Synthesis and characterization of novel silica coated magnetic nanoparticles with tags of β-cyclodextrin: application as an eco-friendly and chiral micro-vessel catalyst in the enantioselective reduction of ketones

Article abstract of DOI:10.1007/s11164-018-3256-9

In this work, we report the synthesis of a novel, green and recoverable organic–inorganic magnetic nanocomposite by grafting β-cyclodextrin on the surface of a silica-coated magnetic nanoparticle, Fe₃O₄@SiO₂/Pr-β-CD. FT-IR spectroscopy, transmission electron microscopy, CHN analysis, thermogravimetric analysis, vibrating sample magnetometer and X-ray diffraction analyses confirmed its structure. The magnetic core–shell structured modified silica microsphere has been successfully used as a chiral micro-vessel catalyst for the enantioselective reduction of ketones by NaBH₄. The described catalyst was regenerated and reused without any significant changes in the yield and enantiomeric excess.

Full text of DOI:10.1007/s11164-018-3256-9

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3256-9
Synthesis and characterization of novel silica coated
magnetic nanoparticles with tags of β‑cyclodextrin:
application as an eco‑friendly and chiral micro‑vessel
catalyst in the enantioselective reduction of ketones
Mina Jafari Nasab · Ali Reza Kiasat2
1
Received: 24 October 2017 / Accepted: 8 January 2018
©
Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract In this work, we report the synthesis of a novel, green and recoverable
organic–inorganic magnetic nanocomposite by grafting β-cyclodextrin on the sur-
face of a silica-coated magnetic nanoparticle, Fe O @SiO /Pr-β-CD. FT-IR spec-
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troscopy, transmission electron microscopy, CHN analysis, thermogravimetric anal-
ysis, vibrating sample magnetometer and X-ray diꢀraction analyses conꢁrmed its
structure. The magnetic core–shell structured modiꢁed silica microsphere has been
successfully used as a chiral micro-vessel catalyst for the enantioselective reduction
of ketones by NaBH . The described catalyst was regenerated and reused without
4
any signiꢁcant changes in the yield and enantiomeric excess.
Keywords Organic–inorganic magnetic nanocomposite · β-Cyclodextrin ·
Enantiomeric reduction of ketones · Modiꢁed silica-coated magnetic nanoparticle
Introduction
The core–shell microstructures comprised of magnetic cores and non-magnetic
shells have recently emerged as an attractive area of research mainly because of
their bifunctionalities. In this regard, a shell of silica around magnetic nanoparti-
cles, known as the Fe O @SiO core–shell, constitutes the basic architecture of
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most multifunctional nanocomposites. The attention of researchers in recent years to
this class of nanoscale magnetic core–shell materials is due to these facts that such
*
Mina Jafari Nasab
minajafarinasab@yahoo.com
1
2
Department of Chemistry, College of Science, Shahid Chamran University of Ahvaz, Ahvaz,
Iran
Petroleum Geology and Geochemistry Research Centre (PGGRC), Shahid Chamran University
of Ahvaz, Ahvaz, Iran
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M. Jafari Nasab, A. R. Kiasat
surface modiꢁcation enhances the biocompatibility and provides quite high density
surface functional end groups which allow for connecting to catalysts [1]. In addi-
tion, a silica shell has low-toxicity, less prone to degradation in a biological environ-
ment [2, 3], plus numerous chemical methods exist to functionalize silica shell with
molecular, biomolecular and polymeric ligands.
The unique structure and properties of cyclodextrins (CDs) have made them a
popular choice for a wide range of applications over the past few decades. They play
major roles in many disciplines such as supramolecular chemistry, analytical chem-
istry, catalysis and biomedicine [4–8]. CDs have hydrophilic surfaces and hydropho-
bic cavities that endow them with a special ability to accommodate many organic
compounds by inclusion complexation, because they can be linked both covalently
and non-covalently [9]. On the other hand, these molecules have been investigated as
chiral receptors [10] because of the chirality of the CD cavity and its carbon atoms.
Furthermore, the functionalization of CDs often improves its enantioselectivity.
Chiral alcohols are important as bioactive compounds and as intermediates for
the synthesis of natural products such as pharmaceuticals and agrochemicals [11].
These optically active molecules are useful building blocks for the synthesis of
complex molecules, because the alcohol functionality can be easily converted to the
other functional groups such as chloride [12], amine [13, 14], azide [15] and ꢂuoride
[
16]. Therefore, the asymmetric ketones reduction to alcohols is useful in organic
synthesis and has attracted considerable interest in the research community.
