Construction of Metal-Imidazole-Based Dicarboxylate Networks with

30 Mar 2012 - Single-crystal X-ray diffraction shows the ligand H3IDC is partially ... a wave-like 2D MOF with 44 topology in 5 based on H3IDC is crea...
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Construction of Metal-Imidazole-Based Dicarboxylate Networks with Topological Diversity: Thermal Stability, Gas Adsorption, and Fluorescent Emission Properties Zhi-Gang Gu, Yi-Ting Liu, Xu-Jia Hong, Qing-Guang Zhan, Zhi-Peng Zheng, Sheng-Run Zheng, Wei-Shan Li, She-Jun Hu, and Yue-Peng Cai* School of Chemistry and Environment, Key Laboratory of Electrochemical Technology on Energy Storage and Power Generation of Guangdong Higher Education Institutes, Engineering Research Center of Materials and Technology for Electrochemical Energy Storage (Ministry of Education), South China Normal University, Guangzhou, 510006, China S Supporting Information *

ABSTRACT: Six new metal−organic frameworks (MOFs), namely, [Pb(H2IDC)2(H2O)]n (1), {[Pb(H2IEDC)2)]·3H2O}n (2), [Ca2(HIDC)2(H2O)]n (3), {[Ca 2(IEDC)2(H2O)2]·H2O}n(4), {[Cd(HIDC)(H2O)]n (5), and {[Cd2(HIEDC)2·3H2O}n (6) (H3IDC = 4,5-imidazoledicarboxylic acid, H3IEDC = 2-ethyl-1H-imidazole-4,5-dicarboxylic acid) were hydrothermally synthesized and characterized. Single-crystal X-ray diffraction shows the ligand H3IDC is partially deprotonated with H2IDC− and HIDC2− forms in three corresponding compounds 1, 3, and 5, meanwhile ligand H3IEDC also presents different deportonated motifs of H2IEDC−, IEDC3−, and HIEDC2− in 2, 4, and 6, respectively. Moreover, these partially or fully deportonated ligands coordinate in the μ1 to μ5 manner to generate one-dimensional (1D) → threedimensional (3D) coordination polymers, displaying 10 different coordination modes. Both 1 and 2 have 3D supramolecular networks assembled via hydrogen bonding interactions but different in structure topologies. Complex 1 is a 1D coordination chain and 2 has a two-dimensional (2D) wave layer-like structure with 44 topology. Complex 3 is a 3D MOF with 310·425·510 topology built up from linear 2-connected HIDC2− as a linker and 10-connected tetranuclear metal unit [Ca4(μ2-O)6] as a node. However, complex 4 presents a 2D MOF with 36·46·53 topology. Different from 3 and 4, with the 4-connected binuclear metal unit [Cd(O−C−O)]2 as a node, a wave-like 2D MOF with 44 topology in 5 based on H3IDC is created. Complex 6 shows a 3D coordination framework containing 1D alternately arranged hydrophilic channels including (H2O)4 clusters and hydrophobic channels partially occupied by ethyl groups from imidazole rings, in which the Cd(II) ion is viewed as a 4-connected node, and HIEDC2− and μ2-O are regarded as two different linkers, 6 showing 42·84 topology. Obviously, these results reveal that the existence of the substitute groups of 2-position in such 4,5-imidazoledicarboxylate ligand plays a critical role in the structural direction of these MOFs. Meanwhile, the adsorption and photoluminescent properties of the selected compounds are investigated.

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INTRODUCTION In recent years, much attention has been paid to the design and synthesis of metal−organic frameworks (MOFs), not only owing to their intriguing variety of architectures but also because of their promising applications in gas storage, catalysis, nonlinear optics, optoelectronics, sensors, magnetism, luminescence, porous materials, and so on.1 Generally, the preparation of such materials can be influenced by many factors, such as the nature of organic ligands, the coordination preference of the central metal ion, the crystallization conditions, the metal/ ligand ratio, the reaction solvent system, etc.2 It is well-known that the multifunctional organic ligands play an important role in directing the construction of MOFs with unique structures and properties. Thus, considerable efforts have been devoted to choose or design various multifunctional bridging ligands.3 Recently, the N-heterocyclic dicarboxylate ligands, for example, 4,5-imidazoledicarboxylic acid (H3IDC), have attracted © 2012 American Chemical Society

increasing attention in preparation of interesting polymeric frameworks due to their outstanding features of versatile coordination fashions as well as potential hydrogen-bonding donors and acceptors under hydro(solvo)thermal conditions.4 Accordingly, imidazole-4,5dicarboxy-lic acid (H3IDC) becomes an excellent candidate for assembling novel MOFs by incorporating appropriate metal ions in different ways, based on many MOFs possessing beautiful and interesting topological structures reported by our group as well as by other research groups,5 for instance, one-dimensional (1D) chains, two-dimensional (2D) sheets, three-dimensional (3D) porous structures. Of further interest, 2-ethyl-1H-imidazole-4,5-dicarboxylic acid (H3IEDC) with an ethyl substituent in the 2-position of the imidazole group, as a derivative of H3IDC, remains largely unexplored Received: February 13, 2011 Revised: February 15, 2012 Published: March 30, 2012 2178

