Journal of Molecular Structure 920 (2009) 284–288

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Synthesis and characterization of B-type Anderson polyoxoanions supportedcopper complexes with mixed ligands

Shiwei Zhang a, Yuxin Li b,*, Ying Liu a, Ruige Cao a, Chunyan Sun a, Hongmei Ji a, Shuxia Liu a,*

a Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun City, JiLin 130024, PR Chinab Institute of Genetics and Cytology, Northeast Normal University, Changchun City, JiLin 130024, PR China

a r t i c l e i n f o

Article history:Received 23 September 2008Received in revised form 8 November 2008Accepted 10 November 2008Available online 27 November 2008

Keywords:Organic–inorganic hybridsAnderson-type polyoxoanionPhotoluminescence propertiesMixed ligands

0022-2860/$ - see front matter Crown Copyright � 2doi:10.1016/j.molstruc.2008.11.031

* Corresponding authors. Tel./fax: +86 431 850995(S. Liu).

E-mail addresses: liyx486@nenu.edu.cn (Y. Li), lius

a b s t r a c t

Two novel compounds constructed by Anderson-type polyoxoanions and copper complexes with mixedligands, (H3O+)[Cu(C6NO2H4)(phen)(H2O)]2[Al(OH)6Mo6O18]�5H2O (1) and (H3O+)[Cu (C6NO2H4)(phen)-(H2O)]2[Cr(OH)6Mo6O18]�5H2O (2) have been isolated by conventional solution method, and characterizedby elemental analyses, IR spectra, thermal stability analyses, X-ray powder diffraction and single-crystalX-ray diffraction. Compounds 1 and 2 are isomorphic and reveal an example of three-dimensional supra-molecular organic–inorganic hybrids based on copper complexes with mixed 1,10-phenanthroline andpyridine-4-carboxylic acid ligands supported on Anderson-type polyoxoanions. Furthermore, both ofthe compounds exhibit photoluminescent properties at ambient temperature, and to elucidate the elec-tronic properties of the metal ions (Cu2+ or Cu2+/Cr3+), EPR studies have been performed, and the resultsare in good agreement with the structural feature of these compounds.

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

1. Introduction

Over the past decades, organic–inorganic hybrid compoundshave been attracting considerable interest as a promising new classof materials owing to the possibility of combining the different fea-tures of the components to get unexpected structures, properties,or applications [1]. Polyoxometalates (POMs) are discrete anionicmetaloxygen clusters, because their electronic versatility andstructural diversity [2] have been extensively employed as inor-ganic building blocks for the construction of organic–inorganic hy-brid materials with various transition-metal complexes as bridgingligands [3]. Among the versatile polyoxoanions, the Anderson-typepolyoxoanions holding charming planar structures has been exten-sively employed as inorganic building blocks in recent years, due toeach Mo (or W) atom of the Anderson-type polyoxoanions has twoterminal oxygen atoms with high reactivity [4], which may facili-tate the construction of novel organic–inorganic hybrid com-pounds. The successful syntheses of hybrid compounds holding1D (one-dimensional), 2D, and even 3D structures [5] have furtherinspired our research interest to construct the novel architecturesbased on this kind of polyoxoanions.

More recently, much attention has been focused on developingcoordination polymers with mixed organoamine and carboxylateligands or two different organoamine ligands incorporating the

008 Published by Elsevier B.V. All

02 (Y. Li), +86 431 85099328

x@nenu.edu.cn (S. Liu).

various polyoxoanion for structural reasons, as well as for theirproperties [6]. But up to now, the examples of hybrid compoundsbased on Anderson-type POM building blocks and coordinationpolymers with mixed organoamine and carboxylate ligands haverarely been reported [7]. Taking into account these, we choosethe 1,10-phenanthroline and pyridine-4-carboxylic acid as organicligands to combine the transition-metal centers because of theirmultifold electronic properties and coordination modes.

In this paper, we isolated two organic–inorganic hybrid com-pounds, namely (H3O+)[Cu(C6NO2H4)(phen)]2[Al(OH)6Mo6O18]�5H2O (1) and (H3O+)[Cu (C6NO2H4)(phen)]2[Cr(OH)6 Mo6O18]�5H2O(2), which are built up of Anderson-type polyoxoanions and coppercoordination polymers with mixed 1,10-phenanthroline and pyri-dine-4-carboxylic acid ligands, extensive p–p interactions andhydrogen bonding interactions form the 3D supramolecular struc-tures. Herein, the syntheses, structures, photoluminescence prop-erties and the X-band powder EPR spectra studies for thecompounds have been reported.

