Immobilization of Gd(III) complex on Fe3O4: A novel and recyclable catalyst for synthesis of tetrazole and S–S coupling

Article abstract of DOI:10.1016/j.poly.2019.04.016

In the present work, a novel catalysts prepared by the anchoring of Gd(III) complex with OH groups on the surface of Fe₃O₄ in which characterized by FT-IR, TGA, XRD, EDX, VSM, and ICP-OES techniques and tested in the synthesis of tetrazoles and S–S coupling. This designed methods indicated several advantages including easily recovered from the reaction mixture by magnetic field, several consecutive cycles without noticeable change in its catalytic activity, the use of green solvent, the use of aspartic acid as green ligand, chemical and physical stability of obtained catalyst, short time reaction and good to excellent isolated yields of all product. Also, up to date, Gd(III) complex don't used for the synthesis of tetrazoles and S–S coupling.

Full text of DOI:10.1016/j.poly.2019.04.016

Accepted Manuscript
Immobilization of Gd (III) complex on Fe3O4: A novel and recyclable catalyst
for synthesis of tetrazole and S-S coupling
Mohammad Nemati, Taiebeh Tamoradi, Hojat Veisi
PII:
S0277-5387(19)30263-3
https://doi.org/10.1016/j.poly.2019.04.016
POLY 13876
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
17 March 2019
11 April 2019
Please cite this article as: M. Nemati, T. Tamoradi, H. Veisi, Immobilization of Gd (III) complex on Fe3O4: A novel
and recyclable catalyst for synthesis of tetrazole and S-S coupling, Polyhedron (2019), doi: https://doi.org/10.1016/
j.poly.2019.04.016
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Immobilization of Gd (III) complex on Fe3O4: A novel and
recyclable catalyst for synthesis of tetrazole and S-S coupling
Mohammad Nemati, a, Taiebeh Tamoradi, b, Hojat Veisib
a Department of Chemistry, Faculty of Science, University of Mazandaran, Babolsar, Iran
e-mail: mohamad_mnd@yahoo.com
b Department of Chemistry, Payame Noor University, 19395-4697 Tehran, Iran
e-mail: t.tabss@yahoo.com
Abstract
In the present work, a novel catalysts prepared by the anchoring of Gd (III) complex with OH
groups on the surface of Fe3O4 in which characterized by FT-IR, TGA, XRD, EDX, VSM, and
ICP-OES techniques and tested in the synthesis of tetrazoles and S-S coupling. This designed
methods indicated several advantages including easily recovered from the reaction mixture by
magnetic field, several consecutive cycles without noticeable change in its catalytic activity, the
use of green solvent, the use of aspartic acid as green ligand, chemical and physical stability of
obtained catalyst, short time reaction and good to excellent isolated yields of all product. Also, up
to date, Gd(III) complex don’t used for the synthesis of tetrazoles and S-S coupling.
Keywords Recoverability, Gd, Fe3O4, Disulfide, Teterazole

Address correspondence to University of Mazandaran, Department of Chemistry, Faculty of Science,
Mazandaran, Iran.; e-mail: mohamad_mnd@yahoo.com (M. Nemati) and Payame Noor University,
Department of Chemistry, Faculty of Science, Tehran, Iran.; e-mail: t.tabss@yahoo.com (T. Tamoradi).

