Inorganic Chemistry Communications 14 (2011) 143–145

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Inorganic Chemistry Communications

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Synthesis, structure and properties of novel 3-D porous lanthanide-3, 4′,5-azobenzenetricarboxylate frameworks

Lingling Zhang a, Cuiying Lu b, Sanping Chen a, Fushan Yu a, Xia Li a, Jinting Tan a, Xuwu Yang a,⁎a College of Chemistry andMaterials Science, Shaanxi Key Laboratory of Physico-inorganic Chemistry, Key Laboratory of Synthetic andNatural FunctionalMolecule Chemistry ofMinistry of Education,Northwest University, Xi'an 710069, PR Chinab School of Chemistry and Chemical Engineering, Yulin University, Yulin 719000, PR China

⁎ Corresponding author. Tel.: +86 29 88302054; fax:E-mail address: yangxuwu@nwu.edu.cn (X. Yang).

1387-7003/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.inoche.2010.10.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 July 2010Accepted 12 October 2010Available online 16 October 2010

Keywords:LanthanidePorous metal-organic frameworkMagnetic property

A novel three-dimensional (3-D) lanthanide metal-organic frameworks, {[Gd2(ABTC)2(DMF)2(H2O)2]·-DMF·H2O}n 1 (L=H3ABTC=3, 4′, 5-azobenzenetricarboxylic acid, DMF=N,N′-dimethylformamide) hasbeen prepared under mild conditions, and characterized by single-crystal X-ray diffraction, IR spectra, PXRD,and TG analysis. The magnetic measurement shows that 1 presents antiferromagnetic behavior.

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In recent years, the design and construction of lanthanide-basedmetal-organic frameworks (LMOFs) have attracted extensive atten-tion owing to their intriguing topologies [1] and potential applicationsin a variety of areas, including magnetism [2], fluorescent probe [3],catalysis [4], and gas storage [5]. Since the lanthanide generally havehigher coordination number and flexible coordination geometry, theyare made from an incorporation of solvent molecules (such as H2Oand DMF) into lanthanide coordination networks more likely andhard to construct porous structures compared with transition metals[6]. Since secondary building units (SBUs) can facilitate the design ofporousMOFs and simplify the network structures, many types of SBUshave been used to construct porous MOFs which demonstrate highsurface areas and significant hydrogen uptake. And transition metalporous MOFs with SBUs have achieved great success, such as MOF-5[7]. Although SBUs have been found to construct some porous LMOFs,such as rod-shaped (SBUs) [8], octahedral SBUs [9], square-planar Ln4

(μ4-H2O) SBUs [10], etc., it is still in the stage of experienceaccumulation. For LMOFs, different dinuclear [Ln2(COO)6] motifscontaining bridging and chelating carboxylate groups have beenrecognized as octahedral SBUs, and the combination of such SBUswith dicarboxylates, such as 4, 4′-azodibenzoate and S, S-dioxodi-benzothiophen-3, 7-dicarboxylic acid [11], has led to several 3-Dporous coordination frameworks. Meanwhile, octahedral SBUs withchelate-bridging carboxylate groups combine with 2-fluorobenzoicacid, which has led to a one-dimensional supramolecular struc-ture [12]. Here we reported a new 3-D noninterpenerative porous

LMOF, {[Gd2(ABTC)2(DMF)2(H2O)2]·DMF·H2O}n. To our knowledge,some MOFs based on H3ABTC still have not been reported so far, andthe complex is firstly reported to construct noninterpenerative porouslanthanide metal-organic framework with the octahedral SBUscontaining chelate-bridging carboxylate groups in our work.

In this communication, reaction of Gd(NO3)3·6H2O and 3, 4′, 5-azobenzenetricarboxylic acid was in the mixture solution of DMF andethanol at 60 °C to prepare a unique three-dimensional porousframework {[Gd2(ABTC)2(DMF)2(H2O)2]·DMF·H2O}n 1 [13]. Thephase purity of the bulk material was independently confirmed bypowder X-ray diffraction (PXRD) (see Fig. S1).

