Chiral and Porous Coordination Polymers Based on an N-Centered

Nov 30, 2010 - Chunying Xu , Qianqian Guo , Xianjuan Wang , Hongwei Hou , and Yaoting Fan. Crystal Growth & Design 2011 11 (5), 1869-1879...
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DOI: 10.1021/cg1011764

Chiral and Porous Coordination Polymers Based on an N-Centered Triangular Rigid Ligand

2011, Vol. 11 231–239

Xiao-Qiang Yao, Da-Peng Cao, Jin-Song Hu, Yi-Zhi Li, Zi-Jian Guo, and He-Gen Zheng* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China Received September 5, 2010; Revised Manuscript Received October 27, 2010

ABSTRACT: Four cadmium and cobalt coordination polymers with unique structures and topologies have been successfully synthesized under solvothermal conditions by employing an elongated triangular rigid N-containing ligand tris(4-(1H-imidazol1-yl)phenyl)amine (TIPA) and 5-hydroxyisophthalic acid (5-OH-H2bdc) as anion coligand. Compounds 1-4 were characterized by single crystal X-ray structure analyses, thermogravimetric analyses, SHG, and photoluminescent measurements. Compound 1 crystallizes in the chiral space group C2 (No. 5) and features a 2D f 3D parallel/parallel inclined polycatenated framework. Compound 2 crystallizes in trigonal symmetry with a high-symmetry space group R3 and features a 2D porous noninterpenetrating coordination network. Compound 3 crystallizes in the monoclinic space group P21/n and shows a 2D f 3D parallel/parallel polycatenation framework, and compound 4 crystallizes in the orthorhombic chiral space group P212121 (No. 19) and shows a very rarely 3D þ 3D heterogeneous 2-fold interpenetration framework built from (3,5)-connected (42 3 65 3 83)(42 3 6) AFUQOH nets and (3,5)-connected (63)(69 3 8) gra nets.

Introduction Metal-organic frameworks (MOFs), especially with chirality, have attracted considerable current attention because of their potential applications in gas storage,1 catalysis, gas separation,2 magnetic materials,3 NLO properties,4 and enantioselective separations.5 To a certain extent, the structures with specific topologies and functions can be assembled by exploiting rigid organic ligands and metal centers with fixed coordination geometries.6 However, it is still hard to be accurately predicted. Compared with other factors such as counteranions, the metal-to-ligand ratio, and the solvent system, the coordination geometry of metal ions or clusters and the structural characteristics of polydentate organic ligands take on a greater importance. The tripodal rigid multidentate ligands with N-donors are prominent for constructing the porous MOFs, owing to their natural characteristics.7 Although there are so many merits, just a few related examples have been reported. One reported example is an expanded sodalite-type framework constructed by the elongated benzene- and triazine-centered triangular ligands: 1,3,5-tri-p-(tetrazol-5-yl)-phenylbenzene (H3TPB-3tz) and 2,4,6-tri-p-(tetrazolo-5-yl)phenyls-triazine (H3TPT-3tz), respectively.8 Another relevant example is a two-dimensional (2D) porous CdI2-type network with high thermostability and potential porosity, which assembled by using a rigid triangular ligand 2,4,6-tris[4-(1-H-imidazole-1yl)phenyl]-1,3,5-triazine (TIPT) and Cd2þ ions.9 In addition, our group has reported an unprecedented three-dimensional (3D) cluster polymer with considerable porosity:10 [WS4Cu6I4(timtz)8/3(H2O)12]n, based on a D3h symmetry trigonal planar ligand 2,4,6-tri(1H-imidazol-1-yl)-1,3,5-triazine (timtz). However, to date, there are very few reported examples of chiral MOFs constructed by highly symmetrical tripodal tris-monodentate rigid ligands. Following this strategy, we have synthesized a newly extended triangular ligand tris(4-(1H-imidazol-1-yl)

