NiO nanoparticles prepared via thermal decomposition of the bis(dimethylglyoximato)nickel(II) complex: A novel reusable heterogeneous catalyst for fast and efficient microwave-assisted reduction of nitroarenes with ethanol

Article abstract of DOI:10.1016/j.poly.2010.11.037

NiO nanoparticles with an average size of 12 nm and a high specific surface area of 88.5 m²/g were easily prepared via the thermal decomposition of the complex Ni(dmgH)₂ and were characterized by TGA, XRD, FT-IR, TEM and BET surface area measurement. This nanosized transition metal oxide was used as a new heterogeneous catalyst for the reduction of nitroarenes under microwave irradiation. The efficient and selective reduction of aromatic nitro compounds into their corresponding amines was observed by using ethanol as a hydrogen donor (reducing agent) and KOH as a promoter under microwave irradiation. This highly regio-and chemoselective method is fast, simple, inexpensive, high yielding, clean and compatible with several sensitive functionalities, such as halogens,-OH,-OCH₃,-CHO,-COCH ₃,-COOH,-COOEt,-CONH₂,-CN,-CHCH₂ and-NHCOCH₃. This method is suitable for the large scale preparation of different substituted anilines as well as other arylamines. In addition, the catalytic activity of nanosized NiO is higher than that of the bulk sample.

Full text of DOI:10.1016/j.poly.2010.11.037

Polyhedron 30 (2011) 606–613
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
NiO nanoparticles prepared via thermal decomposition of the
bis(dimethylglyoximato)nickel(II) complex: A novel reusable heterogeneous
catalyst for fast and efficient microwave-assisted reduction of nitroarenes
with ethanol
⇑
Saeid Farhadi , Maryam Kazem, Firouzeh Siadatnasab
Department of Chemistry, Lorestan University, Khoramabad 68135-465, Iran
a r t i c l e i n f o
a b s t r a c t
2
Article history:
NiO nanoparticles with an average size of 12 nm and a high specific surface area of 88.5 m /g were easily
prepared via the thermal decomposition of the complex Ni(dmgH) and were characterized by TGA, XRD,
Received 28 September 2010
Accepted 25 November 2010
Available online 7 December 2010
2
FT-IR, TEM and BET surface area measurement. This nanosized transition metal oxide was used as a new
heterogeneous catalyst for the reduction of nitroarenes under microwave irradiation. The efficient and
selective reduction of aromatic nitro compounds into their corresponding amines was observed by using
ethanol as a hydrogen donor (reducing agent) and KOH as a promoter under microwave irradiation. This
highly regio- and chemoselective method is fast, simple, inexpensive, high yielding, clean and compatible
Keywords:
Nanoparticles
NiO
Nitroarenes
Reduction
with several sensitive functionalities, such as halogens, –OH, –OCH
3 3
, –CHO, –COCH , –COOH, –COOEt,
CONH , –CN, –CH@CH and –NHCOCH . This method is suitable for the large scale preparation of differ-
–
2
2
3
Microwave irradiation
Arylamines
ent substituted anilines as well as other arylamines. In addition, the catalytic activity of nanosized NiO is
higher than that of the bulk sample.
Ó 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
S
8
/NaHCO
and N
molecular hydrogen and catalysts such as Pd/C, Pd(II) complexes/
Al or Raney nickel have also been reported for the reduction
3
[37], NaBH
4 3
/Raney nickel [38], Fe(acac) /TMDS [39]
2
H
4
2 3
/A O /MWI [40]. Catalytic hydrogenation employing
The reduction of nitroarenes to arylamines is a synthetically
important transformation both in the laboratory and in industry
1]. Arylamines are important starting materials and intermediates
2 3
O
[
of nitroarenes [41–44]. However, each of these methods has its
own advantages and limitations. From a practical viewpoint, the
development of simpler, inexpensive, widely-applicable and envi-
ronmentally-benign procedures is still an active area of research.
