Nano-sized NiLa2O4 spinel-NaBH4-mediated reduction of imines to secondary amines

Article abstract of DOI:10.1016/S1872-2067(15)60921-4

Nano-sized NiLa₂O₄ spinel was produced by thermal decomposition of Ni-La compounds via a sol-gel method. The well-crystallized spinel structure was formed after calcination at 750 °C. The physicochemical properties of the spinel were investigated using differential thermal analysis, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and particle size distribution analysis. The results show that the nanoparticles have regular shapes with well-defined crystal faces and consist of uniform quasi-spherical crystallites of average size 40 nm. The refined unit cell parameters are a = 3.861205 ? and c = 12.6793 ?. This new nano-sized NiLa₂O₄ spinel is an efficient heterogeneous catalyst for the selective conversion of imines to the corresponding secondary amines in the presence of NaBH₄ as a reducing agent, in good to excellent yields. All the reactions were completely chemoselective at room temperature and had relatively short reaction times. Secondary amines with different aryl groups, including those bearing electron-withdrawing or electron-donating groups, were obtained under the optimum reaction conditions. The catalyst was readily recovered and was recycled four times with no significant loss of catalytic activity.

Full text of DOI:10.1016/S1872-2067(15)60921-4

Chinese Journal of Catalysis 36 (2015) 1191–1196
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Nano-sized NiLa
O
2 4
spinel–NaBH -mediated reduction of imines to
4
secondary amines
a,
a
a
b
Ali Shiri *, Faezeh Soleymanpour , Hossein Eshghi , Iman Khosravi
a
Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, 91775-1436 Mashhad, Iran
Department of Chemistry, Qeshm Branch, Islamic Azad University, Qeshm, Iran
b
A R T I C L E I N F O
A B S T R A C T
Nano-sized NiLa
Article history:
O
2 4
spinel was produced by thermal decomposition of Ni–La compounds via a
Received 15 February 2015
Accepted 29 May 2015
Published 20 August 2015
sol-gel method. The well-crystallized spinel structure was formed after calcination at 750 °C. The
physicochemical properties of the spinel were investigated using differential thermal analysis, X-ray
diffraction, transmission electron microscopy, scanning electron microscopy, and particle size dis-
tribution analysis. The results show that the nanoparticles have regular shapes with well-defined
crystal faces and consist of uniform quasi-spherical crystallites of average size 40 nm. The refined
Keywords:
Imine
Reduction
Secondary amine
unit cell parameters are a = 3.861205 Å and c = 12.6793 Å. This new nano-sized NiLa
efficient heterogeneous catalyst for the selective conversion of imines to the corresponding second-
ary amines in the presence of NaBH as a reducing agent, in good to excellent yields. All the reac-
O
2 4
spinel is an
4
tions were completely chemoselective at room temperature and had relatively short reaction times.
Secondary amines with different aryl groups, including those bearing electron-withdrawing or
electron-donating groups, were obtained under the optimum reaction conditions. The catalyst was
readily recovered and was recycled four times with no significant loss of catalytic activity.
NaBH
Nano-sized spinel
NiLa
4
2
O
4
©
2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1.
Introduction
they are not always effective.
Spinel oxides play important roles in various catalytic ap-
plications, e.g., gaseous pollutant decomposition [18] and the
Imine reduction to the corresponding amine is a widely
used and important functional group transformation reaction
in synthetic organic chemistry. Many amines are used in the
pharmaceutical and agricultural industries [1–7]. Two ap-
proaches are commonly used to convert imines to amines. One
is catalytic hydrogenation, which requires high pressure. The
other is hydride reduction, which uses metal hydrides such as
water-gas-shift reaction [19]. In normal spinels, AB
2
O , A is
4
generally a divalent cation occupying tetrahedral sites, and B is
a trivalent cation occupying octahedral sites. In inverse spinels,
half of the B cations occupy tetrahedral sites, and the formula is
B(AB)O .
