Nano-sized La0.5Ca0.5CoO3-mediated reduction by nabh4 of aryl nitriles to bis-(benzyl) amines

Article abstract of DOI:10.4067/S0717-97072017000100005

Nano-sized La_0.5Ca_0.5C_oO₃ perovskite, which was produced via the sol-gel method, was an efficient heterogeneous catalyst in combination with NaBH4 for the rapid chemoselective reduction of aryl nitriles to bis-(benzyl)amines at 40°C in good to excellent yields. The physico-chemical properties of the catalyst were characterized by means of differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX) and particle size distributions images. The results show that nanoparticles have regular shapes with well-defined crystal faces with an average size of 30 nm.

Full text of DOI:10.4067/S0717-97072017000100005

J. Chil. Chem. Soc., 62, Nº 1 (2017)
NANO-SIZED La0.5Ca0.5CoO3-MEDIATED REDUCTION BY NaBH4 OF ARYL NITRILES TO BIS-(BENZYL)
AMINES
HOSSEIN BAVANDI,1 ALI SHIRI1*1 AND HAMAN TAVAKKOLI2
1Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, 91775-1436 Mashhad, Iran.
2Department of Chemistry, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran.
ABSTRACT
Nano-sized La0.5Ca0.5CoO3 perovskite, which was produced via the sol-gel method, was an efficient heterogeneous catalyst in combination with NaBH4 for
the rapid chemoselective reduction of aryl nitriles to bis-(benzyl)amines at 40 ºC in good to excellent yields. The physico-chemical properties of the catalyst were
characterized by means of differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy
(TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX) and particle size distributions images. The results show that nanoparticles
have regular shapes with well-defined crystal faces with an average size of 30 nm.
Keywords: perovskite, nanocatalyst, sec-amines, reduction.
INTRODUCTION
RESULTS AND DISCUSSION
The prevalence and extensive use of amines as starting materials for
plastics, agrochemicals, and dyes in industry make them one of the most
important functional groups in organic chemistry.1-2 Due to their interesting
physiological activities, secondary amines are extremely important
pharmacophores in numerous biologically active compounds.3 Among many
procedures developed to synthesize amines, the reduction of nitriles is an
attractive method because of the commercial availability of nitriles and high
atom efficiency of these reductions.4-7 The most important approaches are use
of strong hydride donors such as metal hydrides and catalytic hydrogenation.8-9
Although great advances have been made in the reduction of nitriles,
however, these methods always suffer from lack of chemoselectivity and need
complicated procedures to obtain the target compounds.10-11 Therefore, the
development of novel and mild methods to reduce nitriles chemoselectively
into their corresponding symmetrical secondary amines remains an attractive
endeavor. On the other hand, sodium borohydride (SBH) in combination with
various metal salts and oxides, such as nickel, cobalt and aluminum have
been reported to use for reducing nitriles.12-16 It is a mild, inexpensive, high
stability and invaluable reagent for applications in a wide range of reduction
processes.17 The reducing behavior of sodium borohydride undergoes a drastic
change with addition of transition metals.17-18 Moreover, perovskite type mixed
metal oxides possess unique physical and chemical properties.19 They have
had many possible applications like in electrical devices,20 in ferromagnetic
materials,21 in membranes for gas separation,22 in sensors,23 as catalysts24-25
and as promising adsorbents.26-27 Perovskites also effectively catalyse organic
reactions at very low catalyst loadings.28-30 Furthermore, these catalysts are
readily removed by filtration and lead to extremely low leaching into solution
which provides them as good alternative to the other heterogeneous catalysts.
An important feature of perovskites is their ability to reversibly adsorb and
desorb hydrogen into the crystal lattice by continuous and spontaneous changes
in transition metal oxidation state, without changing the overall bulk crystal
structure.31 In continuation of our interest in developing organic transformations
using simple reagents, catalysts and reaction media32-34 herein, we report the
direct chemoselective reduction of nitriles to symmetrical secondary amines
by NaBH4 in the presence of catalytic amount of nano-sized La0.5Ca0.5CoO3
perovskite. (Scheme 1)
Thermal characterization
The DTA curve of nano-sized La Ca CoO3 perovskite has been
shown in Fig. 1. The DTA curve has0.5the 0e.5xothermic peak at 155 °C
which is related to the decomposition of the citrate into calcium oxide.
