pH Influence on the Structural Variations of 4,4â²-Oxydiphthalate

Article pubs.acs.org/crystal

pH Influence on the Structural Variations of 4,4′-Oxydiphthalate Coordination Polymers Jin-Xia Yang,†,‡ Xin Zhang,† Jian-Kai Cheng,† Jian Zhang,† and Yuan-Gen Yao*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Five new Zn(II)/ODPT/bpe compounds, namely, {[Zn(bpe)(H2O)2](H2ODPT)·3H2O}n (1), {[Zn4(ODPT)2(bpe)3(H2O)2]2(bpe)·7H2O}n (2), {[Zn2(ODPT)(bpe)2]2· 7H2O}n (3), {Zn4(ODPT)2(bpe)}n (4), and {Zn2(ODPT)(bpe)(H2O)2}n (5) (ODPT = 4,4′-oxidiphthalate, bpe =1,2bis(4-pyridyl)ethane), have been synthesized through a hydrothermal method under different pH conditions, and characterized by single-crystal X-ray diffraction, element analysis, infrared spectra (IR), and thermogravimetric (TG) analyses. Compound 1 shows a 3D network framework constructed by the 1D linear [Zn(bpe)(H2O)4]n2+ cationic chains through extensive hydrogenbonding. Compound 2 displays an unusual three-dimensional (3D) meso-racemic self-penetrating coordination network with distinct chiral information in the interpenetrating networks. Compound 3 features a novel 4-connected (4·62·83)(42·62·82) topology, also exhibiting an intriguing 3D self-penetrating structure formed by triple- and double-stranded helical chain motifs. Compounds 4 and 5 are based upon 3D pillared-layer frameworks constructed from Zn2+ and ODPT4‑, and further consolidated by the bpe ligands as molecular pillars. The most striking feature of 5 is that achiral ODPT4− ligands link the Zn cations into right-handed 21 helical chains, and the chiral information is transferred to the 3D framework with a chiral space group C2221. The diversity of the product structures illustrates the marked sensitivity of the coordination chemistry of the V-shaped multicarboxylate ligand (H4ODPT) to the pH value of the solution. Moreover, the thermal dynamic properties and fluorescent properties of all compounds are also investigated.

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INTRODUCTION In the past decade, great progress has been achieved in understanding the chemistry of metal−organic frameworks (MOFs), as they possess intriguing architectures and valuable properties, such as magnetism, luminescence, gas storage, heterogeneous catalysis, etc.1 Among the reported studies about the construction of such complexes, aromatic multicarboxylate ligands have been extensively employed due to their versatile coordination conformations and strong coordination ability.2 Recently, a significant number of efforts have been contributed to the use of V-shaped organic aromatic multicarboxylate species, such as 4,4′-oxidibenzoic acid,3 3-(4-carboxyphenoxyl)phthalic acid, 4 4,4′-(hexafluoroisopropylidene)diphthalic acid,5 and 3,3′,4,4′-benzophenonetetracarboxylate acid,6 that enable introduction of peculiar topological features into the solid-state products and lead to interesting molecular structures including helices and interpenetrating and self-penetrating networks as well as potential applications in the fields of separation, absorption, catalysts, and sensors. Although the hydrothermal technique has been extensively used to create novel structures, from the crystal engineering point of view, the control of self-assembly processes is still a very challenging work for realizing the target syntheses of the © 2011 American Chemical Society

multidimensional MOFs based on V-shaped polycarboxylate ligands. Many factors, such as the coordination geometry of metal ions, medium, template, metal−ligand ratio, pH value, and counterion, can significantly affect the formation of coordination frameworks.7 The pH value of the reaction, as one of the external stimuli, is especially important in the assembly of such MOFs. It affects not only the coordination ability of the polycarboxylate ligand but also the related connection modes and, consequently, the resulting structures.5a,6b,8 Encouraged by the previous studies, we chose the 4,4′oxidiphthalic acid (H4ODPT) as the bridging ligand, which has eight potential coordination sites, as an example to show the role of the pH value of the reaction in controlling the structure of the supramolecular architecture. Although some quite fascinating 3D structures with a series of multiform helical chains have been documented with H4ODPT,9 a systematic investigation of the effect of the pH value of the reaction condition on the construction of such coordination frameworks has not been carried out. Understanding of the effect may assist the rational design of diversely Received: August 31, 2011 Revised: October 30, 2011 Published: November 14, 2011 333

