Cadmium Coordination Polymers Constructed from in Situ Generated

DOI: 10.1021/cg900505v

Cadmium Coordination Polymers Constructed from in Situ Generated Amino-Tetrazole Ligand: Effect of the Conditions on the Structures and Topologies

2009, Vol. 9 5117–5127

Dongsheng Liu,†,‡,§ Gansheng Huang,‡ Changcang Huang,*,† Xihe Huang,† Jianzhong Chen,*,† and XiaoZeng You*,§ †

State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry & Chemical Engineering, Fuzhou University, Fuzhou, Fujian 350108, P. R. China, ‡College of Chemistry & Chemical Engineering, Jinggangshan University, Ji’an, Jiangxi 343009, P. R. China, and §State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China Received May 10, 2009; Revised Manuscript Received August 14, 2009

ABSTRACT: Five Cd(II) coordination polymers with the in situ generated ligand 5-amino-tetrazolate (atz-) were prepared from the hydrothermal reactions of the corresponding Cd(II) salts, and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis (TGA) and single crystal X-ray diffraction. The results of X-ray crystallographic analysis revealed that compounds {[Cd5(atz)9]Cl}n 3 2nH2O (1) and {[Cd5(atz)9](SO4)0.5}n 3 2nH2O (3) are isostructural with the perfect Kagome layers bridged by [Cd2(μ4-atz)3] clusters to generate a three-dimensional (3D) rare lon (topological type symbol) topological network with a vertex symbol of 66. Its hexagonal channels are filled by Cl- or SO42- anions and water molecules. Compound [Cd5(atz)8(μ2-Cl)2]n 3 3nH2O (2) contains three different kinds of bridging modes of the atz- anion and is an intricate 3D polymer. It possesses a 5,6-connected btv topology with a vertex symbol of (47 3 62 3 8)2(410 3 65), which is rarely observed but only predicted by O’Keeffe in theory in coordination polymers. Compound [Cd7.5(atz)9(μ3-SO4)2(μ3-OH)2]n 3 4.5H2O (4) is a 3D coordination polymer with a 3,4-connected (83)4(86)3 topology, which is built from trinuclear [Cd3(μ3-SO4)(μ3-OH)] clusters and bridging mononuclear Cd centers. Compound [Cd5(atz)4(μ5-SO4)2(μ3-OH)2]n (5) is constructed by a 3D inorganic cationic [Cd5(μ5SO4)2(μ3-OH)2]n4þ network, a 6-connected pcu topology, templated by atz- anions. The anion-exchange experiments were performed successfully for 1 and 3. Moreover, the thermal stabilities and photoluminescent properties of these compounds were investigated. This work markedly indicates that the subtle changes in the synthesis conditions profoundly influence the structures and topologies of the products.

Introduction Current interest in novel functional metal-organic frameworks (MOFs) is rapidly expanding because of their variety of intriguing architectures and topologies,1-9 and their potential applications in anion/guest exchange,10-18 catalysis,19-22 magnetism,23,24 luminescence,25-28 gas storage,29-34 and so on. In particular, when the hydrothermal technique was extensively employed in the field of crystal engineering because of the obvious advantages,2 some unexpected and interesting MOFs have been obtained at an exponential rate. However, the control of products in hydrothermal reactions is still an exciting challenge and becomes an everlasting topic for chemists,35,36 since the final products can be influenced by many reaction variables, such as ligand type, source of the metal ion, counterions, stoichiometry, temperature, the pH value of the solution, etc.37-54 Most of the investigated condition-dependent coordination polymers are multicarboxylate polymers since the diverse aspects of the structural chemistry of polycarboxylate ligands are markedly sensitive to synthesis conditions of each individual factor, such as the pH value, temperature, solvent, coordination modes, and protonation state of these ligands. Compared with bridging multicarboxylate compounds, five-membered aromatic heterocyclic tetrazolates have *To whom correspondence should be addressed. E-mail: changcanghuang@ hotmail.com (C.H.), [email protected] (J.C.), [email protected] (X.Z.Y.). r 2009 American Chemical Society

aroused great interest due to their potential applications in some fields.55-61 In particular, tetrazolates have four electrondonating N-atoms. As illustrated in Chart 1, the tetrazolate heterocycle has nine versatile coordination fashions ranging from κ1 to κ4 and has been widely applied in the construction of MOFs.37,62-81 5-Amino-tetrazolate, as a multifunctional small molecular tetrazolates ligand, is isosteric with the carboxylate group and has five binding sites (one amino group and four imino-nitrogen atoms). It has been used to construct some interesting MOFs.63,66,76,82 The coordination modes of the atz ligand could be influenced by many reaction conditions and ultimately influences the structures of the target products. For example, two Zn-tetrazolates, [Zn2(ATA)3(ATA)2/2] (W1) and [Zn(OH)(ATA)2] (W2) (HATAZ=5-amino-tetrazolate), have been synthesized by Wang’s group under the hydrothermal reactions of Zn(II) salts with HATA at different pH values.76 Compound W1 displays a unique four-connected two-dimensional (2D) bilayer “hc” 43.63 topological network. Recently, Zhengs’ research group synthesized two pairs of novel coordination polymers based on the reaction of 5-HATZ and cadmium(II) ions at room temperature and hydrothermal conditions.63 For the sake of enriching such interesting systems, we chose typical Cd(II) ions to assemble with the in situ generated atz ligand. Five coordination polymers, {[Cd5(atz)9]Cl}n 3 2nH2O (1), [Cd5(atz)8(μ2-Cl)2]n 3 3nH2O (2), {[Cd5(atz)9](SO4)0.5}n 3 2nH2O (3), [Cd7.5(atz)9(μ3-OH)2(μ3SO4)2]n 3 4.5nH2O (4), and [Cd5(atz)4(μ3-OH)2(μ5-SO4)2]n (5), Published on Web 10/28/2009

