Zinc and Cadmium Coordination Polymers with Bis(tetrazole) Ligands

Zinc and Cadmium Coordination Polymers with Bis(tetrazole) Ligands Bearing Flexible Spacers: Synthesis, Crystal Structures, and Properties Xiao-Lan Tong, Duo-Zhi Wang, Tong-Liang Hu, Wei-Chao Song, Ying Tao, and Xian-He Bu*

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 5 2280–2286

Department of Chemistry, Nankai UniVersity, Tianjin 300071, China ReceiVed September 22, 2008; ReVised Manuscript ReceiVed January 13, 2009

ABSTRACT: Seven new metal complexes with the formulas, [Zn2(L1)2(H2O)]n (1), [Cd2(L1)Cl2(H2O)]n (2), [Cd3(L1)3(H2O)12]n (3), [Zn(L2)]n (4), [Cd2(L2)Cl2]n (5), {[Zn(L3)(H2O)4] · (H2O)2}n (6), and {[Cd3(L3)2Cl4] · (H2O)4}n (7), have been synthesized by the reactions of ZnII or CdII salts with three structurally related flexible bis(tetrazol-5-yl)alkanes, 1,2-bis(tetrazol-5-yl)ethane (H2L1), 1,3-bis(tetrazol-5-yl)propane (H2L2), and 1,4-bis(tetrazol-5-yl)butane (H2L3), and characterized by single crystal X-ray diffraction analysis, IR spectroscopy, and thermogravimetric (TG) analysis. Because of the geometrical requirements of different metal ions and the diverse coordination modes of the tetrazolate rings of flexible bis(tetrazole) ligands as well as the different reaction conditions, the obtained complexes show diverse structures from one-dimensional (1D) chain to three-dimensional (3D) coordination polymers. 1, 4, and 6 are ZnII complexes in which the ZnII ions show tetrahedral coordination geometries. 1 and 4 are 3D frameworks, while 6 is 1D chain structure. 2, 3, 5, and 7 are CdII complexes, and the CdII ions show octahedral coordination geometries. 3 is a twodimensional (2D) sheet and 2, 5, 7 are all 3D frameworks. The tetrazolate rings of the structurally related ligands adopt either µ1-tetrazolyl, µ2-tetrazolyl, µ3-tetrazolyl, or µ4-tetrazolyl modes in our obtained complexes. The fluorescent properties of these complexes have also been investigated in the solid state at room temperature. Introduction The investigation of metal-organic coordination polymers have attracted much attention for their potential applications as functional materials.1 In recent years, polydentate aromatic nitrogen heterocyclic ligands with five-membered rings (azoles) have been well used for the construction of supramolecular structures.2 Among polyazoles, imidazoles and triazoles ligands have been extensively used to construct various coordination networks with diverse topologies and interesting properties.2e,3 The 5-substituted tetrazolate group, possessing acidity close to the carboxylate group, its four nitrogen atoms being able to exhibit versatile bridging modes, has been extensively used in medicinal, coordination, and material chemistry.4-8 The formation of coordination polymers is not only influenced by the geometrical and electronic properties of metal ions but also is dependent on other factors such as the rigidity or flexibility of the ligands. The flexibility and conformation freedoms of flexible ligands can offer the possibility for the construction of an unpredictable and interesting framework with useful properties. Furthermore, increasing attention has been paid to the coordination polymers built by flexible ligands in recent years.9 To our knowledge, flexible 5-substituted tetrazolate ligands have been less investigated until now,10 and the bis(tetrazole) ligands separated by alkyl (CH2)n spacers are excellent candidates for flexible bridging ligands. Meanwhile, d10 metal ions present variable coordination numbers and geometries, and their complexes generally exhibit luminescent properties.11 Herein, we report the synthesis of seven new coordination polymers, [Zn2(L1)2(H2O)]n (1), [Cd2(L1)Cl2(H2O)]n (2), [Cd3(L1)3(H2O)12]n (3), [Zn(L2)]n (4), [Cd2(L2)Cl2]n (5), {[Zn(L3)(H2O)4] · (H2O)2}n (6), and {[Cd3(L3)2Cl4] · (H2O)2}n (7), obtained from the reactions of three structurally related flexible bis(tetrazol-5-yl)alkane ligands, 1,2-bis(tetrazol-5-yl)ethane (H2L1), 1,3-bis(tetrazol-5-yl)propane (H2L2), and 1,4-bis(tetrazol-5yl)butane (H2L3) (Chart 1) with ZnII or CdII salts. The fluorescent * Corresponding author. E-mail: [email protected]. Fax: +86-2223502458.

