Converting Unstable Imine-Linked Network into Stable Aromatic Benzoxazole-Linked One via Post-oxidative Cyclization

Article abstract of DOI:10.1021/jacs.9b05244

Efficiently converting unstable linkages into stable linkages is an important objective in the chemistry of covalent organic frameworks (COFs), because it enhances stability and preserves crystallinity. Here, an unstable imine-linked COF was converted int

Full text of DOI:10.1021/jacs.9b05244

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Communication
Converting Unstable Imine-Linked Network into Stable Aromatic
Benzoxazole-Linked One via Post-Oxidative Cyclization
Jeong-Min Seo, Hyuk-Jun Noh, Hu Young Jeong, and Jong-Beom Baek
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05244 • Publication Date (Web): 18 Jul 2019
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Converting Unstable Imine-Linked Network into Stable Aromatic
Benzoxazole-Linked One via Post-Oxidative Cyclization
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Jeong-Min Seo†, Hyuk-Jun Noh†, Hu Young Jeong‡ and Jong-Beom Baek*†
†School of Energy and Chemical Engineering/Center for Dimension-Controllable Organic Frameworks, Ulsan National
Institute of Science and Technology (UNIST), 50 UNIST, Ulsan 44919, Republic of Korea
‡UNIST Central Research Facilities, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST, Ulsan 44919,
Republic of Korea
Supporting Information
(Schiff base) to another stable linkage via post-treatment has been
recently introduced8-10. However, to increase practicality it is still
necessary to develop more solid approaches.
ABSTRACT: Efficiently converting unstable linkages into stable
linkages is an important objective in the chemistry of covalent
organic frameworks (COFs), because it enhances stability and
preserves crystallinity. Here, unstable imine-linked COF was
converted into stable aromatic benzoxazole-linked COF (BO-COF)
via post-oxidative cyclization, based on chemistry used to form
fused-aromatic ladder-like rigid-rod polymers. The structure of the
porous BO-COF was confirmed by transmission electron
microscopy, and infrared and solid-state nuclear magnetic
resonance spectroscopies, powder X-ray diffraction patterns and
nitrogen adsorption-desorption isotherms. The efficient post-
treatment of unstable reversible COF converted it into stable
irreversible COF, which had significantly improved thermal and
chemical stabilities as well as high crystallinity. This strategy can
be universally applied for the synthesis of stable fused-aromatic
COFs, expanding their practical applications.
The key to any new approach is the selection of a stable linkage.
Therefore, we focused on the classic linear fused-aromatic rigid-
rod polymers to find stable linkages. These aromatic heterocyclic
polymers, including polybenzoxazoles (PBOs), had been
intensively studied from the 1960s through the 1980s, since the
dramatic advances of the aerospace industries that demanded high-
temperature and high-performance structural polymers11. Due to
the formation of irreversible aromatic rings as polymer forming
reaction, all fused-aromatic rings structures of PBOs have enable
to demonstrate high thermal and chemical stability along with
ultrahigh mechanical properties12,13. Classically, benzoxazole
linkages are formed by an irreversible process, e.g., the formation
of aromatic rings between ortho-aminophenol (Ph-(NH2)-OH) and
benzoic acid (Ph-COOH) in the presence of acid catalyst at elevated
temperature14,15. The thermodynamic energy gain after forming
aromatic oxazole rings by double condensation (–2H2O per
reaction) prevents the reverse reaction (Scheme 1a). The other for
synthesis of benzoxazole linkage is oxidative cyclization of
phenolic Schiff bases derived from the condensation of 2-
aminophenols and aldehydes16. However, reversibility is required
for the formation of highly crystalline COFs17. Therefore,
reversable reaction should be included during the formation of
benzoxazole linkage. The synthesis of benzoxazole-linked COF by
one-pot protocol has been recently reported18-20. These one-pot
protocol kinetically leads to the rapid formation of oxazole rings by
irreversible oxidation before “defect-correcting” for positional
ordering, hence minimize the reverse reaction. Also, the post-
modification of imine to benzoxazole linkage by heterogeneous
linker exchange has recently been reported21. However, this
approach is fundamentally different the direct conversion in this
work.
Linking organic molecules by covalent bond into extended
structures leads to linear one- (1D), two- (2D) and three-
dimensional (3D) polymer structures. Typically, the formation of
porous 2D and 3D network polymers is governed by kinetically
controlled reactions, which lead to amorphous materials. This
makes it difficult to ascertain their physical properties, which are
related to precisely controlled crystalline structures, and this limits
their specific applications1,2. Crystalline materials can be achieved
with thermodynamically controlled reactions by forming reversible
bonds, a process which is known as dynamic covalent chemistry
(DCC)3. Most current chemistry employs this approach, which
leads to COF materials with high crystallinity and permanent
porosity4. For instance, if the linkages in COFs are not ideally
connected, they can be thermally dissociated and reconnected into
correct conformations, resulting in long-range ordered materials5.
