s-Tetrazine-functionalized hyper-crosslinked polymers for efficient photocatalytic synthesis of benzimidazoles

Article abstract of DOI:10.1039/d0gc03719b

Developing green-safe, efficient and recyclable catalysts is crucial for the chemical industry. So far, organic photocatalysis has been proved to be an environmentally friendly and energy-efficient synthetic technology compared with traditional metal catalysis. As a versatile catalytic platform, hyper-crosslinked polymers (HCPs) with large surface area and high stability are easily prepared. In this report, we successfully constructed two porous HCP photocatalysts (TZ-HCPs) featurings-tetrazine units and surface areas larger than 700 m²g^?1through Friedel-Crafts alkylation reactions. The rational energy-band structures and coexisting micro- and mesopores endow TZ-HCPs with excellent activities to realize the green synthesis of benzimidazoles (28 examples, up to 99% yield, 0.5-4.0 h) in ethanol. Furthermore, at least 21 iterative catalytic runs mediated by TZ-HCP1D were performed efficiently, with 96-99% yield. This study of TZ-HCPs sheds light on the wide-ranging prospects of application of HCPs as metal-free and green photocatalysts for the preparation of fine chemicals.

Full text of DOI:10.1039/d0gc03719b

Green Chemistry
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s-Tetrazine-functionalized hyper-crosslinked
polymers for efficient photocatalytic synthesis
of benzimidazoles†
Cite this: Green Chem., 2021, 23,
1292
Wan-Kai An, *a Shi-Jia Zheng, a Hui-Xing Zhang,a Tian-Tian Shang,a
a
He-Rui Wang,b Xiao-Jing Xu,a Qiu Jin,a Yuchen Qin, a Yunlai Ren,
a
Song Jiang,a Cui-Lian Xu,a Mao-Song Houa and Zhenliang Pan
*
Developing green-safe, efficient and recyclable catalysts is crucial for the chemical industry. So far,
organic photocatalysis has been proved to be an environmentally friendly and energy-efficient synthetic
technology compared with traditional metal catalysis. As a versatile catalytic platform, hyper-crosslinked
polymers (HCPs) with large surface area and high stability are easily prepared. In this report, we success-
fully constructed two porous HCP photocatalysts (TZ-HCPs) featuring s-tetrazine units and surface areas
larger than 700 m2 g−1 through Friedel–Crafts alkylation reactions. The rational energy-band structures
and coexisting micro- and mesopores endow TZ-HCPs with excellent activities to realize the green syn-
thesis of benzimidazoles (28 examples, up to 99% yield, 0.5–4.0 h) in ethanol. Furthermore, at least 21
iterative catalytic runs mediated by TZ-HCP1D were performed efficiently, with 96–99% yield. This study
of TZ-HCPs sheds light on the wide-ranging prospects of application of HCPs as metal-free and green
photocatalysts for the preparation of fine chemicals.
Received 3rd November 2020,
Accepted 14th January 2021
DOI: 10.1039/d0gc03719b
rsc.li/greenchem
systems and then reuse. For this reason, much effort has been
Introduction
devoted to realizing heterogeneity in homogeneous catalysts.7
The vital task of green chemistry is to take reasonable As a consequence, a series of solid catalysts based on porous
measures, e.g., using greener and safer solvents and reagents, frameworks, such as periodic mesoporous organosilicas
searching for suitable synthesis approaches, and exploiting (PMOs),8 metal–organic frameworks (MOFs),9 and porous
non-noxious, highly efficient and selective catalysts, to improve organic polymers (POPs)10 have been successfully developed.
the atomic economy.1 In this context, many environmentally Nevertheless, the precise construction of efficient and highly
friendly catalytic technologies, such as photocatalysis, a power- recyclable heterogeneous organic photocatalysts with favorable
ful synthetic technique for traditional synthesis, have been robustness still remains a challenge.
