3D Covalent Organic Frameworks Selectively Crystallized through Conformational Design

Article abstract of DOI:10.1021/jacs.0c11064

We present a strategy whereby selective formation of imine covalent organic frameworks (COFs) based on linking of triangles and squares into the fjh topology was achieved by the conformational design of the building units. 1,3,5-Trimethyl-2,4,6-tris(4-for

Full text of DOI:10.1021/jacs.0c11064

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3D Covalent Organic Frameworks Selectively Crystallized through
Conformational Design
Ha L. Nguyen, Cornelius Gropp, Yanhang Ma, Chenhui Zhu, and Omar M. Yaghi*
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ABSTRACT: We present a strategy whereby selective formation of imine covalent organic frameworks (COFs) based on linking of
triangles and squares into the fjh topology was achieved by the conformational design of the building units. 1,3,5-Trimethyl-2,4,6-
tris(4-formylphenyl)benzene (TTFB, triangle) and 1,1,2,2-tetrakis(4-aminophenyl)ethene (ETTA, square) were reticulated into
[(TTFB)4(ETTA)3]imine, termed COF-790, which was fully characterized by spectroscopic, microscopic, and X-ray diffraction
techniques. COF-790 exhibits permanent porosity and a Brunauer−Emmett−Teller (BET) surface area of 2650 m2 g−1. Key to the
formation of this COF in crystalline form is the pre-designed conformation of the triangle and the square units to give dihedral
angles in the range of 75−90°, without which the reaction results in the formation of amorphous product. We demonstrate the
versatility of our strategy by also reporting the synthesis and characterization of two isoreticular forms of COF-790, COF-791 and
COF-792, based on other square building units.
ovalent organic frameworks (COFs) are two- and three-
dimensional (2D and 3D) crystalline structures com-
(SI), section S2). The positions of the methyl groups in TTFB
ensure a dihedral angle in the desired range of 75−90°.
Accordingly, we synthesized a molecular model system of the
TTFB linker and found dihedral angles of 74°, 83°, and 90°, as
shown in Figure 1b (see SI, section S3). Encouraged by this
structural validation obtained from the model compound, we
used the TTFB and ETTA linkers to crystallize [(TTFB)4-
(ETTA)3]imine (termed COF-790) having the 3D fjh topology
(Figure 1c). COF-790 was synthesized solvothermally from
TTFB and ETTA in a 4:3 molar ratio in nitrobenzene:
mesitylene (3:1 volumetric ratio) at 85 °C for 72 h. A
modulator, p-toluidine (7 equiv), was added (see SI, section
S2). COF-790 was isolated, solvent exchanged with N,N′-
dimethylformamide, 0.1 M NH4OH in methanol, methanol,
and chloroform, and then activated under dynamic vacuum at
90 °C for 4 h to yield the crystalline compound as a yellow
powder in 22% yield (see SI, section S2). COF-790 was fully
characterized by Fourier transform infrared (FT-IR) and solid-
and solution-state nuclear magnetic resonance (NMR)
spectroscopies, elemental analysis (EA), thermogravimetric
analysis (TGA), scanning electron microscopy (SEM), powder
X-ray diffraction (PXRD), nitrogen sorption, and transmission
electron microscopy (TEM; see SI, sections S3−S10).
C
posed of organic building units linked by covalent bonds.1−7
Control over the spatial arrangement of the linkers and
linkages within the COF backbone has led to frameworks with
pre-determined physicochemical properties.8−11 In the syn-
thesis of crystalline COFs, the geometry and connectivity of
the starting building units dictate the choice of structures
formed.12−15 However, combination of building units, such as
trigonal-planar (3-c) and square-planar (4-c), still presents
great challenges,7,16−18 because they are expected to produce
an array of different high-symmetry topologies: pto, tbo, mhq-
z, fjh, iab, gee, and ffc. The question becomes how a specific
COF topology can be targeted selectively so that the other
possibilities are eliminated from forming and thereby
complicating the purity and crystallinity of the product.
Thus, it is paramount in COF chemistry to develop strategies
to design building units that hold sufficient information to
selectively target a specific topology and only that topology.
In this report, we show how conformational design19 of the
trigonal linkers has led to the fjh topology, heretofore
unknown for 3D COFs. Formation of an amorphous phase
from linkers lacking such conformational information under-
lined the importance of such a strategy in selectively targeting
specific COFs. We further demonstrate the versatility of our
strategy by successful crystallization of isoreticular forms (fjh)
from similar conformationally designed linkers.
