Sophisticated Design of Covalent Organic Frameworks with Controllable Bimetallic Docking for a Cascade Reaction

Article abstract of DOI:10.1002/chem.201601334

Precise control of the number and position of the catalytic metal ions in heterogeneous catalysts remains a big challenge. Here we synthesized a series of two-dimensional (2D) covalent organic frameworks (COFs) containing two different types of nitrogen ligands, namely imine and bipyridine, with controllable contents. For the first time, the selective coordination of the two nitrogen ligands of the 2D COFs to two different metal complexes, chloro(1,5-cyclooctadiene)rhodium(I) (Rh(COD)Cl) and palladium(II) acetate (Pd(OAc)₂), has been realized using a programmed synthetic procedure. The bimetallically docked COFs showed excellent catalytic activity in a one-pot addition–oxidation cascade reaction. The high surface area, controllable metal-loading content, and predesigned active sites make them ideal candidates for their use as heterogeneous catalysts in a wide range of chemical reactions.

Full text of DOI:10.1002/chem.201601334

DOI: 10.1002/chem.201601334
Communication
&
Covalent Organic Frameworks
Sophisticated Design of Covalent Organic Frameworks with
Controllable Bimetallic Docking for a Cascade Reaction
+
[a]
+ [a]
[a]
[a]
[b]
[a]
Wenguang Leng , Yongsheng Peng , Jianqiang Zhang, Hui Lu, Xiao Feng, Rile Ge,
[a]
[b]
[a]
[a]
Bin Dong, Bo Wang, Xiangping Hu, and Yanan Gao*
The strong coordination between the organic ligands and
metal ions is a general method to introduce metal active sites
in catalysts. Given that ligands can be uniformly distributed
throughout COFs, their one-to-one interaction with catalysts
allows for the effective isolation of the active sites of the cata-
Abstract: Precise control of the number and position of
the catalytic metal ions in heterogeneous catalysts re-
mains a big challenge. Here we synthesized a series of
two-dimensional (2D) covalent organic frameworks (COFs)
containing two different types of nitrogen ligands, namely
imine and bipyridine, with controllable contents. For the
first time, the selective coordination of the two nitrogen li-
gands of the 2D COFs to two different metal complexes,
chloro(1,5-cyclooctadiene)rhodium(I) (Rh(COD)Cl) and pal-
[29]
lyst at a molecular level. Furthermore, various ligands with
different contents and functions can also be introduced into
the skeleton of the COFs through sophisticated design. Evi-
dently, these features are not easy to achieve with convention-
al porous supports. With this in mind, we have developed
a novel procedure allowing for the precisely controlled place-
ment of two nitrogen ligands, imine (4,4’,4’’,4’’’-(pyrene-1,3,6,8-
tetrayl)tetraaniline (PyTTA)) and bipyridine (2,2-bipyridyl-5,5-di-
ladium(II) acetate (Pd(OAc) ), has been realized using a pro-
2
grammed synthetic procedure. The bimetallically docked
COFs showed excellent catalytic activity in a one-pot addi-
tion–oxidation cascade reaction. The high surface area,
controllable metal-loading content, and predesigned
active sites make them ideal candidates for their use as
heterogeneous catalysts in a wide range of chemical reac-
tions.
aldehyde (2,2’-BPyDCA)), within 2D COFs (Figure 1a) using
[30]
a three-component condensation system.
Four different
types of 2D imine-linked COFs were fabricated and named as
X% BPy COF (X=25, 50, 75, 100), in which X% represents the
molar percentage of 2,2’-BPyDCA present in the dialdehyde
blends. In this way, although the density of the imine groups
across the series of 2D COFs remained constant, the number
of bipyridine moieties on the channel walls could be varied ac-
cording to the feeding ratio of the 2,2’-BPyDCA monomer. As
such, the selective coordination of the two nitrogen ligands to
two different metal complexes, chloro(1,5-cyclooctadiene)rho-
[1–4]
Covalent organic frameworks (COFs),
an emerging class of
crystalline porous polymers that allow the atomically precise
integration of building blocks into periodic networks, have re-
[
5–12]
ceived much attention in catalysis,
gas storage/separa-
[
13–17]
[18,19]
[20–27]
tion,
sensing,
and energy conversion.
