Synthesis, Crystal Structure and Physical Properties of a Novel Cadmium(II) Supramolecular 3D Framework 1

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1 34 卷 5 期结构化学 (JIEGOU HUAXUE) Vol. 34, No Chinese J. Struct. Chem Synthesis, Crystal Structure and Physical Properties of a Novel Cadmium(II) Supramolecular 3D Framework 1 ZHAO Wei-Xing 2 WEN Pu-Hong WANG Yan JIANG Luan FENG Guo-Dong LI Zong-Xiao (College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Shaanxi , China) ABSTRACT A new inorganic-organic hybrid cadmium(ii) complex [Cd 2 (H 1/2 C 4 BIm) 2 (CH 3 COO)] n has been prepared by a hydrothermal reaction, and characterized by fluorescence, IR, TGA, and single-crystal X-ray diffraction analysis. In its crystal structure, there are two kinds of coordinating forms for those Cd(II) ions. One is that the Cd(II) ion is coordinated by one carboxylate oxygen atom from one acetate oxygen ligand and three benzimidazole nitrogen atoms from three 2,2 -(1,4-butanediyl)bis (1H-benzimidazole) (H 2 C 4 BIm) ligands, and the other is that the Cd(II) ion is coordinated by four benzimidazole nitrogen atoms from four H 2 C 4 BIm ligands to furnish two distorted tetrahedral coordination geometries. It exhibits a beautiful structure for a 2D layer and a 3D network, with its four-connecting nodes provided by the H 2 C 4 BIm ligands. Furthermore, this complex shows intense luminescent property at room temperature. In [Cd 2 (H 2 C 2 BIm) 2 (C 9 H 6 O 4 ) 2 ] n, the H 2 C 2 BIm ligand lost the role as a bridging ligand because of the shorter alkyl chain compared with that of H 2 C 4 BIm ligand. Keywords: cadmium, hydrothermal synthesis, crystal structure, fluorescence; DOI: /j.cnki INTRODUCTION In recent years, there has been an upsurge of research in supramolecular chemistry from their potential applications in microelectronics, fluore- scence, non-linear optics, porous materials and catalysis [1, 2]. Most coordination polymers are constructed by using the appropriate organic ligand particularly that containing multidentate oxygen, nitrogen or sulfur donor to coordinate to transition metal centres [3, 4]. The designed synthesis and characterization of metal-organic frameworks con- taining channels and cavities with various sizes and shapes are of great interest, most motivated by their intriguing structural features and enormous range of application in catalysis, adsorption, separation, and electrical conductivity and magnetism. Bis(2-benzimidazoles) and some substituted bis(2-benzimidazolyl)alkanes are attractive choices for their multifunctional linking groups. Many complexes based on them have been reported because of their wide-ranging antivirus activity [5, 6], importance in selective ionexchange resins [7, 8] and coordination chemistry in the context of mimicking biological systems [9]. In addition, Cd(II)-containing coordination polymers have attracted extensive Received 17 November 2014; accepted 11 February 2015 (CCDC ) 1 This work was supported by the Natural Science Foundation of China (No ), the Scientific Research Project (No. 13JS007) from Shaanxi Key Laboratory of Phytochemistry and Key Research Project (No. ZK12016) from Baoji University of Arts and Sciences 2 Corresponding author. weixingzhao@163.com

2 2015 Vol. 34 结构化学 (JIEGOU HUAXUE)Chinese J. Struct. Chem. 787 interests of us due to the amiability to bond with different donors simultaneously, the large radius, various coordination modes and potential applications in catalysis, fluorescent materials, non-linear optics materials and so on [10]. Hence, we intend to chose H 2 C 4 BIm and carboxylate group as organic ligands combining Cd(II) ion to construct a novel cadmium(ii) supramolecular 3D framework containing asymmetric bridging benzimidazole nitrogen atoms and acetate oxygen atoms by a hydrothermal reaction. These ligands lead to 2D layer and 3D network structures and produce a new coordination polymer. Since this polymer has a structure similar to the above, it indicates that this compound has potential applications in microelectronics, fluorescence, non-linear optics, porous materials and catalysis. 