Chart 1. The Bis(tetrazole) Ligands Used in This Work

properties of these complexes have also been investigated in the solid state at room temperature. Experimental Section Materials and General Methods. All the starting chemicals were commercially obtained and used as received. The ligands used in this work were synthesized according to the reported procedure (Synthesis of ligands, Supporting Information).7h,12,13 Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 240C analyzer, and IR spectra were measured on a Tensor 27 OPUS (Bruker) FT-IR spectrometer with KBr pellets. Emission spectra were taken on an F-4500 spectrofluorometer. Thermal stability (TG-DTA) studies were carried out on a Dupont thermal analyzer from room temperature to 800 °C at a rate of 10 °C · min-1. Preparations of Complexes 1-7. [Zn2(L1)2(H2O)]n (1). A mixture of ZnCl2 (0.6 mmol), H2L1 (0.3 mmol), and water (12 mL) was sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 140 °C for 72 h, then cooled to room temperature at a rate of 10 °C · h-1. Colorless prism-shaped crystals of 1 were isolated and washed with water and ethanol and dried in air (ca. 20% yield based on H2L1). Anal. Calcd for C8H10Zn2N16O: C, 20.14; H, 2.11; N, 46.97. Found: C, 20.16; H, 2.19; N, 47.14. IR (KBr pellet, cm-1): 3330, 2950, 1688, 1486, 1421, 1409, 1224, 1177, 726. [Cd2(L1)Cl2(H2O)]n (2). 2 was synthesized similar to the method for 1 by using CdCl2 · 2.5H2O instead of ZnCl2. Colorless prism-shaped crystals of 2 were isolated, washed with water and ethanol, and dried in air (ca. 25% yield based on H2L1). Anal. Calcd for C4H6Cd2Cl2N8O: C, 10.05; H, 1.27; N, 23.45. Found: C, 9.81; H, 1.48; N, 23.20. IR (KBr pellet, cm-1): 3496, 3278, 2948, 1621, 1484, 1436, 1253, 1167, 728. [Cd3(L1)3(H2O)12]n (3). Complex 3 was prepared by dissolving CdCl2 · 2.5H2O (0.3 mmol) and H2L1 (0.3 mmol) in ammonia aqueous

10.1021/cg8010629 CCC: $40.75  2009 American Chemical Society Published on Web 03/12/2009

Zn and Cd Coordination Polymers

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Table 1. Crystal Data and Structure Refinement Summary for Complexes 1-7 formula formula wt crystal system space group T/K a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z D/g cm-3 µ/mm-1 F(000) measured reflns obsd reflns Ra/wRb

formula formula wt crystal system space group T/K a/Å b/Å c/Å β/deg V/Å3 Z D/g cm-3 µ/mm-1 F(000) measured reflns obsd reflns Ra/wRb a

1

2

3

4

C8H10Zn2N16O 477.03 monoclinic Pc 113(2) 10.813(2) 7.5621(15) 10.064(2) 90 112.47(3) 90 760.5(3) 2 2.083 3.199 476 7042 2515 0.0352/0.1005

C4H6Cd2Cl2N8O 477.87 monoclinic P2(1)/c 113(2) 9.885(2) 9.5013(19) 12.782(3) 90 111.29(3) 90 1118.6(4) 4 2.826 4.275 888 14127 2875 0.0199/0.0571

C12H36Cd3O12N24 1045.80 triclinic P1j 293(2) 8.576(6) 9.430(7) 11.430(8) 82.908(12) 70.653(10) 85.374(11) 864.7(11) 1 2.008 1.915 516 4284 2997 0.0566/0.1376

