Nitrogen ligands in two-dimensional covalent organic frameworks for metal catalysis

Article abstract of DOI:10.1016/S1872-2067(15)61050-6

We introduced bipyridine ligands into a series of two-dimensional (2D) covalent organic frameworks (COFs) using 2,2′-bipyridine-5,5′-dicarbaldehyde (2,2′-BPyDCA) as a component in the mixed building blocks. The framework of the COFs was formed by the link

Full text of DOI:10.1016/S1872-2067(15)61050-6

Chinese Journal of Catalysis 37 (2016) 468–475
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Nitrogen ligands in two‐dimensional covalent organic frameworks
for metal catalysis
Jianqiang Zhanga, Yongsheng Pengb, Wenguang Lengb,*, Yanan Gaob, Feifei Xub, Jinling Chaia,#
a College of Chemistry, Chemical Engineering and Material Science, Shandong Normal University, Jinan 250014, Shandong, China
b Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
A R T I C L E I N F O
A B S T R A C T
Article history:
We introduced bipyridine ligands into a series of two‐dimensional (2D) covalent organic frame‐
works (COFs) using 2,2’‐bipyridine‐5,5’‐dicarbaldehyde (2,2’‐BPyDCA) as a component in the mixed
building blocks. The framework of the COFs was formed by the linkage of imine groups. The ligand
content in the COFs was synthetically tuned by the content of 2,2’‐BPyDCA, and thus the amount of
metal, palladium(II) acetate, bonded to the nitrogen ligands could be manipulated. Both the bipyri‐
dine ligands and imine groups can coordinate with Pd(II) ions, but the loading position can be var‐
ied, with one ligand favoring binding in the space between adjacent COFs’ layers and the other lig‐
and favoring binding within the pores of the COFs. The Pd(II)‐loaded COFs exhibited good catalytic
activity for the Heck reaction.
Received 9 December 2015
Accepted 18 January 2016
Published 5 April 2016
Keywords:
Covalent organic framework
Metal catalysis
Heck reaction
Nitrogen ligand
Palladium
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
“organic zeolites”, are widely recognized as promising supports
for the immobilization of catalysts for use in organic synthesis.
An emerging class of porous materials is covalent organic
frameworks (COFs) made up of light weight elements (e.g., B, C,
H, N, O) [1–4]. COFs have been investigated extensively in re‐
cent years because of their potential applications in a number
of areas, including catalysis [5–9], gas storage/separation
[10–14], sensing [15,16] and energy conversion [17–24]. COFs
can be synthesized using the general principles of reticular
chemistry where organic building blocks with pre‐designed
geometries and radicals are linked together by the formation of
covalent bonds to give an extended periodic network. The gen‐
eral features of this strategy allow for the flexible control of
pore parameters (e.g., size, shape, volume, distribution) and
also favor the introduction of functional active sites onto the
skeleton of the COFs. As a result, COFs, which are also known as
2D COFs are easier to prepare than their 3D counterparts
because of their structural simplicity [25–29], and they have
therefore been explored in much greater detail for their appli‐
cation to catalysis [5–8]. Consideration of a typical 2D COF
structure reveals that the covalently bonded planar sheets are
stacked together by π–π interactions leading to the formation
of eclipsed or staggered columnar arrays [1]. This vertical
alignment of COFs leads to the formation of one‐dimensional
(1D) open channels, which can significantly enhance the diffu‐
sion of substances and also be used to support catalysts
through the modification of the skeleton of the COFs [6,30].
Furthermore, although classical boroxine‐ and boro‐
nate‐ester‐based COFs are sensitive to moisture [31,32], the
more recently developed COFs derived from nitrile trimeriza‐
* Corresponding author. Tel: +86‐411‐84379167; E‐mail: lengwg@dicp.ac.cn
# Corresponding author. E‐mail: jlchai99@163.com
This work was supported by the National Natural Science Foundation of China (21473196, 21403214), the 100‐Talents Program of Chinese Academy
of Sciences and State Key Laboratory of Fine Chemicals, Dalian University of Technology (KF1415).
