Synthesis of 2D Imine-Linked Covalent Organic Frameworks through Formal Transimination Reactions

Article abstract of DOI:10.1021/jacs.7b06913

Covalent organic frameworks (COFs) are crystalline, permanently porous, two-dimensional or three-dimensional polymers with tunable topology and functionality. COFs linked with imines or β-ketoenamines are more chemically stable than their boron-linked cou

Full text of DOI:10.1021/jacs.7b06913

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Communication
Synthesis of 2D Imine–Linked Covalent Organic
Frameworks through Formal Transimination Reactions
Edon Vitaku, and William R. Dichtel
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06913 • Publication Date (Web): 30 Aug 2017
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Synthesis of 2D Imine–Linked Covalent Organic Frameworks
through Formal Transimination Reactions
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Edon Vitaku and William R. Dichtel*
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
Supporting Information
ABSTRACT:
Covalent organic frameworks (COFs) are crystalline,
permanently porous, two-dimensional or three-dimensional polymers
with tunable topology and functionality. COFs linked with imines or b-
ketoenamines are more chemically stable than their boron-linked
counterparts, making them more promising for a broad range of appli-
cations, including energy storage devices, proton-conductive mem-
branes, and catalyst supports. We report a general and scalable method
for synthesizing imine- and b-ketoenamine-linked COFs based on the
formal transimination of N-aryl benzophenone imines. These sub-
strates are often the synthetic precursors of traditional polyfunctional
aryl amine monomers and are more stable, soluble, and easy to handle
and purify. The imine- and b-ketoenamine-linked COFs obtained from
this approach show excellent materials quality, as characterized by x-ray
diffraction and surface area analysis. The most optimized COF exhibit-
2
ed a Brunauer–Emmett–Teller surface area (>2600 m /g) very close to
2
its theoretical value (2830 m /g). This method is amenable to both
conventional solvothermal conditions and microwave heating, provid-
ing similar or even improved materials quality with shorter reaction
times. The high materials quality, scalability, and availability of benzo-
phenone imine monomers are all attractive features of this approach.
Figure 1. Comparison of the synthesis of imine-linked 2D COFs from
polyfunctional aryl amine monomers (left) and the corresponding
benzophenone imines (right).
1
Covalent organic frameworks (COFs) are porous, crystalline net-
works made from light elements such as H, B, C, N, and O. COFs offer
well-defined solid-state structures and tunable pore sizes that emerge
from the topology and shape of their molecular building blocks. The
first reported COFs were linked by boronate esters and boroxines, but
these linkages are easily hydrolyzed or oxidized. Therefore, imine-
linked and other nitrogen-containing COFs show greater promise for
emerging applications, such as energy storage, catalysis, and drug
delivery. Imine-linked COFs are typically prepared through the con-
densation of aryl amines and aldehydes under Brønsted acid-catalyzed
solvothermal conditions. We also recently reported that metal triflate
catalysts also provide imine-linked COFs with excellent crystallinity
benzophenone imines are typically more soluble, less prone to oxida-
tion, and more easily purified. Their use in COF synthesis also pre-
cludes the need for a separate deprotection step. We prepared both
imine-linked and b-ketoenamine-linked two-dimensional (2D) COFs
from benzophenone-imine protected monomers. These 2D COFs
exhibit excellent crystallinity and Brunauer–Emmett–Teller (BET)
surface areas higher than networks derived from the corresponding aryl
amines. Our method is compatible with both traditional solvothermal
conditions and microwave irradiation, which require shorter reaction
times. These results demonstrate that N-aryl benzophenone imines are
desirable monomers, particularly when incorporating synthetically
advanced building blocks into these periodic polymer networks.
2
3
4
5
6
and high surface areas under exceptionally mild conditions.
