Ultra-small and highly dispersed Pd nanoparticles inside the pores of ZIF-8: Sustainable approach to waste-minimized Mizoroki–Heck cross-coupling reaction based on reusable heterogeneous catalyst

Article abstract of DOI:10.1002/aoc.4952

The rapid development of nanomaterials, particularly advanced hybrid nanoparticles, has made new opportunities for the design and fabrication of high-performance metal-based catalysts. However, generating metal nanoparticles of desired size without aggregation is an important challenge for enhancing the catalytic activity of metal nanoparticles supported in the host matrix. In this work, a hybrid nanoporous material, namely Pd nanoparticles@N-heterocyclic carbene@ZIF-8, with a high internal surface area was successfully prepared using a dispersed anionic sulfonated N-heterocyclic carbene–Pd(II) precursor inside the cavities of zeolitic imidazolate framework (ZIF-8) using an impregnation approach followed by reduction with NaBH₄. The anionic sulfonated N-heterocyclic carbene was found to be a superb ligand for the stabilization of Pd nanoparticles in the pores of ZIF-8. The resulting system was applied to the Mizoroki–Heck cross-coupling reaction, in which the catalyst showed high catalytic activity under mild reaction conditions.

Full text of DOI:10.1002/aoc.4952

Received: 20 December 2018
DOI: 10.1002/aoc.4952
Revised: 5 March 2019
Accepted: 6 March 2019
F U L L P A P E R
Ultra‐small and highly dispersed Pd nanoparticles
inside the pores of ZIF‐8: Sustainable approach to
waste‐minimized Mizoroki–Heck cross‐coupling reaction
based on reusable heterogeneous catalyst
Mohammad Azad | Shahnaz Rostamizadeh
| Hamid Estiri | Fatemeh Nouri
Faculty of Chemistry, K. N. Toosi
The rapid development of nanomaterials, particularly advanced hybrid nano-
particles, has made new opportunities for the design and fabrication of high‐
performance metal‐based catalysts. However, generating metal nanoparticles
of desired size without aggregation is an important challenge for enhancing
the catalytic activity of metal nanoparticles supported in the host matrix. In this
work, a hybrid nanoporous material, namely Pd nanoparticles@N‐heterocyclic
carbene@ZIF‐8, with a high internal surface area was successfully prepared
using a dispersed anionic sulfonated N‐heterocyclic carbene–Pd(II) precursor
inside the cavities of zeolitic imidazolate framework (ZIF‐8) using an impregna-
tion approach followed by reduction with NaBH4. The anionic sulfonated
N‐heterocyclic carbene was found to be a superb ligand for the stabilization of
Pd nanoparticles in the pores of ZIF‐8. The resulting system was applied to the
Mizoroki–Heck cross‐coupling reaction, in which the catalyst showed high
catalytic activity under mild reaction conditions.
University of Technology, PO Box 15875‐
4416, Tehran, Iran
Correspondence
Shahnaz Rostamizadeh, Faculty of
Chemistry, K. N. Toosi University of
Technology, PO Box 15875‐4416, Tehran,
Iran.
Email: rostamizadeh@kntu.ac.ir
Funding information
K. N. Toosi University of Technology; Iran
National Science Foundation
KEYWORDS
heterogeneous catalyst, hybrid nanoporous material, Mizoroki–Heck cross‐coupling reaction, ZIF‐8
1 | INTRODUCTION
which are important characteristics for achieving sustain-
able chemical processes.[1–3]
Green chemistry is the design, development and manage-
ment of chemical products and processes to decrease or
eliminate contaminating substances which are hazardous
to health and the environment by generating eco‐friendly
chemicals through green processes. Also, the recovery
and reusability of catalysts is one of the most essential
features for many catalytic processes. As a result, the
use of a heterogeneous catalyst has become an efficient
route for improvement of green chemistry. This is
because it is a favourable way to minimize the problems
of homogeneous catalysts such as chemical wastes.
Therefore, advanced investigations have focused on the
development of highly active and recoverable catalysts
Thus, in the design and synthesis of heterogeneous cat-
alysts, various porous materials including mesoporous sil-
icates,[4] covalent organic frameworks,[5] mesoporous
organic polymers[6] and especially metal–organic frame-
works (MOFs)[7,8] have been widely used as heterogeneous
catalyst supports. Among the various matrixes, porous
MOFs have attracted tremendous academic and industrial
interest in the last few decades due to the extraordinary
combination of features including well‐defined porosities,
tunable crystalline structure, high surface area and chemi-
cal tenability.[9–11] MOFs are crystalline hybrid porous
compounds which are formed by self‐assembly and bridg-
ing of metal ions and organic linkers to form one‐, two‐
Appl Organometal Chem. 2019;e4952.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4952

