WANG Qing-Wei SUN Ming WANG Y-Nn QI Xio-Fei LI Xiu-Mei LIU Bo

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Synthesis, Crystal Structure and Theoretical Calculations of a New Two-dimensional Co(II) Coordination Polymer Based on Oxalic Acid and Bis(imidazol) Ligands①

WANG Qing-Weia②SUN MingbWANG Ya-NanaQI Xiao-FeiaLI Xiu-Meic②LIU Boa

a(()136000)b(132101)c(134002)

A new Co(II) coordination polymer has been successfully synthesized under hydrothermal conditions [Co(C2O4)(mbix)]n(1, H2C2O4= oxalic acid, mbix = 1,3-bis(imidazol- 1-ylmethyl)benzene). Its structure has been determined by elemental analyses, IR, UV spectroscopy and single-crystal X-ray diffraction analysis. Pink crystals crystallize in the triclinic system, space groupwith= 8.8666(7),=9.5859(8),= 10.8537(9) ?,=67.6810(10),= 66.1260(10),= 77.1300(10)o,= 777.77(11), C16H14CoN4O4,M= 385.24,D= 1.645 g/cm3,(000) = 394,= 2,(Mo) = 1.134 mm-1, the final= 0.0482 and= 0.1231 for 2968 observed reflections (> 2()). It shows a two-dimensional (2D) network structure. The intermolecular C–H···O hydrogen bonding andstacking interactions extend complex 1 into a 3D supramolecular architecture andplay an important role in stabilizing 1. In addition, Natural Bond Orbital (NBO) analysis was performed by using the PBE0/LANL2DZ method built inGaussian 09 Program. The calculation results showed obvious covalentinteraction between the coordinatedatoms and Co(II) ion.

hydrothermalsynthesis, crystal structure,Co(II) complex, naturalbond orbital;

1 INTRODUCTION

Recently, studies on the synthesis of coordination polymers (CPs) have received much attention in coordination chemistry because of their interesting molecular topologies and tremendous potential applications in catalysis, molecular selection, non- linear optics, ion exchange and microelectronics[1-5]. Generally speaking, the structural diversity of such crystalline materials dependent on many factors, such as metal ion, templating agents, metal-ligand ratio, pH value, counteranion, and number of coor- dination sites provided by organic ligands[6, 7]. Among the strategies,the rational selection of organic ligands or coligands according to their length, rigidly, and functional groups is important for the assembly of structural controllable CPs, and a great deal of significant workshave has beendone by using this strategy[8]. Usually, the organic ligands with bent backbones, such as V-shaped, triangular, quadrangular, and so on, are excellent candidates for building highly high-connected, interpenetrating, or helical coordination frameworks due to their bent backbones and versatile bridging fashions[9, 10]. Besides, the carboxylate groups are good hydrogen- bond acceptors as well as donors, depending upon the degree of deprotonation. Among them, quadran- gular polycarboxylic acids like 1,2,4,5-benzenete- tracarboxylic acid, 3,3?,4,4?-benzophenonetetra- carboxylic acid, 4,4?-oxydibenzoic acid and oxalic acid are paid much attention due to their rich coor- dination modes[11]. Apart from the carboxylate lin- kers, bis(imidazole) bridging ligands with different length and flexibly, such as 1,3-bis(imidazol-1-yl- methyl)benzene,1,4-bis(imidazol-1-ylmethyl)ben-zene, and 1,4-bis(imidazol-1-yl)-butane, are fre- quently used in the assembly process of CPs as bridging linkers[12].

Thus, these considerations inspired us to explore new coordination architectures with oxalate and (bis)imidazole bridging linkers (mbix). In this paper, we report the synthesis and characterizations of 1,which exhibits a two-dimensional (2D) network structure and isdifferent fromour previous reports for oxalate-bridged complexes[11(d)].

2 EXPERIMENTAL

2. 1 General procedures

All reagents were purchased commercially and used without further purification. Elemental analyses (C, H and N) were measured on a Perkin-Elmer 2400 CHN Elemental Analyzer. IR spectrum was recorded in the range of 4000～400 cm-1on a Nicolet 6700 spectrometer using a KBr pellet. The UV spectrum was obtained on a Shimzu UV-250 spectrometer in the 200～400 nm range.

2. 2 Synthesis of [Co(C2O4)(mbix)]n

The title compound was prepared from a mixture of CoC2O4(0.073 g, 0.4 mmol), pyridine-3,4-dicarboxylic acid (0.033 g, 0.2 mmol), mbix (0.048 g, 0.2 mmol) and H2O (18 mL) in a 30 mL Teflon-lined stainless steel vessel which was then sealed and heated at 140 ℃ for 5 days. After the reaction mixture was slowly cooled down to room temperature at a rate of 5 ℃/h, pink block crystals of compound 1 were obtained. Yield: 35%. Anal. Calcd. for C16H14CoN4O4: C, 49.88; H, 3.66; N, 14.54. Found (%): C, 49.36; H, 3.49; N, 14.21. IR (cm-1): 3151w, 1670m, 1609s, 1525m, 1445w, 1435m, 1406w, 1361w, 1313m, 1228m, 1104w, 1085m, 1036w, 941m, 795m, 733w, 658m, 638w, 491w.

