A semiconducting supramolecular Co(ii)-metallohydrogel: an efficient catalyst for single-pot aryl-S bond formation at room temperature

Article abstract of DOI:10.1039/c9dt03373d

A novel mechanically stable supramolecular Co(ii)-metallohydrogel has been synthesized. Cobalt(ii) nitrate hexahydrate and monoethanolamine, as a low molecular weight organic gelator, are used to get the gel. The mechanical stability of the supramolecular hydrogel was analyzed. The morphology of the supramolecular metallohydrogel was scrutinized. The semiconducting features of the metallohydrogel were studied. The conducting properties of the Co(ii)-metallohydrogel establish a Schottky barrier diode type nature. The catalytic nature of the Co(ii)-metallohydrogel based room temperature single pot aryl-S coupling reaction was explored. Most interestingly, the Co(ii)-metallohydrogel based catalytic aryl-S coupling reaction does not require any column-chromatographic purification protocol to get pure aryl-thioethers. Thus, through this work a semiconducting Schottky barrier diode application and catalytic role in the room temperature single pot aryl-S coupling reaction of a supramolecular Co(ii)-metallohydrogel have been explored.

Full text of DOI:10.1039/c9dt03373d

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A semiconducting supramolecular
Co(II)-metallohydrogel: an efficient catalyst
for single-pot aryl–S bond formation at
room temperature†
Cite this: Dalton Trans., 2019, 48,
17388
Subhendu Dhibar, a Amiya Dey,a Rajkumar Jana,b Arpita Chatterjee,a
a
Gourab Kanti Das,a Partha Pratim Rayb and Biswajit Dey
*
A novel mechanically stable supramolecular Co(II)-metallohydrogel has been synthesized. Cobalt(II) nitrate
hexahydrate and monoethanolamine, as a low molecular weight organic gelator, are used to get the gel.
The mechanical stability of the supramolecular hydrogel was analyzed. The morphology of the supra-
molecular metallohydrogel was scrutinized. The semiconducting features of the metallohydrogel were
studied. The conducting properties of the Co(II)-metallohydrogel establish a Schottky barrier diode type
nature. The catalytic nature of the Co(II)-metallohydrogel based room temperature single pot aryl–S
coupling reaction was explored. Most interestingly, the Co(II)-metallohydrogel based catalytic aryl–S
coupling reaction does not require any column-chromatographic purification protocol to get pure aryl-
thioethers. Thus, through this work a semiconducting Schottky barrier diode application and catalytic role
in the room temperature single pot aryl–S coupling reaction of a supramolecular Co(II)-metallohydrogel
have been explored.
Received 18th August 2019,
Accepted 5th November 2019
DOI: 10.1039/c9dt03373d
rsc.li/dalton
originate through the self-assembly of LMWG-type organic
chemical constituents in polar and non-polar solvents.
1. Introduction
Soft hydrogel scaffolds are esteemed for their increasing Versatile inter-connected supramolecular non-covalent facets
applications1,2 in transport medium, drug delivery, chemical like H-bonding patterns, metal coordination modes, van
sensing, self-healing properties,3 moldability,4 load-bearing der Waals interactions, aromatic π-system mediated non-
properties,5 catalytic properties,6 etc.7 The semi-solid architec- covalent interactions, like a π–π stacking force, solvophobic
ture of hydrogel contains huge water molecules in a three and/or hydrophobic interactions, etc.8–12 are critically respon-
dimensional soft scaffold organized by the gelator molecules.8 sible for the LMWG-based supramolecular gel formation
Low molecular weight gelators (LMWGs) are well-recognized in strategy.
the supramolecular gel chemistry. The supramolecular gels
Recently, metal ion mediated supramolecular metallogels
have been utilized for many potential uses,13 including
tissue engineering,14 drug delivery,15 organic electronics,16
sensing,17,18 biomedical applications,19 energy resources,20
electronic devices,21 etc.22
The exploration of conductive metallohydrogels is also used
for the fabrication of electronic devices.23 The electronic pro-
perties of metallohydrogel like Schottky barrier diodes, solar
cells, LEDs and memory devices24 are also intriguing.
Besides, thioethers having C–S bonds are significant
aDepartment of Chemistry, Visva-Bharati University, Santiniketan 731235, India.
