Two new polyoxoniobosilicate-based compounds: Syntheses, structures, characterizations and their catalytic properties for epoxidation and water oxidation

Article abstract of DOI:10.1016/j.jssc.2021.122029

Two new polyoxoniobosilicates, Na₁₂[Cu(en)₂(H₂O)₂]₅[H₃(SiOH)₂Si₂Nb₁₆O₅₄]₂·52H₂O (1) and K₄[Cu(en)₂(H<

Full text of DOI:10.1016/j.jssc.2021.122029

Journal of Solid State Chemistry 297 (2021) 122029
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Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Two new polyoxoniobosilicate-based compounds: Syntheses, structures,
characterizations and their catalytic properties for epoxidation and
water oxidation
Ting-Ting Zhang, Zi-Qiu Zhao, Ge Tian, Xiao-Bing Cui *
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University, Changchun, 130021, PR China
A B S T R A C T
Two new polyoxoniobosilicates, Na12[Cu(en)2(H2O)2]5[H3(SiOH)2Si2Nb16O54]2⋅52H2O (1) and K4[Cu(en)2(H2O)2]2[Cu(en)2(H2O)]2[Cu(en)2][SiNb18O54]⋅16H2O (2)
(en ¼ ethylenediamine), have been successfully prepared with the combination method of hydrothermal and solvent evaporation methods. Both the two compounds
were characterized by IR, PXRD, single crystal X-ray diffraction analysis, etc. Single crystal X-ray analysis shows that compounds 1 and 2 include a Dawson-like and a
crescent-like cluster, respectively. The catalytic performances of compounds 1 and 2 were evaluated for the oxidation of styrene and the oxygen evolution reaction
(OER). For the oxidation of styrene, under optimized conditions, compound 2 can achieve a 93.3% conversion in 8 h with a 71.1% selectivity toward styrene oxide,
while, for the OER compound 1 exhibited the higher performance, offering a stable current density of 10 mA cmꢀ2 at 533 mV in basic media.
1. Introduction
reported [36,37]. As for the main application of POMs, the catalysis of
POWs and POMos has formed a relatively mature research system [38].
However, many PONb studies still only focused on the property of the
photocatalytic hydrogen production [39–43], and there are few explo-
rations on their other catalytic properties.
Although there is only about a 200-year history for the poly-
oxometalate (POM) [1,2], due to its various structures [3–5] and strong
versatilities [6,7], it is always a crucial objective for inorganic chemistry
development. Up to now, most POM studies have focused on
polyoxo-tungstates (POWs), -molybdates (POMos) and -vanadates
(POVs). Compared with the POMs mentioned-above, the development of
the polyoxoniobates (PONbs) is later and slower, for the {Nb6O19} pre-
cursor as the only niobium source for the synthesis of most PONbs,
seriously hinder PONbs’ development [8]. However, the higher negative
charge and special chemical properties make PONbs own more potential
applications in many fields such as virology [9–11], anticancer [12–14],
nuclear waste treatment [15], catalysis [16–19], photolysis [18,20–22]
and electrochemistry [23–26], and more and more scientists are devoted
to the study of PONbs.
The previous PONb researches mainly concentrated on the iso-
polyniobates (IPONbs), but the unitary structure and element limited one
from exploring the properties of the PONbs to a certain extent [22,
27–29]. With the introduction of heteroatoms and transition metal
complexes (TMCs), the advancement of PONbs became rapid. The first
heteropolyniobate (HPONb) K12[Ti2O2][SiNb12O40]⋅16H2O was synthe-
sized by Nyman et al., in 2002 [30]. After that, a series of HPONbs which
include a variety of TMCs were reported [31–35]. PONbs prepared by the
introduction of lanthanide metal and their fluorescent properties are also
Polyoxoniobosilicates, as one of the subclasses of the PONbs, could be
classified into two groups based on the cluster models: {Si4Nb16O56
}
({Si4Nb16}) and {SiNb18O54} ({SiNb18}). The polyoxoniobosilicates
{Si4Nb16} was first reported by Nyman et al., in 2002 [30], and then they
synthesized another {Si4Nb16} PONbs of which {Si4Nb16} polyanions are
combined with TMCs [44]. Subsequently, three polyoxoni
obogermanates with {Ge4Nb16} which has the identical cluster model to
{Si4Nb16} as the cluster were reported by Zhang and Wang et al. [21].
and Liu et al. [45]., two of them are dipolymers constructed out of
{Ge4Nb16} and TMCs [21,45], while the other one is composed without
the transition metal [45]. In comparison with the {Si4Nb16} model, the
{SiNb18} polyoxoniobosilicate emerges later. Around 2012, Nyman et al.
reported a series of {SiNb18} PONbs with alkali metal ions as counter
cations [46]. And then, organic-inorganic hybrid polyoxoniobates were
consecutively synthesized with {SiNb18} as clusters and [Cu(en)2]2þ as
TMCs [47,48]. Just like the {Si4Nb16} compounds, there were also
{GaNb18} and {AlNb18} which have the identical cluster model to
{SiNb18} as the cluster were reported [49].
Here, we reported two new organic-inorganic hybrid poly-
oxoniobosilicates based on the integrations of the two above-mentioned
* Corresponding author.
E-mail address: cuixb@mail.jlu.edu.cn (X.-B. Cui).
https://doi.org/10.1016/j.jssc.2021.122029
Received 30 October 2020; Received in revised form 6 January 2021; Accepted 27 January 2021
Available online 8 February 2021
0022-4596/© 2021 Elsevier Inc. All rights reserved.

T.-T. Zhang et al.
