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DOI: 10.1021/cg100688z

Syntheses, Structures, and Photoluminescence of Zinc(II) Coordination Polymers Based on Carboxylates and Flexible Bis-[(pyridyl)-benzimidazole] Ligands

2010, Vol. 10 4795–4805

Hai-Yan Liu,†,‡ Hua Wu,† Jian-Fang Ma,*,† Ying-Ying Liu,† Bo Liu,† and Jin Yang*,† †

Key Lab of Polyoxometalate Science, Department of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China, and ‡Department of Chemistry and Pharmaceutical Engineering, Suihua University, Suihua 152061, People’s Republic of China Received May 23, 2010; Revised Manuscript Received August 29, 2010

ABSTRACT: Six coordination polymers based on three related flexible bis-[(pyridyl)-benzimidazole] ligands and different carboxylates, namely, [Zn2(m-BDC)2(L1)] 3 2H2O (1), [Zn2(p-BDC)2(L1)(H2O)2] (2), [Zn2(m-BDC)2(L2)] 3 2.25H2O (3), [Zn2(p-BDC)2(L2)] 3 CH3OH (4), [Zn2(m-BDC)2(L3)] 3 2H2O (5), and [Zn2(p-BDC)2(L3)] 3 2CH3OH (6), where L1 = 1,10 -(1,4-butanediyl)bis[2-(2-pyridyl)benzimidazole], L2 = 1,10 -(1,6-hexanediyl)bis[2-(2-pyridyl)benzimidazole], L3 = 1,10 -(1,10-decanediyl)bis[2-(2-pyridyl)benzimidazole], m-BDC = m-benzenedicarboxylate anion, and p-BDC = p-benzenedicarboxylate anion, have been synthesized under solvothermal conditions. Their structures have been determined by single crystal X-ray diffraction analyses and further characterized by elemental analyses and infrared (IR) spectra. Compound 1 shows a one-dimensional (1D) double chain which extends into a two-dimensional (2D) supramolecular sheet by π-π interactions. Polymers 2-4 display similar 2D layer structures with 3-connected 63 topologies. In 2 and 3, the adjacent layers are further connected through π-π interactions to form three-dimensional (3D) supramolecular structures. However, in 4, two identical layers penetrate each other in parallel modes to give a 2-fold parallel interpenetrating net. Compounds 5 and 6 show 2-fold interpenetrating networks with R-Po topological structures. A systematic structural comparison of these complexes indicates that the different lengths of the bis-[(pyridyl)-benzimidazole] ligands (L1-L3) are important for the formation of the different structures. In addition, the luminescent properties of L1-L3 and 1-6 have been studied in the solid state at room temperature.

Introduction Metal-organic frameworks (MOFs) on the basis of selfassembly of metal ions and multifunctional ligands have been rapidly developed because of their intriguing molecular topologies and potential applications in the fields of catalysis, ion exchange, gas absorption, luminescence, nonlinear optics (NLO), and magnetism.1-6 Generally, the construction of molecular architectures greatly depends on the coordination geometry of central metal and organic ligands.7 In this regard, the design of suitable organic ligands favoring structure-specific self-assembly is one of the keys for the construction of coordination architectures. Among the reported structures, organic ligands with carboxylate groups are particularly interesting because of their various coordination modes to metal ions, resulting from completely or partially deprotonated sites allowing for the large diversity of topologies.8 On the other hand, the flexible N-bridging ligands, such as 1,3-bis(40 -pyridyl)propane,9a 1,4-bis(4-pyridyl)butane,9a 1,2bis(40 -pyridyl)ethane,9b,c 1,2-bis(tetrazol-5-yl)ethane,9d 1,4bis(tetrazol-5-yl)butane,9d and 1,6-bis(tetrazol-1-yl)hexane9e have been investigated as auxiliary ligands for the construction of novel MOFs. Through using these flexible N-bridging ligands, a variety of MOFs with versatile topologies have been obtained.9 In our previous studies, several flexible bis(imidazole) ligands, such as 1,10 -(1,4-butanediyl)bis(imidazole), 1,4bis(imidazol-1-ylmethyl)benzene, 1,2-bis(imidazol-1-ylmethyl)benzene, and 2,20 -bis(1H-imidazolyl)ether, have been widely

Scheme 1. Molecular Structure of Ligands

*Correspondence authors. (J.-F.M.) E-mail: [email protected]. cn. Fax: þ86-431-85098620 . (J.Y.) E-mail: [email protected].

studied and yielded a few fascinating architectures.10 On the other hand, the crystal structures and optical properties of the (pyridyl)-benzimidazole-metal complexes have been well studied. The results indicate that the (pyridyl)-benzimidazole ligands and their complexes are good candidates for optical materials.11 However, the crystal structures and optical properties of bis-[(pyridyl)-benzimidazole]-metal complexes are rarely reported.11 To the best of our knowledge, the coordination chemistry of the alkyl substituted flexible bis-[(pyridyl)benzimidazole] ligand and the optical properties of their complexes have not been investigated.12 On the basis of previous studies, we synthesized three flexible bis-[(pyridyl)-benzimidazole] ligands (L1-L3) (L1= 1,10 -(1,4-butanediyl)bis[2-(2-pyridyl)benzimidazole], L2 = 1,10 -(1,6-hexanediyl)bis[2-(2-pyridyl)benzimidazole], and L3 = 1,10 -(1,10-decanediyl)bis[2-(2-pyridyl)benzimidazole]) based on 2-(2-pyridyl)benzimidazole (Scheme 1). The three ligands have the following structural character: (i) compared with the typical N,N-chelating ligands, such as 2,20 -bipyridine and 1,10-phenanthroline, the L1-L3 ligands can act as both chelating and bridging ligands. (ii) The flexible nature of -CH2- spacers allows the ligand to bend and rotate freely

