Hydrothermal Synthesis, Structures, and Luminescent Properties of

Feb 3, 2009 - Hua-Dong Guo, Xian-Min Guo, Stuart R. Batten, Jiang-Feng Song, Shu-Yan ... Luminescent studies show that 1−7 exhibit strong blue fluor...
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Hydrothermal Synthesis, Structures, and Luminescent Properties of Seven d10 Metal-Organic Frameworks Based on 9,9-Dipropylfluorene-2,7-Dicarboxylic Acid (H2DFDA) Hua-Dong Guo,†,‡ Xian-Min Guo,†,‡ Stuart R. Batten,§ Jiang-Feng Song,† Shu-Yan Song,†,‡ Song Dang,†,‡ Guo-Li Zheng,†,‡ Jin-Kui Tang,† and Hong-Jie Zhang*,†

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 3 1394–1401

State Key Laboratory of Rare Earth Resource Utilizations, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, Graduate School of the Chinese Academy of Sciences, Beijing, P. R. China, and School of Chemistry, Monash UniVersity, Victoria, 3800, Australia ReceiVed June 18, 2008; ReVised Manuscript ReceiVed December 12, 2008

ABSTRACT: A series of Zn(II) and Cd(II) metal-organic frameworks, namely, [Zn(DFDA)] (1), [Cd(DFDA)(C2H5OH)] (2), [Zn2(DFDA)2(L1)2]2 · 3H2O (3), [Cd2(DFDA)2(L1)2] (4), [Zn(DFDA)(L2)] (5), [Cd(DFDA)(L2)(DMF)] (6), and [Zn(DFDA)(L3)] (7) (where DFDA ) 9,9-dipropylfluorene-2,7-dicarboxylate anion, L1 ) 1,4-bis(imidazol-1-ylmethyl)benzene, L2 ) 1,1′-(1,4-butanediyl) bis(imidazole), L3 ) 2,2′-bipyridine) have been synthesized under hydrothermal conditions and structurally characterized. Compound 1 exhibits a three-dimensional (3D) framework containing one-dimensional (1D) Zn(II)-O clusters, with (48 · 67) topology. Compound 2 contains hydrophobic channels built from infinite 1D Cd(II)-O clusters, with (48 · 54 · 63) topology. Compounds 3 and 4 possess isomorphic two-dimensional (2D) layer structures which contain two kinds of rings, and the layers interdigitate extensively. Compound 5 features a 3D diamond-like structure that contains four interpenetrating networks. Compound 6 displays a 2D (4,4) topology. Compound 7 is composed of 1D zigzag chains that are entangled through π-π stacking interactions to generate three interpenetrating diamond-like networks. The potential of ligand H2DFDA to produce interesting metal-organic frameworks is investigated. In particular, the effects of the coordination modes of carboxylates on the formation of different frameworks are evaluated. Thermogravimetric analyses (TGA) for these compounds are discussed. Luminescent studies show that 1-7 exhibit strong blue fluorescent emissions. Introduction In the past decade, metal-organic frameworks (MOFs) have attracted considerable attention, owing to their various intriguing molecular topologies and potential applications as functional materials.1,2 On the basis of the concept of crystal engineering, structure-function relationships with properties such as magnetism, luminescence, and permanent porosity have thereby been developed in a systematic manner.3 But crystallization is a very complicated process, and minor changes in the chemical environment, such as the temperature, pH value, or solvent, can have an unpredictable impact on the composition of the resulting compounds.4 So, the “true” engineering of MOFs remains a difficult challenge, even though the careful selection of metal coordination geometries and organic ligand structures can still play a decisive role in the construction of MOFs. It is well-known that carboxylic acids are excellent building blocks for the construction of MOFs due to a number of advantages. First, carboxylates can adopt versatile coordination conformations, from terminal monodentate to various bridging modes (e.g., syn-syn, syn-anti, anti-anti), which can generate metal-oxygen nodes, chains, or polynuclear clusters.5 Second, modification of the size, shape, and the potential donor sites of the ligands will lead to changes in the structures and properties of the resulting compounds.6 Third, the strong coordinating ability of carboxylates can result in good thermal stabilities of the materials, which increases the possibility of their functionalization.7 With that in mind, we have selected H2DFDA as an organic linker (where DFDA ) 9,9-dipropylfluorene-2,7-dicarboxylate * To whom correspondence should be addressed. Fax: +86-431-85698041. E-mail: [email protected]. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences. § Monash University.