developed.2 Upon exposure of substrates to light, free radicals
Compared with MOFs,9 PMOs,8 covalent organic frame-
or radical ions, etc., will be generated in situ with the assist- works (COFs),11 conjugated microporous polymers (CMPs) and
ance of photocatalysts.3 The classical photocatalysts are mainly porous aromatic frameworks (PAFs),12 hyper-crosslinked poly-
metal complexes,4 organic dyes,5 and metal oxides/sulfides.6 mers (HCPs) are constructed directly from undecorated build-
However, these catalysts are either expensive, highly toxic, have ing blocks.13 With inexpensive Lewis acids (e.g., FeCl3 or AlCl3)
low visible-light utilization, or are unstable.3–6 Meanwhile, as the catalyst, and chloromethyl methyl ether (MOMCl) or for-
most homogeneous molecule catalysts suffer from other pro- maldehyde dimethyl acetal (FDA), or similar, as the cross-
blems; for example, they are not easy to recover from reaction linker, precursors can be easily assembled into HCP networks
via a Friedel–Crafts alkylation reaction. Moreover, HCPs gener-
ally have large specific surface areas, and are stable enough
aCollege of Science, Henan Agricultural University, Zhengzhou, Hennan 450002,
P.R. China. E-mail: anwk@henau.edu.cn, panzhl@henau.edu.cn
bCollege of Food Science and Technology, Henan Agricultural University, Zheng-zhou,
Hennan 450002, P.R. China
under harsh chemical conditions. In this respect, their concise
and compatible synthetic strategy and their significant stability
and porosity endow HCPs with great potential for selection as
catalyst carriers. Studies using HCP catalysts are important for
industrial chemistry. Molecular catalysts or ligands, such as
triphenylphosphine (PPh3), N-heterocyclic carbene, 2,2′-bis
(diphenylphosphino)-1,1′-binaphthalene (BINAP), porphyrin,
†Electronic supplementary information (ESI) available: Synthesis of monomers
and TZ-HCPs, TGA, PXRD, CV of TZ-HCPs, FT-IR and 13C CP/MAS NMR spectra
of the fresh catalyst, recyclability test of TZ-HCP1D, and NMR spectra of pro-
ducts. See DOI: 10.1039/d0gc03719b
1292 | Green Chem., 2021, 23, 1292–1299
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imidazolium/quaternary phosphonium salt, etc., have been porosity. In this context, the rational design started by introdu-
successfully embedded into HCP platforms.10i–l,14 The result- cing p-methoxyphenyl (PMP) into pytz (Scheme 1). The
ing superligands/catalysts exhibit superior catalytic perform- methoxy group in PMP will, to some extent, enhance the
ance for cross-coupling reactions, click reactions, hetero-Diels– activity of benzene and improve the solubility of pytz. TZ-func-
Alder reactions, hydroformylation and epoxidation of olefins. tionalized 2 could be facilely synthesized from PMP-substi-
Recently, several available HCP photocatalysts for organic tuted picolinonitrile via cyclization and oxidation mediated by
transformation have been reported.15 In 2014, two HCPs based hydrazine hydrate and sodium nitrite, respectively.22 As shown
on thioxanthone (TX-Ph and TX-TPM) were constructed by in Scheme 1, we then constructed the photoactive porous
Thomas and co-workers.15a These functional HCPs could be framework TZ-HCPs from violet powder 2 and benzene or 9H-
applied as effective photocatalysts for light-induced free fluorene, through a two-stage synthesis strategy. With FeCl3 as
radical/cationic polymerizations. Subsequently, Zhang et al. catalyst, and 1,2-dichloroethane (DCE) and FDA as solvent
developed benzo[c][1,2,5]thiadiazole-embedded HCP catalysts and/or cross-linking agent, the systems were first reacted at
(MOPs), and realized the photocatalytic selective bromination 45 °C for 5 h to obtain prepolymers, and then heated at 90 °C
of electron-rich aromatic rings.15b In order to promote selective for 19 h to realize depth of polymerization. In general, greyish
photocatalytic aerobic oxidation reactions, the Liu group incor- green, brownish red or brownish black powders, which are in-
porated bis-(N-carbazolyl)-9-fluorenone into HCP networks soluble in common solvents (N,N-dimethylformamide, MeOH,
yielding CF-HCP with highly catalytic efficiency.15c In addition, etc.), could be easily obtained. Experimental results revealed
these photocatalytic transformations could also be conducted that the content of benzene or compound 2 in the resulting
efficiently by PS-HCPs, which were synthesized by Liu et al. in polymers has obvious effects on the porosity of TZ-HCP1 and
2020.15d Furthermore, HCPs and metallic oxide can be its thermostability. Investigation using nitrogen adsorption–
assembled into composite photocatalysts and used for photo- desorption tests indicated that the Brunauer–Emmett–Teller
degradation of organic matter or CO2 conversion.16 Despite (BET) specific surface areas increased with benzene content,
this, available HCP photocatalysts are rare and yet to be from 3 m2 g−1 for TZ-HCP1A to 73 m2 g−1 for TZ-HCP1B to
further developed.
