Novel Structural Diversity of Triazolate-Based Coordination Polymers

DOI: 10.1021/cg9012894

Novel Structural Diversity of Triazolate-Based Coordination Polymers Generated Solvothermally with Anions

2010, Vol. 10 2136–2145

Dong-Ping Li, Xin-Hui Zhou, Xiao-Qiang Liang, Cheng-Hui Li, Chao Chen, Jian Liu, and Xiao-Zeng You* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China Received October 16, 2009; Revised Manuscript Received February 23, 2010

ABSTRACT: Five novel d10 metal coordination polymers, {[Cd3(trz)5(Br)] 3 H2O}n (1), [Cd2(trz)3(NO3)(H2O)]n (2), [Cd(Htrz)(SO4)(H2O)]n (3), [Zn2(trz)2(NO3)2(H2O)]n (4), and [Zn2(Htrz)2(SO4)2]n (5) (Htrz = 1,2,4-triazole), have been synthesized through in situ solvothermal decarboxylation reactions using 1H-1,2,4-triazole-3-carboxylic acid and cadmium/zinc salts as starting materials. The X-ray crystallographic analyses demonstrate the two-dimensional structure of complex 4 and threedimensional ones of complexes 1-3 and 5. The Htrz ligands adopt the μ1,2,4-bridging mode in complexes 1 and 2 and μ2,4bridging mode in complex 3. Complex 5 represents the first example of metal/triazolate complexes, in which the neutral Htrz ligands act as both μ1,2 and μ2,4 bridges. The diversity of architectures of these complexes reveals that anions play important roles in the self-assembly process under in situ solvothermal conditions. The possible mechanism of decarboxylation is discussed, and the thermal stability and fluorescent properties of these complexes are reported briefly.

Introduction Coordination polymers with diverse architectures and useful properties, such as catalysis, optics, ferroelectrics, magnetism, and gas sorption, have been rapidly developed in recent years.1-5 One of the most effective approaches to synthesize coordination polymers is through a solvothermal assembly process.6-8 The high temperature and pressure of solvothermal conditions may facilitate the formation of some unexpected and important in situ ligands and provide one-pot syntheses for some intriguing organic compounds that otherwise require multistep processes. Some functional coordination polymers that are inaccessible or not easily obtained by routine synthetic methods can also be obtained by in situ solvothermal reactions.9-12 Many factors, including the ligands, charge-balance requirement, orientation of ions, control of pH, temperature, and pressure, etc., have been found to influence the structures and properties of coordination polymers under solvothermal conditions.13-15 In particular, the role of anions proves important in the self-assembly processes, which is an interesting and active theme in recent studies.16 However, studies into the influence of anions on the self-assembly processes during in situ solvothermal reactions are less reported.17 During the past decade, coordination polymers based on 1,2,4-triazole and its derivatives have drawn considerable attention in the development of novel functional materials. In the literature, metal/triazolate complexes show diverse topologies and interesting properties due to the flexible bridging modes of five-membered N-heterocyclic ligands.18-26 In contrast to the large and increasing number of works on metal/triazolate complexes, the coordination modes of 1,2,4-triazole in these coordination polymers are quite unitary. Most coordination polymers based on this ligand prefer to adopt the μ1,2,4-bridging mode (Scheme 1), while the μ2,4- and μ1,2-bridging modes are rarely reported.20d,21b *To whom correspondence should be addressed. Fax: þ86-25-83314502. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 04/20/2010

Scheme 1. Three Bridging Modes of Triazole Ligand in Complexes

Herein, five novel d10 (Zn, Cd) metal/triazolate coordination polymers: {[Cd3(trz)5(Br)] 3 H2O}n (1), [Cd2(trz)3(NO3)(H2O)]n (2), [Cd(Htrz)(SO4)(H2O)]n (3), [Zn2(trz)2(NO3)2(H2O)]n (4), and [Zn2(Htrz)2(SO4)2]n (5), were obtained from solvothermal decarboxylation reactions using 1H-1,2,4triazole-3-carboxylic acid (Htrza) and cadmium/zinc salts as starting materials. The diversified architectures and topologies of these complexes reveal that anions play important roles in the self-assembly process under in situ solvothermal conditions. The mechanism of decarboxylation was discussed in this paper. The thermal stability and luminescent properties of these complexes are investigated in the solid state. Experimental Section Materials and General Procedures. All reagents were obtained from commercial suppliers and used as received. The IR spectra were recorded on a vector 22 Bruker spectrophotometer. Elemental analyses of C, H, and N were performed on a Perkin-Elmer 240C analyzer. Fluorescent emission spectra for solid samples were measured on r 2010 American Chemical Society

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Table 1. Crystal Data and Structure Refinements for Complexes 1-5 complexes

1

2

3

4

5

empirical formula formula weight crystal system space group a (A˚) b (A˚) c (A˚) R (°) β (°) γ (°) V (A˚3) Z θ range (°) collected reflections unique R(int) T (K) R1a, wR2b [I>2σ(I)] data/restraints/param GOF

