Polyoxometalate-based metal-organic frameworks for selective oxidation of aryl alkenes to aldehydes

Article abstract of DOI:10.1021/acs.inorgchem.8b00282

Polyoxometalates (POMs) show considerable catalytic performance toward the selective oxidation of alkenes to aldehydes, which is commercially valuable for the production of pharmaceuticals, dyes, perfumes, and fine chemicals. However, the low specific surface area of POMs as heterogeneous catalysts and poor recyclability as homogeneous catalysts have hindered their wide application. Dispersing POMs into metal-organic frameworks (MOFs) for the construction of POM-based MOFs (POMOFs) suggests a promising strategy to realize the homogeneity of heterogeneous catalysis. Herein, we report two new POMOFs with chemical formulas of [Co(BBTZ)₂][H₃BW₁₂O₄₀]·10H₂O (1) and [Co₃(H₂O)₆(BBTZ)₄][BW₁₂O₄₀]·NO₃·4H₂O (2) (BBTZ = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene) for the selective oxidation of alkenes to aldehydes. Compound 1 possesses a non-interpenetrated three-dimensional (3D) cds-type open framework with a 3D channel system. Compound 2 displays a 3D polyrotaxane framework with one-dimensional channels along the [100] direction. In the selective oxidation of styrene into benzaldehyde, compound 1 can achieve a 100% conversion in 4 h with 96% selectivity toward benzaldehyde, which is superior to that of compound 2. A series of control experiments reveal that the co-role of [BW₁₂O₄₀]^5- and Co²⁺ active center as well as a more open framework feature co-promote the catalytic property of the POMOFs in this case. This work may suggest a new option for the development of POMOF catalysts in the selective oxidation of alkenes.

Full text of DOI:10.1021/acs.inorgchem.8b00282

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Polyoxometalate-Based Metal−Organic Frameworks for Selective
Oxidation of Aryl Alkenes to Aldehydes
Yuanyuan Ma, Haiyue Peng, Jianing Liu, Yonghui Wang,* Xiuli Hao, Xiaojia Feng,†,§
†
†
†
,†
†,‡
†
,†
,†
Shifa Ullah Khan, Huaqiao Tan,* and Yangguang Li*
†Key Laboratory of Polyoxometalate Science of Ministry of Education Faculty of Chemistry, Northeast Normal University,
Changchun 130024, China
‡
School of Chemical and Biological Engineering, Taiyuan University of Science and Technology, Taiyuan 030021, China
§College of Science, Shenyang Agricultural University, Shenyang 110866, China
*
S Supporting Information
ABSTRACT: Polyoxometalates (POMs) show considerable catalytic performance
toward the selective oxidation of alkenes to aldehydes, which is commercially
valuable for the production of pharmaceuticals, dyes, perfumes, and fine chemicals.
However, the low specific surface area of POMs as heterogeneous catalysts and
poor recyclability as homogeneous catalysts have hindered their wide application.
Dispersing POMs into metal−organic frameworks (MOFs) for the construction of
POM-based MOFs (POMOFs) suggests a promising strategy to realize the
homogeneity of heterogeneous catalysis. Herein, we report two new POMOFs with
chemical formulas of [Co(BBTZ) ][H BW O ]·10H O (1) and
2
3
12 40
2
[
1
Co (H O) (BBTZ) ][BW O ]·NO ·4H O (2) (BBTZ = 1,4-bis(1,2,4-triazol-
3 2 6 4 12 40 3 2
-ylmethyl)benzene) for the selective oxidation of alkenes to aldehydes. Compound
1
possesses a non-interpenetrated three-dimensional (3D) cds-type open framework
with a 3D channel system. Compound 2 displays a 3D polyrotaxane framework
with one-dimensional channels along the [100] direction. In the selective oxidation
of styrene into benzaldehyde, compound 1 can achieve a 100% conversion in 4 h with 96% selectivity toward benzaldehyde,
5−
2+
which is superior to that of compound 2. A series of control experiments reveal that the co-role of [BW O ] and Co active
12
40
center as well as a more open framework feature co-promote the catalytic property of the POMOFs in this case. This work may
suggest a new option for the development of POMOF catalysts in the selective oxidation of alkenes.