Journal of Solid State Chemistry 297 (2021) 122029
different clusters and the similar transition metal complexes. The cluster
of the Na12[Cu(en)2(H2O)2]5[H3(SiOH)2Si2Nb16O54]2⋅52H2O (1) is a
Dawson-type cluster. However, K4[Cu(en)2(H2O)2]2[Cu(en)2(H2O)]2[-
Cu(en)2][SiNb18O54]⋅16H2O (2) has a crescent-like cluster which is
formed by two Lindqvist {Nb6O19} units and a {SiNb6O26} fragment. For
POMs are widely used as oxidation catalysts [50,51] and recently some
groups also employed some POMs as OER catalysts [52–54], in this
manuscript, we characterized not only the structures of the two new
compounds, but also their catalytic properties for the oxidation of styrene
and oxygen evolution reaction (OER).
measurements were performed with a CHI 760e electrochemical work-
station and electrolyte is the 1 M KOH with saturated oxygen. Before the
LSV test, 50 circles cyclic voltammetry (CV) was performed at the scan
rate of 5 mV sꢀ1 to stable the working electrode and all of the LSV ex-
periments were carried out with 1 mV sꢀ1 scan rate. All potentials re-
ported in this work were against the reversible hydrogen electrode
(RHE), which they were converted from the Ag/AgCl (3 M KCl) scale. The
current densities were calculated using the geometric surface area (0.07
cm2) and the overpotential (ƞ) was obtained using the equation of ƞ ¼
ERHEꢀ 1.23 V.
Catalysis. The epoxidation reactions were carried out in 5 mL two-
neck borosilicate microreactors with a magnetic stirrer and a reflux
condenser. The microreactors containing the olefin and solvent were
immersed in the thermostated oil bath and preheated to the desired re-
action temperature (60, 70, 80 ꢁC) for 10 min prior to the addition of the
oxidant solution; the addition of the oxidant was taken as the initial
instant of the catalytic reaction. Initial molar ratios of catalyst/olefin/
TBHP were typically 1:3704:7408 or 1:2000:4000. CH3CN or EtOH was
added as solvent. In a typical run, the catalyst (compound 1 (2.00 mg,
2. Experimental
2.1. Chemical reagents and instruments
All the chemicals were of reagent grade and used without further
purification. Infrared spectra were recorded as KBr pellets on a Perki-
nElmer Spectrum One FTIR spectrophotometer. Powder XRD patterns
were obtained with a Scintag X1 powder diffractometer system using Cu
K
α
radiation with a variable divergent slit and a solid-state detector. The
0.27 μmol), compound 2 (2.00 mg, 0.50 μmol), 0.114 ml (1.00 mmol) of
styrene, and 2.00 ml of solvent were added to the 5 ml two-neck flask
with stirring and refluxing. The mixture was heated to a certain tem-
perature and then TBHP was injected to start the oxidation reaction. The
evolutions of the catalytic reactions were monitored using a gas chro-
matograph (Shimadzu, GC-8A) equipped with a FID detector and a
electrochemical measurements were carried out on a CHI 760E electro-
chemical workstation. K7HNb6O19⋅13H2O was prepared as described in
the literature [55].
2.2. Synthesis
capillary column (ZHD SE-54, 30 m ꢂ 0.53 mm ꢂ 1.0
μm) and the
content of relative percentage of each ingredient was determined by the
gas chromatography area normalizing. In oxidation reactions of other
organic substrates like cyclohexene, cyclooctene and 1-octene, all the
procedures are identical with the exception of different organic substrate
(with identical moles to styrene) and toluene as internal standard added
in the catalytic system. Reaction products were identified by comparison
with authentic samples and GC-MS (ThermoFisher Scientific Trace Dsq)
with He as carrier gas.
Na12[Cu(en)2(H2O)2]5[H3(SiOH)2Si2Nb16O54]2⋅52H2O (1) Com-
pound 1 was synthesized by a combination method of hydrothermal and
evaporation methods. K7HNb6O19⋅13H2O (0.25g, 0.18 mmol), Cu(CH3
-
COO)2⋅H2O (0.133g, 0.67 mmol), Na2SiO3⋅9H2O (0.033g, 0.12 mmol),
Sb2O3 (0.013g, 0.050 mmol), en (0.33 ml) were dissolved in 8.00 ml
Na2CO3/NaHCO3 buffer solution (1 M, PH ¼ 10). The resulting purple
blue solution was adjusted to pH ¼ 12 with a 3 M KOH solution, and it
was then transferred to a Teflon-lined stainless steel autoclave (18 mL).
The autoclave was heated at 140 ꢁC for 72 h, and cooled to room tem-
perature at a rate of 10 ꢁC hꢀ1, then the resulting solution was filtered
and evaporated at room temperature. Deep purple single crystals were
obtained after 5 days. Yield (40% based on Nb). Anal. Calcd for
2.3. X-ray crystallography
The crystal structures of compounds 1 and 2 were determined by
single crystal X-ray diffraction. The intensity data of compounds 1 and 2
were collected on a Bruker Smart-CCD diffractometer equipped with a
H
214C20Cu5Na12N20Nb32O174Si8: Nb, 40.66; Cu, 4.35; Si, 3.07; Na, 3.77;
C, 3.29; N, 3.83; H, 2.95%; found: Nb, 40.58; Cu, 4.22; Si, 3.01; Na, 3.55;
C, 3.38; N, 3.50; H, 2.66%. IR (KBr, cmꢀ1): 3019 (w), 1584 (w), 1342 (w),
1277 (w), 1048 (w), 929 (w), 881 (m), 811 (m), 719 (m), 502 (m).
