Aminal-Linked Covalent Organic Frameworks through Condensation of Secondary Amine with Aldehyde

Article abstract of DOI:10.1021/jacs.9b08017

New linkage chemistry will endow covalent organic frameworks (COFs) with not only structural diversity but also fascinating properties. However, to develop a new type of linkages has been a great challenge. We herein report the first two COFs using aminal

Full text of DOI:10.1021/jacs.9b08017

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Communication
Aminal-Linked Covalent Organic Frameworks through
Condensation of Secondary Amine with Aldehyde
Shu-Yan Jiang, Shi-Xian Gan, Xi Zhang, Hui Li, Qiao-Yan Qi, Fu-Zhi Cui, Jian Lu, and Xin Zhao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b08017 • Publication Date (Web): 06 Sep 2019
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Aminal-Linked Covalent Organic Frameworks through Condensation
of Secondary Amine with Aldehyde
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†,‡,§
Shi-Xian Gan,†,,§ Xi Zhang, Hui Li, Qiao-Yan Qi, Fu-Zhi Cui, Jian Lu, and Xin
†

†
†
†
Shu-Yan Jiang,
Zhao*
,
†
†
Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Mo-
lecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai
2
‡
00032, China
University of Chinese Academy of Sciences, Beijing 100049, China

Shanghai Engineering Research Center of Green Energy Chemical Engineering, Shanghai Normal University, Shanghai
2
00234, China
Supporting Information Placeholder
1
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bonds. However, while imine has been widely used as linkages
ABSTRACT: New linkage chemistry will endow covalent or-
ganic frameworks (COFs) with not only structural diversity but
also fascinating properties. However, to develop a new type of
linkages has been a great challenge. We herein report the first two
COFs using aminal as the linkages. These two COFs have been
synthesized by condensation of secondary amine and aldehyde.
They crystallize in cpi net, which is a new topology for COFs.
The aminal linkage is found to favor reservation of photophysical
property of the monomers due to its tetrahedral geometry and
non-conjugated feature. These aminal-COFs exhibit good thermal
stability and high chemical stability under neutral and basic condi-
tions.
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9
in the synthesis of COFs, aminal was never observed in COFs.
Although a few examples in literature suggest that aminal linkag-
es are strong enough to produce amorphous microporous poly-
20
mers, to fabricate crystalline materials through aminal chemistry
is a great challenge due to the drastic change of the stereo struc-
ture during the condensation process, as a result of the transfor-
mation from the planar sp hybridization of carbonyl carbon to the
2
3
tetrahedral sp hybridization of aminal carbon. Such a change
undoubtedly increases difficulty of COF synthesis in the mono-
mer design and structure prediction. In this communication, we
report the construction of two aminal-linked 2D COFs based on
an elaborate design. Firstly, non-planar D -symmetric tetraalde-
2
h
hydes were chosen to accommodate the space change in the pro-
2
3
cess of the sp -to-sp transformation. Secondly, since condensa-
1
8
tion of primary amine with aldehyde usually produces imine,
Covalent organic frameworks (COFs) are a class of crystalline
porous organic polymers constructed by linking organic building
blocks via covalent bonds. In this context, linkages not only play
a crucial role in connecting vertexes and linkers together into
well-ordered 2D or 3D network structures, but also have a great
influence on the properties of the materials. Benefiting from their
tailored structures with permanent porosity, COFs have versatile
secondary amine was used to impede the formation of imine link-
ages. Based on this design, two aminal-linked 2D COFs with cpi
net have been successfully synthesized, through which we have
developed not only a new type of linkages but also a new topolo-
gy for COFs. Moreover, unique property associated with the satu-
rated bond and tetrahedral geometry of aminal has also been
demonstrated.
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applications ranging from gas storage, separation, catalysis,
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sensing, drug delivery, energy storage, to photo/electronic de-
The synthesis of the target COFs was implemented through the
condensations of piperazine with D -symmetric tetraaldehydes
A1 and A2, respectively, inspired by the reaction between benzal-
dehyde and piperidine (Scheme 1). Experimentally, the conden-
sation reactions were carried out under solvothermal condensa-
8
9
vices. Since the pioneering work of Yaghi in 2005, this field has
experienced a rapid growth over the past decade, to which two
aspects, developing new structures and exploring new proper-
ties/applications, are mainly focused. With the development of
COFs, their linkages have evolved from B-O and C=N in the
2h
2
1
10
11
12
o
tions at 120 C for 72 h in sealed tubes. The molar ratio between
1
3
14
15
16
early stage to C=C, C-N, C-O, C-C and even mechanical
the aldehydes and piperazine is 1:4 and toluene was found to be
the optimized solvent after screening a series of solvent systems
(Table S1). The COFs were obtained as a pale yellow microcrys-
talline powder with high crystallinity (termed Aminal-COF-1) in
77% yield from tetraaldehyde A1, or white crystallites (termed
Aminal-COF-2) in 62.5% yield from tetraaldehyde A2, respec-
tively.
