CHEN Cheng-Yun SUN Wei-Ming WENG Qing-Hua HUANG Sheng KANG Jie

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Synthesis, Characterization, Optical Property and Bioactivityof the Eu(Ⅲ) Complex with Aromatic Carboxylic Triazole①

CHEN Cheng-Yun SUN Wei-Ming WENG Qing-Hua HUANG Sheng KANG Jie②

(350004)

Eu(Ⅲ) complex, aromatic carboxylic triazole, quantum chemistry, fluorescence, anti-bacterial activity;

1 INTRODUCTION

In recent years, the design and synthesis of new lanthanide organic complexes have been of great interest due to their usage in many areas of medical science not only because of their fascinating network topological structures[1-4]but also on account of their popular applications in a wide variety of fields such as fluorescence[5-8], targeted therapy[9], anti-bac- terial[10, 11]and anti-cancer[12-14]. As a characteristic multi-chelating ligand, aromatic carboxylic triazole [5-(4H-1,2,4-triazol-4-yl) isophthalic acid] plays a crucial role in constructing novel and stable lantha-nide organic frameworks mainly because of the rigid structure, strong coordination ability, multi-function bridge site, molecular hydrogen bonding and other properties.

Lanthanide ions are good candidate central atoms due to their remarkably high affinity for donor atoms, such as nitrogen, oxygen or hybrid nitrogen-oxygen atoms. The organic frameworks activate lanthanide ions to emit strong characteristic fluorescence by the “antenna effect”[15]. The hydrothermal synthesis method is proved to be an effective technique to synthesize organometallic complexes. Taking advan-tage of this method, a new coordination polymer{[Eu(H5TIA)(C2O4)0.5(H2O)3]·H2O}nhas been suc-cessfully synthesized. Herein reported are the synthesis, characterization, optical property and bioactivity of the Eu(Ⅲ) complex with aromatic carboxylic triazole.

2 EXPERIMENTAL

2.1 Materials and physical measurements

All the reagents were obtained from commercial sources and used as received without further puri-fication. IR spectra were obtained using a Nicolet IS50 FTIR spectrometer with KBr pellets in the range of 4000～500 cm-1. Ultraviolet absorption spectra were performed on a Cary 60 UV-Vis spec-trometer. Luminescent properties were recorded on a Cary Eclipase fluorescence spectrometer. Optical rotation was measured with a WZZ-2S automatic polarimeter. Crystal structure data were collected on a Bruker APEX-II CCD diffractometer.

2.2 Synthesis

A mixture of Eu(NO3)3·6H2O (0.1338 g, 0.3 mmol), 5-(4H-1,2,4-triazol-4-yl) isophthalic acid (0.0466 g, 0.2 mmol) and oxalic acid dihydrate (0.0252 g, 0.2 mmol) were dissolved in 10 mL H2O. The resulting solution was added in a 25 mL Teflon-lined reactor and heated under autogenous pressure at 423 K for 3 days and then cooled slowly to room temperature at a rate of 10 K·h-1. The colorless crystals were obtained by filtration of the resultant solution.

2.3 Crystal structure determination

A suitable single crystal of the complex (0.28mm × 0.15mm × 0.12mm) was selected and mounted on a glass fiber for X-ray analysis. Diffraction data were collected on a Bruker APEX-II CCD diffractometer with a graphite-monochromatic Mo-radiation (= 0.71073 ?). A total of 6328 reflections including 3081 independent ones (int= 0.0217,sigma= 0.0325) were collected in the range of 4.146<2<52.616° with anscan mode and corrected byfactors and empirical absoption. The structure was solved by direct methods with SHELXS-97 (Sheldrick 2008) and refined by full-matrix least-squares techniques on2with SHELXL-2014 (Sheldrick 2014). All non-hydrogen atoms were refined with anisotropic temperature factors. Hydrogen atoms were generated geometrically.

2.4 Quantum chemistry calculation

By means of density functional theory com-putation, the ESP, NBO charges and frontier orbital distribution of the complex were analyzed through Gaussian 09W software with the most commonly used B3LYP method of DFT and 6-32G basis set. Furthermore, the views were made by Gauss View 5.0 software[16].

2.5 Inhibition measurement

The anti-bacterial activities against Gram positive (S.) and Gram negative (E.) bacteria were detected by utilizing the agar diffusion method. The results are judged and reported in Section 3. 4.

2 RESULTS AND DISCUSSION

3.1 Crystal structure

Fig. 1. Molecular structure of [Eu(H5TIA)(C2O4)0.5(H2O)3].H2O

(Symmetry codes: (1): 2 –, 2 –, 1 –; (2): 1 –, 2 –, 1 –; (3): 1 –, 1 –, 1 –)

Fig. 2. Cell packing diagram of this complex along theaxis with no disordered atoms considerd here for clarify

In the title complex, the europium atoms show oxidation states +3. Two ends of the deprotonated ligand (H5TIA)2–coordinated to three Eu(Ⅲ) ions with one2mode and another1mode, and oxalic acid ion (C2O4)2–coordinated to two Eu(Ⅲ) ions with the2mode. There also exist several hydrogen bonds of O–H×××N and O–H×××O between the depro- tonated ligands (H5TIA)2–and water molecules (O(3)–H(3B)×××N(2)4, O(4)–H(4A)×××O(9)5, O(5)–H(5B)×××O(9)2) and between the deprotonated oxalate ligand (C2O4)2–and water molecule (O(3)–H(3A)×××O(7)3). These hydrogen bonding interactionsplay an important role in the formation and stability of this compound. Besides, it is observed that two adjacent complexes are combined together by one guest water molecule through two hydrogen bonds (O(4)–H(4B)×××O(8), O(8)–H(8A)×××O(10)2), which also helps to stabilize the supramolecular framework. Experimental details for bond lengths, bond angles and hydrogen bonding parameters are presented in Tables 1 and 2, respectively.