One of the most promising methods for the reduction of nitroa-
renes to their corresponding amines is catalytic transfer hydroge-
nation [45]. This method presents a special variation of catalytic
hydrogenation in which the highly diffusible and flammable
hydrogen gas is replaced with a safe, green and stable hydrogen
donor, such as hydrazine hydrate, formic acid or alcohols. The
use of hydrogen donors has some advantages over molecular
hydrogen since it avoids the risks and the constraints associated
with the highly flammable hydrogen gas as well as the necessity
for pressure vessels and other expensive equipment. In comparison
with homogeneous systems, heterogeneous catalytic transfer
hydrogenation has potential advantages, including operational
simplicity, high chemoselectivity and yield, no highly diffusible
flammable hydrogen gas being used and no special equipment re-
quired. Pt, Pd, Ru and Raney Ni are classical heterogeneous cata-
lysts employed in catalytic transfer hydrogenation [46]. These
catalysts are expensive and require stringent precautions because
for the manufacture of numerous organic chemicals, such as dye-
stuffs, pharmaceutical products, agricultural chemicals, surfactants
and polymers [2,3]. This important transformation is traditionally
performed using metals such as Fe, Zn and Sn in acidic or basic
media, which generate a large amount of metal wastes [4–6]. In re-
cent years, numerous reagents have been developed for the reduc-
tion of nitroarenes to their corresponding amines, including
activated metals [7–10], Al/NH
12], Co (CO) /H O [13], Mo(CO)
liquid [16], N /FeCl [17], N
FeSO [20], B10 14/Pd/C [21], FeCl
Pd/C [23], HCOONH /Mg [24], HCOON
26], HCOOH/Raney nickel [27], N
ONa/K PO [29], N /AlH /AlCl
ÁH
31], N O/Fe /MgO [32], Na
ultrasound irradiation [34], HCOON H
5
4
Cl/MeOH [11], Zn/CaCl
[14], SnCl /H O [15], SnCl
/Zn [18], TiCl [19], NaH
/Zn/DMF/H O [22], HCOONH
/Zn [25], N /Ru(bpy)
/Raney nickel [28], HCO-
[30], HCOONH /Ni[P(OPh
S/NEt Br [33], Sm/NH
/Raney Ni [35], HI [36],
2
/EtOH
[
2
8
2
6
2
2
2
/ionic
2
H
4
3
2
H
4
3
2
PO
2
/
/
4
H
3
2
4
4
2
H
H
2 4
5
2
H
4
2
/hm
[
3
4
2
H
4
3
3
4
3 4
) ]
4
Cl/
[
2
H
4
2
2
O
3
2
4
2
⇑
Corresponding author. Tel.: +98 6612202782; fax: +98 6616200088.
E-mail address: sfarhad2001@yahoo.com (S. Farhadi).
0
277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.11.037

S. Farhadi et al. / Polyhedron 30 (2011) 606–613
607
of their flammable nature in the presence of air. Since this trans
formation requires catalysts with a high surface area and highly
acidic active sites, it seems that nanosized materials which possess
such characteristics could be very suitable for this purpose. In
recent years, inorganic nanomaterials, such as silicate or alumi-
nophosphate molecular sieve-supported transition metal ions
400 °C for 0.5 h in air. The temperature for the decomposition of
the complex was selected from the results of thermogravimetric
analysis (TGA). The decomposition product was cooled to room
temperature and collected for characterization.
2.2. Catalyst characterization
[
47–54], polymer-supported amorphous Ni–B nanoparticles [55],
silica-supported gold nanoparticles (nano-Au/SiO ) [56], Mg–Fe
2
The crystal structure and phase composition of the NiO nano-
Hydrotalcite [57] and Faujasite NaY zeolite [58], have been em-
ployed as safe and green recyclable heterogeneous catalysts for
the catalytic transfer hydrogenation of nitroarenes to arylamines.
However, these reactions are usually conducted under conven-
tional refluxing conditions, a process that requires several hours
of time. Furthermore, the activity of most of the catalysts decreases
with subsequent recycling.
