Palladium oxide-decorated mesoporous silica on graphene oxide nanosheets as a heterogeneous catalyst for the synthesis of β-substituted indole derivatives

Article abstract of DOI:10.1039/d1dt00408e

In this work, an efficient and facile strategy has been adopted for the stepwise synthesis of the RGO-MSiO₂/PdO hybrid nanomaterial (HY-NM). Herein, a hybrid nanostructure of mesoporous silica over graphene oxide (GO) sheets has been developed

Full text of DOI:10.1039/d1dt00408e

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Palladium oxide-decorated mesoporous silica on
graphene oxide nanosheets as a heterogeneous
catalyst for the synthesis of β-substituted indole
derivatives†
Cite this: Dalton Trans., 2021, 50,
5644
Digvijay Sah, Javaid Shabir, Surabhi, Padmini Gupta and Subho Mozumdar
*
In this work, an efficient and facile strategy has been adopted for the stepwise synthesis of the RGO–MSiO2/
PdO hybrid nanomaterial (HY-NM). Herein, a hybrid nanostructure of mesoporous silica over graphene
oxide (GO) sheets has been developed followed by immobilizing palladium oxide nanoparticles (PdO NPs),
and then it has been utilized for catalyzing a multicomponent reaction (MCR). To authenticate the successful
synthesis of the HY-NM and successive immobilization of PdO NPs, various physicochemical characteri-
zation techniques were utilized such as SEM, EDAX, HR-TEM, HR-XRD, TGA, BET, FT-IR, and XPS analysis.
The activity of the HY-NM has been determined by performing the catalyst-mediated synthesis of
β-substituted indole derivatives (yield 90–98%). The excellent catalytic activity of the prepared HY-NM
could be observed due to its high surface area and large porosity, which facilitates the penetration and inter-
action of reactant molecules with the catalytic active species. This protocol eliminates the requirement of
further purification after the isolation of the product from the reaction mixture. The ease of handling, recycl-
ability of the catalyst, and simple work-up procedure are the main features of this protocol. The synthesized
HY-NM could be recycled for multiple catalytic cycles making it a very effective heterogeneous catalyst.
Received 5th February 2021,
Accepted 21st March 2021
DOI: 10.1039/d1dt00408e
rsc.li/dalton
and among them, the major drawback is its aggregation under
harsh reaction conditions. To overcome the exfoliation behav-
1. Introduction
In recent years, numerous reports have been published on ior of GO, the coating of GO sheets with silica could be a very
mesoporous silica1 due to its multidisciplinary functions in useful option to retain the morphology of the material for a
various fields such as adsorption,2,3 separation,4,5 catalysis,6–14 longer period of time. In spite of several tremendous pro-
gene therapy,15 drug delivery,16–18 sensing,19–21 etc., owing to its perties of graphene sheets, GO sheets aggregate very easily due
low density,22 high thermal stability,23 low toxicity,24 facile route to the π–π stacking interactions between the graphene sheets,
of synthesis,25,26 ease of surface modifications,27 and excellent and this limits their applications.41 The coating of GO sheets
possibility for tailoring its shape,28 size,29,30 and pore volume.31 with the silica would enhance the restraining of aggregation
Mesoporous silica with a tailorable pore size and volume is in property of the graphene oxide sheet by weakening the π–π
high demand owing to the greater possibility for variation of its stacking interactions.42,43 The utilization of surfactants during
surface area and pore diameter i.e. specific amount of organic the surface functionalization of GO could create a porous
or inorganic moieties could be immobilized over it. structure over its surface and hence increase the overall
Graphene oxide (GO) gets special attention from all surface area of the material.
material-oriented research fields owing to its special and
Moreover, transition metal-based nanoparticles have also
unique properties including easy functionalization as well as attracted great interest for catalyzing numerous reactions such
the modification of its carbon skeleton,32,33 large surface as epoxidation, reduction–oxidation, C–C coupling reactions,
area,34–36 higher mechanical strength,37,38 and good chemical hydrogenation, hydro-chlorination, etc. owing to their high
stability.39,40 There are certain limitations associated with GO specific surface area and increased density of edge/corner
atoms of metal-based nanoparticles.44–51 The major drawback
of metal-based nanoparticles is their rapid agglomeration. The
agglomeration of metal nanoparticles could decrease their
catalytic activity. The agglomeration of metal nanoparticles
could be inhibited by uniformly immobilizing them over an in-
organic mesoporous support. The mesoporous surface would
Department of Chemistry, University of Delhi, Delhi-110007, India.
E-mail: subhoscom@yahoo.co.in
†Electronic supplementary information (ESI) available: HR-MS, 1H-NMR and
13C-NMR spectra of the synthesized β-substituted indole derivatives.