2 EXPERIMENTAL 2. 1 Materials and methods All chemicals for syntheses and analyses were commercially purchased and used without further purification. The H 2 C 4 BIm ligand was prepared according to the reported method [11]. And H 2 C 2 BIm was prepared according to the reported method [12]. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400CHN Elemental Analyzer. The emission/excitation spectra were recorded on a Varian Cary Eclipse spectrometer. FT-IR spectra were recorded in the range of 400~4000 cm -1 on an Alpha Centaurt FT/IR Spectrophotometer as KBr pellets. TG analyses were performed on a Perkin- Elmer TGA instrument in flowing N 2 at a heating rate of 10 min -1. Powder X-ray diffraction (XRD) analysis of the sample was carried out on a SHIMADZU XRD-6100 X-ray diffractometer with CuKα (λ = nm) radiation Synthesis of complex [Cd 2 (H 1/2 C 4 BIm) 2 (CH 3 COO)] n (1) A mixture of Cd(CH 3 COO) 2 (69.1 mg, 0.3 mmol), H 2 C 4 BIm (59.0 mg, 0.3 mmol), NaOH (24.0 mg, 0.6 mmol), and H 2 O (14.0 ml) was sealed in a 25 ml Teflon-lined stainless steel container, which was heated at 423 K for 72 h and then cooled down to room temperature at a rate of 5 K h 1. Hereafter, some colourless needle-like crystals were obtained with the yield of 68% (based on Cd(II) salts). Anal. Calcd. (%) for C 38 H 36 Cd 2 N 8 O 2 (formula weight ): C, 52.98; H, 4.21; N, Found (%): C, 53.05; H, 4.11; N, Selected IR data (KBr, cm 1 ): 3030, 2947, 2754, 1605, 1572, 1543, 1449, 1410, 1273, 749,730, Synthesis of complex [Cd 2 (H 2 C 2 BIm) 2 (C 9 H 6 O 4 ) 2 ] n (2) A mixture of Cd(CH 3 COO) 2 (69.1 mg, 0.3 mmol), H 2 C 2 BIm (78.7 mg, 0.3 mmol), 2-carboxymethylbenzoic acid (C 9 H 6 O 4, 54.0 mg, 0.3 mmol), NaOH (24.0 mg, 0.6 mmol), and H 2 O (14 ml) was sealed in a 25 ml Teflon-lined stainless steel container, which was heated at 423 K for 72 h and then cooled down to room temperature at a rate of 5 K h 1. After that, some colourless needle-like crystals were obtained with the yield of 42.0% [13]. Anal. Calcd. (%) for C 50 H 40 Cd 2 N 8 O 8 (formula weight ): C, 54.31; H, 3.65; N, Found (%): C, 54.37; H, 3.58; N, X-ray single-crystal structural determination X-ray diffraction measurements were performed using a Bruker APXII CCD diffractometer with graphite monochromated MoKα (λ = Å) radiation at room temperature. An empirical absorption correction was performed by using the SADABS computer program. The structural model was solved using SIR-97 and completed using difference Fourier maps calculated with SHELXL- 97, which was also used for the final refinement. All programs were run under the WinGX system with atomic coordinates and anisotropical thermal parameters for all non-hydrogen atoms. The hydrogen atoms of aromatic rings were included in the structure factor calculation at the idealized positions by using a riding model. The detailed crystallographic data and structure refinement parameters for complex 1 are as follows. This compound crys-

3 ZHAO W. X. et al.: Synthesis, Crystal Structure and Physical 788 Properties of a Novel Cadmium(II) Supramolecular 3D Framework No. 5 tallizes in triclinic, P 1 space group, with a = (5), b = (5), c = (5) Å, α = (5), β = (5), γ = (5), V = (13) Å 3, Z = 2, D c = g cm -3, F(000) = 864.0, R int = and wr ref (F 2 ) = The selected bond lengths and bond angles are listed in Table 1. Those data for complex 2 are given in the supplementary material. Table 1. Selected Bond Lengths (Å) and Bond Angles ( ) for Complex 1 Bond Dist. Bond Dist. Bond Dist. Bond Dist. Cd(1) N(2A) 2.208(4) Cd(2) O(1) 