dx.doi.org/10.1021/cg201143f | Cryst. Growth Des. 2012, 12, 333−345

Crystal Growth & Design

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46.72; H, 2.12; N, 2.48. Found: C, 46.76; H, 2.06; N, 2.43. IR (cm−1): 3427 (m), 3054 (w), 1575 (s), 1538 (s), 1496 (m), 1416 (s), 1253 (m), 1227 (s), 1152 (w), 1069 (w), 1038(w), 996 (w), 874 (w), 845 (w), 826 (m), 722 (w). Luminescent spectrum (nm): λex, 331; λem, 373. {Zn2(ODPT)(bpe)(H2O)2}n (5). The same synthetic procedure as that for 1 was used except that pH was adjusted to 6.0 by addition of 1.8 mL of 0.1 mol·L−1 NaOH, giving colorless block crystals of 5 in a 61.6% yield based on Zn(OAc) 2 ·2H 2 O. Anal. Calcd for C28H22N2O11Zn2 (693.26): C, 48.47; H, 3.17; N, 4.04. Found: C, 48.43; H, 3.21; N, 4.07. IR (cm−1): 3348 (m), 3080 (w), 1615 (s), 1558 (m), 1506 (w), 1427 (m), 1395 (s), 1373 (m), 1254 (m), 1224 (m), 1154 (w), 1071 (w), 1025 (w), 970 (w), 825 (m), 657 (w). Luminescent spectrum (nm): λex, 332; λem, 430. Single-Crystal Structure Determination. Suitable single crystals of compounds 1−5 and were carefully selected under an optical microscope and glued to thin glass fibers. Single-crystal X-ray diffraction data for compounds 1−5 were recorded on an Oxford Xcalibur E diffractometer (Mo Kα radiation, k = 0.71073, graphite monochromator) at 293(2) K. All diffractometers were equipped with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). Empirical absorption corrections were applied to the data using the SADABS program.10 The structures were solved by the direct method and refined by full-matrix least-squares on F2 using the SHELXTL-97 program.11 All non-hydrogen atoms were refined anisotropically, the hydrogen atoms bound to carbon were located by geometrical calculations, and their positions and thermal parameters were fixed during the structure refinement. All calculations were performed with the SHELXL-97 package. Pertinent crystallographic data and structural refinements for 1−5 are listed in Table 1. The selected bond distances of these compounds are listed in Tables S1−S5 of the Supporting Information.

connected MOFs. We also notice that the introduction of N-containing auxiliary ligands 1,2-bis(4-pyridyl)ethane (=bpe) into the metal−ODPT system may lead to the structural evolution of these metal−organic compounds. Fortunately, this has been achieved for five compounds by a simultaneous use of H4ODPT and auxiliary ligands having bridging ability, namely, {[Zn(bpe)(H2O)2](H2ODPT)(H2O)2}n (1), {[Zn4(ODPT)2(bpe)3(H2O)2](bpe)0.5(H2O)5}n (2), {[Zn2(ODPT)(bpe)2](H2O)7}n (3), {Zn2(ODPT) (bpe)0.5}n (4), and {Zn2(ODPT)(bpe)(H2O)2}n (5). Their structures range from a 1D chain to a 3D network, revealing the pH value of the reaction plays a key role in structural control during self-assembly processes.

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EXPERIMENTAL SECTION

Materials and Instrumentation. All reagents were purchased commercially and used without further purification. All syntheses were carried out in 23 mL polytetrafluoroethylene-lined stainless steel containers under autogenous pressure. Elemental analyses were performed on an EA1110 CHNS-0 CE elemental analyzer. FT-IR spectra were measured as KBr pellets on a Nicolet Magna 750 FT-IR spectrometer in the range 400−4000 cm−1. All powder X-ray diffraction (PXRD) analyses were recorded on a Rigaku Dmax2500 diffractometer with Cu Kα radiation (λ = 1.54056 Å) with a step size of 0.05°. The fluorescence spectra were measured on polycrystalline or powder samples at room temperature using an Edinburgh FLS920 TCSPC fluorescence spectrophotometer. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer with a heating rate of 10 °C/min under an air atmosphere. Preparation of 1−5. Syntheses of compounds 1−5. Compounds 1−5 are isolated by the same reaction method, with pH different from that of the starting materials. A mixture that contains all starting materials was stirred for 30 min and then sealed in a 25 mL Teflonlined stainless steel container. The container was heated to 120 °C and held at that temperature for 60 h, then cooled to 100 °C at a rate of 5 °C·h−1, and held for 8 h, followed by further cooling to 30 °C at a rate of 3 °C·h−1. And then the title compounds were isolated. {[Zn(bpe)(H2O)2](H2ODPT)(H2O)3}n (1). The starting materials were 4,4′-oxidiphthalic anhydride (ODPTA) (0.062 g, 0.20 mmol), bpe (0.037 g, 0.20 mmol), Zn(OAc)2·2H2O (0.044 g, 0.20 mmol), H2O (10 mL). Then, the pH was adjusted to 3.0 by addition of 0.2 mL of 0.1 mol·L−1 NaOH. Colorless crystals of 1 were collected in 60.5% yield based on Zn(OAc)2·2H2O. Anal. Calcd for C28H34N2O16Zn (719.96): C, 46.71; H, 4.72; N, 3.89. Found: C, 46.25; H, 4.36; N, 3.97. IR (cm−1): 3412 (m), 1614 (s), 1593 (s), 1558 (s), 1506 (m), 1432 (m), 1365 (s), 1263 (m), 1224 (s), 1071 (w), 968 (m), 846 (m), 775 (w), 846 (w), 653 (w). Luminescent spectrum (nm): λex, 341; λem, 385. {[Zn4(ODPT)2(bpe)3(H2O)2]2(bpe)(H2O)7}n (2). The same synthetic procedure as that for 1 was used except that pH was adjusted to 4.0 by addition of 0.8 mL of 0.1 mol·L−1 NaOH, giving colorless sheet X-rayquality crystals of 2 in a 30.2% yield based on Zn(OAc)2·2H2O. Anal. Calcd for C148H128N14O47Zn8 (3379.67): C, 52.54; H, 3.79; N, 5.80. Found: C, 52.52; H, 3.82; N, 5.77. IR (cm−1): 3413 (m), 1621 (s), 1595 (s), 1507 (w), 1434 (m), 1379 (s), 1259 (m), 1227 (s), 1148 (w), 1070 (w), 1032 (w), 969 (w), 803 (m), 718 (w). Luminescent spectrum (nm): λex, 332; λem, 430. {[Zn2(ODPT)(bpe)2]2(H2O)7}n (3). The same synthetic procedure as that for 1 was used, except that pH was adjusted to 4.5 by addition of 1.0 mL 0.1 mol·L−1 NaOH, giving colorless block crystals of 3 in a 75.8% yield based on Zn(OAc)2·2H2O. Anal. Calcd for C80H74N8O27Zn4 (1840.92): C, 52.17; H, 4.02; N, 6.08. Found: C, 52.20; H, 3.96; N, 6.11. IR (cm−1): 3434 (m), 1620 (s), 1587 (s), 1506 (w), 1431 (m), 1384 (s), 1262 (m), 1228 (s), 1148 (w), 1070 (w), 1032 (w), 964 (w), 826 (m), 653 (w). Luminescent spectrum (nm): λex, 303; λem, 350. {Zn4(ODPT)2(bpe)}n (4). The same synthetic procedure as that for 1 was used except that pH was adjusted to 5.5 by addition of 1.5 mL of 0.1 mol·L−1 NaOH, giving colorless block crystals of 4 in a 53.2% yield based on Zn(OAc)2·2H2O. Anal. Calcd for C22H12NO9Zn2 (565.11): C,