C10H12N15OBrCd3 775.46 orthorhombic Cmc21 10.7383(11) 26.276(3) 7.8516(8) 90.00 90.00 90.00 2215.4(4) 4 2.05 to 24.97 5465 1997 (0.0386) 291(2) 0.0733, 0.1733 1997/107/151 1.099

C6H8N10O4Cd2 509.02 orthorhombic Pcab 8.0222(8) 18.1099(18) 21.954(2) 90.00 90.00 90.00 3189.5(6) 8 2.17 to 25.99 15843 3119 (0.0780) 291(2) 0.0779, 0.1749 3119/48/226 1.284

C2H5N3O5SCd 295.55 triclinic P1 5.5144(10) 8.0021(15) 9.2467(17) 78.487(3) 72.869(3) 87.200(3) 382.06(12) 2 2.35 to 24.99 1916 1327 (0.0362) 291(2) 0.0237, 0.0592 1327/0/112 1.026

C4H6N8O7Zn2 408.91 triclinic P1 8.0316(12) 9.0376(13) 9.7530(15) 77.170(3) 68.834(2) 66.673(2) 603.62(16) 2 2.25 to 25.00 3032 2093 (0.0481) 291(2) 0.0292, 0.0792 2093/3/190 1.029

C4H6N6O8S2Zn2 461.00 triclinic P1 5.1572(8) 7.8010(11) 15.051(2) 88.404(3) 87.843(2) 85.791(3) 603.27(15) 2 2.62 to 24.99 3001 2077 (0.1333) 291(2) 0.0482, 0.1338 2077/0/202 1.061

a

R1 = Σ(|Fo| - |Fc|)/Σ|Fo|. b wR2 = [Σw(Fo2 - Fc2)2/Σw(Fo2)2]0.5.

powder samples at room temperature using an AMINCO Bowman Series2 luminescence spectrometer. The X-ray powder diffraction (XRPD) analysis was performed with a Philips X-pert X-ray diffractometer at a scanning rate of 4°/min in the 2θ range from 5 to 50°, with graphite monochromatized Cu KR radiation (λ = 1.5418 A˚). Thermal stability studies were carried out on a Perkin-Elmer Pyris 1 TGA analyzer by heating the sample from room temperature to 800 °C at a heating rate of 10 °C/min in N2. Synthesis of {[Cd3(trz)5(Br)] 3 H2O}n (1). A mixture of Htrza (0.0565 g, 0.5 mmol), CdBr2 3 4H2O (0.1135 g, 0.33 mmol), NaOH (0.040 g, 0.5 mmol), and H2O (10 mL) was heated in steel autoclaves with Teflon liners at 180 °C for 4 days. After the mixture was cooled to room temperature, the colorless crystals were collected, washed with H2O, and dried in air. Yield: 40%. Anal. Calcd for Cd3C10H12N15BrO: C, 15.49%; H, 1.56%; N, 27.09%. Found: C, 15.56%; H, 1.62%; N, 27.06%. IR (KBr pellets, cm-1): 3409 (s), 1619 (m), 1498 (m), 1396(m), 1385 (m), 1276 (m), 1153 (s), 1108 (m), 1068 (m), 988 (m), 868 (m), 669 (m). Synthesis of [Cd2(trz)3(NO3)(H2O)]n (2). A mixture of Htrza (0.0565 g, 0.5 mmol), Cd(NO3)2 3 4H2O (0.1542 g, 0.5 mmol), isobutanol (0.8 mL), and methanol (0.8 mL) was sealed in a glass tube and heated at 85 °C for 7 days. After the mixture was cooled to room temperature, the colorless crystals were collected, washed with methanol, and dried in air. Yield: 40%. Anal. Calcd for Cd2C6H8N10O4: C, 14.16%; H, 1.58%; N, 27.52%. Found: C, 14.20%; H, 1.67%; N, 27.24%. IR (KBr pellets, cm-1): 3416 (s), 3127 (w), 1745 (m), 1638 (m), 1558 (w), 1502 (s), 1395 (s), 1331(m), 1276 (s), 1198 (w), 1155 (s), 1065 (m), 996 (m), 877 (m), 819 (w), 666 (s). Synthesis of [Cd(Htrz)(SO4)(H2O)]n (3). Complex 3 was prepared similarly to compound 2 but using CdSO4 3 8/3H2O (0.1283 g, 0.5 mmol) instead of Cd(NO3)2 3 4H2O. After the mixture was cooled to room temperature, the colorless crystals were collected, washed with methanol, and dried in air. Yield: 20%. Anal. Calcd for CdC2H5N3SO5: C, 8.13%; H, 1.71%; N, 14.22%. Found: C, 8.32%; H, 1.56%; N, 14.35%. IR (KBr pellets, cm-1): 3384 (s), 3160 (m), 1655 (m), 1513(m), 1388 (m), 1294 (m), 1193 (m), 1150 (s), 1118 (s), 1055 (s), 991 (m), 969 (m), 893 (m), 671 (m), 631 (w), 617 (w), 606 (w). Synthesis of [Zn2(trz)2(NO3)2(H2O)]n (4). Complex 4 was prepared similarly to compound 2 but using Zn(NO3)2 3 6H2O (0.1485 g, 0.5 mmol) instead of Cd(NO3)2 3 4H2O. After the mixture was cooled to room temperature, the colorless crystals were collected, washed with methanol, and dried in air. Yield: 35%. Anal. Calcd for Zn2C4H6N8O7: C, 11.75%; H, 1.48%; N, 27.40%. Found: C, 11.82%; H, 1.62%; N, 27.54%. IR (KBr pellets, cm-1): 3422 (m), 3127 (m), 3106(m), 1532 (s), 1503 (m), 1382 (m), 1303 (s), 1284 (s), 1180 (m), 1100 (s), 1024 (s), 825 (m), 662 (s). Synthesis of [Zn2(Htrz)2(SO4)2]n (5). Complex 5 was prepared similarly to compound 2 using ZnSO4 3 7H2O (0.1437 g, 0.5 mmol) instead of Cd(NO3)2 3 4H2O. After the mixture was cooled to room temperature, the colorless crystals were collected, washed with

