Urea-Linked Covalent Organic Frameworks

Article abstract of DOI:10.1021/jacs.8b10612

2D covalent organic frameworks (COFs) with flexible urea linkages have been synthesized by condensation of 1,3,5-triformylphloroglucinol (TFP) with 1,4-phenylenediurea (BDU) or 1,1′-(3,3′-dimethyl-[1,1′-biphenyl]-4,4′-diyl)diurea (DMBDU). The resulting CO

Full text of DOI:10.1021/jacs.8b10612

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Urea-Linked Covalent Organic Frameworks
Chenfei Zhao, Christian S. Diercks, Chenhui Zhu, Nikita Hanikel, Xiaokun Pei, and Omar M. Yaghi
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Nov 2018
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Urea-Linked Covalent Organic Frameworks
Chenfei Zhao,† Christian S. Diercks,† Chenhui Zhu,‡ Nikita Hanikel,† Xiaokun Pei,† Omar M. Yaghi†,∥,*
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†Department of Chemistry, University of California-Berkeley; Materials Sciences Division, Lawrence Berkeley National Laboratory; Kavli
Energy NanoSciences Institute at Berkeley; Berkeley Global Science Institute, Berkeley, California 94720, United States
‡Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
∥UC Berkeley-KACST Joint Center of Excellence for Nanomaterials for Clean Energy Applications, King Abdulaziz City for Science and
Technology, Riyadh 11442, Saudi Arabia
Supporting Information Placeholder
Scheme 1. Synthesis of Urea-Linked COFsa
ABSTRACT: 2D covalent organic frameworks (COFs) with
flexible urea linkages have been synthesized by condensation of
1,3,5-triformylphloroglucinol (TFP) with 1,4-phenylenediurea
(BDU) or 1,1'-(3,3'-dimethyl-[1,1'-biphenyl]-4,4'-diyl)diurea
(DMBDU). The resulting COF-117 and COF-118 undergo
reversible structural dynamics within their layers, in response to
inclusion and removal of guest molecules, emanating from urea C–
N bond rotation and interlayer hydrogen-bonding interactions.
These compounds are the first urea-linked COFs, serving to expand
the scope of reticular chemistry.
In the chemistry of covalent organic frameworks (COFs), the
linkage between the building blocks impacts the physical and
chemical properties of the material.1,2 For example, the chemical
stability,3 adsorption behavior,4 and catalytic activity5 of this
family of frameworks are profoundly affected by the nature of the
linkages employed. New linkage chemistry has also allowed for the
employment of diverse building units which otherwise would not
be capable of reticulating COFs.6 However, developing new
linkage chemistries is challenging because conditions for obtaining
crystalline COF products, without compromising the integrity of
the building units, have to be explored. Furthermore, most known
COFs are based on rigid linkages and building blocks because
increased molecular flexibility adds to the significant challenge of
obtaining crystalline products.7 Here, we report the use of urea as
linkages
triformylphloroglucinol (TFP) and 1,4-phenylenediurea (BDU) or
1,1'-(3,3'-dimethyl-[1,1'-biphenyl]-4,4'-diyl)diurea (DMBDU)
for
2D
COFs.
Condensation
of
1,3,5-
yields crystalline COF-117 and COF-118, respectively (Scheme 1).
Since this new urea linkage is flexible around the C–N bonds, the
two frameworks exhibit reversible structural dynamics within their
layers upon guest inclusion and removal ‒‒ the first evidence of
such behavior in 2D COFs.
A major challenge in making urea-linked COFs is the flexibility
of such linkages: Various conformations in molecules are known
depending on the specific steric, hydrogen-bonding, and π-π
stacking environments.8 The rotational barriers of urea C–N bonds
are typically lower than that of amides.9 It is therefore anticipated
that phenylenediurea unit might adopt various conformations
during the COF synthesis under solvothermal conditions, resulting
in varied linker lengths and directionalities (Figure 1). This linkage
flexibility increases the potential for crystallographic defects and
undermines the process of dynamic error correction required
aColor code for space-filling diagrams: H, white; C, gray; N, blue;
and O, red.