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RESULTS AND DISCUSSION Syntheses. Hydrothermal methods were employed in this work for the syntheses of the title compouds. These methods can minimize the problems associated with ligand solubility and enhance the reactivity of reactants in favor of efficient molecular building during the crystallization process. There are a variety of hydrothermal parameters, such as reaction time, temperature, pH value, template, and molar ratio of reactants, and small changes in one or more of the parameters can have a profound influence on the final reaction outcome.12 By comparison of numerous parallel experiment results, it is confirmed that the pH value is a crucial factor in this synthesis system, as is the case for some other metal polycarboxylate coordination polymers reported.13 Therefore, our synthetic strategy for the Zn(II)−ODPT−bpe coordination polymers is schematically depicted in Scheme 1. We first carried out the reaction at a pH value of 3.0, and compound 1, with 1D polymeric chains, was obtained because of partial deprotonation of the organic acid. Since the high pH value will favor the deprotonation of the H4ODPT, we changed the pH value of 3.0 in 1 to 4.0 or more in an attempt to assemble the 1D chain into a 2D or 3D network. The successful isolation of 2 when the pH value was raised to 4.0 prompted us to prepare other Zn(II) complexes of ODPT and bpe through altering reaction-influencing factors. When we raised the pH value to 4.5, compound 3 was obtained. Both compounds 2 and 3 are an unusual 3D selfpenetrating network. If the pH value was raised to 5.5 and 6.0, compounds 4 and 5, with pillared-layer 3D structure, were isolated, respectively. It is noteworthy that directed reaction of ODPTA and Zn(OAc)2 under the same conditions generated unidentified white powder products without auxiliary N-donor ligand bpe, which suggests that bpe plays an important role in the formation of these five complexes.9b These observations are a clear indication that the formation of these five 334



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Table 1. Summary of Crystal Data and Structure Refinements for 1−5

a

compd

1

2

3

4

5

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (mg m−3) F (000) reflcn collcd/unique GOF on F2 R1, wR2 [I > 2σ(I)]a R1, wR2 (all data)b

C28H32Zn2N2O14 719.96 monoclinic C2/c 18.883(3) 14.4900(8) 14.237(4) 90 125.30(3) 90 3179.2(1) 4 1.504 1496.0 6498/2800 0.915 0.0393, 0.1098 0.0515, 0.1143

C148H130Zn8N14O47 3379.67 triclinic P i̅ 13.8606(4) 16.9142(5) 18.1588(5) 112.895(3) 96.840(2) 101.840(2) 3743.76(2) 1 1.493 1718.0 27355/13158 1.038 0.0604, 0.1857 0.0815, 0.1960

C80H74Zn4N8O27 1840.92 monoclinic C2/c 21.0705(5) 18.4633(5) 12.0995(3) 90 103.461(3) 90 3704(2) 2 1.325 1864 8531/4014 1.104 0.0505, 0.1526 0.0654, 0.1570

C22H12Zn2NO9 565.11 monoclinic C2/c 27.8969(13) 7.5757(2) 21.3408(9) 90 118.087(6) 90 3979.0(3) 8 1.887 2264 7511/3479 0.884 0.0338, 0.0992 0.0494, 0.1037

C28H22Zn2N2O11 693.26 orthorhombic C2221 9.5741(8) 11.4403(14) 25.674(3) 90 90 90 2812.0(5) 4 1.637 1408 2996/2180 1.192 0.0562, 0.1348 0.0611, 0.1366

