Control Interlayer Stacking and Chemical Stability of Two-Dimensional Covalent Organic Frameworks via Steric Tuning

Article abstract of DOI:10.1021/jacs.8b08452

Layer stacking and chemical stability are crucial for two-dimensional covalent organic frameworks (2D COFs), but are yet challenging to gain control. In this work, we demonstrate synthetic control of both the layer stacking and chemical stability of 2D COFs by managing interlayer steric hindrance via a multivariate (MTV) approach. By co-condensation of triamines with and without alkyl substituents (ethyl and isopropyl) and a di- or trialdehyde, a family of two-, three-, and four-component 2D COFs with AA, AB, or ABC stacking is prepared. The alkyl groups are periodically appended on the channel walls and their contents, which can be synthetically tuned by the MTV strategy, control the stacking model and chemical stability of 2D COFs by maximizing the total crystal stacking energy and protecting hydrolytically susceptible backbones through kinetic blocking. Specifically, the COFs with higher concentration of alkyl substituents adopt AB or ABC stacking, while lower amount of functionalities leads to the AA stacking. The COFs bearing high concentration of isopropyl groups represent the first identified COFs that can retain crystallinity and porosity in boiling 20 M NaOH solution. After postsynthetic metalation with an iridium complex, the 2,2′-bipyridyl-derived COFs can heterogeneously catalyze C-H borylation of arenes, whereas the COF with isopropyl groups exhibits much higher activity than the COFs with ethyl groups and nonsubstituents due to the increased porosity and chemical stability. This work underscores the opportunity in using steric hindrance to tune and control layer stacking, chemical stability and properties of 2D COFs.

Full text of DOI:10.1021/jacs.8b08452

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Article
Control Interlayer Stacking and Chemical Stability of Two-
Dimensional Covalent Organic Frameworks via Steric Tuning
Xiaowei Wu, Xing Han, Yuhao Liu, Yan Liu, and Yong Cui
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08452 • Publication Date (Web): 03 Nov 2018
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Control Interlayer Stacking and Chemical Stability of Two-
Dimensional Covalent Organic Frameworks via Steric Tuning
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Xiaowei Wu, Xing Han, Yuhao Liu, Yan Liu* and Yong Cui*
†School of Chemistry and Chemical Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, China
Supporting Information
ABSTRACT: Layer stacking and chemical stability are crucial for two-dimensional covalent organic frameworks (2D COFs), but are yet
challenging to gain control. In this work, we demonstrate synthetic control of both the layer stacking and chemical stability of 2D COFs by
managing interlayer steric hindrance via a multivariate (MTV) approach. By co-condensation of triamines with and without alkyl
substituents (ethyl and isopropyl) and a di- or trialdehyde, a family of two-, three- and four-component 2D COFs with AA, AB or ABC
stacking is prepared. The alkyl groups are periodically appended on the channel walls and their contents, which can be synthetically tuned
by the MTV strategy, control the stacking model and chemical stability of 2D COFs by maximizing the total crystal stacking energy and
protecting hydrolytically susceptible backbones through kinetic blocking. Specifically, the COFs with higher concentration of alkyl
substituents adopt AB or ABC stacking, while lower amount of functionalities leads to the AA stacking. The COFs bearing high
concentration of isopropyl groups represent the first identified COFs that can retain crystallinity and porosity in boiling 20 M NaOH
solution. After post-synthetic metallation with an iridium complex, the 2,2’-bipyridyl-derived COFs can heterogeneously catalyze C-H
borylation of arenes, whereas the COF with isopropyl groups exhibits much higher activity than the COFs with ethyl groups and non-
substituents due to the increased porosity and chemical stability. This work underscores the opportunity in using steric hindrance to tune
and control layer stacking, chemical stability and properties of 2D COFs.
aldehydes,13 introduce enol-keto tautomerizations that either stack
INTRODUCTION
regularly or offer enhanced planarity,1b or convert imine linkage
into amide,14 dioxin,15 oxa- and thiazole,16 and quinoline17 linkages
by postsynthetic modification methods. Recent study indicated
that alkylation of COFs is also capable of slowing down the rate
of hydrolysis of the imine linkage.18 In this work, we report the
synthetic control of both layer stacking (AA, AB and ABC) and
chemical stability of 2D COFs by managing interlayer steric
Covalent organic frameworks (COFs) are crystalline porous
materials constructed by linking organic building blocks through
covalent bonds.1 Owing to their tunable compositions, structures,
functions and porosity, COFs have shown potential applications in
gas storage,2 separation,3 catalysis,4 sensor,5 and energy storage.6
Among all COFs, two-dimensional (2D) COFs have received
particular attentions for their unique structural and electronic
properties. While the monolayers of a 2D COF can be predesigned
via the geometry of its building blocks, it is the stacking of
monolayers into 3D structures that leads to the generation of 1D
channels orthogonal to the layers, which influences interlayer
electron or charge transport, thereby exerting a profound effect on
their electronic and optic properties.7 Obviously, packing manner
of the layers strongly affects not only the topology but also the
physical and chemical properties of 2D COFs. Strategies have
been explored to control or alter their stacking behaviors,
including for example the use of propeller-shaped building units
that can lock into each other to induce stacking without offset,8
and the utilization of donor and acceptor molecules that stack in
an alternating fashion for self-complementary π-electronic
interactions.7b,9 Nonetheless, no stacking transformation has been
reported for 2D COFs to date. With few exceptions that adopt
AB1a,10 or ABC11 layer stacking, almost all reported 2D COFs
exhibit eclipsed or serrated AA stacking.12 It remains a formidable
synthetic challenge to tune and control interlayer stacking of 2D
COFs.
