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First Stable Nitrate-Encapsulated Sandwich Type Polyoxometalate: Synthesis,Structural Characterization, and Catalytic Performance
Mostafa Riahi Farsani, Bahram Yadollahi, Hadi Amiri Rudbari, AkbarAmini, Tom Caradoc-Davis, Jason R. Price
PII: S1387-7003(14)00048-3DOI: doi: 10.1016/j.inoche.2014.02.008Reference: INOCHE 5450
To appear in: Inorganic Chemistry Communications
Received date: 4 January 2014Accepted date: 6 February 2014
Please cite this article as: Mostafa Riahi Farsani, Bahram Yadollahi, Hadi AmiriRudbari, Akbar Amini, Tom Caradoc-Davis, Jason R. Price, First Stable Nitrate-Encapsulated Sandwich Type Polyoxometalate: Synthesis, Structural Characteriza-tion, and Catalytic Performance, Inorganic Chemistry Communications (2014), doi:10.1016/j.inoche.2014.02.008
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First Stable Nitrate-Encapsulated Sandwich Type Polyoxometalate: Synthesis,
Structural Characterization, and Catalytic Performance
Mostafa Riahi Farsania, Bahram Yadollahi
*a, Hadi Amiri Rudbari
a, Akbar Amini
a, Tom
Caradoc-Davisb, Jason R. Price
b
aDepartment of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
bAustralian Synchrotron, 800 Blackburn Road, Clayton, Melbourne, Victoria 3168, Australia
Keywords: Sandwich type polyoxometalates, Nitrate-encapsulated, Catalysis, Epoxidation,
Hydrogen peroxide
* Corresponding Author. Tel: +98-311-7932742; fax: +98-311-6689732; e-mail: [email protected],
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Abstract
A stable sandwich type Keggin polyoxometalate, Na4K5[K3Cu3(NO3)(A-α-PW9O34)2], has been
synthesized and crystallized by a simple reaction method. Elemental analysis, FTIR, UV-vis, X-
ray single crystal, thermal gravimetry and cyclic voltammetry were used for characterization of
synthesized POM. The TBA salt of [K3Cu3(PW9O34)2]9-
revealed high catalytic activity and 74-
99% selectivity in the liquid phase epoxidation of alkenes with aqueous H2O2. The X-ray
structure of this complex reveals that three Cu(II) ions are sandwiched between two A-α-PW9O34
moieties and that a nitrate monoanion is encapsulated in the same plane as the three Cu(II)
atoms. The nitrate is coplanar with the three copper atoms, which are each in a distorted square-
pyramidal environment. After recrystallization NO3- group in the central belt didn’t exit, this
phenomenon attributed to the type of counter ion in the complex (as a result of mixing Na+ and
K+ as counter ions). This compound showed excellent stability in different pH and also didn’t
dissociate with hydrogen peroxide.
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The oxidation or oxygenation of organic compounds catalyzed by transition metal centers has
been extensively studied and is still subject of high interest [1-5]. One major goal in this field of
research is synthesis of effective, custom-made, and nontoxic catalysts to achieve lowering of
costs and environmentally friendly conditions in industrial processes. Generally, important
requirements for these technologies are stable catalysts with high product selectivity’s without
waste of the applied oxidant [6].
Polyoxometalates (POMs) have been widely used in diverse as catalysis, medicine and material
science
[7-10]. Particular interest has been focused on transition metal-substituted
polyoxometalates because of their unique in its topological and significant catalytic activities
[11-14]. Sandwich type polyoxometalates based on trivacant Keggin moieties represent the
largest subfamily. The first sandwich type polyoxoanion, [Co4(H2O)2(B-α-PW9O34)2]10-
, was
discovered by Weakley et al in 1973 [15]. Over the last 40 years of research activity, many
sandwich type species including Weakley, Hervé [16], Krebs [17], and Knoth [18,19] types have
been identified [20,21].