4
Spinels are prepared using various methods, including hy-
drothermal synthesis [20], modified oxidation processes [21],
forced hydrolysis [22], and ball-milling [23], solid-state [24],
and microemulsion methods [25]. Sol-gel methods are pre-
ferred because they are effective and convenient techniques
and use less energy and materials [26].
LiAlH
solvent in a protective inert atmosphere [9,10]; in contrast,
NaBH is an inexpensive, safe, and environmentally friendly
reducing agent. Various Lewis acids such as LiCl [11], TiCl [12],
TiCl [13], Ni B [14], I [15], ZrCl [16], and CoCl [17] have
been used to reduce C=N bonds in the presence of NaBH , but
4
or NaBH
4
[8]. LiAlH is normally used in a dry organic
4
4
4
3
2
2
4
2
4
In this study, as part of our research on organic transfor-
*
Corresponding author. Tel: +98-51-38805547; Fax: +98-51-38796416; E-mail: alishiri@um.ac.ir
DOI: 10.1016/S1872-2067(15)60921-4 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 8, August 2015

1192
Ali Shiri et al. / Chinese Journal of Catalysis 36 (2015) 1191–1196
H
nano-sized NiLa O spinel
2
4
N
R2
N
R
2
R1
R
1
NaBH , CH Cl , CH OH, r.t.
4
2
2
3
Scheme 1. Imines to the corresponding secondary amines by NaBH
and NiLa
4
2 4
O .
mations using simple reagents, catalysts, and reaction media
27], we investigated the direct reduction of imines to the cor-
responding secondary amines by NaBH in the presence of a
catalytic amount of nano-sized NiLa spinel (Scheme 1).
[
4
O
2 4
2.
Experimental
0
100 200 300 400 500 600 700 800 900 1000
o
Temperature ( C)
2
.1. Preparation of NiLa
O
2 4
powders
Fig. 1. DTA curve of NiLa
2
O gel precursor obtained using the sol-gel
4
method.
Citric acid (18 mmol, 3.50 g) was gradually added to a solu-
tion of LaCl (4 mmol, 1.48 g) and Ni(NO ·4H O (2 mmol, 0.58
g) in deionized water (10 mL), with constant magnetic stirring.
The gel was heated at 80 °C for 3 h and dried at 120 °C for 24 h.
It was then ground to a powder in an agate mortar and fired at
3
)
3 2
2
obtained using the sol-gel method. The DTA curve has a large
endothermic peak at around 100 °C, which is related to the
evaporation of water. The combustion of organic compounds
such as citric acid causes the exothermic peak at around 200 °C.
The small exothermic peak at 350 °C is attributed to the de-
composition of nitrate, which decomposes completely at
around 420 °C, giving a large exothermic peak [28]. The exo-
thermic peak at 750 °C corresponds to crystallization of the
750 °C in air for 4 h.
2.2. Characterization
Decomposition of the dried polymeric gel precursor was in-
NiLa
2
O phase. No obvious change is observed above 800 °C. It
4
vestigated using differential thermal analysis (DTA; NETZSCH,
Germany) at a heating rate of 10 °C/min in the range 25–1000
is therefore concluded that the optimum calcination tempera-
ture is 750 °C.
°C. Crystal structures were examined by X-ray diffraction (XRD;
Xpert 200, Philips, Cu K radiation) at a scanning rate of 0.02°/s
α
3.2. XRD analysis
in the 2θ range 0° to 70°. The morphologies and dimensions of
the nanoparticles were examined using transmission electron
microscopy (TEM; LEO 912AB, accelerating voltage 120 kV).
The morphologies of the calcined powders were examined
using scanning electron microscopy (SEM; LEO 1450VP, accel-
erating voltage 120 kV).
Figure 2 shows the XRD pattern of spinel-type NiLa
2
O ,
4
which was formed when the gel was calcined at 750 °C for 4 h.
