Four d10 Metal Coordination Polymers Containing Isomeric

Four d10 Metal Coordination Polymers Containing Isomeric. Thiodiphthalic Ligands: Crystal Structures and Luminescent. Properties. Yang Su, Shuangquan ...
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CRYSTAL GROWTH & DESIGN

Four d10 Metal Coordination Polymers Containing Isomeric Thiodiphthalic Ligands: Crystal Structures and Luminescent Properties

2007 VOL. 7, NO. 7 1277-1283

Yang Su, Shuangquan Zang, Yizhi Li, Huizhen Zhu, and Qingjin Meng* Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed August 26, 2006; ReVised Manuscript ReceiVed April 8, 2007

ABSTRACT: Two versatile ligands, 2,3,2′,3′-tdpa and its isomer 2,3,3′,4′-tdpa, have been introduced to construct novel metalorganic frameworks with interesting structural motifs and good function. By hydrothermoreactions with d10 metals (Zn and Cd), four coordination polymers were obtained and characterized. 1 is a 3D framework embodying two distinct types of helical chains. 2 features a 2D lamellar structure with helices and 14-membered rings in every layer. 3 is a 3D framework bearing the (62.83.10)(62.8)(6.8.10) topology and has chiral layers in it. 4 shows a quasi-2D structure with 1D binuclear zinc chains and less common monocoordinated 4,4′-bipyridine ligands. All four of these coordination polymers exhibit intense blue fluorescence emissions (1 at 429 nm, 2 at 473 nm, 3 at 451 nm, and 4 at 492 nm) and may be suitable as excellent candidates of blue fluorescent materials. Introduction

Chart 1.

The rational design and assembly of inorganic coordination polymers, or metal-organic frameworks (MOFs), have received remarkable attention and have developed rapidly in recent years. Being easily and efficiently synthesized from relatively simple subunits, these complexes exhibit fascinating structural topologies and potential applications as functional materials.1,2 It is well acknowledged that the kind of metal ions, the geometry, and the number of coordination sites provided by organic ligands are all important parameters for directing the self-assembly processes. Balanced rigidity and flexibility of the ligands are also known to be essential for the given architecture. Since a carboxylate group can bridge metal ions to give rise to a wide variety of polynuclear complexes ranging from discrete entities to three-dimensional systems,3 polycarboxylic acids constitute an important family of multidentate O-donor ligands and have been extensively employed in the preparation of metal-organic complexes. Recently, we have introduced a versatile ligand, 2,3,2′,3′-odpa, to construct chiral helical coordination polymers.4 The interesting results inspire us to investigate its sulfur counterpart 2,3,2′,3′-tdpa (H4L) and the isomer 2,3,3′,4′-tdpa (H4L′) (Chart 1). The introduction of etheric sulfur instead of etheric oxygen somehow enhances the conjugation of the two phenyl rings in the ligand. In spite of this, tdpa is very similar to odpa in terms of its coordination features. Briefly speaking, the twisted conformation of the ligand provides the potential capability to form helical structures, and stepwise deprotonation provides the ligand the variability of coordination modes and hydrogenbonding schemes. We have previously reported the pHdependent behavior of 2,3,2′,3′-tdpa (i.e., the second feature) and how it affects the construction of coordination polymers.5 In this study we then investigate four d10 metal coordination polymers containing the isomeric tdpa ligand (either L or L′): Cd2L(bpy)(H2O)3‚H2O (1), Zn2L(bpy)(H2O)2‚3H2O (2), Cd2L′(bpy)0.5(H2O)3 (3), and Zn(H2L′)(bpy) (4). Their luminescent properties are studied as well. * To whom correspondence [email protected].

should

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2,3,2′,3′-tdpa (H4L) and 2,3,3′,4′-tdpa (H4L′)