R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

bridged by these hydrogen-bonding interactions to form a singlestranded meso helix with a pitch of 15.765(8) Å (Figure 1d). As known by us, such a hydrogen-bonded single-stranded meso helix is the first and only example within helical coordination polymers. {[Zn4(ODPT)2(bpe)3(H2O)2](bpe)0.5(H2O)5}n (2). Compound 2, obtained through tuning the pH to 4.0, features a rare 3D selfpenetrating network. Single-crystal X-ray structural analysis reveals that the asymmetric unit of 2 contains four crystallographically independent Zn(II) atoms, two ODPT4‑ anions, three bpe ligands, two coordinated water molecules, five free water molecules, and half a guest bpe. All Zn(II) cations possess typical four-coordinated, distorted tetrahedral geometries (Figure 2). Zn1 and Zn2 centers have similar coordination environments, both coordinated by two carboxylate oxygen atoms from two different ODPT4− anions and two nitrogen atoms from two independent bpe ligands. Both Zn3 and Zn4 are defined by two different carboxylate oxygen atoms, one nitrogen atom, and one coordinated water molecule. The two kinds of ODPT4− ligands (ODPT1, ODPT2) all adopt a μ4-bridging fashion with each of the four carboxylate groups in a μ1-η1:η0 monodentate mode (Scheme 2a). In compound 2, Zn atoms are bridged by the V-shaped phthalic groups of ODPT4− anions to form the left- and right-handed helical chains with a pitch of 8.789 Å (Figure 3c). These adjacent single-handed helical chains are further interconnected by ether O atoms of ODPT4− anions into a 2D chiral layer (Figure 3b). Interestingly, two types of chiral layers are alternately bridged by three kinds of bpe pillars (namely bpe1(N1−N2), bpe2(N3−N4), and bpe3(N5−N6)) to generate a complicated 3D open framework (Figure 3a) with tubular channels observed along the b axis. Free bpe molecules and uncoordinated water molecules are located in the channels, which further consolidate the structure (Figure S2 of the Supporting Information). Additionally, π−π interactions in compound 2 are also found between different bpe3 ligands (centroid-to-centroid and centroid-to-plane distances: 3.87 Å and 3.761/3.759 Å; dihedral angel: 2.76°) (Figure S3 of the Supporting Information). By further carefully looking into the structure, we surprisingly discovered that 2 is an unusual 3D self-penetrating network which is derived from a 2-fold interpenetrating 3D architecture.

Scheme 1. Synthesis of Compounds 1−5

Zn(II)−ODPT−bpe complexes is typically via pH-dependent reaction patterns. Crystal Structures. {[Zn(bpe)(H2O)2](H2ODPT)(H2O)2}n (1). X-ray crystallographic investigation reveals that compound 1 is made up of 1D linear [Zn(bpe)(H2O)4]n2+ cationic chains, uncoordinated [H2ODPT]2− anions, and free water molecules. In the [Zn(bpe)(H2O)4]n2+ chain, each Zn(II) has a slightly distorted octahedral geometry with four coordinated water molecules in the equatorial sites and two nitrogen atoms from two different bpe molecules in the axial positions (Figure 1a). The Zn−O lengths are in the range 2.090(5)−2.141(8) Å, which is in accordance with the previous reports9 (see Table S1 of the Supporting Information). Notably, the uncoordinated [H2ODPT]2− is partly deprotonated, not only compensating for the charge of the [Zn(bpe)(H2O)4]n2+ cationic chain motif but also providing potential hydrogen-bond acceptors or donors. Therefore, abundant hydrogen bonds among free water molecules, coordinated water molecules, and carboxylate oxygen atoms hold the separate [Zn(bpe)(H2O)4]n2+ cationic chains together into a complicated 3D supramolecular framework (Figure 1b). If the hydrogen-bonding interactions through the [H2ODPT]2− ligands are ignored, the [Zn(bpe)(H2O)4]n2+ cationic units are linked via hydrogen bonds among coordinated water molecules and free water molecules (O2w···O4w = 2.692(6) Å, O3w···O4w = 2.831(9) Å) (see Table S1) into a 3D open framework (Figure S1 of the Supporting Information). As shown in Figure 1c, the coordinated water molecule O2w donates a hydrogen bond to lattice water molecule O4w, which in turn donates hydrogen bonds to lattice water molecule O3w. Interestingly, the Zn centers are 335



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Figure 1. (a) View of the asymmetric unit for 1 (symmetry codes: A = −1.5 − x, −0.5 − y, −2 − z; B = −1 − x, y, −1.5 − z). (b) Perspective view of the 3D supramolecular structure formed by hydrogen bonds between the coordinated water molecules and the O atoms from the [H2ODPT]2−.The hydrogen-bonding interactions are indicated by dashed lines. (c) Perspective view of hydrogen bonds in water trimers (symmetry codes: C = −x, −y, −1 − z; D = −x, −y, −1.5 − z). (d) Perspective view of the single-stranded meso-helix formed by hydrogen bonds in 1.

Figure 2. View of the asymmetric unit for 2. Symmetry codes: A = −1 − x, −y, 1 − z; B = 1 + x, y, z; C = −1 + x, y, 1 + z; D = −1 + x, y, z; E = x, y, −1 + z; F = 2 + x, 1+ y, z.

frameworks are bridged by bpe2 ligands, resulting in the unusual 3D self-penetrating network (Figure 4a). Another fascinating structural feature of 2 is that two interpenetrating 3D networks (Figure 4b: type A, red; type B, green) display distinct chiral information. The 2D chiral layers in type A are left-handed, whereas those layers in type B are right-handed. As shown in Figure 4c, there is another distinct helix with a pitch of 8.418 Å which is exclusively right-handed in type A, whereas those in

If the connections through the bpe2 ligands are ignored, the aforementioned chiral layers are pillared by bpe1 and bpe3 to form a microporous framework with large cavities of size 13.86 Å × 13.28 Å (Figure S4 of the Supporting Information). As is usually found, in order to minimize the big void cavities and stabilize the framework, the potential voids formed by a single 3D network show incorporation of another identical network, thus giving a 2-fold interpenetrating network. The adjacent 3D 336



