Syntheses, Structures, and Photoluminescent Properties of

Article pubs.acs.org/crystal

Syntheses, Structures, and Photoluminescent Properties of Coordination Polymers Based on 1,4-Bis(imidazol-l-ylmethyl)benzene and Various Aromatic Dicarboxylic Acids Xue-Zhi Song,†,‡ Shu-Yan Song,† Chao Qin,† Sheng-Qun Su,†,‡ Shu-Na Zhao,†,‡ Min Zhu,†,‡ Zhao-Min Hao,†,‡ and Hong-Jie Zhang*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China ‡ Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China S Supporting Information *

ABSTRACT: Nine coordination polymers, formulated as {[Zn(pbdc)(bix)](DMF)2}n (1), {[Co(p-bdc)(bix)](DMF)2}n (2), {[Cd(pbdc)(bix)](DMF)2}n (3), {[Zn(bpdc)(bix)](DMF)3}n (4), {[Co(bpdc)(bix)](DMF)3}n (5), {[Cd(m-bdc)(bix)(H2O)](DMF)}n (6), {[Zn(m-bdc)(bix)](solv)x}n (7), {[Zn2(m-bdc)2(bix)2](solv)x}n (8), and {[Zn(m-bdc)(bix)](solv)x}n (9) (p-H2bdc = 1,4-benzenedicarboxylic acid, H2bpdc = biphenyl-4,4′-dicarboxylic acid, m-H2bdc = 1,3benzenedicarboxylic acid), have been prepared by solvothermal reactions of the semirigid neutral ligand 1,4-bis(imidazol-l-yl-methyl)benzene (bix) with metal ions in the presence of various aromatic dicarboxylic acids. These complexes were characterized by Fourier transform infrared (FT-IR) spectroscopy, elemental analysis, thermogravimetric analysis (TGA), and single-crystal X-ray diffraction analysis. Isostructural complexes 1 and 2 reveal a 3-fold interpenetrating threedimensional (3D) framework with dia topological type. Compound 3 exhibits a non-interpenetrating, highly undulating twodimensional (2D) network with (4,4) topology. Compounds 4 and 5 exhibit (6,3) layer structures consisting of hexagonal meshes and loops. Compound 6, from the topological point of view, shows the same (4,4) topology as compound 3; however interestingly all the layers can be divided into pairs in the overall network. Complex 7 also possesses a 3-fold interpenetrated 3D dia framework similar to 1 and 2. Compound 8 comprises two independent 2D (4,82) nets that interpenetrate in a parallel manner. Non-interpenetrating 2D (4,4) layers are also observed in compound 9 that are further packed into a 3D framework featuring one-dimensional (1D) channels. The structural diversity of nine coordination polymers indicates that the structures can be tuned by metal ions, various ditopic carboxylate anions, and changeable conformations of neutral ligand. In addition, photoluminescent properties of four coordination polymers were also investigated in this paper.

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INTRODUCTION Metal−organic coordination polymers (MOCPs) with infinite one-, two-, or three-dimensional (1D, 2D, or 3D) structures are assembled with metal ions or polynuclear clusters as nodes and organic ligands as linkers.1 Recently, chemists have devoted themselves to design and synthesize coordination polymers, not only due to their potential applications in the realm of gas adsorption and separation,2 catalysis,3 magnetism,4 luminescence,5 host−guest chemistry,6 etc., but also for their aesthetic and often complicated architectures and topologies.7 However, it is still a big challenge to predict the final structures of desired crystalline products, since many factors such as metal ions, organic ligands, solvent systems, pH, and temperature may have a great influence on the self-assembly process.8 Up to now, systematic investigation of the relationship between diversified initial conditions and final structures is in its infancy. © 2011 American Chemical Society

Furthermore, the combination of multicarboxylate anions with N-donor auxiliary ligands is a good choice for the construction of novel topology and networks. Previously, representative results reported by Yaghi and et al. indicate that 1,4-benzenedicarboxylic acid (p-H2bdc), biphenyl-4,4′-dicarboxylic acid (H2bpdc), and their derivatives are good candidates as organic linkers in the synthesis of porous coordination polymers1a,9 because of their rigidity in conformation as well as varieties in coordination modes and sensitivity to pH values of the carboxylate groups. As well, in the family of N-donor ligands, imidazole-based ligands play important roles in the construction of coordination polymers.10 Among these, 1,4-bis(imidazol-l-yl-methyl)benzene (bix), first Received: August 7, 2011 Revised: November 12, 2011 Published: November 21, 2011 253

dx.doi.org/10.1021/cg201025u | Cryst. Growth Des. 2012, 12, 253−263

Crystal Growth & Design

Article

Table 1. Crystal and Structure Refinement Data for Complexes 1−9 1 empirical formula formula weight T, K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dcalc/g cm−3 μ/mm−1 F(000) reflections collected independent reflections Rint GOF on F2 R1a, I > σ(I) (all) wR2b I > σ(I) (all) Δρmax, Δρmin/e Å−3 empirical formula formula weight T, K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dcalc/g cm−3 μ/mm−1 F(000) reflections collected independent reflections Rint GOF on F2 R1a, I > σ(I) (all) wR2b, I > σ(I) (all) Δρmax, Δρmin/e Å−3 a

C28H32N6O6Zn 613.97 187(2) monoclinic C2/c 20.5754(14) 11.5480(8) 14.0617(10) 90 116.5300(10) 90 2989.3(4) 4 1.364 0.872 1280 8154 2980 0.0360 1.062 0.0382 (0.0548) 0.0782 (0.0845) 0.417, −0.411 6

