Synthesis of Vinylene-Linked Covalent Organic Frameworks from Acetonitrile: Combining Cyclotrimerization and Aldol Condensation in One Pot

Article abstract of DOI:10.1021/jacs.0c04570

Combining two or more consecutive reactions in one pot is a common approach for process development, as such a method involves cheap starting materials and allows in situ generation of a reactive intermediate, to undergo further reaction, without isolation. Herein, we report the synthesis of a vinylene-linked (-CHa CH-) covalent organic framework, COF-701, directly from acetonitrile, a cheap commodity solvent, by combining/telescoping two consecutive reactions-cyclotrimerization of nitrile and subsequent aldol condensation with aldehydes-in one pot. Acetonitrile is trimerized to generate protonated 2,4,6-Trimethyl-s-Triazine tautomers in situ, which undergo Aldol condensation with 4,4′-biphenyldicarbaldehyde in one pot to form crystalline COF-701. COF-701 is obtained as a polycrystalline powder and possesses permanent microporosity and a BET surface area (SABET) of 736 m2·g-1. This strategy can be further extended to generate other porous vinylene-linked frameworks.

Full text of DOI:10.1021/jacs.0c04570

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Communication
Synthesis of Vinylene-Linked Covalent Organic Frameworks from Acetonitrile:
Combining Cyclotrimerization and Aldol Condensation in One Pot
Amitava Acharjya, Lewis Longworth-Dunbar, Jerome Roeser, Pradip Pachfule, and Arne Thomas
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 17 Jul 2020
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Synthesis of Vinylene-Linked Covalent Organic Frameworks
from Acetonitrile: Combining Cyclotrimerization and Aldol
Condensation in One Pot
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Amitava Acharjya,*§† Lewis L.-Dunbar,§† Jérôme Roeser,† Pradip Pachfule,† Arne Thomas*†
†Department of Chemistry/Functional Materials, Technische Universität Berlin, Hardenbergstraße 40, 10623 Berlin,
Germany
Supporting Information Placeholder
COFs can be synthesized by both acid30,36 and base
ABSTRACT: Combining two or more consecutive reactions in
one pot is a common approach for process development, as such a
catalyzed31,32,35 condensation reactions between active aryl-methyl
group-containing monomers and multitopic aryl aldehydes,
imparting flexibility and diversity over the choice of synthetic
conditions to be used.
method involves cheap starting materials and allows in-situ
generation of a reactive intermediate, to undergo further reaction,
without isolation. Herein, we report the synthesis of a vinylene-
linked (-CH=CH-) covalent organic framework (COF-701)
directly from acetonitrile, a cheap commodity solvent, by
A simple retrosynthetic analysis reveals that the vinylene-
linked frameworks bearing an s-triazine core should be accessible
directly from acetonitrile in two consecutive steps under
appropriate synthetic conditions (Figure 1a). While
multicomponent reactions have been reported for the generation
of crystalline frameworks,37–39 one pot syntheses by combining
two successive reactions have never been reported for COF
synthesis. Multistep sequential reactions are sometimes used for
the industrial synthesis of fine chemicals and drug development,40
as such a procedure, due to the dispensability of intermediate
isolation steps, allow for cheaper and safer production processes.
Here, we report a strategy for preparing vinylene-linked COFs by
a multistep synthesis approach directly from the cheap commodity
solvent acetonitrile. In particular, COF-701,30 the first reported
vinylene-linked COF prepared under acidic condition, attracted
our attention since it was reported to form by acid-catalyzed aldol
condensation involving 2,4,6-trimethyl s-triazine (TMT). We
show that the trimerization of acetonitrile catalyzed by the
Brønsted acid, trifluoromethanesulphonic acid (TfOH), generates
in-situ protonated tautomers of TMT which further undergo aldol
condensation with aryl aldehydes in one pot (Figure 1b).
combining/telescoping
two
consecutive
reactions,
cyclotrimerization of nitrile and subsequent aldol condensation
with aldehydes, in one pot. Acetonitrile is trimerized to generate
protonated 2,4,6-trimethyl s-triazine tautomers in-situ, which
undergo Aldol condensation with 4,4′-biphenyldicarbaldehyde in
one pot to form crystalline COF-701. COF-701 is obtained as
polycrystalline powder and possesses permanent microporosity
and a BET surface area (SABET) of 736 m2.g-1. This strategy can
be further extended to generate other porous vinylene-linked
frameworks.
