Conversion of Imine to Oxazole and Thiazole Linkages in Covalent Organic Frameworks

Article abstract of DOI:10.1021/jacs.8b05830

Imine-linked ILCOF-1 based on 1,4-phenylenediamine and 1,3,6,8-tetrakis(4-formylphenyl)pyrene was converted through consecutive linker substitution and oxidative cyclization to two isostructural covalent organic frameworks (COFs), having thiazole and oxazole linkages. The completeness of the conversion was assessed by infrared and solid-state NMR spectroscopies, and the crystallinity of the COFs was confirmed by powder X-ray diffraction. Furthermore, the azole-linked COFs remain porous, as shown by nitrogen sorption experiments. The materials derived in this way demonstrate increased chemical stability, relative to the imine-linked starting material. This constitutes a facile method for accessing COFs and linkages that are otherwise difficult to crystallize due to their inherently limited microscopic reversibility.

Full text of DOI:10.1021/jacs.8b05830

Communication
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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Conversion of Imine to Oxazole and Thiazole Linkages in Covalent
Organic Frameworks
Peter J. Waller,†,§,‡ Yasmeen S. AlFaraj,†,‡ Christian S. Diercks,†,§,‡ Nanette N. Jarenwattananon,†
and Omar M. Yaghi*,†,§,‡,∥
†Department of Chemistry, University of CaliforniaBerkeley, Berkeley, California 94720, United States
‡Kavli Energy NanoSciences Institute at Berkeley, and Berkeley Global Science Institute, Berkeley, California 94720, United States
§Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
∥King Abdulaziz City for Science and Technology, Riyadh 11442, Saudi Arabia
S
* Supporting Information
we recently demonstrated that imine-linked frameworks can be
ABSTRACT: Imine-linked ILCOF-1 based on 1,4-
phenylenediamine and 1,3,6,8-tetrakis(4-formylphenyl)-
pyrene was converted through consecutive linker sub-
stitution and oxidative cyclization to two isostructural
covalent organic frameworks (COFs), having thiazole and
oxazole linkages. The completeness of the conversion was
assessed by infrared and solid-state NMR spectroscopies,
and the crystallinity of the COFs was confirmed by
powder X-ray diffraction. Furthermore, the azole-linked
COFs remain porous, as shown by nitrogen sorption
experiments. The materials derived in this way demon-
strate increased chemical stability, relative to the imine-
linked starting material. This constitutes a facile method
for accessing COFs and linkages that are otherwise
difficult to crystallize due to their inherently limited
microscopic reversibility.
oxidized to form amide-bonded COFs that retain the structure
of their progenitors.24 Here, we take this approach a step
further, showing that new functionalities can be incorporated
into the framework by postsynthetic linker exchange, in order
to bring about subsequent reactivity that leads to modified
linkages.
For this study, we targeted linker exchanges using imine-
bonded frameworks. These are an attractive starting point
since they are the most abundant and well-studied COFs. In
addition, the imine bond itself is poised for reaction giving rise
to a wide array of chemical motifs. In this case, we aimed to
form oxazole- and thiazole-linked frameworks by adding
appropriately substituted linkers to the imine-based material
after synthesis (Scheme 1). As a platform for this trans-
Scheme 1. Proposed Strategy for Synthesis of Azole-Linked
COFs through Substitution, Cyclization, and Oxidation
ovalent organic frameworks (COFs) are crystalline,
Cporous networks, constructed from small molecular
linkers connected through reversible bond formation.1−6 It is
this reversibility that not only leads to crystallinity through
dynamic error correction during synthesis but also diminishes
the resulting material’s chemical stability. Recently, it has been
shown that the dynamic nature of the bond also allows for the
linkers of a COF to be exchanged postsynthetically, yielding an
isostructural material with different pore metrics while
retaining crystallinity.7,8 In this work, we use the linkage’s
intrinsic reversibility to introduce linkers with functionalities
incompatible with COF synthesis conditions. Notably, such
functionalities cannot be added through established covalent
postsynthetic modifications for COFs.9−14 These functional
groups then incite additional reactivity at the imine bond,
resulting in multiple irreversible linking chemistries.