2.179(3) Cd(1) N(4B) Cd(2) N(1) 2.206(4) Cd(1) N(6B) 2.209(4) Cd(2) N(5) 2.231(4) Cd(1) N(8) 2.201(4) Cd(2) N(7) 2.191(4) Angle ( ) Angle ( ) Angle ( ) Angle ( ) N(6B) Cd(1) N(8) (15) O(1) Cd(2) N(7) (13) N(2A) Cd(1) N(8) (14) O(1) Cd(2) N(1) (13) N(2A) Cd(1) N(6B) 98.83(14) O(1) Cd(2) N(5) (14) N(4B) Cd(1) N(8) 98.74(15) N(1) Cd(2) N(7) (15) N(4B) Cd(1) N(6B) (16) N(5) Cd(2) N(7) (14) N(2A) Cd(1) N(4B) (16) N(1) Cd(2) N(5) 98.57(14) C(7) C(8) C(9) 109.3(4) C(34) C(35) C(36) 115.0(4) C(25) C(32) C(33) 114.4(4) Symmetry transformations used to generate the equivalent atoms: A: 5/2 x, 1/2+y, 1 z; B: 1/2+x, 3/2 y, 1+z; C: 2 x, 2 y, z 3 RESULTS AND DISCUSSION 3. 1 Description of the crystal structures Single crystal X ray diffraction analysis reveals that complex 1 crystallizes in the triclinic space group P1 and the asymmetric unit of complex 1 consists of two Cd(II) ions and three ligands. In the crystal structure, there are two kinds of coordinating forms for those Cd(II) ions. One is that the Cd(II) ion is coordinated by one carboxylate oxygen atom from an acetate oxygen ligand and three benzimidazole nitrogen atoms from three H 2 C 4 BIm ligands, as shown in Fig. 1; and other is that the Cd(II) ion is coordinated by four benzimidazole nitrogen atoms from four H 2 C 4 BIm ligands to furnish two distorted tetrahedral coordination geometries (Fig. 2). (a) (b) Fig. 1. (a) Ball-rod structural models of 2,2 -(1,4-butanediyl)bis(1H-benzimidazole). (b) 2,2 -(1,2-ethanediyl)bis(1H-benzimidazole). Color codes: C, black; N, dark blue; H, dark red Fig. 2. Coordination environment of Cd(II) ion in complex 1 with the ellipsoids drawn at 30% probability level. Hydrogen atoms were omitted for clarity

4 2015 Vol. 34 结构化学 (JIEGOU HUAXUE)Chinese J. Struct. Chem. 789 This complex crystallizes in the triclinic space group P 1 with all unique atoms located on the general positions. Selected interatomic distances and angles are listed in Table 1. There are one Cd O and seven Cd N bonds in the crystal structure. The Cd O (Cd(2) O(2) 2.179(3) Å) and Cd N (Min: Cd(2) N(7) 2.191(4) Å, max: Cd(2) N(5) 2.231(4) Å) bond distances are all in the normal ranges [14-17]. Meanwhile, each Cd(II) ion is attached to four ligands, which can be considered as a fourconnected node, and each benzimidazole nitrogen ligand connects with two Cd(II) ions. Thus, the ligand can be considered as raise cross-curve-linker. Four benzimidazole nitrogen ligands possessing the same coordination mode connect the Cd(1). Three benzimidazole nitrogen ligands with the same coordination mode and one acetate oxygen ligand connect the Cd(2). The acetate oxygen ligand connects with one benzimidazole nitrogen ligand by a hydrogen bond N(3) H(11) O(2) (N(3) O(2) = Å, N(3) H(11) O(2) = ). The carboxyl plays the role of a bridging ligand. Those alkyl chains linking benzimidazole rings from H 2 C 4 BIm ligands form three kinds of different stretched conformations, with their distances to be Å (C(7) C(12)), Å (C(34) C(34A)) and Å (C(25) C(25A)), respectively. Besides, it can be observed from Fig. 2 that the partial superposition between the aromatic ring (C(1) C(6)) and another aromatic ring (C(19) C(24)) produces the π-π interaction of offset edge-to-face. Based on this simplification, four Cd centers are connected by sixteen bridging ligands, generating a diamondoid cage. This structure extends into a 1D chain, as shown in Fig. 3(a). One benzimidazole nitrogen ligand connects two Cd(1) and one Cd(2), and the structure extends into a 2D layer, as shown in Fig. 3(b). The 2D layer is parallel to the (0 1 0) plane, which is a 5-nodal net with point symbol of {4 2.6} 4 { }{ } 2 { }. The C(1) H(1) bond in one layer connects the neighbourhood C(34) atom in another layer, forming a hydrogen bond (bond length: Å and bond angle: ). The hydrogen-bonding interactions also contribute to the formation of a 3D network, as shown in Fig. 3(c). It is accomplished by connecting four curve ligands to the tetrahedral building. (a) (b) (c) (a) (b) (c) Fig. 3. Extension of the structure for complex 1. (a) 1D chain; (b) 2D layer; (c) 3D network. Color codes: Cd, red; N, dark blue; O, dark green