K4[Cu(en)2(H2O)2]2[Cu(en)2(H2O)]2[Cu(en)2][SiNb18O54]⋅
16H2O (2) Compound 2 was synthesized by a combination method of
hydrothermal and evaporation methods, too. K7HNb6O19⋅13H2O (0.25g,
0.18 mmol), Cu(CH3COO)2⋅H2O (0.10g, 0.50 mmol), Na2SiO3⋅9H2O
(0.013g, 0.050 mmol), Sb2O3 (0.020g, 0.070 mmol), en (0.33 ml) were
dissolved in 8.00 ml Na2CO3/NaHCO3 buffer solution (0.5 M, PH ¼ 10).
The resulting purple blue solution was adjusted to pH ¼ 12 with a 3 M
KOH solution, and it was then transferred to a Teflon-lined stainless steel
autoclave (18 mL). The autoclave was heated at 140 ꢁC for 72 h, and
cooled to room temperature at a rate of 10 ꢁC hꢀ1, then the resulting
solution was filtered and evaporated at room temperature. Deep purple
single crystals were obtained after 7 days. Yield (52% based on Nb). Anal.
Calcd for C20H124Cu5K4N20 Si Nb18O76: Nb, 41.44; Cu, 7.87; Si, 0.70; K,
3.88; C, 5.95; N, 6.94; H, 3.10%; found: Nb, 41.22; Cu, 7.97; Si, 0.81; K,
3.60; C, 5.66; N, 6.82; H, 3.05%. IR (KBr, cmꢀ1): 3243 (w), 1601 (m),
1460 (w), 1378 (w), 1107 (w), 1047 (m), 962 (w), 873 (s), 713 (s), 506
(s).
Mo K
α radiation (λ ¼ 0.71073 Å). No crystal showed evidence of crystal
decay during data collections. The two structures were solved by direct
methods and refined using full-matrix least squares on F2 with the
SHELXS-2014/7 and SHELXTL-2014/7 crystallographic software pack-
age via the Olex. 2 interface. In final refinements, all non-hydrogen atoms
were refined anisotropically. The hydrogen atoms of en were placed in
calculated positions and included in the structure factor calculations but
not refined, while hydrogen atoms of water molecules were not added. A
summary of the crystallographic data and structure refinements for
compounds 1 and 2 is shown in Table 1. CCDC number: 2036404 for
compound 1, 2036405 for compound 2.
3. Results and discussion
3.1. Syntheses
Hydrothermal and solvent evaporation are two of the most commonly
used methods for crystal synthesis and growth, and both methods have
their own advantages. As for the hydrothermal synthesis, due to the
particularity of the synthesis conditions (high temperature and pressure),
some crystals with specific structures are usually formed [56–59]. By
contrast, the room temperature evaporation synthesis is easy to generate
some high-nuclear compounds because lower temperature result in a
slower nucleation speed [60]. The combination of the two synthesis
methods can better integrate the advantages of the two methods, thereby
synthesizing more interesting crystals with diversified structures and
Preparation of the working electrode: 12 mg of graphite powder, 4
mg of catalyst and 6 μl Nujol were mixed and ground in an agate mortar
for 10 min (all of the catalysts in the corresponding ratio by weight with
25%). The blank CPE was prepared in the same procedure without the
catalyst using only 16 mg graphite powder. The homogenized paste was
firmly packed into a poly (tetrafluoroethylene) tube (surface area ¼ 0.07
cm2) and the tube surface was wiped with pan paper. Electrochemical
2

T.-T. Zhang et al.
Journal of Solid State Chemistry 297 (2021) 122029
functions. In addition to that, using buffer solution as a solvent is also a
common strategy in crystal synthesis, and it generally serves as a good
controller to make the system in a relatively stable pH environment [61].
In the synthesis of compounds 1 and 2, although the buffering capacity of
the buffer solution was exceeded by adding too much strong alkali, the
introduction of the buffer solution is still very necessary. There is nothing
target crystal formed in the absence of the buffer solution, so we specu-
late that the buffer solution may play an obbligato role in a certain
process of crystal formation. As we can see although the Sb2O3 was added
to the reaction system, it did not participate in the formation of the
crystals, perhaps it may be due to the fact that the Sb2O3 does not have a
good solubility in this reaction solutions.
Table 1
Crystal data and structure refinements for compounds 1 and 2.
X
X
X
X
a: R1 ¼
jjFoj ꢀ jFcjj=
jFoj: b: wR2 ¼ f ½wðFo2?Fc2Þ2ꢃ=
½wðFo2Þ2ꢃg1=2:
Compound 1
Compound 2
Empirical formula
H214C20Cu5Na12N20Nb32O174Si8
C20H124Cu5K4N20
SiNb18O76
4035.95
monoclinic
I2
18.8861(8)
14.7009(6)
20.2083(8)
90
Formula weight
Crystal system
Space group
a/Å
7311.31
orthorhombic
Pnnm
14.7897(9)
25.2546(16)
30.1299(16)
90
b/Å
c/Å
α
(ꢁ)
3.2. Crystal structure of compound 1
β (ꢁ)
90
90
98.941(3)
90
γ (ꢁ)
Volume/Å3
Z
11253.8(12)
2
2.158
2.196
6834.0
62645
5542.5(4)
2
2.418
2.985
3930.0
The single-crystal X-ray diffraction analysis shows that compound 1
crystallizes in the orthorhombic space group Pnnm. As for the cluster, the
slow crystallization process led it to form a middle-size polyanion which
is distinguished from the most common PONb types like Keggin and
Lindqvist ones. According to the atomic numbers and the structure, the
DC (mg mꢀ3)
μ
/mmꢀ1
F(000)
Reflections
collected
Reflections unique
R(int)
Parameters
GOF on F2
11153
cluster [H3(SiOH)2Si2Nb16O54]
11ꢀ in compound 1 is more like a Dawson
10140
0.0360
698
1.067
7920
0.0178
657
1.059
one with 18 addenda atoms and 2 center heteroatoms. There are two Si
atoms replacing two adjoining Nb addenda atoms of the Dawson cluster,
the substitution thus led the whole Dawson-like cluster of compound 1 to
have a marked distortion (Fig. 1(a)). There are two kinds of terminal and
three kinds of bridging oxygens in the Dawson-like [H3(SiOH)
Ra [I > 2
σ
(I)]
R1 ¼ 0.0943
R1 ¼ 0.0257
R
b (all data)
wR2 ¼ 0.2514
wR2 ¼ 0.0723
2Si2Nb16O54]
11ꢀ. The terminal oxygens can be divided into two in terms
Fig. 1. (a) Ellipsoid representation of the Dawson-like cluster in compound 1; (b) ellipsoid representation of the Dawson-like cluster and sodium cluster in compound
1; (c) the 1-D chain structure formed by the cluster and the sodium ions in compound 1.
