Nano-Fe3O4 encapsulated-silica supported boron trifluoride as a novel heterogeneous solid acid for solvent-free synthesis of arylazo-1-naphthol derivatives

Article abstract of DOI:10.1039/c4ra12604a

Nano-Fe₃O₄ encapsulated-silica supported boron trifluoride (Fe₃O₄@SiO₂-BF₃) as a new type of green heterogeneous solid acid was prepared by the immobilization of BF₃·Et₂O on the surface of Fe₃O₄@SiO₂ core-shell nanoparticles and characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), field emission-scanning electron microscope (FE-SEM), energy dispersive X-ray (EDS), and transmission electron microscope (TEM). Then, this super solid acid was used as an acidic reagent for the synthesis of aryl diazonium salts as a starting reactant, followed by its diazo coupling with 1-naphthol in a basic solvent-free medium at room temperature. The main advantages of this clean method were high yields, short reaction times, the possibility of performing it at room temperature, and no need of corrosive and toxic liquid acids and solvents. In addition, the long-term stability of aryl diazonium salts supported on Fe₃O₄@SiO₂-BF₃ magnetic nanoparticles (MNPs) at room temperature was one of the most important results of this procedure. This journal is

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Nano-Fe3O4 encapsulated-silica supported boron trifluoride as a novel
heterogeneous solid acid for solvent-free synthesis of arylazo-1-
naphthol derivatives
Abdolhamid Bamoniri* and Naimeh Moshtael-Arani
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Nano-Fe3O4 encapsulated-silica supported boron trifluoride (Fe3O4@SiO2–BF3) as a new type of green
heterogeneous solid acid was prepared by the immobilization of BF3·Et2O on the surface of Fe3O4@SiO2
core-shell nanoparticles and characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray
diffraction (XRD), vibrating sample magnetometer (VSM), field emission-scanning electron microscope
(FE-SEM), energy dispersive X-ray (EDS), and transmission electron microscope (TEM). Then, this
super solid acid as an acidic reagent was used for the synthesis of aryl diazonium salts as a starting
reactant and following, their diazo coupling with 1-naphthol in a basic solvent-free medium at room
temperature. Main advantages of this clean method were high yields, short reaction times, room
temperature, no need to corrosive and toxic liquid acids and solvents. In addition, long-term stability of
aryl diazonium salts supported on Fe3O4@SiO2–BF3 magnetic nanoparticles (MNPs) at room temperature
was one of the most important results of this procedure.
Introduction
In recent years, solvent-free organic reactions1 have captured
great interest because of their many advantages such as high
efficiency and selectivity, easy separation and purification, mild
reaction condition, reduction in waste, simplicity in progress
and handling, and benefit to the industry as well as the
environment. Aromatic azo compounds constitute a very
important class of organic dyes because of their widespread
applications in many areas of technology and medicine. They
10 are well known for their use as colorants in the textile
industries,2 digital printing and photography.3 They are also
applied as chiral receptors,4 liquid crystals,5 new glassy
materials,6 chiral switches in photochemistry,7 dyes for drug,
food, cosmetic, biomedicine,8 and molecular recognition.9 As
15 synthesis of azo dyes requires some special conditions such as
low temperature and concentrated liquid acid, in addition to
high costs, it leads to corrosion of reactors and equipments,
too.10 Nowadays, solid supported reagents have resolved these
problems and improved activity and selectivity rather than
20 individual reagents.11 Such reagents not only simplify
purification processes but they also help prevent release of
reaction residues into the environment.12 In this regard, nano
structure solid acids exhibit higher activity and selectivity than
their corresponding bulk materials due to their particular
25 physical and chemical properties especially large surface to
volume ratio.13 Recently, Fe3O4 MNPs has appeared as a new
kind of efficient catalyst support due to low toxicity, ease of
surface modification, unique physical properties including the
high surface area and superparamagnetism.14 In order to
30 prevent the aggregation of Fe3O4 nanoparticles, its surface is
usually modified with silica layer, taking advantage of being
biocompatible and hydrophilic and, also, because the surface
silanol groups can easily react with various organic and
inorganic materials to achieve the certain purposes especially in
35 the field of catalysis.15 As a continuation of our efforts on the
development of heterogeneous solid acids in organic
transformations,16 we herein report the preparation and
characterization of a novel and eco-friendly magnetic solid acid
as Fe3O4@SiO2–BF3 and its utility for the synthesis of arylazo-
40 1-naphthol dyes in a solvent-free environment at room
temperature. In this method, it is not require providing special
cold condition for stabilization of aryl diazonium salt. The
reaction easily takes place at room temperature and resulting
diazonium salt can remain on the solid substrate for several
45 months. To the best of our knowledge, this research is the first
report about long-term stability of aryl diazonium salts
supported on the surface of MNPs and their use as the reactant
for synthesis of arylazo-1-naphthols in solvent-free condition.
