Syntheses and catalytic activities of homogenous and hierarchical ZSM-5 grafted Pd(II) dicarbene complex of imidazole based ionic liquids

Article abstract of DOI:10.1016/j.cattod.2012.01.014

Pd(II) dicarbene complex of imidazole based cyclic and acyclic ionic liquids were prepared and their catalytic activities were compared in the Heck and the Sonogashira reactions of deactivated and economical bromobenzene/ chlorobenzene. Acyclic Pd complex was found to be more active than the cyclic one. Geometrically optimized structures of cyclic and acyclic Pd complexes, which was computed using density functional method was able to explain the low activity of cyclic Pd complex. Further, these catalysts were grafted on hierarchical ZSM-5 surface and their catalytic activity and recyclability were investigated. Pd(II) dicarbene complex grafted on hierarchical ZSM-5 were found to be efficient and recyclable catalysts for CC coupling reactions. Catalysts were found to be stable and show negligible loss in activity even after the five cycles.

Full text of DOI:10.1016/j.cattod.2012.01.014

Catalysis Today 198 (2012) 189–196
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Syntheses and catalytic activities of homogenous and hierarchical ZSM-5 grafted
Pd(II) dicarbene complex of imidazole based ionic liquids
Rajkumar Kore, Mahesh Tumma, Rajendra Srivastava∗
Indian Institute of Technology Ropar, Nangal Road, Rupnagar 140001, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Pd(II) dicarbene complex of imidazole based cyclic and acyclic ionic liquids were prepared and their cat-
alytic activities were compared in the Heck and the Sonogashira reactions of deactivated and economical
bromobenzene/chlorobenzene. Acyclic Pd complex was found to be more active than the cyclic one. Geo-
metrically optimized structures of cyclic and acyclic Pd complexes, which was computed using density
functional method was able to explain the low activity of cyclic Pd complex. Further, these catalysts were
grafted on hierarchical ZSM-5 surface and their catalytic activity and recyclability were investigated.
Pd(II) dicarbene complex grafted on hierarchical ZSM-5 were found to be efficient and recyclable cata-
lysts for C C coupling reactions. Catalysts were found to be stable and show negligible loss in activity
even after the five cycles.
Received 9 November 2011
Received in revised form 8 January 2012
Accepted 14 January 2012
Available online 26 February 2012
Keywords:
Pd–NHC complex
Ionic liquids
Hierarchical ZSM-5 grafted ionic liquids
Heck reaction
© 2012 Elsevier B.V. All rights reserved.
Sonogashira reaction
1. Introduction
palladium catalyst for these reactions. The use of heterogeneous
Pd catalysts is a promising solution to this problem because of
Palladium catalyzed carbon–carbon cross-coupling reactions
exemplify one of the important processes in organic chemistry
[1,2]. The Heck, Suzuki, Stille, Sonogashira, Negishi, and Hiyama
reactions are among the most widely used reactions for the for-
mation of carbon–carbon bonds [3,4]. The reaction is powerful
because, along with broad versatility, they generally tolerate the
presence of functional groups in the coupling partners, and thus
do not require protection and de-protection of functional groups in
the reagents. The reaction is catalyzed by Pd(0) formed in situ while
a base is required to neutralize the hydrogen halide produced as a
by-product [1–3]. Costly phosphines based Pd complex are more
suitable to catalyze these reactions because phosphines stabilizes
the in situ formed soluble Pd(0) complexes, which are generally
considered as the catalytically active species [5]. Economic and
environmental concerns makes researchers look for phosphine-
free catalysts. Unfortunately, such catalysts are prepared by costly
multistep synthetic routes and the ligands are sensitive to air and
moisture [6–10]. Furthermore, it may be noted that the practical
limitation of performing homogeneously catalyzed reactions is the
difficulty of recovering and reusing the costly Pd catalyst. This limit
is of significant environmental and economic concern in large scale
organic syntheses. It is desirable to develop a more robust, simple,
cost-effective, non phosphine ligand containing, and a recyclable
their easy separation and recyclability [4,11–13]. To minimize the
cost of production, efforts are in progress to (1) replace aryl iodides
with the cheaper bromide/chlorides, (2) eliminate the use of phos-
phine ligands, (3) employ waste-free production methods by using
aromatic anhydrides as aryl donors, and (4) utilize heterogeneous,
reusable catalysts in place of the present homogeneous systems. In
general, the essential requirement for the Pd catalyzed C C cou-
pling reaction includes a base, a ligand-stabilized active Pd species,
and a reaction medium. This led us to design and synthesize ionic
liquid based Pd catalyst.