The FT-IR spectroscopic traces of COF-790 indicated imine
formation (νC=N = 1628 cm−1), with no identifiable aldehyde
stretches (νC=O = 1692 cm−1) remaining (see SI, section S4).
Complete imine formation of COF-790 was corroborated by
To identify the unique metric information needed for fjh,
this topology was deconstructed into its trigonal-planar and
square-planar building units, as shown in Figure 1a. The
dihedral angles between these units are found to be 75−90°.20
Based on this information, an fjh imine-linked framework
should result by combining 1,3,5-trimethyl-2,4,6-tris(4-
formylphenyl)benzene (TTFB) with 1,1,2,2-tetrakis(4-
aminophenyl)ethene (ETTA); see Supporting Information
Received: October 20, 2020
https://dx.doi.org/10.1021/jacs.0c11064
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
© XXXX American Chemical Society
A

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Figure 1. Synthesis of a 3D COF of the fjh topology through conformational linker design. (a) The fjh deconstructs into 3-c and 4-c building units
and their corresponding chemical equivalents: 1,3,5-trimethyl-2,4,6-tris(4-formylphenyl)benzene (TTFB) and 1,1,2,2-tetrakis(4-aminophenyl)-
ethene (ETTA), respectively. (b) Single-crystal X-ray structure of the trigonal-planar molecular model system with dihedral angles of 74−90° and
(c) crystal structure of [(TTFB)4(ETTA)3]imine, termed COF-790, with yellow spheres placed inside the pores at the van der Waals radii of the
framework atoms.
13C cross-polarization magic angle spinning (CP-MAS) NMR
substantiated by FT-IR spectroscopy and TGA traces. TGA
done under a N2 atmosphere showed a weight loss of ∼5% at
spectroscopy, displaying characteristic CN resonances at
250 °C and an onset in thermal decomposition of COF-790 at
around 400 °C (see SI, section S6). The constitution of COF-
790 was supported by solution-state NMR of its acid-digested
form and demonstrated a 4:3 ratio of TTFB:ETTA (Figure
2a). This result was consistent with the FT-IR and CP-MAS
13C NMR results, pointing to negligible defects within the
crystallites and the absence of unreacted aldehyde and amine
functionalities of the starting building units TTFB and ETTA
(see SI, section S5).
161.7 ppm and the absence of resonances corresponding to
aldehyde groups of the starting material TTFB. The
resonances of the methyl groups located at the center phenyl
of the TTFB unit appeared at 18.9 ppm (see SI, section S5).
The atomic composition of COF-790 was determined by EA
and found to be C198H144N12, corresponding to [(TTFB)4-
(ETTA)3]imine (Calcd for C, 88.36; H, 5.39; N, 6.25%. Found:
C, 83.36; H, 5.65; N, 6.45%; see SI, section S2). The difference
in elemental composition between the calculated and the
experimental values likely originates from remaining water
molecules in the framework (ca. 5.7%), evidence of which was
SEM micrographs of the COF-790 crystallites showed a
single morphological phase with a homogeneous distribution
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Figure 2. Structural and molecular characterization of COF-790. (a) Solution-state NMR spectroscopy of the acid-digested COF-790 indicates a
4:3 stoichiometric ratio of the TTFB:ETTA linkers. (b) N2 adsorption isotherm at 77 K of COF-790 demonstrates a mesoporous structure with a
pore diameter of 20.0 Å and a BET surface area of 2650 m2 g−1.
of needle-shaped crystals of ∼200 nm and aggregated spherical
particles (SI, section S7). PXRD analysis of the microcrystal-
line powder of COF-790 confirmed its crystallinity and
revealed no diffraction peaks that could be attributed to
residual starting materials or reaction additives (Figure 3; see
SI, section S8).
The crystal structure was obtained by comparison of the
experimental wide-angle X-ray scattering (WAXS) pattern of
COF-790 with the simulated PXRD patterns of the model
structures. The experimental diffraction pattern of COF-790
matched well with the simulated pattern obtained from the fjh
net, displaying two characteristic peaks at 1.91 and 2.30° (λ =
1.24 Å), where the first peak can be indexed as the 110
reflection and the subsequent ones as the 200 and 020
reflections. Importantly, alternative structural models of the
pto, tbo, mhq-z, iab, ffc, and gee topologies matched poorly
with the experimental results (see SI, section S8).
