Fe3O4-SiO2-P4VP pH-sensitive microgel for immobilization of nickel nanoparticles: An efficient heterogeneous catalyst for nitrile reduction in water

Article abstract of DOI:10.1002/cctc.201300984

Fe₃O₄ magnetic nanoparticles (MNPs) were modified with (3-aminopropyl)triethoxysilane through silanization. An atom transfer radical polymerization-initiating site immobilized onto amine-functionalized Fe₃O₄ MNPs. The surface-initiated atom transfer radical polymerization of 4-vinylpyridine was then performed in the presence of Fe ₃O₄-SiO₂-Br nanoparticles, which led to the formation of Fe₃O₄-SiO₂-P4VP [P4VP=poly(4-vinylpyridine)] hybrid microgels cross-linked with Fe ₃O₄ MNPs. Our approach uses polymer microgels as templates for the synthesis of nickel nanoparticles (NiNPs). The tunable properties of synthesized NiNPs@Fe₃O₄-SiO₂-P4VP pH-sensitive microgels were used in the catalytic reduction of aliphatic and aromatic nitriles. Moreover, the catalytic activity of metal nanocomposites that can be modulated by the volume transition of microgel structures with changing pH has been evaluated. TEM, X-ray photoelectron spectroscopy, thermogravimetric analysis, atomic absorption spectroscopy, XRD, UV/Vis spectroscopy, and FTIR spectroscopy were used to characterize the resultant catalyst. Mystery solved: Our approach uses polymer microgels as templates for the synthesis of nickel nanoparticles. The tunable properties of synthesized NiNPs@Fe₃O ₄-SiO₂-P4VP [NiNPs=nickel nanoparticles; P4VP=poly(4-vinylpyridine)] pH-sensitive microgels are used in the catalytic reduction of aliphatic and aromatic nitriles. Moreover, the catalytic activity of metal nanocomposites that can be modulated by the volume transition of microgel structures with changing pH has been evaluated. Copyright

Full text of DOI:10.1002/cctc.201300984

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DOI: 10.1002/cctc.201300984
Fe O –SiO –P4VP pH-Sensitive Microgel for Immobilization
3
4
2
of Nickel Nanoparticles: An Efficient Heterogeneous
Catalyst for Nitrile Reduction in Water
[
a]
Mohammad Reza Nabid,* Yasamin Bide, and Mahvash Niknezhad
Fe O magnetic nanoparticles (MNPs) were modified with (3-
synthesized NiNPs@Fe O –SiO –P4VP pH-sensitive microgels
3 4 2
3
4
aminopropyl)triethoxysilane through silanization. An atom
transfer radical polymerization-initiating site immobilized onto
amine-functionalized Fe O MNPs. The surface-initiated atom
were used in the catalytic reduction of aliphatic and aromatic
nitriles. Moreover, the catalytic activity of metal nanocompo-
sites that can be modulated by the volume transition of micro-
gel structures with changing pH has been evaluated. TEM, X-
ray photoelectron spectroscopy, thermogravimetric analysis,
atomic absorption spectroscopy, XRD, UV/Vis spectroscopy,
and FTIR spectroscopy were used to characterize the resultant
catalyst.
3
4
transfer radical polymerization of 4-vinylpyridine was then per-
formed in the presence of Fe O –SiO –Br nanoparticles, which
3
4
2
led to the formation of Fe O –SiO –P4VP [P4VP=poly(4-vinyl-
3
4
2
pyridine)] hybrid microgels cross-linked with Fe O MNPs. Our
3
4
approach uses polymer microgels as templates for the synthe-
sis of nickel nanoparticles (NiNPs). The tunable properties of
Introduction
Amines are important in various industries, such as plastics,
dyes, agrochemicals, surfactants, and auxiliaries in the rubber
and paper industries, and as anticorrosion agents, organic
building blocks, and intermediates for material science. Owing
to their interesting properties, amines are important function-
the formation of colloidal materials upon workup and its high
sensitivity to water. NaBH can be efficient in the presence of
4
[21]
[22]
[23]
[24]
[25,26]
a Lewis acid
such as LiCl,
TiCl4,
TiCl3,
or ZrCl4.
Moreover, the high hydrogen content, safe handling, environ-
mentally benign properties, and stability of boron hydrides
[
1–8]
[27]
alities in academic and industrial research.
For example, the
have increased the attention to these compounds. The use
worldwide production of aliphatic amines in 2000 was several
of a suitable transition metal as a heterogeneous catalyst can
[
9]
lakh tons per annum. Moreover, the growing use of biologi-
cally relevant nitrogenous molecules has necessitated the de-
velopment of new and efficient methods to prepare
increase the reducing power of NaBH and lead to the devel-
4
opment of mild, more selective, and practical methods for the
[28–34]
reduction of the CꢀN multiple bonds to amino groups.
Nanosized catalysts can efficiently improve heterogeneous
[
10–13]
amines.