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when coordinating to metal centers so as to conform to the coordination geometries of metal ions. (iii) The large aromatic system can provide potential supramolecular recognition sites for π-π stacking interactions that can be used to govern the process of self-assembly. Here, six new coordination polymers [Zn2(m-BDC)2(L1)] 3 2H2O (1), [Zn2 (p-BDC)2 (L1) (H2O)2] (2), [Zn2(m-BDC)2(L2)] 3 2.25H2O (3), [Zn2(p-BDC)2(L2)] 3 CH3OH (4), [Zn2(m-BDC)2(L3)] 3 2H2O (5), and [Zn2(p-BDC)2(L3)] 3 2CH3OH (6) have been successfully synthesized under solvothermal conditions (m-BDC = m-benzenedicarboxylate anion and p-BDC = p-benzenedicarboxylate anion). Their structures have been determined by single crystal X-ray diffraction analyses, IR spectra, and elemental analyses. In addition, the luminescent properties of the complexes have also been investigated in the solid state. Experimental Section General Procedures. Chemicals were purchased from commercial sources and used without further purification. The 2-(2-pyridyl)benzimidazole was synthesized according to the literature method.13 Syntheses of L1, L2, and L3. A mixture of 2-(2-pyridyl)benzimidazole (7.8 g, 40 mmol) and NaOH (1.68 g, 42 mmol) in DMSO (20 mL) was stirred at 60 °C for 0.5 h, and then 1,4-dibromobutane (4.32 g, 20 mmol) was added. The mixture was cooled to room temperature being stirring at 60 °C for 12 h, and then poured into 400 mL of ice water. A yellow solid of L1 formed immediately, which was isolated by filtration in 85% yield after drying in air. Anal. Calcd for C28H24N6 (Mr = 444.53) C, 75.65; H, 5.44; N, 18.91. Found: C, 75.58; H, 5.50; N, 18.86. IR (cm-1): 3431(w), 3052(w), 2953(s), 2926(m), 2865(w), 1591(s), 1562(m), 1507(w), 1465(s), 1443(s), 1386(s), 1352(s), 1329(s), 1275(s), 1241(m), 1211(m), 1145(m), 1113(w), 1093(m), 1048(w), 1009(w), 995(m), 901(w), 836(w), 822(m), 800(s), 737(s), 696(m), 632(w), 576(w), 511(w), 434(w). 1H NMR (400 MHz, CDCl3) δ/ppm: 2.01 (2H, triplet, N-C-CH2), 4.86 (2H, triplet, N-CH2-C), 8.56-7.29 (8H, multiplet, aromatic protons). L2 was prepared in the same way as for L1 by using corresponding 1,6-dibromohexane instead of 1,4-dibromobutane. Yield: 80% based on 2-(2-pyridyl)benzimidazole. Anal. Calcd for C30H28N6 (Mr = 472.568) C, 76.24; H, 5.97; N, 17.79. Found: C, 76.15; H, 5.86; N, 17.83. IR (cm-1): 3424(w), 3055(w), 2930(m), 2851(m), 1590(s), 1565(m), 1506(w), 1464(s), 1432(s), 1397(s), 1365(s), 1333(s), 1272(m), 1255(m), 1198(w), 1154(m), 1114(w), 1090(m), 1038(w), 988(w), 895(w), 826(w), 789(s), 771(m), 759(m), 737(s), 698(m), 629(w), 577(w), 508(w), 429(w). 1H NMR (400 MHz, CDCl3) δ/ppm: 1.37 (2H, multiplet, C-C-CH2), 1.86 (2H, triplet, N-C-CH2), 4.79 (triplet, N-CH2-C), 8.59-7.28 (8H, multiplet, aromatic protons). L3 were prepared in the same way as for L1 by using corresponding 1,10-dibromodecane instead of 1,4-dibromobutane. Yield: 82% based on 2-(2-pyridyl)benzimidazole. Anal. Calcd for C34H36N6 (Mr = 528.69) C, 77.24; H, 6.86; N, 15.90. Found: C, 77.17; H, 6.92; N, 15.85. IR (cm-1): 3425(s), 3038(m), 2996(m), 2922(s), 2851(s), 1590(s), 1563(m), 1511(m), 1469(s), 1437(s), 1395(s), 1368(s), 1331(s) 1294(m), 1277(s), 1255(s), 1166(m), 1146(m), 1092(m), 1038(m), 1006(m), 994(m), 895(w), 826(w), 789(s), 762(s), 740(s), 696(m), 629(w), 609(w), 580(w), 432(w). 1H NMR (400 MHz, CDCl3) δ/ppm: 1.23-1.33 (6H, multiplet, C-C-CH2), 1.86 (2H, multiplet, N-C-CH2), 4.81 (2H, triplet, N-CH2-C), 8.67-7.28 (8H, multiplet, aromatic protons). Synthesis of [Zn2(m-BDC)2(L1)] 3 2H2O (1). A mixture of Zn(OAc)2 3 2H2O (0.044 g, 0.2 mmol), m-H2BDC (0.033 g, 0.2 mmol), L1 (0.044 g, 0.1 mmol), water (8 mL), and methanol (2 mL) was placed in a Teflon reactor (15 mL), which was heated at 140 °C for 3 days and then gradually cooled to room temperature at a rate of 10 °C 3 h-1. Colorless crystals of 1 were obtained. Yield: 68% based on Zn(OAc)2 3 2H2O. Anal. Calcd for C44H36N6O10Zn2 (Mr = 939.52) C, 56.24; H, 3.86; N, 8.94. Found: C, 56.40; H, 3.98; N, 8.68. IR (cm-1) 3546 (s), 3073 (ms), 1614 (s), 1576 (s), 1548 (s), 1499 (s), 1481 (s), 1432 (s), 1372 (s), 1290 (s), 1277 (s), 1156 (s), 1105 (ms), 1123 (ms), 1079 (s), 1055 (ms), 1007 (ms), 930 (ms), 914 (ms), 827