1
. INTRODUCTION
Selective oxidation of alkenes to aldehydes is one of the most
important and commercially valuable chemical reactions, since
aldehydes (especially benzaldehydes) have been widely used for
Polyoxometalates (POMs) as a well-defined library of nano-
sized metal−oxo clusters have been extensively explored in the
field of catalysis, owing to their tunable acidity, high thermal
stability, unique redox properties, and inherent resistance to
oxidation. To maximize the benefit of POMs, the design and
synthesis of POM-based metal organic frameworks (POMOFs)
have been developed as an efficient strategy that can realize a
good dispersal of POMs at the molecular level and can expose
more accessible active sites. Meanwhile, the appropriate
porous structures or channels in POMOFs facilitate the
accessibility of reactant molecules to active POM species,
actualizing the homogeneity of heterogeneous catalysis. To
the production of pharmaceuticals, dyes, perfumes, and fine
14−17
chemicals.
In this reaction system, POMs have exhibited
7,18
excellent activity and selectivity as homogeneous catalysts.
However, the poor recyclability of POM catalysts hindered
their further application. The construction of POM-based
heterogeneous catalysts usually decreases the catalytic activity
due to the low specific surface area and the low exposure of
POM active sites. Thus, constructing new POMOF catalysts
becomes a feasible strategy for solving the above problem. In
order to introduce catalytically active POM units into the
POMOFs, we conducted a series of preliminary experimental
studies (Table S1 in the Supporting Information), demonstrat-
ing that polyoxotungstate-based catalysts exhibit potential
catalytic activities on the selective oxidation of styrene to
1
−5
6
,7
8
−13
date, plenty of POMOFs have been reported;
however,
only a few of them were successfully explored as heterogeneous
catalysts, and the reaction models catalyzed by POMOFs are
still limited since the catalysts usually possess specific activity
7
b,19
benzaldehydes.
Among these catalysts, K [BW O ]
5 12 40
11−13
and selectivity for certain reactions.
Therefore, it is still a
exhibits ideal catalytic activity and selectivity compared to
great challenge to design and prepare desired POMOFs
according to the requirement of specific catalytic reactions
and to expand their catalytic reaction scopes.
Received: January 31, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00282
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Inorganic Chemistry
Article
those of Na [PW O ] and K [SiW O ]. Meanwhile, the
3
12 40
4
12 40
high negative charges of K [BW O ] are also conducive to
5
12 40
construct 3D porous frameworks with the metal−organic
7
coordination fragments. Moreover, considering the unique
catalytic activity of the Co element in many oxidation
2
0,21
5−
transformations,
we selected cobalt ions, [BW O ] ,
12 40
and the neutral N-donor bridging ligand (1,4-bis(triazol-1-
ylmethyl)benzene (BBTZ)) (Figure S1) to construct new
POMOF catalysts for the selective oxidation of alkenes. Herein,
we report two new POMOFs: [Co(BBTZ) ][H BW O ]·
2
3
12 40
1
0H O (1) and [Co (H O) (BBTZ) ][BW O ]·NO ·4H O
2 3 2 6 4 12 40 3 2
(2). Compound 1 exhibits a 3D open framework with a non-
interpenetrated cds-type topology. The channels in three
directions are interconnected with each other to form a 3D
channel system. Compound 2 possesses a 3D polyrotaxane
framework with only 1D channels. Compound 1 shows
excellent catalytic activity in the selective oxidation of styrene
to benzaldehyde and realizes a full conversion of styrene in 4 h
with a 96% selectivity toward benzaldehyde, which is superior
to that of compound 2. Furthermore, compound 1
demonstrates good compatibility and recyclability in the
catalytic reaction system. To our knowledge, compound 1
represents the first POMOF catalyst unleashing the advantage
in the oxidation reaction of styrene to benzaldehyde.
Figure 1. (a) Ball-and-stick and schematic views of the basic building
units (1D loop-containing chains and POM chains) in the structure of
compound 1. (b) Schematic view of the cds-type framework in
compound 1. (c) Ball-and-stick and polyhedral views of the 3D open
framework in compound 1 viewed along the c axis.
