Tuning the Dimensionality of Inorganic Connectivity in Barium

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Tuning the Dimensionality of Inorganic Connectivity in Barium Coordination Polymers via Biphenyl Carboxylic Acid Ligands Maw Lin Foo,†,‡ Satoshi Horike,⊥,§ Jingui Duan,† Wenqian Chen,§ and Susumu Kitagawa*,†,‡,§ †

Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ERATO Kitagawa Integrated Pores Project, Kyoto Research Park Building #3, Shimogyo-ku, Kyoto 600-8815, Japan ⊥ Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡

S Supporting Information *

ABSTRACT: The synthesis and characterization of three barium coordination polymers with one-, two-, and three-dimensional (1-D, 2D, 3-D) inorganic connectivity based on biphenyl carboxylic acid ligands are described. Employing biphenyl-3,3′,5,5′-tetracarboxylic acid (H4BTTC) as a ligand, [Ba2(BTTC)(H2O)2]n (1, space group = Pn21a, a = 7.059(1) Å, b = 12.432(2) Å, c = 19.090(3) Å), a coordination polymer with 1-D inorganic connectivity (I1O2), can be synthesized. The coordinated water is strongly coordinated and removed at 270 °C. By using 4,4′-biphenyldicarboxylic acid (H2BPDC), [Ba(BPDC)]n (2, space group = C2/m, a = 6.955(2) Å, b = 5.947(1) Å, c = 13.852 (4) Å, β = 92.399(4)°) a coordination polymer with 2-D inorganic connectivity (I2O1) is obtained. The connection of the Ba−O bonds in each layer is topologically similar to CaF2. Using biphenyl-3,5,5′-tricarboxylic acid (H3BPTC) as a ligand, [Ba3(BPTC)2(NMF)5⊃2NMF]n (3, space group = I4̅2d, a = 25.984(3) Å, c = 13.999(2) Å) (NMF = Nmethyl formamide), a structurally porous coordination polymer with rare 3-D inorganic connectivity (I3O0) can be synthesized. Hence, barium as a metal is extremely malleable with respect to construction of coordination polymers of different inorganic dimensionalities. 2 with I2O1 connectivity demonstrates extraordinary thermal stability and maintains its crystallinity until decomposition at 590 °C. The luminescence behavior of 1, 2, and 3 at room temperature has been investigated and is predominantly intraligand based.

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INTRODUCTION Coordination polymers and their porous counterparts, porous coordination polymers (PCPs) or metal organic frameworks (MOFs) are crystalline solids that consist of metal ions/clusters connected to organic ligands via coordination bonds, forming discrete one-dimensional (1-D) chains, two-dimensional (2-D) layers, or three-dimensional (3-D) networks.1 Because of the wide variety of metals/clusters and organic ligands available, and the high degree of synthetic control possible due to the mild temperature of synthesis, this area has seen rapid growth, both for fundamental studies and possible applications such as gas storage/separations, catalysis, ion conductivity, and drug delivery.2 One way of classifying coordination polymers has been proposed by Rao and Cheetham,3 by considering their inorganic and organic connectivity, i.e., ImOn (m, n = 0, 1, 2, or 3). “I” refers to the dimensionality of the inorganic connectivity as embodied by metal−oxygen−metal (M−O− M) bonds, and “O” refers to the connectivity of the organic (ligand) component, i.e., M-ligand-M connectivity. Most porous coordination polymers (PCPs) or metal organic organic frameworks (MOFs) consist of discrete metal centers/clusters © XXXX American Chemical Society

linked by ligands. Thus, they have zero dimensionality for inorganic connectivity and three dimensionality for organic connectivity, i.e., I0O3 for [Zn4O(1,4-BDC)6]n (H2BDC = benzenedicarboxylic acid) or MOF-5. Although not frequently observed, PCPs with higher inorganic connectivity (m > 0) may be beneficial in obtaining enhanced physical properties such as thermal and chemical stability, magnetism, etc. due to strong interactions between the inorganic components. For example, nickel succinate,4 the first I3O0 framework synthesized, is thermally stable till 425 °C, a relatively high value for coordination polymers. The decrease of band gap with increasing size and dimensionality of the secondary building unit (SBU) for a series of Zn-BPDC (H2BPDC = 4,4′biphenyldicarboxylic acid) coordination polymers has been recently elucidated.5 Although barium coordination polymers have been studied for some time,6 before our work, this area has not been systematically explored. As barium has the largest ionic radii for Received: March 14, 2013 Revised: May 25, 2013