Liu et al. (ms), 758 (s), 744 (s), 721 (s), 695 (s), 633(ms), 570 (s), 505 (ms), 432 (s). Synthesis of [Zn2(p-BDC)2(L1)(H2O)2] (2). A mixture of Zn(OAc)2 3 2H2O (0.044 g, 0.2 mmol), p-H2BDC (0.033 g, 0.2 mmol), L1 (0.044 g, 0.1 mmol), water (8 mL), and methanol (2 mL) was placed in a Teflon reactor (15 mL), which was heated at 140 °C for 3 days and then gradually cooled to room temperature at a rate of 10 °C 3 h-1. Colorless crystals of 2 were obtained. Yield: 46% based on Zn(OAc)2 3 2H2O. Anal. Calcd for C44H36N6O10Zn2 (Mr = 939.53): C, 56.24; H, 3.86; N, 8.94. Found: C, 56.43; H, 3.62; N, 8.76. IR (cm-1) 3297 (ws), 2878 (ws), 2609 (ws), 1597 (s), 1486 (s), 1460 (ms), 1436 (s), 1361 (s), 1302 (s), 1289 (s), 1165 (ms), 1140 (ws), 1124 (ws), 1087 (ws), 1054 (ws), 1010 (ms), 969 (ws), 947 (ws), 932 (ws), 885 (ws), 824 (ms), 786 (ms), 740 (s), 692 (ms), 642 (ms), 570 (ms), 529 (ms), 500 (ms), 427 (ws), 413 (ws). Synthesis of [Zn2(m-BDC)2(L2)] 3 2.25H2O (3). A mixture of Zn(OAc)2 3 2H2O (0.044 g, 0.2 mmol), m-H2BDC (0.033 g, 0.2 mmol), L2 (0.047 g, 0.1 mmol), water (5 mL), and methanol (5 mL) was placed in a Teflon reactor (15 mL), which was heated at 140 °C for 3 days and then gradually cooled to room temperature at a rate of 10 °C 3 h-1. Colorless crystals of 3 were obtained. Yield: 59% based on Zn(OAc)2 3 2H2O. Anal. Calcd for C46H40.50N6O10.50Zn2 (Mr = 972.08): C, 56.83; H, 4.20; N, 8.65. Found: C, 56.61; H, 4.39; N, 8.83. IR (cm-1) 3415 (s), 3068 (ms), 2937 (ms), 2870 (ms), 1612 (s), 1558 (s), 1508 (s), 1484 (s), 1458 (s), 1435 (s), 1360 (s), 1294 (s), 1272 (s), 1162 (ms), 1079 (ms), 1051 (ws), 1019 (ms), 933 (ms), 914 (ws), 833 (ms), 808 (ms), 792 (ms), 745 (s), 727 (s), 695 (s), 662 (ms), 642 (ms), 579 (ms), 502 (ms), 458 (s), 408 (ms). Synthesis of [Zn2(p-BDC)2(L2)] 3 CH3OH (4). A mixture of Zn(OAc)2 3 2H2O (0.044 g, 0.2 mmol), m-H2BDC (0.033 g, 0.2 mmol), L2 (0.047 g, 0.1 mmol), water (5 mL), and methanol (5 mL) was placed in a Teflon reactor (15 mL), which was heated at 140 °C for 3 days and then gradually cooled to room temperature at a rate of 10 °C 3 h-1. Colorless crystals of 4 were obtained. Yield: 65% based on Zn(OAc)2 3 2H2O. Anal. Calcd for C47H40N6O9Zn2 (Mr = 963.59): C, 58.58; H, 4.18; N, 8.72. Found: C, 58.36; H, 4.02; N, 8.59. IR (cm-1) 3450 (ws), 3069 (ws), 2923 (ws), 2856 (ws), 1577 (s), 1560 (s), 1505 (ms), 1485 (s), 1394 (s), 1292 (s), 1169 (ms), 1123 (ws), 1111 (ws), 1093 (ws), 1031 (ws), 1012 (ms), 933 (ws), 903 (ws), 843 (s), 819 (ws), 791 (ws), 765 (ms), 745 (s), 695 (ms), 638 (ws), 533 (ms), 432 (ws), 407 (ws). Synthesis of [Zn2(m-BDC)2(L3)] 3 2H2O (5). A mixture of Zn(OAc)2 3 2H2O (0.044 g, 0.2 mmol), m-H2BDC (0.033 g, 0.2 mmol), L3 (0.053 g, 0.1 mmol), water (2 mL), and methanol (8 mL) was placed in a Teflon reactor (15 mL), which was heated at 140 °C for 3 days and then gradually cooled to room temperature at a rate of 10 °C 3 h-1. Colorless crystals of 5 were obtained. Yield: 51% based on Zn(OAc)2 3 2H2O. Anal. Calcd for C50H48N6O10Zn2 (Mr = 1023.68): C, 58.66; H, 4.73; N, 8.21. Found: C, 58.47; H, 4.56; N, 8.03. IR (cm-1) 3415 (ws), 3062 (ws), 3029 (ws), 2923 (ms), 2851 (ms), 1622 (s), 1571 (s), 1480 (s), 1456 (s), 1431 (s), 1400 (s), 1363 (s), 1300 (s), 1287 (s), 1268 (s), 1176 (ms), 1160 (ms), 1121 (ms), 1109 (ms), 1093 (ms), 1079 (ms), 1036 (s), 1009 (ms), 941 (ms), 839 (ws), 817 (ms), 807 (ms), 789 (ms), 749 (s), 706 (s), 657 (s), 632 (ms), 591 (ms), 545 (ms), 504 (ms), 467 (ms), 430 (s), 412 (s). Synthesis of [Zn2(p-BDC)2(L3)] 3 2CH3OH (6). A mixture of Zn(OAc)2 3 2H2O (0.044 g, 0.2 mmol), m-H2BDC (0.033 g, 0.2 mmol), L3 (0.053 g, 0.1 mmol), water (2 mL), and methanol (8 mL) was placed in a Teflon reactor (15 mL), which was heated at 140 °C for 3 days and then gradually cooled to room temperature at a rate of 10 °C 3 h-1. Colorless crystals of 6 were obtained. Yield: 46% based on Zn(OAc)2 3 2H2O. Anal. Calcd for C52H50N6O10Zn2 (Mr = 1049.72): C, 59.49; H, 4.80; N, 8.01. Found: C, 59.62; H, 4.59; N, 8.18. IR (cm-1) 3054 (ws), 2920 (s), 2851 (s), 1605 (s), 1498 (s), 1479 (s), 1456 (s), 1405 (s), 1386 (s), 1364 (s), 1290 (s), 1172 (ms), 1140 (ws), 1104 (ws), 1070 (ws), 1047 (ws), 1007 (ms), 981 (ws), 932 (ws), 883 (ms), 819 (s), 789 (ms), 761 (s), 749 (s), 695 (ms), 632 (ms), 569 (ms), 515 (s), 432 (ms), 410 (ms). Physical Measurements. Elemental analyses were carried out with a Carlo Erba 1106 elemental analyzer, and the FT-IR spectra were recorded from KBr pellets in range 4000-400 cm-1 on a Mattson Alpha-Centauri spectrometer. The phase purities of the bulk samples were identified by X-ray powder diffraction on a Siemens D5005 diffractometer. 1H NMR spectra were recorded on a Bruker

