1D, 2D, and 3D MetalâOrganic Frameworks Based on Bis(imidazole

DOI: 10.1021/cg900460k

1D, 2D, and 3D Metal-Organic Frameworks Based on Bis(imidazole) Ligands and Polycarboxylates: Syntheses, Structures, and Photoluminescent Properties

2009, Vol. 9 4660–4673

Lai-Ping Zhang, Jian-Fang Ma,* Jin Yang,* Ying-Ying Liu, and Guo-Hua Wei Key Lab for Polyoxometalate Science, Department of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China Received April 25, 2009; Revised Manuscript Received September 10, 2009

ABSTRACT: Ten new coordination polymers constructed from two structurally related ligands, 1,10 -(1,5-pentanedidyl)bis(imidazole) (biim-5) and 2,20 -bis(1H-imidazolyl)ether (BIE), have been synthesized: [Co(L1)(biim-5)] (1), [Co(L2)(biim5)] 3 H2O (2), [Co(L3)(biim-5)] (3), [Co(L4)(biim-5)] 3 4H2O (4), [Co(L5)0.5(biim-5)] (5), [Co2(L6)(BIE)2] 3 1.5H2O (6), [Zn2(L6)(BIE)2] 3 2.5H2O (7), [Cd(L6)0.5(BIE)(H2O)] 3 H2O (8), [Zn2(L7)(BIE)2] 3 H2O (9) and [Cd(L8)0.5(BIE)(H2O)] (10), where H2L1 = 1,2-benzenedicarboxylic acid, H2L2 = 1,3-benzenedicarboxylic acid, H2L3 = 5-OH-1,3-benzenedicarboxylic acid, H2L4 = DL-camphoric acid, H4L5 = 1,2,3,4-butanetetracarboxylic acid, H4L6 = 4,40 -oxidiphthalic acid, H4L7 = 4,40 -(hexafluoroisopropylidene)diphthalic acid, and H4L8=1,2,3,4-benzenetetracarboxylic acid. Compounds 1 and 4 display the same 2D layer structures with 63-hcb nets, but in 4 the water tetramers extend the layers to a 3D supramolecular framework by intermolecular hydrogen bonds. Compound 2 is an uncommon example of 2D double layers with the Schl€ afli symbol of (42 3 63 3 8). 3 shows a 2D sql net with large open windows, while 5 exhibits a rare 3,4-connected (83)2(85 3 10) topology. The crystal structures of 6 and 7 are close to being isostructural with a scarce (32 3 62 3 72)(32 3 4 3 62 3 7)2 topology. 8 contains two kinds of chiral layers, one left-handed and the other right-handed, with a unique topological type of (52 3 64)(53 3 62 3 7)2. Compound 9, related by a pseudocenter of inversion, possesses a 3D porous framework with a (3,4)-connected (4 3 102)2(42 3 104)-dmd-net. 10 shows a 1D chain structure. The structural and topological differences of these ten compounds indicate that the polycarboxylate ligands play important roles in producing novel frameworks and topologies of the coordination complexes. The infrared spectra and thermogravimetric and luminescent properties were also investigated for the compounds.

Introduction Metal-organic coordination polymers have attracted considerable attention because of their potential applications as function materials as well as their structural diversity and intriguing variety of topologies.1,2 In this regard, a great many metal-polycarboxylate compounds displaying various frameworks from 1D to 3D have been designed and characterized.3 Metal-polycarboxylate complexes, in the presence of rigid N-donor bridging ligands, such as pyrazine and 4,40 bipyridine (bpy), have been reported widely.4 However, metal-polycarboxylate compounds with flexible N-donor bridging ligands have not been well investigated.5,6 Among the N-donor bridging ligands, bis(imidazole) ligands, as an important family of flexible N-donor ligands, have attracted great interest.7 In our previous work, we have reported a series of fascinating archetypal structures based on the 1,10 -(1,4-butanediyl)bis(imidazole) (biim-4) ligand.8 On careful inspection of the reported cases, we found that the flexible nature of the alkyl (-CH2-)4 spacer allows the biim-4 ligand to bend and rotate freely so as to conform to the coordination geometries of central metal atoms. Compared with the linear bpy ligand, the bidentate biim-4 ligand can feature various coordination modes, such as cis- and transconformations, during the assembly with the metals. For example, in [Cu(L9)(bpy)] 3 H2L9 (H2L9=1,4-benzenedicarboxylic acid)9 (11), the bpy ligands link the Cu(II) ions to give *To whom correspondence should be addressed. J.-F.M.: e-mail, jianfangma@ yahoo.com.cn; fax, þ86-431-85098620; J.Y.: e-mail, yangjinnenu@yahoo. com.cn. pubs.acs.org/crystal

Published on Web 10/08/2009

a 1D linear chain; however, in [Cu2(biim-4)4(H2O)2] 3 (L9)2 3 14H2O8g (12), the biim-4 ligand, adopting a transconformation, connects the Cu(II) to form a square-planar net. To expand this work, we synthesized two structurally related flexible N-donor bis(imidazole) ligands, 1,10 -(1,5pentanedidyl)bis(imidazole) (biim-5) and 2,20 -bis(1H-imidazolyl)ether (BIE). The biim-5 ligand, bearing a longer methylene (-CH2-)5 skeleton, tends to exhibit more flexible conformations. Compared with biim-5, a heteroatom O was introduced into the BIE ligand through taking the place of a symmetric C center, enhancing the flexibility of the ligand. In this paper, ten metal-bis(imidazole)-poylcarboxylate compounds were obtained under hydrothermal conditions: [Co(L1)(biim-5)] (1), [Co(L2)(biim-5)] 3 H2O (2), [Co(L3)(biim-5)] (3), [Co(L4)(biim-5)] 3 4H2O (4), [Co(L5)0.5(biim-5)] (5), [Co2(L6)(BIE)2] 3 1.5H2O (6), [Zn2(L6)(BIE)2] 3 2.5H2O (7), [Cd(L6)0.5(BIE)(H2O)] 3 H2O (8), [Zn2(L7)(BIE)2] 3 H2O (9), and [Cd(L8)0.5(BIE)(H2O)] (10) (H2L1=1,2-benzenedicarboxylic acid, H2L2=1,3-benzenedicarboxylic acid, H2L3= 5-OH-1,3-benzenedicarboxylic acid, H2L4 = DL-camphoric acid, H4L5 = 1,2,3,4-butanetetracarboxylic acid, H4L6 = 4,40 -oxidiphthalic acid, H4L7 = 4,40 -(hexafluoroisopropylidene)diphthalic acid, H4L8 = 1,2,3,4-benzenetetracarboxylic acid, biim-5=1,10 -(1,5-pentanedidyl)bis(imidazole), and BIE= 2,20 -bis(1H-imidazolyl)ether). The compounds are characterized by elemental analysis, IR spectra, and X-ray crystallography. The crystal structures as well as the topological analysis of these compounds and the systematic investigation of the effect of polycarboxylate anions on the ultimate frameworks r 2009 American Chemical Society

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Crystal Growth & Design, Vol. 9, No. 11, 2009