2
. RESULTS AND DISCUSSION
Crystal Structure of Compound 1. X-ray single-crystal
diffraction analysis revealed that compound 1 crystallizes in the
monoclinic system with the C2/c space group. The crystallo-
graphically asymmetric unit is constitutive of one Co2+ (Co1)
ion, two BBTZ bridging ligands, one protonated α-Keggin-type
2−
polyoxoanion [H BW O ] , and ten lattice water molecules
3
12 40
(Figure S2). Among them, the Co center (Co1) adopts a six-
coordinated mode by connecting to four nitrogen atoms
originating from four BBTZ ligands and two O atoms derived
2−
from two [H BW O ] units (Figure S2). The Co−N bond
3
12 40
distance ranges from 2.057(13) to 2.073(14) Å, and the bond
lengths of Co−O are 2.303(15) Å. The O/N−Co−N/O bond
angles vary from 89.4(6)° to 180° (Table S2). In compound 1,
the BBTZ ligands adopt only a cis-type configuration, which are
coordinated to the adjacent two Co centers into 1D loop-
containing chains (Figure 1a). The ligands in the loop units can
Figure 2. (a) Ball-and-stick and polyhedral views of the 3D open
framework of compound 1 with 3D channels viewed along the c axis.
(b) The channel along the [001] direction in compound 1. The
solvent accessible voids are modeled with a yellow background.
2
−
be considered as linkers. The [H BW O ] units can be also
3
12 40
viewed as linkers which are connected by adjacent Co centers
to form into POM-containing chains (Figure 1a). These loop-
containing 1D chains and POM-containing chains are further
perpendicularly connected with each other, forming a rare non-
dispersion of POMs and exposes more active sites for
heterogeneous catalysis.
5
interpenetrated 3D four-connected {6 .8} net with the CdSO4
Crystal Structure of Compound 2. Compound 2
(cds) topology (Figure 1b). The open framework of compound
crystallizes in the triclinic system with a P1 space group. The
̅
1
contains a 3D channel system (Figure 1c). These channels
crystallographically asymmetric unit is constitutive of one
5−
2+
intersect with each other and extend in three different
directions (Figures 2 and S3−S5). The window size is
[BW O ] polyoxoanion, three Co (one Co1, two Co2)
12 40
ions, four BBTZ ligands, six coordinated water molecules
connected to two Co2 centers, four lattice water molecules, and
a NO3− ion (Figure S6). The valence states of Co and W have
been confirmed by bond valence sum (BVS) calculations and
X-ray photoelectron spectra (XPS) (Table S3 and Figure S7).
In compound 2, two kinds of metal−organic coordination
polymer moieties exist in the structure, that is, the 2D network
units and the 0D loops (Figure 3). The 2D network holds one
Co center (Co1), which displays a hexa-coordinated octahedral
configuration surrounded by two O atoms from two
8.50(7) Å × 8.50(7) Å along the [001] direction, 6.31(1) Å
×
5.47(9) Å along the [110] direction, and 6.31(1) Å ×
.47(9) Å along the [110] direction, as shown in Figures S3−
S5. It is worth noting that the channels with the window size of
.50(7) Å × 8.50(7) Å are hydrophobic and the other two kind
5
̅
8
of channels are hydrophilic, which could be a benefit for the
entrance and reaction of organic substrates and H O2
2
molecules (Figure 2). For compound 1, the solvent accessible
volume is ca. 28.9% based on the calculation of the PLATON
2
2
5−
program. Such an open framework realizes the good
[BW O ] units and four N atoms originating from four
12
40
B
DOI: 10.1021/acs.inorgchem.8b00282
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Inorganic Chemistry
Article
Figure 3. (a, b) Ball-and-stick and schematic views of the 2D metal−
organic network unit and 0D loop unit in compound 2. (c, d) Ball-
and-stick and schematic views of the 2D polyrotaxane architecture in
compound 2.