E-mail: bdeychem@gmail.com
bDepartment of Physics, Jadavpur University, Jadavpur, Kolkata, 700 032, India
†Electronic supplementary information (ESI) available: Synthetic details, influ-
ence of the metal counter anion on the gelation abilities, the gelation process of
the metallohydrogel with different Co(II)-salts in various solvents, an investi-
gation of the minimum critical gelation concentration (MGC) of the CoMEA
metallohydrogel, an investigation of the minimum critical gelation concen-
tration (MGC) of the CoMEA metallohydrogel with varying monoethanolamine
content, a pH-dependent CoMEA metallohydrogel formation strategy, a pH- for biological and pharmaceutically relevant complexes.25
dependent CoMEA metallohydrogel formation strategy with Co(OAc)2·4H2O and
Thioethers also have several important aspects in organic
synthesis, biological chemistry, and materials science.26
CoCl2·6H2O salt, thermogravimetric analysis of the CoMEA metallohydrogel,
FESEM images, EDX spectral analysis, infrared spectroscopy, PXRD, optimiz-
Transition-metal catalyzed carbon–heteroatom coupling reac-
ation studies for the co-catalyzed coupling of aryl iodides and thiophenols,
tions are interesting in synthetic chemistry.25 The literature is
general experimental procedure for the synthesis of diphenylsulfide, and
1H-NMR and 13C-NMR spectra. See DOI: 10.1039/c9dt03373d
overwhelming with different transition metal catalyzed C–S
17388 | Dalton Trans., 2019, 48, 17388–17394
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1
bond formation strategies,26 but the cobalt-catalyzed aryl– Chemical shifts are referenced to CDCl3 (δ = 7.26 for H and
sulfur bond formation reaction to achieve thioethers is really δ = 77.16 for 13C{1H} NMR) as an internal standard.
limited in the literature.25
A Keithley 2635B source meter interfaced with a computer
In this present attempt, we report a supramolecular Co(II)- was used to perform the current–voltage (I–V) characteristics of
metallohydrogel (CoMEA metallohydrogel) which is prepared a CoMEA metallohydrogel material based thin film device.
by the use of monoethanolamine as the LMWG. The CoMEA
2.3. Synthesis of Co(II)-metallohydrogel (CoMEA
metallohydrogel)
metallohydrogel shows a semiconducting application like a
Schottky barrier diode device. Interestingly, the CoMEA metal-
lohydrogel also exhibits an excellent heterogeneous catalysis
towards the C–S coupling reaction to form aryl–sulfur bonds.
Before us, Chien-Hong Cheng and co-workers25 reported a
tedious and tough route for a cobalt catalyzed C–S coupling
reaction which needed an elevated temperature (i.e. 80 °C) and
prolonged reaction time (i.e. 10 hours). But, the CoMEA metal-
lohydrogel is able to rapidly perform a single pot C–S coupling
reaction at room temperature. Most interestingly, the CoMEA
metallohydrogel catalyzed C–S coupling reaction does not
require any column-chromatographic purification protocol to
get the pure aryl-thioethers.
The Co(II)-metallohydrogel (CoMEA metallohydrogel) was syn-
thesized by the instantaneous mixing of 500 µl of an aqueous
solution of cobalt nitrate hexahydrate (0.291 g, 1 mM) and
500 µl of the monoethanolamine ligand at room temperature
instantaneously (Fig. 1). Here, we further scrutinized with
different cobalt salts (i.e. Co(OAc)2·4H2O and CoCl2·6H2O) to
obtain different metallogels. Remarkably, gel formation was
observed only with Co(NO3)2·6H2O salts under the above-men-
tioned condition (see ESI for Table S1†). The gelation ability in
various solvents, like water, methanol, ethanol, ethyl acetate,
DMSO, acetonitrile, dichloromethane, THF, n-hexane, acetone,
toluene, chloroform, benzene and petroleum ether, using
Co(NO3)2·6H2O and monoethanolamine to get a stable CoMEA
metallohydrogel was been studied (see Table S2 in ESI†).
The gelation process of CoMEA metallohydrogel with
Co(OAc)2·4H2O and CoCl2·6H2O salts in various solvents, like
water, methanol, ethanol, ethyl acetate, DMSO, acetonitrile, di-
chloromethane, THF, n-hexane, acetone, toluene, chloroform,
benzene and petroleum ether, was also studied (see Tables S3
and S4 in ESI†).
