BF3-functionalized silica-coated magnetic nanoparticles as a novel heterogeneous solid acid for synthesis of formazan derivatives via a green protocol

Article abstract of DOI:10.1246/bcsj.20140298

A new type of green heterogeneous solid acid was prepared by the immobilization of BF₃·Et₂O on the surface of Fe₃O₄@SiO₂ core-shell nanocomposite (Fe₃O₄@SiO₂-BF₃) 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). The activity of this super solid acid was probed through the synthesis of aryl diazonium salts as the starting reactant and then, their diazo coupling with aldehyde phenylhydrazones for formation of formazan derivatives in a solvent-free medium at room temperature. This clean and environmentally benign methodology has advantages such as: no need for corrosive and toxic liquid acids, solvents, or buffer solutions, room temperature reaction, high yields, and short reaction times. In addition, long-term stability of aryl diazonium salts supported on the surface of Fe₃O₄@SiO₂-BF₃ magnetic nanoparticles (MNPs) at room temperature was one of the most important results of this procedure.

Full text of DOI:10.1246/bcsj.20140298

BF3-Functionalized Silica-Coated Magnetic Nanoparticles
as a Novel Heterogeneous Solid Acid for Synthesis
of Formazan Derivatives via a Green Protocol
Abdolhamid Bamoniri* and Naimeh Moshtael-Arani
Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, Iran
E-mail: bamoniri@kashanu.ac.ir
Received: October 6, 2014; Accepted: January 15, 2015; Web Released: January 22, 2015
A new type of green heterogeneous solid acid was prepared by the immobilization of BF3¢Et2O on the surface of
Fe3O4@SiO2 core-shell nanocomposite (Fe3O4@SiO2-BF3) 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). The activity of this super solid
acid was probed through the synthesis of aryl diazonium salts as the starting reactant and then, their diazo coupling with
aldehyde phenylhydrazones for formation of formazan derivatives in a solvent-free medium at room temperature. This
clean and environmentally benign methodology has advantages such as: no need for corrosive and toxic liquid acids,
solvents, or buffer solutions, room temperature reaction, high yields, and short reaction times. In addition, long-term
stability of aryl diazonium salts supported on the surface of Fe3O4@SiO2-BF3 magnetic nanoparticles (MNPs) at room
temperature was one of the most important results of this procedure.
At the beginning of the new century, synthesis of organic
compounds under solvent-free conditions has received much
attention,1-5 as it provides manipulative simplicity, greater
selectivity, easier workup, shorter reaction times and higher
yields which matches with the green chemistry protocol.
Formazans are colored compounds due to their π-π* electronic
transitions and form a distinct class of organic dyes with certain
properties. These compounds have received much attention for
their applications in analytical chemistry,6 biological applica-
tions,7 and as dyestuffs.8 Their antiviral,9 antimicrobial,10 anti-
inflammatory, analgesic,11 antifungal,12 anticancer, anti-HIV,13
photo- and thermochromic activities14 are also of utmost impor-
tance. Although so far a lot has been known about numerous
synthesized formazans and their structural, spectral features
and reaction mechanisms of their formation,15-25 their difficult
synthetic conditions including the control of temperature
between 0-5 °C and medium pH between 10-12, and problems
such as time-consuming preparation of buffer solutions with
specific concentrations, incompatibility with the environment
due to the use of toxic liquid acids and solvents, instability of
aryl diazonium salts at room temperature, and finally the long
reaction times and modest yields, have prevented formazans
from gaining widespread use in industry. Accordingly, the
development of novel and simple methods for solving the
above mentioned problems and efficient synthesis of formazans
is an interesting challenge. Nowadays, solid acids have
resolved most of these drawbacks and improved activity and
selectivity rather than individual reagents.26-30 In this regard,
nanostructure solid acids exhibit higher activity and selectivity
than their corresponding bulk materials due to their large sur-
face to volume ratio.31-33 Recently, silica-coated MNPs34 have
emerged as a new kind of efficient catalyst support because
of advantages such as biocompatibility, hydrophilicity, low
toxicity, high surface area, and ease of surface modification
with various organic and inorganic materials to achieve certain
purposes especially in the field of catalysis.35-42 Motivated by
the special properties of Fe3O4@SiO2 nanoparticles and high
acidity of BF3¢SiO2, and in continuation of our efforts on the
development of efficient solid acids for useful synthetic organic
transformations43-47 herein we report the preparation and char-
acterization of a novel and eco-friendly magnetic solid acid as
Fe3O4@SiO2-BF3 and its utility for the synthesis of formazan
dyes in a solvent-free environment at room temperature. 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 formazans in solvent-free conditions. The findings of this
research may have implications for an effective synthesis on a
larger scale in dyeing and medical industries.