phenyl)amine (TIPA). Compared with benzene- and triazinecentered triangular ligands, the N-centered tripodal TIPA ligand has a more movable “kink” which allows the ligand to more easily meet the geometric needs of different metal ions. Via the hydrothermal method, we have synthesized four new coordination polymers with either chirality or porosity: [Cd(TIPA) 3 (NO3)2 3 5H2O]n (1), [Co(TIPA)1/3 3 Cl 3 2H2O]n (2), [Co(TIPA) 3 (5-OH-bdc) 3 2H2O]n (3), and [Cd2(TIPA)2(5-OHbdc)2 3 5.5H2O]n (4). Compounds 1 and 2 were synthesized by TIPA ligand with Cd2þ and Co2þ ions, respectively. Compound 1 is a chiral porous 2D f 3D parallel/parallel inclined polycatenated framework, and 2 is a 2D CdI2-type network with large voids. Under the same conditions, compounds 3 and 4 were synthesized by using 5-OH-H2bdc as a coligand. Compound 3 is a 2D f 3D parallel/parallel polycatenation framework, and 4 is a very rarely chiral 3D þ 3D heterogeneous 2-fold interpenetration framework with a considerable solvent accessible volume. Experimental Section

*To whom correspondence should be addressed. E-mail: zhenghg@nju. edu.cn. Fax: 86-25-83314502.

Materials and Methods. The triangular ligand tris(4-(1H-imidazol1-yl)phenyl)amine (TIPA) was synthesized by the copper-catalyzed carbon-nitrogen bond cross-coupling reaction between imidazole and tris(4-bromophenyl)amine using Ullmann condensation methods. All other reagents and solvents were commercially purchased without further purification. The IR absorption spectra of the complexes were recorded in the range 400-4000 cm-1 by means of a Nicolet (Impact 410) spectrometer with KBr pellets (5 mg sample in 500 mg of KBr). C, H, and N analyses were carried out with a Perkin-Elmer 240C elemental analyzer. 1H NMR spectra were obtained in a Bruker 500 MHz NMR spectrometer. Electrospray ionization mass spectra (ESI-MS) were recorded on a Finnigan MAT SSQ 710 mass spectrometer in the scan range 120-1000 amu. XRD measurements were performed on a Bruker D8 Advance X-ray diffractometer using Cu KR radiation (1.5418 A˚), in which the X-ray tube was operated at 40 kV and 40 mA. Luminescence spectra for the solid samples were recorded on an AMINCO Bowman Series2 fluorescence spectrophotometer at room temperature (25 °C). A pulsed Q-switched Nd:YAG laser at a wavelength of 1064 nm was used to

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Table 1. Crystallographic Data and Structure Refinement Details for Complexes 1-4a complex 1 2 3 4 C54H66Cl2CoN14O12 C35H29CoN7O7 C140H122Cd4N28O31 formula C27H31CdN9O11 formula weight 770.01 1233.01 718.58 3142.30 crystal system monoclinic trigonal monoclinic orthorhombic P21/n P212121 space group C2 R3 a (A˚) 27.042(4) 16.0155(10) 9.731(2) 18.1535(12) b (A˚) 15.420(3) 16.0155(10) 18.145(5) 18.3033(14) c (A˚) 8.9800(15) 26.675(3) 18.588(5) 25.2354(19) R (deg) 90.00 90.00 90.00 90.00 β (deg) 104.943(3) 90.00 96.984(5) 90.00 γ (deg) 90.00 120.00 90.00 90.00 3617.9(11) 5925.4(8) 3257.7(14) 8384.9(11) V (A˚3) Z 4 3 4 2 1.414 1.037 1.465 1.245 Dc (g cm-3) 0.668 0.339 0.589 0.572 μ(Mo KR) (mm-1) F(000) 1568 1935 1484 3196 θ min, max (deg) 2.3, 26.0 2.1, 26.0 2.2, 25.0 1.8, 26.0 tot, uniq data 9802, 5520 10852, 2596 15890, 5715 45239, 16464 R(int) 0.050 0.045 0.109 0.049 observed data [I > 2σ(I)] 4062 1956 4340 13146 5520, 438 2596, 146 5715, 451 16464, 991 Nref, Npar absolute structure parameter 0.03(3) 0.033(18) R, wR2 [I > 2σ(I)] 0.0571, 0.1178 0.0529, 0.1203 0.0504, 0.1398 0.0540, 0.1054 S 1.07 1.06 1.00 1.00 -3 -0.76, 0.96 -0.28, 0.27 -0.48, 0.61 -0.75, 0.95 min and max resd dens (e 3 A˚ ) P P P a 2 2 2 2 2 P 2 2 1/2 2 2 2 R1 = ||Fo| - |Fc||/ |Fo|; wR2 = { [w(Fo - Fc ) ]/ [w(Fo ) ]} ; where w = 1/[σ (Fo ) þ (aP) þ bP], P = (Fo þ 2Fc )/3.