hindrance
through
a
multivariate-component
(MTV)
approach.11c,19 A family of two-, three- and four-component 2D
COFs with AA, AB or ABC stacking is prepared by co-
condensation of triamines with and without two ethyl or isopropyl
groups adjacent to each amine and di- or trialdehydes (Scheme 1).
Mediation of different ratios of two triamines with aldehydes for
COFs growth leads to the stacking transformation from ABC/AB
to AA, which has never been reported for COFs before. Chemical
stability of the COFs is also highly relevant to the density of
incorporated alky groups that can protect imine linkage and
hydrolytically susceptible backbones. The COFs decorated with
isopropyl groups can maintain crystallinity in 20 M NaOH
o
solution at r.t. and 100 C for one week, topping any reported
stable COFs against harsh base condition. When metalated with
[Ir(COD)(OMe)]2, the 2,2'-bipyridine-derived COF containing
isopropyl groups can be an efficient active and recyclable
heterogeneous catalyst for arene C-H borylation, whereas the
COFs containing ethyl groups and non-substituents give much low
activities. The addition of bulky alkyl substituents to COFs leads
to greatly enhanced catalytic activity by increasing porosity and
chemical stability.
Imine-linked 2D COFs provide a convenient platform for
structural control and functional design because of their broad
monomer scope, but they are often crystallized with limited
chemical stability.12 Improved stability has been realized in
systems that incorporate methoxy groups adjacent to the
RESULTS AND DISCUSSION
Synthesis and Characterization. The 2D COFs 1-6-H have been
reported previously and adopts a AA stacking mode13,20. As
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Scheme 1. (a) Construction of the binary, ternary and quaternary COFs and (b) their stacking modes
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b)
shown in Scheme 1, COFs 1-6-iPr and 1-6-Et were synthesized
the 13C CP-MAS NMR spectra, isopropyl group signals appeared
at 23 and 31 ppm, ethyl group signals appeared at 14 and 25 ppm
by reacting triamine Pr21 (60 mg, 0.1 mmol) or Et (52 mg, 0.1
i
mmol ) with di- or tri-aldehyde (0.15 mmol or 0.1 mmol )in EtOH
in the presence of acetic acid (9 M, 0.2 mL) at 120 C for three
days, which afforded yellow crystalline solids in 74-95% yields
(Scheme 1). All COFs are stable in common organic solvents.
and the characteristic signals due to C=N bonds were observed at
162 ppm (Figure S2). The aldehyde carbon peaks were barely
present. The chemical shifts of other fragments are consistent with
those of the monomers. Thermal gravimetric analysis revealed that
those COFs exhibited no weight loss under N2 on heating to 380
oC (Figure S3). Transmission electron microscopy (TEM) images
of the COFs reveal that the materials crystallize in the form of
platelets with very large domain sizes of 100-300 nm, indicative
of the 2D nature of these structures (Figure S5).
o
i
In the FT-IR spectra of the series of Et- and Pr-COFs, the
characteristic C=O stretching bands (~1689 cm-1) almost
disappeared, indicative of the consumption of the aldehydes
(Figure S1). Strong stretching vibration bands attributed to the
new generation of C=N linkages were observed at ~1631 cm-1. In
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b)
e)
c)
f)
Figure 1. PXRD patterns of COFs (a) 1-iPr (b) 4-iPr and (c) 5-iPr with the experimental profiles in black, Pawley-refined profiles in red,
calculated profiles in blue, and the differences between the experimental and refined PXRD patterns in dark cyan. Top (left) and side (right)
views of the corresponding refined 2D crystal structures of (d) 1-iPr (e) 4-iPr and (f) 5-iPr.
Crystal Structure. The crystalline structures of the COFs were
determined by powder X-ray diffraction (PXRD) analysis with Cu
Kα radiation. As revealed from PXRD analyses, COFs 1-iPr and
2-iPr show high crystallinity, exhibiting the first intense peak at a
low angle 4.9° (2θ), which corresponds to the (110) reflection
plane, along with minor peaks at 8.4°, 11.4°, 12.5°, 14.5°, and
17.8°, attributed to the (300), (20 ), (211), (131) and (241)
reflection planes, respectively (Figures 1a, S14). For 3-iPr and 4-
iPr, the first and most intense peak corresponding to the (110)
reflection plane appears at ∼4°, with other minor peaks at 7.0°,
8.0°, 10.7°,12.9° and 16.1° attributed to the (300), (220), (20 ),
(131), and (440) reflection planes, respectively (Figures 1b, S14).