1
7
entanglement in recent years and distinct properties associated
with the different linkages have been demonstrated, undoubtedly
indicating that new linkage chemistry becomes one of the strong
driving forces to push this exciting field forward.
Aminal, also termed N, N-acetal which can be regarded as the
product of subsequently attacking a pre-formed imine bond by
another amine, exhibits good reversibility similar to imine
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Scheme 1. Model Reaction and Synthesis of Aminal-linked COFs.
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The as-obtained crystallites were characterized with Fourier-
transform infrared (FT-IR) spectroscopy (Figures S1-S2), solid-
0
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1
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1
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state C and N cross-polarization with magic angle spinning
CP/MAS) NMR (Figures S3-S5), and X-ray photoelectron spec-
0
0
2
(
1
-
1
2
troscopy (XPS) (Figures S6-S7), from which the aminal linkages
in the COFs have been identified. Concretely, no vibration band
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2
0
0
2
-
-
a
b
2
0
4
3
-
1
-
corresponding to N-H group of piperazine (3214 cm ) is observed
-
1
2
4
2
1
1
-
1
and the peak assignable to aldehyde group (1700 cm ) is dramati-
cally attenuated in the IR spectra of both the two COFs, suggest-
ing high degrees of conversions of the monomers. No vibration
peak of C=N shows up, indicating there are no imine linkages in
1
3
c
d
the as-obtained materials. The C NMR spectra of the two COFs
also reveal absence of C=N units in their structures. Instead, sig-
nals around 88 ppm are observed. The peaks are assigned to ami-
nal carbon (N-C-N), providing compelling evidence for the for-
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mation of aminal linkages. The N spectra of the COFs show two
resonances around 50 and 55 ppm, which are different from that
of piperazine (37 ppm) and should be assigned to two kinds of
aminal nitrogen in different environment. In the XPS spectra,
while the XPS survey spectra display strong C1s peaks and N1s
peaks with moderate intensity, the peaks corresponding O1s ex-
hibit very low intensities, suggesting the as-formed materials are
mainly composed of carbon and nitrogen. The oxygen observed in
the XPS spectra, also revealed by the IR data above, is attributed
to the residual carbonyl group at the peripheral of the COFs.
Scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) confirmed phase pure of the COFs, as only
one morphology was observed for each COF (Figure S8-S9).
o
2
Theta/
f
e
Figure 1. (a) Experimental (black) and refined (red) PXRD pat-
terns of Aminal-COF-1. (b) Difference plot between the experi-
mental and refined PXRD patterns. Simulated PXRD patterns for
c) eclipsed and (d) staggered structures. Perspective and top
views of structural representation of Aminal-COF-1 in (e)
After the COFs were confirmed to be produced through aminal
linkages, their crystal structures were elucidated using powder X-
ray diffraction (PXRD). The high quality of the PXRD patterns
offer direct acquisition of crystal lattice parameters of the COFs
based on the abundant peaks after indexing (Figures 1a and 2a).
For Aminal-COF-1, three high intense diffraction peaks appear at
(
eclipsed and (f) staggered stacking.
the optimized parameters of a = 22.74 Å, b = 16.95 Å, c = 6.06 Å,
o
o
o
o
o
4
(
.24 , 5.68 and 8.27 , which are attributed to (100), (010) and
110) facets, respectively. Another six weak peaks assignable to
(1-20), (210), (120), (4-20), (4-30) and (3-21) facets are observed
α = β = 90.00 and γ = 112.37 . The calculated PXRD diffraction
pattern match well with the experimental result (Figure 1c), with
Rwp = 4.00% and R = 3.13%. Staggered (AB) stacking structure
was also simulated (Figure 1f). The comparison of its PXRD pro-
file with the experimental one shows significant deviations (Fig-
ure 1d), ruling out the AB stacking model. This type of network
structure, consisting of periodic distribution of pentagonal and
p
o
o
o
o
o
at 10.49 , 11.87 , 13.53°, 16.40 , 18.95 and 19.98 , respectively.
