Heterogeneous sequential N-arylation of N-heterocycles over copper anchored mesoporous silica catalyst

Article abstract of DOI:10.1016/j.apcata.2015.12.029

N-Arylation reaction of N-heterocyclic compounds has been carried out in excellent yield using copper anchored SBA-15 catalyst. Sequential N-arylation reactions employing single catalyst under heterogeneous condition provides a straightforward access to a

Full text of DOI:10.1016/j.apcata.2015.12.029

Accepted Manuscript
Title: Heterogeneous sequential N–arylation of
N–heterocycles over copper anchored mesoporous silica
catalyst
Author: Soma Das Tanmoy Maity Subratanath Koner
PII:
S0926-860X(15)30306-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.12.029
APCATA 15703
To appear in:
Applied Catalysis A: General
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Revised date:
Accepted date:
3-11-2015
19-12-2015
22-12-2015
Please cite this article as: Soma Das, Tanmoy Maity, Subratanath Koner,
Heterogeneous sequential Nndasharylation of Nndashheterocycles over
copper anchored mesoporous silica catalyst, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2015.12.029
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Heterogeneous sequential N‒arylation of N‒heterocycles over copper anchored
mesoporous silica catalyst
Soma Das, Tanmoy Maity, Subratanath Koner* snkoner@chemistry.jdvu.ac.in
Department of Chemistry, Jadavpur University, Jadavpur, Kolkata‒700 032, India
*Corresponding author. Tel.: +91 33 2457 2778; fax: +91 33 2414 6414.

Graphical Abstract (Synopsis)
A heterogeneous copper(II) catalyst was developed for synthesis of imidazolium based
N and N'‒arylated product via C‒N cross coupling reaction. This catalyst offered a
novel method to synthesize unsymmetrical imidazolium salt. High selectivity for
products, high turnover, and possibility of easy recycle makes the catalyst highly
desirable to address the industrial needs and environmental concerns.

Highlights



A heterogeneous copper(II) catalyst was developed for synthesis of
imidazolium based N and N'‒arylated product.
The catalyst offered a simple method to synthesize some new unsymmetrical
imidazolium salts which have relevance to ionic liquids.
High selectivity for products, high turnover, and possibility of easy recycle
makes the catalyst highly desirable.

Abstract
N‒arylation reaction of N‒heterocyclic compounds has been carried out in excellent yield
using copper anchored SBA‒15 catalyst. Sequential N‒arylation reactions employing single
catalyst under heterogeneous condition provides a straightforward access to a series of
important arylazolium salts. Simplicity of the method and outstanding character of the
catalyst facilitate the reactions to go smoothly which encompass a broad range of functional
groups. Catalyst can be recovered and reused several times without any significant loss in its
catalytic activity.
Keywords: Copper Schiff‒base; heterogeneous catalyst; mesoporous SBA‒15; heterocyclic
N‒arylation; aryl quaternization.

1. Introduction
Copper catalyzed N‒arylation of nitrogen containing heterocycles via the classical
Ullmann‒type coupling had widely been reported in the literature [1‒6]. However further
N'‒arylation of N‒arylazoles without acidic proton which affords quaternized salt has been
attempted scarcely [7‒12]. Quaternized salts are used as important precursor of
N‒heterocyclic carbenes (NHCs) [8], anion receptors [13], fluorophores [14] etc. Various
protocols have been applied for quaternization of N‒arylazoles which includes direct aryl
quaternization of N‒substituted imidazoles catalyzed by copper(II) salt in homogeneous
medium [7‒10]. Nevertheless ligands which afford electron‒rich metal complexes with
increased stability can be a viable strategy to circumvent catalyst deterioration. Such systems
are necessary to reach a high degree of efficiency and/or selectivity [15]. Even though
homogeneous catalytic systems are known to exhibit better reactivity than heterogeneous
systems, for large‒scale liquid‒phase reactions, heterogeneous catalyst is highly desirable
because of ease of handling, simple recovery/recycling of the catalyst and minimization of
unnecessary toxic wastes [16]. Furthermore, designing of heterogeneous catalytic system
with comparable efficiency to that of homogeneous catalyst is a challenge. To our knowledge
there is hardly any report of metal complex based heterogeneous catalyst for synthesis of
arylazolium salts. Hence, we have endeavored to search a suitable heterogeneous system in
this study in respect of its practical application. Schiff‒base transition metal complexes had
been studied widely due to their potential application in catalyzing various catalytic reactions
[17,18]. Ease of loading of Schiff‒base moiety onto the solids supports such as resins [19],
alumina [20], zeolites [21], porous silicas [22] through heterogenization [23], encapsulation
or post‒synthesis modification [24] paved the way for designing of hybrid catalysts.
However, there are certain disadvantages with polymeric supports and zeolites owing to their
vulnerability towards some chemicals/solvents, pore size restriction and severe leaching of