anion), which features two special characteristics: (i) its nonlinear shaped conformation can improve the flexibility of the polymeric frameworks, and (ii) because of its good fluorescent characteristics, the products may exhibit some interesting luminescent properties.8 Furthermore, in order to investigate the influence of neutral ligands on the formation of supramolecular architectures, three N-donor ligands, 1,4-bis(imidazol-1-ylmethyl)benzene (L1), 1,1′-(1,4-butanediyl)bis(imidazole) (L2), and 2,2′-bipyridine (L3), were introduced.9 We have successfully synthesized seven MOFs, namely, [Zn(DFDA)] (1), [Cd(DFDA)(C2H5OH)] (2), [Zn2(DFDA)2(L1)2]2 · 3H2O (3), [Cd2(DFDA)2(L1)2] (4), [Zn(DFDA)(L2)] (5), [Cd(DFDA)(L2)(DMF)] (6), and [Zn(DFDA)(L3)] (7). The structural and luminescent properties of 1-7 were investigated in detail. Experimental Section H2DFDA, L1, and L2 were synthesized according to the literature.10 All other starting materials were of analytical grade and used as received without further purification. IR spectra were obtained from KBr pellets on a Perkin-Elmer 580B IR spectrometer in the 400-4000 cm-1 region (SI). Elemental analyses (C, H, N) were performed with a Perkin-Elmer 240c elemental analyzer. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TG-7 analyzer heated from 25 to 650 °C under nitrogen. The luminescent properties of compounds were measured on a HITACHI F-4500 spectrometer. [Zn(DFDA)] (1). A mixture of Zn(NO3)2 · 6H2O (0.0297 g, 0.1 mmol), H2DFDA (0.0338 g, 0.1 mmol), methanol (5 mL), and H2O (5 mL) was placed in a Teflon reactor (20 mL) and heated at 160 °C for 4 days. After the sample was gradually cooled to room temperature at a rate of 10 °C · h-1, colorless crystals of 1 were obtained with 68% yield based on H2DFDA. Anal. Calcd for C21H20ZnO4: C, 62.78; H, 4.98. Found: C, 62.88; H, 5.06. IR (4000-400 cm-1): 3058vw, 2959m, 2866w, 1584s, 1546s, 1476s, 1392vs, 1115w, 777s, 678m, 462m. [Cd(DFDA)(C2H5OH)] (2). A mixture of Cd(NO3)2 · 4H2O (0.0308 g, 0.1 mmol), H2DFDA (0.0338 g, 0.1 mmol), DMF (5 mL), ethanol

10.1021/cg8006469 CCC: $40.75  2009 American Chemical Society Published on Web 02/03/2009

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Table 1. Crystal and Structure Refinement Data for Compounds 1-7 param

1

2

3

4

5

6

7

formula fw space group a b c R (deg) β (deg) γ (deg) V Z Dcalcd (g cm-3) F(000) reflcns collcd/unique GOF on F2 R1a [I > 2σ(I)] wR2b Flack

C21H20ZnO4 401.74 Pnna 16.429(5) 11.675(5) 20.934(5) 90 90 90 4015(2) 8 1.329 1664 21154/4017 1.024 0.106 0.2577

C23H26CdO5 494.84 C2/c 20.5511(12) 21.5004(13) 14.0681(8) 90 130.5470(10) 90 4723.4(5) 8 1.392 2016 13095/4650 1.081 0.0593 0.1775

C140H142Zn4N16O19 2614.18 P1j 13.7354(7) 13.8485(8) 22.4233(12) 88.4380(10) 85.2750(10) 70.7740(10) 4013.7(4) 1 1.082 1366 22465/15341 1.051 0.0758 0.2153

