Heterogeneous O -arylation of nitroarenes with substituted phenols over a copper immobilized mesoporous silica catalyst

Article abstract of DOI:10.1039/c6ra04409c

Highly porous and robust mesoporous silica, SBA-15 has been subjected to post-synthesis modification for the anchoring of copper through Schiff base moiety formation using the silicon alkoxide route. The hybrid porous material has been fully characterized by powder-XRD, electronic spectra, EPR, thermogravimetric analysis, N₂ sorption measurements, and TEM and SEM/EDS studies. The efficiency of the catalyst has been assessed in the O-arylation reaction using various substituted phenols and nitroarenes in heterogeneous conditions. The catalytic coupling reaction efficiently produces unsymmetrical diaryl ethers. The impressive capability to activate substrates having electron-donating or electron-withdrawing substituents and to have a high turnover frequency in the catalytic reactions made the catalyst highly desirable.

Full text of DOI:10.1039/c6ra04409c

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Heterogeneous O‒arylation of nitroarenes with substituted phenols
over copper immobilized mesoporous silica catalyst#†
Tanmoy Maitya, Susmita Bhuniaa, Soma Dasa and Subratanath Konera*
aDepartment of Chemistry, Jadavpur University, Jadavpur, Kolkata‒700 032, India. Tel.: +91
#
33 2414 6666 Ext. 2778; Fax: +91 33 2414 6414; E.Mail: snkoner@chemistry.jdvu.ac.in. All
authors have equal contribution.
______________________________________________________________________________
†Electronic supplementary information (ESI) available: Results of catalytic reactions in specific
condition in tabular form, PXRD, and TEM and SEM images of virgin and recovered catalyst,
N2 sorption data, graphical presentation of O-arylation reaction of phenol in five successive runs,
1H and 13C NMR spectra and HRMS plots of the isolated products.

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For Table of contents entry
A heterogeneous mesoporous silica based copper catalyst has been designed for O‒arylation of
phenol using nitroarene as electrophile which affords unsymmetrical diarylether. The catalyst
shows high efficiency for catalyzing reaction as reflected in high turnover frequency. Catalyst
can be recycled up to 5 times without any significant loss of catalytic activity.

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Abstract
Highly porous and robust mesoporous silica, SBA‒15 has been subjected for post-synthesis
modification for anchoring of copper through Schiff base moiety formation in it using silicon
alkoxide route. Hybrid porous material has been fully characterized by powder‒XRD, electronic
spectra, EPR, thermogavimatric analysis, N2 sorption measurement, TEM and SEM/EDS study.
Efficiency of the catalyst has been assessed in O‒arylation reaction using various substituted
phenols and nitroarenes in heterogeneous condition. Catalytic coupling reaction efficiently
produces unsymmetrical diarylethers. Impressive capability to activate substrates having
electron‒donating or electron‒withdrawing substituent and to afford high turnover frequency in
catalytic reactions made the catalyst highly desirable.
Introduction
Diarylethers has received much attention because of their natural biological activity and their
importance in polymer industries.1 Among the various synthetic methods of diarylethers,
transition metal catalyzed C–O bond formation has become a general procedure for the direct
synthesis of these compounds.2 Many of these transformations utilize aryl halides,2 arylboronic
acids3 or arenes4 as coupling partners. Recently, Chen and co–workers introduced homogeneous
copper catalyzed C–O bond formation reaction using cheap nitroarene as arylating regent.5 Very
recently, we have reported a homogeneous copper Schiff base complex catalyzed coupling of
nitroarenes with phenols to produce diaryl ethers.6 But for large‒scale applications,
heterogeneous catalyst is highly desirable because of the ease of handling, simple