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group P3 (No. 143) yielded unit cell parameters in good agreement
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with the acetonitrile solvated sample (a = b = 36.1 Å, c = 3.4 Å, Rp
= 3.86%, Rwp = 6.83%) and the activated sample (a = b = 35.3 Å, c
= 3.8 Å, Rp = 6.55%, Rwp = 8.80%). The observed structural change
from activated phases for both COFs can be induced by exposure
to methanol vapor under ambient conditions. In the case of COF-
118, vapor exposure over time resulted in continuous change
between the two phases as evidenced by the change in the PXRD
pattern (Figure 3), confirming the Bragg diffractions at 2.90°,
5.02°, and 5.78° of the guest-free sample gradually shift to 2.81°,
4.86°, and 5.60° in the solvated phase. The nitrogen
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Figure 1. Selected possible conformations of the diurea linker.
for growth of crystalline COFs. We studied the condensation
between TFP and phenylurea as a model reaction for COF
synthesis. X-ray crystallographic analysis indicates the product
undergoes tautomerization to the keto form, therefore each urea
group possesses two N–H bonds rather than an imine moiety
(Supporting Information (SI), Section S10).10,11 The corresponding
urea-linked COF (termed COF-117) was synthesized by
solvothermal reaction between TFP and BDU in a mixed solvent of
N-methyl-2-pyrrolidinone (NMP) and 1,2,4-trichlorobenzene
(TCB) catalyzed by 6 M aqueous acetic acid (Scheme 1, top). The
molecular connectivity of the urea linkage in the resulting COF was
ascertained by Fourier-transform infrared (FT-IR) and 13C cross-
polarization magic angle spinning (CP-MAS) NMR
spectroscopies. Data were collected on the starting materials,
model compound, and the COF (SI, Section S3 and S4). As a result
of an increased double-bond character, the urea C=O stretching
band undergoes a blueshift from 1650 cm–1 in the starting material,
BDU, to 1713 cm–1 in the framework (Figure S2 and S5).12 The
disappearance of the characteristic aldehyde stretching peak at
Figure 2. (a) PXRD pattern of COF-117 immersed with MeCN. (b)
PXRD pattern of activated COF-117. (c) PXRD pattern of COF-
118 immersed with MeCN. (d) PXRD pattern of activated COF-
118.
1633 cm–1 was also noted, indicating a complete reaction. The 13
C
CP-MAS NMR spectrum of the COF has the characteristic
resonance of the newly formed ketone which matches well with the
model compound (Figure S16): The newly formed ketone
functionality is observed at 184.4 ppm. The peak at 149.1 ppm
results from overlapped carbon signals of both the urea and its α-
methine groups, as corroborated by the solution spectrum of the
model compound.
We observed that crystalline COF-117 was rendered amorphous
upon desolvation, possibly due to the flexible nature of the urea
linkages. Nevertheless, it readily regains crystallinity when treated
with solvents such as methanol, acetonitrile, and tetrahydrofuran
(Figure 2a and b).13 Pawley refinement of a structural model based
on conformation 1 was performed against the experimental powder
X-ray diffraction (PXRD) pattern of a sample solvated with
acetonitrile. The unit cell parameters (a = b = 29.0 Å and c = 3.4
Å) were obtained with good agreement factors (Rp = 1.21%, Rwp
=
2.11%) in the trigonal space group P-3 (No. 147). It is noteworthy
that some linker conformations other than 1 can also provide
reasonable structures in good agreement with the observed PXRD
pattern, but current data cannot determine their distribution in the
COF (SI, Section S5). The nitrogen adsorption isotherm measured
on the activated sample at 77 K gives a Brunauer−Emmett−Teller
(BET) surface area of 114 m2/g (Figure S25 and S26). This low
value is likely due to the observed pore deformation upon
activation as a result of framework flexibility.
Figure 3. (a) Comparison of PXRD patterns of activated COF-118,
material exposed to vapor of methanol for 4 or 24 hours, and COF
fully immersed with methanol and acetonitrile. (b) Space-filling
illustration of the pores of activated and methanol immersed COF-
118. Methanol molecules were drawn arbitrarily. Scale indicates
the maximal diameter of the hexagon. Color code: H, white; C,
gray; N, blue; and O, red.