2. Experimental

2.1. Materials

All chemicals were used as purchased without further purifica-tion. Elemental analyses (C, H and N) were performed on aPerkin-Elmer 2400 CHN elemental analyzer. Cr, Al, Cu and Mo weredetermined by a PLASMA-SPEC(I) ICP atomic emission spectrome-ter. IR spectra were recorded in the range 400–4000 cm�1 on an

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Table 1Crystal data and structure refinement parameters for 1 and 2.

1 2

Empirical formula C36H47AlCu2N6Mo6O36 C36H47CrCu2N6Mo6O36

M 1869.49 1894.51Crystal system Triclinic TriclinicSpace group P�1 P�1h range (�) 2.21–28.13 2.11–25.60a (Å) 9.9403(10) 9.9418(9)b (Å) 9.9424(10) 9.9459(9)c (Å) 14.2362(14) 14.2493(12)a (�) 83.1470(10) 81.4570(10)b (�) 81.4550(10) 83.0140(10)c (�) 77.1910(10) 77.2240(10)V (Å3) 1351.3(2) 1353.1(2)Z 1 1F (000) 936 1170Dc (g cm�3) 2.294 2.327Abs coeff., (mm�1) 5.085 8.000Total data collected 7159 7275Unique data 4942 5025Rint 0.0143 0.0163GOF 1.021 1.046R1 [I > 2r(I)]a 0.0257 0.0304wR2 (all data)b 0.0651 0.0781

a R1 =P

| |Fo|�|Fc| |/P

|Fo|.b wR2 ¼ ½

PwðF2

0 � Fc2Þ2=P

wðF20Þ

2�1=2.

S. Zhang et al. / Journal of Molecular Structure 920 (2009) 284–288 285

Alpha Centaurt FT/IR spectrophotometer using KBr pellets. Ther-mal stability analyses were performed on a Perkin-Elmer TGA-7instrument in N2 atmosphere with a heating rate of 10 �C/min.Powder X-ray diffraction measurements were performed on aRigaku D/MAX-3 instrument with Cu-Ka radiation in the angularrange 2h = 3–50� at 293 K. Luminescence measurement was car-ried out on a Hitachi F-4500 Fluorescence Spectrophotometer.The electron paramagnetic resonance (EPR) spectra were recordedon a Japanese JES-FE3AX spectrometer at room temperature andliquid nitrogen temperature, respectively.

2.2. Syntheses of compounds 1 and 2

2.2.1. (H3O+)[Cu(C6NO2H4)(phen)(H2O)]2[Al(OH)6Mo6O18]�5H2O (1)Both the cationic complex [Cu(C6NO2H4)(phen)(H2O)]+ (A) and

the anion [Al(OH)6Mo6O18]3� (B) were prepared in situ. For A,pyridine-4-carboxylic acid (0.073 g, 0.27 mmol) was dissolved in20 mL hot water, after cooling to room temperature, 10 mL meth-anol solution of 1,10-phenanthroline (0.11 g, 0.60 mmol) and10 mL aqueous solution of Cu(NO3)2�3H2O (0.14 g, 0.60 mmol)were added in succession under stirring. For B, Na2MoO4�2H2O(0.90 g, 3.72 mmol) was dissolved in 30 mL water and the pH ofthe solution was adjusted with the dilute HNO3 solution (3 M) toapproximately 4.5, and 10 mL aqueous solution containingAl(NO3)3�9H2O (0.56 g, 1.50 mmol) was added with stirring, thefinal pH of the solution was adjusted to 2.6 with the dilute HNO3

solution (3 M). The solution of A was quickly added into that of Bwith stirring, immediately blue precipitate formed and the pH ofresulting solution was about 2.6. The mixture was stirred for30 min and then filtered. The filtrate was allowed to evaporatefor 10 days at room temperature and blue block crystals of com-pound 1 were isolated (53% yield based on Mo). Anal. Calcd forC36H47AlCu2N6Mo6O36: C, 23.13; H, 2.53; N, 4.50; Al, 1.44; Cu,6.79; Mo, 30.79 (%). Found: C, 23.34; H, 2.38; N, 4.67; Al, 1.65;Cu, 7.13; Mo, 31.05 (%). IR (KBr, cm�1): 3440 (br), 3049 (m), 1606(m), 1558 (m), 1518 (m), 1424 (m), 1370 (m), 1342 (m), 1238(w), 1199 (w), 1150 (w), 1107 (w), 1014 (w), 936 (s), 919 (s), 830(m), 773 (m), 723 (m), 647 (s), and 444 (m).

