Mesoporous ZnS nanospheres: A high activity heterogeneous catalyst for synthesis of 5-substituted 1H-tetrazoles from nitriles and sodium azide

Article abstract of DOI:10.1039/b912284b

Mesoporous ZnS nanospheres is a novel heterogeneous catalyst for synthesis of 5-substituted 1H-tetrazoles from various nitriles and sodium azide with excellent catalytic performance.

Full text of DOI:10.1039/b912284b

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Mesoporous ZnS nanospheres: a high activity heterogeneous catalyst for
synthesis of 5-substituted 1H-tetrazoles from nitriles and sodium azidew
Leiming Lang, Baojun Li, Wei Liu, Li Jiang, Zheng Xu* and Gui Yin
Received (in Cambridge, UK) 23rd June 2009, Accepted 11th November 2009
First published as an Advance Article on the web 26th November 2009
DOI: 10.1039/b912284b
Mesoporous ZnS nanospheres is a novel heterogeneous catalyst
for synthesis of 5-substituted 1H-tetrazoles from various nitriles
and sodium azide with excellent catalytic performance.
Tetrazoles have been attracting increasing attention, due to
1
their wide application, such as in pharmaceuticals as lipophilic
spacers and metabolically stable surrogates for carboxylic acid
in medicinal chemistry, photography and information recording
systems and special explosives in materials science, ligands in
coordination chemistry, and valuable precursors to a variety
of nitrogen-containing heterocycles in organic synthesis.
The most convenient route for the preparation of
5
-substituted 1H-tetrazoles is via [3 + 2] cycloaddition of
azide to the corresponding nitriles. Earlier synthetic methods
suffered from some drawbacks including expensive and toxic
metals, use of strong Lewis acids, and the presence of
hydrazoic acid. In order to overcome these disadvantages,
Fig. 1 SEM (a,b), TEM (c,d) and HRTEM (e,f) images of MZNSS.
observed by scanning electron microscopy (SEM). Fig. 1a is
the low-magnification SEM image of the MZNSS with
average diameter about 70 nm. The enlarged SEM and TEM
images (Fig. 1b and d) show that ZnS nanospheres have rough
surfaces composed of nanoparticles with 3–5 nm diameters.
The TEM image in Fig. 1d indicates the mesoporous structure
of MZNSS. Fig. 1f is the HRTEM image of a ZnS nanosphere
as marked with the black frame in Fig. 1e. The interplanar
spacing is about 0.31 nm corresponding to the (002) lattice
plane of hexagonal ZnS.
2
new pathways have been developed, such as the catalytic
3
method using a stoichiometric amount of inorganic salts
,4
5
and metal complex as a catalyst, and also using TMSN and
3
6
TBAF instead of metal salts under solvent-free conditions or
7
8
in micellar media and ionic liquid. However, these homo-
geneous catalytic processes suffered from a serious trouble-
some problem, namely, the separation and recovery of the
catalyst. Therefore, the heterogeneous alternatives are highly
desirable and have already attracted much attention. Recently,
several heterogeneous catalytic systems were reported using,
The XRD pattern of MZNSS (Fig. S1aw) is attributed to
hexagonal ZnS phase (JCPDS card No. 75-1547). The energy
dispersive spectrum (EDS) in Fig. S1bw shows that the catalyst
only contains zinc and sulfur elements, demonstrating that the
9
a
9b
for example, nanocrystalline ZnO, Zn/Al hydrotalcite, Zn
10
9
hydroxyapatite and Cu
c
2
O
as catalysts. Our group also
prepared tetrazoles with high yield in a heterogeneous catalytic
12
synthesized product is pure ZnS. Fig. S2w shows the N
2
1
1
system using tungstate salts or g-Al O as catalyst. In this
adsorption/desorption isotherm and the pore-size distribution
(inset) of MZNSS. The isotherm belongs to type IV with a
hysteresis loop, demonstrating their mesoporous characteristics.
The BET (Brunauer–Emmett–Teller) surface area and single
2
3
communication, we report mesoporous ZnS nanospheres
(
MZNSS) as a new heterogeneous catalyst for synthesis of
tetrazoles with super catalytic activity.
2
À1
3
À1
MZNSS were prepared according to a previously reported
point total pore volume are 223 m g and 0.25 cm g
respectively.