Figure 4. (a) Ball-and-stick and polyhedral views of the 3D open
framework of compound 2 along the a axis. (b) Schematic view of the
3D structural topology in compound 2. (c) 1D channel in compound
2
. The solvent accessible voids are modeled with a yellow background.
BBTZ ligands (Figure S6). The Co1−N bond distance is in the
range of 2.107(8)−2.120(8) Å, and the Co1−O bond length is
compound 1 as the catalyst and hydrogen peroxide as the
oxidant (Tables 1 and S5). As shown in Table 1, the reactions
2
.155(10) Å. The N(O)−Co1−N(O) bond angles are in the
range of 89.2(4)−180° (Table S4). In such a 2D network, all of
Table 1. Optimization of the Different Reaction Conditions
the BBTZ ligands are in trans configuration, jointing the Co1
for Styrene Oxidation to Benzaldehyde by Using Compound
4
centers into a 2D network with 4 sqp topology (Figure 3a).
a
1
as the Catalyst
The 0D loops also contain one Co center (Co2) which exhibits
an octahedral coordination geometry, defined by two N atoms
5−
from two BBTZ ligands, one O atom from the [BW O ]
12
40
unit, and three coordinated water molecules (Figures 3b and
S6). The bond lengths of Co2−N/O are in the range of
b
b
entry
solvent
T (°C)
conv. (%)
select. (%)
2
.110(10)−2.122(10) Å. The bond angles of N(O)−Co2−
1
2
3
4
5
6
7
8
9
toluene
ethanol
DMF
70
70
70
70
70
80
60
50
40
50 ± 4
73 ± 2
100
77 ± 2
49 ± 3
51 ± 1
62 ± 3
96 ± 3
73 ± 2
96 ± 1
95 ± 1
94 ± 2
N(O) are in the range of 90.3(5)−175.8(6)° (Table S4). In
this fragment, adjacent Co2 ions are jointed by two cis BBTZ
ligands, forming 0D loops (Figure 3b). It is noteworthy that the
H O
2
91 ± 2
>99
2
D network unit penetrates the 0D loops, exhibiting an
CH CN
3
interesting polyrotaxane architecture (Figure 3c and 3d). In
compound 2, each [BW O ] unit can be viewed as a four-
CH CN
>99
3
5−
CH CN
63 ± 3
34 ± 5
12 ± 3
1
2
40
3
connection node that is linked with two Co1 atoms from the
adjacent 2D nets and two Co2 atoms from two 0D loops
CH CN
3
CH CN
3
a
(
Figure S8). Based on such a connection, the 2D polyrotaxane
Reaction conditions: styrene (0.25 mmol), H O (1 mmol), catalyst
2
2
b
networks are linked together by POM units, generating a 3D
open network (Figures 4a and S9). If the 0D loop is viewed as a
linker, while the Co1 center and [BW O ] unit are reduced
(0.028 mmol), and solvent (2 mL) for 4 h. Determined by GC
analysis.
5−
1
2
40
to six-connected and four-connected nodes, respectively, the
whole framework of compound 2 adopts a 4,6-T19 topology
in toluene and ethanol exhibit a low conversion (50% and 73%,
respectively) of styrene and low selectivity (77% and 49%,
respectively) to benzaldehyde (Table 1, entries 1 and 2,
2
3
6 9
with the point symbol {4 .6 .8}{4 .6 } (Figures 4b and S9).
Such an open framework contains 1D channels with a window
size of 10.23(9) Å × 6.62(1) Å along the [100] direction
respectively). In DMF and H O, the catalytic reactions achieve
2
(
2
Figures 4c and S10). The solvent-accessible void in compound
is 17% calculated by using the PLATON program.
Selective Oxidation of Styrene. Selective oxidation of
a good conversion of styrene, but the selectivity (51% and 62%,
respectively) to benzaldehyde is still low (Table 1, entries 3 and
4). In comparison, acetonitrile was the suitable solvent for this
reaction to afford a full conversion of styrene and high
selectivity of benzaldehyde in 4 h (96%; Table 1, entry 5). The
reaction temperature also affects the catalytic performance in
this reaction system (Table 1, entries 5−9). With an increase of
temperature to 80 °C, the selectivity of benzaldehyde dropped
to 70%, which may be due to the over-oxidation of
2
2
alkenes into aldehydes is an important commercial reaction
since aldehydes have emerged as significant chemical materials
and been widely used for the production of pharmaceuticals,
23−27
dyes, perfumes, and fine chemicals.