5 ZHAO W. X. et al.: Synthesis, Crystal Structure and Physical 790 Properties of a Novel Cadmium(II) Supramolecular 3D Framework No. 5 The topological analysis of complex 1 reveals that it is a typically diamondoid framework containing large adamantanoid cages with the topology Schläfli symbol, as shown in Fig. 4. Fig. 4. Diamondoid framework contained in complex 1 Fig. 5 reveals the extensions of the structure for complex 2 viewed along the a-, b-, and c-axes. In the crystal structure of 2 (triclinic, P1), the Cd(II) center is six-coordinated by two nitrogen atoms from one H 2 C 2 BIm ligand, two carboxylic oxygen atoms and two oxygen ions from two homophthalates, respectively. There are two Cd N and four Cd O bonds in the crystal structure. The Cd N (Cd(1) N(2) 2.296(2) Å and Cd(1) N(4) 2.244(2) Å) and Cd O (Min: Cd(1) O(3) 2.289(2) Å, max: Cd(1) O(4) 2.480(2) Å) bond distances are all in normal ranges [14-17]. The two homophthalate ions bridge two Cd(II) ions to form a dimeric complex molecule. These latter are linked by N H O hydrogen bonds between an H 2 C 2 BIm ligand and a neighbourhood homophthalate ligand, forming a one-dimensional chain structure. The N H O hydrogen bonds formed by other N H bonds from the H 2 C 2 BIm ligand link the chains to form a layered structure, as shown in Fig. 5 (a and b). And this layered structure has approximately rectangular apertures along [001] (Fig. 5(c)). (a) (b) (c) Fig. 5. Extensions of the structure for complex 2 viewed down the a-axis (a), b-axis (b), and c-axis (c). Color codes: Cd, red; O, dark green. Dashed lines indicate the hydrogen bonding interactions Comparing the structures of the two complexes, we found that different roles of bis(2-benzimidazolyl)alkanes ligand are a main factor causing the structural difference. In complex 1, the H 2 C 4 BIm ligand plays a role in bridging the Cd(II) ions, and establishes a 13-membered and an 18-membered large rings. Different from that of complex 1, the homophthalate ligand, which substituted the H 2 C 2 BIm ligand, plays a role in connecting the Cd(II) ions in complex 2. The H 2 C 2 BIm ligand lost the role as a bridging ligand because of its shorter alkyl chain compared with that of H 2 C 4 BIm ligand, and it establishes a 7-membered large ring by forming two Cd N bonds.

6 2015 Vol. 34 结构化学 (JIEGOU HUAXUE)Chinese J. Struct. Chem Luminescent properties The photoluminescent properties of free H 2 C 4 BIm ligand and complex 1 were examined in the solid state at room temperature. As shown in Fig. 6, the main emission peak of free H 2 C 4 BIm ligand is at 308 nm (λ ex = 272 nm), which can be assigned to π* n and π* π transitions of the intraligands. Complex 1 shows the maximum emission at 421 nm (λ ex = 411 nm). In comparison with that of free ligand, the maximum emission wavelength of complex 1 shifted slightly to red, probably due to the intraligand charge transitions [18, 19]. The fluorescence emission of complex 1 might be attributed to the ligand-to-metal charge transfer (LMCT) [20, 21]. This observation indicates that complex 1 may be a good candidate for potential photoactive materials. Intensity /a.u. (A) 308 nm (B) 421 nm Wavelength /nm Fig. 6. Solid-state excitation spectra of free 2,2 -(1,4-butanediyl) bis(1h-benzimidazole) ligand (A) and complex 1 (B) 3. 