The second exothermic peak at 250 °C is attributed to the decomposition
of the cobalt citrate into cobalt oxide.35-36 The third exothermic peak
at 380 °C is corresponds to the formation of lanthanum oxide.37-38 No
obvious change is observed above 380 °C.
Fig 1 DTA curve of the nano-sized La0.5Ca0.5CoO3 perovskite precursors
obtained by the metal nitrate polymerized complex.
Fig. 2 shows the TGA curve of the thermal decomposition process
of nano-sized La0.5Ca0.5CoO3 perovskite obtained at room temperature to
1000 °C in the heating rate of 10 ºC/min. The total weight loss of the xerogel
is approximately 77% and the decomposition process can be divided
into two distinct steps. The first weight loss occurs during the heating
º
step from 100 to 260 C (47.5%) which is due to the dehydration and
decomposition of nitrates. In this area, TGA curve shows two exothermic
º
peaks at 155 and 250 C. A weight loss of about 30% is observed from
340 ºC to around 640 ºC which is related to the oxidizing combustion of
the organic compounds such as citric acid (forming carbonate and oxide
with cation). The TGA curve also reveals a strong exothermic peak at
380 °C which is likely due to the oxidation or combustion of the chelate
complex which has been occurred along with forming the metal oxides.
No obvious change was observed above 650 ºC. Therefore, it is plausible
to conclude that the optimum calcination temperature is about 700 ºC.
Scheme 1
3330
e-mail: alishiri@um.ac.ir

J. Chil. Chem. Soc., 62, Nº 1 (2017)
Fig 2 TGA curve of the nano-sized La0.5Ca0.5CoO3 perovskite carried out at
Fig 4 TEM image of La0.5Ca0.5CoO3 perovskite nanoparticles.
room temperature to 1000 °C in air.
The nanoparticle size affects many properties of nanomaterials, and
is a major indicator of quality and performance. Fig. 5 shows the nano-
La0.5Ca CoO3 perovskite particle size distribution; this was determined
from th0e.5TEM image, based on more than 100 particles.
X-ray diffraction studies
Fig.
3 shows the X-ray diffraction pattern of nano-sized
La Ca CoO3 perovskite when the gel was calcinated at 700 ºC for 10 h.
Th0e.5dif0f.r5actograms reveal that the crystalline perovskite structure is the
main phase for the synthesized powders.
Fig. 5 Particle size distribution for calcinated nano-sized La0.5Ca0.5CoO3
perovskite catalyst.
The morphology of the La Ca0.5CoO perovskite nanoparticles
has been displayed in Fig. 6. It 0s.5hows the 3micrograph of the sample
synthesized by the sol–gel modified Pechini method and calcinates at
700 °C. Based on the SEM image, porosity of the surface is evident and
it seems that the nanoparticles have grown with uniform size. The size of
pores varied from 25 to 80 nm. It is better to be mentioned that catalytic
pores of large geometric dimensions are more suitable for application
in the adsorption process in liquid phase due to the simplicity in solid–
liquid separation and thus with much improved versatility for catalytic
reactions. The surface looks nearly fully covered with the particles
grown on it. Furthermore, it can also be seen from the SEM image that
in addition to the larger particles, the surface contains also rather smaller
particles. However, appearance of bigger particles on the surface looks
to be dominant. The aggregation of the smaller particles (in the nm
range) may result in bigger La0.5Ca0.5CoO3 perovskite nanoparticles on
the surface.
Fig 3 The XRD pattern of nano-sized La0.5Ca0.5CoO3 perovskite calcinated
at 700 °C.
As it has been shown in Fig. 3, the presence of (0 1 2), (1 1 0), (2 0 2),
(0 2 4), (1 1 6), (2 1 4) and (2 2 0) crystal planes of metallic hexagonal-
rhombohedral structure for nano-sized La0.5Ca0.5CoO3 perovskite
(JCPDS-36-1390) can be identified. No additional peak is observed in
XRD pattern of the catalyst which reveals the high purity of the prepared
perovskite nanoparticles. The crystallite sizes are calculated using XRD
peak broadening of the (110) peak using Scherer’s formula:
(Scherer’s formula)
where Dhkl (nm) is the particle size perpendicular to the normal line of
(hkl) plane, β is the full width at half maximum, θhkl (Rad) is the Bragg
angle of (hkl)hpkleak and λ (nm) is the wavelength of X-ray. The calculated
particles size of La0.5Ca0.5CoO3 perovskite nanoparticles calcinated at 700
°C is about 30 nm.