The lattice modeling and Pawley refinement of Aminal-COF-1
were conducted using Materials Studio, which output the most
probable structure of Aminal-COF-1 with eclipsed (AA) stacking,
as the one illustrated in Figure 1e. Pawley refinement produced
22
hexagonal pores in a plane, is identified as cpi net. To the best of
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our knowledge, this topology has never been observed in COFs
before.
areas were estimated from the adsorption data in the range of P/P
0
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from 0.003 to 0.05, which afforded BET surface areas of 1169 m
-
1
2
-1
g for Aminal-COF-1 and 1168 m g for Aminal-COF-2 (Fig-
ure S15). The total pore volumes of Aminal-COF-1 and Aminal-
In the experimental PXRD pattern of Aminal-COF-2, the most
o
intense peak appears at 3.86 ,assignable to (100) facet, while
3
-1
3
-1
o
o
o
o
o
COF-2 were calculated to be 0.65 cm
g
and 0.89 cm g
other diffraction peaks at 5.28 , 7.61 , 9.78 ，10.85 , 12.52 ,
(
P/P =0.99), respectively. Their theoretical BET surface areas
o
o
o
0
15.85 , 18.43 and 20.54 are attributed to (010), (110)/(2-10), (1-
0), (210)/(3-10), (120), (030), (121) and (401) facets, respective-
2
-1
were calculated to be 1465 m g for Aminal-COF-1 and 1934
2
2
-1
++
24
m g for Aminal-COF-2 using Zeo calculation. Their pore
size distribution (PSD) analysis was generated based on quenched
solid-state density functional theory (QSDFT). As shown in Fig-
ure 3b and 3d, two narrow peaks are observed in both the PSD
profiles (5.2 and 8.2 Å for Aminal-COF-1, 5.4 and 8.2 Å for
Aminal-COF-2), indicating existence of two kinds of micropores
ly (Figure 2a and Figure S10). The crystal structure of Aminal-
COF-2 was determined to be the one depicted in Figure 2e. Ami-
nal-COF-2 also adopts eclipsed stacking, as supported by the
good agreement between the calculated and experimental PXRD
patterns (Figure 2c). Pawley refinement reproduced the experi-
mental PXRD pattern quite well and gave unit cell parameters of a
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in the COFs. Based on Zeo calculations, theoretical pore sizes
of the pentagonal and hexagonal micropores in Aminal-COF-1
are estimated to be 5.3 and 8.2 Å, while for Aminal-COF-2 are
5.4 and 10.5 Å (Figure S16), which are in good agreement with
the experimental results. The PSD results further collaborate the
formation of the target aminal-linked 2D dual-pore COFs.
=
24.53 Å, b = 17.88 Å, c = 6.60 Å, α = β = 90.00°and γ =
o
1
11.91 , with R = 5.13% and R = 4.13%. A staggered stacking
wp p
structure was also ruled out by the mismatch between the simulat-
ed and experimental PXRD patterns (Figure 2d).
Unusually large interlayer distances are observed in the two
COFs. A close examination on their crystal structures reveals that
the piperazine units in the frameworks adopt a chair conformation,
and the aminal units hold a tetrahedral configuration. As a result,
the distances between phenyl rings (> 6.0 Å) go beyond that of a
typical aromatic stacking and thus the interactions between them
are minimized (Figure S11-S14). It is quite different from the
COFs with planar linkages, in which aromatic stacking usually is
significant.
a
500
400
0.5
0.4
0.3
0.2
b
Adsorption
Desorption
8.2
5
.2
3
00
00
2
0
0
.1
.0
100
0
0.0
0.2
0.4
0.6
0.8
1.0
0
10
20
30
40
50
Relative Pressure (P/P
)
Pore width (Angstrom)
0
0
0
1
0
1
-
c 600
500
d 0.15
a
0
Adsorption
Desorption
2
1
5.4
0
-
&
1
3
8.2
0
0
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&
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00
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c
0
.00
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40
50
0
.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P
)
Pore width (Angstrom)
0
Figure 3. N2 adsorption-desorption isotherms of (a) Aminal-
COF-1 and (c) Aminal-COF-2, and pore size distribution profiles
of (b) Aminal-COF-1 and (d) Aminal-COF-2.