C70H68Cd2N8O8 1374.12 P1j 13.8420(19) 13.928(2) 22.838(3) 87.372(3) 87.465(3) 72.708(3) 4197.5(10) 2 1.087 1408 23027/15907 0.955 0.1071 0.2196

C31H34ZnN4O4 592.01 Pbcm 8.783(2) 24.640(6) 14.613(4) 90 90 90 3162.4(14) 4 1.231 1216 16860/3248 1.126 0.0877 0.1928

C34H41CdN5O5 712.12 P21/c 13.1771(9) 11.5147(8) 22.2995(15) 90 95.6420(10) 90 3367.1(4) 4 1.428 2016 18460/6631 1.051 0.0537 0.1324

C31H28ZnN2O4 557.94 Cc 27.347(5) 8.637(5) 10.972(5) 90 103.309(5) 90 2521.9(19) 4 1.385 1096 6686/3331 0.984 0.0661 0.1579 0.00(4)

a

R1 ) Σ|Fo| - |Fc|/Σ|Fo|. b wR2 ) |Σw(|Fo|2 - |Fc|2)|/Σ|w(Fo2)2|1/2.

(2 mL), and H2O (1 mL) was placed in a Teflon reactor (20 mL) and heated at 80 °C for 2 days. After being cooled to room temperature, colorless crystals of 2 were obtained with 76% yield based on H2DFDA. Anal. Calcd for C23H26CdO5: C, 55.83; H, 5.26. Found: C, 55.79; H, 5.22. IR (4000-400 cm-1): 3427m, 3127m, 2959s, 2859m, 1661s, 1592vs, 1546vs, 1430s, 1384vs, 1238m, 1092s, 939w, 785s, 747m, 662m, 424w. [Zn2(DFDA)2(L1)2]2 · 3H2O (3). A mixture of Zn(NO3)2 · 6H2O (0.0297 g, 0.1 mmol), H2DFDA (0.0338 g, 0.1 mmol), L1 (0.0238 g, 0.1 mmol), DMF (5 mL), ethanol (2 mL), and H2O (1 mL) was placed in a Teflon reactor (20 mL) and heated at 80 °C for 2 days. After being cooled to room temperature, colorless crystals of 3 were obtained with 65% yield based on H2DFDA. Anal. Calcd for C140H142Zn4N16O19: C, 64.72; H, 5.47; N, 8.63. Found: C, 64.77; H, 5.42; N, 8.61. IR (4000-400 cm-1): 3427w, 3112m, 2951m, 2874m, 1622vs, 1430s, 1361vs, 1238s, 1115s, 954s, 869w, 785s, 662m, 562vw, 432m. [Cd2(DFDA)2(L1)2] (4). The synthesis procedure of complex 4 was similar to that of complex 3 except that Zn(NO3)2 · 6H2O was replaced by Cd(NO3)2 · 4H2O. Colorless crystals of 4 were obtained with 68% yield based on H2DFDA. Anal. Calcd for C70H68Cd2N8O8: C, 61.13; H, 5.05; N, 8.15. Found: C, 61.20; H, 5.12; N, 8.21. IR (4000-400 cm-1): 3404w, 2959m, 1645s, 1553s, 1430s, 1384vs, 1115w, 877vw, 777s, 662w, 424m. [Zn(DFDA)(L2)] (5). Complex 5 was synthesized following the same synthetic procedure as that for complex 4 except that L2 was used instead of L1. Colorless crystals of 5 were obtained with 57% yield based on H2DFDA. Anal. Calcd for C31H34ZnN4O4: C, 62.84; H, 5.74; N, 9.50. Found: C, 63.33; H, 4.65; N, 9.64. IR (4000-400 cm-1): 3427w, 3135m, 2959s, 2866m, 1607vs, 1430s, 1353vs, 1238s, 1085s, 954m, 785s, 655s, 432m. [Cd(DFDA)(L2)(DMF)] (6). Complex 6 was synthesized following the same synthetic procedure as that for complex 4 except that L2 was used instead of L1. Colorless crystals of 6 were obtained with 62% yield based on H2DFDA. Anal. Calcd for C34H41CdN5O5: C, 57.29; H, 5.76; N, 9.83. Found: C, 58.14; H, 5.72; N, 9.71. IR (4000-400 cm-1): 3419w, 3058w, 2959s, 2866m, 1599vs, 1546s, 1445vs, 1353vs, 1246m, 1023m, 862m, 777vs, 731s, 655m, 516w, 439m. [Zn(DFDA)(L3)] (7). Complex 7 was synthesized following the same synthetic procedure as that for complex 3 except that L3 was used instead of L1. Colorless crystals of 7 were obtained with 55% yield based on H2DFDA. Anal. Calcd for C31H28ZnN2O4: C, 66.67; H, 5.02; N, 5.02. Found: C, 70.66; H, 5.42; N, 5.40. IR (4000-400 cm-1): 3427w, 3120w, 2951m, 2866m, 1653m, 1592s, 1538s, 1438s, 1376vs, 1238m, 1085m, 939s, 777s, 647m, 424m. X-ray Crystallography. Single-crystal XRD data for compounds 1-7 were recorded on a Bruker Apex CCD diffractometer with graphite monochromatized Mo KR radiation (λ ) 0.71073 Å) at 185(2) K. Absorption corrections were applied using a multiscan technique. All the structures were solved by direct methods using SHELXS-9711 and refined by full-matrix least-squares techniques using the SHELXL-9712 program within WINGX. Non-hydrogen atoms were refined with anisotropic temperature parameters. The disordered methyl group of ethanol in compound 2 was refined using an isotropic C atom split