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Table 1. Crystal Data and Structure Refinements for Compounds 1-6 1

2

3

empirical formula fw crystal size [mm] crystal system space group a [A˚] b [A˚] c [A˚] R [°] β [°] γ [°] volume [A˚3] Z GOF reflns collected/unique Rint R1 [I > 2σ(I)] wR2 (all data)

C44H36N6O10Zn2 939.52 0.22  0.18  0.10 monoclinic C2/c 19.72(2) 12.388(8) 16.56(1) 90 104.9(1) 90 3910(6) 8 0.957 15455/3505 0.0805 0.0708 0.1843

C44H36N6O10Zn2 939.53 0.12  0.10  0.06 triclinic P1 8.712(3) 10.869(5) 12.553(6) 114.25(4) 92.97(4) 111.84(4) 976.1(7) 1 1.088 9208/3350 0.1515 0.0753 0.1887

C46H40.5N6O10.5Zn2 976.08 0.18  0.16  0.10 monoclinic P21/n 8.6457(5) 23.0914(11 11.5675(5) 90 105.197(5) 90 2228.59(19) 2 1.048 11052/4545 0.0612 0.0781 0.2027

4

5

6

empirical formula fw crystal size [mm] crystal system space group a [A˚] b [A˚] c [A˚] R [°] β [°] γ [°] volume [A˚3] Z GOF reflns collected/unique Rint R1 [I > 2σ(I)] wR2 (all data)

C47H40N6O9Zn2 963.59 0.24  0.18  0.16 orthorhombic Pbcn 15.0433(7) 16.9932(6) 18.9513(7) 90 90 90 4844.6(3) 4 0.780 22845/4400 0.1007 0.0568 0.1687

