Fully Conjugated Donor-Acceptor Covalent Organic Frameworks for Photocatalytic Oxidative Amine Coupling and Thioamide Cyclization

Article abstract of DOI:10.1021/acscatal.0c01242

Covalent organic frameworks (COFs) are promising candidates as heterogeneous photocatalysts because of their porosity and tunable light absorption. The photostability and charge separation of COFs are highly important to improve the efficiency of photocatalytic transformation. In this work, a fully conjugated donor-acceptor COF is constructed with a benzothiadiazole unit, which exhibits high stability and enhanced charge separation. The prepared COF can efficaciously produce superoxide radical anions under air and visible light, which mediate the photocatalytic oxidative amine coupling and cyclization of thioamide to 1,2,4-thiadiazole in moderate to high yield and high recyclability (18 examples). This study demonstrates the great capacity of fully conjugated COFs with a D-A structure for light-driven organic synthesis.

Full text of DOI:10.1021/acscatal.0c01242

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Article
Fully Conjugated Donor-Acceptor Covalent Organic Frameworks for
Photocatalytic Oxidative Amine Coupling and Thioamide Cyclization
Shuai Li, Li Li, Yijun Li, Lu Dai, Caixia Liu, Yanze Liu, Jiani Li, Jianning Lv, Pengfei Li, and Bo Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c01242 • Publication Date (Web): 10 Jul 2020
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Fully Conjugated Donor-Acceptor Covalent Organic
Frameworks for Photocatalytic Oxidative Amine
Coupling and Thioamide Cyclization
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*a
Shuai Li, Li Li, Yijun Li, Lu Dai, Caixia Liu, Yanze Liu, Jiani Li, Jianning Lv, Pengfei Li
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and Bo Wang
a
Key Laboratory of Cluster Science Ministry of Education, Beijing Key Laboratory of
Photoelectronic/Electrophotonic, Advanced Research Institute of Multidisciplinary Science,
School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, China
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00081.
Abstract
Covalent organic frameworks (COFs) are promising candidates as heterogeneous photocatalysts
due to their porosity and tunable light absorption. The photostability and charge separation of
covalent organic frameworks are highly important to improve the efficiency of photocatalytic
transformation. In this work, a fully conjugated donor-acceptor COF is constructed with
benzothiadiazole unit, which exhibits high stability and enhanced charge separation. The prepared
COF can efficaciously produce superoxide radical anion under air and visible light, which
mediates the photocatalytic oxidative amine coupling and cyclization of thioamide to 1,2,4-
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thiadiazole in moderate to high yield and high recyclability (18 examples). This study
demonstrates the great capacity of fully conjugated COFs with D-A structure for light-driven
organic synthesis.
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KEYWORDS: covalent organic frameworks, donor-acceptor, superoxide radical anion, organic
synthesis, photocatalysis.
Introduction
Visible-light driven organic transformations offer a highly sustainable and environment friendly
route for the synthesis of important chemicals, which have attracted significant attention in recent
1
-3
years. Heterogeneous visible-light photocatalysts with great recyclability are especially
4
promising for large scale applications. Inorganic semiconductors with well-defined band
structures are intensely investigated as heterogenous photocatalyst, such as TiO and ZnO, which
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, 6
mostly work in the UV range. Subsequently, organic based photocatalysts, such as organic dyes,
are developed to enhance the visible light absorption. To further escalate the catalytic performance,
porous photocatalyst or porous material supported semiconductors with exposed photocatalytic
7, 8
9,
sites have been synthesized, such as metal organic frameworks, covalent organic frameworks,
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conjugated microporous polymers
and g-C
3
N . Among these porous materials, COFs are
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particularly attractive due to their chemical and structural features.
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COFs are a class of polymers with high crystallinity and porosity, which are constructed from
organic building blocks with well-defined pore structure and highly tunable opto-electronic
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properties.
In recent years, COFs have been found a variety of applications, such as gas sorption
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and separation,
energy transformation and storage,
and particularly heterogeneous
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catalysis.
2D COFs are emerging heterogeneous photocatalysts owing to the combination of
strong visible light absorption and mesopore structure. Under visible-light irradiation, the photo-
generated electrons and holes in COFs promote a series of reactions, such as hydrogen
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generation,
oxygen evolution,
carbon dioxide reduction
and organic transformations.