The findings of this research may have implications for
5
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effective synthesis on a larger scale in dyeing and medical
industries.
from the absorption of B–O (Fig. 1c). The absorption band of
B–F was hidden under Si–O band. Also, the ethanolic OH and
45 existing moisture in BF3·Et2O caused a broad O–H stretching
band at wavenumber of 3221 cm-1. Again observation of Fe3O4
absorption peaks in Fig. 1b and Fig. 1c implies that Fe3O4
MNPs do not change chemically or physically after coating and
surface modification processes. According to this information
50 and regarding the reported structure of BF3·SiO2 in the
literature,17 the final structure of nano Fe3O4@SiO2–BF3 was
predicted in Scheme 2.
Results and discussion
This research was performed in three stages. Initially,
Fe3O4@SiO2–BF3 MNPs was synthesized and identified by FT-
IR, XRD, VSM, FE-SEM, EDS, and TEM. In the second stage,
aryl diazonium salts were synthesized in the presence of
Fe3O4@SiO2–BF3 nanoparticles at room temperature and their
structure and stability was investigated. The third stage was
5
10 diazo coupling of aryl diazonium salts supported on
Fe3O4@SiO2–BF3 with basic 1-naphthol under solvent-free
grinding condition.
Synthesis and characterization of Fe3O4@SiO2–BF3
nanoparticles
15 Fe3O4@SiO2–BF3 core–shell nanoparticles, with Fe3O4 spheres
as the core and silica supported BF3 as the shell, was prepared
by
a simple and convenient method. At first, Fe3O4
nanoparticles were prepared by chemical co-precipitation of
FeCl2·4H2O and FeCl3·6H2O in ammonium hydroxide solution.
20 To improve the chemical stability of Fe3O4 and prevent self-
aggregation, its surface engineering was successfully performed
by the suitable deposition of SiO2 onto Fe3O4 surface via the
ammonia-catalyzed hydrolysis of tetraethylorthosilicate
(TEOS). Next, the Fe3O4@SiO2 spheres served as support for
25 the immobilization of BF3 groups by simple stirring of
Fe3O4@SiO2 and BF3·Et2O in ethanol. All three steps were
carried out under sonication condition at room temperature
(Scheme 1).
Fig. 1 FT-IR spectra of a) Fe3O4, (b) Fe3O4@SiO2 and (c) Fe3O4@SiO2–
BF3.
H2O/EtOH, PEG
NH4OH, TEOS
.
FeCl3 6H2O
HCl, NH4OH
+
Sonication, N2
30 min, r.t.
Sonication, N2
6 h, r.t.
.
FeCl2 4H2O
= OBF3 EtOH2
Fe3O4
Fe3O4@SiO2
Scheme 2 The structure of Fe3O4@SiO2–BF3 MNPs.
.
BF3 Et2O, EtOH
Fig. 2 shows the XRD powder diffraction patterns of three
synthesized MNPs. The data for the Fe3O4 nanoparticles at 2θ
55 of 30.22, 35.61, 43.25, 53.58, 57.30, 62.89, and 74.66° (Fig. 2a)
corresponded to the standard Fe3O4 powder diffraction data.
Moreover, the relatively sharp peaks observed indicate phase
purity of Fe3O4 nanoparticles, which are consistent with the
presence of the cubic inverse spinel structure of Fe3O4. The
60 XRD pattern of the Fe3O4@SiO2 (Fig. 2b) was in good
agreement with that of Fe3O4 phase, except for a broad peak
around 2θ of 20-30° corresponding to amorphous phase of
SiO2. This indicates that the MNPs obtained after the coating
process are composed of Fe3O4 core and amorphous SiO2 shell.