The use of ionic liquids (ILs) has received more atten-
tion as eco-friendly, reusable, and alternative reaction media in
organic synthesis because of their unique properties, such as
high thermal and chemical stability, negligible vapour pressure,
no-flammability, high loading capacity and excellent electrical con-
ductivity [14]. Most of the common ionic liquids are based on
the nitrogen-containing heterocyclic cations such as imidazolium,
benzimidazolium and pyridinium [15]. Functionalization ability of
these heterocyclic cations forms new families and generations of
ILs with more specific and targeted properties [16,17]. Recently, the
synthesis of “task-specific” ILs with special functions according to
the requirement of a specific reaction has become an attractive field
[18,19]. The N-heterocyclic carbenes (NHC) ligands have emerged
as an alternative to phosphines as ligands for this transformation
[20,21]. Recent research in the area of ionic liquids prompted us
to develop several imidazole base NHC ligands and utilize them
in the C C coupling reactions. Several studies have appeared that
∗
Corresponding author. Tel.: +91 1881 242175; fax: +91 1881 223395.
E-mail address: rajendra@iitrpr.ac.in (R. Srivastava).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2012.01.014

190
R. Kore et al. / Catalysis Today 198 (2012) 189–196
IL1–Pd: IR (KBr, ꢀ, cm−1) = 3445, 3134, 3094, 3076, 2933, 2854,
1647, 1560, 1454, 1366, 1157, 1096, 838, 740, 713, 638, 467. 1H
NMR (DMSO-d6): ı (ppm) 7.64 (m, 4H), 7.41 (m, 10H), 5.46 (s, 4H)
4.23 (t, 4H) 1.86 (m, 4H) 1.35 (m, 4H).
describe applications of bridged dicarbene Pd(II) complexes in such
C couplings [22–25], but to the best of our knowledge, a com-
bined comparison on the spacer and cyclization in ligand, Pd source,
and surface grafting on mesoporous zeolite have not been reported
yet.
Herein, we report the synthesis and characterization of Pd(II)
dicarbene complexes, prepared from imidazole based cyclic (with
spacer) and acyclic (with and without spacer) ionic liquids, and a
comparison of their catalytic activities in the Heck and the Sono-
gashira reactions of bromobenzene/chlorobenzene. Further, these
ligands (with and without spacer) were grafted on hierarchical
ZSM-5 surface and their catalytic activity and recyclability were
investigated.
C
IL2–Pd: IR (KBr, ꢀ, cm−1) = 3444, 3132, 3096, 3076, 2933, 2855,
1642, 1560, 1455, 1366, 1158, 1096, 838, 740, 713. 1H NMR (DMSO-
d6): ı (ppm) 7.84 (m, 4H), 4.24 (m, 8H), 1.87 (m, 8H) 1.37 (m, 8H).
IL3–Pd: IR (KBr, ꢀ, cm−1) = 3445, 3134, 3090, 3079, 2930, 2853,
1646, 1562, 1498, 1454, 1357, 1163, 1096, 853, 755, 740, 721, 702,
623, 462. 1H NMR (DMSO-d6): ı (ppm) 7.23 (m, 4H), 7.36–7.21 (m,
10H), 5.11 (bs, 4H) 3.85 (bs, 6H).
2.2. Synthesis of hierarchical ZSM-5 grafted ionic liquid based Pd
catalysts
2. Experimental
For the synthesis of hierarchical ZSM-5 grafted ionic liquid based
Pd catalysts, first hierarchical ZSM-5 and triethoxysilane containing
ionic liquids (Scheme 1) were synthesized.
2.1. Synthesis of ionic liquids based Pd catalysts
For the synthesis of imidazolium based ligands, first N,Nꢀ-
hexamethylenebis(imidazole) was prepared (Scheme 1). In a two
necked round-bottomed flask, K2CO3 (75 mmol), 40 ml acetonitrile
and imidazole (50 mmol) were added and the reaction mixture
was refluxed for 30 min. 1,6-Dichlorohexane (25 mmol) was sub-
sequently added drop wise over a period of 30 min and the mixture
was refluxed for another 12 h. After the reaction mixture was cooled
to room temperature and solvent was removed under reduced
pressure. Water was added to the reaction mixture. The reaction
mixture was then extracted three times with dichloromethane.
Organic phases were combined and dried over sodium sulfate and
solvent was removed by distillation (Yield = 84%).
