Anion-Induced Coordination Versatility of 1H-1,2,4-Triazole-3-thiol

CRYSTAL GROWTH & DESIGN

Anion-Induced Coordination Versatility of 1H-1,2,4-Triazole-3-thiol (HtrzSH) Affording a New Hybrid System of Cadmium(II) Polymers: Synthesis, Structure, and Luminescent Properties

2008 VOL. 8, NO. 7 2562–2573

Rui-Bo Zhang,†,‡ Zhao-Ji Li,† Jian-Kai Cheng,† Ye-Yan Qin,† Jian Zhang,† and Yuan-Gen Yao*,† The State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China ReceiVed February 22, 2008; ReVised Manuscript ReceiVed March 28, 2008

ABSTRACT: A new hybrid system of cadmium(II) polymers has been synthesized by using 1H-1,2,4-triazole-3-thiol (HtrzSH) and cadmium salts, affording six novel luminescent complexes, that is, [Cd(H2trzS)2Cl2] (1), [Cd(HtrzS)Br] (2), [Cd(HtrzS)I] (3), [Cd2(HtrzS)2(SO4)] (4), [Cd(H2trzS)(H2Edta)] · (H2O) (5), and [Cd(trzS)2] (6). In these materials, owing to the effective inducement of inorganic or organic anions, coordination versatility of the HtrzSH ligand manifests as many as six bridging modes (type A-F), and five of them (type B-F) are first reported. The structure of complex 1 is a one-dimensional chain constructed from the neutral monodentate and bidentate H2trzS ligand (type A-B), whereas two-dimensional complexes 2-3 and the three-dimensional complex 4 possess the deprotonated tridentate to quadridentate HtrzS ligands (type C-E). Zero-dimensional complex 5 and peristyle-like three-dimensional complex 6 are synthesized by means of organic anions, and in the latter complex a bideprotonated quadridentate trzS ligand is presented (type F). All complexes exhibit strong photoluminescence with fluorescent emissions varying from blue to orange in the solid state. Some structure related red or blue emission shifts compared to the free ligand are studied, as well as several advantageous structural factors to obtain longer fluorescence lifetime. X-ray powder diffraction and thermal studies are used to investigate the bulk properties of these compounds. Introduction In recent decades, the study of metal coordination polymers has witnessed tremendous growth as an attractive interface between synthetic chemistry and material science, which significantly boosts the understanding of the relationship between molecular structure and material function.1 Although a variety of hybrid polymers have been successfully synthesized with intriguing architectures, topologies, and physical properties,2 rational control in the assembly of these materials with desired properties still remains a distant prospect in synthetic chemistry. In particular, it is a great challenge to prepare advanced luminescent materials with predictable structures and properties through the combination of organic ligands (as building blocks) with metal ions (as coordination centers). Therefore, the skillful selection of organic ligands containing appropriate coordination sites and suitable metal ions bearing the right coordination geometries is pivotal for designing novel fluorescent complexes.3 Because the cadmium(II) ions possess variable coordination geometries and their complexes manifest extraordinary fluorescence properties as well as second- or thirdorder nonlinear optical (NLO) properties,4 much research interest has been focused on this metal ion involved inorganic-organic hybrid materials. On the other hand, the organic ligands also play a crucial role in the control of fluorescent characteristics of metal-organic complexes, through tuning their structural dimensionalities and stereochemistry with different coordination sites. As a ligand with multiple coordination sites, 1,2,4-triazole has gained more interest since it can bridge different metal centers to afford coordination polymers that exhibit extraordinary structural * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +86-591-83714946. Tel: +86-591-83711523. † Fujian Institute of Research on the Structure of Matter. ‡ Graduate School of the Chinese Academy of Sciences.

Scheme 1

diversity and facile accessibility of functionalized new magnetic materials.5–9 However, cadmium(II) complexes constructed from this ligand often display a weak or even absent photoluminescence;7 hence a mercapto-substituted 1,2,4-triazole ligand, 1H1,2,4-triazole-3-thiol (HtrzSH), was selected instead because its electron-donating thiol group can obviously enhance the conjugation degree of main chromosphere, that is, the aromatic fivemembered heterocycle. Although the derivatives of the HtrzSH ligand, especially amino-, aryl-, or alkyl-substituted ones as well as their complexes, have been well studied for their biological activities,10 the coordination chemistry and luminescence enhancement of the HtrzSH ligand itself is seldom investigated. To the best of our knowledge, only one X-ray structure of its complex (iron complex) has been reported previously,11 none with cadmium(II) reported thus far. It is noteworthy that the HtrzSH ligand can present a tautomeric equilibrium in solution, which enables two possible configurations of the sulfide group as thione and thiol (Scheme 1), whereas after the coordination reaction, only the thione form exists in solid-state complexes. This tautomerism may engender the proton transfer on the triazole ring, and consequently endow the HtrzSH ligand with variable coordination sites. In addition, it is known that inorganic or organic counteranions can effectively induce the coordination versatility of the organic ligand, and thus influence the architectures and physical properties of the coordination polymers.12 By means of this inducement, the assembly of structure-predictable luminescent complexes can be accomplished via deliberate selection of appropriate inorganic or organic counteranions in a metal-organic hybrid system. Nevertheless, systematic investigation of anion-

10.1021/cg800199x CCC: $40.75  2008 American Chemical Society Published on Web 06/03/2008

