Construction of Flexible Amine-linked Covalent Organic Frameworks by Catalysis and Reduction of Formic Acid via the Eschweiler–Clarke Reaction

Article abstract of DOI:10.1002/anie.202102373

Compared to the current mainstream rigid covalent organic frameworks (COFs) linked by imine bonds, flexible COFs have certain advantages of elasticity and self-adaptability, but their construction and application are greatly limited by the complexity in s

Full text of DOI:10.1002/anie.202102373

Angewandte
Research Articles
Chemie
How to cite:
International Edition: doi.org/10.1002/anie.202102373
German Edition: doi.org/10.1002/ange.202102373
Covalent Organic Frameworks
Hot Paper
Construction of Flexible Amine-linked Covalent Organic Frameworks
by Catalysis and Reduction of Formic Acid via the Eschweiler–Clarke
Reaction
Meicheng Zhang+, Yang Li+, Wenli Yuan, Xinghua Guo, Chiyao Bai, Yingdi Zou,
Honghan Long, Yue Qi, Shoujian Li, Guohong Tao, Chuanqin Xia, and Lijian Ma*
Abstract: Compared to the current mainstream rigid covalent
organic frameworks (COFs) linked by imine bonds, flexible
COFs have certain advantages of elasticity and self-adaptabil-
ity, but their construction and application are greatly limited by
the complexity in synthesis and difficulty in obtaining regular
structure. Herein, we reported for the first time a series of
flexible amine-linked COFs with high crystallinity synthesized
by formic acid with unique catalytic and reductive bifunctional
properties, rather than acetic acid, the most common catalyst
for COF synthesis. The reaction mechanism was demonstrated
to be a synchronous in situ reduction during the formation of
imine bond. The flexibilities of the products endow them with
accommodative adaptability to guest molecules, thus increas-
ing the adsorption capacities for nitrogen and iodine by 27%
and 22%, respectively. Impressively, a novel concept of
flexibilization degree was proposed firstly, which provides an
effective approach to rationally measure the flexibility of
COFs.
and the vast majority of known COFs are linked by imine
bonds.
It is worth noting that more than 80% of the imine-linked
COFs are synthesized in the presence of acetic acid as
a catalyst in literature-reported methods.[8] On the one hand,
acetic acid can be widely used in various mixed solvent
systems under solvothermal conditions. On the other hand,
the synthesized COFs using acetic acid as the catalyst have
=
structures with rigid C N bonds which facilitate to construct
COFs with better crystallinity and regular pores. With these
characteristics, acetic acid has become the most general
catalyst for the synthesis of COFs. In addition to acetic acid,
a small number of other types of catalysts, such as Sc(OTf)
3
and p-toluenesulfonic acid, were also used to synthesize
imine-linked COFs.[9]
=
However, most COF structures with rigid C N bonds
endow the regular pores and channels with fixed sizes which
may not maximize the advantage of each active site and
functional group of COFs in practical applications. In order to
sufficiently utilize the strengths of regular pores and uniform
sites of the materials, several flexible COFs have been
prepared which have the advantages of elasticity, self-
adaptive ability and so on.[5d,10] For example, COF-117 and
COF-118 containing flexible urea-based linkages were re-
ported by Yaghi et al. and the flexible materials undergo
reversible structural dynamics upon the addition and removal
of guest molecules.[11] A series of COFs based on flexible
modules were reported by our previous work which have
elastic and self-adaptive pores, and exhibited superior iodine
adsorption performance than traditional rigid COFs.[5d]
Although a few flexible COFs have been successfully
synthesized, it is still relatively difficult to build flexible
structural frameworks because of the flexibility and varia-
bility of the building units in three-dimensional direction.
And the increased flexibility usually results in irregular
polymerization and worse crystallinity of COFs.[3d,7b,d] Addi-
tionally, the syntheses of flexible building blocks are also
quite complicated and cumbersome.[7b,12] The synthesis of
flexible COFs generally needs monomers with side chains in
a certain length, which increases the complexity of the
monomer synthesis compared with the traditional rigid
monomers. Moreover, the longer and freely rotatable side
chains of the flexible monomers can reduce the probability of
regular combination and arrangement of the monomers, and
may lead to the formation of amorphous polymers even if the
flexible monomers undergo condensation reaction. There-
fore, it remains a great challenge to explore new and
Introduction
Covalent organic frameworks (COFs) are an emerging
class of crystalline porous materials constructed by organic
building units. Owing to their regular pore and abundant
active sites, COFs have been adopted in many different
applications such as adsorption and separation,[1] energy
storage,[2] sensing,[3] and catalysis[4] in recent years.
Current reported COFs are mainly linked by imine
bonds,[5] boron-oxygen bonds[6] and hydrazone bonds.[7]
Among them, imine bonding dominates the bonding types
[*] Dr. M. Zhang,[+] Dr. W. Yuan, Dr. X. Guo, Y. Zou, H. Long, Dr. Y. Qi,
Prof. S. Li, Prof. G. Tao, Prof. C. Xia, Prof. L. Ma
College of Chemistry, Sichuan University, Key Laboratory of Radiation
Physics & Technology, Ministry of Education
Chengdu, 610064 (P. R. China)
E-mail: ma.lj@hotmail.com
Dr. Y. Li[+]
Hefei National Laboratory for Physical Sciences at the Microscale,
University of Science and Technology of China
Hefei, Anhui 230026 (P. R. China)
Dr. C. Bai
Chengdu New Radiomedicine Technology CO. LTD.
Chengdu, 610064 (P. R. China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202102373.