metals [25]. Conversely large pore mesoporous silica SBA‒15, with larger surface area [26]
and higher stability owing to thicker walls in comparison to MCM‒41 [27], is considered to
be good candidate for catalytic support [28]. In addition large pore size facilitates easy
diffusion of bulky reactants and products during catalytic reactions [29].
We report herein, for the first time, a heterogeneous copper Schiff‒base modified
SBA‒15 catalyst, S(C)‒15 (see Experimental) for sequential N‒arylation reactions producing
some new arylazolium salts. Single catalyst can be employed for C‒N cross‒coupling
reaction between aryl halides and N‒heterocyclic compounds (imidazoles, bezimidazoles,
triazoles) and successively N'‒arylation of those N‒arylazoles by using diphenyliodonium
tetrafluoroborate salt under leaching free condition. To compare the catalytic efficacy of
heterogeneous catalyst, S(C)‒15 with an analogous homogeneous system, a copper(II)
Schiff‒base complex 1 (see Experimental) with similar active center configuration has been
designed.
2. Experimental
2.1. Materials
Cu(OAc)2∙H2O, tetraethylorthosilicate (TEOS, 98%), the non-ionic surfactant Pluronic
P123,
3‒((2‒aminoethylamino)propyl)‒trimethoxysilane
(ATMS),
N‒(N‒propyl)ethylenediamine, o‒hydroxyacetophenone 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.
2.2. 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. Powder X‒ray patterns of complex 1 were taken in the 2θ range of 5°
to 50°. Perkin‒Elmer 240C elemental analyzer was used to microanalysis of samples (C, H
and N). IR spectra (KBr pellet) 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. EPR spectra of the catalyst and complex 1 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). 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 on a 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 scattering (EDS) analysis of samples
(with gold coating) was done on OXFORD EDS8100 equipment. 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 and comparing of their
NMR and Mass spectra with the literature data. 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.
2.3. Synthesis of heterogeneous catalyst S(C)‒15
Mesoporous silica material, SBA‒15 was synthesized by following the procedure
described in literature [30]. In a typical experiment (see Scheme S1), SBA‒15 (calcined, 0.1
g), dry chloroform (12 mL) and ATMS (0.1 mL, 0.46 mmol) was stirred at room temperature
for 12 h under N2 atmosphere. The white solid thus formed, named S(A)‒15, was filtered,
washed with dry chloroform for several times followed by dry dichloromethane and dried in
air. This solid was then refluxed with o‒hydroxyacetophenone (0.063 g, 0.46 mmol) in
methanol (12 mL) for 6 h at 65 °C in air. The resulting yellowish solid, S(SB)‒15 was then
collected by filtration and was dried in a desiccator. Finally, the S(C)‒15 was prepared by
refluxing S(SB)‒15 with Cu(OAc)2∙H2O (0.09 g, 0.46 mmol) in methanol (12 mL) at 65 °C
for 12 h under air. The green solid thus formed was filtered, washed with methanol using a
Soxhlet, and dried under vacuum. The Cu content (measured from AAS) of the catalyst found
to be 1.5 (wt.%) (0.02 mol.%). Elemental analysis showed Cu:N was 2.05.
2.4. Synthesis of copper(II) Schiff‒base complex 1
As shown in Scheme S2, the Schiff‒base, HL was synthesized by refluxing equimolar
mixture of N‒(N‒propyl)ethylenediamine ( 0.102 g, 1.0 mmol) and o‒hydroxyacetophenone
(0.136 g, 1.0 mmol) in methanol. Consecutively a solution of Cu(OAc)2·H2O (0.199 g ,1.0
mmol) in methanol was added to the Schiff‒base solution and stirred for few minutes. The
resulted green solution was kept for slow evaporation process. After several days single green
colored block crystals suitable for X‒ray diffraction were obtained. The Cu content
(measured from AAS) of the catalyst found to be 6.2 (wt%) (0.1 mol%). Elemental analysis
calcd. (%) for C15H24CuN2O4: C 50.06, H 6.72, N 7.78; found: C 50.0, H 6.7, N 7.8.