C5H6ZnN8 243.54 monoclinic P2(1)/c 113(2) 5.5424(11) 15.704(3) 9.5651(19) 90 96.04(30 90 827.9(3) 4 1.954 2.937 488 10148 1975 0.0244/0.0517

5

6

7

C5H6Cd2Cl2N8 475.82 orthorhombic Fmm2 293(2) 13.663(5) 37.256(8) 10.932(2) 90 5564(2) 8 2.258 3.431 3536 7910 2961 0.0299/0.0763

C6H20ZnN8O6 365.65 orthorhombic Ima2 113(2) 20.501(4) 9.790(2) 7.8907(16) 90 1583.8(6) 4 1.534 1.589 760 5720 1365 0.0297/0.0734

C12H24Cd3Cl2N16O4 864.56 monoclinic C2/m 113(2) 7.8724(16) 17.206(3) 9.2884(19) 98.10(3) 1245.6(4) 2 2.387 3.015 856 4566 1121 0.0397/0.0996

R ) Σ(|Fo| - |Fc|)/Σ|Fo|. b wR ) [Σ(|Fo|2 - |Fc|2)2/Σ(Fo2)]1/2.

solution (15 mL). Colorless plate-shaped crystals were formed after several days with the evaporation of the solvent (ca. 30% yield based on H2L1). Anal. Calcd for C12H36Cd3O12N24: C, 13.78; H, 3.47; N, 32.14. Found: C, 14.06; H, 3.73; N, 31.83. IR (KBr pellet, cm-1): 3334, 2946, 1623, 1470, 1439, 1403, 1286, 1121, 788. [Zn(L2)]n (4). Complex 4 was prepared using the method similar to that for complex 1 except that H2L1 was replaced by H2L2. Colorless block-shaped crystals were collected and dried in air (ca. 20% yield based on H2L2). Anal. Calcd for C5H6ZnN8: C, 24.66; H, 2.48; N, 46.01. Found: C, 24.75; H, 2.54; N, 45.44. IR (KBr pellet, cm-1): 3447, 2951, 2912, 2883, 1601, 1489, 1429, 1401, 1262, 1111, 766. [Cd2L2Cl2]n (5). A mixture of CdCl2 · 2.5H2O (0.6 mmol), H2L2 (0.3 mmol), and water (12 mL) was sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 160 °C under autogenous pressure for 72 h. Then the reaction mixture was slowly cooled to room temperature at a rate of 10 °C · h-1, and suitable crystals for X-ray diffraction were produced, which were collected by filtration and washed with ethanol and dried in air (ca. 20% yield based on H2L2). Anal. Calcd for C5H6Cd2Cl2N8: C, 12.67; H, 1.28; N, 23.65. Found: C, 12.81; H, 1.53; N, 23.30. IR (KBr pellet, cm-1): 3611, 3382, 2949, 2814, 1620, 1473, 1407, 1271, 1162, 732. {[Zn(L3)(H2O)4] · [H2O]2}n (6). Complex 6 was obtained by a procedure similar to that for 3 except for using ZnCl2 instead of CdCl2 · 2.5H2O and H2L3 instead of H2L1 (ca. 30% yield based on H2L3). Anal. Calcd for C6H20ZnN8O6: C, 19.71; H, 5.51; N, 30.64. Found: C, 20.02; H, 5.73; N, 30.19. IR (KBr pellet, cm-1): 3312, 2954, 2876, 1631, 1488, 1438, 1239, 1127, 727. {[Cd3(L3)2Cl2] · [(H2O)4]}n (7). Complex 7 was obtained by a procedure similar to that for 5 except for using methanol/H2O (v/v, 1:1) instead of water (12 mL), and H2L3 instead of H2L1 (ca. 20% yield based on H2L3). Anal. Calcd for C12H24Cd3Cl2N16O4: C, 16.67; H, 2.80; N, 25.92. Found: C, 17.07; H, 2.21; N, 26.33. IR (KBr pellet, cm-1): 3405, 2941, 2872, 1635, 1488, 1438, 1417, 1255, 1098, 717. X-ray Diffraction Measurements. Complexes 1-7 were collected on a Rigaku RAXIS-RAPID diffractometer (at 293 (2) K for 3 and 5,