d
5
10
15
Theta/
20
25
30
o
2
Thermogravimetric analysis (TGA) revealed almost no weight
o
o
loss before 200 C and 10% weight loss around 320 C for the
aminal-linked COFs, indicating their good thermal stability (Fig-
ure S17-S18). Their chemical stability was examined by compar-
ing the PXRD patterns, FT-IR spectra and residues weights of the
samples before and after they were soaked in organic solvents,
water and aqueous NaOH solutions (1 to 9 M) for a set time, re-
spectively (Figures S19-S26, Tables S2-S3). The experimental
results indicate that the COFs display high chemical stability un-
der these conditions (see Supporting Information for details).
f
e
The composition of Aminal-COF-1 was further analyzed with
1
H NMR spectroscopy after it was completely hydrolyzed under
1
acidic condition. The H NMR spectrum shows the molar ratio
between tetraaldehyde A1 and piperazine is 1:4 (Figure S27),
again confirmed the chemical constitution of the target aminal-
linked COF. Elementary analysis of the COFs indicates the con-
tents of carbon, hydrogen and nitrogen are close to their corre-
sponding values, also indicating that as-obtained polymers are in
line with the predicted structures of the COFs.
Figure 2. (a) Experimental (black) and refined (red) PXRD pat-
terns of Aminal-COF-2. (b) Difference plot between the experi-
mental and refined PXRD patterns. Simulated PXRD patterns for
(c) eclipsed and (d) staggered structures. Perspective and top
views of structural representation of Aminal-COF-2 in (e)
eclipsed and (f) staggered stacking.
Different from imine-linked COFs in which the frameworks are
extendedly conjugated, aminal is saturated and thus aminal-linked
COFs do not have extended -conjugated skeletons. Moreover,
Both the COFs exhibited a typical nitrogen sorption isotherm of
23
microporous materials (Figure 3a and 3c). Their BET surface
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the tetrahedral configuration of aminal makes the interlayer stack-
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ing less favorable than that in the 2D COFs assembled via planar
linkages (for example, imine, boronate, boroxine). The feature of
fewer disturbances is favorable for reservation of properties of
functional monomers after they assemble into frameworks. This
will benefit customized synthesis of COFs with designed and
predicted physical properties. To examine this point, fluorescent
properties of the two COFs, monomers A1 and A2 were investi-
gated. The solid-state fluorescence spectra reveal that emission
wavelengths of the aminal-COFs are almost same as that of their
monomers (Figure 4), clearly indicating that the COFs have re-
tained the photophyscial properties of the monomers. In contrast,
for 2D COFs with unsaturated or planar linkages, monomers and
frameworks are usually reported to exhibit very different optical
features due to the close interlayer packing and/or the dramatic
change in the extent of conjugation after the formation of the
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank the National Science Fund for Distinguished Young
Scholars of China (No. 21725404), Shanghai Scientific and Tech-
nological Innovation Project (18JC1410600), and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(Grant No. XDB20000000) for financial support. We also thank
Mr. Lubing Bai and Prof. Yuping Wu in Nanjing Tech University
for their help in recording the solid-state fluorescence spectra.
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5
b,25
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Aminal-COF-1
Monomer A1
b 1.0
Aminal-COF-2
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0.4
0.2
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.8
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Figure 4. Normalized solid-state fluorescence spectra of the ami-
nal-COFs and monomers.
In summary, for the first time, COFs with aminal linkages have
been synthesized. The aminal frameworks crystallize in cpi net, a
new topology never reported for COFs before. These advances
well demonstrate that linkage chemistry of aminal brings new
development in the field of COFs and will also broaden the scope
of reticular chemistry in terms of new bond connection. The tetra-
hedral geometry and non-conjugated feature of the aminal linkage
distinguishes itself from the unsaturated or planar linkages by
reducing disturbance on the properties of monomers, which may
bring in more precise design and prediction for the properties of
COFs.
Frameworks (COFs) for Efficient CO
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ASSOCIATED CONTENT
Supporting Information
Procedures for the preparation of the monomers and COFs, IR
1
3
15
spectra, solid-state C and N CP-MAS NMR spectra, SEM and
TEM images, high-resolution PXRD profile, BET plots, TGA
traces, additional PXRD profiles, illustration of the crystal struc-
tures of the COFs, and fractional atomic coordinates for the unit
cells of the COFs. This material is available free of charge via the
Internet at http://pubs.acs.org
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AUTHOR INFORMATION
2+
detection of Cu ions. Chem. Commun. 2016, 52, 6613-6616.
Corresponding Author
xzhao@sioc.ac.cn
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Author Contributions
§
S.-Y. Jiang and S.-X. Gan contributed equally.
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