Inorganic Chemistry Communications 14 (2011) 407–410

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Inorganic Chemistry Communications

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A novel metal–organic framework with bifunctional tetrazolate-5-carboxylateligand: Crystal structure and luminescent properties

Yan Li a, Xin-Hua Zhong a, Fa-Kun Zheng b,⁎, Mei-Feng Wu b, Zhi-Fa Liu b, Guo-Cong Guo b

a Key Laboratory for Advanced Materials, Department of Chemistry, East China University of Science and Technology, Shanghai 200237, PR Chinab State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China

⁎ Corresponding author. Tel.: +86 591 83704827; faxE-mail address: zfk@fjirsm.ac.cn (F.-K. Zheng).

1387-7003/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.inoche.2010.12.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 October 2010Accepted 10 December 2010Available online 16 December 2010

Keywords:Metal–organic frameworksTetrazolate-5-carboxylate ligandTrigonal ringTubular structureLuminescent properties

Hydrothermal reaction of NaN3, Na(4-cba) (4-Hcba=4-cyanobenzoic acid) and ZnBr2 afforded a novel 3DZn(II)coordination polymer [Zn2(4-tzba)2(H2O)]n 1 (4-H2tzba = 4-(5H-tetrazol)benzoic acid). In situ [2+3]cycloaddition reaction of nitrile and azide in the presence of ZnBr2 yielded tetrazolate-5-carboxylate 4-tzba2−

ligand. Compound1displays anunprecedented four-connected topological networkwith 1Dhexagonal channelsconstructed from trigonal rings [Zn3(4-tzba)6]. The solid-state photoluminescent studies revealed that 1 exhibitsa strong blue emission mainly originating from 4-tzba2− ligand-centered charge transition.

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Ever-increasing interests have been aroused in the design andconstruction of metal–organic frameworks (MOFs) due to theirintriguing structures and potential applications as functional materials[1]. Carboxylate compounds used in assembling MOFs have beenextensively studied for decades because of their excellent coordinationcapability and possibility of building up novel open architectures [2]. Onthe other hand, tetrazoles with similar pKa values (≈4) to carboxylateacid and their substituted derivatives have attracted great attention andserved as organic linkers in the construction ofMOFs since Sharpless andhis co-workers reported a safe, convenient and environmentallyfriendly synthetic route to 5-substituted 1H-tetrazoles [3]. To revealthe exact role of the metal salt in this reaction, a series of hydrothermalreactions aimed at trapping and characterizing the solid intermediateshave been investigated by Xiong and his co-workers, providing a moreaccessible and simple route to obtain tetrazolate coordination polymers[4]. Then several tetrazolate-based MOFs with novel structures andinteresting physical properties have been generated by in situhydrothermal methods [4b,5]. Carboxylate-containing 5-substitutedtetrazolate ligands (so-called tetrazolate-5-carboxylates), containingfour nitrogen and two oxygen electron-donating atoms, can exhibitdiverse coordination modes and have been used as bridging buildingblocks in the formation of polymeric networkswith charming structuralvarieties and peculiar performance [6–9]. However, in contrast withcarboxylate or tetrazolate, the investigation on chemistryof bifunctionaltetrazolate-5-carboxylate ligands remains less developed though theyare expected to be a versatile bridge ligand.

Recently, we have obtained a high thermally stable Zn(II)coordination polymer [Zn(4-tzba)]n (4-H2tzba = 4-(5H-tetrazol)benzoic acid), which displays a gsi (γ-silicon) topology, interestingheat-induced photoluminescent properties and second-order nonlinearoptical effects [7a]. Thereafter several coordination polymers based on3-tzba2− ligand (3-H2tzba = 3-(5H-tetrazol)benzoic acid), possessinginteresting magnetic and luminescent properties, have been synthe-sized and characterized [7b,c]. In addition, the structurally related tza2−

(H2tza = 1H-tetrazolate-5-acetic acid) and tzf2− (H2tzf = 1H-tetrazolate-5-formic acid) ligands have also been employed in assemblyof polymeric architectures [8]. To extend our researchwork in this field,herein, we present the synthesis, crystal structure and luminescentproperties of a new 3D zinc(II) coordination polymer [Zn2(4-tzba)2(H2O)]n 1. Interestingly, the network of 1 is created by the linkage of thetrigonal rings to produce 1D open-ended and hollow channels, whichare further connected to result in a new four-connected topology.