1. Introduction
The Heterogeneous catalysts have been known for decades and have become an effective strategy
for efficient and environmentally friendly organic chemistry, and their efficiency has become
increasingly effective [1, 2]. There are several types of heterogeneous catalysts, such as zeolites,
mixed metal oxides, solid-supported catalysts, hydrotalcites, resins, polymers, etc [3].
Nanocatalysts, due to the small size (1-100 nm) and the presence of exposed metal atoms on the
surface, are an important and emerging field in catalyst science and also minimizes the cost for
each operation [4-6]. One of the most important and growing arena in heterogeneous catalysts is
solid-supported catalysts [7]. Nanosupports such as ZrO2, MgO, Al2O3, CaO, Fe3O4 and SiO2, have
attracted much attention due to their large surface area, versatile physical surface, good catalytic
properties, inherent adsorptive properties and applications in catalysts [8-14]. In pursuit of green
chemistry efforts, nanocatalysts are another field of exploration that avoids the use of flammable
organic solvents, toxic reactions, hazardous reaction conditions and / or harsh conditions and
extreme separation [15-17]. The use of green solvents, catalysts and alternative energy sources is
essential for the development of a sustainable protocols [18-20]. Magnetite nanoparticles are one
of the most widely used research and applied materials in various fields, including biotechnology,
biomedicine, separation, catalytic, magnetic resonance imaging (MRI) and etc. Magnetite is an
ideal supportable oxide, which is easily prepared, and has a very active surface for adsorption or
immobilization of metals and ligands that can be separated by magnetic decantation after the
reaction, thereby improving its efficiency, and its catalytic stability is maintained for several times
[21]. In recent years, successfully supported magnetite catalysts using organic compounds as
ligand or linker have been used for many important chemical reactions. Many scientists have used
various metal-supported magnetite nanoparticles (Fe3O4-M) in many organic chemical reactions.

(Whether with or without the use of supported ligands) [22]. Supported nanomagnatite catalysts
are emerging and comprehensive catalysts in the heterogeneous catalytic domain that are suitable
for the development of suitable protocols. It opens the gats for a wide variety of research protocols
that scientists work on, such as mannich type reactions, C–C, C–S, and C–O coupling reactions,
coupling of thiophenols, alkylation, oxidation, reductions, click reactions, multicomponent
synthesis, and asymmetric synthesis, among other named reactions. [22]. Supported
nanomagnetite catalysts are an excellent and growing research field in catalytic science that has
harmonies with the goals of green chemistry and the synthesis and development of organic
chemistry. The most important aspect in the magnetic catalyst is the design and support of a
specific ligand or metal as a catalyst in a particular organic reaction, which should be considered
mechanically and the possibility of such a reaction on a laboratory scale with the potential of
industrial application. Also, therecycling capability is one of the prominent features of the
popularity and applications of nanomagnetite catalysts [22]. Selective oxidation of sulfides to
sulfoxide and disulfides is one of the important methods for the synthesis of some chemical and
biological drugs [23]. Sulfoxides play an important role in the activation of enzymes. Other roles
of these compounds include anti-fungal, anti-hypertensive, antibacterial and anti-atherosclerotic
effects, and other important biological functions. [24-25]. Disulfides, as oxidative products of thiol
compounds, are beneficial in biological and chemical fields such as DNA dispersing properties,
peptide stabilization in the three-dimensional structure of proteins, protective groups, and the
effects of vulcanization of rubber [26,27]. To synthesize disulfides, the oxidation of thiols to
disulfide is very common and desirable. Furthermore, there are reports of S-S coupling reactions
with several supported metals heterogeneous catalysts such as MCM‐41-Adenine‐Zr [28], Fe3O4-
AMPD-Cu [29], Fe3O4–adenine–Zn [30], Therefore, the search for the possibility synthesis of

these compounds under mild reaction conditions with recoverable effective heterogeneous catalyst
is still in high demand. On the other hand, 1H-tetrazoles form an important category of heterocyclic
compounds due to their application in pharmaceutical science and materials, including explosives
and photography [31, 32]. These compounds can act as an isosteric replacement for carboxylic
acids in drug design. [31]. Also, mentioned materials have biological activity such as antibacterial
activity, antiviral, antifungal, analgesic, antiulcer, antiinflammatory, and antihypertensive
activities [33-39]. The tetrazolium compounds have the ability to interact with metals and can be
used as agents for the coordination of various metals ions [40]. These compounds are typically
prepared by adding azide salts to organic nitriles or cyanides. [41-43]. Several syntheses of 1H-
tetrazoles have been reported through the [3+2] cycloaddition of nitriles using NaN3 or TMSN3 in
the presence of hetrogenious catalysts such as FeCl3–SiO2 [44], Zn/Al hydrotalcite [45], nano ZnO
[46], montmorillonite K-10 [47], montmorillonite K10-Cu [48], CoY Zeolite[49], Nickel
Zirconium phosphate nanoparticles [50], CoFe2O4@glycine-M (M= Pr, Tb and Yb) [51], MCM-
41@AMPD@Zn [52], and Ni (II) on Fe3O4@tryptophan [53]. So far, there is no report on the use
of gadolinium metal for the synthesis of disulfide coupling and 1H-tetrazoles. Gadolinium has
been used as a catalyst in various reactions such as Lewis acid catalyst for acetylation of alcohols
and phenols [54], catalyst for degradation of organophosphate pesticide [55], Hantzsch reaction
[56], the synthesis of isoprene Rubber [57], polymerization of 1,3-Butadiene [58], heterogeneous
recyclable Lewis acid catalyst for Michael additions [59], catalyst for acetylation of alcohols and
amines [60]. Up to date, various catalysts reported in the the synthesis of N-containing heterocyclic
compounds [61-63]. But, the use of Gd magnetic catalyst has not been shown in previous reports
for the synthesis of N-containing heterocyclic compounds in the short reaction time and good to
excellent yields. Based on mentioned introduction, we try to report of synthesis a novel catalyst