The complex 1 crystallizes in themonoclinic space group C2/c [14].XRD study of 1 shows a 3-D coordination polymer with dinuclearmetal-containing unit. As shown in Fig. 1, the Gd3+ center binds tonine oxygen atoms, seven from six different carboxylate groups of(ABTC)3−, one from the coordinated water molecule, and the otherfrom the coordinated DMF. Moreover, four of these ligands connecttwo metal ions. Two of the ligands have a μ-COO−-bridging moiety,and the other two are triply coordinated. The coordination polyhe-dron around the central Gd3+ can be visualized as a distortedtricapped trigonal prism with one chelating carboxylic (O5D), onechelate-bridging oxygen atom (O2A), and onewater oxygen (O1W) inthe capped positions (see Fig. S2). The carboxylic groups of H3ABTCare completely deprotonated and all the oxygen atoms from thecarboxylate groups of (ABTC)3− take part in coordination withlanthanide ions, which employ three different coordinate modesshown as Scheme 1. Two phenyl rings of the (ABTC)3− ligand are notparallel. The longest Gd–O bond in 1 is Gd1–O2, with a bond length of2.717(5) Å, is much longer than that of Gd1–O1 2.435(5) Å, which istypical for a tridentate carboxylate coordination, similar to the

Fig. 1. Coordination environment of 1. Ball-and-stick representation of 1 with atom labelling of selected atoms. For clarity, all hydrogen atoms are omitted.

144 L. Zhang et al. / Inorganic Chemistry Communications 14 (2011) 143–145

literature values [15], and Gd…Gd interatomic distance is 4.010(7) Å.The remaining Gd–O bonds range from 2.327(5) to 2.506(6) Å.

The complex 1 adopts the octahedral Ln2(COO)6 as secondarybuilding units (SBUs), where terminal DMF and H2O can be removedat high activation temperature. The octahedral SBUs serve as the nodeto connect with (ABTC)3− ligands and extend into a 2-D layernetwork. Four ligands connect four octahedral SBUs to form rhombicgrid sheet planes with an included acute angle of 86.478°, the layer-by-layer separation is ca. 16.578 Å between two parallel 2-D sheets.Furthermore, the ligands act as pillars to support the 2-D rhombicgrids into a 3-D framework with approximately rhombic channelswith dimensions (accounting for the van der Waals radii) of ca.9.938×9.938 Å2 (see Fig. 2). Meanwhile, the complex 1 has arelatively 3-D aperture, ca. 9.938×9.938 Å2 channel along [110]direction (see Fig. 3a) and ca. 16.863×9.322 Å2 along [001] direction(see Fig. 3b). These channels are partially occupied by the coordinated

Scheme 1. Coordinated mode of carboxyl groups involved in this research.

and solvent molecules. The size of the void created is 1689.9 Å3 for 1,which is 30.2% of the unit cell volume [16].

The TG study of 1 is performed under N2 atmosphere from 25 to1000 °C with a heating rate of 10 °C min−1 (see Fig. S3). The gradualweight loss of 21.51% from 25 to 255 °C corresponds to the loss ofsolvent molecules, two terminal DMF molecules and two terminalH2O molecules (calcd. 21.45%). When the temperature is up to ca.255 °C, the product begins to lose H3ABTC ligand.

The magnetic property of 1 is investigated from 2 K to 300 K. Theobserved value of χMT per [Gd2] unit is 16.25 cm3 K mol−1 at roomtemperature, which is close to the value of two noninteracting Gd3+

with the S=7/2. With a decrease in the temperature, χMT decreasessmoothly to a minimum of 15.12 cm3 K mol−1 at 2 K. The plot of χM

−1

versus T over the whole temperature range obeys the Curie–Weisslaw [χ=C/(T−θ)] with C=16.25 cm3 K mol−1 and θ=−0.39 K (seeFig. S4). The decrease of χMT and the negative value of θ indicate that

Fig. 2. The rhombic channels of 1 viewed along [−101] direction. Hydrogen atoms,terminal DMF molecules, H2O molecules, and solvent molecules are omitted for clarity.

Fig. 3. The channel of 1 viewed along [110] direction (a) and [001] direction (b),respectively. Hydrogen atoms, terminal DMF molecules, H2O molecules, and solventmolecules are omitted for clarity.

145L. Zhang et al. / Inorganic Chemistry Communications 14 (2011) 143–145

the antiferromagnetic interaction between the Gd3+ ions dominatesthe magnetic property of 1 (see Fig. 4).

In summary, we have successfully synthesized and characterizedcomplex {[Gd2(ABTC)2(DMF)2(H2O)2]·DMF·H2O}n. It is first exampleto construct noninterpenerative porous lanthanide metal-organicframework with the octahedral SBUs containing chelate-bridgingcarboxylate groups in our work. Combination of a functionallymodified building block with organic carboxylate may be envisionedto produce a new 3-D porous material. Further works on other porousLMOFs are in progress.

Fig. 4. Plots of χMT (□) and χM (○) versus temperature (T) for 1.

Acknowledgement

The author is thankful for “13115” S&T innovation program ofShaanxi Province (No. 2008ZDKG-22).

Appendix A. Supplementary material

CCDC 784391 contains the supplementary crystallographic datafor this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data to this article can be found online atdoi:10.1016/j.inoche.2010.10.007.

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