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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: yadollahi@chem.ui.ac.ir,

yadollahi.b@gmail.com

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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

cages for negative ions with extremely high “coordination number”: an unusual property of V-O

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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Highlights

Nitrate-encapsulated sandwich type polyoxometalate

The X-ray structure of Na4K5[K3Cu3(NO3)(A-α-PW9O34)2]

Catalytic oxidation by hydrogen peroxide

Oxidation of alkenes TBA salt of [K3Cu3(NO3)(A-α-PW9O34)2]9-