Syntheses, Structures, and Photoluminescence of a Series of Three

DOI: 10.1021/cg100467p

Syntheses, Structures, and Photoluminescence of a Series of Three-Dimensional Cd(II) Frameworks with a Flexible Ligand, 1,5-Bis(5-tetrazolo)-3-oxapentane

2010, Vol. 10 4370–4378

Ping Cui, Zhi Chen, Dongliang Gao, Bin Zhao,* Wei Shi, and Peng Cheng* Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China Received April 8, 2010; Revised Manuscript Received August 16, 2010

ABSTRACT: Six novel CdII-based metal-organic frameworks with a flexible ligand 1,5-bis(5-tetrazolo)-3-oxapentane) (H2btz), [Cd5(N3)6(btz)2]n (1), [Cd5X2(N3)4(btz)2]n (X = Cl (2), Br (3)), [Cd3(N3)2(btz)2]n (4), and [Cd3X2(btz)2]n (X = Cl (5), Br (6)), have been successfully synthesized under hydrothermal reactions and structurally characterized by single-crystal X-ray diffraction. The H2btz ligands in the complexes 1-4 can be in situ generated from the [2 þ 3]-cycloaddition reaction of the precursor bis(2-cyanoethyl)ether (BCEE) and NaN3 under CdII-catalyzed. Single-crystal X-ray diffraction analysis revealed that all of the complexes 1-6 displayed rich three-dimensional (3D) frameworks constructed from Cd2þ, btz2- ligand, and bridging anions. It has been found that the ingenious changes of anions finally lead to the structural diversity in 1-6, such as N3- for 1 and 4, N3- and Cl- for 2, N3- and Br- for 3, Cl- for 5, and Br- for 6. Additionally, the X-ray powder diffraction (XRPD), thermal stability, and the photoluminescence properties at room temperature in the solid state for 1-6 have also been studied.

Introduction The currently progressive impetus for the design and fabrication of novel metal-organic frameworks (MOFs) is rooted not only in their fascinating topological structures but also from their promising properties and great potential applications in the fields of gas storage, ion-exchange, catalysis and nonlinear optics, and so on.1 Conventionally, hydro(solvo)thermal synthetic strategy is to construct MOFs through the direct coordination interaction of metal ions and suitable organic ligands such as carboxyl-type and azole-type,2,3 in which the backbone of organic ligands kept structurally intact from beginning to end. In contrast, as one interesting method of constructing novel MOFs, in situ metal/ligand reactions under hydro(solvo)thermal conditions have provoked intense interest,4-8 and up to now, many types of in situ reaction mechanisms already have been documented.9 Meanwhile, in comparison with conventional methods, the MOFs formed by the in situ synthetic strategy may give some unique topological structures and functions not obtained by other methods. On the other hand, it is well-known that the selection of organic ligands plays a crucial role in constructing desired MOFs, in which tetrazole-based ligands as building blocks analogous to that of carboxylate ligands10 have attracted significant attention due to their variety of bridging modes, excellent coordination capacities, and potential applications in advanced materials.11-13 In our work, the bis(2-cyanoethyl)ether (BCEE) was employed as a precursor ligand reacting with NaN3 under the catalysis of the CdII salts to construct some interesting MOFs containing in situ generated ligand 1,5-bis(5-tetrazolo)-3-oxapentane (H2btz). The selection of H2btz was based on the following reasons: (1) As a long flexible ligand with one -O- and four -CH2- spacers, conformational freedom may be better to satisfy the geometric needs *Corresponding authors. E-mail: [email protected] (B.Z.); pcheng@ nankai.edu.cn (P.C.). pubs.acs.org/crystal