C50H48N6O10Zn2 1023.68 0.32  0.28  0.18 monoclinic P21/n 10.811(5) 15.391(5) 15.011(5) 90 110.388(5) 90 2341.2(15) 2 0.841 16395/5578 0.0441 0.0496 0.1526

C52H50N6O10Zn2 1049.72 0.28  0.22  0.16 triclinic P1 10.156(5) 10.933(5) 12.072(5) 107.783(5) 95.133(5) 96.859(5) 1255.9(10) 1 0.920 9709/5714 0.0441 0.0545 0.1292

DPX-400 instrument. The solid-state emission/excitation spectra were recorded on a Edinburgh Fluorescence spectrometer at room temperature. X-ray Crystallography. Single-crystal X-ray diffraction data for compounds 1, 3-6 were recorded on an Oxford Diffraction Gemini R CCD with graphite-monochromated Mo KR radiation (λ = 0.71073 A˚) at 293 K. Single-crystal data for 2 was collected on an Oxford Diffraction Gemini R CCD with graphite-monochromated Cu KR radiation (λ = 1.54184 A˚) at 293 K. The structures were solved with the direct method of SHELXS-9714 and refined with full-matrix least-squares techniques using the SHELXL-97 program15 within WINGX.16 The non-hydrogen atoms of complexes 1, 2, 4, and 5 were refined with anisotropic temperature parameters. The hydrogen atoms attached to carbons were generated geometrically. Some C and O atoms of complexes 3 and 6 (O2W for 3; O5, O50 , C23, C230 , C24, C240 , C25, and C250 for 6) were refined with isotropic temperature parameters and other non-hydrogen atoms were refined anisotropically. The disordered C atoms and O atoms in compound 6 (C23, C230 , C24, C240 , C25, C250 , O5, O50 ) and the disordered O atoms in 3 (O2, O20 , O1W, O1W0 ) and 5 (O1W, O1W0 ) were refined using C and O atoms split over two sites, with a total occupancy of 1. In compound 3, the O2W atom was assigned 0.125 occupancy without H atoms. For compounds 1-6, the hydrogen atoms of the disordered C atoms, water molecules, and methanol molecules were not included in the model. Hydrogen atoms of other water molecules were located from difference Fourier maps and refined with isotropic displacement parameters.

Results and Discussion Selected bond distances and angles and hydrogen bonds for compounds 1-6 are listed in Tables S1 and S2, Supporting Information. The detailed crystallographic data and structure refinement parameters for 1-6 are summarized in Table 1.

Structure of [Zn2(m-BDC)2(L1)] 3 2H2O (1). As shown in Figure 1a, the structure of 1 contains one unique Zn(II) cation, one kind of m-BDC anion, one kind of L1 ligand, and one lattice water molecule. The Zn(II) center is six-coordinated by four carboxylate oxygen atoms from three m-BDC anions (Zn(1)-O(4) = 2.043(5), Zn(1)-O(3)#1 = 2.044(4), Zn(1)-O(1)#2 = 2.436(6), and Zn(1)-O(2)#2 = 2.087(4) A˚) and two nitrogen atoms from L1 ligand (Zn(1)-N(2) = 2.008(6) and Zn(1)-N(3) = 2.392(8) A˚), showing a distorted octahedral coordination geometry. The average Zn-O and Zn-N distances are 2.15 and 2.20 A˚, respectively. The Zn-O and Zn-N distances are quite similar to the normal Zn-O and Zn-N distances.17 The two carboxylate groups of m-BDC show bidentate chelating and bidentate bridging coordination modes, respectively (Chart 1c). Each m-BDC anion connects three Zn(II) cations to form a one-dimensional (1D) double chain (Figure S1, Supporting Information). The L1 ligand exhibits a bis-bidentate chelating coordination mode and a “U-shaped” conformation, attaching to the double chain of m-HBC anions and Zn(II) ions (Figure 1b). The chains are extended into 2D layers through π-π interactions between pyridyl rings from adjacent chains with a face-to-face distance of 3.50 A˚ and centroid-centroid distance of 4.66 A˚ (Figure 1c). Structure of [Zn2(p-BDC)2(L1)(H2O)2] (2). When p-BDC with a different angular character replaced m-BDC anion, a two-dimensional (2D) layer structure of 2 was obtained. There is one kind of unique Zn(II) atom, two kinds of unique p-BDC anions, one kind of L1 ligand, and one coordinated water molecule. The Zn(II) atom is five-coordinated by two

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Liu et al. Chart 1. Coordination Modes of m-BDC2- and p-BDC2Anions in 1-6

Figure 1. (a) Coordination environment of the Zn(II) ion in 1. (b) View of the 1D double chain structure of 1. (c) The 2D supramolecular layer via π-π interactions in 1.