Covalent organic frameworks (COFs) are extended 2D and 3D
crystalline porous solids constructed solely from organic building
blocks linked by covalent bonds.1 An abundant toolkit of covalent
bond formation reactions have been reported for the
crystallization of such fully organic frameworks.1–5 Nevertheless,
the inherent lower chemical stability of commonly used linkages
based on reversible B-O,6–10 B-N,11 Si-O12,13 and C-N14–17 bond
formation remains an issue that drives the further development of
such materials. Different synthetic strategies such as keto-enol
tautomerization,18–20 post-synthetic modification (PSM)21,22 or
interlayer stacking optimization23 have been applied to generate
chemically stable COFs in acidic and basic media. It was
generally believed that reticulating COFs with strong C-C
covalent bonds would address the stability issues of COF linkages
while also leading to materials with improved properties notably
due to the formation of frameworks with extended π-conjugated
domains.24,25 Indeed the recent discovery of cyanovinylene (-
C(CN)=CH-)25–29 and vinylene-linked (-CH=CH-) COFs,30–33
showed that a new class of chemically stable C-C linked COFs
can be developed. Vinylene-linked COFs30–35, in particular, are
desirable due to their high stability30 and structural resemblance to
poly-phenylene vinylenes (PPVs)32, used in light emitting diodes
(LEDs) and organic photovoltaics. Interestingly, vinylene-linked
As for the development of any new COF synthetic strategy,
investigating a model reaction can give precious indication on the
framework formation. In order to control the stoichiometry of the
reaction, a time dependent proton (1H) NMR study was performed
to determine the yield of protonated TMT formed by trimerization
of acetonitrile with TfOH (Supporting Information (SI), Section
S2, Figure S1). Protonated tautomeric TMT species41 were
directly detected upon addition of TfOH and a 20% yield was
calculated (after 1 hour) from the relative integral of TMT species
and acetonitrile as well as by isolation of diprotonated TMT-
triflate salt42,43 from solution (SI, Section S2, Figure S2 and S3).
The model compound 2,4,6-tristyryl s-triazine (TST) was isolated
in 80% yield in presence of benzaldehyde (based on 20%
trimerization yield after 1 hour) and thus confirmed the viability
of our strategy for vinylene-linked COF formation directly from
acetonitrile (Figure 1b, SI, Section S2).
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Figure 1: a. Schematic Illustration of retrosynthetic steps to form a vinylene-linked framework (COF-701) from acetonitrile b. Synthetic scheme of
2,4,6-tristyryl s-triazine (model compound) directly from acetonitrile by combining two consecutive reactions.
The first attempts to crystallize COF-701 from acetonitrile
under the solvothermal reaction conditions reported initially
(mesitylene/dioxane/acetonitrile) proved to be unsuccessful, as
only black amorphous solids were collected (Table S1, entry
COF-701-1). Since TfOH can catalyze unwanted radical reaction
pathways with common solvents44,45 the choice of suitable solvent
for the synthesis of crystalline COF-701 is fundamental. A
thorough screening of different solvents and temperature provided
a comprehensive idea of suitable solvents for the synthesis of
COF-701 directly from acetonitrile (Table S1). COF-701 could be
successfully crystallized under solvothermal conditions in a
methanol and acetonitrile (1:1) mixture at 120 °C (SI, Section
S2). COF-701 was collected as dark red powder by filtration and
further washed with methanol, water and acetone, before being
neutralized with dilute-ammonia solution. After Soxhlet
extraction with methanol overnight and vacuum drying at 100 °C,
COF-701 was obtained as light-yellow powder. Notably, the
reaction of isolated TMT-triflate salt and 4, 4’-
biphenyldicarboxaldehyde (BPDA) under COF-701 formation
condition did not yield any solid product. Therefore, the presence
of excess amount of acid in the reaction is deemed necessary for
the subsequent aldol condensation step.
signals located at δ ~138.1 ppm and 132.1 ppm were
unambiguously assigned within the aromatic carbon signals.
Moreover, two distinct aromatic s-triazine carbon-signals at δ
~170 ppm and 176 ppm reveals the formation of triazine cores
within the COF. That two signals can be differentiated
furthermore points to a partial [2+2]-cycloaddition of vinylene
linkages within the 2D-layers of COF-701, further corroborated
by the presence of a broad signal at δ ~ 43 ppm attributed to the
cyclobutane moieties.31,47
The crystallinity of COF-701 was analyzed by powder X-ray
diffraction (PXRD) analyses. A good match was found between
the experimentally obtained PXRD patterns and the proposed 2D
layered model with hcb topology reported for COF-701 (Figure 3,
SI, Figure S5). Final lattice parameters were extracted after
Pawley refinement and COF-701 synthesized from acetonitrile
was found to crystallize in a hexagonal unit cell (P6/m,
a = b = 29.7405 Å, c = 3.4456 Å, Rp= 2.47 % and Rwp= 3.22 %).
