Stable hydrazone-linked chiral covalent organic frameworks: Synthesis, modi?cation, and chiral signal inversion from monomers

Article abstract of DOI:10.1016/j.cclet.2020.11.063

The designed synthesis of chiral covalent organic frameworks (COFs) featuring intriguing properties is fairly scant and remains a daunting synthetic challenge. Here we develop a de novo synthesis of an enantiomeric pair of 2D hydroxyl-functionalized hydra

Full text of DOI:10.1016/j.cclet.2020.11.063

Chinese Chemical Letters 32 (2021) 107–112
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Stable hydrazone-linked chiral covalent organic frameworks:
Synthesis, modification, and chiral signal inversion from monomers
Yilun Yana, Xinle Lib, Gui Chena, Kai Zhanga, Xihao Tanga, Shuyuan Zhanga,
a,
Shengrun Zhenga, Jun Fana, Weiguang Zhanga, , Songliang Cai
*
*
a
School of Chemistry, South China Normal University, Guangzhou 510006, China
b
Department of Chemistry, Clark Atlanta University, Atlanta, Georgia 30314, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 28 June 2020
Received in revised form 18 November 2020
Accepted 18 November 2020
Available online 1 December 2020
The designed synthesis of chiral covalent organic frameworks (COFs) featuring intriguing properties is
fairly scant and remains a daunting synthetic challenge. Here we develop a de novo synthesis of an
enantiomeric pair of 2D hydroxyl-functionalized hydrazone-linked chiral COFs, (S)- and (R)-HthBta-OH
COFs, using enantiopure 2,5-bis(2-hydroxypropoxy)terephthalohydrazide (Hth) as monomers. The
formation process of hydroxyl-functionalized chiral COFs was monitored using rigorous time-dependent
PXRD, vibrational circular dichroism (VCD), and electronic circular dichroism (ECD) studies. Remarkably,
VCD spectra indicated a unique chiral signal inversion from the positive Cotton effect of (S)-Hth monomer
to the negative Cotton effect of (S)-HthBta-OH COF, which has never been reported in chiral COFs.
Moreover, two unprecedented carboxyl-functionalized chiral COFs, (S)- and (R)-HthBta-COOH, were
constructed by a post-synthetic modification of the corresponding hydroxyl chiral COFs with succinic
anhydride. Notably, carboxyl-functionalized COFs retained homochirality and crystallinity without linker
racemization and structural collapse after the chemical modification due to the chemically robust nature
of pristine hydrazone-linked chiral COFs.
Keywords:
Chiral covalent organic frameworks
De novo synthesis
Post-synthetic modification
Chiral signal inversion
© 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Covalent organic frameworks (COFs), a new class of frontier
crystalline porous materials constructed by linking organic
building blocks through reversible covalent bonds, have garnered
enormous attention owing to their enticing structural uniqueness
such as precise skeleton periodicity, ultralow density, high porosity
and modular pore metrics [1–4], which underpin their widespread
potential applications in gas absorption [5–7], heterogeneous
catalysis [8,9], chemical sensing [10–12], lithium-ion batteries
[13,14], chromatographic separation [15–18], optoelectronics [19–
21], etc. Despite a vast number of multifunctional COFs reported to
date, the construction of chiral COFs is rather scarce and remains a
formidable challenge, largely due to the intrinsic conflict between
crystallinity and asymmetry [1,22]. Furthermore, developing chiral
COFs by utilizing enantiopure building blocks might result in
unwanted achiral polymers owing to the linker racemization under
harsh solvothermal conditions, as being demonstrated in the
synthesis of chiral metal-organic frameworks (MOFs) [23–25].
