A comparative study between heterogeneous stannous chloride loaded silica nanoparticles and a homogeneous stannous chloride catalyst in the synthesis of 5-substituted 1: H -tetrazole

Article abstract of DOI:10.1039/c6ra14352k

Heterogeneous SnCl₂-nano-SiO₂ efficiently catalyzed 5-substituted 1H-tetrazole synthesis with excellent yield. The catalyst was characterized by using FT-IR, TGA, TEM, and EDX. It is widely applicable on aliphatic, aromatic, heteroaromatic and sterically hindered nitriles with five time recyclability. Being simple and an economically viable approach for the synthesis of SnCl₂-nano-SiO₂ are additional advantages.

Full text of DOI:10.1039/c6ra14352k

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A comparative study between heterogeneous
stannous chloride loaded silica nanoparticles and
a homogeneous stannous chloride catalyst in the
synthesis of 5-substituted 1H-tetrazole†
Cite this: RSC Adv., 2016, 6, 75227
Received 2nd June 2016
Accepted 1st August 2016
a
a
b
a
*
Arvind Kumar, Satyanand Kumar, Yugal Khajuria and Satish Kumar Awasthi
DOI: 10.1039/c6ra14352k
www.rsc.org/advances
Heterogeneous SnCl2–nano-SiO2 efficiently catalyzed 5-substituted catalyst, etc. Moreover, most of the above mentioned catalysts
1H-tetrazole synthesis with excellent yield. The catalyst was charac- were not widely applicable on all type of nitriles (aliphatic,
terized by using FT-IR, TGA, TEM, and EDX. It is widely applicable on aromatic, heteroaromatic, sterically hindered aromatic nitrile)
aliphatic, aromatic, heteroaromatic and sterically hindered nitriles with with excellent efficiency. Thus, these drawbacks encouraged us
five time recyclability. Being simple and an economically viable to design an efficient catalyst which is cost effective, recyclable,
approach for the synthesis of SnCl2–nano-SiO2 are additional and equally useful to aliphatic, aromatic, heteroaromatic and
advantages.
sterically hindered aromatic nitriles with various electron with-
drawing or electron donating functionalities.
The literature survey reveals that tin salts show high Lewis
acidity as compared to other transition metals and the order is
Sn2+ [ Zn2+ > Pb2+ z Hg2+.29 To keep it in mind that Sn2+ salt has
higher Lewis acidity in comparison to other metal salts, we have
selected SnCl2 as homogeneous catalyst. Further, in the last few
years, some metal salts viz. Fe, Sb etc. immobilized on silica
support have received considerable attention in tetrazole
synthesis,30–36 but these were suffer with several drawback such as
longer reaction time, lesser recyclability, higher loading of catalyst
etc. To improve upon the catalyst efficiency, catalyst loading,
applicability and recyclability of the catalyst, we choose silica
nanoparticles as it has high surface area, large pore volume, and
recyclable tendency; hence, it may be useful as support for
immobilization of Sn2+ salts. We immobilized SnCl2 by reaction of
hydroxyl groups on silica (Scheme 1), thus homogeneous SnCl2
catalyst converted into heterogeneous catalyst with improved
features for the synthesis of 5-substituted 1H-tetrazole synthesis.
Our research group has been working on diversied area
such, catalysis, asymmetric synthesis, and design, synthesis of
malaria inhibitor.
Tetrazoles are nitrogen rich heterocyclic compounds having
a wide range of applications in the area of medicinal chem-
istry,1–8 agriculture,9 coordination chemistry10 and material
chemistry.11 They are stable against a wide pH range, and
various oxidizing and reducing agents.12
Initially, tetrazoles were synthesized by the reaction of
hydrogen cyanide and anhydrous hydrazoic acid under pressure.