COF 5-iPr gave strong PXRD peaks at 5.5°, 11.1°, 14.7°, 17.6°,
and 19.3°, corresponding to the (100), (101), (20 ), (211) and
(220) reflection planes, respectively (Figure 1c), and 6-iPr gave
strong PXRD peaks at 4.8°, 8.4°, 9.7°, 10.9°, 11.9°, 12.8° and
13.8°, corresponding to the (100), (110), (200), (001), (101), (120)
and (111) reflection planes, respectively (Figure S14).
In order to elucidate the structures of the COFs and calculate
the unit cell parameters, three types of possible 2D structures were
generated for each of them, that is, eclipsed stacking (AA),
staggered stacking (AB and ABC) models were built and
optimized by the Materials Studio Forcite molecular dynamics
module method (Figure S15). It is found that the total crystal
stacking energy of ABC stacking (116.52 kcal mol-1) for COF 1-
iPr is much higher
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c)
20 M NaOH (100 C)
a)
b)
1 M NaOH (100 C)
0.1 M HCl (r.t.)
Boiling water
0.1 M HCl (r.t.)
1 M NaOH (r.t.)
Boiling water
0.1 M HCl (r.t.)
Pristine 1-H
5
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Boiling water
20 M NaOH (100 C)
20 M NaOH (r.t.)
Pristine 1-iPr
Pristine 1-Et
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0.1 M HCl (r.t.)
Boiling water
2 Theta (degree)
2 Theta (degree)
20 M NaOH (100 C)
f)
4 M NaOH (r.t.)
Boiling water
1 M HCl (r.t.)
e)
d)
0.1 M HCl (r.t.)
Boiling water
20 M NaOH (r.t.)
Pristine 5-Et
20 M NaOH (100 C)
Pristine 5-iPr
Pristine 5-H
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2 Theta (degree)
2 Theta (degree)
2 Theta (degree)
h)
i)
g)
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Pristine 5-iPr
5-iPr (NaOH)
5-iPr (Boiling water)
5-iPr (HCl)
Pristine 1-iPr
1-iPr (NaOH)
1-iPr (HCl)
1-iPr (Boiling water)
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400
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Pristine 4-iPr
4-iPr (HCl)
Ir-4-iPr
4-iPr(Boiling water)
4-iPr(NaOH)
After 10 catalytic runs
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Pore Width (nm)
Pore Width (nm)
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Relative Pressure (P/P0)
Relative Pressure (P/P0)
Relative Pressure (P/P0)
Figure 2. PXRD patterns of COFs (a) 1-iPr (b) 1-Et (c) 1-H (d) 5-iPr (e) 5-Et (f) 5-H upon treatment in different solvents for 7 days. N2
adsorption-desorption isotherms (77 K) and pore size distribution profiles (insert) of the COFs (g) 1-iPr (h) 4-iPr(i) 5-iPr after treatment in
boiling water, 0.1 M HCl and 20 M NaOH solutions at 100 oC for 7 days.
than those of the AA (44.67 kcal·mol-1) and AB (70.33 kcal·mol-1)
stacking, and the total crystal stacking energy of AB stacking for
COF 5-iPr (119.40 kcal·mol-1) is much higher than those of AA
(37.51 kcal·mol-1) and ABC (73.21 kcal·mol-1) stacking. Similar
behaviors were found in COFs 2-4-iPr and 6-iPr (Table S4). The
experimental PXRD patterns for 1-4-iPr match well with the
simulated patterns of the staggered stacking (ABC) model in the
trigonal R-3 space group (Figure 1). However, the models
proposed for PXRD patterns of 5- and 6-iPr agreed well with the
simulated pattern generated from the staggered stacking (AB) of
the 2D layers in the trigonal P-3 space group (Figures 1c and S14).
Pawley refinements gave optimized parameters (a = b = 36.40 Å
and c = 9.21 Å for 1-iPr; a = b = 36.23 Å and c = 9.05 Å for 2-iPr;
a = b = 44.03 Å and c = 9.03 Å for 3-iPr; a = b = 43.80 Å and c =
9.04 Å for 4-iPr), which provided good agreement factors (Rp =
4.48% and Rwp = 5.76% for 1-iPr; Rp = 3.57% and Rwp = 5.54%
for 2-iPr; Rp = 2.39% and Rwp = 3.73% for 3-iPr; Rp = 1.03% and
Rwp = 1.60% for 4-iPr). Similarly, unit cell parameters were
obtained for 5-iPr (a = b = 18.39 Å, c = 9.19 Å) and 6-iPr (a = b =
21.09 Å, c = 8.03 Å), with acceptably low residuals (Rp = 5.07%
and Rwp = 7.04% for 5-iPr, Rp = 4.12% and Rwp = 5.66% for 6-iPr).
COFs 1-4-Et showed the isomorphic structures to 1-iPr while 5-
and 6-Et preferred AB stacking (Figures S14 and S15).
