GUO Xing-Gung WU Xio-Yun ZHANG Qi-Ki YANG Wen-Bin LIN Lng YU Rong-Min LU Cn-Zhong② (Stte Key Lortory of Structurl Chemistry, Fujin Institute of Reserch on the Structure of Mtter, Chinese Acdemy of Sciences, Fuzhou 350002, Chin) (Grdute School of the Chinese Acdemy of Sciences, Beijing 100049, Chin)

1 INTRODUCTION

During the past decades, the design and synthesis of novel metal-organic frameworks (MOFs) have provoked great interest owing to their enormous variety of intriguing structural topologies as well as great potential applications as microporous, magnetic, nonlinear optical, heterogeneous asymmetric catalytic and fluorescent materials[1]. Although several factors have great influence on the design and syntheses of these materials, such as the coordination trend of central metal ions, the metal/ligand ratio, the nature of organic ligands, and the reaction conditions[2], deliberate selection of aromatic polycarboxylates, especially the N-heterocyclic carboxylates as multifunctional ligands, is an effective approach to construct MOFs with unique structures and properties because of their versatile coordination modes. For example, a large diversity of structures based on 4,5-imidazoledicarboxylic acid (H3ImDC) ligand has been synthesized. Not only can it be partially or fully deprotonated to H2IDCˉ, HIDC2ˉ, and IDC3ˉ, but also it has various coordination modes and hydrogen-bonding capabilities[3]. The H3ImDC derivative H3PIDC, which introduces additional positions for modulating coordination ability to give more coordination modes and enlarge the conjugation π-system, is used to construct new coordination polymers[4-5].

Here we report the synthesis, X-ray crystallography study and luminescent property of one Y(Ⅲ)coordination polymer with H3PIDC ligand,{[Y(HPIDC)(OX)1/2(H2O)]·2H2O}n(1).

2 EXPERIMENTAL

2.1 Materials and methods

H3PIDC was synthesized via a published procedure[6]. All reagents and chemicals were purchased commercially and used without purification. Elemental analysis (C, H, N) was performed on a Vario EL III elemental analyzer. IR spectra using KBr pellets technique were recorded on a Spectrum-One FT-IR spectrophotometer. The powder X-ray diffraction (PXRD) pattern was recorded on crushed single crystals in the 2θ range of 5–65° using a CuKα radiation. Thermogravimetric analysis was recorded on a NETZSCH STA 449C unit at a heating rate 10 ℃·min-1under nitrogen atmosphere.Fluorescence spectrum was measured with an Edinburgh Analytical instrument FLS920.

2.2 Synthesis

{[Y(HPIDC)(OX)1/2(H2O)]·2H2O}n(1)

A mixture of Y(NO3)3·6H2O (0.2 mmol), H3PIDC(0.3 mmol), H2C2O4·2H2O (0.4 mmol) and 10 mL H2O was sealed in a Teflon-lined stainless steel autoclave (23 mL), heated at 180 ℃ for 3 days, and then cooled to room temperature in 1.5 days. Yellow prismatic crystals were collected (48% based on Y(Ⅲ)). Elemental analysis (wt.%) for YC11N3O9H11:Calcd.: C, 31.58; H, 2.63; N, 10.05%. Found: C,31.63; H, 2.57; N, 10.09%. FT-IR (4000–600 cm-1),3163 (b), 1688 (m), 1608 (s), 1540 (w), 1487 (w),1440 (s), 1391 (w), 1365 (m), 1313 (w), 795 (m).

2.3 Crystal structure determination

A selected yellow single crystal of the title compound with approximate dimensions of 0.20mm ×0.18mm × 0.16mm was mounted on a glass fiber.Crystal data of 1 were collected on a SCXmini CCD diffractometer equipped with a graphite-monochromatic MoKα radiation (λ = 0.71073 ?) using an ω-2θ scan mode in the range of 2.79≤θ≤27.49°(–10≤h≤10, –18≤k≤18, –13≤l≤14) at 293(2)K. A total of reflections 10900 including 3133 unique ones were collected, of which 2770 with I ＞2σ(I) were considered as observed and used for the succeeding refinement. The data sets were corrected for absorption by multi-scan technique. The structure was solved by direct methods using SHELXS-97 program and refined with SHELXL-97[7]by full-matrix least-squares techniques on F2. All of the non-hydrogen atoms were refined anisotropically.The hydrogen atoms of lattice water molecules were found in the electron density map and refined by riding, and the other hydrogen atoms were generated with idealized geometries and treated as riding. The final R = 0.0480, wR = 0.1134 (w = 1/[σ2(Fo2) +(0.0566P)2+ 1.8576P], where P = (Fo2+ 2Fc2)/3),(Δρ)max= 0.684, (Δρ)min= –1.133 e·?–3, (Δ/σ)max=0.000 and S = 1.053. Selected bond lengths and bond angles for 1 are shown in Table 1.