Published on Web 08/31/2010

of different metal ions to construct diversified and intriguing architectures and topologies. (2) It exhibits excellent coordination capacities with nine coordination sites of two tetrazolate rings and one O donor, and may be a good candidate for the construction of coordination polymers with fascinating properties such as in magnetism. (3) To our knowledge, no coordination polymers based on H2btz ligand have been observed so far, although a few coordination polymers associated with flexible 5-substituted tetrazolate ligands were reported.14 Herein, we focused our attention on the coordination polylmers generated in the presence of different cadmium sources and investigated the influence of the inorganic anions on their structures. In this contribution, we report the syntheses and crystal structures of a series of CdII-based coordination polymers, [Cd5(N3)6(btz)2]n (1), [Cd5X2(N3)4(btz)2]n (X = Cl (2), Br (3)), [Cd3(N3)2(btz)2]n (4), and [Cd3X2(btz)2]n (X = Cl (5), Br (6)), which are obtained by hydrothermal reactions of the corresponding CdII salts with BCEE or H2btz. These coordination polymers exhibit three-dimensional (3D) robust frameworks and contain various bridging inorganic anions N3- for 1 and 4, N3- and Cl- for 2, N3- and Br- for 3, Cl- for 5, and Br- for 6. Namely, these anions may be responsible for the structural diversity in 1-6. Furthermore, the X-ray powder diffraction (XRPD) data, thermal and fluorescent properties of the complexes have also been investigated. Experimental Section Materials and General Methods. All chemicals were commercially purchased and used without further purification. The H2btz ligand in 1-4 can be in situ generated, and the one in 5-6 was synthesized in advance according to the literature.15 The elemental analyses (C, H, and N) were carried out on a Perkin-Elmer elemental analyzer. The XRPD data were collected on a Rigaku D/Max-2500 diffractometer at 40 kV and 100 mA, employing a Cu-target tube and a graphite monochromator. Thermogravimetric analyses were performed on a Netzsch TG 209 TG-DTA analyzer from room r 2010 American Chemical Society

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Table 1. Crystallographic Data for Complexes 1-6 empirical formula formula weight crystal system space group a/A˚ b/A˚ c/A˚ R (°) β (°) γ (°) V/A˚3 Z Fcalc/(Mg 3 cm-3) F(000) θ range (°) reflns collected goodness-of-fit on F2 R1 [I > 2σ(I)] wR2 (all data)

1

2

3

4

5

6

C12H16O2N34Cd5 1230.59 triclinic P1 8.2060(16) 8.6920(17) 12.905(3) 92.94(3) 105.46(3) 110.34(3) 821.2(3) 1 2.488 582 2.53-25.01 4945 1.067 0.0452 0.1283

C12H16O2N28Cl2Cd5 1217.43 triclinic P1 8.0489(16) 8.4254(17) 13.213(5) 93.17(3) 107.53(3) 109.10(3) 795.5(4) 1 2.541 574 2.85-25.01 6359 1.074 0.0240 0.0642

C12H16O2N28Br2Cd5 1306.35 triclinic P1 8.0685(16) 8.3110(17) 13.313(3) 93.53(3) 106.74(3) 109.18(3) 795.3(3) 1 2.728 610 2.63 -25.01 8279 1.054 0.0532 0.1397

C12H16O2N22Cd3 837.67 orthorhombic Pbca 14.207(2) 9.918(3) 16.545(3) 90 90 90 2331.3(9) 4 2.387 1608 2.46-25.01 14670 1.116 0.0178 0.0400

C12H16O2N16Cl2Cd3 824.51 orthorhombic Pbca 14.2128(4) 9.8325(2) 16.3309(4) 90 90 90 2282.20(10) 4 2.400 1576 2.49-25.01 5861 1.139 0.0176 0.0417

C12H16O2N16Br2Cd3 913.43 orthorhombic Pbca 14.3329(9) 9.8015(6) 16.5134(10) 90 90 90 2319.9(2) 4 2.615 1720 3.52-25.01 12415 1.120 0.0321 0.0893

Scheme 1. Synthetic Strategy of 1-6

Scheme 2. In Situ Synthesis of the H2btz Liganda

a

X = Cl-, Br-, I-, NO3-, SO42-.