The EDX analysis was performed for further confirmation of the
obtained product composition. Fig. 7 shows EDX spectrum which
indicates the existence of La, Ca, Co and O elements in this nanopowder.
Catalytic application of La0.5Ca0.5CoO3 perovskite nanoparticles in
reduction of nitriles to secondary amines using NaBH
Initially, 4-chlorobenzonitrile as a model was 4investigated in a
reductive hydrogenation reaction using SBH catalyzed by nano-sized
La Ca0.5CoO3 perovskite in MeOH and the results were summarized in
Ta0b.5le 1.
Powder morphology
The size of nanoparticles is evaluated and confirmed by TEM.
Fig. 4 shows representative TEM image of La0.5Ca0.5CoO3 perovskite
nanoparticles. The image of the sample which is calcinated at 700 °C
confirms the particle size of about 30 nm. These values are in agreement
with the results achieved from XRD measurements. As the TEM image
shows, the morphology of nanoparticles is homogeneous.39
The reduction of 4-chlorobenzonitrile with NaBH4 using
La0.5Ca CoO3 perovskite nanoparticles as catalyst was screened in
several0s.5olvents and the results showed MeOH was the best solvent in
3331

J. Chil. Chem. Soc., 62, Nº 1 (2017)
terms of activity among the other hydroxylic ones. In EtOH, the amine
was also obtained in 65% conversion, but this reaction required 180 min
to complete (entry 2).
complete disappearance of the mentioned nitrile and appearance of
the corresponding secondary amine. Generally, excellent preparation
of secondary amines with different aryl groups were obtained under
the optimized reaction conditions, including those bearing electron-
withdrawing or electron-donating groups and the corresponding products
were obtained in relatively short reaction times with excellent yields in
all cases.
Table 1. Effect of variation of solvent, temperature and duration
of reaction and NaBH4/catalyst-loading on the yield of bis(4-
chlorobenzyl)amine (2c; R = 4-Cl-C H ) formed by the reduction of
4-chlorobenzonitrile (1c; R = 4-Cl-C66H44) by NaBH4 in the presence of
nano-sized La0.5Ca0.5CoO3 perovskite (Scheme 1).a
Substrate/
NaBH4/
Catalyst
Time/
min
Temperature
(ºC)
Entry
Solvent
Yield/%b
(mole ratio)
1
2
MeOH
EtOH
1: 20: 0.1
1: 20: 0.1
1: 20: 0.1
1: 20: 0.1
1: 20: 0.1
1: 20: 0
20
180
12 h
12 h
12 h
12 h
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
0
92
65
3
i-PrOH
t-BuOH
H2O
trace
trace
35
Fig. 6 The SEM image of La0.5Ca0.5CoO3 perovskite nanoparticles.
4
5
6
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
trace
91
7
1: 10: 0.1
1: 5: 0.1
8
30
90
9
1: 2: 0.1
40
90
10
11
12
13
14
15
16
1: 1: 0.1
180
25
53
1: 2: 0.05
1: 2: 0.025
1: 2: 0.001
1: 2: 0.0005
1: 2: 0.001
1: 2: 0.001
90
35
89
45
87
180
180
45
46
52
Fig. 7 The EDX analysis results of the La0.5Ca0.5CoO3 perovskite
nanoparticles.
40
92
17
MeOH
1: 2: 0.001
25
60
94
Table 1 also lists the results of reductive reagent optimization
experiments. To increase the yield of secondary amine and at the same
time to restrain the formation of primary amine, various amounts of
NaBH4 was used. We tried various ratio of NaBH and the results showed
that the minimum amount of substrate/NaBH4 rat4io which can be used to
result no obvious decrease of the yield of bis(4-chlorobenzyl)amine as a
secondary amine was 1:2. (entries 7-10)
a
Reaction conditions: (1c; R = 4-Cl-C6H4) (1.0 mmol) was treated under
various conditions in solvent (5 ml).
b Isolated yields.
The reduction of aromatic nitriles containing the functional groups
-F, -Cl and -Br were completed within 40-45 min and the corresponding
sec-amines were isolated in relatively high yields (entries 2-4). 4-Me-,
3-Me- and 2-Me-benzonitrile were also successfully reduced under the
optimized reaction conditions in good yields after 90-120 min (entries
5-7). The reduction of aromatic nitriles containing the electro-donating
groups -OMe, and -OEt were also performed in moderate yields (entries
8-10). However, these reactions required more time than the reduction
of nitriles bearing electron-withdrawing groups. The reduction of the
substrates containing heterocyclic aromatic compounds afforded the
corresponding sec-amines in good yields (entry 11-14). All compounds
were charaterized, as appropriate, by IR, 1H NMR and 13C NMR spectra.
