A template free protocol for fabrication of a Ni(ii)-loaded magnetically separable nanoreactor scaffold for confined synthesis of unsymmetrical diaryl sulfides in water

Article abstract of DOI:10.1039/d0ra02287j

In the present report, an environmentally benign magnetically recoverable nickel(ii)-based nanoreactor as a heterogeneous catalyst has been developedviaa template free approach. The catalytic performance of the synthesized catalyst is assessed in the confined oxidative coupling of arenethiols with arylhydrazines to form unsymmetrical diaryl sulfides under aerobic conditions. The salient features of our protocol include oxidant- and ligand-free conditions, use of water as a green solvent, room temperature and formation of nitrogen and water as the only by-products. Moreover, a broad range of functional groups are tolerated well and provide the corresponding diaryl sulfides in moderate to good yields. Moreover, the heterogeneous nature of the catalyst permits facile magnetic recovery and reusability for up to seven runs, making the present protocol highly desirable from industrial and environmental standpoints.

Full text of DOI:10.1039/d0ra02287j

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A template free protocol for fabrication of a Ni(II)-
loaded magnetically separable nanoreactor
scaffold for confined synthesis of unsymmetrical
diaryl sulfides in water†
Cite this: RSC Adv., 2020, 10, 19390
a
ab
a
a
Gunjan Arora,
Pooja Rana,
Manavi Yadav,
Rashmi Gaur,
Radhika Gupta,
a
ab
a
*
Priya Yadav
and Rakesh Kumar Sharma
In the present report, an environmentally benign magnetically recoverable nickel(II)-based nanoreactor as
a heterogeneous catalyst has been developed via a template free approach. The catalytic performance
of the synthesized catalyst is assessed in the confined oxidative coupling of arenethiols with
arylhydrazines to form unsymmetrical diaryl sulfides under aerobic conditions. The salient features of our
protocol include oxidant- and ligand-free conditions, use of water as a green solvent, room temperature
and formation of nitrogen and water as the only by-products. Moreover, a broad range of functional
groups are tolerated well and provide the corresponding diaryl sulfides in moderate to good yields.
Moreover, the heterogeneous nature of the catalyst permits facile magnetic recovery and reusability for
up to seven runs, making the present protocol highly desirable from industrial and environmental
standpoints.
Received 11th March 2020
Accepted 14th May 2020
DOI: 10.1039/d0ra02287j
rsc.li/rsc-advances
Recently, a rhodium-catalyzed approach has been introduced
where deborylthiolation of borylarenes using thiosulfonates
was done to synthesize diaryl suldes.17 A report by Fu and co-
Introduction
Aromatic suldes represent an important class of organosulfur
compounds that have a wide utility in various elds such as
pharmaceutical chemistry, materials sciences and synthetic and
polymer chemistry (Scheme 1).1–5 The extensive presence of C–S
bonds in several drug molecules used in many targeted
diseases, such as inammation, HIV, diabetes, asthma, cancer,
mood disorders, and Parkinson's and Alzheimer's diseases,
validates the importance of C–S bond-forming reactions.6–11
Over the last few decades, transition metal catalyzed oxida-
tive cross-coupling reactions have emerged as a powerful and
straightforward tool for the construction of carbon–carbon and
carbon–heteroatom bonds.12–14 Eventually, numerous catalytic
systems have been developed for the synthesis of diaryl suldes
via the transition metal mediated formation of C(sp2)–S bond.
For instance, Shi et al. have reported Ni(OAc)2 and N-
heterocyclic carbene catalyzed cross-coupling reaction of aryl
halides with various aliphatic and aromatic thiols.15 In another
study a magnetically recoverable and reusable CuFe2O4 nano-
catalyst was fabricated that facilitates the formation of aryl–
sulfur bond between aryl halides and thiols/disuldes.16
workers described visible-light photoredox arylation of thiols
with aryl halides at room temperature using [fac-Ir(ppy)3] as
catalyst.18 However, these approaches suffer from various
disadvantages such as use of high temperature, longer reaction
time, toxic ligands, reagents and/or solvents. Moreover, most of
these approaches are based on coupling of different electro-
philes such as aryl halides and triates with arenethiols.