To impart chirality in COFs, two main strategies, i.e., post-
synthetic modification [26,27], and de novo approach [15–
18,28,29] have been proved as effective means to integrate chiral
functionalities into two- or three-dimensional (2D or 3D) COFs. For
instance, in 2015, Jiang and co-workers developed the robust chiral
COF by a post-synthetic modification strategy, wherein the chiral
moiety of (S)-pyrrolidine was introduced into the pore wall of an
achiral COF via click reaction [27]. Later in 2016, Yan’s group
reported a de novo synthesis of chiral COFs by utilizing an
enantiopure monomer that was functionalized with (+)-diacetyl-L-
tartaric anhydride [15]. In 2017, Cui’s group developed two chiral
Zn(salen)-based COFs through Schiff-base reaction between
enantiopure 1,2-diaminocyclohexane and trisalicylaldehydes
[29]. Despite significant progress in the design and synthesis of
chiral COFs, the underlying mechanistic study of chiral COFs
formation remains substantially elusive [30]. On the other hand,
achiral COFs bearing carboxyl groups have been synthesized
through a facile post-synthetic modification of hydroxyl COFs
[31,32]. However, the construction of carboxyl-functionalized
chiral COFs has never been realized.
* Corresponding authors.
E-mail addresses: wgzhang@scnu.edu.cn (W. Zhang),
songliangcai@m.scnu.edu.cn (S. Cai).
Herein, we report a de novo synthesis of an enantiomeric pair of
2D hydrazone-linked chiral COFs bearing pre-installed hydroxyl
https://doi.org/10.1016/j.cclet.2020.11.063
1001-8417/© 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.

Y. Yan, X. Li, G. Chen et al.
Chinese Chemical Letters 32 (2021) 107–112
Scheme 1. Synthesis of chiral (S)- and (R)-HthBta-OH COFs as well as the corresponding (S)- and (R)-HthBta-COOH COFs by post-synthetic modification.
groups, termed (S)-HthBta-OH and (R)-HthBta-OH COFs
(Scheme 1), via the Schiff-base reaction of enantiopure 2,5-bis
(2-hydroxypropoxy)terephthalohydrazide ((S)-Hth and (R)-Hth)
with 1,3,5-benzenetricarboxaldehyde (Bta). The formation mech-
anism of such hydroxyl-functionalized chiral COFs was systemati-
cally investigated by means of rigorous kinetic studies.
Remarkably, time-dependent VCD spectra uncovered an inverted
chiral signal from the positive Cotton effect of (S)-Hth monomer to
the negative Cotton effect of (S)-HthBta-OH COF. To the best of our
knowledge, this represents the first report of VCD chiral signal
inversion in chiral COFs. Moreover, post-synthetic modification
allowed for the reaction between hydroxyl groups of (S)- and (R)-
HthBta-OH COFs and succinic anhydride, leading to the first
construction of carboxyl-functionalized (S)- and (R)-HthBta-COOH
COFs, respectively (Scheme 1). Furthermore, the homochirality and
Fig. 1. (a) Experimental (red) and Pawley refined (black) PXRD patterns of (S)-
crystallinity of (S)- and (R)-HthBta-COOH COFs remained intact
during the post-synthetic modification process owing to the
chemical robustness of pristine hydrazone-linked chiral COFs.
(S)- and (R)-HthBta-OH COFs could be synthesized by the Schiff-
base condensation between Bta (3.2 mg, 0.02 mmol) and enantio-
pure (S)-Hth or (R)-Hth (10.3 mg, 0.03 mmol) in a degassed
mixture of 1,4-dioxane (0.5 mL), mesitylene (0.5 mL) and 6 mol/L
aqueous acetic acid (0.1 mL) at 110 ꢀC for 3 days, giving rise to pale-
yellow solids in excellent yields (ꢁ90%). The resulting chiral COFs
were found to be insoluble in common organic solvents such as
acetone, ethanol, tetrahydrofuran, dichloromethane, and N,N-
dimethylformamide (DMF). Powder X-ray diffraction (PXRD) was
employed to assess the crystallinity of (S)- and (R)-HthBta-OH
COFs. (S)-HthBta-OH COF exhibited a strong PXRD peak at 3.4ꢀ, and
two minor peaks at 6.9ꢀ and 26.4ꢀ (Fig. 1a), corresponding to the
(100), (200), and (001) facets, respectively. Furthermore, (S)- and
(R)-HthBta-OH COFs showed identical PXRD patterns (Fig. S1a in
HthBta-OH COF. (b) Simulated PXRD pattern for the eclipsed AA mode. (c) Simulated
PXRD pattern for the staggered AB mode. (d) The difference between the
experimental and the refined PXRD patterns. (e) Eclipsed AA packing and (f)
staggered AB packing of (S)-HthBta-OH COF.