Calculations for green chemistry matrices. See DOI: 10.1039/d1dt00408e
5644 | Dalton Trans., 2021, 50, 5644–5658
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facilitate the uniform confinement of metal nanoparticles all alytically active palladium oxide nanoparticles. The incorpor-
over the surface based upon their sizes and hence lower the ated palladium oxide nanoparticles would play a vital role in
chance of aggregation of the metal nanoparticles in the form the C–H activation of olefin/indole to enhance the electrophili-
of clusters. The uniformly distributed metal nanoparticles over city of an electrophilic centre onto which an appropriate
the mesoporous solid support would have great ability to nucleophile could attack easily. The synthesized hybrid nano-
enhance its catalytic activity. The metal nanoparticles material could be easily recycled from the reaction mixture
anchored over the surface of the support material could be simply through centrifugation and reused multiple times for
beneficial for both the metal nanoparticles and the support. further reactions.
Due to the fixed confinement of metal nanoparticles over the
solid support, the restacking of the support could be restricted
and the aggregation of metal nanoparticles could also be
reduced.
2. Experimental section
2.1. Chemicals and reagents
Nowadays, indoles and many of their derivatives are widely
found in various natural products,52 bio-active alkaloids,53
Natural graphite flakes, hexadecyltrimethylammonium chlor-
agrochemicals,54 and pharmaceuticals.55 The synthetic route
ide (CTAC, 25 wt% in water), triethanolamine (TEA), and tetra-
ethylorthosilicate (TEOS, reagent grade, 98%) were all pur-
chased from Sigma-Aldrich. Sulfuric acid (H2SO4) and potass-
ium permanganate (KMnO4) were purchased from Fischer-
to β-substituted indole derivatives involves the multi-com-
ponent (i.e. aldehydes, active methylene compounds, and
indoles/indole derivatives) reaction in one-pot. Generally, the
Scientific. Ortho-phosphoric acid (H3PO4, Puriss AR) and
reaction involves two successive reactions namely,
Knoevenagel and Michael addition reactions. The major
Emparta™, respectively. All the above chemicals are used
difficulty of a one-pot multi-component reaction is the selecti-
vity of the desired product. In the past, Yonemitsu et al.
adopted the one-pot approach with three-components, namely
hydrogen peroxide (30%) were bought from Spectrochem and
directly, as received without any further purifications. Double
distilled water was used for all the experiments.
Meldrum’s acid, aldehydes, and indoles, to synthesize
2.2. Synthesis of graphene oxide (GO) sheet
β-substituted indole derivatives.56 The main difficulty with this
GO was synthesized from graphite flakes by the improved
Hummer’s method.62,63 Briefly, in a 1000 mL beaker, H2SO4
(180 mL) and H3PO4 (20 mL) were taken in a 9 : 1 ratio.
Graphite flakes (1.5 g) were added to the beaker under constant
stirring and then KMnO4 (9 g) was added slowly. The reaction
mixture was stirred at 45–50 °C for 12 hours. Afterward, the
reaction mixture was cooled to ambient temperature and
crushed ice (150 mL) was added in the next step. Shortly there-
after, H2O2 (∼3–4 mL) was added dropwise to obtain a colour
change from greenish-brown to light yellow. The reaction
mixture was allowed to stir at room temperature for an hour.
Then, the final reaction mixture was centrifuged and washed
several times with double distilled water, hydrochloric acid,
ethanol, and diethyl ether; respectively. The obtained precipi-
tates were dried overnight at room temperature.
reaction was the substitution of Meldrum’s acid with other
active methylene compounds resulting in the trace amount of
desired products.57 In 2014, Gu et al. used a homogeneous
catalyst, L-proline, to synthesize β-substituted derivatives by
choosing different derivatives of indole, dimedone, and alde-
hydes as the reactants.58 The desired product yield was found
to be excellent but the major concern of this reaction was the
recyclability of the catalyst. In consecutive years 2014–15,
Rawat and co-workers developed two different heterogeneous
catalysts for the synthesis of β-substituted indole derivatives by
using different derivatives of indole, active methylene com-
pounds, and aldehydes as the reactants.59,60 These reports
include heterogeneous catalysts which resolved the recyclabil-
ity problem to some extent, but the requirement of high temp-
eratures and longer reaction times are a major concern. In
another report published in 2017 by Mishra and co-workers,
the successful synthesis of the morphologically controlled
phosphate grafted SnO2–ZnO2 oxide nanocomposite via urea
hydrolysis, which was further utilized as the heterogeneous
catalyst to synthesize β-substituted indole derivatives, has been
reported.61 The major drawback associated with this hetero-
geneous catalyst was the longer reaction time and the require-
ment of the larger amount of the catalyst.