Of many methods developed to synthesize
[35–40]
amines, the most general and often benign processes involve
catalytic reactions in organic synthesis.
In the reduction re-
[
14–18]
catalytic reductions.
The reduction of nitriles is an attrac-
actions, the first-row transition metals are much more attrac-
tive as catalysts than the precious metals, owing to their less
cost and abundance in the earth. Nickel-based nanosized cata-
lysts have been used for the reduction of aldehydes and ke-
tive method, owing to the commercial availability of nitriles
and the high atom efficiency of these reductions. Reported
methods for the reduction of CꢀN multiple bonds include cata-
[
41]
[42,43]
lytic hydrogenation with LiAlH4 and NaBH . However, these
tones, for the hydrogenation of olefins
and as supports
4
[44]
methods are not general and have some drawbacks that con-
for hydrogen adsorption, and so on.
fine them to reducing only some of these CꢀN multiple bond
However, to prevent the aggregation of metal nanoparticles
in a solution, they must be stabilized in suitable carrier sys-
[
19,20]
compounds.
For example, often a mixture of primary, sec-
[45,46]
[47]
ondary, and tertiary amines is obtained through the catalytic
tems. Dendrimers,
block copolymer micelles, microgels
[48–51]
[52,53]
hydrogenation of CꢀN multiple bonds. Some disadvantages of
(MGs),
and latex particles
are used as “nanoreactors”
using LiAlH are decreased selectivity and low yields, owing to
in which the metal nanoparticles can be immobilized and used
for the desired purpose. Of these systems, MGs have several
important advantages, such as stability, straightforward synthe-
sis, and facile functionalization to provide stimuli-responsive
behavior (e.g., change in volume in response to a change in
4
[a] Dr. M. R. Nabid, Y. Bide, M. Niknezhad
Faculty of Chemistry
Department of Polymer
Shahid Beheshti University
G.C., P.O. Box 1983963113, Tehran (Iran)
Fax: (+98)21-22431661
[49]
temperature, pH, or ionic strength).
For typical types of organic–inorganic hybrid MGs, the easi-
est synthesis approach that does not need any conventional
chemical cross-linker is in situ formation of hybrid MGs via co-
valent cross-linking with functional inorganic nanoparticles
E-mail: m-nabid@sbu.ac.ir
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300984.
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that actually act as chemical
cross-linkers. At present, only
a few reports exist about this
type of organic–inorganic hybrid
MGs. Magnetic nanoparticles
(MNPs) have the added advant-
age of being magnetically sepa-
rable and thus eliminated the re-
quirement of the catalyst filtra-
tion after the reaction comple-
tion.
Because of our interest in the
use of catalysts in the develop-
ment of useful new synthesis
[
54–57]
methodologies
as well as
the general importance of this
issue, we present the synthesis
and characterization of nickel
nanoparticles immobilized on
Fe O –SiO –P4VP [P4VP=poly(4-
3
4
2
vinylpyridine)] pH-sensitive MGs
NiNPs@Fe O –SiO –P4VP MGs)
(
3
4
2
and their use as heterogeneous
catalysts for the reduction of ali-
phatic and aromatic nitriles to
form primary amines with
a
broad
substrate
scope
(Scheme 1).
Results and Discussion
Synthesis and characterization
of the supported catalyst
Scheme 2. Synthesis of Fe O –SiO –P4VP MGs (N,N,N’,N’,N’’-pentamethyldiethylenetriamine=PMDETA).
3
4
2
The synthesis method of Fe O –
3
4
SiO –P4VP
organic–inorganic
2
[59]
hybrid MGs consists of a “grafting from” strategy via the atom
transfer radical polymerization (ATRP) reaction, which involves
the following steps (Scheme 2): 1) MNPs of 8–12 nm were syn-
thesized with the method described by Sahu et al., which in-
process. 3) The surface silanation of these silica-coated nano-
particles with (3-aminopropyl)triethoxysilane under reflux con-
ꢀ
1
ditions in toluene for 24 h gave 0.37 mmolg of amino
groups (determined by using back titration). 4) 2-Bromoisobu-
tyryl bromide (BIBB), an ATRP-initiating site, immobilized onto
2
+
3+
volves coprecipitation of Fe and Fe with the ammonium
[
58]
hydroxide solution. 2) Owing to the aggregation of nanopar-
ticles and solubility problems in organic media, our initial at-
tempts to synthesize MGs, directly on the MNPs, failed. Coating
MNPs with silica could solve these problems. For this purpose,
after coating with polyvinylpyrrolidone (PVP), MNPs were sus-
pended in 2-propanol and mixed with tetraethoxysilane to
create a silica shell under basic conditions by using a sol–gel
amine-functionalized magnetic Fe O4 MNPs by nucleophilic
3
attack of the amino group to acyl bromide of BIBB. 5) Fe O –
3
4
SiO –P4VP MGs were synthesized with Fe O –SiO –Br nanopar-
2
3
4
2
ticles as cross-linkers and 4VP as the monomer through the
ATRP reaction (Scheme 2). The living radical polymerization en-
ables the coating of the polymer on the Fe O surface in a con-
3
4
trolled fashion, which results in good dispersibility of the parti-
cles in a suitable medium. The synthesized polymeric network
involving Fe O MNPs as cross-linking points is formed through
3
4
interparticle termination reactions of the growing free radicals
situated on neighboring Fe O MNPs (Scheme 3).
3
4
2
+
Then, Ni ions were introduced into Fe O –SiO –P4VP MGs
3
4
2
and subsequently reduced with sodium borohydride to obtain
NiNPs@Fe O –SiO –P4VP MGs (Scheme 4).
3
4
2
The FTIR spectra of Fe O MNPs, PVP-coated Fe O MNPs,
Scheme 1. NiNPs@Fe
3
O
4
–SiO
2
–P4VP MGs as the catalyst system for the
3
4
3
4
reduction of nitriles.
silica-coated Fe O MNPs, Fe O –SiO –NH , and Fe O –SiO –Br
3 4 3 4 2 2 3 4 2
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Scheme 3. Schematic of interparticle termination giving a cross-linked network.
are shown in Figure S1a–e, respectively. Moreover, Fe O –SiO –
spheres, which were Fe O MNPs (Figure 2a and b). The TEM
3
4
2
3
4
P4VP MGs were characterized by using FTIR spectroscopy
images confirmed that Fe O MNPs act as cross-linkers. BIBB-
3 4
ꢀ
1.
(
Figure 1). The pyridine ring signals appeared at 1661 cm In
modified Fe O MNPs are like chemical cross-linkers with multi-
3 4
ꢀ
1
addition, the peak at 1220 cm is assigned to the aromatic
amine of the pyridyl rings. Moreover, the characteristic signals
of pyridyl rings, such as out-of-plane deformation vibrations of
ꢀ
1
4
-monoalkyl pyridine (818 cm ) and hydrogen bending of 4-
ꢀ
1
monosubstituted pyridine (1024 cm ), were also apparent,
which indicated the existence of P4VP. Thus, the FTIR spectra
confirmed the formation of Fe O –SiO –P4VP MGs.
3
4
2
The TEM observation indicates that Fe O –SiO –P4VP MGs
3
4
2
and NiNPs@Fe O –SiO –P4VP MGs contained many small black
3
4
2
3 4 2
Figure 2. TEM images of NiNPs@Fe O –SiO –P4VP MGs.
ple functional groups, which led to the successful formation of
hybrid MGs. Furthermore, the TEM images of NiNPs@Fe O –
3
4
SiO –P4VP MGs (Figure 2b) show that nickel nanoparticles
2
were uniformly dispersed on Fe O –SiO –P4VP MGs, in which
3
4
2
the main average size of nickel nanoparticles is approximately
–4 nm. This observation is attributable to the existence of
2
a large number of nitrogen-containing pyridine groups as an-
choring sites on the Fe O surface.
3 4 2
Figure 1. FTIR spectrum of Fe O –SiO –P4VP MGs.
3
4
The UV/Vis spectra of Fe O –
3
4
SiO2, Fe O –SiO –NH , Fe O –
3
4
2
2
3
4
SiO –P4VP MGs, and NiNPs@-
2
Fe O –SiO –P4VP MGs are shown
3
4
2
in Figure 3a–d, respectively. In
the spectra of Fe O –SiO and
3
4
2
Fe O –SiO –NH , a broad feature-
3
4
2
2
less peak can be observed at
a wavelength of approximately
390 nm, which confirms the
core–shell Fe O –SiO MNPs and
3
4
2
may result from the changes in
band gap caused by the quan-
tum size effect and the surface
[60,61]
effect of nanostructures
and
the FeꢀOꢀSi bonds of the core–
[62]
shell nanoparticles. The UV/Vis
spectrum of Fe O –SiO –P4VP
3
4
2
MGs demonstrates a broad ab-
sorption band at approximately
257 nm. This band was assigned
Scheme 4. Synthesis of NiNPs@Fe
3
O
4
–SiO
2
–P4VP MGs.
to pyridine ring absorption, and
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Figure 3. UV/Vis spectra of a) Fe
c) Fe –SiO –P4VP MGs (yellow), and d) NiNPs@Fe
green).
3
O
4
–SiO
2
(red), b) Fe
3
O
4
–SiO
2 2
–NH (blue),
3
O
4
2
3
O
4
–SiO –P4VP MGs
2
(
its structured feature can be attributed to both p!p* and
n!p* transitions that are located in the same spectral region.
The spectrum of NiNPs@Fe O –SiO –P4VP MGs shows absorp-
3 4 2
Figure 4. XPS spectrum of NiNPs@Fe O –SiO –P4VP MGs.
3
4
2
tion bands at 211 nm and a shoulder at 310 nm, which is prob-
ably due to the effect of quantum size for this particle size (2–
4
nm). These results are in agreement with the values reported
[
63]
by the work of Creighton and Eadon, in which particles with
a size of 10 nm show a band near 210 nm. Curtis et al. found
that the absorption of the bulk nickel presents a band cen-
[
64]
tered near 230 nm.
Thermogravimetric analysis (TGA) was performed for the
quantitative analysis of functionalized Fe O MNPs. Before TGA
3
4
measurements, freely adsorbed P4VP molecules were removed
through Soxhlet extraction with boiling methanol for 36 h. The
TGA curves of silica-coated magnetite nanoparticles, Fe O –
3
4
SiO –P4VP MGs, and NiNPs@Fe O –SiO –P4VP MGs are shown
2
3
4
2
in Figure S2A–C. According to the TGA results, the content of
nickel in the synthesized catalyst is approximately 12 wt%,
which is in good agreement with the atomic absorption spec-
troscopy data.
The XRD patterns of Fe O MNPs, Fe O –SiO , and Fe O –
3
4
3
4
2
3
4
SiO –P4VP MGs are shown in Figure S3, which indicate that the
2
crystal structure of Fe O during the synthesis of the magnetic
3
4
3 4 2
Figure 5. XPS spectrum of the Ni2p core level region for NiNPs@Fe O –SiO –
MG is well maintained.
P4VP MGs.
X-ray photoelectron spectroscopy (XPS) is a powerful tool
for the investigation of hybrid materials. The XPS spectrum of
NiNPs@Fe O –SiO –P4VP MGs (Figure 4) demonstrates the pho-
[
66]
nickel (852.3 and 869.7 eV, respectively), which can be attrib-
3
4
2
[67]
toelectron lines at binding energies of approximately 285, 531,
and 711 eV, which are attributed to C1s, O1s, and Fe2p, re-
spectively. In contrast to the g-Fe O XPS spectrum, it does not
uted to the matrix effect. XPS has been used to determine
the oxidation states of nickel in the catalyst sample. The two
peaks with lower intensities at 868 and 885 eV are related to
2
3
2
+
contain the charge transfer satellite of Fe2p3/2 at 720 eV, which
Ni species that could have been formed during the XPS
[
65]
0
confirms that the oxide in the sample was Fe O . The nitro-
sample preparation because the stabilized Ni nanoclusters are
3
4
gen (N 1s) signal is attributed to P4VP and PVP produced
during the synthesis of NiNPs@Fe O –SiO –P4VP MGs. The
sensitive to aerobic atmosphere.
3
4
2
peak at 103 eV is assigned to Si2p, which indicates that SiO is
2
The catalytic activity and selectivity of NiNPs@Fe O –SiO –
P4VP MGs toward the reduction of nitriles
3
4
2
deposited on the Fe O surface.
3
4
The spectrum of the Ni2p core level of NiNPs@Fe O –SiO –
3
4
2
P4VP MGs is shown in Figure 5, which demonstrates two
With nickel nanoparticles immobilized on Fe O –SiO –P4VP
3
4
2
0
0
bands at 863 and 881 eV assigned to Ni 2p3 and Ni 2p1/2, re-
MGs in hand, we attempted to evaluate their catalytic activity
for the reduction of nitriles. The reduction of benzonitrile was
performed initially as a model reaction. To optimize the
amount of the catalyst, the model reaction was performed
/2
spectively. The relative intensities of the 2p3/2 and 2p1/2 peaks
are approximately 1:2. The 2p3/2 and 2p1/2 binding energies are
shifted to the higher values relative to the values for the bulk
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with various amounts of the catalyst in water at room temper-
ature. Within 2 h and with 1 mol% of nickel, 88% isolated
yield of benzyl amine was obtained (Table 1, entry 4). With 0.4
and 0.7 mol% of nickel, a longer time is required for the reac-
tion to complete even under thermal conditions (808C) and it
3 4
Table 3. Effect of pH on the reduction of benzonitrile with NiNPs@Fe O –
[a]
2
SiO –P4VP MGs as the catalyst system.
Entry
pH
Yield [%]
1
2
3
4
5
4.5
5.5
6.5
7.5
8.5
31
88
22
20
17
Table 1. Effect of the amount of the catalyst on the reduction of benzo-
[
a]
nitrile.
[
a] Reaction conditions: 1 mmol of benzonitrile, 1 mmol of NaBH
4
, 5 mL
Entry
Amount of the catalyst
t
[h]
T
[8C]
Isolated yield
[%]
of water, 258C, NiNPs@Fe –SiO –P4VP MGs as the catalyst system, 2 h.
3
O
4
2
0
(
Ni content) [mol%]
1
2
3
4
5
0.4
0.7
0.7
1
3.5
3
2.2
2
80
25
80
25
25
46
68
77
88
89
1.1
2
[a] Reaction conditions: 1 mmol of benzonitrile, 1 mmol of NaBH
4
, 5 mL
of water, NiNPs@Fe
adjusted to 5.5.
3 4 2
O –SiO –P4VP MGs as the catalyst system, and pH
causes the reaction to occur with decreased yields (Table 1, en-
tries 1–3). In contrast, increasing the amount of the catalyst to
1.1 mol% did not affect the yield considerably (Table 1,
entry 5). In the next step, the effect of the solvent on the reac-
tion was investigated (Table 2). The yield in water is significant-
Figure 6. Fe
catalyst support.
3 4 2
O –SiO –P4VP MGs as both the reaction medium and the metal
[
a]
Table 2. Effect of different solvents on the reduction of benzonitrile.
slows down, which is the result of the marked shrinking of the
network. Thus, the rate of the reaction catalyzed by the nano-
particles decreases considerably. In this way, the network acts
as a nanoreactor that can be opened or closed to a certain
extent. Thus, the synthesized pH-sensitive MG system enables
us to modulate the catalytic activity of MG-stabilized nanopar-
ticles through a thermodynamic transition that takes place
within the carrier system. Consequently, the best system for
the reduction of bezonitrile is water as a solvent at room tem-
perature with 1 mol% of nickel at pH 5.5. Thus, the results indi-
cate that at pH 4.5 and lower pH values, most of the pyridine
groups are protonated and the MG swells whereas at pH 6.5
and higher pH values, owing to the hydrophobic nature of pyr-
idine groups, the hybrid MG deswells. At pH 5.5, pyridine
groups that orient along outer regions of the MG are protonat-
ed to have interaction with water molecules in an aqueous
medium; however, pyridine groups in the inner regions of the
MG are not protonated, although partial swelling occurs and
some water diffuses into the inner region. Partial swelling per-
mits the MG to accept the substrates to the inner hydrophobic
part, whereas in the deswelled form (pH >5.5), the diffusion of
substrates to the inner parts decreases owing to steric interac-
tions. Thus, in the catalytic reaction in an aqueous medium,
the organic substrates were ejected from the water medium
and the hydrophilic outer region of the MG to concentrate in
the inner hydrophobic regions, in which the metal catalysts
exist and therefore an efficient catalytic reaction occurs.
Entry
Solvent
T
Isolated
yield [%]
[h]
1
2
3
4
5
6
acetonitrile
DMF
THF
toluene
water
4
4
5
5
2
2
40
51
38
28
88
62
N-methylpyrrolidone
[a] Reaction conditions: 1 mmol of benzonitrile, 1 mmol of NaBH
4
, 5 mL
of the solvent, 258C, NiNPs@Fe O –SiO –P4VP MGs as the catalyst system,
3 4 2
and pH adjusted to 5.5.
ly higher than other solvents, owing to the hydrophobic
nature of P4VP, which concentrates the substrates in its inner
sites in which the nickel nanoparticles exists. Thus, the catalyst
and substrates collect in the same sites and an efficient catalyt-
ic reaction occurs. Then, to investigate more about the catalyt-
ic event and to study the effect of pH, we used the reduction
of benzonitrile in different pH values (Table 3 and Figure 6). A
buffer solution of (0.1m) KH PO /K HPO was used to maintain
the pH values stable. The pH of the buffer solution was adjust-
ed with HCl and NaOH at a certain amount. The results indicat-
ed that the reaction occurred efficiently at pH 5.5. However, at
higher pH, the catalyst efficiency decreased significantly. At
low pH, metal nanoparticles embedded in such a network are
completely reachable by the reactants. However, above the
transition, the diffusion of the reactants within the network
2
4
2
4
To prove the efficiency of the catalyst, we studied the reduc-
tion reactions of different aliphatic and aromatic nitriles
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1
(
Table 4); the H NMR spectra of
[
a]
Table 4. Reduction of nitriles with NiNPs@Fe
3
O
4
–SiO
2
–P4VP MGs as the catalyst system.
the products are shown in Fig-
ures S4–S20. Reductions of ben-
zonitriles with one or more elec-
tron-withdrawing groups on the
aromatic ring are generally faster
and result in higher yields
[
b]
Entry Nitrile
Amine
T
t
Conversion Isolated yield Selectivity
[
8C] [h] [%]
[%]
88
90
89
99
[%]
98
1
2
3
4
25
2
1
1
2
90
91
100
100
25
98
97
(Table 4, entries 4–11). For exam-
91
ple, 2,4-dicholorobenzonitrile
and 2-chloro-4-bromobenzoni-
trile were reduced to 2,4-dicho-
lorobenzylamine and 2-chloro-4-
bromobenzylamine in 99% yield
>99
>99
5
6
25
25
2
2
93
97
90
94
96
97
(
Table 4, entries 9 and 10). We
found that benzonitriles with
sterically crowded nitrile
Table 4, entry 11) or electron-do-
a
7
25
25
25
25
2
2
2
2
94
93
93
93
99
99
98
>99
>99
>99
(
nating groups (Table 4, entries 2
and 3) required higher tempera-
tures for the complete reduction.
For example, 2,6-dichlorobenzo-
nitrile was reduced to the corre-
sponding amine in 88% yield
after 1 h of refluxing in water
8
9
>99
>99
10
(Table 4, entry 11). The lower
yield and need for increased
temperature is attributed to the
steric hindrance around the re-
duction site. In addition, 4-me-
thoxybenzonitrile is reduced to
the corresponding 4-methoxy
benzylamine in 89% yield after
1
1
100
1
88
88
>99
1
2
100 18
100 12
90
95
87
92
96
97
13
14
1
h reflux in water. The reduction
of 4-cyanobenzaldehyde lead to
4-(aminomethyl) phenyl]metha-
100 12
89
87
98
[
1
1
5
6
100
100
8
7
98
89
97
86
99
96
nol in 90% yield, in which both
the nitrile and the aldehyde
groups were reduced (Table 4,
entry 5). Thus, the selective re-
duction of a nitrile in the pres-
ence of an aldehyde group was
not successful. Aliphatic nitriles
can also be reduced with
[c]
1
1
1
7
8
9
25
25
25
3
2
2
99
79
87
99
74
83
>99
95
[
[
d]
e]
96
NiNPs@Fe O –SiO –P4VP MGs as
3
4
2
[a] Reaction conditions: 1 mmol of nitrile, 1 mmol of NaBH
4
, 5 mL of water, NiNPs@Fe
(1 mol% of Ni ) as the catalyst system, and pH adjusted to 5.5; [b] Calculated selectivity; [c] 1 mmol of dinitrile
and 2 mmol of NaBH ; [d] Alternate conditions: H was used instead of NaBH , 0.4 MPa; [e] Alternate condi-
tions: H was used instead of NaBH , 0.6 MPa.
3 4 2
O –SiO –P4VP MGs
0
the catalyst system. Notably,
many of the current systems
cannot reduce aliphatic nitriles
even under extended reflux con-
4
2
4
2
4
[
68–70]
ditions,
owing to the depro-
tonation of the hydrogen a to the nitrile with these reagents
and thus halting the reduction. However, our catalytic system
does not suffer from this problem and several aliphatic nitriles
were reduced to their analogous amines in refluxing water in
excellent yields (Table 4, entries 12–15). Benzyl cyanides are
hard nitriles to reduce, owing to the extreme acidity of the
protons a to the nitrile and consequently deprotonation in-
stead of reduction with most reducing agents. In contrast, the
presented catalyst reduces several examined benzyl cyanides
to their corresponding phenethylamines in excellent yields.
The reduction of benzyl cyanides is an important issue be-
cause it enables the facile preparation of various biologically
active compounds from similar benzyl nitriles. An aliphatic ni-
trile that contains an alkyne not conjugated with the nitrile
group was tested and showed that the nitrile group can be re-
duced selectively without affecting the CꢀC multiple bond
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(Table 4, entry 16). Terephthalonitrile as a dinitrile was also ex-
amined, which gave 1,4-phenylenedimethanamine with excel-
lent yield (Table 4, entry 17).
In addition, in our catalytic system, the special structure of
the supported catalyst that helps determine the surface of the
catalyst is the reason for the high selectivity of this system.
Steric effects avoid the formation of secondary and tertiary
amines. Similarly, the high retention of the triple bonds can be
attributed to steric effects and the preference that these ef-
fects have for the absorption of the nitrile over that of the
triple bonds. Moreover, with respect to the general applicabili-
ty of the NiNPs@Fe O –SiO –P4VP catalyst, we tested the hy-
3
4
2
Figure 7. Effect of recycling on the catalytic activity and productivity of
drogenation of benzonitrile with molecular hydrogen as
a model reaction, which is especially important from the eco-
nomic and ecological viewpoints, and the results are shown in
Table 4 (entries 18 and 19).
3 4 2
NiNPs@Fe O –SiO –P4VP MGs after a) 30 min (red) and b) 2 h (green).
Notably, magnetic extraction precludes the need for filtra-
tion or centrifugation steps and the workup of the final reac-
tion mixture to recover the catalyst.
Thus, we have achieved several important objectives:
1) having a heterogeneous catalyst bearing the advantage of
being magnetically separable and thus removing the need of
the catalyst filtration after the reaction completion; 2) using
MGs as stabilizers for nanoparticles, which have several impor-
tant advantages such as stability, straightforward synthesis,
and facile functionalization; 3) forming hybrid MGs in situ via
covalent cross-linking with functional inorganic nanoparticles
that does not need any conventional chemical cross-linker;
Conclusions
The pH-sensitive organic–inorganic hybrid microgels [Fe O –
3
4
SiO –P4VP MGs; P4VP=poly(4-vinylpyridine)] were prepared
2
through the surface-initiated atom transfer radical polymeri-
zation with 4VP as the pH-sensitive monomer and Fe O mag-
4) providing the stimuli-sensitive support, which changes in
3
4
volume with the change in pH; in reality, the physicochemical
properties of the polymeric MG significantly affect the catalytic
activity; and 5) developing an efficient synthesis process for
the facile and selective catalytic reduction of aliphatic and aro-
matic nitriles.
netic nanoparticles as cross-linkers. The formation of the syn-
thesized magnetic MG was confirmed by using FTIR, UV/Vis,
thermogravimetric analysis, XRD, and X-ray photoelectron
spectroscopy analysis. The synthesized hybrid MGs were effec-
tively used as substrates for the in situ formation of nickel
nanoparticles (NiNPs). The encapsulation of NiNPs on Fe O –
3
4
SiO –P4VP MGs was studied confirmatively by using atomic ab-
Recycling NiNPs@Fe O –SiO –P4VP MGs
2
3
4
2
sorption spectroscopy, thermogravimetric analysis, XRD, X-ray
photoelectron spectroscopy, and TEM. The prepared NiNPs@
Fe O –SiO –P4VP MGs were used as new nanocatalysts for the
Many reports of stable and recyclable supported homogene-
ous catalysts confirm the catalyst stability and recyclability on
the basis of a single data point in each run and often a reaction
yield after a long reaction time. However, catalysis is complete-
ly a kinetic phenomenon, and thus one can argue that only ki-
netic data should be considered to confirm recyclability, stabili-
ty, and deactivation. The measurement of initial rates obtained
from kinetic plots is a good approach to investigate recyclabili-
ty and deactivation because if a single data point was taken in
each run after a long reaction time, one could incorrectly con-
3
4
2
reduction of various nitriles. Both aromatic and aliphatic nitriles
can be reduced with this catalyst in high yields. The recyclabili-
ty of the catalyst was examined for seven cycles, which
showed no loss in activity. This system has the advantages of
flexibility and practicality, a simple workup process, and easy
recovery of the catalyst, and thus it can be used for the reduc-
tion of many different CꢀN multiple bonds.
[71]
clude that the catalyst is completely stable and recyclable.
Thus, we investigate conversion yields for every run after
0 min for the reduction of benzonitrile as a model reaction. Experimental Section
3
The reaction was monitored by using GC, and the results are
shown in Figure 7.Analysis of NiNPs@Fe O –SiO –P4VP MGs re-
Materials
3
4
2
vealed good stability and recyclability, as the conversion yields
for seven runs after 30 min did not decrease significantly. In
addition, to investigate the productivity of the prepared cata-
lyst, we examined the reduction of benzonitrile for seven
cycles even after the completion of the reaction. The results
shown in Figure 7b demonstrate that after every run, the
product yield does not change significantly, which indicates
the high productivity of the catalyst.
FeCl ·6H O and FeSO ·7H O were purchased from Merck. Ammoni-
3 2 4 2
um hydroxide was received from Merck. (3-Aminopropyl)triethoxy-
silane was purchased from Acros Organics. Toluene was dried over
sodium. Dichlorometane and methanol were dried from calcium
hydride and magnesium, respectively. 4VP (95%, Acros) was frac-
tionally distilled before use. Nickel(II) chloride, PVP (65000 Da), and
BIBB were purchased from Merck. All other solvents and reagents
were purchased from Aldrich or Merck and used without further
purification unless otherwise stated.
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Instruments and characterization
Preparation of BIBB-coated MNPs
The amine-modified Fe O –SiO MNPs were dispersed in anhydrous
The IR spectra were recorded on a BOMEM MB-Series FT-IR spectro-
3
4
2
1
photometer. The H NMR spectra were recorded on a Bruker
dichloromethane (50 mL). After the addition of distilled triethyla-
mine (0.711 mL, 5.0 mmol), the whole solution was kept in an ice
bath under argon atmosphere and cooled to 08C. BIBB (0.528 mL,
4.2 mmol) was added to the above solution, which was mechani-
cally stirred for 2.0 h at 08C and then at RT for 16 h. The synthe-
sized MNPs (labeled as BIBB-coated MNPs) were isolated and
washed three times with dichloromethane, ethanol, and water, re-
spectively, and finally dried in vacuum overnight at 508C.
AVANCE DRX-300 spectrometer, and deuterated chloroform was
used as a solvent. The TEM images were recorded with a LEO
9
12AB electron microscope. Catalysis products were analyzed with
a Varian 3900 GC system (GC conversions were obtained with n-
decane as an internal standard based on the amount of nitriles
used relative to the authentic standard product). Ultrasonic bath
(
Eurosonic 4D ultrasonic cleaner with a frequency of 50 kHz and
an output power of 350 W) was used to disperse materials in sol-
vents. TGA was performed with an STA 1500 instrument at a heat-
ꢀ
1
ing rate of 108Cmin in air. The XRD patterns were collected on
an XD-3A X-ray diffractometer using CuKa radiation. XPS analysis
was performed with a VG MultiLab 2000 spectrometer (Thermo VG
Scientific) in an ultrahigh vacuum. The UV/Vis spectra were ob-
tained with a Shimadzu UV-2100 spectrophotometer. CHN analysis
was done with a Vario EL analyzer.
Surface-initiated ATRP of 4VP from BIBB-coated MNPs
The BIBB-coated MNPs (0.1 g) were sonicated with 4VP (3 mL,
2
8 mmol) in a Schlenk tube for 1 h before the addition of a solution
of copper(I) bromide (0.3 g, 2.1 mmol) and N,N,N’,N’,N’’-pentame-
thyldiethylenetriamine (PMDETA) (0.42 mL, 2.1 mmol) in dioxane
(1 mL). The mixture was degassed by three freeze–pump–thaw
cycles and heated at 808C for 24 h to establish the ATRP reaction.
Fe O –SiO –P4VP MGs were magnetically separated and washed
3
4
2
thoroughly with methanol and dried in vacuum (Scheme 2).
Preparation and coating of the magnetite nanoparticles
with silica
Preparation of NiNPs@Fe O –SiO –P4VP MGs
Fe O MNPs were prepared by coprecipitating iron(II) and iron(III)
3
4
2
3
4
in the alkaline solution according to the method described in the
The aqueous solution of nickel(II) chloride (0.01 mL, 0.15m) and
[58]
literature.
In brief, FeCl ·6H O (0.54 g) and FeSO ·7H O (0.3 g)
3 2 4 2
Fe O –SiO –P4VP MGs (0.1 g in 10 mL) were mixed and placed in
3
4
2
were added to Millipore water (40 mL) under argon atmosphere.
The aqueous ammonia solution (1.5m) was introduced into the re-
action vessel with violent stirring. Then, the reaction mixture was
heated at 808C and the pH of the medium was maintained at 10
with the addition of aqueous ammonium hydroxide (2.5m) during
the reaction. The obtained magnetite was immediately washed
five times with water and then two times with ethanol. Then,
MNPs were separated with an external magnet and were dried in
a vacuum oven at 508C. Coating of the nanoparticles with PVP
an ultrasonic bath (50 kHz) for 10 min to disperse metal ions in the
hybrid material. The mixture was stirred at RT for 8 h, and then re-
duction was performed with the addition of the aqueous solution
of NaBH (0.5 mL, 0.01m) to the mixture and stirring at RT for 1 h.
4
It was filtered under vacuum, washed with ethanol and water (2ꢁ
2
0 mL), and dried under vacuum at 508C for 4 h to obtain NiNPs@-
Fe O –SiO –P4VP MGs (Scheme 3). The atomic absorption spectros-
3
4
2
copy and TGA were used to determine the amount of nickel in the
synthesized catalyst.
[72]
was done according to the method of Lee and co-workers. A so-
lution of PVP-coated magnetite nanoparticles (1 g) was suspended
in 2-propanol (2 L) containing concentrated ammonia (50 mL,
General method for the reduction of a nitrile with NiNPs@-
Fe O –SiO –P4VP MGs as the catalyst system
2
8%).
3
4
2
The solution was divided into three portions, and each one was so-
nicated for 1 h. After combining the solutions, tetraethoxysilane
A nitrile (1 mmol) was added to distilled water (2 mL), and then
the ultrasonically dispersed NiNPs@Fe O –SiO –P4VP MG catalyst
(
5 mL, 22.4 mmol) in 2-propanol (100 mL) was added dropwise,
3
4
2
(0.005 g, 1 mol% of nickel) in water (3.0 mL) was added to this so-
over a 3 h period, to the magnetite solution under mechanical stir-
ring. The stirring was continued for another 3 h, and then the
silica-coated nanoparticles were separated magnetically after the
decantation of the solution and washed three times with time
drinking water (TDW). The final product was obtained after drying
at RT for 24 h under a vacuum of 0.2 mmHg.
lution. Finally, NaBH4 (1 mmol, 0.038 g) was added. The mixture
was stirred. After the reaction completion, the catalyst was re-
moved with an external magnet and washed twice with dichloro-
methane (6.0 mL). Then, the organic phase was combined and the
solvent was removed under vacuum to afford the pure product.
Acknowledgements
Silanation of the silica-coated magnetite nanoparticles
We are grateful to Shahid Beheshti University Research Council
for partial financial support of this work.
Dry silica-coated magnetite powder (10 g) was suspended in dry
toluene (200 mL). After sonication for 35 min, a solution of (3-ami-
nopropyl)triethoxysilane (2.5 mL, 10.68 mmol) was added under
mechanical stirring. The solution was heated at 1058C for 20 h. The
particles were separated with an external magnet after cooling to
RT, washed three times with dry methanol, and dried under
Keywords: nanoparticles · nickel · microgels · nitriles
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Published online on January 16, 2014
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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