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

To be continued

3 RESULTS AND DISCUSSION

3.1 Crystal structure description of 1

X-ray single-crystal diffraction data reveal that compound 1 possesses a 3D open framework with 1D channels along the [100] direction. In the asymmetric unit of 1, there are one Y(Ⅲ) ion, one HPIDC2-ligand, a half of OX2-anion, one coordinated water molecule and two lattice water molecules. The Y(Ⅲ) ion exhibits a dodecahedral configuration coordinated by one N atom (Y(1)–N(3), 2.517(3) ?) and four O atoms (Y(1)–O(4),2.359(3) ?; Y(1)–O(1C), 2.254(3) ?; Y(1)–O(2B),2.351(3) ?; Y(1)–O(3C), 2.326(3) ?) from three HPIDC2–anions, three O atoms (Y(1)–O(5), 2.393(3)?; Y(1)–O(6), 2.392(3) ?) from one OX2–anion and one water molecule (Y(1)–O(7), 2.343(3) ?), as shown in Fig. 1. The Y–O and Y–N distances in 1 are comparable to those of Y(Ⅲ) compounds reported[5b]. In 1, there is one HPIDC ligand, which adopts a μ3-κO:κO′,O′: κ O′′, N mode (Scheme 1)to bridge three Y(Ⅲ) cations in Nimidazole,O-chelating, O-bridge, O,O′-bidentate manner. The imidazole and pyridyl rings in the HPIDC2-ligand are noncoplanar with the dihedral angle of 58.7°.The overall structure of 1 is a three-dimensional framework.

Scheme 1. Coordination mode of HPIDC2ˉ ligand

Fig. 1. Coordination environment of the YIII ion in 1. Symmetry codes: A, x, –y+1/2, z – 1/2;B, –x+1, y – 1/2, –z+1/2; C, 2 – x, –y, –z

Interestingly, the 3D framework of 1 contains 1D channels along the [100] direction (Fig. 2). Excluding the van der Waals radii of the surface atoms,the free channel is sized ca. 11.4? × 5.7? filled with lattice water molecules. The free volume of channels occupies 14.7% of the crystal volume calculated by PLATON[8].

Fig. 2. 3D framework with 1D channels in 1 along the [100]direction (Lattice watermolecules were omitted for clarity)

In order to illustrate the structure of 1 more clearly, the resultant three-dimensional framework can be assigned as a diamondoid network by considering yttrium atoms as nodes. One central yttrium atom can be considered as a four-connected node linking four surrounding atoms to form a tetrahedral geometry (Fig. 3). As shown in Fig. 4a, there exist π-π stacking interactions between the imidazole and pyridine rings of the HPIDC2-ligands (centroidto-centroid distance is 3.539(3) ?, dihedral angle =4.6(2)°). Moreover, abundant hydrogen bond interactions are observed in complex 1. The coordinated water molecule O(7) forms intermolecuar hydrogen bond with O(1W) from the lattice water molecule and N(1) from the pyridine ring of HPIDC2-ligand.The lattice water molecule O(1W) forms O–H··O bond with O(5) from carboxylic group of OX2-ligand, while the lattice water O(1W) forms O–H··O bond with O(4) from carboxylic group of HPIDC2-ligand. In addition, uncoordinated N(2)from imidazole ring of HPIDC2-ligand forms N–H··O bonds with O(2W). Furthermore, the two lattice water molecules O(1W) and O(2W)are connected with each other through O–H··O bonds (Fig. 4b). The hydrogen bond lengths and bond angles are listed in Table 2.

Fig. 3. (a) Illustration of Y unit, viewed as a 4-connected node in 1.(b) Representation of the diamondoid network considering only yttrium atoms as nodes.(c) Three-dimensional diamondiod network of compound 1

Fig. 4. (a) Representation of π-π interactions for complex 1 (indicated by dashed lines).(b) Hydrogen bonding interactions in complex 1 (indicated by dashed lines)

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°) for Complex 1

3.2 Thermogravimetric and PXRD analysis

The thermal stability of compound 1 is investigated through thermogravimetric analysis (TGA)experiments in the temperature range of 30–800 °C under a flow of nitrogen at a heating rate of 10°C·minˉ1(Fig. 5). For 1, there is no obvious weight loss at 30–161 °C. On further heating, TGA for 1 shows a weight loss of 12.82% between 161 °C and about 240 °C, which corresponds to the removal of two lattice water molecules and one coordinated water molecule (calcd.: 12.94%). Further heating results in the gradual decomposition of the framework. The as-synthesized sample of 1 is also characterized by powder X-ray diffraction (PXRD).As shown in Fig. 6, the PXRD pattern is almost consistent with its simulated spectra, indicating the sample has good purity.

Fig. 5. TGA curve for complex 1

Fig. 6. Powder X-ray diffraction patterns of the simulated and as-synthesized sample 1

3.3 Photoluminescent property

The emission spectra of compound 1 in solid state at room temperature were investigated. As shown in Fig. 7, compound 1 exhibits yellow photoluminescence with emission maxima at 584 nm. It should be pointed out that the emission of complex 1 is neither metal-to-ligand charge transfer (MLCT) nor ligandto-metal charge transfer (LMCT) which is mainly based on the luminescence of ligands. Compared with the emission maxima of free H3PIDC ligand at 480 nm, compound 1 exhibits a red-shift with 104 nm. The emission peak for 1 should originate from the intraligand π → π* transitions mainly through the H3PIDC ligand, namely ligand-to-ligand charge transfer (LLCT)[9]. Different delocalization environments may be an important factor on influen- cing different luminescent behaviors for 1 and the free H3PIDC ligand. The different dihedral angles between the imidazole and pyridyl rings between complex 1 and the free H3PIDC ligand may lead to different p-electron systems.

Fig. 7. Solid-state emission spectra for free H3PIDC (red) and complex 1 (black) at room temperature

4 CONCLUSION

In summary, one novel Y(Ⅲ) compound based on multidentate H3PIDC and oxalate ligands was synthesized under hydrothermal condition. Compound 1 is a 3D porous metal-organic framework with 1D open channels along the a axis. In addition, the fluorescent property of 1 was investigated.

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