2.3. Synthesis of RGO–MSiO2
In a 250 mL round bottom flask, GO (100 mg) was dispersed
in distilled water (36 mL) by ultra-sonicating the mixture for
30 minutes. CTAC (24 mL) was added and sonicated again for
15 minutes followed by magnetic stirring. After this, TEA
(180 μL) was added to the solution and the temperature of the
round bottom flask was maintained at 60 °C for an hour.
Then, the mixture of TEOS (4 mL) and cyclohexane (20 mL)
was added gently to the reaction mixture. The precipitates
were washed several times with water followed by ethanol and
then dried overnight in a vacuum.
Herein, we developed a highly efficient and stable hybrid
nanomaterial having a high surface area with large porosity
onto which palladium oxide nanoparticles could be effectively
anchored and utilized for the synthesis of β-substituted indole
derivatives. It would be presumed that the palladium oxide
incorporated into mesoporous silica developed over the gra-
2.4. Synthesis of RGO–MSiO2/PdO
phene sheet enhances the rate of reaction by facilitating the The PdO decorated RGO–MSiO2 was synthesized via an in situ
interaction and penetration of reactant molecules into the cat- approach.64 The solid RGO–MSiO2 (200 mg) was dispersed in
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distilled water (40 mL) through the process of ultra-sonication volume analyses were performed using a Quantachrome
for 30 minutes. Afterward, PdCl2 (20 mg) and KCl (100 mg, to instrument of model ASI-CI-11 equipped with an automatic
enhance the solubility of palladium)64 were added to it fol- error checking system.
lowed by stirring for about an hour at room temperature.
Meanwhile, 1 M NaOH solution was added to the reaction
mixture in a dropwise manner. The pH of the solution was
maintained at ∼13. The resultant reaction mixture was stirred
3. Results and discussion
for 24 hours at room temperature. The synthesized hybrid The complete schematic synthetic procedure of PdO NPs deco-
nanomaterial was washed with distilled water and collected by rated HY-NM of silica over GO has been shown in Scheme 1. A
simple centrifugation at 4000 rpm. The solid product was sol-gel oriented bi-phasic approach65 has been used for the syn-
washed repeatedly with water and ethanol and dried overnight thesis of RGO–MSiO2 and this inorganic support material has
in a vacuum at 50 °C.
been further decorated with PdO NPs (RGO–MSiO2/PdO). The
HY-NM thus synthesized has been characterized by various
physicochemical characterization techniques such as FT-IR,
2.5. Synthesis of β-substituted indole derivatives via MCR
To catalyze an MCR reaction, 10 mg of the as-synthesized cata- SEM, TGA, P-XRD, BET, HR-TEM, EDAX, and XPS analysis.
lyst was dropped into the round bottom flask containing a 1 : 1 The FT-IR spectra of the synthesized materials GO, RGO–
ratio (v/v) of water and ethanol and ultra-sonicated for 10 min MSiO2, and RGO–MSiO2/PdO were recorded to confirm the
to completely disperse the catalyst. Furthermore, to this solu- changes in the functionalities and are shown in Fig. 1(a). Since
tion active methylene compound (1 mmol), aldehyde (1 mmol), the GO sheets contain –OH, –COOH, and epoxide functional-
and indole (1 mmol) were added simultaneously followed by ities, it shows characteristic bands at 3405 cm−1, 1737 cm−1
,
continuous magnetic stirring of the reaction mixture at 35 °C. 1624 cm−1, and 1224 cm−1 for –OH str., –CvO str. of car-
The progress of the reaction was monitored through TLC. After boxylic acid, un-oxidized sp2 CvC bond str. and –C–O str. of
the completion of the reaction, the heterogeneous catalyst was epoxy, respectively. After the successful coating of silica over
separated by a simple centrifugation method. To perform the GO sheets, the above peaks almost disappeared and the
effective extraction of the product from the reaction mixture, appearance of the asymmetric and symmetric stretching of Si–
ethanol was evaporated using a rotatory evaporator. Then, the O–Si at 1061 cm−1 and 808 cm−1; respectively could be
product was extracted in dichloromethane (DCM) and the observed. The PdO NPs decorated HY-NM (RGO–MSiO2/PdO)
organic layer was dried over Na2SO4. Under reduced pressure, show three bands at 1052 cm−1, 965 cm−1, and 799 cm−1 for
DCM was evaporated to obtain the precipitate followed by asymmetric Si–O–Si str., symmetric Si–O str. and symmetric
recrystallization with diethyl ether and hexane. The 1H NMR, Si–O–Si str., respectively.
proton decoupled 13C NMR and mass spectra of the synthesized
products were recorded for characterization purposes.