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AZAD ET AL.
and three‐dimensional networks.[12,13] Because of the
outstanding chemical and physical properties of MOFs,
they have become highly promising candidates for a vari-
ety of applications including gas sensing,[14] gas storage,[15]
chromatographic separation,[16] drug delivery,[17] biosens-
ing[18] and heterogeneous catalysis.[19–21] Among the
MOFs, the zeolitic imidazolate framework (ZIF‐8) is a
subclass, which consists of zinc ions linked by 2‐
described in Scheme 1. We anticipate that this strategy will
prove to be an adequately flexible, attractive and desirable
method for the synthesis of a broad range of catalytic metal
nanoparticle@NHC@MOF hybrid nanocomposites. In
connection with our particular interest in the preparation
of hybrid materials and their use as heterogeneous systems
for the production of fine chemicals, the catalytic behav-
iour of the Pd NPs@NHC@ZIF‐8 nanocomposite was
investigated in the Mizoroki–Heck cross‐coupling reac-
tion. The Pd NPs@NHC@ZIF‐8 catalyst showed various
advantages such as stability in air and moisture, easy prep-
aration, reusability, versatility and high catalytic activity.
methylimidazole ligands (Zn(MeIM)2; MeIM
= 2‐
methylimidazole). ZIF‐8 exhibits various properties
including robust thermal stability (550°C in nitrogen),
high porosity, high surface area, uniform structure cavities
and stability in refluxing organic solvents and aqueous
solution.[22] As a result of these outstanding physical and
chemical properties, ZIF‐8 provides an excellent support
platform for depositing metal nanoparticles.[23–25]
2 | EXPERIMENTAL
2.1 | Materials and methods
It is expected that small metal particles will readily
aggregate under reaction conditions within the pores of
MOFs, which originates from their high surface energy,
forming large particles which leads to a decrease in their
catalytic activity. However, the stabilization of small and
well‐defined metal nanoparticles inside the cavities of
MOFs is a significant challenge. Hence, several strategies
such as the use of functionalized MOFs, ionic liquids, sur-
factants and external ligands can be employed to assist in
the prevention of migration and agglomeration of metal
nanoparticles in the pores of MOFs.[26–29] Among
external ligands, N‐heterocyclic carbenes (NHCs) have
emerged as an essential and highly promising class of
ligands for the development of stable functionalized
materials due to their versatility, stability, accessibility,
feasibility of modification and excellent σ‐donating prop-
erties.[30,31] Accordingly, in recent years, an increasing
number of studies have been focused on the utilization
of NHCs as ligands for stabilizing nanoparticles. In this
regard, NHCs have been successfully used for the stabili-
zation of metal nanoparticles such as those of Ru, Pt, Au,
Ni and Pd.[32,33] NHCs can form strong covalent bonds
with the surface of metal nanoparticles.
Herein, we report the synthesis and characterization of
a negatively charged NHC ligand to stabilize and control
the aggregation of small palladium nanoparticles (Pd
NPs) through the pores of ZIF‐8. Because of the electro-
static interaction of negatively charged NHC, functional-
ized Pd exhibits an excellent environment for stabilizing
Pd NPs, and consequently prevents their undesired
agglomeration. The obtained Pd NPs show the smallest
sizes among metallic nanoparticles stabilized by solid sup-
port materials, especially the common MOFs reported to
date.[34–37] On the basis of transmission electron micros-
copy (TEM) analysis, ultra‐small Pd NPs were well dis-
persed into the cavities of ZIF‐8. The process for the
fabrication of Pd NPs@NHC@ZIF‐8 nanocomposite is
All chemicals were purchased from commercial sources
(Aldrich and Merck) and used without further
purification. These included Zn(NO3)2⋅6H2O (98%,
Sigma‐Aldrich), 2‐methylimidazole (99%, Aldrich), 1‐
methylimidazole (99%, Sigma‐Aldrich), palladium(II)
chloride (PdCl2; 99%, Sigma‐Aldrich), sodium borohyride
(NaBH4; 99%, Sigma‐Aldrich), sodium chloride (NaCl;
99%, Merck), sodium bicarbonate (NaHCO3; ≥99.7%,
Sigma‐Aldrich), N,N‐dimethylformamide (DMF; AR,
Merck), hexane (AR, Merck) and methanol (AR, Merck).
Hexane and DMF were distilled over Na/benzophenone
and CaH2, respectively, and stored over activated molecu-
lar sieves.
2.2 | Characterization
Melting points were determined using a Buchi B‐540 appa-
ratus. Infrared (IR) spectra were recorded with an ABB
Bomem model FTLA200‐100 instrument using KBr discs.
13
1H NMR and
C NMR spectra were recorded with a
Bruker DRX‐300 spectrometer, at 300 and 75 MHz, using
tetramethylsilane as internal standard. Coupling constants
(J) were reported in hertz (Hz). Elemental analysis was
performed using a PerkinElmer Series II. Powder X‐ray dif-
fraction (XRD) data were obtained with a Philips X'Pert
powder diffractometer using Cu Kα radiation in the 2θ
range from 5° to 60°. The surface area, pore volume, pore
size and pore structure of the prepared catalyst were veri-
fied by nitrogen sorption isotherm ([5.0.0.3] Belsorp, BEL
Japan, Inc.). TEM was conducted with a Zeiss‐EM10C
instrument at an accelerating voltage of 100 kV. Induc-
tively coupled plasma (ICP) analysis was performed with
a Varian Vista‐MPX instrument. Thermal behaviour of
the catalyst was investigated according to thermogravimet-
ric analysis (TGA) and differential thermal analysis using a

AZAD ET AL.
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SCHEME 1 Preparation of Pd NPs@NHC@ZIF‐8 catalyst
NETZSCH STA 409 PC/PG instrument with a heating rate
of 10°C min−1. Particle size and shape analyses were per-
formed using a scanning electron microscopy (SEM)
instrument (TESCAN VEGA II‐LMU).
was washed with diethyl ether three times and subse-
quently dried at room temperature to afford the
imidazolium salt as a white solid (2.1 g, 64%); m.p. 182–
183°C. IR (KBr, cm−1): 535, 1050, 1187, 1458, 1568,
1
1637, 3095. H NMR (300 MHz, CH3OD, δ, ppm): 1.74–
1.84 (m, 2H, NCH2CH2CH2CH2SO3), 2.07 (2H, quintet,
CH2CH2CH2SO3), 2.40 (2H, t, CH2CH2SO3), 3.94 (3H, s,
N CH3), 4.28 (2H, t, NCH2CH2), 7.58 (1H, t, Himid), 7.67
(1H, t, Himid), 8.99 (1H, s, Himid).
2.3 | Catalyst preparation
2.3.1 | Preparation of 4‐(1‐
methylimidazolium‐3‐yl)butane‐1‐sulfo-
nate (1)
2.3.2 | Synthesis of sulfonate‐
N‐Methylimidazole (15 mmol, 1.23 g) and 1,4‐
butanesultone (10 mmol, 1.36 g) were mixed in 30 ml of
dry dioxane. The reaction mixture was stirred at ambient
temperature for 1 day. After filtration, the formed solid
functionalized Pd(II)–NHC complex (2)
The NHC was generated by deprotonation of
imidazolium salts with NaHCO3. Compound 1 (1 mmol,