Supporting information), indicating the long-range ordered and
isostructural structures of both two COFs. To elucidate the
crystalline structures of (S)- and (R)-HthBta-OH COFs, two possible
2D layered chiral structures, that is, eclipsed AA stacking with a
space group of P6 (Fig. S1e, Tables S1 and S3 in Supporting
information) and staggered AB stacking with a space group of P63
(Fig. S1f, Tables S2 and S4 in Supporting information), were
simulated by using Materials Studio program [33]. The experi-
mental PXRD patterns of (S)- and (R)-HthBta-OH COFs reproduced
well with the simulated patterns based on the eclipsed AA packing
mode with respect to peak position and intensity (Fig. 1b and
Fig. S1b in Supporting information) but did not match well with
108

Y. Yan, X. Li, G. Chen et al.
Chinese Chemical Letters 32 (2021) 107–112
those generated from the staggered AB packing of 2D layers (Fig. 1c
and Fig. S1c in Supporting information). Pawley refinements based
on the eclipsed AA stacking structures were further conducted and
gave rise to the optimized parameters of unit cell: a = b = 30.32 Å, c
was higher than the reported hydrazone-linked achiral 2D COFs
[12,34]. The total pore volume was determined to be 0.35 cm3/g on
the basis of a single point measurement at P/Po = 0.99. Further-
more, the pore width of (S)-HthBta-OH COF estimated using
nonlocal density functional theory (NLDFT) was 1.5 nm (Fig. 2f),
which was in good agreement with the predicted value for an AA
stacking mode (1.6 nm).
To assess the chirality of Hth monomers and the resulting
hydroxyl-functionalized COFs, electronic circular dichroism (ECD)
spectroscopy was applied in the solid-state at room temperature.
As depicted in Fig. 3a, ECD spectra of (S)- and (R)-HthBta-OH COFs
clearly showed mirror images of each other, indicating that they
= 4.08 Å, α = β = 90ꢀ,
g
= 120ꢀ for (S)-HthBta-OH COF, and a = b
= 120ꢀ for (R)-HthBta-OH COF,
= 30.33 Å, c = 4.07 Å, α = β = 90ꢀ,
g
with good agreement factors (Rwp = 2.80%, Rp = 2.16% for (S)-
HthBta-OH COF and Rwp = 6.25%, Rp = 4.50% for (R)-HthBta-OH
COF).
Considering (S)- and (R)-HthBta-OH COFs were enantiomers,
we selected (S)-HthBta-OH COF as a representative example for
thorough structural characterization by various analytical meth-
ods. Elemental analysis of (S)-HthBta-OH COF revealed that the
experimental C, N and H contents corroborated well with the
theoretical values (Calcd. for C10H11N2O3 (%): C, 57.97; H, 5.35; N,
13.52. Found: C, 51.70; H, 5.82; N, 11.38). The Fourier transform
infrared (FT-IR) spectrum of (S)-HthBta-OH COF (Fig. 2a)
revealed two characteristic vibrational bands at 1618 and
were
a pair of enantiomers. In the wavelength range of
350ꢂ450 nm, (S)-HthBta-OH COF displayed a positive Cotton
effect at 427 nm and negative dichroic signal at 397 nm, whereas
(R)-HthBta-OH COF showed an enantiomeric ECD pattern at the
similar wavelengths. The ECD spectra of (S)- and (R)-Hth
monomers also affirmed their enantiomeric nature, in which
opposite Cotton effects at the same wavelengths (main peaks at
ꢁ285, 314 and 356 nm) were observed (Fig. S2a in Supporting
information). Since vibrational circular dichroism (VCD) spectros-
copy could extend the range of ECD measurements into the
infrared region, it has been widely used as a potent analytical tool
for probing the structure of chiral porous frameworks [35,36]. As
depicted in Fig. 3b and Fig. S2b (Supporting information), the VCD
spectra of both (S)/(R)-Hth monomers and (S)/(R)-HthBta-OH COFs,
which corroborated well with the IR spectra (Fig. S2b), exhibited
satisfactory mirror images of each other, offering another evidence
of their enantiomeric nature. Moreover, (S)- and (R)-HthBta-OH
COFs displayed strong fluorescence emission in the solid-state
(Fig. S3 in Supporting information), which inspired us to explore
their circularly polarized luminescence (CPL) behaviors. As
illustrated in Fig. 3c, (S)- and (R)-HthBta-OH COFs also exhibited
good mirror-image CPL signals centered at 480 nm. Taken together,
the combined results of ECD, VCD, and CPL demonstrated the
successful transformation of chirality from chiral Hth moieties to
HthBta-OH COFs.
