LI Gui-Lian LIU Guang-ZhenHUANG Lei-Lei LI Zhao-Xiao

(College of Chemistry and Chemical Engineering,Luoyang Normal University, Luoyang 471022, China)

1 INTRODUCTION

The advent of desirable coordination polymers based upon the assembly of metal ions and multifunctional organic ligands is of perpetual interest in the field of supramolecular chemistry and crystal engineering. The reasons not only derive from their potential applications as functional materials but also from their fascinating structural diversities[1-5].Though it is currently a great challenge for the directional synthesis of such materials due to their unclear formation mechanisms, many complexes with interesting compositions and structures have been synthesized through taking various factors into account, such as the metal/ligand nature, solvent,templates, counterions and the metal-to-ligand ratio[6-8].

To date, aromatic multicarboxylate ligands have gained more and more attention as ligands in constructing appealing MOFs due to their excellent coordination capability[9-11]. Thereinto, the 1,2-benzenedicarboxylic acid (o-H2bdc) has been universally employed in MOF chemistry[12-14]whereas few researches have been carried out in this synthesis by using H2nbdc[15-17], which can probably provide the potential to enrich the structural and functional diversities of MOFs. It may be attributed to the existence of an electron-withdrawing group (-NO2)on the aromatic backbone, which will have a profound impact on the electron density of such a ligand and therefore different physical and chemical properties[18]. Furthermore, exo-bidentate rod-like 4,4?-bpy ligand, which has been extensively introduced into the metal-polycarboxylate assembled systems, can modify the structures and properties of the resulting materials[19]. Additionally, the metal Cu(II), which contains one unpaired electron, is often used to build novel coordination architectures because some of its polymeric compounds exhibit magnetic properties.

With these considerations in mind, in this work,we adopt such a strategy to prepare a three-dimensional framework and further demonstrate the effect of substituted group and 4,4?-bpy ligand on governing the final structures. Herein, we report the synthesis, crystal structure, and characterization of{[Cu2(nbdc)2(4,4?-bpy)2(H2O)2]·2H2O}n. The thermal degeneration, IR and PXRD of the complex are also given.

2 EXPERIMENTAL

2.1 Reagents and instruments

All reagents were of analytical grade and used as purchased without further purification. Elemental analyses for carbon, hydrogen and nitrogen were performed with a Flash 2000 organic elemental analyzer. The FT-IR spectra were recorded on powdered samples using a NICOLET 6700 FT-IR spectrometer in the 4000～600 cm-1range. Powder X-ray diffraction (PXRD) patterns were recorded with a Bruker D8 ADVANCE X-ray diffractometer.The thermogravimetric analysis (TGA) was performed on a SII EXStar6000 TG/DTA6300 analyzer in flowing N2with a heating rate of 10 ℃ min-1.

2.2 Synthesis of

{[Cu2(nbdc)2(4,4?-bpy)2(H2O)2]·2H2O}n

Cu(Ac)2·4H2O (0.1 mmol), H2nbdc (0.1 mmol),4,4?-bpy (0.2 mmol) and H2O (7 mL) were placed into a 23 mL Teflon-lined stainless steel reactor,heated to 120 ℃ for 4 days, and then cooled at 5℃ h-1to room temperature. Blue block crystals were obtained. Elemental analysis calcd. (%) for C36H30N6O16Cu2: C, 46.53; H, 3.20; N, 8.91. Found(%): C, 46.51; H, 3.25; N, 9.04.

2.3 Crystal structure determination

A blue crystal of the title complex with approximate dimensions of 0.25mm × 0.21mm × 0.20mm was carried out on a Bruker SMART APEX CCD diffractometer equipped with a graphite-monochromated Mo-Kα (λ = 0.71073 ?) radiation by using a φ-ω scan technique at room temperature. Out of the 51217 total reflections collected in the 2.38≤θ≤25.50o range, 6698 were independent with Rint=0.0970, of which 4548 were considered to be observed (I ＞ 2σ(I)) and used in the succeeding refinement. The structures were solved by direct methods with SHELXS-97 and refined on F2by full-matrix least-squares using the SHELXL-97 program package[20-21]. All non-hydrogen atoms in the complex were refined with anisotropic thermal parameters. The hydrogen atoms were added in the riding model. The final R = 0.0591 and wR = 0.1378(w = 1/[σ2(Fo2) + (0.0716P)2+ 15.6852P], where P= (Fo2+ 2Fc2)/3), R = 0.0991 and wR = 0.1581 for all data. (Δ/σ)max= 0.000, (Δρ)max= 1.264 and(Δρ)min= –0.871 e/?3. The goodness-of-fit indicator(S) is 1.043. The selected bond lengths and bond angles are given in Table 1.

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

Symmetry transformation for the compound: a: x, –y + 2, z–1/2; b: –x + 1/2, –y + 3/2, z + 1/2

3 RESULTS AND DISCUSSION

3.1 Description of the crystal structure

X-ray crystallographic analysis reveals that the complex crystallizes in the orthorhombic crystal system of Pbcn space group. The crystal structure of the compound reveals a three-dimensional (3D)framework with bcu topological type. Each molecule fragment consists of two crystallographically distinct Cu(II) ions, two completely deprotonated nbdc dianions, two 4,4?-bpy ligands, two coordination water molecules and two extra framework water molecules (Fig. 1a). The Cu(1) and Cu(2) atoms show similar five-coordinated environments with two nitrogen atoms from two 4,4?-bpy ligands respectively and two oxygen atoms from two nbdc dianions occupying the basal plane, and one oxygen atom from coordination water molecules occupying the apical position. All the Cu–N bond lengths are in the range of 2.037(4)～2.120(4) ? and the Cu–O bond separations are between 1.923(4) and 2.325(4) ?.