New Hybrid System of Cadmium(II) Polymers Scheme 2

induced coordination versatility of the organic ligands in construction of complexes is rarely known to the best of our knowledge. Herein, we report a comprehensive study of a new hybrid system of cadmium(II) polymers, which have been prepared by means of an anion-induced HtrzSH ligand. Six novel materials, namely, [Cd(H2trzS)2Cl2] (1), [Cd(HtrzS)Br] (2), [Cd(HtrzS)I] (3), [Cd2(HtrzS)2(SO4)] (4), [Cd(H2trzS)(H2Edta)] · (H2O) (5), (H4Edta is the ethylenediaminetetraacetate), and [Cd(trzS)2] (6) are synthesized and structurally characterized. In these complexes, with the effective inducement of different anions, the coordination versatility of the HtrzSH ligand manifests as many as six bridging modes (Scheme 2, type A-F), and five of them are first reported (type B-F). Moreover, the structure tuned fluorescent properties of these complexes have also been investigated as well as other physical properties such as X-ray diffraction (XRD), thermal stability, etc. Experimental Section General Considerations. Commercially available reagents were used as received without further purification. All syntheses were carried out in 23 mL Teflon-lined Parr autoclaves under autogenous pressure. The elemental analyses were performed on an EA1110 CHNS-0 CE elemental analyzer. The IR spectroscopy was recorded on a PECO (U.S.A.) SpectrumOne spectrophotometer with pressed KBr pellets. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer. The fluorescence spectra were measured on polycrystalline or powder samples at room temperature using an Edinburgh FLS920 TCSPC fluorescence spectrophotometer. The phase purity and crystallinity of each product were checked by powder XRD using a Rigaku Dmax2500 diffractometer with Cu KR radiation (λ ) 1.54056 Å). A step size of 0.05° and counting time of 1.2 s/step were applied in a 2θ range of 5.00-55.00°. The observed and simulated powder XRD patterns of all compounds are displayed in Figure S1, Supporting Information. Synthesis of [Cd(H2trzS)2Cl2] (1). A mixture of HtrzSH (0.10 g, 1.0 mmol) and CdCl2 · 2.5H2O (0.114 g, 0.5 mmol) were placed in a 23 mL Teflon liner; 10 mL of water was then added. The resulting mixture was stirred for 3 min and was then sealed in a Parr autoclave. The autoclave was then placed in a programmable furnace and heated to 160 °C. The temperature was held for 3 days, and then the reactant mixture was cooled at a rate of 0.5 °C min-1 to room temperature. Upon standing and evaporation of the resulting colorless solution under ambient conditions for two months, prism-shaped crystals of 1 were obtained as the product. Yield: 78% (based on Cd(II) salts). Anal. Calcd (%) for C4H6CdCl2N6S2: C, 12.46; H, 1.57; N, 21.80; S, 16.63. Found: C, 12.45; H, 1.61; N, 21.78; S, 16.65%. IR (solid KBr pellet, v/cm-1) for complex 1: 3125 (s), 1659 (m), 1556 (m), 1479 (m), 1400 (m),

Crystal Growth & Design, Vol. 8, No. 7, 2008 2563 1400 (m), 1336 (w), 1245 (w), 1192 (m), 1047 (w), 967 (m), 953 (m), 875 (w), 784 (w), 706 (w). Synthesis of [Cd(HtrzS)Br] (2). A mixture of HtrzSH (0.15 g, 1.5 mmol) and CdBr2 · 4H2O (0.172 g, 0.5 mmol) were placed in a 23 mL Teflon liner; 2 mL of ethanol and 8 mL of water were then added. The resulting mixture was stirred for 5 min and was then sealed in a Parr autoclave. The autoclave was then placed in a programmable furnace and heated to 150 °C. The temperature was held for 2 days, and then the reactant mixture was cooled at a rate of 0.5 °C min-1 to form orange column crystals of 2. Yield: 86% (based on Cd(II) salts). Anal. Calcd (%) for C2H2BrCdN3S: C, 8.21; H, 0.69; N, 14.37; S, 10.96. Found: C, 8.22; H, 0.67; N, 14.40; S, 10.95%. IR (solid KBr pellet, v/cm-1) for complex 2: 3127 (s), 1628 (m), 1517 (m), 1502 (m), 1476 (s), 1404 (m), 1351 (m), 1312 (s), 1286 (w), 1199 (m), 1140 (w), 1060 (m), 1001 (w), 864 (m), 753 (m), 702 (m), 652 (m). Synthesis of [Cd(HtrzS)I] (3). A mixture of HtrzSH (0.2 g, 2.0 mmol) and CdI2 (0.732 g, 2.0 mmol) were placed in a 23 mL Teflon liner; 11 mL of water was then added. The resulting mixture was stirred briefly and was then sealed in a Parr autoclave. The autoclave was then placed in a programmable furnace and heated to 160 °C. The temperature was held for 4 days, and then the reactant mixture was cooled at a rate of 1.0 °C min-1 to form light deep yellow prism crystals of 3. Yield: 73% (based on Cd(II) salts). Anal. Calcd (%) for C2H2CdIN3S: C, 7.08; H, 0.59; N, 12.38; S, 9.45. Found: C, 7.05; H, 0.61; N, 12.40; S, 9.43%. IR (solid KBr pellet, v/cm-1) for complex 3: 3128 (s), 1627 (m), 1513 (m), 1450 (s), 1400 (s), 1345 (w), 1310 (m), 1196 (m), 1131 (w), 1051 (w), 983 (w), 858 (m), 725 (m), 698 (m). Synthesis of [Cd2(HtrzS)2(SO4)] (4). A mixture of HtrzSH (0.1 g, 1.0 mmol) and 3CdSO4 · 8H2O (0.257 g, 0.33 mmol) were placed in a 23 mL Teflon liner; 1 mL of ethanol and 6 mL of water were then added. The resulting mixture was stirred for 3 min and was then sealed in a Parr autoclave. The autoclave was then placed in a programmable furnace and heated to 170 °C. The temperature was held for 2 days, and then the reactant mixture was cooled at a rate of 1.0 °C min-1 to form colorless prism crystals of 4. Yield: 88% (based on Cd(II) salts). Anal. Calcd (%) for C4H4Cd2N6O4S3: C, 9.22; H, 0.77; N, 16.13; S, 18.46. Found: C, 9.22; H, 0.76; N, 16.12; S, 18.49%. IR (solid KBr pellet, v/cm-1) for complex 4: 3128(s), 1630 (m), 1533 (w), 1516 (w), 1474 (m), 1402 (m), 1367 (w), 1319 (w), 1292 (w), 1277 (m), 1111 (broad, m), 986 (m), 865 (w), 715 (w), 702 (w). Synthesis of [Cd(H2trzS)(H2Edta)] · (H2O) (5). A mixture of HtrzSH (0.1 g, 1.0 mmol), Cd(NO3)2 · 4H2O (0.154 g, 0.5 mmol), and disodium EDTA (0.186 g, 0.5 mmol) were placed in a 23 mL Teflon liner; 10 mL of water was then added. The resulting mixture was stirred for 10 min and was then sealed in a Parr autoclave. The autoclave was then placed in a programmable furnace and heated to 150 °C. The temperature was held for 5 days, and then the reactant mixture was cooled at a rate of 0.5 °C min-1 to form colorless block crystals of 5. Yield: 93% (based on Cd(II) salts). Anal. Calcd (%) for C12H19CdN5O9S: C, 27.62; H, 3.67; N, 13.42; S, 6.41. Found: C, 27.61; H, 3.69; N, 13.43; S, 6.43%. IR (solid KBr pellet, v/cm-1) for complex 5: 3457 (s), 3150 (s), 1622 (broad, s), 1495 (m), 1435 (m), 1406 (m), 1332 (m), 1297 (m), 1270 (m), 1236 (m), 1149 (w), 1125 (w), 1104 (m), 999 (w), 978 (w), 957 (w), 918 (m), 847 (m), 728(m), 705 (w), 683 (w), 642 (m). Synthesis of [Cd(trzS)2] (6). A mixture of HtrzSH (0.1 g, 1.0 mmol) and Cd(NO3)2 · 4H2O (0.154 g, 0.5 mmol), and sodium propionate (or sodium acetate, or sodium butyrate, 1.0 mmol), were placed in a 23 mL Teflon liner; 1 mL of ethanol and 5 mL of water were then added. The resulting mixture was stirred for 20 min and was then sealed in a Parr autoclave. The autoclave was then placed in a programmable furnace and heated to 170 °C. The temperature was held for 2 days, and then the reactant mixture was cooled at a rate of 1.0 °C min-1 to form light yellow block crystals of 6. Yield: 82% (based on Cd(II) salts). Anal. Calcd (%) for C2HCdN3S: C, 11.36; H, 0.48; N, 19.87; S, 15.16. Found: C, 11.38; H, 0.47; N, 19.87; S, 15.14%. IR (solid KBr pellet, v/cm-1) for complex 6: 3132 (s), 1630 (s), 1475 (m), 1401 (s), 1368 (m), 1304 (m), 1273 (m), 1160 (m), 1077 (w), 1026 (w), 890 (w), 717 (w), 657 (w). X-ray Crystallography. Suitable single crystals of 1-6 were carefully selected under an optical microscope and glued to thin glass fibers. Structural measurements were performed on a computercontrolled Siemens Smart CCD diffractometer with graphite-monochromated Mo KR radiation (λMo KR ) 0.71073 Å) at T ) 293.15 K.