DOI: 10.1016/S1872‐2067(15)61050‐6 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 37, No. 4, April 2016

Jianqiang Zhang et al. / Chinese Journal of Catalysis 37 (2016) 468–475
469
tion [33] and Schiff‐base [34–36] reactions exhibit much better
stability at high temperatures and in a range of different sol‐
vents, making them useable at standard catalytic conditions. In
general, 2D imine‐type (i.e., Schiff‐base) COFs that incorporate
Pd ions or Pd nanoparticles have shown superior catalytic per‐
formance in C–C coupling reactions than conventional Pd cata‐
lysts [5,8]. The strong coordination effect between the imine
ligands and noble metal leads to negligible levels of catalyst
leaching after repeated catalytic cycles. Furthermore, given that
the imine groups can be uniformly distributed throughout the
COFs, their one‐to‐one interaction with the catalyst allows for
the isolation of the active sites of the catalyst at a molecular
level [37]. This level of uniform dispersion is not easy to
achieve with conventional porous supports.
methylacetamide, tetrahydrofuran, acetone, dichloromethane,
methanol, ether, chloroform, N,N‐dimethylformamide, toluene,
styrene, hexadecane, hydrochloric acid, fuming nitric acid, po‐
tassium carbonate, anhydrous magnesium sulphate and sodi‐
um periodate were purchased from Sinopharm Chemical Rea‐
gent Co.
2.2. Synthesis
Synthesis of the building blocks. 4,4’,4’’,4’’’‐(pyrene‐1,3,6,8‐
tetrayl) tetraaniline (PyTTA) [30], 2,2’‐bipyridine‐5,5’‐di‐
carbaldehyde (2,2’‐BPyDCA) [40], and 1,3,6,8‐tetrakis(4‐
formylphenyl) pyrene (TFPPy) [41] were prepared according
to literature reports.
Several reports have been published on the application of
2D imine‐linked COFs as catalyst carriers, where the imine
groups were used to bind metallic guests [5,8]. Given that the
density of the imine groups within a certain COF is usually con‐
stant, it is challenging to manipulate the amount of catalyst on
the surface of the support. With this in mind, it was interesting
to introduce different nitrogen ligands into the COFs simulta‐
neously and regulate their contents to allow for controllable
metal loading. Bipyridine units have recently been incorpo‐
rated at the pore edge of metal organic frameworks (MOFs)
[37,38] and COFs [30]. The versatile combination of these ni‐
trogen ligands with metallic components is well recognized in
coordination chemistry. As a result, bipyridine units can be
used as a second type of nitrogen ligand that can be used in
conjunction with imine type ligands in the skeleton of the COFs.
Here, we report the development of a novel procedure for
the control of the nitrogen content of the ligands in 2D COFs
using a pore surface engineering strategy [39]. Imine and bi‐
pyridine groups were both included in the skeleton of the COFs
to allow for controllable metal loading. These two types of ni‐
trogen ligands can both coordinate palladium(II) acetate
(Pd(OAc)2), but the loading position of the Pd(OAc)2 can be
varied. The imine ligand favors binding Pd(OAc)2 in the space
between adjacent layers of the COFs, while the bipyridine lig‐
and would coordinate Pd(II) in the pores of the COF. The cata‐
lytic performances of these COFs were investigated and the
results indicated that the Pd(II)‐based COF exhibited good cat‐
alytic efficiency for the Heck reaction.
Synthesis of 25% BPy COF. 2,2’‐BPyDCA (16 mg, 0.075
mmol), 4,4’‐biphenyl dialdehyde (47 mg, 0.225 mmol) and
PyTTA (85 mg, 0.15 mmol) were placed in a glass ampule ves‐
sel (20 mL), followed by adding a solution of mesity‐
lene/dioxane/3 mol/L acetic acid (3/3/1 by volume; 3.5 mL).