Results and Discussion
This research was performed in two stages. Initially,
Fe3O4@SiO2-BF3 MNPs were synthesized and identified by
FT-IR, XRD, VSM, FE-SEM, EDS, and TEM techniques. In
the second stage, formazan derivatives were synthesized by
solvent-free mixing aryl diazonium salts supported on MNPs
and aldehyde phenylhydrazones at room temperature.
Synthesis and Characterization of Fe3O4@SiO2-BF3
Nanoparticles. Fe3O4@SiO2-BF3 core-shell nanoparticles,
with Fe3O4 spheres as the core and silica-supported BF3 as the
shell, were prepared by a simple, low cost, and convenient
method. At first, Fe3O4 nanoparticles were prepared by the
coprecipitation of FeCl2 and FeCl3 in ammonia solution. To
improve the chemical stability of Fe3O4, its surface engineering
662 | Bull. Chem. Soc. Jpn. 2015, 88, 662–672 | doi:10.1246/bcsj.20140298
© 2015 The Chemical Society of Japan

.
H2O/EtOH, PEG
NH4OH, TEOS
FeCl3 6H2O
HCl, NH4OH
+
Sonication, N2
30 min, r.t.
Sonication, N2
6 h, r.t.
.
FeCl2 4H2O
Fe3O4
Fe3O4@SiO2
.
BF3 Et2O, EtOH
Sonication, N2
1 h, r.t.
Fe3O4@SiO2-BF3
= OBF3 EtOH2
Scheme 1. The schematic diagram for synthesis of Fe3O4@SiO2-BF3 MNPs.
2θ/degree
Figure 2. XRD patterns of (a) Fe3O4, (b) Fe3O4@SiO2, and
(c) Fe3O4@SiO2-BF3.
Figure 1. FT-IR spectra of a) Fe3O4, (b) Fe3O4@SiO2, and
(c) Fe3O4@SiO2-BF3.
tion band of B-F was hidden under the Si-O band. Also, the
ethanolic OH and existing moisture in BF3¢Et2O caused a broad
O-H stretching band at a wavenumber of 3221 cm¹1. Again
observation of Fe3O4 absorption peaks in Figures 1b and 1c
implies that Fe3O4 MNPs do not change chemically or physi-
cally after coating and surface modification processes. Accord-
ing to this information and regarding the reported structure
of BF3¢SiO2 in the literature,48 the final structure of nano
Fe3O4@SiO2-BF3 was predicted (Scheme 1).
Figure 2 shows the XRD powder diffraction patterns of three
synthesized MNPs. The data for the Fe3O4 nanoparticles at
2ª of 30.22, 35.61, 43.25, 53.58, 57.30, 62.89, and 74.66°
(Figure 2a) corresponded to the standard Fe3O4 powder diffrac-
tion data. Moreover, the relatively sharp peaks observed indi-
cate phase purity of Fe3O4 nanoparticles, which are consistent
with the presence of the cubic inverse spinel structure of Fe3O4.
