Twist Building Blocks from Planar to Tetrahedral for the Synthesis of Covalent Organic Frameworks

Article abstract of DOI:10.1021/jacs.9b13824

Rational construction of covalent organic frameworks (COFs) with novel structures still remains a challenge. Herein, we report the designed synthesis of two COFs, 2D-BPTA-COF and 3D-BMTA-COF, starting from biphenyl-based precursors but with different groups at the ortho positions. Both COFs exhibited high crystallinity and large surface area, and interestingly, 2D-BPTA-COF crystallizes into 2D sheets with AB stacking mode while 3D-BMTA-COF adopts a 7-fold interpenetrated structure with pts topology. This structural difference could be ascribed to the introduction of methyl groups in the building blocks, as the dihedral angle of biphenyl rings in 2D-BPTA-COF is a0° while in 3D-BMTA-COF it is a60°. Therefore, it is possible to synthesize COFs with different structures by twisting building blocks from planar to tetrahedral with steric hindrance. We believe this result represents a general and straightforward way to expand the diversity of tetrahedral nodes for constructing 3D COFs in the future, and moreover, a new tetrahedral node for constructing 3D COFs is now available.

Full text of DOI:10.1021/jacs.9b13824

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Communication
Twist Building Blocks from Planar to Tetrahedral
for the Synthesis of Covalent Organic Frameworks
Chao Gao, Jian Li, Sheng Yin, Junliang Sun, and Cheng Wang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b13824 • Publication Date (Web): 12 Feb 2020
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Twist Building Blocks from Planar to Tetrahedral for the Synthesis of
Covalent Organic Frameworks
Chao Gao,1† Jian Li,2,3† Sheng Yin,1 Junliang Sun2,3* and Cheng Wang1*
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1Sauvage Center for Molecular Sciences and Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, College of
Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
2College of Chemistry and Molecular Engineering, Beijing National Laboratory for Molecular Sciences, Peking University,
Beijing 100871, China
3Department of Materials and Environmental Chemistry, Stockholm University, Stockholm 10691, Sweden
Supporting Information Placeholder
Scheme 1. Schematic Representation of the Synthesis of
2D-BPTA-COF and 3D-BMTA-COF.
ABSTRACT: Rational construction of covalent organic frame-
works (COFs) with novel structures still remains a challenge.
Herein, we reported the designed synthesis of two COFs, 2D-
BPTA-COF and 3D-BMTA-COF, starting from biphenyl-based
precursors but with different groups on ortho positions. Both
COFs exhibited high crystallinity and large surface area, and in-
terestingly, 2D-BPTA-COF crystallizes into 2D sheets with AB
stacking mode while 3D-BMTA-COF adopts a seven-fold inter-
penetrated structure with pts topology. This structural difference
could be ascribed to the introduction of methyl groups in the
building blocks, as the dihedral angle of biphenyl rings in 2D-
BPTA-COF is ~0° while in 3D-BMTA-COF is ~60°. Therefore, it
is possible to synthesize COFs with different structures by twist-
ing building blocks from planar to tetrahedral with steric hin-
drance. We believe this result represents a general and straight-
forward way to expand the diversity of tetrahedral nodes for con-
structing 3D COFs in future, and moreover, a new tetrahedral
node for constructing 3D COFs is now available.
Covalent organic frameworks (COFs) represent an emerging class
of crystalline polymers with high porosity and structural tunabil-
ity.1 Since first announced by Yaghi in 2005,2 COFs have gained
intensive attention and shown great potentials in gas storage and
separation,3 heterogeneous catalysis,4 optoelectronics devices,5
sensors,6 energy storage7 and so on. According to the dimension-
ality of the covalent connectivity, COFs can be either categorized
into two-dimensional (2D) layered structures or three-dimensional
(3D) networks. Until now, most of the research efforts have fo-
cused on 2D structures, and a large number of 2D COFs have
been reported.8,9 In contrast, 3D COFs are much less studied
(about only forty examples),10-13 although their inherent character-
istics (e.g. exceptionally high surface areas exceeding 5000 m2/g)
and quadrilateral building blocks connected through [4 + 4] con-
densation reactions.3a,5a,12 However, it should be pointed out here,
some challenges still need to be further explored, especially the
limited availability of molecular building blocks.13 In order to
increase the structural diversity of 3D COFs, the exploration of
novel building units is highly demanded.
