GONG Teng-Fei ZHU Cheng-Feng YE Cheng-Cheng SHENG En-Hong② LIU Bi-Zhn CUI Yong② (College of Chemistry nd Mterils Siene,Anhui Norml University, Wuhu, Anhui 241000, Chin) (Shool of Chemistry nd Chemil Tehnology nd Stte Key Lortory of Metl Mtrix Composites, Shnghi Jiotong University, Shnghi 200240, Chin) (Chin Too, Shnghi 200240, Chin)

1 INTRODUCTION

Metal-organic frameworks (MOFs), also known as coordination polymers, are hybrid crystalline solids composed of organic struts and inorganic nodes and have emerged as one of the most fascinating porous crystalline materials for diverse applications,such as gas storage, catalysis and separation[1-4].Moreover, the structures and properties of MOFs can be readily tuned by the judicious choice of metal ions and organic linkers[4-5]. In particular, the bridging ligand plays a critical role in the performance of pore walls of the framework[6-7]. In deed, some great successes in this regard have been achieved by preparing MOFs with more than one organic component (“mixed component”) or installing specific chemical moieties (such as alcohols, aldehydes,carboxylic acids, nitriles, azides, alkylamines) in organic component[8-10]. In addition, pores and/or channel sizes, shapes can be adjusted by introducing auxiliary bicarboxylate/bipyridine ligands with different lengths[7,11]. We report here the synthesis of a new bipyridine-functionalized ligand L which has been utilized as building blocks to make two Cd-based 3D microporous MOFs.

2 EXPERIMENTAL

2.1 Reagents and apparatus

All of the chemicals are commercially available,and used without further purification. Elemental analyses of C, H and N were performed with an EA1110 CHNS-0 CE elemental analyzer. The IR(KBr pellet) spectra were recorded (400–4000 cm-1region) on a Nicolet Magna 750 FT-IR spectrometer.Powered X-ray diffraction (PXRD) patterns of the samples were recorded by a PANalytical X?Pert Pro diffractometer. Thermogravimetric analyses (TGA)were carried out on a STA449C integration thermal analyzer in a N2atmosphere with a heating rate of 10 ℃·min-1. The fluorescence spectra were carried out on a LS 50B Luminescence Spectrometer(Perkin Elmer, Inc., USA).

2.2 Synthesis of 1

L (13.01 mg, 0.05 mmol), H2BDC (8.31 mg, 0.05 mmol), Cd(NO3)2·4H2O (30.85 mg, 0.10 mmol),MeOH (1.5 mL), DMF (1.5 mL) and H2O (1.0 mL)were added to a small vial that was sealed and heated to 353 K for 24 h and then cooled to room temperature. Colorless block-like crystals of 1 were obtained and collected in 80% yield. Elemental analysis calculated (%) for C28H15CdN3O8: C, 53.06;H, 2.39; N, 6.63. Found (%): C, 53.28; H, 2.15; N,6.58. IR (KBr disk, cm-1) 1: 3419(m), 1703(m),1562(vs), 1398(s), 1224(w), 1181(w), 1103(w),1071(w),811(m), 744(m), 713(w), 508(w).

2.3 Synthesis of 2

L (13.01 mg, 0.05 mmol), H2BPDC (12.11 mg,0.05 mL), Cd(NO3)2·4H2O (30.85 mg, 0.10 mmol),MeOH (0.5 mL), DMF (2.5 mL) and H2O (1.0 mL)were added to a small vial that was sealed and heated to 353 K for 24 h and then cooled to room temperature. Colorless rod-like crystals of 2 were obtained and collected in 82% yield. Elemental analysis calculated (%) for C31H19CdN2O9: C, 55.09;H, 2.83; N, 4.14. Found (%): C, 55.01; H, 2.76; N,4.23. IR (KBr disk, cm-1) 2: 3423(w), 1699(w),1608(m), 1582(s), 1535(s), 1397(vs), 1224(w),1181(w), 1071(w), 1012(w), 962(w), 894(w),840(w), 812(w), 766(w), 563(w).

2.4 Crystallographic measurements and structure determination

Single-crystal XRD data for the compounds were collected on a Bruker SMART Apex II CCD-based X-ray diffractometer equipped with Cu-Kα radiation(λ = 1.54178 ?). The structures were solved by direct methods with SHELXS-97 and refined with SHELXL-97[12-13]. All the non-hydrogen atoms were refined by full-matrix least-squares techniques with anisotropic displacement parameters. The hydrogen atoms were geometrically fixed at the calculated positions attached to their parent atoms, and treated as riding atoms. For 1, the final R = 0.0568, wR =0.1513 (w = 1/[σ2(Fo2) + (0.0656P)2+ 39.0699P],where P = (Fo2+ 2Fc2)/3), S = 1.130, (Δ/σ)max=0.000, (Δρ)max= 1.318 and (Δρ)min= –1.254 e/?3.For 2, the final R = 0.067, wR = 0.1805 (w =1/[σ2(Fo2) + (0.1024P)2+ 16.6703P], where P = (Fo2+ 2Fc2)/3), S = 1.088, (Δ/σ)max= 0.000, (Δρ)max=1.673 and (Δρ)min= –0.868 e/?3. And the selected bond lengths and bond angles are given in Table 1.