The XRD pattern of the Fe3O4@SiO2 (Figure 2b) was in good
agreement with that of Fe3O4 phase, except for a broad peak
around 2ª 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.
The XRD pattern of the modified Fe3O4@SiO2 with BF3¢Et2O
in Figure 2c was nearly the same as Figure 2b, which it seems
that the surface modification by BF3 groups has little effect
was successfully performed by the suitable deposition of SiO2
onto Fe3O4 surface via the ammonia-catalyzed hydrolysis of
tetraethyl orthosilicate (TEOS). Next, the Fe3O4@SiO2 spheres
served as support for the immobilization of BF3 groups by
simple stirring of Fe3O4@SiO2 and BF3¢Et2O in ethanol. All
three steps were carried out under sonication (Scheme 1).
In order to identify the molecular structures of Fe3O4,
Fe3O4@SiO2, and Fe3O4@SiO2-BF3 MNPs, FT-IR analysis
of three mentioned samples was performed (Figure 1). Fe3O4
was identified by a stretching vibration of the Fe-O absorption
peak at 576 cm¹1, O-H stretching vibration at 3429 cm¹1, and
O-H deformed vibration at 1625 cm¹1 in Figure 1a. The FT-IR
spectrum of Fe3O4@SiO2 (Figure 1b) displayed characteristic
peaks at 1093 and 800 cm¹1 corresponding to symmetrical and
asymmetrical vibrations of Si-O-Si, respectively. The weak
band at 466 cm¹1 corresponded to the Si-O-Fe stretching vibra-
tions of the Fe3O4@SiO2 core-shell, overall confirming the
presence 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 cm¹1, which
originates from the absorption of B-O (Figure 1c). The absorp-
Bull. Chem. Soc. Jpn. 2015, 88, 662–672 | doi:10.1246/bcsj.20140298
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Figure 3. Magnetization curves of (a) Fe3O4, (b)
Fe3O4@SiO2, and (c) Fe3O4@SiO2-BF3.
on the XRD pattern of Fe3O4@SiO2 nanoconpmosite, because
of the shielding effect of Fe3O4 and SiO2 peaks. However,
decrease in the signal to noise ratio in Figure 2c compared
to Figure 2b 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
indicating that the binding process did not induce any phase
change.
The magnetic properties of synthesized Fe3O4, Fe3O4@SiO2,
and Fe3O4@SiO2-BF3 nanoparticles were assessed by VSM
at room temperature. The magnetization curve in Figure 3
indicates magnetization as a function of applied magnetic field.
The saturation magnetization of the Fe3O4@SiO2 nanocompo-
¹1
site was about 29.15 emu g (Figure 3b), and this reduced
Figure 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.
¹1
to 27.44 emu g after supporting with BF3¢Et2O (Figure 3c).
Both of these values were much lower than the initial satura-
tion magnetization of Fe3O4 nanoparticles (59.2 emu g¹1) in
Figure 3a. The decrease of the saturation magnetization after
surface coating of Fe3O4 confirms the presence of a diamag-
netic outer shell (SiO2 or SiO2-BF3) at the surface of the Fe3O4
particles.
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
that is SiO2-BF3 shell. About 100 nanoparticles were consid-
ered for each sample to obtain their size distribution histo-
grams. From the size distribution histograms, the average size
of 9 nm for Fe3O4 and 13 nm for Fe3O4@SiO2-BF3 nano-
particles could be estimated.
The high-magnification FE-SEM images of the purified
MNPs are displayed in Figures 4a-4c. These images clearly
showed the surface morphology of three kinds of synthesized
MNPs with a nearly spherical shape. Elemental components of
three MNPs were characterized by EDS analysis. Figure 4d
shows EDS of the Fe3O4 nanoparticles, in which the particles
contain Fe and O elements. The presence of Fe, O, and Si
elements in Figure 4e verified the coating of Fe3O4 core by
SiO2 shell. The appearance of two new signals related to F and
C elements in Figure 4f confirmed supporting of BF3¢Et2O on
Fe3O4@SiO2 core-shell nanoparticles according to the final
structure of Fe3O4@SiO2-BF3 presented in Scheme 1.