2.5. General experimental procedure for the N‒arylation Reactions
A mixture of aryl halide (2.4 mmol) and Cs2CO3 (4.0 mmol, 0.650 g),
nitrogen‒containing heterocycle (2.0 mmol), dry DMF (3 mL) solvent and catalyst was
stirred at 100 °C in an oil bath under air. After cooling to room temperature, catalyst was first
separated out by centrifugation and the liquid part was extracted with water and diethylether
(2 × 15 mL). The organic layers thus collected were combined and washed with brine, dried
over Na2SO4, and concentrated in vacuo. The residue was purified by column
chromatography on silica gel (mesh 60‒120) using an n‒hexane/ethylacetate mixture as the
eluent to collect the desired product. The product was analyzed by 1H and 13C NMR and mass
spectroscopy.
2.6. General experimental procedure for the quaternization of N‒arylated compounds
At first diphenyliodonium tetrafluoroborate was synthesized according to procedure
1
13
reported previously [31] and characterized by H and C NMR and mass spectral analysis
(see supplementary content). A mixture of N‒heteroarene (1.0 mmol), diphenyliodonium salt
(1.5 mmol), catalyst and DMF (3 mL) was stirred at 80 °C for a desired time in air. After
cooled down to room temperature, heterogeneous catalyst was separated out by
centrifugation. The liquid part was then subjected to rotary evaporation and the residue
obtained was passed through a silica gel column (dichloromethane/acetone = 5:1‒2:1 v/v) to
1
afford the desired arylazolium salt. The product was analyzed by H and 13C NMR and mass
spectroscopy.
3. Results and discussion
3.1. Catalyst characterization

Small angle X‒ray diffraction patterns of SBA‒15, S(SB)‒15 and S(C)‒15 are shown in
Fig. 1 Pattern of fresh SBA‒15 demonstrated typical three diffraction lines [32] with a very
strong d100 = 101.32 Å diffraction line at 2θ ≈ 0.88 Å and two small diffraction lines for the
110 and 200 planes at 2θ ≈ 1.46 Å and 2θ ≈ 1.68 Å, respectively confirming a well defined
P6mm hexagonal symmetry and long‒range ordering of structure [33]. Like the parent material
SBA‒15, all modified porous samples S(SB)‒15 and S(C)‒15 showed the similar diffraction
patterns indicating the chemical process involved in modification of mesoporous silica did
not diminish the structural ordering of the SBA‒15, however, some degree of decrease in
crystallinity takes place in the catalyst. A progressive decrease in the intensity and shifting of
the peaks towards the higher angle was observed after 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 [25].
FTIR spectra of S(SB)–15 and S(C)–15 are shown in Fig. 2A. Schiff‒base functionalized
SBA‒15 (Fig. 2Aa) showed a peak at 1618 cm‒1 which may assign to the C‒N stretching
frequency of the C═N imine bond. A shift of ca. 14 cm‒1 in imine C‒N stretching towards
lower frequency region was observed upon metalation (Fig. 2Ab) indicating the coordination
of azo‒methaine nitrogen to the copper ion. Besides, there were asymmetric and symmetric
vibrations of acetate group typical for bridged acetate at ~1561 cm‒1 and ~1440 cm‒1,
respectively [18]. While in complex 1 azomethine stretch appeared at ~1598 cm‒1 and acetate
vibrations appeared at ~ 1549 cm‒1 and 1440 cm‒1 (Fig. 2B) suggesting the formation of
metal complex moiety with similar configuration in S(C)–15.
The solid state diffuse reflectance UV–Vis spectrum of S(SB)–15 showed four absorption
maxima at ca. 215, 250, 319 and 387 nm (Fig. 3Aa) which could be attributed to intra ligand
charge transfer transitions. A similar spectrum was obtained for HL as shown in Fig. 3Ab.