methanol, and dried in vacuum. Yield: 20%. Anal. Calcd for Zn2C4N6H6S2O8: C, 10.42%; H, 1.31%; N, 18.23%. Found: C, 10.39%; H, 1.35%; N, 18.22%. IR (KBr pellets, cm-1): 3416 (s), 3162 (m), 3139 (m), 1638 (m), 1552 (m), 1313 (m), 1297 (m), 1124 (s), 1077 (m), 1054(s), 996 (m), 974(m), 672 (m), 644 (m) 619 (m). X-ray Crystallography. Single-crystal X-ray diffraction measurements for complexes 1-5 were carried out using a Bruker SMART APEX CCD diffractometer with graphite monochromatized Mo KR radiation (λ = 0.71073 A˚) operating at T = 291(2) K. The data integration and empirical absorption corrections were carried out by the SAINT program.27 Absorption corrections were made using SADABS program. The structures were solved by direct methods (SHELXS 97).28 Hydrogen atoms bonded to the carbon atoms were generated geometrically and refined isotropically with the riding mode. The hydrogen atoms of the water molecules in 1, 3, and 4 were located in the difference Fourier map. Details of the crystal parameters, data collection, and refinements for 1-5 are summarized in Table 1. Selected bond lengths, bond angles, and H-bond data are listed in Tables S1 and S2 (Supporting Information).

Results and Discussion Syntheses. In this work, 1,2,4-triazole was formed from decarboxylation reactions of 1H-1,2,4-triazole-3-carboxylic acid under solvothermal conditions. The metal/triazole coordination polymers were formed by self-assembly processes during the in situ formation of 1,2,4-triazole. A systemic study on the reaction of the Htrza ligand and Cd(II) or Zn(II) salts with different counterions (Cl-, Br-, I-, NO3-, SO42-, ClO4-) has been carried out under solvothermal conditions. Some crystal structures we obtained have been reported previously, such as [Cd2(trz)3I]18a and [Zn(trz)Cl].21d From some other reactions, we only got amorphous powders which are not suitable for single-crystal X-diffraction. All these data will not be included here. The five new complexes 1-5 reported here cannot be obtained by simply adding metal salts to the Htrz ligand under the same solvothermal conditions. On the other hand, the mixing of metal salts with Htrza or Htrz ligand in solution at room temperature leads to rapid reaction and immediate precipitation. Therefore, no single crystal can be obtained via a nonsolvothermal route for structure determination in this work. Complexes 1-5 are insoluble in common organic solvents, such as methanol, acetone, dichloromethane, and acetonitrile. The infrared spectra of compounds 1-5 and 1,2,4-triazole ligand are given in Figures S3-S8 (Supporting Information).

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Scheme 2. Possible Mechanism of Decarboxylation for the Htrza Ligand