temperature to 800 °C under a nitrogen atmosphere at a heating rate of 10 °C min-1. Solid-state luminescence spectra were recorded with a Varian Cary Eclipse fluorescence spectrophotometer. Syntheses of 1-3. A mixture of Cd(NO3)2 3 4H2O (0.308 g, 1.0 mmol) for 1, CdCl2 3 2.5H2O (0.228 g, 1.0 mmol) for 2, CdBr2 3 4H2O (0.344 g, 1.0 mmol) for 3, NaN3 (0.081 g, 1.25 mmol), BCEE (0.062 g, 0.5 mmol), H2O (4 mL), and C2H5OH (4 mL) were sealed in a 25 mL Teflon-lined stainless steel autoclave and heated at 130 °C for 3 days, and then slowly cooled to room temperature in 3 days. Finally, colorless crystals of 1-3 suitable for X-ray crystallographic analysis were obtained. Yield: 36% for 1, 38% for 2, and 42% for 3. Anal. Calcd. for 1 C12H16O2N34Cd5: C, 11.71; H, 1.31; N, 38.70. Found: C, 11.85; H, 1.47; N, 38.95. Anal. Calcd. for 2 C12H16O2N28Cl2Cd5: C, 11.84; H, 1.32; N, 32.22. Found: C, 11.95; H, 1.48; N, 32.49. Anal. Calcd. for 3 C12H16O2N28Br2Cd5: C, 11.03; H, 1.23; N, 30.02. Found: C, 11.15; H, 1.05; N, 30.18. Syntheses of 4. A mixture of 3CdSO4 3 8H2O (0.770 g, 1.0 mmol) or CdI2 (0.366 g, 1.0 mmol), NaN3 (0.081 g, 1.25 mmol), BCEE (0.062 g, 0.5 mmol), H2O (4 mL), and C2H5OH (4 mL), or a mixture of Cd(ClO4)2 3 6H2O (0.419 g, 1.0 mmol), H2btz (0.105 g, 0.5 mmol), H2O (4 mL), and C2H5OH (4 mL) were sealed in a 25 mL Teflonlined stainless steel autoclave and heated at 130 °C for 3 days, and then slowly cooled to room temperature in 3 days. Finally, colorless

crystals of 4 suitable for X-ray crystallographic analysis were obtained. Yield: 41% (3CdSO4 3 8H2O), 18% (CdI2) and 35% (Cd(ClO4)2 3 6H2O). Anal. Calcd. for C12H16O2N22Cd3: C, 17.21; H, 1.93; N, 36.79. Found: C, 17.42; H, 1.84; N, 36.98. Syntheses of 5 and 6. A mixture of CdCl2 3 2.5H2O (0.228 g, 1.0 mmol) for 5 and CdBr2 3 4H2O (0.344 g, 1.0 mmol) for 6, H2btz (0.105 g, 0.5 mmol), H2O (4 mL) and C2H5OH (4 mL) were sealed in a 25 mL Teflon-lined stainless steel autoclave and heated at 130 °C for 3 days, and then slowly cooled to room temperature in 3 days. Colorless crystals of 5 (yield 60%) and 6 (yield 55%) suitable for X-ray crystallographic analysis were obtained. Additionally, 5 can also be obtained through a mixture of CdCl2 3 2.5H2O (0.228 g, 1.0 mmol), NaN3 (0.065 g, 1.0 mmol), H2btz (0.105 g, 0.5 mmol), H2O (4 mL), and C 2H5OH (4 mL) under the same reaction conditions with the yield of 51%. Anal. Calcd. for 5 C12H16O2N16Cl2Cd3: C, 17.48; H, 1.96; N, 27.18. Found: C, 17.59; H, 1.86; N, 27.33. Anal. Calcd. for 6 C12H16O2N16Br2Cd3: C, 15.78; H, 1.77; N, 24.54. Found: C, 15.91; H, 1.61; N, 24.68. Caution!. Owing to the potentially explosive nature of NaN3, only a small amount of NaN3 should be used and handled with care. Crystallographic Studies. Suitable single crystals of complex 5 were mounted on a Oxford diffractometer SuperNova TM at 150(2) K

