LI Xiu-Mei WANG Zhi-Tao PAN Ya-Ru

(Faculty of Chemistry, Tonghua Normal University, Tonghua134002, China)

1 INTRODUCTION

The designed synthesis and characterization of metal-organic coordination polymers or metalorganic frameworks (MOFs) have gained important progress in supramolecular chemistry and material chemistry[1-8].The increasing interest in this field is justified not only for their particular beauty andintriguing structural diversities of architecture, but also for their potential applications as catalytic,conductive, luminescent, magnetic, spin-transition,non-linear optical or porous functional materials[9-16].The combination of metal ions with neutral and anionic mixed bridging ligands makes the assembly process more controllable than a single ligand.The prospect of introducing the second or more organic ligands into a reaction system provides further impetus for research on metal-organic supramolecular frameworks.The construction of supramolecular architectures through selective and directional non-covalent forces such as hydrogen bonding, π···π and C–H···π interactions inmetal-organic frameworks arouses considerable contemporary interest owing to their potential applications as functional materials.

The hydrothermal technique is well suited to the preparation of crystals of synthetic minerals, new inorganic materials, and organometallic coordination polymers.Of particular interest to us is the construction of transition metal polymers with new structural features by utilizing hydrothermal synthesis.With this background information, in this work, a new coordination polymers [Cd0.5(nba)(bib)0.5]2n(1) was synthesized based on Hnba and bib ligands.Its synthesis, structure, thermal stability and luminescence properties of complex 1 were studied.

2 EXPERIMENTAL

2.1 Materials and instruments

All the chemicals were of analytical grade and used without further purification.Elemental analyses for C, H and N were performed on an Elementar Vario III Elemental Analyzer.IR spectra were obtained using KBr pellets on a Nicolet 6700 spectrophotometer in the 4000～400 cm-1region.Powder X-ray diffraction (PXRD) patterns were collected in the 2θ range of 5～50o with a scan speed of 0.1 o·s-1on a Bruker D8 Advance instrument using a CuKα radiation (λ = 1.54056 ?) at room temperature.The fluorescent studies were carried out on a computer-controlled JY Fluoro-Max-3 spectrometer at room temperature.TGA was carried out using a Thermal Analyst 2100 TA Instrument and SDT 2960 Simultaneous TGA-DTA Instrument in flowing nitrogen at a heating rate of 10 ℃/min.

2.2 Synthesis

A mixture of Hnba (0.033 g, 0.2 mmol), bib (0.038 g, 0.2 mmol), Cd(OAc)2·2H2O (0.053 g, 0.2 mmol)and 18 mL H2O was adjusted to pH ≈ 8 with 40%NaOH, sealed in a Teflon-lined stainless-steel vessel,heated to 120 °C for 5 days, and followed by slow cooling (a descent rate of 5 °C/h) to room temperature.Colorless block crystals were obtained.Yield of 21%.Anal.Calcd.for C24H22CdN6O8: C, 45.40; H, 3.49; N,13.24%.Found: C, 45.00; H, 3.01; N, 13.01%.IR(cm–1): 3125(w), 2938(w), 1568(m), 1511(s),1468(m), 1443(w), 1405(m), 1317(w), 1296(w),1273(w), 1231(m), 1165(w), 1067(w), 1034(w),1011(w), 944(w), 929(w), 877(w), 835(m), 741(m),724(m), 659(w), 623(w), 566(w), 521(w).

2.3 X-ray crystallography

All diffraction data of complex 1 were collected on a Bruker/Siemens Smart Apex II CCD diffractometer with graphite-monochromated MoKα radiation (λ =0.71073 ?) at 293(2) K.Data reductions and absorption corrections were performed using the SAINT and SADABS programs, respectively.The structure was solved by direct methods with SHELXS-97 program[17(a)]and refined by full-matrix least-squares techniques on F2with SHELXL-97[17(b)].All non-hydrogen atoms were refined anisotropically and the hydrogen atoms of organic ligands were generated geometrically.A total of 2568 reflections were collected in the range of 1.78≤θ≤26.12°, of which 2426 were independent (Rint=0.0154).The final R = 0.0268 and wR = 0.0683 for observed reflections with I ＞ 2σ(I), and R = 0.0287 and wR = 0.0697 for all data with (Δρ)max= 0.371 and(Δρ)min= –0.371 e·?-3.Selected bond lengths and bond angles of complex 1 are shown in Table 1.