An extended framework termed COF-118 was synthesized and
characterized in a similar fashion (Scheme 1, bottom). In stark
contrast to COF-117, this material remains crystalline upon
activation, though a contraction of the unit cell and a decrease in
the number of Bragg diffractions were observed when compared to
the solvated sample (Figure 2c and d). Structure modeling and
Pawley refinement based on conformation 1 in the trigonal space
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adsorption isotherm at 77 K revealed the mesoporous nature of
The high crystallinity and surface area of COF-118 allow for
detection of potential structural degradation under various
conditions. This material displayed excellent stability towards 12
M HCl (aq), boiling water, and saturated NaHCO3 (aq) for 24 hours,
with retained crystallinity and only minor decrease in surface area
(Figure 4). On the other hand, no COF material was recovered after
treatment with 1 M NaOH (aq) for the same duration.
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activated COF-118, and yielded a BET surface area of 1524 m2/g
(Figure S27 and S28). The markedly higher crystallinity and
surface area of activated COF-118 compared to COF-117 can be
partly attributed to an interlayer stabilizing effect14 resulting from
the biphenylene linker used in the COF-118 synthesis, which
constrains the contraction process during activation. Urea is well-
known to aggregate due to favorable hydrogen-bonding
interactions.15 Compared with COF-118, COF-117 is more inclined
to undergo structural deformation caused by hydrogen bonding due
to its higher weight percentage of urea groups.16
On a fundamental level, this report identifies and illustrates the
conditions under which urea-linked COFs can be crystallized.
These materials undergo reversible structural dynamics upon
addition and removal of guest molecules. The observed framework
dynamics are attributed to facile urea linkage rotation and interlayer
hydrogen-bonding interactions. This work also provides an
approach to extend COF dynamics from 3D frameworks to layered
structures.
9
To explore the origin of framework flexibility in response to
guest molecules, FT-IR studies were performed on acetonitrile
treated COFs. Compared with the guest-free samples, the presence
of acetonitrile resulted in blueshifts of the urea C=O and N–H
stretching peaks for both COFs while no significant shift was
observed for the other bands (Figure S7 and S8). The observed
changes in the positions of C=O stretching peaks can be attributed
to diminished hydrogen-bonding interactions that contribute to
urea-urea aggregation.17 Specifically, upon guest removal, urea
groups can potentially engage in interlayer N–H···O hydrogen-
bonding interactions with disordered conformations due to facile
C–N bond rotation and slight layer offset. The addition of
acetonitrile disrupts such intra-framework interactions and
therefore restores or augments the materials’ crystallinity. In
contrast, if acetonitrile merely interacted with free urea N–H bonds
through hydrogen bonding, then a redshift should be observed for
the N–H stretching frequencies instead.
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ASSOCIATED CONTENT
Supporting Information
Methods and additional data (PDF)
AUTHOR INFORMATION
Corresponding Author
Corresponding Author *yaghi@berkeley.edu
ORCID
Omar M. Yaghi: 0000-0002-5611-3325
As noted above, exposure of both COFs to methanol vapor under
ambient conditions can induce structural deviations from their
activated phases. Methanol vapor adsorption isotherms were
performed at 288 K on activated samples and found to exhibit a
moderate step for COF-118 (P/P0 = 0.2–0.3) and no step for COF-
117 (Figure S34). This suggests that structural responses to
methanol dosing are gradual for both COFs.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank Dr. Nanette N. Jarenwattananon for the acquisition of
solid state NMR spectra, Mr. Yu Cao for the assistance of WAXS
data acquisition, Mr. Peter J. Waller, Mr. Robinson W. Flaig, and
Dr. Chang Yan for helpful discussions. C.S.D. would like to
acknowledge the Kavli foundation for funding through the Kavli
ENSI graduate student fellowship. The collaboration with and
funding from King Abdulaziz City for Science and Technology is
gratefully acknowledged. 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.
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