The strong characteristic ν(N-H) bands of the 1,2,4-triazole ligand at 3119 and 3129 cm-1 disappeared for complexes 1, 2, and 4 but shifted to the range of 3160-3162 and 31393140 cm-1 in the infrared spectra of complexes 3 and 5. The ν(C-N) or ν(N-N) bands of the triazole ligand in compounds 1-5 are in the range of 1193-1297 cm-1.18a The characteristic bands of ν(N-O) stretches for 2 and 4 are located at 1502 and 1155 cm-1. The sulfate phases of 3 and 5 show characteristic ν(S-O) bands in the range of 1054-1077 and 606-644 cm-1. Brill29 and Cleland30 investigated the mechanisms of decarboxylation reactions of a series of carboxylic acid derivatives and suggested that the zwitterion structure formed by proton transfer from the carboxylic acid to the nitrogen atom plays a key role in the decarboxylation reactions. On the basis of their results, the decarboxylation process of 1H-1,2,4-triazole-3-carboxylic acid and formation of metal/ triazole coordination polymers are outlined in Scheme 2. The decarboxylating processes differ with and without the base (NaOH). Upon addition of NaOH (such as for preparation of complex 1), the H atom in the carboxyl group is initially deprotonated. The electronegative trz- ligand formed from decarboxylation at high temperature is easily coordinated to metal ions with the μ1,2,4-bridging mode. In contrast, the proton in the carboxyl group is primarily shifted to the N atoms in the triazole ring in the absence of NaOH. Deprotonation of carboxylic acid gives rise to a zwitterionic transition state. The Htrz ligand formed after decarboxylation is difficult to ionize under acidic conditions. Therefore, the Htrz ligands are apt to form complexes adopting the μ1,2or μ2,4-bridging modes. The flexible coordination motif of the trz- ligand in this work (see next section) may thus be explained by the different decarboxylation processes of 1H1,2,4-triazole-3-carboxylic acid under different conditions. Description of Crystal Structures. Structure of 1. The crystal structure of complex 1 is depicted in Figure 1. The asymmetric unit contains three half crystallographically independent Cd atoms, one and two half trzligands, a half of Br- anion, and a half of solvated water molecule (Figure 1). In the asymmetric unit, the three Cd atoms, two trzligands, one Br- anion, and one solvated water molecule are all sited on the special position. The Cd centers are all six-coordinated and show different octahedral coordination environments: Cd1 atom is coordinated to six nitrogen atoms from different trzligands, which forms a [CdN6] octahedron, Cd2 atom to four nitrogen atoms from different trzligands, one Br- anion, and one oxygen

Figure 1. Coordination environments for the trinuclear cadmium unit in 1. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for clarity. Symmetry codes: (a) -x þ 1/2, -y þ 1/2, z - 1/2; (b) -x þ 1, y, z; (c) x, -y þ 1, z - 1/2; (d) -x þ 1, -y þ 1, z - 1/2; (e) x þ 1/2, y þ 1/2, z; (f) -x, y, z; (g) x - 1/2, -y þ 1/2, z - 1/2.

atom from water molecules, Cd3 atom to five nitrogen atoms and one Br- anion in a [CdN5Br] unit. All three nitrogen atoms in the triazolate ligands are coordinated to three different Cd atoms. The bromide anion adopts a μ2-bridging mode to link two separate Cd centers. The Cd-N bond lengths range from 2.31(2) to 2.36(2) A˚, which are comparable to the value of those reported for Cd(II)-triazolate/Br complexes (2.226-2.319 A˚).18 The Cd-Br bond lengths (2.598(9) and 2.879(5) A˚) are in the range of 2.668-2.981 A˚ as reported for [Cd3(trz)3Br3]. The Cd-O bond distances (2.702 A˚) are longer than the mean values quoted for Cd-O bonds (2.36 A˚)31 but clearly shorter than the sum of the van der Waals radii (3.0-3.3 A˚) of the ions. The Cd1 3 3 3 Cd2 and Cd2 3 3 3 Cd3 distances are 6.537(2) and 3.933(3) A˚, respectively. As shown in Figure 2a, the Cd1 atoms are connected by the 1- and 2-positional N atoms of trzligands along the c-axis to form an infinite chain, which is similar to that of Zn2(trz)3Cl24a and [ZnF(AmTAZ)] solvents.25 The Cd2 and Cd3 atoms are linked through the trzligands and Br- anions forming a twodimensional (2D) layer in the ac plane (Figure 2c). The complicated three-dimensional metal-organic framework can be described in terms of two-dimensional layers linked through one-dimensional chains (Figure 2b). This connectivity pattern generates a series of irregular cavities, viewed along the c-axis. With removal of the guest aqua molecules, the potential solvent-accessible space for 1 is calculated to be about 10.1% of the total volume using PLATON.32 In the three-dimensional framework of 1, the Cd1 atom and trz- ligand can be reviewed as six- and three-connected nodes, respectively. For convenience of topological analysis, we take the Cd2 and Cd3 binuclear cluster as one node (Figure 2c). The new node is a seven-connected node as it is connected by four other same nodes and three Cd1 atoms (Figure 3). On the basis of this simplification and according to topological analysis by the TOPOS 4.0 program package,33 the framework of complex 1 can be symbolized as (4 3 52)3(44 3 58 3 62 3 74 3 83)(46 3 54 3 75). Structure of 2. There are two Cd atoms, three trzligands, two half nitrate anions, and two half coordinated water molecules in the crystallographically asymmetric independent unit of 2 (Figure 4). The nitrate anions and the coordinated water molecules are disordered in the crystal lattice. The total occupancy of both nitrate anions and

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Figure 3. Schematic description of the 3,6,7-connected 3D topology of 1.

