Synthesis and catalytic activities of two new extended Preyssler–type tungstophosphates with different cavity centers

Article abstract of DOI:10.1016/j.inoche.2016.10.018

Two new extended Preyssler-type polyoxometalates (POMs) constructed by Fe(III), 2,2′–biimidazole and Preyssler–type tungstophosphate anions with Na and Ag cavity centers respectively, namely {(H₄biim)₅H[Fe(H₂biim)(H₂O)₂(ZP₅W₃₀O₁₁₀)]·17H₂O}_n (Z?=?Na, compound 1; Z?=?Ag, compound 2; H₂biim?=?2,2′–biimidazole), were designed and synthesized under hydrothermal conditions, and were systematically characterized by physico–chemical and spectroscopic methods. The two compounds are isostructural coordination polymers. In compound 1 or 2, there exists interesting infinite 1D chains composed of Preyssler–type [ZP₅W₃₀O₁₁₀]^14? (abbreviated as {ZP₅W₃₀}) polyanions bridged by Fe(III)–complex fragments, and these chains further formed 3D supramolecular frameworks via extensive hydrogen–bonding interactions. Especially for compound 2, two types of transition metals, i.e. one Ag⁺ ion as a center in the inner cavity and two Fe³⁺ ions as modified cations on the outer surface of one {ZP₅W₃₀} unit, existed in the same compound. Additionally, the electrochemical behaviors and acid catalytic activities of compounds 1 and 2 have been investigated.

Full text of DOI:10.1016/j.inoche.2016.10.018

Inorganic Chemistry Communications 73 (2016) 119–123
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Synthesis and catalytic activities of two new extended Preyssler–type
tungstophosphates with different cavity centers
Jing Du, Mei-Da Cao, Zai-Ming Zhu, Fang Su ⁎, Lan-Cui Zhang ⁎
School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2016
Received in revised form 2 October 2016
Accepted 7 October 2016
Available online 08 October 2016
Two new extended Preyssler-type polyoxometalates (POMs) constructed by Fe(III), 2,2′–biimidazole and
Preyssler–type tungstophosphate anions with Na and Ag cavity centers respectively, namely
{
(H
4 5 2 2 2 5 30 2 n
biim) H[Fe(H biim)(H O) (ZP W O110)]·17H O} (Z = Na, compound 1; Z = Ag, compound 2;
2
H biim = 2,2′–biimidazole), were designed and synthesized under hydrothermal conditions, and were system-
atically characterized by physico–chemical and spectroscopic methods. The two compounds are isostructural co-
ordination polymers. In compound 1 or 2, there exists interesting infinite 1D chains composed of Preyssler–type
Keywords:
14−
5 30 110
[ZP W O ]
5
(abbreviated as {ZP W30}) polyanions bridged by Fe(III)–complex fragments, and these chains
Preyssler–type polyoxometalate
Tungstophosphate
Iron (III)–complex
Hydrothermal synthesis
Acid catalysis
further formed 3D supramolecular frameworks via extensive hydrogen–bonding interactions. Especially for com-
+
3+
pound 2, two types of transition metals, i.e. one Ag ion as a center in the inner cavity and two Fe ions as mod-
ified cations on the outer surface of one {ZP 30} unit, existed in the same compound. Additionally, the
electrochemical behaviors and acid catalytic activities of compounds 1 and 2 have been investigated.
5
W
©
2016 Published by Elsevier B.V.
As a kind of well–known metal–oxo–cluster compounds, poly-
oxometalates (POMs) have aroused wide concern due to their
controllable molecular size, inimitable structural features and promis-
ing applications in catalysis, electrochemistry and material science [1].
