Efficient reduction of graphene oxide to graphene nanosheets using a silica-based ionic liquid: Synthesis, characterization and catalytic properties of IMD-Si/FeCl4-@GNS

Article abstract of DOI:10.1039/c9nj05768d

The aim of the present protocol is to develop a facile route for the synthesis of novel ionic liquid (IL)-functionalized graphene nano sheets (IMD-Si/FeCl₄^-@GNS). An imidazolium IL with tetrachloroferrate as an anion (IMD-Si/FeCl₄^-) was used to reduce graphene oxide. IMD-Si/FeCl₄^-@GNS were well characterized using various techniques such as Fourier transform infrared (FTIR), Raman, ultraviolet-visible diffuse reflectance spectroscopy (UV-VIS DRS), scanning electron microscopy/energy dispersive X-ray (SEM/EDX), elemental mapping, transmission electron microscopy (TEM), powder X-ray diffraction (XRD), thermal gravimetry (TG), electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) analyses. The IL-functionalized material was investigated for its catalytic properties and found to be an efficient catalyst for the synthesis of (E)-selective substituted thiosemicarbazone derivatives. The (E)-selectivity was confirmed using DFT calculations which showed that the (E)-isomer was stabilized by 4.90 kcal mol^-1 more than the (Z)-isomer. The catalyst maintained its catalytic activity for six successive runs.

Full text of DOI:10.1039/c9nj05768d

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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DOI: 10.1039/C9NJ05768D
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Efficient reduction of Graphene oxide to Graphene Nanosheets by Silica based Ionic
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Liquid: Synthesis, Characterization and Catalytic properties of IMD-Si/FeCl4ˉ@GNS
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Mohd Umar Khan, Shaheen Siddiqui, Waqas Ahmad Khan and Zeba N. Siddiqui*,
Department of Chemistry, Aligarh Muslim University, Aligarh, 202002, (India)
* Corresponding author. E-mail: siddiqui_zeba@yahoo.co.in, zns.siddiqui@gmail.com
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Abstract
The aim of the present protocol is to develop a facile route for the synthesis of novel ionic liquid
(IL) functionalized graphene nano sheets (IMD-Si/FeCl4ˉ@GNS). An imidazolium IL with iron
chloride as an anion (IMD-Si/FeCl4ˉ) was used to reduce graphene oxide. IMD-Si/FeCl4ˉ@GNS
was well characterized by various techniques such as Fourier Transform Infrared (FTIR),
Raman, Ultraviolet-Visible Diffuse Reflectance Spectroscopy (UV-VIS DRS), Scanning
Electron Microscope/Energy Dispersive X-ray (SEM/EDX), Elemental mapping, Transmission
Electron Microscope (TEM), Powder X-ray diffraction (XRD), Thermal Gravimetric (TG),
Electron Paramagnetic Resonance (EPR), and X-Ray Photoelectron Spectroscopy (XPS)
analyses. The IL functionalized material was investigated for its catalytic property and found to
be an efficient catalyst for the synthesis of (E)-selective substituted thiosemicarbazone
derivatives. The (E)-selectivity was confirmed by the DFT calculations which showed that the
(E)-isomer was stabilized by 4.90 kcal/mol more than the (Z)-isomer. The catalyst maintained its
catalytic activity for six successive runs.
Keywords: Ionic liquid, IMD-Si/FeCl4ˉ, heterogeneous catalyst, thiosemicarbazone
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Introduction
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In recent years, ionic liquids (ILs) have gained significant attention of the researchers because of
their unique properties such as low surface energy, non-volatility, greater selectivity and
environmental compatibility, high chemical and thermal stability, tunable miscibility with water
and organic solvents, etc.1 These valuable properties of ILs assisted most of the chemical and
industrial field of research such as organic synthesis (as catalyst and green solvent)2,3, liquid-
liquid extraction4 and electrochemistry5. Due to the above-mentioned attributes, ILs have been
successfully employed as a sustainable substitute of traditional mineral acids (such as sulfuric
acid and hydrochloric acid) in chemical synthesis and catalysis6. Among the ILs, imidazolium-
based ILs have been proved to be a remarkable ionic system due to their air and moisture
stability7. Hence, these ILs have been extensively applied in many fields of research, especially
in catalysis. Several organic transformations have been catalyzed by these ILs e.g. synthesis of
benzoxanthene8, hydroxyketones9, phenylpropanethioate10, amidoalkyl naphthols11, etc.