The flexible
dium(I) (Rh(COD)Cl) and palladium(II) acetate (Pd(OAc) ), can
2
regulation of the pore parameters (e.g., size, shape, volume,
and distribution) and easy introduction of functional active
sites to the skeleton of the COFs make them promising plat-
forms for the immobilization of catalysts in organic synthesis.
In particular, two-dimensional (2D) COFs possess eclipsed or
staggered columnar arrays with one-dimensional (1D) open
channels, which not only significantly enhance the diffusion of
substances, but can also be used to dock catalysts through the
be realized using a programmed synthetic procedure (Fig-
ure 1b).
Fourier transform infrared (FTIR) spectroscopy provided
direct evidence for the formation of imine linkages in X% BPy
COFs (Figure S1 in the Supporting Information). Elemental
analysis showed that the actual nitrogen contents of the COFs
were close to the calculated values (Figure S2). Thermogravi-
metric analysis (TGA) revealed them to be highly thermostable
(Figure S3). The 100% BPy COF exhibited strong powder X-ray
diffraction (PXRD) peaks at 3.28, 4.68, 6.48, 9.78, 12.98 and 23.88,
corresponding to the (110), (020), (220), (330), (440) and (001)
[6,28]
modification of the building blocks of COFs.
+
+
[
a] Dr. W. Leng, Y. Peng, J. Zhang, Dr. H. Lu, Dr. R. Ge, Dr. B. Dong,
Prof. X. Hu, Prof. Y. Gao
[28]
facets, respectively. The similar PXRD patterns were also ob-
Dalian National Laboratory for Clean Energy
served for the other X% BPy COFs, indicating that they have
a similar crystal structure (Figure 1c). The use of lattice-model-
ing and Pawley refinement processes led to an eclipsed AA
stacking model (Figure S4), suggesting the presence of open
channels (Figure 1d). Nitrogen-adsorption isotherms exhibited
reversible type-IV curves for the X% BPy COFs, indicative of
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
457 Zhongshan Road, Dalian 116023 (P.R. China)
E-mail: ygao@dicp.ac.cn
[
b] Dr. X. Feng, Prof. B. Wang
Key Laboratory of Cluster Science, Ministry of Education of China
School of Chemistry, Beijing Institute of Technology
5
South Zhongguancun Street, Beijing 100081 (P.R. China)
+
a mesoporous character (Figure S5). The pore parameters (SBET,
[
] These authors contributed equally to this work.
volume, size distribution, and interlayer distance) are summar-
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201601334.
ized in Table 1. The high surface area, good thermostability,
Chem. Eur. J. 2016, 22, 9087 – 9091
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Communication
Figure 1. a) Use of a three-component condensation system to modulate the nitrogen content of the 2D imine-type COFs. b) Designed strategies for the
monometallic (Route 1) and bimetallic docking (Route 2). c) PXRD patterns of the X% BPy COFs. d) Open channels of the COFs. e) PXRD patterns of the
II
I
II
Pd @X% BPy COFs. f) PXRD patterns of the Rh /Pd @X% BPy COFs. g) Open channels of the metal loaded COFs.