Herein, the selective
oxidation of styrene was chosen as a model reaction to evaluate
the catalytic activities of POM-based MOF materials. To
optimize the reaction conditions, styrene oxidation reactions
were first carried out in various solvents at 70 °C using
27
benzaldehyde. Decreasing the temperature to 40 °C led to
the decline of the styrene conversion. On the basis of a series of
C
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control experiments, the suitable temperature that can achieve
the best conversion and selectivity is 70 °C.
With the aforementioned optimized reaction conditions, we
systematically evaluated the catalytic activities of compounds 1
and 2 on the selective oxidation of styrene. When compound 1
was used as the catalyst, the full conversion of styrene was
achieved in 4 h with a 96% selectivity to benzaldehyde (Figures
reaction conditions (Figure 5c and Table 2). Using the same
equivalents of POM precursor (K BW O ·11H O) as the
5
12 40
2
catalyst, the conversion of styrene was just 51% after 12 h,
which is lower than that of both compounds 1 and 2. The
relatively low activity of the POM precursor can be ascribed to
its low specific surface area, which hinders the accessibility of
the reaction substrate to the active site. The use of a transition-
metal precursor (Co(OAc) ·4H O) to catalyze this reaction led
5a and S11; Table 2, entry 1). Using compound 2 as the
2
2
to a 33% conversion and a 91% selectivity, suggesting that the
2
+
Co ion also possesses a relatively high selectivity to
benzaldehyde (Figure 5c; Table 2, entries 3−5). Furthermore,
28
Co(BBTZ) Cl
exhibits a catalytic activity and selectivity
2
2
similar to those of Co(OAc) ·4H O, indicating that the BBTZ
2
2
ligand makes no contribution in enhancing the catalytic activity
of the catalysts (Figures 5c and S12; Table 2, entry 6). A blank
experiment in the absence of any catalyst revealed a negligible
conversion (<10%) of styrene (Figure 5c). These results
demonstrate that both POM units and Co2+ ions make an
important contribution to the high activity and selectivity of
compounds 1 and 2. In order to gain further insight into the
structural influence of compounds 1 and 2 on the catalytic
performance, we selected another non-porous POM-based
organic−inorganic hybrid compound with the chemical formula
[
Co (BBTZ) (H O)][BW O ]·4H O as the reference cata-
2.5 4 2 12 40 2
29
lyst 3 for the selective oxidation of styrene. Compound 3
possesses a chemical composition similar to that of compounds
1
and 2 but has no solvent-accessible void in its crystal structure
(
Figure S13). The catalytic reaction with compound 3 achieved
Figure 5. Conversion of styrene and selectivity of products in the
selective oxidation of styrene with (a) compound 1 and (b) compound
a 100% conversion in 12 h and a 92% selectivity to
benzaldehyde, which was obviously slower than the reactions
catalyzed by compounds 1 (4 h) and 2 (6 h), but kept a high
selectivity (Figure 5d and Table 2, entry 7). This result suggests
that styrene oxidation catalyzed by compound 3 just occurs on
the surface of the catalyst. As for compounds 1 and 2, their
inherent porous property exposes more catalytic sites both on
the external surface and in interior channels, thus accelerating
the oxidation reaction.
2
as catalysts. (c) The conversion of styrene with a series of control
samples as catalysts as a function of reaction time. (d) The conversion
of styrene and selectivity of products in the selective oxidation of
styrene catalyzed by compound 3. Reaction conditions: styrene (0.25
mmol), H O (1 mmol), catalysts (0.028 mmol), and acetonitrile (2
2
2
mL) at 70 °C.