2

3

C28H32N6O6Co C28H32N6O6Cd 607.53 661.00 187(2) 187(2) monoclinic monoclinic C2/c P2/c 20.4671(11) 17.3468(8) 11.4539(6) 7.2668(3) 14.1389(8) 24.8936(9) 90 90 116.1950(10) 111.930(2) 90 90 2974.1(3) 2910.9(2) 4 4 1.357 1.508 0.627 0.802 1268 1352 8024 15547 2956 5743 0.0255 0.0288 1.077 1.042 0.0389 (0.0491) 0.0396 (0.0525) 0.1062 (0.1132) 0.1040 (0.1121) 0.417, −0.390 0.700, −0.723 7 (squeezed)

C25H27N5O6Cd 605.92 173(2) triclinic P1̅ 10.1872(5) 10.5478(6) 14.1089(9) 68.1130(10) 73.0210(10) 71.9650(10) 1310.58(13) 2 1.535 0.882 616 7347 5122 0.0259 1.070 0.0417 (0.0509) 0.0957 (0.0998) 1.232, −0.853

C22H18N4O4Zn 467.77 173(2) orthorhombic Pna21 18.7116(9) 10.8977(5) 14.6613(7) 90 90 90 2989.6(2) 4 1.039 0.847 960 16029 5695 0.0407 0.977 0.0376 (0.0500) 0.0873 (0.0918) 0.311, −0.261

4 C37H43N7O7Zn 763.15 187(2) triclinic P1̅ 12.6318(8) 12.8249(8) 13.5305(8) 79.9720(10) 70.9580(10) 70.0140(10) 1942.4(2) 2 1.305 0.688 800 10822 7612 0.0279 1.037 0.0788 (0.1123) 0.2134 (0.2415) 1.048, −0.939 8 (squeezed) C44H36N8O8Zn2 935.55 173(2) monoclinic C2/c 34.7613(14) 13.1219(5) 27.2378(10) 90 105.8150(10) 90 11953.8(8) 8 1.040 0.848 3840 32584 11758 0.0574 0.938 0.0437 (0.0716) 0.0970 (0.1049) 0.409, −0.370

5 C37H43N7O7Co 756.71 187(2) triclinic P1̅ 12.6183(6) 12.8952(6) 13.6128(6) 79.3810(10) 70.7780(10) 69.5720(10) 1953.96(16) 2 1.286 0.494 794 10911 7626 0.0206 1.050 0.0782 (0.0987) 0.2197 (0.2398) 0.964, −0.978 9 (squeezed) C22H18N4O4Zn 467.77 173(2) monoclinic P21/n 9.5136(8) 16.6490(14) 17.5660(15) 90 97.206(2) 90 2760.3(4) 4 1.126 0.918 960 13181 4535 0.0902 0.874 0.0520 (0.0914) 0.1075 (0.1185) 0.317, −0.344

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

architectures and help chemists understand the process of selfassembly. Herein, we successfully apply this strategy and obtain nine novel coordination polymers based on imidazole-based ligand bix and various ditopic carboxylate ligands, namely, {[Zn(p-bdc)(bix)](DMF) 2 } n (1), {[Co(p-bdc)(bix)](DMF)2}n (2), {[Cd(p-bdc)(bix)](DMF)2}n (3), {[Zn(bpdc)(bix)](DMF) 3 } n (4), {[Co(bpdc)(bix)](DMF) 3 } n (5), {[Cd(m-bdc)(bix)(H2O)](DMF)}n (6), {[Zn(m-bdc)(bix)](solv)x}n (7), {[Zn2(m-bdc)2(bix)2](solv)x}n (8), and {[Zn(mbdc)(bix)](solv)x}n (9) Their syntheses, crystal structures, and thermal properties are reported in this paper. In addition, the

introduced to the field of coordination polymers by Robson in 1997,11 seized our attention because the presence of two methylene groups between the benzene ring and imidazole ring would undoubtedly cause the rotation around the C−N bonds to be more flexible with the result that a variety of complicated structures would be obtained. This has been verified by Ma and co-workers who have used it as a coligand and constructed intriguing network architectures.12 Considering all these above-mentioned, we consider that the simultaneous use of N-donor ancillary ligands and ditopic carboxylate anions will contribute to the formation of various 254



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photoluminescent properties of four complexes are discussed in detail.