TEM images of Fe3O4 and Fe3O4@SiO2-BF3 together with
their size distribution histograms are displayed in Figure 5.
As demonstrated in Figure 5a, Fe3O4 nanoparticles have the
spherical morphology. In Figure 5b, two regions with different
electron densities can be distinguished that confirms the Fe3O4
nanoparticles were successfully coated with a thin layer of a
different phase. However, it can be observed that the sample is
Finally, Fe3O4@SiO2-BF3 was identified by using the tech-
niques described above, and applied for synthesis of formazan
derivatives.
Synthesis and Characterization of Formazan Derivatives
3a-3q. The most common synthetic route to the formazan
compounds 3a-3q involves coupling of aryl diazonium salts
1a-1g with aldehyde phenylhydrazones 2a-2i in a basic
medium. As synthesis of formazans is a highly time-consuming
reaction with modest yields, which requires special conditions
such as low temperature, using buffer solutions to adjust pH
and liquid acids to synthesize diazonium salts,15-25 we tried to
solve all these problems by the development a green and facile
procedure for synthesis of formazans.
At first, the efficiency of Fe3O4@SiO2-BF3 MNPs in
diazotization reaction, followed by diazo coupling with alde-
664 | Bull. Chem. Soc. Jpn. 2015, 88, 662–672 | doi:10.1246/bcsj.20140298
© 2015 The Chemical Society of Japan

Figure 5. TEM images of (a) Fe3O4 and (b) Fe3O4@SiO2-BF3 nanoparticles along with size distribution histograms.
hyde phenylhydrazones, was explored via a model reaction. 4-
Chloroaniline was selected as a model substrate and screened
for a range of parameters such as effect of various acids, sta-
bility of 4-chlorophenyl diazonium salt (1d) at room temper-
ature, time of diazotization, and yield of resulted formazan
(3h). The observed results are summarized in Table 1.
According to Table 1, the mild and protic solid acids such
as silica phosphoric acid and silica chloride gave the corre-
sponding formazan (3h) with modest yields of 43% and 48%,
respectively (Table 1, Entries 1 and 2). The stability of 4-
chlorophenyl diazonium salt (1d) supported on the above
mentioned acids was at maximum 2 days. Lewis acids such as
Fe3O4, Fe3O4@SiO2, and BF3¢Et2O sources were also tested
individually. There was no reaction in the presence of Fe3O4,
Fe3O4@SiO2, and BF3¢Et2O (Table 1, Entries 3-5), while the
yield of formazan 3h in the presence of BF3¢SiO2 (Table 1,
Entry 6) as a strong protic acid was obtained 63%. These
results clearly indicated that the presence of Brønsted acid
sites on the solid acid surface is an essential factor for promot-
ing diazotization reaction. In other words, diazotization only
occurs in the presence of Brønsted acids. One of the interesting
points of using BF3¢SiO2, in addition to decrease the reaction
time, was the long-term stability of aryl diazonium salt (1d)
supported on the surface of this solid acid compared to silica
phosphoric acid and silica chloride. So, BF3¢SiO2 showed
better performance than the other above mentioned acids
(Table 1, Entries 1-5). In order to improve the yield of reaction,
we decided to prepare a protic solid acid with similar properties
to BF3¢SiO2 in stabilizing aryl diazonium salts and reducing
the reaction time. So, Fe3O4@SiO2-BF3 nanoparticles as a
novel protic solid acid was synthesized and its performance
tested in diazotization reaction. Fe3O4@SiO2-BF3 nanoparti-
cles with different loadings afforded the improved yield of
79% to 87% for formazan 3h (Table 1, Entries 7-10).
