Inorganic Chemistry Communications 12 (2009) 219–222

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Inorganic Chemistry Communications

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Synthesis and characterization of a mixed-valence hexadecavanadatecluster with half-open framework

Lian Chen, Fei-Long Jiang *, Ning Li, Chun-Feng Yan, Wen-Tao Xu, Mao-Chun HongState Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou 350002, China

a r t i c l e i n f o

Article history:Received 18 October 2008Accepted 24 December 2008Available online 3 January 2009

Keywords:VanadiumHexadecavanadatePolyoxometalateCluster structure

1387-7003/$ - see front matter Crown Copyright � 2doi:10.1016/j.inoche.2008.12.021

* Corresponding author. Fax: +86 591 83714605.E-mail address: fjiang@fjirsm.ac.cn (F.-L. Jiang).

a b s t r a c t

A new mixed-valence hexadecavanadate cluster compound, (Me2NH2)4[H6V16O42(SO4)] � 8H2O (1), hasbeen synthesized via an efficient method in organic media. The hollow cluster shell of 1, with a sulfateion locating in its centre, exhibits four large eight-membered –O–V–O–V– rings on its surface, whichmakes the cluster into a half-open structure distinguished from the common hollow clusters built upwith closed surfaces. The intense blue luminescence of 1 in solution state is scarcely reported for thepolyoxometalate clusters.

Crown Copyright � 2008 Published by Elsevier B.V. All rights reserved.

Research involving polyoxometalates is driven not only by theirremarkable structural and electronic properties, but also because oftheir significance in quite diverse applications including catalysis,coatings, pigments, smart materials, luminescence, electrochemis-try to biochemistry and medicine [1]. Among the polyoxometalates,polyoxovanadates represent a remarkable class of high-nuclearitymixed-valence clusters. These compounds have an appealing redoxchemistry and can serve as redox pools, wherein the electron pop-ulation can be chemically controlled [2,3]. In the recent years, somehigh-nuclearity polyoxovanadate clusters ranging from {V12} to{V19} have been reported [4–18]. Compared to the compact struc-tures, the hollow vanadium oxide clusters are more attractive sincethey can act as host shells for a variety of neutral or anionic guestspecies which may exert templating effects on the electronic andframework structures of the host metal–oxide shells. Most of re-ported hollow clusters exhibit the closed sphere-like structures,however, open or half-open hollow frameworks are really scarce.On the other hand, despite of the fast development of polyoxovana-date systems, the nonaqueous synthetic approach is still underin-vestigated. Herein, we reported a facile synthesis of a newhexadecavanadate complex, (Me2NH2)4[H6V16O42(SO4)] � 8H2O (1),which possesses a novel half-open framework.

Compound 1 was obtained with high yield via a facile syntheticroute by heating the DMF (N,N-dimethylformamide) solution ofVOSO4 [19]. The reaction can easily be reproduced with consistenthigh yields. Compared to the hydrothermal method which is oftenused to construct polyoxovanadate compounds, the route requiresmilder condition with relatively lower temperature and pressure

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and gives the production with high purity. Comparative to our pre-vious research [13a], in which the {V16} clusters were obtained byusing the vanadium(V) complex as starting material, the syntheticroute here is more simple only through a one-step reaction. Sin-gle-crystal X-ray analysis [20] reveals that compound 1 exhibits ahollow anionic hexadecavanadate cage (illustrated in Fig. 1) witha sulfate anion located in the centre of the cluster. The space groupof the compound is P42/nnm. In the cage, the 16 vanadium atomsare all five-coordinated with the VO5 square pyramid coordinationgeometries. These distorted VO5 square pyramids constitute the{V16O42} shell configuration by sharing vertices and edges through14 l2-oxygen and 12 l3-oxygen atoms. Among the 16 vanadiumatoms in the hexadecavanadate cluster, three are independentand are divided into two groups according to their coordinationwith different bridging oxygen atoms (l2-oxygen or l3-oxygenatoms). One of the three atoms (V3) is bonded by four l2-oxygenatoms with V–O distances ranging from 1.7798(17) to 1.971(6) Å.The other two vanadium atoms (V1 and V2) link to one l2-oxygenatom [V–O = 1.728(4) and 1.721(6) Å] and three l3-oxygen atoms[V–O = 1.886(4) to 2.040(6) Å]. The V–O distances for the terminaloxygen atoms all lie within the narrow range of 1.597(6) to1.610(4) Å. As shown in Fig. 2 (in which the half transparent yellowsphere with the diameter of ca. 6.8 Å presents the large space in thehollow cluster), the four large openings are shown on the surface ofthe cluster. Each of the rings is consisted of four vanadium and fouroxygen atoms, where the four vanadium atoms lie almost in thesame plane and constitute an approximate square figure with theV–V–V angles close to 90�. In the front of the half transparentyellow sphere, two eight-membered ring openings (V1F–O2F–V3C–O8–V3–O2A–V1A–O4A and V3C–O2B–V1B–O4B–V1D–O2D–V3–O8) stand by sharing edge V3–O8–V3C. The longest distance

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Fig. 2. The structure of hexadecavanadate host shell {V16} (top) and eight-membered ring openings showing on the surface of the cluster (bottom). Theyellow half transparent sphere represents the large space in the center of thehollow cluster. Symmetry codes: A: x, 3/2 � y, 1/2 � z; B: 1/2 � x, y, 1/2 � z; C: 1/2 � x, 3/2 � y, z; D: �1/2 + y, 1 � x, 1/2 � z; F: 1 � y, 1/2 + x, 2/1 � x. F: 1 � y, 1/2 + x,2/1 � x.

