Squaramide-decorated covalent organic framework as a new platform for biomimetic hydrogen-bonding organocatalysis

Article abstract of DOI:10.1039/c9cc01317b

A squaramide-decorated COF was synthesized and used as a highly efficient heterogeneous catalyst for hydrogen-bonding organocatalysis as exemplified in the context of catalyzing Michael addition reactions under mild conditions. Our work lays a foundation

Full text of DOI:10.1039/c9cc01317b

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C9CC01317B
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Squaramide-Decorated Covalent Organic Framework as a New
Platform for Biomimetic Hydrogen-Bonding Organocatalysis
Xia Li,a Zhifang Wang,a Jiaxing Sun,b Jia Gao,a Yu Zhao,a Peng Cheng,a,b Briana Aguila,c Shengqian
Received 00th January 20xx,
Accepted 00th January 20xx
Ma,*c Yao Chen,*b and Zhenjie Zhang*a, b, d
DOI: 10.1039/x0xx00000x
A squaramide-decorated COF was synthesized and used as a highly
Hydrogen-bonding organocatalysis has been developed as
efficient heterogeneous catalyst for hydrogen-bonding excellent biomimetic alternatives to Lewis acid catalysis.16
organocatalysis as exemplified in the context of catalyzing Michael Recently, bifunctional squaramide moieties have been proven
addition reactions under mild conditions. Our work lays a as excellent biomimetic hydrogen-bonding organocatalysts,17
foundation for the development of functional COF as a new ascribed to their hydrogen-bond accepting and donating
platform for biomimetic organocatalysis.
functionality through their carbonyl and N–H groups,
respectively.18 Structurally, squaramides are related to the
arginine residue via their relationship to the urea and
guanidinium ion.19 However, due to the strong intermolecular
hydrogen-bonding interactions between carbonyl and N-H
groups,20 squaramide derivatives suffer from severe self-
association/aggregation that seriously hinders squaramides’
solubility and catalytic efficiency.21 To tackle this issue, one
facile and effective strategy is to incorporate squaramide
moieties into porous materials to form heterogeneous
catalysts, which can offer many advantages such as good
reusability and enhanced stability.7 For instance, squaramide-
based carboxylic ligands have been used to construct porous
metal-organic frameworks (MOFs) to serve as effective
heterogeneous catalysts towards Friedel-Crafts reactions.21
However, to the best of our knowledge, there is no report for
squaramide-based COFs up to now. Herein, we created a
catalytic squaramide-linked COF with good crystallinity and high
porosity. Moreover, this novel COF can catalyze Michael
addition reactions with high catalytic efficiency and good
reusability. This study not only enriches the library of COF
materials, but also points out a new strategy to design and
synthesize heterogeneous biomimetic organocatalysts.
Covalent organic frameworks (COFs) are a class of designable
crystalline polymers with structural periodicity and inherent
porosity that consist of cross-linked organic building blocks.1
Benefiting from their high crystallinity,2 tunable pore size3 and
large surface area,4 COFs demonstrate diverse applications
including gas storage,5 gas separation,6 catalysis,7 smart
sensors,8 optoelectronics,9 drug delivery,10 energy storage11
and so on. Recently, the exploration of COFs as heterogeneous
catalysts has attracted increasing attention due to the high
catalytic efficiency, excellent stability and good reusability.12
One routine strategy to construct COF catalysts is incorporating
catalytic metal particles or organic species into COFs via post-
synthetic modifications. For example, Wang et al. immobilized
Pd species into COF-LZU1 to catalyze Suzuki-Miyaura Coupling
reactions.13 Another alternate strategy is using catalytic
monomers to directly prepare COFs in which the framework
itself is catalytic. Yaghi and co-workers used porphyrins as
monomers to prepare porphyrin COFs with high efficiency for
electrocatalytic carbon dioxide reduction.14 Cui et al.