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purification; solvents were dried by standard procedures. Elemental analyses for C, H, N were performed on a Perkin-Elmer 240C analytical instrument. IR spectra were recorded on a Nicolet FT-IR170SX spectrophotometer in KBr pellets. Thermogravimetric analyses (TGA) were performed on a Perkin-Elmer TGA7 analyzer with a heating rate of 10 °C/min in flowing air atmosphere. The luminescent spectra for the solid state were recorded at room temperature on Hitachi F-2500 with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra, the pass width is 5.0 nm. Nitrogen and hydrogen adsorption isotherms were taken on a Beckman Coulter SA 3100 surface area and pore size analyzer. Synthesis of the Compounds. For compounds 1−6, six complexes were prepared by a similar procedure. A mixture of H3IDC/H3IEDC (1.0 mmol), M(NO3)2 (1.0 mmol, M = Pb for 1 and 2; Ca for 3 and 4; Cd for 5 and 6), KOH (5.6 mg, 1.0 mmol), and deionized water (10 mL) was sealed in a 20 mL Teflon-lined stainless steel vessel and heated at 125 °C for 96 h. After the mixture was cooled to room temperature at a rate of 10 °C/h, colorless crystals were obtained, washed with distilled water, and dried in air. Finally the target products 1−6 were collected. 1. Yield 54% (based on the Pb). Elemental analysis calcd (%) for C10H8N4O9Pb: C, 22.41; H, 1.49; N, 10.46. Found: C, 22.38; H, 1.51; N, 10.44. IR (KBr, cm−1): 3474(br), 1628(s), 1552(m), 1455(m), 1312(w), 1237(m), 1104(m), 997(w), 841(m), 826(w), 785(w), 662(m), 552(w). 2. Yield 52% (based on the Pb). Elemental analysis calcd (%) for C14H20N4O11Pb: C, 26.77; H, 3.19; N, 8.92. Found: C, 26.75; H, 3.22; N, 8.88. IR (KBr, cm−1): 3485(br), 1635(s), 1561(s), 1460(vs), 1255(m), 1245(s), 1123(s), 1015(w), 863(m), 826(m), 794(m), 540(w). 3. Yield 48% (based on the Ca). Elemental analysis calcd (%) for C10H6N4O9Ca2: C, 29.53; H, 1.48; N, 13.78. Found: C, 29.50; H, 1.49; N, 13.75. IR (KBr, cm−1): 3480(br), 1635(s), 1557(m), 1485(m), 1455(s), 1298(s), 1217(s), 1143(s), 1042(w), 872(m), 837(m), 792(s), 674(m), 545(w). 4. Yield 49% (based on the Ca). Elemental analysis calcd (%) for C14H18N4O11Ca2: C, 33.70; H, 3.61; N, 11.23. Found: C, 33.68; H, 3.64; N, 11.20. IR (KBr, cm−1): 3653(br), 3483(s), 1630(s), 1559(s), 1455(s), 1252(w), 1206(s), 1127(s), 1103(vs), 1002(w), 861(m), 757(s), 537(w). 5. Yield 51% (based on the Cd). Elemental analysis calcd (%) for C5H4N2O5Cd: C, 21.09; H, 1.41; N, 9.84. Found: C, 21.06; H, 1.44; N, 9.82. IR (KBr, cm−1): 3642(br), 3035(s), 1635(m), 1568(s), 1450(m), 1385(m), 1312(w), 1158(s), 1247(s), 1103(m), 1012(w), 834(m), 811(m), 734(m), 654(s), 544(w). 6. Yield 50% (based on the Cd). Elemental analysis calcd (%) for C14H18N2O11Cd2: C, 26.12; H, 2.80; N, 4.35. Found: C, 26.10; H, 2.84; N, 4.38. IR (KBr, cm−1): 3476(br), 1633(s), 1571(m), 1453(m), 1310(w), 1233(m), 1094(m), 999(w), 852(m), 823(w), 783(m), 667(w), 563(m). X-ray Data Collection and Structure Refinement. Data collections were performed at 298 K on a Bruker Smart Apex II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) for six compounds 1−6. Absorption corrections were applied by using the multiscan program SADABS.7 Structural solutions and full-matrix least-squares refinements based on F2 were performed with the SHELXS-978 and SHELXL-97 9 program packages, respectively. All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms on organic motifs were placed at calculated positions, and all water hydrogen atoms were located from difference maps and refined with isotropic temperature factors. The terminal carbon atom C5 of the substituted ethyl group in 6 was disordered into two sites with 0.5 occupancy of per position. Details of the crystal parameters, data collections, and refinements for complexes 1−6 are summarized in Table 1. Selected bond lengths and angles are shown in Table S1, Supporting Information. Further details are provided in Supporting Information. CCDC 805510, 805511, 805512, 805513, 805514, and 805515 are for six new compounds 1−6, respectively.