The limited availability and poor oxidative stability of many poly-
functional aryl amines are major stumbling blocks for the design and
synthesis of new imine-linked COFs. Monomers that are not commer-
cially available are commonly prepared via a Buchwald–Hartwig cou-
pling of aryl halides and benzophenone imine. The resulting N-aryl
benzophenone imines are then hydrolyzed to the corresponding free
amines. Recognizing that imine-linked COFs crystallize under condi-
tions that promote dynamic imine exchange, we hypothesized that
N-aryl benzophenone imines are competent monomers for COF for-
mation (Figure 1). Compared to the corresponding free amines, N-aryl
1
,3,5-Triformylbenzene ( ) and N-benzidine benzophenone imine
1
(
2
) were condensed under solvothermal conditions to obtain an imine-
linked network, BND-TFB COF (Table 1). This COF was selected
2a
7
because it has been prepared previously from benzidine (
prone to oxidation, with a maximum BET surface area (SBET) of 1496
3
), which is
2
5b,9
m /g. A deoxygenated mixture of
1,4-dioxane, and 6M aqueous CH CO
nitrogen atmosphere and heated to 120 °C for 24 h. The resulting
1
and
2
2
in a mixture of mesitylene,
H (3:3:1) was sealed under a
8
3
BND-TFB COF was activated by washing with supercritical CO
2
,
2
10
giving an SBET of 1938 m /g (entry 1, see SI for a pictorial guide).
Extending the reaction time to 5 days (entries 2–4) increased the SBET
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to 2618 m /g, which is among the highest reported values of a 2D
COF. The powder x-ray diffraction pattern of the isolated material is
consistent with an approximately eclipsed model structure (Figure 2A–
Table 1. Evaluation of BND-TFB COF synthetic procedures
2
C), which exhibits a calculated Connolly surface area of 2830 m /g
(Figure 2E and 2F). Scherrer analysis revealed a domain size of approx-
imately 25 nm (see Table S5 and Figure S66). Analysis of the N ad-
sorption isotherm using non-local density functional theory is con-
sistent with a narrow pore size distribution in good agreement with the
2
1
1
calculated pore size of 24 Å (Figure 2D). BND-TFB COF was also
1
3
characterized by infrared spectroscopy, elemental analysis, C cross-
polarization magic angle spinning NMR spectroscopy, and scanning
electron microscopy (see Supporting Information), which were con-
sistent with its expected structure and suggested its phase purity.
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Entry
Deviation From
Above Conditions
Time
S
BET
We evaluated modifications to the newly established conditions
i.e., entries 1–4) based on their impact on the SBET of the isolated
2
1
2
3
4
5
6
7
—
24 h
2 d
3 d
5 d
3 d
3 d
3 d
1938 m /g
(
2
BND-TFB COF, as surface area is a quantitative parameter that is high-
ly sensitive to the polymerization and activation conditions. These
experiments demonstrated that the BND-TFB COF may be activated
under dynamic vacuum instead of supercritical CO
thorough deoxygenation of the polymerization mixture is unnecessary
entry 6). When the reaction scale was increased 15-fold (entry 7), a
COF with no difference in crystallinity or surface area was isolated.
However, polymerizations conducted under reduced pressure, which is
common practice, yielded COFs with decreased surface area (entry 8).
These results suggest that the monomers and reaction intermediates
do not undergo noticeable oxidative degradation, which enables the
use of simple reaction setups that do not require degassing procedures.
We also extended this method to microwave irradiation, which allows
for COF formation of similar materials quality in shorter times (entries
-12). A 1 h reaction time yielded a material with a surface area of only
29 m /g (entry 9). This value increased to 2200 m /g (entry 12) after
h, which indicated an optimal irradiation time between 5 and 7 h.
—
2396 m /g
2
—
—
2314 m /g
2
2618 m /g
2
(entry 5), and that
2
vacuum activation
under air
2313 m /g
(
2
2327 m /g
2
15× scale,
vacuum activation
2321 m /g
2
8
9
under vacuum
3 d
1 h
1677 m /g
2
microwave, under air
microwave, under air
microwave, under air
microwave, under air
629 m /g
1
2
2
10
3
h
1558 m /g
2
1
1
5 h
7 h
3 d
3 d
3 d
2100 m /g
9
6
7
2
2
2
12
13
14
2200 m /g
2
3
3
used (instead of
used with 2 equiv
used, under air
2
)
1501 m /g
2
To determine whether the role of benzophenone is to modulate the
number of available amine functionalities through slow deprotection
or act as a competitor, we performed two control experiments. A
direct condensation between free benzidene (
4
1453 m /g
1
3
2
1
5
3
1381 m /g
14
) and was performed
3 1
in the absence (entry 13) or presence (entry 14) of 2 equiv of
Figure 2. BND-TFB COF. (A) Top view, (B) side view. (C) experimental, Pawley-refined, predicted powder X-ray diffraction patterns, and a differ-
ence plot (experimental pattern minus refined pattern), (D) pore size distribution, and (E) nitrogen isotherms (C-E correspond to Table 1, entry 3).
