A tungstoantimonate coordinated by dmso groups, [Sb2W20(OH)2(dmso)2O66]8−

Inorganic Chemistry Communications 11 (2008) 1184–1186

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A tungstoantimonate coordinated by dmso groups, [Sb2W20(OH)2(dmso)2O66]8 Li-Hua Bi *, Bao Li, Li-Xin Wu* College of Chemistry, State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China

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Article history: Received 28 May 2008 Accepted 15 July 2008 Available online 24 July 2008 Keywords: dmso Antimony(III) Polyoxometalate Tungsten Crystal structure

a b s t r a c t A novel dmso-coordinated heteropolytungstate [Sb2W20(OH)2(dmso)2O66]8 (1) has been successfully isolated as [Ru(bpy)3]2+ salts by routine synthetic reaction in mixed solutions with dmso and water. The compound was characterized by spectroscopic methods as well as by X-ray single crystal structure analysis. The compound represents a tungstoantimonate framework with two W–O–S(CH3)2 bonds. Ó 2008 Elsevier B.V. All rights reserved.

Polyoxometalates (POMs) are inorganic metal–oxygen clusters with an enormous structural and compositional variety [1–3]. Although POMs have been known for about 200 years, a large number of novel polyoxoanions with unexpected structures are still being discovered [4–6]. Recently, the functionalization of POMs by grafting the organic or organic–metal groups and their organic–inorganic hybrid materials have attracted much attention due to the possibility of combining the organic and inorganic components to generate unusual structures, additional properties, or extended applications in different fields including catalysis, materials science, and medicine [7–10]. Therefore, the strong efforts have been devoted to prepare the functionalized POMs to produce novel materials with especial properties in considerable ongoing research. The complex ruthenium(II) tris(bipyridine) fRuðbpyÞ2þ 3 g has been attracting extensive interest for its luminescence and electrochemistry properties and it was used as electrochemiluminescen (ECL) compound for the solid-state ECL detection in capillary electrophoresis (CE) or CE microchip [11–13]. In recent years, much work has been done on the synthesis of hybrid materials containing POMs and RuðbpyÞ2þ 3 : McCormac et al. prepared the compound [Ru(bpy)3]3[P2W18O62] and studied its electrochemistry both in aqueous solution and solid state [14]; Keyes et al. reported on three compounds, [Ru(bpy)3]3.5[P2W17O61(FeOH2)], [Ru(bpy)3]3[P2W17O61(FeBr)] and [Ru(bpy)3]5[P2W17O61], and investigated their photophysic properties [15]; Bond et al. presented the voltammetric, photo-physical and photo-electrochemical behaviors of the * Corresponding authors. Tel.: +86 431 85168499; fax: +86 431 85193421 (L.-H. Bi). E-mail addresses: [email protected] (L.-H. Bi), [email protected] (L.-X. Wu). 1387-7003/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2008.07.008

compounds [Ru(bpy)3]2[S2M18O62] (M = Mo and W) [16]; Wang et al. described two compounds, [Ru(bpy)3]–[PMo12O40] and [Ru(bpy)3]2[S2Mo18O62], which were used as the bifunctional electrocatalyst for the fabrication of the chemically bulk-modified carbon paste electrode [17]. Recently, our group synthesized the nanoparticles of [Ru(bpy)3]2SiW12O40  2 H2O and applied them as electrochemiluminescence sensor [18]. However, all of the above compounds were characterized by elemental analysis, IR spectra and electrochemistry. To the best of our knowledge, there is only one report on the structurally-characterized hybrid compound [Ru(bpy)3]2[W10O32]  3dmso [19]. The above results show that the rational synthesis and structural characterization of hybrid POMs containing RuðbpyÞ2þ 3 are a challenge. Nevertheless, the potential photo-physical, ELC and bifunctional electrocatalytic properties of such species are a strong motivation to undergo this kind of research. Therefore, we initiated a systematic studies on the interaction of Ru(bpy)32+ with the lonepair containing tungstoantimonates. The existence of SbIII-containing POMs has been known for several years. The lone pair of electrons on the heteroatom does not allow to form the closed Keggin unit, resulting in some unexpected structures, e.g. [NaSb9W21O86]18, [Na2Sb8W36O132(H2O)4]22 [20]. The tungstoantimonate [Sb2W22O74(OH)2]12 and its transition metal-substituted derivatives [M2(H2O)6(WO2)2(b-SbW9O33)2](142n) (Mn+ = Fe3+, Co2+, Mn2+, Ni2+, Cu2+ and Zn2+) were reported for the first time by Krebs et al. in 1997 and 1999, which were called Krebs-type structures [21]. Recently, Proust reported the organoRu supported Krebs-type tungstoantimonate [Sb2W20O70{Ru(pcymene)}2]10 [22]. The Krebs-type structures were described as follow: two b  B  SbW9 O9 33 units are formally connected by two WO2 groups to the dimeric structure forming the ‘‘unknown”