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with a graphite-monochromated Mo Ka radiation (λ = 0.71073 A˚) using the ω-scan technique, and suitable single crystals of complexes 1-4 and 6 were mounted on a Bruker Smart CCD diffractometer at 113(2) K with a graphite-monochromated Mo Ka radiation (λ = 0.71073 A˚) using the ω-scan technique. All the structures were solved by direct methods and refined anisotropically by full-matrix least-squares techniques based on F2 using the SHELXS-97 and SHELXL-97 programs.16 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The hydrogen atoms were placed in idealized positions and constrained to ride on their parent atoms. As shown in Figure S1 (Supporting Information), the structures of 1 and 3 were found to be disordered. In 1, disordered components were split into two sites and had site occupancy factors of 0.71 for carbon atoms (C2 and C3), 0.28 for carbon atoms (C4 and C5), and 0.38 for N3- anion (N15, N16, and N17), respectively. Disordered N3- anion (N9, N10, and N11) of 3 was split into two sites similar to that of 1 and possessed a site occupancy factor of 0.82. For 1-6, the important crystal parameters, data collection, and refinements are summarized in Table 1, and selected bond lengths and angles with their estimated standard deviations are given in Table S1 (Supporting Information).

Results and Discussion Syntheses. The complexes 1-6 were generated by hydrothermal reactions of the corresponding CdII salts with BCEE or H2btz and the synthetic strategy of 1-6 was presented in Scheme 1. The in situ generated 1,5-bis(5-tetrazolo)-3-oxapentane

Figure 1. The coordination environments of Cd atoms in 1. The disordered components and hydrogen atoms were omitted for clarity.

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(H 2btz) ligand in 1-4 was from [2 þ 3]-cycloaddition of bis(2-cyanoethyl)ether (BCEE) with NaN3 in the presence of CdII metal salts (Scheme 2), that is, so-called DemkoSharpless [2 þ 3]-cycloaddition reaction.17 We adopted Cd(NO3)2, CdCl2, CdBr2, and CdI2/CdSO4 as cadmium sources to produce complexes 1-4 by in situ reaction, respectively. Among them, we found that the choice of suitable CdII salts was critical in the formation of final structures. In order to enrich the interesting systems and attempt to investigate the influence of the inorganic anions on the structures, we synthesized H2btz ligand according to the literature,15 and obtained two novel 3D frameworks 5 and 6. Remarkably, in this study, complex 4 was also obtained by using the Cd(ClO4)2-NaN3-H2btz system. However, the 3D framework of complex 5 did not change by adopting CdCl2-NaN3H2btz instead of the CdCl2-H2btz system, which was confirmed by single-crystal X-ray diffraction. Therefore, the inorganic anions probably play a certain important role in the formation of 3D frameworks of 1-6. Description of Crystal Structures. [Cd5(N3)6(btz)2] (1). Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the triclinic space group P1 and exhibits a 3D framework with the asymmetric unit of [Cd2.5(N3)3(btz)]. As depicted in Figure 1, the CdII centers (Cd1, Cd2, and Cd3) locate in three types of six-coordinate octahedral environments. The Cd1 center bonds to six N atoms from four btz2anions lying in the equatorial plane and two μ3-N3- anions in the axial position with a N12-Cd1-N12A angle of 180.0°. The subtle difference from that of Cd1 is the coordination geometry of the Cd2 center completed by six N atoms from three btz2- and three N3- anions, respectively. For the Cd3 center, one chelating tridentate btz2- anion (O1, N1, and N5) and three N atoms (N9, N15, and N15A) from three N3anions together form its whole geometry, with the Cd3-O1 and Cd3-N9 bond distances being 2.472(6) and 2.287(7) A˚ [O1-Cd3-N9 = 157.0°]. Each btz2- anion in 1 is coordinated to six separate CdII centers through its tetrazolate rings acting as μ3- and μ4bridges (Model A, Scheme 3), thus leading to the formation of a 2D layer, which can be topologically simplified by the tetrazolate rings as 3- and 4-connected nodes (Figure 2a). Owing to the flexibility around -CH2- and -O- spacers, the

Scheme 3. Coordination Modes of Ligands Observed in Complexes 1-6: A and B for H2btz; C (end-on, EO) and D for N3-

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Figure 2. (a) The topological structure of the 2D layer in 1; tetrazolate rings were simplified as 3- (green) and 4-connected (blue) nodes. (b) The 3D structure of 1 viewed along the b axis. Blue, N3- anions. Hydrogen atoms were omitted for clarity.