Scheme 1. Structures of the biim-5 (up) and BIE (down) Ligands

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Scheme 2. Structures of the Carboxylic Acids Used in This Work

will be discussed. The luminescent properties of compounds 7-10 were also investigated. Experimental Section General Procedures. All reagents and solvents were purchased from commercial sources and used as received. The ligand biim-5 was synthesized with a slight modification of a reported method10, and BIE7c was synthesized in accordance with the procedure reported. Synthesis of 1,10 -(1,5-Pentanedidyl)bis(imidazole) (biim-5). A mixture of imidazole (3.4 g, 50 mmol) and NaOH (2.0 g, 50 mmol) in N,N-dimethylformamide (DMF) (10 mL) was stirred at 60 °C for 2 h, and then 1,5-dibromopentane (5.748 g, 25 mmol) was added. The mixture was cooled to room temperature after stirring at 60 °C for another 5 h. After the evaporation of DMF, 10 mL of water was added and stirred. The resulting mixture was extracted with dichloromethane (3 25 mL), and colorless oil was obtained after evaporation of the solvent. Yield: 65%. Synthesis of [Co(L1)(biim-5)] (1). A mixture containing CoCO3 (35.7 mg, 0.30 mmol), H2L1 (49.8 mg, 0.30 mmol), biim-5 (61.8 mg, 0.30 mmol), and water (7 mL) was sealed in a Teflon reactor (15 mL), which was heated at 140 °C for 3 days, and then it was cooled to room temperature at 10 °C 3 h-1. Purple crystals of 1 were collected in a 64% yield (based on CoCO3). Anal. Calcd for C19H20CoN4O4 (Mr=427.32): C, 53.40; H, 4.72; N, 13.11. Found: C, 53.65; H, 4.55; N, 12.79. IR (cm-1): 3444 (w), 3113 (m), 2932 (w), 1609 (s), 1584 (s), 1374 (s), 1102 (m), 835 (w), 656 (m), 621 (w). Synthesis of [Co(L2)(biim-5)] 3 H2O (2). The preparation of 2 was similar to that of 1 except that H2L2 was used instead of H2L1. Purple crystals of 2 were collected in a 59% yield (based on CoCO3). Anal. Calcd for C38H44Co2N8O10 (Mr=890.67): C, 51.24; H, 4.98; N, 12.58. Found: C, 51.50; H, 4.79; N, 12.79. IR (cm-1): 3386 (w), 3132 (m), 1610 (s), 1523 (w), 1364 (s), 1230 (m), 1090 (m), 950 (w), 721 (m), 562 (w). Synthesis of [Co(L3)(biim-5)] (3). The preparation of 3 was similar to that of 1 except that H2L3 was used instead of H2L1. Purple crystals of 3 were collected in a 56% yield (based on CoCO3). Anal. Calcd for C19H20CoN4O5 (Mr=443.32): C, 51.47; H, 4.55; N, 12.64. Found: C, 51.24; H, 4.75; N, 12.89. IR (cm-1): 3122 (w), 1572 (s), 1368 (s), 1097 (m), 860 (w), 725 (m), 657 (m), 572 (w), 523 (w). Synthesis of [Co(L4)(biim-5)] 3 4H2O (4). The preparation of 4 was similar to that of 1 except that H2L4 was used instead of H2L1. Purple crystals of 4 were collected in a 54% yield (based on CoCO3). Anal. Calcd for C21H37CoN4O8 (Mr=532.48): C, 47.37; H, 7.00; N, 10.52. Found: C, 47.11; H, 7.18; N, 10.21. IR (cm-1): 3460 (m), 2958 (w), 1530 (s), 1401 (s), 1291 (w), 813 (w), 758 (w), 662 (m), 430 (w). Synthesis of [Co(L5)0.5(biim-5)] (5). The preparation of 5 was similar to that of 1 except that H4L5 was used instead of H2L1. Purple crystals of 5 were collected in a 66% yield (based on CoCO3). Anal. Calcd for C15H19CoN4O4 (Mr=378.27): C, 47.62; H, 5.06; N, 14.81. Found: C, 47.33; H, 4.29; N, 14.59. IR (cm-1): 3444 (w), 3120 (m), 2937 (w), 1595 (s), 1386 (s), 1097 (m), 951 (w), 764 (w), 551 (w). Synthesis of [Co2(L6)(BIE)2] 3 1.5H2O (6). A mixture containing CoCO3 (35.7 mg, 0.30 mmol), H4L6 (51.9 mg, 0.15 mmol), BIE (61.8 mg, 0.30 mmol), and water (7 mL) was sealed in a Teflon reactor (15 mL), which was heated at 160 °C for 3 days, and then it was cooled to room temperature at 10 °C 3 h-1. Pink crystals of 6 were collected in a 48% yield (based on CoCO3). Anal. Calcd for C36H35.5Co2N8O12.5 (Mr = 898.08): C, 48.14; H, 3.98; N, 12.48. Found: C, 47.86; H, 4.31; N, 12.75. IR (cm-1): 3443 (w), 3126 (m), 1617 (s), 1362 (s), 1228 (s), 1108 (s), 953 (m), 842 (w), 655 (m), 524 (w). Synthesis of [Zn2(L6)(BIE)2] 3 2.5H2O (7). The preparation of 7 was similar to that of 6 except that ZnCO3 was used instead of

CoCO3. Colorless crystals of 7 were collected in a 57% yield (based on ZnCO3). Anal. Calcd for C36H39ZnN8O13.5 (Mr = 930.49): C, 46.47; H, 4.22; N, 12.05. Found: C, 46.79; H, 4.41; N, 12.39. IR (cm-1): 3436 (w), 3126 (m), 1673 (s), 1366 (s), 1226 (s), 1107 (s), 954 (m), 843 (m), 747 (w), 654 (m), 524 (w). Synthesis of [Cd(L6)0.5(BIE)(H2O)] 3 H2O (8). The preparation of 8 was similar to that of 7 except that CdCO3 was used instead of ZnCO3. Colorless crystals of 8 were collected in a 47% yield (based on CdCO3). Anal. Calcd for C36H42Cd2N8O15 (Mr =1051.58): C, 41.12; H, 4.03; N, 10.66. Found: C, 41.43; H, 4.31; N, 10.93. IR (cm-1): 3415 (w), 3118 (w), 1561 (s), 1398 (s), 1220 (m), 1086 (m), 954 (m), 885 (w), 828 (m), 655 (m), 548 (w). Synthesis of [Zn2(L7)(BIE)2] 3 H2O (9). The preparation of 9 was similar to that of 7 except that H4L7 was used instead of H4L6. Colorless crystals of 9 were collected in a 55% yield (based on ZnCO3). Anal. Calcd for C39H36F6Zn2N8O11 (Mr = 1037.50): C, 45.15; H, 3.50; N, 10.80. Found: C, 45.44; H, 3.66; N, 10.60. IR (cm-1): 3415 (w), 1616 (s), 1425 (s), 1185 (s), 1089 (m), 849 (m), 722 (m), 615 (w), 502 (w). Synthesis of [Cd(L8)0.5(BIE)(H2O)] (10). The preparation of 10 was similar to that of 8 except that H4L8 was used instead of H4L6. Colorless crystals of 10 were collected in a 58% yield (based on CdCO3). Anal. Calcd for C15H17CdN4O6 (Mr =461.73): C, 39.02; H, 3.71; N, 12.14. Found: C, 39.32; H, 3.45; N, 12.32. IR (cm-1): 3418 (w), 1543 (s), 1376 (s), 1205 (w), 921 (m), 828 (m), 781 (w), 708 (w), 600 (w), 524 (m). Physical Measurements. The C, H, and N elemental analysis was conducted on a Perkin-Elmer 240C elemental analyzer. The FT-IR spectra were recorded from KBr pellets in the range 4000-400 cm-1 on a Mattson Alpha-Centauri spectrometer. TGA was performed on a Perkin-Elmer TG-7 analyzer heated from 25 to 600 °C under nitrogen. The photoluminescent properties of the ligands and compounds were measured on a Perkin-Elmer LS55 spectrometer. The emission/excitation spectra and the lifetime were measured on an Edinburgh FLS-920 spectrophotometer equipped with a continuous Xe-900 xenon lamp, an nF900 nanosecond flash lamp. X-ray Crystallography. Single-crystal X-ray diffraction data for compound 2 was recorded on a Bruker Apex CCD diffractometer with graphite-monochromated Mo KR radiation (λ=0.71073 A˚) at 293 K. Crystallographic (diffraction) data for compounds 1 and 3-10 were collected on a Rigaku RAXIS-RAPID single-crystal diffractometer with Mo KR radiation (λ = 0.71073 A˚) at 293 K. Absorption corrections were applied using a multiscan technique. All the structures were solved by Direct Method of SHELXS-9711 and refined by full-matrix least-squares techniques using the SHELXL-97 program.12 The hydrogen atoms attached to carbons were generated geometrically. Some aqua hydrogen atoms of compounds 6, 7, and 9 were not included in the model. Other hydrogen atoms of water molecules and hydroxyl hydrogen atoms were located from difference Fourier maps and refined with