On the other hand, the reversibility of chemical bonds
simultaneously implies the resulting COFs will be unstable. It is a
significant challenge to convert unstable reversible bonds via post-
treatment of unstable bonds in COFs into stable irreversible bonds
while preserving high crystallinity.
Here, we report the conversion of reversible imine-linked COF
(I-COF) to irreversible benzoxazole-linked COF (BO-COF) via
post-oxidative cyclization. Compared to one-pot irreversible
synthesis, this strategy is able to preserve the original topology,
crystallinity and porosity of the resultant COF, while enhancing
thermal and chemical stability. The aim of this work is to improve
the crystallinity of stable COF by stepwise reaction, which fixes the
ordered structure of framework before transforming stable
benzoxazole linkage and to visualize crystalline COF by
transmission electron microscopy. We selected 2,3-dichloro-5,6-
To date, post-treatment methods used to convert reversible into
irreversible bonds have mainly focused on the tautomerization of
the enol-to-keto linkage6. However, this keto linkage is unstable in
alkaline conditions, because of the backward formation of keto-to-
enol7. As an alternative, a method of converting an imine linkage
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Scheme 1. Schematic presentation of the oxidative cyclization of I-COF into BO-COFa
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a(a) Classic formation of benzoxazole ring between TABT (1) and benzoic acid (2) to form benzoxazole compound (6) by one-step
irreversible condensation in a PPA medium. (b) Model reaction between TABT (1) and benzaldehyde (3) to form a reversible imine
compound (4), its tautomeric intermediate benzoxazoline structure (5), and final conversion into a benzoxazole compound (6) by oxidative
cyclization with DDQ. (c) Synthesis of I-COF from the reaction between TABT (1) and TPA (7), then, the oxidative cyclization of I-COF
into BO-COF by DDQ.
dicyano-1,4-benzoquinone (DDQ) as the oxidant, because it not
only promotes the reaction in a straightforward way, but can also
be easily removed after the reaction. Based on the formation of a
model compound (Scheme 1b), our strategy for post-oxidative
cyclization resulted in a BO-COF with high crystallinity and
porosity. As a result of stable rigid benzoxazole linkages, the
resulting BO-COF exhibited excellent thermal and chemical
stability compared to I-COF.
explored the post-modification of I-COF into BO-COF by
oxidative cyclization. As seen in scheme 1b and scheme S1, the
imine (-C=N-) linkages on the ortho-position of the phenolic
hydroxyl moieties (4) were easily cyclized into aromatic
benzoxazoline rings (5). As a result, the selection of the appropriate
oxidant is important to efficiently oxidize benzoxazoline (5) into a
benzoxazole (6) linkage23,24. While optimizing the reaction
condition, DDQ was found to be the best oxidant, and resulted in
the highest degree of crystallinity for the conversion of I-COF to
BO-COF (SI section S4). Hence, BO-COF was synthesized by
treating I-COF with DDQ in dichloromethane at room temperature
for 3 days (see SI section S2).
The synthesis of BO-COF was carried out using a two-step
reaction sequence (Scheme 1c): (1) the formation of I-COF and (2)
the oxidative cyclization of I-COF into BO-COF. First, I-COF was
obtained from the reaction between 1,3,5-triamino-2,4,5-
benzenetriol (TABT, 1) and terephthaldehyde (TPA, 7) by
solvothermal synthesis in methanol and mesitylene at 120 °C for 3
days. To achieve the highest crystallinity and porosity, the reaction
condition was optimized (Supporting Information (SI) section S3).
As-synthesized I-COF has very poor stability, even in ambient
moisture22 (Figure S2), because of the residual pyridinium chloride
in I-COF (see SI section S3). Accordingly, we subsequently
The conversion of the imine-linkage before and after oxidative
cyclization was monitored by Fourier-transform infrared (FT-IR)
and carbon thirteen (13C) cross-polarization magic angle spinning
nuclear magnetic resonance (CP-MAS NMR) spectroscopies. The
FT-IR spectra of the BO-COF exhibited the O-C=N stretching band
of the benzoxazole linkage at 1630 cm–1. The disappearance of the
C=N stretching band of the imine-linkage at 1610 cm–1 indicates
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Figure 1. Structural characterizations of I-COF and BO-COF. (a) 13C CP-MAS NMR spectra. Asterisks in I-COF denote the unremoved
pyridinium chloride. Inset, assignment of the 13C CP-MAS NMR signals in the chemical structure. (b) PXRD patterns, indicating retention
of crystallinity after oxidative cyclization (left). Enlargement of (100) reflection, showing the reduction of unit cell dimensions (right). (c)
The permanent microporosities were measured by nitrogen adsorption-desorption isotherms. Filled circles: adsorption; open circles:
desorption. Inset is the corresponding pore size distribution profile calculated from adsorption data using the cylindrical pore NLDFT model.
the conversion of imine into benzoxazole linkages. Furthermore,
new bands at 1490 cm–1 and 1080 cm–1 appeared in the spectrum
of the BO-COF. The peaks are assignable to the symmetric and
asymmetric C-O-C stretching bands in the benzoxazole linkage
reduction in lattice parameter in the PXRD results (inset of figure
1c), is due to oxidative cyclization from the imine-linkage to the
benzoxazole linkage.