Figure 3. The coordination environments of Cd atoms in 2. Hydrogen atoms were omitted for clarity.

btz2- anion employs a twisted configuration and disorderly crystallizes in the crystal structure of 1. The dihedral angle between the two tetrazolate rings of btz2- and the bond angle of N1-Cd3-N5 are about 77.9° and 95.0°, respectively. The range of Cd-N (tetrazole) bond distances is 2.286(6)-2.441(6) A˚. Three crystallographically independent N3- anions exhibit μ2- and μ3-bridging models (Model C and D, Scheme 3). The separations of the μ2-N3- bridging Cd2 3 3 3 Cd3 and Cd3 3 3 3 Cd3A are 3.925 and 3.625 A˚, and the neighboring Cd 3 3 3 Cd separations of one Cd1 and two Cd2 centers bridged by μ3-N3- are 3.759, 5.810, and 6.034 A˚. The Cd-N (μ3-N3-) and Cd-N (μ2-N3-) bond distances fall in the range of 2.349(6)-2.417(6) A˚, and 2.259(6)-2.328(6) A˚. The N3- [μ2-(N9-N10-N11) and μ3-(N12-N13-N14)] anions are embedded in the former two-dimensional (2D) layer by coordinating to Cd1, Cd2, and Cd3 centers, and then the μ2-N3- (N15-N16-N17) anions further link the adjacent 2D layers to form the final 3D network structure by bridging Cd3 centers (Figure 2b). [Cd5X2(N3)4(btz)2] (X = Cl (2), Br (3)). By substituting Cd(NO3)2 with CdCl2/CdBr2 as the cadmium source in the synthesis of 1, two new 3D supramolecular networks 2 and 3 are isolated in the triclinic space group P1 with isostructural to 1. Compared with those of 1, some bond lengths and angles related to the CdII ions as well as the degree of distortion of the H2btz ligand have been significantly altered due to one of the

Figure 4. (a) The 2D structure bridged by btz2- and N3- anions. (b) The 3D structure of 2 viewed along the b axis.

three N3- anions replaced by Cl-/Br- anions in 2/3, respectively. Therefore, the structure of the complex 2 will be further discussed here. As shown in Figure 3, the Cd1 and Cd2 centers in 2 adopt octahedral coordination environments defined by six N atoms similar to that of 1, whereas the coordination environment of the Cd3 center in 2 is defined by one chelating tridentate btz2- anion (O1, N1, and N5), one N atom (N9) from one N3- anion and two Cl- anions (Cl1 and Cl1A). The Cd-N (tetrazole) [2.292(4)-2.448(4) A˚], Cd-N (azide) [2.318(4)-2.412(4) A˚], Cd-Cl [2.554(2), 2.592(2) A˚], and relatively longer Cd-O with 2.525(3) A˚ [in 3, Cd3-O1= 2.498(8) A˚] bond distances are comparable to the values found in the previously published reports.18a

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The crystallographically independent btz2- anion with two tetrazolate rings adopting μ3- and μ4-bridging modes (Model A, Scheme 3) is connected to six separate CdII atoms. In order to successfully introduce the spherical halide anion (Cl-/ Br-) instead of the long pseudoalide anion (N3-), the btz2- anions have to regulate their steric configurations, which is supported by the variety of dihedral angles (tetrazolyltetrazolyl) from 77.9 (in 1) to 73.9 (in 2) and 73.2 (in 3). Meanwhile, the bond angles of N1-Cd3-N5 are altered from 95.0° (in 1) to 89.9° (in 2) and 89.1° (in 3). As a comparison with the crystal packing of 1, the adjacent 2D layers in 2 constructed from the btz2- and N3- anions (Figure 4a) are further connected by μ2-Cl- anions into a 3D network structure with a Cd3 3 3 3 Cd3A separation of 3.642 A˚ (Figure 4b). [Cd3(N3)2(btz)2] (4). The single-crystal structure analysis of 4 indicates a 3D framework crystallizes in the orthorhombic space group Pbca. There are two types of crystallographically

Figure 6. (a) The 1D chain of Cd1 atoms of 4 viewed along the b axis. (b) The 3D structure of 4 viewed along the a- (right) and b-axis (left) directions, respectively. Color codes: red, O; white, C; blue, N; green, Cd1; yellow, Cd2. (c) The topological structure of complex 4; SBU was simplified as a node (sky blue). Hydrogen atoms were omitted for clarity.

