Post-synthetic modified MOF for Sonogashira cross-coupling and Knoevenagel condensation reactions

Article abstract of DOI:10.1016/j.jcat.2016.10.015

A new heterogeneous NHC catalyst (1-Pd) was obtained by the post-synthetic modification of an azolium-containing metal-organic framework (1) which was synthesized by solvothermal reaction of 1,3-bis(4-carboxyphenyl)imidazolium chloride (H₂L⁺Cl^?) and Zn(NO₃)₂·6H₂O. This new material, 1-Pd, was characterized by PXRD, TGA, SEM, TEM, ¹H and ¹³C NMR, FTIR and XPS measurements. This catalyst exhibited an excellent activity for both Sonogashira cross-coupling and Knoevenagel condensation reaction retaining its catalytic and uniform distribution of the active sites, palladium (II) anchored on N-heterocyclic carbene (NHC-Pd(II)), for at least four cycles without losing its structural integrity.

Full text of DOI:10.1016/j.jcat.2016.10.015

Journal of Catalysis 344 (2016) 445–454
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Post-synthetic modified MOF for Sonogashira cross-coupling and
Knoevenagel condensation reactions
Chizoba I. Ezugwu a,b, Bibimaryam Mousavi a,b, Md. Ali Asraf a,b,e, Zhixiong Luo d, Francis Verpoort a,b,c,d,
⇑
a Laboratory of Organometallics, Catalysis and Ordered Materials, State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,
Wuhan University of Technology, Wuhan 430070, PR China
b School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan 430070, PR China
c National Research Tomsk Polytechnic University, Lenin Avenue 30, 634050 Tomsk, Russian Federation
d Ghent University, Global Campus Songdo, 119 Songdomunhwa-Ro, Yeonsu-Gu, Incheon, Republic of Korea
e Department of Chemistry, Rajshahi University, Rajshahi 6205, Bangladesh
a r t i c l e i n f o
a b s t r a c t
Article history:
A new heterogeneous NHC catalyst (1-Pd) was obtained by the post-synthetic modification of an
azolium-containing metal-organic framework (1) which was synthesized by solvothermal reaction of
1,3-bis(4-carboxyphenyl)imidazolium chloride (H2L+Clꢀ) and Zn(NO3)2ꢁ6H2O. This new material, 1-Pd,
was characterized by PXRD, TGA, SEM, TEM, 1H and 13C NMR, FTIR and XPS measurements. This catalyst
exhibited an excellent activity for both Sonogashira cross-coupling and Knoevenagel condensation
reaction retaining its catalytic and uniform distribution of the active sites, palladium (II) anchored on
N-heterocyclic carbene (NHC-Pd(II)), for at least four cycles without losing its structural integrity.
Ó 2016 Elsevier Inc. All rights reserved.
Received 10 September 2016
Revised 16 October 2016
Accepted 18 October 2016
Keywords:
Heterogeneous catalyst
Knoevenagel condensation reaction
MOF
Post-synthetic modification
Sonogashira cross-coupling reaction
1. Introduction
control are some of the advantages of heterogeneous catalysts over
their corresponding homogeneous counterparts [15]. Interestingly,
Multifunctional materials such as Metal-Organic Frameworks
(MOFs) have experienced rapid and extensive growth since their
emergence in last two decades [1–3] due to their high surface areas
[4], well defined pore architectures [5] and tunable structural fea-
tures [6]. MOFs, a subset of coordination chemistry, are highly
crystalline materials comprising infinite network of metal centers
or clusters bridged by multitopic organic ligands through metal-
ligand coordination bonds [3,7,8]. Their structural tunability is
considerably different from that of zeolites that contain pores con-
fined by rigid tetrahedral oxide skeletons that are not easy to mod-
ify [9]. Therefore, MOFs can undergo Post-Synthetic Modification
(PSM) [10] both at their metal nodes [11] and organic linkers,
and this is an emerging area that opens yet more possibilities
[12–14].
MOFs that are heterogeneous catalysts possess advantageous
cumulative attributes that emerge from both homogeneous and
heterogeneous catalysts [16]. Many diverse and efficient strategies
have been established for construction of MOF for catalysis at the
molecular level. Catalysis in MOFs can occur by utilization of the
unsaturated metal coordination sites [17,18], immobilization of
transition metal nanoparticles inside the pores [19,20], employing
ligand chelated metal complexes as the linkers [21,22] and post-
synthetic modification which usually uses functionalized organic
ligands, such as azolium ligand, for the MOFs synthesis.
NHCs are not only NHC-to-metal
r
? d donors, but also metal-
to-NHC d ? p⁄ and NHC-to-metal
p
? d donations contribute to
their metal bond [23]. Furthermore, MOFs based on azolium link-
ers generate pockets that are suitable for binding anionic guests
and unmasking the protons of the azolium moieties makes them
readily available for tethering catalytically active metal complexes
[24]. Different synthetic routes have been applied to metalate
azolium-based MOFs [25,26]. Pre-formed NHC complexes or met-
alloligands have been used as linkers for MOFs [21] and concomi-
tant generation of NHC complexes during MOFs synthesis [26–28].
More so, PSM of frameworks containing azolium based linkers has
been employed [29,30]. Metalation of these MOFs not only
Limitations of homogeneous catalysts in industry are circum-
vented by employing their heterogeneous counterparts. Thus effi-
cient recycling, easy separation, improved handling and process
⇑
Corresponding author at: Laboratory of Organometallics, Catalysis and Ordered
Materials, State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430070, PR China.