The mixture was sonicated for 5 min and then flash frozen in
liquid nitrogen. The vessel was evacuated to a pressure of ~20
Pa, flame‐sealed, and heated at 120 °C for 3 d. The resulting
precipitate was washed sequentially with tetrahydrofuran (3
times) and acetone (3 times) to give a powder. This was dried
at 120 °C under vacuum for 12 h to give the desired product in
88% yield. Elemental analysis (%) calcd. for (C67H41N5)n: C
87.9; H 4.5; N 7.6; Found: C 82.2; H 4.7; N 6.8.
Synthesis of 50% BPy COF. This material was synthesized in
the same way as the 25% BPy COF, except for the feed ratio of
the two dialdehydes: 2,2’‐BPyDCA (31 mg, 0.15 mmol) and
4,4’‐biphenyl dialdehyde (31 mg, 0.15 mmol). The product was
isolated as a powder in 88% yield. Elemental analysis (%)
calcd. for (C66H40N6)n: C 86.5; H 4.4; N 9.1; Found: C 80.9; H 4.5;
N 7.8.
Synthesis of 75% BPy COF. This material was synthesized in
the same way as the 25% BPy COF, except for the feed ratio of
the two dialdehydes: 2,2’‐BPyDCA (47 mg, 0.225 mmol) and
4,4’‐biphenyl dialdehyde (16 mg, 0.075 mmol). The product
was isolated as a powder in 83% yield. Elemental analysis (%)
calcd. for (C65H39N7)n: C 85.1; H 4.2; N 10.7; Found: C 78.7; H
4.5; N 9.1.
Synthesis of 100% BPy COF. This material was synthesized
in the same way as the 25% BPy COF, except for the feed ratio
of the two dialdehydes: 2,2’‐BPyDCA (64 mg, 0.30 mmol) and
4,4’‐biphenyl dialdehyde (none). The product was isolated as a
powder in 76% yield. Elemental analysis (%) calcd. for
(C64H38N8)n: C 83.7; H 4.1; N 12.2; Found: C 74.8; H 4.7; N 10.3.
Synthesis of Pd(II)@X% BPy COF. Pd(OAc)2 was incorpo‐
rated into X% BPy COFs using a simple solution infiltration
method [5,8]. The resulting composites were named
Pd(II)@X% BPy COFs. In a typical procedure, X% BPy COFs
(0.03 mmol, calculated using the framework repeating unit)
was treated with a solution of Pd(OAc)2 in dichloromethane (15
mL). The resulting suspension was stirred at room temperature
for 12 h. The precipitate was washed with excess dichloro‐
methane to remove any dissociated Pd(OAc)2 and then dried at
120 °C under vacuum for 12 h to give Pd(II)@X% BPy COFs.
2. Experimental
2.1. Materials
All the reagents were used as received without further puri‐
fication. 1,3,6,8‐tetrabromopyrene, 4‐(4,4,5,5‐tetramethyl‐
1,3,2‐dioxaborolan‐2‐yl)aniline, 4‐formylphenylboronic acid,
4,4’‐biphenyl dialdehyde, Pd(OAc)2, and tetrakis(tri‐
phenylphosphine) palladium(0) were purchased from TCI
Chemicals.
5,5′‐dimethyl‐2,2′‐dipyridyl,
tert‐butoxy
bis(dimethylamino)methane, and benzidine dihydrochloride
were purchased from Aldrich. Iodobenzene, 4‐iodotoluene,
4‐iodoanisole, 4‐iodobenzonitrile, and 1,4‐diiodobenzene were
purchased from Aladdin. Mesitylene, dioxane, acetic acid, di‐

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Jianqiang Zhang et al. / Chinese Journal of Catalysis 37 (2016) 468–475
Pd(OAc)2 was added in excess and the exact fed amount was
adjusted based on the total nitrogen content of the COFs.
Pd(OAc)2 was dosed as follows: 28 mg, 0.12 mmol (X = 25); 33
mg, 0.15 mmol (X = 50); 39 mg, 0.17 mmol (X = 75); and 44 mg,
0.20 mmol (X = 100).
pressed pellet. Gas chromatography (GC, Agilent 7890A) was
used to monitor the conversion in the reaction using hexade‐
cane as an internal standard and dichloromethane as solvent.