In another comparative study (Table 1, Entries 7-10), the
effect of Fe3O4@SiO2 loading by BF3¢Et2O on the acidic per-
formance of Fe3O4@SiO2-BF3 was investigated. Although the
time of diazotization and the stability of 4-chlorophenyl diazo-
Bull. Chem. Soc. Jpn. 2015, 88, 662–672 | doi:10.1246/bcsj.20140298
© 2015 The Chemical Society of Japan | 665

Table 1. Comparison of Efficiency of Various Acids for Synthesis of Formazan (3h)
H
H
N
C
N
N
N
anion
NH2
Cl
2a
N
H
N
N
N
NaOH
NaNO2, Acid
Solvent-free, r.t.
Solvent-free, r.t.
Cl
1d
Cl
3h
Entry
Acid (wt %)
Stability at r.t.
Time of diazotization
Yielda)/%
1
2
3
4
5
6
7
8
9
Silica phosphoric acid (10)
Silica chloride (10)
Fe3O4
Fe3O4@SiO2
BF3¢Et2O
³2 days
³2 days
®
35 min
30 min
No reaction
No reaction
No reaction
1 min
12 s
6 s
8 s
8 s
43
48
®
®
®
63
79
87
83
80
®
®
BF3¢SiO2 (10)
>12 months
>12 months
>12 months
>12 months
>12 months
Fe3O4@SiO2-BF3 (5)
Fe3O4@SiO2-BF3 (10)
Fe3O4@SiO2-BF3 (15)
Fe3O4@SiO2-BF3 (20)
10
a) The yields refer to the isolated pure formazan (3h) after adding fresh 4-chlorophenyl diazonium salt (1d) into
basic benzaldehyde phenylhydrazone (2a).
nium salt (1d) supported on Fe3O4@SiO2-BF3 nanoparticles
with different loadings in the same conditions were approx-
imately identical, but 10 wt % Fe3O4@SiO2-BF3 resulted in the
highest yield of formazan 3h (Table 1, Entry 8). In conclusion,
10 wt % Fe3O4@SiO2-BF3 was selected as the most ideal solid
acid for synthesis of formazans 3a-3q among those listed
in Table 1.
As previously mentioned, with more investigations, it was
found that 4-chlorophenyl diazonium salts (1d) supported
on Fe3O4@SiO2-BF3 nanoparticles underwent no significant
change at room temperature for several months. In order to
prove this case, we prepared large amounts of 4-chlorophenyl
diazonium salt (1d) and kept it at room temperature without
Figure 6. The yield of formazan 3h prepared from the
stable 4-chlorophenyl diazonium salt (1d) at different
times.
any special conditions to test at different times. Then, diazo
coupling of this diazonium salt (1d) with benzaldehyde phenyl-
hydrazone (2a) was repeated every month under the same con-
ditions and the yield of formazan (3h) measured. The results
presented in Figure 6 indicate that the yield of formazan (3h)
changes from 87% to 80% after 12 months from preparation
time of diazonium salt (1d).
reveals that in this probable structure, aryl diazonium cations
are located on the surface of negatively charged particles called
Fe3O4-silica triflouroborate anions. So, the presence of bulky
anions and charge-charge interactions between nitrogen and
boron atoms are the possible reasons for the unusual stability of
these salts.
As a result, the excellent conversions of aniline derivatives
to aryl diazonium salts 1a-1g and their long-term stability
showed that the Fe3O4@SiO2-BF3 has strong and sufficient
protic sites, which are responsible for its excellent performance
in synthesis of formazan derivatives 3a-3q.
To explore the reason for aryl diazonium salt stability (1d),
its FT-IR spectrum was studied (Figure 7). In this spectrum,
the ethanolic OH and existing moisture in BF3¢Et2O caused
¹1
a broad O-H stretching band at a wavenumber of 3419 cm
.