Figure 4. Basic coordination environments in 2. Thermal ellipsoids are drawn at the 30% probability level. Symmetry codes: (a) -x þ 3/2, y, z þ 1/2; (b) x, y þ 1/2, -z þ 1/2; (c) -x þ 1, -y þ 1/2, z þ 1/2; (d) x - 1/2, -y þ 1/2, z; (e) x - 1/2, -y, -z þ 1/2.

Figure 2. (a) One-dimensional chain generated by Cd1 atoms and trzligands along the c-axis. (b) Three-dimensional structure constructed by the 1D chains and 2D layers. (c) Two-dimensional layer generated by binuclear cadmium clusters and its (4,4) topology. Hydrogen atoms and water molecules have been omitted for clarity.

coordinated water molecules in the asymmetric unit are 1.0. The Cd centers show different coordination environments: Cd1 atom is surrounded by three 4-positional N atoms from different trzligands in the equatorial plane [Cd1-N = 2.181(9)-2.21(1) A˚] and two oxygen atoms from nitrate anions at the apical sites [Cd1-O = 2.11(1)-2.45(1) A˚] and, therefore, has a trigonal bipyramid coordination sphere; Cd2 atom shows an octahedral coordination geometry, which is defined by six 1- or 2-positional N atoms from six trzligands [Cd2-N = 2.32(1)-2.368(9) A˚]. The trzligands adopt a μ1,2,4-bridging mode in complex 2.

The three-dimensional structure of 2 is constructed from [Cd(trz)3]nn- chains linked by Cd1 atoms (Figure 5b). The adjacent Cd2 atoms in the chains are connected by three pairs of 1- and 2-positional N atoms of different trzligands along the a-axis direction [Cd2 3 3 3 Cd2 10.945(1) A˚] (Figure 5a). Each Cd1 atom links three neighbor [Cd(trz)3]nn- chains through the remaining 4-positional N atoms of trzligands. The charge of the framework is balanced by NO3- anions, which are coordinated to Cd1 atoms. This framework is iso-structural with that of [Cd2(trz)3I], [Zn2(trz)3(OH)], [Zn2(trz)3Cl], and [Co2(trz)3Cl].4d,18a,19a,24a As shown in Figure 5b, the bonding pattern of 2 produces hexagonal sections in the bc plane. The hexagonal section is constructed by six 16-numbered circuits [Cd4N8C4]. However, unlike previous complexes, the cavities formed by the 16-numbered circuits [Cd4N8C4] are partly filled by the terminal nitrate anions. The Cd1 atoms, Cd2 atoms, and the trzligands are threeconnected, six-connected, and three-connected nodes, respectively. Therefore, the topology of 2 can be symbolized as (83)(4 3 82)3(46 3 86 3 103), which is the same as that of Zn2(trz)3Cl (Figure 5c).

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Figure 6. Coordination environments for the binuclear cadmium unit in compound 3. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for clarity. Symmetry codes: (a) x - 1, y, z þ 1; (b) -x þ 1, -y þ 2, -z þ 1; (c) -x, -y þ 2, -z þ 2; (d) x, y, z þ 1; (e) -x, -y þ 2, -z þ 1; (f) -x þ 1, -y þ 1, -z þ 1.

Figure 5. (a) One-dimensional chain generated by Cd2 atoms and trzligands along the a-axis. Hydrogen atoms have been omitted for clarity. (b) View of the 3D metal-organic network along the aaxis direction showing [Cd(trz)3]nn- chains linked by Cd1 atoms. Hydrogen atoms, disordered nitrate anions, and water molecules have been omitted for clarity. (c) 3,6-Connected topology of 2.

Structure of 3. The asymmetric unit of complex 3 consists of two half Cd atoms, one sulfate anion, a half neutral Htrz ligand, and one coordinated water molecule. As shown in Figure 6, Cd1 is coordinated to two 2-positional N atoms of two Htrz ligands and four oxygen atoms of four different sulfate anions. Cd2 atom is coordinated by two 4-positional

