Syntheses, structures and photoluminescence of Cd(II) coordination polymers based on in situ synthesized bifunctional ligands

Inorganic Chemistry Communications 48 (2014) 30–35

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Syntheses, structures and photoluminescence of Cd(II) coordination polymers based on in situ synthesized bifunctional ligands Chongjian Zhao, Chuwen Li, Moyuan Shen, Lanfen Huang, Qianhong Li, Mingyuan Hu, Hong Deng ⁎ School of Chemistry & Environment, South China Normal University, Guangzhou 510006, PR China Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, Guangzhou 510006, PR China Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, Guangzhou 510006, PR China

a r t i c l e

i n f o

Article history: Received 9 July 2014 Received in revised form 31 July 2014 Accepted 6 August 2014 Available online 7 August 2014 Keywords: N ligands Metal–organic frameworks Cadmium Luminescence

a b s t r a c t By employing Cd(II) salt, NaN3, and CN–(CH2)n–NC (n = 1, 2) and with the absence or presence of secondary ligands, four new cadmium coordination frameworks, named, {[Cd4(btm)4(H2O)2]·3H2O}n (1); [Cd2(btm)2(H2O)]n (2); [Cd2(bte)(PMA)0.5(H2O)]n (3); and [Cd(tzp)(2,2′-bipy)]n (4) (H2btm = bis(tetrazole) methane: H2bte = 1,2bis(tetrazole-5-yl)ethane; H2tzp = 1H-tetrazolate-5-propionic acid; bipy = bipyridine; PMA = 1,2,4,5benzenetetracarboxylic acid) were synthesized via in situ hydrothermal reaction. Single crystal X-ray diffraction reveals that compounds 1–3 are all three-dimensional (3D) frameworks. Compound 1 is constructed by Cd1– and Cd3–btm2− layers and large bridging metalloligands. Compound 2 exhibits a 3D framework with twodimensional (2D) Cd–btm2− (adopting μ6:κN1, N1′: κN2: κN3: κN4: κN3′: κN4′coordination mode) layers pillared by μ3:κN1, N1′: κN2: κN4′ btm2−. Compound 3 is built up by the Cd–bte2− layers and the linker PMA, with left- and right-handed helical chains arranged alternately. It is notable that btm2− takes on six different coordination modes in 1 and 2. Compound 4 represents a 2D layered framework, which can be simplified into a Shubnikov plane net (4.8^2) topological network with 3-connected T shape linker tzp2− ligands. In addition, the research results show that compounds 1–4 exhibit different fluorescent behaviors and thermal stabilities. © 2014 Elsevier B.V. All rights reserved.

Increasingly a lot of studies have been done on rational design and fabrication of metal–organic frameworks (MOFs) based on tetrazole and its derivatives due to their fascinating topologies as well as potential applications such as catalysis, adsorption, sensors, and magnetic [1]. Especially since Sharpless developed an environmentally friendly [2 + 3] cycloaddition method which is safe and convenient [2], a considerable number of chemists set their focus on the synthesis of fancy tetrazolebased MOFs via in situ [2 + 3] cycloaddition reaction by employing suitable organic nitrile and azide under the catalysis of transition metal [3]. However, although numerous tetrazole-based coordination networks have been extensively investigated [4], it still remains a large challenge for the chemists to design and construct novel MOFs with desired topologies and specific properties. To address this challenge, several synthetic strategies have been developed such as the option of metal ions and/or the organic ligands [5]. However, a careful review of the literature suggests that another extremely feasible strategy is the introduction of secondary ligands [6]. To the best of our knowledge, the investigation involving secondary ligands, in situ tetrazole ligand synthesis, remains less developed [7]. Meanwhile, the exploration of dinitrile-based in situ synthesis is even rarer [7,8].

⁎ Corresponding author. E-mail address: [email protected] (H. Deng).

http://dx.doi.org/10.1016/j.inoche.2014.08.005 1387-7003/© 2014 Elsevier B.V. All rights reserved.