3

T.-T. Zhang et al.
Journal of Solid State Chemistry 297 (2021) 122029
of Nb-Ot with distances of 1.75(1)-1.78(1)Å and Si-Ot with distances of
1.61(2)-1.63(2)Å, while the bridging oxygens can be grouped as: Si-Ob-Si
with a Si–O distance of 1.660(7)Å, Si-Ob-Nb with Si-Ob distances of
1.591(9)-1.66(1)Å and Nb-Ob distances of 2.02(1)-2.47(1)Å, and Nb-Ob-
compounds are different, leading to two different formulas. Secondly, the
space group of our compound is Pnnm, but the Nyman’s space group is
Pnn2. The higher symmetry space group of our compound indicates that
the two compounds are different. We also tried to solve the crystal
structure of compound 1 using the space group Pnn2. The resulting R
value (0.0891) of the space group Pnn2 is even relatively a little lower
than that of Pnnm (0.0943). However, we still choose Pnnm as the right
one, the reasons are listed as below: (a) we must use many hard restraints
to refine the structure when using Pnn2 as the space group; (2) the Pnn2
structure showed a Flack parameter of 0.46(10); (3) many solvent mol-
ecules cannot be found in the Fourier maps when using Pnn2 as the space
group; and (4) most importantly, Pnnm is a higher symmetry space
group. The cif result of Pnn2 is also supplied in the supporting infor-
mation for comparison.
Nb with Nb–O distances of 1.84(1)-2.17(1) Å.
11ꢀ
The asymmetric unit is composed of half a [H3(SiOH)2Si2Nb16O54
]
,
two and two halves Naþ ions, one and a quarter of [Cu(en)2(H2O)2]2þ, and
13 dissociated water molecules. The two and two halves Naþ ions could be
classified into two groups, Na(1), Na(2) and Na(3) belong to one group,
while Na(4) forms the other group by itself. Na–O distances of the former
group are 2.38(1)-2.95(2)Å. Na(1) is entrapped in the pit of
[H3(SiOH)2Si2Nb16O54
two {SiO4} of [H3(SiOH)2Si2Nb16O54
molecules. The two water oxygens as bridges, at the same time, connecttwo
Na(1) from two neighboring [H3(SiOH)2Si2Nb16O54
11ꢀ, forming a novel
]
11ꢀ, and interacts with four bridging oxygens from
11ꢀ and two oxygens from two water
]
]
dipolymer. Except the Na(1) bridges, a pair of Na(2) as two bridges simul-
taneously connect two terminal oxygens from two {SiO4} of two
3.3. Crystal structure of compound 2
[H3(SiOH)2Si2Nb16O54]
11ꢀ, indicating that the pair of Na(2) plays the same
The single-crystal X-ray diffraction analysis shows that compound 2
role ofthe wateroxygensattachedtoNa(1). Itshould benotedthatthere are
crystallizes in the monoclinic space group I2. There are half
a
11ꢀ
[SiNb18O54]
14ꢀ, a [Cu(en)2(H2O)2]2þ, a [Cu(en)2(H2O)]2þ, a [Cu(en)2]
two such pairs of Na(2) sandwiched by the two [H3(SiOH)2Si2Nb16O54
]
.
2þ, one and two halves of Kþ, and 8 dissociated water molecules in the
asymmetric unit of compound 2. As for the cluster in compound 2,
although both compounds 1 and 2 are prepared from the same sources of
{Nb6O19}, compound 2 reserved two whole Lindqvist units in its cluster.
Each Na(2) is three-coordinated by two terminal oxygens from two
11ꢀ
[H3(SiOH)2Si2Nb16O54
]
as well as one water molecule. In addition,
Na(3) is attached to two terminal oxygens from two {NbO5} of the two
[H3(SiOH)2Si2Nb16O54
11ꢀ, that is to say, the Na(3) also as a bridge links
the two [H3(SiOH)2Si2Nb16O54
11ꢀ. There are four such Na(3), each two of
which are linked by a water oxygen, acting as the two-sodium-bridge con-
necting the two adjoining H3(SiOH)2Si2Nb16O54
11ꢀ. Each Na(3) is six-
]
]
And the two {Nb6O19} units with a unique hexavacant {B–SiNb6O26
Keggin fragment by corner-sharing form the complete [SiN-
18O54
14ꢀcluster which has a crescent-like shape, as depicted in
Fig. 2(a). There are five types of oxygens in the [SiNb18O54 2-O,
14ꢀ: Ot,
3-O, 4-O, and 6-O. Fourteen Ot each are only connected to a Nb with
Nb–O distances of 1.752(5)-1.782(5)Å; thirty six 2-bridging O each
coordinate with two Nb atoms with Nb–O distances of 1.812(5)-2.130(6)
Å; two 3-bridging O each connect two Nb and one Si atoms with Nb–O
distances of 2.191(5)-2.197(5)Å and a Si–O distance of 1.657(5)Å; two
4-O each join three Nb and one Si atoms with Nb–O distances of
2.362(5)-2.514(5)Å and a Si–O distance of 1.623(5)Å, and two central
6-O each are linked to six Nb atoms with Nb–O distances of 2.226(5)-
2.529(5) Å.