Table 2. Catalytic Oxidation of Styrene to Benzaldehyde
a
Furthermore, we expanded the substrate scope in order to
investigate the general applicability of the catalytic system. As
summarized in Table 3, both electron-withdrawing and
electron-donating substrates can be converted smoothly and
selectively into corresponding aldehydes by the use of
compound 1 as catalyst. 4-Fluorostyrene and 4-chlorostyrene
exhibit 95% and 98% conversion after 4 and 6 h, respectively
(Table 3, entries 2 and 3, respectively). 4-Nitrostyrene can also
be fully converted and generate the corresponding aldehyde
with a high selectivity (99%) in 6 h (Table 3, entry 4). 4-
Methylstyrene achieved 97% conversion and a 99% selectivity
in 6 h (Table 3, entry 5). As for 4-methoxystyrene and 4-
isopropylstyrene, a good conversion (>95%) can be obtained
after 7 h, and the selectivities corresponding to benzaldehyde
are 97% and 94%, respectively (Table 3, entries 6 and 7,
respectively). 2-Vinylnaphthalene can also achieve a 94%
conversion and a 96% selectivity after 7 h. It is presumed
that the different reaction times for the oxidation of various
styrene derivatives may be related to the molecular size of the
substrates (Figure S14). With the increasing of substrate size,
achieving the full conversion requires a longer reaction time.
This result suggests that the substrates with a large molecular
size may not match the channels in compound 1, thus
decreasing the conversion efficiency. Additionally, compound 2
can also high-selectively catalyze the oxidation of styrene
derivatives to the corresponding aldehydes with a much longer
with H O2
2
c
c
time
(h)
conv.
(%)
select.
(%)
entry
catalyst
1
2
3
4
5
compound 1
compound 2
4
6
>99
96 ± 3
93 ± 2
89 ± 2
91 ± 3
85 ± 2
>99
K BW O ·11H O
12
12
12
51 ± 2
33 ± 4
64 ± 3
5
12 40
2
Co(OAc) ·4H O
2
2
K BW O ·11H O Co(OAc) ·
5
4
12 40
H O
2
2
b
2
6
7
Co(BBTZ) Cl2
12
12
35 ± 3
94 ± 2
90 ± 2
92 ± 2
2
compound 3
a
Reaction conditions: styrene (0.25 mmol), H O (1 mmol), catalyst
2
2
b
(
0.028 mmol), and acetonitrile (2 mL) at 70 °C. Both POM and
c
cobalt salt catalysts are 0.028 mmol. The values are based on GC
analysis.
catalyst, the reaction required 6 h to afford the 100%
conversion of styrene with a 93% selectivity to benzaldehyde
(
Figure 5b; Table 2, entry 2). Since both compounds possess a
similar composition, the distinction of their catalytic activities
should mainly stem from their structural differences. Com-
pound 1 possesses 3D channels, which can expose more active
sites than compound 2.
To illuminate the catalytic active sites in compounds 1 and 2,
a series of contrast experiments were conducted under the same
D
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a
2
Table 3. Selective Catalytic Oxidation of Styrene Derivatives to the Corresponding Aldehydes with H O
2
a
b
Reaction conditions: styrene derivatives (0.25 mmol), H O (1 mmol), catalyst 1 (0.028 mmol), and acetonitrile (2 mL) at 70 °C. The values are
2
2
based on GC analysis.
5
−
reaction time (Table S6). These results further confirm that the
pore size of the catalysts has an obvious influence on the
catalytic activities in such a selective oxidation of styrene series.
Overall, compounds 1 and 2 exhibit an excellent catalytic
performance for the selective oxidation of styrene to aldehydes
[BW O ] and Co2+ catalytically active centers, together
12 40
with a more open framework of POMOFs, facilitate the
selective oxidation of styrene to aldehyde. This work suggests a
new option for the development of POMOF catalysts.
Engineering large porous POMOFs with catalytically active
POM units and extended organic bridging ligands may generate
more efficient catalysts for the oxidation of alkenes to
aldehydes. This work is ongoing in our group.
(
Table S7).