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EXPERIMENTAL SECTION

General Information. All commercially available chemicals and solvents were of analytical grade and were used as received without further purification. 1,4-Bis(imidazol-l-yl-methyl)benzene (bix) was prepared according to the literature.11 Elemental analyses (C, H, N) were performed on a Perkin−Elmer 2400 CHN elemental analyzer. IR spectra were recorded within the 4000−400 cm−1 wavenumber range using a Bruker TENSOR 27 Fourier transform infrared spectrometer with the KBr pellet technique and operating in the transmittance mode. TGA measurements were performed on a Perkin−Elmer Thermal Analysis Pyris Diamond TG/DTA instrument. The samples were heated in flowing N2 from about 40 to 800 °C with a heating rate of 10 °C/min. The powder X-ray diffraction (PXRD) data were collected on a Bruker D8-ADVANCE diffractometer equipped with Cu Kα1 (λ = 1.5406 Å; 1600 W, 40 kV, 40 mA) at a scan speed of 5° min−1. The fluorescence excitation and emission spectra were recorded at room temperature with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as an excitation source. Solid-state UV−vis diffuse reflectance spectra were obtained at room temperature using a Hitachi U-4100 spectrophotometer and BaSO4 was used as a 100% reflectance standard for all samples. Synthesis of {[Zn(p-bdc)(bix)](DMF)2}n (1). A mixture of Zn(NO3)2·6H2O (0.0148 g, 0.05 mmol), p-H2bdc (0.0084 g, 0.05 mmol), bix (0.0119 g, 0.05 mmol), and DMF (6 mL) was sealed in a 15 mL PTFE-lined stainless steel vessel under autogenous pressure and heated at a constant 90 °C for 72 h, and then it was cooled to room temperature slowly. Colorless block crystals of 1 were collected by filtration, washed with DMF and EtOH in sequence, and dried in air with a yield of 63% based on Zn(NO3)2·6H2O. Elem anal. Calcd for C28H32N6O6Zn: C, 54.77; H, 5.25; N, 13.69. Found: C, 54.48; H, 5.39; N, 13.80. FT-IR (cm−1, KBr): 3438(m), 3121(m), 2925(w), 1670(s), 1620(s), 1524(m), 1501(m), 1438(m), 1387(s), 1359(s), 1232(m), 1110(m), 1089(s), 1024(w), 952(w), 864(w), 824(m), 748(s), 657(m), 579(w), 510(w). Synthesis of {[Co(p-bdc)(bix)](DMF)2}n (2). The preparation of 2 was similar to that of 1 except that Co(NO3)2·6H2O (0.0146 g, 0.05 mmol) was used instead of Zn(NO3)2·6H2O. Purple block crystals of 2 were obtained by being mixed with a purple amorphous powder. Bulk analysis was not possible because of the difficulty of manual separation. FT-IR (cm−1, KBr): 3440(m), 3122(w), 2924(w), 1668(s), 1610(s), 1502(m), 1437(m), 1385(s), 1229(w), 1109(m), 1087(s), 1018(w), 951(w), 885(w), 824(m), 746(m), 656(m), 573(w), 534(w). Synthesis of {[Cd(p-bdc)(bix)](DMF)2}n (3). The preparation of 3 was similar to that of 1 except that Cd(NO3)2·4H2O (0.0152 g, 0.05 mmol) was used instead of Zn(NO3)2·6H2O. Colorless block crystals were obtained and filtered off, washed with DMF and EtOH and dried under ambient conditions to give 3 in 58% yield based on Cd(NO3)2·4H2O. Elem anal. Calcd for C28H32N6O6Cd: C, 50.88; H, 4.88; N, 12.71. Found: C, 50.75; H, 4.71; N, 12.95. FT-IR (cm−1, KBr): 3433(m), 3119(m), 2928(m), 2871(w), 1667(vs), 1570(vs), 1516(s), 1446(m), 1387(vs), 1281(w), 1234(m), 1093(s), 1021(w), 940(m), 837(m), 750(m), 656(m), 622(m), 519(m). Synthesis of {[Zn(bpdc)(bix)](DMF)3}n (4). A mixture of Zn(NO3)2·6H2O (0.0148 g, 0.05 mmol), H2bpdc (0.0124 g, 0.05 mmol), bix (0.0119 g, 0.05 mmol), and DMF (7 mL) was sealed in a 15 mL PTFE-lined stainless steel vessel under autogenous pressure, heated at constant 110 °C for 72 h, and allowed to cool down to room temperature at a rate of 5 °C/h. Yield of the reaction was ca. 44% based on Zn(NO3)2·6H2O. Elem anal. Calcd for C37H43N7O7Zn: C, 58.23; H, 5.68; N, 12.85. Found: C, 57.54; H, 5.39; N, 13.06. FT-IR (cm−1, KBr): 3431(m), 3119(m), 2930(w), 1676(s), 1606(vs), 1546(m), 1521(m), 1370(vs), 1235(m), 1172(w), 1106(m), 1090(m), 1027(w), 950(m), 840(m), 773(m), 657(m), 566(w). Synthesis of {[Co(bpdc)(bix)](DMF)3}n (5). A procedure identical to that for 1 was followed to prepare 5 except that Zn(NO3)2·6H2O and p-H2bdc was replaced by Co(NO3)2·6H2O

Figure 1. (a) Coordination environment of the Zn atom in 1 with the ellipsoids drawn at the 50% probability level. The hydrogen atoms and free DMF molecule are omitted for clarity. Symmetry code: A = −x, y, −z + 1/2. (b) View of a single diamond motif in 1. (c) Schematic representation of the 3-fold interpenetrating adamantanoid cages of 1. (d) Schematic view of a single 3D diamond framework. (e) Schematic representation of the 3-fold interpenetrating 3D diamond frameworks.

Scheme 1. Structures of Aromatic Dicarboxylic Acids and Observed Coordination Modes in Compounds 1−9

(0.0146 g, 0.05 mmol) and H2bpdc (0.0124 g, 0.05 mmol), respectively. A handful of pink plate crystals was obtained in slurry, and bulk analysis was not possible because of the difficulty of manual separation. FT-IR (cm−1, KBr): 3436(s), 3129(w), 2925(w), 1659(s), 1606(s), 1586(s), 1529(m), 1386(s), 1285(w), 1234(m), 1176(w), 1090(m), 1029(w), 948(w), 850(m), 773(m), 724(w), 659(m), 620(w), 573(w). Synthesis of {[Cd(m-bdc)(bix)(H2O)](DMF)}n (6). A mixture of Cd(NO3)2·4H2O (0.0153 g, 0.05 mmol), m-H2bdc (0.0084 g, 0.05 mmol), bix (0.0119 g, 0.05 mmol), and DMF (6 mL) was placed in a 15 mL PTFE-lined stainless steel vessel under autogenous pressure, 255