171 m2 g−1 for TZ-HCP1C (Table 1 ). Similar variation can also
Herein, we report the concise construction of stable HCP be observed in the Langmuir surface areas and pore volume.
networks (TZ-HCPs) embedded with s-tetrazine (TZ) units for Additionally, the microporous area revealed by the t-plot
highly efficient photocatalytic synthesis of benzimidazoles method further demonstrated that fine-tuning of the molar
(BIs). As a representative electron-deficient structure, s-tetra- ratio of benzene to compound 2 (nB/n2 = 0/1 to 5.4/1) could
zine has valuable applications in the synthesis of energetic indeed optimize the porosity of the TZ-HCPs. Considering the
compounds,17 synthetic and metal coordination chemistry,18 porosity and concentration of catalyst sites within the poly-
biorthogonal labeling19 and solar cells, etc.20 Furthermore, 3,6- mers, 20 equivalent knots were finally used and resulted in
disubstituted-s-tetrazines often display bright colors and
exhibit excellent photoactivity and oxidation ability.20b
Specifically, 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) has
always served as a highly efficient catalyst because of its easy
reduction by means of accepting an electron to form an anion
radical.21 Replacing MOMCl with low toxic FDA as cross-linker,
TZ-HCP catalysts could be facilely constructed by fabricating
the modified pytz and structural building blocks. High surface
area, and concomitant micro/mesopores within TZ-HCPs,
greatly accelerate the molecular transport process. The abun-
dant, opened-up nanopores efficiently facilitate contact of the
catalytic sites with light and with substrates. Contributing to
the remarkable visible-light absorption of pytz, TZ-HCPs dis-
played outstanding photocatalytic activity in the green prepa-
ration of BIs.
Results and discussion
As is well known, both pyridine and tetrazine are electron-
deficient units that cannot meet the requirements of the clas-
sical Friedel–Crafts alkylation reaction. Thus, electron-rich aro-
matic rings should be incorporated into the modified pytz.
Scheme 1 Rational design and “bottom-up” synthesis of photocatalytic
hyper-crosslinked polymers (TZ-HCPs) embedded with s-tetrazine.
However, the poor solubility of aromatic ring-substituted pytz
Reaction conditions: (a) N2H4·H2O, EtOH, Ar, 90 °C, 18 h; (b) NaNO2,
is unfavorable to the generation of TZ-HCPs with the desired AcOH, RT, 18 h; (c) DCE, FDA, FeCl3, 45 °C for 5 h, then 90 °C for 19 h.
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Table 1 Pore and surface properties of TZ-HCP1 prepared under
selected conditions
stable up to 429 °C under N2 (Fig. S5†). In comparison with
TZ-HCP1D, TZ-HCP2 exhibited a lower BET surface area
(718 m2 g−1) and smaller pore volume (0.72 cm3 g−1, Fig. 1a
and b). The pore size was mainly centered at 1.7 nm and
5.5 nm, suggesting that micro- and mesopores coexist in
TZ-HCP2. Nevertheless, the microporous contribution was up
to 538 m2 g−1 (75%).