As a part of our continuing work in this field [9], we chose malononitrile, butanedinitrile and Cd(II) salts as the initial reactants due to the following considerations: (1) the d10 metals are able to endure various coordination numbers and geometries, and meanwhile, they possess good luminescent properties when bound to functional ligands [10]; (2) both malononitrile and butanedinitrile contain double – CN groups, which can form three species through in situ ligand reaction respectively: –OOC–(CH2)n–COO–, –OOC–(CH2)n–tetrazole– and –tetrazole–(CH2)n–tetrazole– (n = 1, 2); (3) the species of – OOC–(CH2)n–tetrazole– possess multiple potential coordination sites, which can therefore act as an excellent ligand and exhibit various coordination modes. Notably, the investigation of –OOC–(CH2)n–tetrazole– is quite rare [10], especially for –OOC–(CH2)2–tetrazole–. To the best of our knowledge, only one article on –OOC–(CH2)2–tetrazole based metal–organic compounds has been reported [10b] and we are the first to adopt dinitrile as the precursor to obtain such species; (4) the bis(tetrazole) ligands with abundant N atoms permit a range of versatile bridging modes, what's more, the studies on bis(tetrazole) ligands are quite limited [11]; and (5) the bis(tetrazole) ligand is separated by an alkyl (CH2)n spacer, for which it can add some flexibility to the structure [11b]. In this context, we reported four new coordination polymers via in situ ligand reaction, namely, {[Cd4(btm)4(H2O)2]·3H2O}n (1); [Cd2(btm)2(H2O)]n (2); [Cd2(bte)(PMA)0.5(H2O)]n (3) and [Cd(tzp)

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(2,2′-bipy)]n (4), by the addition of different secondary ligands and controlling the reactant ratio, which are structurally related to bis(tetrazol5-yl)alkane ligands, H2btm and H2bte, and H2tzp (Scheme 1). The synthetic route is illustrated in Scheme S1 [12]. In addition, we are the first to obtain tzp2−-based metal compounds by using butanedinitrile as the initial reactant via in situ hydrothermal method. Single crystal X-ray diffraction structural analysis [13] reveals that compound 1 is a 3D framework crystallizing in space group P21/c. As shown in Fig. 1a, the asymmetric unit of 1 contains four crystallographically independent Cd(II) ions, four types of btm2− ligands and two coordinated water molecules. All the Cd(II) ions are six coordinated and have distorted octahedral coordination geometry. Cd2 is coordinated to five N atoms from 4 different btm2− ligands and one O atom from terminal water molecule, Cd4 is coordinated to five N atoms from 3 different btm2− ligands and one O atom from terminal water, while Cd1 and Cd3 are coordinated to six N atoms from six different btm2− ligands. The Cd–N and Cd–O bond lengths range from 2.265(11) to 2.564(15) Å, and from 2.35(2) to 2.353(16) Å. All of the bond distances of Cd–N and Cd–O are comparable to those in other related articles [14]. The btm2− ligands in compound 1 exhibit four coordination modes (see Supporting Information, modes a–d in Scheme S2). The best approach to interpret and understand the complicated structure of 1 is to take six btm2− ligands together with two Cd2 and Cd4 ions as a large metalloligand, Cd1 and Cd3 centers as six-connected nodes. Consequently, every metalloligand links sixteen Cd(II) ions, as shown in Fig. 1b, and Cd1 and Cd3 ions are connected by btm2 − ligands, which results in the construction of 2D Cd1– and Cd3–btm2− layers on the bc-plane (Fig. 1c). Finally, the layers are further bridged by metalloligands to generate a 3D coordination framework (Fig. 1d). Single crystal X-ray diffraction structural analysis [13] reveals that compound 2 is a 3D framework crystallizing in space group P21/n. The asymmetric unit consists of two crystallographically independent Cd(II) ions, two btm2− ligands and one coordinated water molecule. As shown in Fig. 2a, the Cd1 center is surrounded by five nitrogen atoms from four independent btm2 − ligands and one oxygen atom from the coordinated water molecule, while the Cd2 center is surrounded by five nitrogen atoms from four independent btm2 − ligands. The local coordination geometry around Cd1 ion assumes a slightly distorted [CdN5O] octahedron with the Cd–N bond distances ranging from 2.294(2) to 2.557(2) Å and Cd–O bond distance being 2.315(2) Å. While the coordination environment around Cd2 can be described as a slightly distorted [CdN6] octahedral geometry with the Cd–N bond distances being in the normal range of 2.284(2)– 2.457(2) Å. The deprotonated btm2− ligand adopts two kinds of coordination modes (modes e and f in Scheme S2). The six-connected btm2− ligands connect the Cd1 and Cd2 centers to form 2D layers on the [111] plane (Fig. 2b). Then the adjacent layers further connect to each