65 The XRD pattern of the modified Fe3O4@SiO2 with BF3·Et2O
in Fig. 2c was nearly the same with Fig. 2b, which it seems that
the surface modification by BF3 group has little effect on the
XRD pattern of Fe3O4@SiO2 nanoparticles, because of the
shielding effect of Fe3O4 and SiO2 peaks. However, the changes
70 in peaks intensity in spectra 2b and 2c and increase the noise in
Fig. 2c can verify the linking of BF3 on the surface of
Fe3O4@SiO2 core-shell MNPs. Furthermore, characteristic
peaks of Fe3O4 were observed in three samples, thereby
Sonication, N2
1 h, r.t.
Fe3O4@SiO2-BF3
Scheme 1 Preparation of Fe3O4@SiO2–BF3 MNPs under sonication
condition at room temperature.
In order to identify the molecular structures of Fe3O4,
30 Fe3O4@SiO2, and Fe3O4@SiO2–BF3 MNPs, FT-IR analysis of
three mentioned samples was performed (Fig. 1). Fe3O4 was
identified by a stretching vibration of the Fe–O absorption peak
at 576 cm-1, O–H stretching vibration at 3429 cm-1 and O–H
deformed vibration at 1625 cm-1 in Fig. 1a. The FT-IR
35 spectrum of Fe3O4@SiO2 (Fig. 1b) displayed characteristic
peaks at 1093 and 800 cm-1 corresponding to symmetrical and
asymmetrical vibrations of Si–O–Si, respectively. Weak band
at 466 cm-1 corresponded to the Si–O–Fe stretching vibrations
of the Fe3O4@SiO2 core-shell, overall confirming the presence
40 of SiO2 in the sample. The successful covalent linking of the
BF3·Et2O on the surface of Fe3O4@SiO2 core-shell was proved
by the appearance of a new band at 1457 cm-1, which originates
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indicating that the binding process did not induce any phase
change.
surface morphology of three kinds of synthesized MNPs with a
nearly spherical shape. Elemental components of three MNPs
were characterized by EDS analysis. Fig. 4d shows EDS of the
20 Fe3O4 nanoparticles, in which the particles contain Fe and O
elements. The presence of Fe, O and Si elements in Fig. 4e
verified the coating of Fe3O4 core by SiO2 shell. The
appearance of two new signals related to F and C elements in
Fig. 4f confirmed supporting of BF3·Et2O on Fe3O4@SiO2
25 core-shell nanoparticles according to Scheme 2.
Fig. 2 XRD patterns of (a) Fe3O4, (b) Fe3O4@SiO2, and (c) Fe3O4@SiO2–
BF3.
The magnetic properties of synthesized Fe3O4, Fe3O4@SiO2,
and Fe3O4@SiO2–BF3 nanoparticles were assessed by VSM at
room temperature. The magnetization curve in Fig. 3 indicates
magnetization as a function of applied magnetic field. The
saturation magnetization of the Fe3O4@SiO2 nanoparticles in
Fig. 3b was about 29.15 emu/g, and this reduced to 27.44
emu/g after supporting with BF3·Et2O (Fig. 3c). Both of these
5
10 values were much lower than the initial saturation
magnetization of Fe3O4 nanoparticles (59.2 emu/g) in Fig. 3a.
The decrease of the saturation magnetization after surface
coating of Fe3O4 confirms the presence of a diamagnetic outer
shell (SiO2 or SiO2–BF3) at the surface of the Fe3O4 particles.
Fig. 4 FE-SEM images of (a) Fe3O4, (b) Fe3O4@SiO2, (c) Fe3O4@SiO2–BF3
and EDS spectra of (d) Fe3O4, (e) Fe3O4@SiO2, (f) Fe3O4@SiO2–BF3.