2

3

4

formula C19H20CoN4O4 C38H44Co2N8O10 C19H20CoN4O5 C21H37CoN4O8 fw 427.32 890.67 443.32 532.48 crystal system orthorhombic monoclinic monoclinic monoclinic P21/n space group Pbca Cc P21/n a (A˚) 10.350(2) 15.240(5) 9.539(3) 10.267(4) b (A˚) 13.540(5) 13.155(5) 15.724(7) 16.296(4) c (A˚) 27.867(7) 20.779(8) 13.203(4) 15.769(5) R (deg) 90 90 90 90 β (deg) 90 102.406(7) 90.414(11) 96.032(13) γ (deg) 90 90 90 90 3905.3(18) 4068 1980.4(12) 2623.7(14) V (A˚3) Z 8 4 4 4 -3 1.454 1.454 1.487 1.348 Dcalcd (g cm ) F(000) observed 1768 1848 916 1128 reflection/unique 35010/4450 11920/6326 18987/4440 25346/5974 R(int) 0.0614 0.0403 0.0525 0.0719 2 1.048 1.073 1.057 1.085 GOF on F 0.0370 0.0904 0.0524 0.0781 R1a [I > 2σ(I)] 0.2457 0.1286 0.1972 wR2b [I > 2σ(I)] 0.0840 P P P P a R1 = ||Fo| - |Fc||/ |Fo|. bwR2 = | w(|Fo|2 - |Fc|2)|/ |w(Fo2)2|1/2.

1 C15H19CoN4O4 378.27 monoclinic P21/c 11.908(5) 10.756(3) 14.131(5) 90 114.112(13) 90 1651.9(10) 4 1.521 784 15668/3756 0.0582 1.049 0.0440 0.0915

5 C36H35.5Co2N8O12.5 898.08 monoclinic P21/n 11.159(3) 24.415(7) 14.723(4) 90 96.91(3) 90 3982.0(17) 4 1.498 1846 38180/9038 0.0502 0.987 0.0589 0.1671

6

7 C36H39Zn2N8O13.5 930.49 monoclinic P21/n 11.142(3) 24.401(5) 14.707(4) 90 96.871(10) 90 3969.8(17) 4 1.557 1916 38498/9059 0.0688 1.053 0.0619 0.1587

Table 1. Crystal Data and Structure Refinements for Compounds 1-10 8 C18H21CdN4O7.5 1051.58 monoclinic C2/c 27.635(10) 8.673(2) 21.536(8) 90 125.347(13) 90 4210(2) 8 1.659 2120 19587/4802 0.0359 1.031 0.0281 0.0603

9 C39H36F6Zn2N8O11 1037.50 monoclinic Cc 29.511(8) 12.050(3) 13.908(4) 90 111.457(11) 90 4603(2) 4 1.497 2112 22126/10382 0.0498 1.026 0.0641 0.1656

10 C15H17CdN4O6 461.73 monoclinic C2/c 23.590(11) 11.164(4) 16.943(7) 90 132.018(14) 90 3315(2) 8 1.85 1848 15772/3768 0.0658 1.649 0.0741 0.2113

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Table 2. Photoluminescent Data for Organic Ligands and Complexes 7-10 at Room Temperature

compound λex (nm)

BIE 7 8 9 10 490 322, 343 322 343, 314 353

λem (nm) τ (ns) ligand λex (nm) λem (nm) 545 422 442 401 452 7.08 8.37 7.10 9.00 H4L6 H4L6 L H4L8 395 395 290 280 358 358 336 370

Figure 1. (a) Coordination environment of the Co(II) ion in 1 (30% probability displacement ellipsoids). Symmetry code: #1 = -x þ 3 /2, 9y þ 1/2, z; #2 = -x þ 2, -y, -z þ 1. (b) The 1D wavelike chain constructed by L1 anions and Co(II) atoms along the b axis. (c) Topological representation of the 2D 63-hcb net.

isotropic displacement parameters. Non-hydrogen atoms of complexes 1, 4, 5, 6, 8, and 10 were refined with anisotropic temperature parameters. Some C, N, and O atoms of complexes 2, 3, 7, and 9 (C15 and C18 for 2; C14 for 3, O3W for 7; C35, C36, C360 , C39, N7, O1W, and O2W for 9) were refined with isotropic temperature parameters, and other non-hydrogen atoms were refined anisotropically. The disordered C atoms of compound 4 (C21, C210 ) and compound 9 (C36, C360 ) were refined using C atoms split over two sites, with a total occupancy of 1. For compound 9, the hydrogen atoms of the disordered C atoms were not included in the model. To refine the structure with reasonable C(sp3)-C(sp3), O-H, and H 3 3 3 H bond lengths, the C(sp3)-C(sp3) bond lengths of bis(imidazole) ligands in compounds 2, 3, 4, 6, 7, and 9 were restrained

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Figure 2. (a) Coordination environment of the Co(II) ions in 2; water molecules have been omitted for clarity (30% probability displacement ellipsoids). Symmetry code: #1=x þ 1/2, y þ 1/2, z; #2=x þ 1, y, z; #3=x þ 1/2, y - 1/2, z. (b) The zigzag chain constructed by biim-5 ligands and Co(II) ions. (c) Schematic representation (left) and rod-packing diagram (right) of the 1D chains (constructed by the Co(II) atoms and the L2 anions) spanning two different directions. (d) View of the 2D double layer. (e) Topological representation of the 4-connected (42 3 63 3 8) network. to 1.50 ( 0.03 A˚. And the water H atoms for compounds 2, 4, 7, 8, and 10 were refined with distance restraints of O-H=0.89 ( 0.02 A˚ and H 3 3 3 H=1.44 ( 0.01 A˚. The maximum residual electron density is 2.19 e A˚-3 at 1.3 A˚ from C12 for compound 2 and is 3.78 e A˚-3 at 1.2 A˚ from Cd1 for compound 10. The detailed crystallographic data and structure refinement parameters for 1-10 are summarized in Table 1. Selected bond distances and angles for compounds 1-10 are listed in Tables S1-S10 in the Supporting Information. TGA curves of compounds 2, 4, and 6-10 are shown in Figures S12-S18 in the Supporting Information. The emission and excitation peaks of 4,40 -(hexafluoroisopropylidene)-diphthalic dianhydride (L), H4L8 ligands, and 7-10 are shown in Figures S19 and S20 in the Supporting Information. The luminescence decay cures are shown in Table 2 and Figure S21 in the Supporting Information.

Results and Discussion Structure Description of 1. The summarized eight coordination modes of the biim-4, biim-6, biim-5, and BIE are shown in Scheme S1 (see the Supporting Information). The

networks of compounds 1-9 were analyzed by using the OLEX13 program. As shown in Figure 1a, the structure of 1 contains one Co(II) ion, one L1 anion, and one biim-5 ligand. The Co(II) ion is four-coordinated by two carboxylate O atoms from two L1 anions (Co1-O2#1=1.987(2) and Co1-O4=1.964(2)) and two N atoms from two biim-5 ligands (Co1-N1#2 = 2.026(2) and Co1-N4 = 2.010(2) A˚) in a tetrahedral coordination geometry. Each L1 anion bridges the adjacent Co(II) atoms to yield a wavelike chain (Figure 1b), which is further linked by [Co(biim-5)2Co] dimeric units to form an undulated sheet (Figure S1 in the Supporting Information and Figure 1c). The biim-5 ligand adopts a cis-conformation in mode II (see the Supporting Information) with the dihedral angle between the two imidazole rings of 127.7°. The Co(II) 3 3 3 Co(II) separations across the biim-5 ligand and the L1 anion are 8.414(2) and 6.885(3) A˚, respectively. From a topological perspective, this 2D layer can be described as a 3-connected hexagonal honeycomb 63-hcb net (Figure 1c).

