FENG Guo-Dong JIANG Luan CHEN Qiang YANG De-Suo WANG Min-Juan LI Zong-Xiao LUO Xiao-Lin

(Department of Chemistry and Chemical Engineering,Baoji University of Arts and Sciences, Shaanxi 721013, China)

1 INTRODUCTION

The field of inorganic coordination polymeric compounds continues to move forward at an explosive pace driven, not only by their potential applications in microelectronics, nonlinear optics and catalysis[1-4], but also by their esthetic architectures and fascinating topologies, such as rectangular grids,brick walls, herringbones, ladders, rings, boxes,diamondoids, and honeycombs[5-8]. To establish these molecular structures, the “node-and-spacer”approach has been proven to be excellent effective synthetic approach to produce predictable architectures and topologies of coordination networks[9].Among them, T-shaped ligands with high symmetry have been extensively used to construct coordination networks, especially for brickwall, ladder and herringbone[10]. The aromatic multicarboxylate ligands with complicated configurations, such as 1,3,5-benzenetricarboxylate[11],1,2,4,5-benzenetetracarboxylate[12], and biphenyl-3,3?,4,4?-tetracarboxylic acid[13-21], have been proved to be good candidates of T-shaped ligands in the construction of a rich variety of coordination polymers with high-dimensional networks. Nevertheless, we are more curious about biphenyl-3,3?,4,4?-tetracarboxylic acid (H4BPTC).Owing to a lot of coordination modes with four carboxylic groups, which may be regarded not only as hydrogenbonding acceptors but also as hydrogen-bonding donors depending upon the number of deprotonated carboxylic groups[22], the H4BPTC possesses similar geometric configuration, more rich coordination sites, and the potential to construct porous frameworks with larger pores owing to its extended length. Meanwhile, H4BPTC has a large number of conjunctive aromatic rings that make it very easy to generate π··π interactions and extend the structure into higher dimensionality[23].

On the other hand, the large π-conjugated organic ligands like bis(2-benzimidazole)alkanes and d10transitional metal ions have been frequently used in the preparation of coordination polymers because they are very strong N-ligating donors for d-metal ions and can be readily deprotonated to bridge metal ions into extended higher dimensional (2D and 3D)coordination polymers[24-25]. However, the construction of supramolecular compounds by mixing two types of ligands, such as the substituted bis(2-benzimidazole)alkanes and biphenyl-3,3?,4,4?-tetracarboxylic acid ligands, is relatively rare. Hence, in this work we report the synthesis and crystal structure of a new Zn(II) compound [Zn(BPTC)0.5(H2C2EIm)-(H2O)]nfeaturing a 2D wavy parquet based on the H2C2Eim and H4BPTC mixed ligands.

2 EXPERIMENTAL

2.1 Materials and measurements

All chemical reagents were of analytical grade and used without further purification. The H2C2EIm ligand was prepared according to the reported method[26]. H4BPTC ligand was prepared according to the literature method[27]. The single-crystal structure of 1 was determined on a Bruker Smart Apex II CCD diffractometer.

2.2 Synthesis of compound 1

Compound 1 was prepared from the mixture of Zn(NO3)2·6H2O (0.148 g, 0.5 mmol), H2C2EIm(0.106 g, 0.5 mmol), H4BPTC (0.099 g, 0.3 mmol)and H2O (16 mL), which was heated at 165 ℃ for 3 days in a 25 mL Teflon-lined stainless steel vessel under autogenous pressure. Then the mixture was slowly cooled down to room temperature, resulting in colorless block crystals with the yield of 70%.

2.3 Structure determination

A colorless block single crystal of compound 1 with dimensions of 0.26mm × 0.10mm × 0.18mm was carefully selected under a polarizing microscope and mounted on a glass fiber and used for X-ray diffraction analyses. Single-crystal structure determination by X-ray diffraction measurements was performed using a Bruker APXII CCD diffractometer equipped with a graphite-monochromatic MoKa(λ = 0.71069 ?) radiation in the range of 2.11≤θ≤24.90° (–14≤h≤7, –16≤k≤16,–16≤l≤16) at 293 K. Absorption corrections were applied using the multi-scan technique[28]. A total of 10845 reflections including 3834 unique ones were collected, of which 2630 with I ＞ 2σ(I) were considered as observed and used in the succeeding refinements. The structure was solved by direct methods with SHELXS-97 and refined by full-matrix least-square techniques on F2using SHELXL-97[29]. All of the non-hydrogen atoms were refined anisotropically[30]. The H atoms attached to C atoms were positioned geometrically, with Uisovalues derived from Ueqvalues of the corresponding C atom. The final R = 0.0427, wR = 0.0793, S = 1.038,(Δρ)max= 0.310 and (Δρ)min= –0.288 e/?3.