E-mail address: francis.verpoort@ghent.ac.kr (F. Verpoort).
http://dx.doi.org/10.1016/j.jcat.2016.10.015
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

446
C.I. Ezugwu et al. / Journal of Catalysis 344 (2016) 445–454
immobilizes these homogeneous catalysts but also increases their
stability and performance [31].
solution of paraformaldehyde (635 mg, 21.16 mmol, 1.25 equiv)
in 12 M HCl (2.1 mL, 25.33 mmol, 1.5 equiv) in dioxane (4 mL) at
0 °C. The reaction mixture was stirred for 4 h at room temperature.
The product formed was collected by filtration, washed with Et2O
and dried in Vacuum. Yield: 75%, Mp: >300 °C. 1H NMR
(500 MHz, DMSO-d6, 25 °C, TMS, d): 8.078–8.095 (d, 4H, ArH),
8.234–8.252 (d, 4H, ArH), 8.687–8.690 (d, 2H, ImH), 10.590 (s,
1H, ImH). Anal. Calcd. for C17H13N2O4Cl: C, 59.22; H, 3.80; N,
8.12. Found: C, 59.38; H, 4.057; N, 7.86.
An important CAC coupling reaction known as Sonogashira
reaction has extensively been utilized in the synthesis of natural
products, pharmaceuticals, biologically active molecules,
heterocycles, herbicides, dendrimers and conjugated polymers
[32]. Although, NHC is important in organometallic chemistry for
the synthesis of efficient homogeneous and heterogeneous
catalysts, most of the reported NHC catalysts for Sonogashira
cross-coupling reaction are homogeneous in nature [33]. Heterog-
enization of these catalysts has been achieved by supporting the
NHCs on inert solid materials such as carbon polymorphs, metal
oxides, mesoporous solids and polymers [34]. Nevertheless, some
reported works on employing MOF as catalyst for this application
involved Cu(I)-MOF [35], Pd(II)-MOF [36], and Pd nanoparticle
[37] as active sites.
Conversely, one of the valuable CAC bond forming reaction
known as Knoevenagel condensation reaction which involves the
condensation of aldehydes or ketones with active methylene com-
pounds proves to have a wide range of applications in the synthesis
of pharmaceuticals [38], fine chemicals [39], and carbocyclic in
addition to heterocyclic compounds of biological importance. Most
of the catalysts that have been employed for this condensation
reaction are mainly bases or Lewis acids that sometimes require
high reaction temperature [40].
Herein, we report for the first time post-synthetic modification
of the synthesized azolium based MOF, 1, to generate a novel
heterogeneous N heterocyclic carbene catalyst, 1-Pd. The Pd(II)-
NHC MOF catalyst proves highly efficient for Sonogashira cross-
coupling and Knoevenagel condensation reaction. This is the first
time of using the active site Pd(II)-NHC in MOF for these applica-
tions. 1-Pd has superior catalytic properties having a higher TON
value compared to all reported Pd(II)-MOFs catalysts used for
Sonogashira reactions. The optimum conditions for the perfor-
mance of the 1-Pd were established.
2.3. MOF synthesis
Compound
1 was synthesized according to a procedure
reported in the literature [41]. Zn(NO3)2ꢁ6H2O (594.98 mg,
2.0 mmol, 4 equiv) in 3 mL pre-dried DMF was mixed with H2L+Clꢀ
(172.38 mg, 0.5 mmol) in a Teflon-lined autoclave. The reaction
mixture was heated under autogenous pressure to 120 °C for
48 h followed by cooling to room temperature at the rate of
10 °C/h. The product was collected by filtration and washed with
the pre-dried DMF. Yield: 45%.
2.4. Post-synthetic modification of 1
To a 250 mL two-necked round bottom flask containing a THF
solution (100 mL) of Pd(OAc)2 (336.75 mg, 1.5 mmol),
1
(600.0 mg, 0.1 mmol) was introduced. The reaction mixture was
stirred at ambient temperature for 12 h and then refluxed for
24 h under an inert N2 atmosphere. The light-brown solid formed
was collected by filtration followed by washing with THF, MeOH,
and Et2O; and drying in air.
2.5. Characterization
1H and 13C NMR spectra were recorded at 500 MHz and
100 MHz, respectively, if not otherwise stated using Brüker
500 MHz NMR spectrometer. 1H NMR spectra were referenced to
tetramethylsilane (TMS). Powder X-ray diffraction patterns (PXRD)
were recorded using an Empyrean instrument from PANalytical by
2. Experimental
2.1. Materials and methods
applying a monochromatic Cu Ka radiation at ambient conditions.
Fourier transform infrared (FT-IR) spectra are collected on a
Perkin-Elmer Spectrum One spectrometer. Scanning electron
microscope (SEM) from JEOL (JSM-5610LV, 0.5–35 kV) was used
to investigate the texture of the materials. TEM investigation was
carried out in a Philips CM20 microscope operated at 200 kV. The
solid sample was crushed and dispersed in water-free ethanol.
The solution was then ultra-sonicated for 3 min before dropping
onto a Cu-grid with lacey carbon film. Imaging and diffraction of
the structure was performed at low electron dose in order to min-
imize beam damage to the sample. Thermogravimetric analyses
(TGA–DSC) were conducted on a Netzsch (STA449c/3/G) instru-
ment at a heating rate of 10 °C/min under inert atmosphere (N2-
flow). The element distribution map measurement was performed
on a S4800 field emission scanning electron microscope (FESEM,
Hitachi, Japan) equipped with an X-Max 50 energy-dispersive X-
ray spectroscopy (EDS, Oxford Instruments, Britain) with an accel-
erating voltage of 10 kV. Dynamic Light Scattering DLS analysis
was performed in a ZETASIZER Nano series (ZEN4602) instrument
(particle size distribution from 0.6 to 6000 nm and detection limit
of 0.1 ppm). The surface electronic state was analyzed by X-ray
photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C ESCA).
