Synthesis of -CN- linked covalent organic frameworks: Via the direct condensation of acetals and amines

Article abstract of DOI:10.1039/c6cc00947f

We demonstrate herein a facile approach for constructing -CN- linked COFs from acetals. Three new COFs (imine-linked LZU-20, hydrazone-linked LZU-21, and azine-linked LZU-22) were synthesized by the direct condensation of dimethyl acetals and amines. All

Full text of DOI:10.1039/c6cc00947f

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Synthesis of –CQN– linked covalent organic
frameworks via the direct condensation of acetals
Cite this:DOI: 10.1039/c6cc00947f
and amines†
Received 30th January 2016,
Accepted 12th April 2016
Zhi-Jun Li, San-Yuan Ding,* Hua-Dong Xue, Wei Cao and Wei Wang*
DOI: 10.1039/c6cc00947f
www.rsc.org/chemcomm
We demonstrate herein a facile approach for constructing –CQN–
linked COFs from acetals. Three new COFs (imine-linked LZU-20,
hydrazone-linked LZU-21, and azine-linked LZU-22) were synthe-
sized by the direct condensation of dimethyl acetals and amines.
All the synthesized COFs are highly crystalline and exhibit good
thermal stability.
1
Covalent organic frameworks (COFs), constructed using pure
organic groups with strong covalent bonds, have been attracting
intensive research interest since their first discovery in 2005 by
2
Yaghi and co-workers. Through the reversible condensation
reactions, many types of COFs have been successfully synthe-
3
sized. Among them, the –CQN– linked COFs exhibit good
3
b,4
stability
(
and show great potential for diverse applications
5
6
7
3d,8
Scheme 1 Synthesis of LZU-20 via the direct condensation of 1 and 2.
such as gas adsorption, optoelectronics, catalysis, sensing,
and energy storage ). However, the construction strategies of
9
–
–
CQN– linked COFs are less developed. To date, most of the with amines to synthesize organic compounds via the formation
CQN– linked COFs have been synthesized from the reaction of of –CQN– bonds. All these advantages intrigued us to synthesize
12
aldehydes and amines. Due to the electrophilicity of the formyl –CQN– linked COFs via the direct condensation of dimethyl
groups, aldehydes are prone to oxidation and have to be acetals and amines. For this purpose, a new 2D imine-linked
10
protected during their synthetic processes. These disadvantages COF (denoted as LZU-20) was successfully synthesized via the
0
0
make many functionalized aldehydes less available, which direct condensation of 6,6 -bis(dimethoxymethyl)-3,3 -bipyridine
limits their practical applications. In this regard, the synthesis (1) and 1,3,5-tris(4-aminophenyl)benzene (2) (Scheme 1). The
of –CQN– linked COFs by avoiding the use of aldehydes is synthesized LZU-20 is highly crystalline and exhibits permanent
7c
highly desired. Accordingly, we report herein a new strategy for porosity. Moreover, the generality of this strategy was demon-
the synthesis of –CQN– linked COFs from dimethyl acetals.
strated by the synthesis of another two –CQN– linked COFs.
Dimethyl acetals, which can be facilely prepared from carbonyl
As shown in Scheme 1, in the presence of 3 M aqueous acetic
compounds, possess good stability against nucleophiles, bases, acid, the imine-linked LZU-20 was synthesized by the direct
1
0
0
and most of the oxidants. In addition, compared with the condensation of 1 and 2. The dimethyl acetal, 6,6 -bis(dimethoxy-
0
unprotected aldehydes, dimethyl acetals often have better solubility methyl)-3,3 -bipyridine (1), could be facilely synthesized from the
in common organic solvents, which is important for the COF commercially available compounds (ESI†). Importantly, monomer 1
1
1
synthesis. Furthermore, dimethyl acetals can directly react exhibits better solubility in most of the organic solvents than the
corresponding aldehyde (Fig. S34, ESI†), which makes the synthetic
conditions of LZU-20 mild and flexible (Fig. S11, ESI†). The synthe-
sized LZU-20 material was insoluble and stable in common organic
solvents (such as tetrahydrofuran, acetone, acetonitrile, dimethyl-
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, P. R. China.
