Nano BF3·SiO2: A green heterogeneous solid acid for synthesis of formazan dyes under solvent-free condition

Article abstract of DOI:10.1016/j.molcata.2014.06.024

A solvent-free, efficient and rapid approach for synthesis of formazan dyes was developed by diazotization of aromatic amines with NaNO₂, nano silica-supported boron trifluoride (nano BF₃·SiO₂), then diazo coupling with aldehyde phenylhydrazones by grinding method at room temperature. This study aimed to overcome the limitations and drawbacks of the previous reported methods such as: low temperature, using corrosive and toxic acids and solvents, using buffer solutions, instability of aryl diazonium salts, modest yields, and long reaction times.

Full text of DOI:10.1016/j.molcata.2014.06.024

Accepted Manuscript
Title: Nano BF3.SiO2: a green heterogeneous solid acid for
synthesis of formazan dyes under solvent-free condition
Author: Abdolhamid Bamoniri Bi Bi Fatemeh Mirjalili
Naimeh Moshtael-Arani
PII:
S1381-1169(14)00275-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.06.024
MOLCAA 9165
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
14-2-2014
10-5-2014
18-6-2014
Please cite this article as: A. Bamoniri, B.B.F. Mirjalili, N. Moshtael-Arani,
Nano BF3.SiO2: a green heterogeneous solid acid for synthesis of formazan dyes
under solvent-free condition, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.06.024
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Nano BF3.SiO2: a green heterogeneous solid acid for synthesis of formazan
dyes under solvent-free condition
Abdolhamid Bamoniria,*, Bi Bi Fatemeh Mirjalilib, Naimeh Moshtael-Arania
aDepartment of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, Iran
bDepartment of Chemistry, College of Science, Yazd University, Yazd, Iran
Abstract: A solvent-free, efficient and rapid approach for synthesis of formazan dyes was developed by
diazotization of aromatic amines with NaNO2, nano silica-supported boron trifluoride (nano BF3.SiO2), then
diazo coupling with aldehyde phenylhydrazones by grinding method at room temperature. This study aimed
to overcome the limitations and drawbacks of the previous reported methods such as: low temperature,
using corrosive and toxic acids and solvents, using buffer solutions, instability of aryl diazonium salts,
modest yields, and long reaction times.
Keywords: Formazan; Nano BF3.SiO2; Solid acid; Solvent-free; Diazotization
1. Introduction
Formazans are coloured 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 [1,2], biological applications [3,4] and as dyestuffs [5]. Their antiviral [6,7],
antimicrobial [8,9], anti-inflammatory, analgesic [10], antifungal [11], anticancer, anti-HIV [12],
photochromic and thermochromic activities [13] are also of utmost importance. In recent years, the progress
in the field of solvent-free reactions which are based on grinding has reached significance [14-19] due to
* Corresponding author. Tel.: +98 361 5912384; Fax: +98 361 5912397.
E-mail address: bamoniri@kashanu.ac.ir (A. Bamoniri).
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their high efficiency and selectivity, operational simplicity, mild reaction conditions, reduction in waste and
benefit to the industry as well as the environment. As synthesis of formazan dyes requires some special
conditions such as low temperature and concentrated liquid acids (for synthesis of aryl diazonium salts) [20-
31], in addition to high costs, it leads to corrosion of reactors and equipments, too. However, the main
limitation of such synthetic processes is environmental incompatibility. Nowadays, solid acids have
resolved these problems and improved activity and selectivity [32-46]. In this regard, nano structure solid
acids exhibit higher activity and selectivity than their corresponding bulk materials due to their large surface
to volume ratio [47-50]. BF3.SiO2 is an excellent proton source in terms of convenience, cheapness, ease of
production, and insolubility in all organic solvents, so it can be used for the synthesis of many organic
compounds [51-58]. On the other hand, nano BF3.SiO2, in addition to the mentioned properties, has high
surface to volume ratio that makes the application to catalyze many chemical reactions [59-62].
