The synthesis and structure of an amazing and stable carbonized material Cu-PC@OFM and its catalytic applications in water with mechanism explorations

Article abstract of DOI:10.1039/d1nj00861g

An amazing and stable carbonized octahedral frame material Cu-PC@OFM was synthesized and characterized through HRTEM, SEM, XRD, XPS, and Raman spectroscopy and nitrogen adsorption/desorption analysis. In particular, the carbon matrix carrier loaded with nano-copper not only maintains the original structure, but also the nano copper particles generatedin situsignificantly improve the catalytic performance and stability. It was disclosed that the copper-based catalyst material Cu-PC@OFM showed high catalytic activity in the borrowing hydrogen reaction and the synthesis of 1-benzyl-2-aryl-1H-benzo[d]imidazole derivatives with high yields in water. This copper catalytic system provided a much greener and efficient catalyst for the synthesis of functionalized amines and 1-benzyl-2-aryl-1H-benzo[d]imidazoles with good recovery performance in water, which was the first example for the Cu-PC@OFM material-catalyzed synthesis of 1-benzyl-2-aryl-1H-benzo[d]imidazoles. In addition, a plausible reaction mechanism was proposed through some condition control experiments, deuterium labeling experiments and separation of intermediates experiments.

Full text of DOI:10.1039/d1nj00861g

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The synthesis and structure of an amazing and
stable carbonized material Cu-PC@OFM and its
catalytic applications in water with mechanism
explorations†
Cite this: DOI: 10.1039/d1nj00861g
a
An-Qi Tian,a Xiang-Hao Luo,a Zhi-Lin Ren,b Jun Zhaoa and Long Wang
*
An amazing and stable carbonized octahedral frame material Cu-PC@OFM was synthesized and characterized
through HRTEM, SEM, XRD, XPS, and Raman spectroscopy and nitrogen adsorption/desorption analysis. In
particular, the carbon matrix carrier loaded with nano-copper not only maintains the original structure, but also
the nano copper particles generated in situ significantly improve the catalytic performance and stability. It was
disclosed that the copper-based catalyst material Cu-PC@OFM showed high catalytic activity in the borrowing
hydrogen reaction and the synthesis of 1-benzyl-2-aryl-1H-benzo[d]imidazole derivatives with high yields in
water. This copper catalytic system provided a much greener and efficient catalyst for the synthesis of
functionalized amines and 1-benzyl-2-aryl-1H-benzo[d]imidazoles with good recovery performance in water,
which was the first example for the Cu-PC@OFM material-catalyzed synthesis of 1-benzyl-2-aryl-1H-
benzo[d]imidazoles. In addition, a plausible reaction mechanism was proposed through some condition control
experiments, deuterium labeling experiments and separation of intermediates experiments.
Received 20th February 2021,
Accepted 21st April 2021
DOI: 10.1039/d1nj00861g
rsc.li/njc
materials.4,5 During this process, catalysts usually played a crucial
role and determine the occurrence or direction of the reactions.6
Introduction
Substituted 1H-benzo[d]imidazole derivatives are considered one
In 2017, Wang et al. reported that triazole-phosphine–copper
of the biologically active natural products and alkaloids for the complexes (TAP-Cu) could catalyze the selective and highly efficient
extremely useful antitumor, antibiotic, antiviral, inhibitory, anti- synthesis of 1-benzyl-2-aryl-1H-benzo[d]imidazole and fluoro-
malarial and antidiabetic activities.1 For example, Wang and substituted 2-aryl-1H-benzo[d]imidazole derivatives.2 Later, they
coworkers discovered that 2-aryl-1H-benzo[d]imidazole and discovered that [Cu(binap)I]2@HT and MnO2@PDCS could be used
1-benzyl-2-derivatives possessed superior fungicidal activities to realize the synthesis of 1-benzyl-2-aryl-1H-benzo[d]imidazoles,
against Magnaporthe grisea, Fusarium graminearum, Penicillium but couldn’t catalyze the synthesis of substituted-1H-benzo-
italicum, Rhizoctonia solani and Penicillium digitatum.2 They also [d]imidazoles.7 In 2018, Srimani and co-workers described the
observed that 1-(3,4-difluoro-benzyl)-2-(3,4-difluorophenyl)-1H- synthesis of 1,2-disubstituted and 2-substituted benzimidazoles
benzo[d] imidazole showed excellent biological activity for the by dehydrogenative coupling of primary alcohols and aromatic
agricultural fungicide triadimefon. Although the related synthetic diamines through a tridentate NNS ligand-derived manganese(I)