Besides, organic halides are expensive (organic bromides and
iodides), less reactive (organic chlorides) and toxic. Organic
triates on the other hand are unstable and produce toxic triic
acid as a by-product. Therefore, there is still a need to expand
aGreen Chemistry Network Centre, Department of Chemistry, University of Delhi,
Delhi-110007, India. E-mail: rksharmagreenchem@hotmail.com
bDepartment of Chemistry, Hindu College, University of Delhi, Delhi-110007, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra02287j
Scheme 1 Some important applications of aromatic sulfides.
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the scope of electrophilic coupling partners in transition metal
catalyzed C–S bond forming reactions.
Results and discussion
Catalyst preparation
Arylhydrazines have been recognized as environmentally
benign arylating agents for oxidative cross-coupling reactions as
the only associated by-products are nitrogen gas and water
during C–N bond cleavage.19,20 Zhao and co-workers reported an
efficient catalytic system, Pd(OAc)2-PCy3-Na2CO3, for the oxida-
tive cross coupling of arylhydrazines with arenethiols in toluene
at 100 ꢀC to afford diaryl suldes in good to high yields.21
Although, the protocol has highly improved the choice of elec-
trophilic coupling partner, however, high temperature and use
of non-green solvent limits its large-scale applicability. Besides,
the system is homogeneous in nature, which makes the product
purication difficult. Very recently, Yuan and group disclosed
a surfactant-type catalyst that along with PEG-functionalized
Scheme 2 depicts the design thought for the synthesis of nickel-
based double-shelled hollow magnetic catalyst, Ni@Fe3O4–C.
Firstly, Fe3O4 solid microspheres were formed by solvothermal
approach. Then these solid microspheres were converted into
hollow double-shelled Fe3O4-phenolic polymer using liquid
ammonia as a structure directing agent and resorcinol–form-
aldehyde as a carbon source. Gas-bubble assisted Ostwald
ripening process is responsible for this transformation.31 The
beauty of this protocol was realized in the reaction conditions
where the transformation of solid Fe3O4 to double-shell hollow
magnetic nanoreactor was achieved in a single step at low
temperature and shorter reaction time.43 Liquid ammonia
produce gas bubbles (ammonia gas) in the reaction, which
provides aggregation centers for the reaggregation of Fe3O4
crystals, thus, acts as so template for the synthesis.31 Fe3O4–C
is nally obtained by calcinating it in nitrogen atmosphere.
Next, metalation is done using Ni(OAc)2 to form the desired
catalyst.
ꢀ
amphiphilic nitrogen ligand at 100 C showed admirable effi-
ciency and selectivity for oxidative coupling of hydrazines with
thiols in water.22 Moreover, the mother liquor was catalytically
active aer ve cycles. Nevertheless, the use of high temperature
and external ligand make the protocol less environmentally
benign. Therefore, the development of other recoverable and
reusable catalytic systems which operate under room tempera-
ture and utilize green solvents is a highly desirable goal.
In recent years, use of hierarchical core–shell architectures
as heterogeneous support is considered a promising way to
immobilize catalytically active species due to the unique
combination of functional and structural characteristics.23–25
Some outstanding features of these materials are high surface
area, large cavity volume, low density, excellent loading capac-
ities and ease of surface modication.26–30 Physical and chem-
ical properties of these hollow structures can be easily tuned
and controlled by altering chemical compositions and relative
dimensions of core and shells. Therefore, hollow nano-
composites, especially those comprising of magnetic core/shell
and functional shells, nd possible applications as efficient
nanocatalysts due to ease of separation and recycling.
As we are working on designing and synthesis of sustainable
catalysts,31–42 we herein report the synthesis of nickel-based
hollow magnetic catalyst. The synthesized composite is
comprised of nano-scaled cavity and double shells; inner
magnetite shell that allows easy magnetic separation and
external carbon layer that imparts high stability and protects
the inner shell and metal species from external environment.