hitherto in the field of coordination polymers compared with the well-studied ligand H3IDC, which may introduce additional structural constraint in controlling the assembly of metal−organic networks.6 By doing so, we hope to reveal some structural factors of the ligand H3IEDC for dominating the self-assembly, and this will provide more useful information of the different substituent effects in such imidazole-based dicarboxylate ligands. Similarly, it has been found that the ligand H3IEDC not only can be partially or fully deprotonated to H2IEDC−, HIEDC2−, and IEDC3− like ligand H3IDC, but also exhibits very flexible coordination modes, and hence may also result in a large diversity of supramolecular architectures.6 Furthermore, the two nitrogen atoms in the imidazole ring can gear the coordination orientation, sometimes strengthened by the cooperative coordination of the adjacent carboxyl groups, to produce large stable molecular building blocks which can be reticulated into novel topologies. In this contribution, we apply such imidazole-based dicarboxylate ligands to prepare novel coordination frameworks with interesting topologies and further demonstrate the effect of the substituted group on governing the self-organization structures. The syntheses, structural analysis, thermal stability, adsorption, and photoluminescent properties of six PbII, CaII, and CdII complexes, namely, [Pb(H2IDC)2(H2O)]n (1), {[Pb(H2IEDC)2)]·3H2O}n (2), [Ca2(HIDC)2(H2O)]n (3), {[Ca2(IEDC)2(H2O)2]·H2O}n (4), {[Cd(HIDC)(H2O)]n (5), and {[Cd2(HIEDC)2·3H2O}n (6) will be presented. The change of the substituent group of the imidazolebased dicarboxyl organic ligands leads to a modification of the resulting coordination networks (see Scheme 1 for the ligands used Scheme 1. Construction of Three Pairs of Pb, Ca, and Cd Compounds with Topological Diversity Based on H3IDC and H3IEDC Ligands

in this work). To our surprise, three different framework structures from 1D chain for 1, to 3D MOFs with 310·425·510 topology for 3, and 2D wave-like layer for 5 were constructed when H3IDC was used, which were not available in the corresponding complexes with H3IEDC (2, 4, and 6), leading to the formation of distinct crystalline species with diverse structures, from 2D layer with 44 topology (for 2), to 2D layer with 36·46·53 topology (for 4), and 3D networks with 42·84 topology in 6.

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EXPERIMENTAL SECTION

Physical Measurements. All materials were reagent grade obtained from commercial sources and used without further 2179

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Table 1. Crystal Data and Structure Refinement of 11 Compounds 1−6 chemical formula M crystal system space group a /Å b /Å c /Å α /° β /° γ /° V/Å3 Z T/K F(000) Dcalcd/g cm−3 μ /mm−1 λ /Å Rint data/restraint/parm GOF R1 [I = 2σ(I)]a wR2 [I = 2σ(I)]b a

1

2

3

4

5

6

C10H8N4O9Pb 535.39 monoclinic P2(1)/c 6.8893(7) 13.4831(14) 14.5400(15) 90 103.857(4) 90 1311.3(2) 4 298 (2) 1000 2.712 12.931 0.71073 0.0322 2360/3/226 0.968 0.0232 0.0469

C14H20N4O11Pb 627.53 monoclinic P2(1)/c 9.7166(13) 22.424(3) 11.5627(11) 90 124.873(7) 90 2066.9(4) 4 298(2) 1208 2.017 8.227 0.71073 0.0461 3727/12/308 1.030 0.0381 0.0757

C10H6N4O9Ca2 406.35 orthorhombic Pbca 8.0799(14) 13.294(2) 25.911(4) 90 90 90 3793.3(5) 8 298(2) 1648 1.940 0.882 0.71073 0.0307 2517/0/226 1.053 0.0245 0.0568

C14H18N4O11Ca2 498.48 monoclinic P2(1)/c 18.0131(17) 8.6714(8) 13.4980(13) 90 110.1240(10) 90 1979.7(3) 4 298(2) 1032 1.673 0.644 0.71073 0.0332 3381/3/288 1.040 0.0415 0.0960

C5H4N2O5Cd 284.50 orthorhombic Pbca 7.1721(12) 13.702(2) 14.774(3) 90 90 90 1451.9(4) 8 298(2) 1088 2.603 2.998 0.71073 0.0218 1311/3/125 1.077 0.0175 0.0397

C14H18N4O11Cd2 643.12 tetragonal P4(2)/nmc 16.2019(19) 16.2019(19) 6.8947(8) 90 90 90 1809.9(4) 4 298(2) 1256 2.360 2.424 0.71073 0.0232 887/38/92 1.063 0.0391 0.1067

R1 = Σ||Fo| − |Fc||/|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2, where w = 1/[σ2(Fo2) + (aP)2 + bP]. P = (Fo2 + 2Fc2)/3.