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To obtain higher photocatalytic efficiency, enhanced light absorption, suitable band positions,
and fast charge separation are highly important. All these properties are related to the intrinsic
chemical linkage of particular COFs. Among many different linkages, the construction of fully
conjugated covalent organic frameworks would realize desirable light absorption and charge
separation. Until very recently, several ethylene-linked COFs have been synthesized and applied
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to fluorescence,
lithium-ion battery, supercapacitor and others.
Only a few reports
investigate the photocatalytic performance of fully conjugated COFs. Jiang’s group reported a 2D
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vinylene-linked COF that shows impressive photocatalytic hydrogen production activity.
Zhang’s group introduced pyridine and triazine cores into olefin-based COFs, which grant them
strong light-harvesting characteristics and further improved their photocatalytic hydrogen
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evolution rate.
Wang and co-workers incorporated porphyrin to a 2D vinylene-linked COF,
which shows good photocatalytic activity of oxidative secondary amine to imine due to improved
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photo-stability and enhanced electron delocalization. Li and co-workers introduced nitrogen
atom in the skeleton of TP-COF to modulate its electronic structure and lead to a good simulation
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of artificial photosystem I. Recently, Cooper’s group synthesized a bipyridine-containing sp c-
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COF modified with a rhenium complex to improve photocatalytic carbon dioxide reduction
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performance. Fast charge separation in these fully conjugated COFs is very important and can
further improve their photocatalytic performance. The incorporation of donor-acceptor moieties in
porous organic systems has been demonstrated to be an effective way to promote the charge
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seperation.
Herein, we designed and synthesized a fully conjugated Py-BSZ-COF, containing
electron-donating pyrene unit and electron-accepting benzothiadiazole unit to construct a donor-
acceptor structure, which shows high photo-stability and enhanced charge separation. Under
•–
visible light irradiation, this D-A Py-BSZ-COF can produce superoxide radical anion (O
saturated CH
2
) in air
3
CN, which drives the photocatalytic oxidative amine coupling and cyclization of
thioamide to 1,2,4-thiadiazole in moderate to high yield and high recyclability.
Experimental Section
The preparation of COFs
Py-BSZ-COF: 1,3,6,8-tetrakis(4-formylphenyl)pyrene (20 mg, 0.032 mmol) and 4,4’-
benzothiadiazole-4,7-diyl)diacetonitrile (24 mg, 0.065 mmol) in 2 mL o-dichlorobenzene were
(
added to a 10 mL Pyrex tube. After sonification, 0.2 mL of 1 M tetrabutylammonium hydroxide
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(
TBAH) in methanol and 0.1 mL water were added. After three freeze-thaw cycles, the tube was
flame-sealed and heated at 120 °C for 72 h. The precipitate was washed by DMF, 1 M HCl aqueous
solution, water and THF, successively. The solid was Soxhlet extracted with THF for 24 h and
activated at 120 °C under vacuum for 12 h to afford an orange powder of 34 mg in 81% yield.
Elemental analysis showed that the weight percentage of C, H, N and S to be 74.2%, 3.7%, 7.7%
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and 4.84%, which is in well agreement with the calculated value of Py-BSZ-COF (C44
2.6%, H 3.6%, N 8.8%, S 5.0%).
H
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N S, C
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COF-JLU22 was prepared according to the literature method. After activation at 120 C under
vacuum, a red powder of COF-JLU22 was obtained in 76% yield.
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sp c-COF-3 was prepared according to the literature method. After activation at 120 C under
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vacuum, an orange powder of sp c-COF-3 was obtained in 75% yield.