F) summary of the obtained BET surface areas through microwave heating (Table 1, entries 9-12), conventional heating (Table 1, entries 1-4), and
(
its calculated Connolly surface area.
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Table 2. COF Scope Evaluation under Conventional Heating and
Microwave Irradiation
benzophenone (
4
). Both afforded a COF with a lower SBET than COFs
derived from (entry 3). This result suggests that free benzophenone
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9
1
1
1
1
1
1
1
1
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1
2
2
2
2
2
2
2
2
2
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does not act as a competitor. The presence of air also has a minor effect
on the SBET (entry 15). Thus, we hypothesize that the slow deprotec-
tion of the benzophenone imine protecting groups, which modulates
the available free amine functionalities, is responsible for the enhanced
COF quality.
With the optimized conditions for both conventional heating (en-
tries 3 and 4) and microwave irradiation (entry 11) established, we
evaluated the synthesis of a second imine-linked linked COF (Table
2). The TAPB-PDA COF (entry 2), which has a pore size larger than
that of BND-TFB COF, was also isolated as crystalline network (Fig-
ure 3A) with a higher BET surface area (Figure 3C) than previously
reported.8 The pore size distributions for both samples agreed with
the theoretical values (Figure 3B, top). In addition, Fourier transform
infrared (Figure S70) spectra showed no evidence of benzophenone
contamination in any of the isolated COFs.
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, 15
We also evaluated the application of this method to b-ketoenamine-
linked COFs, which condense via a different mechanism than that for
2
imine-linked COFs. Imine-linked COFs undergo continuous error
correction because of the reversible nature of the imine bond in the
8
presence of aqueous acid. In contrast, b-ketoenamine-linked COFs
undergo two potentially reversible imine-bond-forming steps followed
by tautomerization to the b-ketoenamine structure after the third
1
6
imine is formed. Given the notable stability of the final COF materials
to aqueous acid, it is unlikely that these networks correct defects
through hydrolysis after this tautomerization has occurred, and b-
ketoenamine-linked COFs have not yet exhibited as high crystallinity
or SBET as their imine-linked counterparts. Nevertheless, their outstand-
3a,3c,17
ing stability makes these COFs of great applied interest.
Both
conventional heating and microwave irradiation yielded DAB-TFP and
BND-TFP COFs with BET surface areas higher than those of all previ-
ous reports (entries 3–4; see Figure 3C).1
2b, 18
The powder X-ray dif-
fraction patterns and pore size distributions of these materials were also
in good agreement with modeled structures (see Figure 3A and 3B).
The effectiveness of the benzophenone imine-based procedure for
these materials is consistent with our previous optimized synthesis of a
redox-active b-ketoenamine-linked COF, in which crystalline thin films
were only obtained upon slow addition of TFP to a solution of the
1
3
diamine. It is possible that the continuous deprotection of the benzo-
phenone imine-protected monomers achieves improved materials
quality through a similar effect.
In conclusion, we developed a practical, general, and scalable meth-
od based on benzophenone imine-containing monomers for preparing
nitrogen-linked COFs with BET surface areas superior to those of
traditional syntheses. This approach is amenable to conventional sol-
vothermal conditions or microwave irradiation and allows access to
high-quality COFs within reaction times as short as 5 h. The N-aryl
benzophenone imine monomers are typically more soluble and oxida-
tively stable than the corresponding free amines. This method was used
to obtain a BND-TFB COF with a BET surface area that closely ap-
proaches its theoretical value, narrow pore size distribution, and high
crystallinity. Although improved materials quality in COFs has been
achieved by incorporating fluorines19 or methoxy groups ortho to
aldehyde moieties, our results demonstrate that these design criteria
may be beneficial but not required to obtain high-quality COFs.
4d
ASSOCIATED CONTENT
Figure 3. Characterization data for COFs obtained under conventional
heating. (A) PXRD data, (B) pore size distributions, and (C) nitrogen
isotherms.
The Supporting Information is available free of charge on the ACS
Publications website.