over two sites, so the hydrogen atoms of C22 and C23 could not be added. The hydrogen atoms of the organic ligands were refined as rigid groups. All of the water H atoms in compound 3 could not be positioned reliably. The detailed crystallographic data and structure refinement parameters for 1-7 are summarized in Table 1.

Result and Discussion Synthesis of the Compounds. Solvothermal synthesis is a relatively complex process, and the final products are often unpredictable under a given set of conditions. The reaction variables (metal ions, temperature, solvents, reaction time, etc.) may affect the final products remarkably. In our system, the central metals and the solvents both play an important role in the formation of new compounds. In the mixed reagents of H2O/ CH3OH, we successfully isolated [Zn(DFDA)] by the reaction of metal salts with H2DFDA, but for other metal ions, no crystals were afforded. Similarly, using a mixture of DMF/H2O/C2H5OH, we just isolated the crystals of compound 2 and again obtained no crystals for other metals. Water was found to be crucial in producing good quality crystals in the synthesis of compounds 2-7. All of the compounds 1-7 are stable in air and insoluble in common solvents such as ethanol, acetone, and acetonitrile. The purities of the bulk samples were identified by X-ray powder diffraction (Figures S1-S7,Supporting Information). Structural Description of 1. Compound 1 crystallizes in the orthorhombic space group Pnna. The asymmetric unit consists of one Zn atom and one DFDA2- ligand. There are two crystallographically unique Zn atoms. The Zn1 atom adopts a distorted octahedral geometry coordinated by six O atoms from four different DFDA2- ligands (Zn-O 1.967(4)-2.367(7) Å). The Zn2 atom is six-coordinated by O atoms from six different DFDA2- ligands in a distorted octahedral geometry (Zn-O 1.940(7)-2.386(9) Å). The DFDA2- ligand displays one kind of coordination mode to connect five Zn(II) atoms with two carboxylate groups, adopting µ2-η1:η1-bridging and µ3-η2:η2bridging coordination modes. Thus Zn1 and Zn2 are connected by two O atoms from two µ3-η2:η2-bridging carboxylate groups to generate a 1D infinite Zn(II)-O chain along the b axis (Figure 1a). The Zn1 · · · Zn2 distance is about 3.260 Å. The Zn1 · · · O3 · · · Zn2 and Zn1 · · · O4 · · · Zn2 angles are 98.3(4) and 90.2(2)°, respectively. The neighboring chains were connected by the DFDA2- ligands in the µ5 mode to build a 3D network with 1D rhombic channels along the b axis, where the shortest Zn-Zn interchain distance is 11.086 Å (Figure 1b). The propyls are orientated into the channels to minimize the porosity.

