Synthesis, Crystal Structure, Spectroscopic Characterization and Thermal Stability Properties of a Trinuclear Zinc(II) Complex①

LI Chng-Hong LI WeiHU Hn-Xing HU Bo-Nin

a (College of Chemical Engineering, Hunan Institute of Technology, Hengyang 421002, China)

b (College of Chemistry and Materials Science, Hengyang Normal University, Hengyang 421008, China)

1 INTRODUCTION

Metal-organic frameworks (MOFs) are receiving great attention not only for their intrinsic structural characteristics but also for their possible applications in gas storage, molecular recognition, catalysis,etc[1-5]. The carboxylate group is one of the most widely used bridging ligands for designing polynuclear complexes with interesting structures and photochemistry properties[6]. Its versatility as a ligand is illustrated by the variety of its coordination modes when acting as a bridge, with the most common being the so-called syn-syn, syn-anti, and anti-anti modes[7]. As a rigid aromatic carboxylic acid ligand, 3,5-dimethylbenzoic acid (3,5-DMBA)can also be used in the design and construction of coordination compounds[8]. In order to obtain knowledge about the structure and function of metal organic complexes constructed with carboxylic ligands, we synthesized a new trinuclear zinc complex Zn3(2,2?-bipy)2(3,5-DMBA)6·(H2O)1.5(1)using 3,5-DMBA as the ligand. We also report our preliminary results of the spectroscopic characterization and thermal stability property of the complex.

2 EXPERIMENTAL

2.1 Materials and instrumentation

All reagents were of analysis grade and purchased from Reagent No. 1 Factory of Shang-hai Chemical Reagent Co., Ltd, China and used without further purification. The C, H and N analyses were conducted by means of a PE-2400(II) apparatus. The IR spectrum was recorded on a Bruker Vector22 FT-IR spectrophotometer using KBr discs. A fluorescence spectrum was obtained at room temperature on a RF-5301PC fluorescence spectrophotometer. Thermal stability properties were studied on a PRT-2 pyris1 instrument.

2.2 Preparation of

Zn3(2,2?-bipy)2(3,5-DMBA)6·(H2O)1.5(1)0.2 mmol of 3,5-DMBA (about 30.0 mg) and 0.2 mmol of zinc acetate (about 43.9 mg) were added to the mixed solvents (30 mL) of methanol and water(10:2 in volume), then stirred at 60～65 ℃ for about 6.0 h. After that, 0.1 mmol of 2,2?-bipyridine (about 15.6 mg) was added, and the pH value was adjusted to 5.5～6.5 with dilute sodium hydroxide, and finally stirred at 60 ℃ for about 13 h. Afterwards,the resultant solution was filtrated, and the filtrate was kept untouched and evaporated slowly at room temperature. Weak yellow block-shaped single crystals suitable for X-ray diffraction analysis were obtained in 66.02% yield after three weeks. M. p.:211～213 ℃. Anal. for C74H73N4O13.50Zn3(%):calculated: C, 62.08; H, 5.14; N, 3.92. Found: C,61.90; H, 5.12; N, 3.93. Selected IR data (KBr pellet,cm-1): v = 3427(w), 2914(w), 1566(vs), 1400(vs),1313(m), 1269(s), 1245(m), 1016(m), 868(w),791(vs), 766(s), 682(w), 534(w), 419(w).

2.3 X-ray structure determination

A single crystal with dimensions of 0.18 mm ×0.17 mm × 0.15 mm was put on a Bruker SMART APEX CCD diffractometer equipped with a graphite-monochromatic MoKα radiation (λ =0.71073 ?) using a φ-ω scan mode at 173(2) K. A total of 19000 reflections were collected in the range of 1.49≤θ≤25.01°, of which 5,890 were independent (Rint= 0.0350) and 4,897 were observed (I >2σ(I)). All data were corrected by Lp factors and empirical absorption. The crystal structure was solved directly by program SHELXS-97, and refined by program SHELXL-97[9]. The hydrogen and non-hydrogen atoms were corrected by isotropic and anisotropic temperature factors respectively through full-matrix least-squares method. The final R =0.0667, wR = 0.1917 (w = 1/[σ2(Fo2) + (0.1138P)2+2.5200P], where P = (Fo2+ 2Fc2)/3); (Δ/σ)max= 0.00,S = 1.069, (Δρ)max= 0.768 and (Δρ)min= –1.090 e·?-3.

3 RESULTS AND DISCUSSION

3.1 Crystal structure of 1

X-ray crystal structural analysis reveals that complex 1 contains three zinc ions, six 3,5-DMBA molecules, two 2,2?-bipyridine molecules and one and half water molecules (Fig. 1). The selected bond lengths and bond angles are shown in Table 1. In the Zn(1)N6and Zn(2)N2O3units, there exist different coordination atom types and numbers. Zn(1) is coordinated by six oxygen atoms from six 3,5-DMBA molecules, forming a distorted octahedral geometry, while Zn(2) is coordinated with three oxygen atoms from three 3,5-DMBA molecules and two nitrogen atoms from the 2,2?-bipyridine molecule, forming a distorted square pyramidal geometry. The bond angle O–Zn(1)–O is 84.44(13)～173.15(16)o and O/N–Zn(2)–N/O is 56.94(10)～96.07(11)o. The Zn(1)–O bonds range from 2.118(3)to 2.223(3) ?, while Zn(2)–N/O fall in the 2.103(3)～2.263(4) ? range. From Fig. 1 we can see that three centersymmetric zinc ions are bridged by six μ2-η1:η0-carboxylate groups of 3,5-DMBA-. The bond angle Zn(2A)···Zn(1)···Zn(2) is 159.94o, with the bond lengths of Zn(1)···Zn(2) and Zn(1)···Zn(2A) to be the same as 3.549 ?.

3.2 Fluorescence

As illustrated in Fig. 2, the fluorescence of 1 was studied in the solid state at room temperature. There are emission bands at 438 nm (λex= 374 nm) for 1.Such fluorescence emissions may be assigned to intra-ligand π-π* transitions because the free 3,5-DMBA ligand exhibits a similar broad emission at 416 nm upon excitation at 342 nm. The emission bands of 1 are blue-shifted by 22 nm as compared to the 3,5-DMBA ligand, which is attributed to the coordinative interactions between the metal atom and the ligand. Such emission bands may be tentatively assigned to ligand-to-metal charge transfer(LMCT)[10].

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

Fig. 1. ORTEP-drawing of 1: coordination structure of 1

Fig. 2. Spectra of the title complexA: Emission spectrum (λmax = 438 nm);B: Excitation spectrum (λmax = 374 nm)

3.3 Thermal stability

Thermal stability studies of 1 were performed in air. The TG-DTG curve is shown in Fig. 3. There are three weight-loss stages from room temperature to 700 ℃. The first stage takes place from 110 to 200 ℃ with the weight loss of 1.90%, corresponding to the release of one and half free water molecules(calcd. 1.89%). The second stage occurs at 200 to 260 ℃ with the weight loss of 21.90% due to the removal of two 2,2?-bipy molecules (calcd. 21.84%).There is a strong endothermic peak near 212 ℃attributed to the melting endothermic of 1, which conforms to the melting point of 1. The third stage is observed from 260 to 340 ℃with the weight loss of 59.20%, resulting from the departure of six 3,5-DMBA molecules (calcd. 59.20%), which is in agreement with the crystal structure. In air, the final product is zinc oxide with the residual weight being about 17.07% (calcd. 17.0%). Based on the above judgment, the pyrolytic process of 1 may be divided into the following stages:

Fig. 3. TG and DTG curves of the title complex

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