2564 Crystal Growth & Design, Vol. 8, No. 7, 2008

Zhang et al.

Table 1. Crystal Data and Structure Refinements for Compounds 1-6 param

1

2

3

4

5

6

formula fw temp (K) cryst syst space group a (Å) b (Å) c (Å) R (Å) β (Å) γ (Å) V (Å3) Z Dcalcd (g cm-3) µ (mm-1) GOF R1a (I > 2σ(I)) wR2a (all data)

C4H6CdCl2N6S2 385.60 293.15 triclinic P1j 7.700(3) 8.298(3) 10.059(3) 106.701(2) 96.240(2) 114.743(3) 539.4(3) 2 2.374 2.880 1.071 0.0402 0.1037

C2H2BrCdN3S 292.45 293.15 orthorhombic Pbcn 10.877(3) 7.937(3) 14.299(5) 90 90 90 1234.4(7) 8 3.147 10.237 1.025 0.0256 0.0697

C2H2CdIN3S 339.45 293.15 orthorhombic Pbcn 11.056(3) 8.260(2) 14.512(4) 90 90 90 1325.3(6) 8 3.402 8.164 1.012 0.0211 0.0567

C4H4Cd2N6O4S3 521.16 293.15 monoclinic P21/n 7.0693(19) 12.385(3) 13.337(4) 90 101.550(5) 90 1144.1(5) 4 3.026 4.284 1.057 0.0266 0.1024

C12H19CdN5O9S 521.80 293.15 monoclinic P21/c 15.598(6) 7.684(3) 15.932(6) 90 104.432(6) 90 1849.3(12) 4 1.874 1.352 0.971 0.0192 0.0521

C2HCdN3S 211.54 293.15 monoclinic P21/c 6.1204(17) 11.225(2) 6.811(2) 90 114.250(13) 90 426.64(19) 4 3.293 5.433 1.078 0.0232 0.0636

a

R1 ) Σ||Fo| - |Fc||/Σ|Fo|; wR2 ) {Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]}1/2. w ) 1/[σ2(Fo2) + (aP)2 + bP], where P ) [max(Fo2,0) + 2Fc2]/3 for all data.

Absorption corrections were made using the SADABS program.13 The structures were solved using the direct method and refined by fullmatrix least-squares methods on F2 by using the SHELX-97 program package.14 All non-hydrogen atoms were refined anisotropically, and hydrogen atoms attached to carbon and nitrogen atoms were fixed at their ideal positions. The water hydrogen atoms were located from difference maps and refined with isotropic temperature factors. Crystal data and refinement of 1-6 are summarized in Table 1. Selected bond lengths and angles are given in Table S1, Supporting Information.

Results and Discussion Synthesis and Infrared Spectroscopy. By exploitation of the hydrothermal reactions, compounds 2-6 are synthesized in good yields using the 1H-1,2,4-triazole-3-thiol (HtrzSH) ligand and suitable cadmium(II) salts. Compound 1 is obtained by evaporating the resulting colorless solution after the hydrothermal process under ambient conditions for two months. In this work, it is noted that there are two rather important determinants in formation of these products: one is the pH value, and the other is the cadmium salts. The pH values should be kept in the range of 6.5-8.0 to obtain these materials with high yield and good crystal quality. When the pH value is adjusted beyond this range, that is, in a strong alkaline or acid environment, no product is obtained. The selection of suitable Cd(II) salts is the second crucial factor in synthesizing these complexes. Inorganic anions (Cl-, Br-, I-, SO42-) of the corresponding cadmium salts are commonly incorporated into the final coordination polymers, with the exception of NO3-, which is unsuccessfully introduced into the final product using Cd(NO3)2 · 4H2O. Owing to this characteristic, Cd(NO3)2 · 4H2O is selected as the reactant to afford Cd2+ ions exclusively, and organic counteranions would be thus brought in the final products (in compound 5), or induce the bideprotonation of the HtrzSH ligand (in compound 6). The infrared spectra of all complexes show similarities due to the presence in them of the “thioamide” groups,15 which normally gives rise to four different bands in the region of 1570-1395 (band I), 1420-1260 (band II), 1140-940 (band III), and 800-700 cm-1 (band IV). Bands I and II are mainly caused by CdN stretching vibrations and N-H deformation vibrations, but bands III and IV are caused by a significant CdS content.16 In addition, each band also contains considerable contribution from the strongly coupled CdS stretching mode.15 Therefore, it is difficult to identify these bands unambiguously and the assignment would be multiple alternative. Nevertheless,