Besides that, K(1) is seven-coordinated by four oxygens from a
}
]
b
]
coordinated by two terminal oxygens from two [H3(SiOH)2Si
]
μ
11ꢀ
2Nb16O54
]
as well as four water molecules. Therefore, there are two
μ
μ
μ
Na(1), four Na(2) and four Na(3) are sandwiched by the two
H3(SiOH)2Si2Nb16O54
11ꢀ, forming a novel dipolymer, as depicted in
Fig. 1(b). The two Na(1), four Na(2) and four Na(3) give rise to a {Na10
μ
]
}
μ
bridge joins two {Si4Nb16}, forming the novel dipolymer. In addition, the
shortest distance between two Na(2) is 3.09(2)Å, which is comparable to
the previously reported Na–Na distances [44,62].
Besides that, Na(4) is coordinated by 7 oxygens, four terminal ones
originate from two adjacent dipolymers, and the other three are disso-
ciated water molecules with Na–O distances in the range of 2.54(5)-
2.84(1)Å. As shown in Fig. 1(c), two Na(4) as two inorganic bridges
simultaneously connects two dipolymers to construct a novel 1-D chain
structure.
μ
μ
14ꢀ
[SiNb18O54
]
and three oxygens from three water molecules, indi-
cating that K(1) is supported by the [SiNb18O54]14-, while K(2) is eight-
coordinated by six oxygens originating from a [SiNb18O54]14- and two
oxygens from two water molecule with K–O distances of 2.919(6)-
3.06(2)Å, meaning that K(2) is located in the pit of the crescent-like
Except for the sodium metals, two [Cu(en)2(H2O)2]2þ surround the
cluster. The Cu belongs to [Cu(en)2(H2O)2]2þ which exhibits an octa-
hedral geometry with four nitrogens from two en and two oxygens from
two water molecules with Cu–N distances in the range of 2.00(1)-2.02(1)
Å and Cu–O distances of 2.5856(1)-2.7093(1)Å. Along the a axis, the Cu-
en complexes and cluster present a highly organized arrangement, each
cluster. K(3) is seven-coordinated by three oxygens from
a
14ꢀ
[SiNb18O54
]
14ꢀ, one oxygen from another [SiNb18O54
]
and three
oxygens from three water molecules, that is to say, K(3) acts as a bridge
14ꢀ
linking two neighboring [SiNb18O54
]
to form a 2-D extended frame-
14ꢀ is connected
to four K(3), while each K(3) is linked to two [SiNb18O54
14ꢀ, thus, via
14ꢀ, a novel 2-D frame-
of the clusters was surrounded by eight symmetrical [Cu(en)2(H2O)2]2þ
.
work structures. As shown in Fig. 2(b), each [SiNb18O54
]
Because the last synthesis step is evaporation, there are many disso-
ciated water molecules in compound 1, and thus a large number of
hydrogen bonds exist. There are strong N–H⋯O interactions between
nitrogens of copper fragments and oxygens of water molecules and
PONbs with N⋯O distances in the range of 2.983(19)-3.274(16) Å. And
there also exist many C–H⋯O interactions between carbons of copper
fragments and oxygens of water molecules with C⋯O distances in the
range of 3.25(3)-3.45(4) Å. However, the number of O–H⋯O hydrogen
bonds is the largest, and the O⋯O distances of O–H⋯O interactions
between water molecules and oxygens from PONbs are of 2.59(3)-
3.12(6)Å, while the O⋯O distances of O–H⋯O interactions between any
two close water molecules are of 2.89(3)-2.91(3)Å. Therefore, via the
linking of these strong hydrogen bonds, copper fragments, water mole-
cules and PONbs are connected to form a supramolecular structure.
Nyman et al. reported a compound [44] that is structurally very
similar to compound 1. However, detailed analysis of the two structures
found that there are some differences between the two. Firstly, the
numbers of the sodium and the dissociated water molecules of the two
]
the connections between K(3) and [SiNb18O54
]
work structure was constructed.
The three crystallographically different copper ions form three
different coordination complexes: [Cu(en)2(H2O)2]2þ, [Cu(en)2(H2O)]2þ
and [Cu(en)2]2þ. Cu(1) of [Cu(en)2(H2O)2]2þ exhibits an octahedral
geometry with four nitrogens from two en and two oxygens from two
water molecules with Cu–N distances in the range of 1.987(8)-2.012(9)Å
and Cu–O distances in the range of 2.6296(1)-2.7078(1)Å. Cu(2) of
[Cu(en)2(H2O)]2þ shows a square-pyramidal geometry with four nitro-
gens from two en and one oxygens from one water molecules with Cu–N
distances in the range of 1.996(9)-2.01(1) Å and a Cu–O distance of
2.4816(1) Å. And both the Cu(1) and Cu(2) complexes only served as the
charge compensation and space filling agents. However, the Cu(3) of the
metal complex [Cu(en)2]2þ which is not only linked to four nitrogens
from two en but also axially linked to two oxygens from two clusters
plays a key role in the whole structure with Cu–N distances in the range
of 2.006(9)-2.017(9) Å and a Cu–O distance of 2.4867(1)Å. Each Cu(3)
4

T.-T. Zhang et al.