&&&&
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
convenient synthetic methods of flexible COFs for wider
applications.
Inspired by the catalysis of acetic acid, we consider
ties of formic acid,[13a] we employed formic acid instead of
acetic acid in this study, hoping to establish a highly effective
strategy for constructing flexible COFs connected by secon-
dary amine bonds.
As expected, a series of flexible amine-linked COFs with
high crystallinity were obtained successfully, which confirmed
the effectiveness and universality of the catalytic & reducing
strategy by formic acid to synthesize flexible COFs. This
synthesis strategy can avoid the usual synthetic difficulty of
flexible COFs and construct flexible COFs by just one step,
which presents a new idea and solution for the design and
synthesis of flexible COFs.
whether formic acid, the simplest carboxylic acid in nature
with a similar structure to acetic acid, can play a similar
catalytic effect to acetic acid? Particularly, it is noteworthy
that formic acid has not only the commonality of carboxylic
acid, but also a certain unique reducing ability.[13] For instance,
formic acid can serve as the reducing agent to catalyze the
Eschweiler-Clarke reaction between amines and aldehy-
des.[13b,14] In this regard, could this reducibility of formic acid
work in the catalytic synthesis of COFs?
On the basis of the above conception, we assume that if
formic acid is used as the catalyst in the reaction between
amines and aldehydes, the effect of reduction can be realized Results and Discussion
simultaneously without the addition of other reducing agents.
Similarly, if formic acid is used as the catalyst instead of acetic
acid in the preparation of COFs, it may be possible to reduce
Synthesis of Amine-linked Model System
=
the rigid C N double bond into flexible C-N single bond
As an illustration, a model reaction between benzalde-
hyde and aniline was also performed under the catalysis &
reduction of formic acid (Scheme 1a and b). The experiment
result revealed that the model compound of N-phenylbenzyl-
while ensuring high crystallinity of the materials, so as to
realize fast and convenient preparation of flexible COFs.
Therefore, based on the dual catalytic and reductive proper-
Scheme 1. a) Mechanism of catalysis by acetic acid. b) Mechanism of catalysis and reduction by formic acid via the Eschweiler-Clarke reaction.[14a,d]
c) Synthetic route and simulated structures of FAL-COF-1 and RIL-COF-1 (grey C; pink O; purple N).
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
amine (M1) linked by amine bond was obtained with a yield
of 60% in the presence of formic acid (Scheme S1), implying
the viability of this strategy to prepare amine-linked COFs
directly by formic acid. To confirm the intermediate process
of the amine bond formation, benzaldehyde and aniline were
also used to prepare an imine-linked N-benzylideneaniline
(M2) under the catalysis of acetic acid (Figures S27 and S28).
And then M2 was attempted to be reduced to M1 by formic
acid. It was found that M1 can still be isolated in 28% yield by
this two-step reaction, suggesting that the formation of amine
bond did go through the process of forming imine bond and
then being reduced, which is consistent with the mechanism of
Eschweiler-Clarke reaction. However, compared with the
two-step method to prepare amine-bonded M1, the proposed
one-step strategy can perform the in situ reduction from imine
bond to amine bond as soon as the imine bond is formed,
which is more efficient and can greatly improve the yield of
the reaction (from 28% to 60%).
The cross-polarization magic angle spinning 13C solid-
state nuclear magnetic resonance (CP/MAS 13C NMR) spec-
tra of FAL-COF-1 and RIL-COF-1 are shown in Figure 1b
and a new peak at 43.1 ppm of FAL-COF-1 confirms the
presence of the amine bond (C-N) compared with RIL-COF-
1.[10b] This is consistent with the liquid 13C NMR spectrum of
the model compound M1 (Figures S28 and S29), which
showed the peak at 48.5 ppm corresponds to the C-NH bond.
In the N1s spectra of high-resolution X-ray photoelectron
spectroscopy (XPS), a new peak of binding energy of FAL-
COF-1 at 400.3 eV is observed which is assigned to NH-C
(Figures S1–S6).[15] The Fourier transform infrared (FT-IR)
spectrum of FAL-COF-1 shows that the characteristic peak of
ꢀ1
ꢀ
=
C N bond at 1253 cm appears while the peak of C N bond
at 1629 cmꢀ1 largely disappears (Figure S19), which matches
well with those in FT-IR of the model compounds.[16] These
results indicate that FAL-COF-1 with amine bond was
successfully synthesized.
On the other side, we also tried to reduce the imine-linked
RIL-COF-1 to the amine-linked FAL-COF-1 directly using
formic acid as the reducing agent, but it was found that the
reduction degree (16%) is very low (Figures S7 and S20).
Although imine bond can be partly reduced to amine bond in
the model reaction, the layers of the generated polymer
material has already stacked together at the formation
process of RIL-COF-1, making it difficult for formic acid to
fully contact with the internal structure of RIL-COF-1 and to
effectively reduce imine bond. In contrast, the method of one-
step catalysis & reduction by formic acid is able to fulfill the in
situ reduction of imine during the formation of the imine
bond by the catalyzed imine condensation, thus allowing
Synthesis and Characterization of Flexible Amine-linked COF
A flexible amine-linked COF, named FAL-COF-1, was
synthesized with the condensation of 2,4,6-tris(4-formylphe-
noxy)-1,3,5-triazine (Scheme S3 and Figure S26) and 3,3’-
dimethylbiphenyl-4,4’-diamine by catalysis & reduction of
formic acid (Schemes 1c and S4). In contrast, a rigid imine-
linked COF, named RIL-COF-1, was also synthesized using
the same monomers as FAL-COF-1 by catalysis of acetic acid
as a traditional catalyst (Schemes 1c and S5).