all complexes of this study have corresponding absorption bands of the IR spectra in I-IV regions, yet with somewhat slight fluctuation. Furthermore, a medium to strong intensity band in the range of 3125-3150 cm-1 associated with N-H stretching mode is observed for all compounds as well. Cl- Ions Induced Type A and B H2trzS Ligand in [Cd(H2trzS)2Cl2] (1). The structure of chloride phase [Cd(H2trzS)2Cl2] (1), as shown in Figure 1a,b, exhibits a onedimensional (1D) structure. The coordination geometry of each cadmium site is a slight distorted octahedral {CdNCl3S2} site (Figure 1a), with the equatorial plane defined by two µ2-chloride ligands, a terminal chlorine atom and a thione sulfur atom from a µ2-H2trzS ligand, and the axial sites occupied by another thione sulfur atom from a η1-H2trzS ligand and a N1 nitrogen donor. It is noted that both the monodentate η1-H2trzS and bidentate µ2-H2trzS ligand are incorporated in this compound adopting a neutral form, and the latter’s coordination mode; that is, the N1, thione S-bridging mode, (type B in Scheme 2) has never been reported before this contribution. When viewed along the [1j10] axis, it is shown that these cadmium sites are linked through the µ2-H2trzS ligands and µ2-chlorine atoms in the cisoid disposition, affording a 1D {Cd(H2trzS)2Cl2}n chain propagating along the [110] direction (Figure 1b). In addition, π-π stacking and hydrogen bonding interaction play an important role in forming this structure. In a single chain {Cd(H2trzS)2Cl2}n, five-membered heterocycles of a [Cd(H2trzS)2Cl2] unit are aligned parallel to those of another unit, in an alternately inverse face-to-face π-π stacking mode, as shown in Figure 1b. The plane-to-plane distance of this π-π stacking is 3.062 Å, indicating a relatively strong π-π stacking interaction of this compound. Furthermore, there are two kinds of hydrogen bonding interaction in this complex when viewed along the [1 1 0] direction, as shown in Figure 1c. Protonated N4 nitrogen sites of the H2trzS ligands are hydrogen-bonded to the chlorine atoms of another adjacent chain, via medium to strong N-H · · · Cl hydrogen bonds (N1-H2B · · · Cl1: 3.346(3) Å; N4-H4B · · · Cl2: 3.142(3) Å, black dotted line in Figure 1c). Another kind of hydrogen bonding interaction between adjacent chains is built from the thione sulfur atoms of the µ2-H2trzS ligands bonding to the N1 nitrogen atoms of the η1-H2trzS ligands (N2-H1A · · · S2: 3.417(3) Å, red dotted line in Figure 1c). Therefore, the molecular packing of 1 can be regarded as a three-dimensional (3D) supramolecular network interdigitated by 1D chains, which are enhanced by intramolecular π-π

New Hybrid System of Cadmium(II) Polymers

Crystal Growth & Design, Vol. 8, No. 7, 2008 2565

Figure 1. (a) ORTEP drawing of [Cd(H2trzS)2Cl2] (1) showing the atom-labeling system and 50% thermal ellipsoids. (b) View of 1D chain structure of 1 along the [1j 1 0] direction and the π-π stacking interaction. (c) View of the supramolecular structure of 1 showing two kinds of hydrogen bonds (in black and red dotted lines).

stacking and bridged by intermolecular hydrogen-bonding interactions (Figure 1c). Br- and I- Ions Induced Type C HtrzS Ligand in [Cd(HtrzS)Br] (2) and [Cd(HtrzS)I] (3). In order to enhance the steric hindrance of charge balancing anions, Cl- was then changed into Br-, obtaining a bromide phase [Cd(HtrzS)Br] (2). As shown in Figure 2a, each cadmium site of 2 possesses a distorted square pyramidal {CdN2Br2S}, which is defined by a thione sulfur atom, two nitrogen atoms (N1, N2) from a µ3HtrzS ligand and two µ2-bromide ligands. Through the linkage of these ligands, complex 2 manifests a two-dimensional (2D) corrugated layer structure in the ab plane, as illustrated in Figure 2b. In this structure, the five-membered heterocycles of µ3-HtrzS ligands are aligned approximately parallel to those of adjacent ones (with the dihedral angle of 12.605°), affording an inverse

face-to-face intramolecular π-π stacking mode, and the distance between π-π stacking parallel planes is 3.401 Å. Different from the monodentate and bidentate ligands of 1, HtrzS in 2 adopts a deprotonated tridentate chelating form, that is, the N1, N2, thione S-bridging mode (type C in Scheme 2) which is first reported. In addition, because of the stronger steric hindrance of Br- ions and much longer Cd-Br bonds (2.6228(7), 2.8191(8) Å), only one inorganic anion ligand (µ2-Br-) can be accommodated between adjacent cadmium sites for 2 rather than two ligands (µ2-Cl-) accommodated for 1, and thus the structure dimension changes from 1D to 2D. Another result of this enhanced steric hindrance is that two kinds of hydrogen bonding interaction in 1 have been reduced to only one in 2, that is, the thione sulfur atoms bonding to the N4 nitrogen sites of another molecular unit (N3-H1A · · · S1: 3.362(3) Å). When packing


Zhang et al.

Figure 2. (a) ORTEP drawing of [Cd(HtrzS)Br] (2) showing the atom-labeling system and 50% thermal ellipsoids. (b) View of 2D layer structure of 2 and the π-π stacking interaction. (c) View of the structure packing of 2 showing hydrogen bonding interaction (in black dotted line).

molecules along the c axis, it is presented that parallel 2D corrugated layers are linked by these intermolecular hydrogen bonds without any offset, affording a 3D supramolecular network, as shown in Figure 2c.