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recovery/recycling of the catalyst and minimization of unnecessary toxic wastes.7 Furthermore,
designing of heterogeneous catalytic system with comparable efficiency to that of homogeneous
catalyst is a challenge. Phan et al. reported a heterogeneous Cu‒based metal‒organic framework
(MOF) for the coupling reaction of phenols with nitroarenes.8 Further exploration in this area
allows us to endeavor a new heterogeneous system that is highly active towards C–O coupling
reaction in entirely leaching free condition. The role of the support in heterogeneous catalytic
reaction is a prime factor and for that matter a support may change activity/or selectivity of the
catalyst towards reaction.9 Large pore mesoporous silica SBA‒15, with larger surface area
(BET)10 and higher thermal stability owing to thick walls is considered as good candidate for
catalytic support.11 Large pore size facilitates the synthesis of active metal center inside the pore
and easy diffusion of bulky reactants and products during catalytic reactions. To this end
Schiff‒base transition metal complexes has been studied widely due to their potential application
in catalyzing various catalytic reactions12 and ease of loading of Schiff‒base moiety onto porous
silica.13 Besides ligands which afford electron‒rich metal complexes are necessary to reach a
high degree of efficiency.14
Herein we report, a first example of a heterogeneous copper Schiff‒base modified SBA‒15
catalyst, S@Cu (see Experimental Section) for C–O coupling reaction using nitroarene as
coupling partner. The recoverable and reusable catalyst displays high turnover frequency (TOF)
by employing sub–stoichiometric amount of catalyst.
Experimental section
Materials

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CuCl2ꢀ2H2O, tetraethyl orthosilicate (TEOS, 98%), the nonionoic surfactant Pluronic P123, 3–
[2–(2–(Aminoethylamine)ethylamino]propyl–trimethoxysilane (AEPTS), pyridine–2–aldehyde
and all other reagents were purchased either from Sigma‒Aldrich/Fluka or Alfa‒Aesar and were
used as received without further purification. Solvents were purchased from Merck (India) and
were distilled and dried before use.
Physical measurements
Mesoporous materials were characterized by small angle powdered X‒ray (SAX) diffraction
study. Small angle X‒ray patterns of all powdered samples were taken in the 2θ range of 2° to
10° at a rate of 2° min‒1 in steps of 0.02° (Bruker D8 Advance diffractometer) using Cu‒Kα
radiation. Perkin‒Elmer 240C elemental analyzer was used to collect microanalytical data (C, H
and N). Fourier transform–infrared spectra on KBr pellets spectra were measured on a
Perkin‒Elmer L1600300 FTIR spectrometer. The solid state diffuse reflectance UV‒Vis spectra
were recorded by UV‒2401 PC SHIMADZU spectrophotometer in the wavelength range of 200–
800 nm. EPR spectra of both the catalysts were collected from JEOL JES‒FA200 ESR
SPECTROMETER. The copper content of catalyst was determined by a Perkin Elmer
A‒Analyst 200 atomic absorption spectrometer (AAS). The BET surface area and total pore
volume of the mesoporous samples were determined by an Autosorb‒iQ (Quantachorme Inc.)
gas sorption system. Prior to analyses, all the samples were degassed at 130 ºC under vacuum
until a final pressure of 10‒2 Torr was reached to evacuate the physically adsorbed moisture. The
transmission electron micrograph (TEM) of samples was obtained by JEOL‒TEM‒2010 electron
microscopes with an acceleration voltage of 200 kV. Scanning electron microscopic (SEM)
images were recorded on ZEISS EVO 18 (Special edition) and energy dispersive X‒ray

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scattering (EDS) analysis of samples (with gold coating) was recorded on OXFORD EDS8100.
TG/DTA thermograms were recorded on a Perkin Elmer (Singapore) Pyris Diamond TG/DTA
unit with N2 carrier gas flowing at a rate of 150 mL min−1 and the heating rate was programmed
at 4 °C min−1. All products of the catalytic reactions were collected and purified by column
chromatographic method and characterized by elemental analysis (using Perkin‒Elmer 240C
elemental analyzer) and comparison of their NMR and Mass spectra with those reported in the
literature. Reactions were monitored by TLC and NMR spectra recorded in CDCl3 or in
DMSO‒d6 on a Bruker Avance DPX 300 NMR (300 MHz) spectrometer. Mass spectra were
measured on a Waters XEVO‒G2QTOF#YCA351 high resolution Mass Spectrometer.
Catalyst preparation
Inclusion of amine groups into SBA‒15
Mesoporous silica material, SBA‒15 was synthesized following a method described in
literature.15 In a typical experiment [Scheme S1(a)], SBA‒15 (calcined, 0.2 g), dry toluene (15
mL) and AETPS (0.5 mL) was stirred at 70 °C for 24 h under N2 atmosphere. The white solid
thus formed, named S@Am, was filtered, washed with dry toluene for several times followed by
dry dichloromethane and dried in air.
Generation of Schiff‒base moiety in S@Am
0.05 g of S@Am was refluxed with pyridine‒2‒aldehyde (0.07 g) in methanol (10 mL) for 6 h at
65 °C in air [Scheme S1(b)]. The resulting yellowish solid was then collected by filtration and
was dried in a desiccator. The as‒synthesized products were denoted as S@SB.
Preparation of catalyst S@Cu