1H and 13C nuclear magnetic resonance (NMR) spectra were
recorded by a Bruker Advance III 400 MHz NMR spectrometer
(Bruker BioSpin Corporation, Fällanden, Switzerland).
2.3. Procedure for the Heck reaction
3. Results and discussion
A mixture of aryl iodide (204 mg, 1.0 mmol), styrene (120
mg, 1.1 mmol), potassium carbonate (276 mg, 2.0 mmol) and
N,N‐dimethylformamide (5 mL) was stirred and purged with
N2. Pd(II)@X% BPy COFs were then added as catalyst (5 mg,
containing slightly less than 0.01 mmol of Pd element). The
resulting mixture was heated at 105 °C under reflux for 5 h. The
mixture was then cooled to ambient temperature and filtered,
and the solid (catalyst and potassium carbonate blend) was
washed with dichloromethane (3 × 10 mL). The combined fil‐
trates were washed with water (20 mL). The separated organic
phase was concentrated under vacuum at 90 °C to give a slurry,
which was mixed with water (20 mL) to precipitate the crude
product. This powder sediment was filtered and recrystallized
from chloroform to give the pure product. Iodobenzene was
selected as the substrate for the recyclability tests with
Pd(II)@75% BPy COF as the catalyst. The catalyst was removed
by filtration after each cycle together with the K2CO3, which
was removed by washing with water. The catalyst was then
dried under vacuum at 120 °C and reused for another round of
catalysis.
Two dialdehyde monomers, namely 2,2’‐BPyDCA and
4,4’‐biphenyl dialdehyde, were chosen as the building blocks to
adjust the nitrogen content of the 2D COFs (Scheme 1). These
two monomers were blended together in different molar ratios
and reacted with PyTTA using a Schiff‐base condensation [30].
Four different 2D imine‐linked COFs were synthesized and
named as X% BPy COF (X = 25, 50, 75, 100), where X% repre‐
sented the molar percentage of 2,2’‐BPyDCA present in the
dialdehyde blend. The density of the imine groups in the 2D
COFs was constant, but the number of bipyridine moieties on
the wall of the pores was varied by the feed ratio of the
2,2’‐BPyDCA monomer.
FT‐IR spectra of the four X% BPy COFs showed a C=N
stretching vibration at 1622 cm–1 (Fig. 1), which indicated the
successful formation of the imine linkage. The PXRD patterns of
the X% BPy COFs revealed that there were negligible difference
in the crystalline structure. Diffraction peaks were observed at
2θ = 3.2°, 4.6°, 6.4°, 9.7°, 12.9° and 23.8°, which were attributed
to the (110), (020), (220), (330), (440) and (001) facets, re‐
spectively (Fig. 2). The use of lattice modeling and the Pawley
refinement process led to an eclipsed AA stacking model that
could reproduce the PXRD results in peak position and intensi‐
ty. These results showed that the X% BPy COFs belonged to the
P21212 space group with the unit cell parameters of a = 4.214
2.4. Characterization
Elemental analysis was performed by an organic elemental
analyzer (vario MACRO cube, Elementar, Germany). Inductively
coupled plasma optical emission spectroscopy (ICP‐OES) was
conducted by ICP‐OES 7300DV (PerkinElmer). The sample was
firstly calcined at 1000 °C in air for 12 h to burn off organic
moieties. The residue was dissolved by aqua regia and then
diluted by water for ICP‐OES testing. Fourier transform infra‐
red (FT‐IR) measurements were carried out on a Bruker spec‐
trophotometer (Model TENSOR27) with powder pressed KBr
pellets. Powder X‐ray diffraction (PXRD) analysis was carried
out on a Rigaku RINT D/Max 2500 powder diffraction system,
using a Cu Kα radiation (λ = 0.15432 nm). Thermogravimetric
analysis (TGA, STA449F3, NETZSCH, Germany) was performed
from room temperature to 750 °C at a heating rate of 10
°C/min and a N2 flow rate of 20 mL/min. The nitrogen phy‐
sisorption experiment was conducted at –196 °C on a
QUADRASORB SI gas sorption system (Quantachrome Instru‐