So far, a lot has been known about numerous synthesized formazans and their structural, spectral features
and reaction mechanisms of their formation [20-31]. Based on our knowledge, none of the reported
syntheses of formazan derivatives were carried out by grinding under a solvent-free condition and in the
presence of a solid acid. On the other hand, formazans have not gained widespread use in the industry
because of their hard synthetic methods and modest yields. Thus, the development of novel and simple
synthetic methods for the efficient preparation of formazans is an interesting challenge. All these facts have
prompted us to employ a new and eco-friendly grinding methodology for the synthesis of formazan dyes in
solvent-free environment at room temperature. The findings of this research may have implications for an
effective synthesis on a larger scale in dyeing and medical industries.
2. Experimental
2.1. Materials and apparatus
Chemicals and solvents were purchased from Merck Company. Commercial nano silica gel was prepared
from Sigma-Aldrich Company. Transmission electronic microscopy (TEM) micrographs of nano silica gel
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and nano BF3.SiO2 were taken using a LEO 912 AB OMEGA instrument (Germany) with a LaB6 cathode
and accelerating voltage of 100 kV. X-ray diffraction (XRD) patterns were acquired using a Philips Xpert
MPD diffractometer equipped with a Cu Kα anode (λ=1.54 A˚) in the 2θ range from 10 to 80˚. Ultraviolet-
visible (UV–Vis) spectra of formazan dyes were taken by a double beam Perkin-Elmer 550S
spectrophotometer in the range of 200-600 nm, using spectrophotometric grade ethanol as the blank. Fourier
transform infrared (FT–IR) spectra were run on a Nicolet Magna 550 spectrometer. Proton nuclear magnetic
resonance (1H NMR, 400 MHz) spectra were recorded on a Bruker DRX-400 Avance spectrometer.
Tetramethyl silane (TMS) was used as an internal reference and deuterated chloroform (CDCl3) used as
solvent. Melting points were obtained with a micro melting point apparatus (Electrothermal, Mk3) and are
uncorrected.
2.2. Preparation of 15 mol % nano BF3.SiO2
Nano BF3.SiO2 was easily prepared from nano silica gel and BF3.Et2O according to the literature [59]. In
short, BF3.Et2O (2.65 mL) was added drop-wise to a slurry containing nano silica gel (7 g) and ethanol (30
mL, instead of chloroform). The mixture was stirred for 1h at room temperature. The resulted suspension
was filtered to obtain the white solid named nano BF3.SiO2 (15 mol %). The acidic capacity of nano
BF3.SiO2 was 2.07 mmol.g-1 and determined via titration of 0.2 g of solid acid with standard solution of
NaOH.
2.3. Typical procedure for the synthesis of formazan derivatives (7a-s)
In order to synthesize aryl diazonium salts (2a-g), aromatic amines 1a-g (2 mmol) was ground with
NaNO2 (3 mmol) and 15 mol % nano BF3.SiO2 (0.4 g) in a mortar with a pestle. Aldehyde
phenylhydrazones (5a-k) were also synthesized from solvent-free grinding of phenylhydrazine 3 (2 mmol)
and aldehyde derivatives 4a-k (2 mmol) in another mortar. The excellent yields of aldehyde
phenylhydrazone derivatives caused no need to more purification. In the next step, synthesized aryl
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diazonium salt was added to a ground mixture of aldehyde phenylhydrazone and NaOH to reach pH: 7 or
more. The residual acidic sites on nano BF3.SiO2 in diazotization step can partially decompose aldehyde
phenylhydrazones to the starting materials in non- basic medium. So, NaOH amount should be sufficient
and fully ground with aldehyde phenylhydrazones, before adding of aryl diazonium salts. Medium pH of
10-12 is suitable for formazan formation. If the mixed NaOH amount with aldehyde phenylhydrazone
before diazo coupling is not sufficient to achieve pH of 10-12, we can re-add NaOH after coupling of
aldehyde phenylhydrazone with diazonium salt. Anyway, the mixture was ground at room temperature for
1-2 min. After completion of the reaction (TLC), the mixture was washed by distilled water (50 mL) for
naturalization of additional NaOH and then by acetone (30 mL) for separation of nano BF3.SiO2.
Evaporation of the solvent followed by flash column chromatography (in the case of compounds 7d, 7e and
7j, washing by hot ethanol was performed instead of flash column chromatography) that gave formazans in
high yields. The structures of formazan dyes were characterized by spectroscopic methods such as UV–Vis,
FT–IR and 1H NMR.
