Synthesis, characterization and application of β-cyclodextrin-silica nanocomposite as potential microvessel in nucleophilic substitution reaction of phenacyl halides

Article abstract of DOI:10.1007/s10847-012-0263-0

In the present study, β-cyclodextrin-silica hybrid is synthesized as a novel, efficient and eco-friendly microvessel and solid-liquid phase-transfer catalyst. This molecular host system was applied for nucleophilic substitution reaction of phenacyl halide

Full text of DOI:10.1007/s10847-012-0263-0

J Incl Phenom Macrocycl Chem
DOI 10.1007/s10847-012-0263-0
ORIGINAL ARTICLE
Synthesis, characterization and application of b-cyclodextrin-silica
nanocomposite as potential microvessel in nucleophilic
substitution reaction of phenacyl halides
Ali Reza Kiasat Simin Nazari
•
Received: 28 July 2012 / Accepted: 15 November 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract In the present study, b-cyclodextrin-silica hybrid
is synthesized as a novel, efficient and eco-friendly micro-
vesselandsolid–liquidphase-transfercatalyst. Thismolecular
host system was applied for nucleophilic substitution reaction
of phenacyl halides in water. No evidence was observed for
the formation of by-product for example isothiocyanate or
alcohol. Also the products were obtained in pure form without
further purification. The structure and morphology of the
obtained system were investigated by infrared spectroscopy,
powder X-ray diffraction, scanning electron microscopy,
transmission electron microscopy, thermogravimetric analy-
sis and differential thermal analysis techniques. The surface
porosity of the synthesized catalyst was evaluated from the
nitrogen adsorption isotherm. All the results provided evi-
dence for incorporation of b-cyclodextrin inside silica pores.
with their design, preparation and characterization as well
as the development of potential applications [1].
On the other hand, cyclodextrins (CDs) play major roles
in many disciplines such as supramolecular chemistry,
analytical chemistry, catalysis and biomedicine [14–18].
This compounds are widely used as hosts to form inclusion
complexes with small and medium sized organic molecules
[19]. Cyclodextrins are soluble in water and some organic
solvents (such as DMF and DMSO). It should be note that,
the solubility of apolar guest compounds in water is (in
general) increased when they form inclusion complexes
with CDs [14, 20–23]. Thus, cyclodextrins are potent phase
transfer catalysts (PTC) [24].
In recent years, the design of efficient and recoverable
phase-transfer catalysts has become an important issue for
reasons of economic and environmental impact. Recently
we focus on developing novel PTC systems for organic
transformations [25–28]. In this study, we fabricated a
novel inorganic–organic hybrid phase transfer catalyst by
co-condensation method for incorporation of b-cyclodex-
trin (b-CD) inside silica pores. This type of nanocomposite
materials [29] due to exceptional surface areas and well-
defined pore sizes is suitable for the diffusion of bulky
adsorbate and substrate molecules. With these regards, we
evaluated b-cyclodextrin-silica hybrid (b-CD-silica) as
new solid–liquid phase-transfer catalyst and molecular host
system for nucleophilic substitution reactions.
Keywords b-Cyclodextrin-silica hybrid Á
Nucleophilic substitution Á Phenacyl halide Á
Phenacyl thiocyanate Á Phenacyl azide Á Phenacyl acetate
Introduction
Combining the rigidity of the inorganic support and the
intrinsic properties of the organic moiety has increased the
scope of applications of inorganic–organic hybrid materials
in several fields, such as adsorption, separation [1–4], and
catalysis [5–13]. Besides general advantages of high sur-
face area and large pores, inorganic–organic hybrid com-
posites have been the subject of numerous studies dealing
Experimental
General
A. R. Kiasat (&) Á S. Nazari
Chemistry Department, College of Science, Shahid Chamran
University, 61357-4-3169 Ahvaz, Iran
e-mail: akiasat@scu.ac.ir
Tetraethoxysilane (TEOS), 3-chloropropyltrimethoxysilane,
phenacylhalidesandotherchemicalmaterialswerepurchased
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J Incl Phenom Macrocycl Chem
from Fluka and Merck and used without further purifica-
tion. b-CD was dried under vacuum at 110 °C for 3 h. to
remove traces of moisture. Products were characterized by
stirred at 90 °C for the time shown in Table 1. After com-
plete consumption of starting material as judged by TLC
(using n-hexane–ethylacetate as eluent), the insoluble cata-
lyst was filtered off and the aqueous phase was extracted
with diethyl ether (2 9 10 ml). The extract was dried with
1
13
comparison of their physical data, IR, H and C NMR
spectra with known samples. NMR spectra were recorded