In order to analyze the thermal stability of the as-syn-
thesized nanomaterials RGO–MSiO2/PdO, RGO–MSiO2, and
GO, thermo-gravimetric analysis (TGA) has been performed in
the heating range of 20 °C–800 °C, as shown in Fig. 1(b). An
2.6. Structural and morphological characterization
Fourier Transform Infrared (FT-IR) spectra were recorded in a initial weight loss of 8%, 8%, and 24% up to 150 °C could be
range of 400–4000 cm−1 using a PerkinElmer FTIR spectro- observed in the case of GO, RGO–MSiO2, and RGO–MSiO2/
meter through the KBr pellet method. This spectrometer con- PdO; respectively, attributed to the weight loss of physisorbed
sists of Nicolet iS50 FTIR Tri-detector Gold flex spectrometer and chemisorbed water molecules presents on the surface and
(DLATGS detector with a KBr window) having a resolution of in the channels of the as-prepared nanomaterials. Then, a
0.09 cm−1. SEM analysis was performed on a JEOL JSM 6610LV 36% weight loss (Fig. 1(b)(i)) in the range of 150 °C–250 °C
model SEM instrument consisting of a pre-centered tungsten- could be due to the decomposition of oxygen functionalities.
based filament. The morphological, topographical, compo- An approximately 20% weight loss (Fig. 1(b)(i)) observed up to
sitional, and crystallinity information were more accurately 630 °C could be due to the combustion of the carbon skeleton
recorded through HR-TEM. The HR-TEM images of GO, RGO– of GO sheets. A gradual 8% weight loss (Fig. 1(b) (ii)) above
MSiO2, and RGO–MSiO2/PdO nanomaterials were taken with a 150 °C could be attributed to the decomposition of oxygen
TALOS model CRYO-TEM. The EDAX of RGO–MSiO2 and RGO– functionalities. The TGA curve of RGO–MSiO2/PdO HY-NM
MSiO2/PdO nanomaterials were performed by choosing an also shows an initial 8% weight loss of the adsorbed water
area of a sample. After the successful run of a sample, the molecules up to 150 °C and above this, there was minute
weight and atomic percentage of each element in the selected weight loss up to 800 °C. Based on TGA, it could be concluded
area were obtained. The P-XRD of RGO–MSiO2 and RGO– that GO sheets possessed a large number of oxygen functional-
MSiO2/PdO nanomaterials were carried out using a Bruker (D8 ities which further reacts with the –OCH2CH3 groups of TEOS
Discover) high-resolution X-Ray diffractometer at a scan rate of in order to generate a mesoporous structure on the surface of
2° per min in the 2θ range of 5°–80°. The thermal analysis of the GO sheet. On comparing the TGA curves, it could be
the materials was performed in the temperature range from observed that the finally prepared HY-NM i.e. RGO–MSiO2/PdO
20 °C to 800 °C by using a PerkinElmer Pyris Diamond TGA/ was thermally most stable as compared to RGO–MSiO2 and GO
DTA instrument. BET surface area, pore diameter, and pore nanomaterials.
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Scheme 1 Schematic representation of the synthesis of the RGO–MSiO2/PdO HY-NM.
Fig. 1 (a) FT-IR spectra and (b) TGA curve of (i) graphene oxide, (ii) RGO–MSiO2 and (iii) RGO–MSiO2/PdO and (c) N2 adsorption–desorption iso-
therm and pore size distribution (inset) of (i) RGO–MSiO2 and (ii) RGO–MSiO2/PdO.
BET analysis has been performed to determine the total total pore volume, and total surface area of the HY-NM.43 After
surface area, pore size, and pore volume of the synthesized the successful immobilization of PdO NPs within the meso-
HY-NM. Before performing N2 adsorption–desorption analysis, channels of HY-NM, the total surface area of RGO–MSiO2/PdO
the synthesized HY-NM was degassed to remove all the has also been observed to decrease from 583.3 to 574.2 m2 g−1
.
adsorbed gases under vacuum for 4 hours at 160 °C. The BET The total pore volume and pore diameter are also found to
isotherms are type-IV isotherms, indicating the monolayer or decrease after immobilization of PdO NPs. The pore size
layer by layer adsorption isotherms. The obtained isotherms decreased from 17.26 nm to 12.49 nm and the total pore
have a broad hysteresis loop which were typically a character- volume decreases from 1.978 cm3 g−1 to 1.685 cm3 g−1
.