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AZAD ET AL.
0.218 g), PdCl2 (1.2 mmol, 0.213 g) and NaCl (1 mmol,
0.058 g) were dissolved in 5 ml of dry DMF. The suspen-
sion was stirred at 90°C for 24 h. After this period, solvent
was removed under reduced pressure. The resulting
powder was washed with excess diethyl ether to remove
excess PdCl2. Then, the yellowish green solid which was
formed was further purified by column chromatography
using solvent system of CH2Cl2–MeOH (80:20) to
afford pure complex (61%,0.29 g).[38] IR (KBr, cm−1):
460, 531, 606, 852, 1038, 1167, 1464, 1570, 1652, 2933.
1H NMR (300 MHz, CDCl3, δ, ppm): 1.68
(2H, m, NCH2CH2CH2CH2SO3), 1.97 (2H, quintet,
CH2CH2CH2SO3), 2.8 (2H, t, CH2CH2SO3), 3.8 (1H, s,
NCH3), 4.2 (2H, t, NCH2CH2), 7.35 (1H, d, Himid), 7.50
(1H, d, Himid). 13C NMR (75 MHz; CDCl3, δ, ppm): 20.9
(NCH2CH2CH2CH2SO3), 28.1 (CH2CH2CH2SO3), 35.7
with 16 ml of water four times by centrifugation and
then dried at 100°C overnight to remove the solvent.
Subsequently,
a complete reduction approach was
employed by adding a solution of NaBH4 (0.62 mmol,
0.19 g) in water (5 ml) at 0°C (ice bath) with vigorous
stirring. During this period, the mixture turned grey,
indicating the formation of Pd NPs@NHC in the cavities
of ZIF‐8 (4). Finally, the mixture was collected by centri-
fugation and washed several times with methanol. The
resulting solid was dried at 80°C for 24 h then used for
the catalytic reactions.
2.4 | General procedure for in situ Pd
NPs@NHC@ZIF‐8‐catalysed Mizoroki–
Heck cross‐coupling reaction
(NCH3), 50.9 (NCH2CH2),
51.9
(CH2CH2SO3),
122.2(Himid), 123.7 (Himid), 153.9 (Himid).Anal. Calcd for
C8H13Cl3N2Na2O3PdS (%): C 20.19, H 2.75, Cl 22.34, N
5.88, Na 9.66, O 10.08, Pd, 22.36, S 6.74. Found (%): C
20.09, H 2.73, N 5.75, S 6.76.
The catalytic activity of the Pd NPs@NHC@ZIF‐8 hybrid
nanocomposite was examined in the Mizoroki–Heck
reaction in DMF–H2O. In a typical reaction, a mixture
of olefin (1.2 mmol), aryl halide (1 mmol), Et3N
(2.0 mmol), Pd NPs@NHC@ZIF‐8 (0.08–1 mol%) and
2.0 ml of DMF–H2O (3:2) was transferred to a 10 ml
screw‐capped vial, and the reaction mixture was stirred
at 110 to 130°C (for aryl chlorides, tetrabutylammonium
bromide (TBAB; 1.0 mmol) was also added). The progress
of the reaction was monitored by TLC. After the reaction
reached completion, the mixture was cooled to room tem-
perature, the catalyst was separated by centrifugation and
the solvent was then removed using a rotary evaporator.
Ethyl acetate was next added to the organic layer, washed
with brine, dried over MgSO4, filtered and concentrated
in vacuo. The crude products were purified by preparative
2.3.3 | Preparation of ZIF‐8
Synthesis of ZIF‐8 nanocrystals was carried out based on
a previously published method.[39] Typically, zinc nitrate
hexahydrate (Zn(NO3)2⋅6H2O; 4.13 mmol, 1.17 g) was
dissolved in 40 ml of methanol in a scintillation vial
and in another vial 2‐methylimidazole (29 mmol, 2.4 g)
was dissolved in 40 ml of methanol. At this point the 2‐
methylimidazole solution was mixed with the zinc nitrate
hexahydrate solution under vigorously stirring at room
temperature for 1 h. The solution was then centrifuged
at 6000 rpm for 10 min to obtain ZIF‐8 nanocrystals,
which were subsequently washed three times with meth-
anol to eliminate completely the unreacted reactants. The
product was then dried in an oven at 100°C overnight to
remove the solvent and kept at room temperature until
further use.[39]
2.3.4 | Syntheses of Pd NPs@NHC@ZIF‐8
Encapsulation of the NHC–Pd precursor was performed
by an impregnation technique.[40] To prepare 1 wt% Pd
NPs@NHC@ZIF‐8, 200 mg of dried ZIF‐8 was dispersed
in 12 ml of water by sonication at room temperature
until it became homogeneous (ca 1 h). Subsequently,
NHC–Pd(II) (0.12 mmol, 0.05 g) was dissolved in 2 ml
of water and added dropwise over a period of 20 min
under vigorous stirring to the ZIF‐8 solution. The vial
containing the slurry was stirred at room temperature
for 20 h. The impregnated ZIF‐8 (3) sample was washed
FIGURE 1 XRD patterns of ZIF‐8 and Pd NPs@NHC@ZIF‐8
catalyst

AZAD ET AL.
5 of 13
TLC with a gradient from 1:99 to10:90 hexane–ethyl ace-
tate to afford the pure compounds.
the impregnation method. The prepared Pd
NPs@NHC@ZIF‐8 catalyst was characterized using
XRD, SEM combined with energy‐dispersive X‐ray
(EDX) analysis, ICP analysis, TEM, Brunauer–Emmett–
Teller (BET) and Langmuir surface area measurements,
TGA, IR spectroscopy and X‐ray photoelectron spectros-
copy (XPS).
In order to confirm the crystalline nature of the pre-
pared ZIF‐8 and Pd NPs@NHC@ZIF‐8, powder XRD
analysis was performed (Figure 1). The powder XRD
analyses showed that ZIF‐8 had been successfully
3 | RESULTS AND DISCUSSION
3.1 | Characterization of Pd
NPs@NHC@ZIF‐8
As shown in Scheme 1, ultrafine Pd NPs supported
on NHC@ZIF‐8 were successfully prepared using
FIGURE 2 Nitrogen absorption–desorption isotherms of (a) ZIF‐8 and (b) Pd NPs@NHC@ZIF‐8 catalyst. Pore size distributions of (c) ZIF‐
8 and (d) Pd NPs@NHC@ZIF‐8 catalyst