MAd
PVf MPVg
MAe (%) (cm3 g−1
)
b
c
SBET
SL
Sample
nB/n2
(m2 g−1
)
a
TZ-HCP1A
TZ-HCP1B
TZ-HCP1C 5.4/1
TZ-HCP1D 20/1
0/1
3.1/1
3
73
171
754
—
110
262
—
11
94
—
15
55
50
—
0.23
0.25 0.05
0.91 0.20
—
—
Solid-state 13C cross-polarization magic-angle spinning
(CP/MAS) NMR spectroscopy was used to verify the structural for-
mation of the synthesized materials. As shown in Fig. S6,† the
resonance peaks at δ = 110–147 ppm can be attributed to aro-
matic carbon in (non-)substituted benzene, pyridine and fluor-
ene. In addition, NMR signals at ca. 36 ppm and 55 ppm indi-
cated the existence of methylene carbon and methoxy groups,
while the signal at 159 ppm can be ascribed to the carbon
atoms of CvN in pytz.23,24 FT-IR spectra (Fig. S7†) of TZ-HCP
showed obvious vibration peaks at 1605 cm−1, which were in
agreement with the signals of compound 2 and suggested the
existence of tetrazine rings.20c The signal peaks at 1495 and
1457 cm−1 could be ascribed to vibrations from the aromatic
ring skeleton,15b,24b while that at 1249 cm−1 was the signal for
the C–O stretching vibrations.24c Moreover, alkane signals at
about 2923 and 2850 cm−1 from C–H stretching vibrations can
also be found in the FT-IR spectra of TZ-HCPs.23,24 Scanning
electron microscope (SEM) analysis was performed to investi-
gate the microtopography of the TZ-HCPs. Fig. 1c and d shows
that TZ-HCP1D appears as aggregate porous particles with dia-
meters of ca. 20–50 nm, whereas a loose flake-like shape was
found for TZ-HCP2. This significant difference in morphology
might be due to the larger ring-fused structure of 9H-fluorene
compared with the benzene ring. Transmission electron micro-
scope (TEM, Fig. S8†) results showed that TZ-HCPs consisted
of nanometer-scale pores. Energy-dispersive X-ray spectroscopy
(EDS) elemental mapping images (Fig. S9†) collected from the
TEM analysis demonstrated that C, N and O elements were
well distributed over the networks of TZ-HCPs. In addition,
1139 375
a Molar ratio (nB/n2) of benzene to 2. b BET surface area. c Langmuir
surface area. d Micropore area analysed via the t-plot method.
e Contribution of microporous area. f Total pore volume calculated with
the N2 adsorbed at P/P0 = 0.99. g Micropore volume according to t-plot
analysis.
Fig. 1 (a) Nitrogen sorption isotherms and (b) pore size distribution
(PSD) of TZ-HCP1D and TZ-HCP2. PSD was calculated with non-local
density functional theory (NLDFT). (c and d) SEM images of TZ-HCP1D
(c) and TZ-HCP2(d).
TZ-HCP1D (Table 1, Fig. 1a and b) with a higher BET surface elemental analysis for nitrogen content (2.70% for TZ-HCP1D
area (754 m2 g−1) and larger total pore volume (0.91 cm3 g−1). and 1.53% for TZ-HCP2) confirm that the loading of pytz cata-
It should be noted that the microporous and mesoporous lyst in TZ-HCP1D and TZ-HCP2 is approximately 0.32 mmol
surface areas display equal contributions. Notably, just 22% g−1 and 0.18 mmol g−1, respectively. The results clearly identify
micropore volume was observed in TZ-HCP1D. This implied that the pytz unit has been well embedded into the TZ-HCP
that the abundance of mesopores within the networks pro- networks. As suggested by powder X-ray diffraction analysis,
vides an ideal platform for organic transformations involving TZ-HCPs are amorphous polymers (Fig. S10†).
large-sized guest molecules. Besides, non-local density func-
The light capture capacity of TZ-HCPs was precisely exam-
tional theory (NLDFT) showed that the pore size of TZ-HCP1D ined by UV-visible diffuse reflection (DR) spectra (Fig. 2a).
was approximately 1.7 nm and 5.3 nm, respectively (Fig. 1a Compared with pytz, precursor 2 generates a bathochromic
and b). Furthermore, thermogravimetric analysis (TGA) under shift and exhibits maximum absorption peaks at approxi-
a nitrogen atmosphere showed that the degradation tempera- mately 354 nm and 563 nm for the increase in degree of conju-
ture was 220 °C, 371 °C, and 468 °C for TZ-HCP1B–D, respect- gation (Fig. S11†). The resulting TZ-HCPs display a broader
ively (Fig. S4†). The stability difference above confirmed that photoresponse performance that ranges from 200 to 800 nm.
the degree of cross-linking might also be increased gradually In detail, the minimum reflections (i.e., maximum absorp-
from TZ-HCP1B to TZ-HCP1D and affected the formation of tions) are centered at 300–550 nm and 300–400 nm for
nanopores.23 Thus, 20/1 was set as an appropriate ratio to TZ-HCP1D and TZ-HCP2, respectively. However, TZ-HCP1D
prepare TZ-HCPs. Accordingly, TZ-HCP2, a polymer derived still exhibits strong absorption for visible light beyond 500 nm.