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Finally, the S@Cu was prepared by refluxing S@SB (0.07g) with CuCl2ꢀ2H2O (0.086 g, 0.5
mmol) in methanol (12 mL) at 65 °C for 6 h under air [Scheme S1(c)]. The green solid thus
formed was filtered, washed with methanol using Soxhlet and dried under vacuum. The Cu
content (measured from AAS) of the catalyst found to be 1.58 (wt.%) (0.025 mol.%). Elemental
analysis showed N:Cu was 4.1.
General experimental procedure for the C–O coupling reactions
0.003 g of catalyst S@Cu (0.025 mol%) was added to a round−bottom flask containing a DMF
(3 mL) solution of phenol (0.094 g, 1 mmol), 4‒nitrobenzaldehyde (0.166 g, 1.1 mmol) and
K2CO3 (0.391 g, 1.2 mmol). Reaction mixture was refluxed at 100 °C for 8 h. The conversion
was monitored by TLC (thin layer chromatography) method. After 8 h the reaction mixture was
cooled to room temperature and the mixture was extracted with water and dithyl ether (2 × 15
mL). The organic layers thus collected were combined and washed with brine, dried over
anhydrous Na2SO4, and concentrated in vacuo. The residue was purified by column
chromatography on silica gel (mesh 60−120) using an n‒hexane/ethyl acetate mixture as the
13
eluent to give the desired product. The product was characterized by 1H, C−NMR, HRMS and
elemental analysis then compared with literature data.
Hot filtration test
The reaction mixture of phenol (1.0 mmol), K2CO3 (1.2 mmol), and S@Cu (3 mg) in dry DMF
(3 mL) with p‒nitrobenzaldehyde (1.1 mmol) was refluxed at 100 °C under air and the progress
of the reaction was monitored by TLC. After completion of reaction ~30%, catalyst was
separated at the reaction temperature by filtration (Whatman 1, 10 ꢁm); further K2CO3 (1.2

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mmol) was added to the reaction solution and stirred at 100 °C for another 8 h. Progress of the
reaction was examined by TLC analysis.
Preparation of PNA‒SBA‒15
Anchoring of AEPTS into SBA–15 has been achieved according to method mentioned above
[Scheme S1(a)]. The white solid, S@Am was then refluxed with p‒nitroacetophenone (5 mmol,
0.8 g) in 10 mL methanol for 6 h at 65 °C. The resulting yellowish solid PNA‒SBA‒15 was then
collected by filtration and dried in desiccator (Scheme S2).
Three phase test
A solution of phenol (1 mmol), p‒nitrobenzadehyde (1.1 mmol), and K2CO3 (1.2 mmol) in 3 mL
DMF was stirred in the presence of 0.003 g of S@Cu and 1 g of PNA‒SBA‒15 at 100 °C for 8
h. Reaction mixture was collected by filtration and workup in the usual manner, then the residual
portion was hydrolyzed with 2 N aqueous HCl solution (1.7 mL HCl in 8.5 mL H2O) for 3 h
under a refluxing condition and isolated as p–nitroacetophenone. Both the resulting solutions
were analyzed by GC and 1H NMR.
Results and discussion
Catalyst characterization
Small angle X‒ray diffraction patterns of S@Am, S@SB and S@Cu are shown in Fig. 1.
Patterns demonstrated typical three diffraction lines16 in 2θ range 0.9 to 1.8 with a very strong
d100 = 101.32 Å and two small diffraction peaks for the 110 and 200 planes confirming a
well‒defined 2D hexagonal mesoporous structure.17 A progressive decrease in the intensity and