The porosity of these COFs was examined by measuring N2
sorption isotherms at 77 K on the activated samples. The
adsorption curves of them exhibited type-I isotherm (Figures 2
and S11),
Brunauer-Emmett- Teller (BET) surface areas of them range from
355 to 1197 m2·g-1 (Table 1). The nonlocal density functional
theory (NLDFT) gave rise to a narrow pore size distribution for
COFs 1-6-iPr/Et, respectively, in good agreement with the
simulated values (Table 1 and Figures 2 and S11).
a characteristic of microporous materials. The
Chemical Stability. The chemical stability of the COFs was
examined by PXRD and N2 sorption isotherms after 7 days
treatment in boiling water, 0.1 M HCl at r.t and 20 M NaOH (r.t or
100 oC). All binary COFs are stable in boiling water, as evidenced
by the almost unchanged PXRD patterns (Figures 2 and S6), BET
surface areas (Figure 2 and Table 1) and residue weight
percentage (Figure S13). In 0.1 M HCl solution, all these COFs
retained their original structures, but gave a slightly decreased
crystallinity. A small decrease in signal-to-noise ratio and a slight
to moderate decrease in the surface area of them indicate just
partial structural collapse upon treatment (Table 1). Remarkably,
i
o
all Pr-COFs were stable in 20 M NaOH solution at 100 C and
retained good crystallinity. The as-treated samples showed BET
surface areas close to or higher than the pristine samples. The
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increase in surface areas of 2-4-iPr and 6-iPr may be attributed to
and AB stacking is favorable for the combination of C3- and C3-
1
2
3
4
5
6
7
8
the degradation of oligomers trapped in the networks in strong
alkaline solutions, which were not easy to be removed by common
solvent washing or vacuum pump activation. Therefore, the Pr-
COFs show the ultra-strong alkali-resistance ability and the
modest level of acid-resistance. This is consistent with that
protons have a smaller radius than hydroxyl ions (2.8 vs 3.0 Å)
and can easily attack and decompose dynamic C=N bonds.
symmetric monomers. Take a deep look inside the molecular
i
arrangement in each Pr unit, the two central aromatic cores of
i
i
adjacent layers overlapped in a manner that one Pr core rotated
60° around the c-axis (Figure 3). Each of the three isopropyl-arms
i
in Pr rotated around the central core about 45°, allowing for
enough space between the bulky groups while maintaining a
close-packed orientation of the backbone. Similar behaviors were
observed for the Et-based COF. Confirming our hypothesis, the
steric effect of the building blocks defines the position of each
building unit within adjacent COF layers, which guides the
attachment of successive layers in a AB or ABC stacking
manner.8
Table 1.BET surface areas of the COFsa
BET surface areas (m2/g)
Pore
9
COF
width
(nm)
H2O
HCl
(0.1 M)
NaOHb
(20 M)
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Pristine
(100 oC)
For the C3 + C3 combination, the AB stacking enables the
adjacent layers to be precisely arranged to fulfill the -
interactions, whereas no - interactions are involved in the ABC
stacking. Crystal stacking energy for different stacking modes was
calculated to further validate the stacking preference. The
calculated total crystal stacking energy for the AB stacked 5-iPr
(119.40 kcal/mol) is higher than that of the possible ABC and AA
ones (73.21 and 37.51 kcal/mol, respectively), supporting the
observed AB stacking in 5-iPr. For the C2 + C3 combination,
although - interactions exist in both AB and ABC stacking, the
density of phenyl pairs with - interactions for ABC stacking is
calculated to be 2.84 times higher than that of AB stacking (Figure
S22), in agreement with the stacking mode of 1-iPr . Table S4
showed that the calculated total crystal stacking energy for the
ABC stacked 1-iPr (116.52 kcal/mol) is higher than those of the
possible AB and AA ones (70.33 and 44.67 kcal/mol,
respectively), supporting that ABC stacking instead of AA and
AB was adopted by 1-iPr.
1-iPr
2-iPr
3-iPr
4-iPr
5-iPr
6-iPr
1-Et
2-Et
3-Et
4-Et
970
1000
718
960
942
700
1055
976
797
752
755
342
423
906
956
594
1130
863
786
277
119
133
221
967
1061
835
1437
997
940
792
722
296
481
1.00
0.82
1.29
1.26
0.72
0.81
0.93
0.81
1.15
1.17
1197
1068
824
768
755
355
494
5-Et
6-Et
567
688
513
649
443
344
560
702
0.69
0.68
aAfter treatment under different conditions for 7 days. bTreated at 100 C
o
for 1-6-iPr and at r.t. for 1-6-Et.
Compared with COFs 1-6-iPr, 1-6-Et gave decreased
chemical stability and porosity. For example, both 1- and 5-Et are
stable in boiling water and 0.1 M HCl at r.t., but lost crystallinity
o
when the treated temperature was increased from r.t to 100 C
(Figures 2b, 2e and S7). In sharp contrast, the analogous COFs
containing no alkyl groups gave low chemical stability. For
example, COF 1-H was stable in 0.1 M HCl at r.t, but after
soaking in boiling water and 1 M NaOH solution, the sample was
partly dissolved and got amorphous (Figure 2c). In COFs 1-6-
iPr/Et, the bulky isopropyl and ethyl groups were positioned near
nitrogen atoms that can protect C=N bonds and hydrolytically
susceptible backbones through kinetic blocking, leading to higher
chemical stability than the nonalkylated 1-6-H. However, the
substituted ethyl groups were not so hydrophobic and bulky as
that of the isopropyl groups to protect the susceptible backbones,
and so COFs 1-6-Et gave slightly lower chemical stability than 1-
6-iPr. As far as we know, with structural ultrastability in basic
Figure 3. Top (left) and side (right) views of the overlapping the
iPr cores between adjacent layers in COF 2-iPr.