All starting materials and solvents were obtained from com-
mercial sources and used without further purification. The synthe-
sis of the azolium ligand, 1,3-bis(4-carboxyphenyl) imidazolium
chloride (H2LClꢀ) was achieved in two steps following reported lit-
erature procedures [41,42] with slight modifications.
2.2. Ligand synthesis
2.2.1. Synthesis of N,N0-bis(4-carboxyphenyl)ethylenediimine (LA)
To a 100 mL round bottom flask containing magnetic bar was
dissolved 4-aminobenzoic acid (10 g, 72.92 mmol, 2.0 equiv) in
30 mL dry methanol and the mixture was stirred until all the acid
dissolved. 4 drops of formic acid was added followed by dropwise
addition of 40% W/W aqueous solution of glyoxal (4.18 mL,
36.46 mmol, 1.0 equiv). The reaction mixture was stirred at ambi-
ent temperature for 24 h and the white solid product formed was
collected by filtration, washed with cold methanol and dried in
air. Yield: 67%, Melting point: >300 °C. Anal. Calcd. for
C16H12N2O4: C, 64.86; H, 4.08; N, 9.45. Found: C, 64.89; H, 4.04;
N, 9.22.
All the binding energy values are calibrated by using
C
2.2.2. Synthesis of 1,3-bis(4-carboxyphenyl)imidazolium chloride
(H2L+Clꢀ)
In anhydrous THF (30 mL) under an argon atmosphere was dis-
solved compound LA (5 g, 16.89 mmol) followed by addition of a
1s = 284.6 eV as a reference. The content of Pd, are determined
by inductively coupled plasma optical emission spectra (ICP,
Varian VISTAMPX). UV–Vis spectrometry was obtained using a
UV-3600, Shimadzu, Japan.

C.I. Ezugwu et al. / Journal of Catalysis 344 (2016) 445–454
447
2.6. Adsorption test
Simulated
As-synthesized for 1
1-Pd
After 4th catalytic reaction
The adsorption performance of 1-Pd catalyst is tested by
employing the following procedure: 10 mg of 1-Pd was dispersed
into 10 mL ethanol of dye (100 mg/L). The suspension was stirred
for 1 h in darkness to achieve adsorption equilibrium. The adsorp-
tion process was carried out with continuous magnetic stirring at
room temperature. During the process, 3 mL of the solution was
removed at a constant time (i.e. every 10 min for 60 min), where
the sample was separated from the solution by centrifugation
and the concentration of the remaining clear liquid measured by
change in maximum absorbance at a wavelength of 663 nm by
UV–Vis spectrometry. The percentage of degradation D% was
determined as follows:
10
20
30
40
50
D% ¼ ðA0 ꢀ AÞ=ðA0Þ ꢂ 100%
2 Theta /degrees
where A0 and A are the absorbances of the liquid sample before and
after degradation respectively.
Fig. 1. PXRD pattern for 1 and 1-Pd.
2.7. Activity tests
matched, Fig. 1. Furthermore, the structure of the unmodified MOF,
1, has been reported [41] as illustrated in Fig. 2 and our XRD pat-
tern displays a long-range crystalline structure in which all diffrac-
tion peaks match with the reported ones. The framework afforded
an unusual secondary building unit as shown in Fig. 2 [41] and this
SBU coordinates to the six azolium ligands to generate a three-
dimensional structure.
The crystalline structure of the frameworks was maintained
after PSM as indicated in the identical PXRD patterns of the as-
synthesized and modified MOFs, see Fig. 1. Therefore, the structure
of the as-synthesized MOFs can tolerate such a post-modification
condition without the collapse of the frameworks.
2.7.1. Sonogashira cross-coupling reaction
A 100 mL Schlenk flask was charged with 2-bromopyridine
(1.1 mmol), phenylacetylene (0.92 mmol), Cs2CO3 (1.84 mmol), 1-
Pd (0.003 mmol based on Pd), DMF (6 mL) and the reaction mixture
was stirred at 100 °C under air atmosphere for 12 h. The mixture
was cooled to the room temperature; the solid was removed by fil-
tration and washed twice with DMF (3 mL). The filtrate was col-
lected, dried and the residue was extracted with ethyl acetate
(3 ꢂ 3 mL) followed by purification with silica gel chromatography
(petroleum ether) to give a corresponding product.
TGA of 1 indicates that weight loss takes place in four steps,
Fig. 3. The first total weight loss of 2.59% corresponding to loss of
lattice water molecules occurred at a temperature range 73.4–
120 °C and the second weight loss of 2.63% at the temperature
range 181.2–201.1 °C was assigned to loss of DMF. In the temper-
ature range 239.7–265.5 °C, the weight loss of 8.69% was attributed
to the releasing of more molecules of DMF in the frameworks. Fur-
ther heating resulted to 43% weight loss which was assigned to the
liberation of the remaining molecules of DMF. Finally, the frame-
works collapse beyond 455 °C. Also, as can be observed from
Fig. 3, the framework retained its thermal stability after PSM as
the TGA curve (weight loss) of the modified framework is the same
with the as-synthesized framework.
2.7.2. Knoevenagel condensation reaction
A 10 mL round bottomed flask was charged with benzaldehyde
(1.0 mmol), malononitrile (1.1 mmol), 1-Pd (0.001 mmol based on
Pd), and DMSO-d6 (2 mL). The reaction mixture was stirred at room
temperature under air atmosphere. The percentage rate of the con-
version of the reactants to the product was monitored by 1H NMR.