In 1985 for the first time, Knoth reported the sandwich POMs are based on two A-α-Keggin
fragments, e.g. [A-α-PW9O34]9-
[19]. Knoth et al reported an unstable encapsulation of a
monoanion in the A-type sandwich polyoxometalates with potassium countercations. After
eighteen years, Hill et al reported carbonate encapsulated in the A-type sandwich POM of Y(III)
ions [22]. Unfortunately all of the A-type sandwich complexes are unstable in solution and
eventually isomerize into the B-type sandwich structures based on UV-visible spectroscopic data
[18]. Herve et al, by UV-visible studies on K12[(Co(OH2)2)3(PW9O34)2], showed that in aqueous
solution these compounds were decomposed and two different intermediate species formed [23].
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Perhaps one reason for decomposing of these compounds is type of counter ions or encapsulated
anion in central belts. For example Hill et al had shown that a high concentration of sodium ions
stabilizes lacunary sandwich type species [24-26]. In some of unstable polyoxometalates with A-
type structure, which reported in previous years, only K+ or Na
+ are counter ions and just in
structures with both of Na+ and K
+ as counter ions stability could be seen
[18,19,24,28-30].
To the best of our knowledge, Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] is a stable sandwich POM
with three transition metals in the belt which reported here. Nevertheless, up to now, catalytic
epoxidation using sandwich type phosphotungstates with A-type structure are limited to alkenes
due to complications in the synthesis of the material and instability of these compounds in
solvents [27-30].
Nevertheless, the potentially attractive catalytic properties of A-type sandwich polyoxometalates
cause a strong motivation for this kind of polyoxometalates. In the past decades, much attention
has been devoted to the synthesis of transition-metal substituted POMs without idea for the
simple method for synthesis of this compounds and improved stabilization of A-type sandwich
polyoxometalates.
Herein, for the first time, we report the synthesis of a stable A-type sandwich polyoxometalate
with encapsulated NO3- in central belt [31] and a mixture of two counter ions, K
+ and Na
+, for
stability purpose.
Reaction of Na2WO4 with Cu nitrate salt in an aqueous solution produced Na4K5[K3Cu3(NO3)(A-
α-PW9O34)2] (1) in very good yield (79%). The synthesis of 1 requires the presence of NO3-.
Knoth and co-workers was shown by excess KCl the unstable K12M3(PW9)2 (M = Mn, Fe, Cu,
Zn, Pd, Ce) complexes were formed [19]. But in our synthetic method, Na4K5[K3Cu3(NO3)(A-α-
PW9O34)2] was stable at different conditions.
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The X-ray structure of 1 showed that dimeric polyoxoanion Na4K5[K3Cu3(NO3)(A-α-PW9O34)2]
(1) consists of two [A-α-PW9O34]9-
moieties linked by three Cu2+
and three K+ ions, resulting in a
sandwich type structure (Figure 1). The central belt is composed of three Cu2+
and three K+ ions
in alternating positions leading to a polyanion with idealized D3h symmetry. The three copper
ions are equivalent and exhibit square-pyramidal coordination and the vacancies between copper
ions are occupied by potassium ions. This arrangement was expected because some of the
dimeric, sandwich-type polyoxometalates contain two or three transition metal ions in the central
belt and the vacancies between them are usually occupied by sodium or potassium ions [32-36].
The crystal structure also shows that a nitrate anion is encapsulated inside the central cavity of
the Cu3(A-α-PW9)2 structure (Figure 2). The nitrate is coplanar with the three copper atoms,
which are each in a distorted square-pyramidal environment. The pyramidal apices face inward
and are occupied by the nitrate oxygen atoms. The A-α-PW9 groups are directly over each other;
and there is no “side-slip” such as that observed in Cu3(A-α-AsW9)2 [32].
As Knoth report, this structure with potassium counter ions was unstable [19] He showed that
coordination of nitrate is readily reversible; and the nitrate anion was lost upon recrystallization
of K12Cu3(PW9)2.NO3 from neutral water and regained at lower pH in the presence of nitrate.