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Chart 1. Potential Coordination Modes of Tetrazolatea

a When R= NH2, four kinds of coordination modes for atz ligand were labeled Roman numerals in this paper.

were prepared and investigated to determine the influence of the synthesis conditions on the creation of such MOFs. Interestingly, by careful choice of cadmium sources, stoichiometry, and the reaction temperature, this series of polymers exhibit varied 3D topological networks. The anion-exchange experiments were performed for 1 and 3. Moreover, the photoluminescent and thermal stabilities properties of the compounds have been studied. Experimental Section Reagents and Physical Measurements. All reagents and solvents employed were commercially available and used without further purification. Infrared spectra were recorded in the range 4000400 cm-1 on a Perkin-Elmer FT-IR spectrum 2000 spectrometer using KBr pellets. Elemental analyses were determined with a Perkin-Elmer model 240C instrument. The qualitative analyses (XRF) were carried out using Philips PW2424 spectrometer. The fluorescent spectra were recorded on an Edinburgh Instrument FL/ FS-920 fluorescent spectrometer. Thermal analyses were performed on a Delta Series TGA7 instrument in N2 atmosphere with a heating rate of 10 °C/min from 30 to 800 °C. Powder X-ray diffraction (PXRD) data were obtained by using a Rigaku D/MAX 2500 V/PC diffractometer with Cu Ka (λ=1.54056 A˚) radiation. A step size of 0.05° and counting time of 1.2 s/step were applied in a 2θ range of 8.00-80.00 degree. Caution! Sodium azide is potentially explosive. Only a small amount of material should be prepared, and it should be handled with care. Synthesis of {[Cd5(atz)9]Cl}n 3 2nH2O (1). A mixture of CdCl2 (0.0917 g, 0.5 mmol), NaN3 (0.0651 g, 1.0 mmol), DCDA (DCDA= dicyandiamide, 0.0841 g, 1.0 mmol), and water (8 mL) was stirred for 30 min in air (Scheme 1), and then was sealed in a 23 mL Teflon autoclave and heated at 130 °C for 3 days. After the sample was cooled to room temperature at a rate of 10 °C/h, light yellow prismshaped crystals were obtained in ca. 38% yield based on Cd(II). Elemental analysis for C9H22Cd5N45O2Cl (1390.08): C, 7.72%; H, 1.58%; N, 45.31% (calcd: C, 7.78%; H, 1.60%; N, 45.34%). Synthesis of [Cd5(atz)8(μ2-Cl)2]n 3 3nH2O (2). The hydrothermal procedure of the preparation of compound 2 is similar to that for 1 except that the molar number of CdCl2 was changed to 1.0 mmol (0.1834 g). Colorless block crystals were obtained in ca. 46% yield based on Cd(II). Elemental analysis for C8H24Cd5N40O3Cl2 (1361.50): C, 7.02%; H, 1.75%; N, 41.12% (calcd C, 7.06%; H, 1.78%; N, 41.15%).

Liu et al. Scheme 1. In Situ Hydrothermal Syntheses of Compounds

Synthesis of {[Cd5(atz)9](SO4)0.5}n 3 2nH2O (3). A mixture of CdSO4 (0.1043 g, 0.5 mmol), NaN3 (0.0651 g, 1.0 mmol), DCDA (0.0841 g, 1.0 mmol), and water (8 mL) was stirred for 30 min in air (Scheme 1), and then was sealed in a 23 mL Teflon autoclave and heated at 130 °C for 3 days. After the sample was cooled to room temperature at a rate of 10 °C/h, colorless prism-shaped crystals were obtained in ca. 46% yield based on Cd(II). Elemental analysis for C9H22Cd5N45O4S0.5(1402.66): C, 7.69%; H, 1.56%; N, 44.88% (calcd: C, 7.71%; H, 1.58%; N, 44.94%). Synthesis of [Cd7.5(atz)9(μ3-OH)2(μ3-SO4)2]n 3 4.5nH2O(4). Similarly, compound 4 was prepared in the same manner as that for 3 except the molar number of CdSO4 was changed to 1.0 mmol (0.2085 g). Colorless block crystals were obtained in ca. 42% yield based on Cd. Elemental analysis for C9H29Cd7.5N45O14.5S2 (1906.83): C, 5.62%; H, 1.49%; N, 33.01% (calcd: C, 5.67%; H, 1.53%; N, 33.05%). Synthesis of [Cd5(atz)4(μ3-OH)2(μ5-SO4)2]n (5). Similarly, compound 5 was prepared in the same manner as that for 4 but the reaction temperature was changed to 170 °C. Colorless pillar crystals were obtained in ca. 48% yield based on Cd. Elemental analysis for C4H10Cd5N20O10S2 (1124.44): C, 4.14%; H, 1.03%; N, 24.31% (calcd: C, 4.27%; H, 0.90%; N, 24.91%). Compounds 1-5 are air-stable and insoluble in water and common organic solvents. X-ray Crystallographic Determination. Suitable single crystals of three compounds were mounted on glass fibers for X-ray measurement. Reflection data were collected at room temperature on a Rigaku Saturn 724 CCD diffractometer with graphite monochromatized Mo KR radiation (λ=0.71073 A˚). Crystal structures were solved by the direct method and different Fourier syntheses. All calculations were performed by using the SHELX-97 program.39 All non-hydrogen atoms, except the S and Cl atoms for 1 and 3, were refined by full-matrix least-squares techniques on F2 with anisotropic thermal parameters. Hydrogen atoms associated with disordered atoms were not included in the structural refinements. The hydrogen atoms for the amino groups were fixed at calculated positions and refined by using a riding mode. The hydroxyl hydrogen atoms in 4 and 5 were located from difference maps and refined with isotropic temperature factors. For 1, 3, and 4, one μ3-atz (Chart 1, mode II) ligand is located on a 2-fold axis, causing a 2fold disorder of this ligand and thus two possible orientations of the amino group and the associated C atom of the amino group. The disorder was treated by performing half-occupancies with C and N atoms of the tetrazole ring and the amino groups of the tetrazole ligands. Because of the disorder of the guest water molecules in the channels, the distribution of these peaks was chemically featureless. Only parts of the guest water molecules have been found in difference Fourier maps. Also, because of the high disorder of the counteranions Cl- for 1 and SO42- for 3 in the channels, only parts of the free SO42- and Cl- anions have been found in difference Fourier maps. The presence of SO42- anions in the open channels 3 will be supported by the IR spectrum. Furthermore, in order to validate the existence of Cl- anions in 1, the qualitative analysis (XRF) method was performed on the crystals of 1 to determine the chlorine. Crystal data and details of the data collection and the structure refinement are given in Table 1. Selected bond lengths and