Hydrothermal treatment of NaN3, Na(4-cba) (4-Hcba = 4-cyanoben-zoic acid) and ZnBr2 yielded the mixture of needle-shaped crystals of 1and our previously reported prism-shaped crystals [Zn(4-tzba)]n [10],which were separated manually. Single-crystal X-ray structural analysis[11] shows that 1 crystallizes in the space group R-3c and features a novel3D open-framework exhibiting a tubular assembly. The powder X-raydiffraction (PXRD) pattern of 1 agrees well with the simulated one basedon the single-crystal X-ray data (Fig. S1), indicating that 1 is in a purephase.

As shown in Fig. 1, there are two crystallographically independentZn(II) centers in an asymmetric unit, which display different coordinationenvironments. The Zn1 atom is four-coordinated by two carboxylate Oatoms (Zn1-O1 1.951(3) Å) and two tetrazolate 1-nitrogen atoms(Zn1-N1 2.000(4) Å) from four symmetrically related 4-tzba2− ligands

Fig. 1. Themolecular structure of 1. Symmetry codes: A x−y,−y, 1.5−z; B−x+y,−x, z;C−x,−x+y, 1.5−z; D 2/3−x, 1/3−x+y, 11/6−z; E x−y, x, 2−z; F 2/3−x+y, 1/3+y,−1/6+z; G−y, x−y, z; H y,−x+y, 2−z. Hydrogen atoms have been omitted for clarity.

Fig. 2. A trigonal ring [Zn3(4-tzba)6] formed by three Zn1 atoms doubly bridged by six4-tzba2− ligands.

408 Y. Li et al. / Inorganic Chemistry Communications 14 (2011) 407–410

to form a distorted tetrahedral coordination geometry with the anglesN/O-Zn1-N/O varying from 100.67(15) to 121.6(2)°. Meanwhile, the Zn2atom isfive-coordinated by two carboxylate oxygen atoms (Zn2-O22.105

Fig. 3. (a) Schematic representation of the stack of trigonal rings [Zn3(4-tzba)6] in an–ABAB–mgive a 1D chain. (b) The 1D hexagonal channel viewed along the c direction.

(3) Å) and two tetrazolate 4-nitrogen atoms (Zn2-N4 2.041(3) Å) fromfour symmetrically related 4-tzba2− ligands, and one water molecule(Zn2-O1W 2.026(4) Å), furnishing a distorted trigonal–bipyramidalcoordination sphere. In this arrangement, two tetrazolate 4-nitrogenatoms and one oxygen atom fromwatermolecule reside in the equatorialpositions, and the axial positions are occupied by the two carboxylateoxygen atomswith the axial angle O2-Zn2-O2 171.66(17)° as opposed tothe ideal angle of 180°. The 4-tzba2− ligand adopts a tetradentatecoordination mode μ4-κN1:κN4:κO1:κO2, which is similar to that in[Zn(4-tzba)]n [7a]. The tetrazolate ring and its corresponding phenyl ringin 4-tzba2− ligand are not parallel to each other with a dihedral angle of15.91(40)°, and the carboxylate group also not coplanar with the phenylring with a dihedral angle of 45.86(18)°. The dihedral angle of thetetrazolate ring and carboxylate group is 32.32(32)°.