generate the SHG signal. The backward-scattered SHG light was collected by a spherical concave mirror and passed through a filter that transmits only 532 nm radiation. Thermogravimetric analysis (TGA) data were recorded by a simultaneous SDT 2960 thermal analyzer from 25 to 750 °C with a heating rate of 10 °C min-1 in N2 atmosphere (a flow rate of 100 mL min-1). Synthesis of Tris(4-(1H-imidazol-1-yl)phenyl)amine (TIPA). A mixture of imidazole (50.0 mmol, 3.40 g), tris(4-bromophenyl)methane (15.0 mmol 7.20 g), CuI (2.5 mmol, 0.47 g), 1,10-phenanthroline (6.0 mmol, 1.08 g), and K2CO3 (45.0 mmol, 6.21 g) was suspended in 100 mL of DMF. The mixture was refluxed for 4 days and then cooled to room temperature. Solvent was removed by distillation under a vacuum, and CHCl3 (300 mL) was poured into the resulting brightly brown sticky residue. The filtrate was washed with water (100 mL  3), dried over anhydrous MgSO4, and then evaporated to obtain light brown mud. Then the crude product was separated by column chromatography (CH2Cl2/CH3OH = 3:1) to afford white powder (yield: ∼50%, based on tris(4-bromophenyl)methane). 1H NMR (500 MHz, δ, CDCl3): 7.99 (s, 3H), 7.86 (d, 6H), 7.58 (d, 6H), 7.37 (s, 3H), 7.27 (s, 3H). Anal. Calcd for C27H21N7: C, 73.12; H, 4.77; N, 22.11. Found: C, 73.11; H, 4.73; N, 22.13. IR (KBr, cm-1): 3117w, 1647w, 1514s, 1333w, 1306 m, 1285m, 1257m, 1112w, 1057s, 961w, 911w, 828 m, 738w, 657w. MS (ESI), m/z (%): 444.33 (100) for C27H22N7þ (Figure S9, Supporting Information). Synthesis of [Cd(TIPA) 3 (NO3)2 3 5H2O]n (1). A mixture of TIPA (0.066 g, 0.15 mmol) and Cd(NO3)2 (0.082 g, 0.35 mmol) in N,N0 dimethylformamide (DMF) (4 mL) and H2O (2 mL) was placed in a Teflon-lined stainless steel vessel and heated at 95 °C for 3 days and then cooled to room temperature; light brown prism-shaped single crystals suitable for X-ray diffraction were obtained in 60% yield. Anal. Calcd for C54H61Cd2N18O22: C, 42.14; H, 4.00; N, 16.38. Found: C, 42.17; H, 4.08; N, 16.32. IR (KBr, cm-1): 3443m, 3123w, 2919s, 2850m, 1602w, 1542m, 1508m, 1466w, 1384s, 1320m, 1166w, 1120w, 1055s, 831m, 754w, 621w, 538w. Synthesis of [Co(TIPA)1/3 3 Cl 3 2H2O]n (2). A similar synthetic procedure to that for 1 was employed except that CoCl2 (0.065 g, 0.5 mmol) was added instead of Cd(NO3)2; light orange bulk crystals were obtained 3 days later in 80% yield based on TIPA. Anal. Calcd for C54H66Cl2CoN14O12: C, 52.60; H, 5.40; N, 15.90. Found: C, 52.43; H, 5.42; N, 15.33. IR (KBr, cm-1): 3419m, 3130w, 1621m, 1516s, 1394m, 1339w, 1309m, 1118w, 1064m, 962m, 932w, 831m, 735w, 657w, 542w. Synthesis of [Co(TIPA) 3 (5-OH-bdc) 3 2H2O]n (3). Dark red crystals of 3 were grown from solvothermal reaction of Co(NO3)2 (0.046 g, 0.25 mmol), TIPA (0.066 g, 0.15 mmol), and 5-OH-H2bdc (0.027 g, 0.15 mmol) in the mixed solvents of DMF and H2O (v/v = 1: 2) in