Table 1. The pore parameters of the COFs before and after metal dock-
ing
(Figure 1b Route 1) by using a simple solution-infiltration
[
5,8]
method,
and the resulting composites shall be referred to,
II
hereafter, as Pd @X% BPy COFs. PXRD (Figure 1e) and FTIR
spectroscopy (Figure S6) analysis revealed that the structural
frameworks of the COFs had been well maintained, while the
evident darkening in the color of the solids indicated the suc-
cessful loading of Pd(OAc)2 (Figure S7). X-ray photoelectron
spectroscopy (XPS) was performed to determine the docking
position of the Pd(OAc)2 (Figure 2a). Bipyridine-free
COFs
S
BET
2
Pore size Pore volume Interlayer
À1
3
À1
[a,b]
[
m g
]
[nm]
[cm g
]
distance [Š]
2
5
7
1
5% BPy COF
0% BPy COF
5% BPy COF
538
2.7
0.53
1.26
1.11
0.98
0.30
0.59
0.53
0.45
0.23
0.26
0.19
3.77
3.76
3.74
3.73
3.71
3.70
3.68
3.65
3.74
3.71
3.68
1554
1438
1288
283
2.6
2.7
2.6
2.3
2.2
2.1
2.0
2.4
2.4
2.4
00% BPy COF
II
Pd @25% BPy COF
Pd @50% BPy COF
Pd @75% BPy COF
Pd @100% BPy COF
Rh/Pd @25% BPy COF
Rh/Pd @50% BPy COF 146
II
982
847
731
115
[
31]
II
0% BPy COF exhibited a N 1s signal at 398.8 eV, which shift-
II
ed to 399.2 eV after loading with Pd(OAc) , revealing that
2
I
II
[32]
Pd(OAc) coordinated to the imine groups. The coordination
2
I
II
I
II
between the bipyridine monomers and Pd(OAc) gave a signal
2
Rh/Pd @75% BPy COF
97
at 400.4 eV. In comparison, a broad peak overlapping both sig-
nals at 399.2 eV and 400.4 eV can be seen for all
[
[
a] Calculated by the Bragg equation from the PXRD data in Figure S9.
b] Interlayer distance of the (001) facet.
II
Pd @X% BPy COFs (X=25, 50, 75, 100), and the intensity of the
peak at 400.4 eV increased with increasing 2,2’-BPyDCA con-
tent. This result proves that both nitrogen ligands participate
and the single type of mesopores revealed that the COFs
could be used as catalysts or catalyst carriers.
in the coordination with Pd(OAc) . We also designed and pre-
2
pared another bipyridine-free 2D imine-type COF (TF-BD COF,
from benzidine (BD) and 1,3,6,8-tetrakis(4-formylphenyl)pyrene
(TFPPy)) that could be regarded as the analogue of the
As a preliminary investigation for the bimetallic docking,
monometallic Pd(OAc) was first loaded into the X% BPy COFs
2
Chem. Eur. J. 2016, 22, 9087 – 9091
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Communication
I
Rh @X% BPy COFs. No evident color change was observed for
either 0% BPy COF or TF-BD COF when the samples were
loaded with Rh(COD)Cl. Inductively coupled plasma optical
emission spectroscopy (ICP-OES) measurements also suggested
a negligible Rh content in the two COFs. These results revealed
that the large and rigid Rh(COD)Cl molecules did not fit into
the space between adjacent COFs sheets. However, for the
I
Rh @X% BPy COFs (X=25, 50, 75, 100) a darkening in the color
was observed, which indicated the successful loading of
Rh(COD)Cl within the COFs (Figure S11). PXRD and FTIR spec-
troscopy analysis revealed that the structural frameworks of
the COFs had been well maintained (Figure S12). The theoreti-
cal maximum Rh-loading content of the X% BPy COFs, calculat-
ed based on the amount of bipyridine, was 5.0, 8.9, 12.0, and
1
4.6 wt% for X values of 25, 50, 75, and 100, respectively. How-
ever, the measured values by ICP-OES were lower than those
determined theoretically (4.9, 7.8, 7.5, and 7.9 wt%, corre-
spondingly). It is evident that when the molar percentage of
2
,2’-BPyDCA is 25%, the Rh-loading content (4.9 wt%) is close
to the theoretical value (5.0 wt%), whereas the measured
values remain almost unchanged when X is increased to 50,
7
5, and 100%. Considering that the Rh(COD)Cl molecule is
large and rigid, it can be assumed that the docking of
Rh(COD)Cl reaches its saturation when the molar percentage
of 2,2’-BPyDCA is 50%.