Immobilization of ILs on the surface of carbonaceous materials has recently been introduced as a
strategy to prepare supported heterogeneous IL catalysts for application in different growing
fields12.
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Among the carbonaceous materials, graphene and functionalized graphene have opened up a new
exciting field in science and technology on account of their unique properties such as ultrahigh
carrier mobility at room temperature, high surface area, exceptional thermal conductivity,
excellent mechanical properties, remarkable optical properties etc.13. Graphene nanosheet (GNS)
is a 2D system with very strong sp2 bonds, resulting in a layered structure with a high aspect
ratio and a large π-electronic surface that requires no helicity control14. Different graphene
functionalized materials have been prepared and applied as a catalyst in various organic reactions
such as Pechmann condensation15, Aza-Michael addition reaction16, Knoevenagel
condensation17, and Friedel-Crafts addition18 etc.
Thiosemicarbazone derivatives represent an important class of organic compounds that have
received considerable interest because of appreciated anti-microbial activity against Plasmodium
falciparum and Plasmodium berghei19, Trypanosoma brucei rhodesiense20, Trypanosoma cruzi21
and Toxoplasma gondii etc.22
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In continuation of our previous research work on supported heterogeneous catalysts 23-29, the
present study has described an easy route for the synthesis of IL functionalized GNS (IMD-
Si/FeCl4ˉ@GNS) as a recyclable catalyst. The catalytic activity of IMD-Si/FeCl4ˉ@GNS was
explored by synthesizing a series of (E)-selective substituted thiosemicarbazone derivatives and
proved to be an efficient catalyst by affording excellent yield of the products in very short time
period.
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Experimental section
Synthesis of silica modified imidazolium chloride (IMD-Si/Cl)
Silica modified ionic liquid (IMD-Si/Cl) was synthesized by reported procedure with some
modifications.30 For this, a mixture of 3-chloromethoxypropylsilane (30 mmol, 5.5 mL) and
imidazole (30 mmol, 2.46 g) was placed in a three-necked round-bottom (RB) flask and heated
with magnetic stirring at 110°C for 24 h under N2 atmosphere. After 24 h, the reaction mixture
was kept at room temperature for 1h. The obtained bright yellow product (liquid) was washed
with ethyl acetate (3x15 mL) and dried over anhydrous Na2SO4 to get pure IL (IMD-Si/Cl).
Yield: IMD-Si/Cl = 3.20g
Synthesis of silica modified imidazolium iron chloride (IMD-Si/FeCl4ˉ)
A mixture of IMD-Si/Cl (0.01 mol, 2.8 g) and anhydrous FeCl3 (0.01 mol, 1.62 g) was prepared
in a RB flask containing acetone (35 mL). The reaction mixture was stirred at reflux temperature
(80˚C) for 12 h. After keeping the reaction mixture at room temperature for 1h, a dark yellow
product (IMD-Si/FeCl4ˉ) obtained which was filtered, washed several times with acetone until
the colorless filtrate was obtained and dried at room temperature (30°C) for 24 h for further use.
Yield: IMD-Si/FeCl4ˉ = 4.08g.
Synthesis of graphene oxide (GO)
GO was prepared by modified Hummer’s method31. In a typical run, a mixture of graphite flakes
(1.5 g) and NaNO3 (0.5 g) was placed in an RB flask (250 mL) containing concentrated H2SO4
(25 mL) and stirred for 30 min at 0-5°C in an ice bath. KMnO4 (3 g) was then added very slowly
in portions (at the rate of 1g in 10 min to avoid explosion) with vigorous stirring. After the
addition of KMnO4, the ice bath was removed and the reaction mixture was kept at 35°C for 2 h.