synthesized
a series of BINOL-based COFs to catalyze
asymmetric addition reactions of diethylzinc to aldehydes.15
In order to construct squaramide-linked COFs, we first
designed and synthesized
functionalized with
a
new diamino monomer
a. College of Chemistry, Nankai University, Tianjin, 300071, China. E-mail:
zhangzhenjie@nankai.edu.cn
a
squaramide core, 3,4-bis((4
aminophenyl)amino)cyclo-but-3-ene-1,2-dione (1). Diffusion of
CH2Cl2 into a DMF solution of 1 yielded yellow prism crystals of
1. Single crystal diffraction data revealed that monomer 1 is a
linear molecule, which possesses a similar shape and length as
[1,1':4',1''-terphenyl]-4,4''-diamine (3) (Fig. S1). The C3-
symmetric 1,3,5-triformylbenzene (2) was widely employed as
the basic backbone of COFs because the rigid and triaryl
b. State Key Laboratory of Medicinal Chemical biology, Nankai University, Tianjin
300071, China. E-mail: chenyao@nankai.edu.cn
c. Department of Chemistry, University of South Florida, 4202 E. Fowler Avenue,
Tampa, Florida 33620, United States. E-mail: sqma@usf.edu
d. Key Laboratory of Advanced Energy Materials Chemistry, Ministry of Education,
Nankai University, Tianjin 300071, China
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [PXRD patterns, FT-IR spectra,
SEM images, BET curves, catalytic details and NMR spectra]. See
DOI: 10.1039/x0xx00000x
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benzene group is ideal for constructing COFs of large hexagonal of COF-SQ are essential to heterogeneous catalysts. The
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channels.13 For example, Lei and coworkers synthesized an porosity of COF-SQ was studied by meaDsOurI:in10g.1N0329a/Cd9sCoCrp01t3io17nB-
imine-linked COF (2DP1+5) of ~3.7 nm hexagonal channels via desorption at 77 K on the activated samples. The adsorption
the condensation reaction of monomer 2 and 3.22 Considering curves of COF-SQ exhibited the classic type IV isotherm (Fig. 4),
the similarity of 1 and 3, we successfully prepared a which is characteristic of mesoporous materials.24 The
squaramide-linked COF, COF-SQ, via the condensation reaction Brunauer-Emmett-Teller (BET) and Langmuir surface area of
of 1 (0.15 mmol) and 2 (0.10 mmol) in a mixed solvent of COF-SQ was estimated to be 1195 m2g-1 and 2014 m2g-1,
mesitylene and 1,4-dioxane in the presence of 6.0 M acetic acid respectively (Fig. S7). The nonlocal density functional theory
as catalyst within a flame-sealed glass ampule at 120 °C for 3 (NLDFT) gave rise to a narrow pore size distribution with an
days. Subsequently, the as-prepared powder was collected by average pore width centered at 3.6 nm, in good agreement with
centrifugation or filtration and washed with THF and acetone the related simulated value of 3.8 nm.
using Soxhlet extraction for 24 h, respectively. After drying
under vacuum at 120 °C, a brown-colored powder was
obtained. A molecular analogue of COF-SQ with a squaramide
core, 3,4-bis((4-((E)-benzylideneamino)phenyl) amino)cyclobut-
3-ene-1,2-dione (4), was successfully synthesized and used as a
model compound (Scheme S2). The IR spectrum of 1 shows that
the characteristic N-H stretching vibration at 3413 cm−1 and
3286 cm−1 and C=O stretching vibration of 2 at 1681 cm−1
disappeared after the reaction. Meanwhile, the C=N stretching
vibrations (1618 cm-1) of COF-SQ (1622 cm−1 for 4) (Fig. S2),23
indicating the formation of imine bonds. The 13C CP-MAS NMR
Fig. 2 13C CP/MAS NMR spectra of COF-SQ. The assign-ments of 13C chemical shifts
of COF-SQ were indicated in the chemical structure.
spectra showed the characteristic signal for the C=N and C=O
groups at around 165 ppm and 181 ppm, respectively. These
results further confirmed the formation of imide groups in COF-
SQ. Powder X-ray diffraction (PXRD) analysis was used to
determine the crystalline structure of COF-SQ. To resolve its
crystal structure, a 3D extended sheet holding the structure as
illustrated in Fig. 3a was constructed. The PXRD pattern (Fig. 3a,
red) indicates that COF-SQ is a microcrystalline material with a
long-range structure. COF-SQ exhibited six prominent
diffraction peaks at 2.79˚, 4.69˚, 5.40˚, 7.30˚ and 25.07˚,
corresponding to the (100), (110), (200), (210) and (001) facets,
respectively. The Pawley-refined PXRD profiles matched with
the experimental patterns very well (Fig. 3a and Fig. S3). COF-
SQ adopts the AA stacking mode of a space group of P6 with a
= b = 37.8915 Å, interlayer distance (c) of 3.5912 Å, α = β = 90°
and γ = 120° (Table S2). Scanning electron microscopy (SEM)
images of as-prepared COF-SQ powders showed uniformly
dispersed globular particles with 140-250 nm size (Fig. 3b and
Fig. S4).