After the completion of the reaction, the solid mixture was filtered
and washed with DMSO-d6 (2 mL) to recover the catalyst and then
dried to obtain the products.
2.8. Recycling test
To determine the recyclability of the catalyst, after the first run
of the coupling reaction, the catalyst was isolated by centrifuge,
and washed thoroughly with DMF followed by vacuum drying at
100 °C overnight. Then, the catalyst was reused with fresh charge
of reactant in a 100 mL Schlenk flask under identical reaction con-
ditions. The product was isolated following the procedure as the
first run and the catalyst was collected and used for the next cycle.
For the Knoevenagel condensation reaction, after the comple-
tion of the first cycle of the reaction, the catalyst was recovered
by filtration of the solid mixture, washed with DMSO-d6 and vac-
uum dried at 125 °C. The recovered catalyst was reused for the sec-
ond, third and fourth cycle following the same reaction conditions.
After the synthesis of 1, we investigated whether the imida-
zolium moieties are still available for PSM in order to generate
3. Results and discussion
High-resolution powder X-ray diffraction (PXRD) has been used
to observe the local geometry, and the phase purity of the crys-
talline framework and the resulting spectra are presented in
Fig. 1. A good phase purity and homogeneity of the frameworks
were confirmed since all major peaks of experimental and simu-
lated PXRD patterns from the single-crystal structure data are well
Fig. 2. The structure of the unmodified MOF, 1, showing the imidazolium moieties
and six metallomacrocycles connected to the core Zn8O cluster.

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C.I. Ezugwu et al. / Journal of Catalysis 344 (2016) 445–454
that the black dots are distributed uniformly throughout the sam-
100
80
1-Pd
ple. Therefore, the Pd is coordinated in 1 framework. Some of the
black dots (areas of coordinated Pd) are highlighted using red cir-
cles (Fig. 4(d)). Further evidence that the Pd is anchored on the
framework was obtained from the PXRD result of the modified 1
that shows the two peaks of Pd at 40.1° and 46° 2h/degrees, which
are absent for the as-synthesized sample [43].
60
40
100
80
In addition, the energy dispersive X-ray spectroscopy (EDS)
analysis supports the existence of Pd in the modified framework
as shown in Fig. 5. The elemental mapping also reveals that 1-Pd
contains evenly distributed Pd, Zn, C and O. More so, Fig. 6 shows
the presence of Pd peaks around 2.9 and 3.5 keV [44].
1
60
40
The FT-IR spectra of H2L+Clꢀ, 1 and 1-Pd are presented in Fig. 7.
For H2L+Clꢀ, an important strong absorption peak is observed at
0
100
200
300
400
500
600
Temperature / oC
1702.19 cmꢀ1 which is assigned to the
m(C@O) stretching vibration
of uncoordinated carboxylate acids, ACOOH. Notably, this charac-
teristic band is entirely absent in the FT-IR of the as-synthesized
framework, showing the total deprotonation of the ACOOH groups
in the imidazolium carboxylate ligands. Moreover, two new peaks
at 1621.63 cmꢀ1 (Anti symmetric stretching) and 1315.24 cmꢀ1
(Symmetric stretching) appear in the spectra of 1 and are absent
in that of the ligand. These two new peaks are attributed to the
stretching vibration of coordinated carboxylate anions, ACOOꢀ,
present in the as-synthesized frameworks. The aforementioned
results confirm that the solvothermal reaction of 1,3-bis(4-carbox
yphenyl)imidazolium chloride and Zn(NO3)2 resulted in the coordi-
nation of COOꢀ with Zn(II) for the formation the MOF, 1. In addi-
tion, it is observed that the spectrum of 1-Pd is well matched
with that of 1, supporting that the MOF did not decompose during
PSM. The sharp peak at 1174.66 cmꢀ1 in the spectra of the ligand is
due to the CAH bond at the C2 position of the imidazolium ring
[45] which decreases drastically in the FT-IR spectra of the modi-
fied framework, thus supporting the coordination of the Pd to the
carbenic carbon of the imidazolium moieties.
Having realized that Pd is tethered in the framework, we
repeated the digestion test on the modified framework to ascertain
the actual position of the anchored Pd on the NHC structure. 1H
NMR spectrum in Fig. S5 showed two signals in upfield region at
d = 7.42 and 6.95 ppm, which were absent in the ligand (Fig. S1)
as well as the digested 1 (Fig. S3). These two chemical shifts are
the aromatic protons of the azolium ligand containing NHC-Pd
coordination [46] and these peaks appearing in the downfield
Fig. 3. TGA results of as-synthesized 1 and the modified framework, 1-Pd showing
the retention of thermal property of the as-synthesized framework after
modification.
NHCs by digesting the framework with aqueous 5% HCl solution.
The resulting residue was washed with water (3 ꢂ 10 mL), dried
in vacuum at 90 °C overnight and analyzed in DMSO-d6 for 1H
and 13C NMR spectroscopy (Figs. S3 and S4). The NMR spectrum
showed that the imidazolium proton in 1 remained intact and no
additional peaks were observed from the spectrum when com-
pared with the spectrum of the ligand (Figs. S1 and S2). The imida-
zolium moieties are shown in Fig. 2.