particles were identified with a Bruker D4 Advance X-ray diffrac-
tometer using Cu K radiation (k = 0.15418 nm). Infrared spectra
a
were recorded on a Schimadzu system FT-IR 160 spectrophotome-
ter using KBr pellets. Thermogravimetry analysis (TG) of the pre-
cursor was performed in a Netzsch STA 409 PC simultaneous
thermal analyzer at a heating rate of 5 °C/min. The morphology
of the catalyst was determined by a scanning electron microscope
Nanostructured transition metal oxides have attracted great
interest in recent years due to their particular physical and chemical
properties compared to bulk materials [59]. The properties of these
materials mainly depend on their shape, size and structure, which
are strongly determined by their synthetic processes. Among the
various nanostructures of metal oxides, nanoparticles containing a
high surface area and reactive morphologies have been considered
as effective catalysts for organic reactions such as acetylation,
hydrogenation, oxidation, and cross-coupling [60–69]. Nickel oxide
(
SEM, Philips XL-30). The size of the catalyst particles was deter-
mined by a transmission electron microscope (TEM, LEO-906E) at
an accelerating voltage of 80 kV. TEM samples were prepared by
dropping the ethanol dispersion on a carbon-coated copper grid.
The specific surface area was calculated by the BET method using
N
2
adsorption–desorption experiments, carried out at À196 °C on
a Micromeritics ASAP 2010. Before each measurement, the sample
was outgassed at 150 °C for 1 h.
(
NiO) is one of the most common metal oxides used for catalytic pur-
2
.3. Catalytic tests
poses. It was reported that NiO nanoparticles are much more effec-
tive catalysts than commercial NiO powder for catalytic reduction
of carbon dioxide to methanol [70]. There are many different meth-
ods reported for the synthesis of nanosized nickel oxide [71–74].
Among these methods, the solid-state thermal decomposition of
Ni(II) complexes exhibits many advantages: no need for solvent,
surfactant and complex apparatus, high yields, low energy con-
sumption and simple reaction technology.
In continuation of our interest in exploring nanostractured
catalysts for organic transformations [66–69], in this work NiO
nanoparticles were easily prepared via the thermal decomposition
of the bis(dimethylglyoximato)nickel(II) complex and their cata-
lytic activities were evaluated in the reduction of nitroarenes to
their corresponding amines with ethanol. The catalytic results
indicate that the nanosized NiO oxide is a green, highly selective
and efficient recyclable catalyst for the rapid reduction of nitroa-
renes to their corresponding amines by using ethanol as a hydro-
In a 100 mL Teflon-bottle was placed nitroarene (5 mmol), KOH
(
5 mmol), ethanol (10 mL) and NiO (50 mg) as the catalyst. The
vessel was sealed and irradiated in a laboratory microwave oven
at 180 W. The progress of the reaction was monitored by TLC and
GC–MS. After completion of the reaction, the mixture was cooled
to room temperature and the organic layer extracted with ethyl
acetate (2 Â 10 mL), dried over anhydrous Na
2 4
SO and concen-
trated under a reduced pressure. The residue was subjected to
silica-gel plate or column chromatography using carbon tetrachlo-
ride–ethyl acetate to give the pure product. The reduction of
nitroarenes was also carried out under the conventional heating
in a similar manner except that the reaction mixture was refluxed
for the appropriate time in an oil-bath. The results of initial exper-
iments to obtain the optimized reaction conditions are summa-
rized in Tables 1–3. The general results are presented in Table 4.
The obtained results by using bulk NiO catalyst instead of nano-
sized NiO catalyst are compared in Table 5.
gen donor and potassium hydroxide as
a promoter under
microwave irradiation. To the best of our knowledge, this is the
first report on the catalytic reduction of nitroarenes over a nano-
sized metal oxide in conjunction with microwave irradiation.
Table 1
Effect of the catalyst amount on the reduction of nitrobenzene with ethanol.a
NH2
NO2
2
. Experimental
NiO , KOH
Ethanol , MWI
All the nitroarenes were commercially available and were used
without further purification. All the solvents were spectroscopic
Yield (%)b
Entry
NiO (mg)
Time (min)
grade. A commercial bulk NiO sample with a surface area of
2
1
2
3
4
5
6
7
10
20
30
40
50
75
100
0
30
30
30
20
13
12
12
25
25
20
34
52
61
85
98
98
97
0
8
.5 m /g and an average particle size of 18
l
m was purchased from
Merck. Microwave irradiation was carried out using in a laboratory
modified microwave oven (LG-30L, 900 W, MW frequency
2
.45 GHz). All products of the reduction of nitroarenes are com-
mercially available and were identified by comparing their physi-
cal and spectral data (m.p., TLC, FT-IR, GC–MS and 1H NMR) with
those of authentic samples or reported data.
c
8
9
0
d
50
50
0
0
e
1
a
2
.1. Preparation of nanosized NiO catalyst
Reaction conditions: nitrobenzene (5 mmol), KOH (5 mmol), ethanol (20 mL),
under microwave heating.
b
Isolated yields. Selectivity was >99% in all cases.