Fig. 3 PXRD patterns of the COF-SQ with the experimental profiles in red, Pawley-
refined profiles in black, calculated profiles in green and the differences between
the experimental and refined PXRD patterns in blue (Inset: views from the b-axis)
(a). SEM image of COF-SQ (b).
Fig. 4 Nitrogen adsorption-desorption isotherms of COF-SQ. (Inset) Pore size
distribution of COF-SQ. Adsorption and desorption points are represented by filled
and empty symbols, respectively.
Traditional heterogeneous organocatalysts are mostly
formed by incorporating organocatalysts into nonporous
polymer supports.25 However, such catalysts exhibit low
activities as a result of inefficient accessibility to the catalytic
sites. A promising way to circumvent this problem can turn to
immobilization of the organocatalysts on porous materials
including zeolites, MOFs and COFs26. The open meso-sized
channels and good chemical stability of COFs would allow
Fig. 1 Synthesis of COF-SQ by condensation of monomer 1 and 2.
Thermal gravimetric analysis (TGA) exhibited no weight loss
under N2 atmosphere up to 380 °C (Fig. S5). In addition, COF-SQ
exhibited good solvent stability in common solvents, such as
toluene, acetone and THF, indicated by the unchanged intensity
and position of the PXRD patterns (Fig. S6). These characteristics
2 | J. Name., 2012, 00, 1-3
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efficient access to the uniformly distributed organocatalyst sites of 75%, while the higher temperature (60 °C) provVidieewdAraticsleimOnillianer
DOI: 10.1039/C9CC01317B
and facilitate the transport of reactants and products, which yield (98%) as 50 °C (95%). Moreover, we explored the substrate
may give rise to high activity in catalytic reactions.27
scope of Michael addition reaction with various nucleophile and
electrophile substrates as listed in Table 2. We examined the
possibility to use various 1,3-dicarbonyl compounds as
nucleophiles (Table 2, entries 1-4) which all formed the
conjugate addition products with 98%-71% yields. In addition,
decent yields (92%–63%) were observed for a broad range of
aryl, halogenated aryl and alkyl substituted β-nitroolefins (Table
2, entries 5-9). It was found that substrate bearing electron-
donating groups (Table 2, entry 5) possessed higher yields than
electron-withdrawing groups on the aryl ring (Table 2, entries 6-
8).16
Table 1. Optimization of reaction conditions for the Michael addition reaction between
β-nitrostyrene and 2,4-pentanedionea
O
O
NO2
+
O
O
10 mol% catalyst
NO2
1a
2a
3aa
Entr
y
Temp
(°C)
Time
(h)
Yieldb
(%)
Catalyst
Solvent
1
2
3
4
5
6
7
8
9
10
11
12
13
COF-SQ
COF-SQ
COF-SQ
COF-SQ
COF-SQ
COF-SQ
COF-SQ
COF-SQ
COF-SQ
COF-SQ
50
50
50
50
50
50
50
50
r.t.
60
50
50
50
24
24
24
24
24
24
24
24
24
24
24
24
24
Toluene
THF
Acetone
CH2Cl2
CHCl3
DMSO
MeOH
CH3CN
Toluene
Toluene
Toluene
Toluene
Toluene
95
70
40
99
76
88
33
79
75
98
64
35
trace
Table 2. Michael addition reaction of various Nucleophiles and Electrophilesa
O
O
10mol% catalyst
O
NO2
+
R1
R2
NO2
O
R1
R2
Toluene
R
℃
50
24h
R
Entry
R
R1, R2
(2a)H, H
Yieldb (%)
1
2
3
4
5
6
7
8
(1a)H
(1a)H
95
98
98
71
92
74
63
73
COF-SQc
(2b)H, OCH2CH3
(2c)H, OCH3
(2d)CH3, OCH3
(2a)H, H
Monomer 1
Monomer 2
Model
(1a)H
(1a)H
14
15
50
50
24
24
Toluene
Toluene
63
compound 4
(1b)CH3
(1c)OCH3
(1d)Clc
(1e)Br
2DP1+5
trace
(2a)H, H
aReaction condition: 1a (0.10 mmol), 2a (0.15 mmol), COF-SQ (10 mol%) in 1.0 mL
solvent. bDetermined by 1H MNR spectroscopy of the crude mixture. COF-SQ (5
mol%)
(2a)H, H
c
(2a)H, H
aReactions were performed with the electrophile (0.10 mmol) and the nucleophile
(0.15 mmol) in the presence of catalyst (10 mol%) in toluene (1.0 mL) at 50°C for
24h. bIsolated yield determined by 1H MNR spectroscopy of the
crude mixture. c(E)-1-chloro-3-(2-nitrovinyl)benzene.