by anchoring Gd(III) complex on surface of Fe3O4 in order to one-pot synthesis of 1H-tetrazoles
and S-S coupling. It was be found that obtained catalysts can be reused for several times without
significant degradation in activity.
2. Experimental
2.1. Preparation of Fe3O4@L-aspartic-Gd
Fe3O4 nanoparticles were prepared by a simple and versatile procedure following a previous
reports [30]. In order to synthesis of chloro-functionalized Fe3O4, 1.0 g of Fe3O4 was added to a
solution containing 1.5 mL of 3-chloropropyltrimethoxysilane (CPTMS) and toluene (30 mL) and
stirred under reflux condition for 24 h. The obtain powder was separated be magnetic field, washed
with hexane dried at 60 °C. Then, 1.5 mL of triethylamine was gradually added to a suspension
containing 1g of chloro-functionalized Fe3O4 and 0.27 g of L-aspartic as ligand in which refluxed
for 48 h and washed with ethanol/water to give Fe3O4@L-aspartic nanoparticles. Finally, 1 g of
Fe3O4@L-aspartic nanoparticles and 0.86 g of Gd(NO3)2 was dispersed in ethanol and refluxed for
16 h to obtained Fe3O4@L-aspartic-Gd. In over step, obtained powder was separated with
magnetic field and dried at 60 °C.
2.2. General procedure for the synthesis of disulfides
A mixture of thiol (1 mmol) and H2O2 (37%) was stirred in solvent-free conditions at room
temperature in presence of 0.006 g of Fe3O4@L-aspartic-Gd as catalyst. Progress of the reaction
was monitored by TLC. After completion, the catalyst was separated by magnetic field and
crystallized to obtain pure product.

2.3. General procedure for the synthesis of 5-substituted tetrazoles
A mixture of nitrile (1 mmol), sodium azide (1.2 mmol), Fe3O4@L-aspartic-Gd (0.05 g) in PEG
(2 mL) at 100 °C was stirred in appropriate time. After the completion of reaction, HCl (4 N, 10
mL) was added to mixture. Then, the resultant organic layer was extracted with ethyl acetate and
washed with distilled water to obtain corresponding tetrazoles. It was purifies over preparatory
TLC.
2.4. Selected spectral data
1
5-(4-Nitrophenyl)-1H-tetrazole H NMR (400 MHz, DMSO, ppm): δH= 8.22–8.28 (m,
2H), 8.35–8.42 (m, 2H).
2-(1H-Tetrazol-5-yl)phenol 1H NMR (400 MHz, DMSO, ppm): δH= 8.1 (s, 1H), 7.51-753
(m, 1H), 7.21 (d, J= 6.2, 1H), 7.09–7.13(m, 1H), 11.22 (br, 1H).
5-(4-Bromophenyl)-1H-tetrazole 1H NMR (400 MHz, DMSO, ppm): δH= 7.87–8 (m, 4H).
1
5-(4-Hydroxy)-1H-tetrazole H NMR (400 MHz, DMSO, ppm): δH= 8.12-8.15 (m, 2H), 8.41–
8.44 (m, 2H).
1
1,2-Di(naphthalen-2-yl)disulfane H NMR (400 MHz, DMSO): δH= 7.52–7.57 (m, 4H), 7.66–
7.76 (m, 2H), 7.91–7.95 (m, 4H), 7.96-8.03 (m, 2H), 8.04 (s, 2H) ppm.
1,2-Bis(4-bromophenyl)disulfane 1H NMR (400 MHz, CDCl3): δH= 7.45 (t, J = 8.2, Hz,
4H), 7.55 (t, J = 8.2 ,4H) ppm.
1
1,2-Bis(4,6-dimethylpyrimidin-2-yl)disulfane H NMR (400 MHz, CDCl3, ppm): δH= 2.53 (s,
12H), 6.89 (s, 2H) ppm.
Dibenzyl disulfide 1H NMR (400 MHz, DMSO, ppm): δH= 4.02 (s, 2H), 4.2 (s, 2H), 7.36–
7.37 (m, 4H), 7.38–7.45 (m, 6H).