The oxygen–rich surface of POM can be modified by transition metal–
organic complexes [2]. The Preyssler–type polyanion cluster
POM–based coordination polymer decorated with transition metal
Ag(I) complex has been harvested. In this work, Ag ions exist
both in the inner cavity and the exterior of the Preyssler–type
polyanion [7]. Nevertheless, a Preyssler–type polyanion modified
by different kinds of transition metal ions both on its surface and in
its cavity has not been reported. The main reason is that it is difficult
+
1
4−
[ZP
5
W
30
O
110
]
({ZP
5
W30}) with an inner cavity and 30 terminal O
to isolate and purify the {ZP
metal ions and to find suitable synthesis method to obtain high–
quality single crystals. 2,2′–biimidazole (H biim), a kind of N donor
5
W30} polyanion encapsulated with
atoms, as one of largest phosphotungstate clusters, possesses unusual
doughnut–shaped structure and exhibits remarkable physico–chemical
2
stability [3]. The {ZP
appropriate size, such as various alkali, alkaline–earth, transition metal,
lanthanide and actinide cations on the inner surface of the polyanion
5
W
30} polyanions can capture different cations with
ligand, possessing effective and multiple coordination ability as
well as intricate hydrogen–bond interaction [8], can be introduced
into the skeleton of the Preyssler–type polyanion to form high qual-
ity inorganic–organic hybrid crystalline materials. Herein, two new
extended Preyssler–type polyoxometalates with the same structure
[4], and provide opportunities to bind metal ions or metal-complexes
forming extended hybrid materials [5]. That is to say, Preyssler–type
polyanion can be modified both on its surface and in its inner cavity.
and different inner cavities modified by Fe(III) and 2,2′–H biim/
n
O, {(H biim) H[Fe(H biim)(H O) (ZP 110)]·17H O} (Z =
2
5
Therefore, {ZP W30} polyanions can be regarded as promising building
H
2
4
5
2
2
2
5
W
30
O
2
blocks to construct novel POM–based inorganic–organic hybrid mate-
rials with distinct properties. For example, a series of Preyssler–type
polyanion–based inorganic–organic hybrid materials containing differ-
ent first–row transition–metal ions (e.g., Cu(II), Ni(II), or Co(II) ions)
and N–donor polydentate ligands have been reported by Wang's and
Sun's groups, respectively [6]. However, in the above Preyssler–type
polyanion–based hybrid materials, the introduced transition metal
ions located on their exteriors only. In 2014, a 3D Preyssler–type
Na, compound 1; Z = Ag, compound 2) were successfully harvested
by hydrothermal method from a system inclusion of {NaP 30}/
{AgP 30} polyanion, FeCl ·6H O, H biim and H O (Scheme 1) [9].
The acid catalysis activities of the as–prepared compounds were ex-
plored. In addition, the optimal catalytic conditions and reusability
of the catalysts were researched in detail.
5
W
5
W
3
2
2
2
Single crystal X–ray structural analysis [10] reveals that compounds
1 and 2 are isomorphic compounds, their asymmetric unit contains
three types of subunits, i.e. a {NaP
5
W
30}/{AgP
5
W
30} polyanion, one
biim] cations (Figs. S1 and S2).
30} polyanions in the two compounds are es-
sentially the same as the {NaP 30} derivative previously reported in
3+
2+
[Fe(H
2
biim)(H
2
O)
2
]
and five [H
4
⁎
Corresponding authors.
The structures of the {ZP
5
W
E-mail addresses: sufang861218@163.com (F. Su), zhanglancui@lnnu.edu.cn
(L.-C. Zhang).
5
W
http://dx.doi.org/10.1016/j.inoche.2016.10.018
387-7003/© 2016 Published by Elsevier B.V.
1

120
J. Du et al. / Inorganic Chemistry Communications 73 (2016) 119–123
and 2, respectively. And the bond lengths of Fe1\\O are 1.918(12)–
.116(13) and 1.894(13)–2.102(15) Å for compounds 1 and 2, respec-
tively (see Tables S2 and S3). Bond valence sum (BVS) calculations
12] show all W, P, Ag, Fe atoms are in +6, +5, +1 and +3 oxidation
2
[
states in compounds 1 and 2, respectively, which are consistent with
the results of X-ray photoelectron spectroscopy (XPS) analysis (see
below). In addition, no protonated O atom was noted on the polyanions,
and H protons were added due to charge-balance considerations. As
shown in Fig. 1b, each pair of adjacent {ZP
5
W
30} polyanions are bridged
3+
2 2 2
by one [Fe(H biim)(H O) ]
subunit, forming into infinite 1D linear
chains, and further packed into a 3D supramolecular structure through
hydrogen bonding interactions (N\\H⋯O/OW 2.43(3)–3.27(4) and
2
.51(3)–3.26(3) Å for compounds 1 and 2, see Tables S4 and S5) and
electrostatic attractions (Fig. 2). The discrete protonated H biim mole-
biim]2 cations) (Fig. 2c) fill in the spaces as compensating
cations, and the results indicate that the positive charges of
2
+
cules ([H
4
[H
4
biim]2 cations are important for the formation of the final supra-
molecular network (Fig. 2a, Figs. S3–S5).
