LIU Xi-Hui ZHUO Xin② LIU Chao WANG Bin

a (School of Chemical Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, China)

b (Anhui Key Laboratory of Spin Electron and Nanomaterials,Suzhou University, Anhui 234000, China)

1 INTRODUCTION

Over the past ten years, metal-organic coordination complexes have drawn great research interest in view of their fascinating structural diversities and potential applications in molecular adsorption and separation processes[1-3], magnetism[4], catalysis[5]and fluorescence properties[6-7]. It is known that combining different ligands in a complex offers greater tunability of structural frameworks than using single ligand. Up to now, a large number of mixed-ligand MOFs have been reported[8-11].

Flexible TPOM ligands have four N atoms, based on which many complexes with good frameworks have been produced. But many literatures show that TPOM was used to mix with rigid polycarboxylic acid ligands to fabricate metal-organic frameworks,while flexible catenulate aliphatic polycarboxylate acids have not been applied by now. So, we select TPOM and catenulate aliphatic polycarboxylate acids as the primary ligands to synthesize coordination complexes, receiving a new complex with TPOM and GA ligands coordinated to the Cu nitrate.The new was characterized by elemental analysis, IR spectra, and X-ray crystallography. In this paper, the details of synthesis and crystal structures of the complex are presented.

2 EXPERIMENTAL

2. 1 Materials and instruments

Reagents and solvents employed were commercially available and used as received. IR absorption spectrum of the complex was recorded in the range of 400～4000 cm-1on a Nicolet (Impact 410)spectrometer with KBr pellets (5 mg of sample in 500 mg of KBr). C, H and N analyses were carried out with a Perkin Elmer 240C elemental analyzer.Powder X-ray diffraction (PXRD)measurements were performed on a Bruker D8 Advance X-ray diffractometer using Cu-Kα radiation (0.15418 nm),in which the X-ray tube was operated at 40 kV and 40 mA.

2. 2 Preparation

A mixture of Cu(NO3)2·3H2O (0.2 mmol, 0.0483 g), GA (0.10 mmol, 0.0132 g)and TPOM (0.05 mmol, 0.0226 g)was added slowly to 15 mL DMF/H2O (1:5, v/v). The resultant solution was continuously stirred for 30 min and then filtered.The mixture was transferred to a Parr Teflon-lined reaction vessel (ca. 25 mL)and placed inside a preheated oven at 110 ℃ for 3 days. After reaction,the vessel was allowed to cool slowly to room temperature, resulting in green block single crystals suitable for X-ray diffraction study (yield 64%). Elemental analysis (%): calculated for C35H36Cu2N4O12:C, 50.50; H, 4.33; N, 6.73. Found (%): C, 50.52; H,4.28; N, 6.75. IR data (cm-1): 3427(m), 1579(s),1460(m), 1425(m), 1314(w), 1239(w), 1056(w),880(m), 766(w), 646(w), 463(m).

2. 3 X-ray single-crystal structure determination

Crystallographic measurements were carried out using a Bruker SMART APEX-CCD (Version 5.0)diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 ?)using a φ-ω scan mode at 293(2)K. SMART was used for data collection, SAINT (Version 6.02)for data integration, and SADABS for absorption correction.The structure was solved by direct methods with SHELXS-97[12]and refined by full-matrix leastsquares on F2using the SHELXL-97[13]package.The hydrogen atoms of TPOM, GA ligands and water molecules were placed at the calculated positions, allowed to ride on their respective parent atoms, and assigned by fixed isotropic thermal parameters. The final R = 0.0547, wR = 0.1718,GOOF = 1.089, (Δρ)max= 0.322 and (Δρ)min=–0.451 e/?3. The selected bond lengths and bond angles are given in Table 1.

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

3 RESULTS AND DISCUSSION

3. 1 Crystal structure

X-ray single-crystal diffraction study reveals that the complex crystallizes in monoclinic, space group P2/c. The asymmetric unit contains two independent Cu(II)centers, one half of TPOM ligand, and one GA molecule. As shown in Fig. 1, there are two types of coordination environments around the Cu(II)centers. Cu(1)is in a square planar geometry, which is defined by two carboxylic oxygen (O(1)and O(1)#1)atoms from two different GA ligands and two nitrogen (N(1)and N(1)#1)atoms from two different TPOM ligands. The bond lengths of Cu(1)–O and Cu(1)–N are 1.9697(15)and 1.9992(15)?, respectively. Cu(2)is six-coordinated to adopt a distorted octahedral coordination environment in the equatorial plane. There are three oxygen atoms(O(3)#5, O(4)#4 and O(4)#5)from two different GA anions and a nitrogen atom (N(2)#3)from the TPOM ligand. The apical positions are occupied by one oxygen (O(3)#4)atom belonging to the GA ligand and one nitrogen (N(2))atom from the TPOM ligand. The bond lengths of Cu(2)–O(3), Cu(2)–O(4)and Cu(2)–N(2)are 1.995(3), 2.4537(46), and 2.0030(16)?, respectively. These distances fall in the normal range found in other Cu complexes. In the environment around Cu(1), the angle of O(1)–Cu(1)–N(1)is almost 90°, while for Cu(2), the O(3)#4–Cu(2)–N(2), O(3)#5–Cu(2)–N(2), O(4)#4–Cu(2)–N(2)and O(4)#5–Cu(2)–N(2)angles are 151.879(96), 94.673(96), 96.222(88)and 106.936(87)°, respectively.