from compound 2 and 9H-fluorene, was then prepared using This can be ascribed to the higher content of monomer 2 in
these optimized conditions. The porous network obtained was TZ-HCP1D. In addition, a photocurrent could be detected
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able, reusable catalysts for preparation of BIs remain to be
explored. Accordingly, increasing attention has been drawn
toward green technologies, such as electrocatalysis, bio-
mimetic catalysis, and, especially, photocatalysis.26b,31,38 In
2019, Chu and co-workers reported fluorescein-assisted photo-
catalytic synthesis of BIs in acetonitrile.26b Recently, we
employed thiophene-embedded porous organic photocatalysts
(TP-CMPs) to prepare BIs in methanol.32 However, the toxic
solvents in these two cases remain to be further optimized.
Moreover, the homogeneous fluorescein is non-recyclable, and
the porosities of TP-CMPs are also unsatisfactory. Considering
these problems and the great significance of BIs, we envisage
utilizing TZ-HCPs to realize green synthesis of BIs under light.
To evaluate the activities of TZ-HCPs as robust porous
organic photocatalysts, we selected the condensation-cycliza-
tion reaction between o-phenylenediamine (3a) and p-chloro-
benzaldehyde (4a) as a model reaction (Table 2). The notice-
able influence of solvents on the conversion can be seen from
Table 2. For example, TZ-HCP1D (0.8 mol%) was hardly able to
promote the photoreaction in dichloromethane, acetone, tetra-
hydrofuran and acetonitrile (Table 2, entries 1–5). Fortunately,
the reaction proceeded smoothly in EtOH, a safer solvent, and
Fig. 2 (a) UV-visible DR spectra and (b) Kubelka–Munk plots of
TZ-HCPs. (c) Energy-band positions for TZ-HCP1D and TZ-HCP2. (d and
e) ESR spectra (in dark or under white light for 5 min) of TZ-HCPs
(1.0 mg L−1) in air-saturated EtOH that contained (d) 0.1 M DMPO or (e)
TEMPO.
within the two polymers, but TZ-HCP1D displayed a slightly 90% yield was obtained after 1.5 h. Similar results could also
larger current density than TZ-HCP2 due to its lower electro- be found for reported cases of photocatalytic synthesis of BIs.
chemical impedance (Fig. S13 and S14†). Optical band gaps The possible reason is that the viscosity, polarity and polariz-
(Eg) calculated via the Kubelka–Munk function were 1.92 eV
and 2.43 eV for TZ-HCP1D and TZ-HCP2, respectively. The
cyclic voltammetric curves in Fig. S12† reveal that the conduc-
Table 2 Optimization of the photocatalytic conditionsa
tion band positions (Ec) vs. standard calomel electrode (SCE)
are −0.48 V for TZ-HCP1D and −0.54 V for TZ-HCP2. These Ec
•−
values are unable to induce the formation of O2 from O2
(−0.86 V vs. SCE ).25 However, the estimated valence band posi-
tions (Ev) vs. SCE are 1.44 V (TZ-HCP1D) and 1.89 V (TZ-HCP2)
Entry
Catalystb (mol%)
Light
Solvent
t (h)
Yield %c
and might show strong oxidizing ability.
Benzimidazole skeletons are widespread in pharmaceutical
molecules and exhibit fascinating biological activity for cancer,
viruses, funguses, psychoses and arrhythmias.26 Besides, BI-
derived organic polymers, e.g., polybenzimidazoles (PBI), are
often used in membrane fuel cells or adsorption materials.27
Meanwhile, BIs can be employed as precursors of carbene or
as privileged ligands for palladium, rhenium, etc.10j,28 So far,
instructive synthesis strategies for BIs have been developed.
