Preparation of immobilized hexamine on Fe3O4/SiO2 core/shell nanoparticles: a novel catalyst for solvent-free synthesis of bis(indolyl)methanes

Article abstract of DOI:10.1007/s11164-016-2590-z

Abstract: Hexamethylenetetramine covalently immobilized on silica-coated magnetic nanoparticles is prepared and characterized by XRD, FT-IR, SEM, and VSM. According to SEM pictures, the nanoparticles are mostly spherical in shape, and their size is approximately 46?nm. The catalytic performance of these magnetic nanoparticles for the synthesis of bis(indolyl)methanes was investigated. The catalyst was readily recycled by the use of an external magnetic field and can be reused four times without significant loss of activity. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-016-2590-z

Res Chem Intermed
DOI 10.1007/s11164-016-2590-z
Preparation of immobilized hexamine on Fe O /SiO
2
3
4
core/shell nanoparticles: a novel catalyst for solvent-
free synthesis of bis(indolyl)methanes
1
2
Sahar Kangari
•
Issa Yavari
Received: 3 February 2016 / Accepted: 23 May 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract Hexamethylenetetramine covalently immobilized on silica-coated magnetic
nanoparticles is prepared and characterized by XRD, FT-IR, SEM, and VSM. According
to SEM pictures, the nanoparticles are mostly spherical in shape, and their size is
approximately 46 nm. The catalytic performance of these magnetic nanoparticles for the
synthesis of bis(indolyl)methanes was investigated. The catalyst was readily recycled by
the use of an external magnetic field and can be reused four times without significant loss
of activity.
Graphical Abstract
Keywords Magnetically retrievable Á Nano-hybrid Á Bis(indolyl)methanes Á Solvent-
free Á Silica Á Hexamethylenetetramine
&
Issa Yavari
yavarisa@modares.ac.ir
1
2
Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran
Chemistry Department, Tarbiat Modares University, PO Box 14115-175, Tehran, Iran
123

S. Kangari, I. Yavari
Introduction
Catalytic applications of magnetic nanoparticles (MNPs) have attracted increasing
attention owing to their unique magnetic properties and easy separation from the
reaction system by an external magnetic field rather than filtration or centrifugation
[
1–5]. However, MNPs without coating are often unstable and tend to aggregate
during catalytic transformations, resulting in a remarkable drop of catalytic activity
owing to the decrease of surface area. Thus, MNPs are modified with high surface
coating materials. Silica is usually used as an important coating material because of
its low cost, high surface area, and ease of modification with organic functional
groups [6]. Recently, silica-coated MNPs have been used as appropriate supports for
covalent bond formation with organic materials [7–9].
Indole and its derivatives are an important class of heterocyclic compounds in
medicinal chemistry. In particular, bis(indolyl)methanes (BIMs) are active against
human cancer cells [10–13]. The electrophilic substitution reaction of indoles with
aldehydes affords the corresponding BIMs. Many of these methods still suffer from
drawbacks, for example, expensive reagents, long reaction time, low yields,
corrosive reagents, and high catalyst loading, which eventually results in the
generation of large amounts of toxic waste [14–16]. Therefore, there is a need for an
efficient, recyclable, and eco-friendly catalyst for the synthesis of BIMs.
Herein, we report a synthesis of novel core–shell MNPs having a spherical shape
as a core and porous silica as a shell that is modified with HMTA tribromide. This
Fe O –SiO –HMTA tribromide is used as a heterogeneous nanocatalyst for the
3
4
2
synthesis of BIMs by condensation of indole with aldehydes and ketones.
Experimental
Materials and methods
Iron(III) chloride hexahydrate (FeCl Á6H O), Iron(II) chloride tetrahydrate
3
2
(
orthosilicate (TEOS), 3-chloropropyltrimethoxysilane (CPTMS), hexamethylenete-
FeCl Á4H O), ammonium hydroxide solution (NH ÁH O, 25 %), tetraethyl
2
2
3
2
tramine (hexamine, HMTA), KBr, Br , indole, aldehydes, and solvents were
2
obtained from Merck.
Fabrication of the catalyst
Synthesis of MNPs
MNPs were prepared by co-precipitation method [17]. Therefore, a solution of
FeCl Á4H O (1.27 g) in deionized water (25 mL) was added to a solution of
2
2
FeCl Á6H O (3.80 g) in deionized water (25 mL), and the resulting solution was
3
2
vigorously stirred. Then, 10 mL of ammonia (25 %) was added dropwise into the
1
23

3 4 2
Preparation of immobilized hexamine on Fe O /SiO …
solution and stirred at 80 °C for 2 h. When the black precipitate was formed, the
upper solution was discarded and the precipitate was washed several times with
deionized ethanol and dried.
Synthesis of citrate-coated Fe O MNPs
3
4
For the preparation of citrate-coated iron oxide, 1.0 g of Fe O MNPs was added to
3
4
3
0 mL of 0.1 M citric acid, sonicated for 30 min, and stirred under N for 12 h.
2
Then, the coated MNPs were separated by external magnet and washed twice with
deionized water and dried.
Synthesis of Fe O –SiO NPs
2
3
4
The silica-coated magnetic NPs were prepared by coating of silica layer [18]. For
this purpose, the obtained CMNPs were added to the solution containing 20 mL of
ethanol and 4 mL of deionized water and sonicated for 30 min. Then, ammonium
hydroxide (5 mL) was added followed by 2 mL of TEOS and stirred under N for
2
2 h. The obtained precipitate was washed by ethanol several times and dried in an
oven.
Synthesis of Fe O –SiO –CPTMS
3
4
2
The obtained silica-coated magnetic NPs were dispersed in the solution containing
00 mL of ethanol and 2 mL of toluene and then sonicated for 30 min. Next, 2 mL
of CPTMS was added to the resulting solution, and stirred under N for 48 h. In the
1
2
last stage, the resulting precipitate washed several times by methanol and dried.
Synthesis of Fe O –SiO –HMTA
3
4
2
A stirred suspension of hexamine (1.40 g) and the synthesized Fe O –SiO –CPTMS
3
4
2
in CHCl was refluxed at 60 °C for 24 h. The resulting brown precipitate was
3
washed twice by CHCl and dried under vacuum at 80 °C.
3
Modification of immobilized hexamine by exchanging chloride
with tribromide
For exchange of the chloride counter ion with bromide ion, the precipitate was
stirred with a saturated KBr solution for 48 h. Finally, HMTA bromide salt
immobilized on silica was treated with Br (1 mL) and diluted in CH Cl (10 mL)
2
2
2
to afford the final HMTA tribromide silica support catalyst (Scheme 1).
Identification of the catalyst
Scanning electron microscope (SEM) images were obtained using a LEO 1430VP
instrument. FT-IR spectra of the materials were recorded over the range of
-
00–4000 cm region by using a Bruker Tensor 27 series FT-IR spectrometer.
1
4
123

S. Kangari, I. Yavari
3 4 2
Scheme 1 Schematic representation of the synthesis of Fe O –SiO –HMTA tribromide
Using the KBr disc method with a 1 % sample in 200 mg of spectroscopic-grade
KBr. Powder X-ray diffraction (XRD) patterns were recorded in the range of 2°–70°
˚
on a Siemens D5000 X-ray diffractometer, using CuKa radiation (k = 1.5418 A) at
3
0 kV and 30 mA. The magnetic studies were carried out on a VSM Lake Shore
307 Vibrating Sample Magnetometer.
7
Catalytic applications of Fe O –SiO –HMTA tribromide for the synthesis
2
3
4
of BIMs
A mixture of aldehyde or ketone (1 mmol), indole (2 mmol), and catalyst (0.01 g)
was stirred at 80 °C for 10 min. The reaction was monitored by TLC. After
completion of the reaction, EtOH (3 mL) was added and the catalyst was separated
by an external magnet. In the recycling experiment, the separated catalyst was then
washed with ethanol and dried in oven to remove residual solvent. The results of
synthesis of BIMs under solvent-free conditions at 80 °C, are presented in Table 1.
Spectral data of selected compounds
3
cm ): 3407, 3049, 2845, 1597, 1451, 740, 588; H NMR (400 MHz, DMSO-d ):
-[(1H-Indol-3-yl)(phenyl)methyl]-1H-indole (3a) Mp 150–154 °C; IR (KBr,
-1 1
6
3
d 5.94 (s, 1 H, CH), 6.53 (d, J 1.6 Hz, 2 H, 2 CH), 7.08 (t, 2 H, 2 CH), 7.28–7.36
3
3
m, 7 H, 7 CH), 7.4 (d, J 7.6 Hz, 2 H, 2 CH), 7.46 (d, J 7.9 Hz, 2 H, 2 CH), 7.5 (s,
(
2
H, 2 NH).
-
1
3
-[(1H-indol-3-yl)(p-tolyl)methyl]-1H-indole (3b) Mp 96–99 °C; IR (KBr, cm ):
408, 3044, 2839, 1508, 1451, 1004, 742, 588, 472; H NMR (400 MHz, DMSO-
1
3
3
d ): d 2.23 (s, 3 H, Me), 5.79 (s, 1 H, CH), 6.49 (d, J 1.6 Hz, 2 H, 2 CH), 6.96 (t,
6
3
3
J 7.1, 7.7 Hz, 2H, 2 CH), 7.04 (d, J 7.8 Hz, 2 H, 2 CH), 7.12 (t, 2 H, 2 CH),
3
.17–7.23 (m, 4 H, 4 CH), 7.36 (d, J 7.9 Hz, 2 H, 2 CH), 7.58 (s, 2 H, 2 NH).
7
1
23

3 4 2
Preparation of immobilized hexamine on Fe O /SiO …
Table 1 Synthesis of bis(indolyl)methanes 3 catalyzed by Fe
3
O
4
–SiO
2
–HMTA tribromide
R1 R2
O
Fe O -SiO -HMTA Br
3 3
4 2
N
H
R1
R2 80 C, solvent-free
o
N
H
N
H
1
2
3
1
R
2
Entry
R
Product
Yield (%)
Mp (°C) Observed
Mp (°C) Reported
1
2
3
4
5
6
7
8
9
Ph
4-Me-C
4-MeO-C
4-HO-C
4-O N-C
4-Cl-C
3-Me-C
3-MeO-C
3-HO-C
3-O N-C
3-Cl-C
2-Me-C
2-MeO-C
2-HO-C
2-O N-C
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
H
H
H
H
H
H
H
Me
3a
3b
3c
3d
3e
3f
97
93
91
96
94
93
94
93
95
93
92
93
96
94
94
92
91
93
92
92
93
92
90
92
150–154
96–99
150–152 [20]
94–96 [20]
187–189 [20]
120–123 [21]
219 [21]
6
H
4
6
H
4
187–191
120–124
219–223
74–77
6 4
H
2
6 4
H
6
H
4
76–77 [20]
97–99 [25]
187–180 [25]
136–139 [21]
217–221 [20]
74 [20]
6
H
4
3g
3h
3i
95–99
6
H
4
184–188
132–135
262–265
71–75
6 4
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2
6
H
4
3j
6
H
4
3k
3l
6
H
4
90–94
95–97 [21]
134–136 [21]
134–137 [25]
137–139 [21]
107–109 [21]
189–190 [23]
118–120 [23]
94–97 [22]
67–71 [26]
118–120 [21]
72–74 [27]
150 [22]
6
H
4
3m
3n
3o
3p
3q
3r
3s
132–135
130–135
138–142
105–107
186–188
116–119
91–94
6 4
H
2
6 4
H
6 4
2-Cl-C H
Ph
cHex
Me
n
Pent
3t
63–66
n
Pr
3u
3v
3w
3x
115–119
74–78
n
Bu
H
148–153
63–67
Me
66–69 [23]
Reaction time = 10 min
3
-[(1H-indol-3-yl)(4-methoxyphenyl)methyl]-1H-indole (3c) Mp 187–191 °C; IR
-1 1
KBr, cm ): 3394, 3054, 2356, 1666, 1605, 1455, 1235, 749; H NMR (400 MHz,
(
3
DMSO-d ): d 3.76 (s, 3 H, MeO), 5.82 (s, 1 H, CH), 6.60 (d, J 1.5 Hz, 1 H, 2 CH),
6
3
.71–6.82 (m, 2 H, 2 CH), 7.13–7.17 (m, 2 H, 2 CH), 7.3 (d, J 8.1, 2 H, 2 CH), 7.3
6
3
d, J 7.9, 2 H, 2 CH), 7.88 (s, 2 H, 2 NH).
(
3
-[(1H-indol-3-yl)(4-nitrophenyl)methyl]-1H-indole (3e) Mp 219–223 °C; IR
KBr, cm ): 3442, 3052, 1592, 1502, 1341, 1502, 1454, 1341, 733, 512; H
-
1
1
(
3
NMR (400 MHz, DMSO-d ) d 5.99 (s, 1 H, CH), 6.69 (d, J 8.0 Hz, 1 H, 2 CH),
7
6
3
.01–7.05 (m, 2 H, 2 CH), 7.18–7.20 (m, 2 H, 2 CH), 7.34 (d, J 7.9 Hz, 2 H, 2 CH),
123

S. Kangari, I. Yavari
3
.39 (d, J 8.1 Hz, 2 H, 2 CH), 7.51 (d, J 8.6 Hz, 2 H, 2 CH), 8.02 (s, 2 H, 2 NH),
3
7
3
.14 (dd, J 6.9 Hz, 2H, 2 CH).
8
Results and discussion
Preparation of the catalyst
The magnetic nanoparticles were easily prepared via the co-precipitation method [24].
Citric acid was used as a coating agent, covalently attached on the MNPs surface, and
made stable colloidal dispersion. The choice of citric acid over other commonly
available coating agents is to control the hydrodynamic size and prevent aggregation of
MNPs at physiological condition, coating the magnetic nanoparticles with the silica
matrix should be a good solution. Furthermore, the silica hydroxyl groups(Si–OH) were
modified by CPTMS groups with covalent bonds. Finally, HMTA was immobilized on
the nanoparticle with covalent bonds between Silane groups and HMTA.
Characterization of the catalyst
The Fourier transform infrared (FT-IR) spectra of (a) Fe O , (b) Fe O –SiO , and
3
4
3
4
2
(
c) Fe O –SiO –HMTA tribromide are shown in Fig. 1. The FT-IR spectrum of
3 4 2
-
1
MNPs (Fig. 1a) the absorption bands around 3438 cm
stretching vibrations of hydroxyl group absorbed by MNPs and the peak at
is attributed to the
-
1
-1
1613 cm is assigned to the hydroxyl bending. The absorption band at 565 cm is
attributed to the Fe–O bond vibration of magnetite [17]. The broad high-intensity
-
1
band at 1062 cm
asymmetric stretching bands of Si–O–Si in silica shell. The band at 799 cm is
in spectrum of Fe O –SiO (Fig. 1b) is assigned to the
1
3 4 2
-
-
1
assigned to the Si–O–Si symmetric stretch, while the band at 455 cm corresponds
-
to vibration of the silicate ring structure. The band at 956 cm is assigned to the
1
-
Si–O stretch. The bands at 3425, 1630 cm correspond to the stretching and
1
3 4 3 4 2 3 4 2
Fig. 1 FT-IR spectra of (a) Fe O , (b) Fe O –SiO , and (c) Fe O –SiO –HMTA tribromide
1
23

3 4 2
Preparation of immobilized hexamine on Fe O /SiO …
bending vibrations of Si–OH [18]. The IR spectrum shown in Fig. 1c demonstrates
the characteristic bands of organic group modified on the Fe O –SiO framework.
3
4
2
-
1
There are two characteristic bands including C–H stretching 2925 and 2999 cm
-
and bending bonds at, 1459 and 1374 cm of the hexamine ring and CPTMS, C–
1
?
-1
stretching at 1615 cm [19]. Stretching bonds of the C–N moiety are at
N
-
1
-1
009–1233 cm . The band around 1628 cm is due to the bending vibration of
1
the hydroxyl groups of silica.
Furthermore, tribromide was characterized by biamperometric titration method.
Generally, amperometric titrations can be carried out with relatively simple
equipment. In this work, the platinum wire was used as an electrode. The reference
electrode can be double-compartmented with the Ag/AgC1 (3M KC1) reference in
the inner compartment and an appropriate electrolyte solution in the outer, the outer
being in contact with both the reference and the solution under investigation. The
analyte should be held in a titration vessel of 50–100 mL capacity. With the
availability of advanced electronic instrumentation, including microprocessor- or
computer-controlled titrators with microburettes, amperometric titrations are carried
out on a more or less automatic basis. This is particularly true of biamperometric
titrations. The auxiliary equipment required to convert the usual modern potentio-
metric titrator into one capable of amperometric titration involves a unit called a
‘‘polarizer’’. This unit, linked to the potentiometric titration unit, provides the fixed
applied potential required. It can also set the limit within which the current is
expected to fall during the titration. The current changes are converted to mV values
and the titrator plots these values against the titrant volume. The polarizer/titrator
combination can also be used in dead-stop titrations to stop the titration at a current
representative of the endpoint. Polarizers are also capable of carrying out titrations
by fixing the current allowed to flow and then plotting the potential changes required
to sustain this current flow against titrant volume. Here again, this mode can be used
to permit dead-stop titrations to be carried out.
All of the observed diffraction peaks are indexed by the cubic structure of Fe O
3
4
(
PDF: 00-011-0614). As can be seen, the XRD pattern of Fe O –SiO –HMTA
3 4 2
tribromide organic–inorganic hybrid nanomaterial is in agreement with that of the
standard magnetite (Fe O ) structure. Thus, the particles have phase stability and
3
4
their structural integrity is preserved. The diffraction peaks at 2h = 29.9, 35.4, 43.1,
3.4, 57.3, 62.4, and 74.1 correspond to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1),
4 4 0), and (5 3 3) of Fe O crystalline structure (Fig. 2).
5
(
3
4
To have a better insight into the morphology of magnetite and Fe O –SiO –
3
4
2
HMTA tribromide and composite structural changes, scanning electron microscopy
images were observed. Figure 3a clearly shows the Fe O NPs with uniform
structures. Moreover, as shown in Fig. 3b, the Fe O –SiO –HMTA tribromide
3
4
3
4
2
organic–inorganic hybrid nanomaterials have core–shell NPs.
The magnetic properties of Fe O4 were characterized by vibrating sample
3
magnetometer (VSM). The typical room temperature magnetization curves of bare
Fe O (Fig. 4a) and Fe O –SiO –HMTA tribromide (Fig. 4b) are shown. The
magnetic saturation values of the Fe O and Fe O –SiO –HMTA were 69 and
3
4
3
4
2
3
4
3
4
2
32 emu/g, respectively. The lower magnetic saturation of later NPs may be
attributed to the existence of nonmagnetic materials on the surface of MNPs.
123

S. Kangari, I. Yavari
3 4 2
Fig. 2 The XRD powder diffraction patterns of Fe O –SiO –HMTA tribromide
3 4 3 4 2
Fig. 3 The SEM images of a Fe O and b Fe O –SiO –HMTA tribromide
Fig. 4 Magnetization curves obtained by VSM at room temperature for a Fe
SiO –HMTA tribromide
3 4 3 4
O MNPs and b Fe O –
2
1
23

3 4 2
Preparation of immobilized hexamine on Fe O /SiO …
Table 2 A comparison between this work and similar works for the synthesis of 3-[(1H-indol-3-
yl)(phenyl)methyl]-1H-indole (3a)
Entry
Reagent and conditions
Time
Yield (%)
References
1
2
3
4
5
6
7
8
9
Nano-TiO
ZrOCl
Á8H
In(OTf) /CH
La(PFO) /EtOH
Alum 100 °C
ZrCl CH CN
[bmim] [Br] None r.t.
HPA/TPI-Fe
, Solvent-free/100 °C
HPA/TPI-SBA-15, Solvent-free/100 °C
Fe –SiO
–HMTA tribromide, Solvent-free/80 °C
2
, Solvent-free, 80 °C
O, Solvent-free, 50 °C
CN
30 min
40 min
25 min
30 min
60 min
35 min
8
95
84
71
90
73
91
50
96
84
96
[22]
2
2
[28]
3
3
[29]
3
[30]
[29]
4
3
[31]
[32]
3
O
4
25 min
15 min
10 min
[33]
[34]
10
3
O
4
2
This work
3 4 2
Fig. 5 Recyclability of Fe O –SiO –HMTA tribromide determined for the synthesis of 3a
The superiority of the present protocol over reported methods can be seen by
comparing our results with those of some recently reported procedures, as shown in
Table 2. The synthesis of 3-[(1H-indol-3-yl)(phenyl)methyl]-1H-indole (3a) was
used as a model reaction and the comparison is in terms of mol% of the catalysts,
temperature, reaction time, and yields.
The efficiency of the catalyst was determined for the synthesis of compound 3a.
As shown in Fig. 5, after four times recycling, the yield was reduced to 85 %.
Conclusions
In summary, the surface of silica-coated magnetite NPs has been modified with
3
-chloropropyltrimethoxysilane groups (CPTMS), then for the first time, surface of
silica-coated magnetic NPs has been modified with covalently immobilized between
the hexamine and the silane groups. This novel catalyst was characterized by XRD,
123

S. Kangari, I. Yavari
FTIR, SEM, and VSM. The research found that Fe O –SiO –HMTA tribromide is
3
4
2
an appropriate nanomagnetic catalyst with a high stability for the synthesis of BIMs
under solvent-free conditions. This nanocatalyst also was found to be very
stable after four times reusability. The nanomagnetic catalyst can also be readily
separated from reaction systems by external magnetic field.
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