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slight increase in d‒spacing was observed which is associated with the sequential modification of
the pore of SBA‒15. The intensity reduction may be mainly due to contrast matching between
the silicate framework and organic moieties present in the channels of SBA‒15.18
Fig. 1 Powder XRD patterns of (a) S@Am, (b) S@SB, (c) S@Cu.
Fig. 2 depicts the FTIR spectra of S@SB and S@Cu. IR spectrum of S@SB exhibits a
band at 1645 cm‒1 due to C═N bond stretching frequency of imine bond. A shift of imine C‒N
stretching towards lower frequency region (1632 cm‒1) was observed upon metalation (Fig. 2b)
indicating the coordination of azomethine nitrogen to the copper ion. The solid‒state electronic
spectra (Fig. 3a) of the species S@SB showed three absorption maxima at ca. 215, 250 and 319
nm which could be attributed to intra ligand charge transfer transitions. Upon complex formation
intra ligand charge transfer bands shifted towards higher wavelength region and a relatively
weak band appeared at ca. 648 nm is assigned to d‒d transitions (Fig. 3b).19

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Fig. 2 FTIR spectra of a) S@SB, b) S@Cu.
Fig. 3 Diffuse reflectance UV–Vis spectra of a) S@SB, b) S@Cu.
Fig. 4 displays the X‒band EPR spectrum of S@Cu. The principle g values calculated by
the usual methods from EPR spectra are in agreement with those reported for other copper(II)

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Schiff base complexes.20 The g|| and g⊥ values of S@Cu are ca. 2.283 and 2.077, respectively.
The catalyst features g|| > g⊥ > 2.0023 which clearly suggest the characteristic of square planar
geometry in copper(II) complexes.21
Fig. 4 EPR spectrum of S@Cu.
N2 sorption isotherms and pore size distribution plots for SBA‒15, S@Am, S@SB and
S@Cu are shown in Figs. 5A and B, respectively. All plots are of typical of type IV isotherm
(according to the IUPAC nomenclature), with an H2‒type hysteresis loop, which is characteristic
of the mesoporous materials. BET surface area of SBA‒15 is 1431 m2 g‒1 with mesopore volume
of 1.62 cm3 g‒1. The average pore diameter is calculated to be 31 Å using the NLDFT method. A
gradual decrease in the BET surface area [Fig. 5A], pore volume, and pore diameter [Fig. 5B]
could be observed at each stage of modification (Table S1).22 This indicates that the
immobilization of the copper(II) complex has taken place substantially inside the pore channels
of SBA‒15.

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Fig. 5 A) Nitrogen sorption isotherms of a) SBA‒15, b) S@Am, c) S@Cu and B) pore size
distribution plot of a) SBA‒15, b) S@Am, c) S@Cu.
TEM images of SBA‒15 and S@Cu are shown in Fig. 6. When electron beam falls on the
catalyst perpendicular to the pore axis, the pores are seen to be arranged in patches composed of
regular rows, as has been interpreted by Chenite et al.23 Both the SBA‒15 matrix and catalyst
showed open‒ended lamellar type (Fig. 6a and 6b) arrangement. TEM image provides strong
evidence that the long‒range ordering of the support framework is retained even after the
immobilization of the copper(II) Schiff base complex in the mesoporous silica matrix.

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Fig. 6 TEM Images of A) SBA‒15 and B) catalyst, S@Cu.
All the SEM images shown in Fig. 7A revealed the similar worm‒like morphology typical
for SBA type mesoporous materials.24 This also suggest that no significant changes of the
external surface morphology occurred upon inclusion of Schiff‒base of the copper(II) complex
into porous matrix. The EDS elemental analysis indicated the presence of copper and chlorine in
catalyst whereas those atoms are absent in SBA‒15 (Fig. 7B). The ratio of Cu:Cl obtained from
EDS was close to 1:1 ratio which supports the proposed structure of copper Schiff-base complex
inside the pore as shown in Scheme S1.

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Fig. 7A SEM images of A) SBA‒15, B) S@Cu.
Fig. 7B EDS patterns of a) SBA‒15, b) S@Cu.
TG curve (Fig. 8) of S@Cu shows a weight loss between room temperature and 160 °C
while DTA curve shows a broad endothermic peak in the temperature range 100-200 °C. Upon
further heating, the catalyst showed thermal stability up to ~ 200 °C and then a steady mass loss
was obtained upto ~ 700 °C which corresponds to an exothermic peak centered at ~ 550 °C in its
corresponding DTA curve. The first mass loss seemed to be due to removal of adsorbed water
and solvent molecules, while the second mass loss may be due to decomposition of the
organically modified framework.