Experimental Section Syntheses and Physical Measurements. 2,3,2′,3′-tdpa dianhydride (2,3,2′,3′-tdpda) and 2,3,3′,4′-tdpa dianhydride (2,3,3′,4′-tdpda) were synthesized according to the literature.6 All other starting materials were of reagent quality and were obtained from commercial sources without further purification. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240C elemental analyzer. The metal contents were determined by EDTA titration. The IR spectra were obtained as KBr pellets on a Bruker VECTOR 22 spectrometer. Thermal analyses were performed on a TGA V5.1A Dupont 2100 instrument from room temperature to 600 °C with a heating rate of 10 °C/min in air. Luminescence spectra for the solid samples were recorded with a Hitachi 850 fluorescence spectrophotometer. Synthesis of Cd2L(bpy)(H2O)3‚H2O (1). 1 was synthesized hydrothermally in a 23 mL Teflon-lined autoclave by heating a mixture of 0.1 mmol of 2,3,2′,3′-tdpda, 0.2 mmol of Cd(NO3)2‚6H2O, 0.2 mmol of 4,4-bipyridine, and one drop of Et3N in 10 mL of water at 120 °C for 3 days. Colorless block single crystals were collected in 76% yield based on cadmium. Anal. Found (Calcd) for C26H22Cd2N2O12S: C, 38.51 (38.49); H, 2.75 (2.73); N, 3.40 (3.45); Cd, 27.78 (27.71). IR (KBr, cm-1): 3421(s, br), 1568(vs), 1456(m), 1380(vs), 1220(m), 1069(w), 1007(w), 808(m), 773(m), 729(m), 631(m), 454(w).

10.1021/cg060572g CCC: $37.00 © 2007 American Chemical Society Published on Web 05/26/2007

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Table 1. Crystal Data and Structure Refinement for 1-4

temp (K) empirical formula Mr cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm-1) µ (mm-1) F(000) no. of unique reflns no. of obsd reflns (I > 2σ(I)) Rint R1a wR2b GOF ∆Fmax (e Å-3) ∆Fmin (e Å-3) a

1

2

3

4

293(2) C26H22Cd2N2O12S 811.35 monoclinic P21/n 14.397(3) 9.2579(16) 21.485(4)

298(2) C52H46N4O25S2Zn4 1452.53 monoclinic P21/n 10.332(2) 9.3363(19) 28.822(6)

293(2) C21H16Cd2NO11S 715.21 monoclinic P21/n 7.856(2) 26.525(7) 10.692(3)

106.368(3)

90.210(4)

104.520(5)

2747.6(9) 4 1.961 1.694 1600 5378 4036 0.0617 0.0524 0.0905 1.000 0.814 -0.553

2780.3(10) 2 1.735 1.872 1476 5455 4261 0.0559 0.0615 0.1331 1.074 0.418 -0.533

2156.8(10) 4 2.203 2.137 1396 4213 3698 0.0580 0.0639 0.1660 1.076 0.864 -0.580

293(2) C26H16N2O8SZn 581.84 triclinic P1h 9.6585(7) 10.8284(7) 12.2062(8) 103.7760(10) 99.4150(10) 108.1690(10) 1138.26(13) 2 1.698 1.230 592 4384 3769 0.0483 0.0326 0.0878 1.003 0.573 -0.560

R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.