Additionally, the structural model generated from the fjh
topology displayed one discrete pore size of 21.5 Å (see SI,
section S8). To validate our structural model, we measured the
N2 sorption isotherm at 77 K. The N2 adsorption of COF-790
demonstrated permanent porosity with a mesoporous pore
structure and a Type I behavior. The Brunauer−Emmett−
Teller (BET) surface area was calculated to be 2650 m2 g−1.
The pore size distribution, estimated from the N2 isotherm and
calculated by density functional theory (DFT) using the
cylinder geometry and N2-cylindrical pores−oxide surface
model, indicated a pore diameter of 20.0 Å (Figure 2b; see SI,
section S9). This finding was in good agreement with our
structural model (21.5 Å).
Based on these experimental results, a structural model was
built, and the crystal structure of COF-790 was assigned to the
space group Iba2 (No. 45) with unit cell parameters of a =
62.56 Å, b = 60.32 Å, and c = 30.06 Å. Structural details are
Figure 3. WAXS patterns of COF-790 (a), COF-791 (b), and COF-
given in the SI, section S8. High-resolution TEM (HRTEM;
see SI, section S10) showed two kinds of lattice d-spacings of
20 and 28 Å, corresponding to the lattice planes of 121/211
and 020.
792 (c), indicating a shift of the lowest angle peak to lower 2θ values
with increasing length of the 4-c linker from COF-790 to COF-791
and COF-792.
Under synthetic conditions comparable to those used for the
synthesis of COF-790, but with the TFB lacking the methyl
groups and therefore the conformational information, we
obtained an amorphous solid (see SI, section S8). We
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attributed this to the formation of multiple phases, preventing
selective crystallization of fjh. Alternative synthetic conditions
favor the formation of a 2D COF with a defected tth
topology.21 In this 2D COF, the TFB units adopt dihedral
angles between 32 and 51°.
Synthesis and full characterization of COF-790, COF-
791, and COF-792 including EA, FT-IR spectroscopy,
NMR spectra, PXRD data, computational modeling, gas
uptake measurements, TGA, SEM images, and HRTEM
images (PDF)
To demonstrate the versatility of our design strategy, we also
report a series of isoreticular structures of COF-790, namely
COF-791 and COF-792, where we replaced the square-planar
ETTA unit of COF-790 with 1,2,4,5-tetrakis(4-aminophenyl)-
benzene (TAPB) and 1,2,4,5-tetrakis(4-aminophenyl)-3′,6′-
dimethylbenzene (TAPB-Me), respectively (Figure 3). COF-
791 ([(TTFB)4(TAPB)3]imine) and COF-792 ([(TTFB)4-
(TAPB-Me)3]imine) were synthesized and characterized as
demonstrated for COF-790 (see SI, section S2). Formation
of COF-791 and COF-792 was corroborated by FT-IR and
CP-MAS 13C NMR spectroscopies. Disappearance of the
X-ray crystallographic data for COF-790 (CIF)
X-ray crystallographic data for COF-791 (CIF)
X-ray crystallographic data for COF-792 (CIF)
X-ray crystallographic data for TAPB-Amine (CIF)
X-ray crystallographic data for TTFP-Aniline (CIF)
AUTHOR INFORMATION
Corresponding Author
■
Omar M. Yaghi − Department of Chemistry, University of
California Berkeley; Kavli Energy Nanoscience Institute at
UC Berkeley; and Berkeley Global Science Institute, Berkeley,
California 94720, United States; Joint UAEU-UC Berkeley
Laboratories for Materials Innovations, Berkeley, California
94704, United States; orcid.org/0000-0002-5611-3325;
Email: yaghi@berkeley.edu
aldehyde functionality, assigned to the resonances at νC=O
=
1692 cm−1, indicated successful imine formation (νC=N = 1628
cm−1; see SI, section S4). Solid-state 13C NMR spectra of
COF-791 and COF-792 displayed resonances associated with
the imine functionality at 161.5 and 160.6 ppm, respectively.
Moreover, at 18.7 and 18.9 ppm, we observed the resonances
corresponding to the methyl groups in COF-791 and COF-792
(see SI, section S5).
