LIU Bo LV Cong LI Xiu-Mei PAN Y-Ru CHE Gung-Bo ZHOU Shi

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Two Three-dimensional Supramolecular Complexes Assembled by 2-Chlorinebenzoic Acid and Bis(imidazol) Ligands: Syntheses, Structures and Fluorescent Properties①

LIU BoaLV CongaLI Xiu-Meib②PAN Ya-RubCHE Guang-BoaZHOU Shia②

a(()130103)b(134002)

Two new three-dimensional supramolecular complexes, namely,[Cd(cba)2(bib)]n(1) and [Zn(cba)2(bib)]2n(2)(Hcba = 2-chlorinebenzoic acid, bib = 1,4-bis(imidazol-1-yl)-butane) were hydrothermallydesigned and synthesized. Their structures were determined by elemental analyses, IR spectroscopy, TG, fluorescence spectroscopy, single-crystal and powder X-ray diffraction. Complex 1 exhibits a one-dimensionalchain and complex 2 shows a zero-dimensional structure, which were further extended into three-dimensional supramolecular structures through hydrogen bonds and-stacking interactions.

crystal structure, Cd(II) complex, Zn(II) complex, 2-chlorinebenzoic acid, 1,4-bis(imidazol-1-yl)-butane;

1 INTRODUCTION

Coordination polymers with bridged transitionmetals have received intense interest and attentionfor their fascinating architectures and potential applicationsas materials in optical, electronic, magneticfields, gas storage, catalysis and so on[1-5]. Consequently,numerous new complexes can be speciallydesigned by the careful selection of metal centerswith preferred coordination geometries[6-8].It hasbeen observed thatorganicligands play crucial rolesin the preparation of some interesting coor- dinationnetworks, such as flexibility, donating type, and thegeometry of organic ligands[9,10]. Among variousorganic ligands, aromatic carboxylates have beenextensively used because of their extension ability inboth covalent bonding and supramolecular interactions(H-bonding and aromatic stacking). For example, benzoic acid, 1,3-benzenedicarboxylate, 1,4-benzenedicarboxylate,1,2-benzenedicarboxylate and 1,2,4,5-benzene tetracarboxylate[11, 12]are well used in the construction of CPs due totheir structural rigidity, chemical stability andappropriate connec- tivity. Besides the carboxylate linkers, bis(imidazole) bridging ligands with different length and flexibly, for example, 1,3-bis(imidazol-1-ylmethyl)-benzene, 1,4-bis(imidazol-1-ylmethyl)-benzene and 1,4-bis- (imidazol-1-yl)-butane, are frequently used in the assembly process of CPs as bridging linkers[12, 13].

In view of these factors, we herein report the syn-theses and characteristics of two new complexes containing 2-chlorinebenzoic acid and 1,4-bis(imi- dazol-1-yl)-butane, namely, [Cd(cba)2(bib)]n(1)and[Zn(cba)2(bib)]2n(2),which show supramolecular architecture by hydrogen bonding or-interactions.

2 EXPERIMENTAL

2.1 General procedures

All chemicals were of AR grade and used without further purification. The contents of carbon, hydro- gen and nitrogen were determined using an Ele- mentar Vario EL III elemental analyzer. IR spectrum was recorded in the range of 4000~400 cm-1on a Nicolet 6700 FTIR spectrophotometer using a KBr pellet. Thermogravimetric analysis data were collected on a TG/DTA7300 analyzer at a heating rate of 10oC·min-1. Powder X-ray diffraction (PXRD) patterns were collected in the 2range of 5~50o with a scan speed of 0.1 o·s-1on a Bruker D8 Advance instrument using a Curadiation (= 1.54056 ?) at room temperature. The fluorescent spectrum was obtained on a computer-controlled JY Fluoro-Max-3 spectrometer at room temperature.

2.2 Synthesis

[Cd(cba)2(bib)]n(1) A mixture of Cd(OAc)2·2H2O (0.133 g, 0.5 mmol), Hcba (0.078 g, 0.5 mmol), bib (0.095 g, 0.5 mmol), 1.0 mL C2H5OH and 9 mL H2O was stirred for 15 min at room temperature, then sealed in a 20 mL Teflon-lined stainless-steel vessel, and heated to 130 °C for 5 days, followed by slow cooling (a descent rate of 5oC/h) to room tempera- ture. Colorless parallelogram-shaped crystals were obtained, with the yield of 19% (based on Cd). Anal. Calcd. for C24H22CdCl2N4O4: C, 46.96; H, 3.61; Cl, 11.54; N, 9.13%. Found: C, 46.03; H, 3.02; Cl, 11.05; N, 8.87%. IR (cm–1): 3119(m), 1580(m), 1558(s), 1523(w), 1411(m),1376(m), 1241(m), 1105(w), 1092(m), 1050(w), 941(w), 857(w), 752(m), 663(w), 462(w).

