Synthesis, Crystal Structure and Characterization of a Ni(II) Complex of Constructed,6-Bis(benzimidazol-2-yl)pyridine①

YANG Yn LI Chng-Gui LUO Xu-Jin CEN Bo LUO Zhi-Hui LIU Rong-Jun JIANG Yue-Xiu LIANG Wei-Jing

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Synthesis, Crystal Structure and Characterization of a Ni(II) Complex of Constructed,6-Bis(benzimidazol-2-yl)pyridine①

YANG Yana, b②LI Chang-GuiaLUO Xu-JianbCEN BoaLUO Zhi-HuibLIU Rong-JunbJIANG Yue-XiuaLIANG Wei-Jiangb②

a(530004)b(537000)

nickel(II) complex, thermal behaviour, fluorescence

1 INTRODUCTION

The field of crystal engineering has attracted great interest from chemists, and considerable efforts have been focused on the design, synthesis and characteri- zation of supramolecular structures[1, 2]not only because of their intriguing variety of architectures, but also of their versatile potential applications in the areas of absorption, catalysis, nonlinear optics, molecular magnetization and others[3, 4]. On the other hand, 5-hydroxyi-sophthalic acids are rigid and versatile bridging ligands have been extensively studied for designing new coordination polymers because their two carboxylic groups can bond with metal centres and the hydroxyl group. The electron withdrawing groups coexisting in benzene rings can not only act as a hydrogen bond acceptor but also take on some spatial effects[5, 6]. The planar tridentate ligand 2,6-bis(benzimidazol-2-yl)pyridine (Bibimp, Scheme 1) forms either hexa- or penta- coordinate complexes with single ligand, such as [M(Bibimp)2](A)2series (M2+= Mn2+, Fe2+, Co2+, Ni2+, Cu2+and Zn2+)[7-12]and [M(Bibimp)Cl2] systems (M2+= Mn2+, Co2+, Ni2+and Cu2+)[10, 13, 14]. However, Bibimp still remains largely unexplored for synthesizing nickel(II) complexes with mixed ligands in the presence of a bidentate bridging fashion of 5-hydroxyisophthalic acid. Herein we report a novel nickel(II) complex of [Ni(OH- H2Bdc)(Bibimp)]n·nH2Owith both OH-H2Bdc andBibimp ligands, whose structure motifs will be also illustrated.

Scheme 1

2 EXPERIMENTAL

2.1 Materials and physical measurements

All chemicals were commercial reagents and used without further purification. The elemental analysis was carried out on a PE 1700 CHN auto elemental analyzer. The solid infrared spectra (IR) were obtained from a Bruker IFS66V vacuum-type FT-IR spectrophotometer by using KBr pellets. Thermogra- vimetry (TG) analysis was executed on a Perkin Elmer TG/DTA 6300 thermal analyzer under a flowing N2atmosphere at a heating rate of 10 ℃/min. Fluorescence measurements were performed on a Model RF-5 spectrofluorimeter. The crystal structure was determined by a Bruker APEX area- detector diffractometer and SHELXL crystallogra- phic software.

2.2 Synthesis

2.2.1 Synthesis of the ligand H2bibimp

The ligand was synthesized by the condensation of pyridine-2,6-dicarboxylic acid with-phenylene- diamine in phosphoric acid according to the literature[15].

2.2.2 Synthesis of [Ni(OH-H2Bdc)(Bibimp)]n·nH2O

2,6-Bis(benzimidazolyl)pyridine (0.5 mmol) and 5-hydroxyisophthalic acid (0.5 mmol) were dissol- ved in methanol solution (8 mL), and a solution of Ni(NO3)2·6H2O (0.25 mmol) in H2O (5 mL) was added dropwise. The pH value was adjusted to 8.5 with triethylamine. The mixture was heated at 120 ℃for three days and then cooled to room tempera- ture. Green crystals were obtained(52% yield based on Ni), filtered off, washed with distilled water and dried in air. Anal. Calcd (%) for [Ni(OH- H2Bdc)(Bibimp)]n·nH2O:C, 57.08; H, 3.37; N, 12.33. Found (%): C, 56.96; H, 3.52; N, 12.27. Selected IR data (KBr, cm-1): 3424, 3087, 2917, 1607, 1577, 1536, 1492, 1461, 1382, 1316, 1234, 1146, 968, 847, 762, 746.