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Figure 4. (a) Coordination environment of the Co(II) ion in 4 with the water molecules omitted for clarity (30% probability displacement ellipsoids). Symmetry code: #1=x - 1/2, -y þ 1/2, z - 1/2; #2=-x þ 1, -y, -z þ 1. (b) Schematic representation of the 2D layer of 4. (c) Hydrogen-bonded acyclic water tetramer and its coordination environment. Symmetry code: #3=x þ 1, y, z; #4= x þ 1/2, -y þ 1/2, z - 1/2. Figure 3. (a) Coordination environment of the Co(II) ion in 3 (30% probability displacement ellipsoids). Symmetry code: #1=x þ 1, y, z; #2=-x þ 3/2, y - 1/2, -z þ 5/2. (b) Schematic representation of the 1D chain consisting of the Co(II) atoms and the biim-5 ligands. (c) 2D undulated net. (d) View of the 2D sql net constructed by [Co4(L3)2(biim-5)2] units.

Structure Description of 2. When L1 anion was replaced by L2 anion, a quite different structure of 2 was obtained. As illustrated in Figure 2a, the structure of 2 contains Co1 and Co2 cations, both lying in general positions, two L2 anions, two biim-5 ligands, and two water molecules. Both Co1 and Co2 ions are four-coordinated by two carboxylate O atoms

from two L2 anions (Co1-O1#2 = 1.993(8), Co1-O4 = 2.008(7), Co2-O5=1.981(7), and Co2-O8#3 =2.028(8) A˚) and two N atoms from two biim-5 ligands (Co1-N1 = 2.030(1), Co1-N8#1 = 2.013(8), Co2-N4 = 2.016(9), and Co2-N5=2.003(9) A˚), displaying the CoO2N2 tetrahedral geometry. The biim-5 ligand adopts a cis-conformation in mode I (see the Supporting Information), which is different from that of biim-5 in 1. The dihedral angles between the two imidazole rings are 65.2° and 76.0°, respectively. The two biim-5 ligands connect to the adjacent Co(II) ions to form a zigzag chain (Figure 2b) with the Co 3 3 3 Co distances of 8.264(3) and 10.884(4) A˚. The coordination of L2 anion to Co1 and Co2 atoms results in two independent 1D polymeric

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Figure 5. (a) Coordination environment of the Co(II) ion in 5 (30% probability displacement ellipsoids). Symmetry code: #1=x, -y þ 1/2, z - 1/2; #2=-x þ 1, -y, -z þ 1; #3=-x, -y þ1, -z þ 1. (b) [Co2(BTCA)] network with the rhombic grids. (c) 3D framework of 5 (blue for L5 anions and gray for biim-5 ligands). (d) Schematic representation of the (3,4)-connected net with (83)2(85 3 10) topology.

chains, which arrange on parallel levels in different propagating directions with the same Co 3 3 3 Co distance of 10.066(3) A˚ (Figure 2c). The Co1 3 3 3 Co1#1 chain propagates along [110] in the plane z = 0.571, while the Co2 3 3 3 Co2#3 chain propagates parallel to [110] in the plane z=0.300. The two polymeric chains in different directions are further linked by the biim-5 ligands to construct a 2D double layer (Figure 2d). If the Co(II) center is viewed as the 4-connected node, the structure of compound 2 can be described as a uninodal 4-connected 2D double layer with the Schl€ afli symbol of (42 3 63 3 8) (Figure 2e). It should be noted that, among the compounds with 4-connected frameworks, the diamond nets, CdSO4, PtS, quartz, and sra are commonly encountered;14 nevertheless, only one compound with a 4-connected 2D double layer of this topology has been reported by our group.5f Structure Description of 3. H2L3 was reacted with Co(II) and biim-5 using a preparation procedure similar to that of 2,

resulting in a 2D sql net. As shown in Figure 3a, the structure of 3 contains one Co(II) ion, one L3 anion, and one biim-5 ligand. The Co(II) ion adopts a tetrahedral coordination geometry, coordinating to two carboxylate O atoms from two L3 anions (Co1-O1=1.975(2) and Co1-O3#1=1.986(2) A˚) and two N atoms from two biim-5 ligands (Co1-N1 = 2.013(3) and Co1-N4#2 = 2.026(3) A˚). The biim-5 ligand adopts a trans-conformation in mode I (see the Supporting Information) with the dihedral angle between the two imidazole rings of 37.6°. Each biim-5 ligand bridges the adjacent Co(II) atoms to yield wavelike chains (Figure 3b) with a Co 3 3 3 Co distance of 13.300(2) A˚, which are further linked by L3 anions to form a 2D layer (Figure 3c). The 2D layer can also be considered as an undulated sql net by repeating the [Co4(L3)2(biim-5)2] unit in the ab plane (Figure 3d). Notably, the [Co4(L3)2(biim-5)2] unit has a square window with dimensions of 9.539(2) 13.300(2) A˚2 (Figure S2). The resulting highly undulated layers are packed in a parallel fashion and stacked along the c axis with the rodlike

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uncoordinated hydroxyl groups projecting vertically to the ab plane (Figure 3c). The intermolecular hydrogen bonds between the hydroxyl group and the carboxylate group have been observed (O5 3 3 3 O4 = 2.780(4) A˚), which extends the structure from a 2D sql net to a 3D supramolecular structure (Figure S3). Structure Description of 4. As illustrated in Figure 4a, the structure of 4 contains one Co(II) ion, one L4 anion, one biim-5 ligand, and four solvent water molecules. The Co(II) ion is six-coordinated by two carboxylate O atoms from two L4 anions (Co1-O1 = 2.327(3), Co1-O2 = 2.047(3), Co1-O3#1 = 2.058(3), and Co1-O4#1 = 2.365(4) A˚) and two N atoms from two biim-5 ligands (Co1-N1#2=2.072(4) and Co1-N4=2.043(4) A˚), exhibiting a octahedral coordination geometry. The two carboxylate groups of L4 anion display a bis(chelate-bidentate) coordination mode and bridge the adjacent Co(II) atoms to yield wavelike chains. A pair of biim-5 ligands, in cis-conformation (mode II, see the Supporting Information), link two Co(II) ions to furnish a [Co(biim-5)2Co] dimeric unit with the Co 3 3 3 Co distance of 7.986(2) A˚. The dihedral angle between the two imidazole rings is 51.9°. Four L4 anions and two biim-5 ligands connect six Co(II) centers to form a hexagonal [Co6(biim-5)4(L4)4] ring, which arranges alternatively in bc plane to generate a 2D layer by sharing common edges (Figure 4b). From a topological perspective, this 2D layer can be described as a 3-connected hexagonal honeycomb 63-hcb net (Figure S4). As shown in Figure 4c, the four solvent water molecules in 4 assemble themselves to generate a hydrogen-bonded acyclic water tetramer with an average OW 3 3 3 OW separation of 2.760(7) A˚. O1W acts as a donor only, while O2W, O3W, and O4W act as both donors and acceptors. Small water clusters, including dimer, trimer, tetramer, pentamer, and hexamer, have attracted considerable attention both theoretically and experimentally, since they play a crucial role in contributing to the stability and function of biological assemblies.15 Many of these water clusters are stabilized by organic and inorganic hosts.15a Among them, water tetramer is of particular interest, and there are a large number of examples of the crystallographic observation of the cyclic water tetramers in crystal hosts.15d As compared with cyclic water tetramer, discrete acyclic water tetramer is relatively rare.16 In 4, the hydrogen bonds between the carboxylate O atoms of L4 anion and the water tetramer connect the 2D sheets to form a 3D supramolecular framework (Figure S5 in the Supporting Information). The cooperative association of the water cluster and the crystal host plays important roles in stabilizing the water tetramer and the crystal host in the formation of the 3D supramolecular frameworks. Structure Description of 5. When the flexible H4L5 acid was introduced into the reaction mixture, a complex 3D framework of 5 was obtained. As illustrated in Figure 5a, the structure of 5 contains one Co(II) ion, one-half L5 anion, and one biim-5 ligand. The Co(II) ion is four-coordinated by two carboxylate O atoms from two L5 anions (Co1-O1 = 1.973(2) and Co1-O3#1 = 2.023(2) A˚) and two N atoms from two biim-5 ligands (Co1-N1=2.019(2) and Co1-N4#2= 2.028(2) A˚) in a tetrahedral coordination geometry. The Co-O and Co-N bond distances are all within the normal ranges.17 The four carboxylate groups of the L5 anion all adopt monodentate coordination modes. The tetradentate L5 anions bridge the Co(II) centers to form a rhombic grid structure with the dimensions of 9.926(2) 10.296(2) A˚2 (Figure 5b) based on the distances between opposite Co(II)