The morphology of the frameworks was observed by field emis-
sion scanning electron microscopic (FE-SEM) as shown in Fig. 4
(a) and (b). The FE-SEM image of 1 is composed of relatively uni-
form rod-like particles of average length 2.70–3.75 lm. The modi-
fied framework showed no significant change in length and
morphology. In order to have a closer observation on the morphol-
ogy and to evaluate the level of possible aggregation of the Pd dur-
ing PSM, high-resolution transmission electron microscopy (HR-
TEM) images, Fig. 4(c) and (d) of both 1 and 1-Pd samples were
provided respectively. A judicious comparative observation of the
TEM images of the two samples shows the existence of black dots
(Fig. 4(d)) of dimensions 2.46–2.50 nm that are easily distin-
guished from the surrounding gray regions of 1. A further examina-
tion of HR-TEM image of the modified frameworks demonstrated
Fig. 4. FE-SEM images (a) 1, (b) 1-Pd and HR-TEM images (c) 1, (d) 1-Pd before catalysis and (e) 1-Pd after the fourth catalytic cycle.

C.I. Ezugwu et al. / Journal of Catalysis 344 (2016) 445–454
449
Fig. 5. EDS Mapping of 1-Pd indicating the presence of Zn, O, C and Pd.
C
Pd
Pd
O
Zn
3.0
3.5
-1
1621 cm
-1
1315 cm
N
Pd
Pd
-
H2L+Cl
1
1-Pd
-1
1702 cm
0
1
2
3
4
5
Energy/keV
3500
3000
2500
2000
1500
1000
Fig. 6. EDS spectrum of 1-Pd.
wavenumber cm-1
Fig. 7. FT-IR spectra of H2L+Clꢀ, 1 and 1-Pd.
region may be due to the Pd-to-NHC d ? p⁄ donation. Integration
of the 1H NMR demonstrates that 12.55% of the carbenic protons
in the H2L+Clꢀ were deprotonated.
corresponds to the N 1s of the imidazolium moieties [48–50].
Fig. 8(b) illustrates the best-fitted XPS data of Pd. Two peaks
related to Pd 3d5/2 and 3d3/2 are centered at the binding energies
of 337.9 and 343.2 eV respectively. These binding energies are fea-
tures of divalent Pd cations [51,52,36,53] and are also consistent
with the values reported for Pd(II)–NHC complex [54]. No peaks
were observed around 335 and 340 eV corresponding to the 3d5/2
and 3d3/2 levels of Pd(0) [55,56], which confirms that there are
no traces of Pd(0) nanoparticles in 1-Pd. Furthermore, no trace of
Pd(OAc)2 was identified since the 3d5/2 level of binding energy
peak of 338.7 eV that is characteristic of Pd(OAc)2 was absent [29].
The best fitted XPS results of Zn are shown in Fig. 8(c), which
demonstrate two corresponding peaks of Zn 2p3/2 and 2p1/2 with
binding energies of 1022.8 and 1045.9 eV respectively, that are
characteristic binding energies for divalent Zn(II) cation [57–59].
Furthermore, in the 13C NMR spectrum (Fig. S6) of digested
sample, a new signal in upfield region at d = 206.94 ppm corre-
sponds to carbene carbon coordinated to Pd, thereby confirming
that the Pd is anchored on the carbene carbon generating Pd-
NHC [47]. Additional proof for the presence and composition of
Pd was obtained by performing inductively coupled plasma mass
spectrometry (ICP-MS), which shows that 10.88% of Pd(II) tether
to the carbene and this result is in agreement with the 1H NMR
result.
X-ray photoelectron spectra (XPS) provide valuable information
on the oxidation state of the anchored Pd and Zn in the 1-Pd. Fig. 8
(a) represents the general survey scan of the XPS analysis revealing
the existence of Pd, Zn, C, O and N in the modified framework. From
the survey, the binding energy centered around 400.2 eV

450
C.I. Ezugwu et al. / Journal of Catalysis 344 (2016) 445–454
(a)
1000
800
600
400
200
0
Binding Energy (eV)
Zn 2p
(c)
Pd 3d
(b)
3d
5/2
2p
3/2
3d
3/2
2p
1/2
1050 1045 1040 1035 1030 1025 1020
Binding Energy (eV)
348 346 344 342 340 338 336 334 332
Binding energy (eV)
O 1s
C 1s
1s
(d)
(e)
1s
1s
1s
1s
290
288
286
284
282
280
540 538 536 534 532 530 528 526
Binding Energy (eV)
Binding Energy (eV)
Fig. 8. (a) XPS survey spectra; high resolution spectra of (b) Pd 3d region, (c) Zn 2p region, (d) C 1s region and (d) O 1s region of 1-Pd.
The Zn(II) cations coordinate with the carboxylic groups to gener-
ate the SBUs. In addition, the XPS analysis of carbon is presented in
Fig. 8(d). The peaks at 284.8, 286.32 and 288.2 eV are associated
with binding energies assigned to the 1s of C@C of aromatic, imida-
zolium carbon (CAN) and COOꢀ of carboxylate respectively [48,60–
62]. Also, the best fitted XPS result of oxygen reveals two peaks at
534.82 and 531.72 eV attributed to O 1s of carbon-oxygen single
bonds (CAO) and carbon oxygen double bonds (C@O) Fig. 8(e) [61].
In order to access the porosity of the frameworks, nitrogen
adsorption experiments were carried out. The Brunauer–Emmet
t–Teller (BET) surface areas of the as-synthesized MOF, 1, and the
modified MOF, 1-Pd, are 102.02 and 88.36 m2 gꢀ1, respectively.
Similar behavior has been reported for azolium MOFs [63].