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

3 RESULTS AND DISCUSSION

3.1 Structural description

Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in C2/c space group and consists of a one-dimensional chain-like structure.The coordination environment of Cd(II) in 1 is shown in Fig.1.There are half Cd(II) ion, one nba ligand and half bib molecule in the asymmetric unit.The Cd(1) ion is coordinated by three oxygen atoms from two different nba ligands, one nitrogen atom from bib molecule in the equatorial plane (Cd(1)–O(1)= 2.506(2), Cd(1)–O(1A) = 2.506(2), Cd(1)–O(2) =2.306(2), Cd(1)–N(1) = 2.2303(19) ?) and one oxygen from nba ligand, one nitrogen atom from bib molecule at the axial sites (Cd(1)–O(2A) = 2.320(3),Cd(1)–N(1A) = 2.2303(19) ?), which are in the normal range[18]and the coordination angles round the Cd ion vary from 53.30(8) to 138.25(8)°.

Fig.1. ORTEP drawing of 1 showing the local coordination environment of Cd(II) with thermal ellipsoids at 30% probability.Symmetry code: (A) –x, y, –z+3/2

In the crystal structure of complex 1, the wholly deprotonated nba ligands take a bidentate bridging coordination mode and bib molecule adopts a trans-conformation mode with a dihedral angle between two imidazole rings of 0.66o.As a result, the Cd(II) ions are linked by bib ligands to form a one-dimensional zigzag chain structure with the Cd···Cd distance of 13.678 ?, as depicted in Fig.2.The Cd(1) ion shows a distorted octahedral coordination construction.Further analysis of the crystal packing revealed that there are π-π interactions in complex 1 between C(2)C(3)C(4)C(5)C(6)C(7) benzene ring of nba ligand.The centroidto-centroid distance is 3.749(2) ? and perpendicular distance is 3.5442(14) ?, and the dihedral angle is 0°.Therefore, through π-π interactions, the one-dimensional chains are further extended into a threedimensional supramolecular framework (Fig.3).

Fig.2. View of the one-dimensional zigzag chain in 1

3

Fig.3. View of the 3D supramolecular architecture of 1 formed by π-π interactions along the b axis

3.2 IR analysis of complex 1

The C–N absorption peaks of imidazole can be observed at 1342 cm-1.Asymmetric and symmetric COO–stretching modes of the lattice nba-anion were evidenced by very strong, slightly broadened bands at 1511 and 1468 cm-1[19], which is consistent with the results of X-ray analysis.

3.3 Thermal stability and powder X-ray diffraction (PXRD)

To confirm the phase purity of complex 1, powder X-ray diffraction (PXRD) patterns were recorded for 1, and it was comparable to the corresponding simulated patterns calculated from the single-crystal diffraction data (Fig.4), indicating a pure phase of bulky sample.

Fig.4. PXRD analysis of the title complex: bottom-simulated, top-experimental

In order to better understand the thermal stability of complex 1, its thermal decomposition behaviors were investigated at 50～800 ℃ under nitrogen atmosphere (Fig.5).The TG curve of 1 indicates no obvious weight loss from 50 to 334 ℃.The TG curve presents a platform and the framework starts to decompose at 334 ℃.

3.3 Photoluminescent properties

The emission spectrum of complex 1 in the solid state at room temperature is exhibited in Fig.6.It can be observed that 1 shows green photoluminescence with an emission maximum at ca.535 nm upon excitation at 325 nm.In order to realize the nature of these emission bands, we first studied the photoluminescence properties of free Hnba (λem= 279 nm)ligand, and proved that it does not emit any luminescence in the range of 400～800 nm.And then we researched the emission spectrum of bib (λem= 325 nm) itself and the result revealed that the main emission peak of bib ligand is at 438 nm, which has also been proved previously.Thus, on the basis of the earlier literature[20,21], the emission band could be tentatively assigned to π*→ π transitions of neutral ligand.

Fig.5. TG curve of the title complex

Fig.6. Solid-state emission spectra of 1 and 2 at room temperature

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