2. Experimental
2.1. Materials
Cobalt(II) nitrate hexahydrate, cobalt(II) acetate tetrahydrate,
cobalt(II) chloride hexahydrate, monoethanolamine, solvents
and other chemicals were purchased from commercially avail-
able sources and were used without further purification.
Double distilled water was used throughout.
2.4 Minimum critical gelation concentration (MGC) and gel
2.2. Characterization
melting temperature (Tgel) studies
pH measurements were performed using a Mettler-Toledo pH
meter. The absorption spectral study was done using a
SHIMADZU UV-3101PC spectrophotometer. The rheological
experiment was carried out using a rheometer (TA Instrument)
with a cone plate geometry. Field Emission Scanning Electron
Microscopy (FESEM) images were obtained using a Carl Zeiss
SUPRA 55VP FESEM. Microstructural analyses, EDX studies
and elemental mapping were examined under a ZEISS, EVO
18. Solid-phase IR spectroscopy was measured on a Shimadzu
FTIR-8400S spectrometer between 400 and 4000 cm−1, using
the KBr pellet method. HRTEM images were collected using a
JEOL, 2010EX instrument, with an accelerating voltage of 200
kV. Powder X-ray diffraction (PXRD) patterns were recorded on
a Phillips PAN analytical diffractometer for Cu Kα radiation
(λ = 1.5406 Å), with a scan speed of 1° min−1 and a step size of
0.02° in 2θ. TG measurements were carried out using a TG
analyzer of Netzsch, Germany (Model: STA 449C). The gel
destruction temperature of the non-thermoreversible CoMEA
metallohydrogel was recorded through a sophisticated digital
melting-point measurement apparatus. 1H NMR spectra were
obtained on a 400 MHz spectrometer from CDCl3 solutions.
Chemical shifts are expressed in parts per million (δ) and the
signals are reported as s (singlet), d (doublet), t (triplet), m
(multiplet), and coupling constants (J) are given in Hz. 13C{1H}
In order to determine the minimum critical gelation concen-
tration (MGC) of the CoMEA metallohydrogel, we varied the
concentration of Co(NO3)2·6H2O in a water solvent from 10 to
291 mg mL−1 and, effectively, the CoMEA metallohydrogel was
formed at
a
291 mg mL−1 concentration (see ESI for
Table S5†). The minimum critical gel concentration (MGC) of
CoMEA metallohydrogel was also checked by varying mono-
Fig. 1 The synthetic strategy for the CoMEA metallohydrogel through
the mixing of an aqueous solution of a Co(II) salt and monoethanolamine
NMR spectra were recorded at 100 MHz in CDCl3 solution. at room temperature.
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ethanolamine from 10 to 500 µl and, effectively, the CoMEA
In the gel condition of the materials, G′(ω) > G″(ω) and G′
metallohydrogel was formed at 500 µl where Co(NO3)2·6H2O (ω) ≈ ω° always applies, where ω is the angular frequency.
was 291 mg (see ESI for Table S6†). From both experiments, by Here it is worth mentioning that all cases of the gel samples,
varying Co(NO3)2·6H2O and monoethanolamine to determine the value of the storage modulus (G′) was significantly higher
the minimum critical gel concentration (MGC) of CoMEA than that of the loss modulus (G″), also confirming their gel
metallohydrogel, the MGC was established at 291 mg mL−1 state network and supramolecular behavior.
(see ESI for Tables S5 and S6†). Here, we also studied the pH
The average storage modulus of the CoMEA metallohydro-
dependent CoMEA metallohydrogel formation strategy with a gel (G′ > 104 Pa) was observed to have a considerably higher
water solution of the Co(NO3)2·6H2O salt and monoethanola- magnitude than that of the loss modulus (G″), which suggests
mine where CoMEA metallohydrogel formed at pH = 9.704. a good tolerance limit of the prepared CoMEA metallohydrogel
The variation of pH is maintained by monoethanolamine and (Fig. 2a).
it also supports the minimum critical gelation concentration
The strain sweep experiments of the CoMEA metallohydro-
(MGC), which clearly shows that the CoMEA metallohydrogel gel were performed at a constant frequency of 1 rad per s.
only formed with 500 µl of monoethanolamine and 0.291 g of The moduli versus strain (%) plot are presented in Fig. 2b. It
the Co(NO3)2·6H2O salt (see ESI for Fig. S1†).