2.2.2. (H3O+)[Cu(C6NO2H4)(phen)(H2O)]2[Cr(OH)6Mo6O18]�5H2O (2)The synthesis of 2 was prepared following the procedure de-

scribed above, except that Cr(NO3)3�6H2O (0.42 g, 1.12 mmol)was used instead of Al(NO3)3�9H2O. Yield: 61% (based on Mo). Anal.Calcd for C36H47CrCu2N6Mo6O36: C, 22.82; H, 2.50; N, 4.44; Cr,2.74; Cu, 6.71; Mo, 30.37 (%). Found: C, 22.64; H, 2.36; N, 4.61;Cr, 2.85; Cu, 6.94; Mo, 30.68 (%). IR (KBr, cm�1): 3440 (br), 3049(m), 1605 (m), 1541 (m), 1518 (m), 1425 (m), 1370 (m), 1342(m), 1223 (w), 1201 (w), 1150 (w), 1109 (w), 1014 (w), 934 (s),909 (m), 830 (m), 773 (m), 723 (m), 643 (s) and 414 (m).

2.3. X-ray crystallography

Diffraction intensities for compounds 1 and 2 were collected ona SMART CCD diffractometer equipped with graphite monochro-matic MoKa radiation (k = 0.71073 Å) at 293 K. The linear absorp-tion coefficient, scattering factor for the atom, and anomalousdispersion correction were taken from International Tables forX-ray Crystallography. The structure was solved by the directmethod and refined by the full-matrix least-squares method onF2 using the SHELXTL crystallographic software package [8]. Empir-ical absorption correction by SADABS was applied to the intensitydata. Anisotropic thermal parameters were used to refine allnon-hydrogen atoms. Hydrogen atoms on the 1,10-phenanthrolineand pyridine-4-carboxylic acid ligands were placed on calculatedpositions and included in the refinement riding on their respectiveparent atoms. Details of the crystal data and final structure refine-

ments of compounds 1 and 2 are summarized in Table 1. Selectedbond lengths and bond angles are provided in Table S1 and TableS2 of the Supplementary material. The experimental and simulatedX-ray powder diffraction patterns (XRPD) of both compounds areshown in Fig. S5. The diffraction peaks on both experimental andsimulated patterns match well in position, indicating their phasepurity. Additionally, the XRPD patterns of 1 and 2 are similar,which are in agreement with their isomorphic structures deter-mined by single-crystal X-ray diffraction.

3. Results and discussion

3.1. Synthesis and structure

The compounds 1 and 2 were synthesized by the conventionalsolution method. We propose the following formation processscheme for compounds 1 and 2. (C6NO2H5 = pyridine-4-carboxylicacid, C12H8N2 = 1,10-phenanthroline = phen).

Cu2þ þ phenþH2O=CH3OHþ C6NO2H5

! Hþ þ ½CuðC6NO2H4ÞðphenÞðH2OÞnðCH3OHÞ3�n�þ ð1Þ

6½MoO4�2� þM3þ þ 6Hþ ! ½MðOHÞ6Mo6O18�3� ðM ¼ Al; CrÞ ð2Þ2½CuðC6NO2H4ÞðphenÞðH2OÞnðCH3OHÞ3�n�

þ þ ½MðOHÞ6Mo6O18�3�

þHþ þ ð6� nÞH2O

! ðH3OþÞ½CuðC6NO2H4ÞðphenÞðH2OÞ½MðOHÞ6Mo6O18� � 5H2Oþ ð3� nÞCH3OH ð3Þ

It was noted that, by plenty of parallel experiments, we found thatthe adjustment of the pH of the mixture to 2.6 was crucial for theformation of the final product.

For that compounds (H3O+)[Cu(C6NO2H4)(phen)(H2O)]2-[Al(OH)6Mo6O18]�5H2O (1) and (H3O+)[Cu(C6NO2H4)(phen)(H2O)]2-[Cr(OH)6Mo6O18]�5H2O (2) are isomorphic only with slightdifferences in bond lengths and bond angles, 1 is described as anexample below. The unit cell of 1 contains one Anderson-typepolyoxoanions supported copper complex with mixed 1,10-phe-nanthroline and pyridine-4-carboxylic acid ligands (Fig. 1) andsix lattice water molecules. The building block [Al(OH)6Mo6O18]3–

belongs to B-type Anderson structure, which consists of seven

Fig. 1. ORTEP view of the compound 1 with thermal ellipsoids at 50% level, latticewater molecules are omitted for clarity. Symmetry transformations used togenerate equivalent atoms: #1: �x + 1, �y + 2, �z + 1.