routes to 1H-benzo[d]imidazole derivatives have been explored, complex.8 The Singh group developed the Ir-catalyzed synthesis
most methodologies were the condensation reactions during the of 1,2-substituted benzimidazoles through the complexes of
past several decades through diamines and the corresponding (Z5-CpMe5)Ir(III) with 1-benzyl-3-phenylthio/selenomethyl-1,3-dihy-
aldehydes under strong base, acid or noble metals in organic drobenzoimidazole-2-thione/selenone in good yields.9 Later,
solvents under harsh conditions.3 Borrowing hydrogen and dehy- Hikawa and Azumaya provided a Pd-catalyzed environmentally
drogenation reactions are considered green and efficient methods benign reaction for the synthesis of 2-arylbenzimidazole derivatives
for organic synthesis with simple alcohols as the starting on water in moderate to excellent yields from benzyl alcohols and
o-phenylenediamines.10 The Zhang group described an oxidant-free
a Key laboratory of inorganic nonmetallic crystalline and energy conversion
materials, College of Materials and Chemical Engineering, China Three Gorges
University, Yichang, Hubei 443002, China. E-mail: wanglongchem@ctgu.edu.cn
b College of Chemical Engineering, Hubei University of Arts and Science, Xiangyang,
Hubei, 441053, China
synthesis of benzimidazoles from diamines and alcohols by using
Ru(II) hydride complexes bearing new quinoline-based pincer
ligands in mechanistic studies.11 In 2019, Chen and co-workers
developed several bidentate manganese complexes, which were
tested as catalysts for the synthesis of 1,2-disubstituted benzi-
midazoles and quinolines via acceptorless dehydrogenative
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj00861g
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condensations.12 The Banerjee group reported the Ni-catalyzed Fig. 2a), it was proved that an octahedral organometallic frame-
selective synthesis of substituted quinoxalines and benzimida- work of Cu-MOF-199 of high quality was successfully fabri-
zoles through dehydrogenative coupling of aromatic diamines cated. According to the thermogravimetric analysis (TGA;
with alcohols in high yields.13 Mori and Kimura developed a Fig. 3a), the calcined organic ligands of Cu-MOF-199 began to
simple Pd/Xantphos catalytic system for the synthesis of useful decompose and the organic framework began to collapse after
nitrogen-containing heterocyclic compounds such as benzimi- 400 1C. In order to obtain the carbon matrix frame carrier
dazoles via intramolecular acceptorless dehydrogenative conden- material Cu-PC@OFM, Cu-MOF-199 was calcined at 400 1C. The
sation without oxidizing agents.14 Despite all the aforemen- organic part of Cu-MOF-199 calcined at 400 1C was transformed
tioned good results, the development of a highly efficient, into a carbon matrix carrier with an obvious octahedral frame
recoverable, cheap catalytic system to obtain atomic economic structure, and the product had no obvious amorphous carbon
reactions with higher selectivity under green conditions, such as (Raman spectroscopy; Fig. 4a).
water or solvent-free conditions, remains a challenging and
meaningful topic.15,16
It can be clearly observed by SEM that the sintered product
still maintained a regular octahedral frame structure (Fig. 1b).
Due to the high specific surface area and porosity of MOF And the nano copper particles did not aggregate, and were
materials, this kind of material has been revealed to have good generated in situ after calcination (Fig. 1c). Meanwhile, lattice
catalytic performance, such as in electrocatalysis, hydrogen fringes with d = 0.206 nm were also observed, which could
evolution, oxygen evolution, etc., and so extensive exploration match the copper nanoparticles (Fig. 1d). That is, it was
of these materials has aroused people’s interest.17 Nonetheless, confirmed that such particles were nano-Cu, and not in contact
MOF materials have not been well developed in the field of with Cu2O, but were embedded in a carbon matrix carrier
organic catalysis, which may be because MOF materials do not derived from the calcination of Cu-MOF-199, indicating that
show good tolerance for high temperatures and acid and alkali copper nanoparticles were generated in situ. This result was
in organic catalytic reactions, and the structures of MOFs consistent with the results of XRD (Fig. 2b) and XPS (Fig. 3b).