2.2.1. Synthesis of hierarchical ZSM-5
Hierarchical ZSM-5 was synthesized by following a reported
procedure using [(CH3O)3SiC3H6N(CH3)2C16H33 Cl] (60 wt%
methanol solution) hereafter referred as TPHAC, as a structure
directing agent [27,28]. 0.48 g of sodium aluminate (53 wt% Al2O3,
43 wt% Na2O, Riedel–deHaën) was dissolved in 32 g of water
(solution A). 3.76 g of NaOH was dissolved in 20 g of water and
5.20 g organosilane (70% methanol solution) was added (solution
B). 3.35 g of tetrapropylammoniumbromide (TPABr, Aldrich) was
dissolved in 140 g water and 3.24 g of H2SO4 was added to it
(solution C). 17.8 g Ludox (AS-40 colloidal silica, 40 wt% suspension
in water, Aldrich) and solution B was added simultaneously to
solution A. Then, solution C was added to resultant synthesis
gel and vigorously stirred for several hours until a homogeneous
mixture was obtained. The final molar composition of the synthesis
mixture was 1Al2O3/5TPABr/20Na2O/47.5SiO2/2.5TPHAC/4500
H2O/13H2SO4 (input Si/Al = 25). The resultant mixture was trans-
ferred into a stainless steel autoclave and hydrothermally treated
at 443 K under stirring for 3 days. After cooling the autoclave to
room temperature, the product was suspended in water, filtered
by suction, re-suspended in water and filtered again. Then, the
product was dried at 373 K for 10 h. Organic moieties was removed
by calcinations at 823 K for 4 h.
N,Nꢀ-hexamethylenebis(imidazole) was used as a precursor to
prepare IL1 and IL2 by following a reported procedure [26]. In a typ-
ical synthesis, N,Nꢀ-hexamethylenebis(imidazole) (10 mmol) and
toluene (30 ml) were taken in a round bottom flask. Benzyl chloride
(20 mmol)/1,6-dichlorohexane (10 mmol) was added drop wise to
the reaction mixture. The reaction mixture was refluxed for 24 h
under stirring. Upon completion of the reaction, the solvent was
evaporated under vacuum. The residue was washed with ethyl
acetate 3–4 times and then dried under vacuum at 353 K (Yield:
IL1 = 96%, IL2 = 92%).
IL1: M. P. = 468 K. IR (KBr, ꢀ, cm−1) = 3450, 3128, 3059, 2943,
2860, 1559, 1453, 1317, 1152, 1089, 1027, 851, 521, 765, 719, 646.
1H NMR (D2O) ı = 8.99 (s, 2H), 7.55 (m, 14H), 5.12 (s, 4H), 3.79–3.77
(m, 4H), 2.0–1.98 (m, 4H), 1.44–1.43 (m, 4H). 13C NMR ı = 136.94,
135.93, 135.71, 132.84, 130.42, 127.75, 126.27, 120.13, 119.9, 60.19,
48.00, 28.54, 24.58. Elemental analysis for C26H32N4Cl2: theoretical
(%): C 66.24, H 6.79, N 11.89; experimental (%): C 66.19, H 6.88, N
11.76.
2.2.2. Synthesis of triethoxysilane containing ionic liquids
(SIMIL1 and SIMIL2)
N-methylimidazole (20 mmol) or N,Nꢀ-hexamethylenebis
(imidazole)
(10 mmol)
was
reacted
with
(3-
chloropropyl)triethoxysilane (20 mmol) in toluene (40 ml).
Before heating the reaction mixture, the round bottom flask
containing reaction mixture was purged with N2 for 20–25 min.
The reaction mixture was heated at 373 K for 72 h under stirring.
Upon completion of the reaction, the residue was washed with
ether (3–4 times) and then dried under vacuum at 353 K to afford
SIMILs (Scheme 1) [30] (Yield: SIMIL1 = 89%, SIMIL2 = 82%).
IL2: M. P. = 540 K. IR (KBr, ꢀ, cm−1) = 3415, 3130, 3092, 2979,
2934, 2858, 1598, 1564, 1452, 1321, 1292, 1159, 868, 748, 655.
1H NMR (D2O) ı = 8.72 (bs, 2H), 7.41 (m, 4H), 4.20–4.08 (m, 8H),
1.79–1.77 (m, 8H), 1.26 (m, 8H). 13C NMR ı = 135.07, 123.75, 49.40,
29.03, 24.84. Elemental analysis for C18H30N4Cl2: theoretical (%): C
57.91, H 8.04, N 15.01; experimental (%): C 57.27, H 8.43, N 14.73.
IL3 was synthesized from N-methyl imidazole by following the
reported procedure (Scheme 1) [18].