ments). The sample was degassed at 120 °C under vacuum
before testing. The specific surface areas were calculated by the
Brunauer‐Emmett‐Teller (BET) method. The pore size distri‐
bution was evaluated by the nonlocal density function theory
(NLDFT) method. The morphology of sample was observed by
a transmission electron microscope (TEM, Tecnai G2 F30, FEI
Company, operating at 120 kV). X‐ray photoelectron spectros‐
copy (XPS) was recorded by an ESCALAB 250Xi equipped with
Al Kα radiation (1486.6 eV, 200 W) on a sample powder
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
25% BPy COF
50% BPy COF
X=25
X=50
CHO
CHO
CHO
4,4′-Biphenyl
dialdehyde
(100-X)%
H2
H2
N
N
NH2
N
N
+
+
CHO
2,2'-BPyDCA
NH2
PyTTA
X%
X=100
X=75
75% BPy COF
100% BPy COF
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Scheme 1. Pore surface engineering strategy used to modulate the
nitrogen content of the 2D imine‐type COFs.

Jianqiang Zhang et al. / Chinese Journal of Catalysis 37 (2016) 468–475
471
well‐defined crystalline COFs (named BiPh COFs), which sug‐
gested that the addition of 2,2’‐BPyDCA was activated and act‐
ed in concert with 4,4’‐biphenyl dialdehyde during the for‐
mation of the COFs to give well‐defined crystalline structures
[42]. The synthesized X% BPy COFs were insoluble in water
and a range of common organic solvents. TGA curves of the
different X% BPy COFs revealed they were highly stable, with
decomposition temperatures of almost 500 °C (Fig. 4). These
results revealed that the COFs would be good catalysts or cata‐
lyst carriers for catalytic applications. The porous properties of
the COFs were characterized to give the BET surface areas of
538, 1554, 1438 and 1288 m2/g for X values of 25, 50, 75 and
100, respectively, with pore sizes of 2.6 or 2.7 nm. TEM images
revealed clear 1D channels with a diameter of 2 nm (Fig. 5(a)),
which provided further demonstration of the formation of
well‐ordered mesoporous pores.
100% BPy COF
75% BPy COF
50% BPy COF
25% BPy COF
2000 1800 1600 1400 1200 1000 800 600 400
Wavenumber (cm1)
Fig. 1. FTIR spectra of X% BPy COFs (X = 25, 50, 75, 100).
It was well known that imine and bipyridine units can both
coordinate with a variety of noble metals (e.g., Pd, Pt, Rh, Ru
and Ir). As a readily available catalyst for organic synthesis,
Pd(OAc)2 was selected as the model system to be incorporated
into the different X% BPy COFs using a simple solution infiltra‐
tion method. The resulting composites are referred to as
Pd(II)@X% BPy COFs. It was envisaged that the formation of a
strong interaction between the Pd(II) complex and the nitrogen
ligands would lead to the immobilization of Pd(OAc)2 on the
COFs. The TEM image revealed that there was very little ag‐
gregation of Pd(OAc)2 to block the channels of the COFs (Fig.
5(b)). The thermal stability of the COFs was also well preserved
following the loading of the Pd(OAc)2. XPS was performed to
nm, b = 3.690 nm, c = 0.374 nm and α = β = γ = 90°. These mod‐
eling results were in good agreement with the experimental
profiles and gave Rwp and Rp values of 6.27% and 4.74%, re‐
spectively. An alternative AB staggered model did not match
the observed data. Elemental analysis confirmed that the ni‐
trogen content of the X% BPy COF was proportional to the feed
ratio of 2,2’‐BPyDCA, and that it could be tuned in the range of
6.8 wt% to 10.3 wt% (Fig. 3). The nominal nitrogen content
was higher than the measured value due to residual organic
solvent or adsorbed H2O in the pores [34]. It is noteworthy that
the reaction of only 4,4’‐biphenyl dialdehyde with PyTTA under
the same conditions did not result in the formation of
14
12
10
8
6
4
2
0
Measured values
Theoretical values
20
40
60
80
100
2,2'-BPyDCA feeding ratio (X%)
Fig. 3. Elemental analysis of the nitrogen content in the X% BPy COFs.
