35 卷 4 期 结构化学 (JIEGOU HUAXUE) Vol. 35, No Chinese J. Struct. Chem

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1 35 卷 4 期 结构化学 (JIEGOU HUAXUE) Vol. 35, No Chinese J. Struct. Chem Synthesis, Crystal Structure and Theoretical Calculations of a Zinc(II) Coordination Polymer Assembled by Pyrazine-2,3-dicarboxylic Acid and Bis(imidazol) Ligands 1 JIANG Da-Yu a SUI Wei a LI Xiu-Mei b LIU Bo a WANG Qing-Wei a 2 PAN Ya-Ru b a (Key Laboratory of Preparation and Applications of Environmental Friendly Materials (Jilin Normal University), Ministry of Education, Siping, Jilin , China) b (Faculty of Chemistry, Tonghua Normal University, Tonghua , China) ABSTRACT A new metal-organic coordination polymer [Zn(pzdc)(mbix)] n nh 2 O (H 2 pzdc = pyrazine-2,3- dicarboxylic acid, mbix = 1,3-bis(imidazol-1-ylmethyl)-benzene) 1 has been hydrothermally synthesized and structurally characterized by elemental analysis, IR, TG, fluorescence spectrum and single-crystal X-ray diffraction. Yellow crystals crystallize in monoclinic system, space group P2 1 /n with a = (6), b = (10), c = (11) Å, β = (10)º, V = (2) Å 3, C 20 H 18 N 6 O 5 Zn, M r = , D c = g/cm 3, F(000) = 1000, Z = 4, μ(mokα) = mm -1, the final R = and wr = for 3445 observed reflections (I > 2σ(I)). The structure of 1 exhibits a one-dimensional chain-like structure. In addition, natural bond orbital (NBO) analysis was performed by the PBE0/LANL2DZ method in Gaussian 03 Program. The calculation results show obvious covalent interaction between the coordinated atoms and Zn(II) ion. Keywords: Zn(II) complex, pyrazine-2,3-dicarboxylic acid, crystal structure, natural bond orbital; DOI: /j.cnki INTRODUCTION Recently, studies on the synthesis of coordination polymers (CPs) have received much attention in coordination chemistry because of their interesting molecular topologies and tremendous potential applications in catalysis, molecular selection, nonlinear optics, ion exchange and microelectronics [1-5]. Generally speaking, the structural diversity of such crystalline materials is dependent on many factors, such as metal ion, templating agents, metal-ligand ratio, ph value, counteranion, and number of coordination sites provided by organic ligands [6, 7]. Among the strategies, the rational selection of organic ligands or coligands according to their length, rigidity, and functional groups is important for the assembly of structural controllable CPs, and a great deal of significant work has been done by using this strategy [8]. Usually, the organic ligands with bent backbones, such as V-shaped, triangular, quadrangular, and so on, are excellent candidates for building highly high-connected, interpenetrating, or helical coordination frameworks due to their bent backbones and versatile bridging fashions [9, 10]. Besides, the carboxylate groups are good hydrogen-bond acceptors as well as donors, depending upon the Received 15 September 2015; accepted 9 December 2015 (CCDC ) 1 The project was supported by the Science and Technology Development Project of Jilin Provincial Science & Technology Department ( ) and the Science and Technology Research Projects of the Education Office of Jilin Province (No ) 2 Corresponding author. Wang Qing-Wei, born in Tel: , wqw611223@163.com