Upon complex formation intra ligand charge transfer bands shifted towards higher
wavelength region and a new band at ca. 642 nm appeared which can be attributed to d‒d
transition (Fig. 3Bb). A similar type of shifting was also observed in case of S(C)‒15 and an
expected new band d–d transitions band at ~ 644 nm indicating similar type of coordination
environment of copper(II) Schiff‒base moiety in SBA‒15 (Fig. 3Ba) as is exhibited in
complex 1.
The X‒band EPR spectra were measured at room temperature for both catalyst S(C)‒15
and complex 1 and are shown in Figs. 4A and 4B, respectively. Both S(C)‒15 and 1 feature
g > g > 2.0023 (Table S4) which clearly suggest the characteristic of square planar geometry
in copper(II) complexes [34]. Comparable g values and similarity of EPR spectrum also
suggest the presence of similar type of ligand environment around copper(II) center in
S(C)‒15 and complex 1.
N2 sorption isotherms and pore size distribution plots for SBA‒15, S(SB)‒15 and
S(C)‒15 are displayed in Figs. 5A and 5B respectively. All plots are of typical of type IV
isotherm (according to the IUPAC nomenclature), with hysteresis loops, which is
characteristic of the mesoporous materials. A gradual decrease in the BET surface area, pore
volume, and pore diameter could be observed at each stage of modification as expected
because the organic fragments or metal complexes are entering into the channels. Nitrogen
sorption study exhibits that the BET surface area of SBA‒15 is 1882 m2 g‒1 while the
mesopore volume is 2.589 cm3 g‒1. The average pore diameter is calculated to be 31.132 Å
using the NLDFT method (Table S5). All calculated values are in agreement with those
reported for good‒quality mesoporous silica. S(SB)‒15 shows lower N2 uptake, smaller pore
volume and pore diameter whereas the S(C)‒15 catalyst shows even lower N2 uptake, and
smaller pore volume and pore diameter (Table S5). As the pores become bulkier, hysteresis

occurs at lower P/P0 with a concomitant decrease in pore size [35]. This indicates that the
immobilization of the copper(II) complex has taken place substantially inside the pore
channels of SBA‒15. This kind of reduction in surface area, pore volume and pore diameter
was also reported in the literature [25].
To assess the mesoporosity of SBA‒15 and catalyst, samples were studied with TEM
analysis. Fig. 6 shows top view and a cross‒section of the pore system. Both the SBA‒15
matrix and catalyst showed an open‒ended lamellar type morphology (Fig. 6a and 6c) and
honeycomb‒like hexagonal arrangements (Fig. 6b and 6d) as confirmed by Fast Fourier
Transform (FFT) pattern (inset of Fig. 6b and 6d). 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 [36]. TEM micrograph of SBA‒15
viewed along the pore axis reveals a hexagonal array having a channel dimension of ∼ 9.7
nm, which is consistent with the XRD results. S(C)‒15 also exhibits a similar type of array of
regular rows in TEM micrographs. Therefore, it can be concluded that both SBA‒15 and
S(C)‒15 have a similar type of external morphology. In our case, the morphology of the
tubules in SBA‒15 and in S(C)‒15 is rectilinear. The 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.
Therefore, it can be concluded that SBA‒15 preserves its morphology up to formation of
catalyst even after withstanding all the reaction steps starting from the pore functionalization
to the binding of CuII.
SEM images were captured for the mesoporous samples (Fig. 7A). All SEM images
revealed the similar worm‒like morphology typical for SBA type mesoporous materials [37].
This also suggest that no significant changes of the external surface morphology occurred