These reagents are extremely toxic, water sensitive and low
boiling liquids (HN3, ꢀ37 ꢁC).13 Later, Finnegan et al.14 reported
the improved procedure for synthesis of 5-substituted tetrazoles
by (3 + 2) cycloaddition using inorganic azide in place of toxic
hydrazoic acid. Subsequently, Sharpless et al.15 pioneered tetra-
zole synthesis using Zn2+ salt as a Lewis acid catalyst. More
recently, various homogeneous and heterogeneous catalysts
such as Py HCl,16 Yb(OTf)3$xH2O,17 (NH4)2Ce(NO3)6,18 Zn/Al
hydrotalcite,19 CoY zeolite,20 Cu–Zn alloy nanopowder,21 CAN-
HY zeolite,22 Au NPs,23 PtNPs@Ac,24 FeCl3–SiO2,25 NaHCO3-
$SiO2,26 SiO2$H2SO4,27 Ln(OTf)3$SiO2,28 etc. were used in tetra-
zole synthesis. However, these catalysts suffer one or more
drawbacks such as cost of catalysts, non-recyclability, water
sensitivity, tedious synthesis procedure, longer reaction time,
higher reaction temperature, lower yield, and excess use of
aChemical Biology Laboratory, Department of Chemistry, University of Delhi,
Delhi-110007, India. E-mail: satishpna@gmail.com; skawasthi@chemistry.du.ac.in
bSchool of Physics, Shri Mata Vishno Devi University, Katra, Jammu and Kashmir,
India
† Electronic supplementary information (ESI) available: X-Ray analysis, NMR and
IR data are given. CCDC 999827. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c6ra14352k
Scheme 1 Probable structure of covalently anchored SnCl2–nano-
SiO2.
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Herein, we report an efficient, recyclable, widely applicable
and easy to prepare SnCl2–nano-SiO2 as heterogeneous catalyst
for 5-substituted 1H-tetrazole synthesis and compared its effi-
ciency with homogeneous SnCl2 catalyst.
Preparation of SnCl2–nano-SiO2
SnCl2–nano-SiO2 was synthesized using procedure reported in
literature.37 Briey, 2.0 g of silica nanoparticles of 25 nm size
and (1.2 g, 6.32 mmol) of SnCl2 were suspended in 20 mL of
dichloromethane and the resulting suspension was stirred for
overnight. Aer this period, the resulting suspension was
ltered and the catalyst was washed with CH2Cl2 and the solid
ꢁ
powder was dried in high vacuum for 5 hours at 100 C, until
Fig. 2 TGA plots of SiO2, SnCl2/SiO2 (fresh) and SnCl2/SiO2 (after 5
cycles).
a free ow powder of SnCl2–nano-SiO2 was achieved. SnCl2–
nano-SiO2 was characterized by FT-IR, XRD, TEM, EDX and TGA
analysis. The proposed structure of SnCl2–nano-SiO2 catalyst in
which SnCl2 may have reacted with the surface hydroxyl group
of silica nanoparticles is shown in Scheme 1.
FT-IR analysis of SnCl2–nano-SiO2
To identify the structure of SnCl2–nano-SiO2, we studied IR
spectra of SnCl2–nano-SiO2 (Fig. 1). The FT-IR spectrum
displays characteristic peaks at 1102 and 806 cmÀ1 corre-
sponding to symmetrical and asymmetrical vibrations of Si–O–
Si, respectively while the Sn–Cl and Si–OH resonances were
observed in 1564 and 3428 cmÀ1 respectively.37,38 The successful
Fig. 3 TEM images of SnCl2–nano-SiO2 ((A) is before reaction and (B)
covalent linking of the SnCl2 on the surface of SiO2 was is after reaction).