Figure 1. Molecular structure of 1 showing the geometry of the Cd2+ ions and the coordination modes of the carboxylate groups. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms and the solvent water molecule are omitted for clarity. Circled numbers (2, 3, 2′, and 3′) indicate the numbering scheme for the carboxylate groups. Symmetry codes: #1, 1 - x, -y, 2 - z; #2, 0.5 + x, 0.5 - y, 0.5 + z; #3, 1.5 - x, -0.5 + y, 1.5 - z; #4, -0.5 + x, 0.5 - y, -0.5 + z; #5, 1 - x, -y, 1 - z; #6, 1.5 - x, 0.5 + y, 1.5 - z. Synthesis of Zn2L(bpy)(H2O)2‚3H2O (2). 2 was synthesized hydrothermally in a 23 mL Teflon-lined autoclave by heating a mixture of 0.1 mmol of 2,3,2′,3′-tdpda, 0.2 mmol of Zn(NO3)2‚6H2O, 0.2 mmol of 4,4-bipyridine, and 0.4 mmol of NaOH in 10 mL of water at 120 °C for 3 days. Colorless block single crystals were collected in 80% yield based on zinc. Anal. Found (Calcd) for C52H46N4O25S2Zn4: C, 43.07 (43.00); H, 3.14 (3.19); N, 3.85 (3.87); Zn, 18.08 (18.01). IR (KBr, cm-1): 3423(s, br), 1611(vs), 1571(vs), 1560(vs), 1552(vs), 1458(m), 1376(vs), 1221(m), 1070(m), 822(w), 777(s), 730(s), 706(w), 643(m). Synthesis of Cd2L′(bpy)0.5(H2O)3 (3). 3 was synthesized hydrothermally in a 23 mL Teflon-lined autoclave by heating a mixture of 0.1 mmol of 2,3,3′,4′-tdpda, 0.2 mmol of Cd(NO3)2‚6H2O, 0.2 mmol of 4,4-bipyridine, and one drop of Et3N in 10 mL of water at 120 °C for 3 days. Colorless block single crystals were collected in 65% yield based on cadmium. Anal. Found (Calcd) for C21H16Cd2NO11S: C, 35.24 (35.27); H, 2.32 (2.25); Cd, 31.39 (31.43); N, 1.95 (1.97). IR (KBr, cm-1): 3442(s, br), 1561(vs), 1541(vs), 1482(s), 1457(s), 1429(s), 1397(vs), 1385(vs), 1278(w), 1157(w), 905(w), 863(w), 850(w), 837(w), 793(w), 770(s), 728(m), 701(w), 673(w). Synthesis of Zn(H2L′)(bpy) (4). 4 was synthesized hydrothermally in a 23 mL Teflon-lined autoclave by heating a mixture of 0.1 mmol of 2,3,3′,4′-tdpda, 0.1 mmol of Zn(NO3)2‚6H2O, 0.2 mmol of 4,4bipyridine, and 0.2 mmol of NaOH in 10 mL of water at 120 °C for 3

days. Colorless block single crystals were collected in 90% yield based on zinc. Anal. Found (Calcd) for C26H16N2O8SZn: C, 53.64 (53.67); H, 2.81 (2.77); N, 4.84 (4.81); Zn, 11.27 (11.24). IR (KBr, cm-1): 3448(w, br), 3093(w), 2483(w), 1719(s), 1687(m), 1642(vs), 1589(m), 1574(m), 1561(m), 1520(w), 1488(w), 1453(w), 1400(vs), 1290(m), 1273(s), 1254(m), 1210(m), 1141(w), 1100(w), 1083(w), 1059(w), 920(w), 887(w), 814(s), 783(m), 725(s), 695(s), 602(m). X-ray Crystallography. Single crystals of dimensions 0.3 × 0.2 × 0.3 mm for 1, 0.3 × 0.2 × 0.2 mm for 2, 0.3 × 0.2 × 0.2 mm for 3, and 0.3 × 0.2 × 0.3 mm for 4 were used for structural determinations on a Bruker SMART APEX CCD diffractometer7 using graphitemonochromatized Mo KR radiation (λ ) 0.71073 Å) at room temperature using the ω-scan technique. Lorentz polarization and absorption corrections were applied. The structures were solved by direct methods and refined with the full-matrix least-squares technique using SHELXTL version 5.1.8 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. Analytical expressions of neutral atom scattering factors were employed, and anomalous dispersion corrections were incorporated. The crystallographic data for 1-4 are listed in Table 1. Selected geometric information is listed in Tables S1 and S2 in the Supporting Information.