This complexation of an anion by another anion can be rationalized by postulating that there is
little negative charge density in the region of the belt. The negative charge probably remains
largely localized on the polyoxometalate groups whereas the belt, consisting of three positive Cu
metals, probably has localized positive charge. In our reported structure, we changed the
potassium counter ions with a mixture of potassium and sodium ions, which resulted higher
stability for the nitrate-encapsulated sandwich type complex.
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As mentioned above, complex 1 is the first example of a stable structure containing monoanion
encapsulated in a sandwich type POM. Carbonate (CO32-
), a dianion, encapsulated in the A-type
sandwich complex, K11(YOH2)3(CO3)(A-α-PW9O34)2.22H2O (2), as a POM that is structurally
similar to 1 was reported by Hill [22]. In contrast to complex 1, the complex 2 is stabilized by
potassium as counter ion.
Pope and Müller define these anion-encapsulated sandwich type structures as cryptates since
they have well-defined bonding interactions [9]. In contrast, there are also large classes of
reduced polyoxovanadates that have been referred to as clathrates in which the host guest
interactions are significantly weaker. In these compounds, a variety of anions (including Cl-, Br
-,
I-, and CO3
2-) can act as templates for the induced self-assembly of the cluster shell
[37-39].
The decomposition characteristics and thermal stability of this complex in the solid state were
assessed by thermogravimetric analysis (Figure 3). Two distinct mass loss regions are observed
below 450 °C. First, there is a weight loss of approximately 7.7% between 30 and 200 °C that is
associated with the loss of 25 water molecules (both crystalline lattice solvent molecules and
coordinated aqua ligands) per molecule of Na4K5[K3Cu3(NO3)(A-α-PW9O34)2]. In addition, there
is a weight loss of approximately 0.8% from 400 to 450 °C, corresponding to the loss of 1 equiv
NO3 per molecule of Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] [40].
The infrared spectrum of this complex shows band assignable to NO3- at 1391. The characteristic
vibrational modes of the PO4 unit (1100 and 1066 cm-1
) also show splitting, implying that there
is a loss of local symmetry as expected for the A-type trivacant Keggin unit. In addition, the
terminal W=O and bridging W-O-W stretches characteristic of all heteropolytungstates are
present. This compound indicated similar infrared spectra after recrystallization (see Figure 4)
and characteristic band at 1391cm-1
showed nitrate retention [41]. Loosed of the ν stretching
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frequencies characteristic by FTIR, confirmed NO3 decomposition by heating a solid sample to
450°C (Figure 5).
The cyclic voltammetry (CV) of Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] was recorded in sodium
acetate buffer solutions (0.02 M NaOAc/HOAc with 0.38 M NaNO3). The pattern is showed to
irreversible waves observed in the electro active range of the supporting electrolyte. The cyclic
voltammogram of this complex (Figure 6) shows one peak in the negative potential range
features the redox processes of WVI
centers and is located in the domain where analogous W-
based processes were also obtained for sandwich type POMs [42]. Anodic peak at 0.85 V
attributed to the redox processes of Cu2+
centers. The results indicated that with an increase in the
scan rate, the peak separation potential (Ep) increased. As such, the increasing sizes of the
anodic and cathodic peak currents are almost the same, and the peak currents (I–I′) are
proportional to the square root of the scan rates (Figure 7), which indicates that the redox
processes are diffusion-confined over a specific range of scan rate. The same solution could be
kept and used after 2 days without any change in the cyclic voltammogram of the complex [43].
UV-vis spectrum of this polyoxometalate displays no sharp maxima for the Cu-centered d-d
transition bands in the visible region because the electronic spectrum is dominated by the charge-
transfer (oxygen-to-tungsten) bands of the polyoxoanion framework (Figure 8a).
The stability of Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] in solution was assessed by monitoring its
UV-vis spectrum between pH 1 and 10. All of the spectra were reproducible with respect to
absorbencies and wavelengths (Fig. 8b). For above reason, the absorbance bond was selected in
UV region because these peaks were better defined when the pH decreased. Monitoring of the
UV-vis spectrum for this complex confirmed the stability which obtained by cyclic voltammetry.