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Scheme 2. Versatile Coordination Modes of ODPT4− Observed in Compounds 2−5

Figure 3. (a) Perspective view of the 3D open framework of 2 (solvent ligands have been omitted for clarity). (b) Perspective view of the left-handed (top) and right-handed (bottom) layer in 2, showing rhombic windows with dimensions of 11.45 Å × 20.01 Å. (c) Space-filling diagram of the helical chains in the 2D helical layers.

a 3D Chineseknot-like open framework (Figure 5d). Additionally, between these two kinds of bpe ligands, there are π−π interactions which further consolidate the 3D structure (Figure S5 of the Supporting Information). The centroid-to-centroid and centroidto-plane distances between the pyridyl rings (dihedral angle: 13.45°) are 3.87 and 3.53/3.69 Å, respectively. Tubular channels are observed along the crystallographic c axis and possess a void volume of 1265.6 Å3 (27.6% calculated by PLATON analysis) per unit cell, which is occupied by the intercalated water molecules. From the topological point of view, if all the Zn cations and the ODPT4− anion can be regarded as a four-connected node, the framework of compound 3 can be classified as a (4·62·83)(42·62·82) topology, representing a new topological prototype. A better insight into the nature of this intricate architecture can be achieved if one can imagine removing one of the two types of carboxylate groups of ODPT4− ligands at a time. On removing the 3,3′-carboxylate groups, the remainder is an intriguing 2D 3-connected framework with 63 topology. The single honeycomb network contains large hexagonal meshes and exhibits a highly undulating character because of the bent geometry of the ODPT ligands. As observed in similar spacious (6, 3) networks,14 the large honeycomb windows and the corrugation of the single sheets allow two identical networks to interpenetrate the hexagonal windows in a parallel mode, thus giving a 3-fold 2D → 2D

the neighboring type B are exclusively left-handed. In other words, the helices in adjacent type A and B 3D frameworks have opposite chirality, thus leading to a mesomeric network. To the best of our knowledge, until now, such 3D self-penetrating MOFs with distinct chiral information in interpenetrating networks are never reported, which could help us deeply understand the nature of coordination polymer frameworks and better design new functional materials. {[Zn2(ODPT)(bpe)2](H2O)7}n (3). When the pH value was adjusted to 4.5, a completely different product, of the form {[Zn2(ODPT)(bpe)2](H2O)7}n (3), is isolated. Compound 3 crystallizes in the monoclinic space group C2/c. As depicted in Figure 5a, the crystallographically independent Zn atom exhibits a distorted tetrahedral geometry, coordinated by two oxygen atoms from two different ODPT ligands and two nitrogen atoms from two bpe ligands. Similar to compound 2, all carboxylic groups of the ODPT4− ligand exhibit μ1-η1:η0 monodentate coordination modes, and the Zn centers are linked by V-shaped bridging ODPT4− ligands to form a 1D tubular channel (Figure 5b). Two kinds of bpe ligands show the same bidentate bridging mode with different dihedral angles of the pyridyl rings. One kind of bpe ligand with a dihedral angle of 77.07° bridges Zn−ODPT tubular channels to form a 2D layer (Figure 5c), which is further pillared by the other kind of bpe ligand with a dihedral angle of 0° to give 337



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Figure 4. (a) Perspective view of the self-penetrating 3D open framework of 2. (b) Perspective view of a single 3D network from the self-penetrating 3D framework in 2: (top) type A with left-handed 2D chiral layers, possess a distinct right-handed helix; (bottom) type B with right-handed 2D chiral layers, possess a distinct left-handed helix. (c) Perspective diagram of other helical chains in 3.

Figure 5. (a) View of the asymmetric unit for 3 (symmetry codes: A = −x, y, 2.5 − z; B = 0.5 − x, 0.5 − y, 1 − z; C = 1 − x, y, 2.5 − z; D = x, −y, −0.5 + z). (b) View of the 1D tubular structure in 3. (c) Perspective view of the 2D neutral layer in 3. (d) Perspective view of the 3D open framework of 3 (solvent ligands have been omitted for clarity).

parallel interpenetrating layer which is then further cross-linked into a single 3D network by the 3,3′-carboxylate groups (Figure 6a−c). On the other hand, elimination of 4,4′-carboxylate groups leaves 2-fold parallelly interpenetrating 2D three-connected sheets with (6, 3) topology (Figure 6d−f). Thus, compound 3 can be ascribed to a self-penetrating 3D net containing two kinds of interpenetrating layers. The driving force for the formation of this unusual topology is the formation of triple- and

double-stranded helical chain motifs. As show in Figure 7a, a triple-stranded helix is built from 4,4′-carboxylate groups of ODPT4− ligands and bpe ligands bridged between the Zn centers with a period of 36.298 Å, and the double-stranded helices are constructed by bpe and 3,3′-carboxylate groups of ODPT4− ligands bridged between the Zn centers with a pitch of 24.199 Å (Figure 7b). Apart from the intrinsic interest of two types of molecular braids, another notable feature of 3 is the 338



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Figure 6. (a) Space-filling views of a 3-fold 2D → 2D parallel interpenetrating layer. (b) Schematic views of a 3-fold 2D → 2D parallel interpenetrating layer. (c) Schematic representation of the single self-penetrating 4-connected net in 3. Kinked green networks represent the 3-fold parallel interpenetrating layer, and straight orange bonds represent the 3,3′-carboxylate groups. (d) Space-filling views of a 2-fold 2D → 2D parallel interpenetrating layer. (e) Schematic views of a 2-fold 2D → 2D parallel interpenetrating layer. (f) Schematic representation of the single selfpenetrating 4-connected net in 3. Kinked red, green, and blue undulating networks represent the 2-fold parallel interpenetrating layer, and straight black bonds represent the 3,3′-carboxylate groups.