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Fig. 8 TG‒DTA curve of S@Cu.
Catalytic activity
In order to get optimal conditions for C‒O cross coupling reaction we first examined the
influences of different parameters e.g. solvent, base, reaction temperature, % Cu loading in
catalyst, reaction time etc. Our objective was to develop a protocol that requires minimum
amount of catalyst for smooth C‒O cross coupling reaction and that tolerate a wide range of
functional groups on both coupling partners. We first screen the optimized conditions for
reaction of phenol with p‒nitrobenzaldehyde and the results are summarized in Table S2. As the
catalyst is thermal and air stable, inert atmosphere was not employed. It is noteworthy that the
coupling reaction does not proceed without copper catalyst (Table S2, entry 1). Of the different
solvents investigated, DMF was the most suited for catalytic reaction (Table S2, entry 2). While
finding out the suitable base to be used in this coupling reaction it is noticed that when finely

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powdered Cs2CO3 was used, excellent arylated product was obtained (Table S2, entry 11) due to
the enhanced basicity of Cs2CO3.25 However, the use of K2CO3 will be more economic than that
of Cs2CO3 for large scale production. Other bases such as KOH, Na2CO3, CH3COONa,
tert‒BuOK, and DABCO showed a slower formation of 4‒phenoxybenzaldehyde after 8 hours
(Table S2, entries 9– 14). Cheap K2CO3 showed a satisfactory yield and was chosen for the
O‒arylation reaction (Table S2, entry 2). Reaction temperature and time also has great influence
on the coupling reactions. Reaction at 60 °C produced low yield (Table S2, entry 15) even after
20 h. With increase of temperature yield increased and gave a maximum of product yield at 100
°C whereas upon further increase in temperature yield decreased gradually. This is due to
formation of some unwanted non‒isolable by‒products (Table S2, entry 16). Thus the optimized
reaction conditions were found to be K2CO3 (as the base) and DMF (as the solvent) at 100 °C
under air. To study the catalytic efficacy of S@Cu, we controlled the copper loading in the
catalyst by varying the amount of CuCl2·2H2O in the Schiff‒base complex formation as shown
in Fig. S1. A maximum conversion was achieved with 0.025 mol% of catalyst in 8 hours.
Notably isolated yields were not up to the mark with lower concentrations of catalyst.
Additionally, increasing the catalyst concentration beyond 0.025 mol% did not enhance the
reaction rate significantly. Thus the optimum conditions for the catalytic reaction were as
follows: K2CO3 (base), DMF (solvent), 0.025 mol% catalyst and a reaction temperature of 100
ºC. Reactions in the presence S@Am+CuCl2ꢀ2H2O, S@SB+CuCl2ꢀ2H2O, CuCl2ꢀ2H2O were also
investigated under optimum condition to understand the competence of S@Cu as a catalyst
(Table S2, entries 17–19).
With the optimal condition in hand we investigated the catalytic activity of S@Cu for
various aryl alcohols with nitroarenes containing diverse functional groups. The results obtained

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in test reactions demonstrate that the electronic properties of the phenols affect the product yield
(Table 1). Substituted phenols possessing electron donating methyl groups accelerate the reaction
to yield products in a greater amount in comparison to simple phenols (Table 1, entries 1a,c and
1k,l). However, p‒methylphenol showed a higher yield than m‒ and o‒methylphenol indicating
that the yield is also affected by the steric effects of substitution (Table 1, entries 1l‒n). It is
worth noting here that the TOF reached ca. 165 h−1 in the case of p‒methylphenol which
completes the reaction in 8 h (Table 1, entry 1c). Here the p‒methoxy substituent in phenol acts
similarly like p‒methyl group towards the coupling reaction as shown in Table 1, entry 1d. The
substitution in nitroarenes has an impact on the reaction kinetics. Aryl nitro containing the
p‒NO2 and o‒NO2 group gave 99% and 92% yield, respectively, while reacts with
p‒methylphenol, probably due to electronic and steric hindrance, but TOFs still exceeded 165 h‒1
and 153 h‒1 (Table 1, entries 1c and 1d), respectively. p‒nitrobenzonitriles showed more product
yields than p‒nitrobenzaldehyde (Table 1, entries 1f‒h). The coupling reactions of
electron‒deficient phenols with aryl halides have been challenging as the corresponding
phenolates are weak nucleophiles.26 But the results presented here demonstrate that
p‒nitrobenzonitrile can behave as
a
strong electrophile in the O‒arylation of
p‒hydroxyacetophenone and can afford moderate yields (58%) with TOF 77 h‒1 (Table 1, entry
1j).
Table 1 S@Cu catalysed O‒arylation reaction of nitroarene with aryl alcohols.a