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After that, deionized water (50 mL) was added dropwise to the RB flask containing reaction
mixture. The addition of water produces a huge amount of heat and the temperature rises to 100
°C resulting in a brown colored solution. After 20 min, 30% H2O2 (10 mL) and warm water (100
mL) were added to the brown solution in the RB flask and the mixture was stirred for 5 min
producing a yellowish-brown solution. The resulting mixture was filtered through using
Whatman filter paper and subsequently washed with 5% HCl and double distilled water (3x50
mL) until pH reached ~7. The resulting brown solid, was dried in an oven at 50°C for 12h to get
brown graphite oxide powder (1.85g). To obtain GO, graphite oxide (1.67g) was dispersed in
double-distilled water (500 mL) by ultrasonication for 3 h. The graphite oxide was exfoliated and
turns into a stable brown colored GO dispersion. The suspension was centrifuged at 4000 for 50
min and then dried at 60˚C for 4 h to get GO as brown free flowing powder. Yield: GO = 1.43g
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Synthesis of IMD-Si/FeCl4ˉ@GNS
IL functionalized GO was obtained under N2 atmosphere. GO (1g) was dispersed into absolute
ethanol (250 mL) by sonication (30 min) and IL (IMD-Si/FeCl4ˉ) (0.5 g) added to it under N2
atmosphere. After stirring the resultant mixture for 2 h, it turned black indicating the reduction of
GO and formation of IMD-Si/FeCl4ˉ@GNS. The IMD-Si/FeCl4ˉ@GNS was isolated by
centrifugation, washed with anhydrous ethanol (3x20 mL) and dried in a vacuum oven at 50°C
for 24 h. Yield: IMD-Si/FeCl4ˉ@GNS = 1.27g.
General procedure for the synthesis of substituted (E)-thiosemicarbazone derivatives
Equimolar mixture of carbonyl compounds 1a-l, thiosemicarbazide 2 and IMD-Si/FeCl4ˉ@GNS
(80 mg) was taken in a 50 mL RB flask and the reaction mixture was stirred for 3-5 min at room
temperature (30˚C). The reaction progress was monitored by TLC. After the completion of the
reaction, precipitated product was added to a separating funnel containing ethyl acetate and
water. The organic product was extracted with ethyl acetate, and dried over anhydrous sodium
sulphate. The excess solvent was evaporated under reduced pressure to get crude product which
was then recrystallized with ethanol to get pure products 3a-l. The catalyst (in aqueous phase)
was filtered, washed with ethanol and oven dried for further catalytic runs. Yield: 90-96%
Results and Discussion
Characterization of IMD-Si/FeCl4ˉ@GNS
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The synthesis of IMD-Si/FeCl4ˉ@GNS was outlined in Figure 1. The IMD-Si/FeCl4ˉ@GNS was
well characterized by various techniques such as Fourier Transform Infrared (FTIR), Raman,
Ultraviolet-Visible Diffuse Reflectance Spectroscopy (UV-VIS DRS), Scanning Electron
Microscope/Energy Dispersive X-ray (SEM/EDX), Elemental mapping, Transmission Electron
Microscope (TEM), Powder X-ray diffraction (XRD), Thermal Gravimetric (TG), Electron
paramagnetic resonance (EPR), and X-Ray photoelectron spectroscopy (XPS) analyses. The
detailed characterization of the catalyst was discussed below.
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Figure 1: (a) Synthesis of IMD-Si/FeCl4ˉ (b) Synthesis of IMD-Si/FeCl4ˉ@GNS
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FTIR analysis
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In the FTIR spectrum of GO (Figure 2a), a broad peak centered at 3437 cm-1 was attributed to
the vibration and deformation of –OH group. The stretching vibrations of C=O groups appeared
at 1728, 1625 cm-1, O-H deformation at 1408 cm-1 and C-O epoxy stretching at 1225 cm-1
respectively 32.