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Figure 2. (a) ORTEP drawing of the coordination environments of Cd1 and Cd2 in 3 with the ellipsoids drawn at the 30% probability level. The hydrogen atoms and free DMF molecules are omitted for clarity. Symmetry codes: A = −x, y, −z − 1/2; B = −x + 1, y, −z − 1/2. (b) 1D zigzag chain constructed by p-bdc2− anions and Cd ions (the bix ligands are omitted for clarity). (c) View of the single puckered 2D (4, 4) sheet in 3. (d) Schematic diagram illustrating stacking of non-interpenetrating networks in 3. FT-IR (cm−1, KBr): 3434(w), 3122(m), 2933(w), 1673(s), 1621(s), 1568(s), 1525(m), 1340(s), 1236(m), 1159(w), 1095(m), 1027(w), 953(m), 831(m), 753(s), 724(m), 653(m), 621(w), 567(w). In addition, a great deal of parallel experiments were carried out to synthesize other supramolecular isomers under changeable reaction conditions. Unfortunately, they failed. X-ray Crystallography. The X-ray intensity data for the nine compounds were recorded on a Bruker SMART APEX-II CCD diffractometer with graphite monochromatized Mo−Kα radiation (λ = 0.71073 Å) operating at 1.5 kW (50 kV, 30 mA) at low temperature. Data integration and reduction were processed with SAINT software.13 Multiscans absorption corrections were applied with the SADABS program.14 The structures for compounds 1, 3, 4, 7, and 8 were solved by direct methods, but the structure for 5 was solved by the Patterson method of SHELXS-97.15 The structures for 2, 6, and 9 were solved by direct methods with SIR92.16 All of the structures were refined by the full-matrix least-squares method on F2 using the SHELXTL-97 program.17 All non-hydrogen atoms were refined anisotropically except some solvent molecules, and hydrogen atoms of the organic ligands were generated geometrically and refined with the use of riding model. Because guest solvent molecules of complexes 6, 7, 8, and 9 were seriously disordered, it was impossible to refine by using conventional models appropriately. The contribution of the electron density associated with disordered solvent molecules was removed by the SQUEEZE subroutine in PLATON.18 All of the crystal data and structure refinement details are summarized in Table 1, and selected bond lengths and angles are listed in Table S1, Supporting Information. CCDC reference numbers of crystals 1−9 are 836701−836709, respectively. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk.

heated at a constant 90 °C for 60 h, and allowed to cool down to room temperature slowly. Crystals of 6 were collected, washed with DMF and EtOH, and dried under ambient conditions (yield: 49% based on Cd). Elem anal. Calcd for C25H27N5O6Cd: C, 49.56; H, 4.49; N, 11.56. Found: C, 50.02; H, 4.35; N, 11.71. FT-IR (cm−1, KBr): 3425(w), 3120(m), 2929(w), 1670(s), 1604(s), 1552(s), 1518(m), 1443(m), 1378(s), 1282(w), 1240(w), 1091(s), 1021(w), 935(w), 858(w), 830(w), 735(m), 655(m). Synthesis of {[Zn(m-bdc)(bix)](solv)x}n (7). A mixture of Zn(NO3)2·6H2O (0.0148 g, 0.05 mmol), m-H2bdc (0.0084 g, 0.05 mmol), bix (0.0119 g, 0.05 mmol), and DMF (6 mL) was placed in a 15 mL PTFE-lined stainless steel vessel. It was heated at 95 °C for 72 h and then was cooled down to 30 °C in 12 h. A small amount of crystals suitable to single-crystal X-ray diffraction was obtained along with an amorphous white solid, which makes it impossible to analyze the bulk samples. FT-IR (cm−1, KBr): 3434(m), 3123(m), 2931(w), 1658(s), 1619(s), 1569(m), 1526(m), 1435(m), 1359(s), 1237(w), 1159(w), 1096(m), 1029(w), 953(m), 831(w), 750(m), 722(m), 657(m), 622(w), 572(w). Synthesis of {[Zn2(m-bdc)2(bix)2](solv)x}n (8). The same stoichiometric ratio and reactive condition with compound 7 were used in the preparation of 8, except that 0.5 mL of EtOH was added to the starting mixture. Then, only a small amount of crystals suitable to single-crystal X-ray diffraction was obtained and bulk analysis was not carried out. FT-IR (cm−1, KBr): 3434(w), 3123(m), 2932(w), 1665(s), 1621(s), 1568(m), 1523(m), 1433(m), 1345(m), 1236(w), 1157(w), 1097(m), 1029(w), 953(m), 831(w), 752(m), 725(m), 655(m), 621(w), 569(w). Synthesis of {[Zn(m-bdc)(bix)](solv)x}n (9). The same stoichiometric ratio and reactive condition with compound 7 were used in the preparation of 9, except that 1.0 mL of EtOH was added to the starting mixture. Then, only a small amount of crystals suitable to single-crystal X-ray diffraction was obtained and bulk analysis was not carried out. 256