This signifies a method of bypassing the crystallization
problem; typically, the development of linkages relies on
serendipity in the optimization of reaction conditions that
allow for sufficient reversibility to let crystallinity
emerge.1,15−23 Furthermore, this means that linkages with
limited reversibility pose a particular challenge for the field. To
circumvent this problem, our goal is to separate the
crystallization from the final bond formation. As an example,
formation, we selected the previously reported ILCOF-1, a
mesoporous, two-dimensional layered structure with sql
topology (Scheme 2). The material was solvothermally
synthesized using a modified literature procedure, wherein
1,3,6,8-tetrakis(4-formylphenyl)pyrene (1) and 1,4-phenylene-
diamine (2) were heated in a sealed tube at 120 °C for 4 days
in a mixture of 1,2-dichlorobenzene, 1-butanol, and 6 M
aqueous acetic acid solution.25
Received: June 3, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b05830
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Journal of the American Chemical Society
Communication
Scheme 2. Synthesis of Azole-Linked COF-921 and LZU-192 Materials from the Imine Precursor, ILCOF-1
With this material in hand, we began to investigate the
exchange process by treating ILCOF-1 with 4 equiv of the
functionalized linker. The synthesis of the new thiazole-linked
COF, termed COF-921, was accessed through substitution of
the 1,4-phenylenediamine linkers in ILCOF-1 with 2,5-
diaminobenzene-1,4-dithiol dihydrochloride (3). As solvent, a
mixture of DMF and water was used. During reaction
optimization, it was shown that the presence of water provided
better conversion of the imine functionality. This may be a
result of faster imine hydrolysis and, therefore, more rapid
linker exchange. In addition, literature reports suggest that
water may assist in ring closure in the synthesis of azoles.26,27
Addition of substantial amounts of water, however, lowered
both crystallinity and surface area.
Scheme 3. Proposed Inter-COF Hydrogenation of Imine
Linkages by Saturated Heterocycles, and Control Reaction
with Molecular Reductant 4
Since both molecular and amorphous polymeric benzothia-
zoles have been synthesized from aldehydes and amines using
air as an oxidant, optimization was initially performed under
ambient conditions.27,28 Using these reaction conditions, the
imine functionality was well-converted, as evidenced by both
Fourier-transform infrared (FT-IR) and 13C cross-polarization
magic angle spinning (CP-MAS) NMR spectroscopy. How-
ever, unexpected signals appeared in the FT-IR spectrum at
1510 cm−1 and in the 13C NMR spectrum at 46 ppm
(coincident with the spinning sidebands of the spectrum; see
Supporting Information (SI) section S3). In order to explain
these, we hypothesized a transfer hydrogenation side reaction,
which serves to oxidize the intermediate dihydrothiazole
linkages through the reduction of neighboring imines to
amines (Scheme 3). Such a process would be consistent with
the observation of both azole and amine functionalities in the
material and would allow assignment of the carbon NMR
signal as the benzylic carbon of the amine and the IR signal as
arising from the amine moiety. To support this notion, the
reaction between the molecular species 2-phenylbenzothiazo-
line (4) and imine-linked ILCOF-1 was studied. Others have
shown that 4, when combined with Brønsted acids, is a
powerful reductant of imines via transfer hydrogenation, so we
anticipated it could be used in the synthesis of amine-linked
material.29,30 We found that treating ILCOF-1 with 4 in the
presence of hydrochloric acid in DMF gave rise to a new
material with the same signals in the FT-IR and solid state 13
NMR. This corroborates our understanding that these signals
are a result of benzylic amines formed during the course of the
reaction.
C
In order to preclude the formation of the reduced side
product, a variety of oxidants, including DDQ, p-chloranil,
benzoquinone, and PhI(OAc)2, were applied, but their use
resulted either in poor conversion or in loss of crystallinity.
Ultimately, it was found that performing the reaction under an
atmosphere of oxygen gave the desired product without
structural degradation.
Additionally, we targeted the analogous oxazole-linked
framework via linker exchange. In contrast to the thiazole
case, where no such materials were previously reported,
oxazole-linked COFs have been synthesized de novo.21,31 In
these cases, in order to obtain a crystalline material, the
reaction conditions required substantial modification from
those employed in typical imine COF syntheses. Our
approach, instead, relies on easily crystallized imine frame-
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Figure 1. (a) FT-IR spectra of the COFs under investigation, with diagnostic imine stretch indicated. (b) 13C CP-MAS NMR spectra of the imine-
linked starting material and the two azole-linked products, with iminyl carbon signal highlighted. Spinning sidebands are denoted with asterisks. (c)
Comparison of liquid-state 13C NMR spectra of benzothiazole model, unoxidized benzothiazoline, and the 13C CP-MAS NMR spectrum of COF-
921. (d) Comparison of liquid-state 13C NMR spectra of hydroxyl-substituted imine, benzoxazole, and the solid-state NMR spectrum of LZU-192.
(e) PXRD patterns of these materials, showing retention of crystallinity. (f) Nitrogen sorption isotherms, performed at 77 K, showing permanent
porosity of all materials. Solid and open circles indicate adsorption and desorption, respectively. Uptake is defined as cubic centimeters of N2 at 1
atm and 0 °C per gram of COF sample.
works as bases for subsequent exchange and modification. The
synthesis of oxazole-linked material from ILCOF-1 produces
the previously reported LZU-192.31 The conversion was
performed using 2,5-diaminohydroquinone dihydrochloride
(5, Scheme 2) as the new linker. As in the previous case,
under air the imine was well-converted, but a side product with
the same FT-IR and 13C NMR spectral features was evident.