Journal of Solid State Chemistry 297 (2021) 122029
Fig. 2. (a) Ellipsoid representation of the crescent type cluster [SiNb18O54
[SiNb18O54
different channels viewing along the b axis.
]
14ꢀ in compound 2; (b) the 2-D framework structure via the connections between K(3) and
] ]
14ꢀ; (c) 1-D chain structure running along the a axis formed by Cu(3) complex and [SiNb18O54 14ꢀ; (d) The three-dimensional structure with two kinds of
5

T.-T. Zhang et al.
Journal of Solid State Chemistry 297 (2021) 122029
complex links two neighboring [SiNb18O54]
14ꢀ to form a novel 1-D chain
structure running along the a axis, as depicted in Fig. 2(c). Via the con-
nections of both K(3) and Cu(3), compound 2 shows a three-dimensional
structure with two kinds of different channels viewing along the b axis
(Fig. 2(d)). We can see that one of the channels is a rectangle one with the
cross sectional area about 1.05 ꢂ 0.86 nm2 and the other is an S-shaped
one with the cross sectional area about 1.36 ꢂ 1.47 nm2 (en ligands are
omitted for clarity). Each cluster is surrounded by four S-shaped and two
rectangle channels. Viewing along the a-axis, compound 2 exhibits a new
rectangle channels with the cross sectional area about 1.05 ꢂ 1.04 nm2,
and each polyoxoanion is surrounded by four such rectangle channels
(Fig. s1).
Scheme 1. Main products of the selective styrene oxidation.
There are also three kinds of hydrogen bonds in compound 2. Due to
the number of the dissociated water molecules is lesser than that of
compound 1, the O–H⋯O bonds are not as much as those included in
compound 1. And the distance ranges for N–H⋯O, C–H⋯O and O–H⋯O
to oxidize styrene to styrene oxide in the absence of the catalysts was
determined (Table 2, entry 1).
To achieve the optimum reaction condition, the different solvent,
reaction temperature, and the dosage of the oxidant have been investi-
gated, and all the final results are shown in Table 2. The different solvent
was screened for this reaction. With ethanol as the solvent at 60 ꢁC, the
experiments have a better conversion and selectivity compared with
CH3CN as the solvent (Table 2, entries 2–5). However, when the reaction
temperature increased to 70 ꢁC, the catalytic activities sharply decreased
with the conversions were even lower than 10% (Table 2, entries 8–9).
We speculate that maybe the catalysts had been deactivated in such a
higher temperature with the ethanol as the solvent (Table 2, entries 4, 5,
8 and 9). The reason for the deactivation of the catalysts is still elusive.
We repeated the reaction in three different temperatures (60, 70, 80 ꢁC)
in CH3CN. When the reaction temperature is 80 ꢁC, we get the highest
conversions which are for compound 1 80.8% and for compound 2
82.5% (Table 2, entries 2, 3, 6, 7, 10 and 11). Besides, when the dosage of
the oxidant was increased to 2 mmol we got a better result with a higher
conversion (93.1%) and a better selectivity to styrene oxide (70.4%) over
compound 1, while the conversion and selectivity using 2 mmol oxidant
over compound 2 were increased 93.3% and 71.1%, respectively
(Table 2, entries 14 and 15). Therefore, we concluded that this POM
based catalysis system are strongly dependent on the amounts of the
TBHP oxidant. To better understand which is the active center of the
catalysts, the Cu(Ac)2 and Cu(Ac)2þen were also introduced to the re-
action system as catalysts. The results demonstrated that no matter the
amount of the TBHP is 1 or 2 mmol, the conversions which are lower than
10% over Cu(Ac)2þen are always worse (Table 2, entries 13 and 17). On
the contrary, Cu(Ac)2 has a better performance, which indicated that Cu
perhaps make the significant contribution to the activity of the catalysts
(Table 2, entries 12 and 16). The higher catalytic activity over com-
pounds 1 and 2 whether in term of the conversion or the selectivity
implies that the combination of Cu and Nb-cluster can lead to a positive
synergistic catalytic improvements to the two components. Also the ac-
tivity of the catalyst and the bigger molecular weight make them have a
better TOF.
The homogeneity or heterogeneity of compounds 1 and 2 were
confirmed by hot filtration experiments (Fig. 3). After 4h of the reaction,
the catalyst was removed from the reaction system by hot filtration and
the remaining filtrate was allowed to continue the reaction at the iden-
tical reaction temperature. GC analysis showed that the reaction didn’t
stop after the filtration of compound 1. As for compound 2, in the first 2
h, the reaction didn’t stop but at the latter 2 h there was no further re-
action happened. It is demonstrated that although compounds 1 and 2
are not completely dissolved in the reaction solution, some catalysts
which can allow the reaction to continue still dissolved. And compared
with compound 1, compound 2 has a lower solubility in this reaction
system. The catalytic reaction is considered to be homogeneous in nature
when ΔFilt/Δcat ꢄ1, where ΔFilt is the increment in olefin conversion in
the time interval 4–8h for the reaction carried out using the filtered so-
lution and Δcat is the increment in olefin conversion during the same
time interval for the reaction carried out in the presence of compound 1
or 2 [78]. Here, our ΔFilt/Δcat equals 0.90 for compound 1 and 0.83 for
are 2.892(12)-3.356(14) Å, 3.261(16)-3.494(15)
Å and 2.789(8)-
2.904(16)Å respectively. Based on these synergistic interactions of all the
hydrogen bonds, the PONbs and the coordination fragments are linked
together, therefore increasing the stability of the 3-D framework struc-
ture in compound 2.