100
80
25% BPy COF
50% BPy COF
75% BPy COF
Fig. 2. (a) PXRD patterns of 100% BPy COF: experimental ((1), red line)
Pawley refined ((2), green dotted line) difference between experi‐
mental and calculated data (3), calculated for AA‐stacking (4) and
AB‐stacking (5); (b) Unit cell structure of 100% BPy COF using the AA
stacking model along the z axis; (c) Unit cell structure of 100% BPy COF
using the AA stacking model along the y axis; (d) Unit cell structure of
100% BPy COF using the AB stacking model along the z axis; (e) Unit
cell structure of 100% BPy COF using the AB stacking model along the y
axis.
60
100% BPy COF
100 200 300 400 500 600 700 800
Temperature (oC)
Fig. 4. TGA curves for the X% BPy COFs.

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Jianqiang Zhang et al. / Chinese Journal of Catalysis 37 (2016) 468–475
25
(b)
(a)
20
15
10
5
2 nm
Calculated results
Measured results
80 100
0
20 nm
20 nm
20
40
60
2,2'-BPyDCA feeding ratio (X%)
Fig. 5. TEM images of (a) 75% BPy COF and (b) Pd(II)@75% BPy COF.
Fig. 7. Nominal and measured Pd loading in the Pd(II)@X% BPy COFs.
determine the coordinated position of Pd(OAc)2 (Fig. 6). Free
Pd(OAc)2 showed a characteristic peak for Pd(II) 3d5/2 at 338.3
eV. This peak was negatively shifted to 337.9 eV when
Pd(OAc)2 was coordinated to a 2,2’‐BPyDCA monomer. The
bipyridine‐free BiPh COF mentioned above contained only
imine groups, and the coordination of these groups to Pd(OAc)2
resulted in a signal at 337.7 eV. In contrast, all four of the
Pd(II)@X% BPy COFs gave a Pd(II) 3d5/2 peak at 337.8 eV,
which indicated that the bipyridine and imine groups were
both involved in the binding of Pd(OAc)2.
The nominal maximum Pd loading was calculated to have a
quasi‐linear relationship with the total amount of nitrogen
ligands (Fig. 7). However, the measured values were lower than
these nominal values and did not give a linear relationship with
the total nitrogen content of the ligands (Fig. 7). Higher Pd
contents were found in Pd(II)@50% BPy COF (18.2 wt% Pd
element) and Pd(II)@75% BPy COF (18.7 wt% Pd element).
From consideration of the BET surface areas, it was likely that
the structural regularity of the COF scaffolds had an impact on
the loading of Pd(OAc)2. The insufficient occupation of the ni‐
trogen ligands with Pd(OAc)2 (with a molecular size of 1.06 ×
0.5 × 0.43 nm [43]) was probably caused by steric limitation
since the individual sheets of the 2D COFs were stacked in an
eclipsed mode with an interlayer distance of 0.374 nm (Fig. 2).
This would have resulted in the balancing of the ligand content
and the surface area of the COFs to give the optimum loading.
