A rings-in-pores net: Crown ether-based covalent organic frameworks for phase-transfer catalysis

Article abstract of DOI:10.1039/c9cc07639e

We herein present a new family of crown ether-based covalent organic frameworks (CE-COFs) for the first time. The CE-COFs show excellent phase-transfer catalytic performance in various nucleophilic substitution reactions.

Full text of DOI:10.1039/c9cc07639e

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S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
towards bifunctional catalysts of tunable basicity
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DOI: 10.1039/C9CC07639E
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Rings in Pores-Net: Crown Ether-Based Covalent Organic
Frameworks for Phase-Transfer Catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Ji-Chuang Shen,a Wei-Ling Jiang,a Wen-Di Guo,a Qiao-Yan Qi,b De-Li Ma,b Xiaobing Lou,c Ming
Shen,c Bingwen Hu,c Hai-Bo Yang,*a and Xin Zhao*b
DOI: 10.1039/x0xx00000x
We herein present a new family of crown ether-based covalent
organic frameworks (CE-COFs) for the first time. The CE-COFs show
excellent phase-transfer catalytic performance in various
nucleophilic substitution reactions.
As an emerging kind of crystalline porous materials, covalent
organic frameworks (COFs) are composed of pure organic
building blocks joined by strong covalent bonds.1 Since the first
example discovered by Yaghi and co-workers in 2005, COFs
have gained considerable attention and exhibited significant
potential in various applications such as gas storage and
separation,2 catalysis,3 and ion conduction.4 To date,
constructing functionalized COFs remains a major issue in this
area. To achieve a wider range of application, it is often
necessary to introduce corresponding specific functional
groups into the skeletons of COFs.
Crown ethers (CEs) have attracted increasing attention
during the past decades.5 It is well known that crown ethers
can serve as host platforms for binding with inorganic or
organic guests due to their cyclic cavities and electron
donating properties, thus making them widely useful in
supramolecular chemistry,6 biological science,7 and
nanoscience.8 In particular, developing crown ether-
functionalized materials has generated the general interests in
materials science,9 which have been implemented in polymers
and metal-organic frameworks (MOFs).10 Very surprisingly,
crown ether functionalized COFs has not been explored yet.
Fig. 1. The Construction of CE-COFs by the “Bottom-Up” Strategy
Herein, we successfully synthesized three crown ether
containing COFs (CE-COFs) embedded with different secondary
rings (12-crown-4, 15-crown-5 and 18-crown-6). What’s more,
the CE-COFs exhibited excellent performance as phase-
transfer catalysts (PTC)11 in various nucleophilic substitution
reactions such as esterification, etherification and cyanation.
The chemical synthesis route of the CE-COFs was directed by
the “bottom-up” strategy.
A series of crown ether-
functionalized p-triphenylene diamine monomers (CE-TPDA,
Scheme S1, ESI†) were employed to condensation reaction
with 1,3,5-triformylbenzene under solvothermal conditions to
produce three different crown ether-functionalized crystalline
COF materials (Fig. 1, ESI†), respectively. The corresponding
COFs were denoted as 12C4-COF, 15C5-COF, and 18C6-COF,
respectively (Scheme S2, S3, and S4, ESI†).
The crystalline structures of CE-COFs were determined by
powder X-ray diffraction (PXRD) analysis. The experimental
PXRD patterns (Fig. 2a-c) of all the three CE-COFs showed
strong and sharp PXRD peaks (over 20,000 cps.) at 2.84~2.90°,
which could be assigned to the (100) Bragg degree reflection,
indicating that the as-obtained materials had a high degree of
crystallinity (Fig. S3, S13 and S23, ESI†). In addition, the
diffraction peaks at 5.59 and 8.33° in the PXRD pattern of
12C4-COF were attributed to its (200) and (300) reflections
a. Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineerin,
East China Normal University, Shanghai 200062, China.
E-mail: hbyang@chem.ecnu.edu.cn
b. Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional
Molecules, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, China.
E-mail: xzhao@sioc.ac.cn
c. Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Electronic
Science, East China Normal University, Shanghai 200062, China
†Electronic Supplementary Information (ESI) available: Includes detailed synthetic
procedures, NMR, crystal models simulations, and GC data, See
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C9CC07639E
Fig. 2. The XRD profiles and simulated structures of (a) 12C4-COF, (b) 15C5-COF and (c) 18C6-COF. The experimental patterns (12C4-COF in sky blue, 15C5-COF in orange, and
18C6-COF in purple, respectively), Pawley refined patterns (light grey), difference plot between the observed and refined patterns (black), AA stacking patterns (green), and AB
stacking patterns (red) of CE-COFs. (d) 13C CP/MAS NMR spectra of 12C4-COF (sky blue), 15C5-COF (orange) and 18C6-COF (purple), asterisks denote spinning sidebands. (e) SEM of
18C6-COF, (f, g) TEM of 18C6-COF.