a Teflon-lined stainless steel vessel and heated at 120 °C for 3 days. Anal. Calcd for C35H29CoN7O7: C, 63.73; H, 4.43; N, 14.86. Found: C, 63.71; H, 4.45; N, 14.83. IR (KBr, cm-1): 3410m, 1663m, 1575m, 1514s, 1372w, 1272m, 1059m, 828w, 731s, 654m, 549w. Synthesis of [Cd2(TIPA)2(5-OH-bdc)2 3 5.5H2O]n (4). A similar method as described above for 3 was employed except that Cd(NO3)2 (0.082 g, 0.35 mmol) was added instead of Co(NO3)2. Colorless block single crystals were obtained after 3 days in 30% yield based on TIPA. Anal. Calcd for C140H122Cd4N28O31: C, 53.51; H, 3.91; N, 12.48. Found: C, 53.28; H, 3.78; N, 12.51. IR (KBr, cm-1): 3407m, 1563m, 1514m, 1372w, 1272w, 1118m, 1061m, 965w, 831w, 731w, 654w, 549w. X-ray Crystallography. X-ray crystallographic data of 1-4 were collected on a Bruker Apex Smart CCD diffractometer with graphite-monochromatic Mo KR radiation (λ = 0.71073 A˚). Structures were solved by direct methods, and the non-hydrogen atoms were located from the trial structure and then refined anisotropically with SHELXTL using full-matrix least-squares procedures based on F2 values using the SHELXTL (version 6.14) package of crystallographic software.11 The hydrogen atom positions were fixed geometrically at calculated distances and allowed to ride on the parent atoms. The relevant crystallographic data are presented in Table 1, while the selected bond lengths and angles are given in Table S1, Supporting Information. The topological analysis and some diagrams were produced using the TOPOS program.12

Results and Discussion Crystal Structure of [Cd(TIPA) 3 (NO3)2 3 5H2O]n (1). X-ray diffraction analysis shows that compound 1 crystallizes in the monoclinic chiral space group C2. The cadmium center is coordinated by three TIPA ligands and two nitrate anions, and the coordination environment at cadmium is best described as a {CdN3O4} distorted pentagonal bipyramid, with the axial positions occupied by two N donor atoms from two different TIPA ligands (Figure 1). The equatorial plane consists of two chelating nitrate groups and one N donor atom from TIPA. Both Cd2þ and TIPA ligand act as 3-connectors to form a uninodal (6,3) layer with 63-hcb topology (Figure 2a). Packing of the layers generates two sets of layers oriented toward the [1, -1, -1] and [1, 1, -1] directions, respectively. These two sets of layers catenated to each other in a parallel/ parallel arrangement to form a 2D f 3D inclined polycatenation

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structure. The angle between the two adjacent inclined planes is 60.3° (Figure 2c). Further insight into one circle of a layer shows that each circle is catenated with four other circles from different layers in the other set (Figure 2d). The inclined polycatenation of these 2D sheets leads to a 3D porous architecture which contains large 1D hexagonal channels with a cross section of approximately 13.0  10.2 A˚2 that accom-

Figure 1. Coordination environment of 1 with thermal ellipsoids shown at 30% probability. Symmetry codes: a = 0.5 þ x, 0.5 þ y, z; b = 0.5 þ x, -0.5 þ y, 1 þ z.