6 of 13
AZAD ET AL.
prepared. After the impregnation and reduction of NHC–
Pd(II) in the cavities of ZIF‐8, no obvious changes in the
XRD pattern of Pd NPs@NHC@ZIF‐8 were found which
indicated that the structure of ZIF‐8 was well preserved.
This also showed that Pd NPs@NHC were highly dis-
persed on the cavity windows of ZIF‐8.[41]
The surface areas and pore volumes were measured by
nitrogen gas adsorption–desorption isotherms at 77 K and
calculated using the BET method. Nitrogen adsorption–
desorption isotherms of pure ZIF‐8 typically were of type
I (Figure 2a), which is characteristic of a microporous
structure. The Pd NPs@NHC@ZIF‐8 catalyst displayed
typical type IV isotherm with a type H4 hysteresis loop,
indicating a mesoporous framework (Figure 2b).[42,43]
Table 1 presents the BET surface area, total pore volume
and Barrett–Joyner–Halenda (BJH) pore diameter of syn-
thesized Pd NPs@NHC@ZIF‐8 nanocomposite which
were found to be 1451 m2
g
−1, 0.89 cm3 g−1 and
2.42 cm3 g−1, respectively. The nitrogen sorption analysis
of Pd NPs@NHC@ZIF‐8 also confirms a decrease in sur-
face area, pore diameter and pore volume parameters in
comparison with that of pristine ZIF‐8. This result suc-
cessfully demonstrated incorporation of the palladium
species within the ZIF‐8 matrix. Also, the nitrogen sorp-
tion analysis showed that Pd NPs@NHC@ZIF‐8 has a
highly porous framework, which is suitable for efficient
catalytic activity.
TABLE 1 Surface area, BJH pore diameter and pore volume of
Pd NPs@NHC@ZIF‐8a
Figure 3a,b shows the elemental analysis results for
ZIF‐8 and Pd NPs@NHC@ZIF‐8. Figure 3b shows the
presence of Pd and the other elements of Pd
NPs@NHC@ZIF‐8 which agrees well with the structure
of the catalyst. In addition, EDX mapping analysis of Pd
NPs@NHC@ZIF‐8 reveals that the elements of O, S and
Surface area BJH pore Pore
Adsorbent
(m2 g−1
)
diameter
volumeb
ZIF‐8
1232
758
2.42
0.89
Pd NPs@NHC@ZIF‐8
2.1
0.4024
aPore size calculated by BET method.
FIGURE 3 EDX spectra of (a) ZIF‐8 and (b) Pd NPs@NHC@ZIF‐8. (c) Element mappings of Pd NPs@NHC@ZIF‐8 nanocomposite

AZAD ET AL.
7 of 13
Pd are uniformly distributed within the crystals of ZIF‐8
(Figure 3c). These observations confirmed that chemical
modification of the ZIF‐8 had been achieved. Also, we
used ICP‐MS to measure the actual Pd loading on ZIF‐8
for Pd NPs@NHC@ZIF‐8, in which only 0.9 wt% Pd
was detected.
Also, SEM results confirm the crystalline rhombic
dodecahedron morphology of the synthesized Pd
NPs@NHC@ZIF‐8 and the size of individual Pd
NPs@NHC@ZIF‐8 particles was found to be 50–200 nm
(Figure 4). Figure 5 shows a typical TEM image of Pd
NPs@NHC@ZIF‐8. The TEM images showed that
extremely small Pd nanoparticles were uniformly dis-
persed onto the pores of ZIF‐8.Also the TEM results
clearly demonstrated that ultrafine small Pd NPs were
trapped inside the internal cavities of ZIF‐8.
The thermal stability of Pd NPs@NHC@ZIF‐8 was
investigated through TGA under nitrogen gas feed with
a heating rate of 10°C min−1 (Figure S1). As can be seen,
Pd NPs@NHC@ZIF‐8 displayed five steps of weight loss
in the region 0 to 900°C. The initial weight loss appearing
below 100°C is about 2% which was attributed to loss of
FIGURE 5 TEM image of Pd NPs@NHC@ZIF‐8
water and physically adsorbed solvents from the porous
structure of ZIF‐8. The second weight loss is related to
the organic residues attached to the surface of the
FIGURE 4 SEM images of (a, b) ZIF‐8 and (c, d) Pd NPs@NHC@ZIF‐8