conrmed by the appearance of a new band at 541 cmÀ1, which
originates from the absorption of O–Sn (Fig. 1).39
respectively. This weight loss can be generally attributed due to
the loss of surface physisorbed water. Further, the TGA curves of
SiO2, fresh SnCl2–nano-SiO2 and 5 times recycled SnCl2–nano-
SiO2 showed again weight loss in temperature range of 100–
800 ꢁC, which is calculated as 3.36%, 15.77% and 14.53%
respectively. Fresh SnCl2–nano-SiO2 may showed this weight
loss due to physisorbed and chemisorbed SnCl2 from the
surface of the silica nanoparticles, while 5 time cycle used
SnCl2–nano-SiO2 may showed this weight loss only due to
chemisorbed SnCl2. SiO2 showed weight loss in 100–800 ꢁC
temperature range due to condense of water molecules between
Thermogravimetric analysis (TGA) of
SnCl2–nano-SiO2
The TGA curves are plotted in the temperature range of 25–800
ꢁC with heating at the rate of 5 ꢁC minÀ1 under nitrogen
atmosphere. TGA plots of SiO2, fresh SnCl2–nano-SiO2 and
recycled SnCl2–nano-SiO2 showed initial weight loss up to
100 ꢁC, which is calculated as 3.43%, 7.27% and 6.85%
Fig. 1 FT-IR spectra of (a) SiO2 and (b) SnCl2–nano-SiO2.
75228 | RSC Adv., 2016, 6, 75227–75233
Fig. 4 Particle size of SnCl2–nano-SiO2.
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hydroxyl groups at the surface of silica nanoparticles, as a result
oxo bridges (Si–O–Si) are formed. The comparison of weight loss
of SiO2 and fresh SnCl2–nano-SiO2 revealed that 12.41% SnCl2 is
loaded on silica surface in fresh SnCl2–nano-SiO2 catalyst, while
the comparison of weight loss of SiO2 and 5 times recycled
SnCl2–nano-SiO2 revealed that 11.17% SnCl2 is loaded on silica
surface in recycled SnCl2–nano-SiO2 catalyst. Thus, there is
a marginal loss of activity in 5 times recycled SnCl2–nano-SiO2
as compared to fresh SnCl2–nano-SiO2 catalyst (Fig. 2).
High resolution TEM and EDX analysis
of SnCl2–nano-SiO2
Fig. 7 Recyclability test.
The size of SnCl2–nano-SiO2 catalyst was found to be approxi-
mately 25 nm as determined by the TEM images (Fig. 3). Size
was calculated by image j soware (Fig. 4). The Fig. 5 is about
energy dispersive X-ray analysis of SnCl2–nano-SiO2 catalyst.
Table 2 Effect of solvents in the synthesis of tetrazole 1b by using
SnCl2a (C1) and SnCl2–SiO2b (C2) catalyst
Temp (ꢁC)
(C1/C2)
Time (h)
(C1/C2)
Yield (%)
(C1/C2)
Entry
Solvent
1
2
3
4
5
6
7
8
H2O
EtOH
DMF
DMSO
NMP
Toluene
THF
CH3CN
DCM
Acetone
1,4-Dioxane
100/100
78/78
12/12
24/24
6/4
0/0
25/27
90/92
68/72
65/68
35/36
30/32
0/0
0/0
0/0
0/0
120/100
120/100
100/100
110/100
66/66
80/80
39/39
56/56
100/100
12/12
12/12
24/24
24/24
12/12
12/12
12/12
12/12
9
10
11
a
b
10 mmol% of SnCl2. 5 mmol% SnCl2–SiO2 catalyst with 1 mmol of
nitrile.
Fig. 5 The EDX spectrum of SnCl2–nano-SiO2.
The EDX spectrum gives an idea of elemental composition of
constituent elements like O, Si, Cl and Sn. Analysis conrms the
presence of O, Si, Sn, and Cl elements, in the ratio of 48.78,
37.11, 8.72 and 5.4 atomic%. The Y-axis shows the counts
(number of X-rays received and processed by the detector) and
the X-axis shows the energy level of those counts.