(Fig.2a), respectively. Slightly in difference, the peaks at 4.85 of the aromatic region, which was also consistent with the
and 8.36° in Figure 2b, the peaks at 4.93 and 8.44° in Figure 2c increased content of methylene groups on the crown ether
were corresponded to (110) and (300) reflections of 15C5-COF ring (Fig 2d). The thermogravimetric analysis (TGA) of the
and 18C6-COF, respectively. The lattice modeling and Pawley materials demonstrated that CE-COFs featured a good thermal
refinement gave the optimized parameters for the unit cell stability (Fig. S4, S14 and S24, ESI†). Scanning electron
with the space group of P3 (Fig. S10, S20 and S30, ESI†) in AA microscopy (SEM) and transmission electron microscopy (TEM)
structure with excellent agreement factors (a = b = 37.158 Å, c fully revealed the morphological dimensions of the obtained
= 4.356 Å, α = β = 90°, γ =120°, Rwp = 2.38%, and Rp = 1.55% for CE-COFs (Fig. S8, S18, S28, S9, S19 and S29, ESI†), which
12C4-COF; a = b = 36.888 Å, c = 4.545 Å, α = β = 90°, γ =120°, showed that the CE-COFs had the uniform hollow shell
Rwp = 2.27%, and Rp = 1.50% for 15C5-COF; a = b = 36.769 Å, c = structures(Fig.2e, f). We found that it was similar with that in
4.243 Å, α = β = 90°, γ =120°, Rwp = 3.35%, and Rp = 2.63% for R. Banerjee’s report and could be probably expained by the
18C6-COF). In contrast, the staggered AB model did not match theory of Ostwald Ripening.12,13 We also observed the
the observed data, which thus ruled out AB stacking of the existence of the pore structure in COF-sheets under high-
resultant two-dimensional (2D) COFs.
The atomic-level construction of CE-COFs was firstly assembly of materials on the two-dimensional level.
assessed by Fourier transform infrared spectroscopy (FT-IR) The porosity of the as-obtained materials was characterized
resolution TEM (Fig.2g), which directly reflected the regular
(Fig. S1, S11 and S21, ESI†). The FT-IR spectras of the CE-COFs by nitrogen adsorption measurements at 77 K (Fig. 3a, b).
exhibited vibrations bands at 1624 cm-1, which could be 12C4-COF displayed a typical IV isotherm (Fig. S5, ESI†), which
assigned to the -C=N- stretching vibration. More detailed was indicative of a mesoporous material. Comparatively, 15C5-
information was given by 13C cross-polarization magic-angle COF and 18C6-COF were also this type of isotherm although
spinning (CP/MAS) NMR analysis (Fig. S2, S12 and S22, ESI†). the amount of adsorption was relatively low (Fig. S15 and S25,
The chemical shifts at 158 and 71 ppm were attributed to the ESI†). The Brunauer−Emmett−Teller (BET) surface areas for
carbons of imine and methylene groups, respectively. It was 12C4-COF, 15C5-COF and 18C6-COF were calculated to be 210,
found that the peak at 71 ppm gradually increased from 12C4- 59 and 47 m2/g, respectively (Fig. S6, S16 and S26, ESI†). It was
COF, 15C5-COF to 18C6-COF after normalizing the peak height
speculated that the crown ether units blocked the pores to a
certain extent, which thus resulted in the relatively low specific
surface areas of the materials. Pore size distributions were
calculated by using the Barrett-Joyner-Halenda (BJH) method
(Fig. 3b; Fig. S7, S17 and S27, ESI†). It was found that CE-COFs
were mesoporous with pore sizes around 3.2 nm, which also
well matched the simulated values (2.8 to 3.5 nm).
All above data demonstrated that the crown ethers were
successfully introduced into skeletons of the crystalline COFs.
In this context, the further research was conducted to explore
the functionality of the CE-COFs. It is well known that crown
ethers can well bind with alkali metal ions to form stable
crown ether ion-pair complexes, thereby transferring and
activating the counter anions.5 Thus, crown ethers have been
extensively explored as phase-transfer catalysts (PTC).11c With
the CE-COFs in hand, this “ring in pores-net” model were going
to be investigated by using CE-COFs as phase-transfer catalysts
Fig. 3. (a) Nitrogen adsorption isotherms and (b) Pore size distribution profiles of
12C4-COF (sky blue), 15C5-COF (orange) and 18C6-COF (purple) at 77 K.