Results and Discussion Syntheses. We have used the dianhydride derivatives 2,3,2′,3′tdpda and 2,3,3′,4′-tdpda instead of the carboxylic acids 2,3,2′,3′tdpa and 2,3,3′,4′-tdpa during the synthesis process. These dianhydrides underwent in situ hydrolysis/deprotonation in a basic solution and anticipated in the subsequent self-assembly procedure. Crystal Structures. A single-crystal XRD study has revealed that Cd2L(bpy)(H2O)3‚H2O (1) crystallizes in the monoclinic system, P21/n space group. The asymmetric unit of 1 contains one 2,3,2′,3′-tdpa ligand, two independent Cd2+ ions, one 4,4′bipyridine (bpy) molecule, three coordinated waters, and one solvent water (Figure 1). The Cd1 ion is in a distorted octahedral geometry with O3, O4, O2#6, and O5#4 (from the carboxylate groups) on the equatorial plane and O9 (from the coordinated water) and N1 (from bpy) in the axial position (symmetry codes: #4, -0.5 + x, 0.5 - y, -0.5 + z; #6, 1.5 - x, 0.5 + y, 1.5 - z). The Cd2 ion is in a regular octahedral geometry coordinating to O4, O6#4, and O8#1 (from the carboxylate groups), O10 and O11 (from the coordinated waters), and N2#5

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Figure 3. Molecular structure of 2 showing the geometry of the Zn2+ ions and the coordination modes of the carboxylate groups. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms and the solvent water molecule are omitted for clarity. Circled numbers (2, 3, 2′, and 3′) indicate the numbering scheme for the carboxylate groups. Symmetry codes: #1, -0.5 - x, -0.5 + y, 1.5 - z; #2, -0.5 - x, 0.5 + y, 1.5 - z; #3, x, y - 1, z; #4, 1 - x, 1 - y, 2 - z; #5, x, y + 1, z.

Figure 2. Top: 3D network structure of 1 viewed down the b axis. Hydrogen atoms and water molecules are omitted for clarity. Cd1 helices and Cd2 helices are colored as light green and light blue, respectively. The chirality of each helical chain is labeled P (righthandedness) or M (left-handedness). Bottom-left: Cd1 helices and Cd2 helices are connected through O4 and O5 along the a axis. Bottom right: Cd1 helices and Cd2 helices are connected through S1 along the c axis. Circled numbers (2, 3, 2′, and 3′) indicate the numbering scheme for the carboxylate groups.

(from bpy) (symmetry codes: #1, 1 - x, -y, 2 - z; #5, 1 - x, -y, 1 - z). The 2,3,2′,3′-tdpa ligand employs a twisted conformation, and the dihedral angle between the two phenyl rings is 72.6°. The bond angle around S1 is 101.1°. The dihedral angles between the 2-, 3-, 2′-, and 3′-carboxylate groups and their corresponding phenyl rings are 69.5°, 31.8°, 70.4°, and 32.8°, respectively. The 2- and 3′-COO- groups are monocoordinated, the 2′-COO- group is bicoordinated, and the 3-COOgroup is chelated and further bridges Cd1 and Cd2. The combination of these twisted carboxylate groups and the nonlinear flexibility around the S1 atom results in the formation of metal-tdpa helical chains. Through spontaneous resolution, the Cd1 ions are exclusively bridged by 2- and 3-COO- to give one type of 21-helical chain. At the same time, the Cd2 ions are exclusively bridged by 2′- and 3′-COO- to give another type of 21-helical chain. These two distinct types of 21-helical chains are alternately arranged along the a and c axes (Figure 2, top). The screw axes of these helices are all parallel to the b axis, and the pitch is 9.258(2) Å (the same as the cell period b). Each type of helix only discriminates one kind of Cd atom (Cd1 or Cd2). The chirality of each helical chain is opposite to that of the nearest ones of the same type (they are actually symmetry equivalents related through inversion centers and glide planes). Along the c axis, the two types of helices are arranged alternately with the same chirality, but along the a axis, the two types of helices are arranged alternately with the opposite chirality (Figure 2, top). Adjacent Cd1 helices and Cd2 helices are interconnected through O4 and O5 along the a axis (Figure 2, bottom left), through S1 along the c axis (Figure 2, bottom