Tuning of Layer Stacking and Chemical Stability. Although
the nonalkylated COFs 1-6-H adopted AA stacking,20 their
analogues alkylated 1-4-iPr/-Et adopted ABC stacking and 5- and
6-iPr/-Et adopted AB stacking. To further study the influence of
alkyl substituents on layer stacking, three series of ternary COFs
2-H1-x-iPrx/-Etx and 2-Et1-x-iPrx containing different molar ratios
of two triamine monomers were prepared (Scheme 1).
Remarkably, PXRD patterns of 2-H1-x-iPrx are changed with
i
molar ratios of Pr to H. When x>8/9, the diffraction peaks were
o
solutions up to 20 M NaOH solution at 100 C for 7 days, COFs
found at 4.9°, 8.5°, 11.4°, 12.5°, 14.5°, and 17.8°, attributed to the
(110), (300), (20 ), (211), (131) and (241) reflection planes,
respectively, indicating the generation of the ABC stacked product
inherited from 2-iPr (Figures 4a and 4e). However, when
1/9<x<8/9, two sets of diffraction peaks due to a mixture of AA
1-6-iPr not only top the list of COFs, but also surpass all reported
MOFs22 and molecular sieves (ZSM-5,23 MCM-41,23 SBA-1524
etc) regarding alkali resistance.
Among hundreds of 2D COFs that have been constructed, only
a handful of them adopt AB or ABC stacking.10,11 Influences of
substitutes on stacking modes have been reported in a 2D COF, in
which methyl groups extending out-of-plane of layers and the
inclined stacking mode was obtained.7a In this work, when the
bulky alkyl substituents are attached to the COFs, both AB and
ABC stacking models are more steric favorable than AA stacking,
thus to avoid the close repulsion between adjacent layers. As
shown in Scheme 1, we hypothesized that ABC stacking is
favorable for the combination of C2- and C3-symmetric monomers,
and ABC stacked COFs were detected. Fox example, in 2-H1/2
-
iPr1/2 (Figure 4e), one set of peaks due to the ABC stacked
product was observed at 4.9°, 8.5°, 11.4°, assigned to the (110),
(300), (20 ) facets, respectively. Another set of peaks due to the
AA stacked product was seen at 2.9°, 4.8°, 5.8°, 7.6° and 9.86°,
assigned to the (100), (110), (200), (210) and (220) facets,
respectively. When 0<x<1/9, only one set of peaks due to the AA
stacking product was observed at 2.9°, 4.8°, 5.8°, 7.6° and 9.86°,
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assigned to the (100), (110), (200), (210) and (220) facets (Figures
1
4a and 4e).
2
3
4
5
a)
c)
d)
b)
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f)
e)
Figure 4. PXRD patterns and pore size distributions showing the stacking transformation from AA to ABC/AB stacking in the MTV
COFs. (a) PXRD patterns for 2-H1-x-iPrx and (c) 5-H1-x-iPrx. Pore size distribution of COFs (b) 2-H1-x-iPrx and (d) 5-H1-x-iPrx. PXRD
peaks indexing for (e) COFs 2-Hx-iPr1-x and (f) 5-Hx-iPr1-x
COFs 2-H1-x-Etx revealed a similar tendency (Figure S7). For
x>1/2 and 0<x<1/9, ABC and AA stacking of the layers were
adopted, respectively, and for 1/9<x<1/2, both AA and ABC
stacking of the layers were obtained individually. Pore size
distributions calculated from N2 sorption isotherms were
consistent with the results of PXRD (Figure 4b), SEM (Figure
means the path to one stacking manner may be blocked, and only
other stacking models that can significantly alleviate steric
repulsion can be generated. In the case of 2-H1-x-iPrx/Etx, the
i
portion of Pr (or Et) mainly determine the level of steric
repulsion between layers. In the high level of steric repulsion
(8/9<x<1 and 1/2<x<1, respectively), the COFs adopted the ABC
staking mode inherited from 2-iPrx and 2-Et, since the locking
and docking sites generated from ABC stacking greatly release the
repulsion. In the low degree of steric repulsion (0<x<1/9 for both
cases), they preferred to AA stacking inherited from 2-H, which
can generate big mesopores and provided enough space for the
low density of alkyl groups to rotate flexibly to alleviate steric
repulsion. In the moderate level of repulsion (1/9<x<8/9 for 2-H1-
x-iPrx, 1/9<x<1/2 for 2-H1-x-Etx), the route to generate the single
AA or ABC stacked products was prohibited, because
considerable steric repulsion still existed to prevent the formation
of eclipsed AA stacking of layers. Staggered ABC stacking was
also excluded, due to the fact that, to build well-defined periodic