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Table 1. Crystal Data and Structure Refinement for Compounds 1-5 compound

1

2

3

4

5

empirical formula formula weight temperature (K) crystal system space group a, A˚ b, A˚ c, A˚ R, (deg) β, (deg) γ, (deg) V, A˚3 Z Dcal, g/cm3 μ(Mo KR), mm-1 F(000) 2θ (deg) reflections collected independent reflections data completeness data/restraints/parameters GOF on F2 R1, wR2 [I > 2δ(I)] R1, wR2 [all data] max peak, hole/e A˚3

C9H22Cd5N45O2Cl 1390.08 298(2) hexagonal P63/mmc 13.014(4) 13.014(4) 13.457(6) 90.00 90.00 120.00 1973.8(13) 2 2.289 2.793 1296 3.03, 26.34 16736 800 0.998 800/0/64 1.191 0.0914, 0.2072 0.0944, 0.2082 2.852, -4.1920

C8H24Cd5N40O3Cl2 1361.50 298(2) orthorhombic Pbcm 9.852(2) 26.378(5) 13.548(3) 90.00 90.00 90.00 3520.9(12) 4 2.565 3.209 2592 3.01, 26.37 25117 3750 0.998 3750/1/273 1.339 0.0853, 0.2367 0.0869, 0.2379 1.366, -1.398

C9H22Cd5N45O4S0.5 1402.66 298(2) hexagonal P63/mmc 13.0628(18) 13.0628(18) 13.522(3) 90.00 90.00 120.00 1998.1(6) 2 2.282 2.728 1354 3.01, 24.69 12568 682 0.997 682/0/73 1.108 0.0725, 0.2684 0.0725, 0.2684 1.138, -0.787

C9H29Cd7.5N45O14.5S2 1906.89 298(2) cubic I43d 21.500(3) 21.500(3) 21.500(3) 90.00 90.00 90.00 9938(2) 8 2.549 3.332 7248 3.55, 27.49 37640 1910 0.997 1910/4/131 1.093 0.0251, 0.0622 0.0251, 0.0622 0.902, -0.524

C4H10Cd5N20O10S2 1124.44 298(2) monoclinic P21/c 8.8659(18) 9.5769(19) 13.337(3) 90.00 102.91(3) 90.00 1103.8(4) 2 3.383 5.025 1052 3.30, 27.49 8559 2512 0.996 2521/0/208 1.085 0.0264, 0.0682 0.0275, 0.0689 0.504, -0.655

Scheme 2

bond angles of the compounds are listed in Table S1 (Supporting Information). The CCDC reference numbers are 722714, 722715, 722716, and 722567 for 1-4, respectively. The disordered solvent molecules or counteranions may affect the R values for compounds 1-3, but the structures of the compounds are very clear.

Results and Discussion It is well-known that the product composition depends on critical factors such as the stoichiometry between metal salts and ligands, pH value, temperature, reaction time, and the counterions. By exploitation of the hydrothermal reactions, all compounds were synthesized in good yields by using the in situ generated atz ligand and suitable cadmium salts. Singlecrystal X-ray diffraction analysis indicates that the tetrazolate groups in the five compounds were all deprotonated to form tetrazolate monoanion. It is suggested that the Sharpless83,84 [2 þ 3] tetrazole synthesis method works effectively in the in situ formation of cadmium tetrazolate compounds from the reactions of dicyandiamide (DCDA) and sodium azide (Scheme 2). Here, the C-N cleavage occurring under these circumstances seems rather unusual and the same case has been observed in the latest work.85 In this study, it is noticeable that there are three crucial factors to govern the formation of final products: the first is the stoichiometry of the reactant; the second is the cadmium source, and the third is reaction temperature. In our experiments, the only difference among their synthesis conditions is the different Cd/DCDA ratios for 1 and 2, 3 and 4 under the same temperature of 130 °C. When the ratios of the reactants cadmium metal, dicyandiamide and NaN3 were kept at 1:2:2, positive frameworks were obtained for compounds 1 and 3, and when the ratios were changed to 1:1:1, neutral frameworks were