Most importantly, the conned nano-cavity inside the
composite provides an isolated space for reactions to occur, so,
it acts as a nanoreactor. Further, the catalytic activity of the
synthesized nickel-based nanoreactor has been demonstrated
through the synthesis of diaryl suldes. Mild reaction condi-
tions, use of green solvent, moderate to good yields, effortless
magnetic recovery of catalyst and reusability up to seven cycles
are some of the striking features of the present protocol. To the
best of our knowledge, this is the rst report wherein a hollow
magnetic heterogeneous catalyst is developed for the room
temperature synthesis of diaryl suldes in water with air
Characterization
FT-IR spectra of the materials obtained at each step of the
synthesis of Ni@Fe3O4–C catalyst is shown in Fig. 1. In all the
spectra, the absorption band at around 589 cmÀ1 is attributed
to the Fe–O bond vibration of the magnetite structure. In
Fig. 1a, the peaks at 1389 and 1630 cmÀ1 are originated from
carboxylate groups which are derived from trisodium citrate
used during the synthesis of MNPs.44 The presence of carbon
coating around magnetite shell was conrmed with the occur-
rence of bands at 1618 and 2926 cmÀ1 that correspond to C]C
and –CH2 stretching modes respectively (Fig. 1b). Other peaks
in the range of 1060–1510 cmÀ1 indicate the presence of C–O
stretching and O–H bending vibrations,45 suggesting the pres-
ence of residual hydroxyl groups on the surface of Fe3O4–C that
are accountable for active loading of Ni(II). Further, the spec-
trum of Ni@Fe3O4–C (Fig. 1c) exhibited slight shi in these
values, conrming the successful loading of Ni(OAc)2.
The crystallographic structure of materials is unveiled using
powder XRD analysis (Fig. 2). The characteristic Bragg's
diffraction peaks in the spectrum of Fe3O4 were obtained at
(molecular oxygen) as
conditions.
a sole oxidant under ligand-free
Scheme 2 Synthetic protocol for the fabrication of Ni@Fe3O4–C
catalyst.
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catalyst. Fig. 3d displays the peaks for Ni, C, Fe and O in the
catalyst, indicating its successful formation. To calculate the
amount of Ni in the catalyst, ICP-MS analysis was done. The
approximate metal loading was found to be 0.47 mmol gÀ1
.
To further investigate the morphology and size of the
synthesized materials, TEM analysis was conducted. Fig. 4a
conrmed that the magnetite particles are nearly uniform solid
spheres with the mean diameter of about 360 nm. In Fig. 4b, we
can clearly see a light-contrast cavity of approximately 100 nm,
a dark inner magnetite shell and relatively light outer carbon
shell with thickness of nearly 50 nm and 30–40 nm respectively.
This conrms the successful formation of hollow Fe3O4–C
microspheres. Interestingly, the nanometric cavity inside
Fe3O4–C serves as an isolated chemical space that separate the
guest species from outer bulk surrounding, thereby functioning
as a nanoreactor. Fig. 4c, suggests that the structure of nano-
reactor remained intact aer the incorporation of nickel
acetate.
Fig. 1 FT-IR spectra of (a) Fe3O4 (b) Fe3O4–C and (c) Ni@Fe3O4–C.
XPS analysis of Ni@Fe3O4–C catalyst was done to determine
the oxidation state of nickel present in the catalyst (Fig. 5). The
spectrum displayed peaks with binding energies of 856.2 eV and
862 eV which corresponds to the 2p3/2 and 2p1/2 peaks of nick-
el(II) ion.37,47,48 Thus, results revealed that the Ni ion has an
oxidation state of +2 in Ni@Fe3O4–C catalyst.
Fig. 2 XRD spectra of (a) Fe3O4 and (b) Ni@Fe3O4–C.
Saturation magnetization measurements (Ms) were also
recorded for Fe3O4, Fe3O4–C and Ni@Fe3O4–C catalyst at room
temperature (Fig. 6). The Ms values for Fe3O4 is 60.30 emu gÀ1
which is reduced to 32.9 emu gÀ1 and 29.66 emu gÀ1 in Fe3O4–C
and Ni@Fe3O4–C respectively. This decrease could be attributed
to the coating of non-magnetic carbon layer and incorporation
of nickel acetate. However, even with this decrease in the
30.18ꢀ (2 2 0), 35.57ꢀ (3 1 1), 43.26ꢀ (4 0 0), 56.97ꢀ (5 1 1), and
62.83ꢀ (4 4 0) which were indexed to the inverse spinel structure
of Fe3O4 (JCPDS card no. 19-629).46 Fig. 2b clearly depicts that
there is nearly no notable variation in the pattern of magnetite,
suggesting that the crystalline structure of Fe3O4 remains intact
during the synthesis of the catalyst.