E-mail: wang_wei@lzu.edu.cn, dingsy@lzu.edu.cn
†
Electronic supplementary information (ESI) available: Synthetic procedures,
13
formamide, and dichloromethane). Thermogravimetric analysis
2
FT-IR spectra, C CP MAS NMR spectra, N adsorption measurement, TGA
curves, and SEM images. See DOI: 10.1039/c6cc00947f
(TGA) indicated that LZU-20 was thermally stable up to 448 1C
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Fig. 2
N
2
adsorption–desorption isotherms of LZU-20. Adsorption and
Fig. 1 Observed (a) and Pawley refined (b) PXRD patterns of LZU-20.
Inset: expansion of the observed PXRD pattern. (c) The difference plot
between the observed and Pawley refined patterns. (d) The simulated
PXRD pattern based on the eclipsed structure. (e) Schematic representation
of the simulated structure of LZU-20, C: gray, N: blue, and H atoms are
omitted for clarity.
desorption points are represented by filled and empty symbols, respec-
tively. The pore-size distribution of LZU-20 is shown in the inset.
density functional theory (NLDFT) method. LZU-20 displayed a
narrow pore-size-distribution centered at 3.2 nm (Fig. 2, inset),
which is close to that calculated from the eclipsed model
under a nitrogen atmosphere (Fig. S28, ESI†). Scanning electron (Fig. 1e). The existence of a small amount of micropores
microscopy (SEM) images showed that LZU-20 adopted a uniform (centred at 1.4 nm) should be originated from the formation
flower-shaped morphology (Fig. S31, ESI†). The Fourier transform of the staggered structure. Thus, a –CQN– linked COF material
infrared (FT-IR) spectrum of LZU-20 showed the –CQN– stretching can indeed be formed via the condensation of dimethyl acetal
ꢀ
1
vibration at 1622 cm (Fig. S4, ESI†), indicating the formation and amine.
of the imine bonds. Solid state NMR spectroscopy was utilized In order to assess the direct condensation between dimethyl
acetal 1 and amine 2, the mother liquor left upon preparation
1
3
to further confirm the atomic construction of LZU-20. The
C
cross-polarization magic-angle spinning (CP MAS) NMR spectrum of LZU-20 at different reaction times was examined by the
showed a signal at 160 ppm, which is assigned to the carbon of the electrospray ionization mass spectrometry (ESI-MS). No evidence
imine bonds (Fig. S7, ESI†). The other signals at 154, 148, 139, 133, for the formation of the corresponding aldehyde was observed
127, and 121 ppm are assigned to the carbon atoms of the phenyl (Fig. S3, ESI†), implying that LZU-20 was directly constructed from
and pyridyl groups. dimethyl acetal 1 and amine 2. Furthermore, we performed a
The crystallinity of LZU-20 was determined by powder X-ray model reaction to confirm the direct condensation between
diffraction (PXRD) analysis (Fig. 1). The PXRD patterns of LZU-20 dimethyl acetals and amines. 2-(Dimethoxymethyl)pyridine (3)
showed peaks at 2.31, 4.01, 4.61, 6.11, and 24.71, which correspond and aniline (4) were used as the model compounds for dimethyl
to the (100), (110), (200), (210), and (001) facets, respectively. The acetal 1 and amine 2, and the reaction was monitored by gas
structure modelling was performed using Materials Studio (ver. 6.0) chromatography (GC). As shown in Fig. 3, the corresponding
software packages. Two structural models (eclipsed and staggered) aldehyde, 2-pyridinaldehyde (6), was not detected during the
could be possibly applicable for LZU-20. In comparison with the reaction; while the condensation product (5) was generated
staggered structure, the eclipsed model produced a PXRD pattern along with the consumption of 3 and 4. These results indicated
which matches well with the experimental PXRD pattern (Fig. 1d). that the imine bonds should be formed by the direct condensation
1
2
The unit cell was refined using the Pawley method, affording the of dimethyl acetal and amine via the nucleophilic attack
cell parameters of a = b = 44.879 (ꢁ0.380) Å, and c = 3.552 (Fig. S2, ESI†).