carboxylate oxygen atoms from different p-BDC anions (Zn(1)-O(3) = 1.974(5) and Zn(1)-O(1) = 2.098(5) A˚), two nitrogen atoms from the L2 ligand (Zn(1)-N(1) = 2.046(7) and Zn(1)-N(3) = 2.315(8) A˚), and one water molecule (Zn(1)-O(1W) = 2.092(7) A˚), showing a trigonal bipyramidal geometry (Figure 2a). In 2, the carboxylate groups of the p-BDC anion exhibit the bis-monodentate coordination mode (Chart 1d). In this mode, the p-BDC anions link the Zn(II) ions to give a 1D zigzag chain. The adjacent chains are further bridged by L2 ligands to form a 2D layer (Figure 2b), showing a rectangular window with the dimensions 23.720  11.084 A˚. So six Zn(II) ions are linked by four p-BDC anions and two L1 ligands to form a large hexagonal 58-membered ring [Zn6(p-BDC)4(L1)2]. If each

Zn(II) atom is a three-connected node, the structure of 2 can be described as a 63 topology (Figure 2c). In addition, two kinds of π-π stacking interactions exist associated with benzimidazole rings and pyridyl rings (face-to-face distance of 3.52 A˚, and centroid-centroid distance of 4.12 A˚), and phenyl rings and benzimidazole rings (face-to-face distance of 3.61 A˚, and centroid-centroid distance of 3.84 A˚) in the adjacent layers, respectively. These interlamellar π-π interactions led the 2D layers to a 3D supramolecular structure (Figure 2d). Structure of [Zn2(m-BDC)2(L2)] 3 2.25H2O (3). When L1 ligand is replaced with longer L2 ligand, a similar parallel 2D structure of 3 has been obtained. As shown in Figure 3a, the Zn(II) center exhibits a distorted tetrahedral geometry surrounded by two carboxylate oxygen atoms from two m-BDC anions (Zn(1)-O(1) = 1.959(5) and Zn(1)-O(3)#1 = 1.977(5) A˚), and two nitrogen atoms from the same L2 ligand (Zn(1)N(1) = 2.003(6) and Zn(1)-N(2) = 2.153(6) A˚). Each m-BDC anion coordinates to two zinc ions, with two carboxylate groups adopting bis-monodentate coordination modes (Chart 1a). The adjacent Zn(II) atoms are bridged by the m-BDC ligands to result in 1D chains, which are further linked by L2 ligands to generate a 2D wavelike layer (Figure 3b), showing a rectangular window with dimensions 23.223  9.419 A˚. Similar to compound 2, there is a large 58-membered ring [Zn6(m-BDC)4(L2)2] within the layer, formed by four m-BDC anions, two L2 ligands, and six Zn(II) ions. The structure of 3 also can be described as a 63 topology (Figure 3c). Adjacent layers are linked by π-π interactions between benzimidazole rings and pyridyl rings with a face-to-face distance of 3.43 A˚ and centroid-centroid distance of 3.71 A˚ to give a 3D supramolecular structure (Figure 3d). Structure of [Zn2(p-BDC)2(L2)] 3 CH3OH (4). The p-BDC anion is selected instead of m-BDC anion, and compound 4 is obtained. Compound 4 shows a rare 2D f 2D example with 2-fold parallel interpenetration of (6,3) layers. As shown in Figure 4a, the Zn(II) atom shows an octahedral coordination geometry [ZnO4N2] which is completed by four carboxylate

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Figure 2. (a) Coordination environment of the Zn(II) ion in 2. (b) View of the 2D layer of 2. (c) Schematic view of the 63 topology of 2. (d) The 3D supramolecular structure via π-π interactions in 2.

oxygen atoms from two p-BDC anions (Zn(1)-O(1) = 2.069(4), Zn(1)-O(2) = 2.246(5), Zn(1)-O(3)#1 = 2.176(4), and Zn(1)-O(4)#1 = 2.166(5) A˚), and two nitrogen atoms from

one L2 ligand (Zn(1)-N(1) = 2.053(5) and Zn(1)-N(3) = 2.134(6) A˚). The p-BDC anions bridge adjacent Zn(II) atoms in the bidentate chelating modes (Chart 1e) to give a 1D zigzag

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Figure 3. (a) Coordination environment of the Zn(II) ion in 3. (b) View of the 2D wavelike sheet structure of 3. (c) Schematic view of the 63 topology of 3. (d) The 3D supramolecular structure via π-π interactions in 3.

chain, which is further linked by L2 ligands to generate a 2D (6,3) sheet (Figure 4b,c). Within the layer, there is a large grid, showing the dimensions 18.951  13.478 A˚. Different from 2 and 3, the large grid allows each net to be penetrated by other independent net in parallel mode to form a 2-fold interpenetrating network with 63 topology (Figure 4d). Structures of [Zn2(m-BDC)2(L3)] 3 2H2O (5) and [Zn2(pBDC)2(L3)] 3 2CH3OH (6). In order to investigate the influence of flexible N-donor ligands with different lengths on the

ultimate structures, the longer L3 is selected instead of L2 ligand, and compounds 5 and 6 are obtained. Compounds 5 and 6 show 2-fold interpenetrating 3D network structures. As shown in Figure 5a, the structure of 5 contains one kind of Zn(II) atom, one kind of m-BDC anion, one kind of L3 ligand, and one lattice water molecule. The Zn(II) center is coordinated by three carboxylate oxygen atoms from two m-BDC ions and two nitrogen atoms from one L3 ligand to give a ZnO3N2 square-pyramidal geometry [Zn(1)-O(4) = 1.949(3),

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Figure 4. (a) Coordination environment of the Zn(II) ion in 4. (b) View of the 2D sheet structure of 4. (c) Schematic view of the 63 topology of 4. (d) Schematic description of the 2-fold penetrating nets of 4.