in CDCl on a Bruker Advance DPX 400 MHz instrument
3
calcium chloride (CaCl ) and evaporated in vacuo to give
2
spectrometer using TMS as internal standard. The purity
determination of the products and reactions monitoring
were accomplished by TLC on silica gel polygram SILG/
UV 254 plates. The infrared spectroscopy (FT-IR) spectra
of b-CD-silica hybrid was measured as potassium bromide
discs on a BOMEM MB-Series 1998 FT-IR Spectropho-
tometer. X-ray diffraction (XRD) patterns of the samples
were taken with a Philips X-ray diffractometer Model
PW1840 (Bragg–Brentano configuration) at room temper-
corresponding products. The residual catalyst was washed
and dried and then subjected to the next run directly.
Results and discussion
Investigation around the approach of b-cyclodextrin-
silica hybrid preparation and its structural
and morphological analysis
ature utilizing Cu Ka radiation with the wavelength of
˚
.5418 A. The particle morphology was examined by
1
Organic–inorganic hybrid materials possessing high-con-
tent organic groups are a unique class of materials and they
are widely explored in different fields ranging from
chemistry to materials science and biology. The presence
of organic groups within the porous inorganic materials
help effectively to increase the hydrophobicity, hydro-
thermal, thermal, and mechanical properties.
scanning electron microscopy (SEM) and transmission
electron microscopy (TEM). The TG–DTA (differential
thermal analysis) measurement of the b-CD-silica nano-
composite was recorded on a BAHR, SPA 503 at heating
-
1
rates of 10 °C min . The thermal behavior was studied by
heating 1–3 mg of samples in aluminum-crimped pans
under nitrogen gas flow, over the temperature range of
Moreover, the uniformity of the organic groups inside
the pore channels affects the surface properties, function-
alized organic group reactivity and the accessibility of the
organic–inorganic composites for further manipulations.
With these regards, b-CD-silica was prepared by the co-
condensation method.
2
-1
2
5–500 °C. The specific surface area (m g ) was deter-
mined using BET (Braunauer, Emmet and Teller) method.
Preparation of b-cyclodextrin-silica hybrid
In the first step, to a solution of b-CD (1.136 g, 1 mmol) in
Tetraethoxysilane and (MeO) Si(CH ) (b-CD) as silane
3
2 3
3
0.0 ml of anhydrous DMF, 0.12 g (5 mmol) of NaH was
coupling agents were used to synthesize b-CD-silica.
Organic modification of mesoporous pure silica needed two
steps leading to materials with b-CD residues anchored
covalently to the pore walls as shown in Scheme 1. Since the
b-CD is direct component of the silica matrix, pore blocking
is not a problem in the co-condensation method. Further-
more, in this method, the organic units are generally more
homogeneously distributed in comparison with the synthe-
sized materials which are obtained with the grafting process.
The chemical structure of b-CD-silica hybrid was
characterized using FT-IR, XRD, TEM, AFM, thermo-
gravimetric analysis (TGA), DTA and BET and analyses.
Figure 1 depicts the FT-IR spectra of the silica and b-
added. The mixture was stirred at room temperature until
no gas was emitted [30]. Excessive NaH was removed by
filtration and then 1.5 mmol (0.298 g) of 3-chloropropyl-
trimethoxysilane was added to the filtrate. The reaction
mixture was heated at 90 °C under nitrogen protection for
1
2 h. To the reaction, a mixture of water (0.63 g,
3
5 mmol), 1-butanol (0.265 g, 3.58 mmol), TEOS
(
0.416 g, 2 mmol) and dilute HCl (1 drop) were added and
the mixture stirred at 80 °C for 3 h. The hydrogel thus
obtained was dehydrated slowly at 80 °C for 3 h and dried
in a vacuum at 60 °C for 12 h. The dried gel was calcined
in a vacuum at 150 °C for 2 h to fasten the silica network
and then washed with hot water several times. Finally, the
b-CD-silica was dried in a vacuum at 150 °C for 3 h.
CD-silica hybrid. IR-spectrum of SiO was used as a ref-
2
erence. SiO network can be identified with the presence of
2
-
1
characteristic peaks at around 1080, 800 and 460 cm
,
Typical procedure for the nucleophilic substitution
of phenacyl halides catalyzed by b-cyclodextrin-silica
hybrid
due to the stretching asymmetric, symmetric and defor-
mation vibrations of the siloxane network (Si–O–Si). FT-
IR spectrum of silica exhibits another absorption band at
-
1
970 cm that is attributed to the silanol groups (Si–OH)
To a mixture of the phenacyl halide (1.0 mmol) and MY (Y:
[31]. In addition, the infrared bands at 3,458 and
-
1,635 cm are associated with the OH from bound water,
1
SCN, N , CN, OAc) (2–5 mmol) in water (5 ml), b-CD-
3
Silica (0.2 g) was added. The suspension was magnetically
as well as from the Si–OH groups. The typical feature of b-
1
23