The structure and surface morphology of Fe3O4, Fe3O4–C and
Ni@Fe3O4–C were investigated by eld emission-scanning
electron microscopic analysis. The images shown in Fig. 3,
suggest that all the three materials have nearly uniform spher-
ical morphology. Moreover, it can also be seen that Fe3O4–C and
Ni@Fe3O4–C are porous materials. Fig. 3c indicates that the
incorporation of Ni(OAc)2 did not alter the surface morphology
of Fe3O4–C. SEM-coupled energy dispersive spectrum was also
recorded to check the presence of different elements in the
Fig. 4 TEM images of (a) Fe3O4 (b) Fe3O4–C and (c) Ni@Fe3O4–C.
Fig. 3 FE-SEM images of (a) Fe3O4 (b) Fe3O4–C (c) Ni@Fe3O4–C and
(d) EDS spectrum of Ni@Fe3O4–C.
Fig. 5 XPS spectrum of Ni@Fe3O4–C catalyst.
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Fig. 8 Effect of various solvents. Reaction conditions: 4-cyanophe-
nylhydrazine hydrochloride (1 mmol), 4-bromothiophenol (1 mmol),
K2CO3 (4 equiv.), catalyst (30 mg), solvent (3 mL), rt, 24 h.
Fig. 6 Saturation magnetization curves for (a) Fe3O4 (b) Fe3O4–C and
(c) Ni@Fe3O4–C.
diaryl suldes. As depicted in Table 1, in the absence of any
base, trace amount of the desired product was formed, con-
rming the necessity of base in the synthesis (Table 1, entry 1).
Moreover, results conrmed that the activity of catalyst is
sensitive to the base used (Table 1, entries 2–8). Among all,
K2CO3 was found to be the most suitable base. Moreover,
amount of base also plays an important role in maximizing the
yield of the desired product (Table 1, entries 9–10). 3 equiv. of
K2CO3 was found to be optimum for the reaction. The effect of
catalyst amount on the yield of diaryl sulde is shown in Fig. 9a.
25 mg of the catalyst is required to catalyze the reaction. Next,
we conducted the reaction at different reaction times of 10–30 h
(Fig. 9b). Results state that 20 h is the optimized time for the
reaction.
saturation magnetization, Ni@Fe3O4–C catalyst could be effec-
tively and efficiently removed from the reaction mixture within
30 s by applying an external magnet.
Catalytic activity test
To assess the catalytic efficacy of Ni@Fe3O4–C in the synthesis
of diaryl suldes, the reaction of 4-cyanophenylhydrazine
hydrochloride with 4-bromothiophenol was chosen as a model
reaction. Initially, we performed some control experiments
(Fig. 7). In the absence of catalyst, 14% of product was obtained.
When Fe3O4 and Fe3O4–C were employed as catalysts, 19 and
17% of the desired product was formed. However, when
Ni@Fe3O4–C was used as catalyst, a good yield of desired diaryl
sulde was observed. These results conrmed the signicance
of catalyst for the reaction.
Substrate scope
To study the inuence of various parameters on the reaction
and to maximize the yield of the desired product, model
substrates were treated under various conditions using
different solvents, bases, time, amount of base and catalyst
loading. Fig. 8 shows the effect of different solvents. Fortu-
nately, with water as a solvent, the activity of our catalyst was
found to be comparable or superior than DMSO, DMF, aceto-
nitrile, methanol, THF and toluene, thus, making the protocol
green. Next, we screened the effect of base on the synthesis of
Finally, with these optimized reaction conditions in hand,
a range of Ni@Fe3O4–C catalyzed reactions between different
arylhydrazine hydrochlorides and thiophenols were conducted
(Table 2). Various diaryl suldes are obtained in moderate to
good yields. Arylhydrazines and arenethiols with electron-
donating substituents presented the corresponding products
Table 1 Optimization of reaction conditions for the formation of diaryl
sulfidea
Entry
Base
Amount of base (equiv.)