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Experimental procedures, additional PXRD patterns, nitrogen iso-
therms, BET plots, pore size distributions, FTIR spectra, elemental
analyses, SEMs, NMR spectra, and pictorial guide to covalent organic
framework synthesis. (PDF)
1292-1294. (c) Lin, S.; Diercks, C. S.; Zhang, Y. B.; Kornienko, N.; Nichols, E.
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(
AUTHOR INFORMATION
Corresponding Author
8
P.; Zhao, L. Z.; Gao, Q.; Zhao, Y. L. Chem. Commun. 2016, 52, 4128-4131. (c)
Mitra, S.; Sasmal, H. S.; Kundu, T.; Kandambeth, S.; Math, K.; Diaz, D. D.;
Banerjee, R. J. Am. Chem. Soc. 2017, 139, 4513-4520.
*
wdichtel@northwestern.edu
(
6)
Marder, S. R.; Dichtel, W. R. J. Am. Chem. Soc. 2017, 139, 4999-5002.
7) Wolfe, J. P.; Ahman, J.; Sadighi, J. P.; Singer, R. A.; Buchwald, S. L.
Tetrahedron Lett. 1997, 38, 6367-6370.
8) Smith, B. J.; Overholts, A. C.; Hwang, N.; Dichtel, W. R. Chem.
Commun. 2016, 52, 3690-3693.
9) (a) Gao, Q.; Bai, L. Y.; Zeng, Y. F.; Wang, P.; Zhang, X. J.; Zou, R.
Matsumoto, M.; Dasari, R. R.; Ji, W.; Feriante, C. H.; Parker, T. C.;
ORCID
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Edon Vitaku: 0000-0002-0057-4245
William R. Dichtel: 0000-0002-3635-6119
(
Notes
(
The authors declare no competing financial interest.
(
ACKNOWLEDGMENT
Q.; Zhao, Y. L. Chem. - Eur. J. 2015, 21, 16818-16822. (b) Bai, L. Y.; Gao, Q.;
Zhao, Y. L. J. Mater. Chem. A 2016, 4, 14106-14110.
We acknowledge the Army Research Office for a Multidisciplinary
University Research Initiatives (MURI) award under grant number
W911NF-15-1-0447. W.R.D. was also supported by the Camille and
Henry Dreyfus Foundation through a Camille Dreyfus Teacher-
Scholar Award. E.V. acknowledges Ryan Bisbey and Anton Chavez for
helpful discussions. This study made use of the IMSERC and EPIC at
Northwestern University, both of which have received support from
the Soft and Hybrid Nanotechnology Experimental (SHyNE) Re-
source (NSF NNCI-1542205 and NSF ECCS-1542205, respectively),
the State of Illinois, and the International Institute for Nanotechnology
(IIN).
(10)
The plots chosen for the BET calculcation were distributed in the
range of 0.05-0.1 P/P . Although S values can vary with the relative pressure
0
BET
range chosen for calculation, we consistently found that the highest correlation
coefficients were obtained when the BET plots were distributed in that range.
(
11)
Pawley refined structure yielded a pore size of 24 Å.
12) (a) Campbell, N. L.; Clowes, R.; Ritchie, L. K.; Cooper, A. I. Chem.
The pore size was previously reported to be 19Å (see ref 9). Our
(
Mater. 2009, 21, 204-206. (b) Wei, H.; Chai, S. Z.; Hu, N. T.; Yang, Z.; Wei, L.
M.; Wang, L. Chem. Commun. 2015, 51, 12178-12181.
(
13)
DeBlase, C. R.; Hernandez-Burgos, K.; Silberstein, K. E.;
Rodriguez-Calero, G. G.; Bisbey, R. P.; Abruna, H. D.; Dichtel, W. R. ACS
Nano 2015, 9, 3178-3183.
(
14)
H.; Hartschuh, A.; Auras, F.; Bein, T. J. Am. Chem. Soc. 2016, 138, 1234-1239.
b) Smith, B. J.; Hwang, N.; Chavez, A. D.; Novotney, J. L.; Dichtel, W. R.
Chem. Commun. 2015, 51, 7532-7535.
(a) Calik, M.; Sick, T.; Dogru, M.; Doblinger, M.; Datz, S.; Budde,
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