Construction of Ba (II) Coordination Polymers Based on Imidazole

Article pubs.acs.org/crystal

Construction of Ba(II) Coordination Polymers Based on ImidazoleBased Dicarboxylate Ligands: Structural Diversity Tuned by Alcohol Solvents Song-Liang Cai, Sheng-Run Zheng,* Zhen-Zhen Wen, Jun Fan, and Wei-Guang Zhang* School of Chemistry and Environment, South China Normal University, Guangzhou, 510006, China S Supporting Information *

ABSTRACT: Four novel three-dimensional (3D) barium−organic coordination polymers formulated as [Ba(μ5-H2hmIDC)(μ2H2O)]n (1), [Ba(μ3-H3hmIDC)2(μ1-H2O)]n (2), [Ba(μ5-H2hmIDC)(μ2-O2)0.5]n (3), and [Ba(μ3-H3hmIDC)(μ4-H3hmIDC)]n (4) were synthesized under the solvothermal conditions of 2-(hydroxymethyl)-1H-imidazole-4,5-dicarboxylic acid (H4hmIDC) with BaCl2·2H2O using a combination of water or dimethyl formamide (DMF) with various alcohol solvents (methanol, ethanol, n-propanol, isopropanol, and n-butanol). Complex 1 is a 3D framework based on two-dimensional secondary building units (SBUs) and μ5-H2hmIDC2− pillars. This complex contains a unique alb network topology with the Schläfli symbol (45·6)2(410·614·84). Complex 2 is a (3,10)-connected framework with binuclear [Ba2(COO)2] SBUs as 10-connected nodes and two types of μ3-H2hmIDC2− ligands as 3-connected nodes. Complex 3 is a 3D framework formed with one-dimensional (1D) SBUs and μ5-H2hmIDC linkers, wherein the unprecedented oxygen molecules are coordinated using a linear-μ-η1:η1-peroxo fashion, showing (5,6)-connected network topology with the Schläfli symbol (46·53·6)(46·54·65). Complex 4 is a 3D framework based on 1D SBUs with μ3-H3hmIDC and μ4-H3hmIDC as linkers. This complex exhibits a trinodal (3,4,7)-connected 3D framework with the Schläfli symbol (42·6)(43·63)(48·611·82). The results revealed that the alcohol solvent plays a subtle yet essential role in the crystallization and construction of Ba(II)-organic coordination frameworks with diverse 3D structures, although these alcohol molecules do not appear in the frameworks.

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INTRODUCTION Metal coordination polymers constructed by linking the metal centers with multidentate bridging ligands have received significant attention because they exhibit a variety of intriguing architectures.1 These polymers can be potentially applied in gas storage,2 ion exchange,3 chemical sensor,4 catalysis,5 and separation.6 However, structural modification of these metal− organic frameworks (MOFs) requires further studies because of their complexity and uncertainty. Their structures may be affected by the coordination trend of the metal centers, the nature of ligands, the metal−ligand ratio, and the presence of counterions and solvents.7,8 The effect of solvent has also attracted increasing interest in recent years because the solvent media used in the assembly processes may significantly influence the structures and properties of the resulting coordination frameworks. Solvents used in reaction systems generally affect the crystal structures of coordination complexes © 2012 American Chemical Society

in the following ways: (i) solvents participate in coordination and exhibit a great impact on the coordination environment and geometry of the metal ions; (ii) solvents are not bound to the metal ions, but they exist in the final lattice structures as guest molecules; and (iii) solvents are not found in the products, but they influence crystal growth and induce different structural aggregations. Solvent components acting as ligands or guest molecules during the self-assembly process have been studied extensively.8a−d However, solvent-inducing behaviors in which no solvent molecules exist in the final lattice structures have been limitedly explored.8e On the other hand, imidazole-based dicarboxylate ligands have been widely used for the synthesis of various MOFs with Received: March 27, 2012 Revised: May 8, 2012 Published: May 9, 2012 3575