right), and through the “second ligand” bpy along the [101h] direction (Figure 2, top) to form a 3D network structure. By substituting Cd with Zn in 1, Zn2L(bpy)(H2O)2‚3H2O (2) was obtained. 2 also crystallizes in the monoclinic system, P21/n space group, as does 1. The asymmetric unit of 2 contains one 2,3,2′,3′-tdpa ligand, two independent Zn2+ ions, one bpy molecule, two coordinated waters, and three solvent waters (Figure 3). Zn1 is in a tetrahedral geometry coordinating to O1 and O4#2 (from the carboxylate groups), O9 (from the coordinated water), and N1 (from bpy) (symmetry code: #2, -0.5 - x, 0.5 + y). Zn2 is also in a tetrahedral geometry coordinating to O6#5 and O7#4 (from the carboxylate groups), O10 (from the coordinated water), and N2 (from bpy) (symmetry codes: #4, 1 - x, 1 - y, 2 - z; #5, x, y + 1, z). The dihedral angle between the two phenyl rings of the 2,3,2′,3′-tdpa ligand is 87.4°. The bond angle around S1 is 103.8°. The dihedral angles between the 2-, 3-, 2′-, and 3′-carboxylate groups and their corresponding phenyl rings are 72.7°, 29.3°, 88.8°, and 18.1°, respectively. These four carboxylate groups are all monocoordinated. 2 features a 2D lamellar structure with layers stacking along the [101h] direction. In every layer, Zn1 ions are bridged by 2and 3-COO- exclusively to give 21-helical chains of both handednesses (Figure 4, top and bottom left). The screw axes of these helices are all parallel to the b axis, and the pitch is 9.336(2) Å (the same as the cell period b). These helical chains are symmetry equivalents related through inversion centers and glide planes in essense so that adjacent helices possess the opposite chirality and the layer is achiral in general. Interestingly, apart from the apparent helical structures, large 14membered rings can also be found easily between the helical chains (Figure 4, top and bottom right). These rings lie along the b axis and are formed exclusively by 2′- and 3′-COObridging the Zn2 ions. By substituting 2,3,2′,3′-tdpa with 2,3,3′,4′-tdpa in 1, Cd2L′(bpy)0.5(H2O)3 (3) was obtained. 3 still crystallizes in the monoclinic system, P21/n space group, as do 1 and 2. The asymmetric unit of 3 contains one 2,3,3′,4′-tdpa ligand, two independent Cd2+ ions, half of a bpy molecule, and three coordinated waters (Figure 5). Cd1 is in a pentagonal bipyramidal geometry with O3#5, O4#5, O5, O6, and O7#6 (from the carboxylate groups) on the equatorial plane and O2#3 (from 2-COO-) and O9 (from the coordinated water) in the axial

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Figure 6. The 2,3,3′,4′-tdpa ligand can be abstracted as a Y-shaped connector with an additional side arm. The two phenyl rings are defined as nodes E and F. Cd1 is defined as node D. Circled numbers (2, 3, 3′, and 4′) indicate the numbering scheme for the carboxylate groups.

Figure 4. Top: A layer in 2 viewed along the [1,0,-1] direction. Hydrogen atoms and water molecules are omitted for clarity. P helices (right-handed) and M helices (left-handed) are colored as light blue and light green, respectively, while 14-membered rings are colored as pink. Bottom left: The smallest repeat unit (thick bonds) of a righthanded Zn1 helical chain. Bottom right: Formation of a 14-membered ring (thick bonds). Circled numbers (2, 3, 2′, and 3′) indicate the numbering scheme for the carboxylate groups.

Figure 5. Molecular structure of 3 showing the geometry of the Cd2+ ions and the coordination modes of the carboxylate groups. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Circled numbers (2, 3, 3′, and 4′) indicate the numbering scheme for the carboxylate groups. Symmetry codes: #1, 0.5 - x, -0.5 + y, 0.5 - z; #2, -x, 1 - y, 1 - z; #3, 1 - x, 1 - y, 1 - z; #4, x - 1, y, z; #5, 0.5 - x, 0.5 + y, 0.5 - z; #6, x + 1, y, z.