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

Symmetry transformations: (1): 2 –, 2 –, 1 –; (2): 1 –, 2 –, 1 –; (3): 1 –, 1 –, 1 –

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

Symmetry codes: (1): 1 +, 1 +,; (2): 1 +, 1 +, 1 +; (3): –1 +,,; (4): 1 –, 2 –, 2 –; (5): 1 –, 1 –, 1 –

3.2 Optical characterization

The luminescent spectra of ligand (H7TIA) and the title complex are presented inFig. 3. It is observed that the infrared characteristic peaks O–H stretching vibration (3400～2500) and its in-plane vibration (～1430) of carboxylic acid group in this complex are greatly reduced and shifted to different degree, because the carboxylic group of (H5TIA)2-coor- dinates with the Eu(Ⅲ) cation to form the single tooth oxygen which decreases the electron cloud density of COO-. The high UV absorption wave-length of the complex is 280～315 nm, and the complex exhibits fluorescence wavelength at about 786 nm upon excitation at 312 nm, which is attributed to the “antenna effect”. Herein, the optical rotation angle of the complex is –3.623° under the circumstance of natrium lamp (= 589 nm), T = 285 K, C = 100 ug.mL-1,= 2 dm and 5% NaHCO3solution, reflecting that this studied complex is a left-handed triclinic crystal.

3.3 Quantum chemistry calculation

The NBO charges, ESP and frontier orbital of this complex obtained from quantum chemistry calcula-tion are shown in Fig. 4. In Fig. 4a, N(2), N(3), O(1), O(7), O(9), and O(10) carry negative charges of –0.286 |e| ~ –0.619 |e| while the central Eu(Ⅲ) carries positive charge of +1.388 |e| in this studied unit. This indicates that these six atoms have the tendency to bind with Eu(Ⅲ) cation in the adjacent units, which is further confirmed by its ESP. Besides, as shown Fig. 4c, the highest occupied molecular orbital (HOMO) is mainly composed of theatomic orbitals of O(7), O(9) and O(10), indicating these oxygen atoms are the nucleophilic activity points. Especially, the electron cloud of the lowest unoccu- pied molecular orbital (LUMO) is mainly distributed on the central Eu(Ⅲ), suggesting the high electro- philic activity of Eu(Ⅲ) in this complex (Fig. 4d). Considering the high electrophilic activity of Eu(Ⅲ) in this synthesized compound, it may have the potential to bind with the nucleic acid bases of DNA of RNA, thus serving as anti-cancer of anti-bacterial drugs[20-22].

Fig. 3. Luminescent spectra of the studied complex and its ligand (H7TIA)

Fig. 4. NBO charges, ESP and frontier orbital of this studied complex (a: NBO charge, b: ESP, c: HOMO, d: LUMO)

3.4 Biological activity

The anti-bacterial activities of ligand (H7TIA) and the synthesized complex are presented inFig. 5. Before dosing, bacteria were inoculated in the agar culture dish containing nutrient solution. Then 10 uL (1.5 mg·mL-1) ligand and 10 uL (1.5 mg·mL-1) complex respectively were dropped onto the filter paper with three layers in the petri dish. After training for 24 h, the inhibition zone was observed against S.and E., and the antibacterial circle diameter is about 9~16 mm. The experiment demonstrates that the studied complexpossesses a moderate inhibiting effect, and the inhibition effect on E.is better than that on S.. The bacteriostasis mechanism was presumed as follows: (1) In this complex, the chelation between the ligands and metal ions increases the molecular conjugate surface and lipophilicity to form ground state compounds via combining with membrane proteins, which changes the permeability of cell membrane and destroys the integrity of the cell wall[23]. (2) Electrophilic activity of Eu(Ⅲ) in this complex makes for its combination with the DNA bases containing nitrogen or oxygen atoms with rich electrons (such as adenine and guanine), which destroys the structure of DNA[24, 25].

Fig. 5. Anti-bacterial activity of the title ligand and complex (solvent: 5% NaHCO3solution)

4 CONCLUSION

The synthesized complex{[Eu(H5TIA)(C2O4)0.5-(H2O)3].H2O}nof Eu(Ⅲ)with aromatic carboxylic triazole (H7TIA = 5-(4H-1,2,4-triazol-4-yl) isoph-thalic acid, H2C2O4= oxalic acid) exhibits fluore-scence wavelength at about 786 nm upon the exci-tation at 312 nm, but the fluorescence intensity is very high. The optical rotation angle of the complex is –3.623°, reflecting that it is a left-handed triclinic crystal. This complex possesses a moderate inhi-biting effect, and the inhibition effect on E.is better than that of S.. The Eu(Ⅲ) complexes have potential applications in the field of phar-maceutical chemistry so as to play a role in biology, medicine and other bio-medical fields.

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14 March 2018;

31 May 2018 (CCDC 1842994)

① This work was supported by the Natural Science Foundation of Fujian Province (No. 2015J01597, 2016J05032), and Professor Found of Fujian Medical University (JS14008)

. Kang Jie (1966-), male, professor, mainly engaged in the study of synthesis and biological activity of the complexes. Tel: 15396125563, E-mail: davidkj660825@163.com

10.14102/j.cnki.0254-5861.2011-2004