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X-ray diffraction was obtained using a Rigaku RINT powder diffractometer with Cu Kα anode or Rigaku Ultima IV powder diffractometer with Cu Kα anode. For heating experiments, 10 mg samples were heated under nitrogen flow with a thermogravimetric analyzer at the desired temperature for 30 min. Synthesis of [Ba2(BTTC)(H2O)2]n (1). In a 15 mL vial containing Ba(NO3)2 (52.2 mg) and biphenyl-3,3′,5,5′-tetracarboxylic acid (H4BTTC, 33.3 mg), 10 mL of 1:1 DMF/H2O solution was added to yield a white suspension. The vial was then sealed with a Teflonlined cap and heated at 140 °C for 48 h to obtain colorless blocks. The mixture was then filtered, washed with copious amounts of 1:1 DMF/ H2O, followed by DMF and 1 mL of CH2Cl2 to yield colorless blocks. Weight of product = 26.2 mg (yield 41% with respect to ligand). Elemental analysis calculated for [Ba 2 BTTC(H 2 O) 2 ] n or Ba2C16H10O10: C, 30.17; H, 1.58; N, 0. Found: C 30.04; H, 1.68; N, 0. Synthesis of [Ba(BPDC)]n (2). In a 4 mL vial containing barium nitrate (10.45 mg) and 4,4′-biphenyldicarboxylic acid (H2BPDC, 9.68 mg) was added 1.6 mL of DMF, 0.4 mL of DMSO, and 0.2 mL of H2O to yield a cloudy suspension. The vial was then sealed with a Teflon-lined cap and heated at 140 °C for 48 h to afford colorless plate-like crystals. The mixture was then filtered and washed with copious amounts of DMF and 0.5 mL of CH2Cl2. Weight of product = 9.2 mg (yield 61% with respect to ligand). Elemental analysis calculated for [BaBPDC]n or BaC14H8O4: C, 44.54; H, 2.13; N, 0, Found: C 44.55; H, 2.04; N, 0. Synthesis of [Ba3(BPTC)2(NMF)5⊃2NMF]n (3). In a 15 mL vial containing barium nitrate (98.2 mg) and biphenyl-3,5,5′-tricarboxylic acid (H3BPTC, 72.0 mg), 10 mL of 4:1 N-methylformamide:methanol solution was added to yield a slightly cloudy suspension. The vial was then sealed with a Teflon-lined cap and heated at 140 °C for 48 h to obtain colorless needles. The mixture was then filtered and washed with NMF and 1 mL of CH2Cl2. Weight of product = 0.131 mg (75% with respect to ligand). Elemental analysis calculated for [Ba3(BPTC)2(NMF)5⊃2NMF]n or C 37.97; H, 3.55; N, 7.04. Found: C, 38.21; H, 3.38; N, 6.55 Single-Crystal Structural Determination. Single crystal X-ray diffraction measurements were performed at 213 K with a Rigaku AFC10 diffractometer with Rigaku Saturn Kappa CCD system equipped with a MicroMax-007 HF/VariMax rotating-anode X-ray generator with confocal monochromated Mo Kα radiation. Data were processed using Crystal Clear TM-SM (Version 1.4.0). The crystal structure was solved by a direct method and refined by full matrix least-squares refinement using the SHELXL-97. The routine SQUEEZE was not employed in structural solution. The hydrogen atoms in the phenyl rings were added via a riding model. For 1, the hydrogens of the coordinated water molecules were located in the difference Fourier map and restrained using the DFIX and DANG command. For 2, the phenyl rings were disordered and refined using the PART command. For 3, the carbonyl oxygens belonging to two and a half NMF molecules were located. However, only the one-half NMF molecule could be completed and satisfactorily refined without any damping restraints. All atoms except hydrogen atoms were refined anisotropically. Crystal data, as well as details of data collection and refinements of 1−3, are summarized in Table 1.