2.3.1. Spectroscopic data for some selected formazans
2.3.3.1 3-(3-Methoxyphenyl)-1,5-diphenylformazan (7d): Bright red colour; UV–Vis (ethanol) λmax1 (π–π*):
484 nm, λmax2 (n–π*): 323 nm; FT–IR (KBr) υ (cm-1): 2924 (N–H), 1599 (C=C), 1511 (C=N), 1452 (N=N),
1032-1242 (C–O); 1H NMR (CDCl3, 400 MHz) δ: 15.51 (1H, s, NH), 6.93-7.77 (14H, m, CH), 3.92 (3H, s,
OCH3).
2.3.3.2 1-(2-Chlorophenyl)-3-(3-nitrophenyl)-5-phenylformazan (7j): Red colour; UV–Vis (ethanol) λmax1
(π–π*): 470 nm, λmax2 (n–π*): 341 nm; FT–IR (KBr) υ (cm-1): 2925 (N–H), 1589 (C=C), 1522 (C=N), 1452
1
(N=N), 1348 (NO2), 675 (C–Cl); H NMR (CDCl3, 400 MHz) δ: 15.67 (1H, s, NH), 7.16-9.01 (13H, m,
CH).
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2.3.3.3 3-(4-Pyridyl)-1,5-diphenylformazan (7q): Red-brown colour; UV–Vis (ethanol) λmax1 (π–π*): 470
nm, λmax2 (n–π*): 327 nm; FT–IR (KBr) υ (cm-1): 2924 (N–H), 1636, 1402 (C=C), 1594, 1499 (C=N), 1453
(N=N); 1H-NMR (CDCl3, 400 MHz) δ: 15.80 (1H, s, NH), 7.30-8.65 (14H, m, CH).
2.3.3.4 3-Propyl-1,5-diphenylformazan (7r): Red colour; UV–Vis (ethanol) λmax1 (π–π*): 416 nm, λmax2 (n–
π*): 325 nm; FT–IR (KBr) υ (cm-1): 2963 (N–H), 1626 (C=C), 1491 (C=N), 1460 (N=N); 1H NMR (CDCl3,
400 MHz) δ: 10.84 (1H, s, NH), 7.20-8.04 (10H, m, CH), 2.76 (2H, dt, CH), 1.67 (2H, dm, CH), 1.01 (3H, t,
CH3).
3. Results and discussion
This research was performed in two stages. Initially, nano BF3.SiO2 was prepared through stirring a
mixture of nano silica gel and BF3.Et2O in ethanol. The size and structure of nano particles were identified
by TEM and XRD. In the second stage, formazan derivatives with electron-withdrawing substituents as well
as those with electron-donating groups were synthesized by mixing aryl diazonium salts prepared in the
presence of nano BF3.SiO2 and basic aldehyde phenylhydrazones at room temperature.
3.1. Nano BF3.SiO2 characterization
The final structure of BF3.SiO2 (Scheme 1) [62] and its nano particles morphology determined by
Scanning electron microscopy (SEM) images [59] have been previously reported. Herein, the properties of
BF3.SiO2 nano particles were characterized by two other techniques including TEM and XRD.
F
F
F
B
EtOH2
OH
O
O
O
O
O
O
Si
Si
Si
O
O
n
Scheme 1. The structure of BF3.SiO2.
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Inspection of the TEM images in Figure 1 indicated involvement of nano particles with an average size
distribution of 20 nm for nano silica gel and 25 nm for nano BF3.SiO2.
Figure 1. TEM micrographs of a) nano silica gel, b) nano BF3.SiO2.
Nano silica gel XRD pattern displayed a broad strong peak in 2θ value of 21.8024 with full width at half
maximum (FWHM) equal to 0.1771. Nano BF3.SiO2 XRD pattern showed three main sharp peaks in
28.4516 with FWHM of 0.1771, 27.9718 with FWHM of 0.2952 and 14.5404 with FWHM of 0.2362. In
addition, there was a short broad peak in 21.9895 with FWHM of 0.1771 related to amorphous nano silica
gel. The broadness of peak in Figure 2a verified the amorphous nature of nano silica gel and peaks
sharpness in Figure 2b represented the crystal-like structure of nano BF3.SiO2.
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Figure 2. XRD patterns of a) nano silica gel, b) nano BF3.SiO2.
3.2. Synthesis and characterization of formazan dyes
The most common synthetic route to the formazan compounds involves coupling of aryl diazonium salts
with aldehyde phenylhydrazones in basic medium. As synthesis of formazans is a three-step reaction and
highly time-consuming with modest yields which requires special conditions such as low temperature, using
buffer solutions to adjust pH and liquid acids to synthesize diazonium salts [20-31], we tried to solve these
problems by development a green and facile procedure for the synthesis of formazans.
At first, the efficiency of nano BF3.SiO2 in diazotization reaction, followed by diazo coupling with
aldehyde phenylhydrazones, was explored via a model reaction. 4-Chloro aniline (1d) was selected as a
model substrate and screened for a range of parameters such as effect of various solid acids, stability of
diazonium salt (2d) at room temperature, reaction time of diazotization, and yield of resulted formazan (7h).
The observed results are summarized in Table 1.
Solid acids such as silica phosphoric acid and silica chloride gave the formazan with yields of 43% and
48%, respectively (Table 1, entries 1 and 2). The stability of aryl diazonium salt supported on above
mentioned acids was maximum 2 days. Silica gel and BF3.Et2O sources were also tested individually. There
was no reaction neither in the presence of silica gel nor BF3.Et2O (Table 1, entries 3 and 4), while BF3.SiO2
afforded an improved yield of 63% for formazan (Table 1, entry 5). These results clearly indicated that the
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presence of Brønsted acid sites on the solid acid surface is essential factor for promoting diazotization
reactions. Also, the rate of diazotization in the presence of BF3.SiO2 was much higher than silica phosphoric
acid and silica chloride.
In another comparative study (Table 1, entries 6-10), the effect of nano silica gel loading by BF3.Et2O on
the acidic performance of nano BF3.SiO2 was investigated. Although the time of diazotization and the
stability of diazonium salt supported on nano BF3.SiO2 with different loadings in the same conditions were
approximately identical, but 15 mol % nano BF3.SiO2 resulted in the highest yield of formazan. In
conclusion, 15 mol % nano BF3.SiO2 is the most ideal solid acid for synthesis of formazans among those
listed in Table 1.
Table 1. Comparison of efficiency of various solid acids in synthesis of formazan dyes.
NH2
N2 anion
NaNO2
Solid acid
Solvent-free, r.t.
Cl
1a-g
Cl
2a-g
Time of
Yielda
(%)
43
Entry
Solid acid (mol %)
Stability at r.t.
diazotization
35 min
1
2
3
4
5
6
7
8
Silica phosphoric acid (20)
Silica chloride (20)
Silica gel
~ 2 day
~ 2 day
30 min
48
-----
No reaction
No reaction
1 min
---
BF3.Et2O
-----
---
BF3.SiO2 (20)
~ 25 month
~ 30 month
~ 30 month
~ 30 month
63
Nano BF3.SiO2 (10)
Nano BF3.SiO2 (15)
Nano BF3.SiO2 (20)
13 sec
75
10 sec
81
10 sec
77
8
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9
Nano BF3.SiO2 (25)
Nano BF3.SiO2 (30)
~ 30 month
~ 30 month
10 sec
10 sec
73
70
10
a
The yields refer to the isolated pure formazan (7h) after adding fresh diazonium salt (2d) into basic benzaldehyde
phenylhydrazone (5c).
To explore the reasons for stability of diazonium salt supported on nano BF3.SiO2, its FT–IR spectrum
was studied (Figure 3). In this spectrum, the ethanolic OH and existing moisture in nano silica gel and
BF3.Et2O caused a broad O–H stretching band at wavenumber of 3221 cm-1. The appearance of the band at
2278 cm-1 clearly demonstrated N≡N stretching vibration and verified the formation of diazonium salt. The
absorption bands of B–O and Si–O vibrations were observed at 1377 and 1090 cm-1, respectively. The
absorption band of B–F was hidden under Si–O band. Aromatic C–H bending vibrations and C–Cl
stretching band were revealed at 731 cm-1 and 652 cm-1.
Figure 3. FT–IR spectrum of 4-chlorophenyl diazonium salt (2d) supported on nano BF3.SiO2.
According to this information, the structure of diazonium salts supported on nano BF3.SiO2 was guessed.
Scheme 2 reveals that in this probable structure, aryl diazonium cations are located on the surface of
negatively charged particles called nano silica triflouroborate. So, the presence of bulky anions and
favorable charge–charge interactions between nitrogen and boron atoms are the possible reasons of unusual
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stability of these salts. In addition, the small size of nano particles and large surface to volume ratio in nano
BF3.SiO2 can be the reason of greater stability of diazonium salt in comparison to that supported on the bulk
BF3.SiO2.
O
O Si O BF3 N2
O
n
Scheme 2. The structure of aryl diazonium salt supported on nano BF3.SiO2.
On the other hand, synthesis of aldehyde phenylhydrazone derivatives (5a-k) was carried out by a simple
grinding without any use of solvent and acidic catalyst at room temperature. It is expected that aldehyde
derivatives (4a-k) with electron-withdrawing substituent as a strong electrophile should be more reactive
than those containing electron-donating groups. But, as shown in Table 2, the reaction was done so rapidly
that the substituent type could not influence the reaction time and even yield of aldehyde phenylhydrazones.
Totally, excellent yields and purity, short reaction times, solvent- and catalyst-free medium and practically
of the procedure at room temperature were the main advantages of this new method for the synthesis of
aldehyde phenylhydrazone derivatives.
Table 2. Condensation of phenylhydrazine with various aldehydes for the synthesis of
aldehyde phenylhydrazones.
HN NH2
H
H
C
N
N
R1
R1 CHO
+
Solvent-free, r.t.
4a-k
5a-k
3
Aldehyde
Yielda
(%)
R1
Aldehyde
Time (Sec)
phenylhydrazone
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4-OCH3-phenyl
4-CH3-phenyl
Phenyl
4a
4b
4c
4d
4e
4f
5a
5b
5c
5d
5e
5f
8
98
97
98
96
97
98
97
96
98
95
96
10
12
19
40
17
11
8
3-OCH3-phenyl
3-NO2-phenyl
4-NO2-phenyl
2-Furyl
4g
4h
4i
5g
5h
5i
2-Thienyl
4-Pyridyl
6
Propyl
4j
5j
23
18
1-Phenylethyl
4k
5k
a Isolated yield
Diazo coupling of aldehyde phenylhydrazones (5a-k) with diazonium salts (2a-g) in basic medium was
the final step of formazan dyes (7a-s) formation.
Formazans are usually synthesized around 0-5 °C [20-31], because temperatures above 5 °C causes
decomposition of diazonim salts and conversion to side products before coupling with aldehyde
phenylhydrazones and formation of formazan dyes. This is the main reason for the modest yield of
formazans. To overcome this limitation, we decided to use nano BF3.SiO2 as a solid acid to be able to do the
reaction at room temperature. In conclusion, the stability of diazonium salts supported on nano BF3.SiO2 at
room temperature, significantly increased the efficiency of formazans synthesis. Final results of reaction
times and yields are clarified in Table 3.
Table 3. Diazo coupling of aldehyde phenylhydrazones with diazonium salts for the synthesis of formazan
derivatives.
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R
anion
H
H
N
C
N
N
R1
N
NaOH (pH:10-12)
Solvent-free, r.t.
+
N
N
N
H
N
R
C
R1
2a-g
5a-k
7a-s
Diazonium
salt
Aldehyde
Yielda
(%)
Mp.
R
H
R1
Formazan
phenylhydrazone
Found/Reported
172-174/
Phenyl
2a
2a
2a
2a
2a
2b
2c
2d
2c
5c
7a
7b
7c
7d
7e
7f
81
83
85
81
80
84
83
81
85
172-173 [23]
171-173/
H
4-OCH3-phenyl
4-CH3-phenyl
3-OCH3-phenyl
4-NO2-phenyl
Phenyl
5a
5b
5d
5f
170-171 [23]
153-155/
H
154-157 [23]
159-161/
H
160-162 [23]
196-198/
H
196-198 [24]
154-156/
4-CH3
4-Br
4-Cl
4-Br
5c
5c
5c
154 [30]
188-190/
Phenyl
7g
7h
7i
189 [30]
117-119/
Phenyl
119 [30]
176-178/
3-NO2-phenyl
5e
12
176.5-177.5 [27]
Page 12 of 21

174-176/
175.5-176.5 [27]
175-177/
2-Cl
4-Cl
3-NO2-phenyl
3-NO2-phenyl
2e
2d
2f
5e
5e
5a
5a
5a
5g
5h
7j
7k
7l
78
80
87
84
86
86
83
177.5-179 [27]
187-189/
4-OCH3 4-OCH3-phenyl
187-188 [27]
242-244/
4-CH3
4-OCH3-phenyl
2b
2g
2a
2a
7m
7n
7o
7p
242-243 [27]
204-206/
3-OCH3 4-OCH3-phenyl
205-206.5 [27]
116-118/
H
H
2-Furyl
118 [20]
138-140/
2-Thienyl
138 [25]
191-193/
H
H
H
4-Pyridyl
Propyl
2a
2a
2a
5i
5j
7q
7r
7s
88
84
82
192 [26]
86-88
82-84/
1-Phenylethyl
5k
81-82 [21]
a Overall isolated yield after column chromatography
After purification of formazans, their structures were determined by spectroscopic methods. The FT–IR
spectra of formazan derivatives showed characteristic absorption bands at 2921-3066 (N–H), 1586-1638
1
(aromatic C=C), 1487-1524 (C=N) and 1443-1487 (N=N) cm-1. Based on H NMR spectroscopic data, all
products synthesized from aromatic or heteroaromatic aldehydes (7a-q) exhibited a singlet at 14.28-15.80
ppm for NH proton. Two formazans prepared from aliphatic aldehydes (7r, 7s) showed NH proton at higher
13
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fields (10.84, 11.60 ppm). Also, formazans (7a-s) indicated signals at 6.53-9.01 for aromatic protons
dependent on substituent effect. According to Scheme 3, formazan dyes have two tautomeric forms that
quickly interconvert. The participation of the NH proton in rapid intramolecular tautomeric conversions in
1
formazans has been demonstrated by means of H NMR spectroscopy with isotopically enriched samples
[64,65].
N
N
N
N
N
H
N
H
N
N
Scheme 3. Tautomerization in formazan via intramolecular proton transfer.
According to the proposed mechanism in Scheme 4, for full conversion of intermediate (6) to formazan,
medium pH 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 [20-31], while in our research by using grinding procedure
in the absence of solvent, there is no need to 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.
14
Page 14 of 21

NH2
Nano BF3.SiO2
+
NaNO2
R1
R
N
N
N
H
N
R
O
O
O
N2 anion
anion=
R
Si O BF3
n
R1
N
CH
NH
(pH: 10-12)
(pH: 7)
NaOH
+
H
N
N
R1
N
H
NaOH
N
R
(6)
Scheme 4. The plausible mechanism of formazan formation.
Overall, the advantages of our methodology in comparison to the previous reported methods [20-31]
included rapid and solvent-free synthesis of aryl diazonium salts in high conversion at room temperature,
long-term stability of aryl diazonium salts supported on nano BF3.SiO2, not use of toxic solvents, acid
catalysts, and short reaction times in synthesis of aldehyde phenylhydrazones, no need to more purification
of aldehyde phenylhydrazones because of excellent yields, eco-friendly synthesis of formazan dyes at room
temperature, no need to solvent, buffer solution and control of temperature below 5 °C, short reaction times
and high yields using grinding methodology.
Finally, the full consumed time for all three steps was less of 5 minutes and the yields of formazan
derivatives obtained between 78-88%, regardless of the type and position of substituents.
15
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4. Conclusion
A green and highly effective methodology for the synthesis of formazan derivatives (7a-s) was
developed by solvent-free diazo coupling of diazonium salts (2a-g) with basic aldehyde phenylhydrazones
(5a-k) at room temperature. Using nano BF3.SiO2 in diazotization step and solvent-free procedure caused
the experimental simplicity, compatibility with environment, no use of special conditions such as buffer
solution and low temperature, efficient yields, short reaction times, and made this procedure attractive to
synthesize a variety of these dyes. The structure and stability of diazonium salt supported on nano BF3.SiO2
were studied, too.
Acknowledgments
The authors are grateful to University of Kashan for supporting this work by Grant No. 159189/30.
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Highlights:
Nano BF3.SiO2 efficiency in synthesis of aryl diazonium salts and their stability.
Solvent‐ and catalyst‐free methodology for synthesis of aldehyde phenylhydrazones.
Room temperature synthesis of formazans via coupling of above mentioned reactants.
No need to buffer solution, solvent, and liquid acid for synthesis of formazans.
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*Graphical Abstract (for review)
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