2 JIANG D. Y. et al.: Synthesis, Crystal Structure and Theoretical Calculations of a Zinc(II) 506 Coordination Polymer Assembled by Pyrazine-2,3-dicarboxylic Acid and Bis(imidazol) Ligands No. 4 degree of deprotonation. Among them, polycarboxylic acids, such as 1,2,4,5-benzenetetracarboxylic acid, 3,3,4,4 -benzophenone tetracarboxylic acid, pyrazine-2,3-dicarboxylic and 4,4 -oxydibenzoic acid, are paid much attention due to their rich coordination modes [11]. Apart from the carboxylate linkers, bis(imidazole) bridging ligands with different length and flexibility, like 1,3-bis(imidazol-1-ylmethyl)benzene,1,4-bis(imidazol-1-ylmethyl)benzene and 1,4-bis(imidazol-1-yl)-butane, are frequently used in the assembly process of CPs as bridging linkers [12]. Thus, these considerations inspired us to explore new coordination architectures with pyrazine-2,3- dicarboxylic acid and (bis)imidazole bridging linkers. In this paper, we report the synthesis and characterization of a new CPs, namely, [Zn(pzdc)(mbix)] n nh 2 O (1), which exhibits a one-dimensional chainlike structure and is different from our previous report for the pyrazine-2,3-dicarboxylate-bridged complexes [13, 14]. 2 EXPERIMENTAL 2. 1 General procedures All reagents were purchased commercially and used without further purification. Elemental analyses (C, H and N) were measured on a Perkin-Elmer 2400 CHN Elemental Analyzer. IR spectrum was recorded in the range of 4000~400 cm -1 on an Alpha Centaurt FTIR Spectrophotometer using a KBr pellet. TG studies were performed on a Perkin- Elmer TGA7 analyzer. The fluorescent studies were carried out on a computer-controlled JY Fluoro- Max-3 spectrometer at room temperature Synthesis of [Zn(pzdc)(mbix)] n nh 2 O A mixture of Zn(OAc) 2 2H 2 O (0.2 mmol, g), H 2 pzdc (0.4 mmol, 0.068g), mbix (0.2 mmol, g) and H 2 O (18 ml) was sealed in a 30 ml Teflon-lined autoclave under autogenous pressure at 140 for 5 days. After cooling to room temperature, yellow block crystals were collected by filtration and washed with distilled water in 45% yield (based on Zn). Anal. Calcd. (%) for C 20 H 18 N 6 O 5 Zn: C, 49.24; H, 3.72; N, Found (%): C, 48.97; H, 3.48; N, IR (KBr, cm -1 ): 3516w, 3459w, 3125m, 2984w, 1629s, 1561w, 1457w, 1385m, 1351s, 1289w, 1234w, 1202m, 1169w, 1108w, 1104s, 1065w, 1029w, 984w, 954m, 924w, 867w, 837m, 786w, 746w, 728m, 680w, 655w, 646w, 634w, 580w, 497w, 449w Structure determination A single crystal of the title compound with dimensions of 0.352mm 0.235mm 0.128mm was mounted on a Bruker CCD diffractometer equipped with a graphite-monochromatic MoKα (λ = Å) radiation using an ω scan mode at 293(2) K. In the range of 3.74<2θ<51.98 o, a total of reflections were collected and 3980 were independent with R int = , of which 3445 were observed with I > 2σ(I). The correction for Lp factors was applied. The structure was solved by direct methods with SHELXS-97 program [15] and refined by full-matrix least-squares techniques on F 2 with SHELXL-97 [16]. All non-hydrogen atoms were refined anisotropically and hydrogen atoms isotropically. The H atoms of coordinated water molecules were located from difference Fourier syntheses and the hydrogen atoms of organic ligands were generated geometrically. The final R = and wr = (w = 1/[σ 2 (F 2 o ) + (0.0415P) P], where P = (F o + 2F 2 c )/3). S = 1.034, (Δρ) max = 0.401, (Δρ) min = e/å 3 and (Δ/σ) max = The selected important bond parameters are given in Table 1. 3 RESULTS AND DISCUSSION 3. 1 IR spectrum The COO is coordinated with its asymmetric and symmetric stretching appearing at 1629 cm 1 (v(oco) assym ) and 1385 cm 1 (v(oco) sym ) [17], respectively. The Δν (ν(oco) assym ν(oco) sym ) is 244 cm 1 (> 200), showing the presence of monodentate linkage of carboxylates in the dianions. Thus the carboxylates coordinate to the metal as monodentate ligands via the carboxylic carboxylate