3.4. Characterizations
The IR spectrum of compound 1 is shown in Fig. s2, in which the weak
characteristic band centred at 929 cmꢀ1 is attributed to
ν
(Si-Ob-Si), the
medium bands centred at 881 and 811 cmꢀ1 are ascribed to
(Nb-Ot), and
the medium bands centred at 719 and 502 cmꢀ1 are associated with
(Nb-Ob-Nb). As for compound 2, the weak band centred at 962 cmꢀ1 is
due to
(Si-Oc), the strong bands centred at 873 cmꢀ1 is corresponding to
(Nb-Ot), and the strong bands centred at 713 and 506 cmꢀ1 are char-
ν
ν
ν
ν
acteristic of ν
(Nb-Ob-Nb). The absorption bands at 1683-1048 cmꢀ1 and
1607-1052 cmꢀ1 are related to vibrations of en ligands in compounds 1
and 2, respectively.
The XRD patterns of compounds 1 and 2 are well coincident with the
simulated ones, confirming the crystal phase purity (Fig. s3). The dif-
ferences in the reflection intensity probably originate from the different
orientations in the crystal samples of the two compounds.
3.5. Oxidation of olefins
Due to the important position of the oxidation products of olefins in
synthetic chemistry, how to improve the conversion and selectivity of the
olefin oxidation reaction has been the focus of scientists [63–69]. For this
kind of reaction, the addition of a suitable catalyst undoubtedly is a good
way to reach the purpose. POMs which are a kind of catalysts for common
organic synthetic reactions, no matter in the homogeneous or heteroge-
neous reaction, always have a remarkable performance [38,70]. In
addition, it is widely known that various copper-based cage-like coor-
dination compounds can be employed as common catalysts in a variety of
different oxidation reactions [71–74]. The combination of the copper
coordination compounds and POMs perhaps can integrate the merits of
both the POMs and copper complexes, leading to certain novel catalysts.
In the past few years, because of the particularity of PONbs, there were
only few studies about the properties of polyoxoniobates [75,76], and the
few existing studies concentrated on only specific aspects, such as pho-
tocatalytic H2 evolution activities [42,43,77]. Here, we use the com-
pounds 1 and 2 as catalysts for the reaction of oxidation of olefins
(Scheme 1), and contrastively analyse the influence of various reaction
conditions on the catalytic results. The epoxidation of styrene to styrene
oxide with aqueous tertbutyl hydroperoxide (TBHP 70 wt% in H2O)
using compounds 1 and 2 as the catalysts was carried out in a batch
reactor. The mixture of the catalyst, styrene and solvent was heated to a
certain temperature and then TBHP was injected to start the reaction. The
liquid organic products were quantified by using a gas chromatograph
with the area normalization method. The activity of the reaction system
6

T.-T. Zhang et al.
Journal of Solid State Chemistry 297 (2021) 122029
Table 2
Oxidation of styrene catalyzed by compounds 1 and 2.
c
Entry
Catalysts
T(ꢁC)
Solvent
TBHP (mmol)
Conv(%)
Product selectivity (mol%)
TOF(hꢀ1
)
Soa
Bzaa
Othersa
1
2
3
4
5
6
7
8
No catalyst
60
60
60
60
60
70
70
70
70
80
80
80
80
80
80
80
80
CH3CN
CH3CN
CH3CN
EtOH
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
Trace
46.4
38.0
61.6
65.8
62.1
71.6
7.7
̶
̶
̶
̶
Compound 1
Compound 2
Compound 1
Compound 2
Compound 1
Compound 2
Compound 1
Compound 2
Compound 1
Compound 2
14.4
10.1
36.2
28.5
26.4
21.0
10.8
11.3
42.7
34.2
28.4
47.1
70.4
71.1
48.7
56.4
85.7
75.8
55.8
61.9
71.0
75.0
89.2
88.7
52.6
61.0
65.7
30.1
23.9
23.5
45.5
36.5
0.0
14.1
8.0
9.7
2.6
4.0
0.0
2.0
4.7
4.8
5.9
22.8
5.7
5.3
5.8
7.1
214.7
95.0
285.1
164.4
287.3
179.0
35b5
19.3
374.2
206.3
15.5
̶
EtOH
CH3CN
CH3CN
EtOH
9
EtOH
7.7
10
11
12
13
14
15
16
17
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
80.8
82.5
62.0
6.4
93.1
93.3
82.5
8.2
b
Cu(Ac)2
Cu(Ac)2þenb
Compound 1
Compound 2
431.1
233.2
20.6
̶
b
Cu(Ac)2
Cu(Ac)2þenb
a
So: Styrene oxide, Bza: benzaldehyde, Others: including benzoic acid, phenylacetaldehyde.
b
c
Cu(Ac)2: 1 mg of Cu(Ac)2; Cu(Ac)2þen: Cu(Ac)2 (1 mg)þen(30
μL), the mixture was stirred for 30min and then used as catalyst.
TOF ¼ turnover frequency, moles of substrate converted per mole catalyst added per hour.
Fig. 3. The results of the hot filtration test of compounds 1 (a) and 2 (b).
compound 2, such results suggest that the compounds 1 and 2 are ho-
mogeneous catalysts for the epoxidation of styrene with TBHP [78–82].
For the catalysts are not completely dissolved in the reaction, the reus-
ability and regeneration of compound 2 are investigated (shown in
Fig. 4). Under the identical reaction conditions, we did the experiment
three times with 10 mg of compound 2. At the end of each reaction cycle
the catalyst was recovered by simple filtration and washed with aceto-
nitrile. After the recovered catalyst was dried in air, the catalyst was
directly reused again under the same reaction condition. Each cycle
experimental data reveal that the conversions are almost identical, un-
fortunately the selectivities are not stable. After the last cycle, we carried
out the IR analysis to determine whether the catalyst has changed or not.