[Zn(cba)2(bib)]2n(2) A mixture of Zn(OAc)2·2H2O (0.11 g, 0.5 mmol), Hcba (0.078 g, 0.5 mmol), bib (0.095 g, 0.5 mmol), 1.0 mL C2H5OH and 9 mL H2O was stirred for 15 min at room temperature, then sealed in a 20 mL Teflon-lined stainless-steel vessel, and heated to 120 °C for 4 days, followed by slow cooling (a descent rate of 5oC/h) to room temperature. Colorless block-shaped crystals were obtained in 28% yield (based on Zn). Anal. Calcd. for C48H44Cl4N8O8Zn2: C, 50.86; H, 3.91; Cl, 12.50; N, 9.89%. Found: C, 50.21; H, 3.16; Cl, 12.05; N, 9.17%. IR (cm–1): 3132(w), 2932(w), 1631(s), 1588(w), 1536(w), 1433(w),1356(s), 1246(m), 1112(m), 1061(w), 1050(m), 953(w), 840(w), 760(m), 705(w), 658(w), 585(w), 464(w).

2.3 Structure determination

Single-crystal diffraction data of 1 and 2 were respectively collected on a Bruker SMART APEX- CCD diffractometer equipped with a graphite-mono- chromatic Mo(= 0.71073 ?) radiation at room temperature. The structure was solved by direct methods with SHELXS-97 program[14]and refined by full-matrix least-squares techniques on2with SHELXL-97[15]. All non-hydrogen atoms were refined anisotropically and the hydrogen atoms of organic ligands were generated geometrically. Crystallographic parameters and the data collection statistics for structures 1 and 2 are given in Table 1. Selected bond lengths and bond angles are listed in Table 2.

Table 1. Crystal Data and Structure Refinement for 1 and 2

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

Symmetry codes: 1: A:––1/2,–1/2, –+3/2; 2: A: 1–, –, –

3 RESULTS AND DISCUSSION

3.1 Description of the structure

The asymmetric unit of 1 contains one crystallo- graphically unique Cd(II) ion, two cba ligands and one bib molecule. As shown in Fig. 1, the Cd(1) ion is six-coordinated to display a distorted octahedral {CdN2O4} geometry completed by four carboxylate oxygen atoms from two different cba ligands and two nitrogen donors from two flexible bib molecules. The Cd–O bond distances are in the range of 2.3284(19)~2.644(2) ?, and those of Cd–N are from 2.2580(18) to 2.2657(19) ?, which are in the normal range[16]and the N(O)–Cd–O(N) angles fall in the 51.51(7)~137.88(8)o range.

It is interesting to note that the deprotonated carboxylates of cba anion coordinate with Cd(II) ion in a bidentate bridging mode, yet, the bib ligands take a-conformation bridging mode with a dihedral angle between the two imidazole rings of 30.66o. In these modes, bib ligands link adjacent Cd(II) ions to generate a 1chain structure with the Cd···Cd distance of 14.103 ? (Fig. 2); these neighboring chains are linked together by C–H···O hydrogen bonding interactions (Table 3), forming a 3D supramolecular architecture (Fig. 3).

Complex 2 crystallizes in the monoclinic space group21/and exhibits a zero-dimensional struc- ture. The molecular structure of complex2 is shown in Fig. 4. There are two coordination centers, Zn(1) and Zn(1A), in the crystal with the same coordination modes. The coordination geometry around the Zn(1) center could be described as a slightly distorted tetrahedral geometry generated by two carboxylic oxygen atoms (O(1), O(3)) from two different cba ligands and two nitrogen atoms (N(1), N(4A)) from two different bib ligands. The Zn–O and Zn–N bond distances lie in the ranges of 1.934(2)~1.947(2) and 1.993(3)~2.013(3) ?, respectively, both being normal for such coordination bonds[17]. The N(O)–Zn–O(N) angles fall in the 95.09(10)~ 117.37(10)o range.

Fig. 1. View of the asymmetric unit of complex 1. All hydrogen atoms are omitted for clarity

Fig. 2. View of the one-dimensionalchain along theaxis of complex 1

Fig. 3. View of the 3D supramolecular architecture of complex 1 formed by C–H···O hydrogen bonding

Table 3. Hydrogen Bonds for Complexes 1 and 2

Fig. 4. Molecular structure of complex 2. All hydrogen atoms are omitted for clarity

In complex 2, the Hcba ligand adopts a monoden- tate coordination mode using one oxygen atom of the carboxylate group although suitable amount of NaOH is added to the solution (pH, 7), while the bib ligands take a-conformation bridging mode with a dihedral angle between the two imidazole rings of 69.14o. Based on these, cba and bib ligands linked the Zn(II) center to form a double-nuclear subunit with a 24-membered ring.