and 113 (2) K for others) with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) by ω scan mode. The program SAINT14a was used for integration of the diffraction profiles. All the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL.14b Semiempirical absorption corrections were carried out using SADABS program.14c Metal atoms in the complexes were located from the E-maps, and other non-hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F2. The hydrogen atoms of the ligand were generated geometrically, and the hydrogen atoms of the water molecules were located from difference maps and refined with isotropic temperature factors. Crystal data and structure refinement parameter details for complexes 1-7 are given in Table 1, and the selected bond lengths and angles are given in Tables S1-S3, Supporting Information.

Results and Discussion Synthesis. Sharpless et al. reported a safe, convenient, and environmentally friendly method to prepare 5-substituted-1Htetrazole using organic cyano compounds and NaN3 in the presence of ZnII salts as catalysts in water.12,15 This procedure has shown a good generality, but in the cases of alkyl inactivated nitriles high temperatures (140-170 °C) and acid catalysis are usually required.16 Here we utilize the reaction of NaN3 and fatty nitriles as well as triethylamine hydrochloride for acid catalysis in dry toluene according to a known procedure (Synthesis of ligands, Supporting Information).13 Description of Crystal Structures. Complex 1. 1 crystallizes in Pc space group, and the asymmetric unit contains two crystallographically independent ZnII centers, two L1 ligands and one water molecule. Zn1 ion is surrounded by four N atoms

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Tong et al. Chart 2. Coordination Modes of Tetrazole Found in This Work

six CdII centers and the tetrazolate rings in L1 adopt two bridging modes (modes III and IV in Chart 2), and L1 and Cl- anions link the CdII centers to form a 3D framework (Figure 2b).

Figure 1. View of (a) the local coordination geometry around the ZnII in 1, (b) the 2D framework of 1 along the b axis, in which the two kinds of ZnII tetrahedra were highlighted.

from four independent L1 ligands. Zn2 ion is coordinated by four N atoms from three different L1, and one L1 adopts chelate coordination mode (Figure 1a). The Zn1-N and Zn2-N bond distances range from 1.961(5) to 2.050(7) Å and 1.985(7) to 2.007(6) Å, respectively, which are in the normal range.8b The cis-angles at central Zn1 and Zn2 ions fall in the range of 97.7(2)-117.2(2)° and 102.0(2)-117.0(2)°, which indicate that the tetrahedral geometries around ZnII centers are slightly distorted. The two deprotonated L1 both act as tetradentate bridging ligands, one links four ZnII centers (three Zn1 and one Zn2), while another links three ZnII centers (one Zn1 and two Zn2). The µ4 bridging modes of the deprotonated L1 ligand are similar to the bridging bis-bidentate coordination mode of succinic acid anion.17 The tetrazolate rings of L1 in 1 adopt two coordination modes (modes I and II in Chart 2), and link ZnII ions into a 3D supermolecular network (Figure 1b). Two kinds of layers are represented in the 3D supermolecular structure, one is a Zn1 layer, and another is a Zn2 layer, respectively. Complex 2. 2 crystallizes in space group P2(1)/c and forms a 3D framework. The asymmetric unit consists of two CdII centers, one L1 ligand, two bridging Cl- anions and one water molecule. The Cd1 ion is located in a slightly distorted octahedral coordination geometry formed by four N atoms and two Cl- anions. The atoms (N8E, N6F, N1D, and Cl1H) form a planar arrangement around Cd1 ion, and the other two atoms (N3 and Cl2G) occupy the apical positions. The Cd2 center also assumes a slightly distorted octahedral geometry [CdCl3N2O] (Figure 2a). The bond distances and angles around CdII centers are similar to those reported for CdII-tetrazole complexes.5c,7hThe two symmetry-related Cd2 ions are bridged by two µ3-Cl anions, and the Cd2 ions are bridged by tetrazole rings and Cl- anions to Cd1 ions. Each L1 in 2 is coordinated to

Figure 2. View of (a) the diagram of the coordination mode of CdII in 2, (b) the 2D sheet of 2 formed alternately by two layers along the b axis (one is constructed by tetrazole and CdII, while another is by CdII, O-, and Cl- anions).