L.-H. Bi et al. / Inorganic Chemistry Communications 11 (2008) 1184–1186

bis-decatungstate [Sb2W20O70]14, which is further stabilized either by the addition of two fac-WO3 , or two Mn+(H2O)3, or two Ru(p-cymene) groups, respectively. However, as far as we know, this polyoxoanion [Sb2W20O70]14 has never been isolated as lacunary precursor like SbW9 O9 33 . In this paper, we report on the synthesis and crystal structure of the ‘‘unknown” bis-decatungstate [Sb2W20O70]14 coordinated by the dmso groups as [Ru(bpy)3]2+ salt, [Ru(bpy)3]4[Sb2W20(OH)2(dmso)2O66]  16dmso  2H2O (1a), which was characterized in the solid state by IR, TGA, elemental analysis and electrochemistry. Compound 1a was synthesized by interaction of 0.5 g (0.67 mmol) of Ru(bpy)3Cl2  6H2O with 1.0 g (0.17 mmol) of K6Na4[Sb2W20Ni2O70(H2O)6]  20H2O in 20 mL of H2O/dmso (1:1). The solution was heated to 80 °C for 1 h and filtered after it had cooled. Single crystals suitable for X-ray analysis were obtained by slow evaporation of the filtrate at room temperature. Yield: 0.62 g (41.8%). Anal. Calcd (Found) for 1a: Ru 4.6 (4.7), Sb 2.8 (2.6), W 42.1 (41.7), N 3.8 (3.8), S 6.6 (6.4), C 21.5 (21.6), H 2.4 (2.2). IR. mmax/cm1: 1631w, 1600w, 1463m, 1441m, 1400w, 1312w, 1272w, 1245w, 1157w, 1121w, 1019w, 962s, 878sh, 855sh, 811s, 766s, 682m. Single crystal X-ray analysis [23] reveals that polyoxoanion 1 exhibits a symmetric dimeric structure composed of two b  B  SbW9 O9 33 units linking by two WO2 groups (Fig. 1). The b  B  SbW9 O9 33 unit can be derived from the a-Keggin structure by removing one W3O13 fragment and a 60° rotation of one of the remaining W3O13 subunits around the Sb–O bond vector (Fig. 2) [21]. Interestingly, atom W10 in 1 is bounded to six nonequivalent oxygen atoms: four l2-oxo-groups (O21, O31, O32, and O33) from two b  B  SbW9 O9 33 units, one terminal oxygen (O34) and one oxygen atom (O35) from the terminal dmso ligand. The W10–O bond lengths are in the expected range of around 1.7– 2.2 Å [W10–OT, 1.703 Å; W10–O(W), 1.807–2.188 Å; W10–O(S), 2.147 Å]. To the best of our knowledge, such bonding of dmso to the tungsten-oxo framework of a polyoxoanion has never been observed so far, although the coordination of dmso to tungsten through oxygen has been reported before in WO2Cl2(dmso)2 [24]. However, the paucity of reported structures containing dmso complexed to W(VI) is probably due more to the avoidance of dmso as a crystallization solvent rather than any difficulty in substitution of dmso for water in polyoxotungstates. In 1997 and 1999, Krebs et al. reported on the tungstoantimonate [Sb2W22O74(OH)2]12 and its transition metal-substituted derivatives [M2(H2O)6(WO2)2(b-SbW9O33)2](142n) (Mn+ = Fe3+, Co2+, Mn2+, Ni2+ and Cu2+, Zn2+), which consist of the ‘‘unknown” bis-decatungstate [Sb2W20O70]14 stabilized either by the addition of two fac-WO3 , or two Mn+(H2O)3 groups, respectively. Interestingly,