3 2016 Vol. 35 结构化学 (JIEGOU HUAXUE)Chinese J. Struct. Chem. 507 groups [18]. The absence of characteristic bands around 1700 cm -1 in compound 1 attributed to the protonated carboxylic group indicates the complete deprotonation of pzdc ligand upon reaction with Zn ions [19]. In addition, X-ray diffraction analysis further indicates the monodentate coordination manners of carboxylate groups and the deprotonation of pzdc ligands. Table 1. Selected Bond Lengths (Å) and Bond Angles ( ) for 1 Bond Dist. Bond Dist. Bond Dist. Zn(1) N(1) (14) Zn(1) N(4A) (14) Zn(1) O(2) (12) Zn(1) O(4B) (12) Angle ( o ) Angle ( o ) Angle ( o ) O(2) Zn(1) O(4B) (5) O(2) Zn(1) N(4A) (6) O(2) Zn(1) N(1) (6) O(4B) Zn(1) N(4A) 99.41(6) O(4A) Zn(1) N(1) (6) N(1) Zn(1) N(4A) (6) Symmetry codes: A: 2 x, y, 1 z; B: 2 x, 1 y, 1 z 3. 2 Description of the structure Complex 1 crystallizes in the monoclinic system, space group P2 1 /n and features a one-dimensional chainlike structure. The coordination environment of Zn(II) in 1 is shown in Fig. 1. There are one Zn(II) ion, one pzdc ligand, one mbix ligand and one crystal water molecule in the asymmetric unit. Each Zn(II) ion is four-coordinated by two carboxylate oxygen atoms (O(2), O(4B)) from two different pzdc ligands and two nitrogen donors (N(1) and N(4A)) from two flexible mbix molecules to furnish a distorted tetrahedral coordination architecture. The bond distances of Zn O and Zn N in complex 1 fall respectively in the (12) ~ (12) and (14)~2.0177(14) Å regions, which are in the normal ranges, and the coordination angles around the Zn atom vary from 99.41(6)º to (6)º. Fig. 1. Coordination environment of the Zn(II) center in 1. Symmetry codes: (A) 2 x, y, 1 z; (B) 2 x, 1 y, 1 z In 1, the mbix ligand adopts cis-conformation with a dihedral angle between the two imidazole rings of 86.22º, while each pzdc ligand adopts a μ 2 coordination mode. The mbix and pzdc ligands bridge Zn II atoms into an infinite 1D double chain, which alternately contains 14- and 28-membered rings with the Zn Zn distances of and Å, respectively (Fig. 2), and is different from our reported complex [Zn(pzdc)(bix) 0.5 (H 2 O)] [14] n. Further investigation of the crystal packing of compound 1 suggests that there are two kinds of O H O and C H O hydrogen bonding interactions between carboxylate oxygen atom, crystal water molecule and carbon atoms of mbix and pzdc ligands in complex 1 (Table 2). Moreover, there are π-π interactions in complex 1 between benzene ring of mbix ligand. The centroid-to- centroid distance between adjacent rings is (12) Å for

4 JIANG D. Y. et al.: Synthesis, Crystal Structure and Theoretical Calculations of a Zinc(II) 508 Coordination Polymer Assembled by Pyrazine-2,3-dicarboxylic Acid and Bis(imidazol) Ligands No. 4 C(11)C(12)C(13)C(14)C(15)C(16) and C(11 )C(12 )C(13 )C(14 )C(15 )C(16 ) benzene rings (3 x, y, 1 z). The perpendicular distance is (8) Å. Therefore, through hydrogen bonds and π-π interactions, the one-dimensional chains are further extended into a three-dimensional supramolecular framework (Fig. 3). Table 2. Hydrogen Bonds for Complex 1 D H A d(d H) d(h A) d(d A) (DHA) Symmetry codes O(5) H(5A) O(4) 0.75(3) 2.21(3) 2.974(2) 165 x, y, z O(5) H(5B) O(2) (2) 122 x, y, z O(5) H(5B) O(3) (2) x, y, z C(7) H(7A) O(3) (2) x, y, z C(9) H(9A) O(5) (3) 143 3/2 x, 1/2+y, 1/2 z C(10) H(10B) O(1) (2) x, y, z C(12) H(12A) O(5) (2) 167 3/2 x, 1/2+y, 1/2 z C(16) H(16A) O(1) (2) x, y, z C(17) H(17B) O(1) (3) 173 3/2 x, 1/2+y, 1/2 z C(18) H(18A) O(5) (3) 151 3/2 x, 1/2+y, 1/2 z C(20) H(20A) O(1) (2) 125 x, y, z Fig. 2. 1D chain structure of complex 1 alternatively containing 8- (A) and 16-membered (B) rings Fig. 3. View of the 3D supramolecular architecture of 1 formed by hydrogen-bonding and π-π interactions To investigate whether the analyzed crystal structure is truly representative of the bulk materials, X-ray powder diffraction (PXRD) technology has been performed for the complex at room temperature (Fig. 4). The main peak positions observed are in good agreement with the simulated ones. Although minor differences can be found in the positions, widths, and intensities of some peaks, the bulk synthesized materials and analyzed crystal can still be considered as homogeneous. The differences may be due to the preferred orientation of the powder samples [20, 21] Thermal analysis TG curve of 1 (Fig. 5) shows that the first weight loss of 3.8% from 147 to 181 corresponds to the removal of crystal water molecule (calcd.: 3.7%).