,
1
3
procedure with some modifications. Briefly, ZnS nano-
spheres were obtained by the pyrolysis of zinc acetate and
thiourea in ethylene glycol containing PVP at 190 1C. The
product was separated from the reaction mixture by centrifu-
gation, washed several times with ethanol and treated with
It is worth mentioning that ZnS, as a semiconductor
material, is often used as a photocatalyst in degradation of
contaminated water containing halobenzene derivatives,
1
3,14
organic dyes and toxic metal ions,
but its use as a catalyst
0
.1 M HNO
3
. The size and morphology of the catalyst were
for synthesizing 5-substituted 1H-tetrazoles from nitriles and
sodium azide has not been reported. The catalytic synthesis of
5
-phenyltetrazole was completed and the results including
State Key Laboratory of Coordination Chemistry and Nanjing
National Laboratory of Microstructure, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093,
P. R. China. E-mail: zhengxu@netra.nju.edu.cn;
Fax: (+86) 25-83593133; Tel: (+86) 25-83593133
w Electronic supplementary information (ESI) available: Experimental
details of catalyst preparation, characterization and the synthesis of
yields and turnover numbers (TON) are summarized in
1
Table 1. The H NMR spectrum of as-synthesized product
without purification (Fig. S3w) indicates that it is pure
5
-substituted 1H-tetrazole without any by-products. The effect
of reaction times and temperatures on the reaction was
evaluated (Table 1, entries 1–6). The optimum reaction
5-substituted 1H-tetrazoles. See DOI: 10.1039/b912284b
4
48 | Chem. Commun., 2010, 46, 448–450
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Table 1 Catalytic performance of MZNSS under various reaction
parameters
Table 2 Preparation of 5-substituted 1H-tetrazole with various
catalysts
a
a
Yield of isolated
product [%]
BET surface
area/m g
b
b
2
À1
c
d
Entry
Time/h
T/1C
TON
Entry
Catalyst
ZnCl
Yield [%]
TON
1
2
3
4
5
6
7
8
9
12
24
36
48
24
24
36
36
36
36
36
36
36
120
120
120
120
110
130
120
120
120
120
120
120
120
60
80
96
94
49
79
51
72
20
8
1.46
1.94
2.33
2.28
1.19
1.92
1.24
1.75
0.49
0.19
0
1
2
3
4
5
6
7
8
9
2
—
—
—
35
29
51
15
93
96
95
61
51
75
82
84
96
2.26
2.33
2.30
1.48
1.24
1.82
1.99
2.04
2.33
ZnBr
Zn(OAc)
2
2
commercial ZnO
commercial ZnS
ZnS
e
c
e
ZnWO
4
d
f
MZNSS
MZNSS
216
223
e
g
f
10
11
12
13
a
g
3
Reaction conditions: benzonitrile (2.5 mmol), NaN (5.4 mmol),
0
69
>65
b
2+
h
DMF (5 ml), reaction time 36 h, 120 1C. Amount of Zn catalyst
c d
used is equivalent to that of 0.1 g MZNSS. Isolated yields. The
1.67
1.58
i
e
moles of tetrazole formed per mol catalyst. ZnS and ZnWO
f
prepared by precipitation method. MZNSS untreated with 0.1 M
4
a
Reaction conditions: benzonitrile (2.5 mmol), NaN
MZNSS (0.1 g) treated with 0.1 M HNO
3
(5.4 mmol),
b
, DMF (5 ml). The moles
3
g
HNO
3
.
3
MZNSS treated with 0.1 M HNO .
c
d
of tetrazole formed per mol catalyst. In 7 ml DMF–H
2
O (5/2). In
O. Without catalyst.
Yield for the third cycle in DMF. Yield for the sixth cycle in DMF.