Electrochemical measurements
COF powder (6 mg) was ground with poly(vinylidene fluoride) (PVDF, 2 mg), then ultrasonically
dispersed in 2 mL of acetone. The resultant slurry was then drop-casted onto indium tin oxide (ITO)
2
glass with an area of 0.5 × 0.5 cm . A Pt wire (counter electrode), a Ag/AgCl electrode (reference
electrode), and a coated ITO conductive glass (working electrode) were assembled into a three-
electrode system with 0.2 M Na
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SO aqueous solution was used as the electrolyte. The Mott-
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Schottky plots were collected in dark at different frequencies. The photocurrent measurements
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were conducted under the irradiation of a 300 W xenon lamp (100 mW cm ) with a 420 nm cut-
off filter under a nitrogen or oxygen saturated atmosphere. For cyclic voltammetry (CV) tests, the
working electrode was prepared by dropping the prepared suspension onto a glassy carbon
electrode, then CV experiments were carried on a three-electrode system (saturated calomel
electrode as the reference electrode, platinum wire as the counter electrode, glassy carbon electrode
as the working electrode) in nitrogen saturated 0.1 M tetrabutylammonium hexafluorophosphate
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acetonitrile solution with a scan rate of 50 mV s .
To investigate the possible influence of the Pt diffusion, all the electrochemical tests were
conducted by using a graphite rod as the counter electrode under the same experimental conditions.
Electron spin resonance (ESR) measurements
–1
ESR experiments were conducted in a mixture of 1 mg mL Py-BSZ-COF and 0.1 M DMPO air-
–
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saturated acetonitrile suspension in dark or irradiated by 300 W xenon lamp (100 mW cm ) with
a 420 nm cut-off filter.
Results and Discussion
The highly crystalline Py-BSZ-COF was obtained by the condensation of 1,3,6,8-tetrakis(4-
formylphenyl)pyrene (Py-4CHO) and 4,4’-(benzothiadiazole-4,7-diyl)diacetonitrile (BSZ-2CN)
in a mixture of o-dichlorobenzene and 1 M TBAH at 120 °C for 72 h, which afforded an orange
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powder in 81% yield (Figure 1). The chemical structure of Py-BSZ-COF was first demonstrated
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by the existence of C=C–H and C≡N stretching bands at around 3030 cm and 2214 cm and the
–
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attenuation of the C=O stretching band at 1697 cm in Fourier transform infrared (FT-IR)
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spectrum (Figure 2a). A broad peak with several close shoulders from 120 to 140 ppm can be
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assigned to the aromatic carbons and ethylene carbons of Py-BSZ-COF in C CP-MAS NMR
spectrum. A weak carboxylic carbon at ~180 ppm could be observed, which is due to the partially
oxidation of the residue aldehyde groups in the COF (Figure S1).
Figure 1. a) The synthetic condition of Py-BSZ-COF. b) Top and c) side views of the structural
model of Py-BSZ-COF (C, gray; N, blue; S, yellow; H, white).
o
The PXRD pattern revealed that Py-BSZ-COF is highly crystalline with diffraction peaks at 2.7 ,
o
o
o
o
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.5 , 8.3 , 11.1 and 24.0 , which are assignable to (110), (220), (400), (440) and (001) facets,
respectively. Structural models with AA and AB stacking were constructed by Materials Studio
(Figure 1 and S2). The simulated PXRD pattern of the eclipsed AA stacking model was in good
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agreement with the experimental PXRD, while the AB stacking model did not (Figure 2b). In
addition, the Pawley refinement was performed on the experimental PXRD pattern of Py-BSZ-
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COF, which resulted in cell unit parameters of a = 51.3 Å, b = 42.5 Å, c = 3.8 Å, and α = γ = 90 ,
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β = 96.6 with a negligible difference of Rwp = 3.70% and R
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Figure 2. a) FT-IR spectra of Py-BSZ-COF, BSZ-2CN and Py-4CHO. b) Experimentally observed
PXRD pattern, Pawley refinement and their difference of Py-BSZ-COF (red, purple dots, black),
the simulated PXRD patterns of the AA stacking model (blue) and the AB stacking model (green).
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c) N
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isotherm of Py-BSZ-COF at 77 K, inset: pore width distribution and cumulative pore volume
of Py-BSZ-COF. d) HR-TEM image of Py-BSZ-COF.
The porosity of Py-BSZ-COF was investigated by the N
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adsorption isotherm at 77 K (Figure 2c).
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The Brunauer–Emmett–Teller (BET) surface area was evaluated to be 600 m g with a total pore
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volume of 0.33 cm g at P/P = 0.98. The pore size of Py-BSZ-COF was estimated to be 2.4 nm
0
by nonlinear density functional theory (NLDFT), which consists well with pore width predicted
by the structural model. Scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) images show that Py-BSZ-COF has a belt-like morphology (Figure S3 and S4).