3. Results and discussion
3.1. Preparation of catalyst
In this study, Gd complex immobilized on the surface of Fe3O4 and proposed the mechanism for
synthesis of catalyst (Scheme 1 and 2).
Cl
O
H2
.4
Si O
2
FeCl
O O
NH4OH
N2,80 oC,
30 min
Toluen
Reflux,24h
FeCl3.6H2O
MNPs
MNPs-Si-Pr-Cl
O
MeCN
Reflux,48 h
OH
HO
NH2 O
Gd(NO3)3, Ethanol
Reflux, 16 h
MNPs-Si-Pr-Asp acid
Gd3+ dopated on MNPs-Si-Pr-Asp acid

Scheme 1. Synthesis of catalysts.
OH
HO
OH
HO
HO
O
O
O
Cl
O
HO
OH
OH
Si
HO
Si
O
Cl
+
O
HO
OH
HO
OH
O
NH2
OH
OH
O
H
O
HO
HO
Gd
O
HO
HO
HO
OH
OH
OH
O
O
H
O
O
Si
Gd(NO3)3
Si
N
N
H
OH
OH
O
H
O
O
O
O
HO
O
Scheme 2. Proposed mechanism for synthesis of catalyst.
3.2. Catalyst characterization
FTIR spectra for the Fe3O4 nanoparticles, chloro-functionalized Fe3O4, Fe3O4@L-aspartic and
Fe3O4@L-aspartic-Gd was shown in Fig. 1. All samples were shown the strong absorption at
approximately 580 cm−1 in which attributed to the presence of Fe–O stretching vibration. FTIR
spectra of chloro-functionalized Fe3O4, Fe3O4@L-aspartic and Fe3O4@L-aspartic-Gd was shown
the absorption peaks at approximately 2850–2975cm−1 in which attributed to the presence of C–H
stretching vibration. Absorption band at nearly 1385 cm-1 (C–N stretching vibration) and
absorption peak at approximately 1685cm−1 (C=O stretching vibration) in the Fe3O4@L-aspartic
and Fe3O4@L-aspartic-Gd were confirmed the grafting of L-aspartic on the surface of Fe3O4
nanoparticles.

Fig. 1 FTIR spectrum for bare Fe3O4 nanoparticles (a), chloro-functionalized Fe3O4 (b),
Fe3O4@L-aspartic (c) and Fe3O4@L-aspartic-Gd (d).
The quantitative determination of the organic groups supported on the surface of Fe3O4 magnetic
nanoparticles was investigated by thermo-gravimetric analysis (TGA) in Fig. 2. The small amount
of weight loss below 200 °C was contributed to removal of physically adsorbed solvent on the
surface of Fe3O4. Previously reports confirmed that the weight loss of about 2% between 200 and
600 °C was shown that related to decomposition of functional groups chemisorbed onto the surface

of magnetic nanoparticles [29]. As shown in Fig. 2, weight loss of about 11% between 200 and
600 °C was shown that related to decomposition of organic groups chemisorbed onto the surface
of magnetic nanoparticles such 3-cholorthreemetoxysilan and L-aspartic.
Fig. 2 The TGA diagrams of Fe3O4@L-aspartic-Gd (d).
As shown in Fig. 3, surface morphology, the particle shape and fundamental physical
properties of the Fe3O4@L-aspartic-Gd studied by Scanning Electron Microscopy (SEM) in
which shown synthesized nanocatalyst have uniform nanometer-sized. Based on the
synergistic effects theory, obtained morphology assisted to maximize the simultaneous

increasing of the catalyst efficiency and frequency of molecular interactions (higher yields) or
decreasing activation energy of rate determining step (lower times).
Fig. 3 SEM images of Fe3O4@L-aspartic-Gd at 2 μm (a), 1 μm (b), 500 nm (c) and 200 nm (d).