shows that G′ and G″ cross each other at γ = 25%, indicating
The pH dependent CoMEA metallohydrogel formation strat- the critical strain (minimum strain for gel breaking) of
egy with water solutions of different Co(II)-salts (i.e. CoMEA metallohydrogel materials. This critical strain value
CoCl2·6H2O and Co(OAc)2·4H2O) and monoethanolamine is is required for the collapse of the CoMEA metallohydrogel
given in ESI as Fig. S2 and S3.†
The gel to sol temperature (Tgel) of non-thermoreversible and reinforcing agent in the gel network. The metal probably
CoMEA metallohydrogel was found at ∼124 2 °C. The increases the additional stability of the hydrogel and for this
thermogravimetric analysis also supports the thermal stability reason a larger strain is required to rupture this hydrogel
network to a quasi-liquid state. Here, Co acts as cross linker
of the CoMEA metallohydrogel (Fig. S4†).
network.
3.2. Microstructural study
3. Results and discussion
FESEM microstructural analysis explored the interconnected
nano-flower type of hierarchical morphology of the supramole-
cular CoMEA metallohydrogel (Fig. 3a). The fibers of the
CoMEA metallohydrogel were imaged through TEM analysis.
Possibly, the morphological compactness in the hierarchical
architecture of the CoMEA metallohydrogel might be due to
different supramolecular facets responsible for the assembly
of the Co(II) ions and monoethanolamine ligand along with
the trapped water content in its gel-state. An elemental
mapping investigation of the CoMEA metallohydrogel exhibits
the presence of O, Co, N and C elements from the metal ions
and organic ligand in the CoMEA metallohydrogel network
3.1. Rheological analysis
The viscoelastic semi-solid nature of the CoMEA metallohydro-
gel was confirmed by rheological measurements, including
angular frequency sweep and strain sweep studies (Fig. 2a and b).
The storage modulus (G′) of the hydrogel samples shows
the storage energy in the system during the application of
shear on the viscoelastic region. The loss modulus (G″) indi-
cates a dissolute energy under the application of oscillatory
stress when the sample behavior is liquid-like. The storage
modulus (G′) and loss modulus (G″) of the samples are
defined as
G′ ¼ ðσ0=γ0ÞcosðδÞ
G″ ¼ ðσ0=γ0Þ sinðδÞ
ð1Þ
ð2Þ
where, σ0 = shear stress amplitude, γ0 = strain amplitude, σ0/γ0
= amplitude ratio, and δ = phase angle.
Fig. 3 (a) FESEM microstructural images of the CoMEA metallohydro-
gel, (b) TEM image of the CoMEA metallohydrogel, (c–f) and elemental
mapping of the CoMEA metallohydrogel showing the presence of O, Co,
N, and C elements.
Fig. 2 (a) Angular frequency-dependent storage modulus (G’) and loss
modulus (G’’) of the CoMEA metallohydrogel. (b) Strain-sweep measure-
ments of the CoMEA metallohydrogel.
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(Fig. 3b–e) (see ESI for detailed FESEM images, and an EDX using a DEKTAK surface profiler. To complete the Al/CoMEA
spectrum as Fig. S5 and S6†).
metallohydrogel/ITO structure, aluminum was used as a metal
electrode. The effective area (A) of the devices was maintained
as 7.065 × 10−6 m2.
3.3. Infrared study
3.5.3. Fabrication of Schottky barrier diode. The obtained
direct optical bandgap of the CoMEA metallohydrogel
prompted us to move forward toward probing the inherent
conductivity of our synthesized material. We measured the
conductivity of the material in a thin film structure. Current–
voltage (I–V) measurements of the CoMEA metallohydrogel
were recorded with a Keithley 2635B sourcemeter, interfaced
with a computer. Fig. 4 shows the I–V characteristics of our
material under a bias voltage ranging over 10 V. The conduc-
tivity at 30 °C was measured with the help of the standard
equation σ = (d/A)·(I/V), where d, A and I/V are the thickness of
the thin film, effective area of the device and slope of the I–V
curve (Fig. 5). The measured conductivity of CoMEA metallohy-
drogel was 6.36 × 10−4 S m−1 (Fig. 5) which indicates that it is
a semiconductor.