Without catalyst.
Without KOH.
In the absence of MWI.
In order to prepare nanosized NiO particles, the bis(dimethyl-
c
glyoximato)nickel(II) complex (Ni(dmgH)
porcelain crucible and was then heated in an electric furnace at
2
, 5 g) was placed in a
d
e

6
08
S. Farhadi et al. / Polyhedron 30 (2011) 606–613
Table 2
3
. Results and discussion
Effect of the H-donor (alcohol) on the reduction of nitrobenzene.a
Entry
H-donor
Time (min)
Yield (%)b
3.1. Characterization of the nanosized NiO catalyst
1
2
3
4
5
6
Ethanol
13
45
45
20
45
45
98
35
55
97
14
0
Methanol
1-Propanol
2-Propanol
2-Butanol
tert-Butanol
One of the simplest and lowest cost techniques to prepare
nanosized transition metal oxides with a relatively high specific
surface area is the solid-state thermal decomposition of their com-
plexes. In this work, the Ni(dmgH)
room temperature by mixing aqueous solutions of appropriate
amounts of NiSO O and the H dmg ligand under continuous
Á6H
stirring, and it was used as a precursor for the synthesis of nano-
sized NiO particles. First, the thermal behavior of the Ni(dmgH)
2
complex was synthesized at
a
Reaction conditions: nitrobenzene (5 mmol), KOH (5 mmol), alcohol (20 mL),
under microwave heating.
4
2
2
b
Isolated yields.
2
precursor was studied by thermal analysis. The TG curve in Fig. 1
shows a major weight loss at about 300 °C, which was followed
by a gradual weight loss until 380 °C. The total weight loss is about
77.5%, which is close to the theoretical value for the formation of
Table 3
a
Effect of the promoter (base) on the reduction of nitrobenzene.
Yield (%)b
Entry
Base
Time (min)
NiO from Ni(dmgH)
study, the Ni(dmgH)
sample turned black in color after thermal treatment, indicating
2
. Based on the thermal analysis in the present
1
2
3
4
5
6
KOH
NaOH
Ca(OH)
13
15
30
30
30
30
98
92
22
18
0
2
complex was heated at 400 °C for 0.5 h. The
2
Na
NH
Me
2
CO
3
2
complete decomposition of the Ni(dmgH) precursor into pure
3
NiO nanoparticles, according to Scheme 1.
3
N
0
Fig. 2 shows the XRD pattern of the product from the decompo-
sition of the precursor at 400 °C. The recorded diffraction peaks are
well assigned to the cubic phase NiO structure (space group
Fm3/m; a = 4.176 Å; JCPDS78-0643). Moreover, no other peak is
a
Reaction conditions: nitrobenzene (5 mmol), KOH (5 mmol), ethanol (20 mL),
under microwave heating.
b
Isolated yields.
observed belonging to impurities such as Ni(OH)
3
, NiO
2
, and NiCO
3
,
2.4. Recycling test
indicating the absolute transformation from Ni(dmgH)
2
to NiO. It
can be seen from Fig. 2 that the diffraction peaks are markedly
broadened due to the small size of the particles. The average par-
ticle size is calculated using the Debye–Scherrer equation [75]:
d = (0.9k)/(h1/2cos h), where d represents the grain size, k is the
After completion of the reaction with nitrobenzene, the catalyst
was filtered off, washed with ethyl acetate (2 Â 5 mL) and dried.
The recovered catalyst was reused in the next runs under the same
conditions. The results are shown in Table 6.
wavelength of the X-ray (Cu K
a
, 0.15418 nm), h is the diffraction
Table 4
a
The reduction of various nitroarenes with ethanol over a nanosized NiO catalyst.