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Figure 1. (a) Infinite 1D Zn(II)-O clusters formed by Zn atoms and carboxylate groups. (b) View of the 3D network in 1, highlighting the rhombic channels along the b-axis. All of the alkyls are omitted to highlight the channels. (c) View of the (48 · 67) topology of 1.

In 1, if the Zn2 atoms and DFDA2- ligands are considered as linkers to connect two Zn1 atoms, then based on the secondary building unit (SBU) concept, the two Zn2 atoms and four DFDA2ligands around a Zn1 center generate a six-connected octahedral SBU. On the basis of this simplification, the whole structure can be described as a (48 · 67) net (Figure 1c). Structural Description of 2. Compound 2 crystallizes in the monoclinic space group C2/c. The asymmetric unit consists of one Cd atom, one DFDA2- ligand, and one ethanol molecule. There are two crystallographically unique Cd atoms. The Cd1 atom adopts a distorted dodecahedral geometry coordinated by eight O atoms from three different DFDA2- ligands (Cd-O 2.294(4)-2.548(4) Å). The Cd2 atom is in an octahedral geometry coordinated by two O atoms from two ethanol molecules (Cd-O 2.276(5) Å) and four O atoms from four different DFDA2- ligands (Cd-O 2.258(4)-2.286(4) Å). The DFDA2- ligand acts as a µ4-bridge, linking four Cd(II) centers through two equivalent µ2-η1:η2-bridged carboxylate groups. Thus, one Cd2 atom and a symmetrically related pair of Cd1 atoms are connected by two µ2-O2 and two µ2-O4 from four carboxylate groups to form a linear trinuclear subunit with a Cd1 · · · Cd2 distance of 3.627 Å and a Cd1 · · · Cd2 · · · Cd1 angle of 180°. Each unit is connected through the sharing of two

Figure 2. (a) Infinite 1D Cd(II)-O clusters formed by Cd atoms and carboxylate groups in 2. (b) View of the 3D network in 2, highlighting the rhombic channels along the c-axis. All of the alkyls are omitted to highlight the channels. (c) View of the (48 · 54 · 63) topology of 2.

vertexes of the Cd2 atoms to generate a 1D chain with a Cd2 · · · Cd1 · · · Cd2 angle of 151.7° (Figure 2a). According to the literature, the intrinsic packing arrangement of 1D SBUs can prevent interpenetration to guarantee a porous MOF.14 In 2, the adjoining chains are linked by DFDA2- ligands in the µ4 mode to furnish a 3D MOF with 1D rhombic channels running along the c axis with dimensions of 18.42 × 12.46 Å (excluding van der Waals radii) (Figure 2b). However, the propyls from the DFDA2- ligands and ethyls from the ethanol molecules are all orientated to the channel, which not only reduces its size but makes the channel hydrophobic.15 Furthermore, because the Cd2 atom is located in a symmetrical position to link two Cd1 atoms, it can be regarded as one kind of linker, with the DFDA2-

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Crystal Growth & Design, Vol. 9, No. 3, 2009 1397

Figure 3. (a) View of the infinite 2D layer that contains two kinds of rings. (b) View of the interdigitation of two rings I (red and blue) and one ring II (purple) of adjoining sheets. (c) View of the packing of 2D layers. (d) Schematic representation of the (6, 3) topological net (metal atoms are the nodes).