The material was converted to the desired product by allowing
the reaction to proceed under oxygen, though FT-IR indicates
a small amount of this side product may be present.
Conversion of the material to the desired azoles was
spectroscopically verified in two ways. The first indication of
conversion was the attenuation of the imine stretch in the FT-
IR spectra (1622 cm−1, Figure 1a). In the case of COF-921, a
small amount of residual intensity remains at 1622 cm−1,
implying that a minute amount of the imine linkage may still
exist, perhaps at inaccessible defect sites. On the other hand,
while the FT-IR spectrum of LZU-192 shows broad intensity
in this region, at least some of this can be assigned to the
expected vibrations of the oxazole ring.32,33
These compounds were also characterized using 13C CP-
MAS NMR spectroscopy (Figure 1b). In the converted
materials, the peak corresponding to the imine carbon (156
ppm) is attenuated. In turn, it is replaced by new resonances at
167 ppm for the thiazole and 163 ppm for the oxazole. The
assignment of these signals to C2 of the azoles was confirmed
by comparison to molecular models (Figure 1c,d). Given the
difficulty normally encountered in accomplishing the oxidative
cyclization of the hydroxy-substituted imine compound, LZU-
192 was also contrasted against the uncyclized model to verify
that oxidative cyclization took place (Figure 1d).
The structure of the materials was examined using powder
X-ray diffraction (Figure 1e). Notably, the crystallinity of the
imine COF was preserved and all were assigned to the same
space group (Pmmn, No. 59), implying that the overall
symmetry of the material is maintained. Importantly, the unit
cell parameters do change during the transformation from
ILCOF-1 (a = 30.9 Å, b = 32.5 Å, c = 3.7 Å) to COF-921 (a =
34.1 Å, b = 32.5 Å, c = 3.6 Å) and LZU-192 (a = 33.0 Å, b =
31.0 Å, c = 3.6 Å). Therefore, while the unit cell parameters do
change, the material’s overall structure remains the same.
The porosity of the COFs was examined through nitrogen
sorption experiments (Figure 1f). The benzothiazole- and
benzoxazole-functionalized materials have surface areas of
1550 and 1770 m2 g−1, respectively, versus a surface area of
2050 m2 g−1 for the imine. This represents a loss of 24% for
COF-921 and 14% for LZU-192 versus ILCOF-1. A lower
gravimetric surface area is expected for the converted materials,
In addition, affirmative indications of conversion are
provided by FT-IR spectroscopy. For COF-921, the
benzothiazole ring’s breathing (962 cm−1) mode is present
in the spectrum, implying formation of the desired product.34
In the spectrum of LZU-192, the benzoxazole ring modes
(1590 and 1056 cm−1) are observed, further supporting
synthesis of the product (SI, section S4).32,33
C
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since their crystallographic densities are higher (SI section S5).
However, the observed surface area loss is greater than
expected. We hypothesize that, during the exchange, the imine
starting materials may undergo hydrolysis, resulting in the
formation of insoluble oligomers that may block the pores, or
alternatively, the linker may become trapped in the pores. In
spite of this, the oxazole material has a higher surface area than
that of any currently known oxazole COFs.
In order to probe our hypothesis regarding the irreversibility
of the azole linkages relative to the imine, chemical stability
tests were performed. The materials were submerged in basic
(10 M NaOH) and acidic (12.1 M HCl, 18 M H2SO4, 14.8 M
H3PO4, and 9 M H2SO4 in DMSO) solutions for 1 d, and
subsequently analyzed by PXRD and FT-IR (SI section S12).
Interestingly, all materials, even the imine-linked ILCOF-
1, proved stable in HCl. However, the PXRD patterns of
imine-linked ILCOF-1 treated with solutions containing
H2SO4 showed the emergence of a new phase. Furthermore,
the FT-IR spectra indicated that a large amount of imine
hydrolysis had occurred in the solutions of NaOH, H2SO4, and
H3PO4, as evidenced by the attenuation of the imine signal,
and growth of the aldehyde peak. In contrast, the azole
materials performed well in all conditions, where no changes in
the PXRD pattern, and only minimal changes in the FT-IR
spectra, were observed. This serves as strong evidence for the
increased irreversibility of the azole linkages and shows that
this technique is capable of producing chemically stable
materials.
(1144885). Y.S.F. is supported by the Gifted Student Program
Scholarship through the King Abdullah University of Science
and Technology. C.S.D. acknowledges funding through a Kavli
ENSI Philomathia Graduate Student Fellowship. The authors
thank Dr. Chenfei Zhao, Mary E. Garner and Robinson W.
Flaig for helpful discussions.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b05830.
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Peter J. Waller: 0000-0002-4013-8827
Christian S. Diercks: 0000-0002-7813-0302
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