obtained for compounds 2 and 4. In other words, inorganic anions could not be incorporated into the compounds when the coordination sites of the cadmium were completed by the atz ligand, and the positive frameworks were charge-balanced by the inorganic anions solely. When the ratio of the reactants was kept 1:1:1 and only the reaction temperature was increased to 170 °C, the more dense compound 5 was obtained. Potential coordination modes of tetrazolate ligand are listed in Chart 1. (Four kinds of coordination modes for atz ligand in the five compounds are labeled in Roman Numerals in this paper.) Crystal Structure of Compounds 1 and 3. Crystal structure analyses show that compounds 1 and 3 are isostructural and crystallize in the high-symmetry space P63/mmc. Therefore, the structure of 3 is selected and described in detail to represent their frameworks. Compound 3 is a 3D coordination polymer with the perfect kagome layers and [Cd2(μ4atz)3]3þ cationic clusters. There are two unique Cd2þ centers (Cd1 and Cd2), as shown in Figure 1. Each cadmium(II) is coordinated with six N atoms from six different atz- anions with the Cd-N distances ranging from 2.320(20) to 2.391(16) A˚, which are comparable to those of the Cd(II)tetrazole compounds. The atzligands link the Cd2 atoms through the symmetry-related N5 atoms of the μ3-atz (mode II) ligands, affording along the ab plane a perfect kagome layer featuring vertex-sharing equilateral triangles with the Cd 3 3 3 Cd distance equal to 6.531(1) A˚ (Figure 1a,b). A pair of Cd1 atoms are connected to each other by three μ4-atzligands, forming a [Cd2(μ4-atz)3]3þ cluster with a Cd 3 3 3 Cd distance of 4.029(3) A˚. The neighboring perfect kagome layers are bridged by the [Cd2(μ4-atz)3]3þ clusters to generate a 3D porous cationic framework whose hexagonal channels are filled by the disordered SO42- anions and water molecules (Figure 1a). The SO42- groups act as counterions and occupy the void of the channels to stabilize the lattice. The presence of SO42- anions in the channels of 3 is supported by the IR spectrum, which exhibits the IR bands at 1120 and 617 cm-1 for the asymmetric S-O stretching (ν3) and SO42-bending modes (ν4), respectively.86 The amounts of the disordered guest water molecules can be proven by the

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Figure 1. (Top) View of the coordination environment of Cd2þ in compound 1 at 30%. Hydrogen atoms are omitted for clarity. Symmetry codes: (A) 1 - y, 1 þ x - y, z; (B) -x þ y, 1 - x, z; (C) x, 1 þ x - y, z; (D) -1 þ y, x, 1 - z; (E) 1 - x, -x þ y, 1 - z; (F) y, -x þ y, 1 - z; (G) -x þ y, y, z; (H) 1 - y, 1 - x, z; (I) -x, 1 - y, 1 - z; (J) 1 þ x - y, 1 - y, 1 - z; (K) -x, -x þ y, 1 - z; (L) 1 þ x - y, 1 þ x, 1 - z; (M) y, 1 þ x, 1 - z. (Bottom) (a) View of the packing diagram of compound 3; (b) view of the perfect kagome layer featuring vertex-sharing equilateral triangles along the ab plane (the disordered water molecules and SO42- anions were omitted for clarity). (c) The simplified sketch map. (d) View of the packing diagram of 3 after the cluster was simplified by the tetrahedron unit. (e)The tetrahedron unit was represented by a green ball. (f) Topological view showing the network with an enclosed 65 cage unit highlighted in red for 3 along the approximate [010] direction. Tetrahedron node is represented by a green ball.

thermogravimetric analysis (TGA). After removal of the inclusion of the compound 3, it is exceptionally microporous, with a total potential solvent-accessible volume of 21.2% calculated using PLATON.87

Topologically, three μ3-atz ligands ligated three equivalent Cd2 and one Cd1 atoms that lie at the vertexes of a tetrahedron (Figure 1c). With the fuse of a vertex-sharing Cd2 center, a perfect kagome layer is fabricated along the ab

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Figure 3. Topological view showing the 5,6-connected btv network for compound 2 along the approximate [001] direction with the Schl€ afli symbol of (47 3 62 3 8)2(410 3 6 3 5) network. Collided-nodes and Cd3 atoms are represented by cyan balls and dark yellow balls, respectively.

Figure 2. (Top) View of the coordination environment of Cd2þ in compound 2 at 30%. Hydrogen atoms and disordered water molecules are omitted for clarity. Symmetry codes: (i) -1 þ x, y, z; (ii) 2 - x, 1 - y, 2 - z; (iii) x, y, 1.5 - z; (iv) x, y, 2.5 - z; (v) x, 0.5 - y, 2 - z. (Bottom) (a) View of the single Cd3 chains linked by the atzligand. (b) Packing of the overall complicated 3D network of 2; the Cd3 chains are highlighted in yellow. (c) View of the triplex chain built from a Cd2 wavy chain and double Cd1 chains.