ligand considered as another kind of linker. Thus four DFDA2ligands and two Cd2 atoms surrounding a Cd1 center constitute a six-connected octahedral SBU. According to this simplification, the topology of this 3D network can be described as a (48 · 54 · 63) net (Figure 2c). Structural Description of 3 and 4. Single-crystal XRD analysis studies revealed that compound 3 and 4 are almost identical in structure, and hence only the results of 3 are given in the ensuing discussion. Compound 3 crystallizes in the triclinic space group P1j. The asymmetric unit consists of two Zn atoms, two DFDA2- ligands, and two L1 ligands. The two crystallographically unique Zn atoms have almost the same coordination environments. They both adopt distorted tetrahedral geometries coordinated by two O atoms from two DFDA2ligands and two N atoms from two L1 ligands. Minimum differences exist between the bond distances and angles. It should be noted that although O1 and O7 are not coordinated to Zn1, the bond distances of Zn1 · · · O1 (2.776 Å) and Zn1 · · · O7 (2.707 Å) are shorter than the sum of the van der Waals radii (2.9-3.0 Å), which indicates a weak secondary bonding. Similar interactions exist around the Zn2 atom (Zn2 · · · O3 2.586 Å, Zn2 · · · O5 2.814 Å). This phenomenon may contribute to the distortion of the Zn tetrahedral geometry and result in the variation of coordination enviroments.16 DFDA2acts as a µ2-bridge, linking two Zn atoms through two monodentate carboxylate groups. The L1 ligand adopts a cisconformation and bridges two Zn atoms. In the overall structure of 3, Zn ions are bridged by DFDA2anions and L1 ligands to form a 2D polymeric network. In this network two kinds of rings exist: ring I consists of two Zn centers and two L1 ligands, while ring II contains six Zn centers, four DFDA2- anions and two L1 ligands (Figure 3a). Because of the close packing between the layers, one ring II in one layer incorporates parts of the two rings I from two neighboring layers (Figure 3b). This kind of interdigitation no doubt improves the stability of the overall structure.17 At the same time, although

the network is a 2D net containing large irregular hexagonal windows, there is no interpenetration between the networks, as is often seen in coordination polymers with such structures. This is no doubt due to the aforementioned interdigitation. As often observed in closely packed network structures, two kinds of close π-π contacts exist: one occurs between the imidazole rings of L1 ligands from layer1 and layer3 (with a centroid-tocentriod distance of 3.53 Å); the other occurs between the benzene rings of L1 ligands from layer1 and layer4 (with faceto-face distances of 3.66 Å) (Figure 3c). If the DFDA2- anions and L1 ligands are considered as simple connecters, the Zn(II) centers can be simplified to threeconnecting nodes, making the overall topology a (6, 3) net (Figure 3d).18 Structural Description of 5. Compound 5 crystallizes in orthorhombic space group Pbcm. The asymmetric unit consists of half a Zn atom, half a DFDA2- anion, and half a L2 ligand. The one crystallographically unique Zn atom adopts a distorted tetrahedral geometry, and is coordinated by two O atoms from two DFDA2- ligands (Zn-O 1.953(6)-1.966(6) Å) and two N atoms from two L2 ligands (Zn-N 2.000(5) Å). The bond distance of Zn-O4 (2.676 Å) indicates the existence of weak secondary bonding. In the structure of 5, the Zn center connects to two O atoms and two N atoms to generate a tetrahedral SBU. These SBUs are linked to each other by the organic linkers to build a 3D network. In the framework, Zn(II) atoms are linked by the DFDA2- ligands to build zigzag chains with a Zn · · · Zn distance of 14.78 Å. The L2 ligands display a S-shaped conformation, leading to a Zn · · · Zn distance of 12.06 Å. Topological analysis of 5 reveals that it has a typical diamondoid framework containing large adamantanoid cages (Figure 4a,b). Furthermore, two different channels with different sizes are produced. The largest has dimensions of 25.69 × 26.81 × 29.07 Å, corresponding to the longest intracage Zn · · · Zn distances. Because

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Figure 4. (a) View of a single adamantanoid cage (left) and a schematic representation in which every node represents a metal (right). (b) View of a single diamondoid framework (left) and a schematic representation of the framework (right). (c) Schematic depiction of the four interpenetrating nets.