From the geometry perspective of constructing 3D COFs, it is
essential to have at least one building unit that can be extended
into three spatial directions. So far, tetrahedral nodes are the first
direct choice for making such 3D networks and the reported tetra-
hedral linkers are exclusively based on tetraphenylmethane or
adamantine derivatives. Consequently, this few options have seri-
ously limited the number of attainable 3D COFs and the diversity
of tetrahedral nodes needs to be further expanded. According to
the principles of stereochemistry, it is possible to form tetrahedral
geometry from planar molecules by increasing steric hindrance.
For example, in biphenyl-based molecules, the size of ortho sub-
stituents can strongly affect the dihedral angle of two phenyl
rings.14 When four large groups are introduced onto the ortho
positions, a rigid tetrahedral conformation can be obtained. There-
fore, with proper design, building blocks with tetrahedral geome-
10d
can make them as ideal candidate for excellent adsorption or
catalysis. Fundamentally, such underdeveloped status can be at-
tributed to the existing challenges in 3D COFs,13 including syn-
thetic difficulty, complicated structural determination, very few
network topologies and limited building blocks. To our delight,
some important progress in 3D COFs has been achieved over the
past few years. For example, Wang, Sun and Yaghi developed a
general strategy to grow large single crystal of 3D COFs,11 and
their structures were successfully determined by single-crystal X-
ray diffraction. In another example, we demonstrated a new to-
pology design to synthesize 3D COFs, starting from tetrahedral
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try can be achieved through the introduction of steric hindrance
and consequent construction of 3D COFs with novel structure
might become possible.
respectively. By using quenched solid density functional theory
(QSDFT), 2D-BPTA-COF showed a narrow pore size distribution
centered at 1.27 nm (Figure 1b).
With these considerations in mind, we report herein the de-
signed synthesis of two COFs (scheme 1), 2D-BPTA-COF and
3D-BMTA-COF, staring from biphenyl-based building blocks but
with different groups on ortho positions. Both COFs exhibited
high crystallinity and large surface area, and interestingly, 2D-
BPTA-COF crystallizes into 2D sheets with AB stacking mode
while 3D-BMTA-COF adopts a seven-fold interpenetrated struc-
ture with pts topology. Based on their crystal structures, the dihe-
dral angle of biphenyl rings in 2D-BPTA-COF is ~0°. However,
due to the existence of methyl groups, biphenyl rings in 3D-
BMTA-COF have adopted a tetrahedral configuration with dihe-
dral angle of ~60°. Consequently, introducing steric hindrance in
ortho positions of biphenyl cores can change their geometry and
thus provide a new tetrahedral node for constructing 3D COFs. To
the best of our knowledge, this is the first time to investigate the
effect of reinforced nonplanarity on the synthesis of COFs.
The PXRD pattern of 2D-BPTA-COF exhibited a intense
peak at 5.76˚ and two small ones at 9.46˚ and 11.64˚ (Figure 2a),
indicating the long-range ordering. We then used a 3D electron
diffraction (ED) technique, continuous rotation electron
diffraction (cRED),15 to elucidate the unit cell and crystal
structure. Based on the collected cRED data (Figure 2b), the 3D
reciprocal lattice was reconstructed, which contains a serial of
streaks along the c* axis and forms orthogonal pattern in the a*-
b* plane. Accordingly, 2D-BPTA-COF may have a layered
structure (a = 18.5 Å, b = 26.1 Å and γ = 90°) and the streaks
could be induced by stacking faults along the c-axis. From this
unit cell information, possible 2D structure models for 2D-BPTA-
COF were built by using Materials Studio software package
(Figure 3 and Figure S12), and the simulated PXRD pattern with
AB stacking mode shows excellent agreement with the
experimental pattern in peak positions and relative intensities
(Figure 2a). Finally, a more accurate structure model of 2D-
BPTA-COF with AB stacking mode (unit cell parameters: a
=18.500(0) Å, b = 26.330(9) Å, c = 7.801(4) Å, α = γ = 90°, and β
= 96.088(9)°; space group: C2/m) was obtained by the Rietveld
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a)
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refinement with acceptable low residuals Rp = 6.03% and Rwp
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Figure 1. N2 sorption isotherms of 2D-BPTA-COF (a) and 3D-
BMTA-COF (c) at 77 K. Pore size distributions of 2D-BPTA-
COF (b) and 3D-BMTA-COF (d).