istic of mesoporous materials.66 This could authenticate that
Initially, the shape, size, and surface morphology of the pre-
the nanomaterials RGO–MSiO2 and RGO–MSiO2/PdO were syn- pared material were analyzed using the SEM instrument. The
thesized with a uniform pore texture and large mesopores. The scanning images are depicted in Fig. 2(d and e), which shows
N2 adsorption–desorption isotherms not only be used for con- that silica was uniformly coated over the surface of GO, result-
firming the mesoporosity of the nanomaterials but could also ing in the formation of mesoporous sheets. The pores were
be utilized to authenticate the successful immobilization of generated vertically to the surface creating meso-channels over
PdO NPs within the mesopores of the HY-NM. The immobi- the surface, which were further utilized for the immobilization
lized PdO NPs within the mesopores would significantly of PdO NPs. To obtain very precise information about the
change the textural parameters like the radius of pores, the shape, texture, and distribution of various elements present,
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Fig. 2 (a–c) TEM micrographs of RGO–MSiO2/PdO, (d and e) SEM images of RGO–MSiO2/PdO, and (f) the particle size distribution of PdO NPs.
TEM analysis has been performed. The recorded TEM micro-
Furthermore, to confirm the phase (i.e. crystalline or amor-
graphs of the synthesized RGO-MSiO2/PdO HY-NM are shown phous) and unit cell dimension of the HY-NM, powder XRD
in Fig. 2(a–c). The figure shows the successful growth of meso- (P-XRD) of GO, RGO–MSiO2, and RGO–MSiO2/PdO has been
porous silica over the GO sheets. A dark spherical spot appear- performed, as shown in Fig. 3(a). The characteristic peaks of
ing on the HY-NM clearly authenticates the immobilization of GO have been observed at 8.9° and 42.5°, which could be
PdO NPs without any aggregation. The average size of PdO NPs attributed to the (001) and (100) planes. The corresponding
has been found to be approximately 13 nm, as depicted in d-spacing values are found to be 0.99 nm and 0.21 nm; respect-
Fig. 2(f).
ively. These peaks disappeared after the coating of silica. The
Fig. 3 (a) XRD spectra of (i) GO, (ii) RGO–MSiO2 and (iii) RGO–MSiO2/PdO, and (b) small angle XRD patterns of RGO–MSiO2 and RGO–MSiO2/PdO.
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XRD pattern of RGO–MSiO2 showed a broad peak in the range immobilization of palladium nanoparticles in the as-synthesized
of 15–33°, which could be attributed to the (002) plane having HY-NM.
a d-spacing value of 0.36 nm. This could represent the com-
Now, to identify the chemical composition and oxidation
bined peak of both the amorphous silica and reduced gra- state of the synthesized HY-NMs, X-ray photoelectron spec-
phene oxide. The XRD pattern of HY-NM (RGO–MSiO2/PdO) troscopy (XPS) analysis has been performed.71 This technique
shows five additional peaks at 33.8°, 42.0°, 54.8°, 60.5° and is routinely used to analyze metal alloys, polymers, inorganic
71.4° which corresponds to the (101), (110), (112), (200) and compounds, and inorganic support materials. The XPS wide-
(220) crystal planes of palladium oxide; respectively. The scan spectrum of the surface of the RGO–MSiO2/PdO HY-NM
corresponding d-spacing values are found to be 0.26 nm, has shown peaks for carbon, silica, oxygen, and palladium
0.21 nm, 0.17 nm, 0.15 nm, 0.13 nm; respectively (JCPDS: 00- oxide and the detection of these elements are very consistent
43-1024).67 In addition, the average crystallite size of palla- with the EDAX results. Fig. 5(a) illustrates the C 1s XPS spec-
dium oxide nanoparticles was found to be about 4.56 nm and trum of the as-prepared HY-NM. The peaks at 284.8281 eV,
this value was calculated by using the Scherer equation.68,69 286.5746 eV, 288.9822 eV, and 290.1860 eV could be attribu-
No additional peaks for any kind of impurities were detected ted to binding energies of C–C, C–O, CvO, and C–(O)OH
in the powder XRD pattern. The small-angle powder XRD pat- bonds; respectively. As sown in Fig. 5(b), the spectrum exhi-
terns of RGO–MSiO2 and RGO–MSiO2/PdO were also analyzed bits a peak at 530.96401 eV which could be attributed to O
to determine the mesoporosity of the synthesized HY-NM, as 1s. Fig. 5(c) shows the peak with a binding energy of
illustrated in Fig. 3(b). The reflection plane 100 at 2θ = 2.5°, as 102.0231 eV corresponding to Si 2p. Two peaks appearing at
shown in the small-angle powder XRD patterns of both RGO– 337.3093 eV and 343.0420 eV could be assigned to Pd2+ 3d5/2
MSiO2 and RGO–MSiO2/PdO HY-NM, clearly indicating the and Pd2+ 3d3/2, respectively, as shown in Fig. 5(d). These
mesoporous structure of the synthesized HY-NM.70 On suc- results could reveal that the oxidation state of palladium was
cessful immobilization of PdO nanoparticles inside the meso- (+II) in the as-synthesized HY-NM. The accurate palladium
pores of RGO–MSiO2, the peak intensity of the reflection plane content was analyzed through ICP-AES was found to be
100 at 2θ = 2.5° for RGO–MSiO2/PdO was found to be 1.67 mmol g−1
decreased. This proved the successful immobilization of PdO In order to determine the activity and efficiency of the as-
.