The presence of the kinds of elements in Fe3O4@L-aspartic-Gd studied by the energy-dispersive
X-ray spectroscopy (EDX) analysis in which confirmed the presence of Fe, O, N, C and Gd specie
in the nanocatalyst according to the database of the EDX pattern in Fig. 4.
Also, the loading of Gd on the surface of the synthesized nanocatalyst was calculated by the ICP
atomic emission spectroscopy technique. The exact amount of Gd loaded on modified magnetic
nanoparticles was investigated by the ICP atomic emission spectroscopy technique in which found
be 1.08mmol g−1.
Fig.4 EDX pattern of Fe3O4@L-aspartic-Gd.
The X-ray diffraction (XRD) pattern of Fe3O4@L-aspartic-Gd was investigated and a comparison
has been depicted in Fig. 5. Six characteristic peaks for Fe3O4@L-aspartic-Gd crystal structure
corresponding to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) planes are clearly visible. The

Fe3O4 phase morphology has not been changed during the grafting of Gd complex on the magnetic
nanoparticles as evident from the image [29].
Fig. 5 XRD pattern of Fe3O4@L-aspartic-Gd.
3.3. Catalytic studies
After synthesis and characterization, we focused our attention in the S-S coupling (Scheme 3)
using the reaction of 4-methylthiophen (1 mmol) and H2O2 (37%) as model reaction in the presence
of the Fe3O4@L-aspartic-Gd under the effect of various parameters like solvent, catalyst and H202
concentration. The best conditions were obtained in EtOH at room temperature with in the
presence of 0.006 g of catalyst and 0.5 mL H2O2. The results of this study were summarized in
Table 1.

R3
S
Cat
R3 SH
3a-k
S
R3
EtOH, R.T
4a-k
Cat=
Scheme 3. General scheme for the synthesis of disulfide.
Table 1. Optimization of the reaction conditions for the synthesis of disulfide using Fe3O4@L-
aspartic-Gd.
Entry
Solvent
H2O2 (mL)
Catalyst
Time
(min)
40
Yield (%)a
(mg)
1
2
3
4
5
6
7
8
9
EtOH
0.5
0.5
0.5
0.4
0.4
0.4
0.4
0.4
0.4
7
6
3
5
5
5
5
5
5
95
95
58
84
76
52
45
28
37
EtOH
40
EtOH
40
EtOH
40
Solvent-Free
Ethyl acetate
Acetonitrile
Water
40
40
40
40
n-Hexane
40
a Isolated yield.

We confirmed the generality of this protocol in the synthesis of disulfide derivatives (with the
optimal reaction conditions in hand) by a wide range of aromatic and aliphatic thiols bearing
electron-donating and electron-withdrawing substituents with good compatibility (Table 2).
Finally, a plausible reaction mechanism for the S-S coupling was designed in Scheme 4.
Table 2. Oxidative coupling of thiols into disulfides using H2O2 in the presence of Fe3O4@L-aspartic-Gd.
Entry
1
Substrate
Product
Time
(min)
35
Yield
(%)a
98
TON
151
TOF (h-1)
4.31
M. p (ºC)
[Ref.]
4a
68 [29]
SH
SH
2
4b
40
95
147
3.67
40 [30]
3
4
O
N
4c
30
85
94
84
145
130
4.83
1.53
93 [30]
90 [30]
SH
4d
SH
Br
S
N
5
6
4e
4f
40
92
90
142
139
3.55
0.96
174-176
[29]
SH
N
SH
145
165 [30]
N
7
8
4g
4h
45
50
88
92
136
142
3.02
2.84
102-105
[29]
HS
COOH
58 [29]
SH
9
4i
4j
30
90
92
90
142
139
4.73
1.54
Oil [29]
282 [30]
OH
HS
10
SH
COOH