Authors
Ha L. Nguyen − Department of Chemistry, University of
California Berkeley; Kavli Energy Nanoscience Institute at
UC Berkeley; and Berkeley Global Science Institute, Berkeley,
California 94720, United States; Joint UAEU-UC Berkeley
Laboratories for Materials Innovations, Berkeley, California
94704, United States; orcid.org/0000-0002-4977-925X
Cornelius Gropp − Department of Chemistry, University of
California Berkeley; Kavli Energy Nanoscience Institute at
UC Berkeley; and Berkeley Global Science Institute, Berkeley,
California 94720, United States; orcid.org/0000-0002-
5843-8158
Yanhang Ma − School of Physical Science and Technology,
ShanghaiTech University, Shanghai 201210, China;
orcid.org/0000-0003-4814-3740
Chenhui Zhu − Advanced Light Source, Lawrence Berkeley
National Laboratory, Berkeley, California 94720, United
States
The atomic compositions of COF-791 and COF-792 were
determined by EA (COF-791: Calcd for C210H150N12
corresponding to [(TTFB)4(TAPB)3]imine: C, 88.76; H, 5.32;
N, 5.92%. Found: C, 86.80; H, 5.51; N, 5.97%. COF-792:
Calcd for C216H162N12 corresponding to [(TTFB)4(TAPB-
Me)3]imine: C, 88.67; H, 5.58; N, 5.75%. Found: C, 86.58; H,
6.02; N, 5.55%). COF-791 and -792 showed an onset in
thermal decomposition under a N2 atmosphere at around 400
°C (see SI, section S6). SEM micrographs indicated a
homogeneous size in crystallites of cube (∼300−400 nm)
and needle shapes (∼200 nm) for COF-791 and -792,
respectively.
The crystal structures of COF-791 and -792 were built
analogously to the fjh net of COF-790. Based on the model
structures, COF-791 and COF-792 displayed pore diameters
of 22.8 and 22.4 Å, respectively. N2 adsorption isotherms at 77
K of COF-791 and COF-792 demonstrated permanent
porosity and a Type I isotherm with mesoporous pore
structures. The BET surface area was calculated to be 1920
m2 g−1 for COF-791 and 2250 m2 g−1 for COF-792. Analysis
of the pore size distribution yielded diameters of 19.0 Å for
COF-791 and 22.6 Å for COF-792, both in good agreement
with their theoretical values estimated from their model
structures.
The lowest angle peaks of COF-791 and -792, as measured
by WAXS, appeared at 1.89° (λ = 1.24 Å). A shift to lower d-
spacing was observed for the isoreticular forms of COF-790
due to the slight enlargement of the unit cell parameters with
increasing linker lengths of the TAPB units (Figure 3).
HRTEM analysis of COF-791 indicated lattice fringes of 25 Å,
which were attributed to the 220 lattice plane. Owing to the
higher relative crystallinity of COF-792, lattice fringes with d-
spacings of 44 Å were identified, which corresponds to the 110
lattice plane (see SI, Section S10).
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c11064
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.G. is a Leopoldina postdoctoral fellow of the German
National Academy of Science (LPDS 2019-02) and acknowl-
edges the receipt of a fellowship of the Swiss National Science
Foundation (P2EZP2-184380). We acknowledge King Abdu-
laziz City for Science and Technology as part of a joint
KACST−UC Berkeley collaboration and UAE University as
part of a joint UAEU−UC Berkeley collaboration. We thank
Mr. Hao Lyu (Yaghi group, UC Berkeley) for help in obtaining
SEM images and Dr. Tianqiong Ma (Yaghi group, UC
Berkeley) for initial efforts in growing crystals of COF-791.
This research used beamline 7.3.3 of the Advanced Light
Source, which is a DOE Office of Science User Facility under
contract no. DE-AC02-05CH11231. This work is supported by
the National Natural Science Foundation of China (No.
21875140) and CℏEM SPST, ShanghaiTech University
(#EM02161943) for TEM measurements. We acknowledge
ASSOCIATED CONTENT
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https://pubs.acs.org/doi/10.1021/jacs.0c11064.
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the College of Chemistry Nuclear Magnetic Resonance Facility
for resourcesinstruments are partially supported by NIH
S10OD024998and staff assistance from Dr. Hasan Celik and
Dr. Wei-Chih Liao.
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