No secondary phase is observed. The good agreement between
the experimental and calculated data confirms that the sample
consists of a single phase. Rietveld refinement showed that the
material had a tetragonal unit cell with the I /mmm space
4
2.3. General procedure for imine reduction to amines
group. The refined unit cell parameters were a = 3.861205 Å
and c = 12.6793 Å. The unit cell volume was 189.035 Å , which
3
NaBH
ture to a stirred mixture of the appropriate imine (0.5 mmol)
and nano-sized spinel NiLa (0.3 mol%, 6.0 mg) in CH Cl (1
4
(0.37 mmol, 0.014 g) was added at room tempera-
is in close agreement with the results reported in the literature
[29].
O
2 4
2
2
mL). Methanol (1 mL) was added slowly under constant mag-
netic stirring. On completion of the reaction, which was moni-
tored using thin-layer chromatography (TLC; n-hexane:ethyl
acetate = 8:2), the catalyst was removed by centrifugation and
the solvent was evaporated under reduced pressure. Addition
of water (10 mL) gave an oily liquid; the mixture was extracted
3.3. Powder morphology
Figure 3 shows a TEM image of the NiLa
prepared at 750 °C. It shows that nanoparticles were homoge-
neous and consisted of uniform quasi-spherical crystallites of
2
O nanoparticles
4
with CH
dried over anhydrous Na
2
Cl
2
(2 × 10 mL). The combined organic layers were
SO and the solvent was evaporated
2
4
under reduced pressure to afford the product. The sample pu-
1
rity was confirmed using TLC and H nuclear magnetic reso-
nance (NMR) spectroscopy.
3
.
Results and discussion
.1. Thermal analysis
Figure 1 shows the DTA curve of the NiLa
20
30
40
50o
/( )
60
70
80
3
2
Fig. 2. XRD pattern of NiLa
2
O nanopowder sintered at 750 °C.
4
2
O gel precursor
4

Ali Shiri et al. / Chinese Journal of Catalysis 36 (2015) 1191–1196
1193
Fig. 5. SEM image of nano-sized NiLa
O
2 4
prepared by a sol-gel method.
Fig. 3. TEM image of nano-sized NiLa
2
O spinel calcined at 750 °C.
4
boiling point, and is used in many organic transformations.
The catalytic role of the nano-sized NiLa spinel was clari-
fied by performing the model reaction in the absence of a
nanocatalyst and in the presence of the micro-NiLa spinel
catalyst (Table 2, entries 1 and 2). Neither reaction was com-
plete after 60 min. The reduction of N-(4-chlorobenzyli-
dene)aniline was then performed using different molar ratios
of the nanocatalyst to identify the optimum reaction conditions.
The results show that the minimum amount of nanocatalyst
needed to significantly affect the reduction process was 0.3
average size 40 nm. This is in agreement with the results ob-
tained using XRD [30].
The nanoparticle size affects many properties of nano-
materials and is a major indicator of quality and performance.
O
2 4
O
2 4
Figure 4 shows the nano-NiLa
2
O spinel particle size distribu-
4
tion; this was determined from the TEM image based on more
than 100 particles.
The SEM image in Fig. 5 of the NiLa
2
O nanoparticles pre-
4
pared using the sol-gel method shows the typical morphology
of the obtained powder. It shows that the nanoparticles had
regular shapes with well-defined crystal faces [31].
mol%, with 0.75 equiv. of NaBH (Table 2, entry 9). The results
4
also show that the use of 0.15 mol% of nanocatalyst gave a low
conversion because of the lack of appropriate surface active
sites at low nanocatalyst concentrations (Table 2, entry 10).
3
.4. Catalysis of reduction of imines to amines by NiLa
2
O spinel
4
nanoparticles
Table 1
The imines were synthesized by treatment of the corre-
sponding carbonyl compounds with the appropriate amines,
using a previously reported method [32,33]. N-(4-chloroben-
zylidene)aniline was used as a model compound for optimiza-
tion of the catalytic conditions for reductive hydrogenation.