Owing to the reason that the catalytic performance of a porous
material requires that the catalytic sites are accessible by reactant
substrate molecules to enhance heterogeneous catalysis, we
employed a dye-uptake assay [29] in order to quantify the porosity
of the 1-Pd. The distinct adsorption ability of the catalyst for the
dyes, MB and MO, was monitored at different time by UV–Vis spec-
troscopy as presented in Figs. S7 and S8. The adsorption was mon-
itored from 0 to 60 min. The maximum adsorption of 39.26% for
MB and 63.83% for MO was observed by the catalyst. The dye
adsorption can be due to the host-guest (sorbate–sorbent) interac-
tion which plays a dominant role in dye uptake of MOFs [64]. The
higher capture ability of MO is attributed to the anionic nature of
the dye that induces a higher affinity to the cationic imidazolium
moieties in the framework. Thus, the pores in the catalyst are large
enough to accept both MB and MO, which have estimated molecule
sizes of 1.43 ꢂ 0.61 ꢂ 0.40 nm3 and 1.44 ꢂ 1.09 ꢂ 0.64 nm3,
respectively [65,66].
In order to examine the catalytic activity of 1-Pd, an important
Pd-catalyze coupling reaction known as Sonogashira reaction was
carried out. The representative data are summarized in Table 1.
The coupling reactions were carried out with substituted aryl
halide, substituted phenyl acetylene, Cs2CO3, 1-Pd in DMF at
100 °C under air atmosphere for 12 h to produce the corresponding
products. Under these reaction conditions, 1-Pd catalyzes the reac-
tions of 2-bromopyridine with substituted phenylacetylene to gen-
erate excellent corresponding yields (Table 1, entries 1–3). Lower
yields were obtained for 2-bromopyridine with an ortho-
substituted electron-donating group (Table 1, entries 9 and 10)
when compared with the yield obtained (Table 1 entries 1 and

C.I. Ezugwu et al. / Journal of Catalysis 344 (2016) 445–454
451
Table 1
Sonogashira cross-coupling reaction of different substrates with 1-Pd.a
Entry
R
X
Y
R0
Yield (%)b
1
2
3
4
5
6
7
8
H
H
H
Br
Br
Br
I
I
I
I
I
Br
Br
Cl
I
N
N
N
H
H
H
H
H
H
H
N
H
H
N
N
4-OCH3
4-C(CH3)3
H
84.6
94.0
98.0
93.3
87.7
90.0
91.0
67.7
77.8
73.4
62.0
>99
4-CH2CH3
4-CH2CH3
4-CH2CH3
4-OCH3
4-OCH3
2-CH3
2-CH3
H
H
4-OCH3
4-C(CH3)3
H
4-C(CH3)3
4-OCH3
4-C(CH3)3
4-C(CH3)3
H
9
10
11
12
13
14c
15d
4-NO2
4-COCH3
H
I
Br
Br
H
>99
36.0
24.0
4-CH(CH3)3
4-CH(CH3)3
H
a
Reaction conditions: aryl halide (0.92 mmol), phenylacetylene (1.1 mmol), Cs2CO3 (1.84 mmol), catalyst (0.003 mmol based on Pd) in 6 mL DMF heated at 100 °C under air
atmosphere for 12 h.
b
Isolated yield,
Catalyzed by Pd(OAc)2.
Catalyzed by unmodified MOF, 1.
c
d
After the realization that 1-Pd is active for Sonogashira coupling
reactions, we further tried to optimize the reaction conditions of
the catalyst. The coupling reaction of 1-iodo-4-nitrobenzene with
phenylacetylene was selected since very high yield was obtained
with these substrates (Table 1, entry 12). Firstly, the effect of base
on the reaction was investigated (Table 2). Both organic and inor-
ganic bases were tested. Considering the inorganic bases, the
hydroxyl groups (Table 2, entries 1 and 2) provided an excellent
yield, but lower yields were obtained for the carboxylate bases
(Table 2, entries 3 and 4). For the organic bases, the performance
100
94
93.6
93.1
92.4
80
60
40
20
Table 2
0
Effect of base for Sonogashira cross-coupling reactions of 1-iodo-4-nitrobezene and
phenylacetylene using 1-Pd as catalyst.
1
2
3
4
Recycling Number
Entry
Base
Yield (%)
Fig. 9. Durability tests of 1-Pd in Sonogashira cross-coupling. Reaction conditions:
2-bromopyridine (0.92 mmol), 4-tert-Butylphenylacetylene (1.1 mmol), Cs2CO3
(1.84 mmol), catalyst (0.003 mmol based on Pd) in 6 mL DMF heated at 100 °C
under air atmosphere for 12 h.
1
2
3
4
5
KOH
>99
>99
77.2
85.3
91.8
NaOH
Na2CO3
K2CO3
NEt3
Reaction conditions: 1-iodo-4-nitrobezene (0.92 mmol), phenylacetylene
(1.1 mmol), base (1.84 mmol), catalyst (0.003 mmol based on Pd) in 6 mL DMF
heated at 100 °C under air atmosphere for 12 h.
2). Good yields were observed for substituted iodobenzene (Table 1,
entries 4–8). The yield was somewhat low when tested on 2-
chlorobenzene (Table 1, entry 11). On the other hand, the aryl
halide with electron-withdrawing substituents like nitro, entry
12, and acetyl, entry 13, gave excellent yields. Furthermore, Pd
(OAc)2 as a catalyst was used also but the obtained yield was
low (Table 1, entry 14); thus, Pd(OAc)2 was not just grafted on
the framework, rather NHC-Pd(II) was generated as an active site
of 1-Pd. As shown in Fig. 9, the activity of the catalyst was not
changed significantly after the fourth catalytic cycle. The PXRD pat-
terns of the recovered solid after four catalytic cycles revealed that
the structural integrity of the catalyst was retained (Fig. 1). Fur-
thermore, after four catalyst cycles, the TEM and XPS measure-
ments showed no aggregation of the black dots, Fig. 4(e) and that
Pd maintained the oxidation state of +2, Fig. S9, respectively.