SH
11
4k
65
92
142
2.18
133 [30]
a Isolated yield.
S
R
+ H2O
S
R
H
R
R
O
S
H
O
H
Gd
O
O
Gd
O
S
O
O
H
N
N
H
H
MNPs
MNPs
O
O
H2O2
O
H
R
O
O
O
S
Gd
O
N
N
MNPs
H
MNPs
H
H
O
O
H
O
S
O
Gd
O
H
O
H
O
H
R
H
O
O
Gd
N
O
MNPs
H
O
+ H2O
Scheme 4. Proposed mechanism for oxidative coupling of thiols in presence of Fe3O4@L-aspartic-Gd.
After successefull oxidation of sulfides by using Fe3O4@L-aspartic-Gd, catalyst important
application appeared in the synthesis of 5-substituted 1H-tetrazoles (Scheme 5). We initially
investigated a model reaction of 4-chlorobenzonitrile (1 mmol) and sodium azide (1.2 mmol) to
optimize the reaction conditions in the synthesis of corresponding tetrazole. Different factors like

solvent, temperature and amount of catalyst were examined on the outcome of reaction. It can be
seen that the best performance was achieved in PEG at 100˚C in the presence of 0.05 g of
Fe3O4@L-aspartic-Gd as a catalyst. The results found are summarized in Table 3.
N
N
NH
N
CN
NaN3, Cat
R
R
PEG, 100ºC
1a-h
2a-h
Cat=
Scheme 5. Fe3O4@L-aspartic-Gd catalyzed the one-pot synthesis of 5-substituted tetrazoles.
Table 2. Optimization of reaction conditions for synthesis of 5-substituted 1H-tetrazole
derivatives in the presence of Fe3O4@L-aspartic-Gd
.
Entry
Catalyst (mg)
Solvent
Time (min)
Temp (°C)
Yield (%)a
1
2
3
4
5
6
50
50
50
50
50
50
EtOH
DMSO
H2O
125
125
125
125
125
125
80
80
75
53
75
70
90
120
100
120
80
DMF
PEG
PEG
100

7
8
70
60
40
50
PEG
PEG
PEG
PEG
125
125
125
125
100
100
100
120
93
90
77
92
9
10
a Isolated yield.
We confirmed the generality of this protocol in the synthesis of 5-substituted 1H-tetrazole
derivatives by a wide range of aromatic and aliphatic nitriles under the optimal reaction conditions
in hand and then a plausible reaction mechanism was designed for mentioned reaction in Scheme
6.
Table 4 Synthesis of 5-substituted 1H-tetrazole derivatives in the presence of Fe3O4@L-aspartic-Gd.
Entry
Substrate
Product
Time
(min)
125
Yield
(%)a
90
TON
TOF (h-1)
M. p (ºC)
[Ref.]
1
2
6a
6b
16
17
0.13
261 [29]
Cl
CN
135
45
92
91
0.126
183 [33]
225 [29]
CN
Cl
3
6c
17
0.38
CN
OH
4
5
6
6d
6e
6f
35
90
40
97
91
93
18
17
17
0.51
0.19
0.43
218 [33]
215 [30]
250 [30]
O2N
CN
CN
HO
CN

7
8
6g
6h
130
120
93
91
17
17
0.13
0.14
190 [33]
263 [30]
CN
CN
Br
a Isolated yield.
Gd
O
H
O
H
O
N
N
NH
N
O
N
R
H
N
RCN
+
N
MNPs
N
NaN3
N
R
H+
MNPs
MNPs
O
N
O
O
N
H
H
O
O
O
O
Gd
R
N
H
Gd
N
O
Na
N
H
N N
N
Na
N
N
C
R
Scheme 6. Proposed mechanism for synthesis of 5-substituted tetrazoles in presence of Fe3O4@L-aspartic-Gd.

