Synthesis and Catalytic Properties of Metal- N-Heterocyclic-Carbene-Decorated Covalent Organic Framework

Article abstract of DOI:10.1021/acs.orglett.0c02721

We demonstrate herein that the N-heterocyclic-carbene (NHC)-metal complex (NHC-M)-involved covalent organic framework (COF) can be prepared by the direct polymerization of the NHC-M monomer with its counterpart under solvothermal conditions. The NHC-M-COF with different counterions is readily achieved via solid-state anion exchange. The obtained NHC-AuX-COF (X = Cl- and SbF6-) can be a highly active reusable catalyst to separately promote the carboxylation of the terminal alkyne with CO2 and alkyne hydration under mild conditions.

Full text of DOI:10.1021/acs.orglett.0c02721

pubs.acs.org/OrgLett
Letter
Synthesis and Catalytic Properties of Metal−N‑Heterocyclic-
Carbene-Decorated Covalent Organic Framework
Yue Li, Ying Dong,* Jing-Lan Kan, Xiaowei Wu, and Yu-Bin Dong*
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ABSTRACT: We demonstrate herein that the N-heterocyclic-carbene (NHC)−metal
complex (NHC−M)-involved covalent organic framework (COF) can be prepared by the
direct polymerization of the NHC−M monomer with its counterpart under solvothermal
conditions. The NHC−M−COF with different counterions is readily achieved via solid-
state anion exchange. The obtained NHC−AuX−COF (X = Cl− and SbF6 ) can be a
−
highly active reusable catalyst to separately promote the carboxylation of the terminal
alkyne with CO2 and alkyne hydration under mild conditions.
during the catalytic process could be effectively avoided.
Compared with the reported organocatalysts and metal-
nanoparticle-loaded COF catalysts,12 the synthesis of NHC−
M−COFs in this way is unprecedented thus far.
ince Arduengo et al. reported the first stable and isolable
1
SN-heterocyclic carbene (NHC),
NHC−metal (NHC−
M) complexes have been recognized as an important class of
organometallic catalysts.2 However, the recycling and reuse of
NHC−M catalysts remains challenging because the molecular
NHC−M complexes are typically of single use in catalysis.3 To
meet the multifaceted requirements of low-cost, resource-
saving, and environment-friendly synthesis, molecular NHC−
M complexes are intelligently loaded on solid supports, such as
metal−organic frameworks (MOFs)4 and organic polymers,5
by chemists to realize the catalysis in a heterogeneous way.
Since Yaghi et al. reported the first example of a covalent
organic framework (COF),6 COF-based chemistry has
experienced rapid development.7−11 It is of note that COFs
possess capacious and expedite channels that can not only
facilitate the transport of reactants and products but also
provide efficient access to catalytic sites within the COFs.
These inborn features could endow COFs with more
advantages for their applications in heterogeneous catalysis.9
As we know, COF catalysts can be prepared by introducing
the catalytic species into COFs via direct polymerization or
postsynthetic modifications (PSMs).9 Compared with direct
polymerization, the PSM approach might sometimes result in
less loading of the catalytic moieties with relatively uneven
distributions because not all PSM reactions are quantitative,
especially for those large-sized substrates under solid-state
conditions.12 We wonder if the NHC−M−COFs could be
prepared by the direct polymerization of NHC−M organo-
metallic monomers with their counterparts. In doing so, both
the high catalyst loading and their uniform distribution in the
COF would be realized; furthermore, the catalyst leaching
Continuing with our research on the synthesis of COFs and
their catalytic applications,13 we report herein, for the first
time, NHC−AuCl−COF, which was generated from an
NHC−AuCl-decorated 4,4′′-p-terphenyldicarboxaldehyde
monomer (TPDCA−NHC−Au) and 1,3,5-tris(4-
aminophenyl)triazine (TAPT) via direct polymerization
under solvothermal conditions. By anion exchange, NHC−
AuSbF6−COF with different counterions was also synthesized.