The above obtained conductivity profile of our synthesized
material led to our great interest in the application of this
material in a Schottky barrier diode (SBD). Hence, to verify the
Schottky behavior of our material, we fabricated an Al/CoMEA
metallohydrogel/ITO sandwich structure device. We performed
I–V measurements of the device in the bias voltage range of −2 V
to +2 V. All the electrical characterizations were performed at
30 °C. For the CoMEA metallohydrogel based device, we calcu-
lated the on/off ratio (rectification ratio) of the SBD and the
measured value is 628 at 2 V, which is listed in Table 1. The
obtained high rectification ratio of the material based device
depicts the well the formation of Schottky interfaces between
the CoMEA metallohydrogel (semiconductor) and aluminum
(metal). As the SBD is a majority carrier device, the device per-
formance depends upon the generation of the majority carrier
i.e. electrons. The high rectification ratio reflects the gene-
ration of huge numbers of charge carrier in terms of the fast
switching ability of the devices.
The supramolecular facets constructing the CoMEA metallo-
hydrogel were found by infrared spectral analysis (see ESI for
Fig. S7†).
3.4. Powder X-Ray diffraction study
The amorphous nature of the CoMEA metallohydrogel was
established through a powder X-Ray diffraction study (see ESI
for Fig. S8†).
3.5. Semiconducting application study
3.5.1. UV-Vis absorption study. The optical behavior of the
CoMEA metallohydrogel was studied by UV-VIS absorption.
Fig. 4 shows the UV-Vis absorption spectra of the material. The
absorption spectra were recorded in the wavelength range of
350–1000 nm by a UV-VIS spectrophotometer and the energy
absorption was found in the visible region. From the UV-VIS
absorption spectra, the optical bandgap of our synthesized
material can be calculated using the following standard Tauc’s
equation.27
1=2
αhγ ¼ Aðhγ ꢀ EgÞ
ð3Þ
3.5.2. Fabrication of Schottky barrier diode. To see the
applicability of this material in an active semiconductor device
we fabricated Schottky barrier diodes (SBDs) using our syn-
thesized material (CoMEA metallohydrogel). To fabricate the
Al/CoMEA metallohydrogel/ITO structure, we cleaned a ITO
(indium tin oxide) coated glass substrate using soap solution,
acetone and distilled water consecutively with the help of an
ultrasonication technique. A thin film of our synthesized
CoMEA xerogel material was developed on the ITO coated
glass substrate using a spin coating technique. The thickness
(d) of the deposited active layer was estimated as ∼1 µm by
Fig. 4 UV-vis absorption spectra (inset: Tauc plot) of the CoMEA
metallohydrogel.
Fig. 5 Current–voltage (I–V) characteristics of the CoMEA metallohydrogel.
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Table 1 Electrical parameters of the Al/CoMEA metallohydrogel/ITO
structure
Series resistance
(Rs) (Ω)
Barrier
On/ Ideality
off factor (η) (Φb) (eV) vs. I
height
dv/dln I H(I)
Sample
vs. I
CoMEA metallohydrogel 628 1.16 0.59 9541
12 540
To further analyze the fundamental machinery of the
charge transport properties of our fabricated SBD, we again
analyzed the I–V characteristics with the help of the thermio-
nic emission (TE) theory. In this regard, we considered the fol-
lowing standard equations:27,28
ꢂ
ꢀ
ꢁ
ꢃ
qV
ηkT
I ¼ I0 exp
ꢀ 1
ð4Þ
where q, I, η, k and T are the electronic charge, current at
forward bias voltage, ideality factor of the device as the SBD is
non ideal in nature, Boltzmann constant and the absolute
temperature in Kelvin, respectively. Here, I0 represents the
reverse saturation current and it can be written as follows:27,28
ꢀ
ꢁ
ꢀqΦb
I0 ¼ AA*T2 exp
ð5Þ
kT
Fig. 6 (a) Current voltage (I–V) characteristics (inset: log I vs. V graph)
and (b) dv/dln I vs. I and H(I) vs. I graphs of the Al/CoMEA metallohydro-
gel/ITO structure.
where A is the effective area of the SBD, A* is the Richardson
constant and Φb is the barrier height of the desired device.
The effective diode area is considered as 7.065 × 10−6 m2.