Figure 2. XPS results (N 1s) for the different samples before and after dock-
ing with a) Pd(OAc) and successive docking with b) Rh(COD)Cl and
Pd(OAc)
2
The subsequent loading of Pd(OAc) was carried out and the
2
2
.
I
II
resulting samples were named as Rh/Pd @X% BPy COFs. PXRD
Figure 1 f) and FTIR spectroscopy (Figure S13) results revealed
(
0
% BPy COF, as both have a similar structure, but with
that the skeleton of the COFs remained intact, while the color
of the materials became further darker in appearance (Fig-
ure S14) indicating the successful bimetallic loading. XPS analy-
sis was performed for the different samples after successive
a higher degree of crystallinity of the 0% BPy COF (Figure S8).
The Brunauer–Emmett–Teller (BET) surface area and the pore
diameter of the X% BPy COFs (X=25, 50, 75 and 100) both de-
creased after loading with Pd(OAc) (Table 1). In contrast, the
Rh(COD)Cl and Pd(OAc) treatments (Figure 2b). Bipyridine-free
2
2
I
II
TF-BD COF showed negligible changes in the pore size before
Rh /Pd @0% BPy COF exhibited a similar N 1s peak compared
II
and after the docking of Pd(OAc) (Figure S8). This result indi-
to that of Pd @0% BPy COF, further confirming that Rh(COD)Cl
2
cated that the Pd(OAc)2 molecules coordinate to the imine
groups that are located between adjacent COF sheets, which
could hardly enter the void space between adjacent COF
layers. The signal at 398.8 eV was attributed to the coordina-
tion between Rh(COD)Cl and bipyridine. This signal could also
[5]
could not have an influence on the pore diameter. Instead,
I
II
these resided Pd(OAc) molecules pulled the interlayer distance
be seen in the Rh/Pd @X% BPy COFs (X=25, 50, 75, 100), con-
2
I
of adjacent COF sheets closer (Table 1), since the PXRD peak
assigned to the (001) facet gradually shifted to higher angles
firming that Rh mainly interacted with the bipyridine units.
When the bipyridine ratio was low (X=25), the peak at
399.2 eV indicated that Pd(OAc)2 mainly coordinated with
imine groups. However, as the bipyridine ratio increased, the
peak at 400.2 eV was gradually intensified. This suggests that
a proportion of the bipyridine sites could not be occupied by
Rh(COD)Cl due to steric limitations, and the smaller and more
(
Figure S9a and b). In contrast, coordination of Pd(OAc) to the
2
bipyridine groups could occupy the pore space, and therefore
[28]
lead to a reduction in the pore size. The measured Pd-load-
ing contents were lower than the theoretical maximum values
(
Figure S10), revealing that the structural regularity of the COF
scaffolds was likely to have an impact on the accommodation
of Pd(OAc)2.
flexible Pd(OAc) had a better chance of being immobilized by
2
the residual bipyridine units.
I
II
To realize the bimetallic loading, we used a programmed
synthetic procedure (Figure 1b, Route 2). Rh(COD)Cl (7.4”6.6”
The BET surface areas of the Rh /Pd @X% BPy COFs were
II
much lower than those of the Pd @X% BPy COFs (Figure S15
[
33]
5
4
.3 Š)
is bulky and more rigid than Pd(OAc)2 (10.6”5.0”
and Table 1), again indicating the successful bimetallic loading.