1229 cmꢂ1, which were assigned to the C
N bonds. Furthermore,
¼
the stretching bands arising from amine (3100–3400 cmꢂ1) and
aldehyde (1699 cmꢂ1) largely decreased or even disappeared in
comparison to those of (S)-Hth and Bta, further verifying the
effective condensation reaction between COF precursors. The 13C
cross-polarization magic angle spinning (CP-MAS) solid-state
NMR spectrum of (S)-HthBta-OH COF showed an intense
signal at 150 ppm, corresponding to the carbons in C N bonds,
¼
while the aldehyde carbon signal was barely observed (Fig. 2b).
Thermogravimetric analysis (TGA) demonstrated that (S)-HthBta-
OH COF was stable up to 320 ꢀC under the air atmosphere (Fig. 2c).
The scanning electron microscopy (SEM) showed that (S)-HthBta-
OH COF adopted
a unique hollow rod-shaped morphology
with the length being a few microns (Fig. 2d). The porosity of
(S)-HthBta-OH COF was evaluated by nitrogen adsorption-
desorption isotherms at 77 K, which exhibited a characteristic
Type I shape, indicating the micro-porosity of the obtained COF
material. The Brunauer-Emmett-Teller (BET) surface area of
(S)-HthBta-OH COF was calculated to be 794 m2/g (Fig. 2e), which
Fig. 2. (a) FT-IR spectra of (S)-Hth, Bta and (S)-HthBta-OH COF. (b) Solid-state 13C CP-MAS NMR spectrum of (S)-HthBta-OH COF. (c) TGA curve of (S)-HthBta-OH COF under the
air atmosphere. (d) SEM image of (S)-HthBta-OH COF. (e) N2 adsorption (*) and desorption (ꢀ) isotherm of (S)-HthBta-OH COF and (f) pore size distribution of (S)-HthBta-OH
COF.
109

Y. Yan, X. Li, G. Chen et al.
Chinese Chemical Letters 32 (2021) 107–112
Fig. 3. (a) ECD, (b) VCD and (c) CPL spectra of the (S)- and (R)-HthBta-OH COFs. (d) PXRD patterns, (e) VCD and (f) ECD spectra of the (S)-HthBta-OH COF products obtained at
different reaction times ranging from 15 min to 72 h, respectively.
It is worth noting that the signals in the VCD spectra of (S)- and
(R)-HthBta-OH COFs were inverted compared to those of chiral Hth
monomers, which intrigued us to further elucidate the formation
process of (S)- and (R)-HthBta-OH COFs. Toward this end, the
synthesis of (S)-HthBta-OH COF as a representative example was
undertaken in the identical solvothermal conditions with varied
reaction times. The products were collected by filtration, washed
with 1,4-dioxane and THF, and then subjected to PXRD, VCD, and
ECD measurements. As shown in Fig. 3d, the PXRD pattern of the
product obtained in 15 min exhibited a set of diffraction peaks
arising from the (S)-Hth monomer and two very weak peaks in the
smaller angle region, indicating the incompleteness of Schiff-base
reaction and the amorphous nature of (S)-HthBta-OH COF. When
the reaction time was extended to 30 min, the peak intensity of (S)-
Hth significantly decreased, while those of (S)-HthBta-OH COF
increased. Notably, the (S)-Hth monomer completely disappeared
after 45 min, whereas medium-strong diffraction peaks at 3.5ꢀ, 7.0ꢀ
and 26.4ꢀ were observed, indicative of an amorphous-crystalline
transformation in (S)-HthBta-OH COF. The crystallinity of (S)-
HthBta-OH COF further increased in 75 min, which was compara-
ble to that obtained in 72 h, revealing the rapid formation of (S)-
HthBta-OH COF, which was drastically faster than the conventional
hydrazone-linked COFs [12]. To gain deeper insights into the chiral
signal change from monomers to chiral COFs, we conducted the