Zn(1)-O(1)#1 = 1.975(3), Zn(1)-O(2)#2 = 2.023(3), Zn(1)-N(2) = 2.045(3), and Zn(1)-N(3) = 2.284(3) A˚]. Each m-BDC anion coordinates to three Zn(II) atoms in the monodentate and bidentate bridging coordination modes (Chart 1b). Two neighboring Zn(II) ions are joined together through bidentate bridging carboxylate groups of two m-BDC anions with a Zn 3 3 3 Zn distance of 4.128 A˚ (Figure S2a, Supporting Information). As a result, two Zn(II) ions and two carboxylate groups constitute a bimetallic [Zn2(CO2)2] unit. Each bimetallic unit connects four m-BDC anions, and in turn each m-BDC anion links two bimetallic units to give a 2D sheet, showing a window with dimensions 10.755 A˚  9.372 A˚ (Figure 5b). The L3 ligands bridge the adjacent sheets in the bidentate

chelating coordination mode to generate a 3D network structure (Figure 5c). From the topological view, if each [Zn2(CO2)2] unit is considered as a 4-connected node and the m-BDC anion and L3 ligand are considered as connectors, the structure of 5 is a R-Po network (Figure 5d) with the dimensional sizes of 10.794 A˚  10.794 A˚  21.337 A˚ (Figure S2b, Supporting Information). In order to minimize the big void cavities and stabilize the framework, the potential voids formed by a single 3D framework show incorporation of another identical network, thus giving a 2-fold interpenetrating network (Figure 5e). When the p-BDC anion was replaced by the m-BDC anion, a similar 2-fold interpenetrating 3D structure of 6 was obtained. As shown in Figure 6a, the structure of 6

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Figure 5. (a) Coordination environment of the Zn(II) ion in 5. (b) View of the 2D sheet of 5. (c) Perspective view of 3D framework in 5 (Zn-m-BDC sheets: blue; L3: pink). (d) Schematic view of the R-Po network of 5. (e) View of the 2-fold interpenetrating framework.

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contains one kind of unique Zn(II) atom, two kinds of unique p-BDC anions, one kind of L3 ligand, and one methanol molecule. The coordination environment of Zn(II) anion in 6 is similar to that of 5. The Zn(II) center is fivecoordinated by three oxygen atoms from different p-BDC ions (Zn(1)-O(1) = 1.985(3), Zn(1)-O(4) = 1.987(3), and Zn(1)-O(3)#1 = 2.015(2) A˚), and two nitrogen atoms from the same L3 ligand (Zn(1)-N(1) = 2.071(3) and Zn(1)N(2) = 2.278(3) A˚), showing a distorted square-pyramidal geometry. The average Zn-O and Zn-N bond lengths are 2.00 and 2.17 A˚, respectively. A part of p-BDC anions connect the adjacent Zn(II) atoms with the bidentate bridging

Figure 6. (a) Coordination environment of the Zn(II) ion in 6. (b) View of the 2D sheet of 6.

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modes (Chart 1f) to give a 1D chain sturcture. In this chain, there are the bimetallic [Zn2(CO2)2] units with a Zn 3 3 3 Zn distance of 4.057 A˚ (Figure S3a, Supporting Information). Furthermore, these chains are linked by another part of the p-BDC anions with the bis-monodentate modes (Chart 1d) to form a 2D polymeric sheet, showing a window with dimensions 10.156 A˚  11.008 A˚ (Figure 6b). The packing modes of compounds 5 and 6 are very similar. The 3D framework of 6 is shown in Figures S3b and 3c (Supporting Information). The compound 6 also shows a 2-fold interpenetrating 3D structure with R-Po topology (Figure 5d). Influence of H2O/CH3OH Ratio on the Syntheses of Coordination Polymers. On the basis of the hydrothermal methods, we have tried to synthesize compounds 1-6 by reaction of Zn(OAc)2 3 2H2O, m-H2BDC (p-H2BDC), and bis-[(pyridyl)benzimidazole] ligand. Unfortunately, all trials led to no anticipated products. Consequently, by the reaction of Zn(OAc)2 3 2H2O, m-H2BDC (p-H2BDC), and bis-[(pyridyl)-benzimidazole] ligand at the solvothermal conditions (H2O/CH3OH), the crystals of the compounds were isolated with a higher yield. Interestingly, with an increase of the spacer lengths of the bis-[(pyridyl)benzimidazole] ligands from L1 to L3, if we would obtain the crystals of the compounds, we had to increase the amount of CH3OH under solvothermal reaction. For example, compounds 1 and 2 contain the same flexible L1 ligands, and their crystals were obtained at a H2O/CH3OH ratio of 4:1 under solvothermal conditions. However, the compounds 3 and 4, and 5 and 6, contain flexible L2 and L3 ligands, respectively. The crystals of these compounds were formed at H2O/CH3OH ratios of 1:1 and 1:4, respectively (Scheme 2). The synthetic conditions for 1-6 are the same, except for the H2O/ CH3OH ratio at the solvothermal conditions. Therefore, the amount of CH3OH under solvothermal conditions may improve the solubility of L1-L3 ligands, which favors the formation of the crystals. Influence of the Flexible Bis-[(pyridyl)benzimidazole] Ligands with Different Spacer Lengths on the Dimensionalities of the Coordination Polymers. In this work, we select three kinds of flexible bis-[(pyridyl)benzimidazole] ligands