Ionic liquids (IL1/IL2 (2 mmol), IL3 (4 mmol)) were taken in
25 ml of acetonitrile, which was followed by the addition of 2 mmol
of PdCl2 under nitrogen atmosphere. The reaction was continued
at reflux condition for 24 h under nitrogen atmosphere. The solvent
was removed using rotary evaporator and dried under vacuum to
afford Pd complexes (Scheme 1). Pd complex derived from IL1, IL2,
IL3 using PdCl2 are designated as IL1–Pd, IL2–Pd, IL3–Pd. To study
the influence of Pd source, complex was also prepared using IL3
and Pd(OAc)2, which is designated as IL3–Pd(I).
2.2.3. Synthesis of SIMIL-Pd-grafted hierarchical ZSM-5 catalysts
After full dehydration at 673 K, hierarchical ZSM-5 (1.0 g)
was refluxed for 24 h in 10 ml toluene solution containing
SIMIL1/SIMIL2 (2 mmol). The product was filtered, washed with
CH2Cl2 (10 ml) three times, and dried under vacuum at 353 K. 1 g
of the resultant product was added into 10 ml CHCl3 solution con-
taining PdCl2 (0.1 g) and reacted for 6 h at reflux condition. The
mixture was filtered, washed with CHCl3 (10 ml × 5) and dried at
353 K under vacuum. Amount of Pd grafted on hierarchical ZSM-5
was determined from ICP analysis.

R. Kore et al. / Catalysis Today 198 (2012) 189–196
191
H
N
N
Cl
Pd
N
N
N
N
N
c
Cl
N
N Cl
PhH2C
N
IL1-Pd
a
Cl
CH2Ph
CH2Ph
PhH2C
IL1
N
N
N
N
b
Cl
Pd
Cl
N
N
N
N
c
Br
N
N
N Br
N
IL2-Pd
IL2
Ph
Ph
N
N
N
N
Cl
Pd
Cl
a
c
IL3-Pd
N
N
Cl
N
N
IL3
Ph
O
Si
N
N
O
EtO
OEt
OEt
Hier-
ZSM-5
O
c
d
N
Si
N
Cl Pd Cl
N
Si
EtO
N
N
Cl
OEt
O
N
Cl
O
O
N
N
Si
EtO
SIMIL1
N
Hier-ZSM-5-SIMIL1
Hier-ZSM-5-SIMIL1-Pd
OEt
OEt
O
O
Si
Si
N
N
N
N
N
N
N
N
N
Si
EtO
Si
OEt
O
Cl
O
Cl
EtO
Hier-
ZSM-5
d
c
Cl
Cl Pd
N
O
O
O
O
OEt
OEt
OEt
N
N
N
N
N
Si
EtO
Si
EtO
Cl
Cl
SIMIL2
Hier-ZSM-5-SIMIL2
Hier-ZSM-5-SIMIL2-Pd
OEt
Si
CH2Cl
Br(CH2)6Br
c = PdCl2
a =
b =
d = Cl
OEt
OEt
Scheme 1. Schematic representations of homogenous and hierarchical ZSM-5 grafted Pd(II) dicarbene complexes.
2.3. Catalytic reactions
catalyst and chlorobenzene was 0.01. The reaction mixture was
withdrawn periodically and analyzed by GC.
2.3.1. Reaction condition for the Heck coupling reaction
The Heck reaction was carried out at 413 K under nitrogen
atmosphere using a 50 ml round bottom flask equipped with
condenser. The catalyst (0.02 mmol) was added to a mixture of bro-
mobenzene/chlorobenzene (2.0 mmol), alkene (4.0 mmol), K2CO3
(4.0 mmol) and dimethylformamide (5 ml). The reaction mixture
was withdrawn periodically and analyzed by gas chromatogra-
phy (Yonglin 6100; BP-5; 30 m × 0.25 mm × 0.25 ␮m). The products
were identified by GC-MS and authentic samples obtained from
Aldrich.
2.4. Catalyst characterizations
Fourier transform infrared spectra (FT-IR) were recorded using
Bruker TENSOR-27 spectrometer (spectral resolution = 4 cm−1
;
number of scans = 100). BRUKER DSX 300-Solid State NMR spec-
trometer was used to characterize solid samples. X-ray diffraction
(XRD) patterns were recorded in the 2ꢁ range of 5–50◦ with a scan
speed of 2◦/min on a PANalytical X’PERT PRO diffractometer using
Cu K␣ radiation (ꢂ = 0.1542 nm, 40 kV, 20 mA) and a proportional
counter detector. Nitrogen adsorption measurement at 77 K was
performed by Autosorb-IQ Quantachrome Instruments volumetric
adsorption analyzer. Samples were out-gassed at 573 K/423 K (for
grafted samples) for 2 h in the degas port of the adsorption appara-
tus. The specific surface area was determined by BET method using
the data points of P/P0 in the range of about 0.05–0.3. The pore
diameter was estimated using the Barret–Joyner–Halenda (BJH)
model.