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RESULTS AND DISCUSSION Synthesis and General Characterization of Compounds 1−6. A direct mixing of the solutions of metal salt and organic ligand usually results in white precipitations that are difficult to be structurally characterized, and thus, a hydrothermal method was applied in this study, which has been proven to be a powerful approach for the preparation of various crystalline materials, because at higher temperature the reaction becomes faster, thus leading to a higher degree of reversibility in the process of crystal growth.10 The structures of the complexes were identified by satisfactory elemental analysis and X-ray diffraction. The IR spectra of 1−6 are similar. The strong and broad absorption bands in the range of 3474−3653 cm−1 in compounds may be assigned to the characteristic peaks of the νN−H and the νO−H stretching frequencies of the imidazole ring and coordinated water molecules, respectively. Six complexes exhibit strong characteristic absorptions around 1552−1571 cm−1 (νas(COO−)) and 1450− 1485 cm−1 (νs(COO−)), respectively.11 Thermal stability of the complexes was investigated by the TGA technique (see Figure S1, Supporting Information). The TG curves for complexes 1, 3, and 5 are similar and show two steps of weight loss. The weight loss of coordinated water molecules is observed from 100 to 135 °C for 1, 148 °C for 3, and 153 °C for 5 (calc. 3.36% and exp. 3.48% for 1; calc. 4.43% and exp. 4.52% for 3; calc. 6.33% and exp. 6.40% for 5). On further heating, the compound starts to be decomposed at 314 °C for 1, 405 °C for 3, and 376 °C for 5. For 4, the weight loss of 10.90% from 90 to 190 °C is attributed to the release of one lattice water molecule and two coordinated water molecules (calc. 10.83%). On further heating, the compound starts to be decomposed at 356 °C. The TGA curves of compounds 2 and 6 are also similar and show continuous weight losses in the temperature range of 90−150 °C due to the removal of three solvate water molecules. The observed weight losses of 8.73% for 2 and 8.52% for 6 are in good agreement with the calculated

values (8.61% for 2 and 8.40% for 6). When the temperature reached to ca. 350 °C, the MOFs of 6 were gradually decomposed. Structural Analysis and Discussion. [Pb(H2IDC)2(H2O)]n (1). Single crystal X-ray diffraction shows that complex 1 crystallizes in the monoclinic space group P2(1)/n. The asymmetric unit contains one crystallographically independent Pb(II) ion, two H2IDC− units with two distinctly different coordination modes, and one coordination water molecule (Figure 1). Pb1 is six-coordinated by two nitrogen and four

Figure 1. The coordination environments of the PbII ion in compound 1. Symmetry code: (a) 0.5 + x, 1.5 − y, 0.5 + z; (b) −0.5 + x, 1.5 − y, −0.5 + z.

oxygen atoms from three individual ligands H2IDC− and one coordinated water molecule. The Pb−O distances are in the range of 2.526(5)−2.833(5) Å, and the Pb−N distances are from 2.557(6) to 2.573(6) Å. In this structure, the H2IDC− ligand exhibits two kinds of different coordination modes μ- (mode 1) and μ2- (mode 2) as shown in Scheme 2. Two adjacent PbII centers are linked by a pair of H2IDC− and the coordinated water molecule as a spacer via coordination and hydrogen bonds to afford a dinuclear macrocyclic motif [Pb2(H2IDC)2(H2O)], and such dinuclear structural units are further extended to result in a 1D chain (see Figure 2), in which the two H2IDC− adopt the μ- (mode 1) and μ2- (mode 2) 2180

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Scheme 2. Ten Coordination Modes from μ- to μ5-Forms in Complexes 1−6

Figure 4. Perspective views of the coordination environments of the PbII ion in compound 2, and yellow dotted lines show the intramolecular hydrogen bonds. Symmetry code: (a) 0.5 + x, 0.5 − y, 0.5 + z; (b) 1 + x, y, z. Figure 2. 1D chain constructed by dinuclear macrocyclic motif [Pb2(H2IDC)2(H2O)] via the coordination and hydrogen bonds in 1.

coordination fashion. The Pb···Pb distances separated by two H2IDC− are 8.419 (5) Å. Through interchain O−H···O and interlayer N−H···O hydrogen bonds (Table S2, Supporting Information), these 1D chains are further assembled into a 2D supramolecular layer (Figure 3) and 3D supramolecular network (Figure S2, Supporting Information).

Figure 5. 2D wave-like sheet with 44 topology extended in the ac plane in 2; uncoordinated water molecules have been omitted for clarity.

line in Figure 5) (viewing as two linkers) to form two crosslinked chains with central metal PbII ions as the hinge nodes. Obviously, based on these connections, a 2D wave-like layer with 44 topology is generated as shown in Figure 5. Interestingly, viewing along the c axis direction, the 2D framework in 2 may be also regarded as the layer with thickness of 7.8 Å containing 1D left-/right-hand helical hydrophobic channels, in which the upside-down coupling of two triangular-shaped helical chains with opposite helixes (M: left-hand, P: right-hand) is depicted in Figure 6. Meanwhile, there exists six-membered ring-like water clusters (H2O)6 with chair-configuration between two adjacent layers (see Figure 7). Through hydrogen bonding O−H···O and H···Pb12 interactions among water clusters, lattice water (O9) and adjacent layers, a 3D supramolecular network is assembled as shown in Figure 7. [Ca2(HIDC)2(H2O)]n (3). Complex 3 is a 3D coordination polymer and crystallized in orthorhombic space group Pbca. The asymmetrical unit in 3 contains two crystallographically independent calcium ions, Ca1 and Ca2, along with two HIDC2− anions and one coordinated water molecule. The Ca1 ion is six-coordinated by six oxygen atoms from four HIDC2− anions, while Ca2 is eight-coordinated by two nitrogen and six oxygen atoms from four HIDC2− anions and one water molecule. Ca1 and Ca2 are μ2-bridged by two carboxylate oxygens (O1 and O5) to form the dinuclear moiety with Ca1···Ca2 separation of 3.9585(6) Å, and then further give the

Figure 3. 2D supramolecular layer formed by hydrogen bonding O−H···O interaction along the b axis in 1.