dx.doi.org/10.1021/cg3004068 | Cryst. Growth Des. 2012, 12, 3575−3582

Crystal Growth & Design

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using a Bruker D8 Advance diffractometer at 40 kV, 40 mA with a Cutarget tube, and a graphite monochromator. Solid-state fluorescence spectra were preformed on a Hitachi-2500 spectrophotometer with a 150 W xenon lamp as the light source at room temperature. Synthesis of [Ba(μ3-H3hmIDC)2(μ1-H2O)]n (1). A mixture of H4hmIDC (0.2 mmol), BaCl2·2H2O (0.2 mmol), CH3OH/H2O (1/1, 6 mL), and Et3N (0.1 mL) was sealed in a 15 mL Teflon-lined stainless steel autoclave, heated at 170 °C for 72 h, and then slowly cooled to room temperature at a rate of 5 °C/h. Pink block-shaped crystals of 1 were collected after washing with distilled water and drying in air. Yield, 34% (based on the Ba). Anal. calcd for C6H6BaN2O6: C, 21.23; H, 1.78; N, 8.25. Found: C, 21.18; H, 1.81; N, 8.26%. IR (KBr, ν/ cm−1): 3423s, 1586s, 1510w, 1468 m, 1371s, 1253w, 1187w, 1108w, 1049w, 1015w, 959w, 842w, 797w, 691w, 615w, 582w, 520w, 442w. Synthesis of [Ba(μ5-H2hmIDC)(μ2-H2O)]n (2). The synthesis of 2 was similar to the above description for 1 except that the solvents of CH3OH/H2O (1:1, 6 mL) were replaced by EtOH/H2O (1:1, 6 mL). Pale-brown block crystals of 2 were isolated, washed with distilled water, and dried in air. Yield, 46% (based on the Ba). Anal. calcd for C12H12BaN4O11: C, 27.42; H, 2.30; N, 10.66. Found: C, 27.37; H, 2.34; N, 10.63%. IR (KBr, ν/cm−1): 3429s, 2975w, 2924w, 1569s, 1379 m, 1256 m, 1119w, 1052 m, 1018w, 968w, 943w, 920w, 862w, 778 m, 618w, 590w, 523w, 420w. Synthesis of [Ba(μ5-H2hmIDC)(μ2-O2)0.5]n (3). Compound 3 was prepared by a similar method used for 1 except that the solvents of CH3OH/H2O (1:1, 6 mL) were replaced by (CH3)2CHOH/H2O (1:1, 6 mL). Colorless block crystals of 3 were collected by filtration and washed with distilled water for several times. Yield, 18% (based on the Ba). Anal. calcd for C6H4BaN2O6: C, 21.36; H, 1.19; N, 8.30. Found: C, 21.33; H, 1.22; N, 8.34%. IR (KBr, ν/cm−1): 3664s, 2924w, 2851w, 1667 m, 1578s, 1552w, 1510w, 1466 m, 1371s, 1345w, 1256w, 1197w, 1105w, 1049w, 1018 m, 959w, 839w, 797s, 730w, 685w, 666w, 609w, 520w. Synthesis of [Ba(μ3-H3hmIDC)(μ4-H3hmIDC)]n (4). Compound 4 was obtained by a similar method used for 1 except that the solvents of CH3OH/H2O (1:1, 6 mL) were replaced by EtOH/DMF (1:1, 6 mL). Colorless block crystals of 4 were collected by filtration and washed with ethanol several times with a yield of 65% (based on the Ba). Anal. calcd for C12H10BaN4O10: C, 28.40; H, 1.99; N, 11.04. Found: C, 28.43; H, 2.02; N, 11.08%. IR (KBr, ν/cm−1): 3433s, 2970w, 2361 m, 2341w, 1691w, 1549s, 1440w, 1391 m, 1368w, 1260s, 1181w, 1057 m, 1025w, 854w, 779w, 669w, 614w, 522w, 497w, 443w, 407w. X-ray Data Collection and Structure Refinement. Data collections were performed at 298 K on a Bruker Smart Apex II

different structures and useful properties because of their flexible coordination modes.9 We have recently successfully prepared a new multidentate ligand, 2-(hydroxymethyl)-1Himidazole-4,5-dicarboxylic acid (H4hmIDC) by introducing a hydroxymethyl group on the 2-position of H3IDC. This ligand aroused our interest mainly because of the diverse reaction behavior of its hydroxymethyl group under hydrothermal conditions when assembled with different transitional metal ions.10 The current study reports the coordination chemistry of H4hmIDC based on Ba(II) ions as a continuation of our previous work. Four novel compounds, [Ba(μ5-H2hmIDC)(μ2H2O)]n (1), [Ba(μ3-H3hmIDC)2(μ1-H2O)]n (2), [Ba(μ3H3hmIDC)(μ4-H3hmIDC)]n (3), and [Ba(μ3-H3hmIDC)(μ4H3hmIDC)]n (4), are formed in water/alcohol (complexes 1− 3) and DMF/ethanol (complex 4) solvent systems (Scheme 1). Scheme 1. Schematic Drawing of the Reactions between Ba(II) and H4hmIDC in Different Solvent Systems