Various solvents were screened for the reduction of
Solvent optimization for chemoselective reduction of N-(4-chloroben-
zylidene)aniline to N-(4-chlorobenzyl)aniline.
a
Solvent
DMF
THF
Time (min)
Conversion (%)
30
30
5
90
88
97
96
85
CH
CHCl
n-Hexane
Cl
2 2
N-(4-chlorobenzylidene)aniline with NaBH
4
using NiLa O
2 4
spi-
3
5
nel nanoparticles as a catalyst (Table 1). The results show that
5
CH
2
Cl
Cl
2
2
was the best solvent in terms of activity. The polarity of
is appropriate for dissolving the reactant, and appropri-
Reaction conditions: N-(4-chlorobenzylidene)aniline 1 mmol, nano-
sized NiLa O
2 4
spinel 1 mg, solvent 2 mL, NaBH 4 eq., room tempera-
4
CH
2
ture.
ate interactions between the hydrogenated surface of the
nanocatalyst and the substrate are triggered. Furthermore,
a
1
Based on H NMR spectroscopic integrations.
CH
2
Cl is a solvent with a low moisture content, toxicity, and
2
Table 2
Data for reduction of N-(4-chlorobenzylidene)aniline to N-(4-chloro-
benzyl)aniline.
4
3
3
2
2
1
1
0
5
0
5
0
5
0
5
0
Conversiona
Nano-sized NiLa
2
O
4
NaBH
(eq.) (min) (°C)
4
4
4
2
1
4
Time
t
Entry
(mol%)
(%)
40
63
95
96
98
97
50
97
95
50
73
96
1
2
3
4
5
6
7
8
9
None
60
60
10
10
10
10
10
10
10
60
35
6
25
25
25
25
25
25
25
25
25
25
0
Micro-NiLa O
2 4
(1 mol %)
1
1
1
1
1
0.6
0.3
0.15
0.3
0.75
0.50
0.75
0.75
0.75
0.75
0.75
20-30
30-40
40-50
Particle diameter (nm)
50-60
1
1
0
1
Fig. 4. Particle size distribution for calcined nano-sized NiLa
2
O
4
spinel
12
a
0.3
40
catalyst.
According to TLC monitoring and H NMR spectroscopic integration.
1

1194
Ali Shiri et al. / Chinese Journal of Catalysis 36 (2015) 1191–1196
Table 3
Substrates tested in reduction of imines to amines with NaBH
4
in the presence of nano-sized NiLa
O .
2 4
a
b
Yield a TOF
b
Time
Yield TOF
Time
(min)
Entry
1
Imine
Sec-amine
Entry
Imine
Sec-amine
−1
−1
(min)
(%) (h )
(%) (h )
H
N
H
N
H
N
N
H
N
Ph
Ph
10
98 1963 10
90 1202 11
N
N
Ph
Ph
15
2
Ph
Ph
92 1228
H
N
N
Ph
Ph
Ph
N
H
2
3
15
15
92 9212
88 8812
O
O
H
N
N
O
O
Ph
97 1295 12
97 971 13
2
5
H
N
N
O
Ph
Ph
O
OH
OH
H
N
N
N
Ph
Ph
Ph
H
4
20
85 3405
N
N
Ph
Ph
H
N
Ph
Cl
Cl
5
6
5
2
90 3605 14
95 9513 15
15
5
H
95 1268
97 3885
N
N
Ph
Ph
F
F
H
N
N
Ph
Ph
Ph
Cl
Cl
H
N
Ph
N
H
7
2
98 9813 16
N
Ph
15
N
Ph
91 1215
86 1155
Br
Br
H
N
N
N
Ph
Ph
Ph
Ph
H
8
9
5
92 3685 17
86 1722
N
Ph 15
N
Ph
H
N
10
Reaction conditions: Substrate 0.5 mmol, NiLa
2
O
4
spinel nanoparticles 6 mg, CH
2
Cl
2
1 mL, MeOH 1 mL, NaBH 0.75 eq., room temperature.