Entry
Substrate
Product
Microwave method
Conventional method
Time (min)
Yield (%)b
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
C
6
H
5
NO
2
C
6
H
5
NH
2
13
8
8
11
9
98
98
95
97
94
96
97
98
97
94
94
70
94
93
85
97
95
95
90
93
92
92
90
91
93
88
92
93
89
94
60
40
45
40
35
70
75
75
70
80
80
95
70
80
90
35
35
40
90
100
90
80
100
90
80
100
90
50
80
180
96
98
94
96
98
96
97
98
96
95
96
80
96
94
93
95
96
98
80
90
88
90
87
91
90
85
89
95
88
90
p-MeC
m-MeC
m-MeOC
o-MeOC
m-ClC
o-ClC
p-BrC
m-BrC
6
H
4
NO
NO
2
p-MeC
m-MeC
m-MeOC
o-MeOC
m-ClC
o-ClC
p-BrC
m-BrC
6
H
4
NH
NH
2
6
H
4
2
6
H
4
2
6
H
4
NO
2
6
H
4
NH
2
6
H
4
NO
NO
NO
NO
NO
2
6
H
4
NH
NH
NH
NH
NH
2
6
H
4
2
6
H
4
2
14
14
14
14
13
14
20
15
17
15
10
10
11
18
14
12
10
15
16
16
14
15
10
12
24
6
H
4
2
6
H
4
2
6
H
4
2
6
H
4
2
6
H
4
2
6
H
4
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
2,5-(Cl)
2,3-(Cl)
2
C
C
6
H
H
3
NO
NO
2
2
2,5-(Cl)
2,3-(Cl)
2
C
C
6
H
H
3
NH
3
NH
2
2
2
6
3
2
6
3-NO
m-NO
p-NO
p-CNC
p-NH
m-NH
o-NH
3,4-(NH
p-HOC
m-CHOC
p-MeCOC
p-HOOC–C
p-EtOOC–C
p-NH CO–C
p-MeCONHC
p-CH @CHC
2
,4-ClC
NO
NO
6
H
3
NO
2
3-NO
m-NO
p-NO
p-CNC
p-NH
m-NH
o-NH
3,4-(NH
p-HOC
m-CHOC
p-MeCOC
p-HOOC–C
p-EtOOC–C
p-NH CO–C
p-MeCONHC
p-CH @CHC
2
,4-ClC
NH
NH
6 3 2
H NH
2
C
6
H
4
2
2
C
6
H
4
2
2
C
6
H
4
2
2
C
6
H
4
2
6
H
4
NO
2
6
H
4
NH
2
2
C
6
H
4
NO
2
2
C
6
H
4
NH
2
2
C
6
H
4
NO
NO
2
2
C
6
H
4
NH
NH
2
2
C
6
H
4
2
2
C
6
H
)
4
2
2
)
2
C
6
H
3
NO
2
2
2
C
6
H
3
NH
2
6
H
4
NO
2
6 4 2
H NH
6
H
4
NO
2
6
H
4
NH
2
6
H
4
NO
NO
NO
NO
NO
NO
2
6
H
4
NH
NH
NH
NH
NH
NH
2
6
H
4
2
6
H
4
2
6
H
H
4
2
6
H
H
4
2
2
6
4
2
2
6
4
2
6
H
4
2
6
H
4
2
2
6
H
4
2
2
6
H
4
2
1-Nitronaphthalene
2-Nitronaphthalene
C
1-Naphthylamine
2-Naphthylamine
C H NH
6 5 2
9
c
3
0
6
H
5
NO
2
a
b
c
Reaction conditions: nitroarene (5 mmol), KOH (5 mmol), ethanol (20 mL), catalyst (50 mg) under microwave and conventional heating.
Yields are for isolated pure products. Selectivity was >99% in all cases.
The reaction was carried out on a 100 mmol scale of nitrobenzene.

S. Farhadi et al. / Polyhedron 30 (2011) 606–613
609
Table 5
Comparison of the reduction of several nitroarenes to their corresponding amines over nanosized- and bulk NiO.
a
Entry
Nitroarene
Nanosized-NiO catalyst
Time (min)
Bulk-NiO catalyst
Time (min)
3
Yield (%)b
Yield (%)b
1
2
3
4
5
Nitrobenzene
13
14
8
11
14
98
96
98
97
93
45
45
30
35
45
50
55
60
55
40
m-Chloroonitrobenzene
4-Methylnitrobenzene
3-Methoxynitrobenzene
4-Nitrophenol
a
Reaction conditions: nitrobenzene (5 mmol), KOH (5 mmol), ethanol (20 mL), catalyst (50 mg) under microwave heating.