Figure 3. (a) Schematic view of the coordination mode of CdII in 3, (b) the 2D network of 3 with the trinuclear units connected by ligand L1.

Complex 3. 3 is a 2D framework constructed by L1 and CdII centers associated with water molecules, and crystallizes in space group P1j. The asymmetric unit consists of two CdII centers, one and a half L1 ligands and six water molecules. Three of the water molecules are coordinated to CdII center, while the others act as free molecules in the crystal lattice. Both the CdII centers are hexacoordinated; the Cd1 ion is coordinated by six N atoms from six different tetrazolate rings of each L1, showing a slightly distorted octahedral coordination environment. Cd2 ion also locates in a distorted octahedral coordination geometry formed by three N atoms from three tetrazolate rings of L1 and three O atoms of water molecules (Figure 3a). Interestingly, the 2D structure of 3 contains trinuclear CdII cluster formed through Cd-N-N-Cd bonds, and the tetrazolate rings adopt a µ2 bridging mode (mode V in Chart 2) to link three CdII centers. The adjacent trinuclear CdII clusters are connected by ligand L1 in a and b axes to complete the overall 2D network (Figure 3b). Complex 4. 4 is a 3D framework crystallized in space group P2(1)/c, and the asymmetric unit consists of a ZnII center and half of tetrazole ligand L2. The ZnII center features a tetrahedral geometry, coordinated by four N atoms from four different L2 (Figure 4a). Two bridging modes of tetrazolate rings exist in L2, one adopts µ1,3, while another adopts µ1,4 bridging mode (mode I and II in Chart 2). The Zn-N bond distances are in the 1.9806(15)-2.0152(15) Å range, and the N-Zn-N angles are in the range of 102.73(6)-120.67(16)°, which are in the normal range.8b Furthermore, the L2 in complex 4 bridges four ZnII centers through two sides of tetrazolate rings leading to the formation of a 3D polymer (Figure 4b), and the distances of the two adjacent ZnII centers are 6.158 and 5.542 Å with the µ1,4 and µ1,3 bridging modes, respectively. Meanwhile, the 3D framework of complex 4 can be rationalized to be a (62 · 8)(6 · 82)(62 · 8 · 10) topological net

with L2 acting as T-shaped and Y-shaped nodes and ZnII acting as a tetrahedral node (Figure 4c). Complex 5. 5 crystallizes in space group Fmm2. There are two independent CdII centers (Cd1 and Cd2) in the unit cell (Figure 5a). Each Cd1 ion takes a distorted octahedral coordination geometry (CdCl2N4) with the Cd-Cl distances being 2.633 and 2.526 Å and the Cd-N distances from 2.206 to 2.397 Å, being close to those reported for CdII-tetrazole complexes.5c,7h The Cl1 anion acts as a µ3 bridging mode linking two equivalent Cd2 ions and one Cd1 ion, while Cl3 anion acts as a µ2 bridging mode linking two equivalent Cd1 ions. The Cd2 ion can also be described as a distorted octahedron with three N atoms from L2 and three bridging Cl- anions. The tetrazolate rings in 5 adopt III and VI coordination modes (Chart 2). Ligand L2 and the Cl- anions link the CdII centers to form a 3D network (Figure 5b), in which the layers in the network are bridged by the ligands and Cl- anions alternately. Complex 6. The local coordination geometry around the ZnII center in 6 adopts a slightly distorted tetrahedron. The ZnII center is coordinated to two N atoms from tetrazolate rings and two O atoms from two water molecules (Figure 6a). Each adjacent two ZnII centers are bridged by L3, which results in an infinite Zn-Zn chain along the crystallographic a axis (Figure 6b). Consequently, each L3 acts as a bidentate ligand bridging two ZnII centers, and the distance of the two adjacent ZnII centers in this infinite molecular chain is 10.403 Å. Complex 7. 7 crystallizes in space group C2/m. An analysis of the local symmetry of the two nonequivalent CdII centers reveals that Cd1 ion is coordinated to four N atoms from four tetrazolate rings and two bridging Cl- anions, while Cd2 is coordinated by four N atoms of four tetrazolate rings and two O atoms from water molecules. Both CdII centers show slightly distorted octahedral CdCl2N4 and CdN4O2 geometries, respectively (Figure 7a). The

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Figure 5. (a) Schematic view of the coordination mode of CdII in 5, (b) top of view showing the packing of 5 with the same grid layers connected by the Cl- anions and L2, alternately.