However, high temperature, acid conditions, and long reaction
times are needed for traditional methods of preparation from
carboxylic acids and aromatic diamines. Some new
approaches, developed later, addressed these problems, but
ionic liquids, oxidants, Lewis acids, molecular iodine, acid/
base additives, or metal catalysts are often necessary for the
conversion of aromatic diamines and aldehydes/alcohols.29
These catalytic systems are either difficult to purify/prepare, or
unstable in air/moisture, or are homogeneous and have less re-
usability and recyclability. A sustainable G-SO3H catalyst was
exploited to synthesize BIs efficiently by Suresh et al., yet the
preparation is still tedious and less environmentally friendly,
with large-scale use of H2SO4 and KMnO4.30 Green synthetic
methods and non-noxious, metal-free, easily prepared, recycl-
1
2
3
4
5
6
7
8
TZ-HCP1D (0.8)
TZ-HCP1D (0.8)
TZ-HCP1D (0.8)
TZ-HCP1D (0.8)
TZ-HCP1D (0.8)
TZ-HCP1D (1.6)
TZ-HCP1D (2.4)
TZ-HCP1D (1.6)
TZ-HCP1D (1.6)
TZ-HCP1D (1.6)
TZ-HCP1D (1.6)
None
TZ-HCP1D (1.6)
TZ-HCP1D (1.6)
TZ-HCP1D (1.6)
TZ-HCP1D (1.6)
TZ-HCP1D (1.6)
TZ-HCP1D (1.6)
TZ-HCP2 (1.6)
2 (1.6)
White
White
White
White
White
White
White
Red
CH2Cl2
acetone
THF
1.5
1.5
1.5
1.5
1.5
1.0
1.0
0.8
0.8
0.8
0.8
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
Trace
Trace
Trace
Trace
90
MeCN
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
98 (90)d
88
31
58
70
92
Trace
Trace
Trace
38
26
Trace
92
9
Yellow
Green
Blue
10
11
12e
13f
14g
15h
16i
17j
18k
19
20
21
White
White
White
White
White
White
White
White
White
White
97
86
80
pytz (1.6)
a General conditions: 3a (0.2 mmol), 4a (0.2 mmol), solvent (4 mL), RT,
air, 6 W white LEDs. b Catalyst dosage were based on pytz. c Isolated
yields after chromatography. For entries 1–4, 12–13 and 17, almost no
products are detected by thin layer chromatography (TLC). d 0.8 h. e No
catalyst. f Under N2 atmosphere. g In the dark. h C2O4(NH4)2 as hole sca-
venger. i KIO3 as electron scavenger. j NaN3 as 1O2 scavenger.
k Benzoquinone as superoxide radical scavenger.
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Table 3 Green synthesis of BIs catalyzed by TZ-HCPs under visible
ability of EtOH may influence the surface interaction between
the porous photocatalysts and the substrates. In comparison
with other solvents used here, EtOH might also have signifi-
cant influence on the generation of light-generated electrons
and holes, contributing to the electron transfer process and
stabilizing the reactive intermediates.33 TZ-HCP1D displayed
significant advantages (98% yield, 1.0 h) over fluorescein and
TP-CMPs after increasing the catalyst loading to 1.6 mol%
(Table 2, entry 6).26b,32 However, the higher dosage of
TZ-HCP1D did not have a positive impact on the catalytic
efficiency (Table 2, entry 7), which could be attributed to the
ineffective use of light because of blocking from the higher
concentration of solid catalyst. Thus, the photocatalyst dosage
here was set as 1.6 mol%. The influence of the nature of the
light was examined precisely (Table 2, entries 6 and 8–11).
With irradiation with 6 W white or blue LEDs for 0.8 h,
TZ-HCP1D could provide 5a in 90–92% yield. However, unsatis-
factory yields (31–70%) were obtained under red, yellow or
green light within the same reaction time. In consequence, the
photocatalytic effects really depend on the degree of matching
of the irradiation with the light reflection/absorption of
TZ-HCP1D (Fig. 2a). Considering that white light is easily avail-
able and similar to natural light, it was finally selected as the
universal light source to promote the photocatalytic reaction.
Control experiments showed that visible light and air (O2) were
essential for the photocatalytic synthesis of BIs (Table 2,
entries 12–14). To our delight, TZ-HCP2 also displayed excel-
lent catalytic effects (97% yield, Table 2, entry 19) under white-
light irradiation. Moreover, TZ-HCPs showed preferable activity
compared with the homogeneous counterparts pytz21a and 2
(entries 6, 19–21). This information implies that it is practic-
able to prepare high efficiency, non-noxious and green hetero-
geneous photocatalysts from homogeneous pytz.
lighta
a Reaction conditions:
3 (0.2 mmol), 4 (0.2 mmol), TZ-HCP1D
(1.6 mol%, 10 mg), EtOH (4.0 mL). Isolated yields after chromato-
graphy. b TZ-HCP2 (1.6 mol%, 17.8 mg) as photocatalyst. c Scale-up
experiment: 3a (10 mmol), 4s (10 mmol), TZ-HCP1D (1.1 mol%,
350 mg), EtOH (200 mL).