¹1
The appearance of a new band at 2289 cm clearly demon-
strated N¸N stretching vibration and verified the formation of
diazonium salt (1d). The absorption bands of B-O and Si-O
vibrations were observed at 1442 and 1085 cm¹1, respectively.
Aromatic C-H bending vibrations, C-Cl and Fe-O stretching
bands were revealed at 829, 634, and 586 cm¹1, respectively.
According to this information, the structure of aryl diazo-
nium salts 1a-1g supported on MNPs was surmised. Scheme 2
Diazo coupling of aldehyde phenylhydrazones 2a-2i with
aryl diazonium salts 1a-1g in basic medium was the final step
of formazan dyes 3a-3q formation. According to Scheme 3,
for full conversion of intermediate to formazan, medium pH
666 | Bull. Chem. Soc. Jpn. 2015, 88, 662–672 | doi:10.1246/bcsj.20140298
© 2015 The Chemical Society of Japan

should be changed from 7 to 10-12. In order to control and
fixate the medium pH, different buffer solutions have been used
in the literature,17,18,21-25 while in our research by using grind-
ing procedure in the absence of solvent, there is no need to use
buffer solution for pH adjustment. Hence, one of the significant
results of our methodology compared to previous methods,
there was no requirement for buffering solution to adjust
medium pH.
Formazans are usually synthesized around 0-5 °C, because
temperatures above 5 °C cause decomposition of diazonium
salts and conversion to side products before coupling with
aldehyde phenylhydrazones.15-25 This is the main reason for the
relatively low yield of formazans. To overcome this limitation,
we decided to use Fe3O4@SiO2-BF3 MNPs as a solid acid to
be able to do the reaction at room temperature. In conclusion,
the stability of aryl diazonium salts supported on Fe3O4@SiO2-
BF3, significantly increased the efficiency of formazans synthe-
sis. Final results of reaction times and yields were clarified in
Table 2.
It is important to note that Fe3O4@SiO2-BF3 in diazotization
reaction acts as a two-function reagent, so that not only its
¹
acidic protons convert the nitrite anions (NO2 ) in NaNO2 to
nitrosonium cations (NO+) to promote diazotization, but also
its bulky anions cause the stability of aryl diazonium cations.
In total, we hope that Fe3O4@SiO2-BF3 as an environ-
mentally benign inherent solid acid catalyst, in addition to the
application for the synthesis of formazans, would have a
promising potential for many industrially important catalytic
processes.
Figure 7. FT-IR spectrum of 4-chlorophenyl diazonium salt
(1d) supported on Fe3O4@SiO2-BF3 MNPs.
Conclusion
A green and highly effective methodology for the synthe-
sis of formazan derivatives 3a-3q was developed by solvent-
free diazo coupling of aryl diazonium salts 1a-1g with basic
aldehyde phenylhydrazones 2a-2i at room temperature. Using
Fe3O4@SiO2-BF3 as a novel heterogeneous super solid acid
in the diazotization step and solvent-free procedure provided
experimental simplicity, compatibility with the environment,
no use of special conditions such as buffer solution and low
= OBF3 N2Ar
Scheme 2. The probable structure of aryl diazonium salt 1a-
1g supported on Fe3O4@SiO2-BF3 nanoparticles.
H
H
N
C
N
R1
2a–2i
H
N
N
R1
ArNH2, NaNO2
Solvent-free, r.t.
NaOH (pH: 7)
N
H
Solvent-free, r.t.
N
Ar
= OBF3 EtOH2
= OBF3 N2Ar
1a–1g
Intermediate
N
R1
NaOH (pH: 10-12)
Solvent-free, r.t.
N
H
N
N
Ar
3a–3q
Scheme 3. Synthesis of formazans 3a-3q in the presence of Fe3O4@SiO2-BF3 under solvent-free condition.