of the spacious nature of the single network, four identical diamondoid networks interpenetrate (Figure 4c).19 Structural Description of 6. Compound 6 crystallizes in the monoclinic space group P21/c. The asymmetric unit consists of one Zn atom, one DFDA2- anion, one L2 ligand, and one DMF molecule. There is one crystallographically unique Cd atom, which is five-coordinated by three O atoms from two DFDA2ligands (Cd-O 2.295(3)-2.395(4) Å) and two N atoms from two L2 ligands (Cd-N 2.250(4)-2.282(4) Å). There are also two weak secondary bonds (Cd · · · O3 2.608(3) Å, Cd · · · O5 2.818 Å). The DFDA2- ligand bridges two Cd(II) centers through one monodentate carboxylate group and one chelated carboxylate group. The L2 ligand acts as a linker to connect two Cd(II) centers. In the overall structure of 6, the Cd(II) centers are linked by the DFDA2- anions to form a 1D wave-like chain with a period of 22.30 Å (Figure 5a). These chains are then linked by the L2 ligands to generate a 2D layer (Figure 5b). If all of the ligands are regarded as the linkers, the metal centers are considered as nodes, the topology of 6 is that of the common 2D (4,4) net (Figure 5c). Between adjoining layers, two kinds of C-H · · · O bonds exist (C25-H · · · O4 3.200 Å, C23-H · · · O1 3.362 Å),20 but there are not obvious π-π stacking interactions, as often observed in compounds with 2D-layered structures. Structural Description of 7. Compound 7 crystallizes in the monoclinic space group Cc. The asymmetric unit consists of one Zn atom, one DFDA2-, and one L3 ligand. The one crystallographically unique Zn atom adopts a distorted trigonal

Figure 5. (a) View of the infinite polymeric wave-like chains of {Cd(II)DFDA2-}. (b) View of the 2D layer structure. (c) Schematic representation of the (4, 4) topological net (metal atoms are the nodes).

bipyramid geometry coordinated by three O atoms from two DFDA2- ligands (Zn-O 1.950(5)-2.322(5) Å) and two N atoms from one chelating L3 ligand (Zn-N 2.059(7)-2.108(6) Å). DFDA2- acts as a µ2-bridge, linking two Zn(II) centers through one monodentate carboxylate group and one chelating one. In the overall structure of 7, [ZnL3]2+ nodes are connected by DFDA2- linkers to generate a 1D zigzag chain with a period of 27.02 Å (Figure 6a). The chains, with the terminal L3 ligands directed outward, are arranged in such a way that π-π stacking interactions are formed between pyridine rings of adjacent L3 ligands from different chains. The adjacent pyridine rings are related by inversion centers, and the centroid-centroid distance (3.53 Å), plane-to-plane distance (3.34 Å), and displacement angle (0.87°) indicate strong π-π stacking interaction.21 These

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Figure 6. (a) View of the structure of the 1D zigzag chain in 7. (b) View of the 3D network that features 1D nanosized channels. (c) Schematic view of a single diamond-like framework (nodes for Zn atoms, solid lines for DFDA2- ligands, and dashed lines for the interacting pyridines). (d) View of the 3-fold interpenetrating framework.

Chart 1. Coordination Modes of the DFDA2- Ligand in 1 (a), 2 (b), 3, 4 (c), 5, 6, 7 (d)

interactions arrange the chains into a 3D supramolecular network (Figure 6b). If the DFDA2- ligands and the π-π stacked pyridine pairs are considered as linkers, and metal centers are considered as nodes, the architecture can be regarded as a 3D diamond net (Figure 6c). The network features 1D nanosized pillar-like channels along the c axis with dimensions of 20.13 × 27.42 × 29.23 Å (corresponding to the longest intracage Zn · · · Zn distances). The Zn · · · Zn distance (8.54 Å) separated by the π-π stacked pyridine rings is much shorter than those (14.95 Å) by the DFDA2- ligands, and three identical nets interpenetrate each other, with the metal coordination spheres of two nets located at the channel centers of the other net (Figure 6d). In 1 and 2, DFDA anions adopt µ5- and µ4-bridging coordination modes (Chart 1a,b), respectively. These two kinds of coordination modes are essential in chelating metal ions and locking their positions into M-O-M clusters, which have been widely employed by Yaghi, Fe´rey, and co-workers in the construction of stable MOFs with large pores.14,22 In 3 and 4, the DFDA anion adopts a bridging syn-anti bis(monodentate) coordination mode (Chart 1c), which leads the metal atoms to form isolated mononuclear motifs. In 5, 6, and 7, the DFDA