Table 3
Effect of solvent for Sonogashira cross-coupling reactions of 1-iodo-4-nitrobezene and
phenylacetylene using 1-Pd as catalyst.
Entry
Solvent
Yield (%)
1
2
3
4
5
1,4-Dioxane
Toluene
Acetonitrile
Ethanol
33.1
21.4
43.8
96.7
>99
Methanol
Reaction conditions: 1-iodo-4-nitrobezene (0.92 mmol), phenylacetylene
(1.1 mmol), Cs2CO3 (1.84 mmol), catalyst (0.003 mmol based on Pd) in 6 mL solvent
heated at 100 °C under air atmosphere for 12 h.

452
C.I. Ezugwu et al. / Journal of Catalysis 344 (2016) 445–454
of the catalyst was better for triethylamine (Table 2, entry 5) than
for the carboxylate bases.
100
96
92
88
84
80
Next, the effect of solvent on the reaction was examined to
explore the best solvent for 1-Pd catalyst. The effect of non-polar
solvent (1,4-dioxane &toluene), polar aprotic (acetonitrile) and
protic (ethanol & methanol) solvents was investigated as shown
in Table 3. For non-polar solvent, very poor yield of the product
was obtained, but 1,4-dioxane performed better than toluene.
Although the polar aprotic solvent, acetonitrile, proved better than
non-polar solvents, but the obtained yield was still low (Table 3,
entry 3). Nevertheless, high yields were obtained with polar protic
solvents (Table 3, entries 4 and 5) with methanol performing best
among the solvents studied.
0
20
40
60
80
100
120
Moreover, we tried to study the time dependence of 1-Pd
catalyst on the Sonogashira cross-coupling reaction of
1-iodo-4-nitrobezene and phenylacetylene as presented in
Fig. 10. The initially observed catalyst performance is fairly low
in the beginning of the reaction (0 h), but after 4 h, the percentage
yield increased appreciably. The optimum reaction time for 1-Pd
was observed around 8 h, after which the percentage yield
remained constant (Fig. 10).
Temperature (°C)
Fig. 11. Effect of reaction temperature for Sonogashira cross-coupling reactions of
1-iodo-4-nitrobezene and phenylacetylene using 1-Pd as catalyst.
Table 4
Effect of the concentration of 1-Pd on the Sonogashira reaction at different time.
Entry
Catalyst (mmol based on Pd)
Time (h)
Yield (%)
As presented in Fig. 11, the catalytic performance of 1-Pd was
examined from ambient temperature to 110 °C. Low product yield
was obtained at ambient temperature up to 60 °C, after which
there is a drastic increase in the percentage yield. Very high yield
of the product was achieved after 80 °C.
Furthermore, it is very crucial to acquire the optimum concen-
tration of 1-Pd catalyst for the Sonogashira cross-coupling reaction.
As shown in Table 4, low yields of products are achieved at 1-Pd
catalyst concentration of 0.0020 and 0.0025 mmol based on Pd as
shown in Table 4, entries 1 and 2 respectively. Notably, decreasing
the reaction time for the catalyst loading at 0.0025 and 0.0030
from 12 h to 4 and 6 h respectively revealed that the percentage
yield decreased as presented in Table 4 entries 8 and 6. Hence,
the observed optimum performance of the catalyst is detected at
a catalyst concentration of 0.0030 mmol based on Pd.
Dynamic light scattering (DLS) measurements were performed
to ascertain the possible formation of nanoparticles during the
coupling reaction. Firstly, we heated (100 °C) a blank sample con-
sisting of 2-bromopyridine, phenylacetylene, and Cs2CO3 in DMF
for 12 h, followed by conducting the DLS measurement (Fig. S10)
of the filtrate, which showed a peak with particle size of about
1000 nm. Also, the DLS analysis of the filtrate from Table 1, entries
2, 5 and 7 were carried out and the results showed the same
particle size of 1000 nm with no extra peaks as that of the blank.
1
2
3
4
5
6
7
8
0.0020
0.0025
0.0030
0.0040
0.0030
0.0030
0.0025
0.0025
12
12
12
12
8
6
6
4
56.6
79.4
>99
>99
>99
94.7
75.1
70.3
Reaction conditions: 1-iodo-4-nitrobezene (0.92 mmol), phenylacetylene
(1.1 mmol), base (1.84 mmol), catalyst (mmol based on Pd) in 6 mL DMF heated at
100 °C under air atmosphere.
Furthermore, under the same reaction conditions, the DLS mea-
surements of the reaction mixtures (before catalysis, without Cs2-
CO3, and without Cs2CO3 and 1-Pd) were analyzed (Fig. S10);
showing peaks less than 1 nm, which indicate that no formation
of nanoparticles was observed [67]. Therefore, it can be deduced
that the peak with particle size of about 1000 nm may be attribu-
ted to the formation of inorganic micron-sized particles from the
Cs2CO3 and no Pd(0) nanoparticle was formed after the catalytic
reaction.
The heterogeneous character of the catalyst was examined by
heating a mixture of the catalyst and the base at the reaction
temperature for 12 h followed by filtration. To the hot filtrate,
2-bromopyridine, phenylacetylene, and more base was added,
followed by heating at reaction temperature for another 12 h, but
no product was detected. Furthermore, no trace of Pd was detected
in the ICP analysis result of the filtrate. The ICP of 1-Pd after four
catalytic runs showed no appreciable change in the Pd content.
These results show the heterogeneous catalytic behavior of 1-Pd.
To the best of our knowledge, 1-Pd has the highest reported
TON value of 300 [36,51,53] among Pd(II)-MOF catalysts used in
Sonogashira cross-coupling reaction [36,51,53]. This high value
may be attributed to the efficient Pd-NHC active site that has been
reported as among the most stable and active Pd complexes to
promote cross-coupling reactions [68].