{[Pb(H2IEDC)2)]·3H2O}n (2). When the ligand H3IEDC with an ethyl substituent group in the 2-position of the imidazole ring replacing the ligand H3IDC reacted with Pb(NO3)2 under the same reaction conditions, the corresponding compound 2 was obtained. X-ray single crystal diffraction reveals that complex 2 also crystallizes in the monoclinic P2(1)/n space group, while it exhibits a 2D wave-like layer with 44 topology differing from 1 with a 1D chain-like structure. From Figure 4, the asymmetric unit of 2 consists of one PbII atom, two H2IEDC¯ ligands, and three uncoordinated water molecules. Apparently, Pb1 in 2 is also six-coordinated by two nitrogen and four oxygen atoms from four individual H2IDC¯ moieties. The Pb−O distances are in the range of 2.516(3)−2.844(3) Å, and the Pb−N bond lengths are from 2.603(3) to 2.644(3) Å, which are slightly longer than those of complex 1. In this structure, the H2IEDC¯ ligand connects two PbII ion with the Pb···Pb distances of 9.339 Å for mode (3) (Scheme 2 and green line in Figure 5) and 9.717 Å for mode (4) (Scheme 2 and blue 2181

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Figure 8. Perspective views of the coordination environments of the CaII ions in compound 3. Symmetry code: (a) −0.5 + x, y, 0.5 − z; (b) 1 − x, −0.5 + y, 0.5 − z; (c) 1 − x, −y, −z; (d) 1 + x, y, z.

Figure 6. 2D layer with thickness of 7.8 Å containing 1D left-/righthand helical hydrophobic channels along the c axis in 2; uncoordinated water molecules have been omitted for clarity.

Figure 7. 3D supramolecular network constructed by hydrogen bonding O−H···O and Pb···H interactions containing 1D left-/righthand helical hydrophobic channels and chair-conformation (H2O)6 clusters along the c axis in 2.

tetranuclear repeating unit via the μ2-bridges of two carboxylate oxygen O6 atoms as shown in Figure 9a. The average Ca−O/ Ca−N distance is 2.358(5) Å/2.556(5) Å, which are within the normal range. As for the HIDC2− ligand, each HIDC2− group bonds to four Ca atoms in coordination modes (7) and (8) (Scheme 2) to generate a 2D layer-like structure (Figure S3, Supporting Information). And then through Ca1−O4 coordination interaction from mode (7) of the HIDC2− ligand in adjacent layers, the 3D MOF is constructed as depicted in Figure 9b, in which each tetranuclear unit connects another 10 identify units around and each HIDC2− ligand links two tetranuclear units. Moreover, O9 of the coordinated water molecule forms strong hydrogen bonds with the coordinated carboxylate oxygen atoms O3 and O7 with separations of 2.017(5) and 2.113(5) Å (Table S2, Supporting Information), respectively; the nondeprotonated imidazole nitrogen (N2) and carbon (C5) atoms form strong hydrogen bonds with coordinated carboxylate oxygen atoms O2 and O8 (Table S2, Supporting Information) with separations of 2.053(6) and 2.528(6) Å, which further

Figure 9. In 3, (a) the tetranuclear repeating unit [Ca4(μ2-O)6]; (b) 3D coordination network extended in the bc plane; (c) schematic illustrating the 3D 10-connected 310·425·510 topological network.

stabilize the final 3D architecture. A better insight into the nature of 3 can be achieved by the application of a topological approach (involving Topos40 program),13 namely, reducing multidimensional structures to simple node-and-linker nets. As discussed above, each tetranuclear Ca(II) center acts as a 10-connected node and each HIDC2− ligand as a 2-connected linker, and the resulting structure of 3 is symbolized as a new 310·425·510 topological net (Figure 9c). {[Ca2(IEDC)2(H2O)2]·H2O}n (4). In the structure of 4, two independent Ca(II) ions are also found (see Figure 10). The 2182

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Figure 10. Perspective views of the coordination environments of the CaII ions in compound 4. Symmetry code: (a) x, 1.5 − y, 0.5 + z; (b) x, 1 + y, z; (c) 2 − x, 1 − y, 2 − z.