Figure 4. (a) Schematic view of the coordination environments of ZnII in 4 (H atoms omitted for clarity), (b) the 3D structure of 4 in which the ZnII is highlighted as a tetrahedron, and (c) topological view of 4 (ZnII acts as four-connected node, while L2 acts as three-connected node).

Cl- anions all adopt a µ2 bridging mode in complex 7, and the tetrazolate ring in each L3 adopts IV coordination mode (Chart 2). Consequently, ligand L3 and the Cl- anions link the CdII centers to form a 3D network, with the hexagonal configuration units viewed down the b axis (Figure 7b). Comparison of Complexes Structures. Complexes 1-3 are based on the same ligand, but their structures are quite different. In complex 1, the ZnII ion takes tetrahedral coordination geometry, while in 2 and 3, the CdII ions all take octahedral coordination geometry. This difference and the differences in coordination modes of the tetrazolate rings of H2L1 in the two complexes are probably the main reasons for the formation of 1 and 2 with quite different structures. In complex 2, there is an additional Cl- anion ligand with two bridging modes, µ3 mode to connect two Cd2 centers and one Cd1, and µ2 mode to connect one Cd1 and one Cd2, respectively, and this is different from that found in complex 1. Complexes 2 and 3, synthesized under different conditions, share the same ligands and metal center (CdII), but their structures and the coordination modes

Figure 6. (a) The coordination environment of ZnII in 6, and (b) 1D chain of 6 (the dissociative water and H atoms omitted for clarity).

of ligands are quite different. This suggests the influences of reaction conditions on the complex formations. Similar to the case of complexes 1 and 2, in complexes 4 and 5, the difference in the 3D frameworks of the two complexes is also due to the influences of the nature of the metal ions and the differences in coordination modes of the tetrazole of H2L2. Complexes 6 and 7 also share the same ligand but differ in metal ions, and the conditions for preparing them are also different. Therefore, the structure differences between them come from the coeffect of the central metal ions and synthetic conditions. The number of -CH2- groups in the alkyl spacers of the bis(tetrazole) ligands greatly affects the coordinating properties of the ligands. Increasing the number of -CH2- groups in their alkyl chains of the ligands does not lead to simple network expansion in the metal complexes, but instead, it may lead to



Figure 7. (a) Schematic view of the coordination mode of CdII in 7, (b) view down the c axis of 7 with the hexagonal configuration units.

Chart 3. (a-c) Conformations of the Ligands L1-L3 in Metal Complexes 1-5 and 7

dramatic changes in complexes structures. For example, although H2L1 and H2L2 differ only by a -CH2- group in their spacer length, they form quite different coordination polymers 1 and 4 with ZnCl2. The change of the spacer length of flexible bis(tetrazole) ligands is also the main reason for the different frameworks of complexes 2, 5, and 7. The difference in the bridging mode of the Cl- anions between complexes 7 and 5 is that the Cl- anions all adopt a µ2 bridging mode in 7, while µ2 and µ3 modes in 5. Coordination Modes and Conformations of the Ligands in Metal Complexes. The tetrazole rings of H2L1-H2L3 can act as either monodentate or multidentate ligands when coordinating to metal ions under different conditions, and seven coordination modes for tetrazolate rings have been observed in these complexes (Chart 2). The ligands are all deprotonated to form bivalent anionic derivatives. Interestingly, a chelate coordination mode of L1 exists in complex 1 because of the short length of the flexible chain between the two tetrazolate rings in L1. The flexible alkyl spacers of the ligands take various conformations in different complexes. As shown in Chart 3, the following conformations have been found in our complexes: the “Ω” conformation (Chart 3a), the “Z” conformation (Chart 3b), and the zigzag conformation (Chart 3c). The L1 adopt “Ω”