8 of 13
AZAD ET AL.
support. The observed weight loss (24.38%) between 300
and 450°C is due to thermal decomposition of the weakly
linked guest 2‐methylimidazole molecules and 4‐(1‐
methylimidazole linker during the formation of ZIF‐8.
The peak at 1584 cm−1 is due to the C═N stretch, and
also two bands at 1426 and 1308 cm−1 were associated
with C─N stretching vibration, which were typically
characteristic for ZIF‐8 (Figure S2a). The characteristic
peaks for Pd(II)–NHC at around 427, 526, 608, 1049,
1186, 1464, 1571 and 1621 cm−1 are attributed to NHC–
Pd(II), S═O asymmetric and symmetric stretching vibra-
methyl‐1H‐imidazol‐3‐ium‐3‐yl)butane‐1‐sulfonate.
580 to 900°C the decomposition of ZIF‐8 took place. As
result, the Pd NPs@NHC@ZIF‐8 composite is
At
a
extremely stable for organic processes even at high
temperatures.
−
Moreover, in order to identify all functional groups in
the resulting materials, IR spectroscopy was performed
after each step of the preparation of the catalyst (Figure
S2). In the case of the IR spectrum of ZIF‐8, the peak at
420 cm−1 is related to Zn─N stretching as the zinc atoms
in the ZIF‐8 structure connect to nitrogen atoms of 2‐
tions of the ─SO3 group and C═C, C═N stretching
vibrations of the imidazole ring (Figure S2b). After mod-
ification of Pd NPs@NHC (Figure S2c), a series of bands
appeared around 531–619 cm−1 associated with Pd─C
stretch mode. The bands emerging at 1030–1182 cm−1
are ascribed to asymmetric and symmetric stretching
TABLE 2 Optimization of reaction conditions for Heck reaction of bromobenzene with styrenea
Entry
Amount of catalyst (mol%)
Solvent
Base (1.5 mmol)
Temperature (°C)
Time (h)
Yield (%)b
1
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.08
0.1
Toluene
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
120
90
24
24
24
12
12
24
12
24
6
<5
<5
<10
73
69
<5
75
49
70
53
53
34
33
84
76
80
83
90
92
74
82
91
45
0
2
THF
3
Dioxane
4
DMSO
5
NMP
6
EtOH
7
DMF
8
H2O
9
DMF–H2O (1:1)
DMF–H2O (1:1)
DMF–H2O (1:1)
DMF–H2O (1:1)
DMF–H2O (1:1)
DMF–H2O (1:1)
DMF–H2O (2:1)
DMF–H2O (3:2)
DMF–H2O (3:2)
DMF–H2O (3:2)
DMF–H2O (3:2)
DMF–H2O (3:2)
DMF–H2O (3:2)
DMF–H2O (3:2)
Toluene–H2O
DMF–H2O (3:2)
Et3N
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
K2CO3
Na2CO3
NaOAc
K3PO4
DIPEA
Et3N
12
12
12
12
8
3
Et3N
3
Et3N
2.5
2
Et3N
Et3N
0.1
2
0.1
Et3N
5
0.1
Et3N
100
110
110
110
4
0.15
0.1
Et3N
2
Et3N
5
0
Et3N
24
aReaction conditions: 4‐bromotoluene (1 mmol), styrene (1.2 mmol), base (2 mmol), Pd NPs@NHC@ZIF‐8 and solvent (2 ml).
bIsolated yield after preparative TLC.

AZAD ET AL.
9 of 13
TABLE 3 Catalytic activity of Pd NPs@NHC@ZIF‐8 in Mizoroki–Heck cross‐coupling reaction of aryl halides with olefinsa
Entry
R1
R2
X
Time (h)
Temperature (°C)
Yield (%)b
1
H
Ph
I
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
3.5
3.5
1
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
100
110
110
110
130
130
130
130
130
94
93
94
90
95
89
88
90
88
80
86
75
81
87
86
79
89
86
90
89
64
67
69
90
76
92
89
93
84
67
68
67
68
68
66
64
54
62
2
H
CO2CH3
CO2Et
CO2Bu
CN
I
3
H
I
4
H
I
5
H
I
6
p‐CH3
p‐CH3
p‐CH3
m‐CH3
o‐CH3
o‐CH3
m‐CH3O
m‐CH3O
p‐COMe
p‐COMe
m‐CH3O
p‐CN
p‐CN
H
Ph
I
7
CO2Bu
CO2CH3
Ph
I
8
I
9
I
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33c
34c
35c
36c
37c
38c
CO2Et
Ph
I
I
Ph
I
CO2Et
CN
I
I
CO2Bu
CO2Bu
Ph
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
3
3.5
2.5
4.5
2.5
3.5
12
CO2Bu
Ph
H
CO2Bu
Ph
p‐CH3O
p‐CH3O
p‐CH3O
p‐CH3
p‐CH3
p‐NO2
p‐CN
p‐CN
p‐COMe
p‐HO
o‐Me
o‐Me
H
CO2Et
CO2Bu
Ph
12
12
2
CO2Bu
Ph
5
4.5
4.5
4.5
4
CO2Bu
Ph
Ph
CO2Bu
Ph
12
6
CO2Bu
Ph
6
24
H
Ph
12
H
CO2Bu
Ph
16
p‐Me
p‐MeO
p‐MeCO
18
Ph
24
Ph
12
aReaction conditions: Pd NPs@NHC@ZIF‐8 (0.08 mol%), aryl halide (1 mmol), olefin (1.2 mmol), Et3N (2 mmol) and DMF–H2O (3:2).
bIsolated by preparative TLC.
cPhase transfer reagent (TBAB) (1 mmol).