The recyclability of compound 1 as a heterogeneous catalyst
has also been investigated. The catalyst can be easily recovered
from the reaction system by centrifugation and reused seven
times without significant loss of catalytic activity (Figures S15
and S16). These results demonstrate the good stability and
recyclability of compound 1. PXRD patterns and IR spectra
after recycling disclose that compound 1 retains its structural
integrity (Figures S17 and S18). The insoluble nature of
compound 1 was also examined by a hot filtration experiment
4. EXPERIMENTAL SECTION
Materials and Methods. All commercially available reagents for
syntheses were used without further purification. 1,4-Bis(triazol-1-
ylmethyl)benzene (BBTZ) as a ligand was synthesized according to a
reported procedure. K [α-BW O ]·11H O was prepared according
to literature methods. [Co2.5(H
has been reported by our group earlier and synthesized based on
literature methods. Compound Co(bbtz) Cl was synthesized with
reference to the literature methods.
3
0
5
12 40
2
3
1
O)
(BBTZ) ][BW12O40]·4H O (3)
2
2
4 2
(Figure S19). After 2 h of reaction, the catalyst was removed
2
9
from the reaction system and the filtrate was kept under its
original conditions. No further reaction was detected,
confirming the heterogeneity of this material. Moreover, the
UV−vis spectra of the filtrate also showed that no signal of
POM was detected, further confirming the stability of
compound 1 (Figure S20).
2
2
2
8
Synthesis of [Co(BBTZ) ][H BW O ]·10H O (1). BBTZ (0.205
g, 0.85 mmol) was dispersed in CH CN/H O (10 mL, v/v = 3:3) at
2
3
12 40
2
3
2
room temperature, and then K [α-BW O ]·11H O (1.29 g, 0.4
5
12 40
2
mmol) and Co(OAc) ·4H O (0.127 g, 0.5 mmol) were added to the
2
2
solution and stirred for 0.5 h. Then, concentrated HNO (30 μL) was
3
added dropwise to adjust the pH value of the hybrid solution to 1.0.
The turbid liquid was transferred to a 23 mL Teflon-lined autoclave
and heated at 130 °C for 4 days. After the solution was slowly cooled
to room temperature, orange block crystals were obtained and filtered
(56% yield based on W). The final pH value of the solution was 2.5.
Anal. Calcd for C H N O BW Co: C 8.05, H 1.32, N 4.69, B 0.30,
3
. CONCLUSION
In summary, we reported two new POMOFs which were
utilized in catalyzing the selective oxidation of styrene to
aldehydes for the first time and which showed excellent
catalytic activity and high selectivity. Compound 1 displays a
24 47 12 50
12
W 61.68, Co 1.65. Found: C 8.17, H 1.41, N 4.72, W 61.64, Co 1.68.
−1
Selected IR (solid KBr pellet, cm ): 3370(m), 3111(m), 1621(w),
3D non-interpenetrated cds-type framework with a 3D open
1
9
523(s), 1426(m), 1371(w), 1284(s), 1217(w), 1132(s), 995(m),
54(s), 914(s), 823(s). TG analysis demonstrates that the first weight
channel system. Compound 2 exhibits a 3D polyrotaxane
framework with 1D channels. Compound 1 can achieve a 100%
conversion of styrene in 4 h with a 96% selectivity toward
benzaldehyde, which is superior to that of compound 2. A
series of control experiments demonstrate that both
loss ranging from 50 to 200 °C can be identified as the loss of ten
lattice water molecules (Figure S21).
Synthesis of [Co (H O) (BBTZ) ][BW O ]·NO ·4H O (2).