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are filled via mutual interpenetration of the independent frameworks, leading to the formation of a 3-fold interpenetrating 3D architecture with dia topological type (Figure 1c,e). Notably, even with this interpenetration, the total free void space value is calculated by PLATON to be 1210.7 Å3 per unit cell (after lattice DMF molecules have been hypothetically removed),20 approximately 40.5% of the crystal cell volume. Crystal Structure of {[Cd(p-bdc)(bix)](DMF)2}n (3). When the Cd2+ cation was utilized in place of Zn2+ and Co2+ cations under the reaction conditions similar to that of compounds 1 and 2, compound 3 was obtained, which possesses an entirely different structure from 1 and 2. Compound 3 crystallizes in the monoclinic space group P2/c. The asymmetric unit consists of two crystallographically unique Cd ions with half occupancy, one p-bdc2− anion, two halves of bix ligands, and two DMF solvent molecules. As illustrated in Figure 2a, two crystallographically independent Cd ions display distorted octahedral coordination geometry, defined by four carboxyl oxygen atoms from two symmetry-related p-bdc2− ligands and two nitrogen atoms from two symmetry-related bix ligands. The p-bdc2− ligand acts in the chelating bidentate coordination mode (Scheme 1b); meanwhile, the bix ligand adopts a cisconformation with two different Ndonor···N−Csp3···Csp3 torsion angle values of 107.11° and 117.89° and dihedral angles between imidazole and phenyl rings of 64.49° and 68.21°. As shown in Figure 2b, neighboring Cd ions are connected by pbdc2− ligands into a zigzag chain. Adjacent chains are further cross-linked by bix ligands, giving rise to a 2-D puckered sheet with intragrid Cd···Cd separations of 11.165 × 12.230 Å across each p-bdc2− ligand and bix ligand, respectively (Figure 2c). A topological analysis of this structure was performed with the TOPOS program,21 which reveals a non-interpenetrating uninodal four-connected (44.62) topology. Furthermore, the neighboring 2-D networks are parallel to each other with a separation of 7.27 Å and stack in an ...AAA... fashion along the b axis to form a 3-D supramolecular framework via the π−π interactions between the aromatic rings (the face-to-face distance 3.565 Å, Figure 2d). The non-interpenetrating nature provides a relatively large solvent-accessible volume of 1013.2 Å3 per unit cell (after lattice DMF molecules have been hypothetically removed), and the space ratio was approximately 34.8%. Crystal Structures of {[Zn(bpdc)(bix)](DMF)3}n (4) and {[Co(bpdc)(bix)](DMF)3}n (5). Single crystal X-ray analysis indicates that compounds 4 and 5 are isostructural, and hence only the results of 4 are given in the ensuing discussion. The crystal structure of 4 was solved in the space group P1.̅ There are one Zn atom, one bix ligand, one bpdc2− anion, and three lattice DMF molecules in the asymmetric unit. Each Zn atom is four-coordinate by two oxygen atoms from two bpdc2− ligands and two nitrogen atoms from two bix ligands, showing a slightly disordered tetrahedral geometry (τ4 ≈ 0.94), as portrayed in Figure 3a. Each bpdc2− anion bridges two Zn atoms in a bis-monodentate coordination mode (Scheme 1c) to form a one-dimensional zigzag chain with the angle between bpdc2− ligands of 115.97°, which approaches 120° (Figure 3b). Meanwhile, two cis-bix ligands, with two different Ndonor···N− Csp3···Csp3 torsion angle values of 92.57° and 107.78° in the same bix ligand, link two adjacent Zn atoms to give rise to a closed loop B, {Zn2(bix)2}. As a result, the zigzag chains are connected by Type-B loops to form an interesting 2D (6,3) layer consisting of the larger hexagonal meshes (Type-A, 24.5 × 21.5 × 25.8 Å) and the smaller loops (type-B, 11.4 × 11.0 Å)

Figure 3. (a) Coordination environment of the Zn atom in 4 with the ellipsoids. The hydrogen atoms and free DMF molecules are omitted for clarity. (b) The 1D zigzag chain constructed by bpdc2− ligands and Zn ions. (c) The illustration of single 2D layer containing hexagonal meshes and loops in the ac plane (hydrogen atoms and solvent molecules are omitted for clarity). (d) Schematic representation of stacking honeycomb nets. (e) The illustration of π−π interactions between adjacent imidazole rings and benzene rings (hydrogen atoms are omitted for clarity).

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RESULTS AND DISCUSSION Structure Description. Crystal Structures of {[Zn(pbdc)(bix)](DMF)2}n (1) and {[Co(p-bdc)(bix)](DMF)2}n (2). Single crystal X-ray analysis reveals that compounds 1 and 2 are isostructural. For the convenience of depiction, the structure of compound 1 is described as a representative example in detail here. Compound 1 crystallizes in the monoclinic system, space group C2/c. Each asymmetric unit of 1 contains one-half Zn atom, one-half p-bdc2−, one-half bix, and one lattice DMF molecule. As shown in Figure 1a, Zn1 displays a slightly distorted tetrahedral geometry defined by two N atoms and two O atoms from different bix ligands and p-bdc2− anions. The distortion of tetrahedron can be indicated by the calculated value of the τ4 parameter introduced by Houser (τ4 ≈ 0.93 in 1).19 The p-bdc2− anion in 1 adopts a bis-monodentate coordination mode (Scheme 1a), whereas the bix ligand adopts a trans-conformation with a Ndonor···N−Csp3···Csp3 torsion angle of 83.97° and the dihedral angle between imidazole and phenyl rings of 82.64°. Tetrahedral nodes of Zn(II) are connected by six bix ligands and six p-bdc2− anions into an adamantanoid cage containing four cyclohexane-like windows, each in a chair conformation (Figure 1b). The Zn···Zn distances across bix and p-bdc2− ligands are 13.752 Å and 10.997 Å, respectively. The adamantanoid cages are elongated significantly along the c axis and exhibit maximum dimensions (corresponding to the longest intracage Zn···Zn distances of 34.644 × 26.407 × 20.575 Å), as described in Figure 1d. From another point of view, the zigzag chains along the (101) direction constructed by Zn cations and p-bdc2− anions were further connected to form 3D dia frameworks with bix ligands. With the spacious nature of the single framework, the large cavities within this structure 257



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Figure 4. (a) Coordination environment of Cd atom in 6 with the ellipsoids. The hydrogen atoms are omitted for clarity. Symmetry codes: A = x, y − 1, z + 1; B = x − 1, y, z. (b) View of the single layer in 6. The O1w and hydrogen atoms are omitted for clarity. (c) Schematic representation of π−π interactions between two layers in distinct pairs with a space-filling mode (yellow: single layer in one pair; green: couple layers in the other pair). (d) The illustration of π−π interactions between adjacent m-bdc2− ligands (hydrogen atoms are omitted for clarity). (e) Perspective view of the completely flat (4, 4) network.

