吳晶晶 韓占剛 王彥娜 郝青華 翟學(xué)良

(河北師范大學(xué)化學(xué)與材料科學(xué)學(xué)院，石家莊 050016)

研究簡報

新型一維鉬氧鏈超分子化合物[(H2bpe)Mo4O13]的合成與光學(xué)性質(zhì)研究

吳晶晶 韓占剛＊王彥娜 郝青華 翟學(xué)良＊

(河北師范大學(xué)化學(xué)與材料科學(xué)學(xué)院，石家莊 050016)

鉬氧鏈；晶體結(jié)構(gòu)；光學(xué)性質(zhì)

Polyoxometalates (POMs)have been widely studied for their structural versatility and potential application in many kinds of fields,such as medicine,catalysis,analytical chemistry,and materials science[1-3].POMs have been employed as important inorganic building blocks for constructing novel hybrid materials with various organic molecules[4-8].In comparison to the classical Keggin-and Dawson-type heteropolyanions that have attracted more attention in hybrid materials,the research regarding the small sized isopolyanions is expected[9-11].Up to now,a large number of polymers based on di-,tri-,tetra-,penta-,heptamolybdates with 1D chain,2D layer and 3D open-framework structures have been studied[11-16].However,the solid materials of one-dimensional structure with only molybdenum oxide frameworks have been reported rarely by comparison with them[17].

Molybdenum oxide frameworks can be easily modified because of its high reactivity and structural flexibility[18-20].So,here we aim to synthesize new structures of molybdenum oxides under hydrothermal conditions using simple molybdate as the starting materials and bpe as the template,and compound 1 was obtained.

1 Experimental

1.1 General

All chemicals were commercially purchased and used without further purification.Elemental analyses(C,H,and N)were performed on a Elemental Vario EL elemental anayzer.IR spectra were recorded in KBr pellets with a FTIR-8900 IR spectrometer in the range of 400～4000 cm-1region.UV spectra were obtained on a Unican UV-2501PC spectrometer.Thermogravimetric analysis (TG-DTA)was carried out using a Perkin-Elmer TGA-7 instrument in flowing N2with a heating rate of 10 ℃·min-1.Fluorescent spectra were measured with a Hitachi F-4500 luminescence spectrometer.

1.2 Preparation of[(H2bpe)Mo4O13](1)

A mixture of Na2MoO4·2H2O (480 mg,2 mmol),bpe(45 mg,0.25 mmol)and H2O (10 mL)was stirred for 30 min and the pH was adjusted to 4 by 4 mol·L-1HCl.Then the mixture was transferred to a Teflon-lined stainless steel autoclave(25 mL)and kept at 170℃for 5 days.After the autoclave was cooled to room temperature at a rate of 10℃·h-1,yellow block-like crystals were obtained in a yield of 52%(based on Mo).The crystals were picked out,washed with distilled water,and dried at air atmosphere.Anal.Calcd for C12H12Mo4N2O13:C,18.56;H,1.55;N,3.61%.Found:C,18.41;H,1.60;N,3.72%.

1.3 Crystal structure determination

The data of compound 1 was collected with the approximate dimensions 0.25 mm×0.21 mm×0.12 mm onaSMARTAPEXⅡCCDAreaDetectordiffractometer at 298(2)K using graphite-monochromated Mo Kα radiation(λ=0.071073 nm)and oscillation scans technique in the range 2.03°～25.02°.Empirical absorption correction was applied.A total of 9690 (3438 unique,Rint=0.0262)reflections were measured.The structure was solved by direct method using the program SHELXS-97[21-22]and refined by full-matrix least-squares methods on F2using the SHELXL-97 program package.Anisotropic thermal parameters were used to refine all non-hydrogen atoms.Protonated hydrogen atoms attached to nitrogen atoms were located from the difference Fourier map.Hydrogen atoms attached to carbon atoms were fixed at their ideal positions.All the crystal data and structure refinement details for the compound 1 are given in Table 1.

CCDC:739954.

Table 1 Crystal data and structure refinements for compound 1

2 Results and discussion

2.1 Crystal structure of compound 1

A single-crystal X-ray diffraction analysis revealed that this compound is constructed from 1D [Mo4O13]2-anion chains integrated by protonated bpe ligands into a 3D framework.This tetramolybdate subunit(Fig.1)consists of four {MoO6}octahedral which are further connected through edge-or corner-sharing to form a 1D tetramolybdate chain running along the a-axis(Fig.2).

Fig.1 ORTEP drawing of 1 with 50%probability level,showing the coordination environments around Mo atoms

Fig.2 1D[Mo8O26]n4n-inorganic chain in 1

Fig.3 2D supramolecular structure of 1

Also,the tetramolybdate chain may be described as constructed from octamolybdate units fused at two common vertices.The structural feature of this[Mo8O26]4-subunit is similar to that of γ-[Mo8O26]4-which contains(N-H…O=Mo)hydrogen bonds of which distances are shown in Table 2,and π-π stacking interactions of bpe molecules with the distance of 0.3507～0.3848 nm(Fig.4).Then,the 2D layers are stacked to a 3D framework in parallel staggering fashion.six{MoO6}octahedraand two{MoO5}squarepyramids[23].The {MoO6}octahedra are all distorted and the Mo-O bond lengths are in the range of 0.1686(3)～0.248 3(3)nm.The octamolybdate unit in 1 can also be found in K2Mo4O13[24],(NH4)2Mo4O13[25]and(NH4)6[Mo8O27]·4H2O[26],[H2enMe]2[Mo8O26][27].In these reported compounds,octamolybdate units are only isolated cluster and don′t extend into one-dimensional structure.To the best of our knowledge,the similar octamolybdate chain once appeared in(Me-NC5H5)4n[Mo8O26]n[28]and(H2bbi)2[Mo8O26][29].