e
f
g
5
h
ml DMSO. In 5 ml THF. In 5 ml H
2
i
More interestingly, experimental results reveal that the
acid treatment has a significant influence on the catalytic
performance of MZNSS. MZNSS treated with 0.1 M HNO3
exhibits high catalytic activity with yield of 96% (Table 2,
entry 9), while the untreated MZNSS shows lower activity
with 84% yield (Table 2, entry 8). In order to get an insight
into the role played by acid treatment, we first inspect which
conditions are 36 h and 120 1C. Further prolonging the
reaction time or increasing the reaction temperature will not
improve the yield of the reaction. The solvent has a prominent
influence on the yield (Table 1, entries 3, 7–10). The experi-
mental results show that DMF and DMSO are good solvents
with 96% and 72% yields, respectively, whereas water and
tetrahydrofuran (THF) are not suitable for the reaction. A
control experiment showed that no reaction took place
without MZNSS (Table 1, entry 11). Recyclability is
important for industrial application of a catalyst. Therefore,
the reuse performance of MZNSS was investigated (Table 1,
entries 12, 13). The experimental results indicate that the
performance of MZNSS shows some degradation after three
times reuse, such as the yield of tetrazoles and specific surface
II
II
one is an active site: Zn or S . To this end, several control
experiments were completed. The CdX, and ZnX (X = S, Se)
were prepared by a precipitation method and the results of
catalytic reaction (Table S1w) indicated that the catalytic
performance of ZnX was much better than that of CdX and
the anions (S and Se) have no obvious influence on their
II
catalytic performance. It indicates that Zn is the catalytic
active site. Secondly, as we well know, HNO can react with
3
ZnS to produce H S, meantime some S vacancies remained,
2
II
which made a coordination number of Zn unsaturation and
2
À1
II
resulted in more Zn being exposed to reactants, therefore
area of MZNSS which decrease to 69% and 170 m
g
,
respectively. But it is almost constant during another three
cycles such that the yield is B65%. The reason for this might
be that the mesoporous structure shows some variation after
first time reaction.
2
increasing the catalytic activity. On the other hand, H S in the
II
solution or S in solid ZnS in air is easily oxidized by oxygen
to form sulfur sol (in solution) or nanparticles (in air), which
were deposited on the surface of ZnS to cover the catalytic
active sites, thus degrading the catalytic activity of ZnS treated
by acid. Whereas in the case of MZNSS, the situation is quite
different, PVP playing an important role: in solution, PVP
prevents sulfur sol particles from being deposited on the
surface of ZnS, but reactant molecules (which are much
smaller than sulfur sol particles) can pass through PVP to
reach the catalytic active sites; in air, PVP protects ZnS from
oxidation. So, acid treated MZNSS has a higher catalytic
activity. Further evidence comes from the colour of the ZnS
catalysts. All of as-prepared ZnS and MZNSS and acid treated
MZNSS are white in colour, but acid treated ZnS prepared by
a precipitation method and ZnS stored in air (including
commercially available ZnS) are light yellow–brown, due to
sulfur anion being oxidized to form sulfur. Also, the catalytic
activity of the former is much higher than that of the latter.
For comparison, various catalysts were investigated for
synthesis of tetrazoles under our experimental conditions.
Although zinc salts exhibit high catalytic activity with yields
of >90% (Table 2, entries 1–3) in our reaction system, the
separation and reuse of the catalyst are troublesome problems
due to dissolving in DMF completely. Therefore, MZNSS
shows unique virtues, that is, not only is the catalytic perfor-
mance comparable with homogeneous catalysts, but also the
separation and reuse of the catalyst is easily achieved. As
expected, commercial ZnO and ZnS show low catalytic activity
(
Table 2, entries 4, 5). ZnS and ZnWO
4
prepared by a
precipitation method exhibit relatively high yields of 75% and
11
82% (Table 2, entries 6, 7), respectively. Our previous work
demonstrated that the catalytic active site was quite different and
tungstates were the active sites in ZnWO catalyst.
4
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a
Table 3 The synthesis of 5-substituted 1H-tetrazoles with MZNSS
In conclusion, a simple and effective method for the synthesis
of 5-substituted 1H-tetrazoles from nitriles and sodium azide
has been developed using mesoporous ZnS nanospheres as a
catalyst. The heterogeneous catalyst MZNSS with high
surface area and fine mesoporous structure shows excellent
catalytic performance for various nitriles, and also is easy to
separate from the reaction mixture to be reused, the first time
this has been reported. The MZNSS catalyst is a good
candidate to substitute metal salts or other heterogeneous
catalysts and has potential value for industrial applications.
Financial support from the National Natural Science
Foundation of China under the Major Research Project
b
c
À3 À1d
Entry
1
Product
Yield [%]
TON
TOF/10
h
96
2.33
64.7
2
3
86
99
2.09
2.40
58.1
66.7
(
No. 90606005) and the Jiangsu Province Foundation of
Natural Science (BK2006717) is greatly appreciated.
Notes and references
4
5
98
93
2.38
2.26
66.1
62.8
1
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7
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c
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(
(
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and the highest yield >99% was obtained in the case of
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4
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50 | Chem. Commun., 2010, 46, 448–450
This journal is ꢀc The Royal Society of Chemistry 2010