Inorganic Chemistry Communications 29 (2013) 11–13

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Inorganic Chemistry Communications

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Organically templated (3,8)-connected microporous heterometallic Zn(II)–Sr(II)coordination polymer

Xin-Ping Kang, Li-Hua Zhu, Ying-Shuang Hu, Zhe An ⁎School of Chemistry and Life Science, Guangdong university of Petrochemical Technology, Maoming, Guangdong 525000, People's Republic of China

⁎ Corresponding author.E-mail address: anzhe6409@sina.com (Z. An).

1387-7003/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.inoche.2012.11.030

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 November 2012Accepted 28 November 2012Available online 16 December 2012

Keywords:Heterometallic compoundTrinuclear clusterTemplateLuminescence

This communication describes a new (3,8)-connected microporous heterometallic coordination polymer,[Zn2Sr(pbdc)3(μ2-O1w)2(DMF)]n·n(DMF) (1 H2pbdc=terephthalic acid, DMF=N,N′-dimethylformamide).Crystallographical analysis reveals that it features an unusual heterometallic trinuclear building unit [Zn2-

Sr(COO)4(μ2-OH2)]. These trinuclear clusters are further interconnected by pbdc ligands, giving rise to thefinal three-dimensional (3D) microporous framework. In 1, DMF molecules not only act as terminal ligandsbut also act as templates filling in the one-dimensional (1D) channel further consolidating the whole frame-work. Moreover, this compound also exhibits strong ligand-centered violet luminescence at room tempera-ture in the solid state.

© 2012 Elsevier B.V. All rights reserved.

In recent years, crystalline microporous metal–organic frame-works (MOFs) have attracted tremendous attention owing to theirpotential application in the areas of catalysis, gas storage, chemicalseparation, ion exchange, and so on [1]. One of the feasible strategiesfor the construction of crystalline microporous MOFs, now known, isthe template-directed method [2]. Usually, typical templates used inthe synthesis of MOFs are polyoxometalates, water clusters, andsome organic species (such as DMF, DMA, etc.). Notable examplesare Ag14(trz)10[SiW12O40] and Ag10(Htrz)2(trz)6[SiW12O40] templatedby [SiW12O40] [3], [Co4(dpdo)12][H(H2O)27(CH3CN)12][PW12O40]templated by [PW12O40] and [H(H2O)27] clusters [4], and two-fold interpenetrated (Et3NH)[Zn4(ip)3(bta)3] templated by organic(Et3NH) cations [5]. However, it is still a great challenge to constructcrystalline microporous materials through template-directed meth-od because such results remain a tiny part of the already reportedMOFs.

According to previous reports in the literature, small organic mole-cules (such as DMF and DMA) as solvent can not only increase the sol-ubility of the organic ligand, but can also act as terminal ligands or guestmolecules playing a template role and filling in the channels. Moreover,DMF or DMA also can in situ decompose into dimethylamine cationsunder suitable conditions, which also can act as a template to directthe synthesis [6]. Considering that in this work we adopted mixedDMF and H2O as reaction media, we also introduced a second metalion (Sr(II)) owing to its high affinity for oxygen donors, especially forwater molecules [7]. The main purpose of using the above strategy isfrom two aspects. The first is to construct a new microporous frame-work templated by DMF or its decomposition product. The second is

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to fabricate a new heterometallic cluster as building subunit to con-struct a new topological framework. Successfully, a new heterometallicmicroporous framework templated by DMF molecules was obtainedexpectedly. It features a new heterometallic trinuclear cluster [Zn2-Sr(COO)4(μ2-OH2)] as building subunit, and can be simplified into anew (3,8)-connected topological net with the Schläfli symbol of{42.6}2{44.614.78.82}.

Compound 1 was synthesized hydrothermally by the reaction ofZn(NO3)2, Sr(NO3)2, H2pbdc and e-urea (2-imidazolidinone) in themixed solvent of DMF and H2O [8,9]. X-ray crystallographical analysisrevealed that [Zn2Sr(pbdc)3(μ2-O1w)2(DMF)]n·n(DMF) (1) crystal-lizes in C2/c space group. The fundamental unit of 1 consists of onecrystallographically independent Zn(II) ion, half of Sr(II) ion, oneand half pbdc ligands, one μ2-water molecule, one disordered coordi-nated DMF molecule and one disordered free DMF molecule. As rep-resented in Fig. 1a, each Zn(II) ion is tetrahedrally coordinated bythree carboxylate oxygen atoms (O1, O5 and O4d) from three differ-ent pbdc ligands and one bridged water molecule (O1w), and eachSr(II) ion lying in a crystallographic 2-fold axis with a half occupationis nine-coordinated by six carboxylate oxygen atoms (O2, O6, O2a,O3b, O3c and O6a) from six different pbdc ligands, two bridgedwater molecules and one coordinated DMF molecule. The distancesof Zn–O and Sr–O are in the range of 1.930(6)–1.998(6) Å, and2.510(1)–2.787(6) Å, respectively, which are comparable with thosepreviously reported Zn(II)-/Sr(II)-based polymers [10]. It should benoted that two Zn(II) ions and one Sr(II) ion are bridged by fourbis-monodentate carboxylate groups and two μ2-water molecule, giv-ing rise to a new heterometallic trinuclear building block [Zn2-

Sr(COO)4(μ2-OH2)] with the Zn–Sr distance of 3.884 Å (Fig. 1b),which is shorter than the distance of Zn–Sr in [ZnSr(PBDC)2(DMA)2]n(H2PBDC=terephthalic acid, DMA=N,N′-dimethylacetamide) [11].