Ca1 center is seven-coordinated by five oxygen atoms and two nitrogen atoms from four HIEDC2− ligands, while Ca2 center is eight-coordinated by eight oxygen atoms from four HIEDC2− ligands and two coordinated water molecules. Similar to compound 3, in compound 4, Ca1 and Ca2 are also μ2-bridged by three carboxylate oxygens (O4, O5, and O8) to form the dinuclear moiety with Ca1···Ca2 separation of 3.6495(9) Å, and then further give the tetranuclear repeating unit via the μ2-bridges of two carboxylate oxygen O6 atoms as shown in Figure 11a. The average Ca−O distance is 2.428 (2) Å and the Ca−N bond length is 2.541(3) Å, which are within the normal range. Only one type of HIEDC2− anion is found in this case, and the coordination fashion of the HIEDC2− ligand in 4 is modes (5) and (9), namely, μ3- and μ5-, respectively (see Scheme 2). As a result, the CaII ions are extended by HIEDC2− to afford a 2D layer-like coordination framework (see Figure 11b) with the Schläfli symbol of 36·46·53 (see Figure 11c), which represents a 6-connected topology type. In this case, a tetranuclear repeating unit [Ca4(μ2-O)8] can be viewed as the 6-connected node and HIEDC2− anion as the 2-connected linker. And such layers are further connected by interlayer hydrogen bonding N1−H1···O2 interactions to constitute a 3D supramolecular network (Figure S4 and Table S2, Supporting Information). Moreover, intralayer hydrogen bonding O10− H10A···O3, O10−H10B···O9, and N3−H3···O1 interactions further stabilize the supramolecular network (Figure S4 and Table S2, Supporting Information). {[Cd(HIDC)(H2O)]n (5). Single crystal X-ray diffraction shows that complex 5 was reported previously by Zhang and co-workers.14 In 5, Cd(II) ion has a distorted octahedral environment (Figure S5, Supporting Information) and the doubly deprotonated HIDC2− unit with one coordination mode (6) coordinates in the μ3-mode (6) (Scheme 2) to generate a 2D wavelike (4,4) topological layer similar to compound 2 (Figure S6, Supporting Information). Moreover, interlayer hydrogen bonding O5−H5A···O4, O5−H5B···O2, and N2−H2···O2 interactions also serves as a spring to link the adjacent coordination layers to form a 3D network (Figure S7, Supporting Information). {[Cd2(HIEDC)2·3H2O}n (6). Complex 6 crystallizes in the tetragonal space group P4(2)/nmc, which is assembled in a 3D framework with a half Cd(II) atom, a half HIEDC2− anion, and three-quarter lattice water molecule in each asymmetric unit. As is shown in Figure 12, each Cd(II) ion is six-coordinated by two nitrogen atoms and four oxygen atoms from four individual

Figure 11. In 4, (a) the tetranuclear repeating unit [Ca4(μ2-O)8]; (b) 2D layer extended in the bc plane; (c) schematic illustrating the 2D 6connected 36·46·53 topological network.

Figure 12. Perspective views of the coordination environments of the CdII ion in compound 6. Symmetry code: (a) −0.5 + y, 0.5 + x, 0.5 − z; (b) 1.5 − y, 1.5 − x, −0.5 + z; (c) 1 − x, 2 − y, 1 − z; (d) 0.5 − x, y, z.

μ4-HIEDC2− anions, forming a [CdN2O4] unit with a slightly distorted octahedral coordination geometry. The Cd(1)−L (L = O, N) distances are in the range of 2.268 (5)−2.450(5) Å, and the trans L−Cd−L bond angles are in the range of 66.46(17)−163.6(2). In this structure, each HIEDC2− anion adopts a μ4-κN,O:κO:κO′:κN,O coordination mode (9) to bridge four Cd(II) atoms in N,O-chelating, O-bridging, O-bridging, and N,O-chelating fashion as depicted in Scheme 2, and each Cd(II) node also acts as a 4-fold connector in the resulting 3D MOF with 1D square open hydrophobic and 2183

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hydrophilic channels alternately arranged along the c-axis (Figure 13). As illustrated in Figure 13, the cross-section of

Figure 14. The 4-connected 42.84topology of the 3-D framework in 6 viewed along the a-axis.

in which the K+ are located in the channels along the c-axis (Figure S10, Supporting Information). Meanwhile, the agreement between the experimental and simulated XRD patterns indicated the phase purity of the product 6 (Figure S11, Supporting Information). Effect of the Carboxylate-Ligands on Network Assemblies. From the above description and discussion, it can be seen that the different carboxylate-involved ligands will have a great influence on the framework construction of these complexes.15 Compared with three pairs of Pb (1 and 2), Ca (3 and 4), and Cd (5 and 6) compounds in present text, three corresponding complexes 1, 3, 5 present 1D to 3D polymers based on the organic ligand H3IDC without any substitute group in the 2-position of the imidazole ring. However, when ligand H3IEDC with an ethyl in 2-position of the imidazole ring was used under similar synthetic conditions, the resulting complexes 2, 4, and 6 show high-dimensional networks such as 2D and 3D (Scheme 1), which possibly derives from the fact that existence of the bulky hydrophobic ethyl group in the 2-position of the imidazole ring badly prevents or reduces water molecules coordinating to the central metal, in particular, when the imidazole nitrogen atom coordinated to the metal center, and thus the donor atoms in the ligand can be fully utilized, easily resulting in high-dimensional coordination frameworks. However, complexes 3 (3D MOFs) and 4 (2D MOFs) in the present system is an exception, which can be attributed to diverse coordination modes of dicarboxylic groups in the imidazoledicarboxylate-involved ligands. Moreover, although coordination of water molecules to central metal ions possibly reduces the dimension of the coordination frameworks in the present case, rich hydrogen bonds may generate a lot of interesting supramolecular structures as those in 1 and 5. As indicated above, it is noted that the selection of the metal ions with different coordination geometries or radii, as well as various coordination modes of imidazoledicarboxylate-involved ligands give rise to the different MOFs though ligands and reaction conditions employed are the same. In other words, directing coordination of these imidazoledicarboxylate-involved ligands to metal centers is the synergy of various factors, but the substitute group in the 2-position of the imidazole ring indeed plays an important role in inducing the arrangement and coordination modes of the ligands and finally leading to the