time-dependent VCD measurements. As illustrated in Fig. 3e, the
VCD spectrum of (S)-Hth exhibited three intense positive peaks at
aromatic ring skeleton vibration of the resulting chiral COF. These
VCD results were in line with the PXRD and IR analyses (Fig. S4 in
Supporting information). Remarkably, the VCD spectra revealed a
unique VCD chiral signal inversion from the positive Cotton effect
of (S)-Hth monomer to the negative Cotton effect of (S)-HthBta-OH
COF, which has never been documented in chiral COFs. Given that
the VCD chiral signals could be profoundly affected by the
conformations of chiral compounds [37,38], we postulated that
such a rare chiral signal inversion from monomer to COF could be
attributed to the conformation change of (S)-Hth moieties in the
resulting (S)-HthBta-OH COF. While the conformation change of
(S)-Hth moieties was due to the
p-p stacking and hydrogen
bonding interactions between the adjacent COF layers. Moreover,
the time-dependent synthesis of (S)-HthBta-OH COF was moni-
tored using ECD spectroscopy (Fig. 3f). The ECD spectra of the
products obtained from 15 min to 72 h presented strong positive
peaks centered at 416, 419, 421, 424, 426 nm, respectively. Such
bathochromic shifts could be due to the formation of the more
extended
p-systems in (S)-HthBta-OH COF.
On account of the high crystallinity, large surface area, good
stability, and abundant free hydroxyl functionalities in the
homochiral COFs, we rationally transformed (S)- and (R)-
HthBta-OH COFs into carboxyl-functionalized chiral COFs through
the post-synthetic modification. Specifically, an enantiomeric pair
of chiral COFs bearing carboxyl groups, namely (S)- and (R)-
HthBta-COOH COFs, were synthesized by a facile reaction between
the hydroxyl groups on the backbones of (S)- and (R)-HthBta-OH
COFs and succinic anhydride in DMF at 80 ꢀC for 4 days (Scheme 1).
The resulting carboxyl-functionalized chiral COFs were thoroughly
characterized by a number of analytical techniques. The PXRD
pattern of (S)-HthBta-COOH COF displayed an intense diffraction
peak at 3.4ꢀ and two weak peaks at 6.8ꢀ and 26.4ꢀ, which matched
well with those of the parent (S)-HthBta-OH COF (Fig. 4a),
indicating the packing mode of (S)-HthBta-COOH COF did not
change after the post-synthetic modification of (S)-HthBta-OH
COF. SEM images of (S)-HthBta-COOH COF displayed similar
morphology to that of (S)-HthBta-OH COF (Fig. S5 in Supporting
information), further confirming no obvious morphological and
1645, 1614 and 1526 cmꢂ1, which were ascribed to the C
O
¼
stretching vibration, NꢂꢂH bending vibration of -NH2 group, and
the skeleton vibration of the aromatic ring, respectively. The
intensities of the three positive peaks continuously decreased as
the reaction times were kept at 15 and 30 min. These three positive
peaks disappeared after 45 min, whereas new negative peaks at
1616, 1526 cmꢂ1 arose, indicating the complete consumption of
(S)-Hth and the formation of (S)-HthBta-OH COF. In particular, the
VCD spectrum of (S)-HthBta-OH COF obtained in 75 min (similar to
the one synthesized in 72 h) displayed three obvious negative
peaks at 1652, 1616, 1526 cmꢂ1, corresponding to the C
stretching vibration, the stretching vibration, and the
¼O
C¼N
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Y. Yan, X. Li, G. Chen et al.