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modate abundantly disordered water molecules (Figure 2b); the total void value of the channels without water guests is estimated (by PLATON13) to be 1037.6 A˚3, approximately 28.7% of the total crystal volume, 3617.9 A˚3. Compound 1 represents the rare examples of 2D f 3D assembled by trigonal rigid ligand and metal. The 3,3connected (6,3) nets based on trigonal rigid ligand are apt to form large voids; because of this, they have aroused many group’s interest. However, until recently, the related examples reported are still rare.14 The previous examples exhibiting 2D f 3D parallel/parallel inclined polycatenation based on (6,3) layers are commonly formed by 3-connecting metal centers and biconnecting linear ligands.15 Crystal Structure of [Co(TIPA)1/3 3 Cl 3 2H2O]n (2). Compound 2 crystallizes in trigonal geometry with a highsymmetry space group R3. The asymmetric unit contains one-third of a TIPA ligand, one Cd2þ ion which is sited at the crystallographic symmetry center with 1/6 site occupancy, one-third of Cl-, and two lattice water molecules. Clcounteranions are located on a site with crystallographically imposed 3 symmetry. Two water molecules are disordered over five positions. The coordination geometry of Co2þ is an ideal octahedron with all the Co-N bond distances 2.1699(17) A˚, surrounded by six imidazole rings of distinct TIPA ligands, and the TIPA ligand in 2 shows D3 symmetry (Figure 3a). A topological analysis of the cationic networks in 2 revealed the CdI2-type net, as shown in Figure 3b. The CdI2-type net is one of the Catalan nets, which are known in inorganic compounds such as metal alkoxides and hydroxides.16 The

Figure 2. (a) View of the 2D 63-hcb layer in 1. (b) Packing diagram of compound 1 with the channels extending along the b axis. (c) Scheme showing interesting frameworks of 2D f 3D inclined polycatenation with dihedral angle 60.3°. (d) Portion of the topological net showing one circle is catenated by four other six-membered circles.

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Figure 3. (a) Scheme showing the 6-connecting Co2þ coordination geometry in 2; the atoms of the asymmetric unit are labeled. Hydrogen atoms are omitted for clarity. (b) CdI2-type net topology of 2 with the two vertex symbols indicated.

Figure 4. (a) Bowl shaped structural unit of 2. (b) Side view of the capsule in 2 encapsulating gust water molecules and two Cl- counteranions.

Cd2þ ions act as 6-connecting nodes (vertex symbol: 46 3 66 3 83), and the TIPA ligands are also simplified as 3-connecting nodes (vertex symbol: 43). There are twice as many 3-connecting nodes as 6-connecting nodes; obviously, this is a 2D binodal net. Therefore, as determined by TOPOS software, the Schl€ afli symbol for this 3,6-connected binodal network is {46 3 66 3 83}{43}2, and the topological type is kgd. There are spacious voids in compound 2 which can accommodate abundant small guest molecules. The capsules-like void consists of two face-to-face bowl-shaped structural units (as depicted in Figure 4). The incipient voids of 2 are occupied by the guest molecules showing no voids (estimated by PLATON). If we just consider the cationic framework alone, the solvent accessible void can reach 3127 A˚3 (52.8%). Similar CdI2-type MOFs have been reported by Su9 and coworkers which have been assembled from Cd2þ ions and a triangular rigid TIPT ligand (TIPT = 2,4,6-tris(4-(1Himidazol-1-yl)phenyl)-1,3,5-triazine). However, the porosity of this above-mentioned CdI2 network is occupied by bulky counteranions, and the authors expect that they would have obtained the CdI2 topological networks with large voids without bulky counteranions. In spite of this, their exploratory works provide a practicable strategy to fabricate genuine

porous coordination polymers. At first, we try to use metal center Cd2þ and bulky triangular ligand TIPA to construct CdI2-type MOFs. However, we fail to get this CdI2-type network, and we obtain a 2D f 3D inclined polycatenation structure of compound 1 instead. Considering Co2þ also has an octahedral coordination geometry, when we replace Cd2þ with Co2þ, the CdI2-type 2D porous framework has readily been synthesized in good yield. Compared with the TIPT ligand and other trigonal ligands, the TIPA ligand has a more movable “kink”, making the ligand able to easily satisfy the geometrical requirement of the octahedral metal centers. Crystal Structure of [Co(TIPA) 3 (5-OH-bdc) 3 2H2O]n (3). Compound 3 crystallizes in the monoclinic space group P21/n. The asymmetric unit consists of one Co atom, one TIPA ligand, and two water molecules. The local coordination geometry around the Co2þ cation is depicted in Figure 5. The Co1 atom adopts a distorted trigonal bipyramid coordination sphere that is defined by two oxygen atoms from two distinct 5-OH-bdc2- anions and three N atoms from three different TIPA ligands; thus, the Co2þ atoms can be considered as 5-connecting nodes. Each TIPA links three Co atoms, acting as a 3-connecting node to form a 1D ladder-like chain