In order to prove our idea, we decided to construct biphenyl-
based COFs as discussed above. As shown in Scheme 1, we first
designed and synthesized a linker, 3,3’,5,5’-tetra(p-aminophenyl)-
biphenyl (BPTA) (Figure S1), which has hydrogen atoms at ortho
positions. After condensation reaction of BPTA and 1,2,4,5-
tetrakis-(4-formylphenyl)-benzene (TPB-H) in a mixture of o-
dichlorobenzene, n-butanol and aqueous acetic acid for 7 days,
2D-BPTA-COF was obtained (Figure S3) as yellow powder in-
soluble in common organic solvents and water. The structure of
2D-BPTA-COF was then investigated by several techniques.
From the Fourier transform infrared (FT-IR) spectrum, 2D-
BPTA-COF exhibited a stretching vibration band at 1626 cm−1
(Figure S5), indicating the formation of imine bonds. According
to the solid-state nuclear magnetic resonance (ssNMR) experi-
ment (Figure S6), a new resonance at ~158 ppm was found, con-
firming again the presence of imine bonds. In addition, thermo-
gravimetric analysis (TGA) showed 2D-BPTA-COF has a high
thermal stability up to 530 °C under nitrogen (Figure S7), and
scanning electron microscopy (SEM) revealed a flake-like mor-
phology (Figure S8). Furthermore, we measured the nitrogen
adsorption isotherms of 2D-BPTA-COF at 77 K. As can be seen
in Figure 1a, 2D-BPTA-COF exhibited a type I sorption isotherm,
indicating the existence of microporous structure. The
Brunauer−Emmett−Teller (BET) surface area and the total pore
volume were calculated to be 1000 m2 g−1 and 1.16 cm3 g−1,
Figure 2. Powder X-ray diffraction patterns of 2D-BPTA-COF (a)
and 3D-BMTA-COF (c). The experimental XRD patterns are
shown in black, the refined patterns in red, the difference between
the observed and refined profiles in blue, Bragg position from
crystal structures in green. 3D reciprocal lattice of 2D-BPTA-
COF (b) and 3D-BMTA-COF (d).
According to the structure of 2D-BPTA-COF, the biphenyl
rings exhibited a planar conformation with a D2h symmetry and
have a dihedral angle of ~0°. Since the introduction of steric hin-
drance at the ortho positions can render the two rings orthogonal
with a tetrahedral symmetry, we predicted the resulting building
units may be suitable for constructing 3D COFs. To assess this
concept, force-field simulation was then applied to calculate the
framework energy for possible 2D structure models (Figure S15)
after incorporation of methyl groups into the biphenyl core as
steric hindrance. In addition, as it is reasonable to construct 3D
COFs starting from tetrahedral and quadrilateral precursors
through [4 + 4] connection,3a,5a,12 we also built a 3D structure with
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non-interpenetrated pts topology (Figure S16). After geometry
4). As expected, due to the existence of methyl groups, the dihe-
dral angle of biphenyl rings was found to be ~60°, which resulted
in a tetrahedral conformation with D2 symmetry. Finally, the
structure of 3D-BMTA-COF (unit cell parameters: a = 41.216(3)
Å, b = 4.927(2) Å, c = 40.237(4) Å, and β = 89.943(3)°; space
group: C2/c) was obtained by the Rietveld refinement with ac-
ceptable low residuals Rp = 6.31% and Rwp = 7.95% (see Tables
S3 and S4 for details). We should mention here, due to the pres-
ence of sp2 hybridised bridge carbon atoms in biphenyl nodes,
3D-BMTA-COF is a conjugated 3D COF. Based on these results,
it is possible to synthesize COFs with different structures through
steric-reinforced nonplanarity.
optimization, the energy of 2D framework with methyl groups
(949.02 Kcal/mol for AA stacking and 930.27 Kcal/mol for AB
stacking) was found to be much higher than that of the 3D struc-
ture with non-interpenetrated pts topology (859.72 Kcal/mol).
The detail calculated information can be found in section S6.
Therefore, a 3D framework can be expected after introducing
methyl groups as steric hindrance to BPTA.
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Figure 3. Structural representations of 2D-BPTA-COF. (a) Con-
formation of BPTA unit in 2D-BPTA-COF, (b) the structure of
adjacent two-layer, (c) side view, (d) top view.