Table 1 Effect of tin catalysts in synthesis of tetrazole 1b
Entry
Catalysta,b
Sna
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
6
4
6
6
6
6
60
90
92
85
82
86
12
a
SnCl2
SnCl2–SiO2
SnCl4
b
a
a
Me2SnCl2
a
SnI4
Table 3 Effect of temperature and catalyst loading in the synthesis of
tetrazoles 1b in DMF
a
SiO2
a
b
10 mmol% of catalyst at 120 ꢁC. 5 mmol% of catalyst at 100 ꢁC.
Entry
Catalyst (mmol%)
Temp (ꢁC)
Time (h)
Yielda (%)
1
2
3
4
5
6
7
8
SnCl2 (5)
SnCl2 (5)
SnCl2 (5)
SnCl2 (5)
SnCl2 (10)
SnCl2 (10)
SnCl2 (10)
SnCl2 (15)
SnCl2–SiO2 (5)
SnCl2–SiO2 (5)
SnCl2–SiO2 (5)
SnCl2–SiO2 (10)
SnCl2–SiO2 (10)
100
100
120
120
100
120
120
120
100
120
120
100
120
6
12
6
12
12
12
6
6
4
4
6
66
73
78
82
83
90
90
92
92
92
92
93
93
9
10
11
12
13
4
4
a
Isolated yield.
Fig. 6 X-Ray analyzed unit cell packing of compound 6b.
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Table 4 Synthesis of 5-substituted 1H-tetrazoles
Time (h)
Yieldc (%)
Entry
Substratea
Product
SnCl2/SnCl2–SiO2
SnCl2/SnCl2–SiO2
1
2
6/4
6/4
90/92
92/95
3
4
6/4
6/4
91/94
87/90
5
6
7
6/4
6/4
6/4
94/97
86/89
88/90
8
6/4
6/4
6/4
6/4
91/94
93/98
92/97
91/95
9
10
11
12
6/4
92/96
13
14
6/4
6/4
86/89
82/86
15
8/6
80/84
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Table 4 (Contd.)
Time (h)
Yieldc (%)
Entry
16
Substratea
Product
SnCl2/SnCl2–SiO2
SnCl2/SnCl2–SiO2
8/6
8/6
8/6
8/6
78/82
80/85
82/86
84/88
17
18
19
20
6/4
8/6
82/88
90/93
21b
a
The reaction of nitriles with NaN3 (1.5 equiv.) was conducted in DMF in the presence of 10 mmol% of SnCl2 at 120 ꢁC and in presence of 5 mmol%
of SnCl2–nano-SiO2 at 100 ꢁC. b The reaction of nitrile with NaN3 (3 equiv.) was conducted in DMF in the presence of 20 mmol% of SnCl2 at 120 ꢁC
and in presence of 10 mmol% of SnCl2–nano-SiO2 at 100 ꢁC. c Isolated yield (%).
We know that the selection of the catalyst is of utmost concentrated our efforts in the comparative study of tetrazole
important to achieve good yield of a reaction. Therefore, synthesis using homogeneous SnCl2 and heterogeneous SnCl2–
initially we compared the catalytic activity among all viz. Sn(0), nano-SiO2 catalysts.
SnCl2, SnCl2–nano-SiO2, SnCl4, SnI4 and Me2SnCl2 in the
SnCl2–nano-SnCl2 catalyst was easily synthesized in cost
synthesis of tetrazole 1b. Sn(0) gave poor yield (60%, Table 1, effective manner which is recyclable and reusable catalyst.
entry 1). SnCl2–nano-SiO2 catalyst showed highest catalytic Recycling of the catalyst is an important factor in catalyst design
activity followed by SnCl2 in the synthesis of tetrazole 1b (Table which reduces the overall cost of catalyst. We tested this feature
1, entry 2 and 3). Catalysts SnCl4, Me2SnCl2 and SnI4 also gave in case of SnCl2–nano-SiO2 in ve subsequent cycles and
good yield in the range of 82–86% (Table 1, entry 4–6). However, showed minimum decrease in the isolated yields, hence
handling of SnCl4 is difficult due to its highly volatile nature, minimum loss of catalytic efficiency (Fig. 7).