plane. Simultaneously, two Cd1 vertexes of two tetrahedral are connected together by three pairs of μ4-atz ligands N atoms (Figure 1c), propagating a 3D complicated structure with tetrahedron units (Figure 1d). Therefore, the tetrahedron can be regarded as a 4-connected node (Figure 1e). On the basis of this simplification, the structure of 3 can be described as a 4-connected lon topological network (Figure 1f, Figure S1), with the short (Schl€ afli) vertex

symbol88 of 66 and the long topological (O’Keeffe) vertex symbol89 of 62 3 62 3 62 3 62 3 62 3 62; it is not a cristobalite structure (diamond net with 64 cage unit) but a tridymite structure (lonsdaleite net with 65 cage unit).90 Within the lon topological network, the nodes are linked into boat and chairlike 6-rings, which form an enclosed 65 cage (see the highlighted unit in Figure 1f, two chairlike rings and three boat-like rings). This is an interesting net, which was first announced by O’Keeffe et al. in 199291 and is demonstrated here in real crystal structures for the first time. Crystal Structure of Compound 2. The asymmetric unit of compound 2 contains two and a half crystallographically independent Cd atoms, two and four half atz ligands, two half chlorine atoms, and three disordered free water molecules. In the asymmetric unit, one Cd atom, four atz ligands, and two chlorine atoms are all sited on the special positions (Figure 2). The Cd1 and Cd2 atoms locate at the general position and adopt distorted octahedral coordination geometry, which are formed by five nitrogen atoms from five atz ligands [Cd1-N 2.294(11) ∼ 2.436(10) A˚, Cd2-N 2.329(11) ∼ 2.462(10) A˚] and one chlorine atom [Cd1-Cl1 2.564(2) A˚, Cd2-Cl2 2.578(3) A˚], respectively. Cd3 atom (in the crystallographic center of inversion) adopts a quasi perfect octahedral coordination geometry, formed by four N atoms located at an equatorial plane and two nitrogen atoms at the axial positions [Cd3-N 2.314(10) ∼ 2.395(11) A˚]. It is noticeable that the bond angles of N-Cd-N around the Cd3 center are all 180°. The Cd-N bond distances are well in the range of the value of those reported Cd(II)-tetrazole compounds. The atz ligands link the Cd3 atoms through the symmetryrelated N16 atoms of μ4-atz ligands (mode IV), forming a nearly linear chain along the c-axis with a Cd 3 3 3 Cd distance of 6.774(2) A˚ (Figure 2a). A pair of Cd2 atoms are connected to each other by the symmetry-related N6 atom and N17 atom of two μ4-atz ligands and a μ2-Cl2 atom (Cd 3 3 3 Cd distance, 3.802(2) A˚), and then the adjacent Cd2 atom pairs are further linked together by the μ4-atz ligands to propagate a wavy Cd2 chain with the Cd 3 3 3 Cd separation of

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Figure 4. (Top) View of the coordination environment of Cd2þ and the ligand in compound 2 at 30%. Hydrogen atoms and disordered water molecules are omitted for clarity. Symmetry codes: (A) 0.5 þ y, 0.5 - z, -x; (B) -z, 0.5 þ x, 0.5 - y; (C) -0.25 þ y, -0.25 þ x, 0.25 - z; (D) 0.25 - x, 0.75 - z, -0.25 þ y; (E) 0.25 þ y, 0.75 - x, -0.25 þ z; (F) 0.5 þ y, 0.5 þ z, -0.5 þ x; (G) 0.25 - z, 0.75 - y; -0.25 þ x; (H) 0.25 þ z, 0.75 - y, 0.25 - x; (I) 0.5 - x, y, -z. (Bottom) (a) View of the coordination environment of Cd1, each Cd1 linked to four [Cd3(μ3-OH)(μ3-SO4)] clusters. (The atz ligands were partly drawn for clarity; Cd1 and Cd2 atoms were colored in green and cyan, respectively.). (b) View of the 3D (3,4)connected topological network with the Schl€ afli symbol of (83)4(86)3 network for 4. Cd1 node and the center of the trinuclear Cd2 cluster are represented by the green ball and cyan ball, respectively.

6.589(2) A˚ (see Figure S2, Supporting Information). Cd1 atoms are linked by μ2-atz and μ4-atz ligands alternately to form a Cd1 chain along the c-axis . The neighboring Cd1 chains are bridged by μ2-Cl1 atoms and μ4-atz ligands N atoms, generating a Cd1 double chain (see Figure S3, Supporting Information). The Cd1 double chains and the wavy Cd2 chains are connected through the μ4-atzligands to form triplex chains (Figure 2c). Each triplex chain further links four Cd3 linear chains through two different μ3-atzligands resulting in the overall complicated 3D network (Figure 2b). The disordered free water molecules are resided in the cavity of 2. Interestingly, 2 contains four kinds of bridging atzligands (as shown in Chart 1: μ2-atz mode (I), two different μ3-atz modes (II and III) and μ4-atz mode (IV)), representing the first example of a metal-atz system

containing four different modes of atz linker, which has never been reported to date. For better insight into its intricate polymeric framework, a topological analysis of compound 2 was performed. The Cd1 and Cd2 atoms are bridged by three pairs of N atoms from three atz ligands in the triplex chains of Cd1 and Cd2 chains; we can collide Cd1 and Cd2 into one node, which is connected with three other same nodes and two Cd3 atoms, so the collided-node can be viewed as a 5-connected node. The Cd3 atoms in the Cd3 chain link four collided-nodes and two symmetry-relatived Cd3 atoms through the atz linkers; the Cd3 atom can be considered as a 6-connected node. On the basis of this simplification, the quite complicated 3D network of 2 can be represented as an unprecedented (5,6)connected btv topology (Figure 3). In the (5,6)-connected