J Incl Phenom Macrocycl Chem
Table 1 Nucleophilic substitution reaction of phenacyl halides with thiocyanate, azide, cyanide and acetate anions in water catalyzed by b-CD-
silica hybrid
Entry
1
Phenacyl halide
Product
Phenacyl halide: nucleophile
1:3
Time (min)
75
Yield
a
(%)
94
90
88
81
83
2
3
4
5
1:3
1:2
1:5
1:3
75
60
180
50
6
7
8
1:3
1:2
1:5
60
30
80
89
78
110
-
CD is the peak around 2,930 cm . In addition, all the
1
structures which is reduced in FT-IR spectrum of b-CD-
silica hybrid. These results are attributed to this fact that b-
CD bonded to the silica matrix.
-
1
significant peaks of b-CD in the range of 900–1,100 cm
are present in the spectrum of b-CD-silica hybrid with a
small shift. Also, bands appeared at about 2850, 1413 and
Figure 2 shows powder X-ray diffraction (XRD) pat-
terns of b-CD and b-CD-silica hybrid.
-
1
1
,250 cm
are assigned to stretching and deformation
vibration of C–H groups [32]. The intensity of Si–O
The XRD patterns of b-CD, silica and b-CD-silica
hybrid gave different patterns. The XRD profile of SiO2
-
vibrations at 1,078 cm shows the presence of dense SiO2
1
123

J Incl Phenom Macrocycl Chem
-
CH OH
CH O
CH O
Si(OCH3)3
2
2
2
Cl
Si(OCH3)3
2 h/ 90 °C
NaH /DMF
1
(
I)
O
O Si
O
O
H C
O
2
OH
O
Si
Si
O Si
O
Si
O
O
O
O
Si
O
O
O
Si
O
OH
O
O
Si
O
Si
O
HO Si
O
O
O
O
Si
Si
OH O
HO
R = Cyclodextrin
Scheme 1 Synthesis of b-CD-silica hybrid
partial hydrogen bonding between composite building
blocks.
In addition, TGA–DTA techniques were applied to
verify the presence of b-CD into silica network were
applied (Figs. 3, 4). From the TGA curve, three stage
degradation pattern can be obviously found. The first step
of weight loss (about 11.7 %) from 40 to 230 °C could be
attributed to the evaporation of physically absorbed water
and alcohol originating from the co-condensation reaction,
which is an endothermic process observed from the DTA
curve (around 230 °C).
The greatest weight losses are at over 240 and 410 °C,
respectively. Between 240 and 410 °C, weight loss
(
*54 %) is attributed to the decomposition of organic
Fig. 1 FT-IR spectra of a silica and b b-CD-silica hybrid
compounds that are part of the silica network. It could be
confirmed with the presence of a sharp endotherm tran-
sition which is found around 330 °C in DTA curve. The
latest weight loss around 440 °C (*14 %) is due to the
complete carbonization of organic compounds in the
functionalized sample and collapse of the silicon back-
bone. The thermal stability of the silica hybrid is higher
than that of the pure b-CD and this was due to the
inorganic skeleton attached to the organic part through
perfect covalent bonding. Furthermore, silica component
induces a protective barrier against thermal degradation
for organic species. Hence, a delay was observed in the
degradation of b-CD.
consists of a broad diffraction peak at 2h = 23.65 °
whereas b-CD displays intense and sharp diffraction peaks
due to the high degree of crystallinity and hydrogen
bonding between samples. It can be observed that when b-
CD is incorporated into silica matrix it almost disrupts the
fine structure crystalline of the b-CD. A few distinct
characteristic peaks of the b-CD crystals were observed for
the silica composite which indicated that the b-CD-silica
hybrid have the semi-crystalline nature. This semi-crys-
talline nature of the b-CD-silica network is attributed to
1
23