S4o-4r) and 1H NMR (Figure S10) obtained from digested 2-H1-x
-
iPrx/Etx confirmed that both Pr (or Et) and H existed in the
products. In contrast, all three COFs 2-Et1/9-iPr8/9, 2-Et1/2-iPr1/2
and 2-Et8/9-iPr1/9 exhibited almost the same PXRD patterns as
those of 2-iPr and 2-Et, indicating the formation of the ABC
stacked products (Figure S8). Therefore, through mediation of
i
i
different ratios of Pr (or Et) to H with M2, the resulting 2-H1-x
-
iPrx/-Etx can evolve to different stacking manners, from ABC, via
a transitional period of mixtures of ABC and AA, to the single AA
stacking. To the best of our knowledge, this is the first report on
the stacking mode transformation in 2D COFs.10-12
Previous studies showed that, if suitable substituents are
introduced into the skeleton of 2D COFs, steric repulsion between
the substituents of layers will affect the COF structures.7a,25 That
i
docking sites, more Pr (or Et) groups were needed to maintain a
close-packed orientation of the periodic and intact backbone. As a
result, two COFs with AA and ABC stacking were obtained
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individually. The SEM images of 2-H1-x-iPrx (Figures S4o-4r) are
phases AA (0<x<1/9) and ABC (8/9<x<1), or the existence of two
1
informative towards the phase purity, further confirming the pure
separated phases of AA and ABC (1/9<x<8/9).
2
3
4
5
6
7
a)
c)
b)
3M HCl (r.t.)
3 M HCl (r.t.)
Boiling water
3 M HCl (r.t.)
Boiling water
Boiling water
8
9
20 M NaOH (r.t.)
Pristine 2-H8/9-Et1/9
20 M NaOH (r.t.)
20 M NaOH (r.t.)
10
11
12
13
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Pristine 2-H8/9-iPr1/9
Pristine 2-H8/9-Et1/18-iPr1/18
5
10
15
20
25
30
5
10
15
20
25
30
5
10
15
20
25
30
2 Theta (degree)
2 Theta (degree)
2 Theta (degree)
d)
f)
e)
1 M HCl (r.t.)
1 M HCl (r.t.)
Boiling water
1 M HCl (r.t.)
Boiling water
Boiling water
20 M NaOH (r.t.)
20 M NaOH (r.t.)
20 M NaOH (r.t.)
Pristine 5-H8/9-iPr1/9
Pristine 5-H8/9-Et1/9
Pristine 5-H8/9-Et1/18-iPr1/18
5
10
15
20
25
30
5
10
15
20
25
30
5
10
15
20
25
30
2 Theta (degree)
2 Theta (degree)
2 Theta (degree)
Figure 5. PXRD patterns the MTV-COFs (a) 2-H8/9-iPr1/9 (b) 2-H8/9-Et1/9 (c) 2-H8/9-Et1/18-iPr1/18 (d) 5-H8/9-iPr1/9 (e) 5-H8/9-Et1/9 and (f) 5-
H8/9-Et1/18-iPr1/18 upon treatment in different solvents for 7 days.
To further study the influences of steric effects on the 2D
structure, we prepared ternary COFs 5-H1-x-iPrx/-Etx and 5-Et1-x
interactions. In addition, COFs 2-H1/9-iPr8/9/Et8/9 and 2-H1/9-Et4/9
iPr4/9 gave similar chemical stability to the binary COF-2-iPr/Et
(Figure S9). Meanwhile, both COFs 5-H8/9-iPr1/9/Et1/9 and 5-H8/9
-
-
iPrx by varying the molar ratios of different triamine monomers
undersolvothermal reaction conditions (Scheme 1). PXRD showed
that, the ternary layer structures adopted AA and AB stacking for
0<x<1/5 and x>4/5, respectively, and a mixture of AA and AB
stacking for 1/5<x<4/5 (Figures 4c, 4d, 4f and S7), whereas, as
expected, all COFs 5-Et1-x-iPrx (Figure S8) adopted only AB
stacking. Also, we prepared quaternary COFs 2- and 5-H1-x-y-Etx-
iPry. Depending on the ratios of different monomers, the MTV
COFs 2-H1-x-y-Etx-iPry can adopt AA, ABC or a mixture of AA
and ABC stacking, and 5-H1-x-y-Etx-iPry adopt AA, AB or a
mixture of AA and AB stacking (Figure S8). Taken together, the
above findings indicated the stacking modes of the 2D COFs are
determined by the interlayer repulsion and can be controlled by
carefully tuning the concentration of steric monomers.
-
-Et1/18-iPr1/18 exhibiting comparable chemical stability to 5-Et, but
much improved water and base stability compared with 5-H
(Figures 5d, 5e, 5f). For example, these MTV COFs retained high
crystallinity and porosity in boiling water and 20 M NaOH
solution at r.t., whereas the binary COF 5-H became amorphous
quickly in boiling water and 4 M NaOH solution (Figure 2f). This
enhanced stability may be ascribed to the fact that the bulky
isopropyl or ethyl groups in the MTV COFs can protect
hydrolytically susceptible imine bonds through kinetic blocking. It
is likely that the MTV strategy can be used not only to guide the
stacking mode of 2D COFs, but also to control the chemical
stability, which was determined by the nature of the main
component of building blocks.