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Figure 5. (a) View of the packing diagram of the inorganic network of 5 along the a-axis; (b) the inorganic chain molecular structure and pentanuclear cadmium cluster atomic labeling of 5, showing a 6-connected metal center. (Cd1, Cd2, and Cd3 atoms were colored in green, cyan, and dark yellow, respectively.) (c) The 6-connected pcu topological network for compound 5.

framework, the ratio of 5-connected nodes to 6-connected Cd3 atoms is 1:2. Therefore, the short (Schl€ afli) vertex symbol88 is (47 3 62 3 8)2(410 3 65) and the long topological (O’Keeffe) vertex symbol89 of (4.4.4.4.4.4.4.6.6. *)(4.4.4.4.4.4.4.4.4.4.6.6.*.*.*). It is a rarely observed topological net but only predicted by O’Keeffe in theory in coordination polymers. Although a few (5, 6)-connected networks with two types of vertexes have been identified and categorized by O’Keeffe et al., such as btv, cai, fab, fsx, mog-e, thp-a, and yav, only one binodal with (5,6)-connected network MOFs has been reported to date,92 while structures containing btv topological network MOFs have never been reported. Crystal Structure of Compound 4. Single crystal X-ray analyses showed that compound 4 is a 3D coordination polymer built from trinuclear [Cd3(μ3-OH)(μ3-SO4)] clusters and mononuclear Cd(II) centers (Figure 4). As illustrated in Figure 4, Cd2 adopts a pseudo-octahedral coordination geometry that is formed by four nitrogen atoms from four atz ligands [Cd1-N 2.314(4)-2.387(4) A˚], one oxygen atom

from sulfate anion [Cd1-O2 2.272(5) A˚], and one OH-group [Cd1-O3 2.281(2) A˚]. The hydroxyl acts as a μ3-bridge linking three equivalent Cd2 atoms, generating an equilateral triangle with Cd 3 3 3 Cd distances equal to 3.754(1) A˚. The Cd-O2-Cd angle is 110.72(13)° and the OH- group is displaced out of the Cd3 plane, which results in the formation of a non-coplanar [Cd3(μ3-OH)] trimetric unit. The sulfate anion resides on a 3-fold axis, acting as an architectural truss to support the trimetric unit to form a stable [Cd3(μ3-OH)(μ3-SO4)] cluster. The cluster is surrounded by three identical μ4-atz ligands, each of which sets up an N-N bridge between two Cd2 ions via two neighboring nitrogen atoms (N2 and N3) from one tetrazole ring. The ligand is also bonded to Cd1 and Cd2 ion via N4 and N5 atom, respectively. Simultaneously, the neighboring Cd1 and Cd2 are linked through two neighboring nitrogen atoms of the other μ3-atz(mode II) and μ4-atz ligands, and hence constitutes a paddle-wheel. The shortest distance of Cd1 3 3 3 Cd2 distance is 4.062(1) A˚. Each Cd1 ion locates at the special position and adopts the octahedral coordination

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geometry with six N atoms from six atzligands. The equatorial positions are occupied by four different but symmetry-related N4 atoms [Cd1-N4 2.348(4) A˚] from four atzligands, and the axial positions are occupied by two symmetry-related N8 atoms [Cd1-N8 2.322(6) A˚] of the other two atzligands. Thus, each Cd1 linked to four [Cd3(μ3-OH)(μ3-SO4)] clusters (Figure 4a), and each cluster is connected to three Cd1 centers, giving rise to a complicated 3D network. Taking the clusters as 3-connected nodes and the Cd1 centers as 4-connected nodes, the 3D network represents a (3,4)-connected topological 3D network. In the (3,4)-connected framework (Figure 4b), the ratio of 3-connected [Cd3(μ3-OH)(μ3-SO4)] clusters to 4-connected Cd2 atoms is inversely proportional to their connectivity (i.e., 4:3). Therefore, the short (Schl€ afli) vertex symbol88 is 3 6 (8 )4(8 )3. Among the known (3,4)-connected nets in coordination polymers,93 the typical examples are boracite, Pt3O4,94 cubic-C3N4,95,96 and the twisted boracite. Recently, another two new (3,4)-connected nets with (4 122)2(42 124) afli symbol were reported by and (62 10)2(64 102) Schl€ Moorthy’s97 and Hong’s group,98 respectively. Therefore, the structure of 4 provides another valuable prototype of 3,4connected nets which may be important for the design of MOFs since three- and four-connected centers are readily available in coordination chemistry. Crystal Structure of Compound 5. Compound 5 was prepared by in situ generated atzligand with cadmium salt, which is different from the procedure by Yao.63 It is suggested that the Sharpless [2 þ 3] tetrazole synthesis method works effectively in the in situ formation of cadmium tetrazolate compounds from the reactions of dicyandiamide and sodium azide. The architecture of compound 5 is confirmed by X-ray analysis to be equal to that formerly reported,63 which is a 3D coordination polymer constructed by inorganic cationic [Cd5(μ5-SO4)2(μ3-OH)2]n4þ network (Figure 5a) templated by atz- anions. From a topological view, each pentanuclear cadmium (which was labeled in the inorganic chain, shown in Figure 5b) cluster is connected to six adjacent clusters through six μ5-SO4 units resulting in a 3D purely inorganic cationic [Cd5(μ5-SO4)2(μ3-OH)2]n4þ subnet, so the pentanuclear cadmium cluster can be regarded as a sixconnected node (Figure 5a). On the basis of this simplification, the subnet can be depicted as a uniform 6-connected pcu topological network with the long topological (O’Keeffe) vertex symbol89 of (4.4.4.4.4.4.4.4.4.4.4.4.*.*.*) (Figure 5c). A similar example of metal-organic polymers constructed by bridging sulfate group has been reported, such as [Cd2(pzta)(OH)(SO4)]n [pzta = pyrazinyl tetrazolate]99 and [Cd(bpethy)(SO4)] [bpethy=bis(4-pyridyl)ethyne].100 Powder X-ray diffraction. PXRD experiments were carried out on 1-5 in order to establish their crystalline phase purity. As shown in the PXRD patterns (see Figure S4, Supporting Information), the major peak positions of the PXRD patterns of the bulk solids of 1-5 matched well with that of the simulated patterns obtained from respective single-crystal data, indicating the presence of mainly one crystalline phase in the corresponding coordination polymers 1-5. IR Spectroscopy. The IR spectra of compounds 1-5 (see Figure S5, Supporting Information) show a medium strong intensity band in the range of 3300-3490 cm-1, which can be assigned to the ν(NH) characteristic stretching frequency of the amino groups (for 1, 3441 and 3367 cm-1; 2, 3411 and 3335 cm-1; 3, 3444 and 3383 cm-1; 4, 3465 and 3370 cm-1; 5,