Figure 7. Perspective views of (a) triple-stranded helices and (b) the double-stranded helix in 3. Space-filling views of (c) the first type of helices and (d) the second type of helices in 3. (e) Perspective view of the single-stranded meso-helix in 3.

existence of two types of helical chains. The first type of helix is also built from 4,4′-carboxylate groups of ODPT4− ligands and bpe bridging between the Zn centers with a pitch of 27.071 Å (Figure 7c), whereas the second type of helix is formed by 3,3′carboxylate groups of ODPT4− ligands and bpe bridging the Zn atoms with a pitch of 21.718 Å (Figure 7d). Furthermore, a careful examination of the crystal structure of 3 shows the bpe ligands and 3,3′-carboxylate groups of ODPT4− ligands link Zn

centers to generate an interesting single-stranded meso helix (Figure 7e) with a pitch of 26.628 Å. 3 and the reported compound {[Zn4(bptc)2(bpy)4]·(C5H3N)· 4H2O}n15 (bpy = 4,4′-bipyridine) have similar composition and structural moieties, but the biggest difference between them is the use of a different auxiliary ligand. {[Zn 4(bptc)2(bpy) 4]· (C5H3N)·4H2O}n is a self-penetrating network containing a quintuple-stranded molecular braid, 9-fold meso helices, and 339



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Figure 8. (a) View of the asymmetric unit for 4 (symmetry codes: A = 0.5 − x, −0.5 + y, 0.5 − z; B = −x, y, 0.5 − z; C = 0.5 + x, 0.5 − y, 0.5 + z; D = −x, −y, −z). (b) Perspective view of the 2D layer in 4 (top). Side view of the 2D layers parallel to the crystallographic c axis (bottom). (c) Perspective view of the 3D neutral network of 4 formed by Zn2+ cations and ODPT4− ligands. (d) Perspective view of the helical chains in 4. (e) Perspective view of the 3D structure of 4 along the b axis, highlighting the channels occupied by bidentate pillared coordinated bpe ligands.

ligand in 4 adopts a μ6-bridging mode with one of the four carboxylate groups in a μ2-η1:η2 chelating−bridging mode and three of them in a μ2-η1:η2 bridging mode with a syn−syn conformation, respectively (Scheme 2b). Such a coordination mode has not been observed in other metal−ODPT complexes.9,16 On the basis of this connection mode, one kind of phthalate group of ODPT4− links Zn(II) cations to form a Zn−O helical chain with a pitch of 7.576 Å, and the adjacent opposite handed chains are connected by the other kind of phthalate group to form mesomeric layers (Figure 8b). These layers are further connected by ether O atoms of ODPT4− anions to generate a 3D pillared-layer structure (Figure 8c). By careful inspection of structure 4, there still is another kind of helical chain running along the crystallographic b axis that coexists in the 3D network. The neighboring Zn1 and Zn2 atoms are bridged by the

17-fold interwoven helices using rigid auxiliary ligand bpy, whereas 3, using a flexible bpe ligand, exhibits two types of 2D → 2D parallel interpenetrating layer. A comparison of 3 with compound {[Zn4(bptc)2(bpy)4]·(C5H3N)·4H2O}n indicates that the bpe ligand which has a longer and more flexible backbone may play a crucial role in the formation of such 2D interpenetrating layers. {Zn2(ODPT)(bpe)0.5}n (4). There are two crystallographically independent Zn centers in the asymmetric unit of 4 (Figure 8a). The Zn1 center is coordinated in a distorted octahedral geometry to six carboxylate oxygen atoms from four different ODPT4− ligands. The Zn2 atom is coordinated by one nitrogen atom from a bpe ligand and three oxygen atoms from three different ODPT4− ligands, exhibiting a distorted tetrahedral geometry. In contrast to compounds 2 and 3, the ODPT4− 340



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Figure 9. (a) View of the asymmetric unit for 5 (symmetry codes: A = x, 2 − y, −z; B = −0.5 + x, 1.5 − y, −z; C = −0.5 + x, 0.5 + y, z). (b) Perspective view of the 2D chiral layer in 5. (c) Space-filling views of the right-handed 21 helical chain in the chiral layer. (d) Perspective view of the 3D neutral network of 5 formed by Zn2+ cations and ODPT4− ligands. (e) Perspective view of the 3D structure of 5 along the b axis, highlighting the channels occupied by bidentate pillared coordinated bpe ligands.