conformation in complex 1, while “Z” conformations in both complexes 2 and 3. In complex 4, the L2 ligand also adopts “Ω” conformation, but it takes zigzag conformation in complex 5. The L3 ligand in complex 7, also takes a “Ω” conformation. The L3 ligand in complex 6 is disordered, so here we cannot give its exact conformation. The centroid-centroid separations of tetrazolate rings in the ditopic ligands may be shorter when the tetrazolate rings are coordinated to metals with µ1,4 bridging modes (I, IV, and VI in Chart 2) than those with µ1,2 bridging modes (III, IV, V, and VI in Chart 2). Different conformations in each ligand may lead to different centroid-centroid separations of the tetrazolate rings within each ligand, and the centroid-centroid separations of the two tetrazolate rings in each ligands with “Ω” conformation, may be shorter than the other two conformations (Table S4, Supporting Information).18 Luminescence Properties. The photoluminescent spectra of complexes 1-7 were studied in the solid state at room temperature, and their emission spectra are depicted in Figure S1 (see Supporting Information). For complexes 1, 2 and 3, there are main emission bands (475, 485, and 496 nm for 1, 475 and 493 nm for 2, 473 and 492 nm for 3, respectively), whereas a strong emission peak at 323 nm was observed for the free ligand H2L1, which indicate that the emission peaks of 1-3 may derive from the metal-to-ligand charge transfer (MLCT) and/or ligand-to-metal charge transfer (LMCT).8b Complexes 4 and 5 exhibit main emission bands at 473 and 495 nm, and these emissions may probably be assigned to the intraligand (n-π* or π-π*) transfer since similar emission was observed at ca. 480 nm for the free ligand H2L2, which are in reasonable agreement with literature examples on such zinc or cadmium coordination polymers.11,19 It can be observed that both complexes 6 and 7 exhibit emission peaks at 472 and 488 nm for 6, while at 472 and 496 nm for 7, respectively. The emissions of complexes 6 and 7 are neither MLCT nor LMCT in nature, and

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can probably be assigned to intraligand (n-π* or π-π*) transfer since similar emissions 472 nm are observed for free H2L3.20 Thermogravimetric Analysis. In order to determine the thermal stability of the seven complexes, thermogravimetric analysis (TGA) for 1-7 was performed by heating the corresponding complexes from 20 to 800 °C in air at a rate of 10 °C · min-1. The TGA results (see Supporting Information, the TGA analysis and Figure S2) show that normally the 3D complexes are more stable than the 1D and 2D complexes. In fact, 3D MOFs normally show higher robustness than the 1D and 2D ones.21,22 Furthermore, the low density value (the calcld density: F ) 1.534 g · cm-3 for 6 and F ) 2.008 g · cm-3 for 3, respectively) also support the lower thermal stability of the 1D and 2D frameworks than the 3D ones. In summary, seven new coordination polymers 1-7 with three structurally related flexible bis(tetrazole) ligands (H2L1, H2L2, and H2L3) have been synthesized and characterized. The structural features exhibited by all the complexes show the diverse coordination modes of the tetrazolate groups. At the same time, the variation of the ligand spacers of the flexible ligands as well as the reaction conditions lead to the formation of complexes with different structures. The nature of the metal ions also has a great influence in the formation of coordination polymers. Acknowledgment. This work was supported by the NNSF of China (Nos. 20531040 and 50673043) and the Natural Science Fund of Tianjin, China (07JCZDJC00500). Supporting Information Available: Crystallographic information files (CIF), the synthesis of ligands, the TGA analysis, the selected bond lengths and angles (Tables S1-S3), the centroid-centroid separations and the angles of the two tetrazolate rings of each complex (Table S4), the emission spectra (Figure S1) and TGA plots (Figure S2) for complexes 1-7. This information is available free of charge via the Internet at http://pubs.acs.org.

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CG8010629

Zinc and Cadmium Coordination Polymers with Bis(tetrazole) Ligands

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