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Figure 2. FTIR spectrum of (a) GO (b) IMD-Si/FeCl4ˉ (c) IMD-Si/FeCl4ˉ@GNS and (d)
Recycled catalyst
The FTIR spectrum of IMD-Si/FeCl4ˉ (Figure 2b) displayed characteristic bands of imidazole
ring and alkyl chain containing silicon functional group. In imidazole ring, the bands at 3382 cm-
1 and 3134 cm-1 were due to the stretching vibrations of N–H and C–H respectively whereas two
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sharp bands at 1578 cm-1 and 1454 cm-1 were assigned to ring in-plane symmetric/anti-
symmetricstretch CH2(N)33 and symmetric stretching of C–N respectively34a. In alkyl chain, C–H
stretching vibration appeared at 2837 cm-1 34b while other bands at 1224 cm-1, 1091 cm-1 and 819
cm-1 were ascribed to Si-O-C stretching, asymmetric Si-O-Si stretching and symmetric Si-O-Si
stretching mode of vibrations respectively35. The FTIR spectrum of GO supported IL, IMD-
Si/FeCl4ˉ@GNS, (Figure 2c) showed a broad peak at 3406 cm-1 due to O–H stretching of GNS
and N–H stretching of imidazole ring. The intensity of C=O band (1726 cm-1) was reduced in
catalyst showing the formation of GNS. The characteristic peaks at 1726 cm-1 (C=O), 1629 cm-1
(C=C), 1580 cm-1 CH2(N), 1452 cm-1 (C–N), 1220 cm-1 (Si-O-C), 1090 cm-1 (Si-O-Si) indicated
the formation of IMD-Si/FeCl4ˉ@GNS.
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UV-VIS DRS analysis
The UV-visible DRS spectrum of GO showed absorption peak at 230 nm and a shoulder at 305
nm ascribing a characteristic feature of π–π* transition of C–C bond and n–π* transition of C–O
bond respectively (Figure S1a) 36. A red shift was observed in the UV-visible spectrum of IMD-
Si/FeCl4ˉ@GNS from 230 nm to 268 nm confirming the change in electronic configuration of
honeycomb lattice of GO to highly reduced GNS (Figure S1b). In addition, an absorption band
at 476 nm appeared due to d-d transition of FeCl4ˉ 37.
SEM and EDX/elemental mapping analyses
The surface morphologies of GO and IMD-Si/FeCl4ˉ@GNS were examined by SEM analysis
(Figure 3). The SEM images of GO indicated the layered structure (Figure 3a) and IMD-
Si/FeCl4ˉ@GNS showed presence of IL on the surface of GNS (Figure 3b). The EDX analysis
of GO displayed the presence of C and O (Figure 4a) whereas that of IMD-Si/FeCl4ˉ@GNS
confirmed the presence of C, O, Si, Cl and Fe (Figure 4b). The percentage of O in the GO and
IMD-Si/FeCl4ˉ@GNS was found to be 23.73% and 21.96 % respectively. The decrease in
percentage of O in the catalyst was due to removal of oxygen functionalities during reduction of
GO. The elemental mapping images of GNS and IMD-Si/FeCl4ˉ@GNS were depicted in Figure
5. The presence of C and O was observed in mapping image of GNS (Figure 5a-c) whereas
IMD-Si/FeCl4ˉ@GNS was constructed with C, O, Si, Cl and Fe elements as confirmed from
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Figure 5d-i. These results revealed that each element in synthesized catalyst (IMD-
Si/FeCl4ˉ@GNS ) was uniformly distributed.
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Figure 3: SEM images of (a) GO and (b) IMD-Si/FeCl4ˉ@GNS
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Figure 4: EDX analysis of (a) GO and (b) IMD-Si/FeCl4ˉ@GNS
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Figure 5: Mapping images of (a) mixed elements in graphene nannosheets (b) carbon and (c)
oxygen; (d) mixed elements in IMD-Si/FeCl4ˉ@GNS (e) carbon (f) oxygen (g) silicon (h)
chlorine and (i) iron
TEM analysis
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TEM images of GO, GNS and IMD-Si/FeCl4ˉ@GNS were shown in Figure 6. The morphology
of both GO and graphene showed folded sheets (Figure 6a, 6b). In the TEM image of IMD-
Si/FeCl4ˉ@GNS (Figure 6c), folded graphene sheets along with silica IL on its surface was
clearly discernible. The electron diffraction pattern of IMD-Si/FeCl4ˉ@GNS displayed well-
defined hexagonal array indicating that the synthesized IMD-Si/FeCl4ˉ@GNS had insignificant
number of superimposed sheets (Figure 6d).