2.3.2. Reaction condition for the Sonogashira coupling reaction
The Sonogashira reaction was carried out at 343 K under
nitrogen atmosphere using
equipped with condenser. Catalyst (0.02 mmol) was added
to mixture of chlorobenzene (2.0 mmol), phenylacetylene
a
50 ml round bottom flask
a
(2.4 mmol), CuI (0.04 mmol), K2CO3 (4.0 mmol) and solvent
(H2O/dimethylformamide = 2/2 in ml). The molar ratio between

192
R. Kore et al. / Catalysis Today 198 (2012) 189–196
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
400
Hier-ZSM-5-SIMIL2-Pd
(after 5th recycle)
350
300
250
200
150
100
0
10
20
30 40
50
Pore diameter (nm)
Hier-ZSM-5
0.0
0.2
0.4
0.6
0.8
1.0
10
20
30
40
50
Relative pressure (P/P0)
2θ (degree)
Fig. 1. XRD pattern of calcined hierarchical ZSM-5 and recovered Hier-ZSM-5-
Fig. 2. N2-adsorption desorption isotherm of hierarchical ZSM-5. Inset shows pore
SIMIL2-Pd after 5th recycle.
size distribution.
with silanol groups at the wall surface. Due to the large meso-
pore surface area, the amount of silanol groups is very high when
compared with the external surface of conventional ZSM-5. Reac-
tions between zeolite silanol groups and silane contained ionic
liquids were carried out by refluxing in toluene solution, which are
similar to the conventional functionalization of mesoporous silica
reported previously [29,31]. To prevent moisture contamination,
zeolites were immediately used after calcinations in air at 673 K
and anhydrous toluene was used as a solvent. SIMILs grafting on
hierarchical ZSM-5 was confirmed by elemental analysis (Table 1),
FT-IR, and 13C MAS NMR. FT-IR of hierarchical ZSM-5 functional-
ized SIMILs shows characteristic bands at 3155, 3095, 2930, 2855,
1565, 1460 cm−1, which signifies the presence of imidazole based
ionic liquids grafted on hierarchical ZSM-5 (Fig. 3). Strong FT-IR
band at 2960 and 2855 cm−1 further signify that more numbers
of CHx groups are present in the Hier-ZSM-5-SIMIL2 compared to
3. Results and discussion
Pd based ionic liquid catalysts were prepared from N-methyl
imidazole as starting materials (Scheme 1). Ligands were char-
acterized using various spectroscopic tools such as FT-IR, NMR,
elemental analysis etc. All Pd(II) complexes have been obtained as
air and moisture-stable orange–yellow solids. Their formation as
Pd-complex is generally indicated by the absence of the downfield
signal (≈9 ppm) typical for their precursors in the 1H NMR spectra.
Organo-functionalization of mesoporous silica is well doc-
umented in the literature whereas organo-functionalization of
zeolite is less known due to small external surface area of micro-
porous zeolites [27]. Hierarchical ZSM-5 is having large external
surface area, which makes it ideal support for surface functional-
ization through silanol groups. Very recently, it has been reported
that more amount of organic compound can be functionalized on
hierarchical ZSM-5 surface when compared with SBA-15 [13]. This
study encouraged us to choose hierarchical ZSM-5 as support for
the functionalization of ionic liquids. Our current research work is
focused on the synthesis and applications of hierarchical zeolites
[26,30], hence through this study, we demonstrate the applica-
bility of hierarchical ZSM-5 for organo-functionalization, which
is not much explored. It may be noted that zeolite was not con-
verted into H-form (acidic form) of zeolite. Calcined (Na form
of) ZSM-5 was chosen for the study. Triethoxysilane containing
ionic liquids were prepared from N-methylimidazole and were
grafted on hierarchical ZSM-5 (Scheme 1). Hierarchical ZSM-5 had
the MFI framework structure with high phase purity, which was
confirmed by using XRD (Fig. 1). The XRD pattern of hierarchi-
cal ZSM-5 is broad in nature, which confirms the nanocrystalline
nature of material. The N2-adsoption isotherm of hierarchical ZSM-
5 showed a typical type-IV isotherm with H1 hysteresis loop similar
to the mesoporous materials (Fig. 2). The isotherm of hierarchical
ZSM-5 exhibits a distinct increase of N2 adsorption in the region
0.4 < P/P0 < 0.85, which is interpreted as capillary condensation in
mesoporous void spaces. The mesopore walls are composed of
microporous crystalline zeolite frameworks, which are terminated
Hier-ZSM-5-SIMIL2
Hier-ZSM-5-SIMIL1
4000
3500
3000
2500
2000
1500
Wavenumber (cm-1)
Fig. 3. FT-IR of SIMILs grafted on hierarchical ZSM-5.