The obtained NHC−AuX−COF (X = Cl−, SbF6 ) can be a
−
highly active heterogeneous catalyst to promote terminal
alkyne carboxylation with CO2 and alkyne hydration under
mild conditions, respectively (Scheme 1).
The NHC−Au-decorated monomer TPDCA−NHC−Au
was prepared from 4,7-dibromo-1-ethyl-1H-benzo[d]imidazole
via multiple synthetic steps (Supporting Information). NHC−
AuCl−COF was synthesized as yellow solids through Schiff
base condensation between TPDCA−NHC−Au and TAPT
under solvothermal conditions (Scheme 1, Supporting
Information). IR and 13C CP-MAS (Figure S1) were used to
establish the connectivity of the COF, and the existence of
Received: August 14, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02721
© XXXX American Chemical Society
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A

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a
Scheme 1. Synthesis of NHC−AuX−COF (X = Cl−, SbF6 ) and Its Catalytic Reactions
−
a
Photographs of COF samples are inserted.
TPDCA−NHC−Au and TAPT monomers was directly
evidenced.
showed no weight loss up to ∼400 °C, indicating that NHC−
AuCl−COF possesses excellent thermal stability (Figure 1d).
The structural modeling of NHC−AuCl−COF was
conducted with the Materials Studio (ver. 5.0) software
based on the collected powder X-ray diffraction (PXRD) data
(Figure 2).14 Its most probable structure was a 2D staggered
AB-stacking mode, and the observed diffraction peaks at 2.0,
3.4, 4.0, 6.9, and 8.9° were, respectively, assignable to the
(100), (110), (200), (220), and (140) facets. The Pawley
Inductively coupled plasma (ICP) analysis showed that the
Au content in NHC−AuCl−COF was 23.40 wt % (calcd.
24.13 wt %). The oxidation state of Au was determined by X-
ray photoelectron spectroscopy (XPS) measurement, and the
binding energies for Au 4f7/2 and 4f5/2 were 85.38 and 88.92
eV, respectively, indicating that the Au in NHC−AuCl−COF
is monovalent (Figure 1a).5b NHC−AuCl−COF is highly
Figure 1. (a) XPS spectrum, (b) TEM image, (c) SEM image, and
(d) TGA trace of NHC−AuCl−COF.
crystalline, and no Au NP was detected, which was well
supported by its TEM image (Figure 1b). SEM showed that
NHC−AuCl−COF features micrometer-scale lamellar mor-
phology (Figure 1c), and the mappings of C, N, Cl, and Au
indicate that their distributions are uniform (Figure S2). TGA
Figure 2. PXRD pattern of NHC−AuCl−COF: experimental (red
line) and Pawley-refined (black) profiles and the simulated pattern
(orange) for the AB stacking mode. The refinement differences (blue)
and the observed reflections (green) are also shown. Insets are the
single and AB stacking layers in NHC−AuCl−COF.
B
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a
Table 1. Optimization of the NHC−AuCl−COF-Catalyzed Carboxylation of Phenylacetylene with CO2
b
entry
cat. (mol % Au)
base
t (h)
solv.
T (°C)
yield (%)
1
2
3
4
5
6
7
NHC−AuCl−COF (0.5)
NHC−AuCl−COF (0.5)
NHC−AuCl−COF (0.5)
NHC−AuCl−COF (0.5)
NHC−AuCl−COF (0.5)
NHC−AuCl−COF (0.3)
NHC−AuCl−COF (1.0)
NHC−AuCl−COF (0.5)
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
16
16
16
16
16
16
16
16
DMSO
DMF
50
50
80
20
50
50
50
50
50
96
96
96
11
43
78
96
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
c
8
9
16
a
b
c
CO2 (1 atm), phenylacetylene (1 mmol), base (1.1 mmol), solvent (4 mL). Isolated yields. Without CO2. The products are determined by 1H
NMR, 13C NMR, and MS spectra (Supporting Information).
a
−
Table 2. Optimization of the NHC−AuX−COF-Catalyzed (X = Cl−, SbF6 ) Model Alkyne Hydration Reaction
b
entry
cat. (mol % Au)
t (h)
solv.