The catalytic activity of COF-SQ was examined in the context
of Michael addition reactions.28 Michael addition reaction is one
of the basic C-C bond formation reactions and provides a
powerful synthetic tool for the formation of synthons of many
important natural and biologically active products.29 In our
initial investigation, we selected β-nitrostyrene and 2,4-
pentanedione as model substrates to explore the optimal
reaction conditions (Table 1). Firstly, the reaction was
performed using β-nitrostyrene and 2,4-pentanedione as
reactants at 50°C in toluene in the presence of 10 mol% COF-
SQ for 24h. The Michael addition reaction occurred and gave a
yield of 95% (TON of 9.5 and TOF of 0.396 h-1). To optimize the
reaction conditions, we screened various solvents on this
catalytic system (Fig. S8). As summarized in Table 1, we found
CH2Cl2 and toluene were the superior solvents in comparison
with other solvents. For protic solvents such as MeOH, CH3CN
and acetone, a strong hydrogen-bonding effect may exist
between the catalytic sites and protic solvent which can
influence the catalytic effect.18 However, for aprotic solvents,
such as CH2Cl2 and toluene, they won't interact with the
catalytic sites of COF-SQ, which thus exhibited high catalytic
activity. In addition, we also studied the influence of
temperature to the catalytic activity of COF-SQ. We found the
lower temperature (e.g. room temperature) gave a lower yield
In order to study if the catalytic activity is originated from the
squaramide or other sites, monomer 1, 2 and model compound
4, along with 2DP1+5, which possesses a similar structure as COF-
SQ but without squaramide moiety, were chosen as controls30.
As expected, no catalytic activity was observed for 2 and 2DP1+5
due to the lack of catalytic sites (Table 1, entries 12-15 and Fig.
S9).22 It is noteworthy that squaramide monomer 1 and model
compound 4 possessed much lower activity than COF-SQ. These
results indicate that the high porosity of COF-SQ and uniformly
distributed catalytic sites may contribute to the high activity of
COF-SQ. To further explore the advantage of heterogeneous
catalyst, we tested the reusability of COF-SQ and found it could
be easily recovered by centrifugation and reused for at least 4
times without significant loss of catalytic activity (Fig. S10). The
PXRD patterns (Fig. S11) of the recycled COF-SQ revealed that
its crystallinity was retained.31 A hot filtration test was further
performed to study the heterogeneous catalyst nature for COF-
SQ (Fig. S12). We found the catalysis reaction was stopped once
COF-SQ catalyst was removed from the reaction mixture by hot
filtration, suggesting COF-SQ indeed catalyzed the reaction.
This journal is © The Royal Society of Chemistry 20xx
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Huang, G. E. Wang, Z. H. Fu, M. S. YaoViaewndArticGle.OnXlinue,
CrystEngComm., 2016, 18, 4259–4263.DOI: 10.1039/C9CC01317B
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These results verified the heterogeneous nature of COF-SQ as a
catalyst.21
In summary, we developed a straightforward strategy for the
construction of catalytic squaramide-linked COFs (COF-SQ) via
direct employment of catalytic squaramide building blocks.
COF-SQ exhibited high crystallinity, permanent porosity and
good
thermal/solvent
stability.
Moreover,
COF-SQ
demonstrated good catalytic performance and good
recyclability compared to the homogeneous counterparts in
Michael addition reaction. Our discovery paves a new way for
the construction of squaramide-based heterogeneous catalysts
and broadens the highly-valued applications of functional COFs.
Further investigations regarding the design and synthesis of
new catalytic COFs toward high value-added reactions are
ongoing in our lab.
The authors acknowledge the financial support from the
National Natural Science Foundation of China (21601093).
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There are no conflicts to declare.
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