Scheme 2. Schematic View of the Syntheses of Compounds 1-6

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reveal that the heteroatom can effectively decrease π and π* orbital energies.20 The similarity of emissions for 2, 4, and 5 are in agreement with this explanation, which indicates that ligand centered π f π* excitation is responsible for these emissions. For compounds 1 and 3, the emission bands appear at 453 nm (λex = 360 nm) and 451 nm (λex = 370 nm), respectively, which are highly red-shifted compared to the free bis-[(pyridyl)benzimidazole] ligands. This result indicates that the photoluminescence between 1 and 3 may be attributed to a mixture characteristics of intraligand and ligand-to-ligand charge transition (LLCT), as reported for other Zn(II) complexes with N-donor ligands.21 Conclusion

Figure 7. The excitation-emission spectra of 1-6, L1-L2, m-H2BDC, and p-H2BDC at room temperature.

L1-L3, intending to observe their effect on the assembly of the coordination polymers by alteration of the spacer lengths of the organic ligands. Compounds 1, 3, and 5 have the same parent compound. From their structures, it can be seen that the different spacer lengths of the L1-L3 ligands resulted in drastic structural changes. The compound 1 containing the L1 ligand showed a 1D chain structure. However, when L2 was used as a N-donor ligand, the 2D sheet structure of 3 was obtained. Finally, the longer L3 ligand was selected instead of L2 ligand, and the 3D framework structure of 5 was formed. Like compounds 1, 3, and 5, compounds 2, 4, and 6 are also based on the same parent compound, and the difference rests on the -(CH2)n- (n = 4, 6, and 10) spacers of bis-[(pyridyl)benzimidazole] ligands, which leads to the structures of these compounds from 2D to 3D. These results indicate that the number of -CH2- groups in the alkyl spacers of the bis-[(pyridyl)benzimidazole] ligands greatly affects the framework structures of the complexes. Photoluminescent Properties. The photoluminescent spectra of compounds 1-6, together with those of the ligands L1-L3, m-H2BDC, and p-H2BDC were investigated in the solid state at room temperature. The emission and excitation peaks of the compounds are shown in Figure 7. The photoluminescent spectra of the m-H2BDC and p-H2BDC show a similar emission peak at 390 nm (λex = 350 nm), which is similar to those of the reported free benzenecarboxylic acids.18 Emission bands were observed at 378, 403, and 403 nm for the free L1, L2, and L3 ligands with excitation at 345 nm, respectively. These emission peaks of the carboxylic acids and the free bis-[(pyridyl)benzimidazole] ligands may be assigned to n f π* or π f π* transitions of the ligands. It is clear that there are emission bands at 418 nm (λex = 360 nm) for 2, 418 nm (λex = 370 nm) for 4, 415 nm (λex = 356 nm) for 5, and 405 nm (λex = 355 nm) for 6, respectively. Such emission of complex 6 can be tentatively assigned to the intraligand transition of the L3 ligand, since a similar emission was observed for the free L3 ligand. The emission bands of compounds 2, 4, and 5 are slightly red-shifted with respect to the bands shown by the L1-L3 ligands. These emissions are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in nature since Zn2þ ions are difficult to oxidize or to reduce due to its d10 configuration.19 The luminescence in Zn(II) heterocylic complexes has been reviewed by Chen,20a,b and their studies

Six new Zn(II) carboxylate polymers with three new flexible bis-[(pyridyl)-benzimidazole] ligands have been synthesized under solvothermal conditions. These compounds display intriguing and versatile coordination features with 1D, 2D, and 3D frameworks. The influences of the spacer lengths of the bis-[(pyridyl)-benzimidazole] ligands on the structures of the compounds have been investigated. It is found that different alkyl spacers of the N-donor ligands greatly affect the framework structures of the complexes. The photoluminescent emissions show that these complexes may be good candidates for optical materials. This study will give impetus to the investigation of the coordination chemistry of bis[(pyridyl)-benzimidazole] ligands. It is believed that more metal complexes containing such flexible bis-[(pyridyl)-benzimidazole] ligands with interesting structures as well as physical properties will be synthesized in the future. Acknowledgment. We thank the Program for Changjiang Scholars and Innovative Research Teams in Chinese University, the Fundamental Research Funds for the Central Universities, the Specialized Research Fund for the Doctoral Program of Higher Education, China Postdoctoral Science Foundation, the Postdoctoral Foundation of Northeast Normal University (NENU), the Training Fund of NENU’s Scientific Innovation Project and the Analysis and Testing Foundation of NENU for support. Supporting Information Available: X-ray crystallographic files (CIF); PXRD patterns of the compounds; selected bond lengths and angles; hydrogen-bond geometries; the Zn-m-BDC double chain in 1; the bimetallic unit [Zn2(CO2)2] and dimensional sizes of the 3D framework of 5 and 6; schematic view of the R-Po network of 6. This material is available free of charge via the Internet at http:// pubs.acs.org.

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