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All of these observations suggested that Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] is stable in solution
but K salts of this complex haven’t stability in different range of pH in solvent (Fig 8c) [44].
Catalytic studies of tetra-n-butylammonium (TBA) salt of [Cu3K3(A-PW9O34)2]9-
in the
epoxidation of representative organic substrates with H2O2 as oxidant and MeCN as solvent was
also performed [45]. The catalytic oxidation results are presented in Table 1. By 2 equivalent of
H2O2 and 0.5 mol% TBA salt of [Cu3K3(A-PW9O34)2]9-
, styrene, cyclohexene and cyclooctene
resulted mainly corresponded epoxides with high selectivity at 47-65% substrate conversion
[46].
In the case of styrene, 65% conversion with 74% selectivity for styrene epoxide and 26% for
benzaldehyde was obtained. Especially for cyclooctene, only cyclooctene oxide was obtained
selectively. TBA7H2[Cu3K3(PW9)2] showed 47% conversion for cyclohexene with 83%
selectivity towards cyclohexene oxide and 17% for 1-cyclohexene-3-on. Also, these oxidation
reaction with H2O2 and TBA salt of [Cu3K3(A-PW9O34)2]9-
as catalyst were carried out in
different solvents. It was found that CH3CN at the same condition present the best activity and
selectivity. At the reaction condition, the presence of catalyst was crucial for progress of
oxidation and in the absence of catalyst the progress of reaction was very low. In comparison
with previous similar catalysts in epoxidation of alkenes, our catalytic system shows very good
results in both of conversion and selectivity [27-30,47]. The infrared spectra of the [Cu3K3(A-
PW9O34)2]9-
which obtained at the end of reactions, indicated that stability of the catalysts had
occurred.
Here, the first example of a stable central belt nitrate encapsulated A-type sandwich
polyoxometalate was reported. This synthetic procedure opens up a new route to synthesis of the
stable sandwich type polyoxometalate structures directly from their Keggin analogues. All of
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observations showed that Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] was stable in aqueous and in
MeCN solution. The TBA salt of [Cu3K3(A-PW9O34)2]9-
revealed good catalytic activity and 74-
99% selectivity in the liquid phase oxidation of model organic compounds with aqueous H2O2.
The superiority of the catalyst lies in its good conversion with high selectivity.
Appendix A. Supplementary material
The single crystal X-ray diffraction measurement was carried out at MX1 beam lines at the
Australian Synchrotron, Melbourne. Diffraction data were collected using Si <111>
monochromated synchrotron X-ray radiation [λ=0.71080 (MX1)] at 100 (2) K with BlueIce
software [48] and were corrected for Lorentz and polarization effects using the XDS software
[49]. The structures were solved by direct methods and the full-matrix least-squares refinements
were carried out using SHELXL [50] Details concerning collections and analyses are reported in
Table 2. CCDC 955495 contains the supplementary crystallographic data for this structure. This
data can be obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article could be
found, in the online version, at http://dx.doi.org/10.1016/j.ica.2012.10.030.
Acknowledgments
Support for this research by the University of Isfahan is acknowledged.
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[31] Synthesis of Na4K5[K3Cu3(NO3)(A-α-PW9O34)2]: Na2WO4·2H2O (5 g, 15.2 mmol) and
Na2HPO4 (0.24, 1.7 mmol) were dissolved in 100 mL H2O followed by an addition of
Cu(NO3)2·6H2O (0.31 g, 1.01 mmol), resulting in a cloudy suspension. The pH was adjusted to
7.5 by drop wise addition of 6 M HCl, and a green solution formed. The produced green solution
heated at 90 ºC for 1 h and then allowed to cool to room temperature. Powder KCl (0.6 g,
8.0mmol) was added, and the solution was kept at room temperature. After several weeks, dark
green needle crystals were formed (yield 79% based on W). Synthesis of
K12[Cu3(PW9O34)2(NO3)][19]: Na8HPW9O34.xH2O (30 g, 11 mmol) and copper nitrate three
hydrate (5.06 g, 21 mmol) were added simultaneously to 240 mL of water. The mixture was
stirred for 2 min to obtain a clear green solution. This was filtered; potassium chloride (34 g) was
added to the filtrate, and the mixture was then stirred for another 2 min and filtered to obtain of
K12[Cu3(PW9O34)2(NO3)]. FTIR (KBr pellets) (cm-1
): 1100(s), 1065(s), 1043(sh), 945(sh),
871(m), 810(m), 741(s), 640(s). Elemental analysis calcd (%) for K12[Cu3(PW9O34)2(NO3)]: Cu,
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3.68; K, 9.06; N, 0.27; P, 1.20; W, 63.87. Found (%):Cu, 3.65; K, 9.0; N, 0.27; P, 1.23; W,
63.62.