10.187 × 14.120 Å across each m-bdc2− ligand and bix ligand, respectively (Figure 4b). However, a more careful examination leads to the exceptional finding that all the layers can be divided into pairs in the overall network. Each coupled layer is stabilized by significant C−H···π interactions between the aromatic rings and imidazole rings; the edge-to-face separations are about 3.71 and 3.73 Å. The m-bdc2− ligands point outside each coupled layers and hence play an important role in subsequent packing into a 3D framework. Adjacent pairs of layers are further extended into a 3D network under the direction of strong π−π stacking interactions between the mbdc2− ligands; the face-to-face distance is 3.61 Å (Figure 4c,d). Crystal Structure of {[Zn(m-bdc)(bix)](solv)x}n (7). X-ray diffraction result reveals that compound 7 crystallizes in noncentrosymmetric space group Pna21 with Flack parameter 0.0391. One Zn atom, one m-bdc2− anion and one bix ligand are contained in the fundamental asymmetric unit. As depicted in Figure 5a, the Zn cation is coordinated by two O atoms from two different m-bdc2− anions, two N atoms from two distinct bix ligands to furnish a distorted tetrahedral geometry (τ4 ≈ 0.91). The m-bdc2− ligand adopts a monodentate-bridging coordination mode (μ1:η1, Scheme 1e) to link the adjacent Zn atoms into a 1D zigzag chain along the a axis (Figure 5b), while

(Figure 3c). These 2D layers stack in an ...ABAB... sequence in the ac plane into a 3D supermolecular architecture via the π−π stacking interactions between neighboring imidazole and benzene rings that further stabilize the crystal structure. The dihedral angle α, defined by the stacked imidazole and benzene rings, is 11.66°, and the interplanar distance between the centers of two rings is 3.936 Å. The slipping angles β and γ are 31.50° and 23.67°, respectively, indicating a weak stacking interaction (Figure 3e).22 Crystal Structure of {[Cd(m-bdc)(bix)(H2O)](DMF)}n (6). Single crystal X-ray diffraction result reveals that compound 6 crystallizes in space group P1̅. The asymmetric unit contains one crystallographically independent Cd atom, one m-bdc ligand, one bix ligand, one aqua ligand, and one lattice DMF molecule. Each Cd atom is coordinated by two nitrogen atoms from two different bix ligands, three carboxyl oxygen atoms from two symmetry-related m-bdc2− anions, and one aqua ligand (Figure 4a). The m-bdc2− ligand adopts a chelatemonodentate coordination mode (Scheme 1d). The layered structure of 6 is similar to that of 3, in which neighboring Cd ions are also connected by m-bdc2− ligands into a 1D chain and then adjacent chains are further cross-linked by bix ligands, giving rise to a 2D sheet with intragrid Cd···Cd separations of 258



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Figure 5. (a) Coordination environment of Zn atom in 7 with the ellipsoids. The hydrogen atoms are omitted for clarity. Symmetry code: A = x + 1 /2, 3/2 − y, z. (b) 1D chain constructed by m-bdc2− anions and Zn atoms (the bix ligands are omitted for clarity). (c) Schematic representation of the 3-fold interpenetrating adamantanoid cages. (d) Schematic view of a single 3D coordination framework. (e) Schematic representation of the 3fold interpenetrating 3D diamond frameworks.

the bix ligand with nearly trans-conformation coordinates two zinc atoms by its ditopic nitrogen atoms. Similar to compound 1, bix ligands and m-bdc2− ligands bridge Zn atoms to form an adamantanoid unit. Furthermore, three independent diaframeworks interpenetrate mutually to promote the stability of the overall crystal, since large empty voids weaken the stability (Figure 5c−e). In contrast to compound 1, the Zn···Zn distances across bix and m-bdc2− are 13.882 Å and 10.407 Å, respectively; the subtle changes may be due to the obviously different angles of carboxyl groups and bix conformation. The Ndonor···N−Csp3···Csp3 torsion angles and dihedral angles between imidazole and phenyl rings are 89.02° and 90.08°, 85.94°, and 86.06° in the bix ligand. The adamantanoid cages are elongated significantly along the c axis too, with maximum dimensions of 32.693 × 23.771 × 23.771 Å (corresponding to the longest intracage Zn···Zn distances). The solvent molecules, which have been squeezed, probably occupy the potential volume that is approximately 40.5% of the crystal cell volume (1190.9 Å3 out of the 2989.6 Å3). Crystal Structure of {[Zn2(m-bdc)2(bix)2](solv)x}n (8). X-ray diffraction analysis reveals that the asymmetric unit of 8 consists of two crystallographically unique Zn atoms, two deprotonated m-bdc2− anions, and two bix ligands. As shown in Figure 6a, both Zn centers present distorted tetrahedral

geometry, coordinated by two oxygen atoms from different m-bdc2− anions and two nitrogen atoms from different bix ligands. The τ4 parameters introduced by Houser19 of Zn1 and Zn2 are 0.94 and 0.90. Adjacent zinc atoms are connected by m-bdc2− and bix ligands into a unique 2D layer in which the mbdc2− ligand adopts a monodentate coordination mode (Scheme 1e) and the bix ligands present two kinds of conformations. A remarkable feature of this single 2D layer is that three kinds of rings coexist in the overall network (Figure 6b). Two bix ligands with cis- and trans-conformations bridge two Zn atoms into the smallest C-type ring (9.57 × 11.54 Å). If C-type ring is simplified as a line, two m-bdc2− ligands connect them to form a D-type ring that is nearly perpendicular to the C-type ring (10.24 × 11.54 Å). The largest E-type ring is composed of six m-bdc2− ligands and two C-type rings (11.54 × 27.81 Å). In other words, the C-type rings separate and support the 1D Zn - m-bdc2− chains to form D-type rings and E-type rings. A topological analysis of this 2D net was performed with the TOPOS program,21 which reveals a three-connected (4,82)-type topology. Interestingly, the windows of the E-type ring in a single layer are large enough to allow the D-type rings of another layer to penetrate them, thus giving a 2-fold parallel interpenetrated network (Figure 6c).23 This kind of entanglement no doubt further promotes the 259