4
a
1
Determined by H NMR spectroscopy.
b
Turnover frequency defined as mmoles of products obtained per mmoles of catalyst per hour.
Table 3 shows the general applicability of the current
method in the synthesis of a wide variety of secondary amines.
The reactions were performed by mixing the synthesized
of unsaturated double bonds, particularly on the nanoscale
[34−36]. A suggested mechanism for the reduction of imines to
their corresponding secondary amines over the nano-sized
imines with NaBH
4
in methanol in an ambient atmosphere until
NiLa
schematically in Scheme 2.
The nano-sized NiLa
2
O
4
spinel catalyst in the presence of NaBH
4
is shown
TLC showed the complete disappearance of the imine and for-
mation of the corresponding secondary amine. In all cases, ex-
cellent yields of secondary amines with different aryl groups,
including those bearing electron-withdrawing or electron-do-
nating groups, were obtained in short reaction times under the
optimum reaction conditions.
The mechanism of heterogeneous hydrogenation is unclear,
partly because the reactive sites on the metal surface cannot be
as precisely described as can small molecule reagents in solu-
tion. However, it is probable that hydrogen is adsorbed onto
the surface of the catalyst, forming metal–hydrogen bonds; this
type of intermediate has been implicated in the hydrogenation
2 4
O
spinel catalyst is insoluble in or-
ganic solvents, therefore it can easily be recovered. The catalyst
reusability was examined under the optimum reaction condi-
tions, using N-(4-chlorobenzylidene)aniline reduction as a
model reaction. In each cycle, the catalyst was recovered,
washed with ethyl acetate, and dried before use in the next
reaction. It is important to note that the catalyst was reused
four times without significant loss of activity and selectivity, as
shown in Fig. 6.
The SEM and TEM images of the nano-sized NiLa
after the fourth cycle are shown in Fig. 7. The surface rough-
2
O spinel
4

Ali Shiri et al. / Chinese Journal of Catalysis 36 (2015) 1191–1196
1195
4. Conclusions
H
H
NiLa2O4
40 nm)
We developed a novel, highly efficient, and easy protocol for
the synthesis of secondary amines from the corresponding
imines in the presence of NaBH . The procedure has several
(
R'HC NR
H2 (not adsorbed)
4
advantages over other methods, including improved yields,
cleaner reactions, a simple experimental procedure, and the
use of easily prepared and inexpensive starting materials. This
approach is therefore a useful and attractive strategy in syn-
thetic organic chemistry.
MeOH
R'H2C NHR
NaBH4
NaB(OMe)4
H2O
+
H2
NiLa2O4
40 nm)
(
NaBO2
+
4MeOH
Scheme 2. Proposed mechanism for the reduction of imines to second- Acknowledgments
ary amines.
The authors gratefully acknowledge Ferdowsi University of
Mashhad and Qeshm Branch, Islamic Azad University for par-
tial support of this project (Grant No. 3/28973).
1
00
8
6
4
2
0
0
0
0
0
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Catalyst
BO
Amount of catalyst (eq.)
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(eq.)
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3
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r.t.
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1196
Ali Shiri et al. / Chinese Journal of Catalysis 36 (2015) 1191–1196
Graphical Abstract
Chin. J. Catal., 2015, 36: 1191–1196 doi: 10.1016/S1872-2067(15)60921-4
Nano-sized NiLa spinel–NaBH -mediated reduction of imines to
O
2 4
4
secondary amines
NR'
NHR'
Ali Shiri *, Faezeh Soleymanpour, Hossein Eshghi, Iman Khosravi
Ferdowsi University of Mashhad, Iran; Islamic Azad University, Iran
R
R
NaBH4
The well-crystallized nano-sized NiLa
applies as an efficient heterogeneous catalyst for selective conversion of
imines to the corresponding secondary amines in the presence of NaBH
as reducing agent.
2
O spinel structure is formed and
4
8
5%-98%
2 4
nano-sized NiLa O spinel
4
2-20 min
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