Figure 1. Characterization of FAL-COF-1 and RIL-COF-1. a) PXRD patterns of RIL-COF-1 (blue) and FAL-COF-1 (pink: experimental; purple: Pawley
refined; yellow: calculated for AA stacking; green: the difference between experimental and calculated data). b) Solid-state 13C NMR spectra RIL-
COF-1 (blue) and FAL-COF-1 (pink). c) N2 sorption isotherm curves (inset: corresponding pore size distributions). d) HRTEM image and lattice
distance (inset: the interlayer distance of simulated structure) of FAL-COF-1.
&&&&
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
COFs with flexible amine bond prepared efficiently. Addi-
COF-1 (3.4 nm) is smaller than that of RIL-COF-1 (3.7 nm),
tionally, NaBH4 was also used to reduce RIL-COF-1 to
FALCOF-1, but the content of amine bond in the product
(48%) was still much lower than that obtained by this method
(Figures S8 and S21) and the surface area (99 m2 gꢀ1, Fig-
ure S52) was also lower than that of the original RIL-COF-
1 (132 m2 gꢀ1).
which further confirms that the flexible structural framework
of FAL-COF-1 does shrink the aperture of the material.
Moreover, to demonstrate the flexible characteristics of
FAL-COF-1, the interactions between the host framework of
FAL-COF-1 and several guest organic solvent molecules were
investigated. The PXRD patterns of FAL-COF-1 after adding
a few drops of ethyl alcohol, tetrahydrofuran, acetone, and
acetonitrile solvents, were tested, respectively (Figure S41).
We found that after dropping the solvents, the (100) peak of
all the samples shifted to the direction of high degree,
indicating that FAL-COF-1 performed a pore shrink after the
solvent molecules entered to the framework, and as the
solvents volatilized, the (100) peak gradually returned to the
original position. In contrast, the (100) peak of RIL-COF-
1 only slightly shifted to the direction of high degree after the
different solvents dripped (Figure S41). These data clearly
indicate that the pore and channel of FAL-COF-1 have
a certain elasticity and self-adaptability, and FAL-COF-
1 shows strong host-guest affinity for organic solvents which
can also be demonstrated by the density functional theory
(DFT) calculation (Table S11). Thermal gravimetric analysis
under N2 atmosphere demonstrated a thermal stability of up
to 4008C for both RIL-COF-1 and FAL-COF-1 (Figures S56
and S57), indicating that the flexible FAL-COF-1 still has very
good thermal stability compared with the rigid RIL-COF-1.
The mechanism for the catalysis by formic acid and the
catalysis & reduction by formic acid was shown in Scheme 1a
and 1b (see Supporting Information for details).[14a,d] Accord-
ing to the Eschweiler-Clarke reaction, the aldehyde and
amino groups are firstly catalyzed by the acid to form a Schiff
base intermediate, which is similar to the mechanism of
catalysis by acetic acid (Scheme 1a).[14a] Secondly, the Schiff
base intermediate was reduced by formic acid to form
a transition state of six-membered ring. After a hydride
transfer, the amine bond was formed.[14a,d]
Crystallinity and Pore Shrinkage Effect of FAL-COF-1
The crystallinity of FAL-COF-1 and RIL-COF-1 (Fig-
ure 1a) were confirmed by powder X-ray diffraction (PXRD)
analysis with Cu Ka radiation. As shown in Figure 1a, both of
the materials have high crystallinity. To determine the crystal
structures of FAL-COF-1 and RIL-COF-1 and to calculate
the unit cell parameters, eclipsed (AA) stacking and stag-
gered (AB) stacking models were built and optimized by the
Forcite module of Materials Studio (Figures S33,S34 and
Tables S12,S13). Pawley refinement of the AA stacking
model against the experimental PXRD data of FAL-COF-
1 gave a unit cell with parameters (a = 42.60 ꢀ, b = 42.60 ꢀ,
c = 3.53 ꢀ) with the P6/m space group, providing good
agreement factors (Rp = 3.18% and Rwp = 4.46%). The
diffraction peak of FAL-COF-1 at 2.398 corresponds to the
(100) reflection, of which the value is larger than that of RIL-
COF-1 (2.298), indicating that the pore size of FAL-COF-
1 (3.69 nm) is smaller than that of RIL-COF-1 (3.85 nm). The
pore sizes of the two materials are also coincident with the
values calculated by the simulated structures (3.95 nm for
FAL-COF-1 and 3.34 nm for RIL-COF-1). This difference
probably is caused by the shrinkage of the flexible framework
of FAL-COF-1, which is thought to be related to the
ꢀ
flexibility and flippability of the C N bond and the shorter
length of the linking units. High-resolution transmission
electron microscopy (HRTEM, Figures 1d and S42) image
shows that the interlayer distance of is 3.56 ꢀ, which is larger
than that of RIL-COF-1 (3.32 ꢀ).
FAL-COF-1 and RIL-COF-1 were dispersed in ethanol
and the dispersion of FAL-COF-1 shows weaker fluorescence
intensity than that of RIL-COF-1 (Figures S43 and S44). This
is due to that the p-p conjugation interaction of FAL-COF-
1 with the C-N single bond is weaker than that of RIL-COF-
Calculation of the flexibilization degree (DF)
In general, the flexibility of COF materials is mainly
caused by the rotation and torsion of the chemical bonds,
especially the single bonds in their structural frameworks,
which also directly leads to the change of corresponding bond
angles formed by these bonds. By comparing the structural
features of FAL-COF-1 and RIL-COF-1, we found that the
flexibility of FAL-COF-1 comes from the rotation of the
chemical bonds in not only the knot unit (forming bond angles
of q1–q3), but also the linker unit (forming bond angles of q4–
q7, Scheme 2 and S12).