Once the standard reaction conditions were established, we
then studied the substrate scope of the photocatalytic reaction
(Table 3). With TZ-HCP1D as photocatalyst, most BIs could be yields (up to 98%). A scale-up experiment to synthesize 5s was
synthesized efficiently within 0.5–4.0 hours. Various aromatic conducted, to investigate the potential of TZ-HCPs in indus-
aldehydes with electron-donating/withdrawing substituents trial catalysis. Interestingly, the photocatalytic reaction of
could react very well with unsubstituted or methyl/halogen 10 mmol of substrate in 200 mL EtOH worked well (1.87 g,
(F, Cl, Br)-substituted aromatic diamines, and produced BIs 96% yield, 0.8 h) when mediated by 1.1 mol% TZ-HCP1D.
(28 examples) in 82–99% yield. Noticeably, pyrene-1-carbaldehyde Therefore, TZ-HCPs are indeed efficient photocatalysts for the
was well suited, giving 5p in a decent yield (99%), except that a synthesis of BIs at large scale.
longer reaction time was needed. The reason for this may be
Catalytic recyclability and stability are the critical factors for
the slower mass transport process of 5p in the pores caused by heterogeneous catalysts in industrial applications. Therefore,
poor solubility and larger steric hindrance. Experiments the photocatalytic performances and intrinsic structures of
showed that TZ-HCP1D was tolerant of aliphatic aldehydes TZ-HCPs were examined closely. The catalytic recycling experi-
and produced four 2-alkyl-substituted BIs (5u–5x, 82–98% ments to synthesize 5a were performed according to the stan-
yield). In addition, heterocyclic aromatic aldehydes, e.g., fur- dard conditions. TZ-HCP1D could be easily recovered via cen-
furaldehyde, picolinaldehyde, thiophene-2-carbaldehyde and trifugation or filtration from the reaction system, and then
1H-imidazole-4-carbaldehyde, could be assembled efficiently used in the next catalytic cycle. As revealed by Fig. 3,
with aromatic diamines (up to 99% yield) and led to BIs TZ-HCP1D could afford equivalent yields and maintained high
(5q–5t) with double heterocycles. Specifically, 5s and 5t might catalytic activity even after 21 iterative runs. However, nitrogen
be used as alternative metal ligands to promote Heck or sorption measurements (Fig. S17†) after the 21st run demon-
Suzuki–Miyaura coupling reactions. TZ-HCP2 also showed strated that the BET surface area and pore volume of
impressive applicability in the green synthesis of BIs; for TZ-HCP1D decreased from 754 m2 g−1 to 302 m2 g−1 and
instance, 5a–5d, 5j–5m, 5u and 5aa could be prepared in high 0.91 cm3 g−1 to 0.37 cm3 g−1, respectively. The decreased poro-
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Fig. 4 Possible mechanism for the condensation–cyclization reaction
between 3 and 4 catalyzed by TZ-HCPs under light.
Fig. 3 Catalytic recycling test of TZ-HCP1D for the reaction of 3a and
4a.
As shown in Fig. 4, upon irradiation with light, the h+ and e−
could initiate the following reaction: h+ (Ev = 1.44 V) could
capture an electron from imine I (E(I/I•+) = 0.45 V),26b,32 and
generate I•+, which then released a proton to provide II.
Cyclization of the free radical intermediate took place and was
reduced by photoexcited e− to give III. Meanwhile, partial
singlet electrons translated into excited triplet electrons via
sity within the network further led to the mass transport
slowing down, which was confirmed by the variation in the
reaction time from 60 to 110 min. Thus, perfect porosity is
crucial for superior POP catalysts. In fact, the polymeric nano-
pores might be partially blocked by the residual products.
Thus, small changes could also be found in the FT-IR spectra
of the photocatalyst used after 21 runs (Fig. S18†). In addition,
the structure of the porous catalyst might also be gradually
affected by the reactive oxygen species generated in the photo-
catalysis. However, the remaining highly efficient catalytic
results after 21 runs implied that the as-synthesized framework
was robust enough to recycle and reuse.