When the I- ion, a ligand with a much bigger size and stronger steric hindrance, was used instead of the Br- ion, the iodide phase [Cd(HtrzS)I] (3) was obtained (Figure 3), and this compound is isomorphous with that of [Cd(HtrzS)Br] (2).



Figure 3. ORTEP drawing of [Cd(HtrzS)I] (3) showing the atomlabeling system and 50% thermal ellipsoids.

Similar to 2, compound 3 also adopts a 2D corrugated layer structure in the ab plane through the linkage of µ3-HtrzS ligands and µ2-iodide ligands, and parallel layers are connected by intermolecular hydrogen bonds affording a 3D network (Figures S12 and S13, Supporting Information). The first difference between 3 and 2 is that the former has I- ions with much stronger steric hindrance and longer Cd-I bond (2.7934(5), 3.0287(7) Å), which leads somewhat to expansion of the structure, exhibiting an increase in volume (about 6.86%) compared to 2. Secondly, due to the expansion effect, the intramolecular π-π stacking interaction of 3 is further weakened, with the plane-to-plane distance of 3.510 Å longer than that of 2. In addition, the intermolecular hydrogen bonding interaction between parallel 2D layers of 3 is also decreased, because the hydrogen bond length of 3.454(3) Å (N1-H1A · · · S1) in 3 is longer than that of 3.362(3) Å in 2. Therefore, the supramolecular 3D network of 3 is less tightly connected than is 2. It is noteworthy that by increasing the steric hindrance of halogen anions (Cl- to I-), the coordination mode of HtrzSH ligand is changed from the neutral η1-H2trzS monodentate (type A) or µ2-H2trzS bidentate (type B) to the deprotonated µ3-HtrzS tridentate chelating (type C). And the structure dimension of these compounds is also varied from 1D to 2D correspondingly. So we believe that halogen anions Cl-, Br-, and I- can effectively induce the coordination versatility of the HtrzSH ligand. SO42- Ions Induced Type D and E HtrzS Ligand in [Cd2(HtrzS)2(SO4)] (4). To further investigate the inducement of inorganic anions, sulfate ions were chosen instead of halogen anions, obtaining a complex 3D sulfate phase [Cd2(HtrzS)2 (SO4)] (4). The HtrzS ligands in this compound adopt two unprecedented types of chelating mode: one is a deprotonated tridentate N1, N4, thione S-bridging mode (type D), and the other is a deprotonated quadridentate mode, with N1, N2 nitrogen atoms and a µ2-thione sulfur atom serving as coordination sites (type E). As shown in Figure 4a, this complex exhibits two distinct cadmium geometries. The first (Cd1) is constructed from a slightly distorted trigonal bipyramidal {CdN3OS} site, with the equatorial plane defined by N1 and N2 nitrogen atoms from the µ4-HtrzS ligand and a N4 nitrogen atom from the µ3HtrzS ligand, and the axial sites occupied by a µ2-thione sulfur atom and a sulfate oxygen donor. The coordination geometry of the second cadmium site (Cd2) is a distorted square pyramidal {CdNO2S2}, with the basal plane occupied by two sulfate oxygen atoms, a N1 nitrogen atom and the aforementioned µ2thione sulfur atom, and another thione sulfur site in the apical position completing the coordination geometry. It is noted that these two distinct geometries assemble two kinds of 1D chain substructures, respectively. As shown in Figure 4b, HtrzS ligands bridge the trigonal bipyramidal Cd1 sites constructing a 1D chain

Figure 4. (a) ORTEP drawing of [Cd2(HtrzS)2(SO4)] (4) showing the atom-labeling system and 50% thermal ellipsoids. (b) View of 1D chain substructure built from Cd1 sites. (c) View of 1D chain substructure built from Cd2 sites. (d) View of 3D structure of 4.

propagating along the a axis, and its secondary building unit (SBU) can be described as a binuclear {Cd2(HtrzS)4(SO4)2}. Similarly, the 1D chain built from the Cd2 sites also extends along the a axis, whereas its SBU is represented as a mononuclear {Cd(HtrzS)2(SO4)2} (Figure 4c). When observed along the a axis (Figure 4d), we note that these two parallel 1D chains are alternately connected through the µ3-HtrzS ligands, the µ2thione sulfur sites, and the sulfate groups which adopt a η2, µ3 coordination mode. Therefore, an interlocked 3D network is produced.


Figure 5. (a) ORTEP drawing of [Cd(H2trzS)(H2Edta)] · (H2O) (5) showing the atom-labeling system and 50% thermal ellipsoids. (b) View of the structure packing of 5 showing π-π stacking interaction and hydrogen bonds (in black dotted lines).

H2Edta2- Ions Induced Type A H2trzS Ligand in [Cd(H2trzS)(H2Edta)] · (H2O) (5). When inorganic anions are changed into organic ones, that is, H2Edta2- ions, the zerodimensional (discrete) Edta phase [Cd(H2trzS)(H2Edta)] · (H2O) (5) is obtained, as shown in Figure 5a. The coordination polyhedron around the cadmium center is a distorted trigonal prismatic {CdN2O3S}, with one basal plane defined by two nitrogen donors and a carboxylate oxygen atom from the H2Edta ligand, and the other basal plane is defined by a thione sulfur atom from the H2trzS ligand, a carboxylate oxygen atom and a carbonylic oxygen atom from the H2Edta ligand. Analogous to compound 1, the H2trzS ligand in this structure also adopts a neutral form without any deprotonated nitrogen sites (type A). Furthermore, as shown in Figure 5b, the five-membered heterocycles of the H2trzS ligands from different molecular units are parallel to each other in an alternately inverse face-to-face π-π stacking mode. The plane-to-plane distance of this π-π stacking is 3.226 Å, indicating a medium strong π-π stacking interaction of 5. In addition, rich hydrogen bonding interactions also play an important role in forming its supramolecular structure. As shown in Figure 5b, N4 nitrogen atoms of the H2trzS ligands are hydrogen-bonded to carboxylate oxygen atoms (N1-H1A · · · O8: 2.7354(19) Å) and carbonylic oxygen atoms(O6-H6A · · · O8:2.5914(19)Å;O6-H6A · · · O7:3.0568(19)

Zhang et al.