upon inclusion of Schiff‒base of the copper(II) complex into SBA‒15. EDS elemental
analysis (Fig. 7B) indicates the presence of copper and nitrogen in the catalyst whereas these
atoms were absent in SBA‒15.
3.2. Catalytic activity
In order to get optimal conditions for C‒N cross coupling reaction we first examined the
influences of different parameters e.g. solvent, base, reaction temperature, % Cu loading in
catalyst, reaction time etc. These parameters can influence the progress on C‒N cross
coupling reaction [38]. Our objective was to develop a protocol that requires minimum
amount of catalyst for smooth C‒N 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 imidazole with p‒bromonitrobenzene and the results are summarized in Table S6.
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 S6, entry 3). Of
the different solvents investigated, DMF was the most suited for catalytic reaction (Table S6,
entry 1). While finding out the suitable base for coupling reaction it is noticed that the
performance of insoluble inorganic bases, such as K2CO3, K3PO4, Cs2CO3, Na2CO3, and
Na3PO4 were much better in comparison to soluble organic base like Et3N (Table S6, entries
1‒11). Notably, when fine powdered Cs2CO3 was used, excellent arylated product was
obtained (Table S6, entries 1 and 2) due to the enhanced basicity of Cs2CO3 [39]. Reaction
temperature and time also has great impact on N‒arylation of imidazole. Reaction at 60 °C
produced low yield (Table S6, entry 13) even after 30 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‒isolated by‒products (Table S6, entry 14). Generally, aryl iodide shows greater

reactivity than aryl bromide in the cross‒coupling reactions. However, in present case both
aryl iodide and aryl bromide (Table S6, entries 1 and 15) showed comparable yields. In a
recent study, it had been proposed that the I¯ or I2 generated from iodobenzene can act as an
inhibitor during the catalytic process [40]. Thus the optimized reaction conditions were found
to be Cs2CO3 (as the base) and DMF (as the solvent) at 100 °C under air. To study the
catalytic efficacy of S(C)‒15, we controlled the copper loading in the catalyst by varying the
amount of Cu(OAc)2˙H2O in the Schiff‒base complex formation as shown in Figure S1 (see
supplementary content).
With the optimal condition in hand we examined reaction of various N‒heterocyclic
amines with bromobenzenes containing diverse functional groups. Evidently the reaction
tolerates varieties of functional groups. p‒nitrobromobenzene delivered an excellent yield
showing a TOF value 165 h‒1 while o‒nitrobromobenzene also gives comparable yield (Table
1, entries 1 and 2). It was observed that bromoarenes containing electron‒withdrawing groups
reacted at a faster rate and affords higher yield than bromoarenes with electron‒donating
substitution groups (Table 1, entries 9 and 10). Benzimidazole and triazoles reacted fairly
well with substituted bromobenzenes to give the corresponding N‒arylated products in good
yield (Table 1, entries 6‒15). Benzimidazole showed a little difficulty to react with
arylbromide compared to imidazole indicating the sensitivity of reaction towards steric
hindrance of the nucleophiles (Table 1, entries 1 and 7).
After successful demonstration of the insertion of aryl groups into imidazole via C‒N
cross coupling, we tried to incorporate another aryl group in N‒substituted imidazoles using
same catalyst which yielded quaternized salt of imidazoles. In the standardization process we
started by reacting N‒p‒nitrophenylimidazole with diphenyliodonium salts in different
solvents at different temperature. Reaction does not proceed without employing copper

catalyst (Table S7, entry 1). Reaction in acetonitrile, DMSO, toluene or dioxane afforded
lower yields than in DMF (Table S7, entries 7−10). Changing Ph2IBF4 with Ph2ICl keeping
other reaction condition unaltered produced negative result (Table S7, entry 6). However,
lowering of temperature to 80 °C the catalyst showed excellent activity in DMF with 95%
yield in open atmosphere (Table S7, entry 11).
Evidently we were succeeded in inserting subsequent aryl group into N‒arylazoles to
synthesize an array of unsymmetrical N,N′‒diarylatedimidazolium compounds under the
optimized reaction condition and the results are summarized in Table 2.
N‒p‒nitrophenylimidazole showed excellent activity to afford the corresponding quaternized
salt within 6 h (Table 2, entries 1) with a TOF value of 158 h‒1. Nevertheless, o‒nitro variety
showed a comparable yield with more or less equal value of TOF (Table 2, entries 2). Other
substituents such as cyano, bromo at different positions of the phenyl ring were also
successfully N'‒arylated to afford the corresponding unsymmetrical imidazolium salts in high
yields (Table 2, entries 3‒5). N‒arylated benzimidazoles gave moderate to high yield within
6 h (Table 2, entries 7‒10). It was found that electron‒donating substituents yielded less
compared to electron‒withdrawing substituents while non‒substituted benzimidazole
demonstrates a middling yield (Table 2, entries 6‒10). Between N‒arylated 1,2,3‒triazoles
and 1,2,4‒triazoles later showed more product selectivity (Table 2, entries 11‒15).
To ascertain the catalytic efficiency of S(C)‒15 we have undertaken few control
experiments. It is known that Cu(OAc)2·H2O catalyzes aryl quarternization reaction
efficiently [7,8]. Cu(OAc)2·H2O catalyzed the reaction between N‒p‒nitrophenylimidazole
and diphenyliodonium salt in DMF at 100 ºC (Table S7, entry 2) showing a TOF value of ca.
1.92 h‒1. In contrast the catalyst S(C)‒15 efficiently catalyzed the same reaction at much
lower temperature (at 80 °C). Though S(C)‒15 requires little longer time to complete the