Moreover, we calculated some important electrical para-
meters, such as the ideality factor (η), barrier height (Φb) and
the series resistance (Rs), which are important parameters for
the investigation of the electrical behavior of the device. The
ideality factor (η) and the series resistance (Rs) of the device
can be evaluated by the following equation:
rectification with the ratio 628 at 2 V. The ideality factor
(1.16) of the device indicates that the device is almost ideal.
The series resistance and barrier height (0.59 eV) of the device
were estimated from the dv/dln I vs. I and H(I) vs. I graph
(Fig. 6(b)). Fig. 6(b) shows the H(I) vs. I curve of the Schottky
barrier diode. The graph of H(I) vs. I is a linear curve whose
intercept and slope provide the barrier height (Φb) and series
resistance (Rs). The series resistance measured from the H(I)
vs. I curve is a second order determination of the series resis-
tance of the devices. The evaluated values of the barrier height
and series resistance are presented in Table 1. The values of
the on/off ratio and barrier height indicate that the CoMEA
metallohydrogel based material can be used for thin film
Schottky device.
dv
d lnðIÞ
ηkT
¼
þ IRs:
ð6Þ
q
The ideality factor and the series resistance were determined
from the intercept and slope of the dv/dln I vs. I plot, respectively.
The measured value of the ideality factor is 1.16 for the CoMEA
metallohydrogel based Schottky diode. The ideality factor indi-
cates that there is a small deviation from ideality. This may be
due to the presence of the inhomogeneities in barrier height,
existence of the series resistance and the interface states.29 The
series resistance was calculated from the dv/dln I vs. I plot as 9541
Ω for the Al/CoMEA metallohydrogel based device. The barrier
height can be measured from the following standard equations:
3.6. CoMEA metallohydrogel catalyzed aryl-sulfur bond
formation
The possibility of using the CoMEA metallohydrogel as an
economically viable catalyst towards the aryl–S coupling reac-
tion was explored through the work. Thiophenol and diverse
aryl halides were exploited for the study of the catalytic role of
CoMEA metallohydrogel towards the different aryl–S coupling
reactions. The CoMEA metallohydrogel was used in the metal-
logel form to perform the catalytic reaction in the presence of
ꢀ
ꢁ
ꢀ
ꢁ
ηkT
q
I
HðIÞ ¼ V ꢀ
ln
ð7Þ
AA*T2
Eqn (7) can be written in the following manner
HðIÞ ¼ IRs þ ηΦb
ð8Þ
Fig. 6(a) represents the (I–V) characteristics of the Al/ different aryl halides and thiophenol. Initially, 4-bromoben-
CoMEA metallohydrogel/ITO device which gives a very high zene and thiophenol were reacted in the CoMEA metallohydro-
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gel network in the presence of zinc powder and a minimum
volume of ethyl acetate solvent for 6 h in an open single reac-
tion vessel at room temperature (∼30–35 °C). The coupling
product of diphenylsulfane (1) (Table 2, entry 1) was obtained
with an 89% yield following the optimized reaction condition
(see ESI for Table S8,† entry 1 and details of the methodology,
including the optimization of the CoMEA metallohydrogel
catalyzed reaction in Table S7†). To explore the role of the
CoMEA metallohydrogel as the catalyst in several aryl–S coup-
ling reactions, halide substituted benzene substrates, like
4-bromobenzaldehyde, 4-bromobenzonitrile, 1-iodo-4-methox-
ybenzene, 2-bromopyridine, 1-(4-iodophenyl)ethanone, 1,2,4,5-
tetrabromobenzene, 1,2,4-tribromobenzene, 1,3,5-tribromo-
benzene, 1,4-dibromobenzene, 9-bromophenanthrene and
4,4′-dibromobiphenyl, (Table S8,† entries 2–12), were used
and, interestingly, the CoMEA metallohydrogel was extremely
successful to act as a catalyst towards the aryl–S coupling reac-
tions to get numerous aryl sulfanes (Table 2), like 4-(phe-
nylthio)benzaldehyde (entry 2, yield 84%), 4-(phenylthio)
benzonitrile (entry 3, yield 83%), (4-methoxyphenyl)(phenyl)
sulfane (entry 4, yield 87%), 2-(phenylthio)pyridine (entry 5,
yield 85%), 1-(4-(phenylthio)phenyl)ethanone (entry 6, yield
82%), 1,2,4,5-tetrakis(phenylthio)benzene (entry 7, yield 79%),
benzene-1,2,4-triyltris(phenylsulfane) (entry 8, yield 80%),
1,3,5-tris(phenylthio)benzene (entry 9, yield 78%), 1,4-bis(phe-
nylthio)benzene (entry 10, yield 82%), phenanthren-9-yl
(phenyl)sulfane (entry 11, yield 90%) and 4,4′-bis(phenylthio)
Scheme 1 Recycling of the CoMEA metallohydrogel catalyst.