3
2
6
4
12 40
3
2
K [α-BW O ]·11H O (1.29 g, 0.4 mmol) and BBTZ (0.144 g, 0.6
5
12 40
2
E
DOI: 10.1021/acs.inorgchem.8b00282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
mmol) were dispersed in CH CN/H O (10 mL, v/v = 2:4) at ambient
details), PXRD, FTIR, TG, UV−vis absorption spectra,
selected bond lengths and angles, additional structures,
and additional catalytic experiments (PDF)
3
2
temperature, and then a mixture of K [α-BW O ]·nH O (0.44 g,
5
12 40
2
0
.15 mmol) and Co(OAc) ·4H O (0.102 g, 0.4 mmol) was added and
2 2
stirred for 0.5 h. The pH value of the hybrid solution was regulated to
.8 by concentrated HNO (10 μL). Then, the final suspension was
1
Accession Codes
3
transferred to a 23 mL Teflon-lined autoclave and heated at 130 °C for
CCDC 1587314−1587315 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
4
days. After the solution was chilled, pink block crystals were obtained
and filtered (82% yield based on W). The final pH value of the
solution was 3.6. Anal. Calcd for C H N O BW Co : C 13.59, H
48
68 25 53
12
3
1
8
3
1
.60, N 8.26, B 0.25, W 52.08, Co 4.17. Found: C 13.67, H 1.71, N
−1
.35, W 52.10, Co 4.15. Selected IR (solid KBr pellet, cm ): 3445(w),
126(m), 1610(m), 1523(s), 1423(m), 1381(w), 1349(w), 1280(s),
207(m), 1128(s), 1016(m), 960(s), 912(s), 821(s). TG analysis
AUTHOR INFORMATION
Corresponding Authors
E-mail: wangyh319@nenu.edu.cn.
E-mail: tanhq870@nenu.edu.cn.
E-mail: liyg658@nenu.edu.cn.
reveals that the first weight loss ranging from 50 to 400 °C
corresponds to the loss of four lattice and six coordinated water
molecules (Figure S20).
■
*
*
*
Procedure for Selective Oxidation of Styrene to Benzalde-
hyde. In a typical case, styrene (0.25 mmol) and the catalyst (0.028
mmol) were dispersed in CH CN (2 mL) and stirred for 5 min. Then,
3
3
0% H O (1 mmol) was injected into the above admixture and
ORCID
2
2
heated at 70 °C for 4−12 h. A small amount of the mixture was
Yangguang Li: 0000-0002-9696-8192
Notes
The authors declare no competing financial interest.
regularly removed and centrifuged. The filtrate was ascertained by GC
1
and H NMR analyses. After reaction, the catalyst was retrieved by
centrifugation, washed with CH CN, H O, and C H OH alternately,
3
2
2
5
and then dried in an oven at 70 °C for 24 h. For the recyclability
experiments, the reactions were conducted under the same reaction
conditions.
X-ray Crystallography. Crystallographic data for this paper have
been deposited at the Cambridge Crystallographic Data Center:
CCDC 1587315 for 1, 1587314 for 2. Crystal data and structural
refinement for 1 and 2 are listed in Table 4. Selected bond lengths and
angles of 1 and 2 are listed in Tables S2 and S4.
ACKNOWLEDGMENTS
The authors gratefully acknowledge financial support from the
National Natural Science Foundation of China (21671036 and
1771033), Fundamental Research Funds for the Central
Universities (2412016KJ018), and the Opening Project of Key
Laboratory of Polyoxometalate Science of the Ministry of
Education (130028721).
■
2
Table 4. Crystal Data and Structure Refinement for 1 and 2
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formula
Mr
C H N O BW Co
C H N O BW Co
3
1
4
2
4
47 12 50
12
48 68 25 53
12
3579.67
298(2)
monoclinic
C2/c
4237.07
(
T (K)
298(2)
triclinic
crystal system
space group
a (Å)
1
P1
̅
17.9972(7)
17.9935(6)
22.9940(13)
90
14.6466(5)
14.6578(7)
14.7089(7)
118.132(5)
100.992(3)
109.294(4)
2389.92(18)
1
b (Å)
c (Å)
α (deg)
β (deg)
101.509(5)
90
γ (deg)
V (Å3)
7296.5(6)
4
2
Z
μ (mm−1)
F(000)
reflns
19.153
6380
14.982
(
1929
14432
21419
Rint
0.0438
1.059
0.0409
GOF
1.006
a
R1 [I > 2σ(I)]
0.0802
0.0664
b
wR2 (all data)
0.1955
0.1659
a
R = ∑∥F | − |F ∥/∑|F |. wR = ∑[w(F − F ) ]/∑[w(F ) ]1/2.
b
2
2
2
2
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DOI: 10.1021/acs.inorgchem.8b00282
Inorg. Chem. XXXX, XXX, XXX−XXX