N atoms from different Htrz ligands, four oxygen atoms from two coordinated water molecules, and two sulfate anions. The average Cd-N and Cd-O bond lengths are 2.285 and 2.306 A˚, respectively. The neighboring Cd1 and Cd2 atoms are linked by the Htrz ligands adopting a μ2,4bridging mode [Cd1 3 3 3 Cd2 = 6.6168(8) A˚]. Unlike in complexes 1 and 2, the Htrz ligand did not deprotonate in complex 3. This may be due to the fact that the bivalent anion (SO42-) can more easily satisfy the charge balance of the framework than the univalent anions (Br- and NO3-). Similar to complexes [Zn2(trz)(SO4)(OH)] and [Cd8(trz)4(OH)2(SO4)5(H2O)],19a the sulfate anions also act as μ3 bridge ligands in complex 3. The three-dimensional structure of 3 can be viewed as 2D inorganic layers linked by Htrz ligands through the Cd1 and Cd2 sites (Figure 7b). Within the 2D layer, there are cornersharing corner chains34 constructed by Cd1 atoms and trzligands along the a-axis. These chains are further linked through the Cd2 sites via the μ3-bridging sulfate anions in the bc plane (Figure 7a). The framework contains two kinds of 16-numbered circuits, [Cd4N4C2S2O4] and [Cd4S4O8], in the framework. However, the cavities of [Cd4N4C2S2O4] are partly filled by the coordinated water molecules and sulfate anions. The 2D layer can be simplified as a 3,4-connected topological structure. The Cd1 nodes in one 2D layer connects to the Cd2 sites in the other neighboring layer through the linker (Htrz ligands), forming a 3,4,6-connected 3D grid with (4 3 62)(42 3 68 3 83 3 102)(64 3 82) topology (Figure 7c). Structure of 4. The asymmetric unit of 4 contains two Zn atoms, two trzligands, two nitrate anions, and one coordination water molecule (Figure 8). Zn1 is coordinated to three nitrogen atoms from three trzligands and one oxygen atom from nitrate anion, giving a distorted [ZnN3O] tetrahedral geometry [Zn1-N = 1.983(2)-1.992(2) A˚, Zn1-O = 1.995(2) A˚]. Zn2 sits in a distorted octahedral coordination environment defined by three nitrogen atoms of different trzligands and three oxygen atoms from one nitrate anion and one water molecule [Zn2-N = 2.045(3)-2.073(2) A˚, Zn2-O = 2.173(2)-2.283(2) A˚]. The trz- ligand adopts a μ1,2,4-bridging mode in complex 4. Unlike in complex 2, the nitrate anions show mono- and bidentate coordination modes in complex 4. Complex 4 is a 2D layer structure, which is constructed by different binuclear zinc units and the bridging trzligands (Figure 9a). Within the 2D layer, each binuclear zinc unit is built by two adjacent Zn1 or Zn2 atoms through the 1- and 2-positional N atoms of trzligands. The 2D layer consists

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Figure 9. (a) View of the 2D layer of 4 along the a-axis. Nitrate anions and coordinated water molecules have been omitted for clarity. (b) The (4,4) topology of 4.

Figure 7. (a) Two-dimensional inorganic layer in the ac plane and its 3,4-connected topology. Hydrogen atoms have been omitted for clarity. (b) View of the 3D network along the a-axis direction constructed by the inorganic layers and Htrz ligands. (c) Threedimensional grid of 3.

Figure 8. Coordination environments for the binuclear zinc unit in 4. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for clarity. Symmetry codes: (a) -x, -y þ 2, -z; (b) -x þ 1, -y þ 1, -z þ 1; (c) x, y, z þ 1.

of two kinds of six-numbered circuits, [Zn1Zn1N4] and [Zn2Zn2N4], and one kind of 16-numbered circuit, [(Zn1Zn2)2N8C4]. The Zn1 3 3 3 Zn1 and Zn2 3 3 3 Zn2 distances between the different six-numbered circuits are 3.6187(8) and 3.5998(8) A˚, respectively. However, the cavities of the 16-numbered rings are filled with the nitrate anions. The 2D layer structure has a (4.82)4 topology by taking Zn atoms and trzligands as 3-connected nodes. If we collide each binuclear zinc unit into one node, 4 is simplified as a simpler (4,4) topological network (Figure 9b). Two similar 2D layers based on Ag(I)/Cd(II)-trz- (or its derivatives) have been reported previously.21a,24a Ouellette and co-workers have systematically prepared a series of metal/ triazole polymers and pointed out that no conditions could be found to provide Zn/trz-/NO3- phases in their research.18a However, our present work successfully obtained the Zn/trz-/ NO3- complex 4 through a self-assembly process under the in situ solvothermal conditions. Structure of 5. Complex 5 contains a trinuclear unit with one and two half Zn atoms, two Htrz ligands, and two sulfate anions in the asymmetry unit. As shown in Figure 10, Zn atoms have different coordination environments. Zn1 and Zn3 atoms have similar six-coordinated [ZnN2O4] octahedral coordination geometry, while Zn2 has a distorted [ZnN2O3] tetragonal pyramid polyhedron. The two [ZnN2O4] units are bridged by the sulfate anions and Htrz ligands through the Zn2 site. The bond angle (N-Zn-N or O-Zn-O) around Zn1 and Zn3 atoms is 180°. The average Zn-N and Zn-O bond lengths are 2.083 and 2.093 A˚, respectively. Sulfate anions act as μ3-bridging ligands in this complex. It is noteworthy that there are two kinds of neutral Htrz ligands (1H-1,2,4-triazole and 4H-1,2,4-triazole) that occurred during the in situ decarboxylation of the Htrza ligand in this compound. To the best of our knowledge,