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independent Cd2þ ions (Cd1 and Cd2 having 1 and 1/2 site occupancy, respectively), one btz2- and one N3- anion in an asymmetric unit. As shown in Figure 5, Cd1 center is fivecoordinated and locates in a square pyramidal geometry. The basal plane of this square pyramid is occupied by one chelating tridentate btz2- anion (O1, N1, and N5) [Cd1O1 = 2.651(2), Cd1-N1/N5 = 2.226(2)/2.218(2)A˚ ] and one N atom (N9) from one N3- anion [Cd1-N9 = 2.245(2) A˚], and one N atom (N4A) from another btz2- anion is in the apical position [Cd1-N4A = 2.325(2) A˚]. The coordination geometry of the Cd2 center can be best described as a slightly distorted octahedron completed by six N atoms from four btz2- anions (N2, N2B, N8C, and N8D) [Cd2-N, 2.469(2) and 2.301(2) A˚] and two N3- anions (N9 and N9A) [Cd2-N9 = 2.317(2) A˚]. All of the bond distances in 4 are comparable to reported values.18 The two tetrazolate rings of btz2- anion in 4 adopt the μ2- and μ3-bridging modes (Mode B, Scheme 3) to together bridge four separate CdII atoms, of which the dihedral angle (tetrazolyl-tetrazolyl) is 46.7°. The N3- anion adopts μ2-bridging (end-on, EO) mode linking Cd1 and Cd2 centers with the Cd1 3 3 3 Cd2 separation of 3.743 A˚. The btz2- and N3- anions together link the Cd1 centers to form a 1D chain (Figure 6a), and the adjacent chains are held together to give a novel 3D network structure through the connectivity of the btz2-, N3- anions and Cd2 centers (Figure 6b). In 4, through the bridging of the two N3- anions, two Cd1 and one Cd2 centers are connected into a second building unit (SBU), which is connected to adjacent eight SBUs through eight tetrazolate rings. Therefore, each SBU and tetrazolate ring in 4 can be regarded as an eight-connected node and a line in the network topology, respectively. On the basis of the above simplification, the MOF structure of 4 can be abstracted as eight-connected 3D frameworks with (424 3 64) topological network (Figure 6c). [Cd3X2(btz)2] (X = Cl(5), Br(6)). Although the synthetic methods of complexes 5 and 6 differ from that of 4, it is interesting to note that complexes 5 and 6 crystallize in the orthorhombic Pbca space group isomorphous to 4. The introduction of the Cl- anion causes the partial bond lengths and angles involving the CdII ions as well as the degree of distortion of the H2btz ligand change. Therefore, only the structure of 5 is further described here. As shown in Figure 7, the asymmetric unit of 5 contains one and a half Cd2þ, one Cl- anion, and one btz2- anion. The introduced Cl- anion substitutes for N3- anion to participate in the building of the coordination environments of the CdII centers. The Cd1 center lies in a five-coordinated square pyramidal geometry by coordinating to one chelating tridentate btz2- anion (O1, N1, and N5), one Cl- (Cl1) anion and one N atom (N4A) from another btz2- anion. The bond distances of Cd1-N [2.209(2)-2.323(2) A˚], Cd1-Cl [2.496(6) A˚] and Cd1-O [2.763(2) A˚] are comparable to reported values.18 As a comparison, the Cd-O distance of 2.763(2) A˚ (2.784(4) A˚ in 6) is obviously longer than the corresponding Cd-O bond distance of 2.651(2) A˚ in 4. The coordination geometry around the Cd2 center is a slightly distorted octahedron with the Cd2-N distances being 2.269(2) and 2.439(2) A˚, which is shorter than that of in 4. The crystallographically independent btz2- ligand connects four separate CdII atoms in Model B (Scheme 3). In comparison to that of 46.7° in 4, the dihedral angle (tetrazolyl-tetrazolyl) of the btz2- ligand in 5 is adjusted into 52.2° (51.2° in 6) in order to successfully substitute the


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Figure 8. The 3D structure of 5 viewed along the b axis. Hydrogen atoms were omitted for clarity.