A plausible mechanism of the reduction of nitriles with nano-sized
La Ca0.5CoO3-NaBH system has been suggested to involve the followed
ste0p.5s: reduction of a4nitrile to the corresponding imine following the
nucleophilic addition of it to another nitrile. The latter intermediate
is further reduces to amine as well as formation of the corresponding
imines as the result of the removing of NH3. The latter intermediate was
eventually reduced to the corresponding sec-amine due to the treatment
with reducing agent. According to the similar literature,44 it seems all
reactions were conducted on the surface of the nano catalyst (Fig. 8).
The catalytic role of the nano-sized La0.5Ca0.5CoO was clarified
by performing the model reaction in the absence of th3e nanocatalyst
(entry 6). No reaction was observed even after 12 h and only a trace
of secondary amine was obtained. Consequently, the model reaction
was performed in different molar ratios of the catalyst in the presence
of reductive reagent in order to obtain the optimum reaction conditions.
The amount of the nano catalyst (0.1-0.001 mol ratio) in the reduction of
4-chlorobenzonitrile was also efficient; allowing the complete conversion
of 4-chlorobenzonitrile into the corresponding sec-amine, but 0.001 mol
ratio was selected as the optimum amount of nano catalyst (entry 11-14).
Temperature variations also affected the yields. (entries 15-17) When
the reaction was carried out at 40 ºC, the yield is higher than the reaction
held at 60 ºC, but slightly longer reaction time was needed. So the best
choice of molar ratio of substrate /NaBH4 /La0.5Ca CoO3 perovskite
nanoparticles selected for the reduction of nitriles to 0s.5econdary amines
was 1: 2: 0.001 at 40 ºC.
To explore the scope and substrate limitations of the reaction, the
applicability of this method for the synthesis of a wide variety of diverse
secondary amines has been demonstrated in Table 2. The reactions
were carried out by mixing the various substituted nitriles with NaBH4
in methanol under normal atmosphere at 40 ºC, until TLC showed
3332

J. Chil. Chem. Soc., 62, Nº 1 (2017)
Table 2. Yields of various bis(benzyl)amines 2a-n formed by the reduction of aryl nitriles 1a-n by NaBH4 in MeOH (5 ml) in the presence of
nano-sized La0.5Ca0.5CoO3 perovskite (Scheme 1).a
Entry
1
Product No.
Time (min)
60
Product (RCH2)2NH, R =
C6H5
Yield (%)b
TOF (h-1)c
mp (ºC)
oil
mp (ºC)[lit.]d
2a
2b
2c
2d
2e
2f
85
80
92
90
82
80
75
72
70
65
92
90
85
80
850
[40]
2
40
4-F-C6H4
1200
1227
1200
547
oil
[40]
3
45
4-Cl-C6H4
oil
[40]
4
45
4-Br-C6H4
47-49
34-35
oil
46-48 [41]
34-36 [41]
[42]
5
90
4-CH3-C6H4
3-CH3-C6H4
2-CH3-C6H4
4-CH3O-C6H4
4-CH3CH2O-C6H4
3,5-di-CH3O-C6H4
pyrid-4-yl
6
90
533
7
2g
2h
2i
120
150
150
160
20
375
oil
[42]
8
300
35-36
oil
34 [41]
[40]
9
280
10
11
12
13
14
2j
244
oil
[42]
2k
2l
2760
1800
1457
533
oil
[40]
30
pyrid-3-yl
oil
[40]
2m
2n
35
pyrid-2-yl
oil
[40]
90
thiophen-2-yl
oil
[43]
a Reaction conditions: NaBH4 (2.0 mmol) was added to a stirred solution of 1a-n (1.0 mmol) in MeOH (5 ml) at 40 ºC in the presence of 1mol% nano-sized
La0.5Ca0.5CoO3 perovskite.
b Isolated yields.
c Turnover frequencies (TOFs) defined as mmoles of products reacted per mmoles of catalyst per hour.
d The products are also characterized by comparing their spectral data with literature.