286 S. Zhang et al. / Journal of Molecular Structure 920 (2009) 284–288

edge-sharing octahedra, six of which are {MoO6} octahedral, ar-ranged hexagonally around the central {Al(OH)6} octahedron. Thehydrogen atoms of the hydroxyl groups around the Al3+ ion arelocated from difference Fourier maps. The Al–O bond lengths andO–Al–O bond angles summarized in Table S1 show only slightdistortion of the {Al(OH)6} octahedron. Molybdenum–oxygendistances as expected are divided into three groups: Mo–Ot,1.69–1.74 Å; Mo–Ob, 1.90–1.96 Å; Mo–Oc, 2.23–2.29 Å, which aresimilar to those reported in other compounds containing the[Al(OH)6Mo6O18]3� anion [9]. In the compound 1, the Andersonanion [Al(OH)6Mo6O18]3� acts as a bidentate ligand, coordinatingtwo copper fragments with mixed 1,10-phenanthroline and pyri-dine-4-carboxylic acid ligands.

As can been seen in Fig. 1, there is a crystallographically indepen-dent copper center in compound 1, residing in the center of a dis-torted square-pyramidal polyhedron, which is completed by oneoxygen atom from the pyridine-4-carboxylic acid (Cu1–O13 =1.927(2) Å), two nitrogen atoms from the 1,10-phenanthroline(Cu1–N1 = 2.021(3) Å, Cu1–N2 = 2.019(3) Å), and a water moleculeat the equatorial sites (Cu1–O15 = 1.973(2) Å) and one terminaloxygen atom from the Anderson-type polyoxoanions at the apicalpositions (Cu1–O11 = 2.368(2) Å).

Further studies reveal that there exists extensive p–p interac-tions of the 1,10-phenanthroline and pyridine-4-carboxylic acidligands in compound 1 (the centroid–centroid distance of the1,10-phenanthroline and pyridine-4-carboxylic acid rings is3.397 Å), which lead to the 2D layer structure (Fig. S1), and furtherlink into a 3D supramolecular network, also by hydrogen bondsinteractions among surface of oxygen atoms of polyoxoanions,coordinated waters, and lattice water molecules (Fig. 2).

For compounds 1 and 2, the bond valence sum calculations [10]indicate that all Al and Cr atoms are in the +3 oxidation state, all Cuatoms are in the +2 oxidation state and all Mo atoms are in the +6

Fig. 2. Packing of three-dimensional hybrid framework of compound 1.

oxidation state, and the bond valence sum calculations for all the Oatoms suggest that all the l3-briging O atoms around the Al3+ andCr3+ are protonated. The overall charges of all atoms in the formulaobtained from the single-crystal structure determination are �1.The difference Fourier map reveals that no Na ion presents, whichis further confirmed by elemental analyses. For the acidic syntheticcondition of 1 and 2, we presume that there is a protonated latticewater molecule to compensate for charge balance.

3.2. IR Spectroscopy

The IR spectra of compounds 1 and 2 are similar (Fig. S2). In thelow-wavenumber regions of the IR spectra, they display the char-acteristic patterns of the Anderson-type structure [4], with bandsbetween 400 and 600 cm�1 are attributed to the Mo–Oc stretchingvibration, between 640 and 800 cm�1 belongs to the Mo–Ob

stretching modes, and ranging from 890 to 950 cm�1 are assignedto the Mo–Ot characteristic vibration. The characteristic bands inthe region from 1014 to 1606 cm�1 are assigned to the featuresof the 1,10-phenanthroline and pyridine-4-carboxylic acid ligands[5b,11], which are of low intensity compared with those of thepolyainons. A broad band around 3281 cm�1 is attributed to theabsorptions of water molecules.

3.3. Thermogravimetric analysis

The thermal gravimetric (TG) analysis was performed with N2

atmosphere at 20–600 �C for the compounds. The TG curve of 1shows a total weight loss of 43.17% in the range of 25–483 �C,which agrees with the calculated value of 42.94% (Fig. S3). Theweight loss of 10.43% at 25–230 �C corresponds to the loss of sixlattice water molecules, two coordinated water molecules, andsix OH groups of [Al(OH)6Mo6O18]3� (calc. 10.65%). The weight lossof 32.74% at 300–483 �C are due to the decomposition of 1,10-phe-nanthroline, one proton and pyridine-4-carboxylic acid ligands(calc. 32.32%). The TG curve of 2 is similar to that of 1 (Fig. S4).