The IR spectra of {NaP 30}, {AgP 30}, compounds 1 and 2 are pre-
sented in Figs. S6 and S7. They all exhibit the characteristic vibrations of
+
5
W
5
W
−
1
the {ZP
925–918 cm ), and νas(W—Ob/c) vibrations (801–762 cm ) (O
central oxygen; O : terminal oxygen; Ob/c: bridging oxygen) [6,8]. The
5 a t
W30} polyanion: νas(P—O ) (1168–1076 cm ), νas(W–O )
−
1
−1
(
a
:
t
−
1
wide frequency ranges of 3479–3450 cm are associated to ν(O—H)
of water molecules. The weak bonds in the range of 3147–2931 cm−
1
Scheme 1. Designation and synthetic process of compound 1/2.
2
can be assigned to ν(C—H) of H biim molecules. The absorption in the
−
1
1
985 [11]. Compared with compound 1, there are two types of transi-
range of 1604–1596 cm can be regarded as the stretching vibrations
of the imidazole ring [8]. To investigate the thermal stability, thermo-
gravimetric analysis (TG/DTA) was applied, and the result curves of
compounds 1 and 2 were shown in Fig. S8. The first weight loss step
at 35 to 200 °C is 3.8% (calcd. 3.5%) for compound 1 and 3.6% (calcd.
3.5%) for compound 2, which are ascribed to the loss of all lattice
water molecules. From 200 to 1000 °C, a main weight loss of 13.3% for
compound 1 (11.9% for compound 2) corresponds to the loss of all or-
ganic composites, coordinated water molecules and sublimable phos-
+
tion metals in the structure of compound 2, one transition metal Ag
cation as a center is trapped in the inner cavity of a {AgP
5
W
30} unit,
O as a complex fragments
30} polyanion. Crystal data
and structural refinement parameters of compounds 1 and 2 were listed
in Table S1. As shown in Fig. 1a, the {ZP 30} cluster acts as a bidentate
ligand and coordinates with two [Fe(H
through the terminal oxygen atoms of two equivalent WO
3+
and two Fe ions combined with H
2 2
biim/H
are hung on the outer surface of a {AgP
5
W
5
W
3
+
2
biim)(H
2
O)
2
]
subunits
octahedra.
6
All Fe centers exhibit the same six–coordination environments, coordi-
nating with two oxygen atoms (O1W, O2W) from two coordinated
water molecules, two oxygen atoms (O38, O40) from two symmetrical
5 30
phorus oxide species originating from partly collapse of the {ZP W }
polyoxoanion skeleton. The two exothermal peaks in DTA curves ob-
served at 460 and 605 °C for compound 1 and 472 and 620 °C for com-
{
ZP
ecule (Fig. 1c). The bond lengths of Fe1\\N are in the range of
.097(15)–2.105(15) and 2.106(15)–2.110(15) Å for compounds 1
5
W
30} clusters, and two nitrogen atoms (N1, N2) from a H
2
biim mol-
2
pound 2 respectively are assigned to the combustion of the H biim
molecules and the sublimation of phosphorus oxide species [13]. In ad-
dition, there are apparent weight additions appear at 600 °C in TG
curves, which could be due to the decomposition of complex
2
3+
2 2
[Fe(H biim)(H O)
2
]
and the formation of iron oxides [14]. In order
to further verify the oxidation states of elements in the compounds,
XPS analysis was carried out. As shown in Figs. S9 and S10, the XPS spec-
tra exhibit the characteristic peaks of C 1s, N 1s, O 1s, P 2p, W 4f, Fe 2p
and Na 1s for compound 1, and the characteristic peaks of C 1s, N 1s,
O 1s, P 2p, W 4f, Fe 2p and Ag 3d for compound 2, respectively. These
results are consistent with the BVS calculation and elemental analysis,
which further confirm that all W, P, Ag and Fe atoms are in +6, +5,
+
1 and +3 oxidation states, respectively [15]. The above results indi-
cate that the {NaP 30}/{AgP 30} polyanion skeleton in compound
/2 are thermally stable at lower than 400 °C. The experimental and
5
W
5
W
1
simulated results in powder X–ray diffraction (PXRD) patterns of com-
pounds 1 and 2 are presented in Fig. S11, the main diffraction peaks po-
sitions are accordant, indicating the products are in a pure phase. The
different intensities of peaks may be caused by the diverse preferred ori-
entations of the powder samples.