Figure 9. TG curves of complexes 1-6.

pseudoalide anion (N3-) with the halide anion (Cl-/Br-). Each Cl- anion in μ2-bridging fashion bridges two crystallographically independent CdII centers with the Cd1 3 3 3 Cd2 separation being 3.828 A˚. Apparently, the crystal packing is insignificantly different in 4 and 5. In 4, the btz2- anions connect the SUBs constructed from three CdII atoms and two μ2-N3- anions to give rise to the 3D framework, whereas

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in 5, the μ2-Cl- anions substitute of μ2-N3- anions as linkers participating in the building of a 3D framework (Figure 8). XRPD Patterns and Thermal Stability Analyses. The experimental and simulated XRPD patterns of the compounds 1-6 have been made to verify the purities of the bulk crystalline samples. (See Figure S2 in Supporting Information.) The thermal stability of complexes 1-6 has been investigated and their thermogravimetric analysis (TGA) curves are shown in Figure 9. The results suggest that the 3D frameworks of complexes 1-6 exhibit high thermal stability, which may be excellent candidates for potential luminescence materials. It should be noted that the weight of 1-6 at 800 °C are nearly close to zero, which can be ascribed to the sublimation of newly formed CdX2 (X = Cl, Br) and/or CdO.19 Below the decomposition temperatures of 1-6, no weight losses observed from the curves are due to no solvent molecules in 1-6. The TG curves for complexes 1 and 2 reveal that they are stable up to about 335 °C. With further heating, a gradual weight loss occurs, which is due to the departure of organic ligands and N3- anions. As for complexes 3 and 4, a very rapid weight loss occurs at about 325 °C for 3 and 353 °C for 4, whereupon the processes should be assigned to the explosive decomposition of organic ligands and N3- anions. The framework structures of 5 and 6 maintain stability up to 375 °C, and then as the temperature increases, a series of the decomposition of organic ligands commenced. The complexes 5 and 6 exhibit the highest thermal stability among 1-6, perhaps arising from the absence of N3- anions.

Figure 10. Solid-state photoluminescence spectra of H2btz and complexes 1-6 at room temperature. Inset: detailed photoluminescence spectra of H2btz.

Luminescent Properties. The solid-state emission spectra of complexes 1-6, as well as free ligand H2btz, have been studied at room temperature, as shown in Figure 10. Excitations at 390 nm for 1, 2, 5, 6 and 356 nm for 3, 4 lead to maximum emission bands at 474 nm for 1 and 2, 473 nm for 3, 464 nm for 4, 493 nm for 5 and 494 nm for 6. Upon excitation at 290 nm, the free H2btz ligand exhibits two weak emission peaks at 358 and 472 nm, which are ascribed to the π* f π or π* f n electronic transitions. Compared with the luminescence of ligand, the emissions of complexes 1-6 are neither metal-to-ligand charge transfer (MLCT) nor ligandto-metal transfer (LMCT) in nature since the CdII ions are difficult to oxidize or reduce due to their d10 configuration.20 Thus, they may be assigned to intraligand (n-π* or π-π*) emission. Further, it can be observed that the emission spectra for complexes 1-6 exhibit light shifts with respect to the free H2btz ligand, which may be ascribed to the deprotonation of the H2btz ligand and the cocoordination effects of the inorganic ions (N3-, Cl-, Br-) and the btz2ligand to the CdII ions.21 And that, the difference of the emission peaks intensity for complexes 1-6 mainly derives from the differences of the coordination mode of btz2ligand, the coordination environments of the metal ions, and the rigidity of solid-state crystal packing. Effects of Anions. From the discussion and investigation mentioned above, the self-assembly process of complexes 1-6 exhibits the effects of the coordination ability, the size, and the steric hindrance of the inorganic ions (N3-, Cl-, Br-, I-, NO3-, ClO4-, SO42-) on the structures. A general relationship among anions, the structures, and TG/luminescent properties was demonstrated in Table 2. Complex 1 affords a 3D framework containing N3- bridges, whereas the 3D framework of complex 2 benefits from the connection of μ-N3- and μ-Cl-, and the topological fabrication of complex 3 is associated with both N3- and Br- bridges. Compared with the structure of 1, the successful substitution of Cl- in 2 and Br- in 3 for one N3- reveals that the halide anions (Cl-, Br-) have a stronger coordination ability than that of N3-, which is further supported during the synthesis and structure determination of 5. Nevertheless, the spherical halide anions (Cl-/Br-) may possess a larger size and steric hindrance than the long pseudoalide anion (N3-), resulting in the case that N3- anions in 2 and 3 were only partly replaced by Cl-/Br-. In preparing 4, different anions (SO42-, ClO4-, and I-) in CdII salts and synthetic methods were employed; however, the substitution of N3- was not observed, maybe originating from the larger steric hindrance and/or weaker coordination ability of these anions than N3-. It should be noted that for 1 and 4, NO3- and SO42-/I-/ClO4- from reagents Cd(NO3)2 in 1 and CdSO4/CdI2/Cd(ClO4)2 in 4 may play a certain