[
33]
I
II
.3 Š), but could still be readily immobilized by the bipyri-
The pore diameters of all the Rh /Pd @X% BPy COFs were mea-
sured to be 2.4 nm (Figure S16). This value is lower than those
of their parent X% BPy COFs (2.6 or 2.7 nm) but higher than
[
34]
dine units. However, it is difficult for Rh(COD)Cl to enter the
space between adjacent COFs sheets (the interlayer distance is
about 3.74 Š, as measured by PXRD or calculated by simula-
tion). To this end, the docking of Rh(COD)Cl was carried out
first and the resulting samples were named as
II
that of the Pd @X% BPy COFs (2.3, 2.2, 2.1, and 2.0 nm with X
values of 25, 50, 75, and 100, respectively). The Rh-loading con-
I
II
tents of the Rh /Pd @X% BPy COFs were measured to be 4.8,
Chem. Eur. J. 2016, 22, 9087 – 9091
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9089 ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
7
.1, 6.1 and 7.7 wt% with X values of 25, 50, 75 and 100, re-
Table 2. One-pot cascade reactions using different homogeneous/heteroge-
neous catalysts
I
spectively, which is close to those of the Rh @X% BPy COFs
4.9, 7.8, 7.5, 7.9 wt% with X values of 25, 50, 75 and 100, re-
(
spectively), suggesting that Rh was apparently not lost after
I
the Pd(OAc)2 loading. The Pd-loading contents of the Rh /
II
Pd @X% BPy COFs were determined to be 8.5, 9.1, 8.0 and
1
0.7 wt% with X values of 25, 50, 75 and 100, respectively,
II
which is close to that of bipyridine-free Pd @0% BPy COF
II
(
(
11.1 wt%), but much lower than that of the Pd @X% BPy COFs
14.3, 18.2, 18.7 and 16.4 wt% with X values of 25, 50, 75 and
Catalyst
Yield of
Yield of
Yield of
addition
reaction [%]
oxidation
reaction [%]
cascade
reaction [%]
1
00, respectively). These results further indicate that Pd(OAc)2
[a]
[b]
[c]
molecules mainly occupy the imine sites in the space between
adjacent COF layers. The PXRD patterns reveal that after load-
I
II
Rh/Pd @75% BPy COF
90
88
–
99
99
–
90
87
88
87
85
0
I
ing with Rh , the peak assigned to the (001) facet was evidently
I
II
Rh/Pd @75% BPy COF
not shifted compared to their corresponding X% BPy COFs
[d]
2
nd cycle
I
II
(
Figure S9c), consistent with the argument that Rh(COD)Cl
Rh/Pd @75% BPy COF
rd cycle
3
mainly coordinated with bipyridine units. In contrast, evident
I
II
II
Rh/Pd @75% BPy COF
th cycle
–
–
shifting of this peak was observed after subsequent Pd dock-
4
ing (Figure S9d), which again supports the assumption that
the later added Pd(OAc)2 mainly coordinated to the imine
groups and pulled the COF layers closer. To sum up, the selec-
tive coordination of bipyridine and imine ligands to the metal
I
II
Rh/Pd @75% BPy COF
5th cycle
–
–
I
Rh@75% BPy COF
88
0
0
II
Pd @75% BPy COF
99
–
0
complexes Rh(COD)Cl and Pd(OAc) has been realized using
2
[e]
the described programmed synthetic procedure (Figure 1 g).
To characterize the catalytic performance of the COFs, the
Rh(COD)Cl+Pd(OAc)2
–
3
I
II
Rh(COD)Cl+Pd(OAc)
bipyridine ligand
2
–
–
99
bimetallic Rh/Pd @X% BPy COFs were used as catalysts for
[f]
+
[35]
I
a cascade reaction. As reported by Sakai et al., the Rh -cata-
lyzed addition of phenylboronic acid to benzaldehyde can pro-
duce diphenylmethanol in high yield. On the other hand, di-
phenylmethanol can be further oxidized to generate benzo-
[
a] Phenylboronic acid (2.0 mmol), benzaldehyde (1.0 mmol), potassium car-
bonate (3.0 mmol), catalyst (containing 0.01 mmol metal), toluene/H
15 mL/5 mL), 1208C, 24 h, N protection. [b] Diphenylmethanol (1.0 mmol),
potassium carbonate (3.0 mmol), catalyst (containing 0.01 mmol of metal),
toluene/H O (15 mL/5 mL), ambient air, 1008C, 12 h. [c] After the addition re-
2
O
(
2
II
[36]
phenone under Pd catalysis.