J Incl Phenom Macrocycl Chem
Fig. 2 XRD pattern of a Silica,
b b-CD and c b-CD-silica
hybrid
The total weight loss over the full temperature range is
estimated to be 80 % due to the loss of the adsorbed water
and kept constant throughout the scan and the topography of
the surface was obtained. Three dimensional AFM images
(Fig. 5) and two dimensional profile of the synthesized
hybrid nanocomposite revealed regular distribution of the
particles. In this sample the average size of particles is
20–40 nm. In addition, aggregates with height from 40 to
65 nm and width from 40 to 100 nm were present in nano-
composite surface. In addition, the AFM images shows that
b-CD-silica hybrid have distinct orientation of the aggre-
gates on surfaces, this being an important condition for
nanocomposites used in sensing and catalytic processes, to
maintain continuity. On the whole, AFM revealed that the
synthesized hybrid exhibited a porous surface that has the
ability for the inclusion of an organic guest.
and organic units. The amount of silica (SiO ) was evalu-
2
ated by deducting the total weight loss, whose value was
obtained from the TGA curve. From these data, the amount
of SiO is estimated to be 20 %. The TGA analysis of the
2
as-prepared catalyst showed a CD moiety loading of
-
1
approximately 0.6 mmol g
.
High-resolution imaging using atomic force microscope
AFM) has been applied to directly observe the topography
(
of the surface, shape, morphology, dimension and roughness
of the prepared hybrid material. Surface roughness param-
eters are significant for various applications of hybrids. The
force between the tip and the sample surface was detected
123

J Incl Phenom Macrocycl Chem
Fig. 4 DTA curve of b-CD-silica hybrid
Fig. 3 TGA curve of b-CD-silica hybrid
The morphology and particle size of the b-CD-silica
hybrid were performed by measuring SEM using a Philips
XL30 scanning electron microscope. As can be seen in
Fig. 6, the SEM images show the amorphous hybrid par-
ticles and presence of voids and pores, which can be
attributed to the silica network. SEM micrograph of the
synthesized hybrid nanocomposite also shows that the
particles have a spongy structure and uniform distribution
of the hybrid matrices. The particle size distribution has an
average of 62 nm.
Transmission electron micrographs (TEM) of the synthe-
sized functionalized silica were taken on a LEO-906E
instrument operated at the accelerating voltage of 80 kV and
shown in Fig. 6. The TEM images show that there are some
ordered pores but the majority of the sample is unordered
which is confirmed by the pore size distribution. The shapes
Fig. 5 AFM images of b-CD-
silica hybrid
1
23

J Incl Phenom Macrocycl Chem
Fig. 6 a SEM images of b-CD-silica hybrid b TEM images of b-CD-silica hybrid
Fig. 7 a Nitrogen sorption isotherm of b-CD-silica hybrid b BJH-plot of b-CD-silica hybrid
123