The structural diversity tuned using a series of alcohol molecules that do not appear in the final frameworks is a unique case. In addition, the compounds are also examples of Ba(II) compounds based on imidazole-based dicarboxylate ligands that are still rarely explored.9m,n

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EXPERIMENTAL SECTION

Materials and Measurements. The organic ligand H4hmIDC was prepared according to literature procedure.10a All the other reagents were of grade quality obtained from commercial sources. The C, H, and N microanalyses were carried out on a Thermo FlashEA112 elemental analyzer. IR spectra were recorded by using a Shimadzu IR Prestige-21 spectrometer with KBr pellets in the 400−4000 cm−1 region. X-ray powder diffraction measurements were measured by

Table 1. Crystal Data and Structure Refinement of Complexes 1−4

a

complex

1

2

3

4

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z D (g cm−3) μ (mm−1) T (K) Ra/wRb total/unique/Rint

C6H6BaN2O6 339.47 monoclinic P2(1)/c 7.1034(9) 16.639(2) 7.4341(10) 90 104.069(2) 90 852.32(19) 4 2.645 4.673 298(2) 0.0202/0.0495 4580/1661/0.0215



C12H10BaN4O10 507.58 orthorhombic Pna2(1) 15.4200(2) 10.7220(5) 9.1370(2) 90 90 90 1510.65(8) 4 2.232 2.700 298(2) 0.0547/0.1465 7948/2945/0.0507

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. 3576



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Figure 1. (a) Coordination environment of Ba(II) in complex 1. (b) The 1D chain in complex 1. (c) The 2D SBU in complex 1 (the μ2-H2O molecules are highlighted in yellow). (d) The 3D framework in complex 1. diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71073 Å) for four complexes 1−4. Absorption corrections were applied by using the multiscan program SADABS.11 Structural solutions and full-matrix least-squares refinements based on F2 were performed with the SHELXS-9712 and SHELXL-9712 program packages, respectively. All of the nonhydrogen atoms were refined anisotropically. All of the hydrogen atoms were placed at calculated position and refined with isotropic temperature factors. Details of the crystal parameters, data collections, and refinements for complexes 1− 4 are summarized in Table 1. Selected bond lengths and angles are shown in Table S1 in the Supporting Information. Further details are provided in the Supporting Information. CCDC 871828−871831 are for complexes 1−4, respectively.

Scheme 2. Coordination Modes of the H4hmIDC in Complexes 1−4

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RESULTS AND DISCUSSION Crystal Structure of 1. Complex 1 crystallizes in the monoclinic space group P2(1)/c, and the asymmetric unit comprises one Ba(II) ion, one μ5-H2hmIDC2− dianion ligand, and one coordinated water molecule. The coordination environment of Ba(II) ion is shown in Figure 1a. Each Ba(II) atom exhibits a distorted [BaNO8] geometry, being coordinated with one imidazole nitrogen atom [Ba−N, 2.969(3) Å], one hydroxymethyl oxygen atom [Ba−O, 2.897(3) Å], and five carboxylate oxygen atoms [Ba−O, 2.666(2)−2.830(2) Å] from five different μ5-H2hmIDC2− anions, and two oxygen atoms from two μ2-H2O ligands [Ba−O, 2.916(3)−3.125(3) Å]. The bond angles around the central Ba(II) ion range from 56.00(7) ° to149.19(7)°. In 1, each μ5-H2hmIDC2− ligand adopts a complicated μ5-kN,O:kO′,O″:kO″:kO′″:kO″″ coordination mode to bridge five Ba(II) ions in N,O-chelating, O′,O″-chelating, O″bridging, and monodentate fashions (Scheme 2a). In complex 1, the μ3-η2:η1 carboxylate groups bridge Ba(II) ions into a 1D chain extend along the a-direction (Figure 1b), where the adjacent nonbonding Ba···Ba distances are 4.7825(5)