position (symmetry codes: #3, 1 - x, 1 - y, 1 - z; #5, 0.5 x, 0.5 + y, 0.5 - z; #6, x + 1, y, z). Cd2 is in a regular octahedral geometry coordinating to O1#2, O3#2, and O5 (from the carboxylate groups), O10 and O11 (from the coordinated waters), and N1 (from bpy) (symmetry code: #2, -x, 1 - y, 1 - z). The dihedral angle between the two phenyl rings of the 2,3,3′,4′-tdpa ligand is 72.3°. The bond angle around S1 is 101.4°. The dihedral angles between the 2-, 3-, 3′-, and 4′-carboxylate groups and their corresponding phenyl rings are

Figure 7. The Y-shaped part forms a 2D honeycomb-like layer by coordinating to Cd1 ions. The side arm points up or down alternately from this layer.

88.8°, 53.1°, 66.1°, and 15.6°, respectively. 2-COO- is bicoordinated. 3- and 3′-COO- are chelated and further bridge Cd1 and Cd2. 4′-COO- is monocoordinated. Although the actual assembly process may happen all at once rather than stepwise, it is more convenient to describe the 3D structure of 3 in a two-step procedure. First, the 2,3,3′,4′-tdpa ligand bridges Cd1 to form a 3D host framework. Then the bpy ligand bridging two Cd2 ions as a whole acts as an SBU guest to coordinate to the Cd1-L′ framework by use of the rest of the coordination sites on the Cd2 ions. To analyze and understand more clearly the complicated structure of the Cd1-L′ framework, we take advantage of the “network approach” or topological approach, which provides a powerful tool for the analyses of complex network structures.9 By reducing the multidimensional structure to a simple nodeand-connection reference net, the description of the coordination polymer architecture can be attributed to structural simplification and subsequent systematization. As shown in Figure 6, 3-, 3′-, and 4′-COO- are almost coplanar and 2-COO- is nearly perpendicular to this plane. Therefore, the 2,3,3′,4′-tdpa ligand can be abstracted as a Y-shaped connector with an additional side arm. This connector defines two nodes, E and F, and contains one edge, EF. The Y-shaped part of the connector forms a 2D honeycomb-like layer by coordinating to Cd1 ions (defined as node D), leaving the side arm pointing up or down alternately from the layer (Figure 7). Interestingly, helical structures are found in this layer (Figure 8). The screw axes of these 21-helical chains are all parallel to the b axis. The pitch of each helical chain is equal to the cell period b. 3- and 3′-COO- bridge the Cd1 ions to form the helical chain, and 4′-COO- coordinates to the Cd1 ions on the

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Figure 10. Cd2-bpy-Cd2 linear connector in 3.

Figure 8. Left: A chiral layer in 3 containing helical chains. All the helices are left-handed. Right: Structure and interconnection of the helical chains in a layer. 3- and 3′-COO- bridge the Cd1 ions to form the helical chain. 4′-COO- coordinates to (emphasized by the black arrows) the Cd1 ions on the neighboring helical chains to form 2D layers. Circled numbers (2, 3, 3′, and 4′) indicate the numbering scheme for the carboxylate groups.

Figure 9. Schematic illustration of the ABAB packing style and the (6.28.310)(6.28)(6.8.10) topology of the Cd1-L′ host framework.

neighboring helical chains to form 2D layers. All the helical chains in the layer bear the same handedness, making the layer chiral. The whole three-dimensional Cd1-L′ host framework can then be achieved by letting the side arms in one layer coordinate to the Cd1 ions in adjacent layers, giving an ABAB packing style (Figure 9). It should be noted that although each layer itself is chiral, neighboring layers are essentially enantiomers related through inversion centers located between the layers. Therefore, the Cd1-L′ host framework is a 3D mesomeric network in general. From Figure 9, one can easily identify that node D is 4-connected while nodes E and F are 3-connected. The Cd1-L′ host framework can then be classified topologically as a (6.28.310)(6.28)(6.8.10) network. The bpy molecule bridges two Cd2 ions to form a linear connector (Figure 10). The rest of the coordination sites on Cd2 are coordinated to O1 (from 2-COO-), O3 (from 3-COO-), and O5 (from 3′-COO-) in the Cd1-L′ host framework and two aqua ligands (O10 and O11, not shown in Figure 10). As previously stated, the Cd1-L′ host framework shows an ABAB packing style. Every Cd2 bridges neighboring layer A and layer B like a trielcon. The Cd2-bpy-Cd2 linear connector as a whole bridges every adjacent (AB) double layer. In this way, these linear connectors embed themselves into the host framework and reinforce the stability of the whole structure. By substituting Cd with Zn in 3, Zn(H2L′)(bpy) (4) was obtained. 4 crystallizes in the monoclinic system, P1h space group. The asymmetric unit of 4 contains one 2,3,3′,4′-tdpa