Fig. 1. Combined polyhedral/ball-and-stick representation of 1. The color codes are as follows: WO6 octahedra, red; Sb, green spheres; S, yellow spheres, and C, cyan spheres. Hydrogens not shown for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 2. Combined polyhedral/ball-and-stick (top) and ball-and-stick (bottom) representations of the asymmetric unit of 1 with atom labeling. The color codes are the same as in Fig. 1. Hydrogen atoms have been omitted for clarity. Bond lengths (Å): S1–C61, 1.733(7); S1–C62, 1.762(8); S1–O35, 1.528(4); Sb1–O11, 1.994(4); Sb1–O15, 1.979(4); Sb1–O6, 1.976(4); W1–O1, 1.707(5); W1–O2, 1.898(4); W1–O3, 2.009(4); W1–O4, 1.897(5); W1–O5, 1.835(4); W1–O6, 2.300(4); W2–O2, 1.893(4); W2–O7, 1.711(4); W2–O8, 1.855(4); W2–O9, 1.921(4); W2–O10, 2.050(4); W2–O11, 2.348(4); W3–O8, 1.996(4); W3–O12, 1.709(4); W3–O13, 1.838(4); W3–O14, 1.956(4); W3–O15, 2.295(4); W3–O28, 1.893(4); W4–O3, 1.846(4); W4–O14, 1.900(4); W4–O15, 2.339(4); W4–O16, 1.706(4); W4–O17, 1.997(4); W4–O18, 1.907(4); W5–O4, 1.935(4); W5–O6, 2.277(4); W5–O18, 1.930(4); W5–O19, 1.693(5); W5–O20, 1.927(4); W5–O21, 1.924(4); W6–O5, 2.084(4); W6–O6, 2.307(4); W6–O20, 1.914(4); W6–O22, 1.717(4); W6–O23, 1.777(4); W6–O24, 1.993(4); W7–O10, 1.830(4); W7–O11, 2.267(4); W7–O24, 1.857(4); W7–O25, 1.699(4); W7–O26, 2.096(4); W7–O27, 1.977(4); W8–O9, 1.908(4); W8–O11, 2.238(3); W8–O27, 1.882(4); W8–O28, 1.951(4); W8–O29, 1.703(4); W9–O13, 2.057(4); W9–O15, 2.304(4); W9–O17, 1.891(4); W9–O30, 1.699(4); W9–O31, 2.035(3); W9–O32, 1.798(4); W10–O21, 1.892(4); W10–O31, 1.807(3); W10–O32, 2.186(4); W10–O33, 1.966(4); W10–O34, 1.703(4); W10–O35, 2.146(4). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the ‘‘unknown” bis-decatungstate was isolated by now and it was coordinated to two dmso groups, indicating the bis-decatungstate can exist alone. Bond valence sum calculations [25] indicate that the terminal oxygen atom O26 is monoprotonated on 1 and therefore, the charge of the polyanion must be 8. On the other hand, the W7 is considered as bounding to OH because W7–O26 distance of 2.096 Å is rather long for W@O double bonds. The conclusion is supported by the analogous anions [Bi2W22O74(OH)2]12 and [Sb2W22O74(OH)2]12 which reveal the same bonding models [21]. In the solid state, the negative charge of 1 is balanced by four [Ru(bpy)3]2+ cations, located by X-ray diffraction. Elemental analyses of 1a further confirmed the chemical composition. In addition, thermogravimetric analysis allowed us to estimate the number of crystal waters and dmso (see Supp. Info). As expected, the IR spectrum of 1a (see Supp. Info) includes the tungsten-oxo framework and Ru(bpy)3Cl2 superimposed on each other followed by the slight shift and splitting of some bands for the vibration bonds of W–O bonds, which are similar to that of [M2(H2O)6(WO2)2(b-SbW9O33)2](142n) [21]. This implies that the tungsten-oxo framework is maintained in 1a. In addition, the IR spectrum of 1a also shows the expected S–O stretching vibrations of the dmso groups at 1121 and 1029 cm1 [26]. Synthesis of 1 was accomplished by reaction of [Ru(bpy)3]2+ and [Sb2W20Ni2(H2O)6O70]10 in the mixed media of H2O/dmso (1:1). Therefore, formation of 1 involves the lose of Ni2(H2O)6 and the coordination of dmso to the tungsten-oxo framework. In order to