Fig. 1. (a) Repeat unit of the resulting polymer. Symmetry codes: A = x, –y+2, z–1/2; B = –x+1/2, –y+3/2, z+1/2;(b) View of the 1D straight chain, gray: Cu1, linear: Cu2; (c) The extended 3D network of the complex with nitro groups omitted for clarity; (d) A diagrammatic sketching of π··π interactions with nitro groups omitted for clarity;(e) Schematic view of the bcu net topology. All hydrogen atoms of carbon atoms are omitted for clarity

Each nbdc dianion along the c axis adopts a μ1-η1:η0bridging mode to connect the Cu(1) and Cu(2)atoms forming infinite chains with the Cu(1)··Cu(2)distances of 7.1273(17) and 7.6678(16) ?, as shown in Fig. 1b. The adjacent chains are further crosslinked by 4,4?-bpy ligands to produce a 3D selfpenetrated framework (Fig. 1c).

In addition, kinds of O–H··O hydrogen bonding interactions are observed in an overall structure: (a)hydrogen bonds between coordinated water oxygen atoms and crystallization water oxygen atoms(O(13)–H(2W)··O(14): d = 3.088(17) ?, θ = 167.0°;O(18)–H(4W)··O(15): d = 2.701(6) ?, θ = 163.5°);(b) hydrogen bonds between crystallization water oxygen atoms and uncoordinated oxygen atoms of nbdc dianion motif (O(15)–H(7W)··O(2): d =2.708(6) ?, θ = 175.2°; O(15)–H(8W)··O(7): d =2.831(6) ?, θ = 167.7°); (c) hydrogen bonds between uncoordinated water molecule and oxygen atom of nitro group (O(14)–H(6W)··O(6): d =2.797(11) ?, θ = 132.0°; O(14)–H(6W)··O(5): d =3.130(11) ?, θ = 169.4°; O(15)–H(8W)··O(11): d =3.033(7) ?, θ = 110.7°); (d) hydrogen bonds between coordinated water molecule and uncoor-dinated oxygen atoms of carboxylate motif (O(13)–H(1W)··O(9): d = 2.690(6) ?, θ = 139.2°). There exist relatively weaker π··π stacking interactions between the parallel benzene rings of nbdc dianions between two adjacent chains (phenyl-phenyl distance: 3.888 and 3.944 ?), as indicated in Fig. 1d.The H-bonding bonds and π··π interactions play important roles in forming by self-assembly and enhancing the stability of the resultant 3D framework.

Topological analysis reveals that each Cu(1)/Cu(2)center can be regarded as the four-connected network nodes and the nbdc and 4,4?-bpy as two kinds of linkers. Thus, the architecture of the compound can be considered as bcu topology with the Schl?fli symbol of (46?48) (Fig. 1e).

The previously reported compound [Cu(nbdc)-(H2O)2]nshows a 2D layer structure[22], which possesses crystallographically distinct copper ion in the unit, quite different from the structural motif observed here. This structural difference may be ascribed to the synergetic effect of 4,4?-bpy ligand.

3.2 TGA, IR and PXRD

Thermogravimetric experiment of the compound was carried out under flowing nitrogen atmosphere at a heating rate of 10 ℃·min?1in order to probe the thermal stability of the 3D framework, as shown in Fig. 2. In the TGA diagram of the complex, weight loss of 7.69% between 65 and 180 ℃ corresponds to the removal of coordinated and crystallization water molecules, which is consistent of the corresponding calculated value of 7.74%. The second gradual weight loss above 200 ℃ corresponds to the decomposition of nbdc anions and 4,4?-bpy ligands,and does not stop until the heating ends at 900 ℃.

Fig. 2. TGA curves for the compound

Fig. 3. PXRD patterns for the compound

IR spectrum of the compound corresponds with their single-crystal structures. IR spectra show several characteristic bands: Broad bands observed in the region of 3000～3600 cm-1represent O–H stretching modes within the coordinated or free water molecules. The strong absorptions at 1576 and 1346 cm-1are attributed to the asymmetric and symmetric stretching vibration (vas(COO-) and vs(COO-)), respectively. Medium intensity bands at 1604 and 1221 cm-1can be ascribed to the stretching modes of pyridyl rings of 4,4?-bpy ligands. Puckering modes of the pyridyl rings are observed at 831 and 667 cm-1.

Powder X-ray diffraction (PXRD) of the compound has been studied at room temperature, which is in good agreement with the pattern simulated from the single-crystal data, implying the good phase purity of the complex (Fig. 3).

4 CONCLUSION

In conclusion, a new three-dimensional coordination polymer of Cu(II) has been synthesized by hydrothermal reaction adopting 4-nitrobenzene-1,2-dicarboxylic acid and 4,4?-bpy ligands, in which 4,4?-bpy has played an important role in increasing the dimensionality of the resultant structure. The compound displays high thermal stability at normal temperature as confirmed by TGA analysis. Further work in this area is in progress. We anticipate that this ligand will result in a variety of coordination polymers with stable microporosity and fascinating construction.

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