A Copper-Containing Polyoxometalate-Based Metal-Organic Framework as an Efficient Catalyst for Selective Catalytic Oxidation of Alkylbenzenes

Article abstract of DOI:10.1021/acs.inorgchem.0c03741

A copper-containing polyoxometalate-based metal-organic framework (POMOF), CuI12Cl2(trz)8[HPW12O40] (HENU-7, HENU = Henan University; trz = 1,2,4-triazole), has been successfully synthesized and well-characterized. In addition, the excellent catalytic ability of HENU-7 has been proved by the selective oxidation of diphenylmethane. Under the optimal conditions, the diphenylmethane conversion obtained over HENU-7 is 96%, while the selectivity to benzophenone is 99%, which outperforms most noble-metal-free POM-based catalysts. Moreover, HENU-7 is stable to reuse for five runs without an obvious loss in activity and also can catalyze the oxidation of different benzylic C-H with satisfactory conversions and selectivities, which implied the significant catalytic activity and recyclability.

Full text of DOI:10.1021/acs.inorgchem.0c03741

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Article
A Copper-Containing Polyoxometalate-Based Metal−Organic
Framework as an Efficient Catalyst for Selective Catalytic Oxidation
of Alkylbenzenes
Baijie Xu, Qian Xu, Quanzhong Wang, Zhen Liu, Ruikun Zhao, Dandan Li, Pengtao Ma,
Jingping Wang,* and Jingyang Niu*
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ABSTRACT: A copper-containing polyoxometalate-based metal−organ-
ic framework (POMOF), CuI12Cl2(trz)8[HPW12O40] (HENU-7, HENU
= Henan University; trz = 1,2,4-triazole), has been successfully
synthesized and well-characterized. In addition, the excellent catalytic
ability of HENU-7 has been proved by the selective oxidation of
diphenylmethane. Under the optimal conditions, the diphenylmethane
conversion obtained over HENU-7 is 96%, while the selectivity to
benzophenone is 99%, which outperforms most noble-metal-free POM-
based catalysts. Moreover, HENU-7 is stable to reuse for five runs
without an obvious loss in activity and also can catalyze the oxidation of
different benzylic C−H with satisfactory conversions and selectivities,
which implied the significant catalytic activity and recyclability.
catalysts did have praiseworthy catalytic performance for
selective oxidation of DPM to BP.27−34 However, POMs suffer
from some intrinsic drawbacks, including low specific surface
area, high water solubility, and difficult separation (reuse),
which shackle the further industrial applications of
POMs.32,33,35−38 Therefore, a promising solution is to select
a proper carrier and load the POMs.39 Simultaneously, metal−
organic frameworks (MOFs), which have tunable nanospaces
providing satisfactory accommodation for multitudinous guest
species, have been regarded as a new class of porous
materials.40−45 Thanks to their inherent tunability, MOFs
have cumulative application in gas storage, magnetism,
fluorescence, and catalysis. Moreover, it is a good choice to
synthesize polyoxometalate-based metal−organic frameworks
(POMOFs), which not only skillfully combine the strength of
POMs and MOFs but also circumvent the aforementioned
limitations of POMs,1,46−52 by building POMs into the entire
framework as a part of the skeleton. Aside from this way,
POMs also could be encapsulated in the pores of
MOFs46,53−56 to support the skeleton of POMOFs.27 It is
well-known that POMOFs can be tailored by judicious choice
INTRODUCTION
■
The selective oxidation of benzylic C−H to the corresponding
CO bond has recently attracted increasing attention in both
academic and industrial research.1−3 In this connection, the
oxidation product of diphenylmethane (DPM), especially
benzophenone (BP), is considered as a key intermediate for
the manufacture of many high-value chemicals such as
pharmaceutical, dyes, agrochemical, and perfumes.4−6 Tradi-
tionally, BP was mainly produced by the Friedel−Crafts
acylation reaction of aromatic compounds using a stoichio-
metric amount of homogeneous strong protonic acids (for
example, HF, H2SO4, etc.) or Lewis acids (such as AlCl3,
FeCl3, and BF3), which suffered from the generation of
corrosive and poisonous wastes.7,8 Aside from the traditional
route, a variety of stoichiometric quantities of oxidizing agents,
such as CrO3 and KMnO4, have been widely used in this
transformation, but they are gradually phased out due to the
formation of pollution.9−11 As a result, it is really highly desired
to develop a new catalytic system with environmentally
friendly hydroperoxide oxidants (e.g., H2O2
3,12,13
or tert-butyl
hydroperoxide14−21) that have outstanding catalytic perform-
ance along with easy separation in this oxidation reaction.