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(Figure 6a). Two carboxylate groups of the 5-OH-bdc2anions adopting monodentate coordination mode chelate two Co2þ ions, and 5-OH-bdc2- joins adjacent parallel chains as a pillar connector to form a 2D gridlike (4,4) bilayer (Figure 6b). The Schl€ afli symbol for this binodal net is

Figure 5. Coordination environment of 3 with thermal ellipsoids shown at 30% probability. Symmetry codes: a = 1 þ x, y, z; b = x, 1 þ y, z; c = -1 þ x, y, z; d = 1.5 - x, 0.5 þ y, 1.5 - z.

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(42 3 67 3 8)(42 3 6). Viewed from the b axes, there exist rectangular channels (Figure 6c) with a cross section of approximately 20  11 A˚ (excluding van der Waals radii). The channel is so large that the two other equivalent bilayers can be accommodated in that channel. This may be the mainly reason to form the 2D f 3D parallel entangled structure. Upon interpenetration, compound 3 just contains a small solvent accessible void space of 9.1% of the total crystal volume, according to a calculation performed using PLATON. The topological feature of compound 3 is most unusual because it is a rarely observed bilayer motif which is parallel/ parallel catenated with two other equivalent adjacent ones to form a 3D superamolecular structure (Figure 7). To our knowledge, this 3,5-connected bilayer is yet to be reported. Compared to 2D f 3D polycatenation systems in parallel/ parallel inclined fashion or parallel/parallel highly undulating fashion, fewer examples of 2D f 3D parallel entangled structures have been observed.17 Crystal Structure of [Cd2(TIPA)2(5-OH-bdc)2 3 5.5H2O]n (4). Single crystal X-ray diffraction reveals that 4 crystallizes in the chiral orthorhombic space group P212121 with a Flack parameter of 0.033(18), confirming that 4 is a chiral coordination polymer despite the use of achiral starting materials. The asymmetric unit of 4 contains two independent fragments

Figure 6. (a) View of the ladder-like chain. (b) Perspective view of the 2D gridlike (4,4) bilayer in 3 (the TIPA ligand is simplified to a 3-connecting node for clarity). (c) Top view of the bilayer along the b axis; the approximate width and height of one channel unit are marked.

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(A and B) (Figure 8), which result in two networks (net A and B) with different topologies. The coordination environments of Cd1 and Cd2 are almost alike; each Cd2þ atom is coordinated by three imidazole nitrogen atoms from three different TIPA ligands and four oxygen atoms from two different 5-OH-bdc2- ligands. As we previously described in compound 1, the coordination environment of Cd2þ in compound 4 can also be described as a {CdN3O4} distorted pentagonal bipyramid. In net A, the Cd2 atoms are bridged by TIPA, giving rise to the two-dimensional hexagonal grid planar layers (Figure 9a), and such layers are further connected by 5-OH-bdc2-, leading to the formation of a 3D framework. The distance between adjacent layers is ∼9.8 A˚ (Figure 9b). Better insight into the present 3D frameworks can be accessed by the topological method. The Cd2 atom connecting to two 5-OH-bdc2and three TIPA ligands can be viewed as a five-connected node, the TIPA ligand can be considered as a three-connected node (connecting to three Cd atoms), and each 5-OH-bdc2can be considered as a two-connected node, thus forming a (2,3,5)-connected network. However, 5-OH-bdc2- acts as a pillar connecting two Cd2- anions, which can be further simplified to a line, and net A can be further simplified to a rare (3,5)-connected binodal 3D framework with gra topology (Figure 11a). The Schl€ afli symbol for this net is (63)(69 3 8). One character of the gra nets is that they usually have hexagonal helical channels. There is also a channel with a cross section of approximately 13.2 A˚  11.6 A˚ in net A. Because the hydroxyl group of 5-OH-bdc2- is orientated to the channel, the total void value of the channel is relatively decreased (Figure S1, Supporting Information). Each channel

Figure 7. Topological representation of the 2D f 3D parallel enangled structures.