and 5.0434(5) Å. These 1D chains are further linked by the μ2H2O molecules to produce a 2D layer in the ac plane (Figure 1c), which can be seen as 2D SBU (Figure S1 in the Supporting Information). In the 2D layer, two Ba(II) ions are joined by two water molecules to form a [Ba2(μ2-H2O)2] dimer with the nonbonding Ba···Ba distance 5.2863(6) Å. Finally, these 2D layers are further connected by using μ5-H2hmIDC2− ligands as pillars to construct a complicated 3D framework (Figure 1d). From a topological perspective, each of the dinuclear [Ba2(μ2-H2O)2] building blocks are bound to eight μ5H2hmIDC2− ligands and can be denote as a 8-connected node (Figure S2a in the Supporting Information). Likewise, 3577



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Figure 2. (a) Coordination environment of Ba(II) in complex 2. (b) The 1D chain based on binuclear SBUs in complex 2. (c) The 3D framework of complex 2. (d) The 3D (3,10)-connected network topology in complex 2 (the 3-connected nodes are shown in cyan balls, and the 10-connected nodes are shown in pink balls).

Figure 3. (a) Coordination environment of Ba(II) in complex 3. (b) The 1D SBU in complex 3. (c) Polyhedral view of the 1D SBU in complex 3. (d) The 3D framework based on 1D SBUs in complex 3 (the μ2-O2 molecules are highlighted in red balls).

type 3D framework with the Schläfli symbol (45·6)2(410·614·84) (Figure S2c in the Supporting Information). Crystal Structure of 2. Complex 2 also crystallizes in the monoclinic P2(1)/c space group. The asymmetric unit of the

each of the μ5-H2hmIDC2− ligands is linked to four [Ba2(μ2H2O)2] building blocks and acts as a 4-connected node (Figure S2b in the Supporting Information). Thus, the resulting structure of compound 2 is a bimodal (4,8)-connected alb 3578



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Figure 4. (a) Coordination environment of Ba(II) in complex 4. (b) The 1D SBU in complex 4. (c) Polyhedral view of the 1D SBU in complex 4. (d) The 3D framework based on 1D SBUs in complex 4.

complex contains one Ba(II) ion, two unique H3hmIDC− anions with two different coordination modes, and one coordination water molecule. The coordination polyhedron around the central Ba(II) ion can be visualized as a slightly distorted dodecahedral geometry with a [BaNO7] coordination mode: one imidazole nitrogen atom, one hydroxymethyl oxygen atom, five carboxylate oxygen atoms from six individual H3hmIDC− anions, and one oxygen atom comes from the coordinated water molecule (Figure 2a). The Ba−N bond length is equal to 2.944(5) Å, while the Ba−O distances span from 2.720(5) to 2.999(5) Å, all of which are similar to that in complex 1 and comparable to those observed for the other Ba(II) complexes based on N-heterocyclic carboxylic acids.9m,n There are two types of H3hmIDC− in complex 2: one links to three Ba1 atoms in a μ3-kN,O:kO′:kO″ mode (Scheme 2b), while the other joints three Ba1 atoms in a μ3-kO:kO′:kO″ mode (Scheme 2c). As illustrated in Figure 2b, two μ2-η1:η1 carboxylate groups (each oxygen atom coordinates to one metal atom, and the carboxylate group coordinates to two metal atoms) from the H3hmIDC− in μ3-kN,O:kO′:kO″ mode bridge two neighboring Ba(II) ions into a binuclear [Ba2(COO)2] SBU with a Ba···Ba separation distance of 5.025 Å. These SBUs connect each other via the H3hmIDC− in μ3-kO:kO′:kO″ coordination mode, resulting in an infinite one-dimensional (1D) chain running along the a-axis. Moreover, each 1D chain links to four adjacent and parallel chains by both types of μ3H3hmIDC− ligand in two different orientations to generate a 3D framework (Figure 2c). The binuclear [Ba2(COO)2] SBU connected to 10 μ3H3hmIDC− ligands can be regarded as a 10-connected nodes, while the μ3-H3hmIDC− ligand can be regarded as a 3connected node; therefore, the overall framework is a binodal (3,10)-connected network from a topological view (Figure 2d and Figure S3 in the Supporting Information). The Schläfli