The chemical stability of the MTV COFs was also tested in
acid, base and boiling water by PXRD and N2 sorption isotherms.
As shown in Figure 5, both ternary COFs 2-H8/9-iPr1/9/Et1/9 and
quaternary COFs 2-H8/9-Et1/18-iPr1/18 can retain their crystallinity
and porosity in boiling water, 20 M NaOH solution at r.t and 3 M
HCl solution at r.t (Figures 5a, 5b and 5c), which is about 30 times
in resistance against HCl solution than that of COFs 2-iPr/Et. The
improved acid stability may be due to the introduction of H
monomers, which induce the layer stacking from ABC to AA to
strengthen the interlayer interactions. However, the acid stability
is lower than that of COF 2-H (stable in 12 M HCl), probably as a
consequence of steric interactions between the bulky pendent
isopropyl/ethyl and methoxyl groups that decrease interlayered
Heterogeneous Catalysis. We have employed COFs 4-iPr/-Et/-H
for heterogeneous catalysis by taking advantage of their 2, 2’-
bipyridine open sites. 2,2’-bipyridine and its derivatives have been
widely used as chelating ligands for forming metal complexes in
coordination chemistry.26 For example, Iridium–bipyridine
complexes are highly active catalysts for many meaningful
reactions including C–H borylation of arenes and heteroarenes.27
However, such homogeneous catalysts have very open
coordination environments and are prone to deactivation via
intermolecular pathways. Therefore, to inhibit deactivation
pathways, Ir-bipyridine complexes have been immobilized on
diverse porous solid supports such as silica28 and metal-organic
frameworks (MOFs).29
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Metalation of a microcrystalline powder of 4-iPr/-Et/-H with
respectively, and catalyzed by Ir-4-H produced 76%, 81% and
1
2
3
4
5
6
7
8
[Ir(COD)(OMe)]2 in THF at r.t affording light brown powders as
Ir-4-iPr/-Et/-H. Inductively coupled plasma optical emission
spectrometry (ICP-OES) analyses revealed that the Ir loadings
were 4.1 wt % for Ir-4-iPr, 6.5 wt % for Ir-4-Et, and 4.9 wt % for
Ir-4-H, which are consistent with Ir coordination to 13.24, 19.83,
and 11.36% of the total bipyridine sites, respectively. This
indicates that only part of pores in the frameworks is occupied by
Ir, ensuring the open channels for catalysis. After optimization of
reaction conditions, Ir-4-iPr was found to be an active catalyst for
dehydrogenative borylation of aromatic C-H bonds using B2(pin)2
(pin = pinacolate) as the borylating agent to provide aryl
boronates,which are versatile reagents in organic synthesis.28 As
shown in Table 2, in the presence of 0.4 mol% Ir-4-iPr, benzene
and thiophene were borylated to give products in 100 % yield at
100 °C for 24 h (Table 2, entries 1 and 14). Pure borylated
products can be readily obtained by removing the solid catalyst
via centrifugation followed by removal of the volatiles. Halogen
and alkoxy functional groups were well tolerated under the
reaction conditions (Table 2, entries 5, 9, 13). The borylation
occurred at the least sterically hindered C-H bonds of arenes
(Table 2, entries 5, 9, 13) and tended to activate 2-position of C-H
bonds in heteroarenes (Table 2, entries 15 and 16).
79% yields. All yields are much lower than those obtained with
Ir-4-iPr, as shown in Table 2 (entries 2, 3, 6, 7, 10 and 11).
Therefore, the above results showed that the catalytic activities of
the three COFs are in the order Ir-4-iPr > Ir-4-Et > Ir-4-H. This
trend could be explained in the following two reasons. Firstly,
compared with the eclipsed stacked Ir-4-H, the ABC packing of
Ir-4-Et/iPr has divided the large hexagonal open channels into
smaller triangular micropores with more suitable pore widths.
Indeed, higher surface areas were recorded for Ir-4-Et/iPr (336
and 1084 m2/g, respectively) compared with Ir-4-H (217 m2/g).