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in net A consists of one right-handed helix and one lefthanded helix, because the right and left handed helices are alternatively arranged; the whole chirality of net A is therefore racemic (Figure 9c). In net B, the network also can be simplified to a 3,5connected binodal 3D framework, as determined by TOPOS software; the Schl€ afli symbol for this binodal net is (42 3 65 3 83)(42 3 6), and the topology type of this net is AFUQOH (Figure 11b). Obviously, net B is significantly different from net A. The reasons for the difference will be discussed briefly in the ensuing paragraphs. Net B has a distorted narrow hexagonal channel, viewed along the a axis, with a cross section of approximately 26.3 A˚  9.6 A˚ (Figure 10b). Each channel in net B has a right-handed helix, and therefore, the whole net is chiral (Figure 10c,d). We believe that the overall chirality of compound 4 is imposed by net B. Though fragments A and B are almost alike, net B is totally different from net A. The primary cause of the enormous variations of nets A and B is derived from the different assembly methods. As depicted in Figure 9a, in net A, the TIPA ligands are joined together by Cd2 to form 2D (6,3) hexagonal layers, and then the 5-OH-bdc2- join adjacent equivalent layers to form a 3D framework resembling graphite. While in net B, we adopt the same method to analyze this structure. Setting the 5-OH-bdc2- aside, Cd1 atoms assemble with TIPA ligands to form a ladder-like chain (Figure 10a), the adjacent parallel chains were further linked by two sets of parallel 5-OH-bdc2- to form a distorted hexagonal channel (Figure 10b,c). Each hexagonal channel connects to six adjacent ones by sharing common edges, thus establishing a 3D framework. This may be the primary reason for the two distinct nets with different topology coexisting in this framework (Figure 11c,d). This 3Dþ3D heterogeneous 2-fold interpenetration framework still has a considerable solvent accessible volume; the abundantly disordered water molecules permeate through the frameworks of compound 4. Considering the coordination framework alone, the total void is estimated to be 2886.2 A˚3, approximately 34.4% of the total crystal volume 8384.9 A˚3. The 3D/3D interpenetration of heterogeneous nets with different topology is very rare. To our best knowledge, it is unprecedented in the field of organic-inorganic hybrid materials, and just numbered pure inorganic compounds have been reported, which are summarized by Batten.18 Luminescent Properties. Compounds 1-4 are insoluble in common organic solvents; their photoluminescence properties

Figure 8. Two distinct asymmetric units (fragment A and B) of 4 with atom labels; the hydrogen atoms and solvent molecules are omitted. Symmetry codes: a = -0.5 þ x, 0.5 - y, 1 - z; b = 1 - x, -0.5 þ y, 0.5 - z; c = -1 þ x, y, z; d = 1.5 - x, 1 - y, - 0.5 þ z; e = x, 1 þ y, z; f = -0.5 þ x, 1.5 - y, 1 - z.

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Figure 9. (a) Schematic representation of a 63 layer in net A. (b) Perspective view of net A, showing adjacent equivalent layers are linked by 5-OH-bdc2- to form a 3D framework resembling graphite (the TIPA ligand is simplified to a 3-connecting node for clarity). (c) Schematic representation of a channel which consists of a left-handed helix (left) and a right-handed helix (right).