The Pd(II)@X% BPy COFs gave different Pd loadings in the
range of 14.3 to 18.7 wt%, which are among the highest re‐
ported values with a similar porous polymer matrix [44]. Alt‐
hough the Pd(II)@X% BPy COFs still have mesoporous pores,
their BET surface areas were decreased significantly to 283,
982, 847 and 731 m2/g and the pore diameter of the COFs de‐
creased to 2.3, 2.2, 2.1 and 2.0 nm, respectively, for X values of
25, 50, 75 and 100 because of the presence of the guest mole‐
cules. To further elucidate the coordination position of the
Pd(OAc)2 molecules, another bipyridine‐free 2D imine‐type
COF, named as TF‐BD COF, was also synthesized. TF‐BD COF
can be regarded as the analogue of the BiPh COF as the TF‐BD
COF has a similar structure to BiPh COF but with much better
crystallinity. The TF‐BD COF showed negligible change in its
pore size both before (2.6 nm) and after (2.6 nm) the loading of
Pd(OAc)2. This result indicated that the Pd(OAc)2 molecules
were coordinated to the imine groups located between adja‐
cent COF sheets, since they did not have an influence on the
pore diameter [5]. In contrast, the Pd(OAc)2 molecules coordi‐
nated to bipyridine groups occupied the pore space, and there‐
fore led to a reduction in pore size (Fig. 8) [30]. As the percent‐
age of bipyridine units in the COFs increased, their loading ratio
with Pd(OAc)2 also increased, which led to the gradual de‐
crease in the pore size.
338.3
Pd2+ 3d5/2
Pd2+ 3d3/2
To characterize the catalytic performance of the COFs, the
Pd(II)@X% BPy COFs were used as heterogeneous catalyst for
the classical Heck reaction, which is a powerful and versatile
337.9
337.8
Pd(OAc)2
N
337.8
Pd(II)@2,2'-BPyDCA
337.8
337.8
OAc
Pd(II)@100% BPy COF
Pd(II)@75% BPy COF
Pd(II)@50% BPy COF
N
N
N
Pd
Pd
OAc
OAc
OAc
AcO
337.7
Pd(II)@25% BPy COF
Pd(II)@BiPh COF
Pd
OAc
N
350 348 346 344 342 340 338 336 334 332
Binding energy (eV)
N
N
N
Pd
OAc
AcO
Fig. 6. XPS results (Pd 3d) for the samples showing the coordination of
Pd(OAc)2. Pure Pd(OAc)2 and the complex formed between Pd(OAc)2
and 2,2’‐BPyDCA (Pd(II)@2,2’‐BPyDCA) were tested for comparison.
Fig. 8. Scheme for the regulated Pd(OAc)2 coordination on bipyridine
and imine groups.

Jianqiang Zhang et al. / Chinese Journal of Catalysis 37 (2016) 468–475
473
Table 1
100
The series of Pd(II)@X% BPy COFs catalysts for the Heck reaction.
80
60
40
20
0
R
Pd(II)@X% BPy COF
R
I
-1
-2
-3
-4
-5
Entry
1
X%
25
50
75
100
25
50
75
100
25
50
R
H
Conversiona (%) Yieldb (%)
98
99
99
99
98
99
99
99
95
97
97
96
79
80
83
81
92
94
96
95
92
94
96
95
93
95
96
95
90
93
94
91
73
77
80
76
88
91
92
90
2
3
4
H
H
H
0
1
2
3
4
5
6
5
6
7
8
CH3
CH3
CH3
CH3
OCH3
OCH3
OCH3
OCH3
CN
CN
CN
CN
I
Time (h)
0
1
2
3
4
5
6
Time (h)
9
Fig. 9. Reaction kinetics of Pd(II)@75% BPy COF as catalyst in the Heck
reaction. Reacting conditions: iodobenzene (1.0 mmol), styrene (1.1
mmol), potassium carbonate (2.0 mmol), Pd(II)@75% BPy COF (5 mg,
containing slightly less than 0.01 mmol of Pd element),
N,N‐dimethylformamide (5 mL), 105 °C, 5 h, N2 atmosphere. The con‐
version was monitored by GC using hexadecane as internal standard.
The reaction was completed in 5 h. Inset: plot of log of the remaining
iodobenzene concentration versus time which exhibited a first‐order
rate constant of k = 0.542 ± 0.034 h–1.