The tetra-n-butylammonium (TBA) salt of [Cu3K3(NO3)(PW9O34)2]9-
was prepared by a
metathesis reaction. To 0.555 g (0.1 mmol) of Na4K5[Cu3K3(PW9)2](K1) in 30 mL of H2O was
added with stirring 0.322 g (10 mmol) of TBABr. CH2Cl2 (250 mL) was added to this cloudy
solution, and the mixture was shaken in a separatory funnel. After standing for 30 min, the
mixture separated into a cloudy colorless upper layer and a clear yellow lower layer. The bottom
organic layer was collected and concentrated to thick oil with a rotary evaporator. A yellow solid
was then precipitated from the concentrated organic layer by addition of diethyl ether (100 mL).
The resulting solid was collected on a medium frit and dried under vacuum for 24 h (yield 80%).
IR and elemental analysis established that Na4K5[Cu3K3(NO3)(PW9O34)2] had not changed
during the conversion of Na4K5[Cu3K3(PW9O34)2(NO3)] to TBA7H2[Cu3K3(PW9O34)2]. FTIR
(2% KBr pellet, 1300-400 cm -1
): 1100(s), 1065(s), 1041(sh), 943(sh), 870(m), 808(m), 744(s),
642(s). Elemental analysis calcd for TBA7H2[Cu3K3(PW9O34)2]: C, 20.92; H, 3.98; Cu, 2.96; K,
1.82; N, 0.22; P, 0.96; W, 51.46 [MW = 6430.18]; Found: C, 20.88; H, 3.93; Cu, 2.91; K, 1.86;
N, 0.20; P, 0.94; W, 50.40.12.
[32] A.C. Stowe, S. Nellutla, N.S. Dalal, U. Kortz, Magnetic properties of lone pair containing,
sandwich-type polyoxoanions: a detailed study of the hetero atom effect, Eur. J. Inorg. Chem.
2004, 3792-3797.
[33] H. Liu, C. Qin, Y.G. Wei, L. Xu, G.G. Gao, F.Y. Li, X.S. Qu, Copper-complex-linked
polytungsto-bismuthate (-antimonite) chain containing sandwich Cu(II) ions partially modified
with imidazole ligand, Inorg. Chem. 47 (2008) 4166-4172.
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[34] U. Kortz, S. Nellutla, A.C. Stowe, N.S. Dalal, J. van Tol, B.S. Bassil, Structure and
magnetism of the tetra-copper(II)-substituted heteropolyanion [Cu4K2(H2O)8(α-AsW9O33)2]8-
,
Inorg. Chem. 43 (2004) 144-154.
[35] U. Kortz, N.K. Al-Kassem, M.G. Savelieff, N.A. Al Kadi, M. Sadakane, Synthesis and
characterization of copper-, zinc-, manganese-, and cobalt-substituted dimeric heteropolyanions,
[(α-XW9O33)2M3(H2O)3]n-
(n = 12, X = AsIII
, SbIII
, M = Cu2+
, Zn2+
; n = 10, X = SeIV
, TeIV
, M =
Cu2+
) and [(α-AsW9O33)2WO(H2O)M2(H2O)2]10-
(M = Zn2+
, Mn2+
, Co2+
), Inorg. Chem. 40
(2001) 4742-4749.