Figure 11. Molecular structure of 4 showing the geometry of the Zn2+ ions and the coordination modes of the carboxylate groups. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are represented by balls of arbitrary size. Hydrogen atoms on the aromatic rings are omitted for clarity. Circled numbers (2, 3, 3′, and 4′) indicate the numbering scheme for the carboxylate groups. Symmetry codes: #1, 1 - x, -y, 1 - z; #2, x - 1, y - 1, z; #3, 2 - x, 1 - y, 1 - z; #4, 1 + x, 1 + y, z.

ligand, one Zn2+ ion, and one bpy molecule (Figure 11). Zn1 is in an octahedral geometry with O3#1, O4#4, O7, and O8#3 (from the carboxylate groups) on the equatorial plane and N1 (from bpy) and Zn1#3 in the axial position (symmetry codes: #1, 1 - x, -y, 1 - z; #3, 2 - x, 1 - y, 1 - z; #4, 1 + x, 1 + y, z). The dihedral angle between the two phenyl rings of the 2,3,3′,4′-tdpa ligand is 86.3°. The bond angle around S1 is 103.8°. The dihedral angles between the 2-, 3-, 3′-, and 4′-carboxylate groups and their corresponding phenyl rings are 88.1°, 14.6°, 49.5°, and 31.8°, respectively. 3- and 4′-COOare bicoordinated. 2- and 3′-COO- remain protonated. The zinc(II) ions are bridged by two 3- and two 4′-COOgroups and show a typical paddle wheel motif. The Zn-Zn distance is 2.948 Å. The rest of the coordination site of the zinc ion is occupied by the bpy ligand (Figure 12). These binuclear zinc units are interlinked by the 2,3,3′,4′-tdpa ligands to form 1D chains (Figure 13). Intrachain hydrogen bonds (O6-H6B‚‚‚O2) are found to further stablize these chains. In 4, the bpy ligand adopts an uncommon coordination mode. Only N1 coordinates to Zn1, and N2 acts as a interchain hydrogen-bonding acceptor (O2-H2A‚‚‚N2′; symmetry code: -x + 1, -y + 1, -z) (Figure 14). Compared to the bicoordinated mode which is commonly adopted, this monocoordinated mode of bpy is relatively rare.10 Apparently, it is the two protonated carboxyl groups that make it possible to stabilize this monocoordinated mode by affording hydrogen bonds to the bpy molecules. Otherwise, the bpy ligand would prefer the

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Figure 12. A binuclear zinc(II) arrangement in 4 showing a typical paddle wheel motif. Circled numbers (3 and 4′) indicate the numbering scheme for the carboxylate groups. Figure 15. Excitation (thin line) and emission (thick line) spectra of 1 (red), 2 (green), and 2,3,2′,3′-tdpda (blue) in the solid state at room temperature.

Figure 13. A 1D chain in 4. Dashed lines stand for hydrogen bonds.

Figure 16. Excitation (thin line) and emission (thick line) spectra of 3 (red), 4 (green), and 2,3,3′,4′-tdpda (blue) in the solid state at room temperature.