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other through the tridentate-bridging btm2− ligands to generate a 3D framework (Fig. 2c). Single crystal X-ray diffraction structural analysis [13] reveals that compound 3 is a 3D framework crystallizing in space group P21/c which is built up by the Cd–bte2− layers and the linker PMA. The asymmetric unit of compound 3 consists of two crystallographically independent Cd(II) ions, one bte2 − ligand, half of an PMA ligand and one coordinated water molecule. As shown in Fig. 3a, Cd1 ion is surrounded by four nitrogen atoms from three independent bte2− ligands and two oxygen atoms from two independent PMA ligands. The coordination environment around Cd1 can be described as a slightly distorted [CdN4O2] octahedral geometry with the Cd–N and Cd–O bond distances being in the normal range of 2.278(3)–2.643(3) Å and 2.280(2)–2.329(2) Å, respectively. And the seven-coordinated Cd2 atom is surrounded by two nitrogen atoms from two independent bte2 − ligands, five oxygen atoms from two independent PMA ligands, one coordinated water molecule with the Cd–N bond length of 2.322(3) Å and the Cd–O bond distance ranging from 2.276(2) to 2.489(2) Å. The deprotonated bte2 − ligand acts as μ5-κN1, N1′: κN2: κN3: κN4: κN2′ bridging ligand which is quite similar to btm2− in mode c of Scheme S2 with μ4 and μ2 tetrazole ring, linking three Cd1 and two Cd2 atoms (mode g of Scheme S2). While the PMA ligand supplies one oxygen atom to connect two Cd atoms to form infinitely -Cd1-O-Cd2-O-Cd1- helical chains extending along b direction with the shortest Cd···Cd distance of 3.909(3) Å and the pitch of screws of 8.281 Å, as shown in Fig. 3b, and simultaneously the μ4 tetrazole ring links four Cd atoms (two Cd1 and Cd2 atoms) to reinforce the chain. The adjacent chains which are right-handed and lefthanded further connect to each other through the μ2 tetrazole ring of bte2− ligands to generate a 2D layer on the bc-plane (Fig. 3d). The adjacent layers were finally bridged by the PMA ligands, resulting in a 3D framework (Fig. 3c). Single crystal X-ray diffraction structural [13] analysis reveals that compound 4 is a 2D framework crystallizing in space group Pbca and the asymmetric unit consists of one Cd(II) ion and one 2,2′-bipy and tzp2− ligand. The Cd(II) ion features a distorted octahedral coordination geometry. As shown in Fig. 4a, the center Cd(II) ion is coordinated by four nitrogen atoms (from one 2,2′-bipy and two tzp2 − ligands) and two oxygen atoms (from the carboxylate group of another tzp2 − ligand). The selected bond lengths (Å) and angles (°) are shown in Table S1. The Cd–N and Cd–O bond distances range from 2.285(3) to 2.393(4) Å and from 2.288(3) to 2.484(3) Å. In this structure, each 2,2′-bipy ligand chelates one Cd(II) ion and all the tzp2− ligands act as T-shaped linkers to link three Cd(II) ions (Fig. S1 and mode h in Scheme S2). Each tetrazole group takes the μ2-κN2: κN6 bridging mode to connect two Cd(II) ions generating an infinite –Cd–tetrazole–Cd– zigzag chain extending along the a direction (Fig. 4b). And the adjacent chains further interconnect to each other through the carboxylate group of

Scheme 1. The tetrazole ligands studied in this study.

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Fig. 1. (a). View of the asymmetric unit of compound 1; (b). View of the large metalloligand constructed by six btm2− ligands and two Cd2 and Cd4 ions; (c). View of the Cd1– and Cd3– btm2− layers through bc-plane; (d). View of the 3D structure of compound 1. (All H atoms and free water molecules were omitted for clarity; N atoms were represented as green, C as black, O as red and Cd as pink spheres.) Symmetry codes: (a): −x, 0.5 + y, 0.5 − z; (b): x, 1 + y, z; (c): 1 − x, 1 − y, 1 − z; (d): −x, 1 − y, 1 − z; and (e): −x, −y, 1 − z. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. (a). View of the asymmetric unit of compound 2; (b). View of the 2D layers on [111] plane; (c). View of the 3D structure of compound 2. (All H atoms were omitted for clarity; N atoms were represented as green, C as black, O as red and Cd as pink spheres.) Symmetry codes: (a): −x, −y, 2 − z; (b): −0.5 + x, 0.5 − y, −0.5 + z; (c): 0.5 − x, 0.5 + y, 1.5 + z; and (d): −x, 1 − y, 2 − z. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