reaction, the TOF of the reaction exceeds 158 h‒1. For a similar set of catalysts and catalyzed
reactions generally homogeneous system performed better than its corresponding
heterogeneous counterpart. With this view we have prepared complex 1 to compare the
activity of copper center in homogeneous and heterogeneous conditions. Coupling of
N‒p‒nitrophenylimidazole and diphenyliodonium salt can be catalyzed by complex 1 under
homogeneous condition in shorter period of time (Table S7, entry 3).
3.3. Hot filtration test
To ascertain the catalysis is indeed heterogeneous we have performed hot filtration test
(see supplementary content) [38]. Liquid phase of the reaction mixture was collected by
filtration at the reaction temperature when the catalytic reaction (both for N‒arylation and
quaternization) was 30% complete and the residual activity of the supernatant solution was
studied. Coupling reactions do not proceed further after filtration of catalyst (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 for
both the cases. The above results suggest that Cu was not being leached out from the solid
catalyst during the reactions.
3.4. Three phase test
To get deeper insight into the heterogeneity of the system, we performed three phase tests
[38] for N‒arylation reaction using a designed aryl halide in usual condition as described in
Scheme S4 (see supplementary content). In the case of the solid phase immobilized aryl
bromide, no reaction was observed on support even after 12 h at 100 °C and no
N‒p‒acetylphenylimidazole was detected. We found p‒bromoacetophenone remained
unreacted, while the soluble p‒bromonitrobenzene substrate converted to
N‒p‒nitrophenylimidazole as expected for N‒arylation reaction. If there were any leaching of

Cu from the catalyst, solid phase anchored p-bromoacetophenone would have also
participated in the above-described reactions. However, this has not happened for the present
catalyst. Therefore, all the above tests convincingly demonstrate that there was no leaching of
Cu species occurring in the S(C)‒15 catalyzed N‒arylation reactions.
3.5. Separation and catalyst reuse
For recycling study the first N‒arylation reactions were performed with
p‒bromonitrobenzene, imidazole and Cs2CO3 (Table 1, entry 1), maintaining the optimized
reaction conditions. For quaternization we used N‒p‒nitrophenylimidazole and
diphenyliodoniun tetrafluoroborate maintaining the same reaction condition (Table 2, entry
1). As 5 mg of solid catalyst was not enough to recover, reuse and finally characterize,
therefore, catalyst was collected from different batches of same catalytic cycle and an average
values were given here. After the every cycle, the catalyst was recovered by centrifugation
and then washed thoroughly with (9:1) acetonitrile‒water mixture. The recovered catalyst
was dried under vacuum at 100 °C overnight. Atomic absorption spectrometric analysis of
the recovered catalysts from both reactions confirmed the copper content of the S(C)‒15
remained virtually the same. The performance of the recycled catalyst up to five successive
runs is shown in Fig. 8.
In order to check the stability of the recovered catalyst, it was thoroughly characterized
by X‒ray powder diffraction, gas adsorption study and SEM image. Comparison of powder
X‒ray diffraction patterns (Fig. S3), gas adsorption studies (Fig. S4), transmission electron
microscopic measurement (Fig. S5) and SEM images (Fig. S6) of the pristine and recovered
catalyst convincingly demonstrated that the structural integrity of the catalysts retained after
reaction. TEM images of the recovered catalyst showed no sign of formation of Cu
nanoparticles on the surface of the catalyst.