collapse after one reaction. Carbonization is the solution to An obvious peak of copper nanoparticles was obtained by XRD,
the above problems and to overcome these disadvantages. Xu which matched the standard card JCPDS NO. 04-0836. Two
and Jiang are the pioneers in this field. Xu has reported lots of peaks were also clearly seen in XPS images, and the binding
excellent achievements in the fields of MOF-derived catalysts.18 energies were 932.4 eV (2p3/2) and 952.3 eV (2p1/2), respec-
Jiang has successively reported his outstanding achievements tively, both of which correspond to the total content of Cu+ and
in the fields of MOF-derived catalytic nitro reduction, alcohol Cu0, the total content being up to 95%. However, there was no
oxidation, and carbon dioxide fixation.19 In addition, Li et al. peak value of Cu2O as shown by XRD images at 400 1C,
developed Cu-MOF-74-derived Cu–Cu2O–C nanocomposites indicating that the content of Cu2O on the surface of the
to catalyze coupling of phenylacetylene to form 1, product was too low or was highly dispersed. And the CuO
4-diphenylbutadiyne under visible light, which was due to the formed by the coordination of Cu2+ with oxygen decomposed by
synergy of the three components Cu–Cu2O–C.20 Our group organic ligands in Cu-MOF-199 might be completely reduced to
realized a series of applications of MOF electrocatalysis in Cu2O and CuO by the carbon matrix carrier derived from
hydrogen/oxygen evolution and methanol oxidation.21 Based calcination. In addition, the two peaks observed correspond
on our recent discovery of heterocyclic synthesis22 herein, we to Cu2+, which were caused by the partial oxidation of the
synthesized a carbonized octahedral frame material (OFM) Cu-
PC@OFM, which was characterized through HRTEM, SEM,
XRD, XPS, and Raman spectroscopy and nitrogen adsorption/
desorption analysis. This catalytic system was successfully
applied in the borrowing hydrogen reaction and in the synth-
esis of 1-benzyl-2-aryl-1H-benzo[d]imidazole derivatives in
water with high yields. This catalytic system provided a much
greener and efficient catalyst for the synthesis of functionalized
amines and 1-benzyl-2-aryl-1H-benzo[d]imidazoles with good
recovery performance in water.
Results and discussion
Characterization of composite Cu-PC@OFM
To better understand the material Cu-PC@OFM, X-ray diffrac-
tion analysis (XRD), X-ray photoelectron spectroscopy (XPS) and
a series of tests were conducted. Firstly, by performing a series
of characterization studies of MOF-199, using scanning elec-
tron microscopy (SEM; Fig. 1a) and X-ray diffraction (XRD;
Fig. 1 SEM of MOF-199 (a), SEM of Cu-PC@OFM (b), HRTEM of
Cu-PC@OFM (c and d).
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Table 1 Optimization of reaction conditionsab
Entry
Catalyst
Additives
Ratio (4a/3a)b
Yield (4a) [%]c
Fig. 2 XRD spectrum of MOF-199 (a), XRD of Cu-PC@OFM (b).
1
2
3
4
5
6
7
8
CuO
Cu2O
CuCl2
AgNO3
Cu-PC@OFM
Cu-PC@OFM
Cu-PC@OFM
Cu-PC@OFM
—
—
—
K2CO3
KOH
KOH
—
—
o5
o5
53
12 : 1
—
11 : 1
3 : 1
420 : 1
420 : 1
—
—
–
—
KOH
o5
t-BuOK
Cs2CO3
KOH
KOH
TBAF
TBAI
38
21
82d
75e
o5
o5
o5
o5f
9
10
11
12
KOH
KOH
Cu-PC@OFM
a
Reaction conditions: 1a (1 mmol), 2a (2.6 mmol), catalyst (10 mg),
b
base (1.5 equiv.), H2O (4 mL), 100 1C or reflux, 48 h. Determined by
1H NMR. Isolated product. TBAF (15 mol%). TBAI (15 mol%).
c
d
e
Fig. 3 TGA of MOF-199 (a), XPS of Cu-PC@OFM (b).
f
No solvent.
should be noted that the reaction couldn’t take place under
solvent-free conditions (Table 1, entry 15).