Isolated yields.
b
Table 6
a
Reusability of the nanosized NiO catalyst.
Run
1
2
3
4
Time (min)
Yield (%)b
13
98
13
96
14
96
14
95
a
Reaction conditions: nitrobenzene (5 mmol), KOH (5 mmol), ethanol (20 mL),
catalyst (50 mg) under microwave heating.
b
Isolated yields.
Fig. 2. XRD pattern of the NiO nanoparticles obtained at 400 °C for 0.5 h.
angle of the peak and h1/2 stands for the full-width at half-height of
the peaks (in radian). The average particle size calculated using the
most intense reflection (2 0 0) at 2h = 43.5° is 11.8 nm. This value is
in accordance with the TEM observation (vide infra).
The FT-IR spectra of the precursor complex and its decomposi-
tion product are shown in Fig. 3. In the FT-IR spectrum of the pre-
2
cursor (Fig. 3a), the characteristic bands of the Ni(dmgH) complex,
with a square-planar structure, appeared in the range 450–
À1
1
500 cm [76]. Also, the broad bands related to the stretching
and bending vibrations of HÁ Á ÁO–H were observed around 3500
À1
and 1620 cm , respectively. As can be seen in Fig 3(b), all of these
bands disappeared after the thermal decomposition of the com-
plex. In the FT-IR spectrum of the product (Fig. 3b), only a strong
À1
Fig. 1. TGA curve of the Ni(dmgH)
2
complex.
band around 445 cm
was observed, which is related to the
Ni–O stretching vibration of NiO
ture [77].
6
octahedra in the cubic NiO struc-
The catalytic activity of metal oxides is strongly dependent on
the shape and size of their particles. The shape and size of the
NiO powder were investigated by SEM and TEM, as shown in
Fig. 4. The SEM image in Fig 4(a) reveals that the NiO powder is
a crystalline aggregate of nanoparticles. The SEM observation con-
firms that the NiO nanoparticles have uniform spherical shapes
with weak agglomeration. The size of the NiO particles were ob-
served by TEM. The TEM sample was prepared by dispersing the
powder in ethanol using ultrasonic vibrations. The TEM image of
the product is shown in Fig. 5(b). It can be seen that the powder
was formed from extremely fine spherical particles which were
loosely aggregated. From this image, it is evident that the particles
have a semi-spherical morphology with a narrow size distribution
and homogeneous shape. From the data obtained from the TEM
H C
CH3
3
H C
3
O
N
N
N
Ni
O
NOH
NOH
Ni2+(aq) + 2
H
H
O
N
O
H C
3
H C
CH3
3
O
400
C
Nanosized NiO particles
Scheme 1.

6
10
S. Farhadi et al. / Polyhedron 30 (2011) 606–613
2
Fig. 3. FT-IR spectra of (a) Ni(dmgH) complex and (b) NiO nanoparticles.
micrograph, a particle size histogram can be drawn and the mean
size of the particles can be determined. The inset of Fig. 4(b) shows
the particle size distribution of the NiO nanoparticles. It can be
seen that the particle sizes possess a narrow size distribution in
a range from 8 to 16 nm, and the mean particle diameter is about
1
2 nm. Actually, the mean particle size determined by TEM is very
close to the average particle size calculated using the Debye–
Scherrer formula and the XRD pattern.
Fig. 5 shows the nitrogen adsorption–desorption isotherm of
nanosized NiO, indicating the isotherms are of type IV. The sharp
decline in the desorption curve at 0.4 6 P/P 6 0.7 suggests charac-
0
teristics of mesoporous materials. From the linear portion of this
curve, the specific surface area of the nanosized NiO particles cal-
2
culated by the BET method was 88.5 m /g. However, that of the
2
bulk NiO purchased was 8.5 m /g. Hence the BET area of the nano-
sized NiO was almost ten times more than that of the bulk NiO
sample. This suggests that the as-prepared NiO nanoparticles have
a better surface adsorption property. Meanwhile, the high specific
surface area of the nanosized NiO product also indicates the possi-
bility of its application as an efficient catalytic material.