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Table 1. Phase Transfer Catalytic Activity Test of 18C6-COF in the Iodination of
in various nucleophilic substitution reactions. Before the
catalytic experiments, we also investigated the adsorption of
different alkali metal cations by using CE-COFs as adsorbents
(Section 6, ESI†), which showed a strong size-matching effect
between the alkali cations and crown ethers, which was
consistent with the reports in literatures.5 For example, 18C6-
COF featured the decreasing adsorption capacity of 1.2, 0.7
and 0.2 mmol/g for K+, Na+ and Li+, respectively.
View Article Online
1-bromooctane under Solid-Liquid-Solid Condition[a]
DOI: 10.1039/C9CC07639E
Controlled
yield (%)[c]
25
12
N.D.[d]
N.D.
Entry
Catalyst
Solvent
Yield (%)[b]
1
2
3
4
5
6[e]
7
8
9
10
18C6-COF
18C6-COF
18C6-COF
18C6-COF
18C6-COF
18C6-COF
18C6-TPDA
DB18C6
THF
EtOAc
n-Hexane
Toluene
o-DCB
THF
THF
THF
THF
THF
96
96
22
31
54
95
46
66
94
99
In the catalytic section, 18C6-COF was firstly employed to
catalyze the iodination of 1-bromooctane. Since KI was difficult
to dissolve in THF and the COF was present as a solid powder
as well, a solid-liquid-solid phase-transfer catalysis system (S-L-
S PTC) was obtained.11c,14 Fortunately, through the kinetic
tracking (Fig. S35, ESI†), it was found that the addition of 5.0
mol % of 18C6-COF had a significant catalytic effect (96% yield
after 10 hours) in THF at 70 ℃ compared with the controlled
experiment without any catalyst (25% yield). In addition,
kinetic analysis was performed on the reaction (Figure S38,
ESI†). The rate constants were determined to be k1’= 0.341 s-1
and K0’= 0.029 s-1 according to the quasi-first-order reaction
fitting, of which the correlation coefficient reached R = 0.99
and R = 0.98, respectively. These results indicated that the
reaction was a first-order reaction under the S-L-S system. In
addition, an orthogonal test were explored to study the effect
of the compatibility of crown ether size and alkali metal ion
size on catalytic performance (Figure S39, ESI†). When KI (Fig.
S39a, ESI†) was used as the fixed substrate, the ionic radius of
K+ (2.66 Å) was matched with the cavity size of 18-crown-6
(2.60~3.20 Å).15 Therefore, K+ could be efficiently trapped by
the 18C6-COF. As a result, anion I- was activated by weakening
the binding between K+ and I-, which thus led to an efficient
iodination (96% in yield). By contrast, the use of 15C5-COF
resulted in the decrease of the capture ability to K+ due to the
smaller cavity of 15-crown-5 (1.20~2.20 Å).14 Therefore, in this
case, I- was more likely to bind to K+ than to participate in the
substitution reaction (63% in yield). The size of the 12-crown-4
(1.20~1.50 Å)15 was so small that it did not work at all (25% in
yield). On the other hand, 15C5-COF was employed as the
invariant catalyst with different iodide salts as substrates to
study the activation relationship between the same host and
different guests (Fig. S39b, ESI†). In line with expectations,
15C5-COF featured the best activation for NaI (1.90 Å for Na+)
due to the size effect (100% in yield), followed by LiI (1.36 Å for
Li+, and 81% in yield) and KI (2.66 Å for K+, and 63% in yield).15
Along with the ion adsorption date, these results
demonstrated that the matching relationship between the size
of crown ethers and ionic radius affected the phase-transfer
catalysis.
N.D.
Ph4PBr
(n-Bu)4NBr
[a] Unless otherwise noted, the reaction conditions are: 1-bromooctane (0.25
mmol), KI (1.25 mmol), and catalyst (5 mol %, calculated by crown ether
contents in CE-COFs), 2 mL solvent, 70 °C for 10 hours, GC-MS and NMR
determined the structure. [b] GC yield. [c] Controlled experiment in the
absence of catalyst. [d] N.D. = Not detected the product. [e] Yield at the fifth
catalytic cycle.