symbol of this framework is (3·4·5)2(34·46·518·614·72·8) as analyzed by TOPOS.13 Highly connected networks with connectivity larger than 10 are still rarely reported, and one successful strategy is to employ ploynuclear SBUs as connecting nodes.14 Up to now, the construction of highly connected network based on polynuclear Ba(II) SBU is still rare.15 Crystal Structure of 3. When using water/isopropanol or water/butanol as the solvents, complex 3 can be successfully obtained. The asymmetric unit contains one barium ion, one μ5-H2hmIDC2− dianion ligand, and a half of μ2-O2 molecule. Each Ba(II) ion is nine-coordinated and locates in a distorted [BaNO8] environment, which is completed by one imidazole nitrogen atom, one hydroxymethyl oxygen atom, and six carboxylate oxygen atoms from five unique μ5-H2hmIDC2− anions, and one oxygen atom from one μ2-O2 bridging ligand (Figure 3a). The Ba−N bond length is 2.873(4) Å, and those of Ba−O are in the range of 2.639(4)−3.138(4) Å. The bond angles around the central Ba(II) atom vary from 42.97(15) to 163.52(16)°. Similar to that in 1, the H2hmIDC2− ligand also displays a μ5-coordination mode (μ5-kN,O:kO′,O″:kO″,O′″:kO′″:kO″″) to bridge five Ba(II) ions in N,Ochelating, O′,O″-chelating, O″,O″-chelating, and monodentate modes (Scheme 2d). Because the coordination mode of H2hmIDC2− ligand resembles that in 1, a similar 1D chain composed of Ba(II) ions and μ3-η2:η2 carboxylate groups is also formed in complex 3 (Figure 3b,c). The additional coordination bonds between Ba(II) and oxygen atoms [the Ba···O distance is 3.142 Å in 3, while the corresponding oxygen atoms in 1 is not coordinated directly to Ba(II) ion with a Ba···O distance of 3.414 Å] make this chain more contractive than that in 1. Therefore, the adjacent nonbonding Ba···Ba distance (4.584 and 4.587 Å) is shorter than that in 1. Each 1D chain acts as a 1D SBU, which 3579



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is connected to four neighboring chains by the μ5-H3hmIDC− ligands and gives rise to a 3D framework structure with small 1D channel along the a direction (Figure 3d). The oxygen molecules are captured in these channels via the coordination bonds between Ba(II) ions and oxygen atoms. The coordinated oxygen molecules in the small 1D channels use μ2-kO:kO′ coordination mode to link two diagonal Ba(II) ions together with an O,O′-bridging fashion. The μ2-O2 molecule exhibits an O−O distance of 1.186(17) Å, only slightly smaller than the O−O distance in free O2. The Ba−O−O angle is equal to 172.64°, indicating that the oxygen molecule ligand and two Ba atoms are almost collinear. The reported μ-k1:k1-peroxo complexes always contain either cis-μ-η1:η1-peroxo or trans-μη1:η1-peroxo,16 whereas no linear-μ-η1:η1-peroxo has been documented. The coordination bond distance and coordination geometry of O−O group in compound 3 are quite different from the peroxo group. Therefore, O−O is considered as an oxygen molecule rather than a peroxo group. The topological analysis approach is also employed for complex 3. As shown in Figure S4 in the Supporting Information, this network contains two kinds of nodes: the barium ion can be considered as a 6-connected node, while the μ5-H3hmIDC− ligand can be considered as a 5-connected node, and the oxygen molecule as a linker. Thus, this network can be described as a (5,6)-connected 3D framework with the Schläfli symbol of (46·53·6)(46·54·65). Crystal Structure of 4. Complex 4 crystallizes in the orthorhombic form with space group Pna2(1) and exhibits a 3D framework. There are only one independent Ba(II) ion and two different coordination modes of H3hmIDC− liagnds in the asymmetrical unit. As presented in Figure 4a, the Ba(II) ion is 10-coordinated with one nitrogen atom and four carboxylate oxygen atoms from three different μ3-H3hmIDC− ligands, as well as one nitrogen, one hydroxymethyl oxygen atom, and three carboxylate oxygen atoms come from four individual μ4H3hmIDC− ligands. The Ba−N bond lengths are in the range of 2.949(6)−3.016(9) Å, and the Ba−O distances vary from 2.729(7) to 3.096(7) Å. The bond angles around each Ba(II) ion range from 43.04(19) to 154.3(2)°. The H3hmIDC− ligands in complex 4 show two kinds of coordination modes: one acts as μ3-bridge to link three Ba(II) ions in N,O-chelating, O′,O″-chelating, and O″-bridging (Scheme 2e), and the other adopts μ4-mode to bridge four Ba(II) ions in N,O-chelating, Obridging, and monodentate fashions (Scheme 2f). In complex 4, the bridging carboxylate oxygen atoms, O8 and O1 from the μ3-H3hmIDC− ligands and the μ4-H3hmIDC− ligands, respectively, link the two adjacent Ba(II) atoms into an 1D chain parallel to the c-axis, with a Ba···Ba separation of 4.779 Å (Figure 4b,c). Similar to that in complex 3, each 1D chain acts as a 1D SBU link to four adjacent 1D SBUs via the μ3-H3hmIDC− and μ4-H3hmIDC− ligands, generating a 3D framework structure (Figure 4d). We can consider each μ3-H3hmIDC− ligand as 3-connected node with vertex symbol of (42·6) and each μ4-H3hmIDC− ligand as a 4-connected node with vertex symbol of (43·63), as well as the Ba1 atom as 7-connected node (links to three μ3H3hmIDC− ligands and four μ3-H3hmIDC− ligands) with vertex symbols of (48·611·82). As a result, a trinodal 3D coordination framework with (3, 4, 7)-mixed connectivity is formed, as depicted in Figure S5 in the Supporting Information, which has the Schläfli symbol of (42·6) (43·63) (48·611·82). Role of the Solvents in Self-Assembly Process. The structural analysis of complexes 1−4 directly indicated that the