Figure 13. 3D coordination framework containing 1D square open hydrophobic and hydrophilic channels alternately arranged extended in the ab plane in 6; uncoordinated water molecules have been omitted for clarity.

each symmetrical square channel consists of four Cd(II) centers with N2O4 donor set and four μ4-HIEDC2− anions. These four Cd atoms are in the same plane with a distance of 6.375(2) Å between two adjacent Cd···Cd and four Cd−Cd−Cd angles of 90°. The [CdN2O4] octahedron units are connected with each other through two bridging oxygen atoms O1 and O1′ atoms from HIEDC2− ligands, forming infinite 1D chains along the c-axis as four corner posts of 1D square channels, and such 1D chains are further connected by 4-IEDC2− bridges (as the sides) to form 1D channels (Figures 13 and S8). In these 1D channels, the hydrophilic channels are filled with (H2O)4 clusters from uncoordinated water molecules through intermolecular hydrogen-bonding O−H···O interactions (Figure S9 and Table S2, Supporting Information). However, those hydrophobic channels are partially filled with the ethyl groups from HIEDC2− ligands, forming a series of the connected caves with about 6 Å of internal diameter along the c axis. Topologically, the HIEDC2− ligand and Cd2+ ion can be viewed as four-connected nodes. In this way, the structure of 6 can be rationalized as a 4connected net. The molar ratio of two kinds of nodes is 1:1. Thus, the framework of 6 is symbolized as a 42.84 topological network (Figure 14). The 3D 42.84 topological network of 6 is similar to the 3D structure in reported compound [KCd(IDC)]n,15a 2184

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different topological structures of metal complexes in the present reaction system and under the reaction conditions employed. Luminescent and Adsorption Properties. The luminescent behaviors of compounds 1−6 were investigated in the solid state at room temperature. The emission spectrum of all compounds at λex = 360 nm is shown in Figure 15; it has been

Figure 16. Isotherms for the adsorption and desorption of nitrogen within 6 at 77 K.

Figure 15. Luminescent behaviors of compounds 1−6 in the solid state at room temperature (all compounds at λex = 360).

observed that the free ligands H3IDC and H3IEDC exhibit emission around 410 and 416 nm, respectively. All the emission intensity of the complexes is stronger than those of the ligands. Complexes 1−6 exhibit different MOFs though ligands and reaction conditions employed are the same. In other words, directing coordination of these imidazoledicarboxylate-involved ligands to metal centers is the synergy of various factors, but the substitute group in 2-position of imidazole ring indeed plays an important role in inducing the arrangement and coordination modes of the ligands and finally leading to the different topological structures of metal-complexes in the present reaction system and under the reaction conditions employed. To check permanent porosities of complex 6, gas adsorption studies were performed utilizing a fully activated sample. TGA revealed that the free water molecules in 6 may be removed between 100 and 200 °C, and framework structure was still maintained. So the fresh sample was evacuated under a vacuum at 180 °C overnight to remove the guest molecules. Measured nitrogen adsorption of the compound at 77 K (Figure 16) shows a typical type I sorption behavior as expected for microporous materials. Compound 6 adsorbs 83 cm3/g of nitrogen at 77 K and 1 atm, corresponding to apparent Brunauer−Emmett−Teller and Langmuir surface areas of 468 and 507 m2/g, respectively. The average pore size is about 9.2 Å, which is in perfect agreement with what could be anticipated from the crystallographic data. The nitrogen adsorption shows good reversibility. To evaluate the hydrogen storage performance of this MOF, hydrogen sorption isotherm of sample 6 was also measured from vacuum to 1 atm at 77 K with the similar method. As shown in Figure 17, gravimetric hydrogen uptake of complex 6 can adsorb about 1.55 wt % with noticeable hysteresis at 77 K and 1 atm, which can be ascribed to both a switch effect of the hydrophobic cavity from the different conformations of ethyl groups and small H2 molecules compared with N2 molecules.

Figure 17. Isotherms for the adsorption and desorption of hydrogen within 6 at 77 K.