Fig. 1. ORTEP view of the {V16} cluster anion with parts of atoms labeled. Thermalellipsoids are at the 30% probability level. Symmetry codes: A: x, 3/2 � y, 1/2 � z; C:1/2 � x, 3/2 � y, z; F: 1 � y, 1/2 + x, 2/1 � x.

220 L. Chen et al. / Inorganic Chemistry Communications 12 (2009) 219–222

measured from the diametrically opposed atoms in the openings is4.9750 (14) Å. Different from the common hollow polyoxovanadatehost shells which exhibit closed surface and prevent the guest mol-ecules from coming out, the large openings in 1 may offer a path forthe guests to exchange with other ions and lead the hollow clusterinto a half-open structure. This type of structure can be viewed asintergradation from open bowl-like to closed cage structures. Thecompound 1 is different from the previous report [13] not only inthe composition, the vanadium valence state, encapsulated anions,and the solvents contained, but also in macroscopical properties,such as solubility and air stability.

The negatively charged {V16O42} hosts a disorder sulfate anionat its center. There is no covalent interaction of this guest ion withthe atoms in the vanadium oxide shell since the distance of theclosest vanadium atom (V1) to the oxygen atom of the sulfate an-ion (O9) is ca. 2.554 Å. The negatively charged {V16O42} shell is sur-rounded and charge balanced by the dimethylammonium cationswhich are formed from the DMF solvent [21]. These dimethylam-monium cations not only act as conterions but also form hydrogenbonds to O6 of the host shell with the N1� � �O6 distance of ca.2.920 Å, generating a three-dimensional supramolecular frame-work (Fig. 3).

The phase purity of compound 1 is confirmed by X-ray powderdiffraction analyses (XRD, see Fig. S1). The peak positions of simu-lated XRD patterns match with that observed, indicating that thecomplexes are obtained as pure phases. The FT-IR spectrum(Fig. S2) of the compound shows characteristic vibrational featuressimilar to other polyoxovanadates reported [7,10,13]. Symmetricand asymmetric stretching of different kinds of V–O bonds are ob-served: The peaks of 965 and 1022 cm�1 are ascribed to terminalV–O bonds; strong bands at 798, 747 and 576 cm�1 are assignedto the antisymmetric stretching vibrations of V–O–V features.

The assignment of the oxidation states for the vanadium atomsis consistent with the electric charge and confirmed by bond va-lence sum calculations. Bond valence calculations [22](Rs = expR[(1.803 � d)/0.37], d = V–O distance in Å) show thatthe valence sums for the bridging O atoms are in the range 1.6–

2.0, except 1.222 for O4 and 1.011 for O8, indicating that the clus-ter is a hexaprotonated core, protonation occurring at O4 and O8.In addition, the valence sums the three independent vanadiumatoms are 4.683, 4.862 and 4.610, respectively. The average valueof 4.71 is in good agreement with the expected bond valence of4.75 calculated from the composition of [Me2NH2]4[H6V4

IVV12-VO42(SO4)] � 8H2O. X-band EPR (electron paramagnetic resonance)spectrum (Fig. S3) of 1 was recorded to further investigate the va-lence state. Compound 1 gives a paramagnetic signal withg = 1.9559 in the solid state at room temperature, suggesting theexistence of the V4+. The UV/vis spectrum of compound 1(10 mM in H2O) shows a broad absorption band around 893 nm(Fig. 4a), which may be assigned to intervalence charge transfer(IVCT, V4+ ? V5+) [9,23].

The polyoxometalate clusters emitting luminescence have beenscarcely reported in literature. When illuminated with the wave-length of 378 nm at room temperature, compound 1 exhibits blueluminescence in solution state (DMF), giving broad and strongemissions at 414 and 436 nm (Fig. 4b). According to the similaremission occurs in the tetradecavanadate cluster our previously re-ported [9], we tentatively assign the origin of the emission to anoxide-to-vanadium charge transition [23]. Thermogravimetricanalysis (TGA) in a flow of nitrogen atmosphere was carried out

Fig. 3. The hydrogen bonding interactions in 1 and a schematic view of the three-dimensional supramolecular framework of the compound.

Fig. 4. (a) Uv/vis absorption spectrum of compound 1 in solution. (b) Luminescenceexcitation and emission spectra of 1 in DMF solution at room temperature.

L. Chen et al. / Inorganic Chemistry Communications 12 (2009) 219–222 221

for polycrystalline sample of 1 over the temperature range 30–900 �C (Fig. S4). Compound 1 is thermally stable until ca. 180 �C,and then the weight loss is 7.2% in the temperature range of180–220 �C, corresponding to the loss of eight lattice water mole-cules (calcd. 7.5%).

In summary, a new mixed-valent hexadecavanadate clustercompound with half-open framework was successfully synthe-sized in high yield through a simple route. The hollow cluster shellencapsulates a sulfate ion in its centre and exhibits four largeeight-membered –O–V–O–V– rings on the surface. Since the non-aqueous synthetic methodology is still underinvestigated in thepolyoxovanadate chemistry, our result introduces a facile synthesisof the novel type of half-open clusters. Efforts to its ion exchangeexperiments and to prepare new polyoxovanadate clusters by var-iation of temperature, templating ions and countercations areunderway.

Acknowledgements

This work was supported by the grants of 973 Program(2006CB932900), National Nature Science Foundation of China(20571074) and Nature Science Foundation of Fujian Province,NSF for Young Scientists of China (20801056), Young ScientistFunds of Fujian Province (No. 2007F3111).

Appendix A. Supplementary material

CCDC 704744 contains the supplementary crystallographic datafor this paper. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via http://www.ccdc.ca-m.ac.uk/data_request/cif. Supplementary data associated with thisarticle can be found, in the online version, at doi:10.1016/j.inoche.2008.12.021.

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