C. Zhao et al. / Inorganic Chemistry Communications 48 (2014) 30–35

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Fig. 3. (a). View of the asymmetric unit of compound 3; (b). View of the infinite –Cd1–O–Cd2–O–Cd1– helical chains extending along b direction; (c). View of the 3D structure of compound 3; (d). View of the 2D layer through bc-plane. (All H atoms were omitted for clarity; N atoms were represented as green, C as black, O as red and Cd as pink spheres.) Symmetry codes: (a): 2 − x, 0.5 + y, 0.5 − z; (b): 2 − x, 1 − y, 1 − z; (c): 1 − x, −0.5 + y, 0.5 − z; (d): 1 − x, 1 − y, 1 − z; and (e): 1 + x, y, z. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

tzp2− ligands to form 2D wavelike layers (Fig. 4c). Meanwhile, the presence of intermolecular C(10)–H(10)…N(5) (3.232(7)Å and 142(1)°) and C(13)–H(13A)…O(1) (3.450(5)Å and 147°) interactions provides extra source of stability of the 2D sheets (Fig. S2). Taking tzp2− ligands as 3-connected T shaped linkers and the Cd(II) ions as 3-connected nodes, then the 2D layer can be simplified into a Shubnikov plane net (4.8^2) topological network (Fig. S3). Despite the fact that there are plenty of 2,2′-bipy, the π–π interaction does not occur obviously between adjacent layers for the long distance of every two pyridyl rings. However, the 3D supramolecular structure finally generates via C(4)– H(4)…O(1) (3.361(6)Å and 137(1)°) intramolecular hydrogen bonding interactions between the adjacent sheets (Fig. 4d). These four compounds are all tetrazole-based compounds derived from dinitrile, however, their structure is quite different from each other. 1 and 2 are btm2−-based compounds and their central ions are both six-coordinated Cd(II) ions, but their structures are quite different. The differences in coordination modes of btm2− ligands in these two compounds are probably the major reason for their structural differences. As for bte2-based compound 3 and tzp2 −-based compound 4, which are both derived from the same precursor butanedinitrile, there are great differences between their structures. The main reason for their difference in dimension can be ascribed to the nature of the secondary ligands. The presence of PMA contributes to the formation of – Cd1–O–Cd2–O–Cd1– helical chains and further helps to conduct the construction of three-dimensional framework in compound 3. While

in the two-dimensional compound 4 the introduction of chelating secondary ligand 2,2′-bipy is the predominant reason for the formation of its 2D network. The existence of chelating 2,2′-bipy often results in 2D network or even lower dimensional network, which has been demonstrated by some related reports [15]. Besides the formation of its carboxylate group in compound 4 is mainly ascribe to a 1:1 NaN3:CN(CH2)nNC molar ratio, while in the other three compounds which are all ditetrazole based compounds, the molar ratio of NaN3:CN(CH2)nNC is 1:2 and thus two cyano groups formed tetrazole groups. In addition, the increase of the number of –CH2– groups will add to the flexibility of the ligands which may result in great changes in their compounds' structures. For instance, H2btm and H2bte only differ by one –CH2– group in their chains, but they can form quite structurally different compounds 1 and 3. The thermal stability of all the products has been examined. The results (see Supporting Information, the TGA analysis and Fig. S4) show that all the compounds have relatively high thermal stability. Metal–organic compounds based on d10 transition metals often exhibit good luminescence properties [16]. Thus, in this context, the luminescence properties of 1–4 are investigated in the solid state at room temperature. As shown in Fig. 5, the emission spectra have maxima at 452, 461, 470 and 441 nm for compounds 1–4, respectively, with excitations at 382, 390, 340 and 398 nm. As reported in previous articles [11b,17], there are emission peaks at about 323 nm and 340 nm for free ligand H2bte and for free ligand PMA at room temperature,

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C. Zhao et al. / Inorganic Chemistry Communications 48 (2014) 30–35