High resolution transmission electron microscopy (HR-TEM) of Py-BSZ-COF reveals a clear
lattice fringe of 2.41 nm, which is close to the pore size calculated from the nitrogen isotherm
(Figure 2d). In addition, the energy dispersive spectroscopy (EDS) mapping of Py-BSZ-COF
shows a homogeneous distribution of carbon, nitrogen and sulfur elements in the particles (Figure
S5).
Due to the irreversible nature of ethylene linkage, Py-BSZ-COF offers high thermal-, chemical-
and photo- stability, which is stable over 400 °C and remains crystallinity, structural intact and
porosity after immerse in 12 M HCl, 12 M NaOH and boiling water or illuminate under 15 W
white LED bulb in a CH
3
CN suspension for at least 3 days as determined by the comparison of the
PXRD patterns, FT-IR spectra and N
2
isotherm at 77 K before and after the treatments (Figure S6–
S9). The high stability of Py-BSZ-COF makes it a promising candidate in photocatalysis.
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Figure 3. a) Solid state UV−vis diffuse reflectance spectra (red), inset: Tauc plot of Py-BSZ-COF
(blue). b) The Mott-Schottky plots of Py-BSZ-COF. c) The band energy diagram of Py-BSZ-COF.
d) Photocurrent responses spectra of Py-BSZ-COF under nitrogen or oxygen atmosphere.
The UV−vis diffuse reflection spectrum of Py-BSZ-COF exhibits a broad visible adsorption range
up to 600 nm. The optical band gap of Py-BSZ-COF is calculated to be 2.24 eV by using the Tauc
plot (Figure 3a). The cyclic voltammetry of Py-BSZ-COF shows a reversible half wave potential
+
(E1/2) at 1.47 V vs. NHE (Figure S10), which can be assigned to the Py-BSZ-COF /Py-BSZ-COF
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couple. To determine the positions of the conduction band (CB) and valence band (VB), the
electrochemical Mott-Schottky spectra and VB X-ray photoelectron spectroscopy (VB-XPS) of
Py-BSZ-COF were measured. The Mott-Schottky plot of Py-BSZ-COF has a positive slope, which
suggests an n-type semiconductor behavior. For an n-type semiconductor, the CB position is close
1
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1
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to its flat band potential. Therefore, the CB position of Py-BSZ-COF is estimated to be –0.68 V
vs. NHE at pH = 7 (Figure 3b and S11). The VB edge of Py-BSZ-COF is determined by the VB-
XPS spectrum, which is located at 1.53 eV (Figure S12). The band gap of Py-BSZ-COF calculated
from the Mott-Schottky plots and VB-XPS spectrum is 2.21 eV, which is quite closed to the optical
band gap. To rule out the possible of Pt diffusion during the electrochemical experiments, we also
tested the cyclic voltammetry, Mott-Schottky spectra of Py-BSZ-COF by using a graphite rod as
the counter electrode under the same conditions. The reversible half wave potential (E1/2) and CB
position of Py-BSZ-COF determined from the graphite counter electrode are quite close to that of
the Pt electrode, which are 1.45 V and –0.67 V vs. NHE at pH = 7, respectively (Figure S10 and
S11). Combine all the evidence, the CB of Py-BSZ-COF (–0.68 V vs. NHE) is negative than the
potential required for the reduction of O
Py-BSZ-COF a potential photocatalyst in generating critical superoxide radical (O
Figure 3c). The photo-generation of superoxide radical in solution with Py-BSZ-COF was probed
by electron spin resonance (ESR) spectra. Upon light irradiation of Py-BSZ-COF with 5,5-
dimethyl-1-pyrroline-N-oxide (DMPO) in an air-saturated CH CN, the intensity of the
2
to superoxide radical (–0.33 V vs. NHE), which makes
•
–
2
) intermediate
(
3
•
–
characteristic peaks of typical DMPO−O
2
adduct were increased by prolonging the illumination
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•
–
time (Figure S13). The existence of O
2
in an irradiated Py-BSZ-COF suspension is also confirmed
7
0
by the reduction of a yellow colored nitro blue tetrazorium (Figure S14). In addition, the transient
photocurrent responses of Py-BSZ-COF over several on-off photoirradiation cycles show high
photo-induced charge separation under both nitrogen and oxygen atmosphere. The photocurrent
of Py-BSZ-COF under nitrogen is higher than that under oxygen atmosphere, which could arise
from the quenching of the photo-generated electrons by oxygen to generate superoxide radical
anion. (Figure 3d and S15).