Fig. 1. Coordination environment of the Cu(II)ion in complex (The hydrogen atoms are omitted for clarity). Symmetry codes: #1: ?x+1, ?y+2, ?z; #2: ?x+1, y, ?z+1/2;#3: ?x, y, ?z+1/2; #4: ?x, y ?2, ?z+1/2; #5: x, y?2, z; #6: x, y+2, z

Each square planar and octahedral Cu atom coordinates to the TPOM ligands to form infinitely wave-like 2D sheet network along the b axis (Fig. 2),and the lengths and angles of the two sides are 11.556(4), 16.7644(50)? and 89.944(2),90.056(20)°, respectively. In this complex, the shape of TPOM is shown in Fig. 3a, where four pyridyl groups are in an irregular orientation. The angles of N(2a)–C(11)–N(2b), N(2a)–C(11)–N(1a)and N(2a)–C(11)–N(1b)are 86.201(19), 120.916(20)and 121.812(20)°, respectively. Between two layers,there are two actions linked with each other to form a 3D framework (Fig. 3b, 3c), which make it very special. On one hand, the nitrogen atoms of pyridyl in the TPOM ligands link Cu atoms belonging to adjacent 2D sheets; on the other hand, each GA ligand links two Cu atoms between adjacent 2D layers through the carboxylate oxygen to generate a 3D framework. In particular, the linking between two carboxylate radicals from one GA ligand and the Cu center is different. As shown in Fig. 3b, one carboxylate radical acts as a bidentate bridging ligand in which two oxygen atoms coordinate to one Cu center, while the other carboxylate radical is a monodentate ligand, in which one oxygen atom coordinates to one Cu center. In order to stabilize the framework during the assembly process, other identical networks are filled in the cavities giving a 2-fold interpenetrating 3D architecture. In Fig. 3c,these reticulate channels are filled with GA and pyridyl from TPOM ligands. The schematic of 2-fold interpenetrating 3D framework is represented in Fig. 3d.

Fig. 2. View of the wavelike 2D sheet by TPOM and Cu anions along the b axis (The hydrogen atoms are omitted for clarity)

Fig. 3. (a)Shape of TPOM in the complex; (b)View of the Cu(II), TPOM and GA forming the 3D network structure; (c)3D network of the complex; (d)Schematic representation of 2-fold interpenetrating 3D framework of complex (The hydrogen atoms are omitted for clarity)

3. 2 XRD results and IR spectroscopy analysis

The phase purity of the as-synthesized complex was confirmed by X-ray powder diffraction (XRD)pattern, consistent with the simulation (see Fig. 4).

Fig. 4. Experimental (lower trace)and simulated (upper trace)X-ray powder diffraction patterns of the title complex

The infrared spectrum of the complex was recorded and some important assignments are shown in the experimental section. The strong broad peak at 3427 cm-1was assigned to the stretching vibration of hydroxyl in GA. The absence of the peak at 1730～1700 cm-1indicated the coordination of carboxylic group in GA[14]. The very strong band at 1579 cm-1was ascribed to the asymmetrical stretching mode of COO-groups, while the shoulder at 1425 cm-1corresponded to the symmetrical COO-stretching modes. These IR results are coincident with the crystallographic structural analyses.

4 CONCLUSION

In summary, we have successfully synthesized and characterized one new complex by the selfassembly of TPOM, GA, and Cu(II)ions under solvothermal conditions. The complex is a 2-fold interpenetrating 3D network structure. In some literatures, TPOM were mixed with aromatic polycarboxylate acid ligands[15-16]. In this complex,aliphatic GA is first used to coordinate with the TPOM ligand, and there are two actions between the 2D sheets to form a 3D structure, which is rare in coordination complexes with TPOM ligand.Subsequent work will be focused on the structures and properties of a series of coordination complexes constructed by the TPOM ligand with more auxiliary polycarboxylate acid ligands and metal ions.

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