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Figure 6. TEM images of (a) GO (b) GNS (c) IMD-Si/FeCl4ˉ@GNS (d) electron diffraction of
IMD-Si/FeCl4ˉ@GNS
XPS analysis
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The surface information of the GO and IMD-Si/FeCl4ˉ@GNS was further investigated by XPS
analysis (Figure 7). The XPS spectrum of GO showed two major peaks at 287.2 eV and 532.4
eV corresponding to binding energy of C 1s and O 1s respectively (Figure 7a). The IMD-
Si/FeCl4ˉ@GNS was composed of six peaks ascribing to C 1s, O 1s, Si 2s, N 1s, Fe 2p and Cl
2p (Figure 7b). The high resolution XPS spectrum for each elements of IMD-Si/FeCl4ˉ@GNS
were also observed to confirm the oxidation state and nature of bonding in the catalyst (Figure
8). The C 1s peak of IMD-Si/FeCl4ˉ@GNS was deconvoluted into three major peaks, at 284.6
eV, 286.5 eV and 288.7 eV, attributing to the binding energies of C=C, C–O and C=O
respectively (Figure 8a)34. The binding energies of O 1s, N 1s and Si 2p were found to be 532.4
eV, 401.7 eV and 102.6 eV respectively as shown in Figure 8b-d. Two peaks were obtained in
the XPS spectrum of Fe2p, at 712.1 eV and 725.2 eV, ascribing to the binding energies of Fe
2p3/2 and Fe 2p1/2 respectively (Figure 8e). The presence of Fe3+ was shown by the peak located
at 712.1 eV 38. The binding energies of Cl 2p3/2 and Cl 2p1/2 were appeared at 197.9 eV and 199.8
eV respectively (Figure 8f)34. These results revealed the successful incorporation of
imidazolium IL on the surface of GNS. The Fe content was found to be 5.76% (w/w) which
corresponded to loading amount of 1.04 mmol/g of catalyst.
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Figure 7. XPS analysis of (a) GO and (b) IMD-Si/FeCl4ˉ@GNS
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Figure 8: High resolution XPS spectra of IMD-Si/FeCl4ˉ@GNS showing the presence of
individual elements (a) C 1s (b) O 1s (c) N 1s (d) Si 2p (e) Fe 2p (f) Cl 2p
Raman analysis
The Raman spectra of GO and IMD-Si/FeCl4ˉ@GNS was obtained in the range of 500 cm-1 to
2500 cm-1. The presence of carbon skeleton in GO (Figure 9a) and IMD-Si/FeCl4ˉ@GNS
(Figure 9b) was confirmed by the appearance of two characteristic bands at 1364 cm-1 and 1596
cm-1 corresponding to D and G bands respectively. These two bands (D and G) confirmed the
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defects, edges and structural disorders of solid carbon (graphite) and in-plane vibration of carbon
atoms arising from the first-order scattering of E2g mode of sp2 hybridized carbon 39.
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Figure 9. Raman analyses of (a) GO and (b) IMD-Si/FeCl4ˉ@GNS
XRD analysis
XRD patterns of natural graphite, GO, IMD-Si/FeCl4ˉ@GNS and recycled catalyst were shown
in Figure 10. XRD pattern of graphite showed sharp peak at 2θ = 26.7o corresponding to (002)
reflection plane (Figure 10a). The peak for exfoliated GO sample appeared at 2θ = 13.1o (002)
(Figure 10b) suggesting the intercalation of water molecule in graphite galleries and generation
of other oxygen functional groups in between the graphite layers upon oxidation35. After the
reduction of GO with IL, the peak at 2θ = 13.1o disappeared due to removal of most of the
oxygen functionalities (C-OH, C-O and C=O)40. Further, appearance of a broad peak at 2θ =
23.4o (Figure 10c) corresponding to (222) reflection plane could be due to the presence of silica
in the synthesized catalyst.
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Figure 10. XRD pattern of (a) Graphite (b) GO (c) IMD-Si/FeCl4ˉ@GNS and (d) Recycled
catalyst
TG analysis
TGA curves of GO, IMD-Si/FeCl4ˉ@GNS and recycled catalyst were shown in Figure S2. All
samples showed weight loss (9.67-11.23%) below 100°C due to evaporation of absorbed water
molecules in between the galleries of carbon materials. However, IMD-Si/FeCl4ˉ@GNS showed
less weight loss as compared to GO showing that less moisture was trapped in IMD-
Si/FeCl4ˉ@GNS (Figure S2b). The total weight loss in GO was found to be 38.45% starting
from 190°C (12.11%) due to the decomposition of oxygen-containing functional groups (CO,
CO2 and steam etc.) introduced during the oxidation of graphite (Figure S2a) 41. The total
weight loss (32.76%) in IMD-Si/FeCl4ˉ@GNS at 600°C was due to decomposition of oxygen-
containing functional groups and surface attached IL confirming that IMD-Si/FeCl4ˉ@GNS was
thermally more stable than GO. This may be due to formation of large amount of Si-O-Si bond
introduced by imidazolium IL which prevented the thermal decomposition of catalyst even at
higher temperature.34
EPR analysis
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The X-band EPR spectrum of IMD-Si/FeCl4ˉ@GNS showed sharp singlet at g = 2.03264 which
was due to the presence of Fe3+ ion of IMD-Si/FeCl4ˉ incorporated in IMD-Si/FeCl4ˉ@GNS
(Figure S3).