The stacking mode of the layers could not be derived from the
PXRD pattern alone, nevertheless an eclipsed stacking
arrangement can be concluded from the main pore size derived
from porosity analysis (vide infra), further supported by the
partial cycloaddition of vinylene-linkages (favorable in eclipsed
stacking arrangement) as revealed by the 13C CP-TOSS NMR
analysis.
Fourier transform infrared (FT-IR) spectroscopy analyses were
applied to assess the structural features of the crystalline
framework (Figure 2a). Complete disappearance of -C=O-
stretching frequency of starting aldehyde monomers at 1689 cm-1
and appearance of a new band at 1627 cm-1 attributed to -
CH=CH- stretching indicated complete condensation of the
starting building blocks and the successful formation of the
vinylene linkage in COF-701. Moreover, presence of the s-
triazine ring was also confirmed by characteristic C-N (aromatic)
stretching frequencies of s-triazine heterocycle at 1366 cm-1 and
1507 cm-1.46 Elemental analysis of COF-701 powders also
matched the reported values (SI, Section S2)
Formation of vinylene (-HC=CH-) linkages as well as s-
triazine cores were confirmed by 13C cross-polarization Total-
sideband-suppression (CP-TOSS) NMR spectroscopy analysis of
polycrystalline COF-701 powders (Figure 2b). Vinylene carbon
Figure 2: a. FT-IR spectra of COF-701 (red), model compound TST
(black) and BPDA (blue). b. 13C CP-TOSS NMR spectra of COF-701
(red) and model compound 2,4,6-tristyryl s-triazine (TST) (black).
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Figure 3: Synthesis and structural characterization of COF-701. a. Synthesis of COF-701 from acetonitrile by combining cyclotrimerization of
nitrile and Aldol condensation in one pot. b. PXRD pattern of COF-701: Pawley refined pattern (black), experimental pattern (red), Simulated
eclipsed (AA) stacking pattern (blue), difference (orange), Bragg diffraction positions (green) and simulated eclipsed (AA) model (inset). c. N2
sorption isotherm of COF-701.
The permanent porosity of the COF-701 framework was
evaluated by low-pressure nitrogen (N2) sorption studies on
evacuated samples, at 77 K. A steep gas uptake in the low relative
pressure range (p/p0 < 0.05) of the N2 adsorption branch shows
the microporous nature of the framework (Figure 2c). The
Brunauer-Emmett-Teller surface area (SABET) calculated from the
N2 adsorption was found to be 736 m2.g-1, which is lower
compared to the reported value (1366 m2.g-1) and could be
attributed to the lower degree of crystallinity. The corresponding
pore size distribution was found to be centered at 1.81 nm (SI,
Section S4, Figure S6), derived by fitting the Ar adsorption
branch data at 87 K with quenched-solid density functional theory
(QSDFT) cylindrical pore model,48 which closely matched well
with the calculated pore size of 2.1 nm of the proposed eclipsed
structural model.
Architectural stability of COF-701 was investigated by
thermogravimetric analysis (TGA) under N2 atmosphere. After an
initial weight loss of ~2 wt% due to desorption of adsorbed
solvents below 100 °C, the frameworks remained stable until 400
°C (SI, Section S5, Figure S7). Scanning electron microscopy
analysis revealed the formation of spherical particles of ~1 µm
Figure 4: a. Synthetic scheme of V-COF-1 b. PXRD pattern of V-
COF-1: experimental pattern (black), simulated eclipsed stacking
pattern (red).
(SI, section S6, Figure S8).
Subsequently, framework formation with 1,3,5-tris(4-
While synthesis of several vinylene-linked COFs were reported
formylphenyl)benzene
(TFPB)
and
1,3,5-tris(4-
by base-catalyzed aldol condensation, acid-catalyzed pathway
remains largely unexplored. We therefore attempted to verify the
versatility of our methodology and targeted several other
frameworks. First, we performed the synthesis of V-COF-131
from acetonitrile and terepthalaldehyde (TA) (SI, Section S2).