To evaluate the versatility of the Cu-PC@OFM system pro-
moted process, the optimized conditions were applied to sub-
stituted phenylenediamine and various benzyl alcohols, and
the desired benzyl-2-aryl-1H-benzo[d]imidazole derivatives were
achieved in good to high yields. As summarized in Table 2, the
substituents on the phenyl moiety of benzyl alcohols were
examined and the results disclosed that high yields of the
corresponding 1-benzyl-2-aryl-1H-benzo[d]imidazole derivatives
were obtained in most cases, whereas the substrate with a steric
hindrance such as the o-MeO or o-Me group proved to be more
Fig. 4 Raman spectra of Cu-PC@OFM (a), N2 adsorption–desorption of
Cu-PC@OFM (b).
surface of copper nanoparticles. The product Cu-PC@OFM was challenging. The substrate containing a thienyl group could
calcined at 400 1C to bring high porosity into the corresponding undergo this dehydrogenation cyclization process with only
carbon matrix carrier with a specific surface area of 33.164 m2 gÀ1 moderate yield.
(N2 adsorption–desorption; Fig. 4b).
Encouraged by the promising synthesis of benzyl-2-aryl-1H-
benzo[d]imidazoles through the Cu-PC@OFM system, the clas-
sical borrowing hydrogen reaction of amines and alcohols was
further explored as the borrowing hydrogen reaction was a
useful and atom-efficient transformation. The results showed
that the reaction could proceed well under the catalyst and
water conditions. As summarized in Table 3, it was interesting
to find that alcohols bearing an electron-donating or electron-
withdrawing group could be used in this Cu-PC@OFM system
and the corresponding substituted amines were achieved with
good yields in most cases.
Catalytic activity
With Cu-PC@OFM in hand, the synthesis of 1-benzyl-2-aryl-1H-
benzo[d]imidazole was attempted to examine the catalytic
activity. The reaction of phenylenediamine and benzyl alcohol
was carried out in water with Cu-PC@OFM as the catalyst.
As shown in Table 1, it was observed that the desired
1H-benzo[d]imidazole product (3a) was separated (Table 1,
entry 3). For a higher yield, the screening conditions were next
surveyed and the results showed that potassium hydroxide was
the best base. The blank experiment revealed that only potas-
sium hydroxide couldn’t promote the formation of 1-benzyl-2-
Mechanism exploration
aryl-1H-benzo[d]imidazole. After a series of screening condi- The prepared heterogeneous Cu-PC@OFM catalyst revealed
tions, it was found that the phase transfer catalyst TBAF could excellent reactivity in the synthesis of various benzyl-2-aryl-
enhance the yield of desired product and the highest yield 1H-benzo[d]imidazole derivatives and substituted amines with
achieved was up to 82%. Several control experiments disclosed high yields in water. It was necessary to put in some effort
that this reaction couldn’t happen only with TBAF or a base. It to better understand this catalytic system and the possible
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Table 2 Reaction of phenylenediamine and benzyl alcoholsa
Scheme 1 The catalyst explorations.
couldn’t promote this reaction. When Cu-PC@OFM was
selected as a catalyst, the best results could be obtained. It
was believed that 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)
usually served as a radical scavenger to test the free radical
mechanism of a reaction. Therefore, the reaction of phenyle-
nediamine and benzyl alcohol with one equivalent of TEMPO
was set up and the results showed that the desired
1H-benzo[d]imidazole was obtained in nearly the same
yield, which could rule out the free-radical pathway for this
borrowing hydrogen and the synthesis of benzyl-2-aryl-1H-
benzo[d]imidazole derivatives (Scheme 1).
The proposed mechanism
Considering the importance and usefulness of benzyl-2-aryl-1H-
benzo[d]imidazole derivatives, the possible reaction mecha-
nism was next proposed (Scheme 2). After careful analysis
together with the literature studies, we believed that the first
step was the dehydrogenation of benzyl alcohol to produce
aldehyde (A), which underwent the condensation with diamine
to give imine (B). The intermediate (C) was formed by cyclisa-
tion. Intermediate (C) could react with another equivalent of A
to gain an intermediate (E), which is supported by ref. 7,13.
After cyclisation and rearrangement, the final product (4g) was
released to finish the reaction.
a
Conditions: 1 (1 mmol), 2 (2.6 mmol), Cu-PC@OFM (10 mg), KOH
(1.0 equiv.), TBAF (15 mol%), H2O (4 mL), 100 1C, 48 h. Isolated yields.