The particle size of the NiO product was also calculated from
the data of the specific surface area, by the equation: DBET
000/(
Á SBET) (in nm), where DBET is the average diameter of a
spherical particle,
=
6
q
3
q
is the theoretical density in g/cm and SBET rep-
resents the measured BET specific surface area of the powder in
2
m /g. The particle size calculated from the surface area data,
assuming spherical particles, is about 12.5 nm. We noticed that
the particle size obtained from the BET method agrees very well
with the results given by the X-ray line broadening and TEM obser-
vations. The results of the TEM observation and the BET method
further confirmed the relevant results obtained by XRD as men-
tioned above.
Fig. 4. (a) SEM and (b) TEM images of the NiO nanoparticles.
3.2. Catalytic activity of the NiO nanoparticles
The aim of the present study was to investigate the activity of
NiO nanoparticles as a novel heterogeneous catalyst for the reduc-
tion of nitroarenes to the corresponding amines under microwave
irradiation. Initial attempts to optimize the reaction conditions
were performed with nitrobenzene as the model substrate. In a
typical reaction, a mixture of nitrobenzene (5 mmol) and KOH

S. Farhadi et al. / Polyhedron 30 (2011) 606–613
611
NiO nanoparticles (cat.)
Ar NO2 EtOH, KOH, MW
Ar NH2
Scheme 2.
of the reactions and the usual side products of nitro reduction, such
as azoxy, azo and hydrazo compounds, were not observed in the fi-
nal product. The present method was highly chemoselective in the
presence of other hydrogenolysable or reducible groups. Sensitive
2 3
functional groups such as halogens, –OH, –NH , –OCH , –CN,
–
COMe, –CHO, –COOH, –COOEt and C@C did not undergo any
change under the reaction conditions. Halogenated nitro com-
pounds were reduced to the amino compounds without losing ha-
lides (Table 4, entries 6–12). The catalyst also shows promise for
the regioselective reduction of dinitro compounds, such that from
dinitrobenzene under these conditions only one nitro function was
reduced (Table 4, entries 12–14). Nitro compounds containing
carbonyl groups were also reduced to the corresponding amino
compounds and reduction of the carbonyl groups was not observed
Fig. 5. Nitrogen adsorption–desorption isotherm for the NiO nanoparticles.
(Table 4, entries 21–26). Even the aldehyde group remained intact
(
5 mmol) dissolved in ethanol (20 mL) was stirred in the presence
under the reaction conditions (Table 4, entry 21). This new reduc-
tion system was easily scaled-up and served for synthesis of aniline
on a several gram scale. The reduction of nitrobenzene into aniline
was performed on a 100 mmol scale. The yield of product was only
slightly lower (94%) comparing to the laboratory scale (98%), and
chemical purity was also maintained as high (Table 4, entry 30).
In all cases, to check the specific effect of microwaves, the
reduction of substrates was also conducted under conventional
heating under the same conditions. The results obtained were
compared with the microwave method in Table 4. Although the
products were obtained under both conditions, the reaction times
were considerably shortened in the case of the microwave
irradiation.
of different loadings of the NiO catalyst under microwave irradia-
tion (180 W). As indicated in Table 1, the yield of aniline increased
with the increase in the catalyst amount from 10 to 50 mg per
5
mmol of nitrobenzene, and no improvement in the reaction time
or the yield was observed by increasing the amount of the catalyst
further (Table 1, entries 1–7). The optimal catalyst load was
showed to be 50 mg per 5 mmol of nitrobenzene, which resulted
in aniline as the only product in 98% yield after 13 min irradiation
(Table 1, entry 5). Hence, all further experiments were carried out
at this catalyst loading. Blank experiments show that no reaction
occurs when nanosized NiO catalyst or KOH was omitted in the
system (Table 1, entries 8 and 9), indicating that NiO and base
are essential for this reaction. Further, no reaction was observed
in the absence of microwave irradiation at room temperature
For a comparison and to investigate the small size effect of the
NiO nanoparticles on the reaction, the reduction of several sub-
strates was investigated over a commercially available bulk NiO
(Table 1, entry 10).