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Table 1. Control experiments of the photocatalytic oxidative amine coupling of benzylamine.
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a
Reaction conditions: benzylamine (0.2 mmol), photocatalyst (5 mg), 2 mL CH
3
CN, 1 atm air, 12
–
2
b
1
c
h and 15 W 520 nm LED bulb (5 mW cm ). Determined by H NMR spectroscopic analysis.
d
e
1
f
In the dark. Under Ar environment. L-histidine used as O radical scavenger. Catalase used as
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g
h
•–
H
2
O
2
scavenger. Isopropanol used as •OH radical scavenger. p-benzoquinone used as O
2
radical
i
j
k
scavenger. AgNO
3
used as electron scavenger. KI used as hole scavenger. Partly decomposed
during the photocatalysis.
Oxidative amine coupling
Imines are important intermediates in organic synthesis and widely used to synthesize dyes, drugs,
7
1
and agrochemicals. The direct synthesis of imine from amine under photocatalytic condition has
7
2-75
attracted great attention in recent years.
Considering the significant superoxide radical
generating ability and the ideal band gap, the activated Py-BSZ-COF was first tested for the
oxidative amine coupling reaction by using benzylamine as the substrate to optimize the reaction
condition. When benzylamine (0.2 mmol) and Py-BSZ-COF (5 mg) in 2 mL of CH CN was
3
–
2
irradiated over a 15 W 520 nm LED bulb (5 mW cm ) under air at room temperature, the formation
of N-benzyl-1-phenylmethanimine was observed with 99% conversion (Table 1, entry 1). A series
of control experiments demonstrated that the photocatalyst Py-BSZ-COF, oxygen and light are all
indispensable reaction conditions, otherwise little or trace conversion can be observed (Table 1,
entries 2–4). To determine the key intermediate in the photocatalytic transformation, a series of
trapping agents with different quenching targets has been added to the reaction mixture. The
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addition of L-histidine (L-His), catalase (CAT) and isopropanol (IPA) shows negligible decrease
1
of the reaction yield, which rules out the existence of O
2
, H
2
2
O and •OHduring the photocatalysis
•
–
(Table 1, entries 5–7). After adding p-benzoquinone (Bq), a typical O
2
scavenger, the reaction
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•–
yield was significantly reduced from 99% to 28%. These quenching experiments suggest that O
2
is the predominant radical in this reaction (Table 1, entry 8). The addition of AgNO can fully
3
quench photo-generated electrons, which completely inhibits the reaction (Table 1, entry 9).
Furthermore, by using KI as the hole scavenger, only 38% of benzylamine converted to the
corresponding imine, which indicated the photo-generated holes are also important in the amine
coupling reaction (Table 1, entry 10).
To study the donor and acceptor behavior of Py-BSZ-COF, two iso-reticular COFs with
benzothiazole in the imine linked COF-JLU22 and without benzothiazole in the olefin bridged
2
sp c-COF-3 were synthesized and fully characterized (Figure S16–S25). The UV-vis diffuse
2
reflection spectrum of Py-BSZ-COF is significantly red-shifted as compared to sp c-COF-3 with
maximum absorption and absorption onset quite close to the benzothiazole incorporated COF-
JLU22 (Figure S26 and S27). Photoluminescence (PL) spectra reveal a weaker fluorescence
2
intensity of Py-BSZ-COF as compared to sp c-COF-3 and COF-JLU22 (Figure S28). Time-
resolved PL spectra demonstrate a longer fluorescence lifetime of Py-BSZ-COF (1.6 ns) as
2
compared with sp c-COF-3 (1.2 ns) and COF-JLU22 (1.1 ns). The weaker PL emission and longer
fluorescence lifetime of Py-BSZ COF clearly demonstrated that the recombination of photo-
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generated excitons was inhibited (Figure S29). In addition, the introduction of benzothiazole in
the fully conjugated Py-BSZ-COF provides an improved charge separation, which generates
2
higher transient photocurrent responses compared to both sp c-COF-3 and COF-JLU22 under
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nitrogen or oxygen atmosphere (Figure S30 and S31). Similarly, the photocurrent responses were
also conducted with a graphite counter electrode to eliminate the possible influence of Pt electrode.