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Figure 7. (a) ORTEP drawing of the coordination environment of Zn atom in 9 with the ellipsoids. The hydrogen atoms are omitted for clarity. Symmetry codes: A = −x + 1/2, y + 1/2, −z + 1/2; B = x + 1/2, −y + 1/2, z − 1/2. (b) 2D grid network of 9. (c) Schematic diagram of the 2D sql topological net. (d) 3D supramolecular framework containing 1D channels.

a 4-connected node, and the resulting net exhibits a uninodal 2D layer with sql topology (Figure 7c).26 Remarkably, there are two morphologically distinct parallelogrammic windows in the layer: one is relatively outstanding with an acute angle of 81.75°, and the other is more contracted with an acute angle of 39.39°. These 2D layers lie in parallel and stack in an ...AAA... sequence into a 3D supramolecular framework containing 1D channels (Figure 7d), which extend along the a axis. PLATON analysis showed that the effective free volume of 9 is 33.6% of the crystal volume (926.5 Å3 out of the 2760.3 Å3 unit cell volume). Significant interlayer π−π (interplanar distance 3.50 Å) and C−H···π (edge-to-face separation 3.35 Å) supramolecular interactions undoubtedly help to stabilize the resulting structure. Synthetic Chemistry and Structural Diversity. As we know, in the generation of ternary coordination polymer, central metal ions and organic linkers play important roles in the formation of the resulting structures. So we herein have prepared nine coordination polymers employing three kinds of metal salts, three types of aromatic dicarboxylate ligands, and an identical neutral ligand in order to investigate their distinct effects on structural diversity. As far as compounds 1−3 are concerned, the same ligand combination but different metal ions are used. Comparisons of their structures in view of the relationship between the structures and metal ions, two matters were observed: one is that their isostructural behavior of 1 and 2 can be attributed to the same coordination number and coordination environment of the Zn atoms and Co atoms, while the four-coordinated Co center is not common in comparison with the six-coordinated ones; the other is that complete difference between 3D dia frameworks and 2D (4,4) layers is probably attributed to the coordination nature of central metal ions, such as coordination configurations, metal

Figure 6. (a) Coordination environments of Zn atoms in 8 with the ellipsoids. The hydrogen atoms are omitted for clarity. (b) Schematic representation of three types of rings in a single layer. (c) Schematic view of 2-fold parallel interpenetrating (4,82) topological diagram.

stability of the overall structure.24 Worth mentioning here is that the entanglement of layers with (4,82) topology is not very common if compared to layers that show the more common square (44) or hexagonal (63) topologies, and only a few examples are known that include 2-fold and 3-fold parallel interpenetrated sheets.25 Crystal Structure of {[Zn(m-bdc)(bix)](solv)x}n (9). The Xray structural determination indicates that complex 9 crystallizes in the monoclinic space group P21/n, and its asymmetric unit contains one independent Zn(II) ion, one mbdc2− ligand, and one bix ligand. Zn(II) exhibits a slightly distorted tetrahedral coordination geometry (τ4 ≈ 0.96), being coordinated by two O atoms from two monodentate carboxyl groups of two m-bdc2− anions and two N atoms from bix ligands (Figure 7a). In complex 9, the bix ligand shows a cisconformation bridging two Zn atoms with a Zn···Zn distance of 12.334 Å. The Ndonor···N−Csp3···Csp3 torsion angle values are 112.82° and 52.92°, respectively. The dihedral angles between phenyl ring and two imidazole rings are 77.91° and 71.45° in the bix ligand. As shown in Figure 7b, each Zn(II) is connected to four neighboring Zn atoms through two m-bdc2− anions and two bix ligands. Therefore, the Zn(II) center can be regarded as 260



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Scheme 2. Synthetic Route and Structural Diversity of Compounds 1−9

Table 2. Structural Diversity of bix Ligand in Compounds 1−9 compound

conformation

1 and (2)a 3

trans cis

4 and (5)a

cis

6

trans

7

trans

8

cis trans

9 a

cis

Ndonor···N− Csp3···Csp3 torsion angle

dihedral angles between imidazole and phenyl ring

dimensions

83.97° 107.11° and 117.89° 92.57° and 107.78° 84.08° and 151.20° 89.02° and 90.08° 100.05° and 115.76° 48.89° and 101.90° 52.92° and 112.82°

82.64° 64.49° and 68.21°

3D 2D

70.91° and 73.11°

2D

75.60°and 84.45°

2D

85.94° and 86.06°

3D

86.00° and 88.00°

2D

71.72° and 85.41° 71.45° and 77.91°

Figure 8. Photoluminescent spectra of compounds 1, 3, 4, and 6 in the solid state.

2D

they have been squeezed in structural refinement. The difference in their structures may trace back to a discrepancy in the solvent system. The synthetic route and structural diversity of these coordination polymers are shown in Scheme 2. To the best of our knowledge, semirigid ligands can relatively freely rotate to meet the requirement of coordination geometries of metal ions in the assembly process. In this paper, bix ligand was selected by us as an ancillary ligand. It did exhibit rich conformational diversity and afford structural diversity of the final structures. In order to illuminate the influence of bix on the structural diversity more directly, the conformations, the Ndonor···N−Csp3···Csp3 torsion angle values and dihedral angles between imidazole and phenyl rings in these complexes are listed in Table 2. From a crystal engineering perspective, crystals could be thought of as the sum of a series of molecular recognition events and self-assembly, in which many factors can have an

The torsion angles and dihedral angles of 2 and 5 are not listed.