The related hydrogen-bonding geometries with symmetry codes are given in Table 3. All values involved with hydrogen bonding fall in the normal range. The intermolecular C–H···O hydrogen-bon- ding interactions between the C atoms and carboxylic oxygen atoms stabilize the structure of complex 2. In addition, there are-interactions in2 between the imidazole rings of bib ligands. The centroid-to- centroid distance between adjacent rings is 3.425(3) ? for N(3)C(22)N(4)C(24)C(23) and N(3)C(22)-N(4)C(24)C(23) (Symmetry code: –, –, –1–) imidazole rings. The perpendicular distance is 3.3496(12) ? for N(3)C(22)N(4)C(24)C(23) and N(3)C(22)N(4)C(24)C(23) (symmetry code: –, –, –1–) imidazole rings. Therefore, by C–H···O hydrogen bonding and-interactions, a 3D supra- molecular structure is formed (Fig. 5).

3.2 IR spectrum

In 1 the C–N absorption peaks of imidazole can be observed at 1241 cm-1. Asymmetric and symmetric COO–stretching modes of the lattice cba anion were evidenced by very strong, slightly broadened bands at 1580 and 1411 cm-1[18], which is consistent with the results of X-ray analysis.

In 2, the C–N absorption peaks of imidazole can be observed at 1246 cm-1. Asymmetric and sym- metric COO–stretching modes of the lattice cba anion were evidenced by very strong, slightly broa- dened bands at 1631 and 1356 cm-1[18], which is consistent with the results of X-ray analysis.

3.3 TGA analysis and PXRD results

To determine the thermal stability of complexes 1 and 2, their thermal behaviors were investigated under nitrogen atmosphere by thermogrametric analysis (TGA). As depicted in Fig. 6, the TG curve and then decomposes upon further heating. The TG curve of 2 shows that the complex is stable up to 260 ℃, and then decomposes upon further heating.

Fig. 5. View of the 3D supramolecular structure of complex 2 formed by C-H···O hydrogen bonding and-interactions

Fig. 6. TG curves of complexes 1 and 2

The patterns for the as-synthesized bulk material closely match the simulated ones from the single- crystal structure analysis, which points out the pure solid-state phase (Fig. 7).

Fig. 7. PXRD patterns of complexes 1 and 2 at room temperature

3.4 Photoluminescent properties

Metal-organic coordination polymers, especially10metal centers, such as AgI, AuI, ZnIIand CdII, and conjugated organic linkers have been researched because of their fluorescent properties and potential applications as fluorescent-emitting materials, che- mical sensors and electroluminescent displays[19]. Therefore, in the present work, the photoluminescent properties of Hcba, bib and complexes 1, 2 have been investigated in the solid state at room temperature. The emission and excitation peaks are shown in Fig. 8.

Fig. 8. Solid-state emission spectra of 1 and 2 at room temperature

When excited at 325 nm for 1 and 312 nm for 2, complexes 1 and 2 both exhibit intense blue lumine- scence and show emission peaks at 394 nm (1) and 415 nm (2), respectively. The free ligands Hcba and bib show photoluminescence with the emission maximum at 419 and 433 nm respectively (ex= 368 nm), which can be assigned to intraligand (→*) transition[20]. Because the Zn(II) and Cd(II) ions are difficult to oxidize or reduce due to the10configura- tion, the emission of these compounds is neither MLCT nor LMCT in nature[21, 22]. Thus, the emission bands of complexes 1 and 2 could be assigned to the emission of ligand-to-ligand charge transfer[23-25]. Owing to their strong fluorescent intensity, they appear to be good candidates for novel hybrid inor- ganic-organic photoactive materials.

4 CONCLUSION

In conclusion, two new complexes have been hydrothermallysynthesized, and structurally charac- terized byelemental analysis, IR spectrum, TG, fluorescence spectroscopy, single-crystal and powder X-ray diffraction. Complexes1 and 2 have highthermal stability and are worthy of further study ascandidates of potential photoluminescence materials.

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13 March 2018;

15 June 2018 (CCDC 1828812 for 1 and 1828813 for 2)

① The project was supported by the Science and Technology development plan of Jilin Province (2015052006JH), Science and technology research project of Education Department of Jilin Province (2016219), Center for Science and Technology Innovation on Target recognition and photocatalytic degradation Materials of Jilin Province (20180623042TC) and Science and technology development plan of Siping city (2013055)

Li Xiu-Mei, born in 1969, E-mail: lixm20032006@163.com; Zhou Shi, E-mail:zs@qq.com

10.14102/j.cnki.0254-5861.2011-2003