2
The photocurrents of Py-BSZ-COF are higher than sp c-COF-3 and COF-JLU22 under both
nitrogen and oxygen atmosphere. All three COFs also exhibit decreased photocurrents under
oxygen atmosphere, which were in accordance with the results by using a Pt electrode (Figure
2
S15b, S30b and S31b). As a result, the iso-reticular sp c-COF-3 only affords a moderate yield of
6
7% (Table 1, entry 11). Similar D-A structure in an imine linked COF-JLU22 has a comparable
yield with the Py-BSZ-COF (Table 1, entry 12). However, COF-JLU22 partly decomposed in the
process of photocatalysis due to the dynamic imine exchange between COF-JLU22 and
1
benzylamine, and the exchange product was observed in H NMR (Figure S32).
With the optimal photocatalyst, we expand the substrate scope to various benzylamine with
different functional groups. High conversion (> 99%) and selectivity (> 98%) were obtained for
4-substituted benzylamine with electron donating groups such as Me-, MeO- and electron
withdrawing groups like F-, Cl-, Br-, CF
3
- and CN- (Table 2). CN- in 4-substituted benzylamine
slows down the reaction rate and lowers the conversion, which may due to the strong electron-
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withdrawing effect of the cyano group in destabilizing the cationic radical intermediate. The
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conversion of 4-cyanobenzylamine to 2h can increase from 52% to 84% by prolonging the reaction
time to 24 h. Moreover, heterocyclic amine like 2-thiophenemethanamine can completely convert
to imine 2i in nearly quantitative yield. The Py-BSZ-COF catalyst has a high recyclability with
conversion and selectivity both over 98% after four consecutive runs (Figure S33 and S34). The
PXRD and FT-IR spectra of Py-BSZ-COF after three cycles remain the same as the pristine one,
which suggest high photostability of Py-BSZ-COF during the catalysis (Figure S35).
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a
Table 2. Heterogeneous photocatalytic oxidative coupling of diverse amines.
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Reaction conditions: amine (0.2 mmol), photocatalyst (5 mg), 2 mL CH
3
CN, 1 atm air and 15 W
–
2
1
5
20 nm LED bulb (5 mW cm ). Conversion (red) and selectivity (blue) are determined by H
NMR spectroscopic analysis.
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Oxidative cyclization of thioamide to 1,2,4-thiadiazole
Compounds containing 1,2,4-thiadiazole are biologically and pharmaceutically important.
7
7
Pharmaceuticals with 1,2,4-thiadiazole as the key scaffold exhibit promising activity in
suppressing inflammation, central nervous system activity and antibiotic action, such as the most
7
8
famous commercially available antibiotic cefozopram. Most of the synthetic routes available for
1,2,4-thiadiazoles rely on the oxidative cyclization of thioamides by various oxidants such as
7
9
80
81
DDQ, iodate and phosphovanadomolybdic acid. Here, Py-BSZ-COF was explored as a
photocatalyst to oxidative cyclization of thioamide due to its outstanding capability to generate
•
–
O . When the thioamide (0.4 mmol) and catalytic amount of Py-BSZ-COF (10 mg) in 4 mL of
2
–
2
DMF was illuminated with a 15 W white LED bulb (11 mW cm ) under air at room temperature,
,5-diphenyl-1,2,4-thiadiazole was obtained in 90% yield (Table 3, entry 1). Control experiments
showed that the light, oxygen, and Py-BSZ-COF are all essential (Table S1, entries 2–4). In
3
•
–
addition, radical scavenger experiments suggest that O
2
is also the predominant radical in the
oxidative cyclization of thioamide (Table S1, entries 5-8). A variety of thioamides can be
converted to 1,2,4-thiadiazoles in moderate to good yield under mild condition. The oxidative
cyclization of thioamides with electron donating groups like Me- and MeO- provides good yields
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over 87%, while the electron withdrawing groups lead to a slightly lower yield (Table 3).