a +2 metal (1.47 Å) and is strongly oxophilic, it will thus possess a high coordination number with carboxylic acid ligands. Thus, the resultant barium coordination polymers are anticipated to exhibit rich structural topologies with m > 0 and are thermally robust as well. By using suitable rigid aromatic carboxylic acids, porosity can be obtained: we have synthesized the first porous barium carboxylate PCP, [Ba(HBTB)(DMF)]n (H3BTB = 1,3,5-tris(4-carboxyphenyl)benzene) by using a bulky tritropic carboxylic acid as a ligand.7 This I1O2 PCP consists of 1-D barium oxide chains linked by BTB3‑ anions to form 1-D channels. Subsequently, [Ba2(TMA)(NO3)(DMF)]n (H3TMA = trimesic acid or 1,3,5-benzenetricarboxylic acid),8 a rare I3 O 0 PCP which possesses a three-dimensionally connected inorganic backbone, was synthesized using a smaller tritopic ligand consisting of a single benzene ring. This is probably due to the large size of the barium ion and the relative proximity of adjacent carboxylate groups in the TMA anion. Another polymorph of barium and trimesate anion which possess I3O0 connectivity has been recently reported, although it is nonporous.9 The next question is what happens when a tritopic ligand with a biphenyl backbone instead of a single phenyl ring is used? Will an I3O0 network still result? For barium carboxylate coordination polymers, ligands with a biphenyl backbone are still unexplored. Biphenyls are important building blocks in various natural products, liquid crystals, and many chiral ligands.10 With respect to crystal engineering, it is envisaged that the longer length of the biphenyl ligand, and the range of torsion angles available between the two phenyl rings, will lead to rich coordination chemistry. In this report, we synthesize barium coordination polymers with biphenyl ligands possessing different numbers of carboxylic functional groups, i.e., 4,4′-biphenyldicarboxylic acid (H2BPDC, two −COOH), biphenyl-3,5,5′-tricarboxylic acid (H3BPTC, three −COOH), and 3,3′,5,5′-tetracarboxylic acid (H4BTTC, four −COOH) to discover novel crystal structures with 1-D, 2-D, and 3-D inorganic connectivity. Scheme 1. Ligands Employed in This Study and the Resultant Inorganic/Organic Connectivity of the Frameworks

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RESULTS AND DISCUSSION

Synthesis. The barium coordination polymers were obtained via solvothermal synthesis with mixed amide solvents. It was experimentally observed that the presence of DMF (or NMF for 3) and another oxygen containing cosolvent, i.e., water, methanol, or DMSO was essential in obtaining diffraction quality single crystals. This is probably due to the strong affinity of barium, an alkaline earth metal for the oxygen atoms in the cosolvent. The relatively high temperature (140 °C) employed was also essential in overcoming the insolubility of the biphenyl ligands at room temperature. Crystal Structure of [Ba2(BTTC)(H2O)2]n (1) with 1-D Ba−O−Ba Chains. 1 crystallizes in a non-centrosymmetric

EXPERIMENTAL SECTION

Materials and Instrumentation. All reagents were purchased commercially and used without purification. All syntheses were carried out in glass vials with Teflon lined screw caps. Infrared spectroscopy was obtained using a Nicolet ID5 ATR operating at ambient temperature. Thermogravimetry analysis was obtained using a Rigaku TG8120 under flowing nitrogen with a 10 K min−1 ramp rate. Powder B