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In search of efficient heterogeneous catalysts for the C‒O cross coupling reactions, we
have undertaken few comparison studies. Recently, Li et al. reported the O‒arylation of
p‒hydroxyacetophenone with p‒fluorobenzonitrile in DMF media at a temperature of 90 ºC
under a N2 atmosphere.27 Buchwald and his group also studied the reactivity of some
electron‒deficient phenols in C–O coupling reactions at 100–110 ºC.26 However, in all the above
reported processes aryl halides were used as electrophiles. In contrast catalyst S@Cu efficiently
catalyzed the same reaction using nitroarene as electrophile at 100 °C and the TOF of the
reaction becomes 77 h‒1. Phan et al. reported for O‒arylation of phenol with
p‒nitrobenzaldehyde using 3 mol% of heterogeneous Cu2(BDC)2(DABCO) manifested TOF ca.
17 h‒1.8 While in our case reaction could proceed with 0.025 mol% of S@Cu and procure TOF
ca. 152 h‒1.

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Hot filtration test
To ascertain the catalysis is indeed heterogeneous we have performed hot filtration test.28 After
hot filtration the residual activity of the supernatant solution was studied. It was found that
coupling reactions do not proceed further as the catalyst was removed by filtration (Fig. S2).
Atomic absorption spectrometric analysis (sensitivity up to 0.001 ppm) of the supernatant
solution collected by filtration also confirmed the absence of copper in the liquid phase. The
above results suggest that copper was not being leached out from the solid support and the
catalyst is indeed heterogeneous.
Three phase test
Three phase test is one of the important tests to ascertain the true heterogeneity of the copper
based catalyst19,22 for O‒arylation reaction using a designed aryl nitro in usual condition as
described in Scheme S3. In the case of the immobilized aryl nitro, no reaction was observed on
support and no p–phenoxyacetophenone was detected. If there be any leaching of Cu from the
catalyst, anchored p–nitroacetophenone would have also participated in the above–described
reaction. We found remained p‒nitroacetophenone as unreacted, while the soluble
p‒nitrobenzaldehyde substrate converted to p–phenoxybenzaldehyde as expected for O‒arylation
reaction, indicating the presence of an active heterogeneous catalyst.
Separation and reuse of catalyst
For recycling study the first O‒arylation reactions were performed with phenol,
p‒nitrobenzaldehyde and K2CO3, maintaining the optimized reaction conditions. After the every
cycle, the catalyst was recovered by centrifugation and then washed thoroughly with (9:1)

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acetonitrile‒water mixture. Recovered catalyst was dried under vacuum at 100 °C overnight.
Atomic absorption spectrometric analysis of the recovered catalysts confirmed that the copper
content of the S@Cu remained virtually the same (~1.56 wt%). Performance of the recycled
catalyst up to five successive runs is shown in Fig. S3. In addition, recovered catalyst was
thoroughly checked by X‒ray powder diffraction (Fig. S4), FTIR spectra (Fig. S5), TEM (Fig.
S6) and SEM study (Fig. S7). All these studies convincingly supported the structural integrity of
the compound.
Conclusion
In essence, we have successfully synthesized a new mesoporous silica based copper Schiff base
catalyst by post‒synthetic modification method. The catalyst facilitates the C‒O coupling
reaction to go smoothly encompassing a broad range of functional groups. Notably, the catalysts
are efficient enough to activate electron‒deficient phenols towards O‒arylation reactions. This
leaching resistant catalyst shows high TOF values for synthesis of diarylethers using cheap
nitroarene as electrophile. O‒arylation can be undertaken in open flask reaction which makes the
procedures easily accessible for synthesis of a variety of important compounds.
Supporting information
Supporting information may be found in the online version of this article.
Acknowledgments
Financial assistance received from UGC through a project (to SK) is gratefully acknowledged. T.
M. and S. D. thanks the Council of Scientific and Industrial Research (CSIR), New Delhi and

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University Grant Commission (UGC), respectively for awarding them fellowship. Authors thank
Department of Science and Technology (DST) for providing funds to the Department of
Chemistry, Jadavpur University in procuring NMR spectrometer, HRMS and powder XRD
system.
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