[36] G. Zhu, Y.V. Geletii, J. Song, C. Zhao, E.N. Glass, J. Bacsa, C.L. Hill, Di- and tri-cobalt
silicotungstates: synthesis, characterization, and stability studies, Inorg. Chem. 52 (2013) 1018-
1024.
[37] A. Müller, M. Penk, R. Rohlfing, E. Krickemeyer, J. Döring, Topologically interesting
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clusters, Angew. Chem. Int. Ed. 29 (1990) 926-927.
[38] G.K. Johnson, E.O. Schlemper, Existence and structure of the molecular ion 18-
vanadate(IV), J. Am. Chem. Soc. 100 (1978) 3645-3646.
[39] A. Müller, E. Krickemeyer, M. Penk, H.J. Walberg, H. Bögge, Spherical mixed-valence
[V15O36]5-
, an example from an unusual cluster family, Angew. Chem. Int. Ed. 26 (1987) 1045-
1046.
[40] Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer TG50
in air from 25 to 800 °C at a heating rate of 5 °Cmin−1
.
[41] Infrared spectra (KBr pellets) were recorded on a JASCO, FT/IR-6300 instrument.
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[42] D. Jabbour, B. Keita, I.-M. Mbomekalle, L. Nadjo , U. Kortz, Investigation of multi-nickel-
substituted tungstophosphates and their stability and electrocatalytic properties in aqueous
media, Eur. J. Inorg. Chem. 2004, 2036-2044.
[43] The CV studies are performed at 25±2°C using a three-electrode assembly in 10 mL glass
cell including a Ag/AgCl (3 M KCl) electrode as the reference, a Pt plate as the counter
electrode, and the glassy carbon electrode (GCE) as the working electrode. All the potentials
were measured and reported vs. Ag/AgCl (3 M KCl). The CV measurements were carried out on
Autolab/Potentiostat/Galvanostat-302N, and controlled by Nova 1.8 software (Eco Chemie,
Utrecht, Netherlands).
[44] UV-vis spectra were recorded on JASCO V-670 UV-vis spectrophotometer (190-2700 nm).
[45] Typical procedure for catalytic epoxidation of alkenes: The oxidation reaction was carried
out as follows: TBA7H2[Cu3K3(PW9O34)2] (0.005mmol), CH3CN (3 mL), substrate (1 mmol),
and H2O2 30% (2mmol) were charged in the reaction flask. The reaction was carried out at 363 K
and detected with GC by the internal standard method.
[46] The oxidation products were quantitatively analyzed by gas chromatography (GC) on a
Shimadzu GC-16A instrument using a 2m column packed with silicon DC-200 and FID detector.
[47] X. Zhang, Q. Chen, D.C. Duncan, R.J. Lachicotte, C.L. Hill, Multiiron polyoxoanions:
Synthesis, characterization, X-ray crystal structure, and catalytic H2O2-based alkene oxidation by
[(n-C4H9)4N]6[FeIII
4(H2O)2(PW9O34)2], Inorg. Chem. 36 (1997) 4381-4386.
[48] T.M. McPhillips, S.E. McPhillips, H.J. Chiu, A.E. Cohen, A.M. Deacon, P.J. Ellis, E.
Garman, A. Gonzalez, N.K. Sauter, R.P. Phizackerley, S.M. Soltis, P. Kuhn, Blu-ice and the
distributed control system: software for data acquisition and instrument control at
macromolecular crystallography beamlines, J. Synchrotron Radiat. 9 (2002) 401-406.
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[49] W. Kabschi, Automatic processing of rotation diffraction data from crystals of initially
unknown symmetry and cell constants, J. Appl. Crystallogr. 26 (1993) 795-800.
[50] G.M. Sheldrick, A short history of SHELX, Acta Crystallogr. A 64 (2008) 112-122.