biphenyl (entry 12, yield 84%), with considerable high yields
(∼78–90%, Table 2) at room temperature (∼30–35 °C) through
a single pot reaction in open air. By using the same protocol,
we were also able to couple aryl halides with various substi-
tuted thiophenols (Table 2, entries 13 and 14). When 4-methyl-
benzenethiol was coupled with 4-bromobenzonitrile it gave the
corresponding aryl sulfides i.e. (4-(p-tolylthio)benzonitrile
(Table 2, entry 13, yield 82%), and 4-methoxybenzenethiol also
gave the corresponding aryl sulfides i.e. 4-(4-methoxyphe-
nylthio)benzonitrile (Table 2, entry 14, yield 85%) in good
yields when reacted with 4-bromobenzonitrile in the presence
of the CoMEA metallohydrogel (5 mol%) and Zn powder at
room temperature (∼30–35 °C) (see ESI† for the general experi-
mental procedure for the synthesis of diphenylsulfide).
Table 2 CoMEA metallohydrogel catalyzed aryl–S coupling reaction of
aryl halides and thiophenola
3.7 Catalytic efficiency of the CoMEA metallohydrogel
Finally, the catalyst was verified to estimate its recycling compe-
tences. The catalyst was separated from the reaction mixture by
a simple filtration process followed by drying to reuse. The reac-
tion between thiophenol and the aryl halide led to the for-
mation of the thioether with quantitative yields by using the
CoMEA metallohydrogel catalyst. The same CoMEA metallo-
hydrogel catalyst was reused to produce thioether, and this was
achieved without decreasing the yield for at least up to four
cycles. The first run achieved a 98% conversion, and the
second, third and fourth runs using the recovered CoMEA
metallohydrogel catalyst showed the conversion extents as 97,
95 and 90%, respectively (Scheme 1). Even, a remarkable stabi-
lity of the CoMEA metallohydrogel catalyst was observed at least
up to six months. The catalytic activity of the CoMEA metallo-
hydrogel could be preserved and stored under open atmosphere
at room temperature for a period of about six months.
4. Conclusions
a Reaction conditions: CoMEA metallohydrogel (5 mol%), 0.5 mmol of
aryl halide, 0.5 mmol of thiophenol, 1.5 equiv. of Zn powder in 2 mL
of ethyl acetate at room temperature for 6 h (see the ESI for 1H-NMR
and 13C-NMR spectra, Fig. S7–S36†).
In summary, a supramolecular Co(II)-metallohydrogel of mono-
ethanolamine was achieved. The mechanical stability of the
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CoMEA metallohydrogel was tested. The morphology and
elemental construction of the CoMEA metallohydrogel gel
were critically investigated. Based on electrical experiments
(i.e., I–V characteristics), the CoMEA metallohydrogel acts as
an efficient Schottky barrier diode device.
7 (a) F. L. Buchholz and A. T. Graham, Modern superabsorbent
polymer technology, Wiley-VCH, New York, 1998.
8 P. Dastidar, Chem. Soc. Rev., 2008, 37, 2699–2715.
9 A. Harada, R. Kobayashi, Y. Takashima, A. Hashidzume
and H. Yamaguchi, Nat. Chem., 2011, 3, 34–37.
Most interestingly, the CoMEA metallohydrogel catalyzed 10 T. Park and S. C. Zimmerman, J. Am. Chem. Soc., 2006, 128,
the C–S coupling reaction at room temperature. The method 11582–11590.
does not need any column chromatographic purification pro- 11 M. Shirakawa, N. Fujita and S. Shinkai, J. Am. Chem. Soc.,
tocol. This might be a unique citation of a room temperature 2003, 125, 9902–9903.
single pot aryl–S coupling reaction. This work is possibly an 12 J. R. Moffat, G. J. Seeley, J. T. Carter, A. Burgessb and
instigation of a metallohydrogel catalyzed ‘green’ synthetic
approach towards carbon–heteroatom coupling reactions.