2. 3 Structure determination

A single crystal of the title compound with di- mensions of 0.301mm× 0.260mm × 0.210mm was mounted on a Bruker CCD diffractometer equipped with a graphite-monochromatic Mo(= 0.71073 ?) radiation using anscan mode at 293(2) K. In the range of 4.34<2<52.10o, a total of 4055 reflec- tions were collected and 2968 were independent withint= 0.0203, of which 2432 were observed with> 2(). The correction forfactors was applied. The structure was solved by direct methods with SHELXS-97 program[13]and refined by full-matrix least-squares techniques on2with SHE- LXL-97[14]. All non-hydrogen atoms were refined anisotropically and hydrogen atoms isotro- pically. The H atoms of coordinated water molecule were located from difference Fourier syntheses and the hydrogen atoms of organic ligands were generated geometrically. The final= 0.0482 and= 0.1231 (= 1/[2(F2) + (0.0931)2+ 0.0143], where= (F2+ 2F2)/3).= 1.128, (Δ)max= 0.519, (Δ)min= –0.518 e/?3and (Δ/)max= 0.000. The selected important bond parameters are given in Table 1.

3 RESULTS AND DISCUSSION

3. 1 IR spectrum

The COO–is coordinated with its asymmetric and symmetric stretching vibration appearing at 1609 cm–1((OCO)asym) and 1435 cm–1((OCO)sym)[15], respectively. The Δ((OCO)asym–(OCO)sym) is 174 cm–1(< 200), showing the presence of bidentate linkage of carboxylates in the dianions. Thus the carboxylates coordinate to the metal as bidentate ligandsthe carboxylate groups[16]. The absence of characteristic bands around 1700 cm-1in com- pound 1 attributed to the protonated carboxylic group indicates the complete deprotonation of H2C2O4ligand upon reaction with Co ions[17]. In addition, X-ray diffraction analysis further indicates the bidentate coordination manners of carboxylate groups and the deprotonation of H2C2O4ligands.

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

Symmetry transformations used to generate the equivalent atoms: A:,+1, z–1; B: 1–, –, 1–; C: 1–, 1–, 1–

3. 2 Description of the structure

A single-crystal X-ray diffraction study reveals that compound 1 crystallizes in triclinic space groupand features a two-dimensional (2D) network structure. The coordination environment of Co(II) in 1 is shown in Fig. 1. There are one Co(II) ion, one C2O42-ligand and one mbix ligand in the asymmetric unit. Each Co(II) ion is six-coordinated by four carboxylate oxygen atoms (O(1), O(2B), O(3), O(4C)) from two different C2O42-ligands and two nitrogen donors(N(1) and N(4A)) from two flexible mbix molecules to furnish a distorted octahedral coordination architecture. The bond distances of Co–O in compound 1 fall in the 2.087(3)～2.155(3) ? range, and the Co–N bond lengths vary from 2.113(3) to 2.130(3) ?, which are in normal ranges[18]and the coordination angles around Co atom are in the 77.72(10)～172.86(11)o region. In the coordination environment, the three carboxylate oxygen atoms (O(2B), O(3), O(4C)) and one imidazole nitrogen atom (N(1)) are located in the basal plane, whereas one carboxylate oxygen atom (O(1)) and one imidazole nitrogen atom (N(4A)) occupy the axial positions from the opposite directions. In the crystal structure of complex 1, the mbix ligand adopts a-conformation bridging mode with a dihedral angle between the two imidazole rings of 46.27o. The completely depro- tonated C2O42-ligands display one kind of coor- dination mode, namely bidentate bridging mode, and link the Co(II) ions to form a one-dimensional chain. The distances of neighboring Co(II) ions are about 5.471 and 5.559 ?, as illustrated in Fig. 2. The neighboring chains are bridged by mbix ligands to afford a 2D network with (4,4) topology (Fig. 2). The Co···Co distances separated by mbix ligands are 11.431 ?. The resulting networks are packed in a parallel fashion and stacked alongaxis.

Hydrogen bonding interactions are usually important in the synthesis of supramolecular archi- tecture[19]. There are persistent C–H···O hydrogen bonding interactions between carboxylate oxygen atom and carbon atoms of mbix ligands in complex 1 (Table 2). Therefore, the two-dimensional net- works are further extended into a three-dimensional supramolecular framework through hydrogen bon- ding interactions (Fig. 3).