Figure 14. 2D supramolecular network in 4. Interchain hydrogen bonds and π-π interactions are represented as dashed lines.

bicoordinated mode as it did in 1-3 where all four carboxyl groups were deprotonated. In addition to those interchain hydrogen bonds, π-π interactions between the bpy ligands (centroid to centroid distance 3.937 Å) are also found (Figure 14). The 1D chains in 4 form a 2D supramolecular network by both the interchain hydrogen bonds and the π-π interactions. So far, we have reported seven coordination polymers constructed from the tdpa ligands (four in this paper and three previously5). Among them six contain helical structures, which proves that the tdpa ligands are really suitable for the construction of helices. As far as this is concerned, the tdpa ligands behave much like the odpa ligand (among the six odpa

complexes4 five contain helices) since they have a similar twisted conformation. On the other hand, the inherent chirality of the helical structure does not guarantee the chirality of the entire framework as the framework may contain additional inversion centers, etc. Considering this, the tdpa ligands show dramatic distinction against the odpa ligands: five of the six tdpa complexes are achiral, whereas all five odpa complexes are chiral. This interesting though unexpected phenomenon suggests that there exist some fundamental differences between tdpa and odpa that are covered up by their seeming analogy and thus need further exploration. Thermal Analyses. Thermal analyses for 1-4 were carried out from room temperature to 800 °C under a nitrogen atmosphere (Figure S5 in the Supporting Information). All four coordination polymers are quite stable. 1 loses all its solvent and coordinated waters at 244 °C, and the framework begins to collapse above 285 °C. 2 loses all its solvent and coordinated waters at 312 °C, and the framework begins to collapse above this temperature. 3 loses all its solvent and coordinated waters at 298 °C, and the framework begins to collapse above 330 °C. The framework of 4 begins to collapse above 300 °C. Fluorescence Properties. The excitation and emission spectra of 1, 2, and 2,3,2′,3′-tdpda in the solid state at room temperature

Polymers Containing Thiodiphthalic Ligands

are shown in Figure 15. These spectra of 3, 4, and 2,3,3′,4′tdpda are shown in Figure 16. These complexes exhibit intense blue fluorescence emission bands at ca. 429 nm for 1, 473 nm for 2, 451 nm for 3, and 492 nm for 4. These emissions are assigned to the intraligand π-π* fluorescence emission since the corresponding ligand also shows a similar emission (2,3,2′,3′tdpda at ca. 426 nm and 2,3,3′,4′-tdpda at ca. 451 nm). Compared with the corresponding ligands, a baseochromic shift of emission occurs in these complexes, which is probably due to the differences of the metal ions and the coordination environment around them because the photoluminescence behavior is closely associated with the metal ions and the ligands coordinated around them.11 All of these complexes may be suitable as excellent candidates of blue fluorescent materials. Conclusion In summary, two versatile ligands, 2,3,2′,3′-tdpa and its isomer 2,3,3′,4′-tdpa, have been introduced to construct novel MOFs with beautiful structures and excellent fluorescence functions. The two isomers show a similar twisted conformation which provides the potential capability to form chiral structures as helices. The slight positional differences of the carboxylate groups between these two isomers, however, may lead to delicate geometric diversification of the resulting coordination polymers. We have successfully isolated and characterized four coordination polymers containing d10 metals (Cd and Zn) varying from 1D chains to 3D frameworks with helical substructures. It is worth noting that the second ligand 4,4′bipyridine adopts a relatively rare monocoordinated mode in 4 besides the common bicoordinated mode as in 1-3. All four coordination polymers exhibit intense blue fluorescence emissions and may be suitable as excellent candidates of blue fluorescent materials. Subsequent works will be focused on the construction of novel coordination polymers by reacting these and other related ligands12 (including another isomeric ligand, 3,4,3′,4′tdpa) with more metal ions. Acknowledgment. We thank Prof. M.-X. Ding and Dr. X.Z. Fang (Polymer Chemistry Laboratory, State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, People’s Republic of China) for their kind help. This work was supported by the State Basic Research Project and the National Science Foundation of China (Grant NSFC 20490218). Supporting Information Available: Selected geometric information for 1-4, coordination geometries of the metal ions, TGA curves for 1-4, and X-ray data files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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