nanoparticles inside the mesopores of RGO–MSiO2. The synthesized HY-NM, it has been employed in the synthesis of
obtained result was in good agreement with the TEM and BET biologically active β-substituted indole derivatives. To achieve
results.
the best results from the as-synthesized HY-NM, it is necessary
Elemental mapping is a colour representation of spatially to optimize the reaction conditions, such as temperature,
distributed elements of a sample. A detailed elemental ana- time, solvent, and catalyst amount. The benchmark reaction
lysis of the HY-NM (RGO–MSiO2/PdO) has shown that PdO with 4-nitrobenzaldehyde (1 mmol), malononitrile (1 mmol),
NPs were uniformly distributed all over the surface of RGO– and indole (1 mmol) were chosen to optimize the reaction con-
MSiO2, which was also confirmed by TEM micrographs. ditions. We proceeded with our optimization and examination
Further, the weight and atomic percentage of the various process by testing the scope of solvents, utilizing a series of
elements present in the prepared nanomaterials, RGO–MSiO2 polar and non-polar solvents, such as water, ethanol, CH2Cl2,
and RGO–MSiO2/PdO, were analyzed by EDAX. From Fig. 4, DMSO, toluene, THF, CHCl3, and the mixture of water and
the well-defined peaks of C, Si, O, and Pd could be observed. ethanol (1 : 1 v/v ratio). In conclusion, it was observed that the
Finally, it could be concluded that there has been successful RGO–MSiO2/PdO HY-NM gives a good yield of the desired
Fig. 4 (a–e) The color mapping display represents the distribution of various elements and (f and g) EDAX of RGO–MSiO2 and RGO–MSiO2/PdO,
respectively.
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Fig. 5 The XPS of RGO–MSiO2/PdO HY-NM for (a) carbon (C), (b) oxygen (O), (c) silicon (Si), and (d) palladium oxide (PdO), and (e) full survey spec-
trum of all elements.
product in a mixture of water and ethanol (1 : 1 v/v ratio) in the interaction between the active sites of RGO–MSiO2/PdO
15 minutes at 35 °C. When various pure solvents were utilized (i.e., Lewis acidic sites (Pd2+), Brønsted basic sites (O2− and
for the reactions, the yields were not satisfactory at 35 °C or OH−)) and reactants is restricted to some extent. Additionally,
there was a requirement of a higher temperature. Without the comparison of catalytic activities of RGO–MSiO2/PdO in
solvent, it was found that the yield of the reaction was in trace different solvents at the same reaction temperature has also
amounts. The yield of the reactions has been found to be 15% been performed (Table 1). The maximum yield of the reaction
and 23% even after a very prolonged reaction time when GO could be obtained with 10 mg of RGO–MSiO2/PdO HY-NM in
and RGO–MSiO2 were chosen as the catalyst. However, the the mixture of water and ethanol (1 : 1 v/v ratio) at room temp-
yield of the reaction was found to be good to excellent when erature. The reaction without the catalyst did not proceed. The
different amounts of RGO–MSiO2/PdO HY-NM were utilized as obtained results clearly indicate that the palladium oxide
the catalyst in the reaction at different temperatures (ESI: present inside the mesopores of the as-synthesized RGO–
Table D1†). It has also been observed that the yield of the MSiO2/PdO HY-NM plays an important role in catalyzing the
product was found to be lower (76%, 60 min) at a high temp- reactions. Moreover, a comparative study of RGO–MSiO2/PdO
erature in comparison with the yield (92%, 30 min) obtained with previously published diverse reports based on GO/SiO2
at a lower temperature as mentioned in ESI: Table D1.† This catalysts (ESI: Table D2†) as well as based on other GO nano-
could be due to the increased kinetic energy of the reactant composites for the synthesis of β-substituted indole derivatives
molecules at a higher temperature which ultimately leads to (ESI: Table D3†) has also been conducted.