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

3 RESULTS AND DISCUSSION

3.1 Synthesis and characterization

As outlined in Scheme 1, L was synthesized in 93% yield by the coupling reaction of pyridin-4-ylboronic acid with 2,5-dibromobenzaldehyde. Two Cd(II) complexes, 1 and 2, were synthesized under mild reaction conditions. Heating Cd(NO3)2·4H2O,L and H2BDC (2:1:1 molar ratio) in DMF, MeOH and water (3:3:2 volume ratio) at 80 ℃ afforded colorless block-like crystals 1 in good yield. For 2,the similar method was followed as for 1, but H2BPDC was used instead of H2BDC.

The phase purity of 1 and 2 was supported by the powder X-ray diffraction (PXRD) patterns of their bulk samples, which are consistent with the calculated patterns (Fig. 1). The thermal stability of the two complexes was investigated on crystalline samples under N2atmosphere from 40 to 800 ℃.TGA curve shows that 1 loses 11.7% of its total weight upon heating to 240 ℃, which corresponds to the loss of DMF guest molecules per formula unit(calculated 12.0%). Above 325 ℃, the complex is decomposed with the collapse of the structure (Fig. 2).TGA analysis indicates that 2 loses 10.6% of its total weight upon heating to 185 ℃, which ascribes to the release of all water guest molecules (calculated 10.5%). And the framework is stable up to 320 ℃,above which it is also decomposed (Fig. 2). The solid-state luminescence properties of complexes 1 and 2 were studied at room temperature, as shown in Fig. 3. Upon excitation at 275 nm, 1 and 2 exhibit intense photoluminescence band ranging from 325 to 575 nm and 350 to 550 nm, respectively.

Scheme 1. Synthesis of L, 1 and 2

Fig. 1. Experimental and simulated powder XRD patterns of 1 and 2

Fig. 2. Thermal analysis curves of 1 and 2

Fig. 3. Fluorescent emission spectra of 1 and 2 in the solid state

3.2 Structural description

A single-crystal X-ray diffraction study reveals 1 adopts a neutral 3D microporous framework and crystallizes in the orthorhombic space group Pcca with one whole formular unit in the asymmetric unit.As shown in Fig. 4a, the Cd center is coordinated by two oxygen atoms of one chelating bis-bidentate BDC ligand, two oxygen atoms from different bridging bis-bidentate BDC ligands and two trans-related N atoms from two L ligands. The Cd–N bond lengths range from 2.316(2) to 2.318(2)? and Cd–O bond lengths from 2.284(4) to 2.388(5)?. Thus the Cd atom is located at the center of a distorted octahedron, and all interatomic distances and interbond angles around the Cd atom can be considered as normal[14–16](see Table 1). The dinuclear Cd2(CO2)4motif, with Cd··Cd distance of 3.966(2) ?, is linked by four BDC units to form a 2D sheet. Then the sheet is further pillared by L to generate an infinite 3D network, which possesses a large parallelogram channel with diagonal distances of about 13.3? × 17.6? along the b-axis (Fig. 5a).And eight dinuclear Cd2(CO2)4units are engaged in a distorted porous rectangular parallelopiped with a cavity dimension of 11.3? × 10.7? × 16.0?. The overall structure of 1 is a pair of identical nets of[CdL(BDC)], which are mutually interpenetrated with each other to form a doubly interpenetrating 3D framework. The channels of 1 along the b direction are fully filled with Cd2(CO2)4motifs from another net, thus the pores are significantly reduced by double interpenetration of the 3D frameworks (Fig.6a). However, calculations using the PLATON program[17]indicate that 1 has 22.1% of total volume that is accessible for the solvent molecules.

Similar to complex 1, the structure of 2 is constructed of dinuclear Cd2(CO2)4building blocks bridged by four BPDC units and L pillar linkers to form a 3D network. 2 crystallizes in the orthorhombic space group Pbcn and the asymmetric unit contains one formular unit. In 2, coordination environment around the Cd atom can be best described as a greatly distorted pentagonal bipyramid since the chelating/bridging bis-bidentate carboxylate groups take the place of BDC in 1, as shown in Fig. 4b. And all the bond lengths and bong angles around the Cd centers are given in Table 1. Due to the elongated BPDC ligand compared with BDC unit, 2 possesses a larger parallelogram channel with diagonal distances of ～14.0? × 26.7? similar to 1 along the b-axis (Fig. 5b). Although the overall structure of 2 is a pair of identical nets of [CdL(BPDC)], which are mutually interpenetrated with each other to form a doubly interpenetrating 3D framework, there still exists a larger 1D channel of 0.7nm × 0.5nm along the c direction (Fig. 6b). The volume occupied by the lattice solvent molecules in 2 is 3421?3per unit cell, which is 44.0% of the total crystal volume(calculated by the program PLATON).

Fig. 4. View of the bimetal unit of 1 (a) and 2 (b). H atoms are omitted for clarity.

Fig. 5. View of the parallelogram channel in 1 (a) and 2 (b)(The bimetal unit is highlighted by a blue polyhedron)

Fig. 6. View of the space-filling representations of 1 (a) and 2 (b) along the c direction

4 CONCLUSION

In conclusion, we have synthesized a new aldehyde-functionalized pyridine ligand and utilized it as building blocks to construct two Cd-based 3D microporous MOFs. Their structures have been cha-racterized by TGA, single and power X-ray diffrac- tion, respectively.

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