10 of 13
AZAD ET AL.
vibrations of S═O bonds. The characteristic band of Pd
NP@NHC@ZIF‐8 at 419 cm−1 was due to Zn─N stretch
mode. Also the peak observed at 1584 cm−1 can be attrib-
uted to C═N stretching. The peaks at around 1142 and
1424 cm−1 were related to C─N and C═C stretching,
respectively. In addition, an increase in the intensity of
the absorption bands of Pd NP@NHC@ZIF‐8 as com-
pared to ZIF‐8 suggested that the encapsulation of Pd
NPs@NHC nanoparticles inside the framework had been
successful.
Additionally the binding energy of the Pd
NP@NHC@ZIF‐8 catalyst was determined using XPS
(Figure S3). XPS results suggest that Pd is in the zero oxi-
dation state. The observation of peaks at 335.2 and
340.6 eV in the XPS spectrum, corresponding to two dis-
tinct spin–orbit pairs of 3d5/2 and 3d3/2, respectively, dem-
onstrated the reduction from Pd(II) to Pd(0).[44]
combination. Thus, we started screening a variety of
organic and inorganic bases such as Et3N, K2CO3,
DIPEA, NaOAc and Na2CO3 in various solvents including
nonpolar, protic and aprotic, and mixtures of solvent with
water. The yield of the Heck product was decreased when
inorganic bases were used. Among the examined bases,
Et3N afforded an almost complete conversion in aqueous
DMF medium in short reaction time (Table 2, entry18).
As is evident from Table 2, other bases were substantially
less effective. After optimization of the base, various sol-
vent systems were tested. When dioxane, tetrahydrofuran
(THF) and toluene were used as aprotic solvents, a poor
yield of product was obtained (Table 2, entries 1–3). Reac-
tion in pure water had lower yields (Table 2, entry 8).
3.2 | Catalytic function of Pd
NPs@NHC@ZIF‐8 in Mizoroki–Heck cross‐
coupling reaction
In order to evaluate the catalytic activity of the Pd
NPs@NHC@ZIF‐8 nanocomposite, it was used in the in
C–C bond forming reaction. To do so, the reaction condi-
tions (base, solvent, temperature and amount of catalyst)
were first optimized. The coupling reaction of 4‐
bromotoluene with styrene was initially used as a model
reaction. As a first step of exploring the performance of
the catalyst in Heck reactions, the effects of various con-
ditions such as base, temperature, solvent and catalyst
loading were compared to find the best possible
FIGURE 6 Reusability of Pd NPs@NHC@ZIF‐8 catalyst in
Mizoroki–Heck cross‐coupling reaction. Reaction conditions: Pd
NPs@NHC@ZIF‐8 (0.1 mol%), bromobenzene (1 mmol), styrene
(1.2 mmol), Et3N (2 mmol) and DMF–H2O (2:3). Isolated yield after
preparative TLC
TABLE 4 Comparison of results for Mizoroki–Heck cross‐coupling reaction using Pd NPs@NHC@ZIF‐8 catalyst with those for previously
reported catalystsa
Entry
Catalyst
Temp. (°C)
Time (h)
Solvent
Yield (%)
Ref.
1
2
3
4
5
6
7
8
9
Pd NPs@NHC@ZIF‐8
TRGO‐NPy‐Pd
110
140
130
90
0.75
DMF–H2O
DMF
90
97
96
85
96
16
63.6
96
88
This work
[45]
5
2
[46]
[47]
[48]
[49]
[50]
[51]
[52]
NHC‐Pd/MCM‐41
MCM‐41‐Crown.Pd
Pd/Met/GO
DMA–H2O
H2O
1
110
130
130
120
95
3
DMF
Pd(II)‐Kryptofix 5
Pd/N‐CNTs
24
1
DMF
DMF
Fe3O4@OA–Pd
7
DMF
(Pd(II)–NHCs)n@nSiO2
12
DMF

AZAD ET AL.
11 of 13
When a mixture of water and DMF was used, a consider-
able increase in product formation was observed (Table 2,
entries 9–22), probably because of the better solubility of
organic reagents. By comparing the results, a 2:3 ratio of
water to DMF was found to be optimal for the cross‐
coupling reaction (Table 2, entry 18). Then, various
amounts of catalyst between 0.02 and 0.15 mol% were
investigated (Table 2). Among the different amounts of
Pd NPs@NHC@ZIF‐8, we observed the highest catalytic
activity by using 18 mg (0.1 mol% Pd) of the catalyst in
the Heck reaction (Table 2, entry 24). In order to show
the unique activity of the catalyst a blank test was also
carried out without using any catalyst, in which no prod-
uct was detected (Table 3, entry 1). Finally, to determine
the effect of temperature on catalytic activity, the reaction
was carried out at 90, 100, 110 and 120°C (Table 2). When
the temperature was below 110°C, the reaction proceeded
slowly (Table 2, entry 20). A quick screening of the reac-
tion conditions led to the choice of DMF–H2O (3:2) as the
best solvent mixture, with Et3N as the base at 110°C, and
catalyst with 0.1 mol% Pd as optimal reaction conditions
(Table 2, entry 18).
poor yield after 24 h (Table 3, entry 30). Based on this
result, increasing the amount of catalyst from 0.1 to
15 mol% had a positive effect on the product yields in
the case of chlorobenzene (Table 3, entry 31). Further
studies showed that increasing the temperature from
110 to 130°C could improve the product yield. When
TBAB (1 mmol) was used for the Mizoroki–Heck cross‐
coupling of chlorides with olefins, the yield and speed
of the reaction showed an obvious effect under these
improved conditions. As is evident (Table 3, entries
31–36), acceptable yields of the cross‐coupling products
were obtained using various aryl chlorides. A comparison
between the efficacy of our catalyst and that of some
other previously reported palladium‐based catalysts for
aryl bromides and chlorides is presented in Table 4.
According to the comparison, our catalytic system is char-
acterized by short reaction times, excellent conversions
and mild reaction conditions.
4 | CATALYST REUSABILITY
Having optimized the reaction conditions for the
Mizoroki–Heck reaction, we then examined the general
scope of this coupling reaction by applying the reaction
to other olefinic substrates such as styrene, ethyl and
methyl methacrylate and acrylonitrile with a variety of
substituted aryl halides including aryl iodides, bromides
and chlorides carrying either electron‐withdrawing or
electron‐donating substituents in aromatic rings
(Table 3). The yields of the in Mizoroki–Heck cross‐
coupling reaction depend on position and electronic
nature of the substituents on the aromatic ring.
Electron‐withdrawing substituents showed higher con-
version as compared to those with electron‐donating
ones. Aryl iodides with electron‐withdrawing or
electron‐donating groups and aryl bromides containing
electron‐withdrawing substituents efficiently reacted with
vinylic substrates to give highly satisfactory yields
(Table 3). However, the reaction of sterically hindered
aryl bromides and iodides such as 2‐bromotoluene and
2‐iodotoluene with styrene and methyl acrylate gave
lower yields than those without sterically hindered
ortho‐substituent (Table 3, entries 10–11 and 28–29).
Also, the coupling reaction of 2‐ and 4‐bromoanisole
and olefins failed to provide the target product even at
high temperature and longer reaction time (Table 3,
entries 19–21). A further investigation was conducted to
develop an effective heterogeneous and eco‐friendly cata-
lytic system for the Mizoroki–Heck cross‐coupling reac-
tion between aryl chlorides and styrene. Under the
same optimal reaction conditions as for aryl bromides
and iodides, coupling of chlorobenzene with styrene gave
For practical applications of a solid catalytic system, the
stability and reusability of the catalyst are very important
from the sustainability viewpoint. To gain an insight into
this issue, recycling studies of Pd NPs@NHC@ZIF‐8 were
performed using the Mizoroki–Heck reaction of p‐
bromotoluene and iodotoluene with styrene, using the
recycled catalyst under the same conditions. After com-
pletion of the reaction, the catalyst was separated from
the reaction mixture by filtration, washed with acetone
and dried in an oven. As for the cross‐coupling reaction
of p‐bromotoluene with styrene, the activity of Pd
NPs@NHC@ZIF‐8 decreases after being used six times
(Figure 6). Also the catalyst is highly reusable and was
FIGURE 7 Reusability of Pd NPs@NHC@ZIF‐8 catalyst in
Mizoroki–Heck cross‐coupling reaction. Reaction conditions: Pd
NPs@NHC@ZIF‐8 (0.1 mol%), iodobenzene (1 mmol), styrene
(1.2 mmol), Et3N (2 mmol) and DMF–H2O (2:3). Isolated yield after
preparative TLC