Therefore, in order to visually and quantitatively describe
the degree of flexibility of FAL-COF-1, seven bond angles
formed by the chemical bonds at the above key positions in
the structural frameworks of FAL-COF-1 and RIL-COF-
1 were measured, and a novel concept named the flexibiliza-
tion degree (DF) was defined as the sum of the angle
variations of these seven bonds, which is expressed as the
following equation
=
1 with the rigid C N double bond. The weakened p-p
conjugation effect of FAL-COF-1 also led to an increase of
interlayer distance which can be observed from TEM images
(Figure S42).
The N2 adsorption analyses of FAL-COF-1 and RIL-
COF-1 (Figure 1c) show that the Brunauer-Emmett-Teller
(BET) surface area and pore volume of FAL-COF-1 are
168 m2 gꢀ1 and 0.32 cm3 gꢀ1, larger than those of RIL-COF-
1 with the values of 132 m2 gꢀ1 and 0.24 cm3 gꢀ1 (Figures S52
and S55).The results could be attributed to (1) the structural
change from the imine to the amine bonding significantly
weakened the interlayer p-p stacking interaction of the COF;
(2) the twisting and stretching of the flexible amine bond in
space gave rise to an increase of interlayer distance, and thus
led to the relatively larger surface area and pore volume. As
shown in the pore size distribution, the pore size of FAL-
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Scheme 2. Calculation of DF of FAL-COF-1. DF is the sum of the angle variations from RIL-COF-1 to FAL-COF-1 of seven bonds in knot unit
(forming bond angles of q1–q3) and linker unit (forming bond angles of q4–q7).
7
X
peak gradually shifted from 2.118 to 2.398 with the increase of
DF
¼
j Dqi j¼ DFꢀknot þ DFꢀlink
ð1Þ
i¼1
reaction time, indicating that the increasing flexibility of the
material over time led to the decreasing pore diameter
(Figure 2a). The N2 adsorption of samples in different
reaction time were also measured. The BET surface areas
of FAL-COF-1 at 1, 2 and 3 days are 117, 145, and 168 m2 gꢀ1,
respectively (Figures S53–S55). The BET surface area in-
creased continuously along with the increase of reaction time
and the decrease of pore size (Figure 2b, insert), which is
consistent with the PXRD analysis and also proves that the
formation of flexible amine bond results in the shrinkage of
pore diameter. The XPS spectra of N1s (Figures S2–S4) show
that the intensity of NH-C peak kept increasing as the
reaction time was prolonged from 1 to 3 days and the yield of
the amine bond increased from 14% to 61%. When the
reaction time was further extended to 10 days, the yield of the
amine bond increased to only 67% showing the slowing down
of the reduction after 3 days (Figures S5,6, S13,14 and
S31,32). The DF of FAL-COF-1 at 1, 2 and 3 days calculated
by Equation (1) were 0, 21 and 34.38, respectively
(Scheme S14). The structural change and the variation trend
of DF show the increasing flexibility of the material with the
extension of the reaction time (Figure 2c and Video S1).
where DF-knot is defined as the knot flexibilization degree and
DF-link is the linker flexibilization degree (see Supporting
Information in detail). It is important to note that the DF of
the material is based on the comparison of the flexible amine-
linked COF with the corresponding imine-linked material.
Based on Equation (1), the DF of FAL-COF-1 was
calculated to be 34.38 (Schemes 2 and S13, and Table S2),
among which DF-knot was 6.48 and DF-link was 27.58 (4 times
more than DF-knot), showing that the flexibility of the C-N
single bond in FAL-COF-1 played a decisive role in the
degree of the materialꢁs flexibility.
DF of three other common COFs, COF-LZU1, COF-I and
N-COF, as well as their flexible counterparts, FAL-COF-
LZU1, FAL-COF-I and FAL-N-COF, were determined using
the proposed method, which proves that the concept of
flexibilization degree proposed in this paper has certain
generality for other COF materials (see Figures S58–S60 and
Tables S6–S8).[17]
Flexibility variations of FAL-COF-1 with different time
Based on the study of reaction mechanism mentioned
above, we have known that the formation of FAL-COF-1 first
Application for Iodine Uptake
=
goes through a process of forming the rigid C N bond, and
Since both FAL-COF-1 and RIL-COF-1 are rich in N
element and have ordered and abundant active sites in their
porous structures, the performances of the two materials on
iodine adsorption were investigated (Figure 3a and b).
Compared with RIL-COF-1, the iodine adsorption capacity
of FAL-COF-1 was significantly increased despite the slower
adsorption rate (Figure 3a). The reason could be that the
flexible framework of FAL-COF-1 presents a certain elastic-
ity and self-adaptive ability which offers a self-accommoda-
tion and adaptive structural adjustment for the entry of the
guest molecules, thereby enhancing the adsorption capacity.
However, the self-accommodation and adaptive structural
ꢀ
then undergoes a reduction to the flexible C N bond.