Liu et al.

3421 and 3342 cm-1). All of them show the blue shift from the bands observed at 3485 and 3382 cm-1 for the free ligand. In 2, a strong band was observed at 3626 cm-1, which can be attributed to the hydrogen bonding interactions of amino groups and the water molecules. Besides these, strong bands around 1630 cm-1 and 1440 cm-1 are also observed, and all these bands associate with ν(CdN)/ring stretching vibrations plus δ(N-H)NH, NH2 of the atzligand.101 In 3, the SO42- is a free ligand, so it is the perfect Td symmetry. The IR bands at 1120 and 617 cm-1 may be attributable to the asymmetric S-O stretching (ν3) and SO42- bending modes (ν4), respectively.85 In 4, the SO42- adopts a μ3 coordination mode and leads to a low site symmetry C3v; the bands show medium to strong intensity at 985 and 448 cm-1, and these may be attributable to the symmetric S-O stretching mode (ν1) and the symmetric SO42- bending mode (ν2). The band at 627 cm-1 can be attributable to the ν4 mode. The strong band around 1160 cm-1 splits into two bands 1162 and 1075 cm-1 and may be assigned to the ν3 mode. In 5, the SO42- adopts a μ5 coordination mode and leads to a low site symmetry C1, and the bands at 987 and 457 cm-1 associate with the symmetric S-O stretching and the symmetric SO42bending vibrations. The bands at 1076, 1123, and 613 cm-1 correspond to the ν3 and ν4 mode, respectively.102-104 After ion exchanging with nitrate anions for the compound 3, the presence of nitrate anions of the ion exchanging product of 3 is supported by the IR spectrum (see Figure S6, Supporting Information), which exhibits an intense peak at around 1385 cm-1 for the N-O stretching vibration. The characteristic bands of the SO42- almost disappeared, which indicated the SO42- locating in the channels of 3 could be exchanged by nitrate anions. Thermal Stability Analyses. The thermal stability of the compounds in N2 was examined by the TG techniques in the temperature range of 30-800 °C. The TG curves (see Figure S7, Supporting Information) for compounds 1-5 are at a heating rate of 10 °C/min under N2 atm. As shown in Figure S4, a weight loss of 2.8% (calcd, 2.6%) for 1 and 2.7% (calcd, 2.5%) for 3 occurred in the range of 30-200 °C, corresponding to the loss of two free water molecules. A weight loss of 6.2% between 30 and 170 °C accorded with the corresponding calculated values of 6.0% for 4, due to the loss of 4.5 free water molecules and two hydroxyl groups. Weight loss of 4.1% was observed in the wide temperature range of 30-320 °C for 2, which was attributed to the loss of the three free water molecules. Then there was no weight loss for 1, 3, and 4 until about 300 °C. And after that, the continuous weight loss above 300 °C for 1, 3, 4 and 320 °C for 2 correspond to the decomposition of atzligands and anions. The relatively wide temperature range for loss of the free water molecules demonstrates their strong hydrogen bonding interactions for 2. The TGA study of 5 shows no weight loss from 30 to 390 °C, indicating that the framework is thermally stable. Above 390 °C, the framework of 5 begins to collapse due to the decomposition of atz ligands and anions. Finally, the probable residues are metal-oxide from the calculating value of the weight loss. Furthermore, the high density value (3.383 g/cm3) also supports the high thermal stability of the 3D framework. Ion exchange Properties. The ion exchange properties of the compound 1 was studied as a result of single crystal X-ray data showing disordered chloride anions, indicating that they are loosely residing in the channels. In a typical experiment, the addition of a slight excess of NaNO3 (aq) to a

Article

Figure 6. (a) Solid-state fluorescence spectra of 1-5 at room temperature. (Excitation wavelength for 1, 360 nm; 2, 395 nm; 3, 355 nm; 4, 365 nm; 5, 378 nm.) (b, c) Solid-state fluorescence spectra of the ion-exchanged compounds 1 and 3 at room temperature. (Lines 1, 3 and 1a, 3a represent the pre-ion-exchanged and post-ionexchanged compounds’ fluorescence spectra, respectively. Excitation wavelength for 1 and 1a, 360 nm; 3 and 3a, 355 nm.)