Yieldb (%)
1
2
3
4
5
6
7
8
—
Cs2CO3
Et3N
—
4
4
4
4
4
4
4
2
3
3
Trace
60
39
84
51
27
41
48
76
K2CO3
NaOH
DBU
Li2CO3
Na2CO3
K2CO3
K2CO3
K2CO3
9
10
84
Trace
11c
a
Fig. 7 Control experiments for the formation of diaryl sulfide. Reac-
tion conditions: 4-cyanophenylhydrazine hydrochloride (1 mmol), 4-
bromothiophenol (1 mmol), K2CO3 (4 equiv.), catalyst (30 mg), H2O (3
mL), rt, 24 h.
Reaction
conditions:
4-cyanophenylhydrazine
hydrochloride
(1 mmol), 4-bromothiophenol (1 mmol), base, catalyst (30 mg), H2O
(3 mL), rt, 24 h. GC-MS yield. c Nitrogen atmosphere.
b
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and morphology of the catalyst remained nearly unaltered even
aer seven consecutive cycles (Fig. S2 and S3†). Fig. S4† repre-
sents the VSM curve of recovered catalyst that suggest a slight
decrease in saturation magnetization. Moreover, the ICP-MS
analysis of recovered catalyst conrmed negligible leaching of
nickel acetate.
Fig. 9 (a) Effect of catalyst amount. Reaction conditions: 4-cyano-
phenylhydrazine hydrochloride (1 mmol), 4-bromothiophenol
(1 mmol), K2CO3 (3 equiv.), catalyst (10–30 mg), H2O (3 mL), rt, 24 h
and (b) Effect of reaction time. Reaction conditions: 4-cyanophe-
nylhydrazine hydrochloride (1 mmol), 4-bromothiophenol (1 mmol),
K2CO3 (3 equiv.), catalyst (25 mg), H2O (3 mL), rt, 10–30 h.
Experimental section
Details of the materials, reagents and instruments used can be
found in ESI.†
Synthesis of Fe3O4 microspheres
Solid Fe3O4 microspheres were synthesized using solvothermal
method.31 Briey, 4.0 g of sodium acetate is dissolved in 40 mL
of diethylene glycol. This solution is then transferred to a ask
containing a stirred solution of 1.95 g FeCl3$6H2O, 0.1 g triso-
dium citrate and 40 mL ethylene glycol. The resulting mixture
was further stirred for additional 30 min, and nally transferred
to a teon-lined stainless-steel autoclave which was then heated
in good yields. Chloro-substituted arylhydrazine coupled with
various arenethiols to afford diaryl suldes in good yields. Even
the strong electron-withdrawing group (–CN) on arylhydrazine
gave the corresponding coupling product in good to high yields.
Moreover, the electron-withdrawing group is well tolerated on
thiophenols as p-bromothiophenol also furnished the desired
products in moderate to high yields. On the grounds of mech-
anistic studies (Scheme S1†) and previous reports, a plausible
mechanism has been proposed which is outlined as Scheme
S2.† Next, we compared the outcomes of our work with various
homogeneous and heterogeneous catalysts that have been re-
ported in literature for the C–S coupling reaction to form diaryl
suldes. The comparison results are summarized in Table S1.†
To our delight, the catalytic activity and reaction conditions of
our catalyst was better than other catalytic systems.
ꢀ
at 200 C for 10 h. Aer completion of the reaction, the auto-
clave was le to cool to room temperature. The obtained black
powder of Fe3O4 microspheres was separated using an external
magnet, washed with water and ethanol, and dried under
ꢀ
vacuum at 50 C.
Synthesis of Fe3O4–C microspheres
The formation of Fe3O4–C microspheres is based on gas bubble
assisted Ostwald ripening process.31 200 mL of ethanol/water
(3 : 1) mixture was taken in a round bottom ask. Next, 0.25 g
of solid Fe3O4 microspheres were added to it. The ask was then
placed under sonication. Aer 15 min, resorcinol (0.4 g) and
aqueous ammonia (4.0 mL) were added and solution was stirred
at room temperature for 10 min. Next, 0.6 mL of formaldehyde
solution was added dropwise. The resultant reaction mixture
was stirred at room temperature for 1 h and then heated at 80 ꢀC
for 5 h. The hollow Fe3O4-phenolic polymer was removed
through magnetic separation, washed with water and ethanol
several times and nally dried under vacuum at 50 ꢀC. The
Heterogeneity test
To validate the heterogeneous nature of the catalyst, a hot
ltration test was performed with Ni@Fe3O4–C for oxidative C–S
coupling reaction using model substrates, 4-cyanophenylhy-
drazine hydrochloride and 4-bromothiophenol, under opti-
mized conditions. At the end of the reaction, the catalyst was
separated using an external magnet. The resulting ltrate was
examined by ICP-MS analysis, which showed negligible metal
leaching. This result conrmed the true heterogeneous nature
of the catalyst.