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thiosemicarbazone derivatives
To identify the optimum reaction conditions, a representative model reaction of acetyl 1,3-
dimethylbarbituric acid 1c (2 mmol) and thiosemicarbazide 2 (2 mmol) affording (E)-2-(1-(1,3-
dimethyl-2,4,6-trioxohexahydropyrimidin-5-yl)ethylidene)hydrazinecarbothioamide 3c was
investigated (Scheme 1) with several parameters such as different catalysts and solvents, catalyst
loading on different supports, catalyst amount and temperature.
Scheme
1.
Synthesis
of
(E)-2-(1-(1,3-dimethyl-2,4,6-trioxohexahydropyrimidin-5-
yl)ethylidene)hydrazinecarbothioamide
In order to examine the effect of catalysts on model reaction, various catalysts had been used.
We started the reaction with different imidazolium ILs and observed that reaction went well with
these ILs and among them, IMD-FeCl4ˉ afforded moderate yield in relatively lesser reaction time
(Table 1, Entries 1-7). When silica modified imidazolium ILs were used, product yield was
improved in lesser time period (Table 1, Entries 8-11). After finding out these results and
considering the effect of surface area on reaction rate, we used IMD-Si/FeCl4ˉ with different
support materials (Table 1, Entries 12-15). Among the support materials used, GNS was found
to be the most appropriate support affording 96% product yield in 3 min. Thus, IMD-
Si/FeCl4ˉ@GNS was chosen as best catalyst for the desired reaction(Table 1, Entry-15). The
-
high catalytic activity of prepared sample; IMD-Si/FeCl4 @GNS, might be due to some factors
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such as presence of imidazolium IL which provide sufficient acidity to activate the carbonyl
carbon of aldehyde and ketones which in turn facilitate the nucleophilic attack of NH2 group of
thiosemicarbazide.42 Moreover, the presence of graphene as support provides enough surface
area to the catalyst to enhance the catalytic activity.43 These factors could be responsible for high
-
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catalytic activity of IMD-Si/GeCl4 @GNS.
After selecting the suitable catalyst, we, then inspected the model reaction with different solvents
such as methanol, ethanol, isopropanol, PEG (200,400,600), acetone, hexane, acetonitrile, THF
and in solvent-free condition. It was observed that with all solvents, the reaction afforded
moderate to good yields in practically same reaction time (Figure 11, Entries 1-10). The
solvent-free reaction condition managed to afford excellent yield in minimum reaction time
(Figure 11, Entry 11).
To obtain the suitable loading amount of IMD-Si/FeCl4ˉ on support, varying amount of IMD-
Si/FeCl4ˉ (5% w/w to 30 % w/w) was used and corresponding data collected (Figure 12, Entries
1-4). A linear relationship between IMD-Si/FeCl4ˉ loading and product yield was
Table 1: Effect of different catalysts on model reactiona
Entry
Catalyst
Timeb
Yieldc (%)
1
2
IMD-Clˉ
IMD-HSO4ˉ
18 h
12.5 h
14 h
30
33
39
42
57
45
48
54
59
64
72
80
85
90
3
IMD-CH3COOˉ
IMD-AlCl4ˉ
4
13 h
5
IMD-FeCl4ˉ
8 h
6
IMD-NiCl4ˉ
10 h
7
IMD-ZnCl4ˉ
10 h
8
IMD-Si/AlCl4ˉ
IMD-Si/ NiCl4ˉ
IMD-Si/ ZnCl4ˉ
IMD-Si/ FeCl4ˉ
IMD-Si/FeCl4ˉ@SiO2
IMD-Si/FeCl4ˉ@ZrO2
IMD-Si/FeCl4ˉ@Al2O3
9 h
9
8 h
10
11
12
13
14
6 h
4.5 h
54 min
48 min
39 min
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IMD-Si/FeCl4ˉ@GNS
3 min
96
aReaction conditions: acetyl 1,3-dimethylbarbituric acid 1c (2 mmol), thiosemicarbazide 2 (2
mmol), catalysts (80 mg), T = 30˚C. bReaction progress monitored by TLC. cIsolated yield.