1
intersystem crossing, and this led to the generation of O2.
Aminal IV, derived from intermediate III by acquiring a
proton, further reacted with 1O2 to produce intermediate V.
After losing hydrogen peroxide (Fig. S20†),36 the final product
5a could be obtained.
Several control experiments (Table 2, entries 15–18) were
conducted to study the reaction mechanism. The reactions
Conclusions
were severely suppressed when photogenerated hole (h+) sca- In summary, we incorporated a versatile photocatalyst with
venger C2O4(NH4)2 or photoexcited electron (e−) scavenger high nitrogen content, namely, pyzt, into porous polymers
34
KIO3 was added into the reaction system, respectively (Table 2, (TZ-HCPs) through a bottom-up fabrication strategy. The
entries 15 and 16). These results implied that h+ and e− are TZ-HCPs were easily formed via Friedel–Crafts alkylation pro-
both crucial for the photocatalytic reaction. Meanwhile, trace moted by inexpensive and safe FeCl3. The high BET surface
product was observed in the presence of NaN3 (1O2 scavenger), areas (>710 m2 g−1) and coexisting micro- and mesopores of
•−
but the existence of benzoquinone (O2 scavenger) did not TZ-HCPs increased the accessibility of photocatalytic sites and
have an adverse effect on the yield (Table 2, entries 17 and facilitated molecular transport. On account of the oxidation
18).35 Thus, the major working oxygen species here might be ability of electron-deficient s-tetrazine, TZ-HCPs exhibited
1O2 rather than O2•−, which is in accordance with the energy- rational energy-band structures and excellent activities to
band structures of TZ-HCPs. Furthermore, DMPO and TEMPO induce the formation of singlet oxygen under visible light. It
were selected as spin-trapping agents in electron–spin reso- was shown that TZ-HCPs displayed powerful photocatalytic
nance (ESR) experiments to detect 1O2 and O2
within efficiency for the green synthesis of benzimidazoles (up to
•−
TZ-HCPs, respectively.25a,34–36 Weak signals (Fig. 2d) for 99% yield, 0.5–4.0 h) in EtOH. Benzimidazole products that
DMPO-O2 showed that O2 was not easy to generate in contain pyridine or imidazole (91–99% yield) could also be
•−
•−
TZ-HCP1D and TZ-HCP2. Upon illumination, however, obvious easily prepared at gram-scale and acted as potential ligands.
1
peaks of TEMPO-1O2 were formed, which proved that O2 was TZ-HCPs showed preferable recyclability (at least 21 iterative
produced easily within the pytz-based organic polymers runs, 96–98% yield) and stability in the reaction system. The
(Fig. 2e). It should be noted that traces of 2-methyl benzimid- catalytic performance of TZ-HCPs was comparable with fluor-
azole could be found in the reaction system (Fig. S19†). The escein and TP-CMPs reported in the literature. Additionally,
results indicated that a small amount of EtOH could be oxi- the heterogeneous photocatalytic system is consistent with the
dized into acetaldehyde by h+ or 1O2, and then reacted with principles of green chemistry because of its high efficiency
o-phenylenediamine to yield 2-methyl benzimidazole.
and reusability, especially avoiding the use of high tempera-
Based on the information obtained above, a possible photo- ture, metals, strong oxidants and acid. We believe that
catalytic reaction mechanism to prepare 5a was proposed.21a,37 TZ-HCPs will facilitate the extensive application of HCPs as
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Green Chemistry
environmentally friendly photocatalysts in the green-safe
preparation of fine chemicals.
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Author contributions
Z. Pan and W.-K. An conceived the scientific research project.
W.-K. An, S.-J. Zheng, H.-X. Zhang, T.-T. Shang, H.-R. Wang,
X.-J. Xu and M.-S. Hou prepared the TZ-HCPs and performed
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key experiments to capture 2-methyl benzimidazole, analysed
the results and revised the catalytic mechanism. H.-X. Zhang,
S.-J. Zheng and W.-K. An wrote the original manuscript. Z. Pan
and C.-L. Xu revised the manuscript. All authors discussed the
comments and contributed to generation of the manuscript.
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Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
The authors gratefully acknowledge the National Natural
Science Foundation of China (No. 21702049 and 21702047),
the Key Scientific Research Projects in Henan Colleges and
Universities (No. 18A150006).
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