ꢀ
obtained Fe3O4-phenolic polymer was further heated at 600 C
under N2 atmosphere for 3 h to obtain double-shelled hollow
Fe3O4–C nanoreactor.
Recyclability
Fast and easy recovery and recyclability of a catalyst are the
prime features that stimulate the design of clean processes and
technologies. So, we investigated the recyclability of Ni@Fe3O4–
C catalyst in oxidative C–S coupling reaction under optimized
parameters. At the end of each reaction cycle, the catalyst was
separated from the reaction mixture with the assistance of an
external magnet, washed with ethanol, dried under vacuum and
reused for subsequent reactions. Fig. S1† depicts that the
activity of the catalyst remained nearly unchanged even aer 7
cycles. We also checked the stability of the catalyst aer 7 runs
Preparation of Ni@Fe3O4–C catalyst
100 mg of Fe3O4–C were sonicated in 40 mL methanol for
15 min. Then, 25 mg Ni(OAc)2 was added. Finally, the reaction
mixture was stirred at room temperature for 5 h. The catalyst
was collected using external mꢀagnet, washed with water and
ethanol, and nally dried at 50 C under vacuum.
General experimental procedure for the synthesis of diaryl
suldes catalyzed by Ni@Fe3O4–C catalyst
by FT-IR, SEM, VSM and ICP-MS analyses. The close resem- In an oven dried round bottom ask, thiol (1 mmol), arylhy-
blance of the FT-IR spectrum and SEM image of fresh and drazine (1 mmol), K2CO3 (3 equiv.), Ni@Fe3O4–C (25 mg) and
reused catalyst indicates that the functional groups, structure H2O (3 mL) were added. The reaction mixture was sonicated for
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Table 2 Scope of the catalytic activity of Ni@Fe3O4–C catalyst in the synthesis of diaryl sulfides derivativesab
a
Reaction condition: arylhydrazine hydrochloride (1 mmol), arenethiol (1 mmol), K2CO3 (3 equiv.), Ni@Fe3O4–C catalyst (25 mg), H2O (3 mL), rt,
b
20 h. GC-MC yield.
15 minutes. Then, it was stirred at room temperature under industries to synthesize medicinally important diaryl suldes in
open atmosphere for 20 h. Aer completion of the reaction, the an effective and sustainable way.
reaction mixture was extracted with ethyl acetate and organic
phase was dried over anhydrous Na2SO4. The obtained products
were analyzed using GC-MS.
Conflicts of interest
There are no conicts to declare.
Conclusions
Acknowledgements
In conclusion, we have developed an efficient nickel-based
magnetic double-shelled nanoreactor for the conned oxida-
tive C–S cross-coupling of arenethiols and arylhydrazines under
aerobic conditions to form unsymmetrical diaryl suldes. A
Gunjan Arora, Radhika Gupta, Pooja Rana and Priya Yadav
gratefully acknowledge CSIR and UGC for awarding the research
fellowships. Also, the authors thank USIC, University of Delhi,
India and TERI, India for providing instrumentation facilities.
diverse range of diaryl suldes are synthesized using water as
green solvent at room temperature under oxidant- and ligand-
free conditions. Interestingly, the use of arylhydrazines as
electrophilic counterpart highly promotes the principles of
Notes and references
green chemistry as only environmentally safe by-products;
nitrogen and water are formed. Besides, the catalyst can be
easily recovered from the reaction medium using external
magnet and can be recycled up to seven consecutive cycles
without appreciable loss in catalytic activity. We expect that the
present protocol could be extended to pharmaceutical
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