Similar to COF-701, a light-yellow polymer was obtained upon
neutralization of the bright red solid collected from the one pot
reaction with acetonitrile and TA. FT-IR and solid state 13C CP-
TOSS NMR spectra of V-COF-1 confirmed the formation of
vinylene-linkages as well as the presence of the s-triazine
heterocycle (SI, Section S7, Figure S9, S10). Powder XRD
measurements confirmed the crystallinity of V-COF-1, however
the broad peaks also show a lower structural order than for COF-
701. (Figure 4). Permanent microporosity of V-COF-1 was
confirmed by the low-pressure nitrogen (N2) sorption studies at
77 K (SI, Section S8, Figure S11) as V-COF-1 exhibited a SABET
of 790 m2.g-1. The corresponding pore size distribution maximum
located at 1.48 nm (SI, Section S8, Figure S12) matched well with
the calculated pore size of 1.6 nm of the proposed eclipsed
structural model.31
formylphenyl)triazine (TFPT) was investigated (SI section S2).
Both frameworks, coined V-COF-2 and V-COF-3, were
predominantly amorphous featuring small crystalline domains, as
revealed by PXRD analyses (SI, Section S9, Figure S13). FT-IR
analyses also confirmed the formation of vinylene linkages as
well as the presence of triazine heterocycle (SI, Section S10,
Figure S14). Despite being predominantly amorphous, both the
frameworks exhibited permanent microporosity and SABET values
of 760 m2.g-1 and 350 m2.g-1, respectively (SI, section S11, Figure
S15). The corresponding pore size distribution maxima of
1.41 nm and 1.52 nm (SI, Section S11, Figure S16) closely
matched with the calculated pore size of proposed eclipsed
structural model.31 It should be noted that the preparation of well-
defined COFs is highly dependent on the used reaction
conditions, which need to achieve the right balance between
monomer/oligomer solubility and reversibility of the COF
forming reactions. An upstream reaction before COF formation,
like the here presented trimerization reaction (see Scheme S1),
demand even more control on the reaction conditions to finally
obtain the COFs with high crystallinity. Moreover, unlike the
imine- or boroxine-linked COFs, where often a binary solvent
mixture of mesitylene and 1,4 dioxane is required to yield
crystalline frameworks, synthesis of vinylene-linked frameworks
seem to be more delicate and strongly depends on the choice of
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solvent mixtures and temperature used for the solvothermal
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condensation with BPDA. We believe, this is not only the first
report of a telescoping synthesis combining cyclotrimerization of
nitrile and aldol condensation, but also the first application of
such a two step-one pot reaction for COF synthesis. Furthermore,
general applicability of our method was established by extending
the reactions with other multitopic aryl-aldehyde building units
(TA, TFPB and TFPT) to generate microporous frameworks (V-
COF-1, V-COF-2 and V-COF-3 respectively). 2,4,6-trimethyl s-
triazine, (TMT), the starting monomer for vinylene-linked COF
syntheses, is an expensive chemical (1 g cost nearly 1000 $) ,
which is usually prepared from ethyl acitimidate hydrochloride in
a rigorous and time-consuming two-step reaction involving
several isolation and purification steps. The here shown approach
applying directly the commodity chemical acetonitrile therefore
enables significant cost and time savings and thus a scalable COF
synthesis. Moreover, our approach provides a complete account
on the acid catalyzed synthetic pathway of vinylene-linked COFs
and avoids the side reactions (Cannizzaro reaction of aldehydes)
otherwise associated with the base catalyzed synthesis of V-
COFs. This method will thus serve as a cheap and scalable
alternative for the synthesis of vinylene-linked frameworks
directly from acetonitrile.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on
the ACS Publications
Full synthetic procedures and characterization data, PXRD
analysis (PDF)
AUTHOR INFORMATION
Corresponding Author
*amitava-_acharjya5@mailbox.tu.berlin.de
*arne.thomas@tu.berlin.de
Author Contributions
§These authors contributed equally.
ACKNOWLEDGMENT
This work is funded by the Deutsche Forschungsgemeinschaft
(DFG, German Research Foundation) under Germany´s
Excellence Strategy – EXC 2008 – 390540038 – UniSysCat. The
DFG is furthermore acknowledged for funding within the project
TH 1463/12-1. AA thanks BIG-NSE and UniCat for the doctoral
scholarship. Dr. Andreas Schäfer is acknowledged for ss-NMR
measurements. Ms. Meng-Yang Ye is acknowledged for the SEM
analysis. We thank Ms. Christina Eichenauer and Ms. Maria
Unterweger for their assistance and Mr. Nicolas Chaoui for
fruitful discussions.
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