anion adopts a µ2-bridging coordination mode with one monodentate and one chelating carboxylate group (Chart 1d), which tends to connect metal ions into 1D zigzag chains. Therefore, the coordination modes of the carboxylate ligands influence the metal nuclearity and hence the connectivity of the resulting net.21b,e,23 Thermal Analysis. To characterize the thermal stabilities of compounds 1-7, their thermal behaviors were investigated by TGA (Figures S16 andS17, Supporting Information). The experiments were performed on samples consisting of numerous single crystals of 1-7 under a nitrogen atmosphere with a heating rate of 10 °C/min. For compound 1, the decomposition of the compound occurs at ca. 370 °C, indicating high thermal stability of the frameworks. The remaining weight corresponds to the formation of ZnO (obsd 22.4%, calcd 19.8%). For 2, the weight loss in the range of 76-142 °C is attributed to the gradual release of coordinated ethanol molecules (obsd 11.88%, calcd 9.06%). The destruction of the frameworks occurs at ca. 300 °C. The remaining weight corresponds to the formation of CdO (obsd 26.23%, calcd 25.94%). Compounds 3 and 4 exhibit similar thermal behaviors. They are stable up to ca. 290 °C.

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The remaining weights correspond to ZnO (obsd 13.22%, calcd 12.40%) and CdO (obsd 17.02%, calcd 18.69%), respectively. Compound 5 remains stable up to ca. 330 °C, finally leading to the formation of a stoichiometric amount of ZnO as the residue (obsd 13.40%, calcd 13.68%). For 6, the weight loss in the range of 76-188 °C is attributed to the gradual release of coordinated DMF molecules (obsd 9.05%, calcd 10.26%). The decomposition of the frameworks occurs at ca. 300 °C, with a residue of CdO (obsd 17.74%, calcd 18.03%). Compound 7 remains stable up to 320 °C, and the remaining weight corresponds to ZnO (obsd 14.33%, calcd 14.52%). Photoluminescence Properties. The solid-state photoluminescent spectra of H2DFDA and 1-7 were investigated (Figures S8-S15,Supporting Information). Excitation at 362 nm leads to strong blue fluorescent emission bands at 405 and 422 nm for 1 and 418 nm for 2. Excitation at 375 nm leads to strong blue fluorescent emission bands at 405 and 426 nm for 3, 407 and 428 nm for 4, 405 and 427 nm for 5, 407 and 428 nm for 6, and 411 and 427 nm for 7. These emissions are neither metalto-ligand charge transfer (MLCT) nor ligand-to-metal transfer (LMCT) in nature since the Zn2+ or Cd2+ ions are difficult to oxidize or reduce due to their d10 configuration. Rather, they can probably be assigned to intraligand (π-π*) fluorescent emission because almost similar emissions are observed for the free H2DFDA at 411 nm and 427 nm.25 The N-donor ligands do not show obvious contributions to the fluorescent emission of the seven compounds. Complexes 1-7 may be suitable as candidates of blue-fluorescent materials, since they are highly thermally stable and insoluble in common solvents.25

Guo et al.

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Conclusions In this study, seven new MOFs with interesting architectures have been synthesized and characterized using the carboxylic acid H2DFDA and three kinds of N-donor ligands as auxiliary ligands by controlling the solvothermal synthesis conditions. This is the first time that H2DFDA has been used to construct MOFs. The results of this study demonstrate that the coordination modes of the carboxylate ligands and the nature of the neutral ligands play an important role in the construction of MOFs. These seven compounds not only show an aesthetic diversity of coordinative network chemistry, but can also, for example, be used for the design of luminescent materials. Our future work will be based on the modification of H2DFDA to synthesize new functional MOFs.

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Acknowledgment. The authors are grateful to the financial aid from the National Natural Science Foundation of China (Grant Nos. 20490210, 206301040, and 20602035) and the MOST of China (Grant Nos. 2006CB601103, 2006DFA42610). Supporting Information Available: Seven X-ray crystallographic files (CIF), selected bond distances and angles, simulated and experimental powder XRD patterns, TGA curves of compounds 1-7, IR spectra and the solid state of emission and excitation spectras of H2DFDA and 1-7. This material is available free of charge via the Internet at http://pubs.acs.org.

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