Polyoxometalates (POMs), as a class of nanosized discrete
anionic metal-oxide clusters, have drawn enormous attention
and are applied widely in many fields (e.g., magnetism,
medicine, electrochemistry, and especially in catalysis) because
of their unique physicochemical properties.22−26 In recent
years, many researchers have demonstrated POM-based
Received: December 22, 2020
Published: March 14, 2021
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of metal ions, bridging organic ligands, and POMs. Numerous
studies in the literature reported that transition metals,57−60
especially copper atoms,34,61−66 could be considered as active
catalytic centers in many important oxidation processes and
the introduction of transition metals may facilitate the
formation of novel POMOFs. Herein, the introduction of
copper atoms is significant from the viewpoint of both catalytic
effect and synthesis strategies. In addition, considering that the
precise choices of bridging organic ligands may help us to
obtain the desired framework, which have satisfactory size,
channel, and so on, the logical choice and design of organic
ligands are crucial.49,67−77 Owing to its diverse coordination
modes (Scheme 1), 1,2,4-triazole (trz), as a tridentate ligand,
Taking these into account, the classic POM {H3PW12O40·
nH2O} (abbreviated as PW12) was handpicked to assemble
with trz and copper acetate to prepare novel POMOFs, which
could be employed as efficient catalysts to oxidize DPM. As a
result, using the one-pot self-assembly method, we have
successfully synthesized a copper-containing POMOF based
on PW12, formulated as CuI12Cl2(trz)8[HPW12O40] (HENU-
7), in which PW12 anions as guest species are encapsulated in
the cavities of frameworks. Furthermore, HENU-7 not only
exhibits excellent reactivity in selective oxidation of DPM to
BP with high conversion and selectivity, which presents similar
or better performance than known noble-metal-free POM-
based catalysts in the literature (Table S4), but also could be
recycled for at least five runs without an obvious loss in
activity. It is worth mentioning that, in the scale-up
experiment, the DPM conversion obtained over HENU-7 is
89%, while the selectivity to BP is 96%, which shows that it is a
very promising alternative catalyst in industrial applications.
Scheme 1. Diverse Coordination Modes of trz
EXPERIMENTAL SECTION
■
Synthesis of HENU-7. H3PW12O40·nH2O (0.587 g, 0.195 mmol)
was dissolved in 5 mL of deionized water, followed by the addition of
Cu(OAc)2·2H2O (0.256 g, 1.28 mmol) with stirring the solution for
15 min. Stirring for another 30 min is essential after adding trz (0.091
g, 1.32 mmol) into the above-mentioned solution. Afterward, 2 M
HCl was added dropwise into the solution to maintain the pH value at
1.57. The mixture was transferred into a 25 mL Teflon-lined autoclave
and further heated at 160 °C for 96 h under autogenous pressure, and
finally cooling down to room temperature at the rate of 10 °C/h. The
dark block crystals could be obtained and further collected by
filtration, washed with deionized water, and dried. Yield: 48% based
on W. Elemental analysis (%) calcd for CuI12Cl2(trz)8[HPW12O40]
has recently attracted increasing attention in synthetic
chemistry.70,78−84 Besides, as the most stable and available
POMs, Keggin-type POMs have a quasi-spherical architecture
and plenty of surface oxygen atoms, which have been widely
studied.50,72,85−87
Figure 1. Ball-and-stick and polyhedral representations of HENU-7. (a) Ball-and-stick representations of {Cu4Cl(trz)4} macrocycle. (b) Four
{Cu4Cl(trz)4} macrocycles combine to form a bigger macrocycle. (c) The frameworks in HENU-7. (d) Ball-and-stick and polyhedral
representations of PW12 anions. (e, f) 3D framework of HENU-7. (color code: WO6, sea green; P, pink; W, green; C, black; N, blue; Cu, turquiose;
O, red; H, gray-25%; Cl, yellow; green and pink dotted lines in (a) and (b) represent the size of the window, respectively.)