as well as the free TIPA ligand were investigated in the solid state at room temperature. An intense emission of the free TIPA ligand was observed with wavelength from 374 to 490 nm (λmax = 405 nm upon excitation at 388 nm; Figure 12). Compound 2 shows no detectable fluorescence at room temperature. However, compounds 1 and 4 exhibit slightly hypsochromic shifted emission bands in comparison with the free ligand. Compound 1 exhibits an intense emission with a maximum at 399 nm upon excitation at 386 nm, and compound 4 exhibits an intense emission with a maximum at 394 nm upon excitation at 384 nm. The two emission bands can be ascribed to π-π* or π-n transitions within the molecular orbital manifolds of imidazolyl and phenyl rings of TIPA moieties. The maximum emission band of compound 3 upon excitation at 332 nm is observed at 507 nm. This low energy emission at 507 nm is assigned to the metalto-ligand charge transfer (MLCT). Thermal Analysis and XRPD Results. The thermal stabilities of 1-4 were analyzed on crystalline samples from 25 to 750 °C min-1, under a nitrogen atmosphere with a flowing rate of 100 mL min-1 (Figure S4, Supporting Information). For 1, dehydration commenced at ∼25 °C and was completed by 88 °C. The 5.7% observed mass loss was consistent with the loss of the five lattice water molecules (5.8% calcd); then its weight is stable up to 180 °C, from which the framework

begins to collapse. For 2, the architecture is stable up to 287 °C; the weight loss from 30 to 287 °C (24.8%) can be attributed to the loss of all the lattice water molecules and Cl- counteranions (25.6% calcd), whereupon the framework begins to collapse. For 3, dehydration commenced at ∼100 °C and was completed by ∼140 °C. The 4.98% observed mass loss was consistent with the loss of the lattice water (5.04% calcd). The plateau region in the temperature range 140-390 °C indicates that the molecular architecture of 3 is stable up to 390 °C in the absence of guests. For 4, a weight loss of 5.7% was observed in the temperature range 22-120 °C, which corresponds to the loss of the lattice water molecules (6.3%). The framework of 4 remains intact up to 368 °C, from which the organic components are decomposed. Obviously, compounds 3 and 4 are more thermally stable than compounds 1 and 2, owing to the incorporation of 5-OH-H2bdc. The purity of compounds 1-4 is confirmed by powder XRD analyses, in which the main peaks of the experimental spectra of 1-4 are almost consistent with its simulated spectra (Supporting Information, Figures S5-S8). Powder Second Harmonic Generation Results. Considering that 1 and 4 both crystallize in the chiral space group (C2 and P212121, respectively), their nonlinear optical properties were studied. The strength of the second harmonic generation (SHG) efficiency of compounds 1 and 4 was tested by

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Figure 10. (a) View of a ladder-like chain built from Cd2þ ions and TIPA ligands. (b) Packing diagram of net B showing narrow distorted hexagonal channels along the c axis (the TIPA ligand is simplified to a 3-connecting node for clarity). (c) Perspective view of one chiral channel in net B. (d) Right-handed homochiral helix motif existing in each channel.

Figure 11. (a) Net A with gra topology. (b) Net B with AFUQOH topology. (c) Schematic representation of 2-fold interpenetration of 3D AFUQOH (42 3 65 3 83)(42 3 6) net þ 3D (63)(69 3 8) gra nets. (d) Portion of the topological net showing the detail of the interpenetration.

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Figure 12. Luminescence spectrum of 1-4 at room temperature. (6)

measuring the microcrystalline powder samples. Preliminary examinations indicate that 1 and 4 are SHG-active and the SHG efficiency is approximately 1.0 and 0.5 times that of urea, respectively, which indicates that 1 and 4 have potential application in optical material. Conclusion In summary, four new coordination polymers with unique topology have been synthesized based on an N-centered bulky tripodal ligand. The ligand TIPA has a more movable “kink” than benzene- and triazine-centered triangular ligands, which can more readily satisfy the coordination geometry of metal ions. The enormous variations of the four compounds suggest that the structures are greatly affected by metal ions and coligand. Further study about this rigid tripodal ligand is in progress. Acknowledgment. This work was supported by grants from the Natural Science Foundation of China (Nos. 20971065; 91022011; 20721002) and the National Basic Research Program of China (2010CB923303; 2007CB925103). Supporting Information Available: Crystallographic data in CIF format, selected bond lengths and angles, TGA, and PXRD in PDF format. This information is available free of charge via the Internet at http://pubs.acs.org.

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