The UV–Vis diffuse reflectance spectra of {NaP
compounds 1 and 2 were performed at 200–800 nm at room tempera-
ture (Fig. S12). Comparing with the spectra of parents {NaP 30} and
AgP 30}, the strong absorption bands at 265 and 350 nm of com-
5 5
W30}, {AgP W30},
5
W
{
5
W
pounds 1 and 2 respectively are assigned to the pπ → dπ charge–trans-
fer transitions of the Ob/c → W bonds (LMCT). The wide absorption
bands at 560 nm for compound 1 and 570 nm for compound 2 respec-
tively are attributed to the d–d transitions of Fe(III) [5b], indicating
Fig. 1. The polyhedral and ball–and–stick view of the coordination pattern of {ZP
the 1D chain (b) and the coordination pattern of Fe(III) (c) in compound 1/2.
5
W30} (a),

J. Du et al. / Inorganic Chemistry Communications 73 (2016) 119–123
121
Fig. 2. The polyhedral and ball–and–stick view of the 3D packing arrangement (a), the 1D chain packing arrangements (b) and the [H
/2 along a direction.
4
biim]2+ fragments (c) fill in the spaces of compound
1
that the transition metal Fe(III) is successfully introduced into com-
pound 1/2. The fluorescent properties of {NaP 30}, {AgP 30}, com-
tested in the optimum conditions for comparison. From the results
listed in Table 1, it is found that the conversion of the cyclohexanone
5
W
5
W
pounds 1 and 2 were characterized in the solid state at room
5 5 3
is 70, 72, 99.9, 91 and 93% for {NaP W30}, {AgP W30}, FeCl , compound
temperature, and the results were shown in Fig. S13. The parent
1 and compound 2, respectively. Though FeCl
3
provided a higher cyclo-
{
4
ZP
52–453 and 470 nm with λex = 265 nm due to LMCT (O → W) [5b].
This result indicates the structures of the parent {ZP 30} polyanions
5
W
30}, compounds 1 and 2 all exist three emission peaks at 422,
hexanone conversion than compound 1/2, the former was completely
dissolved in the reaction mixture for bringing the trouble of catalyst
5
W
reuse. Compound 1/2 exhibits higher catalytic activity than {NaP
and {AgP 30}, which means that the acid catalytic activity of the
{ZP 30} catalyst was effectively improved after introducing the transi-
tion metal–H
5 30
W }
have been kept in compounds 1 and 2. The POMs can proceeded
multi–electron oxidation–reduction process, the electrochemical be-
haviors of compounds 1 and 2 were carried by cyclic voltammograms
5
W
5
W
biim complexes. It can be conjectured that the Fe3 moi-
+
2
(
1
CV) in the potential range of +0.25 to – 0.70 V at the scan rate of
00 mV s under the carbon paste electrodes (1–CPE and 2–CPE). As
eties, as Lewis acids, are the catalytically active centers, which play the
most important roles in the catalytic reaction. Besides, the protons in
−
1
shown in Fig. S14, there are two reversible redox peaks at −0.367 (І),
4 5 2 2 2 5 30 2 n
the molecule {(H biim) H[Fe(H biim)(H O) (ZP W O110)]·17H O}
−
−
0.489 (І′) and −0.210 (II), −0.203 (ІІ′) V for compound 1 and
0.391 (І), −0.474 (І′) and −0.195 (II), −0.194 (ІІ′) V for compound
provide Brönsted acid centers [16b]. In a word, the acid catalysis test re-
sults suggest that these kind of POM–based inorganic–organic hybrid
2. Redox peaks І–І′ and II–II′ for compounds 1 and 2 correspond to two
5
compounds containing {ZP W30} with different center are excellent
consecutive redox processes of W centers [8].