The most fascinating structural feature of 1 is that the skeleton ofthe tubular framework, which is built up by trigonal rings assecondary building units. As shown in Fig. 2, three symmetricallyrelated Zn1 atoms are double-bridged by six 4-tzba2− ligands throughO1 and N1 to give a trigonal ring [Zn3(4-tzba)6], in which the Zn1atoms and 4-tzba2− ligands occupy the vertices and sides of thetriangle, respectively. The Zn1···Zn1 separation is 8.1313(17) Å. Thetrigonal rings stack in an –ABAB– mode along the c direction. Each ofthe six Zn2 atoms is coordinated to two 4-tzba2− ligands from theadjacent trigonal rings through O2 and N4 to give a 1D chain with atubular structure, in which 4-tzba2− ligands are arranged along thecurved surface (Fig. 3). The 1D chain is further connected to sixadjacent chains by sharing Zn2 atoms (Fig. S2), leading to a 3D open-framework showing a tubular assembly (Fig. 4). All the tubes lineup togenerate a pipeline structure that extends along the c direction, andthese tubular assemblies constitute a hexagonal array, similar to thatin the reported compound [12]. The 1D tube can be considered ahexagonal channel viewed down the c direction. The total solvent-accessible volume of the channel in the unit cell is 272 Å3, whichaccounts for 3.4% of the total cell volume as calculated by PLATON[13]. Notably, compared with inorganic or organic tubular ensembles,the present coordination framework-based tubular structure, whichstands for one important type of porous MOFs, remains largelyunexplored [14]. To the best of our knowledge, only two compoundswith tetrazolate-5-carboxylate ligands exhibit permanent porositybut both of them were directly synthesized by the reactions of Zn(NO3)2 with tetrazolate-5-carboxylate ligands [9e,f]. Moreover, if theZn(II) centers and 4-tzba2− ligands are treated as four-connectednodes, the overall structure of 1 can be defined as a 4-connected net(Fig. 5). The short and long Schläfli symbols are (42.64)(62.84)(4.63.82)

ode along the c direction. The Zn2 atoms are bonded to O2 andN4 from4-tzba2− ligands to

Fig. 4. Infinite 3D polymeric framework of 1 viewed along the c direction.

Fig. 6. Solid-state emission spectra of H2(4-tzba) (λex=318 nm) and 1 (λex=365 nm)at room temperature.

409Y. Li et al. / Inorganic Chemistry Communications 14 (2011) 407–410

and (4.4.63.63.62.62)(62.62.82.85.85.85)(4.6.6.6.84.83), respectively,which are different from those in 4-connected [Zn(4-tzba)]n with agsi net 66 [7a]. To the best of our knowledge, the MOF with such atopology has not been reported to date though many 4-connectednets have been well documented [15].

The solid-state photoluminescent spectra of 1 and free neutral4-H2tzba ligand at room temperature are illustrated in Fig. 6. The4-H2tzba compound exhibits an emissionmaximumat 378 nmexcited at318 nm, while 1 displays a strong blue emission at 438 nm uponphotoexcitation at 370 nm under the same condition. The emissiondecay lifetime of 1 is 2.83 ns, and the external quantum yield is 1.05%.Obviously, there exists a red-shift emission for 1 relative to 4-H2tzba. InZn(II)/Cd(II) coordination complexes, ligand-to-ligand charge transfer

Fig. 5. The topological network of 1.

(LLCT) and ligand-to-metal charge transfer (LMCT) are commonlyobserved [16]. Notably, if the ligands areheterocylic aromatic compounds,the heteroatom can effectively decrease π and π* orbital energies. ThusHOMO and LUMO of complexes may lack the contribution from metalatoms, and LMCT emission can be excluded and the nature of organicligand will play a critical role in the photoluminescent mechanism ofZn(II) and Cd(II) complexes [6c,14]. The emission of 1may be assigned to4-tzba2− ligand-centered charge transition, in agreement with those forother 4-tzba2− and 3-tzba2− complexes [6,7].

In summary, we have successfully prepared and structurallycharacterized a novel 3D Zn(II) coordination polymer [Zn2(4-tzba)2(H2O)]n 1 with a new four-connected topology. Compound 1possesses 1D open-ended and hollow channels arranged in ahexagonal array with trigonal rings [Zn3(4-tzba)6] as secondarybuilding units. Photoluminescent spectra demonstrate that 1might bea good candidate for an efficient blue emission material.

Acknowledgements

This work was financially supported by National Nature ScienceFoundation of China (20871115) and Fujian Provincial Key Laboratoryof Nanomaterials (NM10-06).

Appendix A. Supplementary material

CCDC 794982 contains the supplementary crystallographic datafor 1. These data can be obtained free of charge from The CambridgeCrystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.inoche.2010.12.013.

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