T (°C)
yield (%)
1
2
3
4
5
6
7
NHC−AuCl−COF (0.5)
NHC−AuSbF6−COF (0.5)
NHC−AuSbF6−COF (0.5)
NHC−AuSbF6−COF (1.0)
NHC−AuSbF6−COF (0.5)
NHC−AuSbF6−COF (0.3)
20
20
20
20
20
20
20
H2O
H2O
H2O
H2O
H2O
H2O
H2O
60
60
90
60
30
60
60
20
99
99
99
48
73
a
b
Phenylacetylene (1 mmol), solvent (1 mL). Isolated yields. The products are determined by 1H NMR, 13C NMR, and MS spectra (Supporting
Information).
refinement (black) showed a negligible difference between the
simulated (orange) and experimental PXRD pattern (red)
(Figure 2). NHC−AuCl−COF was assigned to the space
group P3 with optimized parameters of a = b = 52.0050 Å, c =
8.0140 Å, α = β = 90°, and γ = 120° and residuals wRp =
5.88% and Rp = 4.11% (Table S1). Notably, NHC−AuCl−
COF herein, with other types of possible stacking modes, gave
a PXRD pattern that significantly deviated from the measured
profiles (Figure S3 and Table S1).
The porosity of NHC−AuCl−COF was demonstrated by
the gas adsorption−desorption measurement (Figure S4). The
N2 absorption amount of NHC−AuCl−COF at 77 K is 148
cm3 g−1, and the corresponding surface area on the basis of the
BET model is 139 m2 g−1. The pore-size distribution by
nonlocal density functional theory (NLDFT) analysis indicates
that it possesses a narrow pore diameter distribution centered
at ∼2.7 nm (Figure S4).
Next, we examined the catalytic activity of NHC−AuCl−
COF for the direct carboxylation of terminal alkynes with CO2,
which is an economical approach for the synthesis of carboxylic
acids.15 On the contrary, the rational utilization of CO2 is an
attractive and straightforward method to reduce the green-
house effect.16
As shown in Table 1 (entries 1−8), catalytic reactions were
performed under different conditions, including different
possible solvents, bases, and catalyst amounts at different
temperatures. The best result was observed when the reaction
was conducted in DMSO or DMF at 50 °C for 16 h (entries 1
and 2) with NHC−AuCl−COF (0.5 mol % Au equiv) and
Cs2CO3 to give the carboxylation product in 96% yield (TON
= 192, TOF = 12 h−1). A reaction temperature >50 °C was not
conducive to increase the product yield (entry 3); meanwhile,
a lower temperature (20 °C) would lead to a very low 11%
yield (entry 4). In addition, when the reaction was performed
in the presence of K2CO3, the product was obtained in a
decreased 43% yield (entry 5). With the aid of less catalyst
loading, 0.3 mol % instead of 0.5 mol %, the product was
isolated in a slightly lower 78% yield (entry 6). Also, more
catalyst loading could not enhance the yield (entry 7). It is of
note that no desired carboxylation product was detected in the
absence of CO2, indicating that the formed carboxyl group
indeed came from CO2 (entry 8). In addition, no product was
detected without NHC−AuCl−COF (entry 9).
As a heterogeneous catalyst, NHC−AuCl−COF can be
reused, and the yield of alkyne carboxylation is still up to 94%,
even after five catalytic cycles, without a loss of its crystallinity
or structural integrity (Figure S5). The leaching amount of Au
in NHC−AuCl−COF is only ∼1% (Au, 23.08 wt %), as
determined by inductively coupled plasma atomic emission
spectroscopy (ICP-AES), and the oxidation state of Au is
intact based on the XPS (Figure S5). The postulated
mechanism is believed to be the same as that of the molecular
NHC−M-catalyzed alkyne carboxylation (Figure S6).16,17
The scope of NHC−AuCl−COF-catalyzed carboxylation
was investigated utilizing various substrates. Besides phenyl-
acetylene, the substituted phenylacetylenes with either
electron-donating (−CH3, −C2H5, −OCH3, and −Ph) or
electron-withdrawing groups (−Cl, −NO2, and −Br) at
C
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Letter
different substituted positions furnished the excellent 95−99%
yields (Table S2), implying that NHC−AuCl−COF could
tolerate various functional groups under the given reaction
conditions. It is of note that almost no desired products were
detected from the protic functional-group-substituted (−OH)
substrate or an aliphatic alkyne.