hexane, toluene, or o-dichlorobenzene (o-DCB), the solubility
of KI was extremely poor, and the effect of solubility on the
reaction was greater than that of the anion activity on the
reaction. More significantly, by comparing the catalytic
performance of 18C6-COF with 18C6-TPDA monomer and
dibenzo-18-crown-6 (DB-18C6) (Table 1, Entry 7-8), it was
found that the 18C6-COF exhibited an enhanced catalytic
activity. The proposal reason was that the open channels of
COF could accumulate the reactants and promote the
reactions in the confined nanopore space.3b Furthermore, the
catalytic ability of 18C6-COF was comparable to that of the
commercially available quaternary ammonium salt or
quaternary phosphonium salt phase-transfer catalysts (Table 1,
Entry 9-10). Notably, unlike the small molecule catalysts, the
resultant 18C6-COF-catalyst was recyclable. It was found that
the crystalline structure of the COF material was not
significantly damaged after five cycles of the efficient catalysis
as determined by 13C CP/MAS NMR and PXRD (Table 1, Entry 6
and Fig S34, S35 and S36, ESI†).
Finally, in order to investigate the range of catalytic
applications in addition to the simple Halex reaction, we have
explored the phase-transfer catalytic activity of 18C6- COF in
some nucleophilic substitution reactions such as the
esterification of various halohydrocarbons, the thiocyanation,
aryl etherification, azidation, cyanidation and fluorination
reactions of benzyl bromide, and even the fluorination of aryl
chloride (Table 2 and Table S5, ESI†). These types of reactions
have been widely used in organic synthesis in lab, chemical
production and pharmaceuticals in industry. In the
esterification between CH3COOK and 1-halohydrocarbons, the
18C6-COF achieved a good to excellent yield for both straight-
chain 1-bromohydrocarbons and benzyl halides in the refluxed
CH3CN after 5 hours without obvious weight loss (Fig. S43,
ESI†), which had a poor yield in the absence of a catalyst (Table
We next studied the catalytic universality of 18C6-COF for
this halogen exchange reaction (halex reaction) in different
solvents (Table 1). The experimental results showed that 18C6-
COF had a certain catalytic ability in the reaction compared
with the absence of catalyst. Moreover, it was found that the
reaction was performed well in the polar solvents such as
tetrahydrofuran or ethyl acetate (Table 1, Entry 1-2) mainly
because the solubility of KI increased in a polar solvent.
Conversely, in non-polar solvents (Table 1, Entry 3-5), e.g.
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Table 2. Catalytic Activity Test of 18C6-COF in Different Nucleophilic Substitution [a]
financial support. X. Lou, M. Shen, and B. Hu are grateful for
supports from the National Natural Science Foundation of
China (21703068).
View Article Online
DOI: 10.1039/C9CC07639E
10% mol 18C6-COF
Solv. reflux
-
X-
+
+
RNu
R-X
Nu
Yield (%)[b]
/Controlled
yield(%)[c]
85/27
91/26
99/34
81/23
96/25
99/31
94/23
97/36
82/N.D. [e]
84/N.D.
47/N.D.
Entry
RX
n-C4H9Br
Nu-
Solvent
T(℃)/t(h)
Conflicts of interest
There are no conflicts to declare.
1
2
3
4
5[d]
6
7
8
9
CH3COOK
CH3CN
CH3CN
CH3CN
CH3CN
NB/H2O
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
85/5
85/5
85/5
n-C8H17Br CH3COOK
PhCH2Br
PhCH2Cl
PhCH2Br
PhCH2Br
PhCH2Br
PhCH2Br
PhCH2Br
PhCH2Br
CH3COOK
CH3COOK
PhCOOK
KSCN
PhOK
NaN3
KCN
KF
KF
References
85/5
100/10
85 /1
85/5
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[a] Unless otherwise noted, the reaction conditions are: halides (0.25 mmol),
nucleophile (1.25 mmol), and 18C6-COF (10 mol %, 14 mg), 2 mL solvent, GC-MS
determined the structure. [b] GC yield. [c] Controlled experiment in the absence
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4
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fluoroarenes.17 Fortunately, the 18C6-COF exhibited good
catalytic effects on the fluorination between 2,4-
dinitrochlorobenzene and KF, especially in polar solvents such
as DMF (Table 2, Entry 11-12).
In summary, we synthesized three isostructral 2D crown
ether-based covalent organic frameworks for the first time by
introducing the discrete covalent organic macrocycle into the
continuous pores-net of COFs. In particular, the crown ethers
were able to realize the applications of COFs in phase-transfer
catalysis in many nucleophilic substitution reactions for the
first time to the best of our knowledge. We hope this complex
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applications such as phase-transfer asymetric catalysis in COFs
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H.-B. Yang thanks Innovation Program of Shanghai Municipal
Education Commission (No. 2019-01-07-00-05-E00012) and
ECNU Academic Innovation Promotion Program for Excellent
Doctoral Students (40500-20102-222000/003/009) for
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