acid catalysts. In addition, to confirm the stability of these compounds
after catalytic processes, the PXRD patterns and IR spectra of samples
after four runs were tested and shown in Figs. S20 and S21. As seen
from Fig. S20, the main diffraction peaks positions are consistent with
the simulated values of compounds 1 and 2, the different intensity of
peaks may be caused by the diverse preferred orientation of the powder
samples. As shown in Fig. S21, the IR characteristic peaks of the powder
samples after catalysis are consistent with those of crystalline com-
pounds 1 and 2 before catalysis. These results indicate that the
The acid catalytic synthesis of ketal is more important for the protec-
tion of the carbonyl group in multi–steps organic synthesis. To evaluate
the catalytic activity of as–prepared compounds, ketalization of cyclo-
hexanone with glycol was selected as a model reaction (Scheme S1)
[16]. It is well known that the high chemical stability in catalysis is crit-
ical to the applications of catalysts. To evaluate the stability of the cata-
lysts in different solvents, compounds 1 and 2 were soaked in glycol,
cyclohexanone and water respectively at the temperature required for
the catalytic reaction, and in cyclohexane at the reflux temperature,
and then the XRD and IR spectra of these solid samples were tested be-
fore and after being soaked in the solvents (Figs. S15 and S16). As seen
from Figs. S15 and S16, it is found that compounds 1 and 2 are still stable
2
in glycol, cyclohexane, cyclohexanone and H O, respectively. Subse-
quently, taking compound 1 as an example, the optimal catalytic condi-
tions of the above model reaction, such as the reaction time, molar ratio
of starting materials and the amount of catalyst were tested and select-
ed systemically (Figs. S17–S19). It is found that the optimum conditions
for the synthesis of cyclohexanone ethylene ketal are: the reaction time
is 2.5 h, the molar ratio of the catalyst (based on W) to cyclohexanone is
1
:200, the cyclohexanone (0.05 mol)/glycol molar ratio is 1:1.4, reaction
temperature is 95–100 °C. Furthermore, compounds 1 and 2 can be re-
covered by simple filtration without any treatment, and reused for three
cycles without distinct decrease of the cyclohexanone conversion under
the optimum reaction conditions (Fig. 3). In addition, the acid catalytic
activities of FeCl
3
, {NaP
5
W30}, {AgP
5
W30} and compound 2 were also
Fig. 3. The catalytic activity of compounds 1 and 2 used after four runs.

122
J. Du et al. / Inorganic Chemistry Communications 73 (2016) 119–123
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2
3
4
5
6
–
–
2.5
2.5
2.5
2.5
2.5
2.5
9
FeCl
{NaP
{AgP
Compound 1
Compound 2
3
Soluble
99.9
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91
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W
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}
}
Partially soluble
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61
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9] Syntheses of K14[AgP
NaP 30} (1.000 g, 0.12 mmol) was dissolved in 15 mL of redistilled water, and
AgNO (0.040 g, 0.24 mmol) was added to this solution under stirring. The resulting
2
8
[
5 30 2 5
W O110]·18H O({AgP W30}): In the synthetic experiment,
{
5
W
3
Appendix A. Supplementary material
mixture was transferred into a Teflon–lined autoclave (30 mL) and kept at 160 °C
for 3 days. After slow cooling to room temperature, the precipitated product was
3
isolated by the addition of 1.000 g of solid KNO . After filtration, the precipitate
CCDC No. 1491919 for compound 1 and 1491920 for compound 2
can be obtained free of charge from the Cambridge Crystallographic
Data. Supplementary data to this article can be found online at doi:10.
was dissolved in hot water (10 mL), and then the solution was gradually cooled,
colourless block crystals formed. Finally, the crystals were filtered and dried in air.
The amount of product was 0.75 g. Elemental analysis (%), found: H 0.46, K 6.64,
1
016/j.inoche.2016.10.018.
Ag 1.20, P1.88, W 65.80. Calcd. for K14
36 5
H O128AgP W30: H 0.43, K 6.51, Ag 1.28,
P1.84, W 65.58. FT IR: 3460 (s), 2931 (w), 1620 (s), 1168 (m), 1.087 (w), 933
1 31
(
m), 767 (s) cm−
of {NaP 30} (0.200 g, 0.024 mmol), FeCl
0.035 g, 0.26 mmol) and distilled water (10 mL) was stirred for 1.0 h. The resulting
.
P NMR: −10.29 ppm. Synthesis of compound 1: A mixture
·6H O (0.150 g, 0.56 mmol), H biim
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W
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(
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