12 of 13
AZAD ET AL.
FIGURE 8 XRD pattern Pd
NPs@NHC@ZIF‐8 after five runs
used for nine runs without observation of a significant
decrease in activity for the cross‐coupling reaction of
iodotoluene with styrene (Figure 7). After the first and
second runs, ICP analysis showed that the amount of Pd
is almost the same as that of the fresh catalyst, which
ORCID
Shahnaz Rostamizadeh
5736
https://orcid.org/0000-0002-2940-
indicates
the
marked
stability
of
the
Pd
REFERENCES
NPs@NHC@ZIF‐8 catalyst. The stability of the recovered
catalyst was also determined using powder XRD analysis
(Figure 8). The powder XRD data of recycled Pd
NPs@NHC@ZIF‐8 after five cycles clearly indicate that
the matrix of the ZIF‐8 does not change during the
reaction.
[1] P. Anastas, N. Eghbali, Chem. Soc. Rev. 2010, 39, 301.
[2] C.‐J. Li, B. M. Trost, Proc. Natl Acad. Sci. USA 2008, 105, 13197.
[3] R. A. Sheldon, Green Chem. 2017, 19, 18.
[4] a)W. Tang, Y. Deng, Y. Chen, Catal. Commun. 2017, 89, 86.
b)N. Gurbuz, S. Vural, S. Yasar, I. Ozdemir, T. Seckin,
J. Inorg. Organometal. Polym. 2010, 20, 19.
[5] H.‐C. Ma, J.‐L. Kan, G.‐J. Chen, C.‐X. Chen, Y.‐B. Dong, Chem.
Mater. 2017, 29, 6518.
[6] Z. Guo, Q. Jiang, Y. Shi, J. Li, X. Yang, W. Hou, Y. Zhou,
5 | CONCLUSIONS
J. Wang, ACS Catal. 2017, 7, 6770.
[7] D. Reinares‐Fisac, L. M. Aguirre‐Díaz, M. Iglesias, N. Snejko,
E. Gutiérrez‐Puebla, M. A.n. Monge, F. Gándara, J. Am. Chem.
Soc. 2016, 138, 9089.
In summary, we have developed new methodology for
the synthesis of a highly active, recyclable and promising
heterogeneous catalyst from readily accessible materials,
using a general and effective impregnation method. The
synthesized Pd NPs@NHC@ZIF‐8 material was further
applied to catalyse the Mizoroki–Heck cross‐coupling
reaction. The anionic sulfonated NHC is effectively capa-
ble of stabilizing and preventing aggregation of the Pd
NPs in the pores of ZIF‐8. The results indicated that this
innovative material represents a highly active heteroge-
neous catalytic system for the Mizoroki–Heck cross‐
coupling reaction. Also, the catalyst can be recovered
and reused at least six times without a significant loss in
its activity and selectivity.
[8] S. F. Amarante, M. A. Freire, D. T. Mendes, L. S. Freitas, Appl.
Catal. A 2014, 469, 320.
[9] O. K. Farha, I. Eryazici, N. C. Jeong, B. G. Hauser, C. E.
Wilmer, A. A. Sarjeant, R. Q. Snurr, S. T. Nguyen, A. O. Z. R.
Yazaydın, J. T. Hupp, J. Am. Chem. Soc. 2012, 134, 15016.
[10] H.‐C. Zhou, J. R. Long, O. M. Yaghi, Chem. Rev. 2012, 112, 673.
[11] H. Furukawa, K. E. Cordova, M. O'Keeffe, O. M. Yaghi, Science
2013, 341. 1230444
[12] T. R. Cook, Y.‐R. Zheng, P. J. Stang, Chem. Rev. 2013, 113, 734.
[13] B. Chen, S. Ma, E. J. Hurtado, E. B. Lobkovsky, C. Liang,
H. Zhu, S. Dai, Inorg. Chem. 2007, 46, 8705.
[14] M. Zhang, C. Chen, Q. Wang, W. Fu, K. Huang, W. Zhou,
J. Mater. Chem. A 2017, 5, 349.
[15] H.‐L. Jiang, Y. Tatsu, Z.‐H. Lu, Q. Xu, J. Am. Chem. Soc. 2010,
132, 5586.
ACKNOWLEDGEMENTS
[16] H. Zheng, Y. Zhang, L. Liu, W. Wan, P. Guo, A. M. Nyström,
We gratefully acknowledge the Iran National Science
Foundation and K. N. Toosi University of Technology
for financial support in developing this work.
X. Zou, J. Am. Chem. Soc. 2016, 138, 962.
[17] A. C. McKinlay, R. E. Morris, P. Horcajada, G. Férey, R. Gref,
P. Couvreur, C. Serre, Angew. Chem. Int. Ed. 2010, 49, 6260.