Bull. Chem. Soc. Jpn. 2015, 88, 662–672 | doi:10.1246/bcsj.20140298
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Table 2. Diazo Coupling of Aryl Diazonium Salts 1a-1g with Aldehyde Phenylhydrazones 2a-2i for Synthesis of
Formazans 3a-3q under Solvent-Free Condition
R1
N
N
N
N
N
H
anion
H
H
N
NaOH (pH: 10-12)
Solvent-free, r.t.
N
C
N
R1
+
R
2a–2i
R
1a–1g
3a–3q
Entry
Diazonium salt
Aldehyde phenylhydrazone
Formazan
Yielda)/%
N
N
N
H
N
H
C
anion
N
H
N
N
N
1
86
2a
1a
3a
OCH3
N
N
N
H
N
H
C
N
H
anion
N
N
N
2
OCH3
88
2b
1a
3b
CH3
N
N
N
H
H
C
N
anion
anion
anion
N
N
H
N
N
3
CH3
87
2c
1a
3c
OCH3
N
N
H
N
H
C
N
N
OCH3
N
N
H
N
4
84
2d
1a
3d
NO2
N
N
N
H
H
C
N
H
N
N
N
N
5
NO2
85
2e
1a
3e
Continued on next page:
668 | Bull. Chem. Soc. Jpn. 2015, 88, 662–672 | doi:10.1246/bcsj.20140298
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Continued:
Entry
Diazonium salt
Aldehyde phenylhydrazone
Formazan
Yielda)/%
N
N
N
anion
N
H
N
H
C
H
N
N
N
6
7
8
89
CH3
2a
CH3
1b
3f
N
N
N
N
H
anion
N
H
N
H
C
N
N
86
Br
2a
Br
1c
3g
N
N
N
N
N
H
anion
H
N
H
C
N
N
87
Cl
2a
Cl
1d
3h
NO2
N
N
N
N
anion
N
H
H
N
H
C
NO2
N
N
9
90
Br
2f
Br
1c
3i
NO2
N
N
H
N
H
C
N
N
anion
Cl
NO2
N
H
N
N
10
82
Cl
2f
1e
3j
Continued on next page:
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Continued:
Entry
Diazonium salt
Aldehyde phenylhydrazone
Formazan
Yielda)/%
NO2
N
N
N
anion
N
H
N
H
C
NO2
H
N
N
N
11
84
Cl
2f
Cl
1d
3k
OCH3
N
N
N
N
H
anion
H
H
C
N
N
N
N
OCH3
12
92
OCH3
2b
OCH3
1f
3l
OCH3
N
N
N
H
anion
N
H
N
H
C
N
N
N
13
OCH3
88
CH3
CH3
2b
1b
3m
OCH3
N
N
N
H
N
H
C
N
H
anion
N
N
N
14
OCH3
91
OCH3
OCH3
2b
1g
3n
O
N
N
N
N
N
H
H
N
H
C
anion
O
N
N
15
89
2g
1a
3o
S
N
N
N
N
N
H
H
N
H
C
anion
S
N
N
16
87
2h
1a
3p
Continued on next page:
670 | Bull. Chem. Soc. Jpn. 2015, 88, 662–672 | doi:10.1246/bcsj.20140298
© 2015 The Chemical Society of Japan

Continued:
Entry
Diazonium salt
Aldehyde phenylhydrazone
Formazan
Yielda)/%
N
N
N
N
H
N
H
C
anion
N
H
N
N
N
N
17
90
2i
1a
a) Overall isolated yield after purification.
3q
temperature, efficient yields, short reaction times and made this
procedure attractive to synthesize a variety of these com-
pounds. The structure and stability of aryl diazonium salts 1a-
1g supported on Fe3O4@SiO2-BF3 MNPs were studied, too.
continued for 30 min. Finally, the precipitates were collected
using an external magnetic field and washed 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.
Modified Stöber sol-gel process50 was used for coating
magnetite nanoparticles with a silica shell. Typically, 50 mL of
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 of 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 again sonicated for 6 h. Fe3O4@SiO2 nanocomposite 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.