J Incl Phenom Macrocycl Chem
O
The BET total surface area for b-CD-silica hybrid was
O
2
-1
Nu
4.32 m g and the mean pore diameter for this nanocom-
posite was found to be 26.7 nm. The nitrogen adsorption
isotherms confirm that the introduction of b-CD in the silica
network. In the contrast to general silica particles with por-
ous properties, b-CD-silica hybrid particles have nonporous
surface. It is shown that this low surface area is because the
compatibility between silica and b-CD is so good that b-CD
closes the pores among primary silica particles.
X
-
β CD-Silica
O/ 90 °C
Nu-
+
H
2
R
R
-
, CN , OAc-
-
Nu = SCN , N
3
Scheme 2 Nucleophilic substitution reaction of phenacyl halides
with different anions catalyzed by b-CD-silica hybrid
2
H C O
Br
OH
Application b-CD-silica hybrid as phase transfer
catalyst for nucleophilic substitution reaction
of phenacyl halides in water
O
Si
Si
O Si
O
Si
O
O
O
O
O
Si
O
O
O
Si
O
OH
O
Si
O
O
O
Si
HO Si
O
O
O
Br
O
H
O
Si
O
Although phenacyl cyanides, acetates, thiocyanates and
azides are important compounds for various synthetic
applications [33, 34], there are few reported practical syn-
thetic routes in the literature for these class of compounds
Si
OH
HO
Y
Scheme 3 Postulated Roles of b-CD-silica hybrid in the nucleophilic
substitution reaction of phenacyl halides
[
35–37]. As a continuation of our research on using water as
reaction medium [38–41] and our interest in developing a
true water tolerant catalyst using inexpensive and nonpol-
luting reagents, herein we report the synthetic applicability
of b-CD-silica hybrid as a novel phase transfer catalyst for
the rapid, and efficient preparation of phenacyl thiocyanates,
cyanides, azides and acetates in water by nucleophilic sub-
stitution reaction (Scheme 2). To evaluate the catalytic
ability of b-CD-silica hybrid in the nucleophilic substitution
reaction, the reaction of p-bromo phenacyl bromide with
thiocyanate anion in water was chosen. This catalyst acted
very efficiently and it was determined that the use of 2 equiv
of KSCN per p-bromo phenacyl bromide in the presence of
the catalyst (0.2 g) in water was the best condition and after
stirring for 1 h at 90 °C, the clean formation of a product
was observed (Scheme 2; Table 1). It is noteworthy that no
evidence for the formation of by-products such as alcohols
or isothiocyanates was observed and the product obtained in
Graph 1 Recyclability of the catalyst
1
3
and dimensions of the b-CD-silica particles are not uniform
but their sizes are in the nanometer range. These nanoparticles
have a narrows size distribution (mostly between 10 and
pure form without further purification. C resonance of the
SCN and NCS groups at *111 and *145 ppm, respec-
tively, are very characteristic for thiocyanate and isothio-
cyanate functionalities .
1
6 nm)withanaverage diameterof13 nm, muchsmallerthan
the sizes obtained from the SEM measurements.
Nitrogen adsorption/desorption isotherm of the prepared
catalyst displayed a characteristic type III adsorption iso-
therm with a narrow hysteresis loop at a relatively high
pressure range, implying the presence of typical meso-
porous solids (Fig. 7a).
It should be pointed out that in the absence of b-CD-
silica hybrid, the reaction was sluggish and even after
prolonged reaction time a considerable amount of starting
material was remained.
After the success of the conversion of p-bromo phenacyl
bromide to p-bromo phenacyl thiocyanate using b-CD-
silica hybrid as solid–liquid phase-transfer catalyst and
molecular host system, the reaction was extended to other
phenacyl halide, p-nitro phenacyl bromide, and anions. The
scope and generality of this process is illustrated with
several examples and the results are summarized in
Table 1. In all cases, a very clean reaction was observed
and desired products were formed in good isolated yields.
The specific surface area of the porous silica hybrid was
also calculated by the BET equation and pore size distri-
bution was calculated from the desorption branch using
the Barrett–Joyner–Halenda (BJH) method (Fig. 7b). The
sharp peaks between 2 and 30 nm in the plot of pore size
distribution indicated that the synthesized hybrid is
mesoporous.
1
23

J Incl Phenom Macrocycl Chem
The reusability of the catalyst is important for the large-
scale operation and industrial point of view. Therefore, the
recovery and reusability of b-CD-silica hybrid was exam-
ined. It was noted that this catalyst did not suffer from
extensive mechanical degradation and was quantitatively
separated and reused after washing with water and methanol
and drying at 70 °C. The reusability of the catalyst was
investigated in the nucleophilic substitution reaction of p-
bromo phenacyl bromide with thiocyanate anion (Graph 1).
The results illustrated in Graph 1 showed that the catalyst
can be used five times with consistent yield. The color of the
catalyst remains same even after the fifth cycle.
1-(4-Bromo-phenyl)-2-thiocyanato-ethanone
-
1 13
,
t
max = 2157, 1674, cm
C NMR (400 MHz, CDCl ):
3
d = 42.61, 111.49, 129.85, 130.38, 132.60, 132.69,
189.89 ppm.
Acetic acid 2-(4-bromo-phenyl)-2-oxo-ethyl ester
-
1 13
,
t
max = 1745, 1692, cm
C NMR (400 MHz, CDCl ):
3
d = 20.56, 65.93, 128.23, 129.05, 132.25, 132.90, 170.40,
191.32 ppm.
Acknowledgments We are grateful to the Research Council of
Shahid Chamran University for financial support.
The catalytic activity of b-CD-silica hybrid in nucleo-
philic substitution reaction could be attributed to the fact
that, in water, hydrophobic central cavities of b-CD units can
act as microvessel and accommodate nonpolar aromatic
guest molecules and resulting in modified reactivities. On the
other hand, the hydrophilic exterior due to the outer OH of
the b-CD cavity formed complexes with cation and these
complexes cause the anion to be activated (Scheme 3).
Furthermore in some cases, water can form a bridge
between the hydroxyl groups of adjacent molecules of
cyclodextrin to form a cage to assist in trapping the guest in
the complex.
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