2
The effect of the H-donor on the reaction was studied using var-
sample with a surface area of 8.5 m /g and an average particle size
ious alcohols, such as methanol, 1-propanol, 2-propanol, 2-butanol
of 18 lm under the optimized conditions. The results in Table 5
and tert-butanol. Among the alcohols used instead of ethanol, only
clearly indicate that the nanosized NiO is much more active than
the bulk NiO, which can be mainly attributed to the higher specific
surface area and the much higher density of active sites in the
nanosized NiO catalyst.
2
-propanol was found to be effective (Table 2, entries 2–5). In an
experiment when tert-butanol, which possesses no -H atom,
a
was used, no observable reduction occurred and the starting nitro
compound was recovered unchanged (Table 2, entry 6). These
observations indicate that the kind of alcohol as a hydrogen source
plays a significant role in the reduction of nitro compounds.
According to the experimental results, ethanol was found to be
the hydrogen donor of choice in terms of reaction time and product
yield.
The reusability of the catalyst was also tested in the reduction
of nitrobenzene. After the completion of reaction, the catalyst
was separated from the reaction mixture by centrifugation,
washed with acetone and dried. The recovered catalyst was reused
in the next runs under the same conditions. The results indicate
that there is no appreciable difference in the yields of product even
after a fourth run (Table 6). This observation confirms that the
nanosized NiO oxide is stable under the reaction conditions and
was not affected by the reactants. Hence, it is confirmed that the
nanosized NiO particles behave truly as a heterogeneous solid
catalyst.
Although the exact mechanism for this reaction over nanosized
NiO particles is not very clear, we think that it is similar to that of
other heterogeneous catalysts, as proposed in the literature
[47–54]. A plausible pathway for the reduction of nitrobenzene is
shown in Scheme 3. As shown in Scheme 3, catalytic reduction is
initiated by coordinating the nitro group of the substrate and the
hydroxyl group of the alcohol on the surface of the nanoparticles
at Lewis acidic sites. The alcohol molecule can be easily be con-
verted to alkoxide species in the presence of KOH base, which is
well known as an activated H-donor [47–54]. The adsorbed nitro
group is attacked by the hydride ion of the alcohol, leading to
direct hydride transfer from the alcohol to adjacent substrate
The effect of promoter (base) on the reaction was also investi-
gated. NaOH provided comparable results to KOH (Table 3, entry
2
). Ca(OH)
their lower basicity than KOH (Table 3, entries 3 and 4) while
NH and Me N did not give any product (Table 3, entries 5 and
). Hence, all further reactions were carried with the ethanol/
2 2 3
and Na CO yielded a lesser amount of aniline due to
3
3
6
KOH system.
To evaluate the scope of this method, the reduction of a variety
of nitroarenes was studied under the optimized reaction condi-
tions. The results listed in Table 4 show that nitroarenes containing
various electron donating/withdrawing groups were converted to
the corresponding amines (Scheme 2). In most cases, the corre-
sponding amines were obtained in excellent yields (>90%) within
very short reaction times of 8–16 min.
As can be seen in Table 4, electron withdrawing/donating
groups do not have a significant influence on the reaction times
and yields. In all cases, amines were found to be the only product

6
12
S. Farhadi et al. / Polyhedron 30 (2011) 606–613
Scheme 3. The plausible pathway for the reduction of nitrobenzene over the nanosized NiO catalyst.
molecules. Furthermore, the hydride transfer from the surface alk-
oxide species is promoted by the presence of KOH. Thus, the forma-
tion of aniline from nitrobenzene proceeds via the formation of
nitrosobenzene (b) and N-phenylhydroxylamine (c) as the inter-
mediates. It is worthwhile mentioning that in all reactions, acetal-
dehyde was found by GC–MS analysis, which is in consistence with
this mechanism.
According to the above mechanism, since the nitro group can
pull electrons more strongly from the aromatic ring compared to
other functional groups, it can easily be adsorbed on the catalyst
surface, leading to the amine products. This may be the reason
for the chemoselective reduction of a nitro group ahead of a car-
bonyl group.
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advantages of this method are: highly selective reduction of nitro
compounds in the presence of other reducible or hydrogenolysable
groups, ready availability and ease of operation, rapid reduction,
high yields of the substituted amines, avoidance of strong acid
media, no equipment of pressure apparatus and cost affectivity.
The present method offers an economical, safe and environmen-
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