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Figure 6. (a) Coordination environment of the Co(II) ions in 6 with water molecules omitted for clarity (30% probability displacement ellipsoids). Symmetry code: #1= x, y, z - 1; #2= x þ 1/2, -y þ 1/2, z þ 1/2; #3= x - 1/2, -y þ 1/2, z - 1/2. (b) View of the neutral 2D double layer composed of Co(II) ions and L6 anions. (c) View of the 2D double layer of 6 stabilized by BIE ligands (green for BIE ligands and black for L6 anions). (d) Schematic representation of the 4-connected net with (32 3 62 3 72)(32 3 4 3 62 3 7)2 topology (the BIE ligand is given in green).

centers. The biim-5 ligand, as a bidentate bridging ligand, adopts a cis-conformation (mode II; see the Supporting Information) with the dihedral angle between the two imidazole rings of 146.2°. Two Co(II) atoms are linked by two biim-5 ligands to form a [Co(biim-5)2Co] unit with the distance between neighboring Co(II) centers of 8.279(3) A˚. The rhombic grid 2D layers are connected by the [Co(biim-5)2Co] units to furnish a 3D framework (Figure 5c). From the topological view, if the four-coordinated Co(II) center can be considered as a 3-connected node, the tetradentate L5 anion as a square-planar 4-connected node, and double bridges from two BIE ligands as one linker, respectively, then the framework of 5 becomes a binodal (3,4)-connected net with a Schl€ afli symbol of (83)2(85 3 10) (Figure 5d). It is noticeable

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Figure 7. (a) Coordination environment of the Cd(II) ion in 8 with the solvent water molecule omitted for clarity (30% probability displacement ellipsoids). Symmetry code: #1= -x þ 3/2, y - 1/2, -z þ 3/2; #2= -x þ 3/2, y þ 1/2, -z þ 3/2; #3= -x þ 1, y, -z þ 3/2. (b) 1D wavelike chain constructed by BIE ligands and Cd(II) ions. (c) View of the 2D right-handed and left-handed chiral layers stacked in an ABAB- sequence viewing along the b axis. (d) Schematic representation of the 4-connected net with (52 3 64)(53 3 62 3 7)2 topology (the BIE ligand is given in blue).

that, among the known (3,4)-connected nets in coordination polymers, most of the structures have focused on the ones with tetrahedral four-connected centers;18-20 however, the (3,4)connected MOFs with square planar four-connected nodes are relatively scarce.6e,7c,21 Structure Description of 6. In 6, the flexible N-donor ligand BIE and the aromatic acid H4L6 were utilized as reactants. Compared with biim-5, a heteroatom O was introduced into the BIE ligand through taking the place of the symmetric C center. Compounds 6 and 7 are close to being isostructural, and therefore, only the structure of 6 is described here in detail. The figures for the structure of compound 7 are shown in the Supporting Information (Figures S7-S9). As shown in Figure 6a, the structure of 6 contains two Co(II) ions lying in general positions, one L6 anion, two BIE ligands, and one and a half water molecules. Co1 and Co2 ions show similar tetrahedral coordination geometries CoO2N2, which are surrounded by two carboxylate O atoms from two L6 anions (Co1-O1#1 = 1.949(3), Co1-O9 = 1.957(3), Co2-O4 = 1.964(3), and Co2-O6#3 = 1.969(3) A˚) and two N atoms from two BIE ligands (Co1-N4 = 2.020(4), Co1-N5 = 2.017(4), Co2-N1 = 2.018(3), and Co2-N8#2 = 2.003(4) A˚). Four carboxylate groups of L6 anion exhibit the same

monodentate coordination mode. Each L6 anion coordinates to four Co(II) centers with its four carboxylate groups, forming a 2D double layer (Figure 6b and Figure S6 in the Supporting Information). The symmetry-related Co1 atoms lie in the layer, whereas the symmetry-related Co2 atoms are located between the two layers. The two BIE ligands adopt a cis-conformation (mode I, see the Supporting Information) with the dihedral angles between the two imidazole rings of 47.6° and 106.3°, which coordinate with the Co(II) ions and fill the voids of the 2D layer (Figure 6c). Although the two crystallographically independent BIE ligands adopt the same cis-conformation, there are two different Co 3 3 3 Co distances of 10.508(3) and 6.328(2) A˚, respectively, separated by two types of BIE ligands. This may be attributed to the flexibility of the long spacer between the two imidazole rings. Topologically, both Co1 and Co2 atoms can be reduced to tetrahedral 4-connecting nodes, and L6 anion can be regarded as a square-pyramidal 4-connecting node. The BIE ligand acts as a linker between the Co(II) centers. Therefore, the framework of 6 can be described as a rare binodal 4-connected network (Co2 and L6 nodes are topologically equivalent), with a Schl€ afli symbol of (32 3 62 3 72)(32 3 4 3 62 3 7)2 (Figure 6d).

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Figure 8. (a) Coordination environment of the Zn(II) ions in 9 with solvent water molecules omitted for clarity (30% probability displacement ellipsoids). Symmetry code: #1 = x, -y, z þ 1/2; #2 = x þ 1/2, y - 1/2, z; #3 = x - 1/2, y þ 1/2, z. (b) 1D chain constructed by L7 anions and Zn(II) ions. (c) 3D framework of 9. (d) View of the 3D framework of 9 with solvent waters located in the channels. (e) Schematic diagram showing the (4 3 102)2(42 3 104)-dmd-net of 9 (3-connected nodes, blue; 4-connected nodes, purple).

Structure Description of 8. Compared with compound 6, Co(II) ion is replaced by Cd(II) ion in 8, resulting in a 2D network. As illustrated in Figure 7a, the structure of 8 contains one Cd(II) ion, one-half L6 anion, one BIE ligand, and two water molecules. The Cd(II) ion is seven-coordinated by four carboxylate O atoms from two L6 anions (Cd1-O2 = 2.459(2), Cd1-O3 = 2.456(2), Cd1-O4#2 = 2.496(2), Cd1-O5#2 =2.385(2), and Cd1-O1W=2.417(2) A˚), two N atoms from two BIE ligands (Cd1-N1#1 = 2.280(2) and Cd1-N4=2.286(2) A˚), and one water molecule to furnish a distorted pentagonal bipyramidal geometry with O1W and N1#1 atoms located at the apical position. The dihedral angle between the two imidazole rings of the BIE ligand is 112.8°. The BIE ligands connect the Cd(II) ions in a cis-comformation (mode I, see the Supporting Information), generating a wavelike chain with the Cd 3 3 3 Cd distance of 12.482(3) A˚ along the b axis (Figure 7b). These wavelike chains are linked by chelating-bidentate carboxylate groups of L6 anions, resulting in the formation of infinite 2D layers. It is interesting to note that homohelical chains are found in the layers, leading to

right-handed and left-handed chiral layers, respectively (Figure 7c). The right- and left-handed layers are stacked alternatively in an -ABAB- sequence viewing along the b axis, generating a mesomeric compound. In addition, the existing hydrogen-bonding interactions between the water molecules and the carboxylate O atoms (O1W-H1A 3 3 3 O2#1 =2.783(3), O1W-H1B 3 3 3 O4=2.717(3), O2W-H2A 3 3 3 O3#4 =2.777(3), and O2W-H2B 3 3 3 O1W#5 = 2.892(4) A˚) stabilize the 2D framework of 8 (Figure S10). 8 and the reported compounds, [Cd2(L6)(L1)0.5(H2O)] 3 4H2O (11), [Cd(H2L6)(L2)] 3 2H2O (12), and [Cd1.5(HL6)(L3)0.5] (13) (where L1 = 1,2-bis[2-(2-pyridyl)imidazol1-ylmethyl]benzene, L2 = 1,3-bis[2-(2-pyridyl)-imidazol1-ylmethyl]benzene, and L3=1,4-bis[2-(2-pyridyl)-imidazol1-ylmethyl]benzene),22 have similar composition and structural moieties, with the only difference being in using different flexible bis(imidazole) ligands. However, compounds 11-13 do not have chiral layers. A comparison of 8 with these three compounds indicates that the more flexible BIE ligand may play crucial roles in the formation of the chiral layers.