10
11
12
13
14
15
16
17
18
19
20
75
100
25
50
75
100
25
50
75
100
I
I
I
rate constant of k = 0.542 ± 0.034 h–1 when Pd(II)@75% BPy
COFs was used as catalyst with iodobenzene as the substrate
(Fig. 9). The good catalytic properties of the COFs prepared in
the current study can be attributed to their higher Pd loadings
(14.3–18.7 wt%) and the larger surface areas of the catalyst
carriers (283–982 m2/g) compared to those reported in the
literature [44]. The Pd(II)@75% BPy COF afforded the best
catalytic performance of the four candidates tested, and was
used further to evaluate its recyclability. Superior catalytic ac‐
tivity was maintained (>90% isolated yield) by this catalyst for
up to four cycles (Fig. 10(a)). The crystalline structure of the
scaffold of the COF was also found to be well preserved after
each cycle (Fig. 10(b)), which highlighted the high stability of
this catalyst in the catalytic environment. The heterogeneous
nature of the catalyst was also confirmed using a leaching test.
In a typical procedure, the supernatant was isolated after 1
cycle of the catalytic experiment and mixed with freshly pre‐
Reaction conditions: aryl iodide (1.0 mmol), styrene (1.1 mmol), potas‐
sium carbonate (2.0 mmol), Pd(II)@X% BPy COF (5 mg, containing
about 0.01 mmol of Pd), N,N‐dimethylformamide (5 mL), 105 °C, 5h, N2
protection. A larger charge of styrene (2.2 mmol) was added when
1,4‐diiodobenzene containing twice as many functional groups was
used as substrate. a Monitored by gas chromatography using hexade‐
cane as an internal standard and dichloromethane as a solvent. b Isolat‐
ed yield.
method for the synthesis of arylated alkenes. A model reaction
between aryl iodide and styrene was selected and carried out
under general C–C coupling reacting conditions [8,37]. All the
Pd(II)@X% BPy COFs exhibited high catalytic efficiency for the
Heck reaction of a variety of different substrates and gave the
desired products in high yields. The results of these reactions
are summarized in Table 1. Kinetic studies also showed a large
100
(a)
(b)
80
60
40
20
0
Fresh Pd(II)@75% BPy COF
After the 1st cycle
After the 4th cycle
1
2
3
4
5
10
15
20
25
30
2/(o )
Cycle number
Fig. 10. (a) Recyclability test of the Pd(II)@75% BPy COF catalyst for the Heck reaction. (b) PXRD patterns of the Pd(II)@75% BPy COF before and
after the recycling test. Iodobenzene was selected as the model substrate for this reaction. All other reaction conditions see Table 1.

474
Jianqiang Zhang et al. / Chinese Journal of Catalysis 37 (2016) 468–475
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ICP‐OES measurements also confirmed that no Pd element was
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4. Conclusions
We synthesized a series of 2D X% BPy COFs (X = 25, 50, 75,
100) containing two different nitrogen ligands (imine and bi‐
pyridine groups) as a support for the immobilization of
Pd(OAc)2. The total nitrogen content can be tuned from 6.8 to
10.3 wt% by varying the feed ratio of 2,2’‐BPyDCA. Both bipyr‐
idine and imine groups behaved in a similar manner except for
the binding location: imine groups favored binding in the space
between adjacent COFs layers and bipyridine groups favored
binding in the pores of the COFs. The Pd loading was deter‐
mined by both the ligand content and surface area of the COFs,
and could be varied between 14.3 and 18.7 wt%. Due to the
high surface area and high Pd loading, the Pd(II)@X% BPy
COFs exhibited outstanding catalytic efficiency in the Heck re‐
action. Pd(II)@75% BPy COF was the best catalyst of the four
and its good recyclability and stability were also confirmed.
These results offer a unique opportunity for the tuning of the
structural parameters of COF‐based heterogeneous catalysts.
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Graphical Abstract
Chin. J. Catal., 2016, 37: 468–475 doi: 10.1016/S1872‐2067(15)61050‐6
Nitrogen ligands in two‐dimensional covalent organic frameworks for metal catalysis
Jianqiang Zhang, Yongsheng Peng, Wenguang Leng*, Yanan Gao, Feifei Xu, Jinling Chai*
Shandong Normal University; Dalian Institute of Chemical Physics, Chinese Academy of Sciences
I
+
R
X=50
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