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Figure 1. Combined polyhedral/ball-and-stick representation of the anions [Cu3K3(PW9O34)2]9-
(1): The WO6 octahedral are shown in purple, and the balls represent copper (green), potassium
(pale blue), nitrogen (blue), phosphor (yellow) and oxygen (red)
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Figure 2. Ball-and-stick representation of the central belt in 1
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Figure 3. Thermogravimetric curve showing the loss of crystalline water molecules (from 30 to
200 °C) and NO3 (from 350 to 400 °C) in complex
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Figure 4. Infrared spectra of Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] (red = fresh complex, blue =
recrystallized complex)
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Figure 5. Infrared spectra of Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] (red = fresh complex, blue = NO3
decomposed after heating to 450 °C)
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Figure 6. Cyclic voltammogram of 1 mM Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] obtained in the
solution of 0.02 M (pH 4.8) NaOAc/HOAc buffer with 0.38 M NaNO3, scan rate 25mVs−1
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Figure 7. Cyclic voltammogram of 1 mM Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] obtained in the
solution of 0.02 M (pH 4.8) NaOAc/HOAc buffer with 0.38 M NaNO3, at different scan rates
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Figure 8a. Electronic absorption spectrum of 3 mM Na4K5[K3Cu3(NO3)(A-α-PW9O34)2]
(dilution-corrected).
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Figure 8b. Electronic absorption spectra obtained during acid/base titration of 3 mM
Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] (dilution-corrected).
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Figure 8c. Electronic absorption spectra obtained during acid/base titration of 3 mM
K12[Cu3(NO3)(A-α-PW9O34)2] (dilution-corrected).
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Table 1. Epoxidation of alkenes in the presence of TBA7H2[Cu3K3(PW9)2]a
Entry Alkenes products Conversion (%)b Selectivity (%)
b
1
47 83
2
54 99
3
65 74
aReaction condition: alkene (1 mmol), catalyst (0.005 mmol), acetonitrile (3
mL), reaction time 7 h and H2O2 30% (2 mmol) at 60ºC.
bConversions and yields were determined by GC analysis using an internal
standard.
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Table 2. Crystal structure data for Na4K5[K3Cu3(NO3)(A-α-PW9O34)2].22H2O
Empirical formula Cu3K9NNa4O93P2W18H44
Formula weight 5551.73
Colour Light green
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal system, space group Hexagonal, P6(3)/m
a[Å] 12.2440(17)
b[Å] 12.2440(17)
c[Å] 32.560(7)
α[°] 90
β[°] 90
γ[°] 120
Volume [A3] 4227.3(17)
Z 2, 4.327 Mg/m3
Absorption coefficient 25.743 mm-1
F(000) 4830
Theta range for data collection 0.63 to 26.99 deg.
Limiting indices -15 ≤ h ≤15, -15 ≤ k ≤ 15, -41 ≤ l ≤ 41
Reflections collected / unique 65703 / 3076 [Rint = 0.0606]
Completeness to theta = 26.99 97.7
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Refinement method Full-matrix least-squares on F2
Data / restraints / parameters 3076 / 0 / 203
Goodness-of-fit on F2 1.087
Final R indices [I >2(I)] R1 = 0.0373, wR2 = 0.0961
R indices (all data) R1 = 0.0381, wR2 = 0.0966
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Graphical Abstract:
First Stable Nitrate-Encapsulated Sandwich Type Polyoxometalate
Mostafa Riahi Farsani, Bahram Yadollahi*, Hadi Amiri Rudbari, Akbar Amini, Tom Caradoc-
Davis, Jason R. Price
The X-ray structure of stable Na4K5[K3Cu3(NO3)(A-α-PW9O34)2] reveals three Cu(II) ions
sandwiched between two A-α-PW9O34 moieties and a nitrate monoanion which encapsulated in
the same plane as the three Cu(II) atoms. The stability of this compound in different pH and in
the presence of hydrogen peroxide was very high. The TBA salt of [K3Cu3(PW9O34)2]9-
revealed
high catalytic activity and selectivity in the liquid phase epoxidation of alkenes with aqueous
H2O2.
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