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Figure 9. (a) Coordination environment of the Cd(II) ion in 10 (30% probability displacement ellipsoids). Symmetry code: #1 = -x þ 1/2, -y þ 1/2, -z þ 1; #2 = -x þ 1, y, -z þ 3/2. (b) View of the 1D chain of 10.

From the topological point of view, the seven-coordinated Cd(II) ion can be considered as a 4-connected node, and the tetradentate L6 anion can also be regarded as a 4-connected node. Consequently, this 2D network of 8 can be reduced to a binodal 4-connected net with the topological type of (52 3 64)(53 3 62 3 7)2 (Figure 7d), which, to the best of our knowledge, has never been reported before. Structure Description of 9. Compared with 7, the L6 anion is replaced by the L7 anion in 9 and a complex 3D structure is obtained. As illustrated in Figure 8a, the structure of 9 contains two Zn(II) ions lying in general positions, one L7 anion, two BIE ligands, and one solvent water molecule. Both Zn1 and Zn2 are four-coordinated by two carboxylate O atoms from two L7 anions (Zn1-O2=1.949(5), Zn1-O5#1= 1.974(5), Zn2-O6#1 =1.963(5), and Zn2-O9=2.002(5) A˚) and two N atoms from two BIE ligands (Zn1-N4=2.018(6), Zn1-N8#2=2.006(6), Zn2-N1#3=1.988(6), and Zn2-N5= 2.009(7) A˚) in a tetrahedral coordination geometry. The four carboxylate groups of L7 anions bridge four Zn(II) ions to generate a 1D chain along the c axis (Figure 8b). Each BIE ligand displays a cis-conformation (mode II, see the Supporting Information) with the dihedral angle between the two imidazole rings of 110.6°. A pair of BIE ligands further linked the 1D chains to provide a 3D framework (Figure 8c). The Zn 3 3 3 Zn distance separated by the BIE ligand is 7.362(2) A˚. Interestingly, there are channels of approximately 12.269(0) 9.544(2) A˚2 dimensions viewing along the c axis occupied by solvent water molecules (Figure 8d and Figure S11). PLATON23 calculations show that the solventaccessible void is 4604.0 A˚3 per unit-cell volume. Better insight into such an intricate framework can be accessed by reducing multidimensional structures to simple node-and-connecting nets. L7 anion tends to act as a 8-connected node;24 however, in 9, L7 anion coordinates to


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four Zn(II) ions simultaneously and acts as a 4-connected node. By considering the double bridges from the two BIE ligands as one linker, the framework is composed of 3-connecting nodes (Zn1 and Zn2) and a 4-connecting L7 node. Wells25 classified such a network as a nonuniform (3,4)-connected net where 3 and 4 indicate the connectivity of the nodes. As shown in Figure 8e, the combination of nodes and connectors provides the (3,4)-connected dmd-net of compound 9 with the topological notation of (4 3 102)2(42 3 104). Structure Description of 10. In compound 10, the rigid L8 anion was introduced to replace the flexible L6 anion. As illustrated in Figure 9a, the structure of 10 contains one Cd(II) ion, one-half L8 anion, one BIE ligand, and one water molecule. The Cd(II) ion is seven-coordinated by four carboxylate O atoms from two L8 anions (Cd1-O1=2.459(6), Cd1-O2 = 2.516(6), Cd1-O3#1 = 2.625(9), Cd1-O4#2 = 2.456(9), and Cd1-O1W=2.345(6) A˚), two N atoms from two BIE ligands (Cd1-N1#2 = 2.228(7) and Cd1-N3 = 2.283(7) A˚), and one water molecule in a distorted pentagonal bipyramidal geometry. O1W and N1 from the BIE ligand are located at the apical position of the pentagonal bipyramid. Each carboxylate group of the L8 anion chelates to one Cd(II) center to form a 1D chain along the a axis. A pair of BIE ligands, adopting a cis-conformation in mode II (see the Supporting Information), hang on both sides of the 1D chain by sharing the Cd(II) ions (Figure 9b). The dihedral angle between the two imidazole rings is 123.8°, and the Cd 3 3 3 Cd separation across the BIE ligand is of 6.356(3) A˚. There are intra- and intermolecular hydrogen bonds between O1W and carboxylate O atoms (O1W-H1A 3 3 3 O2#3 = 2.705(9) and O1W-H1B 3 3 3 O3=2.68(1) A˚), which further stabilized the 1D chain. To the best of our knowledge, H4L8 has never been introduced into the preparation of MOFs to date, although it is expected to be a versatile ligand in the construction of functional materials. From the structure description above, we can see that both the biim-5 and the BIE ligands can bend and rotate freely when coordinating to the central metals due to the flexible nature of the spacers between the two imidazole rings. As a comparison, the bpy is generally considered as a rigid linear ligand. For example, in [Co(bpy)(H2O)4] 3 (L1) 3 2H2O26a (13), the rigid bpy ligands bridge the neighboring Co(II) ions to generate a 1D linear chain, while in [Co(L1)(biim-5)] (1) a pair of flexible biim-5 ligands, in the cis-conformation, link two adjacent Co(II) ions to yield a [Co(biim-5)2Co] dimeric unit. Further, the L1 anions bridge the [Co(biim-5)2Co] dimeric units to give a 2D 63-hcb net. In [Co(bpy)(H2O)4]2 3 (L5) 3 2H2O26b (14), the bpy ligands connect the Co(II) ions to form a 1D rigid chain; however, in [Co(L5)0.5(biim-5)] (5) a couple of biim-5 ligands, in the cisconformation, bridge two Co(II) ions to furnish a [Co(biim5)2Co] dimeric unit. The [Co(biim-5)2Co] dimeric units are further extended by L5 anions to furnish a 3D framework with a Schl€ afli symbol of (83)2(85 3 10). The same as in the above compounds 13 and 14, the bpy ligands in both [Co(bpy)(H2O)4] 3 (L3) 3 H2O26c (15) and [Co(L3)(bpy)(H2O)2] 3 bpy 3 DMF26c (16) connect the central Co(II) ions to give linear chains. In contrast, the flexible biim-5 ligands in [Co(L3)(biim-5)] (3) adopt the trans-conformation and bridge the Co(II) ions to form a wavelike chain, which is further linked by the L3 anions to construct a 2D sql net. On the basis of the above results, it can be seen that the alkyl spacer between the two imidazole rings makes the skeleton of