AZAD ET AL.
13 of 13
[18] A. Dhakshinamoorthy, A. M. Asiri, H. Garcia, Chem. Soc. Rev.
2015, 44, 1922.
[40] C. Z Guo, R. V. Xiao, L. Maligal‐Ganesh, T. Zhou, W. Goh,
X. Li, D. Tesfagaber, A. T. Hiel, W. Huang, ACS Catal. 2014,
4, 41340.
[19] D.‐W. Kim, H.‐G. Kim, D.‐H. Cho, Catal. Commun. 2016, 73,
69.
[41] Y. Zhao, M. M. Liu, B. B. Fan, Y. F. Chen, W. M. Lv, N. Y. Lu,
R. F. Li, Catal. Commun. 2014, 57, 119.
[20] X. Li, R. Van Zeeland, R. V. Maligal‐Ganesh, Y. Pei, G. Power,
L. Stanley, W. Huang, ACS Catal. 2016, 6, 6324.
[42] J. Liu, J. He, L. Wang, R. Li, P. Chen, X. Rao, L. Deng, L. Rong,
J. Lei, Sci. Rep. 2016, 6, 23667.
[21] K. S. Park, Z. Ni, A. P. Côté, J. Y. Choi, R. Huang, F. J. Uribe‐Romo,
H. K. Chae, M. O'Keeffe, O. M. Yaghi, Proc. Natl Acad. Sci. USA
2006, 103, 10186.
[43] T. Zhang, B. Lia, X. F. Zhang, J. S. Qiu, W. Han, K. L. Yeung,
Micropor. Mesopor.Mater. 2014, 197, 324.
[22] Q. Yang, Q. Xu, S. H. Yu, H. L. Jiang, Angew. Chem. Int. Ed.
2016, 128, 3749.
[44] A. S. Roy, J. Mondal, B. Banerjee, P. Mondal, A. Bhaumik,
S. M. Islam, Appl. Catal. A 2014, 469, 320.
[23] Z. Liu, W. Dong, S. Cheng, S. Guo, N. Shang, S. Gao, C. Feng,
[45] Z. Hezarkhani, A. Shaabani, RSC Adv. 2016, 6, 98956.
C. Wang, Z. Wang, Catal. Commun. 2017, 95, 50.
[46] S. M. Sarkar, M. M. Yusoff, M. L. Rahman, J. Chin. Inst. Chem.
2015, 62, 33.
[24] S. Xue, H. Jiang, Z. Zhong, Z.‐X. Low, R. Chen, W. Xing,
Micropor. Mesopor. Mater. 2016, 221, 220.
[47] M. Azaroon, A. R. Kiasat, Appl. Organometal. Chem. 2018, 32,
[25] H. Liu, Y. Liu, Y. Li, Z. Tang, H. Jiang, J. Phys. Chem. C 2010,
114, 13362.
e4271.
[48] H. Veisi, N. Mirzaee, Appl. Organometal. Chem. 2018, 32,
[26] W. Wu, T. Zhou, S. Zhou, Chem. Mater. 2009, 21, 2851.
e4067.
[27] L. Peng, J. Zhang, S. Yang, B. Han, X. Sang, C. Liu, G. Yang,
[49] B. Movassagh, S. Ranjbari, Appl. Organometal. Chem. 2018, 32,
Green Chem. 2015, 17, 4178.
e4224.
[28] F. Glorius, N‐Heterocyclic Carbenes in Transition Metal
Catalysis, 1st ed., Springer‐Verlag, Berlin 2007.
[50] L. Wang, L. Zhu, N. Bing, L. Wang, J. Phys. Chem. Solids 2017,
107, 125.
[29] L. Y. Chen, X. D. Chen, H. L. Liu, Y. W. Li, Small 2015, 11,
[51] E. Rafiee, M. Kahrizi, J. Mol. Liq. 2016, 218, 625.
2642.
[52] M. Khajehzadeh, M. Moghadam, J. Organometal. Chem. 2018,
863, 60.
[30] E. Peris, Chem. Rev. 2017, 117, 9988.
[31] A. V. Zhukhovitskiy, M. J. MacLeod, J. A. Johnson, Chem. Rev.
2015, 115, 11503.
SUPPORTING INFORMATION
[32] J. B. Ernst, S. Muratsugu, F. Wang, M. Tada, F. Glorius, J. Am.
Chem. Soc. 2016, 138, 10718.
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
[33] A. Ferry, K. Schaepe, P. Tegeder, C. Richter, K. M. Chepiga,
B. J. Ravoo, F. Glorius, ACS Catal. 2015, 5, 5414.
[34] S. Turner, O. I. Lebedev, F. Schroder, D. Esken, R. A. Fischer,
G. Van Tendeloo, Chem. Mater. 2008, 20, 5622.
[35] Z. Jin, H. Yang, Nanoscale Res. Lett. 2017, 12, 1.
[36] W. T. Koo, S. Yu, S. J. Choi, J. S. Jang, J. Y. Cheong, I. D. Kim,
How to cite this article: Azad M, Rostamizadeh
S, Estiri H, Nouri F. Ultra‐small and highly
dispersed Pd nanoparticles inside the pores of ZIF‐
8: Sustainable approach to waste‐minimized
Mizoroki–Heck cross‐coupling reaction based on
reusable heterogeneous catalyst. Appl Organometal
Chem. 2019;e4952. https://doi.org/10.1002/aoc.4952
ACS Appl. Mater. Interfaces 2017, 9, 8201.
[37] S. Ding, Q. Yan, H. Jiang, Z. Zhong, R. Chen, W. Xing, Chem.
Eng. J. 2016, 296, 146.
[38] R. Garrido, P. S. Hernández‐Montes, A. Gordillo, P. Gómez‐Sal,
C. López‐Mardomingo, E. de Jesús, Organometallics 2015, 34,
1855.
[39] C.‐W. Tsai, E. H. Langner, Micropor. Mesopor. Mater. 2016,
221, 8.