Heterocyclic thioamide like 2-thiophenecarboxamide can also be cyclized in satisfying yield of
8
4%. The recyclability of Py-BSZ-COF in oxidative cyclization of thioamide was also tested in
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four consecutive cycles, after which the isolated yield of 3,5-diphenyl-1,2,4-thiadiazole reduced
slightly from 90% to 84% possibly due to the loss of photocatalyst during the recycling
experiments (Figure S28). Same as in the photocatalytic amine coupling, Py-BSZ-COF also
exhibits high photostability in the oxidative cyclization of thioamide as the PXRD and FT-IR
spectra of Py-BSZ-COF remain unchanged after three reuses (Figure S36).
Table 3. Heterogeneous photocatalytic oxidative cyclization of diverse thioamides to 1,2,4-
a
thiadiazoles.
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a
Reaction conditions: thioamide (0.4 mmol), photocatalyst (10 mg), 4 mL DMF, 1 atm air, and 15
–2
W white LED bulb (11 mW cm ). Isolated yield after chromatography over silica.
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Figure 4. The plausible photocatalytic mechanisms of Py-BSZ-COF mediated oxidative amine
coupling and cyclization of thioamide to 1,2,4-thiadiazole.
Reaction mechanism
72, 82-87
Based on the above experiments and literature reports,
tentative reaction mechanisms for the
oxidative amine coupling and cyclization of thioamide to 1,2,4-thiadiazole were proposed (Figure
). Py-BSZ-COF is first photo-excited to Py-BSZ-COF* under visible light. The photo-generated
4
electrons in Py-BSZ-COF* reduce molecular oxygens to superoxide radicals. At the same time,
+
the resulted Py-BSZ-COF oxidizes amine to amine radical anion 1 and regenerates the Py-BSZ-
•
–
+
COF. The O
2
abstracts a H and a hydrogen atom from 1 to give the key intermediate
phenylmethanimine 2. After that, two paths are possible. In path A, intermediate 2 is attacked by
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another molecule of benzylamine to give an aminal 3. After the loss of NH , the final product N-
3
benzyl-1-phenylmethanimine is produced. In path B, intermediate 2 is hydrolyzed to afford the
benzaldehyde 4, which subsequently condensed with another molecule of amine to yield the final
product. We could detect the existence of benzaldehyde 4 with gas chromatography/mass
spectrometer (GC/MS), thus path B is more likely to happen in our system (Figure S37). Similarly,
1
1
1
1
1
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1
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+
the Py-BSZ-COF could also oxidize thioamide to a thioamide radical cation 5. Two radical
isomers 6 and 7 are formed with the proton removal and linked to give a dimer 8. The final 1,2,4-
thiadiazole was obtained through the intramolecular cyclization of dimer 8 and aromatization of 9
with the help of superoxide radical anion.
Conclusion
A benzothiazole-containing D-A covalent organic framework with fully conjugated ethylene
linkage was designed and synthesized in high crystallinity and high yield. The introduction of
benzothiazole to the framework induces the red-shift of the visible light absorption and the fast
separation of photo-generated charges. The resultant D-A Py-BSZ-COF can produce superoxide
•–
radical anion (O
2
) for the photocatalytic oxidative amine coupling and cyclization of thioamide
to 1,2,4-thiadiazole with broad substrate scope, good yield and high recyclability.
AUTHOR INFORMATION
Corresponding Author
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*
*
lipengfei@bit.edu.cn
bowang@bit.edu.cn
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Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
ASSOCIATED CONTENT
Supporting Information
This information is available free of charge on the ACS Publication website.
Materials and instruments; Synthesis of monomers; Characterizations of COFs; Photocatalytic
procedures and cycle performance of aerobic oxidation coupling of benzylamine and oxidative
1
13
cyclization of thioamide; H and C NMR spectra of monomers and reaction products. (PDF)
ACKNOWLEDGMENT
The financial support from the National Natural Science Foundation of China (No. 81601549,
6
1933002, 21674012 and 21625102); the 1000 Plan (Youth); Beijing Municipal Science and
Technology Project (Z181100004418001), Beijing Institute of Technology Research Fund
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Program for Young Scholars is acknowledged. The authors acknowledge the Analysis and Testing
Center of BIT for technical supports.
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