Fig. 1. (a) The fundamental unit of 1 (free DMF molecules were omitted for clarity). Symmetry codes: (A) 2−x, y, 4.5−z; (B) 2−x, 1−y, 4−z; (C) x, 1−y, 0.5+z. (b) Heterometallictrinuclear cluster [Zn2Sr(COO)4(μ2-OH2)]. (c) 3D framework of 1with the 1D channels occupied by disordered DMFmolecules. (d) Schematic representation of (3,8)-connected topologyof 1.

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These isolated heterometallic trinuclear building blocks are furtherconnected together by the pbdc ligands to furnish the final 3D openframework, in which the 1D channel is occupied by the disorderedlattice DMF molecules (Fig. 1c). In 1, DMF molecules not only act asterminal ligands but also act as templates filling in the 1D channelfurther consolidating the whole framework. PLATON calculation indi-cates that the solvent-accessible volume of 1without free and coordi-nated DMFmolecules is approximately 932.8 Å3 per unit cell, equal to28.4% of the crystal volume.

All pbdc ligands in 1 adopt the same coordination mode linking twoZn(II) ions and two Sr(II) ions using its two carboxylate groups in uni-form bis-monodentate mode, However, there are still two differentkinds of pbdc ligands: one links two different heterometallic trinuclearbuilding blocks and the other links three heterometallic trinuclear build-ing blocks. In order to get better insight into the intricate framework of1, topological methodwas used to analyze this framework. According tothe above description, pbdc ligands linking two different trinuclear clus-ters can be looked as linkers, and pbdc ligands linking three differenttrinuclear clusters can be reduced into 3-connected nodes (Fig. S1a). Inaddition, each heterometallic trinuclear cluster links six 3-connectednodes and two different trinuclear clusters through linkers, and it canbe looked as an 8-connected node (Fig. S1b). Therefore, the intricateframework of 1 can be simplified into a binodal (3,8)-connected netwith the Schläfli symbol of {42.6}2{44.614.78.82}(Fig. 1d). Although(3,8)-connected topological framework has been widely documented,such 8-connected node in 1 constructed by heterometallic trinuclearclusters is rarely reported in a metal–organic framework. To the bestof our knowledge, such a topology has not been observed in MOFsaccording to the Reticular Chemistry Structure Resource Database [12].

To confirm the phase purity of compound 1, PXRD patterns wererecorded. It matched well with the simulated ones calculated fromthe single-crystal diffraction data (Fig. S2), indicating a pure phaseof the sample.

To characterize the compound more fully in terms of thermal sta-bility, TG analysis was performed on polycrystalline samples under N2

atmosphere with a heating rate of 10 °C/min in the temperaturerange of 30–900 °C. The TGA curve of 1 shows that compound 1 canbe stable up to 175 °C (Fig. S3). The first weight loss occurs between175 and 350 °C, corresponding to the loss of coordinated water mol-ecules, coordinated DMF molecules and free DMF molecules. The ob-served weight loss of 20.41% is a little higher than the calculated one(19.67%), probably due to the hydrate of the power crystal sampleswhen exposed in the air. After this, the skeleton of compound 1 grad-ually collapses because of the decomposition of the organic ligand.

Considering the excellent luminescent properties of metal–organiccoordination polymers constructed from d10 transition metal ions andconjugated organic ligands, the solid-state luminescent emission spec-tra of 1 and free ligand (H2pbdc) were studied at room temperature.As shown in Fig. 2, the free H2pbdc ligand exhibits maximum intenseemissions at 436 nmupon excitation at 349 nm, which can be probablyassigned to theπ*→n orπ→π* electronic transfer [13]. Comparedwiththe luminescent properties of free ligand, compound 1 exhibits a similarmaximum emission at 447 nm upon excitation at 322 nm. However,the maximum emission of 1 is red-shifted compared with the freeligand, which may be ascribed to the enhanced conjugated degreeof the ligand when it coordinated with metal ions. Zn(II) ion is diffi-cult to oxidize or reduce due to its d10 electronic configuration.Therefore, the origin of the luminescent emissions for 1 is neither

Fig. 2. The emission spectra of 1 andH2pbdc ligand in the solid state at room temperature.

13X.-P. Kang et al. / Inorganic Chemistry Communications 29 (2013) 11–13

ligand-to-metal charge transfer (LMCT) nor metal-to-ligand chargetransfer (MLCT), and it might be attributed to the intraligand emis-sion from H2pbdc ligands [14].

In summary, a new (3,8)-connectedmicroporous heterometallic co-ordination polymer has been successfully obtained from solvothermalsynthesis. It features an unusual heterometallic trinuclear unit [Zn2-Sr(COO)4(μ2-OH2)] as building block. In the construction of this com-pound, DMF molecules not only act as terminal ligands but also act astemplates filling in the 1D channel further consolidating the wholeframework. The results reported here also demonstrate that organicmolecules' (such as DMF, DMA, etc.) templating strategy will generatenew crystalline microporous framework expectedly.

Acknowledgments

This work was financially supported by the Research Fund for theDoctoral Program of Guangdong University of Petrochemical Technology(grant no. 512116), the Program for Talent Introduction in GuangdongHigher Education Institutions (grant no. 201191) and the ScientificResearch Start-up Funds for Talent Introduction in Guangdong Universityof Petrochemical Technology (grant no. 208058).

Appendix A. Supplementary material

CCDC no. 888602 contains the supplementary Crystallographicdata for this paper. These data can be obtained free of charge fromThe Cambridge Crystallographic data Center Via htt://www.ccdc.cam.ac.uk/data_request/cif. TGA and powder X-ray patterns can befound in the supporting file. Supplementary data to this article canbe found online at http://dx.doi.org/10.1016/j.inoche.2012.11.030.

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