To investigate whether the analyzed crystal structure is truly representative of the bulk materials, X-ray powder diffraction (PXRD) technology has been performed for the complex at room temperature (Fig. 4). The main peak positions observed are in good agreement with the simulated ones. Although minor differences can be found in the positions, widths, and intensities of some peaks, it still can be considered that the bulk synthesized materials and the analyzed crystal are homogeneous. The dif- ferentces may be due to the preferred orientation of the powder samples[20, 21].

Table 2. Hydrogen Bonds for Complex1

Fig. 1 . Coordination environment of the Co(II) center in1 (Symmetry codes: A: x, y+1, z–1; B: 1–x, –y, 1–z; C: 1–x, 1–y, 1–z)

Fig. 3 . 3D Supramolecular framework viewed along the b axis in 1

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

3. 3 UV spectrum

The UV spectra for the title compound, H2C2O4, and mbix ligands have been investigated in the solid state (Fig. 5). For H2C2O4ligand, there is no absorption band, while both the title compound and mbix have one absorption band at about 278 nm, which should be assigned to the n→*[22]transition of mbix. However, after mbix coordinating to the Co2+ion, the absorption intensity slightly increases. It is clear that the absorption band in mbix remains in the same position with that in the title compound, showing they are not affected basically by the metal coordination.

4 THEORETICAL CALCULATIONS

All calculations in this work were carried out with the Gaussian03 program[23]. The parameters of the molecular structure for calculation were all from the experimental data of the complex. Natural bond orbital (NBO) analysis was performed by density functional theory (DFT)[24]with the PBE0[25]hybrid functional and the LANL2DZ basis set[26].

Fig. 5 . UV spectrum of 1 at room temperature

The selected natural atomic charges and natural electron configuration for the complex are shown in Table 3. It is indicated that the electronic con- figuration of Co(II) ion, N and O atoms are 40.2237.6450.43, 21.66~1.6824.95~4.96and 21.35~1.3624.10~4.12, respectively. Based on the above results, one can conclude that the Co(II) ion coor- dination with N and O atoms is mainly on 3, 4and 5orbitals (The electron number of 4and 4is so small that it can be omitted). N atoms form coor- dination bonds with Co(II) ion using 2and 2orbitals (The electron number of 3and 3isso small that they can be omitted). Four O atoms supply electrons of 2and 2(The electron number of 3and 3isso small that it can be omitted) to the Co(II) ion and form the coordination bonds. There- fore, the Co(II) ion obtained some electrons from two N atoms of mbix and four O atoms of C2O42-ligands. Thus, according to valence-bond theory, the atomic net charge distribution and the NBO bond orders of complex 1 (Table 3) show obvious cova- lent interaction between the coordinated atoms and Co(II) ion. The differences of NBO bond orders for Co–O and Co–N make their bond lengths different[27, 28], which is in good agreement with the X-ray crystal structural data of complex 1.

Table 3. Natural Atomic Charges, Natural Valence Electron Configurations, Wiberg Bond Indexes and NBO Bond Orders (a.u) for 1

Symmetry codes:A:,+ 1,– 1; B: 1 –, –, 1 –; C: 1 –, 1 –, 1 –

As can be seen from Fig. 6, the HOMO is mainly composed of Co(II) ion and ligands, and the LUMO of Co(II) ion and ligands, too. So, the charge transfer from ligand-to-metal and ligand-to-ligand may be inferred from some contours of molecular obitals of complex 1.

Fig. 6 . Frontier molecular orbitals of complex 1

5 CONCLUSION

In summary, we have reported a new cobalt com- plex formed by oxalate and bis(imidazol) ligands. In compound 1, the oxalate ligands function in a bidentate bridging coordination mode and link the Co(II) ions to form a one-dimensional chain. The neighboring chains are bridged by mbix ligands to afford a 2D network with (4,4) topology. It is worthy to note that the intermolecular hydrogen bonds of C–H···O and-interactions play an important role in the supramolecular structure. Here we study the synthesis, structure and UV properties of the coor- dination polymer 1. This material will give new impetus to the construction of novel functional material with potentially useful physical properties. In addition, Natural Bond Orbital (NBO) analysis was performed by using the PBE0/LANL2DZ me- thod built inGaussian 09 Program. The calculation results showed obvious covalent interaction between the coordinatedatoms and Co(II) ion.

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14 July 2014; accepted5 January 2015 (CCDC 979113)

①The project was supported by the Science and Technology Development Project of Jilin Provincial Science & Technology Department (201205080) and the Science and Technology Research Projects of the Education Office of Jilin Province (No. 2013. 384)

. Wang Qing-Wei, born in 1961. Tel: 0434-3291973, E-mail: wqw611223@163.com; Li Xiu-Mei, born in 1969. Tel: 0435-3208077, E-mail: lixm20032006@163.com

10.14102/j.cnki.0254-5861.2011-0452