We then designed and synthesized a new linker 3,3’,5,5’-
tetra(p-aminophenyl)-bimesitylene (BMTA) (Scheme 1 and Fig-
ure S2), by introducing the methyl groups in BPTA. After con-
densation with TPB-H, 3D-BMTA-COF was finally isolated
(Figure S4) as gray powder insoluble in common organic solvents
and water. Same as before, FT-IR spectra (Figure S5) and ssNMR
spectra (Figure S6) of 3D-BMTA-COF provided solid evidence
for the formation of imine bonds, and TGA plot exhibited high
thermal stability of ~510 °C (Figure S7). However, unlike 2D-
BPTA-COF, the SEM images of 3D-BMTA-COF exhibited gran-
ular morphology (Figure S8). Moreover, from the nitrogen sorp-
tion measurement (Figure 1c), 3D-BMTA-COF showed a higher
BET surface area (1650 m2 g-1) and smaller pore size (0.85 nm,
Figure 1d) compared to 2D-BPTA-COF.
Figure 4. Structural representations of 3D-BMTA-COF. (a) Con-
formation of BMTA unit in COF, (b) single pts network of 3D-
BMTA-COF, (c) seven-fold interpenetrated pts topology, (d) the
porous structure of 3D-BMTA-COF.
In summary, we have described the designed synthesis of two
biphenyl-based COFs with high crystallinity and large surface
area. By combining PXRD and cRED techniques, 2D-BPTA-COF
was determined to adopt a 2D layered structure with AB stacking
mode, while 3D-BMTA-COF showed a seven-fold interpenetrated
structure with pts topology. From the crystal structure, the dihe-
dral angle of biphenyl rings in 2D-BPTA-COF is ~0° while in 3D-
BMTA-COF is ~60°. Therefore, their framework difference can
be explained by this different conformation of biphenyl cores,
which should be ascribed to the introduction of steric hindrance
with methyl groups. Consequently, by twisting building blocks
from planar to tetrahedral with steric hindrance, COFs with dif-
ferent structures can be obtained. We believe this result represents
a general and straightforward way to expand the diversity of tet-
rahedral nodes, and moreover, a new tetrahedral node for con-
structing 3D COFs is now available. Considering the sp2 hybrid-
ised bridge carbon atoms in biphenyl nodes, more conjugated 3D
COFs with interesting applications can be imagined.
In order to determine the crystal structure of 3D-BMTA-COF,
PXRD and cRED techniques were applied. The PXRD pattern
(Figure 2c) clearly demonstrated the crystalline nature of 3D-
BMTA-COF, but due to the low intensity of the high angle peaks,
it was very difficult to resolve the structure directly. As the crystal
size of 3D-BMTA-COF is too small to collect 3D ED data, we
were not able to solve the crystal structure, even for the unit cell.
After many synthesis trials, 3D-BMTA-COF with bigger crystal
size was successfully obtained for cRED studies (Figure 2d) by
adding aniline as a modulator under optimized conditions (Section
S4). Although the resolution of 3D ED dataset was only 2 Å, the
central carbon atoms of tetrahedral and quadrilateral building
blocks can be located by SHELXT16. Based on the atom coordi-
nates determined from cRED data, we successfully built a struc-
ture model with a seven-fold interpenetrated pts topology (Figure
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ASSOCIATED CONTENT
Supporting Information
Synthetic procedures, FT-IR, solid state 13C NMR, TGA, SEM,
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found in the Supporting Information. This material is available
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Eur. J. 2017, 23, 5668.
Corresponding Author
*chengwang@whu.edu.cn (C. W.)
*junliang.sun@pku.edu.cn (J. S.)
Author Contributions
†C.G. and J. L. contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This paper is dedicated to Prof. Jean-Pierre Sauvage on occasion
of his 75th birthday. C.W. gratefully acknowledge financial sup-
port from the National Natural Science Foundation of China
(21772149 and 21572170), the Funds for Creative Research
Groups of Hubei Province (2017CFA002) and the Fundamental
Research Funds for Central Universities. J.S. also acknowledge
financial support from the National Natural Science Foundation of
China (21871009, 21621061 and 21527803), the Swedish Re-
search Council (VR), and the Knut and Alice Wallenberg Founda-
tion (KAW) for financial supports.
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