3.4. Reusability of catalyst
In the final step, the reusability of Fe3O4@L-aspartic-Gd catalyst was also investigated in the
synthesis of 5-(4-nitroxyphenyl)-1H-tetrazole and dibenzyldisulfide in which shown no
discernible change in reactivity and particle morphology (Fig. 6).
Fig. 6 Reusability of Fe3O4@L-aspartic-Gd in the synthesis 5-(4-nitroxyphenyl)-1H-tetrazole and dibenzyldisulfide.
3.5. Catalyst leaching study
The Gd leaching in during reaction was studied by checking the amount of Gd after recycling of
the catalyst in synthesis of 5-(4-nitroxyphenyl)-1H-tetrazole and dibenzyldisulfide by ICP-OEIS
technique. It was be observed that only a trace of Gd has been leached from the initial load, 1.8
mmol/g, as confirmed by ICP-OES analysis. In justification to this, two reactions have been carried
out under optimized reaction conditions. 4-nitrobenznitril (1 mmol), NaN3 (1.2 mmol) in presence
of Fe3O4@L-aspartic-Gd (0.05 g) for synthesis of 5-(4-nitroxyphenyl)-1H-tetrazole. The desired

product was obtained after 63 min (in the half time of the reaction) in 65% yield in this case. In
the second reaction, 5-(4-nitroxyphenyl)-1H-tetrazole catalyst was isolated from the same reaction
mixture in half time of the reaction and was continued without catalyst. Interestingly, in this case
the yield of reaction (after 125 min) was 71 %. These results are a definite proof that the leaching
of metal wasn’t significant and the leached material was catalytically inactive.
3.6. Comparison of catalyst
In order to justify the efficiency of the catalytic protocol, the reaction data were compared with
previously reported results. As shown in Table 5, our method offers significant advantages such
as the recoverability and recyclability by simple filtration, low price, short reaction time and high
reaction yield, non-toxicity, stability in comparison with the previously reported ones.
Table 5. Comparison of prepared catalysts in the synthesis of 5-phenyl-1H-tetrazole and
dibenzyldisulfide with previously reported procedure.
Entry
Substrate
Catalyst
Time (h)
Yield (%)a
5
3
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
p-MePh-SH
p-MePh-SH
p-MePh-SH
p-MePh-SH
Fe3O4/ZnS HNSs
CoY zeolite
24
14
8
81 [61]
90 [62]
1
B(C6F5)3
94 [63]
4
FeCl3–SiO2
12
36
12
12
1.5
2
79 [64]
6
Mesoporous ZnS
Fe3O4
86 [65]
7
Trace [ this work ]
Trace [ this work ]
91 [ this work ]
55 [66]
8
Fe3O4@L-aspartic
Fe3O4@L-aspartic-Gd
SSA, KBr, wet SiO2
VO-salen-MCM-41
Ni-salen-MCM-41
Cd-salen-MCM-41
9
11
12
13
14
2
95 [67]
2.6
2.5
95 [68]
98 [68]

15
p-MePh-SH
p-MePh-SH
p-MePh-SH
Fe3O4
12
12
Trace [ this work ]
Trace [ this work ]
95[ this work ]
16
Fe3O4@L-aspartic
Fe3O4@L-aspartic-Gd
17
0.66
aIsolated yields.
4. Conclusion
In this study, a novel catalyst was designed by anchored of Gd (III) complex on the surface of
Fe3O4 in which shown high activity in the synthesis of tetrazole and S-S coupling. Also, the
structure of this reusable and novel catalyst characterized by TGA, EDX, FT-IR, XRD, VSM, and
ICP-OES. More importantly, the prepared catalysts shown attractive and significant features such
as easy separation from reaction mixture by magnetic field, chemical and physical stability, high
catalytic activity, easy work-ups, short time reaction and good to excellent isolated yields.
Acknowledgements
We are thankful to Payame Noor University (PNU) for financial supports.
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Graphical Abstract
Initially, anchored Gd(III) onto the Fe3O4 was successfully performed and then prepared catalysts
as a novel and reusable catalyst investigated for the one-pot synthesis of 5-substituted tetrazoles
and S-S coupling. Then, FT-IR, SEM, TEM, EDX, ICP-OES, XRD, TGA and VSM were used for
characterization of the obtained catalysts. Easy separation, chemical and hydrothermal stability,
excellent reusability of the nanocatalyst, the use of inexpensive materials, and short reaction time
are outstanding advantages of this method.