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Figure 11: Effect of solvents on model reactiona
aReaction conditions: acetyl 1,3-dimethylbarbituric acid 1c (2 mmol), thiosemicarbazide 2 (2
b
mmol) IMD-Si/FeCl4ˉ@GNS (80 mg), solvents, T = 30˚C. Reaction progress monitored by
TLC. cIsolated yield.
observed i.e. on increasing the IMD-Si/FeCl4ˉ loading from 5% w/w to 25 % w/w, yield of the
product also increased. Further increase in loading amount did not affect the product yield. Thus,
the optimum loading amount of IMD-Si/FeCl4ˉ on support material that efficiently catalyzed the
model reaction was found to be 25% (w/w).
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Figure 12: Effect of IMD-Si/FeCl4ˉ loading on support for model reactiona (aReaction
conditions: acetyl 1,3-dimethylbarbituric acid 1c (2 mmol), thiosemicarbazide 2 (2 mmol), IMD-
Si/FeCl4ˉ@GNS (80 mg), T = 30˚C. bReaction progress monitored by TLC. cIsolated yield)
The effect of catalyst amount on model reaction was obtained by carrying out the experiment
with different amount of catalyst (20-100 mg) and corresponding results were shown in Figure
S4. It was observed that 80 mg of catalyst was sufficient to catalyze the model reaction affording
96% of product yield in just 3 mins.
Catalytic property
In order to check the catalytic property of IMD-Si/FeCl4ˉ@GNS, a series of biologically relevant
thiosemicarbazone derivatives were synthesized (Scheme 2). Thiosemicarbazones (3a-l) were
prepared by treating alicyclic/aromatic/heteroaromatic carbonyl compounds (1a-l),
thiosemicarbazide 2 and 80 mg of IMD-Si/FeCl4ˉ@GNS (25% w/w of IMD-Si/FeCl4ˉ and GNS)
at 30˚C under solvent-free condition for specified time period. In all cases, reaction proceeded
smoothly affording high yield in short reaction time. Besides aldehydes, some heteroaromatic
acetyls were also tested for the same reaction. It was found that yield was slightly decreased
which might be due to less reactivity of acetyls than aldehydes.
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Scheme 2: General procedure for the synthesis of substituted thiosemicarbazone derivatives
(aReaction conditions: Carbonyl compounds 1a-l (2 mmol), thiosemicarbazide 2 (2 mmol), IMD-
Si/FeCl4ˉ@GNS (80 mg), T = 30˚C. bReaction progress monitored by TLC. cIsolated yield), TOF
was calculated in h-1.
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The high turnover frequency (TOF) in the range of 114 h-1 to 230 h-1 of catalyst confirmed that
catalyst was very efficient for the conversion of thiosemicarbazide to corresponding E-selective
thiosemicarbazone derivatives.
2.4 Reaction mechanism
The plausible mechanism for the synthesis of substituted thiosemicarbazone derivatives using
IMD-Si/FeCl4ˉ@GNS as a catalyst was shown in Scheme 3. The carbonyl carbon 1a-l was
activated by the catalyst (IMD-Si/FeCl4ˉ@GNS) to generate A. The intermediate A is then
attacked by the nucleophilic reagent 2 (thiosemicarbazide) to form B. Dehydration of B yielded
final products 3a-l.42,52a The geometry of compounds was found to (E) as confirmed from the
DFT calculation. (ref) Matsa et. al. Also reported similar type of study for the synthesis of (E)-
thiosemicarbazone derivatives.52b
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Scheme 3. Plausible mechanism for synthesis of compound 3a-l
Density functional theory (DFT) of thiosemicarbazone (3e)
In order to prove the formation of preferred E-isomer over Z-isomer, DFT calculations were
53
performed on two isomers (Z and E) of the compound 3e using Gaussian 09 software
.