D. K. Smith, Chem. Commun., 2008, 4601–4603.
13 K. J. Skilling, F. Citossi, T. D. Bradshaw, M. Ashford,
B. Kellam and M. Marlow, Soft Matter, 2014, 10, 237–
256.
14 P. P. Kalshetti, V. B. Rajendra, D. N. Dixit and P. P. Parekh,
Int. J. Pharm. Pharm. Sci., 2012, 4, 1–7.
15 D. T. Eddington and J. B. David, Adv. Drug Delivery Rev.,
2004, 56, 199–210.
Conflicts of interest
The authors declare no competing financial interests.
16 S. S. Babu, S. Prasanthkumar and A. Ajayaghosh, Angew.
Chem., Int. Ed., 2012, 51, 1766–1776.
17 D. Buenger, F. Topuz and J. Groll, Prog. Polym. Sci., 2012,
37, 1678–1719.
18 H. R. Culver, J. R. Clegg and N. A. Peppas, Acc. Chem. Res.,
2017, 50, 170–178.
19 E. Caló and V. V. Khutoryanskiy, Eur. Polym. J., 2015, 65,
252–267.
Acknowledgements
B. D. thankfully acknowledges CSIR (New Delhi, India) for
research project (Project No.: 01(2866)/16/EMR-II) financial
support. S. D. is grateful to UGC, New Delhi, for awarding him
a RGNSRF.
20 T. B. H. Schroeder, A. Guha, A. Lamoureux,
G. VanRenterghem, D. Sept, M. Shtein, J. Yang and
M. Mayer, Nature, 2017, 552, 214–218.
Notes and references
1 S. I. Stupp and L. C. Palmer, Chem. Mater., 2014, 26, 507–
518.
21 S. Lin, H. Yuk, T. Zhang, G. A. Parada, H. Koo, C. Yu and
X. Zhao, Adv. Mater., 2016, 28, 4497–4505.
22 A. Y.-Y. Tam and V. W.-W. Yam, Chem. Soc. Rev., 2013, 42,
1540–1567.
23 G. Givaja, P. Amo-Ochoa, C. J. Gómez-García and
F. Zamora, Chem. Soc. Rev., 2012, 41, 115–147.
24 (a) S. M. Sze and K. K. Ng, Physics of Semiconductor Devices,
John Wiley & Sons. Inc., 2006; (b) J. Bardeen, Phys. Rev.,
1947, 71, 717–727.
25 Y.-C. Wong, T. T. Jayanth and C.-H. Cheng, Org. Lett., 2006,
8, 5613–5616.
26 C.-F. Lee, Y.-C. Liu and S. S. Badsara, Chem. – Asian J., 2014,
9, 706–722.
27 R. Jana, A. Dey, M. Das, J. Datta, P. Das and P. P. Ray, Appl.
Surf. Sci., 2018, 452, 155–164.
28 E. H. Rhoderick, Metal Semiconductors Contacts, Oxford
University Press, Oxford, 1978.
29 R. K. Gupta and F. Yakuphanoglu, Sol. Energy, 2012, 86,
1539–1545.
2 (a) T. Nishikawa, K. Akiyoshi and J. Sunamoto, J. Am. Chem.
Soc., 1996, 118, 6110–6115; (b) Y. Osada and J.-P. Gong,
Adv. Mater., 1998, 10, 827–837; (c) S. J. Novick and
J. S. Dordick, Chem. Mater., 1998, 10, 955–958.
3 (a) N. Holtem-Andersen, M. J. Harrington, H. Birkedal,
B. P. Lee, P. B. Messersmith, K. Y. C. Lee and J. H. Waite,
Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 2651–2655;
(b) M. Burnworth, L. Tang, J. R. Kumpfer, A. J. Duncan,
F. L. Beyer, G. L. Fiore, S. J. Rowan and C. Weder, Nature,
2011, 472, 334–337.
4 C. K. Karan and M. Bhattarcharjee, ACS Appl. Mater.
Interfaces, 2016, 8, 5526–5535.
5 S. Saha, J. Bachl, T. Kundu, D. D. Díaz and R. Banerjee,
Chem. Commun., 2014, 50, 3004–3006.
6 J. Bachl, A. Hohenleutner, B. B. Dhar, C. Cativiela,
U. Maitra, B. Konig and D. D. Díaz, J. Mater. Chem. A, 2013,
1, 4577–4588.
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