One of the special features of nano-sized La0.5Ca0.5CoO3 perovskite
catalyst is its insolubility in organic solvents which makes its recovery
very convenient. Therefore, the reusability of the catalyst was examined
under the optimized reaction conditions with 4-chlorobenzonitrile as
a model reaction. For each of the repeated reactions, the catalyst was
recovered, washed with ethylacetate, consecutively and dried before
being used for the next reductive hydrogenation reaction. The catalyst
was reused four times without any significant loss of activity, the yields
of product being, respectively, 97, 95, 91 and 90%.
A literature assay for the comparison of this method with some other
previously reported ones including NaBH4 reductive hydrogenation of
benzonitriles demonstrates the efficiency of nano-sized La0.5Ca0.5CoO
perovskite catalyst for the preparation of sec-amines as shown in Table 33.
Fig. 8 A plausible mechanism for the reduction of nitiles to sec-
amines.
Table 3. A comparison of some other catalyst systems with nano-sized La0.5Ca0.5CoO3 perovskite/NaBH4 for the chemoselective reduction of aryl
nitriles 1 to bis-(benzyl)amines 2 (Scheme 1).
Entry
Reaction conditions
Temperature (ºC)
Time
20-30 h
solvent
HOAc
Yield (%)
70-95
[Lit.]
45
1
2
3
Rh (5 mol%) /HOAc /H2 (1 atm)
rt
rt
Nickel(II) (1 mmol)/ NaBH4 (3 mmol)
15 min
EtOH(dry)
MeOH
3-11
16
La0.5Ca0.5CoO3 nano particles (1 mol %)/
a
40
20-160 (min)
65-92
-
NaBH4 (2 mmol)
athis work.
º
Furthermore, this methodology avoids the formation of primary
amines which often occurs in the reduction of nitriles by catalytic
hydrogenation. The many advantages of this method include high isolated
yields, good chemoselectivity, easy work-up and also the stability of the
catalyst towards air and moisture, allowing the reaction to be carried out
in simple, readily available laboratory equipment in a protic solvent in
the open laboratory.
110 C was studied using NETZSCH-402EP thermoanalyzer at 20 to
º º
1000 C (5 C/min) within a dynamic air atmosphere. X-ray diffraction
(XRD) measurements were carried out for structural investigation of
calcinated powder at 650 ºC in the region of (2θ = 20 to 70º) using CuKα
radiation on a Rigaku D/MAX RB XRD diffractometer equipped with a
curved graphite monochromator. The microstructure of the powder was
examined using LEO 912AB TEM under a working voltage of 120 kV.
The SEM of the type LEO 1450 VP (V = 30 kV) was equipped with
an EDX spectrometer on an Inca 400 Oxford Instruments. Differential
thermal analysis (DTA) was recorded on an apparatus manufactured by
STA 503 (Germany).
EXPERIMENTAL
Thermogravimetric analysis (TGA) of the gel precursor dried at
3333

J. Chil. Chem. Soc., 62, Nº 1 (2017)
La(NO3) .9H2O, Ca(NO3)2.4H2O and Co(NO3)3.6H2O were obtained
from Merck,3Germany; citric acid was purchased from Aldrich, USA. All
the reagents were analytical grade and thus used as received. Deionized
water was used throughout the experiments.
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To a solution of La(NO3)3.6H2O (3 mmol, 1.29 g), Ca(NO )2.4H O
(3 mmol, 0.71 g) and Co(NO )2.6H O (6 mmol, 1.75 g) in d3eioniz2ed
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solution was refluxed for 2 h and then, stirring was continued at 70 ºC for
3 h. The obtained sol was placed in an oven and heated slowly at 110 ºC
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La Ca0.5CoO3 were obtained by calcinations of the powders at 700 ºC for
100h.5. The annealing of the amorphous precursor allows removing most of
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General procedure for the reductionofnitriles intotheircorresponding
sec-amines.
To a stirring mixture of the appropriate nitrile (1 mmol) and nano-
sized La0.5Ca0.5CoO3 perovskite (1 mol%, 1.9 mg) in MeOH (5 ml) at 40
ºC, NaBH4 (2 mmol, 0.08 g) was added portion wise. After the completion
of the reaction which was monitored by TLC using CHCl : MeOH (30:1)
as eluent, the catalyst was centrifuged off and the solvent3was evaporated
under reduced pressure. To the resulting oily liquid, water (20 ml) was
added and the mixture was extracted with CH Cl2 (2 × 20 ml). The organic
layer was dried over anhydrous Na2SO4 and2the solvent was evaporated
under reduced pressure to afford the pure products.
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The authors gratefully acknowledge the Research Council of
Ferdowsi University of Mashhad for financial support of this project
(3/30980).
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