coordination number and geometry of Ba(II) ion (from 8- to 10-coordinated), as well as the coordination modes of ligands (from μ3 to μ5), may contribute to the structural diversity. However, the synthesis method revealed that the solvent plays an essential role in determining the network structures of such Ba−organic frameworks. As shown in Scheme 1, complexes 1− 3 were obtained from the different water−alcohol solvent systems (Scheme 1). The reaction in water/methanol or water/ n-propanol solvent produced complex 1. Complex 2 was prepared using the water/ethanol solvent, and complex 3 was formed in either water/isopropanol or water/n-butanol solvent. The alcohols used in the reaction systems were not incorporated into the final crystal lattices of all of the complexes. Instead, they acted as important reaction modulators, which may have greatly affected the coordination behavior of the water molecules in these complexes. Water molecules that acted as terminal monodentate ligands were coordinated to the Ba(II) ion in complex 2, whereas water molecules that served as bridging ligands tightly linked two neighboring Ba(II) ions in complex 1. Neither terminal solvent water ligands nor bridging solvent ligands were found in complex 3. Instead, the unprecedented oxygen molecules participated in the coordination using O,O′-bridging. The water molecules likely exhibit a more direct effect on the structure through various coordination modes in 1 and 2. Water was replaced with DMF solvent to eliminate the effects of the water molecule and determine the effect of alcohols in the final structure. However, crystals suitable for single X-ray diffraction (XRD) were obtained only when DMF/ethanol was used as solvent. The other alcohols (methanol, n-propanol, and isopropanol) did not lead to the formation of crystals suitable for single XRD. Powder XRD of the unknown products shows that using DMF/methanol easily results in inorganic salts, whereas using DMF/propanol, DMF/isopropanol, or DMF/nbutanol solvents will lead to similar unknown products (Figure S6−S9 in the Supporting Information). Because the alcohol solvent molecules are not observed in the final product, their effects cannot be explained based on the structural analysis. The polarity of the solvent systems that we used have no significant difference, and the final structures do not relied on the polarity of the solvent (MeOH and PrOH lead to complex 1, while EtOH that possesses polarity between MeOH and PrOH leads to complex 2), so the structural difference cannot be explained by polarity differences, either. Considering that the coordination water molecules have great influence on the final structures as discussed above, we may tentatively propose that the different alcohol molecules lead to different solvation of Ba(II) ions, induce different structural aggregates in the solution, and thus affect the final structures. However, the exact reasons for such solvent-induced behavior are still under exploration. X-ray Powder Diffraction and Thermal Analyses. To confirm whether the crystal structures are truly representative of the bulk materials, powder XRD experiments were carried out for complexes 1−4 at room temperature. As described in Figure S10−S13 in the Supporting Information, all of the peaks displayed in the measured patterns closely match those in the simulated patterns generated from single-crystal diffraction data, thus indicating that the bulk crystal samples are pure. The difference in reflection intensity between the simulated and the measured patterns may be attributed to a certain degree of preferred orientation of the powder samples during data collection. 3580



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To estimate their thermal stabilities of the 3D frameworks, thermogravimertric analyses (TGA) of all of the compounds have been measured in the range of 30−700 °C with a heating rate of 10 °C/min under an air atmosphere (Figure 5). For 1, it

Figure 6. Emission spectra of complexes 1−4 and ligand H4hmIDC in the solid state.