(
ꢁ0.029) Å (with Rwp = 7.14% and R
p
= 5.29%). Therefore, the
The generality of this strategy was further demonstrated by
preferred structure of LZU-20 could be determined as the eclipsed the synthesis of another two –CQN– linked COFs. As shown in
arrangement shown in Fig. 1e. Scheme 2, the hydrazone-linked COF (denoted as LZU-21) and
The porosity of LZU-20 was determined by nitrogen adsorption– the azine-linked COF (denoted as LZU-22) could be synthesized
desorption measurements at 77 K (Fig. 2). LZU-20 exhibited a typical by the direct condensation of 1,3,5-tris(6-(dimethoxymethyl)-
type-IV adsorption isotherm, which is characteristic of mesoporous pyridin-3-yl)benzene (7) with 2,5-diethoxyterephthalohydrazide
structures. The hysteresis in the desorption curve of LZU-20 should (8) or hydrazine hydrate (9). The successful formation of
1
3
be derived from the interparticle adsorption, the case of which has hydrazone or azine linkages was confirmed via FT-IR and
C
13
been found for other 2D COFs. The Brunauer–Emmett–Teller CP MAS NMR measurements. The FT-IR spectra of LZU-21
2
ꢀ1
(
BET) surface area of LZU-20 was calculated to be 200 m g
(Fig. S5, ESI†) and LZU-22 (Fig. S6, ESI†) showed strong
–CQN– stretches at 1618 cm and 1622 cm , respectively.
.99). The pore-size-distribution was derived by the nonlocal Meanwhile, the C NMR spectra of LZU-21 (Fig. S8, ESI†) and
3
ꢀ1
ꢀ1
ꢀ1
(0.10 o P/P
0
o 0.25) and the pore volume was 0.15 cm g (P/P
0
=
14
13
0
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Fig. 4 PXRD patterns (a) and N
2
adsorption–desorption isotherms (b) of
LZU-21 (black) and LZU-22 (red). Adsorption and desorption points (b) are
represented by filled and empty symbols, respectively.
adsorption–desorption measurement. Both the adsorption
Fig. 3 Gas chromatography (GC) analysis for the model reaction of isotherms (Fig. 4b) exhibited type-IV shapes, indicating that
2-(dimethoxymethyl)pyridine (3) and aniline (4).
both LZU-21 and LZU-22 are mesoporous materials. The BET
2
ꢀ1
surface areas were calculated to be 882 m
g
for LZU-21
2
ꢀ1
(Fig. S23, ESI†) and 305 m g for LZU-22 (Fig. S26, ESI†). The
SEM images showed that LZU-21 exhibited a uniform fiber-like
morphology (Fig. S32, ESI†), while LZU-22 displayed a coral-like
morphology (Fig. S33, ESI†).
In conclusion, we have developed a new strategy to synthe-
size –CQN– linked COFs directly from dimethyl acetals and
amines, through which three new COFs (imine-linked LZU-20,
hydrazone-linked LZU-21, and azine-linked LZU-22) have been
successfully constructed. All these COFs are highly crystalline
and exhibit good thermal stability. Our strategy not only allows
the facile construction of –CQN– linked materials via the direct
1
5
use of dimethyl acetals, but also opens up a new approach for
constructing application-oriented COFs due to the improved
stability and availability of acetals. Furthermore, the synthesized
COFs are embedded with pyridine units and, therefore, could be
potentially used as catalysts, sensors, or adsorbents. Efforts
towards these applications are actively underway in our lab.
This work was supported by the National Natural Science
Foundation of China (No. 21425206 and 21502081) and the
Specialized Research Fund for the Doctoral Program of Higher
Education (20120211130004).
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carbon atoms of the –CQN– bonds at 150 and 163 ppm,
respectively. PXRD analysis verified their crystalline structures
(Fig. 4a) and the preferred structures of both COFs were the
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1
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