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CONCLUSION In summary, three pair of new different dimensional Pb-, Ca-, and Cd-organic MOFs with architectural diversity have been successfully synthesized based on multidentate ligands, H3IDC and H3IEDC under hydrothermal conditions, in which two ligands H3IDC and H3IEDC display plentiful coordination modes. Complex 1 is a 1D coordination chain and 2 has a 2D wave layer-like structure with 44 topology. However, both 1 and 2 have 3D supramolecular networks assembled via hydrogen bonding interactions. Complex 3 is a 3D MOF with 310·425·510 topology built up from a linear 2-connected HIDC2− (linker) and 10-connected tetranuclear metal unit [Ca4(μ2-O)6] (node), and 4 presents a 2D MOF with 36·46·53 topology. Different from 3 and 4, with the 4-connected binuclear metal unit [Cd(O−C−O)]2 as a node, a wave-like 2D MOF with 44 topology in 5 based on H3IDC is created. Complex 6 shows a 3D coordination framework containing 1D alternately arranged hydrophilic and hydrophobic channels with 42·84 topology, in which Cd(II) ion is viewed as a 4-connected node and the HIEDC2− and μ2-O are regarded as two different linkers. This study clearly indicates the important role that the substitute groups of 2-position in such a 4,5-imidazoledicarboxylate and the coordination geometry of the introduced metal centers can 2185

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(3) (a) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629. (b) Li, X.; Wu, B.-L.; Niu, C.-Y.; Niu, Y.-Y.; Zhang, H.-Y. Cryst. Growth Des. 2009, 9, 3423. (4) (a) Wang, Y.-L.; Yuan, D.-Q.; Bi, W.-H.; Li, X.; Li, X.-J.; Li, F.; Cao, R. Cryst. Growth Des. 2005, 5, 1849. (b) Lu, W.-G.; Su, C.-Y.; Lu, T.-B.; Jiang, L.; Chen, J.-M. J. Am. Chem. Soc. 2006, 128, 34. (5) (a) Gurunatha, K. L.; Uemura, K.; Maji, T. K. Inorg. Chem. 2008, 47, 6578. (b) Gu, Z.-G.; Cai, Y.-P.; Fang, H.-C.; Zhou, Z.-Y.; Thallapally, P. K.; Tian, J.; Liu, J.; Exarhos, G. J. Chem. Commun. 2010, 46, 5373. (c) Gu, Z.-G.; Fang, H.-C.; Yin, P.-Y.; Tong, L.; Yin, Y.; Hu, S.-J.; Li, W.-S.; Cai, Y.-P. Cryst. Growth Des. 2011, in press. (6) (a) Zhang, F.-W.; Li, Z.-F.; Ge, T.-Z.; Yao, H.-C.; Li, G.; Lu, H.-J.; Zhu, Y.-Y. Inorg. Chem. 2010, 49, 3776. (b) Wang, S.; Zhang, L. R.; Li, G. H.; Huo, Q. S.; Liu, Y. L. CrystEngComm 2008, 10, 1662. (7) Sheldrick, G. M. SADABS, Version 2.05; University of Göttingen: Göttingen, Germany. (8) Sheldrick, G. M. SHELXS-97, Program for X-ray Crystal Structure Determination; University of Göttingen: Göttingen, Germany, 1997. (9) Sheldrick, G. M. SHELXS-97, Program for X-ray Crystal Structure Refinement, University of Göttingen: Göttingen, Germany, 1997. (10) (a) Feng, S. H.; Xu, R. R. Acc. Chem. Res. 2001, 34, 239. (b) Cundy, C. S.; Cox, P. A. Chem. Rev. 2003, 103, 663. (c) Zhang, X. M. Coord. Chem. Rev. 2005, 249, 1201. (11) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; John Wiley and Sons: New York, 1997. (12) Izod, K.; McFarlane, W.; Wills, C.; Clegg, W.; Harrington, R. W. Organometallics 2008, 27, 4386. (13) (a) Well, A. F. Further Studies of Three-Dimensional Nets; American Crystallographic Association (distributed by Polycrystal Book Service): New York (Pittsburgh, PA), 1979. (b) Well, A. F. Three-Dimensional Nets and Polyhedra; Wiley: New York, 1977. (14) Fang, R.-Q.; Zhang, X.-M. Inorg. Chem. 2006, 45, 4801. (15) (a) Wang, S.; Zhang, L.-R.; Li, G.-H.; Huo, Q.-S.; Liu, Y.-L. CrystEngComm 2008, 10, 1662. (b) Lin, J. B.; Zhang, J. P.; Chen, X. M. J. Am. Chem. Soc. 2010, 132, 6654.

play important roles in control for MOFs topologies. On the other hand, the different adsorption and desorption behaviors of the compound 6 on N2 and H2 illustrate it can be used as a potential hydrogen storage material. Moreover, compared with two ligands, complexes 1−6 exhibit strong solid-state fluorescence properties at room temperature. Obviously, the systematic investigation of a series of IDC/IEDC-based MOFs should provide a rational synthetic strategy for constructing new photoluminescent and porous materials by selecting functional organic ligands and appropriate metal ions.

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ASSOCIATED CONTENT

S Supporting Information *

Additional structural figures for the related compounds, and the TG curves of compounds 1−6, Tables of selected bond lengths, bond angles and the related hydrogen bonds, as well as X-ray crystallographic files in CIF format for compounds 1−6 are available in supporting material section.This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-20-39310383. Fax: +86-20-39310187. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial aid from the National Natural Science Foundation of P. R. China (Grant Nos. 20772037, 21071056, and 91122008), Science and Technology Planning Project of Guangdong Province (Grant No. 2010B031100018) and the N. S. F. of Guangdong Province (Grant No. 9251063101000006 and 10351063101000001).

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REFERENCES

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