3 RESULTS AND DISCUSSION

3.1 Crystal structure

The ORTEP drawing and 2D layer structures of compound 1 are illustrated in Figs. 1 and 2, respectively. Selected bond distances and bond angles are shown in Table 1. The single-crystal X-ray analysis shows that the asymmetrical unit of compound 1 contains one Zn(II) ion, one H2C2EIm ligand, one half H4BPTC molecule and one free water molecule.As shown in Fig. 1, the Zn site shows the distorted tetrahedral geometry, being chelated by two O atoms from one half H4BPTC ligand and another half H4BPTC ligand. The remaining two coordination sites are chelated by two N atoms from one H2C2EIm ligand. The Zn–O bond lengths are in the range of 0.1942(2)–0.1955(2) nm and the Zn–N bond lengths are from 0.1983(3) to 0.1999(3) nm.As shown in Scheme 1, each H4BPTC ligand bridges four zinc(II) ions in a μ4-fashion to form a 2D parquet network paralleling to the ab plane. Two carboxylate groups of the H4BPTC ligand extend horizontally and the other two remaining carboxylate groups extend vertically upwards and downwards, coordinating to the four Znic(II) ions, respectively (Scheme 1a). Consequently, the whole H4BPTC ligand can be described as a noncoplanar double-T-shaped spacer with the dihedral angles of 46.049(11)o between Zn–ZnA/C(19)–C(24) and ZnB–ZnC/C(19)–C(24) planes (Scheme 1b).

Scheme 1. Schematic representation for the assembly of a 2D wavy parquet motif. (a) Black arrows showing the coordination sites; (b) Schematic representation of the double-T-shaped ligand (H4BPTC) (Light orange ball representation of phenyl ring from H4BPTC and node (Zn ion) (Symmetry codes: A: –x+2, –y+1, –z+1; B: x+1/2, –y+1/2,z+1/2; C: –x+3/2, y+1/2, –z+1/2); (c) 2D wavy parquet topology of compound 1 along the ab plane (The bis(2-benzimidazole) ligands are omitted for clarity); (d) Structure of the two-dimensional wavy layer along the crystallographic [-1 0 -2] direction (The bis(2-benzimidazole) ligands are shown with sticks mode for clarity). Color codes:Cd, teal; N, blue; C, gray; O, red (For interpretation of the references in color in this figure legend, the reader is referred to the web version of this article)

Fig. 1. Coordination environment of the Zn(II) atom in compound 1 (50% thermal ellipsoids).Symmetry codes: A: –x+2, –y+1, –z+1; B: x+1/2, –y+1/2, z+1/2;C: –x+3/2, y+1/2, –z+1/2; E: x–1/2, –y+1/2, z–1/2

Fig. 2. Structure of the three-dimensional [Zn(BPTC)0.5(H2C2EIm)]n network along the crystallographic [-1 0 -2]direction (bis(2-benzimidazole) ligands are shown with sticks mode for clarity). Color codes: Cd, teal; N,blue; C, gray; O, red (For interpretation of the references in color in this figure legend,the reader is referred to the web version of this article)

Table 1. Selected Bond Lengths (nm) and Bond Angles (°)

Table 2. Hydrogen Bond Lengths (nm) and Bond Angles (°) for Compound 1

The simplest cyclic unit of the 2D-network is noncoplanar parquet motif with the dimensions 17.248 ×8.938 (based on the separation of metal ions),consisting of four Zn(II) ions and four H4BPTC ligands (Scheme 1c). Furthermore, the H2C2EIm ligand in compound 1 acts in a chelating-bridging coordination mode and interacts with the Zn atom through two nitrogen atoms of benzimidazole.Owing to the multiple nitrogen atoms in the H2C2EIm ligand, hydrogen-bonding interactions are formed. As a result, the 2D layer-like structure is further linked to form a three-dimensional network by the interlayer hydrogen bonding interactions between free carbonyl oxygen atoms of H4BPTC ligands in one layer and amine hydrogen atoms of the H2C2EIm ligands in a neighboring layer (Fig. 2,Table 2). Moreover, the π···π stacking interactions(dfacetoface= 3.769(3)) among H2C2EIm ligands play important roles in stabilizing the whole 3D supramolecular structure. Interestingly, owning to the large steric hindrance of the H2C2EIm ligand, the H4BPTC is not able to produce coplanar lattice species as previously documented[13]. Comparatively, the layer of compound 1 is like a wavy shap extending along the crystallographic [–1 0 –2] direction and each wave consists of two H4BPTC ligands with the dihedral angles of 74.320(85)o sharing four ZnH2C2EIm units (Scheme 1d). The hydrogen binding of compound 1 is detailed in Table 2.

4 CONCLUSION

In summary, a new Znic(II) ladder layer compound was synthesized from imidazole-carboxylate ligands including H4BPTC and H2C2EIm chelate ligands, which has been performed in the hydrothermal reaction condition, and its structure was determined to be [Zn(BPTC)0.5(H2C2EIm)(H2O)]n(C24H18ZnN4O5, Mr= 507.79) using single-crystal X-ray diffraction analysis. The successful syntheses of the compounds indicate that H4BPTC is a multifunctional ligand potentially able to produce open lattice species and the H2C2EIm ligand plays important roles in the formation of 3D unusual architectures.

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