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identify the crucial components /conditions for the formation of 1 we modified the reaction conditions systematically. For example, we tested if 1 can also be synthesized in the absence of [Ru(bpy)3]2+, but our results were negative. We also tested a two-step procedure: (i) reaction of [Sb2W20Ni2(H2O)6O70]10 with [Ru(bpy)3]2+ and isolation of [Ru(bpy)3]–[Sb2W20Ni2(H2O)6O70] (1b); (ii) redissolution of 1b in the mixed media of H2O/dmso with heating. This time we obtained 1a as based on IR. These observations allow the conclusion that the presence of [Ru(bpy)3]2+ is crucial for the formation of 1. Also we were interested to find out if 1a can be synthesized from the reaction of [Ru(bpy)3]2+ with other tungstoantimonate analogues such as [Sb2W22(OH)2O74]12 and its derivatives [M2(H2O)6(WO2)2(b-SbW9O33)2](142n) (Mn+ = Fe3+, Co2+, Mn2+, Cu2+ and Zn2+), respectively. Surprisingly, we obtained several different dmso-coordinated tungstoantimonates, which will be published elsewhere. We tried to study the electrochemical property of 1a in the solutions. Unfortunately, the compound 1a cannot be dissolved in any solutions including pure dmso, buffer solutions and the mixed solutions of H2O/dmso. Therefore, the cyclic voltammetry (CV) study of 1a was performed in the solid state at pH 2.5 (0.5 M Na2SO4 + H2SO4) on a GCE in the potential range between 0.75 and 1.45 V (Fig. 3). The voltammogram of 1a displays three redox waves at +1.23, 0.31 and 0.59 V, respectively. The first wave is a one-electron redox process, assigned to the Ru3+/2+ couple, while the latter two waves are multi-electron redox processes attributed to the redox of the WVI/V in tungstoantimonate(III) framework. As expected, this CV is similar with that of [Sb2W20Ni2(H2O)6O70]10 in a buffer solution at the same pH (see Supp. Info). In summary, the title polyanion 1 is of interest for several reasons: (i) it represents the first example of dmso-coordinated polyoxotungstate in POM chemistry; (ii) it reveals the presence of the (Sb2W20O70) fragment as a lacunary framework of the Krebs-type tungstoantimonates [Sb2W22O74(OH)2]12 and its transition metal-substituted derivatives [M2(H2O)6(WO2)2(b-SbW9O33)2](142n) (Mn+ = Fe3+, Co2+, Mn2+, Ni2+, Cu2+ and Zn2+); (iii) it adds a new member to tungstoantimonates. The structure of 1a allows for a multitude of studies including organic catalysis, bifunctional electrocatalyst, photo-chemistry, and solid-state ECL detection. Some of this work is currently in progress, and the results will be reported in due time.

15 10

I / μA

5 0 -5 -10 -15 1.5

1.0

0.5

0.0

-0.5

-1.0

E / V vs. Ag/AgCl Fig. 3. Cyclic voltammogram of 1a in the solid state at pH 2.5 (0.5 M Na2SO4 + H2SO4) on the GCE with scan rate 100 mV s1.

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