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Table 1. Effects of the Different Factors in the Catalytic Oxidation of DPM
b
c
d
entry
cat./mol %
TBHP /mmol (equiv)
T/°C
solvent
acetonitrile
conv. /%
sel. /%
a
1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.2
0.3
0.4
0.6
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.25
3 (3)
3 (3)
3 (3)
3 (3)
3 (3)
3 (3)
3 (3)
3 (3)
3 (3)
3 (3)
3 (3)
3 (3)
2 (2)
2.5 (2.5)
3.5 (3.5)
4 (4)
3 (3)
3 (3)
3 (3)
3 (3)
30 (3)
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
65
85
75
75
75
96
67
70
40
50
57
48
35
85
92
94
96
83
88
90
92
82
96
89
91
89
99
95
99
92
98
98
99
89
98
98
99
99
93
99
98
99
98
99
98
97
96
2
3
4
5
6
7
8
9
dichloromethane
distilled water
ethanol
acetone
n-hexane
methanol
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile (1.5 mL)
acetonitrile (1 mL)
acetonitrile (5 mL)
10
11
12
13
14
15
16
17
18
19
20
e
21
a
b
Reaction conditions: DPM (1 mmol), catalyst (0.5 mol %), solvent (0.5 mL), TBHP (3 mmol), T = 75 °C, t = 24 h. TBHP = tert-butyl
c
hydroperoxide. GC conversion for target product BP was based on naphthalene as internal standard. All of the products were identified by GC−
MS spectra and GC spectra. Selectivity for BP. The scale-up experiment, reaction conditions: DPM (10 mmol), catalyst (0.25 mol %),
d
e
acetonitrile (5 mL), TBHP (30 mmol), T = 75 °C, t = 24 h.
(Mr = 4256.08): C, 4.52; H, 0.40; N, 7.90; P, 0.73; W, 51.84; Cu,
17.92; Cl, 1.67. Found: C, 4.53; H, 0.27; N, 7.97; P, 0.70; W, 51.86;
Cu, 17.90; Cl, 1.62. IR (KBr, cm−1): 3388 (w), 3120 (w), 1740 (w),
1601 (w), 1514 (s), 1286 (s), 1167 (w), 1084 (s), 968 (s), 893 (s),
801 (s).
General Procedure for the Oxidation of Alkylbenzenes. The
substrates (1 mmol), TBHP (5.5 mol L−1 in decane, 3 mmol), catalyst
(0.5 mol %), and acetonitrile (0.5 mL) were added into a glass
reaction tube; after that, the above-mentioned suspension liquid was
stirred and heated at the designed conditions in a WP-TEC-1020
parallel reactor from WATTCAS (Figure S5). Details of the reaction
conditions are described in each result. The products were
qualitatively detected by GC−MS. The conversion and selectivity
were monitored by GC based on naphthalene as the internal standard.