the crowding of reactant molecules. Due to steric hindrance,
Based on the above optimizations, the as-prepared HY-NM
the approach of the reactant molecules towards the catalytic has been employed for the synthesis of β-substituted indole
active moieties is reduced comparatively. As a result of this, derivatives (Scheme 2). It is noteworthy that the catalyst gives
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Table 1 Comparison of catalytic activities of RGO–MSiO2@PdO in different solvents at the same temperature
S. N.
Solvent
Catalyst (mg, mol%)
Temp. (°C)
Time (min)
Yielda (%)
1
2
3
4
5
6
7
8
9
C2H5OH
CH2Cl2
DMSO
H2O
CHCl3
Toluene
H2O + C2H5OH (1 : 1)
CH2Cl2 + C2H5OH (1 : 1)
CHCl3 + CH2Cl2 (1 : 1)
RGO–MSiO2/PdO (10, 0.17)
RGO–MSiO2/PdO (10, 0.17)
RGO–MSiO2/PdO (10, 0.17)
RGO–MSiO2/PdO (10, 0.17)
RGO–MSiO2/PdO (10, 0.17)
RGO–MSiO2/PdO (10, 0.17)
RGO–MSiO2/PdO (10, 0.17)
RGO–MSiO2/PdO (10, 0.17)
RGO–MSiO2/PdO (10, 0.17)
35
35
35
35
35
35
35
35
35
30
80
90
50
90
90
15
50
90
90
58
47
65
65
42
98
73
64
Reaction conditions: active methylene compound (1 mmol), 4-nitrobenzaldehyde (1 mmol), indole (1 mmol). a Isolated yield.
Scheme 2 Synthesis of β-substituted indole derivatives catalyzed by RGO–MSiO2/PdO HY-NM.
good to excellent yields of the desired products, as illustrated from the reaction mixture. Then the reaction was allowed to
in Table 2. Under optimal reaction conditions, it could be proceed for an appropriate time without the catalyst. It could
observed that (i) the aldehydes containing electron-withdraw- be observed that there was no further progress of the reaction.
ing functionalities proceeded smoothly to give a high yield of Additionally, the average pore size of RGO–MSiO2 (∼17 nm)
the products in less time as compared to either an aldehyde and the average size of PdO nanoparticles (∼13 nm) are com-
with electron-donating functionalities or without any function- parable. The PdO nanoparticles fit into the mesopores of silica
alities, (ii) the reactivity of the reaction was faster when based upon their sizes and available pore sizes of mesoporous
malononitrile was utilized as the active methylene compound silica. Therefore, it could be suggested that there was no leach-
as compared to dimedone and (iii) the reactivity of salicylalde- ing of catalytically active PdO NPs into the reaction mixture.
hyde derivatives was also found to be comparable to those of This could provide evidence for the heterogeneity of the
aldehydes with different functionalities. HY-NM. Powder XRD and EDAX data were provided in order to
The potential of the HY-NM mainly depends on the stabi- support the heterogeneity.
lity, and recyclability which could be assessed by reusing the
The green chemistry matrix calculations were performed for
synthesized HY-NM multiple times and investigating the entry 2 (Table 2) under optimal reaction conditions. The green
changes in the yield. The prepared heterogeneous HY-NM chemistry matrix calculations have shown values close to the
shows the feature of recyclability and reusability, as illustrated ideal values as mentioned in Table 3.72,73
in Fig. 6. On completion of the reaction, the HY-NM was separ-
3.1. Plausible reaction mechanism
ated simply through centrifugation. The separated HY-NM was
washed several times with ethanol, dried in a vacuum at 60 °C,
and reused for further reactions. The HY-NM shows good cata-
lytic activity without any noticeable loss in activity up to five
catalytic cycles. Also, the leaching tendency of the prepared
catalyst was investigated through a hot water filtration test
under optimal reaction conditions. On completion of almost
50% of the reaction, the HY-NM was filtered hot and separated
A plausible mechanism for the catalyzed reactions has been-
diagrammatically represented in Fig. 7. The synthesized hybrid
nanomaterial (RGO–MSiO2) possesses mesopores with excel-
lent surface area (i.e., 583.3 m2 g−1). These mesopores were uti-
lized for the successful incorporation of palladium oxide nano-
particles. Furthermore, the fact can be explained that the high
surface area will lead to the immobilization of more catalyti-
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Table 2 RGO–MSiO2/PdO catalyzed synthesis of β-substituted indole derivatives
Entry Aldehyde
1
Active methylene group Indole
Product
Time (min) Yielda (%) TON (TOF (h−1))
30
93
547.1 (1094.2)
2
15
98
576.5 (2306)
3
4
5
15
15
25
96
96
93
564.7 (2258.8)
564.7 (2258.8)
547.1 (1302.6)
6
7
8
9
30
35
20
25
90
90
94
92
529.4 (1058.8)
529.4 (912.8)
552.9 (1675.5)
541.2 (1288.6)
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Table 2 (Contd.)