3.4. Photoluminescence properties

We have measured the photoluminescence properties of com-pounds 1 and 2 and the free 1,10-phen ligand in solid state at roomtemperature, the emission spectra are shown in Fig. 3. Excitation of

Fig. 3. Normalized solid-state photoluminescence spectra of compounds 1 and 2and 1,10-phen in the solid state at room temperature. (kex = 247 nm for all the threecompounds).

S. Zhang et al. / Journal of Molecular Structure 920 (2009) 284–288 287

both samples 1 and 2 at 247 nm produces similar luminescencewith the peak maximum at 398 nm. To understand the nature ofthe emission bands, we analyzed the photoluminescence propertyof the 1,10-phen ligand and found that the emission peak for 1,10-phen was at 383 nm. The emission peak for compounds 1 and 2 canbe assigned to photoluminescence signal of the 1,10-phen ligand.In the emission spectra, the emission peak 383 nm for 1,10-phenshifts to the peak maximum 398 nm in compounds 1 and 2, whichmay be attributed to ligand-to-metal charge transfer.

3.5. EPR spectroscopy

The X-band powder EPR spectra of 1 and 2 were recorded atroom temperature and liquid nitrogen temperature, respectively.It is noteworthy that the EPR spectra of 1 and 2 at room tempera-ture and liquid nitrogen temperature are almost the same. For 1,the spectrum consists of a broad signal centered at about 3200 G,as expected for copper(II) system at liquid nitrogen temperature(Fig. 4). The values for gk and g\ are 2.239 and 2.027, respectively,which exhibit the axial features as expected for a square-pyramidalCu2+ centers. The EPR parameters are consistent with those of Cu2+

(d9) systems in the literature [12].The spectrum of 2 at liquid nitrogen temperature is complicated

in comparison with those of 1 as a result of the presence of two dif-ferent paramagnetic resonance centers of Cr3+ and Cu2+ ions

Fig. 4. EPR spectra of compound 1 (top) and 2 (bottom) at liquid nitrogentemperature.

(Fig. 4). The coordination geometry around Cr3+ (3d3, 4F, L = 3,S = 3/2) is a distorted octahedron, which usually gives rise to thezero-field splitting [13]. In the spectrum, the Cr3+ centers of com-pound 2 show an anisotropic axial signal with gk = 4.2 andg\ = 1.23. It is noteworthy that there is another feature resonanceat the g value centered about g � 2, indicating that it arises fromthe presence of copper (II) square pyramids. The entire EPR dataof compound 2 are in good agreement with the reported literature[5b,14]. The above data indicate no obvious strong exchange cou-pling could be found between chromium (III) and copper (II) spinsbecause of the bulkiness of the Anderson polyoxoanion.

4. Conclusions

In conclusion, we have successfully introduced the copper com-plexes with mixed 1,10-phenanthroline and pyridine-4-carboxylicacid ligands into Anderson-type polyoxoanions, forming two novelorganic–inorganic hybrid compounds. Luminescent information onboth compounds exhibits emission peaks maximum at 398 nm,which can be assigned to photoluminescence signal of the 1,10-phen ligand. EPR studies of 1 exhibit emission peak a signal attrib-uted to the Cu2+ ions, and that of compound 2, containing both Cr3+

and Cu2+ ions, show large zero-field spilling of the Cr3+ ion, and asignal for the Cu2+ ion. It is hoped that more organic–inorganic hy-brid compounds with mixed ligands will be synthesized by choos-ing different kinds of ligands, metal centers and POMs.

Acknowledgements

This work was supported by the National Science Foundation ofChina (Grant No. 20871027), the Program for New Century Excel-lent Talents in University (NCET-07-0169), and the Analysis andTesting Foundation of Northeast Normal University.

Appendix A. Supplementary data

Crystallographic data have been deposited with the CambridgeCrystallographic Data Centre, with deposition number CCDC697202–697203. Copies of the data can be obtained free of chargeon application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK,fax: +44 1223 336 033, e-mail: deposit@ccdc.cam.ac.uk; or onthe web: http://www.ccdc.cam.ac.uk. Supplementary data associ-ated with this article can be found, in the online version, atdoi:10.1016/j.molstruc.2008.11.031.

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