After the end of the reaction, the catalysts were washed by using
acetonitrile (3 × 5 mL) and then dried in an oven at 75 °C.
this way, each trz ligand bridges two adjacent Cu atoms to
form a {Cu4Cl(trz)4} macrocycle with a 5.025 × 5.025 Å
window (Figure 1a), in which the Cl atom is sited at the center
of this macrocycle. Moreover, N atoms outside trz ligands
bridged four proximal {Cu4Cl(trz)4} through Cu atoms, and
further extended to propagate a gridlike two-dimensional plane
structure with a size of 15.661 × 15.661 Å (Figure 1b). With
the help of Cu−O bonds, PW12 anions (Figure 1d) intercalate
into the cavity and connect two parallel adjacent planes
(Figure 1c), which form a three-dimensional network (Figure
1e). Crystallographic data and structural refinements of
HENU-7 are listed in Table S1. A similar structure has been
reported in 2016; Zhang et al. synthesized three pillar-layer CuI
coordination polymers, which used three different classic
POMs as the raw materials. The only difference is that HENU-
7 has one additional Cl atom and corresponding proton.84
Selective Oxidation of Alkylbenzenes. According to the
early studies,34,61−66 copper-containing POMOFs have ex-
hibited good catalytic performance for many oxidation
reactions, especially for selective oxidation of DPM, in which
copper atoms could act as catalytically active centers to drive
the oxidation process. Hence, HENU-7 was employed to
catalyze the selective oxidation of DPM to BP to evaluate its
catalytic ability. In order to optimize the reaction conditions to
obtain satisfactory conversion and selectivity, the impacts of
solvents, catalyst loading, dosage of oxidants, and reaction
temperature were investigated using HENU-7 as the catalyst.
The results are summarized in Table 1. First, through the
screening of solvent, it could be clearly observed the
conversion of DPM which was performed in acetonitrile is
RESULTS AND DISCUSSION
■
Structural Description. At a high temperature of 160 °C,
the self-assembly of PW12, copper acetate, and trz under
hydrothermal conditions generated the POMOF HENU-7.
The BVS results confirm the oxidation states of all P, W, and
Cu atoms (Table S2). Single-crystal X-ray diffraction analysis
reveals that HENU-7 crystallized in the monoclinic space
group C2/m. The asymmetric unit of HENU-7 is composed of
a quarter of [PW12]3−, four Cu+ ions (Cu1 and Cu3 site half-
occupied), two fourth Cl− ions, and two trz ligands. Copper
atoms have three types of coordination modes, which are
coordinated with nitrogen atoms, chlorine atom, and oxygen
atoms to structure a five-coordinated distorted triangular
bipyramidal/four-coordinated distorted tetrahedral/three-co-
ordinated “T”-type coordination configuration (Table S3). In
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much higher than that of other solvents, while all the
selectivities range from 92% to 99% (Table 1, entries 1−7).
Different catalyst loadings (0.2−0.6 mol %) were conducted to
study the influence of dosage in the oxidation process. From
0.2 to 0.5 mol % of catalyst dosage, the conversions showed an
upward trend, while selectivity approximatively remained
unchanged (Table 1, entries 1 and 9−11). However, the
conversion remained unchanged when 0.6 mol % HENU-7
was used, which is probably due to the agglomeration of
catalysts (Table 1, entry 12).88,89 As a consequence, 0.5 mol %
was chosen as the final catalyst dosage. As is known to all,
oxidants play a crucial role in the oxidation process, and their
dosage has a significant effect on the conversion and selectivity.
Therefore, a range of equivalents of TBHP have been
evaluated, and 3 mmol of TBHP presented good performance
(Table 1, entries 1 and 13−16). Under the condition of the
above parameters, the temperature effect was investigated, and
the selective oxidation of DPM over HENU-7 at 75 °C
afforded a 96% conversion and a 99% selectivity. With the
further rise in temperature to 85 °C, a slight increase of
conversion could be observed. Hence, the optimal temperature
was locked at 75 °C for the further investigations (Table 1,
entries 1, 18, and 19). In addition, the dependence of catalytic
performances on the volume of solvent has been also studied
(Table 1, entries 1, 19, and 20), and the results demonstrated
that, by improving the solvent volume, the conversions
decreased gradually, which is presumably due to the
concentration decline of the substrate (oxidant).90,91 Notably,
the scale-up experience was also conducted; DPM (10 mmol)
could be transformed into BP with 89% conversion and 96%
selectivity at 75 °C for 24 h in the presence of 0.1 g of HENU-
7, which further implied the excellent catalytic performance
and the potential application of HENU-7 (Table 1, entry 21).