Table 3 The borrowing hydrogen reaction of amines with alcoholsa
The separation or detection of intermediates was an effective
method to verify and better explain the proposed mechanism.
The blank experiment for benzyl alcohol with optimal condi-
tions was carried out and the intermediate A was obtained
smoothly in 74% yield (Scheme 3). Meanwhile, the intermedi-
ate D was detected by using GC-Mass, which was supported by
ref. 13.
Additionally, the deuterium labeling experiment of benzyl
alcohol 2a in D2O showed that deuterium incorporation in the
imidazole core part occurred while benzyl alcohol (2a-d2)
a
Conditions: 2 (1.2 mmol), 5 (1.0 mmol), Cu-PC@OFM (10 mg), KOH
(1.0 equiv.), TBAF (15 mol%), H2O (4 mL), 100 1C, 48 h. Isolated yields.
mechanism. As showed in Table 1, CuO or Cu2O was not
effective for this reaction. Meanwhile, it was observed that only
the phase transfer catalyst TBAF or potassium hydroxide Scheme 2 The proposed mechanism.
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Scheme 3 The separation and detection of intermediates.
Fig. 5 XRD patterns before and after the reaction.
Scheme 4 The deuterium labeling experiments.
Scheme 6 The synthesis of fluoro-substituted 1H-benzo[d]imidazole on
a large scale.
indicated that deuterium incorporation was the only product
just as expected, which well explained the participation of the
benzylic C–H bond of alcohols for this transformation
(Scheme 4). This was also proved by the reported results.
could be maintained even when the Cu-PC@OFM composite
was reused five times. At the same time, the reusability of the
borrowing hydrogen of amine and benzyl alcohol was also
explored and nearly the same results were obtained
(Scheme 5). Meanwhile, we performed XRD characterization
for this catalyst. As shown in Fig. 5, XRD did not change before
and after this reaction, which verified that the catalyst of the
octahedral frame material Cu-PC@OFM was stable in the
reaction system. Then the leaching of the metal-catalyst experi-
ments were carried out, so as to further confirm that the
reaction is a heterogeneous catalytic process. The ICPMS data
showed that the copper content in the recovered reaction
solution is only 0.0008%, which is almost negligible. This
shows that there is almost no loss of the catalyst Cu-
PC@OFM after recovery and repetition, which further confirms
Recyclability of the catalyst
Subsequently, the heterogeneous Cu-PC@OFM catalyst was
washed and centrifuged five times with water and EtOH,
separately. After drying, the recovered composite was recycled
for the reaction of benzyl alcohol and phenylenediamine under
the optimal conditions. It was observed that the yield of the
product (4a) gradually decreased with increased recycling time
(Scheme 5). It was interesting and joyful for us that a good yield
that the reaction is
(Scheme 5).
a heterogeneous catalytic process
Finally, the large scale synthesis of fluoro-substituted 1-(3,5-
difluorobenzyl)-2-(3,5-difluorophenyl)-1H-benzo[d]imidazole was
carried out and more than twenty grams of the product was
obtained using this Cu-PC@OFM catalyst (Scheme 6). This Cu-
PC@OFM catalytic system provided an efficient method for the
synthesis of 1H-benzo[d]imidazole derivatives.
Conclusions
In conclusion, we reported the synthesis and application of the Cu-
PC@OFM material, which was characterized through scanning
Scheme 5 The recycling experiments.
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fully applied in the synthesis of 1-benzyl-2-aryl-1H-benzo-
[d]imidazole derivatives in water with high yields. This catalytic
system provided a much greener and efficient catalyst and
method for the synthesis of functionalized amines and 1-benzyl-
2-aryl-1H-benzo[d]imidazoles in water.
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Author contributions
Anqi Tian: writing – original draft, investigation, data curation.
Xianghao Luo: writing – original draft, visualization, data
curation. Zhi-Lin Ren: data curation. Jun Zhao: writing – review
& editing. Long Wang: conceptualization, methodology, Fund-
ing acquisition, writing – original draft.
¨
(k) L. Neubert, D. Michalik, S. Bahn, S. Imm,
Conflicts of interest
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Acknowledgements
We gratefully acknowledge the financial support of this work by
the National Natural Science Foundation of China (21602123,
21971143) and the 111 Project (No. D20015).
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