Recently, the “greening” of global chemical processes by the use of solid sup-
ported catalysts has become an important task of modern synthetic chemistry. With
this goal in mind and in continuation of ongoing research into the employment of
grafted β-cyclodextrin as a stationary micro-vessel and heterogeneous catalyst in
organic transformations, herein, we report our results on the preparation and char-
acterization of a novel organic–inorganic magnetic nanocomposite, Fe O @SiO /
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Pr-β-CD, and we explored its application in the asymmetric reduction of ketones as
a mild, proꢁcient, recyclable, harmless to the environment, non-toxic and magnetic
powerful solid catalyst with good stability.
Experimental
Preparation of Fe O @SiO
2
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Superparamagnetic nanoparticles (MNPs) were prepared via an improved chemi-
cal coprecipitation method [17]. According to this method, FeCl ·4H O (6.35 g,
2
2
3
1.9 mmol) and FeCl ·6H O (15.14 g, 56 mmol) were dissolved in 640 mL of deion-
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ized water. The mixed solution was stirred under N at 90 °C for 1 h. Then, 80 mL
2
of NH ·H O (25%) was injected into the reaction mixture rapidly, stirred under N
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2
2
for another 1 h and then cooled to room temperature. The precipitated magnetic par-
ticles were washed several times with hot water, separated by magnetic decantation
and dried under vacuum at 70 °C.
Then, Fe O @SiO microspheres were synthesized through a modiꢁed Stö-
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ber method [18]. In a typical process, 1 g of the as-prepared Fe O particles were
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Synthesis and characterization of novel silica coated magnetic…
dispersed in a mixture of ethanol (250 mL), water (65 mL), and NH ·H O (4 mL)
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with the help of ultrasonication. Afterward, 10 mL TEOS was added dropwise, and
the reaction was allowed to proceed for 6 h under stirring. The coated particles were
ꢁ
nally separated by magnetic decantation, washed several times with deionized
water and dried under vacuum at 60 °C overnight.
Preparation of Fe O @SiO /Pr‑β‑CD
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The as-prepared Fe O @SiO microspheres (1 g) were suspended in ethanol
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2
(
100 mL). After addition of CPTES (0.2 g), the mixture was reꢂuxed at 80 °C for
1
6 h. Fe O @SiO /Pr–Cl was ꢁnally separated by a magnetic decantation, washed
3 4 2
several times with distilled water and ethanol and dried under vacuum at 60 °C
overnight.
After that, 0.12 g (5 mmol) of NaH was added to a solution of β-CD (1.14 g,
1
mmol) in 30.0 mL of anhydrous DMF. The mixture was stirred at room tempera-
ture until no gas was emitted [19]. Excessive NaH was removed by ꢁltration and
then 1 g of Fe O @SiO /Pr–Cl was added to the ꢁltrate. The reaction mixture was
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heated at 90 °C under nitrogen protection for 12 h and then washed with hot water
(
80–90 °C) several times. Finally, the Fe O @SiO /Pr-β-CD was dried in a vacuum
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at 110 °C for 5 h.
General procedure for the prochiral ketone reduction
The magnetic nanocomposite, Fe O @SiO /Pr-β-CD (0.02 g), was suspended in
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2
5
mL dry THF, then NaBH (0.06 g, 1.5 mmol) was added and stirred for 0.5 h. To
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the mixture, ketone (1 mmol) was added drop-wise at room temperature. Progress
of the reaction was monitored by TLC, after the completion of the reaction; it was
quenched by HCl (2 N). The nanocomposite was magnetically removed from the
mixture and washed several times with ethanol for reuse. Then the organic phase
was extracted with CH Cl .
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Results and discussion
The synthetic route for the preparation of the core–shell magnetic nanocompos-
ite, Fe O @SiO /Pr-β-CD, is outlined in Scheme 1. As shown in Scheme 1, the
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organic–inorganic nanocomposite was as prepared with a multistep reaction. Ini-
tially, the nano-sized iron oxide was prepared via the cost eꢀective co-precipitation
method from ferrous and ferric ion solutions in basic media and coated by silica
layer through the conventional sol–gel process. To graft β-CD onto the surface of
the core–shell MNPs, ꢁrst, 3-chloropropyltriethoxysilane was reacted with silanol
groups of Fe O @SiO . Then, β-CD was immobilized onto the surface by a simple
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nucleophilic substitution reaction in basic media.
The chemical structure of Fe O @SiO /Pr-β-CD was characterized using FT-IR,
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VSM, TEM, CHN, XRD and TGA-DTA analyses.