In conclusion, four novel 3D Ba(II)−organic frameworks with different structures and topologies from the assembly of H4hmIDC ligand and barium chloride were successfully constructed by regulating the choice of solvents under solvothermal conditions. Complex 1 was obtained from water/methanol or water/n-propanol solvent system, showing a 3D framework based on two-dimensional SBUs. Complex 2 was obtained from water/ethanol solvent system and showed a well-defined 3D, highly (3,10)-connected network. Complexes 3 and 4 were obtained from water/isopropanol (or water/nbutanol) and DMF/ethanol solvent system, respectively, giving rise to a 3D framework based on 1D SBUs with different topologies. The results revealed that the alcohol solvents play a subtle yet essential role in the crystallization and construction of these coordination frameworks with diverse 3D structures, although they are not involved in the final crystal lattices.

Figure 5. TGA curves of complexes 1−4.

is stable up to 110 °C and then reveals a weight loss of 5.6% from 110 to 290 °C for the removal of one coordinated water molecule (calculated, 5.3%) in the asymmetric unit. The major weight loss occurs in next step above 290 °C, which may be ascribed to the decomposition of the coordination framework. Seen from the TGA curve of 2, there is no obvious weight change before 100 °C, and then, it loses weight from 100 to 145 °C (observed, 2.3%; calculated, 3.4%), corresponding to the release of one coordinated water molecule. This compound is stable up to 190 °C, and the 3D framework starts collapsing after that temperature. The temperature for losing the coordinated water molecules in 1 is higher than that in 2 and may be due to the fact that μ2-H2O is more stable than μ1-H2O. TGA data show that 3 is stable up to about 380 °C, and the gradual weight loss of 4.6% before 380 °C may be attributable to the release of half an oxygen molecule (calcd, 4.7%) in the asymmetric unit. As for compound 4, no apparent weight lose can be observed below 230 °C, indicating that the framework of 4 is thermally stable up to 230 °C. When the temperature is higher than 230 °C, the organic ligand of 4 begins to decompose, and it keeps continuous weight loss up to 560 °C. Photoluminescent Properties. Luminescent properties of Ba(II) complexes are not well-studied as compared with those of transition or rare-earth metal complexes.15 The photoluminescent behaviors of complexes 1−4 as well as free ligand H4hmIDC are studied in the solid state at room temperature. As illustrated in Figure 6, the intense blue emission for complexes 1−4 and H4hmIDC can be observed, where the maximum emission wavelength at 461 nm (λex = 370 nm) for H4hmIDC, 414 nm (λex = 352 nm) for 1, 422 nm (λex = 355 nm) for 2, 413 nm (λex = 361 nm) for 3, and 449 nm (λex = 368 nm) for 4. Apparently, the emission spectra of all of the complexes 1−4 are similar to that of the ligand H4hmIDC in terms of the position and the band shape, indicating that the emission bands of 1−4 may be attributed to the emission of intraligand π−π* transition.9m As compared with the emission spectrum of H4hmIDC, slight blue shifts of 47, 39, 48, and 12 nm for 1−4 have been respectively given, which are considered to mainly arise from the different coordination modes of μ3H3hmIDC, μ4-H3hmIDC, and μ5-H2hmIDC (Scheme 2) and coordination environments around Ba(II) ions.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Additional structural figures for the related compounds, tables of selected bond lengths and angles, PXRD, and X-ray crystallographic files in CIF format for compounds 1−4. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author

*E-mail: [email protected] (S.-R.Z.). E-mail: wgzhang@ scnu.edu.cn (W.-G.Z.). Funding

This work was financially supported by the National Natural Science Foundation of P. R. China (grant nos. 21003053 and 21171059), the Natural Science Foundation of Guangdong Province (grant no. 10451063101004667), and Science and Technology Project of Guangdong Province (grant no. 2011B010400021). Notes

The authors declare no competing financial interest.

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Construction of Ba (II) Coordination Polymers Based on Imidazole

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