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Figure 10. Coordination environments for the trinuclear zinc unit in 5. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for clarity. Symmetry codes: (a) -x - 1, -y þ 3, -z þ 2; (b) x - 1, y, z; (c) -x, -y þ 3, -z þ 2; (d) x - 1, y þ 1, z; (e) -x, -y þ 2, -z þ 2; (f) -x þ 1, -y þ 2, -z þ 1; (g) -x, -y þ 2, -z þ 1; (h) x þ 1, y, z.

the coexistence of neutral μ1,2- and μ2,4-Htrz ligands in one coordination polymer was unreported before. As shown in Figure 11, the three-dimensional structure of complex 5 can be viewed as the 2D inorganic layers in the ac plane linked by the μ2,4-bridging 1H-1,2,4-triazole ligands. The 2D layer contains the chains consisting of the Zn2 3 3 3 Zn3 3 3 3 Zn2 trinuclear units along the a-axis, which are linked through Zn1 atoms via the oxygen bridges of sulfate anions (Figure 11a). This connectivity pattern generates 16numbered [Zn4N4C2S2O4] circuits and unusual 30-numbered [Zn8N8C2S4O8] circuits in the complex (Figure 11b). However, the cavities of both kinds of circuits are filled with the Htrz ligand and sulfate anions. Both complexes 3 and 5 show similar 3D structure constructed by 2D layers and the bridging Htrz ligands (μ2,4bridging mode). However, in the 2D topological structure of complex 5, the collided node of the Zn2 3 3 3 Zn3 3 3 3 Zn2 trinuclear cluster replaces the same site of Cd2 atom in complex 3, as shown in Figure 7a and Figure 11a. The 2D layer of complex 5 shows a 3,4,4-connected topological structure if we regard the Zn2 3 3 3 Zn3 3 3 3 Zn2 trinuclear cluster as one collided node, sulfate anion as three-connected node, Zn1 atom as four-connected node, respectively (Figure 11a). The collided node is further linked to two other neighboring collided nodes and four Zn1 nodes in the 3D structure, while the Cd2 atom is a four-connected node in complex 3. On the basis of this simplification, regarding the sulfate anions as three-connected nodes, Zn1 atoms as six-connected nodes, and the Zn2 3 3 3 Zn3 3 3 3 Zn2 cluster as a six-connected nodes, the 3D network of complex 5 can be represented as 3,6,6connected Cr2F5 topology symbolized as (4 3 52)2(42 3 510 3 6 3 72)(58 3 64 3 83). Structural Diversity of Complexes 1-5. As depicted in Scheme 3, complexes 1-5 are described as 2D or 3D frameworks with diverse architectures. Complex 1 is a complicated 3D network structure and presents a 3,6,7-connected topology. Complex 2 adopts a 3,6-connected topology that is built from binuclear cadmium units. Complex 3 is of a 3,4,6connected topology based on binuclear cadmium units. Complex 4 exhibits a 2D grid with (4,4) topology and forms a three-dimensional supramolecular structure via hydrogen bonding. Complex 5 contains trinuclear zinc units and presents a 3,6,6-connected Cr2F5 topology symbolized as (4 3 52)2(42 3 510 3 6 3 72)(58 3 64 3 83). The coordination modes of Htrz ligands are also different among complexes 1-5. The Htrz ligands in complexes 1, 2,

Figure 11. (a) Two-dimensional inorganic layer in the ac plane and its 3,4,4-connected topology. Hydrogen atoms have been omitted for clarity. (b) Three-dimensional metal-organic framework of 5. (c) 3,6,6-Connected topology of 5.

and 4 adopt the μ1,2,4-bridging mode and μ2,4-bridging mode in complex 3. However, both μ2,4- and μ1,2-bridging modes were found in complex 5. These results indicate that the Htrz ligand produced under the decarboxylation solvothermal

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Scheme 3. Schematic Showing the Coordination Diversified Structures in 1-5

conditions would take feasible bridging modes to fulfill the charge-balance requirement of different anions. Thus, the Htrz ligand deprotonated in complexes 2 and 4 under acidic conditions to fulfill the charge-balance requirement of the monovalent nitrate anions but only acted as neutral linkers in complexes 3 and 5 corresponding to the bivalent sulfate anion. In addition, the different coordination modes of anions have an important effect on the structures of coordination polymers in this system. For example, the Br-, NO3-, and SO42- anions adopt monodentate, bidentate, and tridentate coordination modes, respectively, in the cadmium/triazolate coordination polymers. The incorporating inorganic anions with different coordination modes give different structures. On the other hand, coordination polymers with the same anion but different metal centers have different topologies. The diversities in structural features of 1-5 clearly indicate that the architectures of the metal/triazolate coordination polymers are affected by anions. As a result, selection of appropriate anions may be considered an ideal way to construct novel metal/triazolate coordination polymers. PXRD and Thermogravimetric Analyses. Powder X-ray diffraction (PXRD) patterns of complexes 1-5 are in agreement with the simulated ones from the respective single-crystal X-ray data (Figures S9-S13, Supporting Information). It suggests that the crystal structures are truly representative of the bulk materials. The differences in intensity are due to the preferred orientation of the powder samples. The thermal properties of complexes 1-5 were examined by the method of thermogravimetric analysis under nitrogen atmosphere (Figure S14, Supporting Information). Complex 1 gives a gradual weight loss of 2.43% in the range of 35130 °C, which corresponds to the loss of the crystalline water molecules (calculated 2.31%). A quick weight loss that occurs over 365 °C can be assigned to the decomposition of