Table 2. The Effects of Anions on Frameworks and Properties of 1-6

anions in framewrok 1

N3-

2

N3Cl-

3

N3Br-

4 5 6

N3ClBr-

bridging modes of anions

coordination modes of H2btz (Scheme 3)

dihedral angle (tetrazolyl-tetrazolyl) of H2btz (°)

bond length of Cd-O (A˚)

dimensionality

λem (nm)

decomposition temperature (°C)

μ2-N3μ3-N3μ2-N3μ3-N3μ2-Clμ2-N3μ3-N3μ2-Brμ2-N3μ2-Clμ2-Br-

A

77.9

2.472(6)

3D

474

335

A

73.9

2.525(3)

3D

474

335

A

73.2

2.498(8)

3D

473

325

B B B

46.7 52.2 51.2

2.651(2) 2.763(2) 2.784(4)

3D 3D 3D

464 493 494

353 375 375

Article


important role in crystallizing and give rise to a significant structure divergence between them, although these anions did not emerge in final structures of 1 and 4. Indeed, N3- in 1 exists in two coordination modes of μ2-N3- and μ3-N3-, while the anions in 4 have only one μ2-N3- mode. Similarly, btz2- anions are used modes A in 1 and B in 4. Among complexes 1-6, different bridging anions cause the partial bond lengths and angles involving the CdII ions and the dihedral angle between tetrazolyl rings of btz2anions to be obviously changed. However, the impact on the decomposition temperature and maximum emission peaks was not all observed (Table 2). For 1-3, the maximum emission peaks and the decomposition temperature of 1 and 2 are almost identical, and slightly above those of 3. For 4-6, the large differences of these properties between 4 and 5/6 may be observed due to the different structural features with the bridging of μ2-N3- in 4, μ2-Cl- in 5, and μ2-Br- in 6. Conclusion In summary, we have successfully designed and constructed six novel CdII-based coordination polymers with a flexible ligand H2btz and rich bridging anions, such as N3- for 1 and 4, N3- and Cl- for 2, N3- and Br- for 3, Cl- for 5, and Br- for 6. The results of investigation confirm that inorganic anions play a tuning role in the construction of complexes 1-6. The effective introduction of different anions induces the coordination versatility of H2btz ligand and the coordination environmental variability of cadmium centers, and further leads to differences in the final structure. The studies of TGA and luminescence reveal that the frameworks of complexes 1-6 exhibit high stability and may be potential candidates as luminescence materials. Acknowledgment. We gratefully acknowledge the NSFC (No. 20971074), FANEDD (200732), NCET-07-0463, MOE (IRT-0927), and the NSF of Tianjin (10JCZDJC21700). Supporting Information Available: Crystallographic information files in CIF format; the disordered components in 1 and 3; experimental and simulated XRPD patterns for 1-6; tables of selected bond lengths and angles in 1-6. This material is available free of charge via the Internet at http://pubs.acs.org.

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