To ascertain a scope for this COF-based catalysis, the
reactivity of a series of terminal aromatic alkynes was
investigated with NHC−AuSbF6−COF (Table S3). The
substituted terminal aromatic alkynes with both electron-
donating (−OCH3, −Ph, −C2H5,) and electron-withdrawing
(−Cl, −Br, −NO2) groups could be highly efficiently
converted to their corresponding acetophenones (95−99%
yields). However, for the internal aromatic alkyne of 4-phenyl-
diphenylacetylene, the corresponding hydration product was
isolated in only 50% yield, indicating that the internal alkynes
are more reluctant participants than their terminal counterparts
toward hydration, which is consistent with the results reported
by Hayashi and Tanaka.21 Additionally, 3-hydroxyphenyl
acetylene and aliphatic alkyne phenyl propargyl ether could
be converted into 3-hydroxyacetophenone and 1-phenoxypro-
pan-2-one in 63 and 76% yields, respectively.
In summary, we have successfully realized the synthesis of
NHC−M-involved covalent organic framework heterogeneous
catalysts via a direct polymerization approach. In addition, the
type of concomitant counterions in the obtained NHC−AuX−
COF can be readily changed according to the requirements of
different catalytic reactions. It is of note that the metal−
NHC−COFs herein indicate that they showed comparable
catalytic activity to that of the reported heterogeneous metal-
loaded catalysts (Tables S4 and S5). We believe that our
synthetic strategy provided herein is general and, moreover,
significantly broadens the scope of COF catalysts.
To demonstrate the generality of NHC−Au−COF in
catalysis, we also evaluated its activity for alkyne hydration,
which is an important reaction for the syntheses of aldehydes
and ketones.18 To meet the requirement of green synthesis, we
performed the reaction in water. Table 2 reveals that NHC−
AuCl−COF is unsatisfactory for the phenylacetylene hydra-
tion, and the acetophenone yield obtained from this model
reaction is only 20% under the given conditions (0.5 mol %
Au, H2O, 60 °C, 20 h). Nolan et al. reported that the
molecular NHC−AuCl can effectively promote alkyne
19
−
hydration but with the aid of SbF6 . Relying on these
findings, together with our own results (entry 1), we then
prepared NHC−AuSbF6−COF from NHC−AuCl−COF by
replacing Cl− with SbF6 (MeOH/H2O, 30 min, r.t., 96%
−
yield, Supporting Information). Besides a visual color change
(Scheme 1), ICP analysis showed that the Au/Sb ratio is
1:0.97 after anion exchange, implying that almost all of the Cl−
−
in NHC−AuCl−COF has been replaced with SbF6 . The Au
content in NHC−AuSbF6−COF is 18.99 wt % based on the
ICP-AES measurement. The PXRD pattern and SEM and
TEM images after anion exchange are identical to those of
NHC−AuCl−COF, suggesting that the COF structure and
morphology are well maintained upon anion exchange (Figure
ASSOCIATED CONTENT
* Supporting Information
■
sı
−
S7). In addition, the existence of SbF6 was further evidenced
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02721.
by the scanning electron microscopy−energy-dispersive X-ray
(SEM-EDX) spectrum (Figure S7). Its corresponding surface
area calculated on the basis of the BET model is 112.7 m2g−1,
which is slightly less than that of NHC−AuCl−COF (Figure
S8).