Fig. 4. (a). View of the asymmetric unit of compound 4; (b). View of the infinite –Cd–tetrazole–Cd– zigzag chain extending along a direction; (c). View of the 2D wavelike layers through ab-plane; (d). View of the 3D supramolecular structure generates via C(4)–H(4)…O(1) and intramolecular hydrogen bonding interactions. (All H atoms were omitted for clarity; N atoms were represented as green, C as black, O as red and Cd as pink spheres.) Symmetry codes: (a): 0.5 + x, 0.5 − y, 1 − z; and (b): 0.5 − x, −0.5 + y, z. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

respectively, which indicate that the luminescence mechanism of compound 3 may be assigned to the ligand-to-metal charge transfer (LMCT) and/or metal-to-ligand charge transfer (MLCT) [17,18]. Compounds 1 and 2 are also ditetrazole based metal compounds like compound 3. Thus, the luminescence mechanism of 1 and 2 may be the same as that of compound 3. For compound 4, it exhibits obvious red shift compared with other tetrazolate-5-carboxylate compounds [19], which may be attributed to the cooperative effects of intraligand emission and ligand-to-metal charge transfer (LMCT). The results indicate that

compounds 1–4 may have potential application as blue light emitting materials. In summary, four Cd-tetrazole coordination frameworks were synthesized by in situ ligand synthesis using hydrothermal methods and structurally characterized. The result shows that the choice of secondary ligands and the molar ratio of the reactant have great effect on the construction of novel architectures. Complexes 1–4 exhibit intriguing two/ three-dimensional (2/3D) frameworks constructed from cadmium octahedrons and tetrazole linkers. These compounds not only have high thermal stabilities, but also exhibit relatively strong solid luminescent intensities and are thus potential candidates for applications as photoactive materials. Acknowledgments This work was supported by the National Natural Science Foundation of China (21171060), the Program for New Century Excellent Talents in University (NCET-12-0643) and the Natural Science Foundation of Guangdong Province (S2013010012678). Appendix A. Supplementary material

Fig. 5. Luminescence curves of compounds 1–4 and PMA.

CCDC 996007–996010 contain the supplementary crystallographic data for 1–4. These data can be obtained free of charge via http:// www.ccdc.cam.ac.Uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.inoche.2014.08.005. This data

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[19]