R. Kore et al. / Catalysis Today 198 (2012) 189–196
193
Table 1
Physico-chemical properties of hierarchical ZSM-5 and hierarchical ZSM-5 grafted ionic liquids based Pd complexes.
Material
SBET (m2/g)
Total pore
volume (ml/g)
Mesopore
volume (ml/g)
Elemental analysis
(wt%) (C, H, N)
Organic content in
(mmol/g of material)
Pd content (mmol/g)
Hierarchical ZSM-5
Hier-ZSM-5-SIMIL1
Hier-ZSM-5-SIMIL2
Hier-ZSM-5-SIMIL1-Pd
Hier-ZSM-5-SIMIL2-Pd
Hier-ZSM-5-SIMIL2-Pd (after 5th reuse)
580
375
330
–
–
335
0.49
0.38
0.35
–
–
0.36
0.37
0.24
0.21
–
–
0.20
(0.7, 1.5, 0.2)
(13.7, 3.5, 3.5)
(16.3, 4.0, 3.7)
–
–
–
1.20
0.59
–
–
–
0.40
0.58
0.57
–
(15.8, 3.8, 3.6)
Hier-ZSM-5-SIMIL1. Confirmation of SIMILs grafting on hierarchi-
cal ZSM-5 was also investigated using 13C MAS NMR (Fig. 4). Peak
positions of different kind of carbons present in SIMILs are labeled
in 13C MAS NMR spectra (Fig. 4). Surface area and pore volume of
surface grafted hierarchical ZSM-5 were found to be less than the
parent hierarchical ZSM-5 (Table 1), which signify the grafting of
SIMILs through the silanol group present in the sample.
Catalytic activity of ionic liquid based Pd complexes and hier-
archical ZSM-5 grafted ionic liquid based Pd complexes were
evaluated in the Heck and the Sonogashira coupling reactions
(Scheme 2). It is well known that activation of bromoben-
zene/chlorobenzene is difficult compared to costly iodobenzene.
The low reactivity of chlorides/bromides is usually attributed to
IL3–Pd were almost equally active, whereas the activity of IL2–Pd
was ½ times lower than the activity of other two catalysts (IL1–Pd
and IL3–Pd) (Table 2). To understand the reason behind the low
activity of IL2–Pd, structure of IL2–Pd and IL1–Pd were geomet-
rically optimized using B3LYP/SDD basis set method (Fig. 5 and
Table 3) [33]. Accessibility of the reactant molecules to the close
pack structure of IL2–Pd seems to be more difficult than the open
chain IL1–Pd catalysts, which is responsible for its low activity. It
may be further noted that IL3–Pd(I) was found to be less active
than IL3–Pd, which signifies that the complex prepared with PdCl2
was more active than Pd(OAC)2. It was also observed that IL3–Pd(I)
is sticky solid whereas IL3–Pd is powder in nature, which makes
IL3–Pd(I) less user friendly. Bromobenzene can be easily converted
to C C coupled product using these catalysts by following the Heck
reaction. Activities of these catalysts were very low for chloroben-
zene due to high C Cl bond dissociation energy (Table 2). It was
very difficult to recycle these ionic liquid based Pd complexes. To
make the system recoverable and recyclable, ionic liquid complex
was grafted on to the mesopore wall of the hierarchical ZSM-5.
For comparative study, progress of the Heck reaction between bro-
mobenzene and styrene over IL1–Pd and Hier-ZSM-5-SIMIL2-Pd
the strength of the C X bond (bond dissociation energies for Ph
X
– Cl: 96 kcal mol−1; Br: 81 kcal mol−1; I: 65 kcal mol−1) [32], which
leads to reluctance by aryl chlorides/bromides to oxidatively addi-
tion to catalytically active Pd centers, a critical initial step in
palladium-catalyzed coupling reactions. In this study, the Heck and
the Sonogashira coupling reactions were carried out using cheap
and deactivated arylhalides such as bromobenzene and chloroben-
zene. Among the three ionic liquid based Pd complexes, IL1–Pd and
4
5
1
3
O
O
N
N
9
Si
2
6
10
Cl
O
11
7
8
O
N
N
Si
O
Cl
C6
C1
O
200
180
160
140
120
100
80
60
40
20
0
4
3
1
O
5
N
Si
2
O
N
O
6
C6
C1
C9
7
9
Cl
8
200
180
160
140
120
100
80
60
40
20
0
δ (ppm)
Fig. 4. 13C MAS NMR of SIMILs grafted on hierarchical ZSM-5.