The transformation of pore shapes via steric tuning may expose
more readily accessible active sites for reactants and more tailored
channels for the diffusion of substrates and products, thereby
leading to increased activities. Secondly, it is suggested that the
different crystallinity of the COFs may contribute to the contrast
catalytic performances. After catalysis, Ir-4-iPr remained highly
crystalline, as confirmed by the slight drop of surface area to 972
m2/g (Table S8). On the other hand, with the surface areas being
only 26 and 68 m2/g, respectively, Ir-4-H/Et exhibited obviously
decreased crystallinity compared to the pristine samples (Figure
S6). Control experiments showed that amorphous Ir-4-iPr/Et
indeed afforded much lower yields than the crystalline
counterparts in catalyzing the borylation reactions (Table 2,
entries 1, 2, 5 and 6). It is not surprising that the solids with low or
no crystallinity interrupt the uniform distribution of the Ir-bipy
units and provide a decreased number of accessible active sites for
the reactants, thus showing less effectiveness for the
transformations.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 2. Ir-COF Catalyzed C-H Borylation of Arenesa
O
O
0.4 mol% Ir-COF
H2
Ar Bpin
Ar
H
B B
n-heptane
O
O
100 o
catalyst
C
B2(pin)2
product
yield (%)b
entry
time (h)
Significantly, Ir-4-iPr is much more active than the
1
2
3
4
Ir-4-iPr
Ir-4-Et
Ir-4-H
24
24
24
48
100 (51)c
85 (48)c
76
homogeneous counterpart in borylation of arenes. For example, at
Bpin
o
100 C, 0.4 mol% Ir(bipy)(COD)(OMe) (Ir-bipy) afforded 61%
yield of the product in the borylation of benzene after 48 h (Table
2, entry 4) and then no further conversion of arene was detected
with prolonged heating time. The higher activity of Ir-4-iPr is
likely due to active site isolation which prevents any
intermolecular deactivation pathways. Moreover, Ir-4-iPr
displayed catalytic activities and regioselectivities that are
comparable to or surpass those reported Ir(bpy)-based
heterogeneous catalysts (Table S5).30 The catalyst Ir-4-iPr can be
reused at least ten times in the borylation of 3-(trifluoromethyl)
anisole without loss of catalytic activity (96-100% yield, Table
S7). No further conversion of arene was detected after removal of
the solid catalyst from the reaction mixture. ICP-OES analysis of
the filtrate after the reaction revealed almost no leaching of Ir ions
(~0.0016%). After ten cycles, the recovered catalysts remained
high crystallinity and porosity (BET = 750 m2·g-1, Figure 2h).
Ir-bipy
61d
24
24
24
24
100 (76)c
88 (74)c
81
MeO
Ir-4-iPr
Ir-4-Et
Ir-4-H
5
6
7
8
Bpin
80d
F3C
Cl
Ir-bipy
Ir-4-iPr
Ir-4-Et
24
24
100
84
9
10
11
12
Bpin
Bpin
Ir-4-H
24
24
79
63d
Cl
Ir-bipy
Cl
13
Ir-4-iPr
24
100
100
Cl
S
Bpin
14
15
Ir-4-iPr
Ir-4-iPr
24
24
CONCLUSION
O
100
We have designed and synthesized a series of two-, three- and
four-component 2D COFs decorated with sterically hydrophobic
groups by imine condensation of triamine and di- or trialdehyde
building blocks. Modulating the interlayer steric hindrance
through controlling concentrations of alkyl groups, which can be
readily tuned by using a MTV approach, allows access to 2D
COFs with AA, AB or ABC stacking and moderate to high
chemical stability. Ir-4-iPr exhibits much higher catalytic activity
than Ir-4-Et/H in C-H borylation of arenes because of the
increased porosity and chemical stability. Further studies on the
mechanism that how and why steric hindrances can guide the
COFs sheets lock into a position during crystal growth to generate
a certain stacking mode and deep insight into stacking mode
transformation are greatly needed. This work provides a simple
Bpin
Bpin
o:m = 92:8
N
16
Ir-4-iPr
24
95
o:m = 90:10
aReaction conditions: 0.4 mol % Ir loading, 0.52 mmol B2pin2, 1.0 mmol
of arene, 2.0 mL of n-heptane, 100 °C, reflux under N2. For entries 1-4, 14
b
1
c
and 15, neat arene was used. Calculated by H NMR. Catalyzed by 0.4
mol% the amorphous COF. dCatalyzed by 0.4 mol% Ir(bipy)(COD)(OMe)
as the homogeneous control.
Compared with Ir-4-iPr, Ir-4-Et/-H displayed obviously
decreased catalytic activities in borylation of arenes. For example,
under otherwise identical conditions, borylation of benzene, 3-
(trifluoromethyl)anisole and 1,3-dichlorobenzene catalyzed by Ir-
4-Et afforded 85%, 88% and 84% yields of the products,
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approach to control layer stacking and chemical stability for 2D
S.; Perepichka, D. F. J. Am. Chem. Soc. 2017, 139, 2421. (c) Peng,
1
2
3
4
5
6
7
8
COFs, holding great promise for efficiently preparing a wide
range of novel 2D COFs that will display interesting electronic,
optic and catalytic properties.
Y.; Huang, Y.; Zhu, Y.; Chen, B.; Wang, L.; Lai, Z.; Zhang, Z.;
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ASSOCIATED CONTENT
Experimental procedures and characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
9
Corresponding Author
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*yongcui@sjtu.edu.cn
*liuy@sjtu.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was financially supported by the National Science
Foundation of China (Grants 21431004, 21522104 and
21620102001), the National Key Basic Research Program of
China (Grant 2016YFA0203400), Key Project of Basic Research
of Shanghai (17JC1403100 and 18JC1413200), and the Shanghai
“Eastern Scholar” Program.
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