Given the above results, the optimal conditions (Table 1, entry
1) have been determined for the subsequent catalytic
evaluation.
Cu(OAc)2 behaved more active than PW12 and trz, which
implied the vital roles of the copper atoms within HENU-7 as
the efficient catalytic sites in this transformation (Table 2,
entries 2−4). Moreover, the significant difference between
HENU-7 and its raw materials for their catalytic trans-
formation of DPM to BP distinctly indicated the great
contribution of hierarchical structures of HENU-7 (Table 2,
entries 5 and 8).33,57 As far as we know, HENU-7 presents
similar or better performance than known noble-metal-free
POM-based catalysts in the literature (Table S4). In addition,
one of the earlier reported catalysts has been synthesized and
employed to catalyze this transformation (Table 2, entries 6
and 7; Table S4, entry 5).32 And under the optimal conditions,
the DPM conversion obtained over P2W15Co3 is 51%, while
the selectivity to BP is 98%. However, when the 0.3 mmol of
pyridine was added as the additive, the conversion increased a
lot (51% to 83%), which was still lower than that of HENU-7.
The excellent catalytic performance of HENU-7 has been
demonstrated by the comparison result (Table 2, entries 6−8).
Scope of Substrates. Encouraged by the above good
results of DPM oxidation by using TBHP as the oxidant, the
scope of substrates were also further surveyed, which
demonstrated the excellent catalytic ability of HENU-7. The
results are summarized in Table 3. In general, the variation of
Table 3. Oxidation Reaction of Different Substrates
To highlight the excellent catalytic performance of HENU-7,
some reactions were carried out at the optimal conditions. GC
analysis after the blank experience indicated that the desired
BP could be obtained in the absence of any catalysts with a low
conversion (31%), while the corresponding selectivity is 99%
(Table 2, entry 1). Furthermore, the raw materials of HENU-7
were also used as the catalysts, and results showed that
Table 2. Catalytic Activity of Different Samples for
Oxidation Reaction of DPM
a
Reaction conditions: substrates (1 mmol), TBHP (3 mmol), catalyst
b
(0.5 mol %), acetonitrile (0.5 mL), T = 75 °C, t = 24 h. GC
conversion for target product BP was based on naphthalene as
internal standard. All of the products were identified by GC−MS
spectra and GC spectra. Selectivity for ketone.
b
c
entry
cat.
reaction system
conv. /% sel. /%
a
1
31
35
81
34
63
51
83
96
99
93
98
99
90
98
99
99
c
2
3
4
5
PW12
Cu(OAc)2
trz
PW12/Cu(OAc)2/trz
P2W15Co3
P2W15Co3/pyridine
HENU-7
homogeneous
homogeneous
homogeneous
homogeneous
heterogeneous
homogeneous
heterogeneous
catalytic efficiency for the substrates is mainly attributed to the
diverse C−H bond reaction activities.33,92 Both electron-
acceptor −NO2 and electron-donor −OCH3 substituted
substrates showed satisfactory conversions (84% and 93%)
and selectivities (98% and 97%) (Table 3, entries 2 and 3),
however, which were lower than that of ethylbenzene under
the equal reaction conditions (Table 3, entry 1). Moreover, a
good result was obtained for 4-ethyltoluene with a 90%
conversion and full selectivity (Table 3, entry 4). In addition,
alkylbenzenes with a large volume or steric hindrance, such as
fluorine, tetralin, and 2-benzylpyridine, can also be transformed
into the corresponding ketone products by HENU-7, with the
d
6
7
d
8
a
Reaction conditions: DPM (1 mmol), TBHP (3 mmol), catalyst (0.5
b
mol %), acetonitrile (0.5 mL), T = 75 °C, t = 24 h. GC conversion
for target product BP was based on naphthalene as internal standard.