radii and coordination environments, which may induce the connective directions. For the Zn-bix system, three types of aromatic dicarboxylic acid were introduced; fortunately, all carboxyl groups are deprotonated and adopt the monodentate coordination mode. The carboxylate anions bridge Zn(II) ions to form 1D zigzag chains; the lengths of the spacer and the angles between ditopic carboxyl groups will induce the formation of these different secondary structures, which have an important effect on the final structures, while in the Cd-bix system, the coordination modes and the geometries of the carboxyl groups will also impact the structural differences. In compounds 7−9, the components of their networks remain the same but three different architectures exit. As categorized by Moulton and Zaworotko,27a they can be classified as structural superstructures. A plausible reason accounting for this phenomenon is the presence of guest molecules, although 261



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indicates that their different mechanisms are probably due to the intrinsic natures of the carboxylate anions, such as the aromaticity and negative charge density, while in compounds 1 and 3, the electronic nature and coordination environment of different central ions may result in distinct luminescent mechanisms. For the d10 metal complexes, HOMOs are associated with the π-bonding orbital from the aromatic rings of organic ligands, while LUMOs are mainly associated with the M-O (from the carboxylate group) σ*-antibonding orbital, so the luminescent properties are complicated.31 The organic ligands and central metal ions make contributions to the luminescent properties simultaneously.

effect. The results here reported undoubtedly reveal that the structural diversity of coordination polymers can be adjusted by the judicious choice of metal ions, carboxylate anions, and Ndonor ligands. As well, subtle differences of the synthetic solvent system can also have a profound effect on the structural variation. The systematic investigation of the relationship between experimental conditions and final structures provides a promising pathway to rational design of aimed metal−organic coordination polymers. Thermal Analysis and PXRD Results. Powder X-ray diffraction experiments were carried out for compounds 1, 3, 4, and 6. The patterns for the as-synthesized bulk materials closely match the simulated ones from the single-crystal structure analysis, which are indicative of the pure solid-state phases (Figures S3−S6, Supporting Information). To further determine the thermal stability of these compounds, their thermal behaviors were investigated under nitrogen atmosphere by TGA (see Figure S7 in Supporting Information). For complex 1, a rapid weight loss of the lattice DMF molecules of 23.27% (calcd 23.79%) was observed from 70 to 206 °C. The framework started to collapse accompanied by the decomposition of the organic components at 310 °C, leading to the formation of a stoichiometric amount of ZnO as a residue (obsd 13.51%, calcd 13.25%). The TGA curve of 3 showed a weight loss of 19.95% in the range 45−190 °C, which was attributed to the release of DMF molecules in the network. The network of complex 3 started to decompose when the temperature was higher than 248 °C. The remaining weight corresponded to the formation of CdO (obsd 19.73%, calcd 19.43%). Compound 4 had a profile of the TG curve very similar to complex 1, while the loss of solvent molecules was observed from 50 to 174 °C (obsd 25.8%, calcd 28.7%), and the decomposition of the compound occurred at ca. 335 °C, with the residue of ZnO (obsd 11.75%, calcd 10.58%). In the case of 6, a continuous weight loss of 15.69% (calcd 15.05%) was observed from 34 to 270 °C, which was attributed to the loss of H2O and DMF molecules in the structure. The network collapsed at 325 °C and ended at about 530 °C. Photoluminescent Properties. Luminescent compounds are of great interest because of their potential applications in photochemistry, chemical sensors, and electroluminescent display.28 The luminescent properties of complexes 1, 3, 4, 6, as well as free ligands p-H2bdc, H2bpdc, m-H2bdc, and bix ligand were investigated in the solid state at room temperature, as illustrated in Figure 8, Figures S8 and S9 in Supporting Information. The free organic p-H2bdc, H2bpdc, m-H2bdc, and bix ligands show intense emission bands at 382 nm (λex = 314 nm), 400 nm (λex = 345 nm), 360 nm (λex = 310 nm), and 401 nm (λex = 341 nm). All of the ligand emission peaks could be probably attributed to the π*-n or π*−π transitions.29,22 Obviously, the fluorescent emission bands of 1 (λem = 390 nm, λex = 339 nm) and 6 (λem = 404 nm, λex = 330 nm) are probably due to the intraligand charge transitions of the neutral ligand because peaks similar to the free bix ligand appear. Complexes 3 and 4 show broad emission bands centered at 423 nm (λex = 341 nm) and 371 nm (λex = 331 nm), respectively. However, the emissions arising from the free ligands are not observed. The absence of ligand-based emissions suggests energy transfer from the ligands to the central metal ions during photoluminescence. Thus, the photoluminescence can be assigned to the emission of ligand-to-metal charge-transfer (LMCT).30 Comparison of the luminescent properties of the two Zn-based compounds, namely, compounds 1 and 4,

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CONCLUSIONS In summary, we have successfully synthesized nine coordination polymers by self-assembly of an identical neutral bix ligand, various aromatic dicarboxylic acids, and different metal ions under suitable solvothermal conditions. Structures of compounds 1−9 vary from non-interpenetrating layers, interpenetrating layers to 3D interpenetrated frameworks. Structural diversity reported here indicates that length, shape, and the conformation of the organic ligand play significant roles in molecular tectonics of the coordination polymers. In addition, the experimental conditions also have profound effects on the final structures. The synthesis of these complexes may provide the possibility to controllably design and synthesize desired crystalline material with predictable structures. Further research is ongoing to prepare novel coordination polymers with tunable structures based on bix and its derivatives in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files for all the structures in CIF format and other information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *Tel: 86-431-85262127. Fax: 86-431-85698041. E-mail: [email protected].

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ACKNOWLEDGMENTS The authors are grateful to the financial aid from the National Natural Science Foundation of China (Grant No. 21071140) and National Natural Science Foundation for Creative Research Group (Grant No. 20921002).

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REFERENCES

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