Although in the proposed one-step strategy, the freshly
formed imine bonds can be reduced by formic acid at the
initial period as the material is forming, there is actually
a speed competition between the reduction and the formation
of the imine bonds. Therefore, we speculate that the extension
of reaction time will be beneficial to the reduction effect. To
confirm this speculation, FAL-COF-1 with reaction time of 1,
2 and 3 days were, respectively characterized by PXRD
(Figure 2a), XPS (Figures S2–S4) and N2 adsorption (Fig-
ure 2b). The PXRD spectra reveal that the (100) diffraction
&&&&
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Figure 2. PXRD patterns (a), N2 sorption isotherm curves (b) and structural changes (c) of FAL-COF-1 at different reaction time.
Figure 3. a) Gravimetric iodine uptake of FAL-COF-1 and RIL-COF-1 as a function of time at 758C and ambient pressure. b) Controlled release of
iodine upon heating the FAL-COF-1@I2 and RIL-COF-1@I2 at 1258C.
adjustment for guest molecules requires some response time,
which delays the adsorption rate of FAL-COF-1 toward guest
molecules. In addition, the higher pore volume and specific
surface area of the flexible FAL-COF-1 is also one of the
reasons for its larger adsorption capacity. On the other hand,
the energy differences between the model compounds and
some guests were calculated by DFT (Table S11) and the
Synthesis of a series of FAL-COFs and RIL-COFs
As a proof of the universality of our method to prepare
flexible amine-linked COFs in one-step, a series of COFs
(FAL-COF-2, FAL-COF-3 and FAL-COF-4) were designed
and synthesized by catalysis & reduction of formic acid using
2,4,6-tris(4-formylphenoxy)-1,3,5-triazine as the knot mono-
mer and benzidine, 2,5-dimethyl-1,4-benzenediamine and 1,4-
benzenediamine as the linkers, respectively (Figure 4a,
Schemes S6, S8 and S10). As a control, three imine-linked
COFs, named RIL-COF-2, RIL-COF-3, and RIL-COF-4,
were synthesized by catalysis of acetic acid using the same
monomers as FAL-COF-2, FAL-COF-3 and FAL-COF-4
(Schemes S7, S9 and S11).
ꢀ
results showed tighter combination of the flexible C N bond
=
than the rigid C N bond. And the stronger host-guest
interaction also makes the desorption rate of FAL-COF-
1 slower than that of RIL-COF-1 (Figure 3b). The species of
iodine in FAL-COF-1@I2 and RIL-COF-1@I2 were detected
by Raman spectroscopy (Figure S64). It was found that two
peaks appeared at 106.3 and 167.3 cmꢀ1 in the Raman spectra
after iodine adsorption, which correspond to the form of
pentaiodide ion, and thus proved that the adsorbed iodine
exists as pentaiodide ion in the materials.[18]
In high-resolution XPS spectra of N1s, the binding energy
peak at 400.3 eV for all the flexible FAL-COFs was observed
which is assigned to NH-C (Figures S9, S12 and S15). The FT-
ꢀ
IR spectra of FAL-COFs also confirmed the presence of C N
bond at 1255 cmꢀ1 compared with rigid RIL-COFs (Figur-
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Figure 4. a) Synthesis and structure diagram of FAL-COF-2, FAL-COF-3, and FAL-COF-4. PXRD patterns b)–d) of FAL-COF-2 and RIL-COF-2, FAL-
COF-3 and RIL-COF-3, and, FAL-COF-4 and RIL-COF-4. e) Simulated structures and aperture variation (purple) between RIL-COFs and the
corresponding FAL-COFs, and DF of FAL-COFs (blue).
es S23–S25). Similar to FAL-COF-1, the fluorescence inten-
sities of the three FAL-COFs are weaker than those of the
corresponding RIL-COFs (Figures S45–S47) and the yields of
amine bonds in FAL-COFs increased with the extension of
the reaction time (Figures S9–S17). Also, it was found that the
yields of amine bonds in different flexible products by the
catalysis and reduction of formic acid are different due to the
difference of reactants.
The PXRD spectra show that all the above six materials
have high crystallinity (Figures 4b–d, S35–S40 and Ta-
bles S14–S19). Among them, three flexible FAL-COFs show
an enhanced value of the (100) peaks than the corresponding
three RIL-COFs, indicating the pore shrinkage effect of the
flexible COF materials (Figure 4e). The DF of the three FAL-
COFs were also calculated by Equation (1) (Figure 4e and
ties were found for the cases of FAL-COF-2, FAL-COF-3 and
FAL-COF-4 (Figures S61–S63 and Tables S9 and S10). Itꢁs
worth noting that the adsorption capacity of FALCOF-4 is
4.36 ggꢀ1 while that of RIL-COF-4 is 3.57 ggꢀ1, showing
a 22% increment. This increase trend of adsorption capacity
for guest molecule is similar to that of FAL-COF-1 for
nitrogen which can reach 27% from 1.35 to 1.72 mmolgꢀ1.
The preparation of the flexible COF materials and the iodine
adsorption experiments further proved that the strategy of
synthesizing flexible FAL-COFs through one-step catalysis &
reduction by formic acid has general applicability and
provides an effective way for the rapid synthesis and
application of flexible COFs.
Tables S2–S5) and the trends of the DF variations are similar Conclusion
to those of pore shrinkages, which also proved the rationality
of the concept of DF.
In summary, due to the advantages of flexible COFs in
The hydrophilicity of the materials were investigated by
the water contact angel measurements (Figures S48–S51) and
the results shows that the water contact angels of FAL-COFs
are smaller than those of the corresponding RIL-COFs, which
reveals that the introduction of amine bond enhanced the
hydrophilicity of the materials.
In order to verify the advantages of flexible materials for
iodine adsorption, the series of flexible FAL-COFs and the
corresponding rigid COFs were employed in iodine adsorp-
tion experiments. Similar trends of iodine adsorption proper-
elasticity and self-adaptability, a series of flexible amine-
linked FAL-COFs were synthesized in this study by one-step
catalysis & reduction of formic acid inspired by Eschweiler-
Clarke reaction. The presented synthesis strategy fulfilled the
in situ reduction of imine bond while it was forming by the
catalyzed imine condensation, which can efficiently and
readily fabricate the FAL-COFs with flexible amine bonds.
The mechanism of the catalysis & reduction process by formic
acid was demonstrated and explained by the confirmatory
syntheses of the model compounds. It was confirmed that the
&&&&
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Nie, Q. Li, F. Wu, W. Sun, X. Zhang, R. Vajtai, P. M. Ajayan, L.
Chen, Y. Wang, Adv. Mater. 2019, 31, 1901640.
as-prepared FAL-COFs have a certain pore shrinkage effect,
ꢀ
and the elasticity and self-adaptability of their flexible C N
[3] a) V. Lakshmi, C. H. Liu, M. Rajeswara Rao, Y. Chen, Y. Fang,
A. Dadvand, E. Hamzehpoor, Y. Sakai-Otsuka, R. S. Stein, D. F.
Perepichka, J. Am. Chem. Soc. 2020, 142, 2155 – 2160; b) T.
Jadhav, Y. Fang, W. Patterson, C. H. Liu, E. Hamzehpoor, D. F.
Perepichka, Angew. Chem. Int. Ed. 2019, 58, 13753 – 13757;
Angew. Chem. 2019, 131, 13891 – 13895; c) L. Ascherl, E. W.
Evans, J. Gorman, S. Orsborne, D. Bessinger, T. Bein, R. H.
Friend, F. Auras, J. Am. Chem. Soc. 2019, 141, 15693 – 15699;
d) S. Roy, J. Kim, M. Kotal, R. Tabassian, K. J. Kim, I. K. Oh,
Adv. Funct. Mater. 2019, 29, 1900161.
[4] a) Q. Sun, Y. Tang, B. Aguila, S. Wang, F. S. Xiao, P. K.
Thallapally, A. M. Al-Enizi, A. Nafady, S. Ma, Angew. Chem.
Int. Ed. 2019, 58, 8670 – 8675; Angew. Chem. 2019, 131, 8762 –
8767; b) M. Li, S. Qiao, Y. Zheng, Y. H. Andaloussi, X. Li, Z.
Zhang, A. Li, P. Cheng, S. Ma, Y. Chen, J. Am. Chem. Soc. 2020,
142, 6675 – 6681; c) P. F. Wei, M. Z. Qi, Z. P. Wang, S. Y. Ding, W.
Yu, Q. Liu, L. K. Wang, H. Z. Wang, W. K. An, W. Wang, J. Am.
Chem. Soc. 2018, 140, 4623 – 4631; d) M. Bhadra, S. Kandam-
beth, M. K. Sahoo, M. Addicoat, E. Balaraman, R. Banerjee, J.
Am. Chem. Soc. 2019, 141, 6152 – 6156.
[5] a) T. Ma, J. Li, J. Niu, L. Zhang, A. S. Etman, C. Lin, D. Shi, P.
Chen, L. H. Li, X. Du, J. Sun, W. Wang, J. Am. Chem. Soc. 2018,
140, 6763 – 6766; b) C. Franco, D. Rodriguez-San-Miguel, A.
Sorrenti, S. Sevim, R. Pons, A. E. Platero-Prats, M. Pavlovic, I.
Szilagyi, M. L. Ruiz Gonzalez, J. M. Gonzalez-Calbet, D. Bo-
chicchio, L. Pesce, G. M. Pavan, I. Imaz, M. Cano-Sarabia, D.
Maspoch, S. Pane, A. J. de Mello, F. Zamora, J. Puigmarti-Luis, J.
Am. Chem. Soc. 2020, 142, 3540 – 3547; c) X. T. Li, J. Zou, T. H.
Wang, H. C. Ma, G. J. Chen, Y. B. Dong, J. Am. Chem. Soc. 2020,
142, 6521 – 6526; d) X. H. Guo, Y. Tian, M. C. Zhang, Y. Li, R.
Wen, X. Li, X. F. Li, Y. Xue, L. J. Ma, C. Q. Xia, S. J. Li, Chem.
Mater. 2018, 30, 2299 – 2308; e) Q. Sun, C. W. Fu, B. Aguila, J.
Perman, S. Wang, H. Y. Huang, F. S. Xiao, S. Ma, J. Am. Chem.
Soc. 2018, 140, 984 – 992.
bonds contribute to greater adsorption capacities to guest
molecules, such as nitrogen and gaseous iodine with the
maximum increases of 27% and 22% respectively, than the
traditional imine-linked COFs. In particular, to better em-
body the flexibilities of materials, we proposed for the first
time the concept of flexibilization degree which visually and
quantitatively describes the flexibility of COFs. The synthesis
strategy in this study realizes the one-step construction of
COFs with flexible linkages, which opens a new route for the
design and synthesis of flexible COFs. And the proposed
concept of flexibilization degree has the potential to be
a quantitative parameter and an important reference for the
synthesis of flexible materials in the future.
Acknowledgements
The financial support from Science Challenge Project
(TZ2016004), National Natural Science Foundation of China
(Grants 21771128 and 21976125), Sichuan Science and
Technology Program (2020JDRC0014) and China Postdoc-
toral Science Foundation (2020M671906). We thank Xiaoyan
Wang for the NMR analysis of the Analytical & Testing
Center, Sichuan University. And we are grateful for the
support from the Comprehensive training platform of speci-
alized laboratory, College of chemistry, Sichuan University.
Conflict of interest
The authors declare no conflict of interest.
[6] a) G. Zhan, Z. F. Cai, M. Martinez-Abadia, A. Mateo-Alonso, S.
De Feyter, J. Am. Chem. Soc. 2020, 142, 5964 – 5968; b) S. Park,
Z. Liao, B. Ibarlucea, H. Qi, H. H. Lin, D. Becker, J. Melidonie,
T. Zhang, H. Sahabudeen, L. Baraban, C. K. Baek, Z. Zheng, E.
Zschech, A. Fery, T. Heine, U. Kaiser, G. Cuniberti, R. Dong, X.
Feng, Angew. Chem. Int. Ed. 2020, 59, 8218 – 8224; Angew.
Chem. 2020, 132, 8295 – 8301; c) I. Castano, A. M. Evans, H. Li,
E. Vitaku, M. J. Strauss, J. L. Bredas, N. C. Gianneschi, W. R.
Dichtel, ACS Cent. Sci. 2019, 5, 1892 – 1899.
[7] a) X. Li, J. Qiao, S. W. Chee, H. S. Xu, X. Zhao, H. S. Choi, W.
Yu, S. Y. Quek, U. Mirsaidov, K. P. Loh, J. Am. Chem. Soc. 2020,
142, 4932 – 4943; b) X. Guo, T. Mao, Z. Wang, P. Cheng, Y. Chen,
S. Ma, Z. Zhang, ACS Cent. Sci. 2020, 6, 787 – 794; c) F. J. Uribe-
Romo, C. J. Doonan, H. Furukawa, K. Oisaki, O. M. Yaghi, J.
Am. Chem. Soc. 2011, 133, 11478 – 11481; d) X. He, Y. Yang, H.
Wu, G. He, Z. Xu, Y. Kong, L. Cao, B. Shi, Z. Zhang, C. Tongsh,
K. Jiao, K. Zhu, Z. Jiang, Adv. Mater. 2020, 32, 2001284.
[8] a) V. A. Kuehl, J. Yin, P. H. H. Duong, B. Mastorovich, B.
Newell, K. D. Li-Oakey, B. A. Parkinson, J. O. Hoberg, J. Am.
Chem. Soc. 2018, 140, 18200 – 18207; b) C. Yuan, X. Wu, R. Gao,
X. Han, Y. Liu, Y. Long, Y. Cui, J. Am. Chem. Soc. 2019, 141,
20187 – 20197; c) T. Ma, E. A. Kapustin, S. X. Yin, L. Liang, Z.
Zhou, J. Niu, L. H. Li, Y. Wang, J. Su, J. Li, X. Wang, W. D. Wang,
W. Wang, J. Sun, O. M. Yaghi, Science 2018, 361, 48 – 52; d) Y.
Hu, N. Dunlap, S. Wan, S. Lu, S. Huang, I. Sellinger, M. Ortiz, Y.
Jin, S. H. Lee, W. Zhang, J. Am. Chem. Soc. 2019, 141, 7518 –
7525; e) T. Sick, A. G. Hufnagel, J. Kampmann, I. Kondofersky,
M. Calik, J. M. Rotter, A. Evans, M. Doblinger, S. Herbert, K.
Peters, D. Bohm, P. Knochel, D. D. Medina, D. Fattakhova-
Rohlfing, T. Bein, J. Am. Chem. Soc. 2018, 140, 2085 – 2092.
Keywords: covalent organic frameworks (COFs) · Eschweiler–
Clarke reaction · flexible amine-linked · formic acid ·
pore-shrink effect
[1] a) A. K. Mohammed, S. Usgaonkar, F. Kanheerampockil, S.
Karak, A. Halder, M. Tharkar, M. Addicoat, T. G. Ajithkumar,
R. Banerjee, J. Am. Chem. Soc. 2020, 142, 8252 – 8261; b) S.
Zhang, Q. Yang, C. Wang, X. Luo, J. Kim, Z. Wang, Y. Yamauchi,
Adv. Sci. 2018, 5, 1801116; c) H. Wang, C. Qian, J. Liu, Y. Zeng,
D. Wang, W. Zhou, L. Gu, H. Wu, G. Liu, Y. Zhao, J. Am. Chem.
Soc. 2020, 142, 4862 – 4871; d) Y. Ma, Y. Wang, H. Li, X. Guan, B.
Li, M. Xue, Y. Yan, V. Valtchev, S. Qiu, Q. Fang, Angew. Chem.
Int. Ed. 2020, 59, 19633 – 19638; Angew. Chem. 2020, 132, 19801 –
19806; e) Q. Sun, B. Aguila, L. D. Earl, C. W. Abney, L. Wojtas,
P. K. Thallapally, S. Ma, Adv. Mater. 2018, 30, 1705479; f) Y.
Wang, M. Xie, J. Lan, L. Yuan, J. Yu, J. Li, J. Peng, Z. Chai, J. K.
Gibson, M. Zhai, W. Shi, Chem 2020, 6, 2796 – 2809.
[2] a) S. Gu, S. Wu, L. Cao, M. Li, N. Qin, J. Zhu, Z. Wang, Y. Li, Z.
Li, J. Chen, Z. Lu, J. Am. Chem. Soc. 2019, 141, 9623 – 9628; b) Y.
Yusran, H. Li, X. Guan, D. Li, L. Tang, M. Xue, Z. Zhuang, Y.