100
90
80
70
60
50
40
Besides study on CAC coupling reaction, Sonogashira reaction,
the catalytic performance of 1-Pd was further evaluated on one
of the useful CAC bond forming reactions, Knoevenagel condensa-
tion reaction. The Knoevenagel reactions of various kinds of ben-
zaldehyde with malononitrile in DMSO-d6 at room temperature
were monitored in the presence of 1-Pd as catalyst. The percentage
conversion was monitored and calculated with time from the inte-
gration of the 1H NMR of the aldehyde proton as illustrated in
0
2
4
6
8
10
12
14
Time (h)
Fig. 10. Effect of reaction time for Sonogashira cross-coupling reactions of 1-iodo-
4-nitrobenzene and phenylacetylene using 1-Pd as catalyst.

C.I. Ezugwu et al. / Journal of Catalysis 344 (2016) 445–454
453
Fig. 12. 1H NMR spectra for monitoring the percentage conversion from Table 5, entry 1; (a) 5 min, (b) 10 min and (c) 15 min.
Table 5
Knoevenagel condensation reaction of different benzaldehyde and malononitrile
100
catalyzed by 1-Pd.a
99.5
98.9
98.1
100
80
60
40
20
0
Entry
1
R
Time (min)
Conversion (%)b
H
5
10
15
42.5
73.7
100.0
2
3
4
5
6
7c
Cl
<1
4
8
3.3
38.3
100.0
1
2
3
4
F
<1
4
6
62.3
93.4
100.0
Recycling Number
Fig. 13. Durability tests of 1-Pd for the Knoevenagel condensation reaction.
NO2
CH3
OCH3
NO2
<1
2
4
86.3
97.5
100.0
10
25
55
12.3
56.5
98.6
Fig. 12. The three stages progressive 1H NMR spectra of the con-
densation process are shown in Figs. S18–S23. The condensation
products result in the chemical shift of the aldehyde proton from
around 10 ppm to around 8.5 ppm.
60
120
240
17.4
64.8
94.7
The reusability tests of the catalyst for Knoevenagel condensa-
tion reaction show that there was no significant change in the cat-
alytic activity of 1-Pd after four runs as illustrated in Fig. 13.
For the un-substituted benzaldehyde, up to 42.5% conversion
was observed after 5 min of reaction and complete conversion,
100%, was achieved after 15 min (Fig. 12 and Table 5, entry 1). Fas-
ter conversion rates were achieved for benzaldehyde with para-
substituted electron-withdrawing groups, Table 5, entries 2–4,
when compared with un-substituted substrate. 1-Pd was able to
catalyze 100% of nitrobenzaldehyde in 4 min (Table 5, entry 4).
Notably, benzaldehyde with para-substituted electron donating
groups (Table 5, entries 5 and 6) experiences slower conversion
rate when compared with substituted electron withdrawing
groups, Table 5, entries 2–4. This trend could be that the electron
donating groups enhance the electron density of the aldehyde moi-
eties thus discouraging their coordination with the activated
60
120
240
8.2
24.5
31.1
a
Reaction condition: Benzaldehyde (1.0 mmol), malononitrile (1.1 mmol), 1-Pd
(0.001 mmol based on Pd), and DMSO-d6 (2 mL) at room temperature under air
atmosphere.
b
Conversion by 1H NMR.
Catalyzed by unmodified MOF, 1.
c
4. Conclusions
In summary, we synthesized a bent aromatic azolium ligand,
1,3-bis(4-carboxyphenyl)imidazolium chloride,
employed to construct MOF, 1. The carbene protons in the 1 were
still intact; therefore, the framework was post-synthetically mod-
ified to design a novel and efficient heterogeneous NHC catalyst
which was
malononitrile.
A very slow conversion rate of 240 min was
observed for methoxybenzaldehyde.

454
C.I. Ezugwu et al. / Journal of Catalysis 344 (2016) 445–454
[21] K. Oisaki, Q. Li, H. Furukawa, A.U. Czaja, O.M. Yaghi, J. Am. Chem. Soc. 132 (27)
(2010) 9262–9264.
(1-Pd) for both Sonogashira cross-coupling and Knoevenagel con-
densation reactions. 1-Pd presents a high density and uniform dis-
tribution of the active site, NHC-Pd(II), in the framework and
retains its catalytic activity for at least four cycles without losing
its structural integrity. In addition, optimization of the reaction
conditions of 1-Pd catalyst for the Sonogashira cross-coupling reac-
tion reveals that an excellent product yield is obtained at the con-
centration of P0.003 mmol based on Pd when hydroxide and
cesium carbonate base is used in polar protic solvents and DMF
at 80 °C for at least 8 h. Furthermore, the catalyst, 1-Pd, achieved
fast (4 min) percentage conversion (100%) for nitrobenzaldehyde
with malononitrile.
Notwithstanding the surging pace of research in the use of
NHCs as catalyst, there is still missing research area on construct-
ing MOFs with these ligands for this vital CAC coupling, Sono-
gashira coupling, and CAC bond forming reactions, known as
Knoevenagel condensation. The new catalyst, 1-Pd, bridges this
gap in research and has promising applications in industry.
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Acknowledgments
The authors would like to express their deep accolade to ‘‘State
Key Lab of Advanced Technology for Materials Synthesis and
Processing” for financial support. F.V. acknowledges the Chinese
Central Government for an ‘‘Expert of the state” position in the pro-
gram of ‘‘Thousand talents”. C.I.E. and M.A.A. express their deep
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