5 2016 Vol. 35 结构化学 (JIEGOU HUAXUE)Chinese J. Struct. Chem. 509 Upon further heating, an obvious weight loss (78.0%) occurs in the temperature range of 293~840, corresponding to the release of pzdc and mbix ligands (calcd.: 76.3%). Above 840, no weight loss is observed, which means the complete decomposition of 1. The residual weight should be ZnO. Fig. 4. PXRD analysis of the title complex: bottom-simulated, top-experimental Fig. 5. TG curve of complex Photoluminescent properties The emission spectrum of complex 1 in the solid state at room temperature is shown in Fig. 6. It can be observed that 1 exhibits blue photoluminescence with an emission maximum at ca. 455 nm upon excitation at 365 nm. In order to understand the nature of these emission bands, we first analyzed the photoluminescence properties of free H 2 pzdc ligand, and confirmed that it does not emit any luminescence in the range of 400~800 nm. And then we investigated the emission spectrum of mbix only, indicating that it does not emit any luminescence in the range of 400~800 nm, which has also been confirmed previously. Thus, according to the previous literature [22], the emission band could be assigned to the emission of ligand-to-metal charge transfer (LMCT). As 1 possesses strong fluorescent intensity, it appears to be good candidates for novel hybrid inorganic-organic photoactive materials. 4 THEORETICAL CALCULATIONS All calculations in this work were carried out with the Gaussian03 program [23]. The parameters of the molecular structure for calculation were all from the experimental data of the complex. Natural bond orbital (NBO) analysis was performed by density functional theory (DFT) [24] with the PBE0 [25] hybrid functional and the LANL2DZ basis set [26]. The selected natural atomic charges, natural electron configuration, wiberg bond indices and NBO bond orders (a.u) for the complex are shown in

6 JIANG D. Y. et al.: Synthesis, Crystal Structure and Theoretical Calculations of a Zinc(II) 510 Coordination Polymer Assembled by Pyrazine-2,3-dicarboxylic Acid and Bis(imidazol) Ligands No. 4 Table 3. It is indicated that the electronic configurations of Zn(II) ion, N and O atoms are 4s d p 0.38, 2s p 4.24~4.26 and 2s 1.68~1.69 2p 5.09~5.12, respectively. Based on the above results, one can conclude that the Zn(II) ion coor- dination with N and O atoms is mainly on the 4s, 3d, and 4p orbitals. N atoms form coordination bonds with Zn(II) ion using 2s and 2p orbitals. All O atoms supply electrons of 2s and 2p to the Zn(II) ion and form coordination bonds. Therefore, the Zn(II) ion obtained some electrons from two N atoms of mbix ligand and two O atoms of pzdc ligand [26, 27]. Thus, according to valence-bond theory, the atomic net charge distribution and the NBO bond orders of complex 1 (Table 3) shows the obvious covalent interaction between the coordinated atoms and Zn(II) ion. The differences of the NBO bond orders for Zn-O and Zn-N make their bond lengths different [27], which is in good agreement with the X-ray crystal structural data of 1. Table 3. Natural Atomic Charges, Natural Valence Electron Configurations, Wiberg Bond Indexes and NBO Bond Orders (a.u) for Complex 1 Atom Net charge Electron configuration Bond Wiberg bond index NBO bond order Zn(1) [core]4s(0.31)3d(9.97)4p(0.38) O(2) [core]2s(1.68)2p(5.12) Zn(1) O(2) O(4B) [core]2s(1.69)2p(5.09) Zn(1) O(4B) N(1) [core]2s(1.37)2p(4.26) Zn(1) N(1) N(4A) [core]2s(1.37)2p(4.24) Zn(1) N(4A) * Symmetry codes: A: 2 x, y, 1 z; B: 2 x, 1 y, 1 z Fig. 6. Solid-state emission spectrum of 1 at room temperature As can be seen from Fig. 7, the lowest unoccupied molecular orbital (LUMO) is mainly composed of pzdc ligand, whereas the highest occupied molecular orbital (HOMO) mainly consists of the mbix ligand. So, the charge transfer from ligand to ligand may be inferred from some contours of molecular orbital of complex 1. 4 CONCLUSION In summary, we have reported a new cobalt complex formed by pyrazine-2,3-dicarboxylate and bis(imidazol) ligands. In compound 1, the pyrazine- 2,3-dicarboxylate ligands function in a monodentate bridging coordination mode, while mbix ligands show a bidentate bridging coordination mode and link the Zn(II) ions to form a one-dimensional double chain. It is worthy to note that the intermolecular hydrogen bonds of O H O, C H O and π-π interactions play an important role in the

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