{Zn2(ODPT)(bpe)(H2O)2}n (5). Interestingly, when the hydrothermal reactions of ODPT4−, bpe, and Zn(II) were carried out at about pH 6.0, compound 5, with a chiral space group C2221, was obtained though a spontaneous resolution process involving only achiral precursors. As illuminated in Figure 9a, compound 5 has

carboxylic group of ODPT4− to generate Zn1−O−C−O−Zn2− O−Zn1 chains with the Zn1···Zn1 distance 7.576 Å (Figure 8d). As can be seen from Figure 8e, adjacent helical layers are linked by bpe ligands as molecular pillars which further consolidate the structure of compound 4. 341



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the c-axis with a pitch of 25.674 Å. The forth type of helix is built from V-shaped O2C−C−C−CO2 bridges and carboxylic groups between the Zn1 and Zn2 centers with a pitch of 7.459 Å (Figure 10d), displaying the same helical orientation as that of the former helix. As far as we know, this case is rather rare even though a few elegant coordination networks containing four types of helices have previously been characterized.6a Influence of the pH Value of the Reaction on the Structure of the Complexes. Since the only difference in the synthetic conditions in compounds 1−5 is that of the pH value of the reactions (Scheme 1), it is clear that the pH value of the reaction plays a key role in controlling the structures of the complexes. On the basis of the structures of 1−5, we found that the pH influence on the structure of the complexes is, in fact, its effect on the coordinated mode of the multicarboxylate ligand (H4ODPT). Due to the existence of protonated and/or deprotonated carboxyl groups, the ODPT ligand exists in partially and fully deprotonated forms in solution, depending on the pH. Thus, [H2ODPT]2− adopts the uncoordinated pattern in compound 1, which consists of a 1D linear cationic chain at low pH value, whereas, at high pH value of the reaction, 3D frameworks of 2−5 are obtained in which the ODPT ligands are fully deprotonated. Furthermore, the connecting pattern of the ODPT4− ligand also depends on the pH. In a comparison of compounds 2−5 (Scheme 2), it was found that the increase in pH resulted in a higher connectivity level of ODPT4− ligands, which in turn affects the formation of the final structure. The ODPT4− ligand in 2 and 3 at lower pH value adopts relatively simple coordination modes and connects four Zn(II) atoms, which may favor the formation of self-penetrating networks, while for 4 and 5 at the pH values 5.5 or 6.0, respectively, due to the complicated bridging fashion of the ODPT4− ligand, reagents tend to form pillared-layer frameworks. Luminescent Properties. The solid-state luminescent properties of 1−5 have been investigated at room temperature, and their emission spectra are given in Figure 11. Compound 1

two different Zn(II) atoms in the asymmetric unit. Zn1, with a distorted tetrahedral coordination geometry, is coordinated by four O atoms from ODPT4− ligands, while the Zn2 atom is six coordinated and resides in an octahedral coordination environment. Four O atoms, O2 and O2a belonging to two different ODPT4− groups and O1w and O1wa from two coordinated water molecules, form the equatorial plane, and the apical positions are occupied by N1 and N1a from two different bpe ligands with an N1−Zn1−N1a bond angle of 173.23(30)°. Different from the cases of compounds 2−4, the ODPT4− ligand in 5 adopts a μ6-bridging fashion with two of the carboxylate groups in a μ1-η1:η0 monodentate mode and the additional two in a μ2-η1:η1 bridging mode with a syn−anti conformation (Scheme 2c). On the basis of these connection modes, Zn1 and Zn2 atoms are bridged by a pair of carboxylate groups from two different ODPT4− ligands to form a Zn2O4C2 dimeric unit (Figure S6 of the Supporting Information) with Zn−Zn separations of 4.28 Å. The dinuclear Zn2 units are linked together by organic ODPT4− ligands to furnish a 3D pillared-layer architecture (Figure 9d). The most fascinating feature of 5 is that the phthalic groups from achiral ODPT4− ligands link Zn2 cations to form a righthanded 21 helical chain with a pitch of 9.574 Å (Figure 9c), and adjacent helical chains are further interconnected through Zn2 atoms to generate a 2D chiral layer (Figure 9b). The chiral information may be transferred to the 3D framework by helical units with 2-fold rotation. In addition, bpe molecules, using their pyridine arms, further join the Zn1 atoms to finish the coordination sphere of the metal atoms and give a more stabilized 3D structure, as shown in Figure 9e. Moreover, the 3D framework is further consolidated via π−π interactions between pyridyl rings of bpe ligands (centroid-to-centroid and centroid-to-plane distances: 3.84 Å and 3.76/3.63 Å; dihedral angel: 13.7°) (see Figure S7 of the Supporting Information). Interestingly, besides the aforementioned right-handed helical chains in the 2D helical layer, there are four distinct helical chains running along the crystallographic c axis that coexist in the 3D network (Figure 10). The first type of helix is

Figure 11. Emission spectra of 1−5 and organic ligand (4,4′oxidiphthalic anhydride) in the solid state at room temperature. Figure 10. Views of the four types of helices in 5: (a) the first, (b) the second, (c) the third, and (d) the forth.

exhibits a strong blue emission with a maximum at 382 nm upon excitation at 341 nm. The strong blue emission spectra of compound 2 resemble that of compound 5 with a maximum at 430 nm upon excitation at 332 nm, excluding the emission intensity. Compound 3 also displays blue fluorescent emission with a maximum at 444 nm upon excitation at 340 nm, and compound 4 shows a strong violet fluorescent emission with a maximum at 444 nm upon excitation at 340 nm. This implies

built from 4,4′-carboxylate groups of ODPT4− ligands that bridge between the Zn2 centers (Figure 10a). The second type of helix is formed by 3,3′-carboxylate groups of ODPT4− ligands bridging the Zn2 atoms (Figure 10b). The third type of helix is constructed by bpe bridged between the Zn1 centers (Figure 10c). These three kinds of right-handed helix are all running along 342