Å) from another molecule unit. In addition, oxygen donors of lattice H2O molecules are also hydrogen-bonded to carboxylate oxygen atoms on adjacent [Cd(H2trzS)(H2Edta)] units (O1WH1WA · · · O1: 3.237(4) Å; O1W-H1WB · · · O2: 3.239(4) Å). Therefore, these intermolecular hydrogen bonding interactions bridge discrete molecule units of 5 into a 3D supramolecular network, as shown in Figure 5b. Uncoordinated Organic Anions Induced Type F trzS Ligand in [Cd(trzS)2] (6). When we used other organic anions such as MeCO2-, EtCO2-, BuCO2-, etc., instead of H2Edta2-, all products obtained are exclusively the 3D complex [Cd(trzS)2] (6); namely, these organic anions cannot serve as charge balancing components in assembly of this series of complexes and cadmium cations are only bridged and charge balanced by the trzS ligands. As shown in Figure 6a, in this structure each cadmium site adopts a peculiar tetrahedral {CdN3S} geometry (slightly distorted), which has never been reported in cadmium coordination geometries to our knowledge. Interestingly, in this tetrahedral geometry, Cd-N and Cd-S bond lengths are shorter than those of other compounds in this study (except a Cd-S bond of 2), and as a result of this, a tighter connected structure of 6 has been constructed. It is noteworthy that the organic ligand of 6 exhibits an unprecedented µ4-trzS form, that is, the N1, N2, N4, thione S-bridging mode (type F), which is bideprotonated and enables all nitrogen sites to participate in the coordination. The substructure of 6 can be described as a 2D corrugated layer propagating along the b axis, as shown in Figure 6b. Parallel 2D layers are further linked to each other through the thione sulfur atoms from the µ4-trzS ligands, affording a peristyle-like 3D network with Cd-S bonds as “columns”, as illustrated in Figure 6c. Through exploring the structure of 6, we note that although organic anions are not incorporated in the HtrzSH-as-ligand complexes as easily as inorganic anions, they can effectively induce the bideprotonation of the HtrzSH ligand to provide a novel quadridentate chelating trzS ligand. If we consider HtrzSH ligands and cadmium centers as 4-connected nodes, as shown in Figure 7, the 3D network of 6 exhibits a non-interpenetrated sra net (or SrAl2 net) with Schla¨fli symbol 42.63.8 and vertex symbol 4.6.4.6.6.82. Since it contains 4-membered rings, it is obviously distorted from the ideal tetrahedral symmetry. Although those nets are common in molecular chemistry, the non-interpenetrated sra net constructed from binary metal-organic complexes is seldom reported.17 Anion-induced coordination versatility of the HtrzSH ligand in compounds 1-6 is summarized in Table 2. We conclude that inorganic anions Cl-, Br-, I- as well as a particular organic anion H2Edta2- can virtually induce low dimensional structures (from 0D to 2D), within which the HtrzSH ligand adopts monodentate to tridentate chelation. However, SO42- and other organic anions which cannot be incorporated in the structure may afford high dimensional structures and induce the HtrzSH ligand offering more coordination sites, in other words, adopting the deprotonated or bideprotonated quadridentate mode. Thermogravimetric Analysis (TGA). In this study, the thermal stabilities of all six compounds were analyzed on crystalline samples by TGA/DTA from 40 to 900 °C at a rate of 10 °C min-1, under an air atmosphere with a flowing rate of 20 mL min-1. TGA profiles for compounds 1-6 are shown in Figure S2, Supporting Information. Compound 1 exhibits a sharp weight loss of decomposing organic components between 190 and 375 °C (exptl, 45.67%; calcd, 45.71%). And the infrared spectrum of the thermolysis product of 1 at 350 °C manifests bands at ca. 2200 and 670 cm-1, which are associated with



Figure 6. (a) ORTEP drawing of [Cd(trzS)2] (6) showing the atom-labeling system and 50% thermal ellipsoids. (b) View of 2D corrugated layer substructure of 6. (c) View of peristyle-like 3D network of 6.

Figure 7. Schematic representation of the 4-connected sra net in 6 with ligands and cadmium atoms represented by red and teal spheres, respectively.

ν(C≡N) and ν(Cd-C) respectively, corresponding to the formation of Cd(CN)2.7 Then a gradual weight loss process is

observed between 375 and 730 °C, associated with the sublimation of newly formed CdCl2 (exptl, 32.74%; calcd, 32.96%), remaining the finally product Cd(CN)2. The low thermal stability of 1 may be ascribed to its low dimensional structure (1D chains) and loose supramolecular connectivity constructed from hydrogen bonds with low energy. Otherwise, other two halide phase 2 and 3 with their supramolecular networks built from 2D structures show a higher thermal stability. The thermogravimetric curve of 2 is unchanged up to 300 °C. Between 300 and 700 °C, a sharp weight loss of combusting organic components is observed simultaneously with the sublimation of newly formed CdBr2, remaining Cd(CN)2 like 1. The total weight loss of this process is 76.77%, in good accordance with the calculated value 77.35%. Being isomorphous to 2, compound 3 also manifests a comparatively high thermal stability with its TG curve unchanged from room temperature to 285 °C, and then a steady weight loss of decomposing organic components and sublimating newly formed CdI2 is observed without a welldefined plateau between 285 and 730 °C (exptl, 77.30%; calcd, 75.77%). We note that the thermal stability of 3 is little lower


Zhang et al.