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comparison to other previously published compounds which feature the same coordination mode for every carboxylate: either monodentate chelating (μ1:η2) in [Me2NH2][In(BPTC)]n11 or bidentate bridging (μ2:η1:η1) in [Cu2(BTTC)(H2O)2(DMF)3(H2O)]n (MOF-505)12 and [Mg2(H2O)2(BTTC)]n (CPF-1).13 In 1, it is coordinated to 12 barium ions with both bridging and chelating modes. The carboxylate at C13 is mondentate bridging to two barium ions and monodentate chelating to one (μ3-η1: η2: η1). This is a common mode of coordination for barium coordination compounds and has been observed in previous barium coordination polymers8 and clusters.14 The carboxylate at C14 is monodentate bridging to three barium ions and bidentate chelating to one (μ4-η1: η1: η2: η1). This is an extremely rare coordination mode for metals (only 21 examples in the CCDC) but has been previously observed in a discrete barium cluster.15 The carboxylate at C15 is bidentate chelating to one barium ion (μ1:η2), and the carboxylate at C16 is monodentate bridging to four barium ions (μ4-η1: η1: η1: η1) (Scheme 1). This last mode of coordination has been previously observed in a discrete barium cluster16 and is typical for discrete silver clusters.17 Ba1 is 8-coordinated (Ba−O bond distance 2.671(3)−2.966(3) Å), coordinating to six different BTTC ligands and one monodentate water molecule, and Ba2 is 9-coordinated (Ba−O bond distance 2.761(3)−3.002(3) Å) coordinating to six different BTTC ligands and one monodentate water molecule. The Ba2O9 polyhedra forms 1D zig-zagged edge-sharing chains (Ba2−Ba2 distance: 4.396(0) Å) along the a-axis, which is further connected by face-sharing and edge-sharing with Ba1 polyhedra (Ba1−Ba2 distance: 4.289(0), 4.265(0) Å) (Figure 1c). The BTTC ligands are located along the bc plane, and each ligand binds to four different 1-D Ba−O chains along the a-axis. The closest distance of approach between the benzene rings of adjacent layers is ∼3.5 Å, suggestive of π−π interactions. The structure is dense (no solvent accessible volume calculated by PLATON), and thus it is concluded that the noncarboxylate oxygens coordinated to Ba1 and Ba2 belong to water, not DMF from the solvent used for synthesis, which is further substantiated by the absence of nitrogen in elemental analysis . This is further corroborated by a strong sharp peak at 3558 cm−1 in the IR, which suggests tightly coordinated water in a hydrophobic environment. From bond distances (Table 2), hydrogen bonding can be detected between water molecules (O1W−O2W) and from water molecules to carboxylate oxygens (O1W−O5, O2W−O6, O2W−O7). Crystal Structure of [Ba(BPDC)]n (2) with 2D Ba−O−Ba Layers. 2 crystallizes in a C2/m (7) space group with an asymmetric unit consisting of 0.25 Ba and a half benzene ring and a half carboxylate group. The benzene rings are approximately equally disordered over two positions (0.49:0.51) with a dihedral angle of 37.7°. Ba, C1, C2, and C5 are on a mirror plane and in addition, a 2-fold axis passes through Ba. An inversion center is located at the midpoint on the bond connecting the two phenyl rings of the ligand. Each carboxylate group in the fully deprotonated BPDC2− ligand binds two barium ions in a monodentate bridging and one barium ion in a monodentate chelating mode (μ3-η1: η2: η1) (Figure 2a), similar to [Ba2TMA(NO3)(DMF)]n. Ba is 8coordinated, connected to six different BPDC ligands with two unique Ba−O distances: 2.689(2) and 2.894(1) Å. If one considers the BaO8 polyhedra, a distorted square antiprismatic structure is obtained. Each polyhedra is linked to four others by

Table 1. Summary of the Crystal Data and the Structure Refinements for 1, 2, and 3 compound

1

2

3

empirical formula crystal system space group a (Å) b (Å) c (Å) V (Å3) α (°) β (°) γ (°) Z Dcal (g/cm3) μ (mm−1) reflections collected reflections unique Rint restraints/ parameters GOF on F2 R1 indices [I > 2σ(I)] R1 indices (all data) wR2 (all data) Flack parameter

Ba2C16H10O10 orthorhombic Pn21a (No. 33) 7.0587(12) 12.4321(23) 19.0901(35) 1675.24(52) 90 90 90 4 2.51 4.73 12505

Ba0.5C7H4O2 monoclinic C2/m (No. 7) 6.9551(19) 5.9467(15) 13.8525(37) 572.44(26) 90 92.399(4) 90 4 2.19 3.47 3195

Ba1.5C19H18.5N1.5O8.5 tetragonal I4̅2d (No. 122) 25.9843(28) 25.9843(28) 13.9986(15) 9451.63(76) 90 90 90 16 1.55 2.53 35034

3029

709

4890

0.0222 7/265

0.0250 0/69

0.0260 21/258

1.133 0.0156

1.138 0.0129

1.244 0.0396

0.0162

0.0129

0.0440

0.0414 0.04(2)

0.0345 NA

0.1483 0.03(4)

Scheme 2. Coordination Modes of Carboxylate Ligands: (a, b, c, d) for 1, (d) for 2, (d, e) for 3. (b) and (c) Represent Uncommon Modes of Coordination for Barium