As expected, NHC−AuSbF6−COF exhibited excellent
activity. As indicated in Table 2, the treatment of phenyl-
acetylene with water at 60 °C afforded acetophenone in
quantitative yield after 20 h (TON = 198, TOF = 9.9 h−1,
entry 2) in the presence of NHC−AuSbF6−COF (0.5 mol %
Au equiv). Higher temperature (90 °C, entry 3) or higher
catalyst loading (1.0 mol % Au equiv, entry 4) could not
enhance the hydration yield, whereas lower temperature (30
°C, entry 5) or lower catalyst loading (0.3 mol % Au equiv,
entry 6) led to a modest 48 or 73% yield. Again, no product
was observed without the gold catalyst (entry 7).
Instruments and methods; synthesis and additional
characterization of TPDCA−NHC−Au monomer,
NHC−AuCl−COF, and NHC−AuSbF6−COF; cata-
lytic product characterization; catalyst hot leaching and
catalytic cycles; and crystallographic information for
COFs (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Ying Dong − College of Chemistry, Chemical Engineering and
Materials Science, Collaborative Innovation Center of
Functionalized Probes for Chemical Imaging in Universities of
Shandong, Key Laboratory of Molecular and Nano Probes,
Ministry of Education, Shandong Normal University, Jinan
250014, P. R. China; Email: dongyinggreat@163.com
Yu-Bin Dong − College of Chemistry, Chemical Engineering and
Materials Science, Collaborative Innovation Center of
Functionalized Probes for Chemical Imaging in Universities of
Shandong, Key Laboratory of Molecular and Nano Probes,
Ministry of Education, Shandong Normal University, Jinan
250014, P. R. China; orcid.org/0000-0002-9698-8863;
Email: yubindong@sdnu.edu.cn
Again, the hot leaching test demonstrated that NHC−
AuSbF6−COF was a typical heterogeneous catalyst for alkyne
hydration, and the hydration yield was up to 94% yield after
five catalytic cycles (Figure S9). The analysis of the NHC−
AuSbF6−COF after multiple recycling by ICP-AES and XPS
showed that no Au content (Au content 18.92 wt % after
reuse) or valence change was detected (Figure S9). In
addition, its PXRD pattern indicated that its crystallinity and
structure are well maintained after multiple reuses (Figure S9).
Compared with NHC−AuCl−COF (water contact angle
Authors
(WCA) = 123
2°), NHC−AuSbF6−COF is much more
Yue Li − College of Chemistry, Chemical Engineering and
Materials Science, Collaborative Innovation Center of
Functionalized Probes for Chemical Imaging in Universities of
Shandong, Key Laboratory of Molecular and Nano Probes,
Ministry of Education, Shandong Normal University, Jinan
250014, P. R. China
hydrophilic (WCA = 0°), so it is more suitable to promote
organic reactions in aqueous phase under the applied reaction
conditions (Figure S10). The mechanism of NHC−AuSbF6−
COF-catalyzed alkyne hydration was supposed based on the
reported molecular NHC−Au catalyst (Figure S11).20
D
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J. Covalent Organic Frameworks Comprising Cobalt Porphyrins for
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(d) Sun, Q.; Aguila, B.; Perman, J.; Nguyen, N.; Ma, S. Q. Flexibility
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and Threaded Ionic Polymer. J. Am. Chem. Soc. 2016, 138, 15790−
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(g) Pachfule, P.; Acharjya, A.; Roeser, J.; Langenhahn, T.;
Jing-Lan Kan − College of Chemistry, Chemical Engineering and
Materials Science, Collaborative Innovation Center of
Functionalized Probes for Chemical Imaging in Universities of
Shandong, Key Laboratory of Molecular and Nano Probes,
Ministry of Education, Shandong Normal University, Jinan
250014, P. R. China
Xiaowei Wu − School of Chemical Engineering, Nanjing
University of Science and Technology, Nanjing, Jiangsu 210094,
P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02721
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the NSFC (grants
21971153, 21671122, and 21802091), Shandong Provincial
Natural Science Foundation (grant ZR2018BB006), a Project
of the Shandong Province Higher Educational Science and
Technology Program (J18KA066), the Taishan Scholar’s
Construction Project, and the Changjiang Scholar project of
China at SDNU.
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