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M. Jafari Nasab, A. R. Kiasat
Scheme 1 The synthetic route for the preparation of the core–shell magnetic nanocomposite, Fe O @
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4
SiO /Pr-β-CD
2
The grafting of β-CD to the network magnetic mesoporous silica was conꢁrmed
by FT-IR spectroscopy. Figure 1 shows FT-IR spectra of Fe O @SiO and Fe O @
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2
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4
−
1
SiO /Pr-β-CD. A characteristic peak is shown at about 575 cm in both spectra that
2
corresponds to Fe–O stretching vibration. The bands in the range of 1050–1100 and
−1
8
00 cm corresponded to the asymmetric and symmetric stretching vibration of
−
1
Si–O–Si. In addition, peaks at about 2850, 1413 and 1250 cm are relevant to C–H
groups’ vibrations in the cyclodextrin structure.
The morphology and architecture of Fe O @SiO /Pr-β-CD were investigated
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2
using TEM (Zeisss-EM10C at 80 kV). The TEM images (Fig. 2) conꢁrmed the
Fig. 1 The FT-IR spectra of (a) Fe O @SiO and (b) Fe O @SiO /Pr-β-CD
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2
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2
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Synthesis and characterization of novel silica coated magnetic…
Fig. 2 The TEM images of Fe O @SiO /Pr-β-CD microspheres with core–shell structure
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uniform silica coating on the Fe O hybrid nanostructure and a distinct core–shell
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structure was observed for the Fe O @SiO /Pr-β-CD hybrid systems.
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2
Figure 3 displays the SEM images of Fe O and Fe O @SiO microspheres. As
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2
it is shown, these nanoparticles have spherical shapes with approximately 15 nm
diameters.
The elemental analysis test was used to conꢁrm the grafting of organic groups to
the surface of the Fe O @SiO . The percentages of C and H were assessed by ele-
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mental analysis and are presented in Table 1. CHN elemental analysis indicates that
β-cyclodextrin groups were successfully functionalized on the surface of Fe O @
3
4
SiO . According to the results of this analysis, as well as the TGA analysis, about 6
2
percent of the organic groups were on the surface of Fe O @SiO
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2.
Fig. 3 The SEM images of a Fe O and b Fe O @SiO
2
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4
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Table 1 Elemental compositions of Fe O @SiO /Pr-β-CD nanocomposite analyzed using the CHN
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technique
Sample
C (wt%)
5.17
H (wt%)
1.44
Pr-βCD/Fe O @SiO
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M. Jafari Nasab, A. R. Kiasat
Fig. 4 Room temperature VSM magnetization curves of the a Fe O nanoparticle (67 emu/g) and b
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Fe O @SiO /Pr-β-CD (5 emu/g) nanocomposite
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The magnetic properties of Fe O @SiO /Pr-β-CD were studied with a vibrating
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sample magnetometer at room temperature (Meghnatis Daghigh Kavir Co., Kashan,
Iran). Due to the superparamagnetic property, the Fe O @SiO /Pr-β-CD exhibited
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a fast response to an external magnetic ꢁeld and quickly redispersed under slight
shaking once the magnetic ꢁeld was removed (Fig. 4). In addition, the magnetiza-
tion value for Fe O was 67 emu/g so that, with increasing silica and β-CD, the mag-
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netization value of the Fe O @SiO /Pr-β-CD decreased to 5 emu/g, which further
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conꢁrmed the silica coating on Fe O .
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The XRD patterns of Fe O and Fe O @SiO /Pr-β-CD indicate characteristic
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peaks at 2θ = 30.1°, 35.5°, 43.1°, 53.4°, 57° and 63.1° as shown in Fig. 5. These are
related to their corresponding indices (220), (311), (400), (422), (511), and (440),
respectively [20]. This reveals that the resultant nanoparticles, Fe O have a spinel
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structure and its surface modiꢁcation with β-CD does not result in the phase change
of Fe O .
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The TGA-DTG of Fe O @SiO /Pr-β-CD is shown in Fig. 6. According to the
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thermal gravimetric analysis, it shows a three-stage degradation pattern over the
range of 30–900 °C. The ꢁrst weight loss is about 3.2% and occurs at temperatures
below 250 °C, which is attributed to desorption of physically adsorbed water mole-
cules on the surface of silica and the cyclodextrin cavity. The thermal decomposition
of about 6.8% over the range of 250–800 °C is due to the cleavage of the organic
unities grafted to the Fe O @SiO .