(Figure S4)19,25
.
Clear indication of the structural characterization was also
observed by CP-MAS NMR analysis. In the 13C CP-MAS NMR
spectrum of BO-COF, the 5 signal at 159 ppm indicating the imine
carbon in I-COF was attenuated after oxidative cyclization. Indeed,
a new 5’ signal was observed at 162 ppm, attributed to the
formation of a benzoxazole linkage (Figure 1a). The assignment of
these signals was confirmed by comparing with the model
compound (Figure S5).
The crystallinity of I-COF and BO-COF was measured by
powder X-ray diffraction (PXRD). Small shifts in the (100)
reflection of the powder patterns of I-COF and BO-COF support
the reduction in pore size caused by the oxidative cyclization of
imine linkages into benzoxazole linkages (Figure 1b). Extended
structures based on a hexagonal layered hcb topology were
modeled for the I-COF (space group P6, No. 168) and BO-COF
(space group P6/M, No. 175). The experimental patterns for I-COF
and BO-COF were in good agreement with the energetically most
favorable eclipsed models. On the other hand, the staggered model
gave a PXRD pattern that deviated widely from the experimental
one. Pawley refinement26,27 of the experimental patterns provided
unit cell parameters of I-COF (a = b = 22.37 Å, c = 3.55 Å) and
BO-COF (a = b = 21.12 Å, c = 3.38 Å), led to low residuals values
in the I-COF (wRp = 3.73%, Rp = 2.85%) and BO-COF (wRp =
3.31%, Rp = 2.46%), respectively (Figure S6).
Figure 2. Morphologies of BO-COF. (a,b) TEM images of BO-
COF. Inset: corresponding fast Fourier transform of the marked
area (cyan dash line). Bottom: corresponding height profiles of the
marked line (yellow line). Scale bars: 20 nm.
The morphologies of I-COF and BO-COF were visualized by
electron microscopy. Scanning electron microscopy (SEM)
revealed elongated rod shapes composed of small flakes (Figure
S8). Further investigations were conducted using high-resolution
transmission electron microscopy (HR-TEM) but only for BO-
COF, because I-COF had low-electron beam stability28. HR-TEM
images of BO-COF show an ordered, hexagonal porous structure
oriented along the <001> direction with a periodicity of 2.1 ± 0.05
nm, in precise agreement with the pore-to-pore distance of 2.1 nm
in the refined eclipsed model (Figure 2). Both HR-TEM images and
corresponding fast-Fourier transformation (FFT) of the BO-COF
confirmed its crystallinity, and the calculated lattice distance of 1.8
± 0.05 nm corresponds to the (100) plane of the eclipsed model,
and matched the refined PXRD pattern well (Figure S9).
The permanent porosities of the I-COF and BO-COF were
confirmed by low-pressure nitrogen adsorption-desorption
isotherms at 77 K. Both COFs exhibited a high nitrogen uptake at
relatively low pressures, which is indicative of microporous
materials (Figure 1c). The Brunauer-Emmett-Teller surface areas
(SBET) were estimated to be 1906 m2 g–1 for I-COF (theoretical
surface area: 2319 m2 g–1) and 1687 m2 g–1 for BO-COF (theoretical
surface area: 1995 m2 g–1) (Figure S7). The loss in surface area of
BO-COF can be attributed to a decrease in pore size and
crystallinity after oxidative cyclization. The pore size distributions
were calculated from the nitrogen isotherms using a non-local
density functional theory (NLDFT) model. The results showed a
reduction in pore size from I-COF (1.70 nm) to BO-COF (1.65 nm).
The reduction in pore size, which corresponds to the observed
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had high thermal and chemical stability, suitable for practical
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In summary, we were able to synthesize stable fused-aromatic
benzoxazole-linked COF with high crystallinity and porosity. The
key strategy was to form irreversible aromatic benzoxazole rings
from reversible imine linkages using oxidative cyclization. This
approach allows the resulting COF to not only retain its topology,
crystallinity and porosity, but also results in significantly improved
thermal and chemical stability, suitable for practical applications.
This simple reaction protocol is a synthesis of other fused-aromatic
frameworks with crystallinity and porosity, and thus widens their
applications from wet chemistry to device applications.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website.
Synthesis and characterizations (PDF)
AUTHOR INFORMATION
Corresponding Author
*jbbaek@unist.ac.kr
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This research was supported by the Creative Research Initiative
(CRI, 2014R1A3A2069102), BK21 Plus (10Z20130011057), and
Science Research Center (SRC, 2016R1A5A1009405) programs
through the National Research Foundation (NRF) of Korea.
Figure 3. Thermal and chemical stability of I-COF and BO-COF
after treatment in various conditions for 3 days. (a) PXRD patterns.
(b) Nitrogen adsorption-desorption isotherms at 77 K. Filled circles:
adsorption, open circles: desorption.
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a Combined
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was associated with minor structural deformation and adsorbed
small molecules. Overall, the results demonstrated that BO-COF
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