Experimental
Materials and Apparatus. Chemicals and solvents were
purchased from Merck and Sigma-Aldrich Companies. Melt-
ing points were obtained with a micro melting point apparatus
(Electrothermal, Mk3) and are uncorrected. UV-vis (Ultra-
violet-visible) spectra were taken with a double beam Perkin-
Elmer 550S spectrophotometer in the range of 200-600 nm,
using spectrophotometric grade ethanol as the blank. FT-IR
spectra were run on a Nicolet Magna 550 spectrometer.
1H NMR spectra were recorded on a Bruker DRX-400 Avance
spectrometer. Tetramethylsilane (TMS) was used as an internal
reference and deuterated chloroform used as solvent. 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 working frequency of
40 KHz. XRD patterns were acquired using a Philips Xpert
MPD diffractometer equipped with a Cu Kα anode (- =
1.54 ¡) in the 2ª 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 magne-
tometer device (Meghnatis Daghigh Kavir Co., Kashan Kavir,
Iran). The morphology of the synthesized samples was studied
with 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). Fe3O4 and
Fe3O4@SiO2-BF3 average size and distribution were analyzed
by TEM using a Philips CM120 with a LaB6 cathode and
accelerating voltage of 120 kV.
In the final stage, BF3¢Et2O (0.45 mL) was added dropwise
to 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 resulting suspension was filtered
and dried at room temperature to obtain a brown solid named
nano Fe3O4@SiO2-BF3 (10 wt %). The acidic capacity of
¹1
Fe3O4@SiO2-BF3 nanoparticles was 0.72 mmol g and deter-
mined via titration of 0.5 g of solid acid with standard solution
of NaOH.
Typical Procedure for the Synthesis of Formazan Deriv-
atives 3a-3q. In order to synthesize aryl diazonium salts
1a-1g, aromatic amines (2 mmol) were ground with NaNO2
(3 mmol) and 10 wt % Fe3O4@SiO2-BF3 (0.3 g) in a mortar
with a pestle. Then, synthesized aryl diazonium salt 1a-1g was
added to a ground mixture of aldehyde phenylhydrazone 2a-2i
and NaOH (necessary amount to reach pH of 10-12 after diazo
coupling). Upon mixing two reactants in basic medium, the
color of the mixture changed to red. Anyway, the mixture was
ground at room temperature for 1-2 min. After completion of
the reaction (TLC), the mixture was washed with distilled water
(50 mL) and then acetone (30 mL). Evaporation of the solvent
followed by flash column chromatography (in the case of
compounds 3d, 3e, 3j, washing by hot ethanol was performed
instead of flash column chromatography) that gave formazans
in high yields. The structures of formazans 3a-3q were char-
acterized by spectroscopic methods such as UV-vis, FT-IR, and
1H NMR.
Synthesis of Fe3O4@SiO2-BF3 MNPs.
The synthesis
of Fe3O4 nanoparticles was carried out according to a known
procedure using chemical coprecipitation by a little modifica-
tion of methodology already reported in the literature.49 Briefly,
FeCl3¢6H2O (8.0 g, 0.029 mol) and FeCl2¢4H2O (3.5 g, 0.017
mol) with a molar ratio of approximately 2:1, were dissolved in
38 mL of deoxygenated 0.4 M HCl solution. Then, 375 mL of
deoxygenated 0.7 M ammonia solution was quickly 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
The authors are grateful to University of Kashan for
supporting this work by Grant No. 159189/40.
Bull. Chem. Soc. Jpn. 2015, 88, 662–672 | doi:10.1246/bcsj.20140298
© 2015 The Chemical Society of Japan | 671

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672 | Bull. Chem. Soc. Jpn. 2015, 88, 662–672 | doi:10.1246/bcsj.20140298
© 2015 The Chemical Society of Japan