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the biim-5 ligand more flexible than the bpy ligand when coordinating to the central metals. This flexible nature makes biim-5 ligands display various coordination modes and finally lead to novel structures different from the ones based on the bpy ligands. In addition, the flexible nature of the bis(imidazole) ligands can be tuned by changing the length of the spacer between the two terminal imidazole groups. For example, in [Co(L1)(biim-5)] (1), a pair of biim-5 ligands connect two Co(II) to form a [Co(biim-5)2Co] dimeric unit, which is further linked by L1 anions to yield a 2D 63-hcb net. Nevertheless, in [Co(L1)(biim-6)]7a (biim-6 = 1,10 -(1,6-hexanedidyl)bis(imidazole)) (17), when the biim-6 ligand was utilized to take the place of the biim-5 ligand, a wavelike chain arising from the biim-6 ligands and the Co(II) ions is obtained. The chains are further linked by L1 anions to yield an undulated (4,4) net. These structural differences caused by the length of the bis(imidazole) ligands can also be supported by the four structurally different polymers [Zn(L1)(biim-5)] (18) and [Zn(L1)(biim-6)] (19), as well as [Mn(L10)(biim-4)] (20) (H2L10 = oxalic acid) and [Mn(L10)(biim-6)] 3 H2O (21) reported by our group.8e In 18, two biim-5 ligands connect two Zn(II) ions to form a [Zn(biim-5)2Zn] dimeric unit, while in 19 the biim-6 ligands bridge the central Zn(II) ions to furnish a wavelike chain. In 20, the Mn(II) anions are bridged by biim-4 ligands to form a wavelike chain with two kinds of Mn-biim-4-Mn distances of 12.460(3) and 14.055(3) A˚. The L10 anions further link these wavelike chains to generate a 3-fold interpenetrating diamondoid framework. Nevertheless, when the biim-6 was employed to replace biim-4, a wavelike chain of 21 based on biim-6 ligands and Mn(II) ions is formed with only one kind of Mn-biim-4-Mn distance of 12.344(2) A˚. Different from the 3D framework of 20, the wavelike chains in 21 are further bridged by L10 anions to form a 2D layer structure. Another example is the 3D framework of [Co4(L11)2(biim-4)2(OH)2(H2O)] 3 4.5H2O8d (22) (H3L11 =1,3,5-benzenetricarboxylic acid) and the 2D net of [Co(HL11)(biim-6)]7a (23). In 22, The Co(II) ions are connected by the biim-4 ligands to form a [Co(biim-4)Co] dimeric unit, whereas, in 23, the Co(II) ions are linked by the biim-6 ligands to furnish a wavelike chain. The structural discrepancy of these compounds based on the same carboxylate anions and central metals indicates that the changes of the length of the alkyl skeleton play significant roles on the resultant structures. Besides the length of the alkyl spacer between the two imidazole rings, the introduction of a heteroatom may also affect the flexibilities of the bis(imidazole) ligands. In [Co2(L1)2(BIE)2(H2O)2] 3 H2O7b (24) and [Co(L1)(biim-5)] (1), both the BIE and the biim-5 ligands link the Co(II) ions to furnish dimeric units. However, the Co(II) 3 3 3 Co(II) separations in 24 across the BIE ligands are greatly longer than the ones across the biim-5 ligands in 1. As a result, this slight change causes a significant impact on the final structures of 24 and 1. The [Co(biim-5)2Co] dimeric units in 24 are bridged by the L1 anions to give a 1D chain, while the ones in 1 are linked by L1 anions to generate a 2D 63-hcb net. The different flexible abilities of bis(imidazole) ligands caused by the different lengths of the alkyl skeleton, as well as the introduction of heteroatom O, can also be found in the reported compounds [Zn(L5)0.5(biim-4)] 3 H2O8f (25) and [Zn (L5)0.5(BIE)]7c (26). In 25, the biim-4 ligands link the Zn(II) ions to form a 1D chain, whereas, in 26, two BIE ligands connect two Zn(II) ions to furnish a [Zn(BIE)2Zn]

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dimeric unit. The 1D -Zn-biim-4- chains in 25 are further linked by L5 anions to generate an R-Po topology; however, the [Zn(BIE)2Zn] dimeric units in 26, associating with the L5 anions, lead to a (3,4)-connected net with (83)2(85 3 10) topology. From the above discussion, the introduction of a C atom and a heteroatom O into the alkyl skeleton may cause a dominant impact on the flexibilities of bis(imidazole) ligands and further affect the frameworks of the resultant polymers. Besides the N-bridging ligands, the carboxylate anions also have a crucial effect on the construction of the complex framework. Generally, the role of carboxylate anions can be explained in terms of the differences in the number of the carboxylate groups, the positions of the carboxylate groups, the nature of the substituting groups, and the flexible abilities of the carboxylate anions. Take compounds [Co2(biim-5)3(H2O)6] 3 (L12)2 3 8H2O8e (H2L12 = fumaric acid) (27) and [Co(L5)0.5(biim-5)] (5); for example, in 27, each L12 anion, possessing two functional carboxylate groups, acts as a counteranion, whereas, in 5, each tetradentate L5 anion connects the [Co(biim-5)2Co] unit to produce a complex 3D framework. The structural discrepancy caused by the differences of the number of the carboxylate groups can also be found in the 1D structure of [Co(L13)2(biim-4)]26d (28) (HL13=4-methoxylbenzoic acid) and the 3D framework of [Co4(L11)2(biim-4)2(OH)2(H2O)] 3 4.5H2O8d (22). In 28, the L13 anion, as a terminal ligand, attaches to both sides of the helical chains constructed from Co(II) anions and biim-4 ligands, without changing the dimension of the entire structure. In 22, the tridentate L11 anions extend the 2D layers based on the Co(II) ions and the biim-4 ligands into a 3D network. The comparison result reveals that increasing the number of carboxylate groups can result in a higher dimensionality, which in turn leads to novel frameworks and topologies. The structural diversities of compounds [Co(L1)(biim-5)] (1) and [Co(L2)(biim-5)] 3 H2O (2) show the effects of the distinctive positions of the carboxylayte groups on the construction of the polymers. Although both L1 and L2 act as dicarboxylate anions with the existence of benzene rings, the positions of the carboxylate groups are different. In 1 the [Co(biim-5)2Co] dimeric units are linked by the L1 anions to form a 2D 63-hcb net, while in 2 the 1D -Cobiim-5- chains are connected by L2 anions to give a unique 2D double layer with the Schl€ afli symbol of (42 3 63 3 8). The structural differences caused by the positions of the carboxylate groups can also be supported by compounds [Cu(L1)(BIE)]7d (29), [Cu5(L2)4(μ3-O)2(BIE)2(H2O)2]7d (30), and [Cu(L9)(BIE)]7d (31), as well as by compounds [Zn2(L9)2(BIE)2] 3 2.5H2O7c (32) and [Zn2(L2)2(BIE)2]7c (33). In addition, the intruduction of an extra substituted group may also influence the construction of the resulting frameworks. Compared with the L2 anion, the L3 anion possesses an extra hydroxyl group, which tends to form hydrogenbonding interactions. In [Co(L2)(biim-5)] 3 H2O (2), the 1D -Co-biim-5- chains are connected by L2 anions to give a unique 2D double layer. In [Co(L3)(biim-5)] (3), when the L3 anion was used instead of the L2 anion, a 2D sql net arising from the L3 anions and the 1D -Co-biim-5- chains is obtained. Furthermore, the extra hydroxyl groups in the L3 donor hydrogen bond to carboxylate O atoms and extend the 2D layer structure of 3 into a 3D supramolecular framework. The different structures of compounds [Zn2(L6)(BIE)2] 3 2.5H2O (7) and [Zn2(L7)(BIE)2] 3 H2O (9) also show the effects of the substituent groups on the resultant frameworks. The common feature of L6 and L7 anions is that both