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1 mmol), PMA (0.076 g, 0.3 mmol) for 3]; [CdSO4 (0.253 g, 1 mmol), NaN3 (0.065 g, 1 mmol), butanedinitrile (0.080 g, 1 mmol), 2,2′-bipy (0.156 g, 1 mmol) for 4]}, H2O(10 ml) was sealed in a 23 mL Teflon reactor and kept under autogenous pressure at 160 °C for 3 days. Then cooled to room temperature at a rate of 4 °C/h, and colorless crystals were obtained in a yield of 39% based on Cd for 1, 37% based on Cd for 3, 35% based on Cd for 4; light yellow crystals were obtained in a yield of 59% based on Cd for 2. 1: C12H18Cd4N32O5 (1140.23): calcd(%). C 12.64, H 1.59, N 39.31; found: C 12.78, H 1.69, N 39.24; IR (KBr, cm−1): 3400(w), 3101(m), 1613(s), 1499(s), 1406(s), 1299(m), 1243(m), 1149(m), 1042(w), 779(m), 657 (w); 2: C6H6Cd2N16O (543.09): calcd(%). C 13.27, H 1.11, N 41.27, found: C 13.19, H 1.16, N 41.21; IR (KBr, cm−1): 3340(w), 3102(m), 1620(s), 1506(s), 1478(s), 1414(m), 1377(m), 1149(m), 1120(w), 785(m). 650(w); 3: C9H7Cd2N8O5 (532.05): calcd(%). C 20.32, H 1.33, N 21.06, found: C 20.37, H 1.39, N 20.97; IR (KBr, cm−1): 3438(w), 2956(m), 1570(s), 1548(s), 1506(s), 1449(m), 1377(m), 1213(m), 1142(w), 764(m). 678(w); 4: C14H12CdN6O2 (408.71): calcd(%). C 41.14, H 2.96, N 20.56; found: C 41.05, H 2.89, N 20.49; IR (KBr, cm−1): 3424(w), 3110(m), 1587(s), 1573(s), 1480(s), 1441(m), 1334(m), 1163(m), 1007(w), 764(m). 678(w). Crystallographic data for complex 1: Monoclinic P21/c with a = 16.538(7) Å, b = 9.904(4) Å, c = 21.902(9) Å, α = 90°, β = 109.804(5)°, γ = 90°, V = 3375(2) Å3, Z = 4, Dc = 2.244, F(000) = 2184, GOF = 1.102, R1(I N 2σ(I)) = 0.1089, and wR2 (all data) = 0.2495; 2: Monoclinic P21/n with a = 11.1784(7) Å, b = 10.0778(7) Å, c = 12.7696(12) Å, α = 90°, β = 90.8540(10)°, γ = 90°, V = 1438.38(19) Å3, Z = 4, Dc = 2.508, F(000) = 1032, GOF = 1.102, R1 [(I N 2σ(I)] = 0.0206, and wR2 (all data) = 0.0504; 3: Monoclinic P21/c with a = 8.7566(10) Å, b = 8.2814(9) Å, c = 19.113(2) Å, V = 1386.0(3) Å3, α = 90°, β = 90.3790(10)°, γ = 90°, Z = 4, Dc = 2.550, F(000) = 1012, GOF = 1.047, R1 [(I N 2σ(I)] = 0.0235, and wR2 (all data) = 0.0554; 4: Orthorhombic Pbca with a = 9.7636(15) Å, b = 16.297(2) Å, c = 19.006(3) Å, V = 3024.2(8) Å3, α = 90°, β = 90.3790(10)°, γ = 90°, Z = 4, Dc = 1.795, F(000) = 1616, GOF = 1.132, R1 [(I N 2σ(I)] = 0.0348, and wR2 (all data) = 0.0841. L.F. Ma, C.P. Li, L.Y. Wang, M. Du, Zn(II) and Cd(II) coordination polymers assembled from a versatile tecton 5-nitro-1,2,3-benzenetricarboxylic acid and N, N′-donor ancillary coligands, Cryst. Growth Des. 10 (2010) 2641–2649. (a) A.H. Yang, Y.P. Quan, H.L. Gao, S.R. Fang, Y.P. Zhang, L.H. Zhao, J.Z. Cui, J.H. Wang, W. Shi, P. Cheng, ds-Block metal ions catalyzed decarboxylation of pyrazine2,3,5,6-tetracarboxylic acid and the complexes obtained from hydrothermal reactions and novel water clusters, CrystEngComm 11 (2009) 2719–2727; (b) Z. Hulvey, E. Ayala, J.D. Furman, P.M. Forster, A.K. Cheetham, Structural diversity in coordination polymers composed of divalent transition metals, 2,2′bipyridine, and perfluorinated dicarboxylates, Cryst. Growth Des. 9 (2009) 4759–4765. L.L. Wen, Z.D. Lu, J.G. Lin, Z.F. Tian, H.Z. Zhu, Q.J. Meng, Syntheses, structures, and physical properties of three novel metal–organic frameworks constructed from aromatic polycarboxylate acids and flexible imidazole-based synthons, Cryst. Growth Des. 7 (2007) 93–99. Q. Hua, Y. Zhao, G.C. Xu, M.S. Chen, Z. Su, K. Cai, W.Y. Sun, Synthesis, structures, and properties of zinc(II) and cadmium(II) complexes with 1,2,4,5-tetrakis(imidazol-1ylmethyl)benzene and multicarboxylate ligands, Cryst. Growth Des. 10 (2010) 2553–2562. (a) J.Y. Zhang, A.L. Cheng, Q. Sun, Q. Yue, E.Q. Gao, Syntheses, structures, and properties of honeycomb and square grid coordination polymers with in situ formed 5-(2′-pyrimidyl)tetrazolate, Cryst. Growth Des. 10 (2010) 2908–2915; (b) X.Q. Liang, J.T. Jia, T. Wu, D.P. Li, L. Liu, Tsolmon, G.S. Zhu, A spontaneously resoluted zinc–organic framework with nonlinear optical and ferroelectric properties generated from tetrazolate-ethyl ester ligand, CrystEngComm 11 (2010) 3499–3501. M.F. Wu, M.S. Wang, S.P. Guo, F.K. Zheng, H.F. Chen, X.M. Jiang, G.N. Liu, G.C. Guo, J.S. Huang, Photoluminescent and magnetic properties of a series of lanthanide coordination polymers with 1H-tetrazolate-5-formic acid, Cryst. Growth Des. 11 (2011) 372–381.