Entry Aldehyde
10
Active methylene group Indole
Product
Time (min) Yielda (%) TON (TOF (h−1))
30
94
552.9 (1105.8)
11
12
20
98
576.5 (1747)
25
97
570.6 (1358.6)
13
14
35
35
91
93
535.3 (973.3)
547.1 (943.3)
15
16
30
35
96
90
564.7 (1129.4)
529.4 (912.8)
Reaction conditions: active methylene compound (1 mmol), aldehyde (1 mmol), indole (1 mmol), catalyst (10 mg, 0.17 mol%), turnover number
(TON) = yield (%)/catalyst (mol%), turnover frequency (TOF) = TON per hour, water + ethanol (1 : 1), 35 °C. a Isolated yields.
cally active moieties (i.e., PdO NPs) which are ultimately gistically facilitating the synthesis of substituted indole deriva-
responsible for the high catalytic activity of the catalyst. tives. Initially, the Lewis acidic sites (Pd2+) and Brønsted basic
Moreover, the mesoporous silica over graphene oxide sheets sites (O2− and HO−) coordinated with aldehydes and active
contains a large number of hydroxide moieties (i.e., Lewis methylene compounds to increase the electron deficiency of
basic site), and also the incorporated palladium oxide nano- the carbonyl carbon of aldehydes and to abstract the acidic
particles contain both Lewis acidic (i.e., Pd2+) and Brønsted proton from active methylene compounds; respectively.56 The
basic sites (i.e., O2−). Therefore, it could be said that the syn- hydrogen abstraction from active methylene compounds
thesized RGO–MSiO2/PdO HY-NM is a combination of Lewis results in the carbanion formation which could further attack
acidic sites and Brønsted basic sites. These sites help in syner- the carbonyl carbon of aldehydes. This could lead to the for-
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Fig. 6 Catalyst life test for RGO–MSiO2/PdO HY-NM. (a) EDAX of the recycled catalyst (after 5 catalytic cycles), (b) XRD spectra of the (i) fresh cata-
lyst and (ii) recycled catalyst (after 5 catalytic cycles) and (c) recyclability of the catalyst.
Table 3 Calculations of green chemistry matrices
mation of Knoevenagel products (i.e. alkene product). The elec-
trophilic palladium viz. Pd(II) could play the main role in C–H
S. N. GC matrices
Ideal value Calculated for entry 2
activation for further alkenylation reactions of indoles.79 The
Pd(II) coordinated with an olefin to activate it, which was
attacked by the C3-nucleophilic position of indoles to syn-
thesize β-substituted indole derivatives. On the other hand,
the alkenylation reaction of indoles also proceeds through C–
1
2
3
4
5
E-factor
0
100
100
1
0.077
94.61%
98%
1.077
92.81%
Atom economy
Carbon efficiency
Process mass intensity
Reaction mass efficiency 100
Fig. 7 Plausible reaction mechanism for the synthesis of β-substituted indole derivatives by employing RGO–MSiO2/PdO in the reaction.
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Table 4 Comparative study of catalytic activity of RGO–MSiO2/PdO HY-NM with some of the previously reported catalysts for the synthesis of
β-substituted indole derivatives
S. N. Reactants
1
Catalyst (amount)
Reaction Conditions
80 °C, 15 min
Yield (%) Recyclability Ref.
L-Proline, (20 mol%)
99
No
58
2
3
4
TiCl4 (1 equiv.)
0 °C – RT, 30 min
49
No
No
No
74
57
75
Yb(OTf)3, (10 mol%)
Glycine (10 mol%)
Ultra-sonication, 12 h 67
70 °C, 3 h
83
92
97
85
90
5
6
7
8
KH2PO4, (100 mol%)
RT, 3 h
No
No
No
Yes
76
77
78
60
Cu–salen ligand (1 mol%), KH2PO4(1 equiv.) 60 °C, 6 h
TBAF(10 mol%)
60 °C, 2 h
80 °C, 1 h
RS-HM, (10 mol%)
9
RGO/ZnO, (5 wt%)
PSn20Zr, (50 mg)
50 °C, 30 min
95
92
Yes
Yes
59
61
10
60 °C, 3 h
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Table 4 (Contd.)
S. N. Reactants
11
Catalyst (amount)
Reaction Conditions
35 °C, 15 min
Yield (%) Recyclability Ref.
RGO–MSiO2/PdO(10 mg)
98
Yes
This work
H activation of indoles by electrophilic palladium, which was very thankful to SAIF of AIIMS, New Delhi for providing the
further liable to attack by olefin to generate β-substituted TEM facility and are also thankful to USIC, University of Delhi,
indole derivatives. Finally, it could be concluded that RGO– Delhi, for providing instrumentation facilities.
MSiO2/PdO HY-NM works in a synergistic manner: (i) the
proton abstraction from the active methylene compound
which further attacks the carbonyl carbon of aldehydes to
generate an olefin and (ii) the C–H activation of olefin which
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