All of the products were identified by GC−MS spectra and GC
spectra. Selectivity for BP. P2W15Co3 = [Na(H2O)5](NH4)7-
[P2W15O56Co3(H2O)3(OH)3Re(CO)3]·13H2O. Pyridine as the
additive (0.3 mmol).32
c
d
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conversion ranging from 82% to 99% (Table 3, entries 5−7).
Notably, tetralin could be transformed into the corresponding
product (1-tetralone) with a 89% conversion and a 83%
selectivity. And the slight decrease of selectivity is probably due
to the formation of the byproduct (2,3-dihydro-1,4-naphtho-
quinone). Such good results demonstrated that HENU-7 was
applicable to selective oxidation of alkylbenzenes.
Catalytic Stability. Another important indicator to
evaluate the performance of a heterogeneous catalyst is its
reusability and reproducibility. For this purpose, a series of
consecutive reactions were performed over HENU-7 in the
transformation of DPM to BP. HENU-7 can be easily collected
by centrifugation after each catalytic run, washed with
acetonitrile, and dried in an oven. The recovered HENU-7
was applied in the subsequent runs, as shown in Figure 2b. In
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03741.
Structures, BVS, PXRD, IR, TG, catalytic experiments
mass spectrum for the corresponding product, catalytic
device, and kinetic study (PDF)
Accession Codes
CCDC 2026862 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Jingping Wang − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, China;
orcid.org/0000-0001-8079-5939; Email: jpwang@
henu.edu.cn
Jingyang Niu − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, China;
orcid.org/0000-0001-6526-7767; Email: jyniu@
henu.edu.cn
Figure 2. (a) The line graphs of heterogeneous test of HENU-7. (b)
The histogram of conversion and selectivity for reused HENU-7 in
the oxidation of DPM.
its fifth catalytic run, the conversion is 94%, only 2% less than
its initial conversion value, which might be due to mass loss of
the catalysts during its rinsing procedure, indicative of good
stability of the catalyst HENU-7. The good stability of HENU-
7 is probably due to the robust framework of the classic POM
PW12, which supports the construction of POMOFs.27
Moreover, the recovered catalysts kept the integrity of the
framework after the oxidation process, which is proved by
PXRD (Figure S2). In addition, the hot filtration experiment
confirmed its heterogeneous system (Figure 2a). After the
removal of the catalyst, the increase of conversion was unsharp,
which proved the heterogeneous nature of this catalytic system.
Authors
Baijie Xu − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, China;
orcid.org/0000-0002-0481-8614
Qian Xu − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, China
Quanzhong Wang − Henan Key Laboratory of
Polyoxometalate Chemistry, College of Chemistry and
Chemical Engineering, Henan University, Kaifeng, Henan
475004, China; orcid.org/0000-0002-2571-0995
Zhen Liu − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, China
Ruikun Zhao − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, China
Dandan Li − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, China
Pengtao Ma − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, China;
orcid.org/0000-0002-0587-8635
CONCLUSION
■
In summary, through a one-pot self-assembly method, a
{Cu4Cl(trz)4}-based POMOF has been successfully prepared,
which encapsulated classic Keggin-type POM PW12 anions as
guest species. With TBHP as the oxidant, HENU-7 showed
excellent catalytic activity toward the selective oxidation of
DPM, where a 96% conversion and 99% selectivity could be
obtained. Moreover, HENU-7 is stable to duplicate the
oxidation reaction five times without an obvious loss in
activity, and HENU-7 also could catalyze the scale-up
experiment with a satisfactory conversion and selectivity. To
our best knowledge, HENU-7 outperforms most reported
noble-metal-free POM-based catalysts. In future work, much
efforts will be devoted to tailor novel POMOFs with excellent
stability and remarkable catalytic performance for oxidation
reactions.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03741
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the Natural
Science Foundation of China (Grants 21571050, 21573056,
21771053, and 21771054).
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