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of them contain four carboxylate groups. The only structural difference between L6 and L7 anions is the substitute groups bridging the two benzene rings. In 7, the L6 anions bridge the 1D -Zn-BIE- chains to generate a 2D double layer; however, in 9, the L7 anions link the [Zn(BIE)2Zn] dimeric units to give a 3D porous framework with a (3,4)-connected (4 3 102)2(42 3 104)-dmd-net. Besides the factors described above, the flexible abilities of the carboxylate anions also have significant influence on constructing the resultant structures. L6 and L7 anions bear flexible skeletons, which can freely twist around the -O- and [-C(CF3)2-] groups to meet the requirements of the coordination geometries of metal ions in the assembly process. In contrast, the short L1 and L2 anions are rigid and not twisted. As a result, compounds [Co2(L6)(BIE)2] 3 1.5H2O (6) and [Zn2(L7)(BIE)2] 3 H2O (9) display a 2D double layer and 3D porous framework, while compounds [Co2(L1)2(BIE)2(H2O)2] 3 H2O7b (24) and [Zn2(L2)2(BIE)2]7c (33) possess a 1D chain and 2D 44-sql structures. Finally, by comparing the structures of all the carboxylate polymers reported in our paper with those previous ones, it can be seen that the polycarboxylate anions, possessing more carboxylate groups and bearing flexible abilities, are good candidates for design of specific topological structures. Thermal Analysis. To characterize the compounds more fully in terms of thermal stability, the thermal behaviors of 2, 4, and 6-10 were examined by TGA. The experiments were performed on samples consisting of numerous single crystals under N2 atmosphere with a heating rate of 10 °C/min (Figure S12-S18 in the Supporting Information). For 2, the weight loss corresponding to the release of water molecules is observed from room temperature to 163 °C (obsd 4.7%, calcd 4.04%). The decomposition of the residual composition occurs from 321 to 442 °C. The TGA curve of 4 shows that the first weight loss from room temperature to 84 °C corresponds to the loss of water molecules (obsd 14.6%, calcd 13.52%). The second weight loss of 39.0% (calcd 38.33%) in the temperature range 209-370 °C can be assigned to the release of biim-5 ligand. The removal of L4 ligand occurs in the temperature range 370-416 °C (obsd 30.1%, calcd 34.96%), leading to the formation of CoO as the residue (obsd 14.4%, calcd 14.07%). As expected, the TGA curves of compounds 6 and 7 exhibit similar weightloss stages. The first weight loss, corresponding to the water molecules, is observed from room temperature to 109 °C (obsd 4.2%, calcd 3.01%) for 6 and from room temperature to 111 °C (obsd 5.8%, calcd 4.84%) for 7. The second weight loss is attributed to the removal of organic components, from 283 to 505 °C for 6 and from 267 to 487 °C for 7. For compound 8, the weight loss in the range of 73 to 93 °C corresponds to the departure of lattice and coordination water molecules (obsd 3.5%, calcd 3.42%), and the anhydrous compound begins to decompose at 249 °C. 9 releases its water molecule gradually from 34 to 178 °C (obsd 2.7%, calcd 1.73%). The removal of the organic components occurs in the range of 357 to 560 °C. The framework structure of compound 10 decomposes between 338 and 441 °C. However, it is difficult to determine these weight losses accurately. Luminescent Properties. Luminescent properties of compounds which contain zinc or cadmium as the metal centers have been attracting more interest because of their potential applications in chemical sensors, photochemistry, and electroluminescent display.27 In this paper, the solid-state


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photoluminescent properties of L, H4L8, and compounds 7-10 have been investigated in the solid state at room temperature, and the emission peaks as well as the luminescence decay cures are shown in Table 2 and Figures S19-S21 in the Supporting Information. The photoluminescent spectrum of liquid BIE ligand has been investigated and shows the emission maxima at 545 nm (λex=490 nm).7c The emission and excitation peaks of L and H4L8 are at 336 (λex =290 nm) and 370 nm (λex =280 nm), respectively. The emission bands of these free ligands are probably attributable to the π* f n or π* f π transition.28 On complexation of these ligands with Zn(II) and Cd(II) ions, the emissions arising from the free ligands were not observable. Interestingly, the emission spectra for the compounds 7 and 8 show the main peaks at 422 and 442 nm, exhibiting a red-shift with respect to the free H4L6 (395 nm, λex =358 nm).29 The emission spectrum for compound 9 is located at 401 nm (λex=314 and 343 nm) and is red-shifted by 65 nm with respect to the band shown by the L ligand. For 10, this band appears at 452 nm (λex=353 nm), which is also redshifted by 88 nm. As a comparison, it can be seen that all the emission peaks of the four compounds are red-shifted with respect to the free carboxylic acids and blue-shifted related to the BIE ligand. Since the Zn(II) and Cd(II) ions are difficult to oxidize or to reduce because of the d10 configuration, the emission of these compounds is neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in nature.30 Moreover, the replacement of the hydrogen proton by Zn(II) or Cd(II) ions decreases the π* f n or π* f π gap of the carboxylate ligands, resulting in the red-shift of the emission peaks.31 The hypsochromic shift of the emission peaks of the compounds, related to the BIE ligand, suggests that the coordination of the BIE ligand to central Zn(II) or Cd(II) ions increases the rigidity, leading to less vibrations of the skeleton and reducing the loss of energy by radiationless decay of the intraligand emission excited state.32 For the reasons above, the emission can be assigned to the intraligand transitions of the carboxylate anions and the BIE ligands. The emission discrepancy of these compounds is closely associated to the differences of the metal ions and the ligands coordinated around them. The decay curves of 7-10 are well fitted into a singleexponential function as I=A exp(-t/τ) þ y0, suggesting only one luminescence center. This indicates that the energy transfer may occur between the carboxylate ligands and the BIE ligands.33 The luminescence lifetimes [τ(7)=7.08 ns, τ(8) =8.37 ns, τ(9) =7.10 ns, and τ(10)=9.00 ns] are much shorter than the lifetime of the emission resulting from a triplet state (>10-3 s), indicating that it arises from the singlet state.31,34 The nanosecond range of lifetime in the solid state at room temperature reveals that the emission is fluorescent in nature. Conclusion We have successfully isolated a series of 1D, 2D, and 3D metal-organic coordination polymers based on different polycarboxylate anions and bis(imidazole) ligands. The variety of the structures indicates that the polycarboxylate anions play dominant roles in the assembly of the final frameworks. Compounds 1-5, with the same Co(II) center and biim-5 N-donor bridging ligand, display various 2D and 3D frameworks. Although both L1 and L2 act as dicarboxylate anions, the positions of the carboxylate groups are different in the two

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anions. Compound 1 containing L1 anion displays a 2D hexagonal honeycomb 63-hcb net, while 2, including L2 anion, exhibits an uncommon 4-connected 2D double layer with the Schl€afli symbol of (42 3 63 3 8). L2, L3, and L4 anions possess the similar position of the two carboxylate groups. The only difference between L2 and L3 anions is the presence of a substituting hydroxyl group in L3. Compared with the 2D double-layer structure of 2, 3, possessing the L3 anion, displays a 2D sql net with large open windows, which is further linked by hydrogen-bonding interactions between the hydroxyl group and carboxylate O atoms, generating a 3D supramolecular framework. Obviously, the differences of the frameworks between compounds 2 and 3 are mainly caused by the substituting hydroxyl group. Compared with 2, when a fatty acid H2L4 with three substituted methyl groups was used, a structurally different compound 4 was obtained. Perhaps, the substituted methyl groups and the relatively flexible 5-membered fatty ring are responsible for the differences of the architectures between 2 and 4. When a tetradentate L5 anion with flexible alkyl skeleton was utilized to replace dicarboxylate anions, a complicated 3D framework of 5 with a rare (83)2(85 3 10) topology was obtained. Polycarboxylate anions also play important roles in the construction of compounds 7-10. The introduction of the [-C(CF3)2-] group in L7 instead of the symmetric center O atom in L6 results in the formation of distinct architectures of 7 and 9. The employment of the relatively rigid L8 anion replacing the flexible L6 anion leads to the structural discrepancy between 8 and 10. The photoluminescent emissions of coordination polymers 7-10 indicate that these four complexes may be good candidates for optical materials. Acknowledgment. We thank the National Natural Science Foundation of China (Grant No. 20471014), the Program for New Century Excellent Talents in Chinese University (Grant NCET-05-0320), the Program for Changjiang Scholars and Innovative Research Teams in Chinese University, the China Postdoctoral Science Foundation (20080431050 and 200801352), the Postdoctoral Foundation of Northeast Normal University, the Training Fund of NENU’S Scientific Innovation Project, and the Analysis and Testing Foundation of Northeast Normal University for support.

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Supporting Information Available: X-ray crystallographic files (CIF); diagrams of the structures; selected bond distances and angles; coordination modes of the reported flexible bis(imidazole) ligands; TGA curves of compounds 2, 4, and 6-10; and the emission and excitation spectra of free ligands and compounds 7-10. This material is available free of charge via the Internet at http://pubs. acs.org.

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