organic moieties (observed 40.06%, calculated 39.76%). A plateau exists between 510 and 620 °C, followed by partial sublimation of the surplus.19a For 2, the weight loss between 65 and 105 °C can be attributed to the loss of coordinated water molecules (observed 3.68%, calculated 3.54%). The further weight loss belongs to the decomposition of triazole and nitrate components (observed 45.58%, calculated 45.86%). Complex 3 began to lose the coordinated water molecules at 80 °C (observed 6.05%, calculated 6.09%) and then decomposed at 180 °C (observed 23.61%, calculated 23.35%). The loss of coordinated water molecule is observed between 60 and 125 °C for 4 (observed 4.74%, calculated 4.40%), followed by a quick weight loss in the range of 350-450 °C. The total weight loss at 700 °C is 63.41%. No weight loss was observed in the TGA curve of 5 before 365 °C. The weight loss between 365 and 590 °C (observed 30.09%, calculated 29.96%) can be assigned to the decomposition of the Htrz ligand. The remainder is partly sublimated until 800 °C. The thermal stability of 1-5 was verified by PXRD analyses. With removal of solvent water molecules, PXRD patterns of 1 and 2 are inconsistent with those of the pristine samples, suggesting the collapse of their structures. The porous framework of 3 remains unchanged after the removal of guest molecules. However, the desolvented complex 3 exhibits no N2 adsorption at 77 K owing to the limited pore opening. The PXRD pattern reveals collapse of the complex 4 after heating at 200 °C for 0.5 h. Complex 5 is stable over 300 °C until the decomposition of the Htrz ligand (Figure S13, Supporting Information). Photoluminescent Properties. Metal/triazole-based coordination polymers with Cd(II) and Zn(II) ions usually exhibit photoluminescent properties.9a,18a,b,19a,20c,24a The photoluminescent properties of complexes 1-5 were studied in the solid state at room temperature (Figure S15, Supporting

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Information). Upon excitation at 370 nm, complexes 1-5 show broad and unstructured emission bands with the maximum peak at 452 nm for 1, 445 nm for 2, 397 nm for 3, 443 nm for 4, and 422 nm for 5, respectively. By comparison of the emission energies and profiles of the 1,2,4-triazole (421 nm)18a and complexes 1-5, the luminescence behavior of complexes 1-5 is considered to originate from intraligand π-π* transitions. The red shift of the emission energies among these complexes probably results from the different coordination environments of metal ions. The reflectance spectra of complexes 1-5 (Figure S16, Supporting Information) show low reflectivity below 400 nm, which is in agreement with the excited wavelength of photoluminescent spectra. Conclusions Five novel metal/triazole-based coordination polymers with diversities of architectures have been successfully synthesized using 1H-1,2,4-triazole-3-carboxylic acid with cadmium or zinc salts under solvothermal conditions. The structure of these coordination polymers have been characterized by single-crystal X-ray diffraction. The anions play important roles in the construction of the high dimensional structure of these coordination polymers. The anionic components (Br-, NO3-, and SO42-) all participate in structures with different coordination modes, which give rise to different connectivity. On the other hand, the Htrz ligand adopts diverse coordination modes to fulfill the different charge-balance requirement for different anions. These results enrich the structure diversity of metal/triazole-based coordination polymers. Complexes 1-5 exhibit blue fluorescence emission bands due to the intraligand π-π* transitions in the solid state at room temperature. Further investigations on this system, such as the template effect of salts in the decarboxylation, are still in progress. Acknowledgment. This work was supported by the National Basic Research Program of China (2006CB806104 and 2007CB925100), the National Natural Science Foundation of China (20721002), and the funding from enterprise academician workstation in Changzhou Trina Solar Energy Co. Ltd., Jiangsu Province. Supporting Information Available: PXRD patterns, TGA, luminescent properties, and reflectance spectra of 1-5. The IR spectra of triazole ligand and complexes 1-5. X-ray crystallographic files in CIF format for 1-5. This material is available free of charge via the Internet at http://pubs.acs.org.

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Novel Structural Diversity of Triazolate-Based Coordination Polymers

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