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R. Kore et al. / Catalysis Today 198 (2012) 189–196
R
X
R
Catalyst (1 mol %)
K2CO3, DMF, 413 K
R = -Ph, -COOCH3
X = Br, Cl
Heck Reaction
Cl
Catalyst (1 mol %)
K2CO3, DMF/H2O,
CuI, 343 K
Sonogashira Reaction
Scheme 2. Heck and Sonogashira reaction of halo benzene using homogenous and hierarchical ZSM-5 grafted Pd(II) dicarbene complexes.
Table 2
Heck reaction of ionic liquid based Pd complexes and hierarchical ZSM-5 grafted heterogeneous ionic liquid based Pd catalysts.
E. No.
Catalyst
Aryl halide
Alkene
Product
Yield (%)
TON
1
2
3
4
5
6
7
8
8
IL1–Pd
IL1–Pd
IL1–Pd
IL2–Pd
IL2–Pd
IL2–Pd
IL3–Pd
IL3–Pd(I)
IL3–Pd
IL3–Pd
Hier-ZSM-5-SIMIL1-Pd
Hier-ZSM-5-SIMIL1-Pd
Hier-ZSM-5-SIMIL2-Pd
Hier-ZSM-5-SIMIL2-Pd
Bromobenzene
Bromobenzene
Chlorobenzene
Bromobenzene
Bromobenzene
Chlorobenzene
Bromobenzene
Bromobenzene
Bromobenzene
Chlorobenzene
Bromobenzene
Bromobenzene
Bromobenzene
Bromobenzene
Styrene
Methyl acrylate
Styrene
Styrene
Methyl acrylate
Styrene
Styrene
Styrene
Methyl acrylate
Styrene
Styrene
Stilbene
Methyl cinnamate
Stilbene
Stilbene
Methyl cinnamate
Stilbene
Stilbene
Stilbene
Methyl cinnamate
Stilbene
Stilbene
92
90
6
45
42
2
99
85
99
7
55
52
80
76
92
90
6
45
42
2
99
85
99
7
55
52
80
76
9
10
11
12
13
Methyl acrylate
Styrene
Methyl acrylate
Methyl cinnamate
Stilbene
Methyl cinnamate
Reaction conditions: halobenzene (2 mmol), alkene (4 mmol), K2CO3 (4 mmol), DMF (5 ml), catalyst (0.02 mmol), temperature (413 K), reaction time (12 h for bromobenzene
and 36 h for chlorobenzene).
Table 3
were monitored using GC (Fig. 6). Though the activity of Hier-ZSM-
The geometry parameters of IL1–Pd and IL2–Pd calculated at B3LYP/SDD.
5-SIMIL2-Pd was found to be less compared with homogeneous
catalyst (IL1–PdIL1–Pd), but they were found to be recyclable.
Among the two catalysts, Hier-ZSM-5-SIMIL1-Pd was found to
be less active. However, it may be noted that in the homoge-
nous medium, both catalysts exhibited identical activity (Table 2).
Elemental analysis and ICP analysis (amount of Pd) were able to
explain the difference in their activity. Elemental analyses revealed
that SIMIL1 was grafted two times more compared to SIMIL2 on
the hierarchical ZSM-5. However, the amounts of Pd were found to
be less in case of Hier-ZSM-5-SIMIL1-Pd compared to Hier-ZSM-5-
SIMIL2-Pd. This observation indicates that almost all surface grafted
SIMIL2 was able to form Pd–NHC complex, whereas some of the
SIMIL1 grafted on hierarchical ZSM-5 was not able to form Pd–NHC
complex. It may be noted that for SIMIL1 ligand to form Pd–NHC
complex, two SIMIL groups should be present in their vicinity. How-
ever, it seems that all SIMIL1 were unable to find partner in their
vicinity, and hence they were left alone without forming a complex.
Further, when their activities were compared based on the amount
of Pd present in these two solid catalysts, they were found to be
equally active (Table 2).