Pn21a (33) space group, which has been previously observed for other frameworks such as [Ba(HBTB)(DMF)]n7 and [Ba3(BTC)2(H2O)4]n.9 The asymmetric unit of 1 consist of two crystallographically unique barium ions, one fully deprotonated BTTC4− ligand, and two oxygens from water molecules. The BTTC ligand is almost planar with a dihedral angle of 12.8° between the two benzene rings. The coordination modes of BTTC in this case is very rich in C

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Figure 1. (a) Coordination environment of barium in 1. Ba1 is denoted by lavender spheres and Ba2 by brown spheres. The ligand has been truncated and hydrogen atoms omitted for clarity. (b) Crystal structure along the a-axis with an emphasis on the Ba1O8 (magenta) and Ba2O9 (cyan) polyhedra. (c) View along the b-axis showing the 1-D Ba−O chain consisting of edge sharing Ba2O9 polyhedra and face/edge sharing Ba1O8 polyhedra.

Table 2. Hydrogen Bonding Data for 1 D−H···A

angle of D−H···A (°)

d(D···A) (Å)

O1W−H15···O5 O1W−H16···O2W O2W−H13···O6 O2W−H14···O7

169 169 158 146

2.708(5) 3.076(5) 2.765(5) 2.865(5)

symmetry transformation −x −x −x −x

+ + + +

1/2, y + 1/2, z + 1/2 1, y + 1/2, −z + 1 3/2, y − 1/2, z + 1/2 2, y − 1/2, −z + 1

Figure 2. (a) Coordination environment of 2 showing the coordination environment of barium. For simplicity, hydrogens are omitted and only one set of carbon atoms for the disordered benzene rings are shown. (b) Layered structure of 2 with emphasis on the BaO8 polyhedra (magenta). (c) View of the edge-sharing BaO8 polyhedra along the ab plane, showing similarity to (d) CaF2.

fluorite topology for 3-D coordination polymers has been previously reported.21 Crystal Structure of [Ba3(BPTC)2(NMF)5⊃2NMF]n (3) with 3D Ba−O−Ba Network. 3 crystallizes in the noncentrosymmetric tetragonal I4̅2d (122) space group. It is noted that only crystals can be obtained with N-methylformamide (NMF) as the solvent22 and not with other more commonly used amide solvents such as N,N-dimethylformamide (DMF) or N,N-diethylformamide (DEF). This is probably due to its smaller molecular volume and more importantly the ability of the amide hydrogen in NMF to form hydrogen bonds (vide

sharing edges along the c-axis, to form Ba−O layers (Figure 2b,c). Formation of a layered structure (i.e., I2O1 structure) with heavy metal ions such as Pb2+, Eu2+, and Ba2+ using a similar, but shorter 1,4-benzenedicarboxylate ligand has been previously observed.18−20 However, the connections of these M-O polyhedra are more complicated with a mixture of face and edge-sharing compared to 2, which has only edge-shared polyhedra. The edge-shared polyhedra are topologically similar to fluorite, CaF2 (Figure 2d). This fashion of sharing polyhedra has not been previously reported, although it is noted that the D