From Fig. s4, we can see that, after the three cycles, the intensities of the
IR have a few changes but the main peak positions are just the same.
Combined with the above views, it is indicated that compound 2 has
good recoverability and chemical stability for this catalytic reaction. The
catalytic characterization results for the as-prepared and recovered cat-
alysts are similar, and thus we can conclude that it is possible that the
homogeneous contribution is due to the dissolution of a fraction of the
Fig. 4. The recycle experiments of compound 2.
7

T.-T. Zhang et al.
Journal of Solid State Chemistry 297 (2021) 122029
Fig. 5. Electrochemical water oxidation performances of typical samples in a strong alkali solution of 1 M KOH with a traditional three-electrode system (modified
CPE, Pt wire, and Ag/AgCl (3 M KCl) electrodes as the working, counter and reference electrodes, respectively. a) LSV curves of various catalysts in 1.0 M KOH without
iR-compensation. b) LSV curves for per formula μmol. C) Tafel plots derived from Fig. 5(a).
catalyst 2 rather than to its decomposition into soluble, active metal
species [79].
catalyst for OER. Here, we take the compounds 1 and 2 as the catalysts for
the OER process. Without the IR-compensation in a strong alkali solution
of 1 M KOH solution, the test was taken with a traditional three-electrode
system (modified CPE, Pt wire, and Ag/AgCl (3 M KCl) electrodes as the
working, the counter and the reference electrodes, respectively). Their
linear sweep voltammetry (LSV) curves at the scan rate of 1 mV sꢀ1 are
shown in Fig. 5, the current densities showed a rapid increase deviating
from the flat response of the pure CP, indicating the appearance of cat-
alytic currents. As we can see, compound 1 has a superior performance
compared with compound 2. The overpotential values at 1 mA/cm2 and
10 mA/cm2 for compound 1 are 222 mV and 533 mV, respectively. And
at the same current density of 1 mA/cm2, compound 2 has a higher
overpotential of 308 mV. Cu(Ac)2 exhibits a comparable performance to
that of compound 1 with the overpotential of 542 mV at 10 mA/cm2. It is
worth mentioning that the comparisons between compound 1 and
Cu(Ac)2 by weight are unfair to the HPONbs catalysts. At the same
weight, the number of active sites in the Cu(Ac)2 electrode is about one
order of magnitude higher due to its lower molecular weight and higher
density of active sites. If we normalize the current density data taking
into account the total number of Cu centers, the activity of compound 1 is
remarkably better than that of Cu(Ac)2 (Fig. 5(b)). Beyond that, the lower
slope of corresponding Tafel plots which were drawn using the LSV data
for compound 1 (234 mV⋅decꢀ1) also indicates that it has a better activity
Encouraged by the good catalytic performances of the compounds 1
and 2 for the oxidation of styrene, under the above-mentioned optimized
reaction conditions the scope of substrates for this reaction system was
further surveyed. As shown in Table 3, unlike the styrene oxidation
catalysis, the very poor selectivities illustrate that compounds 1 and 2 did
not have a good performance in the epoxidation for cyclohexene and 1-
octene. However, both of the two catalysts have a relatively better ac-
tivity for the cyclooctene oxidation with 60.3% conversion and 44.4%
selectivity for compound 1, and 62.7% conversion and 39.7% selectivity
for compound 2. Unfortunately, the reason for the differences among the
different substrates is still elusive.
3.6. Oxygen evolution reaction
It is well known that the development and use of fossil fuels have led
to many resource and environment problems [83]. So, the research and
preparation of new energy have become a scientific hotspot in various
disciplines for many years [84–87]. As a kind of renewable clean energy
with high combustion value, the exploitation and utilization of hydrogen
energy have always been the direction of chemists’ efforts. However, so
far, electrolysis of water is still the most effective way to obtain high
purity hydrogen. In comparison with the hydrogen evolution reaction
(HER), another half-reaction, oxygen evolution reaction (OER), more
determines the efficiency of water electrolysis, it can be said that the
higher efficiency of oxygen evolution reaction we get the better speed
and ability of water electrolysis we gain [88–91]. Because of their
outstanding chemical properties and lower cost, POMs have also been
used as catalysts for oxygen evolution reaction [52–54], but until now
there is not a systematic study for heteropolyoxoniobates(HPONbs) as a
for water oxidation compared with compound 2 (314 mV⋅decꢀ1
)
(Fig. 5(c)). The electrochemical impedance spectroscopy (EIS) test was
always seen as a strong evidence for the performance of OER. We can see
compound 1 has a lower resistance (Fig. s5), indicating a good conduc-
tivity that is conducive to electron transfer which is consistent with the
Tafel slope, too. We speculated that the chain structure of cluster might
give compound 1 a better electrical conductivity. As for the stability
experiment, both the two compounds have a significant current loss in
8

T.-T. Zhang et al.
Journal of Solid State Chemistry 297 (2021) 122029
Table 3
Catalytic oxidation of different olefin substrates.a
Entry
1
Substrate
Catalyst
Product
Conv./%
Sel./%
1
2
91.7
90.3
1.8
1.3
2
3
1
2
60.3
62.9
44.4
39.7
1
2
69.4
71.1
1.8
1.5
a
Reaction condition: substrate (1 mmol), catalyst (2 mg, 0.27
μmol for compound 1, 0.50 μmol mmol for compound 2), TBHP (2 mmol), CH3CN (2 mL), t ¼ 8 h.
the first 1 h (Fig. s6), however after that, compound 2 get a better sta-
bility result and the 3D structure of compound 2 may play an important
role for its relative stability.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://do
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9

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