Table 2. Coordination Diversity of the HtrzSH Ligand (L) for 1-6 complex 1

anion Cl

chemical form of L

-

neutral

-

deprotonated deprotonated deprotonated deprotonated neutral bideprotonated

2 3 4

Br ISO42-

5 6

H2Edtanull

bridging modes and no. of L (type label) thione S-bridging mode ×1 N1, thione S-bridging mode ×1 N1, N2, thione S-bridging mode ×1 N1, N2, thione S-bridging mode ×1 N1, N4, thione S-bridging mode ×1 N1, N2, µ2-thione S-bridging mode ×1 thione S-bridging mode ×1 N1, N2, N4, thione S-bridging mode ×1

than that of 2, and this can be presumably attributed to their slight structure variation. As for compound 4, a multiple and tight-connected 3D structure endows it with a high thermal stability. There is very little weight loss up to 340 °C, indicating the framework of this compound is quite robust. Then a rapid weight loss occurs between 340 and 480 °C associated with the combustion of HtrzS ligands (exptl, 28.47%; calcd, 28.44%). During this process, new phase Cd(CN)2 is formed similarly to aforementioned compounds. From 480 to 675 °C, the second sharp weight loss is observed, corresponding to the loss of SO4 (exptl, 15.80%; calcd, 15.36%). The TG curve of 5 illustrates that no large weight loss is observed up to 230 °C, manifesting a comparatively low thermal stability of this material. Then a weight loss between 230 and 265 °C corresponds to the dehydration process of the crystallization water (exptl, 3.53%; calcd, 3.45%). Above 265 °C, significant weight loss is observed continually up to 725 °C, indicating the complete decomposition of organic ligands (exptl, 71.62%; calcd, 71.94%). It is noteworthy that 6 displays the highest thermal stability among all compounds of this study, with its TG profile unchanged up to 425 °C, indicating the framework of 6 is quite sturdy and thermally stable at least to 425 °C. From this temperature to 700 °C, a steady and rapid weight loss of decomposing organic components is observed, with the weight loss value of 30.35% in good agreement with the calculated value 30.77%. In addition, the thermodiffraction profile of 6 (Figure S3, Supporting Information) shows that the structure framework of this compound is generally unchanged up to 550 °C, despite its initially decomposing organic components at 425 °C. This observation manifests that partial loss of organic ligands does not surely lead to the structure collapse for 6, and its structure integrity can preserve far beyond the initial decomposition temperature. The remarkably high thermal stability of 6 may be presumably ascribed to its peculiar molecular structure. In detail, each cadmium site of 6 adopts an unprecedented tetrahedral {CdN3S} geometry, in which Cd-N and Cd-S bond lengths are much shorter than other compounds of this study, thus more bond energy stored in this tight-connected 3D framework, which needs much energy to break down. Photoluminescence Properties. Recently, polymeric Zn(II) and Cd(II) complexes with their metal cations adopting d10 configuration have been intensively investigated for attractive fluorescence properties and potential applications as new luminescent materials;18 for example, some zinc complexes have been used as organic light-emitting diodes (OLEDs).19 It is known that the synthesis of desired luminescent materials is still a challenge in this area; however, the method of appropriately incorporating conjugated organic ligands and anionic components into a coordination polymeric system is undoubtedly an efficient way to adjust luminescent properties, such as fluorescent excitation/emission wavelength, intensity, lifetime, and so forth. In the present work, photoluminescence properties

chelating no. of L

structural dimension

monodentate bidentate tridentate tridentate tridentate quadridentate monodentate quadridentate

one

(A) (B) (C) (C) (D) (E) (A) (F)

two two three zero three

Table 3. Summary of Solid-State Photoluminescent Data for of the HtrzSH Ligand and Compounds 1-6 λem (nm)

ligand/complex

λex (nm)

HtrzSH

356

443

1

336

419

2

371

453, 616

3

414

622 (495)

4

380

458

5

367

431

6

380

467

a

τ, ns; (weight)

χ2

τ1 ) 1.04 (0.63) τ2 ) 2.12 (0.37) τ1 ) 111.7 (0.82) τ2 ) 41.95 (0.18) τ1a ) 1.48 (0.47) τ2a ) 3.00 (0.53) τ1b ) 1.75 (0.58) τ2b ) 4.22 (0.42) τ1 ) 3.13 (0.61) τ2 ) 1.28 (0.33) τ3 ) 10.68 (0.06) τ1 ) 2.99 (0.65) τ2 ) 1.15 (0.35) τ1 ) 0.92 (0.86) τ2 ) 5.02 (0.14) τ1 ) 0.85 (0.70) τ2 ) 3.06 (0.30)

1.149 1.006 1.035a 1.646b 1.148 0.900 0.982 1.630

Measured at λem ) 453 nm. b Measured at λem ) 616 nm.

of compounds 1-6 have been explored at room temperature in the solid state, since all six compounds are virtually insoluble in most common solvents such as ethanol, acetone, chloroform, benzene, water, etc. Strong fluorescence emission is observed from all six compounds as well as the free HtrzSH ligand, and photoluminescent characteristics of them are summarized in Table 3. At the ambient temperature, the free HtrzSH ligand exhibits a broad emission with maximum at 443 nm upon excitation at 356 nm in the solid state (Figure S4, Supporting Information). The main chromosphere of this ligand is the aromatic fivemembered hetero ring (triazole ring), and its conjugation degree is further enhanced by the electron-donating thiol group.20 This conjugation enhancement results in that the maximum emission wavelength of the HtrzSH ligand is red-shifted compared with that of the 1,2,4-triazole,7 and its photoluminescence is assigned as originating from the π-π* transitions. Excitation and emission spectra for compounds 1-3 are shown in Figure 8. Compound 1 exhibits a strong violet fluorescent emission band at 419 nm with excitation at 336 nm. In this compound, the highest occupied molecular orbital (HOMO) is presumably associated with the π-bonding orbital from the five-membered hetero ring of the HtrzSH ligand, which is increasingly conjugated by the electron-donating thiol group. And the lowest unoccupied molecular orbital (LUMO) may be dominated by the ligand π* character rather than Cd-Cl σ* orbital, because heteroatoms in an aromatic ligand can effectively decrease the π and π* orbital energies, and consequently metal atoms may lack a significant contribution to the HOMO and LUMO.21 Therefore, the luminescence behavior of 1 may be assigned as the intraligand π-π* transitions, and this assignment is also consistent with its broad and unstructured emission profile.20 It is noteworthy that compound 1 exhibits a blue-shift of 24 nm for its emission band in contrast to the free ligand, which can be tentatively ascribed to the protonation of


Figure 8. Excitation and emission spectra for 1-3 in the solid state at room temperature.