TEM images of Fe3O4 and Fe3O4@SiO2–BF3 are displayed in
Fig. 5. As demonstrated in Fig. 5a, Fe3O4 nanoparticles have
the spherical morphology. In Fig. 5b, two regions with different
electron densities can be distinguished that confirms the Fe3O4
30 nanoparticles were successfully coated with a thin layer of a
different phase. However, it can be observed that the sample is
nearly in core-shell structure. An electron dense region (black
colour) which corresponds to Fe3O4 cores and a less dense or
more translucent region (ash colour) surrounding these cores
35 that is SiO2–BF3 shell. From the size distribution histograms,
the average size of 9 nm for Fe3O4 and 13 nm for Fe3O4@SiO2–
BF3 nanoparticles could be estimated.
Fig. 3 Magnetization curves of (a) Fe3O4, (b) Fe3O4@SiO2, and (c)
Fe3O4@SiO2–BF3.
15 The high magnification FE-SEM images of the purified MNPs
are displayed in Fig. 4(a-c). These images clearly showed the
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20 diazotization of 4-chloro aniline as a model substrate. A range
of parameters such as stability of 4-chlorophenyl diazonium salt
at room temperature, reaction time of diazotization, and yield of
resulted 4-chlorophenylazo-1-naphthol were screened in Table
1.
25 Solid acids such as silica phosphoric acid and silica chloride
gave the 4-chlorophenylazo-1-naphthol with modest yields of
57% and 61%, respectively (Table 1, entries 1 and 2). The
stability of aryl diazonium salt supported on above mentioned
solid acids was maximum 2 days. Fe3O4@SiO2 and BF3·Et2O
30 sources were also tested individually. There was no reaction in
the presence of neither Fe3O4@SiO2 nor BF3·Et2O (Table 1,
entries 3 and 4), while Fe3O4@SiO2–BF3 with different
loadings afforded the improved yields from 87% to 95% for 4-
chlorophenylazo-1-naphthol (Table 1, entries 5-8). These
35 results clearly indicated that the presence of Brönsted acid sites
on the solid acid surface is essential for promoting
diazotization. Also, the rate of diazotization in the presence of
Fe3O4@SiO2–BF3 MNPs was much higher than silica
phosphoric acid and silica chloride.
40 In another comparative study (Table 1, entries 5-8), the effect
of Fe3O4@SiO2 loading by BF3·Et2O on the acidic performance
of Fe3O4@SiO2–BF3 was investigated. Although the time of
diazotization and the stability of aryl diazonium salt supported
on Fe3O4@SiO2–BF3 nanoparticles with different loadings in
45 the same conditions were approximately identical, but 10 wt%
Fe3O4@SiO2–BF3 resulted in the highest yield of the
corresponding azo dye (Table 1, entry 6). In conclusion, 10
wt% Fe3O4@SiO2–BF3 was selected as the most ideal acid for
synthesis of arylazo-1-naphthol dyes among those listed in
50 Table 1.
In addition, with more investigations, it was found that aryl
diazonium salts supported on Fe3O4@SiO2–BF3 nanoparticles
underwent no significant change at room temperature for
several months. To explore the reason of this unusual stability,
55 FT-IR spectrum of 4-chlorophenyl diazonium salt was studied
(Fig. 6).
Fig. 5 TEM images of (a) Fe3O4, and (b) Fe3O4@SiO2–BF3 nanoparticles.
Table 1 Comparison of efficiency of various acids in synthesis of 4-
chlorophenyl diazonium salt.
Finally, Fe3O4@SiO2–BF3 was identified by using the
techniques described above, and applied for synthesis of
arylazo-1-naphthol derivatives.
anion
NH2
N
N
NaNO2, Acid
Synthesis and characterization of aryl diazonium salts
Solvent-free, r.t.
Cl
5
Aryl diazonium salts are usually synthesized in the presence of
a liquid acid dissolved in water at low temperatures between 0
to 5 °C, because temperatures above 5 °C generally promote
phenol formation in aqueous media. Thus, as synthesis of aryl
diazonium salts has limitations and drawbacks such as the
Cl
Time of
Yielda
diazotization (%)
Entry Acid (wt %)
Stabilityat r.t.
1
2
3
4
5
6
7
8
SPAb (10)
~ 2 days
~ 2 days
----
35 min
30 min
No reaction
No reaction
12 sec
6 sec
8 sec
57
61
---
---
89
95
90
87
SCc (10)
10 control and maintenance of the low-temperature, using toxic
liquid acids that are incompatible with environment, and most
important of all, instability of aryl diazonium salts at room
temperature, we tried to resolve these problems by the
development a green and simple procedure for synthesis of aryl
15 diazonium salts and in continue, their diazo coupling with 1-
naphthol.