As a result, it is anticipated
2
I
II
action, the temperature was directly decreased to 1008C and O was purged
into the reaction system with other experimental parameters being identical
to each stepwise reaction. [d] The COF catalyst was filtrated and reused for
another round of the cascade catalysis. [e] Rh(COD)Cl (0.01 mmol) and
2
that the separated loading of Rh and Pd leads to bimetallic
catalysts that could be used to catalyze this cascade reaction,
as illustrated in Table 2.
As expected, a 90% yield of the final product was realized
2
Pd(OAc) (0.01 mmol) as homogeneous catalyst. [f] Rh(COD)Cl (0.01 mmol),
Pd(OAc)2 (0.01 mmol), and bipyridine ligand (0.02 mmol) as homogeneous
catalyst.
I
II
with the Rh/Pd @75% BPy COF as a representative catalyst in
a one-pot reaction. To illuminate the respective functions of
II
I
II
Pd and Rh in the cascade reaction, COFs only loaded with Pd
I
or Rh were investigated. We found that the monometallic
II
Pd @75% BPy COF did not catalyze the addition of phenylbor-
Additionally, the well-preserved crystalline structure of the bi-
metallic docked COFs also verified their high resistance to the
conducted catalytic conditions. The heterogeneous nature of
the catalyst was verified by a leaching test. In a typical experi-
ment, the supernatant after one cycle of the catalytic reaction
was isolated and mixed with fresh reactants for another cycle
of the cascade reaction without the heterogeneous catalyst. As
expected, no signal of the target product was observed under
such conditions. We also attempted to catalyze the cascade re-
actions by homogeneous catalysts under identical experimen-
tal conditions. In the absence of nitrogen ligands, the combi-
onic acid to benzaldehyde, as no diphenylmethanol was ob-
tained in the separated reaction. On the other hand, the
II
Pd @75% BPy COF exhibited an excellent catalytic activity in
the oxidation of diphenylmethanol to benzophenone with
I
a yield of 99%. Likewise, the monometallic Rh @75% BPy COF
failed to catalyze the oxidation of diphenylmethanol to benzo-
phenone, but exhibited a high catalytic activity in the addition
of phenylboronic acid to benzaldehyde to produce diphenyl-
methanol with a yield of 88%, which is comparable to homo-
I
[35]
geneous Rh catalysts (92%) reported by Sakai et al. These
I
results indicate that Rh catalyzed the addition reaction and
nation of Rh(COD)Cl with Pd(OAc) could hardly catalyze the
2
II
I
Pd catalyzed the oxidation reaction, as expected. The Rh /
cascade reaction (only 3% yield). In contrast, the yield reached
II
Pd @75% BPy COF was recycled by filtration and its outstand-
99% when both complexes, Rh(COD)Cl and Pd(OAc) , together
2
ing catalytic activity could be maintained (>85% isolated
yield) for up to five cycles of the cascade reaction. ICP-OES re-
sults also confirmed that only limited leaching of Rh (ca. 3%)
and Pd (ca. 5%) could be observed after the 5th reaction cycle.
with the bipyridine ligand mixture were used as homogeneous
catalysts. This result suggests the necessity of the coordination
between the metals and the nitrogen ligands to ensure effec-
tive catalysis. This, in turn, reveals that the carefully designed
Chem. Eur. J. 2016, 22, 9087 – 9091
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Communication
coordination of the two nitrogen ligands to the two different
metal complexes in this work is of significance to guarantee
both, precise docking of the metal sites and high catalytic ac-
tivity of the loaded catalysts.
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[
ent metal complexes, Rh(COD)Cl and Pd(OAc) , can be realized
2
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eties in the pores of COFs, whereas the smaller and flexible
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I
II
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addition–oxidation cascade reaction. We believe that such
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Rh/Pt, Ir/Pd) in catalyzing other sorts of cascade reactions or
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Received: March 21, 2016
Published online on May 25, 2016
Chem. Eur. J. 2016, 22, 9087 – 9091
www.chemeurj.org
9091
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim