A facile solution-phase synthetic approach for constructing phenol-based porous organic cages and covalent organic frameworks

Article abstract of DOI:10.1039/c9gc04033a

Porous crystalline materials, such as porous organic cages (POCs) and covalent organic frameworks (COFs), are important for a wide range of applications such as gas storage/separation, sensing and catalysis. A conventional synthetic approach for imine-based POCs and COFs usually involves the use of organic solvents/acid catalysts (such as AcOH or TFA). However, synthesizing and growing POC crystals and COF crystallites with enhanced crystallinity is not an easy-to-perform process, in which many reaction parameters need to be screened. To overcome these problems, we have proposed herein a facile and green synthetic strategy by adopting H₂O/organic solvents/MOH (M = Na, K, and Cs) as reaction media to produce a series of phenol-based POCs and COFs. X-ray crystallography and molecular simulations have been used to confirm their structures in addition to NMR, mass and FT-IR spectra, and gas sorption experiments reveal their porosity and sorption properties. This new approach provides not only an efficient and alternative route to synthesize highly crystalline POCs and even COFs, but also insight into the crystallization process of these two kinds of porous materials, especially in the hydrous media.

Full text of DOI:10.1039/c9gc04033a

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DOI: 10.1039/C9GC04033A
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A facile solution-phase synthetic approach for phenol-based
porous organic cages and covalent organic frameworks
Lei Zhang, a c Rongran Liang, b Cheng Hang, a Haiying Wang, a Lin Sun, c Lei Xu, a Dairong Liu, a Zhenyi Zhang, d Xingmin Zhang, e
Feifan Chang, a Shengyu Zhao, a and Wei Huang* a f
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Abstract: Porous crystalline materials, such as porous organic cages (POCs) and covalent organic frameworks (COFs), are
important in a wide range of applications such as gas storage/separation, sensing and catalysis. The conventional synthetic
approach for imine-based POCs and COFs usually involves the use of organic solvents/acid catalysts (such as AcOH or TFA).
However, synthesizing and growing POCs crystals and COFs crystallites with enhanced crystallinity are not an easy-to-
perform process, in which many reaction parameters need to be screened. To overcome these problems, we have
proposed herein a facile and green synthetic strategy by adopting H2O/organic solvent/MOH (M = Na, K, Cs) as the
reaction media to produce a series of phenol-based POCs and COFs. X-ray crystallography and molecular simulations have
been used to confirm their structures in addition to NMR, mass and FT-IR spectra, and gas sorption experiments reveal
their porosity and sorption property. This new approach provides not only an efficient and alternative way to synthesize
high crystalline POCs and even COFs but also an insight into the crystallization process of these two kinds of porous
materials especially in the hydrous media.
www.rsc.org/
sensitive to solvent molecules12-13 as well as molecular
recognition interactions.14-15 In general, the synthesis of POCs
through imine condensation reactions is performed in suitable
organic solvents with or without acid catalysts. However,
screenings of suitable organic solvents remain tedious and
some organic solvents are not environmentally friendly. It is
still a common challenge to obtain high-quality single crystals
of POCs suitable for X-ray diffraction characterization in some
cases. Thus, developing environment-friendly and more facile
method is an interesting field of research.
Introduction
Porous organic cages (POCs) or porous molecules as a new
type of porous materials have emerged and made rapid
developments in the past decade.1-11 In contrast to porous
extended frameworks which are defined by strong and
directional coordination or covalent bonds, these discrete
organic cage molecules usually assemble into well-defined
porous structures via weak intermolecular interactions to
produce interconnected pores. And porous molecular
crystallization and crystal packing are generally strongly
The same situation also exists in the synthesis of covalent
organic frameworks (COFs) which are two- or three-
dimensional porous crystalline frameworks held together by
covalent bonds.16-24 The most extensively used synthetic route
for the preparation of COFs is still solvothermal conditions, in
which a variety of experimental parameters such as solvent
combinations and ratios, the concentration of catalysts and
reaction temperature are crucial to obtain COFs with
enhanced crystallinity. In most cases, it is a tedious procedure
to optimize the parameters and it is not easy to find suitable
a. State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of
Microstructures, School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing, Jiangsu Province, 210093, P. R. China. E-mail:
whuang@nju.edu.cn
b. Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional
Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, P. R. China
c. Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu
Province, Yancheng Institute of Technology, Yancheng 224051, P. R. China
d. Bruker Scientific Technology Co. Ltd, 66 Xixiaokou Road, Beijing 100081, P. R.
China
synthetic conditions. From
a
materials perspective,
e. Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics,
Chinese Academy of Sciences, 239 Zhangheng Road, Pudong New Area, 201204,
P. R. China
solvothermal reactions still remains big challenges such as
scalability and processability in order to achieve practical
applications of COF-based materials. To address the challenge,
some other methods have been explored recently including
f. Shenzhen Research Institute of Nanjing University, Shenzhen, Guangdong
Province, 518057, P. R. China
Electronic Supplementary Information (ESI) available: Detailed synthetic procedure
for four POCs and four COFs, NMR and FT-IR spectra, TGA diagrams, optical
microscope images, additional gas adsorption data, PXRD patterns, structural
simulations and characterization data for related compounds (PDF).
X-ray crystallographic data in CIF format for CCDC 1573819-1573821.
See DOI: 10.1039/x0xx00000x
metal
triflates-catalization,25
aniline-modulated
crystallization,26 seed growth&second polymerzation,27 and p-
toluenesulfonic acid-catalization.28
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Recently, green synthesis via mild conditions and cycloimination (Figure 2). The syntheses were peVrifeowrmArteicdle Oinnlinae
DOI: 10.1039/C9GC04033A
harmfulless or nontoxic solvents especially aqueous systems, mixture of ethanol and water in the presence of MOH (M = Na, K,
as an easy-to-operate, sustainable, scalable and eco-friendly Cs). Firstly, tetraaldehyde and 2 equiv. of KOH were dissolved in a
protocol, has drawn extensive attention in the laboratory an mixture of EtOH/H2O (in the rang of 1:1~3:1) and then 2 equiv. of
industry. Both POCs and COFs are needed to be synthesized by (1S,2S) or (1R,2R)-cyclohexanediamine was added. The mixture was
using a facile, mild and green condition. Inspired by the green refluxed for additional 24 h. The resulting clear solution was
synthesis, some research groups have exploited a fast, simple allowed to slow evaporation in air at room temperature over
and scale-up green synthesis of COFs.29-32 In addition, it is well- several hours to weeks to afford single crystals of the organic cage.
known that both POCs and COFs have been established based It is noted that both of the monomers were fully soluble in
on the dynamic covalent chemistry (DCC) and many building EtOH/H2O (pH ≈ 6~8) during the imine condensation and the
units used in the synthesis of POCs can also be applied to COFs. homogeneous reaction conditions gave organic cages as red or
In this regard, there should be no significant difference for yellow crystals (See ESI for details).
them. So, is there a facile and green synthetic approach
suitable for both POCs and COFs at the same time?
Fig. 1 Schematic representation for the synthetic processes of
the POCs and COFs via organic solvent/water/KOH approach.
In this work, we have developed a general, facile and green
solution-phase synthetic strategy for constructing a family of highly
crystalline POCs and COFs, particularly including three reported -
ketoenamine linked COFs. We adopt a wholly new reaction
condition, organic solvent/water/MOH (M = Na, K, Cs), which is
different from the typical systems comprising of organic solvents
and acidic catalysts (Figure 1). The synthesis of phenol-based POCs
or COFs can be facilely achieved (via solvent evaporation or in-situ
precipitation, respectively) by refluxing an organic solvent/water
Fig. 2 Schematic illustration of the synthesis of the POCs and their
structures. (a) Synthesis of the POCs from tetraaldehydes (1-3) and
cyclohexanediamine. (b) Chemical structures of the four POCs. (c)
Crystal structures for NC1-S, NC2-R and NC4-R.
solution of monomers in the presence of MOH (M = Na, K, Cs).
Particularly, this method is believed to offer an easy and efficient
way to grow single crystals of some POCs and COFs in a large scale,
which are hard to be obtained by using the conventional methods.
NC1-S was obtained by the imine condensation reaction between
tetraaldehyde 1 and (1S,2S)-cyclohexanediamine in the presence of
EtOH/H2O/MOH (M = K, Cs). Red block single crystals of NC1-S were
isolated from a mixture of ethanol and water in a yield of 43% (see
Section S-2 and Figure S27) without any other by-products. Its ESI-
MS spectrum displays the [3+6] cage structure with a molecular-ion
peak at m/z = 1364.17 (Figure S21). Crystallographic analysis shows
that the [3+6] tubular cage NC1-S crystallizes in the tetragonal
space group of P43212 (Figure 2c and Table S-1), which is different
from our previously reported racemic NC1.33 Pore size distributions
(5.0, 5.7, and 6.5 Å) calculated by non-local density functional
Results and discussion
Synthesis and characterization of four POC
crystals.
To better illustrate our synthetic approach, we have adopted o-
hydroxyaryl arylaldehydes (1-3) and 1,3,5-triformylphloroglucinol
(Tp) as monomers to construct four POCs and four COFs,
respectively. Four POCs were synthesized by the condensation
reactions of three o-hydroxy arylaldehydes (1-3) with (1S,2S) or
(1R,2R)-cyclohexanediamine in
a
[3+6], [2+4] or [4+8]
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theory from Ar sorption analysis for NC1-S (Figure S40) are only depend on the angles or chemical functioVnieawlitAyrticolefOntlhinee
consistent with the structural measurement (Figure S32).
precursors, but also on multiple parameterDsOsuI:c1h0.1a0s3s9o/Clv9eGnCt0e4f0fe3c3tA,
NC2-R and NC3-R were synthesized by the condensation reaction temperature, and molecular interactions in the system.
between tetraaldehyde 2 and (1R,2R)-cyclohexanediamine in [2+4] Additionally, no keto-enol tautomerism is observed in the
and [3+6] cycloimination in the presence of EtOH/H2O/MOH (M = structures of four imine cages, in which all compounds adopt the
Na, K, Cs). They can be directly collected from their respective same enol form in their solid state.
mother solutions in pure phase by controlling evaporation
temperature (see SI for detailed experimental methods). NC2-R was
Synthesis and characterization of COFs.
isolated directly as yellow block crystals in 35% yield (Figure S27). It
We also successfully expand our stratege to synthesize four
adopts [2+4] configuration in which the two tetraaldehyde
phenol-based COFs including one new COF (COF-NJU-1) and three
components are nearly perpendicular to each other and four
reported -ketoenamine linked COFs (TpPa-1, TpBD and
(1R,2R)-cyclohexanediamine units lie on every corner of the cage
TpTph),25,34-35 as seen in Figure 3. The synthetic protocol for the
(Figures 2b-c). NC2-R exhibits compressed tetrahedral configuration
COFs was the same as that described aboved for the above POCs
similar to that of CC3,14 although the cage window and cavity
except the organic solvent component (Figure S33). Taking COF-
diameters are relatively smaller. In contrast to polyhedral
NJU-1 as an example, tetraaldehyde 1 and KOH (1:2, molar ratio)
morphology of NC2-R crystals, NC3-R crystallizes as needle-like
were dissolved in water to form a clear solution, and a N,N’-
crystals in a yield of 44% within several hours (Figure S27). However,
diethylformamide (DEF) solution of benzidine (2 equiv.
no single crystals of NC3-R suitable for X-ray crystallographic
corresponding to tetraaldehyde 1) was added under stirring to
analysis could be obtained. As an instead, the structure of NC3-R
afford a crystalline red-brown precipitate (73% yield) after 24 h (see
was confirmed by the presence of a clear positive [3+6] molecular-
SI for detailed data). It is noted that ethanol may be not a suitable
ion peak at m/z = 1407.00 in the ESI-MS (Figure S23), as well as
choice of solvent in this case because EtOH/H2O/KOH combination
1
related FT-IR, H and 13C NMR spectra (see SI), which indicate that
did not result in the formation of crystalline COF-NJU-1. We
NC3-R may be analogous to the configuration of NC1-S (Figure 2b).
therefore screened several organic solvents including acetonitrile,
NC4-S crystals were obtained in a yield of 30% by the condensation
1,4-dioxane, N,N’-dimethylformamide (DMF) and DEF for preparing
reaction between tetraaldehyde 3 and (1R,2R)-cyclohexanediamine
COF-NJU-1 and found that DEF/H2O/KOH was the optimal
in the presence of EtOH/H2O/MOH (M = Na, K, Cs). X-ray structural
combination (Figure 3a).
analysis (Figure 2) and ESI-MS spectrum (Figure S24) of NC4-R
To determine the crystal structure of COF-NJU-1, both
reveal that it is a [4+8] organic cage in the ‘Z’ geometry, like a
experimental and theoretically simulated PXRD patterns were
double-[2+4] cage. So it comes to a conclusion that subtle
collected. The simulations were carried out by using Materials
alterations of the tetraaldehyde precursors could result in totally
Studio version 7.0. Similar to the previously reported 2D dual-pore
different cage configurations. To control the formation of these
COF-BTA-BZ,36 the condensation of
1 and benzidine could
cage structures, the design of the tetraaldehedes, in particular the
angle between the two adjacent phenyl planes, is very important.
The ideal angles for the construction of [3+6] configuration may lie
in the range 116~180o. However, The resulting cage topologies not
theoretically produce two possible frameworks. One holds a dual-
pore structure and the other possesses a single-pore skeleton
(Scheme S1). The formation of a single-pore COF could be ruled out
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Fig. 3 Synthetic scheme for the construction of four COFs and their PXRD patterns. (A) Synthesis of COF-NJU-1 using tetraaldehydes 1 and
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benzidine under DEF/H2O/KOH condition. (B) PXRD patterns of COF-NJU-1 experimentally observed (black), refinDeOdI:(r1e0d.1)0,3s9im/Cu9lGaCte0d40X3R3DA
patterns of dual-pore AA , dual-pore AB and difference plot between the experimental and refined PXRD patterns. (C) The structure of COF-
NJU-1 in AB and AA stacking modes ， respectively. (D) Schematic representation of the synthesis of TpPa-1, TpBD and TpTph under
DMF/H2O/KOH condition. (E-G) PXRD patterns of TpPa-1, TpBD and TpTph experimentally observed (black), refined (red), simulated XRD
patterns of dual-pore AA (orange) , dual-pore AB (blue) and difference plot between the experimental and refined PXRD patterns (gray).
(Inset images showing the photographs of four COFs).
by comparing the experimental PXRD pattern of the as-obtained for TpPa-1, TpBD and TpTph, which could be attributed to the
crystallites with the simulated one of the single-pore isomer, which reflection from (100) planes (Figures 3e-g). The experimental PXRD
clearly indicated significant difference between them (Figure S46). patterns are in good agreement with the simulated AA eclipsed
For the dual-pore COF, simulated PXRD patterns of two stacking models, which match well with the previous reports in
distinguishable packing fashions (eclipsed (AA) and staggered (AB)) literature.28 FT-IR spectra of the four COFs also agree well with the
were generated, as shown in Figure 3b. Comparison between them reported ones (Figure S37), in which the strong peaks at ~1580 cm-1
and the experimental PXRD profile revealed that COF-NJU-1 should (–C–N) and 1250 cm-1 (–C=C) show the formation of the -
adopt AA stacking. The experimental PXRD exhibited diffraction ketoenamine linked COFs. In addition, the SEM and TEM images
signals at 2.70o, 4.72o, 5.46o, 7.24o, 9.89 o and 26.14o, which were exhibit the ribbon-like morphology for all three Tp series COFs
assigned to 100, 110, 200, 210, 130 and 001 facets, respectively. In (Figure S34).
order to further corroborate the stacking model of COF-NJU-1, pore
size distribution profile was generated from HK theory based on N2
sorption data (Figure S41). It revealed the presence of two main
pore size distributions at ~10.6 and 24.3 Å , which were consistent
with the calculated values of 9.9 and 23.2 Å for the dual-pore COF
with eclipsed packing structure, suggesting that the 2D layers of
COF-NJU-1 adopted eclipsed stacking. Pawley refinement was
performed to determine the unit cell parameters of COF-NJU-1 with
AA stacking model, which yielded a = b = 39.10 Å, c = 3.38 Å, α = β =
90°, and γ = 120°, with factors of RP = 1.56% and RWP = 2.26%. The
difference plot indicates that the experimental PXRD pattern is well
reproduced by the refined pattern (Figure 3b). The as-synthesized
COF-NJU-1 was characterized by Fourier transform infrared
spectroscopy (FT-IR). The spectrum of COF-NJU-1 showed the
nearly complete disappearance of C=O resonance, indicating that
tetraaldehyde 1 was mostly consumed during the reaction. In
addition, a strong stretching vibration band at 1625 cm-1 was
observed, confirming the formation of C=N linkage (Figure S36). The
solid-state cross-polarization with magic angle spinning (CP/MAS)
Fig. 4 (a) PXRD patterns of COF-NJU-1 before and after the 7-day
13C NMR spectrum exhibited a resonance signal at 168.4 ppm,
treatment with organic solvents, HCl and NaOH. (b) N2 adsorption
corresponding to the chemical shift of imine carbon atoms in COF-
and desorption isotherms of three POCs (NC4-R was measured by
NJU-1 (Figure S18). Scanning electron microscopy (SEM) of as-
Ar) and four COFs. PXRD patterns of COF-NJU-1 synthesized in
prepared COF powders showed that the COF crystallites have
different ratios of (c) KOH, and (d) DEF/H2O.
flower-like morphology. (Figure S34a). In addition, the transmission
electron microscopy (TEM) images showed rod-like crystals with a
diameter of ~30 nm (Figure S34e).
Porosity
To further demonstrate the generality of this solution-phase
All POCs show good hydrolytic stability except for NC3-R and
approach, three reported -ketoenamine linked COFs were
NC4-R, which can be evidenced by their consistent PXRD patterns
prepared via the same synthetic conditions. As far as we are aware,
before and after the immersion in pure water up to one week
-ketoenamine linked COFs have been synthesized via various
(Figure S25). COF-NJU-1 also shows good stability in water, organic
approaches28,34-35 and investigated as proton-conductive
solvents and even 1 M HCl (Figures 4a, S26). All of the desolvated
membranes,37 catalyst supports38 and energy storage devices,39 due
POCs were obtained by using supercritical carbon dioxide activation,
to their high hydrolytic and chemical stability. In this work, TpPa-1,
and PXRD measurements were performed to check the structural
TpBD and TpTph can be obtained as red or orange crystallites with
stability of all samples before and after desolvation (Figure S19).
high yields of 89, 83 and 86% under the same condition of
The thermogravimetric analysis (TGA) of all POCs and COFs in N2
DMF/H2O/KOH, respectively (Figures 3d, S33). It is worth
atmosphere showed thermal stabilities up to 300 C°, indicating the
emphasizing that the organic solvent component is DMF in these
high thermal stability of theses materials (Figures S38, S39), similar
three cases. The obtained PXRD patterns of the Tp series COFs show
to those reported by other groups. The four COFs were activated at
relatively high intensive first peaks at 4.63°, 3.44°, 2.69° (±0.02, 2θ)
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180°C under a high vacuum to provide the activated COFs, and solubility in water/ethanol. Namely, sodium phenolate has the
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PXRD measurements were performed to check the structural lowest solubility, while cesium phenolaDteOI:h10a.s103t9h/eC9bGeCs0t40a3n3Ad
stability of all samples before and after gas sorption. (see Section S- potassium phenolate has the middle. Such a trend in solubility is the
7 in SI). The porosity of the materials has been investigated. As same as that of their basicity (NaOH < KOH < CsOH).
displayed in Figure 4b, the four POCs show type-I N2 (or Ar) sorption
isotherms, where the Brunauer-Emmett-Teller (BET) surface areas POC crystals and COF crystallites. We also carried out control
of NC1-S, NC2-R, NC3-R, and NC4-R (Ar) were calculated to be 779.0, experiments to prepare POCs by strring and heating
3.7, 213.1 and 455.6 m2 g-1, respectively. Argon was used as the DMF/EtOH/KOH
solution of tetraaldehydes and
Water is also found to be important for accessing high quality
a
probe molecule for the sorption measurement of NC4-R because of cyclohexanediamine but without the presence of water (DMF was
its weak N2 adsorption at 77 K (Figure S38), which may be used owing to its better dissolving capacity for alkali metal
attributed to the existence of structural changes or slow N2 phenoxides). However, no single crystals could be formed by slow
diffusion.40-41 The four COFs also exhibit type-I N2 sorption evaporation even over a period of long time. Similar results were
isotherms (Figure 4b), from which the BET surface areas of COF- found in the synthesis of COFs. In addition, the PXRD results of the
NJU-1, TpPa-1, TpBD and TpTph were calculated to be 1080.8, different products obtained from varying the ratios of DEF/H2O
1247.4, 768.7 and 658.4 m2 g-1, respectively. Those values of BET for suggest that the optimal ratio of DEF/H2O may lie in the rang of
TpPa-1, TpBD and TpTph are lower than those of reported TpPa-1 1:1~3:1 for the formation of COF-NJU-1 (Figure 4d), in accordance
(1432), TpBD (1400) and TpTph (1020) synthesized via an organic with aformentioned results in the synthesis of POCs.
terracatta process.28 The possible reason is attributed to the
discrepancy of activation process during solvent removal from the equilibrium was also realized in some COFs’ synthesis. Wan et
To our knowledge, water as an agent to regulate the chemical
pores or the COF crystallinity.
al.42,43 demonstrated that the introduction of water into the
reaction system could greatly improve the formation of highly
ordered surface covalent organic frameworks (SCOFs). They
Discussion
thought that
a small amount of water could improve the
Many attempts to produce all the four cages via traditional reversibility of reaction and self-healing abilities of the SCOFs.
syntheses (organic solvents/acid catalysis) have also been Dichtel and co-workers44 also reported that the addition of water
performed in
a series of organic solvents. Only flower-like was necessary to generate the imine COF, and the high volume
polycrystals NC1-S could be crystallized from DMF/MeOH or percentage of water (up to 17%) could reliably induce better
DMF/1,4-dioxane, but such polycrystals were hard to be crystallinity and maintain high yields. In contrast, the same
characterized by single-crystal XRD (Figure S28). Attempts to phenomenon was observed in our reaction system but the water
crystallize NC2-R, NC3-R and NC4-R under conventional contents could reach as high as 50% volume percentage (see
experimental conditions always led to amorphous powders, even section S2 in SI). Actually, water in our synthetic system plays
after careful screening of a series of organic solvents (Table S-2). In important roles as a solvent to dissolve tetraaldehyde salts besides
contrast, the single crystals of four POCs could be easily obtained by as an agent to adjust the reversibility of imine condensation. More
using the organic solvent/H2O/MOH (M = Na, K, Cs) method. importantly, the introduction of water is vital and indispensable for
Obviously, alkali hydroxide and water played important roles during the crystallization process of our POCs and COF with high
the consensation synthesis especially in the crystallization process. crystallinity.
To further support our hypothesis, some control experiments have
Actually, the POCs and COFs herein are constructed under the
been performed. Reactions of tetraaldehyde monomers 1 and basic (or weak basic) and aqueous solution, which is different from
(1S,2S) or (1R,2R)-cyclohexanediamine in EtOH/H2O without adding the typical acid-catalyzed imine condensation reactions. In the
KOH resulted in a cloudy solution since tetraaldehyde monomers synthesis of imine-involved crystalline materials, Lewis or Bronsted
and the resultant cages were hard to dissolve in EtOH/H2O. It is acid (such as CF3CO2H or CH3CO2H) is generally employed to
hard to grow high-quality single crystals under such
a
catalyze and facilitate the imine formation and exchange, and a
heterogeneous condition. When equivalents of potassium small amount of water, coming from the reaction product or even
2
hydroxide was introduced into the afore-mentioned suspension, external addition, is found to modulate the reaction equilibrium. In
the mixture became clear quickly and could give rise to POC crystals our phenol-based imine condensation system, the possible process
via evaporation over a period of time. We think this dissolution could be divided into two steps: First, tetraaldehde monomers and
process is a phenol-involved acid and alkali neutralization (salt- NaOH (or KOH, CsOH) could form the ‘tetraaldehyde salts’, which
forming reaction), and the resultant salts are easy to dissolve in the make them easily soluble in organic solvent/H2O to produce a
mixture of ethanol and water. However, some procuors with large homogeneous solution; Second, the ‘tetraaldehyde salts’ react with
chromophore which tend to aggregate may be not soluble in such diamines to produce crystalline cages or framework structures in
polar reaction solvents. To further investigate the optimal amount the presence of a large amount of water. In this dynamic covalent
of KOH during the the COF synthesis, COF-NJU-1 was chosen as a equilibrium, we think that the introduction of alkali hydroxide and
model compound. As shown in Figure 4c, PXRD patterns of COF- water would mitigate the imine condensation rate, there are two
NJU-1 obtained from varied KOH ratios indicate that the optimal possible reasons to explain this process: (i) the solvated
amount may lie in the rang of 2~6 equiv. In addition, NaOH and ‘tetraaldehyde salts’ surrounded by water molecules provide
CsOH had the same effects as KOH in some cases, and the only “barriers” to somehow weaken the attack of diamines; (ii) the
difference is that their corresponding phenolates have different formation of imines proceeds not smoothly in water-containing
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Journal Name
system due to the risk of hydrolysis. However, the decrease of 8. M. W. Schneider, H.-J. Siegfried Hauswald, R. Stoll and
View Article Online
DOI: 10.1039/C9GC04033A
imine condensation rates in our case is believed to be beneficial for
the formation of high crystalline POCs and COF since it allows for
the sufficient rearrangement and self-correction. As for the removal
of alkali ions in the final products, it is likely that the alkali
phenolates are neutralized by the carbon dioxide from air during
the slow evaporation and crystallization process.
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Conclusion
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In conclusion, we have demonstrated an efficient and green
solution-phase synthetic method for the crystal growth of phenol-
based POCs and COFs. The discovery of organic solvent/water/MOH 13. T. Hasell, J. L. Culshaw, S. Y. Chong, M. Schmidtmann,
(M = Na, K, Cs) synthetic method offers not only a simple solution-
phase strategy suitable for POCs and COFs simultaneously, but also
uncovers a corner about crystallization process of POCs and COFs
especially in the hydrous media. It also opens up new possibilities
for the exploration of more POCs and COFs as well as some other
phenol-based materials.
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Conflicts of interest
The authors declare no competing financial interests.
17. X. Han, J. Zhang, J. Huang, X. Wu, D. Yuan, Y. Liu and Y.
Cui, Nat. Commun., 2018, 9, 1294.
Acknowledgements
18. S.-L. Cai, Y.-B. Zhang, A. B. Pun, B. He, J. Yang, F. M.
Toma, I. D. Sharp, O. M. Yaghi, J. Fan, S.-R. Zheng, W.-G.
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We thank Prof. Xin Zhao from Shanghai Institute of Organic
Chemistry (Chinese Academy of Sciences) for his very helpful
advices. This work was supported by the National Natural Science
Foundation of China (Nos. 21871133 and 21801127), the Natural
Science Foundation of Jiangsu Province (Nos. BK20191024 and
BK20171334), and Science, Technology and Innovation Commission
of Shenzhen Municipality (No. JCYJ20180307153251975).
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Green Chemistry
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DOI: 10.1039/C9GC04033A
A facile solution-phase synthetic approach for phenol-based porous
organic cages and covalent organic frameworks
Lei Zhang, Rongran Liang, Cheng Hang, Haiying Wang, Lin Sun, Lei Xu, Dairong Liu, Zhenyi
Zhang, Xingmin Zhang, Feifan Chang, Shengyu Zhao, and Wei Huang*
Graphic Abstract
A general solution-phase synthetic strategy is developed to construct both porous organic cages
(POCs) and covalent organic frameworks (COFs) by simply stirring and heating organic
solvent/water solutions of aldehydes and amines in the presence of MOH (M = Na, K, Cs). The
generality and transferability of this approach are demonstrated by successful growing of
high-quality crystals for four POCs and four COFs.