Optimized geometry of these two isomers were obtained at Becke's three parameter hybrid
functional for the exchange part and the Lee– Yang–Parr (LYP) correlation function along with
6-311G (d,p) basis set (B3LYP) /6-311G (d,p) level of theory54. The geometry of the compound
(E or Z) in Cartesian representation was completely optimized in ground electronic state with
tight convergence criterion. Optimized geometry of Z- and E-isomer was shown in Figure 13. In
that state, the harmonic vibrational frequencies calculations were carried out at same level of
theory without any imaginary frequency. Hence, on potential energy surface, a true and
significant minima was located. Vibrational assignments were made with great accuracy using
animated modes by visualization program GaussView 555. The minimum molecular energies of
Z- and E-isomer were found to be -582587.4390 and -582592.3436 kcal/mol respectively. It was
found that the (E)-isomer was stabilized by 4.9046 kcal/mol more than the (Z)-isomer. Therefore,
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some theoretical results were presented only for (E)-isomer. Dipole moment value for (E)-isomer
was estimated to be 6.76 D with dipole moment vector pointed along the geometry. Spatial plot
of HOMO and LUMO along with corresponding energy values and HOMO-LUMO gap was
illustrated in Figure 14. In this Figure, HOMO and LUMO were localized mainly over the chain
but also partially over the ring of the molecule. HOMO-LUMO transition showed the charge
delocalization within the molecule.
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Figure 13. Ground electronic state optimized structures of the (E)-isomer and (Z)-isomer
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Figure 14. Spatial plots of HOMO and LUMO with their energy values and HOMO-LUMO gap
for the (E)-isomer.
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The recycling experiment of catalyst was done using a representative reaction of acetyl 1,3-
dimethylbarbituric acid 1c, thiosemicarbazide 2 and 80 mg of catalyst at 30˚C. After completion
of the reaction (monitored by TLC) the reaction mixture was added to a separating funnel
containing ethyl acetate (10 mL) and water (10 mL). The organic product (3a-l) was extracted
with ethyl acetate, and the remaining catalyst in aqueous layer, was filtered and washed with
ethanol for further catalytic purpose. The activity of catalyst was examined by successive runs
and found that the catalyst retained good activity for minimum of six cycles (Figure 15). To
investigate the structural change and thermal stability of the catalytic system after six successive
runs, FTIR, SEM, TEM, XRD and TG analyses were performed. The FTIR spectrum of recycled
catalyst after six runs was found similar to that of fresh catalyst retaining all characteristic
vibrational frequencies in the same region as compared to fresh catalyst (Figure 2d). The SEM
and TEM images of recycled catalyst also showed no morphological change in the catalytic
system during reaction course after six runs (Figure S5). The XRD of reused catalyst was also
same showing peak at 2θ = 23.6o which was approximately same as in fresh catalyst (2θ = 23.4o)
(Figure 10d). The TG curve of recycled catalyst showed total weight loss of 34.12% at 593oC
confirming that the recycled catalyst was thermally stable after six runs (Figure S2c).
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Figure 15: Recycling data of catalyst
Conclusion
In summary, a facile route to reduce GO by incorporation of IL to GO has been developed. IL
modified GNS, IMD-Si/FeCl4ˉ@GNS, has been investigated for catalytic potential by
synthesizing a library of (E)-thiosemicarbazone derivatives. The synthesized catalyst showed
excellent catalytic activity affording 93-96% product yield in 3-6 min. Moreover, recyclability of
catalyst up to six cycles was one of the main attracting features of this study. The present energy
sustainable methodology can help green developments by avoiding hazardous chemicals. This
study revealed that IL modified catalyst could be most promising catalyst for various organic
transformations.
Conflict of Interest
There are no conflicts of interest to declare.
Acknowledgements
The authors gratefully acknowledge the financial assistance in the form of Major Research
Project (No. CST/SERPD/D-283) from Council of Science and Technology, Lucknow, U.P. The
authors would also like to thank DRS II, New Delhi (for providing necessary research facilities),
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USIF, A.M.U. (for SEM/EDX and TEM analyses), Department of Physics, A.M.U. Aligarh (for
XRD analysis), and IIT Roorkee (for XPS analysis).
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