Observed parameters
IL1–Pd
IL2–Pd
Pd Cl (Å)
N2(CH) Pd (Å)
Pd H (Å)
2.382, 2.378
2.041, 2.060
Pd H19 (2.672)
Pd H17 (2.489)
−2274.796
4.241
2.44, 2.315
2.00, 2.00
Pd H17 (2.889)
Pd H44 (2.539)
−1969.354
2.617
Energy (a.u.)
Dipole moment (Debye)
activity of various catalysts investigated in this study for the Sono-
gashira and the Heck reaction were found to be similar (Table 2 and
Table 4).
For practical applications of heterogeneous systems, the life-
time of the catalyst and its level of reusability are very important
Table 4
Sonogashira reaction of chlorobenzene using homogenous and hierarchical ZSM-5
grafted Pd(II) dicarbene complexes.
E. No.
Catalyst
Yield (%)
TON
1
2
3
4
5
IL1–Pd
IL2–Pd
IL3–Pd
Hier-ZSM-5-SIMIL1-Pd
Hier-ZSM-5-SIMIL2-Pd
76
30
80
50
72
76
30
80
50
72
The utility of the homogenous and heterogeneous Pd catalysts
was also explored for the Sonogashira reaction using chloroben-
zene (Scheme 2). We found that the IL1–Pd is most efficient for
Sonogashira reaction in presence of K2CO3 as a base and DMF/water
(1:1) as a solvent. Under the optimized reaction conditions, the effi-
ciency of all catalysts were studied for the Sonogashira reaction and
the results are summarized in Table 4. The trends in the catalytic
Reaction conditions: chlorobenzene (2 mmol), phenylacetylene (2.4 mmol), K2CO3
(4 mmol), CuI (0.04 mmol) DMF/H2O (2 ml/2 ml), catalyst (0.02 mmol), temperature
(343 K), reaction time (12 h).

R. Kore et al. / Catalysis Today 198 (2012) 189–196
195
100
80
60
40
20
IL1-Pd
Hier-ZSM-5-SIMIL2-Pd
0
0
2
4
6
8
10
12
Time (h)
Fig. 6. Catalytic activity of IL1–Pd and Hier-ZSM-5-SIMIL2-Pd in the Heck reaction
of bromobenzene and styrene.
conversion) followed by an additional 6 h of reaction time gave a
final conversion of 47%. This experiment confirms that the catalyst
was not leached out during the reaction. Pd leaching was also con-
firmed by the ICP analysis of the reaction mixture containing no
catalyst. These two results confirm that the catalyst does not leach
during the reaction.
4. Conclusions
In conclusion, new concepts for the design and synthesis of
highly active and recyclable heterogenized Hier-ZSM-5-SIMIL-Pd
catalysts have been developed. The catalyst is highly efficient
and active for cheap and deactivated aryl halides such as bro-
mobenzene and chlorobenzene. Activity in the Heck reaction using
bromobenzene is very high compared to chlorobenzene. This is
due the high dissociation energy of Ph Cl compared to Ph Br.
Catalysts exhibited remarkably high activity for the Heck and the
Sonogoshira reaction. Homogenous catalysts were not possible
to recycle whereas heterogeneous Hier-ZSM-5-SIMIL-Pd catalysts
exhibited high stability during the recycling experiments. DFT cal-
culation was able to establish the structure activity relationship.
Fig. 5. Optimized molecular structure of IL1–Pd and IL2–Pd using B3LYP/SDD.
factors. To clarify this aspect, a set of experiments were established
using the recycled catalyst for the Heck reaction of bromobenzene
and styrene. The reactions were conducted under similar condi-
tions (413 K/12 h) in DMF using triethylamine as base (instead of
K2CO3). After 12 h of the first reaction, 88% yield of stilbene was
obtained. The catalyst was recovered by centrifugation, washed
with DMF and acetonitrile, and finally dried at 373 K for 4 h. A fresh
reaction was then performed under same conditions. Although the
color of catalyst was changed from orange to orange-brown, the
Hier-ZSM-5-SIMIL2-Pd could be used for at least five times with-
out significant change in activity and stilbene yield of 82% was
obtained after five cycles. It may be noted that due to loss of catalyst
amount in recovery, successive runs were done using less amount
of bromobenzene, but by keeping the bromobenzene to catalyst
ratio constant during fresh and recycling reactions. No significant
change in the physico-chemical characterizations of the recovered
catalyst after 5th recycle was observed (Fig. 1 and Table 1).
Acknowledgments
The authors thank the Department of Science and Technology,
New Delhi, for financial assistance (SR/S1/PC/31/2009). Authors
thank Sophisticated Instrumental Facility, Indian Institute of Sci-
ence Bangalore for NMR analysis of solid samples.
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