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bpe ligands, and coordinated water molecules (obsd 11.07%, calcd 11.31%). The decomposition of ODPT and bpe ligands occurs in the temperature range 260−860 °C (obsd 69.7%, calcd 73.84%), leading to the formation of ZnO as the residue (obsd 19.05%, calcd 19.27%). For 3, first, a weight loss of 6.84% is observed between 40 and 100 °C, which can be assigned to the release of the free water molecules per formula unit (obsd 6.92%, calcd 6.84%). Then, the compound reaches a plateau with no further weight loss up to 260 °C. Upon further increasing the temperature, the ligand molecules (ODPT and bpe) start decomposing, thus forming an unidentified product. For compound 4, there is no obvious weight loss from 30 to 390 °C in the TGA curve; above 390 °C, rapid weight loss occurred owing to the decomposition of the organic ligands. Compound 5 releases its coordinated water molecules gradually from 90 to 175 °C (obsd 5.15%, calcd 5.19%). The decomposition of the organic components occurs at 260 °C. Additionally, to confirm the phase purity and stability of these compounds, compounds 4 and 5 were selected to examine the experimental variable-temperature PXRD after calcination in the temperature ranges 30−450 and 30−300 °C, respectively. As shown in Figure 13, at 400 °C, the experimental

that these polymeric compounds could be potentially used as luminescent materials. In order to understand the origin of the emissions, the photoluminescence of the free ODPTA and bpe ligands was also investigated under the same experimental conditions. The photoluminescent spectra of ODPTA show the emission maxima at 371 nm upon photoexcitation at 339 nm, and the bpe ligand does not show obvious luminescence in the range 400−800 nm at room temperature. The emission of the ODPTA may be assigned to the π* → n and π* → π transitions of the intraligands. The emission maxima of compounds 1−5 have changed and show blue or red shifts. It is possible that a combination of several factors is acting together,9b−e,g,17 including a change in the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of deprotonated ODPT4− anions and neutral bpe ligand coordinating to metal centers, a charge-transfer transition between ligands and metal centers, and a joint contribution of the intraligand transitions or charge-transfer transitions between the coordinated ligands and the metal centers. However, since the Zn(II) ions are difficult to oxidize or reduce,18 these bands should also be assigned to the intraligand fluorescent emissions9d,e that are tuned by the differences of coordination environment around the metal ions and the deprotonated effect of the multicarboxylate ligands.19 The corresponding decay lifetimes for compounds 1−5 and ODPTA ligand are 2.28, 11.88, 7.93, 3.56, 5.96, and 1.65 ns, respectively. PXRD Patterns and Thermal Properties. The synthesized products of compounds 1−5 have been characterized by PXRD (Figure S8−S12 of the Supporting Information). The experimental PXRD patterns’ peak positions correspond well with the results simulated from the single crystal data, indicating the high purity of the synthesized samples. The difference in reflection intensities between the simulated and experimental patterns was due to the variation in preferred orientation of the powder samples during the collection of the experimental PXRD data. The thermal behaviors of 1−5 were studied by TGA. The experiments were performed on samples consisting of numerous single crystals under N2 atmosphere with a heating rate of 10 °C/min, as shown in Figure 12. For compound 1, the

Figure 13. Variable-temperature powder X-ray diffraction patterns for compound 4.

PXRD patterns of 4 are still in good agreement with the simulated PXRD patterns of 4, indicating that the crystal lattice of 4 remains intact at 400 °C. As illuminated in Figure 14, the experimental variable-temperature PXRD of 5 after calcination

Figure 12. TGA curves for compounds 1−5.

weight loss in the range 55−160 °C corresponds to the departure of lattice and coordination water molecules (obsd 14.45%, calcd 17.50%), and the anhydrous compound begins to decompose at 200 °C, forming an unidentified product. The TGA curve of 2 shows that the first weight loss from 30 to 180 °C corresponds to the loss of lattice water molecules, uncoordinated

Figure 14. Variable-temperature powder X-ray diffraction patterns for compound 5.

at elevated temperature in the range 30−260 °C is still in good agreement with the calculated one, although minor differences 343



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can be seen in the positions, intensities, and widths of some peaks, which indicates that the framework of compound 5 is retained at 260 °C. When compound 5 is heated at 300 °C, the structures of 5 begin to change and form an amorphous phase.

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CONCLUSION Our investigation of the Zn(OAc)2·2H2O, 4,4′-oxidiphthalic anhydride, and 1,2-bis(4-pyridyl)ethane systems under different pH conditions resulted in a series of compounds ranging from 1D to 3D coordination polymers containing two rare selfpenetrating networks and two pillared-layer frameworks. Comparing these complexes, we have found that the various coordination conformations and deprotonation of the H4ODPT ligand can be influenced by pH values easily, and represent a nice example of the acidity-controlled polymeric architecture construction. Furthermore, luminescent measurements reveal that compounds 1−5 are good candidates for photoactive materials, owing to their strong luminescent emissions. Thus, our present research further enriches the crystal engineering strategy and offers new possibilities for controlling the formation of the desired framework structures.

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ASSOCIATED CONTENT * Supporting Information Crystallographic data in CIF format, table of selected bond lengths, simulated and experimental powder X-ray diffraction patterns, and additional structural plots. CCDC reference numbers 813745−813749. This material is available free of charge via the Internet at http://pubs.acs.org. S

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENTS This work was supported by 973 Program of China (2011CBA00505), the Chinese Academy of Sciences (KJCX2-YW-H30, KGCX2-YW-222, and KJCX2-YW-M10), and the Science Foundation of the Fujian Province (2009HZ0005-1 and 2006L2005).

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