2.3 Structure determination

A green single crystal of the title compound with dimensions of 0.30mm ×0.26mm×0.20mm was mounted on a glass fiber. X-ray diffraction intensity data were collected on a Bruker APEX area-detector equipped with a graphite-mono- chromatized Mo-radiation (= 0.71073 ?) by using an-scan mode in the range of 3.08≤≤25.01o at 293(2) K. A total of 9847 independent reflections were measured, of which 4199 were unique (int= 0.2046) and 1591 were observed (> 2()) and used in the subsequent structure determi- nation and full-matrix least-squares refinements. Absorption correction was performed by the SHELXS-97 program[16]. The structure was solved by direct methods and subsequent difference Fourier syntheses, revealing the positions of all non-hydro- gen atoms. The hydrogen atoms were located geometrically. All non-hydrogen atoms were refined anisotropically. All calculations were performed by using SHELXTL package[17]. The final= 0.1232 and0.2955 (= 1/[2(F2) + (0.1010)2+ 0.0000], where= (F2+ 2F2)/3) for 1591 observed reflections with> 2(),= 0.920, (Δ/)max= 0.114, (Δ)max= 0.542 and (Δ)min= –0.544 e·?-3. The selected bond lengths and bond angles are listed in Table 1. The hydrogen bond lengths and bond angle are given in Table 2.

3 RESULTS AND DISCUSSION

3.1 Structure

Single-crystal X-ray diffraction analyses reveal that there are one Ni(II) atom, one Bibimp ligand, one Bdc ligand and one water molecule in the asymmetric unit of 1.The local coordination geo- metry around each Ni(II) center in 1 adopts bridging monodentate and chelating bidentate coordinated modes (Scheme 2(a)) of two carboxyl groups of one Bdc ligand (Ni–O = 2.023(8)～2.366(8) ?) and three N atoms from the chelating tridentate coor- dinated mode (Scheme 2(b)) of Bibimp ligands (Ni–N = 2.127(9)～2.189(9) ?) to complete the coordination sphere (Fig. 1). With its chelating carboxyl groups and bridging Bdc moiety, each Bdc ligand is bound to two Ni atoms, resulting in the formation of a 1D chain (Fig. 2). The adjacent chains are stacked offset with respect to each other in anfashion by van der Waals interactions, only a weak interlayer nonclassical C–H···O hydro- gen bond (C(12)…O(6) 3.16 ?) has been observed. A drawing of the 2D network of 1 viewed along thedirection is presented in Fig. 3.

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

Table 2. Hydrogen Bond Geometry (?,°)

Symmetry codes: (a) 1–,1–, –; (b) –, 2–, –; (c) 1–, 1–, 1–; (d) 1+,,; (e) –1+,,

Scheme 2. Coordination mode of the two ligands

Fig. 1. Coordination environment of Ni(II) metal ion in 1

Fig. 2. 1D chain in 1 along theaxis

Fig. 3. ABAB parallel structure of 1 along theplane

3.2 Thermal analysis

Thermogravimetric analysis was carried out to examine the thermal stability of complex1(Fig. 4). The crushed single-crystal sample was heated up to 1000 ℃ in N2at a heating rate of 10 ℃/min. The TG and DTG curves for 1 show that a total weight loss of ca. 3.3% (calc. 3.2% ) occurs over the tempe- rature range of 30～80 ℃, corresponding to the loss of one guest water molecule per formula. The pyrolysis of mixed-ligands occurred in the 100～800 ℃range. The above thermal behaviors may be attributed to the structural features, the presence of abundant coordination bond of carboxylate ligands and the chelating effect of ?ve-membered rings, which change the bite angles for chelating, resulting in the formation of a rich variety of polymeric structures.

3.3 Photoluminescence properties

The emission spectrum of complex 1 in the solid state is investigated at room temperature. Excitation at 365 nm leads to a strong blue-fluorescent emis- sion band at 483 nm for 1 (Fig. 5). The emission is neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal transfer (LMCT) in nature since the Ni(II) ions are difficult to oxidize or reduce due to their8configuration[18]. For the free H2Bdc ligand, the emission band at 387 nm (ex= 351 nm) can be assigned to-* transition. In addition, for the free Bibimp, the main emission band at 562 nm (ex= 490 nm) is ascribed to the intraligand-* transi- tion[19]. Therefore, we assign the emissions described above for 1to LLCT (ligand-ligand charge transfer) excited states.

Fig. 4. TG-DTG curve of 1

Fig. 5. Fluorescence spectrum of compound1 in the solid state at room temperature

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6 April 2014;

10 July 2014 (CCDC 851816)

① This work was supported by the NNSFC (No. 21341005)

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