suspension of crystalline {[Cd5(atz)9]Cl}n 3 2nH2O in water at room temperature showed that the NO3- anions begin to exchange with Cl- anions after 3 h, as evidenced by the infrared data (see Supporting Information, Figure S6). Here, the characteristic bands at 1791, 1385, 835, and 726 cm-1 for free NO3- begin to appear.105 On the basis of IR data and elemental microanalysis, it is estimated that after 12 h nearly 85% exchange has occurred. Inspection of the crystals under an optical microscope during the exchange process revealed that the crystals could still keep the original shape after ion


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exchange. Furthermore, they still give a sharp X-ray powder diffraction pattern coinciding with that of the original unexchanged material. Analogous ion exchange of NO3- was achieved for compound 3; the intense NO3- bands at 1790, 1382, 833, and 724 cm-1 for free NO3- begin to appear as those intense bands at 1120 and 617 cm-1 due to free SO42beginning to disappear. Photoluminescent Properties. The solid-state luminescent properties of the compounds 1-5 were investigated at room temperature. As illustrated in Figure 5, it can be observed that the intense broad emission bands were at about 459 and 479 nm (λex =360 nm) for 1, 452 and 473 nm (λex =395 nm) for 2, 450 and 471 nm (λex =355 nm) for 3, 452 and 475 nm (λex =365 nm) for 4, and 419 nm (λex =378 nm) for 5, respectively. The luminescent decay profile of 1-5 can be fitted with a double-exponential decay function with τ1 =5.03 ns (39.25%), τ2=1.12 ns (60.75%) for 1, τ1=5.04 ns (53.94%), τ2=1.41 ns (46.06%) for 2, τ1=9.69 ns (83.14%), τ2=1.46 ns (16.86%) for 3, τ1=9.84 ns (88.34%), τ2 = 1.76 ns (11.66%) for 4 and τ1=4.20 ns (48.82%), τ2 = 1.27 ns (51.18%) for 5, respectively. From the values of the luminescent decay of five compounds, these peaks may be assigned to blue-light fluorescence. As previously reported, the free Hatz ligand presents a weak photoluminescence emission at 325 nm at ambient temperature.76 Therefore, the fluorescent emission of 1-5 is tentatively attributed to metal-to-ligand charge transition (MLCT) and/or ligand-to-metal charge transition (LMCT). Further, compared with the emission of compounds, the emission maxima for 2 (452, 473 nm) are blueshifted compared to that of 1 (459, 479 nm). In addition, the emission maxima blue-shift occurred in 4 and 5 compared to that of 3. The variations of photoluminescence of these compounds may be rationalized by the differences in their local coordination environments. Furthermore, the crystal densities of 1-5 are also markedly different (2.289, 2.565, 2.282, 2.549, 3.383 g/cm3 for 1-5, respectively), which imply the interligand contacts and/or the network rigidities in the order of 5 > 4 > 3, 2 > 1. Therefore, the emission blue shifts from 3, 4 to 5 and 1 to 2 may be rationalized by a decrease in HOMO-LUMO gaps by interligand contacts. Along with the high thermal stability and insoluble in common inorganic or organic solvents properties, the emission in the blue region makes the compounds potential blue-light emitting materials. In order to study the fluorescent property changes of the post-ion-exchanged products, the solid-state fluorescent properties of the post-ion-exchanged products were also investigated at room temperature. As depicted in Figure 6b-c, it can be observed that the broad emission bands all changed after ion-exchange with the nitrate anions. For 1, the emission band of the post-ion-exchanged products is widened compared to that of the pre-ion-exchanged products. For 3, the emission band is blue-shifted after ion-exchange, which indicated that the changes of the anions in the channels could affect the photoluminescence of the compounds. This property of the compounds may be a potential material for the sensing of some anions. The photoluminescent changes of the compounds also suggested the ion-exchanged experiment was successful. Conclusion In summary, we have successfully constructed five coordination polymers with the in situ generated ligand 5-amino-tetrazolate.

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When the node was applied to the multinuclear metal cluster for the five compounds, some novel topological networks were presented by us, but compound 5 is a common 6-connected pcu topological net. Compound 4 is a 4-connected with vertex symbol of (83)4 (86)3 topological net. A rarely observed lon topological network, which was only found in cristobalite structure, was found in compounds 1 and 3. Noticeably, compound 2 possesses a btv topology, which was only predicted by O’Keeffe in theory in coordination polymers. Different spatial structural features in compounds 1-5 result from subtle reaction condition changes. The formations of these structures provide a good example of subtle changes in the synthesis conditions that can form different structures with different topological nets. From the investigation of the fluorescent properties, compounds 1-5 may be potential blue fluorescence material. 1 and 3 also may be useful for the sensing of some anions due to photoluminescent changes after ion-exchange. Further investigations on such an interesting system are still in progress. Acknowledgment. This work is supported by the National Natural Science Foundation of China (50772023, 20721002 and 20531040), the Natural Science Foundation of Fujian Province (2007J0148), the Open Fund of National Photonic Crystal Materials Engineering and Technology Research Centre (07h3561xaa), the Innovation Fund for Yong Scientist of Fujian Province (2008F3059), and the Project of Doctoral Visiting Study of the Nanjing University. Supporting Information Available: X-ray crystallographic data in CIF format, XRD, IR spectra for compounds 1-5. This information is available free of charge via the Internet at http://pubs.acs.org/.

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