while SnI4 and Me2SnCl2 are very expensive, hence omitted from
We optimized reaction solvent and DMF was found to be the
our present study. Silica showed very poor catalytic activity (12% best solvent for tetrazole synthesis (Table 2, entry 4), whereas
yield) in tetrazole synthesis (Table 1, entry 7). Since silica dimethylsulfoxide and N-methylpyrrolidone provided moderate
showed poor catalytic activity, we exploited this property and yields (Table 2, entries 4 and 5). Aer solvent optimization
immobilized SnCl2 in order to get synergetic effect of SnCl2 with conditions, we investigated the effects of temperature and
SiO2 due to the multiple Lewis acid catalytic centers. Moreover, catalyst loading on the reaction yields and results are summa-
by immobilizing SnCl2 on SiO2, the resultant catalyst shows rized in Table 3. We have found that 5 mmol% of SnCl2–nano-
better catalytic activity in compare to SnCl2. Thus, we SiO2 at 100 ꢁC or 10 mmol% of SnCl2 at 120 ꢁC gives better
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yields. Further increase in the catalyst loading has insignicant 09 program and found that calculated data was in good agree-
effect on yield (Table 3, entries 8 and 9).
ment with the experimental data (see 5–7 pages of ESI†).
We performed reaction by using SnCl2–nano-SiO2 (5
ꢁ
mmol%) at 100 C for 4 h to get 92% yield (Table 3, entry 10).
Conclusions
Further increase in catalyst loading and reaction temperature or
reaction time did not increase the yield (Table 3 entries 11–14).
Thus, 5 mmol% catalysts loading at 100 ꢁC for 4 h reaction time
is the optimal condition for SnCl2–nano-SiO2 catalyst while 10
mmol% catalysts loading at 120 ꢁC for 6 h is the optimal
condition for SnCl2 catalyst. To authenticate the methodology,
various aromatic and aliphatic nitriles were treated with sodium
azide under optimized conditions using both catalysts (Table 4).
The presence of heteroatom in the ring increases the reactivity
of heteroaromatic nitriles (Table 4, entries 5, 8 & 9). Electron
withdrawing group also increases the reactivity of aromatic
nitriles so that electron withdrawing group with aromatic nitrile
(Table 4, entries 2, 3, 10, 11 & 21) gave better yield than electron
donating group with aromatic nitriles (Table 4, entries 4, 6, 7, 13
In the present study, SnCl2 and SnCl2–nano-SiO2 have been
used for the synthesis of 5-substituted 1H-tetrazoles. Among
these two catalysts, SnCl2 and SnCl2–nano-SiO2 catalyst, later
was most efficient in converting nitrile into tetrazole with high
yield. Lesser reaction time, temperature, and lower loading of
catalyst are additional advantages. Moreover, aromatic nitriles
having electron withdrawing group are more reactive than
having electron donating group. In addition, aromatic nitriles
are more reactive than aliphatic nitrile toward tetrazole
synthesis. This approach also works very well in conversion of
sterically hindered nitriles to their respective tetrazoles.
& 14). Aromatic nitriles gave slightly better yields of corre- Acknowledgements
sponding tetrazoles (Table 4, entries 1–14), as compared to
AK and SK are thankful to UGC, New Delhi, India for Senior
aliphatic nitriles (Table 4, entries 15–19). This approach is also
useful for the synthesis of heteroaromatic tetrazoles (Table 4,
entries 5–9) and sterically hindered tetrazoles (Table 4, entries
20 and 21). To authenticate this methodology, we report herein
crystal data of compound 6b (Fig. 6).
Research Fellowship and to USIC for analysis of our data. SKA is
thankful to DU-DST Purse Grant & University of Delhi, Delhi-
110007 for nancial support.
Notes and references
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