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in the pores; thus the empirical formula of 3 is [Ba1.5(BPTC)(NMF)2.5⊃NMF]n. The BPTC ligand is coordinated to seven barium ions: the carboxylates at C14 and C15 are both monondentate bridging to one and bidentate chelating to another barium ion (μ2η1:η2); this coordination mode has been observed for [Mg3(BPTC)2(H2O)4]n.23 The carboxylate at C1 is monodentate bridging to two barium metals and bidentate chelating to one barium ion (μ3-η1:η2:η1). Ba1 has an occupancy of 1, and coordination of 11 (Ba−O bond distances (2.678(8)-3.096(23) Å), binding to five different carboxylates from five different BPTC ligands, μ2-O1 from coordinated NMF and two μ2-O9 (O9, O9(iii)) from coordinated NMF. The bonds to O1 and O9 are relatively long: 3.054(18) Å and 3.078(16) Å respectively (Ba−O bond length upper quartile = 2.923 Å from Cambridge Crystallographic Database). Ba2 has an occupancy of 0.5 with a coordination of 8 (2.687(5)− 2.849(19) Å), binding to four different carboxylates from four different BPTC ligands and two O1 from μ2-coordinated NMF and two O8 from μ1-coordinated NMF. If one considers only the barium ions and examines closest contacts, Ba1 forms nonplanar squares (4.421(0) Å) along the ab plane, and each corner of the square is connected to another square via Ba2 (Ba1−Ba2 4.319(0) Å) to form nonplanar bowties (Figure 4a). Each bow-tie is rotated 90 degrees with respect to the next. Next, if the BPTC3− ligands are added, the previous geometry is essentially preserved as the ligands are located along the c-axis. Two types of 1-D channels along the c-axis are observed in the crystal structure. The first consist of square hydrophilic channels (∼5.7 Å) defined by four edge-sharing Ba1O9 polyhedra (from the square barium motif) with oxygens lining the channel. Strong hydrogen bonding exists between the N−H of the NMF molecule and O4 with a distance of 2.254(9) Å and angle of 142°. This is reminiscence of the templating effect of amines in inorganic open-framework materials.24 The second, larger hydrophobic channels (∼11.4 × 12.8 Å) surrounding the square channels are defined by face-sharing

infra). The asymmetric unit consists of 1.5 crystallographically unique barium ions and one fully deprotonated BPTC3− ligand with a dihedral angle of 33.3° between the benzene rings (Figure 3). NMF with two different coordination modes are

Figure 3. Coordination environment of Ba1 (lavender) and Ba2 (brown) in 3. The long (>3 Å) Ba−O bonds are indicated in dashed lines. Hydrogen atoms are omitted for clarity, and the ligand has been truncated for clarity.

found: μ1-coordinated NMF and μ2NMF. One μ2-coordinated NMF molecule with an occupation number of 0.5 can be located near a 2-fold rotation axis, and the whole molecule can be completed and satisfactorily refined. However, for O1 and O8, which corresponds to μ2 and μ1-coordinated NMF, the nonoxygen portions can only be refined satisfactorily with the presence of damping restraints during refinement. Thus, only the oxygens (O1 and O8) are considered in the final model. From TGA and elemental analysis, there is free NMF present

Figure 4. (a) Simplified crystal structure of 3 showing only Ba1 and Ba2 and their nearest neighbor contacts. (b) Complete crystal structure of 3 along the c-axis with emphasis on the Ba1O9 (magenta) and Ba2O8 (cyan) polyhedra. Note the 2-fold disordered NMF molecule occupying the square channels. The bonds from Ba to the carbonyl oxygen of the NMF solvent have been omitted for clarity. Inset: view of one square channel showing the hydrogen bonding between the amide hydrogen and O4. (c) View of 3 along the b-axis. Hydrogen atoms from aromatic rings have been omitted for clarity. E

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between the Ba1O9 and Ba2O8 polyhedra with BPTC ligands lining the pore channel. Because of the presence of these hydrophobic channels, the structure contains solvent accessible voids which are occupied by NMF molecules. When the connection of the Ba−O polyhedra is considered, and looking at two orthogonal views (Figure 4b,c), it is obvious that the inorganic Ba−O polyhedra are linked in all three dimensions; thus this is another example of a rare I3O0 barium coordination polymer with solvent accessible voids in addition to [Ba2TMA(NO3)(DMF)]n. However, repeated attempts to solvent exchange and evacuate the sample and obtain permanent porosity as evidenced by gas adsorption were not successful. This is probably because Ba2 is coordinated to four solvent molecules, and thus the framework is not stable to their removal. The topological representation of 3 can be obtained by considering the connections of the Ba ions since for this I3O0 framework, the ligands only decorate the net and no new connectivity is created. As Ba2 is two-connected, it is disregarded. The Ba1 connections can be simplified by considering the centroid of the Ba1 square channels as a four-connected node. If the nodes are connected, one obtains a dia structure (Figure 5).