Fe3O4@SiO2
BF3·Et2O
Fe3O4@SiO2–BF3 (5)
Fe3O4@SiO2–BF3 (10)
Fe3O4@SiO2–BF3 (15)
Fe3O4@SiO2–BF3 (20)
----
˃ 13 months
˃ 13 months
˃ 13 months
˃ 13 months
8 sec
a
The yields refer to the total isolated yield of 2-(4-chlorophenylazo)-1-
naphthol and 4-(4-chlorophenylazo)-1-naphthol after adding fresh 4-
At first, in order to evaluate the efficiency of Fe3O4@SiO2–BF3
MNPs in diazotization reaction, an initial optimization of the
kind and amount of acidic reagent was performed via
b
chlorophenyl diazonium salt into basic 1-naphthol. Silica phosphoric
acid. c Silica chloride.
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In this spectrum, the ethanolic OH and existing moisture in
BF3·Et2O caused a broad O–H stretching band at wavenumber
of 3419 cm-1. The appearance of a new band at 2289 cm-1
clearly demonstrated N≡N stretching vibration and verified the
formation of 4-chlorophenyl diazonium salt. The absorption
bands of B–O and Si–O vibrations were observed at 1442 and
1085 cm-1, respectively. Aromatic C–H bending vibrations, C–
Cl and Fe–O stretching bands were revealed at 829, 634, and
586 cm-1, respectively.
diazotization.
NaNO2
Solvent-free, r.t.
ArNH2
+
5
= OBF3 EtOH2
= OBF3 N2Ar
Scheme 4 Diazotization of aromatic amines in the presence of
Fe3O4@SiO2–BF3 nanoparticles.
It is important to note that Fe3O4@SiO2–BF3 in diazotization
reaction acts as a two-function reagent, so that, on one side, its
−
acidic protons convert the nitrite anions (NO2 ) in NaNO2 to
35 nitrosonium cations (NO+) to promote diazotization and on the
other side, its bulky anions cause the stability of aryl diazonium
cations.
Synthesis of arylazo-1-naphthol dyes
The most common synthetic route to the arylazo-1-naphthol
40 compounds involves coupling of aryl diazonium salts with 1-
naphthol in basic solution. Although using of water for
preparation of 1-naphthoxide salt seems to be necessary, but
like the previous step, using of water cause the formation of
some phenoxide salt and its attack to diazonium salt to form
45 phenolic azo dyes. To prevent this problem, the reaction was
performed under solvent-free condition toward green
chemistry. So, we ground solid 1-naphthol with some NaOH in
a mortar. The moisture absorbed by the reaction mixture during
the grinding seems to be sufficient for the formation of a
50 homogeneous mixture. Moreover, the higher concentration of
reactants in the absence of solvent at room temperature usually
leads to more favourable kinetics than in solution.18
Fig. 6 FT-IR spectrum of 4-chlorophenyl diazonium salt supported on
Fe3O4@SiO2–BF3 MNPs.
10 According to this information, the structure of aryl diazonium
salts supported on MNPs was guessed. Scheme 3 reveals that in
this probable structure, aryl diazonium cations are located on
the surface of negatively charged particles called Fe3O4–silica
trifluoroborate nanoparticles. So, the presence of bulky anions
15 and charge–charge interactions between nitrogen and boron
atoms are the possible reasons of unusual stability of these
salts.
In this reaction, diazo coupling of aryl diazonium salts with 1-
naphtoxide led to two products in which 4-arylazo-1-naphthol
55 derivatives were the major products and 2-arylazo-1-naphthol
derivatives got as the minor products. The resulted two main
products were easily separated by flash column
chromatography and identified by 1H-NMR and FT-IR
spectroscopic methods. The molar ratios of these two dyes were
1
60 determined from integral intensities of OH/NH signals in H-
NMR spectrum (ratio 5:1 for 4-arylazo-1-naphthol to 2-arylazo-
1-naphthol). Total yields of two products are shown in Table 2.
It is also important that the electronic effects of aryl diazonium
salt substituents do not play significant role in the rate of diazo
65 coupling step. However, coupling reaction occurred rapidly
regardless of substituent effects.
Totally, in this research we investigated the application of
Fe3O4@SiO2–BF3 MNPs as a strong and useful solid acid
reagent for synthesis of azo dyes based on 1-naphthol via a
70 green procedure.
= OBF3 N2Ar
Scheme 3 The probable structure of aryl diazonium salt supported on
Fe3O4@SiO2–BF3 nanoparticles.
As a result, the high conversions of aniline derivatives to aryl
diazonium salts and their long-term stability showed that the
20 Fe3O4@SiO2–BF3 has strong and sufficient acidic sites, which
are responsible for excellent performance in synthesis of aryl
diazonium salts.
After optimization of the conditions, aniline derivatives
including
electron-withdrawing
and
electron-donating
25 substituents were ground with NaNO2 and Fe3O4@SiO2–BF3
nanoparticles. Aryl diazonium salts were obtained in a very
short time with an excellent conversion at room temperature
(Scheme 4). Indeed, due to the high acidic strength of
Fe3O4@SiO2–BF3 nanoparticles, the reaction was done so rapid
30 that the substituent type could not effect on the time of
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Table 2 Solvent-free synthesis of arylazo-1-naphthol derivatives at room temperature.
HO
Ar
N
OH
OH
NaOH
N
+
+
Solvent-free, r.t.
N
N
Ar
= OBF3 N2Ar
major
minor
MPfound (Lit.) in a′/ found
(Lit.) in b′ (°C)
Entry Amine
2-Arylazo-1-naphthol (a′)
4-Arylazo-1-naphthol (b′)
Yielda (%)
HO
NH2
NH2
NH2
181-183 (180.5-
181)19/ 176-178
(177-178)20
OH
N
N
OCH3
1
2
92
OCH3
OCH3
N
N
HO
OH
N
OCH3
116-118/ 157-159
(159)20
89
93
N
N
N
OCH3
OCH3
HO
OCH3
127-129 (127.5–
128)19/ 173-175
(173)20
OH
N
N
N
N
3
OCH3
OCH3
HO
HO
HO
NH2
157-159 (156)19/
160-162 (162-
163)20
OH
N
N
CH3
4
5
91
90
CH3
CH3
N
N
NH2
116-118 (117-
118)21/ 201-203
(200)20
OH
N
N
CH3
N
N
CH3
CH3
CH3
NH2
148-150 (151)19/
211-213 (211.5)20
OH
N
N
N
N
6
96
CH3
CH3
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HO
CH3
NH2
CH3
184-186 (186)21/
138-140
OH
N
N
CH3
CH3
N
N
7
87
CH3
CH3
NH2
HO
137-139 (138)22/
207-209 (208)20
OH
OH
OH
N
N
8
89
92
90
N
N
HO
NH2
N
N
Cl
172-174/ 188-190
(189.5-190.5)20
9
Cl
Cl
N
N
N
N
N
N
HO
NH2
N
N
Cl
152-154/ 228-230
(227.5-228)20
10
Cl
Cl
HO
Cl
NH2
OH
N
N
187-189 (187)21/
229-231 (229.5)20
11
95
Cl
Cl
HO
NH2
216-218 (216)23/
258-260 (256-
257)20
OH
N
N
NO2
12
85
NO2
NO2
N
N
a Total isolated yield of 2-arylazo-1-naphthols and 4-arylazo-1-naphthols after chromatography.
working frequency of 40 KHz. XRD patterns were acquired
using a Philips Xpert MPD diffractometer equipped with a Cu
15 Kα anode (λ=1.54 Å) in the 2θ range from 10 to 80°.
Magnetization of the samples was recorded as a function of the
applied magnetic field sweeping between 10 kOe at room
temperature. All measurements were performed on a vibrating
sample magnetometer device (Meghnatis Daghigh Kavir Co.;
20 Kashan Kavir; Iran). The morphology of the synthesized
samples was studied by a Mira II LMU Tescan FE-SEM made
in Czech Republic. Elemental composition of three above
mentioned MNPs was investigated by EDS spectroscopy
(SAMX, France). The average size of Fe3O4 and Fe3O4@SiO2–
25 BF3 MNPs was analyzed by TEM using a Philips CM120 with
Experimental
Materials and apparatus
Chemicals and solvents were purchased from Merck and
Sigma-Aldrich Companies. Melting points were obtained with a
micro melting point apparatus (Electrothermal, Mk3) and are
uncorrected. 1H-NMR spectra were recorded on a Bruker DRX-
400 Avance spectrometer. Tetramethyl silane (TMS) was used
as an internal reference and CDCl3 and DMSO-d6 used as
solvents. FT-IR spectra were run on a Nicolet Magna 550
10 spectrometer. The ultrasonic equipment used for the synthesis
of MNPs (Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2–BF3) was a
Sonica 2200ETH S3 SOLTEC ultrasonic bath (Italy) with a
5
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a LaB6 cathode and accelerating voltage of 120 kV.
aniline derivatives in the presence of Fe3O4@SiO2–BF3 MNPs
and their diazo coupling with 1-naphthol at room temperature.
Using Fe3O4@SiO2–BF3 and solvent-free procedure caused the
experimental simplicity, no use of special conditions such as
60 liquid acids and low temperature, compatibility with
environment, efficient yields, short reaction times and made
this procedure attractive to synthesize a variety of these
important dyes. The structure and stability of aryl diazonium
salt supported on Fe3O4@SiO2–BF3 MNPs were studied, too.
Synthesis of Fe3O4@SiO2–BF3 MNPs
The synthesis of Fe3O4 nanoparticles was carried out according
to the known procedure using chemical co-precipitation method
by a little modification of the methodology already reported in
the literature.24 Briefly, FeCl3·6H2O (8.0 g, 0.0216 mol) and
FeCl2ꢀ4H2O (3.5 g, 0.0108 mol) with molar ratio of 1:2, were
dissolved in 38 mL of deoxygenated 0.4 M HCl solution. Then,
375 mL of deoxygenated 0.7 M ammonia solution was quickly
5
10 added into the reaction mixture under sonication and nitrogen
atmosphere. This resulted in immediate formation of a black
precipitate of Fe3O4 (magnetite). The sonication of magnetite
dispersion was continued for 30 min. Finally, the precipitates
were collected using an external magnetic field and washed for
15 several times with distilled water and ethanol. The synthesized
Fe3O4 MNPs were suspended in 50 mL of distilled water for
use in the next steps.
Acknowledgments
The authors are grateful to University of Kashan for supporting
this work by Grant No. 159189/41.
65 Notes and references
Department of Organic Chemistry, Faculty of Chemistry, University of
Kashan, Kashan 87317-51167, Iran.
E-mail: bamoniri@kashanu.ac.ir
Modified Stöber sol-gel process,25 was used for coating
magnetite nanoparticles with a silica shell. Typically, 50 mL of
20 magnetite suspended in water was added to 250 mL ethanol and
sonicated at room temperature for 20 min under nitrogen flow.
Then 11.85 mL PEG 200, 50 mL distilled water, 25 mL NH3
(28%) were added respectively, and after 15 min, 5 mL of
TEOS was introduced into the suspension and the mixture was
25 again sonicated for 6 h. Fe3O4@SiO2 nanoparticles was
centrifuged at 3000 rpm for 10 min, the solvent was discarded
and nanoparticles were washed three times with water and then
ethanol and dried in vacuum at room temperature.
In the final stage, BF3·Et2O (0.45 mL) was added drop-wise to
30 a slurry containing Fe3O4@SiO2 core-shell nanoparticles (4.5 g)
and ethanol (15 mL). The mixture was sonicated for 1 h at
room temperature. The resulted suspension was filtered and
dried at room temperature to obtain the brown solid named
nano Fe3O4@SiO2–BF3 (10 wt%).
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Nano-Fe3O4 encapsulated-silica supported boron trifluoride as a
novel heterogeneous solid acid for solvent-free synthesis of
arylazo-1-naphthol derivatives
Abdolhamid Bamoniri and Naimeh Moshtael-Arani
Fe3O4@SiO2-BF3 nanoparticles was prepared as a novel solid acid, and effectively applied
for solvent-free synthesis of arylazo-1-naphthols at room temperature.