4. Conclusion
In summary we have successfully developed a novel heterogeneous catalyst for synthesis of
some important unsymmetrical N,N‒diarylated imidazolium compounds via a facile
procedure. Single catalyst can activate consecutively two step of N‒ and N'‒arylation of
imidazoles and some of its congeners. Simplicity of the method and outstanding character of
the catalyst facilitates the reaction to go smoothly encompassing a broad range of functional
groups. The catalyst is resistant to leaching under the reaction condition. Both the arylation
can be undertaken in open flask reaction which makes the procedures easily accessible for
synthesis of a variety of important compounds.
Acknowledgments
Financial assistance received from UGC through a project (to SK) is gratefully
acknowledged. S. D. thanks the University Grant Commission (UGC), New Delhi, for
awarding her a fellowship. Indian Association for the Cultivation of Science is acknowledged
for giving access to TEM facility. Authors thank DST for providing funds to the Department
of Chemistry, Jadavpur University in procuring single crystal X‒ray and NMR spectrometer
under FIST program. We are also grateful to Dr. Kausikisankar Pramanik, Jadavpur
University for allowing us to avail high vacuum rotary evaporator.

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Figure Captions
Fig. 1 Powder XRD patterns of (a) S‒15, (b) S(SB)‒15, (c) S(C)‒15
Fig. 2 A) FTIR spectra of a) S(SB)‒15, b) S(C)‒15 and B) FTIR spectra of complex 1

Fig. 3 A) Diffuse reflectance UV–Vis spectra of a) S(SB)‒15, b) HL and B) Diffuse
reflectance UV–Vis spectra of a) S(C)‒15, b) complex 1
Fig. 4 EPR spectra of A) S(C)‒15 and B) complex 1

Fig. 5 A) Nitrogen sorption isotherms of a) SBA‒15, b) S(SB)‒15, c) S(C)‒15 and B) pore
size distribution plot of a) S‒15, b) S(SB)‒15, c) S(C)‒15
Fig. 6 TEM Images of a),b) SBA‒15 and c), d) catalyst, S(C)‒15

Fig. 7A SEM images of a) SBA‒15, b) S(A)‒15, c) S(SB)‒15 and d) S(C)‒15
Fig. 7B EDS patterns of a) SBA‒15, b) S(A)‒15, c) S(SB)‒15 and d) S(C)‒15

Fig. 8 Bar diagram showing recyclability of the catalyst for (1) N‒arylation of imidazole (2)
N'‒arylation of N‒arylimidazole

Tables
Table 1 S(C)‒15 catalyzed C‒N cross‒coupling reactions of various aromatic halides with
N‒heterocyclic amines
Entrya
1
R
Amine
Product
Time (h) Yieldb (wt%) TOFd (h‒1)
p‒NO2
12
99, 96c
165, 160c
2
3
o‒NO2
p‒CN
12
12
95
97
158
162
4
p‒Br
12
85
142

5
6
m‒Br
12
24
78
88
130
73
H
7
p‒NO2
12
86
143
8
9
o‒NO2
p‒CN
12
12
72
83
120
138
10
11
p‒OCH3
24
12
66
97
55
p‒NO2
162

12
p‒CN
12
95
158
13
14
15
p‒NO2
p‒CN
12
12
12
98
97
82
163
162
137
o‒NO2
a Reaction Condition: Aryl bromide (2.4 mmol), heterocyclic amine (2.0 mmol), Cs2CO3 (4.0 mmol),
DMF (3 mL), 5 mg of S(C)‒15 (Cu content: 0.02 mol%), temperature = 100 °C. b Isolated yields. c
Fifth cycle. d TOF (turnover frequency) = moles converted/(mol of active site × time).

Table 2 S(C)‒15 catalyzed aryl quaternization of N‒substituted heterocyclic amines with
diphenyliodonium tetrafluroborate
Entrya
1
Reactant
Product
Yieldb (wt%)
95, 93c
TOFd (h‒1)
158, 155c
2
3
92
94
153
157
4
5
85
83
142
138