the five-membered hetero ring when the thiol group (in free ligand) is tautomerized to the thione group (in complex 1, affording H2trzS), and this protonation may raise the HOMOLUMO energy gap of 1. Compound 2 displays a broad blue fluorescent emission with maximum at 453 nm upon excitation at 371 nm, and this emission may be assigned as the intraligand π-π* transitions similar to 1. A red-shift of approximately 10 nm compared to the free ligand is observed from the emission spectrum of 2,


and this may be because the deprotonated HtrzS ligand in 2 is more negatively charged, and its HOMO energy is thus increased compared to that of the neutral free ligand, whereas the LUMO energy is virtually unchanged, resulting in a reduction in the HOMO-LUMO gap.21 In addition, besides the blue emission at 453 nm, an unusual and broad orange emission at 616 nm is observed as well. Although the plotted spectrum is normalized, the latter emission is brighter than the former, and significantly (7×) brighter than the violet emission of 1 as well. This large red-shift (173 nm) may suggest that the ligandto-metal charge transfer (LMCT) may be evoked in 2, resulting in the rearrangement of energy levels.18e,22 On the other hand, compound 2 adopts a rigid 2D layer structure which can offer more advantage of energy transfer than the 1D chain structure of 1, and as a result, a stronger emission with longer wavelength is observed for 2. Though compound 3 is isomorphous with 2, their fluorescent bands are obviously different. A broad and very strong orange to red emission with maximum at 622 nm is observed as well as another weak green emission at 495 nm, upon excitation at 414 nm. And we note that the former emission is much brighter (5×) than the orange emission of 2 and the brightest in all six complexes, though the plotted emission bands are normalized. This quite strong emission is assigned as the ligand-to-metal charge transfer (LMCT) and promoted by the rigid 2D layer structure of 3 as discussed before, and the weak green emission is attributed to the intraligand π-π* transitions similar to 1. These two emission bands are red-shifted comparing to 2, and this may be assumably ascribed to the higher ground electronic state of 3. In detail, the better overlap orientation of fivemembered hetero rings can further stabilize the ground electronic state of the complex, and thus decrease of its transition energy.23 This kind of orientation in 3 is not as good as that of 2 (the dihedral angle of 12.65° for 2, 13.39° for 3), leading to a reduction in the HOMO-LUMO gap for 3. Compounds 4 and 6 manifest broad blue fluorescent emission bands at 458 nm and 467 nm, with the excitation at 380 nm, as shown in Figure 9. Their luminescence behaviors may be likewise assigned as the intraligand π-π* transitions. It is revealed that complexes with 3D structure of this study may not much influence their photoluminescence properties. And the red-shifts of their emission bands compared to the free ligand can be also ascribed to the reduction of the HOMO-LUMO gap caused by the ligand deprotonation like 2. A difference that should be noted is the organic ligands of 6 adopt a more negatively charged form, which decrease the HOMO-LUMO energy gap further. Therefore, the fluorescent emission of compound 6 shifts to the red region. For compound 5, a broad and strong violet emission is shown with maximum at 431 nm (λex ) 367 nm), which may be assigned as the intraligand π-π* transitions. And the blue shift of the emission band (12 nm) in contrast to the free ligand may be associated with the increased HOMO-LUMO energy gap as previously discussed. The fluorescence lifetime τ values of complexes 1-6 and the free ligand are on the nanosecond timescale at room temperature, as shown in Table 4 and Figure S6-S11, Supporting Information. The τ values of these compounds are fitted to two components by a biexponential decay curve except that of 3, which is fitted to three components triexponentially. The fluorescence lifetime of 1 is the longest among all complexes, and this may be attributed to its rigid 1D chain structure, within which µ2-H2trzS and µ2-chlorine ligands tighten the skeleton and furnish the strongest interligand π-π stacking interaction in this series of complexes, thus affording much weaker vibrations.21,24


Zhang et al.

comprise various inorganic or organic anions such as Cl-, Br-, I-, SO42-, and H2Edta-. Owing to the effective inducement of different anions, the coordination versatility of the 1H-1,2,4triazole-3-thiol (HtrzSH) ligand in these materials manifests as many as six kinds of bridging modes (type A-F) varying from monodentate to quadridentate chelation, and five of them (type B-F) are first reported. In addition, the coordination polyhedra variability of cadmium sites is revealed as well, such as tetrahedron (for 6), trigonal bipyramid (for 4), square pyramid (for 2, 3, 4), trigonal prism (for 5), and octahedron (for 1). Consequently, by means of anion control, this new hybrid system manifests a quite rich structural chemistry, and this will make a good contribution to design and synthesize new molecular materials. Furthermore, structure-related physical properties of these complexes are also studied. All compounds exhibit strong photoluminescence with fluorescent emissions varying from blue to orange in the solid state at room temperature, which may be differently assigned to the intraligand π-π* transitions or ligand-to-metal charge transfer. Some structure-related red or blue shifts of their emission bands compared to the free ligand are discussed, as well as several advantageous structural factors of obtaining longer fluorescence lifetime and stronger emission. Most compounds of this study display a medium to high thermal stability, and compound 6 shows the highest one with its structure framework generally unchanged up to 550 °C. We believe that these properties make this new hybrid system of complexes useful as thermally stable luminescent materials. On the basis of this work, further syntheses, structures and properties studies of the HtrzSH ligand with other metals are also under way in our laboratory. Acknowledgment. This work was supported by the State Key Basic Research and Development Plan of China (2007CB815302), the Chinese Academy of Sciences (KJCX2-YW-M05), the NSF (E0620005) of Fujian Province, the Major Special Foundation of Fujian Province (2005HZ1027, 2005HZ01-1), the Fund of Fujian Key Laboratory of Nanomaterials (2006L2005), and the Knowledge Innovation Program of the Chinese Academy of Sciences. Supporting Information Available: Crystallographic data in CIF format, X-ray powder diffraction patterns, thermogravimetric profiles for compounds 1-6, thermodiffraction pattern for 6, excitation and emission spectra for the free HtrzSH ligand, emission decay traces with experimental fits for 1-6, selected bond lengths and angles for 1-6, and additional structure illustrations for 3. This material is available free of charge via the Internet at http://pubs.acs.org.

References Figure 9. Excitation and emission spectra for compounds 4-6 in the solid state at room temperature.

From this work, it is revealed that in this hybrid system the complex adopting 1D rigid structure may have the advantage of a longer fluorescence lifetime than that adopting 2D or 3D structure. And Br- and I- ions incorporated complexes will induce the ligand-to-metal charge transfer (LMCT), producing much stronger fluorescence emission than that associated with intraligand π-π* transitions. Conclusions We have investigated a new hybrid system of six luminescent cadmium(II) coordination polymers, and these obtained materials

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