Chinese Chemical Letters 32 (2021) 107–112
Fig. 4. (a) PXRD patterns, (b) FT-IR spectra, (c) VCD spectra and (d) ECD spectra of (S)-HthBta-OH and (S)-HthBta-COOH COFs. (e) VCD and ECD spectra of (S)- and (R)-HthBta-
COOH COFs.
structural damage occurred during the modification process. TGA
revealed that (S)-HthBta-COOH COF possessed comparable ther-
mal stability (325 ꢀC) to the pristine (S)-HthBta-OH COF (Fig. S6 in
Supporting information). The BET surface area and pore size of (S)-
HthBta-COOH COF were determined to be 223 m2/g (Fig. S7 in
Supporting information) and 1.2 nm (Fig. S8 in Supporting
information), respectively, which were smaller than those of the
(S)-HthBta-OH COF owing to the successful grafting of bulky
carboxyl groups onto the pore wall of the pristine COFs.
installed hydroxyl functionalities. VCD, ECD and CPL measure-
ments unambiguously proved that (S)- and (R)-HthBta-OH COFs
were enantiomers. Moreover, the formation process of hydroxyl-
functionalized chiral COFs was monitored using rigorous time-
dependent PXRD, VCD and ECD studies. Remarkably, the VCD
measurement showed an unprecedented chiral signal inversion
from the positive Cotton effect of (S)-Hth monomer to the negative
Cotton effect of (S)-HthBta-OH COF, which was ascribed to the
possible conformation change of (S)-Hth moieties in (S)-HthBta-
OH COF. Thanks to their excellent crystallinity, good chemical
stability, high porosity, and abundant hydroxyl functionalities, we
post-synthetically converted robust (S)- and (R)-HthBta-OH COFs
into carboxyl-functionalized (S)- and (R)-HthBta-COOH COFs with
the retention of homochirality and crystallinity, indicating no
linker racemization and structural damage during the post-
synthetic modification process. This work not only paves a new
avenue to the construction of homochiral COFs possessing distinct
functionalities but also sheds a deeper light on how the chiral
signals changed from chiral monomers to homochiral COFs. We
envision that the resulting stable hydrazone-linked homochiral
COFs with abundant hydroxyl and carboxyl functional groups may
be employed as good candidates for chiral chromatographic
separation and asymmetric catalysis. These potential applications
are currently under study in our group.
The FT-IR spectrum of (S)-HthBta-COOH COF showed an
emerging peak at 1730 cmꢂ1, corresponding to the stretching
vibration of C O in the newly formed ester and free carboxylic
¼
acid, suggesting the effective ring-opening reaction between
succinic anhydride and the hydroxyl groups of (S)-HthBta-OH
COF (Fig. 4b). The VCD spectrum of (S)-HthBta-COOH COF
presented the same trend to that of the pristine (S)-HthBta-OH
COF (Fig. 4c), where a set of negative peaks at similar wave-
numbers were observed, indicating the preserved chirality of COFs
and intact conformations of (S)-Hth moieties in (S)-HthBta-COOH
COF. It is worth noting that a new negative peak at 1733 cmꢂ1 was
also found in the VCD spectrum of (S)-HthBta-COOH COF, further
validating the formation of the ester and carboxylic acid groups.
Furthermore, the ECD spectrum of (S)-HthBta-COOH COF resem-
bled that of (S)-HthBta-OH COF, wherein a positive peak at the
wavelength of 427 nm was observed (Fig. 4d). More importantly,
both VCD and ECD spectra of (S)- and (R)-HthBta-COOH COFs were
mirrored images of each other, confirming their enantiomeric
nature (Figs. 4e and f). The successful construction of carboxyl-
functionalized chiral COFs without linker racemization under
harsh modification conditions could be attributed to the excep-
tional chemical robustness of the hydroxyl-functionalized hydra-
zone-linked COFs. No reversal of the chiral signals was observed in
both VCD and ECD spectra of (S)-HthBta-COOH COF compared with
those of (S)-HthBta-OH COF, illustrating the retained conformation
of the (S)-Hth moieties in the resulting (S)-HthBta-COOH COF.
In summary, we have developed a de novo synthesis of a pair of
2D hydrazone-linked (S)- and (R)-HthBta-OH chiral COFs with pre-
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgments
We are grateful for the financial support from the National
Natural Science Foundation of China (Nos. 21603076 and
21571070) and the Natural Science Foundation of Guangdong
Province (No. 2018A030313193).
111

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Chinese Chemical Letters 32 (2021) 107–112
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