Figure 6. Thermogravimetric analysis of 1, 2, and 3 in flowing nitrogen.

of the framework is preserved at 280 °C, upon heating the framework higher to 315 °C, the framework collapses as evidenced by a marked decrease in crystallinity of the powder XRD (Supporting Information S7). This indicates that although 1 decomposes at 525 °C from TGA, the crystallinity of the compound is not retained upon loss of coordinated water. For 2, no weight loss was observed until 590 °C, upon which a sharp drop corresponding to decomposition occurs. This extremely high decomposition temperature is unprecedented for a coordination polymer where the decomposition of the framework typically occurs 500 °C in nitrogen. Although 1 with a 1-D inorganic connectivity has the lowest decomposition temperature, for 2 and 3 there appears to be no straightforward correlation between decomposition temperature and dimensionality of inorganic connectivity. Indeed, 2 with 2-D inorganic connectivity had the highest thermal stability. This is probably due to 2 being a dense structure with no coordinated solvents and π−π stacking of the biphenyl ligands. From powder XRD of calcined samples (under nitrogen), we show that the crystal structure of 2 is retained even close to its decomposition temperature at 590 °C. Thus, it has the highest thermal stability among the three barium frameworks. However, it is not the case for 1 and 3; the crystal structure is not preserved upon loss of solvents. These results suggest that the temperature of decomposition as evidenced by TGA may not be an absolute proof of thermal stability, and any assertions should be substantiated by supplementary structural methods such as powder XRD. The luminescence of 1, 2, and 3 at room temperature is predominantly intraligand. We have also confirmed that it remains possible to obtain an I3O0 coordination polymer that possesses solvent-accessible void volume by using a longer tritopic biphenyl carboxylate to obtain 3. The templating effect of NMF on the formation of frameworks is observed directly for the first time, as demonstrated by 3.

steady weight drop of 8.2% till 405 °C (theoretical 8.5%) corresponding to the loss of another coordinated NMF per formula unit, and decomposition of the framework results at 570 °C. Separate heating experiments in nitrogen (Supporting Information S8) show that 3 is unstable with respect to loss of solvent and undergoes structural transformation to an unknown phase from 150 °C. Thus, even though 3 possess a high decomposition temperature as evidenced from thermal gravimetric analysis, the original crystal structure had been lost at a much lower temperature.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 8. Luminescence spectra of 1 (λex = 330 nm), 2 (λex = 332 nm), 3 (λex = 325 nm).

Crystallographic data in CIF format, CCDC reference numbers 928398−928400. Table of selected bond lengths, simulated and experimental powder X-ray diffraction patterns, and IR spectra of 1, 2, and 3 are presented. This material is available free of charge via the Internet at http://pubs.acs.org.

The luminescent properties of 1, 2, and 3 and their respective ligands in solid state were examined at room temperature (Figure 7). For the ligands, their luminescent behavior (Supporting Information S9a−c) is attributed to (n or π) → π* transitions with broad luminescence peaks centered at 372 (H4BTTC), 395 (H2BPDC), and 389 nm (H3BPTC). The wavelength of the main emission peak of 1, 2, and 3 is similar to that of the corresponding free ligand, suggesting that charge transfer from metal or ligand is not significant, and the luminescence exhibited is predominantly intraligand. For 1 and 3, the main emission peaks are blue-shifted to 356 and 389 nm, respectively, suggesting that in these two coordination polymers, there is a decrease in interligand π−π interactions compared to the free ligand.25 For 2, the intensity of the emission is significantly attenuated compared to its free ligand, in contrast to 1 and 3. This indicates that the rigidity of the BPDC ligand is less in the coordination polymer 2. In addition, a new secondary peak is observed at 448 nm for 1; this could be due to emission from barium as reported for [Ba(BDCD)(H2O)2]n (H2BDCD = 2,2′-bipyridine-3,3′-dicarboxylic acid1,10-dioxide).26

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Science and Technology Agency ERATO program and Grants-in-Aid for Scientific Research, the Japan Society for the Promotion of Science (JSPS), and the Japan Science and Technology Agency PRESTO program. The iCeMS is supported by the World Premier International Research Initiative (WPI), MEXT (Japan). We thank Dr. Charlotte Bonneau for topological calculations.

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CONCLUSION By using different biphenyl carboxylic acids as ligands and barium as the coordinating metal, we have obtained coordination polymers with different dimensionalities (1-D, 2D, and 3-D) in inorganic connectivity. This suggests that due to its large size and rich coordination modes, barium is a structurally malleable metal with respect to constructing frameworks. Structurally, barium carboxylates tend to form non-centrosymmetric compounds consisting of two crystallographically different bariums with nearest barium distances of