Yan, V. Valtchev, S. Qiu, Q. Fang, Adv. Mater. 2020, 32, 1907289;
c) J. Lv, Y. X. Tan, J. Xie, R. Yang, M. Yu, S. Sun, M. D. Li, D.
Yuan, Y. Wang, Angew. Chem. Int. Ed. 2018, 57, 12716 – 12720;
Angew. Chem. 2018, 130, 12898 – 12902; d) G. Das, T. Prakasam,
M. A. Addicoat, S. K. Sharma, F. Ravaux, R. Mathew, M. Baias,
R. Jagannathan, M. A. Olson, A. Trabolsi, J. Am. Chem. Soc.
2019, 141, 19078 – 19087; e) X. Chen, Y. Li, L. Wang, Y. Xu, A.
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
[9] a) S. A. Ahmed, Q. B. Liao, Q. Shen, M. M. F. Baig Ashraf, J.
Zhou, C. F. Shi, K. Wang, Chem. Eur. J. 2020, 26, 12996 – 13001;
b) M. Matsumoto, L. Valentino, G. M. Stiehl, H. B. Balch, A. R.
Corcos, F. Wang, W. R. Dichtel, Chem 2018, 4, 308 – 317; c) H. L.
Qian, F. L. Meng, C. X. Yang, X. P. Yan, Angew. Chem. Int. Ed.
2020, 59, 17607 – 17613; Angew. Chem. 2020, 132, 17760 – 17766.
[10] a) M. Zhang, J. Chen, S. Zhang, X. Zhou, L. He, M. V. Sheridan,
M. Yuan, M. Zhang, L. Chen, X. Dai, F. Ma, J. Wang, J. Hu, G.
Wu, X. Kong, R. Zhou, T. E. Albrecht-Schmitt, Z. Chai, S. Wang,
J. Am. Chem. Soc. 2020, 142, 9169 – 9174; b) H. Liu, J. Chu, Z.
Yin, X. Cai, L. Zhuang, H. Deng, Chem 2018, 4, 1696 – 1709.
[11] C. Zhao, C. S. Diercks, C. Zhu, N. Hanikel, X. Pei, O. M. Yaghi,
J. Am. Chem. Soc. 2018, 140, 16438 – 16441.
Lemaire, Tetrahedron Lett. 1994, 35, 3313 – 3314; d) A. C. Cope,
W. D. Burrows, J. Am. Chem. Soc. 1966, 31, 3099 – 3103.
[15] N. Graf, E. Yegen, T. Gross, A. Lippitz, W. Weigel, S. Krakert, A.
Terfort, W. E. S. Unger, Surf. Sci. 2009, 603, 2849 – 2860.
[16] M. R. Rao, Y. Fang, S. De Feyter, D. F. Perepichka, J. Am. Chem.
Soc. 2017, 139, 2421 – 2427.
[17] a) S. Y. Ding, J. Gao, Q. Wang, Y. Zhang, W. G. Song, C. Y. Su,
W. Wang, J. Am. Chem. Soc. 2011, 133, 19816 – 19822; b) Q. Gao,
L. Bai, Y. Zeng, P. Wang, X. Zhang, R. Zou, Y. Zhao, Chem. Eur.
J. 2015, 21, 16818 – 16822; c) J. Dong, Y. Wang, G. Liu, Y. Cheng,
D. Zhao, CrystEngComm 2017, 19, 4899 – 4904.
[18] a) P. H. Svensson, L. Kloo, Chem. Rev. 2003, 103, 1649 – 1684;
b) C. Pei, T. Ben, S. Xu, S. Qiu, J. Mater. Chem. A 2014, 2, 7179 –
7187; c) T. Hasell, M. Schmidtmann, A. I. Cooper, J. Am. Chem.
Soc. 2011, 133, 14920 – 14923.
[12] Z. Wang, Q. Yu, Y. Huang, H. An, Y. Zhao, Y. Feng, X. Li, X.
Shi, J. Liang, F. Pan, P. Cheng, Y. Chen, S. Ma, Z. Zhang, ACS
Cent. Sci. 2019, 5, 1352 – 1359.
[13] a) S. Jothilingam, Synlett 2004, 0910 – 0911; b) H. W. Gibson,
Chem. Rev. 1969, 69, 673 – 692.
[14] a) H. T. Clarke, H. B. Gillespie, S. Z. Weisshaus, J. Am. Chem.
Soc. 1933, 55, 4571 – 4587; b) A. C. Cope, W. D. Burrows, J. Am.
Chem. Soc. 1965, 30, 2163 – 2165; c) F. Fache, L. Jacquot, M.
Manuscript received: February 16, 2021
Accepted manuscript online: March 7, 2021
Version of record online: && &&
,
&&&&
&&&&
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Covalent Organic Frameworks
Using the Eschweiler–Clarke reaction,
a series of flexible amine-linked COFs was
synthesized by catalysis and reduction of
formic acid. The unique bifunctional
properties of formic acid can not only
M. Zhang, Y. Li, W. Yuan, X. Guo, C. Bai,
Y. Zou, H. Long, Y. Qi, S. Li, G. Tao, C. Xia,
L. Ma*
&&&& — &&&&
=
catalyze the formation of rigid C N bond
Construction of Flexible Amine-linked
Covalent Organic Frameworks by
Catalysis and Reduction of Formic Acid
via the Eschweiler–Clarke Reaction
like acetic acid, the most common cata-
lyst in COF synthesis, but also reduce and
ꢀ
transform it to flexible C N bond simul-
taneously, giving COFs the accom-
modative adaptability to guest molecules.
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!