Non-covalent interactions are essentially necessary for the basis of the order of inorganic and organic components in supramolecular frameworks[30].In compound 1,π-π interactions and hydrogen bonds both play important roles in the formation of crystal framework.As shown in Fig.3,the two adjacent molybdenum oxide chains are linked up together to form a 2D layer through

Fig.4 A view showing π-π stacking interactions among bpe molecules in 1

Aromatic nitrogen heterocycles (pyridine,bipyridines,etc.)are usually used to construct π-π interactions because of their low π-electron density.In our previous work,bpy,bpp and pbpy (bpy=2,4′-bipyridine;bpp=1,3-bis(4-pyridyl)propane;pbpy=5-phenyl-2-(4-pyridinyl)pyridine)have all been selected to assemble POMs.A Keggin-based hybrid(Hbpy)4[SiMo12O40][31]was once reported by using bpy molecule,in which protonated bpy cations exhibited interesting organic double helical chains by N-H…N interaction.Recently,we reported a new compound (H2bpp)2[β-Mo8O26][32]constructed from one β-[Mo8O26]4-anionic cluster and two protonated bpp cations through the intermolecular hydrogen bonds(N-H…O-Mo and C-H…O-Mo)and π-π interaction.In another compound(pbpy)8H3[PW12O40]·2H2O[33],the pbpy molecules are approximately parallel to each other so as to create π-π stacking interactions.While in compound 1,there is a rather extensive π-π network.The py groups of bpe are almost parallel and this aromatic distances of the planes of the aromatic moieties fall into a range of 0.350 7～0.3848 nm,indicating that these interactions are of the face-to-face π-stacking type.In comparison with bpy,pbpy and bpp,these results indicate that bpe is easier to form the intermolecular face-to-face π-stacking interaction induced by POMs.A reasonable explain may be ascribed to the structure of bpe with a double bond contacting two pyridine rings,which makes the molecule a large conjugated system and creates a more balanced distribution of electron density to further increase the tendency of the face-to-face π-stacking.

Table 2 Hydrogen bond distances in compound 1

2.2 FTIR spectroscopy and TG analysis

The IR spectrum of 1 reveals the characteristic strong bands in the range 956～619 cm-1attributed to ν(Mo=O)and ν(Mo-O-Mo).The absorption bands in the range of 1628～1193 cm-1can be assigned to characteristic peaks of bpe.The broad peak at 3461 cm-1can be assigned to the stretching vibration of N-H bond.The thermal stability is investigated on crystalline sample under an air atmosphere from 25 to 850℃.It exhibits two steps of continuous weight losses of 23.35%between 265 to 520℃which attributes to the loss of protonated bpe and agrees with the calculated value(23.71%).

2.3 UV-Vis spectroscopy and luminescent property

The UV-Vis spectrum of 1 in methanol(Fig.5a)displays two main peaks at 302 and 310 nm,which are attributable to the O-Mo ligand-to-metal charge-transfer(LMCT)bands[34-35].

The photoluminescence properties of 1 and the free bpe at room temperature are shown(Fig.5b).It can be observed that the maximum emission wavelength of 1 occurs at 540 nm(λex=490 nm)which is red-shifted as compared to that of the pure bpe ligands(λem=510 nm and λex=460 nm).The origin of the emission for 1 might be attributable to the intraligand π-π*transitions of bpes[36-38].The result shows that 1 may be a candidate for potential photoluminescence materials.

Fig.5 UV-Vis spectroscopy(a)and luminescent property(b)

3 Conclusion

In summary,we have hydrothermally prepared and structurally characterized a new compound[(H2bpe)Mo4O13]constructed from 1D [Mo4O13]2-anion chains integrated by protonated bpe ligands.The π-π interactions and hydrogen bonds both play important roles in the formation of its framework.It exhibits luminescent property caused by π-π*intraligand transitions.Future research will focus on the study of the reaction between molybdates and other different organic ligands in an effort to obtain novel compounds with diverse dimensionalities and structural features combined with interesting luminescent properties.

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Hydrothermal Synthesis,Characterization and Luminescence Property of a New 1D Molybdenum Oxide Chain:[(H2bpe)Mo4O13]

WU Jing-Jing HAN Zhan-Gang＊WANG Yan-Na HAO Qing-Hua ZHAI Xue-Liang＊
(College of Chemisty and Material Science,Hebei Normal Universyt,Shijiazhuang 050016)

A new 1D compound[(H2bpe)Mo4O13](1)(bpe=trans-1,2-Di-(4-pyridyl)-ethylen)washydrothermally synthesized and characterized.Compound 1 comprises 3D supramolecular network constructed from 1D [Mo4O13]2-anion chains and protonated bpe layers via hydrogen bonds and π-π stacking interactions.The crystal data are the following:C12H12Mo4N2O13,Monoclinic,space group P21/n,a=0.96864(13)nm,b=1.34968(18)nm,c=1.5149(2)nm,β=99.766(2)°,Z=4.The inorganic chain built up from only molybdenum oxide building blocks is interesting.The luminescent property of 1 was studied.CCDC:739954.

molybdenum oxide chain;crystal structure;luminescent property

O614.61+2

A

1001-4861(2010)05-0921-05

2009-11-30。收修改稿日期：2010-01-08。

國家自然科學(xué)基金(No.20701011)，河北省教育廳自然科學(xué)基金基金(No.Z2006436)，河北師范大學(xué)博士啟動基金(No.L2005B13)資助項目。＊

。 E-mail：hanzg116@126.com，Tel：0311-86268311

吳晶晶，女，24歲，碩士研究生；研究方向：：多酸化學(xué)。