3 Thermal properties The TGA analysis of complex 1 was carried out under N 2 atmosphere from 44 to 800 with a heating rate of 10 min 1. The TG curve of the complex is shown in Fig. 7. It is found that the framework collapsed in the temperature range of 360 ~535, corresponding to the release of organic components. The remaining weight of 29.91% corresponds to the percentage (calcd %) of Cd and O components, indicating that the final product may be CdO. 120 TG 100 Weight /% Temperature / Fig. 7. TG curve of complex Powder X-ray diffraction properties The simulated and experimental powder XRD patterns of complex 1 are shown in Fig. 8. The simulated and experimental powder XRD patterns are in good agreement with each other, indicating the phase purity of the product. The differences in intensity may be due to the preferred orientation of the powder sample IR spectrum properties A typical FT-IR spectrum of complex 1 sample is shown in Fig. 9. In this spectrum, the sharp bands located at 1543 and 1273 cm 1 can be assigned to the ν(c=n) and ν(c N) respectively, the bands located at 1572 and 1449 cm 1 to the framework vibration of benzimidazole ring, the broad band at 3030 cm 1 to ν(c H) of the ring, the two bands at 2947 and 2754 cm 1 to ν(c H) of methylene, and that at 730 cm 1 to the framework vibration of -(CH 2 ) 4 -, respec- tively [22]. The two bands at 1605 and 1410 cm 1 are assigned to the asymmetric (-COO - as) and symmetric (-COO - s) stretches of the carboxylate group, respec- tively. The wavenumber difference between ν(-coo - as) and ν(-coo - s) bands (1950 cm 1 ) indi- cates that the complex shows a typical coordination

7 ZHAO W. X. et al.: Synthesis, Crystal Structure and Physical 792 Properties of a Novel Cadmium(II) Supramolecular 3D Framework No. 5 mode for the carboxyl group, which is consistent with the single-crystal X-ray analysis [5, 23]. Intensity /a.u. (B) (A) θ /degree Fig. 8. Powder XRD patterns of complex 1 (A: simulated; B: experimental) Transmittance /a.u Wavenumber /cm Fig. 9. IR spectrum of complex 1 4 CONCLUSION In conclusion, by selecting H 2 C 4 BIm ligand and synthetic strategy, we have hydrothermally synthesized a novel 3D network interpenetrating diamondoid array coordination polymer and it contains large adamantanoid cages. Complex 1 exhibits in- tense fluorescence at about 421 nm in the solid state. Therefore, the photoluminescent analysis re- sult indicates this complex may be a good can- didate for fluorescence material. It can be found that the H 2 C 2 BIm ligand in complex 2 lost the role as a bridging ligand because of its shorter alkyl chain compared with that of the H 2 C 4 BIm ligand. REFERENCES (1) Rao, C. N. R.; Natarajan, S.; Vaidhyanathan, R. Metal carboxylates with open architectures. Angew. Chem. Int. Ed. 2004, 43, (2) Ockwig, N. W.; Olaf, D.; Michael, O. K.; Yaghi, O. M. Reticular chemistry: occurrence and taxonomy of nets and grammar for the design of frameworks. Acc. Chem. Res. 2005, 38, (3) Tranchemontagne, D. J.; José, L. M. C.; Michael, O. K.; Yaghi, O. M. Secondary building units, nets and bonding in the chemistry of metal-organic frameworks. Chem. Soc. Rev. 2009, 38, (4) Yoon, S.; Lippard, S. J. Water affects the stereochemistry and dioxygen reactivity of carboxylate-rich diiron(ii) models for the diiron centers in dioxygen-dependent non-heme enzymes. J. Am. Chem. Soc. 2005, 127, (5) Ashiry, K. O.; Zhao, Y. H.; Shao, K. Z.; Su, Z. M.; Fu, Y. M.; Hao, X. R.; Wang, Y. Synthesis and characterization of three Cu(II), Zn(II), Cd(II) supramolecular complexes bridged by biphenyl 2,2 -dicarboxylate. Solid State Sci. 2007, 9, (6) Nandini, M. S.; Vidyanand, K. R.; Vinayak, B. M. Oxomolybdenum(V) complexes of bis-(2-benzimidazolyl) alkanes. Trans. Metal Chem. 1994, 19, (7) Hoorn, H. J.; Joode, P. D.; Dijkstra, D. J.; Driessen, W. L.; Kooijman H.; Veldman, N.; Spek, A. L.; Reedijk, J. Metal-binding affinity of a series of bis-benzimidazolesimmobilised on silica. J. Mater. Chem. 1997, 7,

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