Kα radiations. SEM images were recorded using
JEOL Model JSM-6390LV Scanning Electron
aReaction conditions [C-H bond oxidation]: ethyl benzene (1 mmol), 70% t-
BuOOH (1.5 mmol), catalyst (0.05g, 1.5 mol% Pd) and water (5 mL) at
60oC; [Reductive amination]: benzaldehyde (1 mmol), nitrobenzene (1.2
mmol), catalyst (0.05g, 1.5 mol% Pd), ethanol:water (3:1, 5 mL) under H2
atmosphere at room temperature. bIsolated Yield.
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Microscope, High Resolution Transmission Electron was continued for another 4 h. The cellulose chloride (CelCl)
Micrographs (HRTEM) were recorded on JEOL, JEM- obtained was washed with several aliquots of dilute
2100F. EDX analysis was carried out on JEOL Model ammonium hydroxide solution (0.5 mol/L NH4OH) and the
JED-2300 and the amount of metal in catalyst was supernatant after each treatment was removed to bring the pH
determined by ICP-AES analysis using ARCOS from to neutral. To complete the washing, the suspension was
M/s Spectro, Germany. XPS spectra of the catalyst were
recorded on a KRATOS ESCA model AXIS 165
(Resolution).VSM was measured using a vibrating
sample magnetometer of Model: 7410 series, Lakeshore
at room temperature from −10 000 to +15 000 Oe.
exhaustively treated with distilled water (2100 mL). The
solid was then separated by filtration and dried in vacuum at
room temperature.
A mixture of CelCl (3 g) and ethylene-1,2-diamine (16
o
mL) was stirred at 116 C for 3 h. The reaction mixture
Synthesis of magnetite nanoparticles (Fe3O4 NPs)
was filtered through a sintered glass crucible and the
solid was dried in vacuum at room temperature to afford
EDAC as yellow powder.
FeCl3 (5.0 g) and FeCl2 (2.0 g) were successively dissolved in
distilled water (30 mL). The resulting solution was then added
dropwise into aqueous NaOH solution (1 M, 200 mL) under
vigorous stirring for 30 minutes. The precipitation and
formation of nanoferrites took place by a conversion of the
metal salts into hydroxides and that, in turn, immediately
transformed into ferrites. After 15 min, the precipitated
ferrites were separated with a magnet and washed to pH < 7.5
by distilled water which were then dried under vacuum at 60
ºC over night.
Synthesis
of
ethylene
diamine
functionalized
Fe3O4@silica/cellulose (Fe3O4@SiO2/EDAC)
To a solution of Fe3O4@SiO2 (2 g) in CHCl3 (17 mL),
EDAC (2 g) and triethyl amine (0.4 mL) were added, and
the reaction mixture was stirred at 60 oC for 12 h.
Fe3O4@SiO2/EDAC was seperated with magnet and
washed with chloroform (3×10 mL) followed by water
(3×10 mL and dried under vacuum at 60 oC over night.
Synthesis of silica coated magnetic nanoparticles
(Fe3O4@SiO2)
Synthesis of palladium nanoparticles onto ethylene
diamine
functionalized
Fe3O4@silica/cellulose
Fe3O4 (2 g) was dispersed in water (30 mL) followed by the
addition of ethanol (120 mL), ammonia (6 mL) and TEOS i.e.
tetraethyl orthosilicate (6 mL). The mixture was then refluxed
for 6 h under vigorous stirring. The resulting Fe3O4@SiO2
colloid was separated with magnet and washed repeatedly
with distilled water (3×10 mL) and ethanol (3×10 mL). Then,
the final product, Fe3O4@SiO2 was dried in vacuum at 60 ºC
over night.
[Fe3O4@SiO2/EDAC-Pd(0)]
To a dispersed solution of Fe3O4@SiO2/EDAC (2 g) in
ethanol (6 mL), aqueous solution of PdCl2 (0.17 g, 1.0
mmol, 3 mL) was added, and the reaction mixture was
stirred at room temperature for 6 h. Then, aqueous
solution of NaBH4 (1.2 mmol, 5 mL) was added slowly
during 8 h. Finally, Fe3O4@SiO2/EDAC-Pd(0) was
seperated with magnet and washed successively with
water (3×10 mL) and ethanol (3×10 mL). Finally, it was
dried under vacuum at room temperature to give the dark
Synthesis of ethylene diamine functionalized cellulose
(EDAC)
polymeric Pd(0) nanoparticles (Scheme 1)
.
o
Cellulose (5 g) previously activated at 80 C for 12 h was
General procedure for the benzylic C-H bond oxidation
suspended in N,N-dimethylformamide (100 mL), followed by
o
the slow addition of thionyl chloride (16 mL) at 80 C, under
mechanical stirring. After the addition was complete, stirring
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To a mixture of arylalkane (1 mmol), Fe3O4@SiO2/EDAC- We thank the Head, SAIF, IIT Bombay for HRTEM and ICP-
Pd(0) (0.05 g, 1.5 mol% Pd) and 70% t-BuOOH (0.2 mL, 2 AES studies; Head, SAIF, Punjab University Chandigarh for
mmol) in a round bottom flask (25 mL), water (5 mL) was XRD; Head, SAIF, Kochi for SEM, and EDX studies, IICT,
added, and the reaction mixture was stirred at 60 °C for an Hyderabad for XPS analysis; and Head, CIF, Guwahati for
appropriate time (Scheme 2). After completion of the reaction VSM analysis. Financial assistance from UGC, New Delhi
(monitored by TLC), the catalyst was separated using an (SRF to one of the authors, MB and major research project, F
external magnet and the reaction mixture was diluted with 41-281/2012 SR) is gratefully acknowledged.
ethyl acetate and filtered. The organic layer was washed with
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Graphical Abstract
Fe3O4@SiO2/EDAC-Pd(0) as a novel and efficient inorganic/organic
magnetic composite: Sustainable catalyst for the benzylic C-H bond
oxidation and reductive amination under mild conditions
Madhvi Bhardwaja, Harsha Sharmaa, Satya Paula* and James H. Clarkb
aDepartment of Chemistry, University of Jammu, Jammu 180006, India. Fax: +91-191-
2431365; Tel: +91-191-2453969; Email: pual7@rediffmail.com.
bGreen Chemistry Centre of Excellence, Department of Chemistry, University of York, York,
UK. Fax: +44 (0)1904 432705; Tel: +44 (0)1904 432567; E-mail: james.clark@york.ac.uk.
Magnetically recyclable ethylene diamine functionalized inorganic/organic composite,
Fe3O4@SiO2/EDAC-Pd(0) for selective C-H bond oxidation and reductive amination under
sustainable reaction media.