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2
In order to explore the catalytic activity of the Fe O @SiO /Pr-β-CD in asym-
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2
metric reduction of ketones, the reaction of acetophenone with NaBH in THF at
4
room temperature was considered. Checking the progress of the reaction by TLC
showed formation of a product.
Due to the success of the above mentioned tests, in order to optimize the reac-
tion conditions, 1 mmol acetophenone with diꢀerent amounts of NaBH (1–2 mmol)
4
and the diꢀerent weight of the nanocomposite (0.01–0.1 g), reacted in THF at room
temperature. The results of preliminary tests are shown in Table 2. As shown in
Table 2, 0.02 g of the catalyst and the molar ratio NaBH :ketone 1.5:1 were the best
4
conditions.
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Synthesis and characterization of novel silica coated magnetic…
Fig. 5 X-ray powder diꢀraction patterns of (a) Fe O and (b) Fe O @SiO /Pr-β-CD from ~ 5° to 70°
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2
angles
Fig. 6 TGA-DTG analysis of Fe O @SiO /Pr-β-CD over the range of 30–900 °C
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2
Table 2 Optimization of the
reaction conditions for the
reduction of acetophenone
in the presence of Fe O @
Entry Catalyst (g) NaBH (mmol) Time (min) Yield (%)
4
1
2
3
4
5
0.01
0.02
0.02
0.05
0.1
2
1
1.5
2
2
30
20
20
20
20
84
80
93
94
94
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4
SiO /Pr-β-CD in THF at room
2
temperature
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M. Jafari Nasab, A. R. Kiasat
O
OH
CH3
Fe
3
O
4
@SiO
/Pr-B-CD
2
CH
3
NaBH , THF
4
not racemic
Scheme 2 Reduction of acetophenone derivatives in the presence of Fe O @SiO /Pr-β-CD at room tem-
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2
perature
Table 3 The results of the reduction reaction of various ketones using NaBH in THF in the presence of
4
Fe O @SiO /Pr-β-CD at room temperature
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2
Entry
1
Ketone
Time (min)a
Yield (%)b
93
[α]c
%eed
O
20
− 2.10
87 (S)
O
2
3
4
5
30
40
25
40
96
98
99
92
− 2.30
− 0.90
− 3.32
− 2.51
88 (S)
81 (S)
92 (S)
90 (S)
HO
O
MeO
O
2
O N
O
6
7
60
25
95
99
− 0.7
80 (S)
82 (S)
O
− 1.02
O
GC condition with column Cydex-B: carrier gas N ꢂow = 0.7 mL/min, inlet temp 200 °C, initial col-
2
umn temp 50 °C, ꢁnal column temp 150 °C, sleep 10 °C/min. 1-phenylethanol
a
Reaction conditions: 1 mmol substrate, catalyst (0.02 g), NaBH (1.5 mmol)
Yields were determined by GC-FID
Optical rotation was determined by Polarimeter P1000-LED
4
b
c
d
%ee were determined by a Chiral GC
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Synthesis and characterization of novel silica coated magnetic…
Fig. 7 Reusability of the mag-
netic nanocomposite, Fe O @
3
4
nSiO /Pr-β-CD, in the reduction
2
of acetophenone by NaBH in
4
THF
After optimizing the reaction conditions, the reduction reaction of various
ketones done in the presence of catalyst with NaBH and the corresponding alco-
4
hol derivatives were obtained in a short time with high eꢃciency (Scheme 2 and
Table 3).
According to the chiral environment inside cavities of the β-cyclodextrin, it is
possible that β-cyclodextrin unites in the nanocomposite acting as a chiral micro-
vessel and by creating a complex with ketone, producing a chiral complex that
aꢀected orientation addition of the hydride anion to the carbonyl group. For this
purpose, the optical rotation was determined after separation.
Fe O @nSiO /Pr-β-CD has the advantages of both magnetic separation tech-
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niques and nano-sized materials, which can be easily recovered or manipulated with
an external magnetic ꢁeld. As shown in Fig. 7, after four runs using the catalyst in
the reduction of acetophenone, there is no eꢃcient decrease of the yield.
Conclusions
In this work, we have successfully synthesized a novel, green and recoverable
organic–inorganic magnetic nanocomposite by grafting β-cyclodextrin on the sur-
face of a silica-coated magnetic nanoparticle, Fe O @SiO /Pr-β-CD. It was eꢃ-
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2
ciently used as a chiral micro-vessel catalyst for the asymmetric reduction of aro-
matic ketones.
Acknowledgements We are grateful to the Research Council of Shahid Chamran University for ꢁnan-
cial support.
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