YAN Li LIU Wei WANG Mi-Ji XU Yue SHI Ke-Zhuo

a (Key Laboratory of Preparation and Applications of Environmental Friendly Materials, Ministry of Education, Jilin Normal University, Siping, Jilin 136000, China)

b (College of Computer Science, Jilin Normal University, Siping, Jilin 136000, China)

ABSTRACT We have synthesized one novel Schiff-base ligand by modifying the aromatic aldehyde: H2L1 (H2L1 = N,N?-bis(2-oxy-acetate-3-methoxyl)benzylpropylene-ethanediamine). [Co(II)L1]2·2EtOH (1) was prepared by the reaction between H2L1 and CoCl2·6H2O in the solvent of C2H5OH. The title compound was structurally characterized by elemental analysis, IR, H NMR and single-crystal X-ray diffraction. Complex 1 crystallizes in monoclinic, space group C2/c with a = 29.472(3), b = 13.4842(13), c = 15.1848(15) ?, β = 115.626(1)°, V = 5441.0(9) ?3, C28H22CoN2O8.50, Mr = 581.41, Dc = 1.420 g/cm3, μ(MoKα) = 0.685 mm-1, F(000) = 2392, Z = 8, the final R = 0.0541 and wR = 0.1565 (I > 2σ(I)). The Co(II) atom is hexa-coordinated, furnishing a triangular prism geometry. It is interesting that the H-bond intersections formed a one-dimensional chain structure. In this paper, we research the synthesis, characterization, oxygen respiratory, antibacterial activity, and photoluminescent property of complex 1.

Keywords: Schiff-base, HNMR, oxygen respiratory, antibacterial activity, photoluminescent property;

1 INTRODUCTION

In recent years, the transition metal complexes containing Schiff-base have attracted the attention of chemical workers for their unique anti-cancer, antibacterial, oxygen-carrying and optical properties[1-6]. Therefore, it is of great significance to design and synthesize new Schiff base complexes and study their properties. There are many organisms containing transition metal ion proteins in nature, which can absorb or release oxygen under certain conditions to meet the needs of life activities, such as hemoglobin containing iron, myoglobin and serum protein containing copper. Similar phenomena can be observed in some inorganic complexes (such as complexes [CoII(Salen)], glycyl glycine cobalt complexes, etc.)[7-9]. This kind of inorganic coordination polymer has been widely used as a model compound for the study of oxygen carriers. In this paper, Co(II) complex 1 was obtained by step-by-step synthesis. The structure of Co(II) complex was characterized by X-ray single-crystal diffraction, IR, NMR and elemental analysis. The data show that the central ion cobalt is a hexagonal coordination configuration, forming a distorted triangular prism configuration centered on cobalt. In addition, the oxygen-carrying and luminous properties of the complexes were studied. For the synthesis route of complex 1, please see Scheme 1.

Scheme 1. Synthesis route of complex 1

2 EXPERIMENTAL

2. 1 Measuring and test instruments

Element analyzer (Perkin-Elmer PE 2400), infrared spectrometer (Vertex 70 FTIR); 1H NMR spectra (Bruker AVANCE AV 400MHz, TMS); Mass spectra (VGZAB-HS, ESI); X-ray diffractometer (Bruker CCD Area Detector); digital melting point instrument (WRS-1A); luminescent spectra (Japan F-4500 fluorescence spectrometer).

2. 2 Synthesis of the ligand H2L1

The deionized aqueous solution (10 mL) dissolved with NaOH (4.0 g, 0.1 mol) was cooled in an ice bath, theno-vanillin (3.8 g, 25 mmol) and chloroacetic acid (4.7 g, 50 mmol) were added, stirred and refluxed for 3 h. The solution was acidified with concentrated hydrochloric acid, and white powder 2-(2-formyl-6-methoxy) phenoxyacetic acid C10H10O5(4.2 g, yield 71%) was obtained after drying. m.p: 120～121 °C. Elemental analysis (%): C10H10O5, calculated value: C, 57.14; H, 4.79; measured value: C, 57.31; H, 4.72. IR (KBr, cm-1):v(Ar-O-C): 1277,vsym(COO): 1429,vas(COO): 1584,v(OH): 2933. The ligand C10H10O5(2.10 g, 10 mmol) was dissolved in absolute ethanol of 10 mL. Ethanediamine (0.30 g, 5 mmol) solution was added under stirring, heated to boiling and refluxed for 3 h. After cooling to room temperature, filtration, washing, precipitation with cold ethanol solution and drying, yellow solid powder was obtained (C22H24N2O8(H2L1), 1.2 g. The yield is 54%). m.p.: 177～178 °C.1H NMR(CDCl3, ppm):δ10.30 (s, 2H, -COOH), 8.28 (d, 2H,J= 8.4, -N=CH), 7.60 (d, 6H,J= 8.4, Ar-H), 4.82 (s, 4H, O-CH2), 3.89 (s, 4H, CH2-CH2). MS (ESI):m/z= 445.2 [M+1]+, 467.2 [M+Na]+. Elemental analysis (%): C22H24N2O8, calculated value: C, 59.45; H, 5.44; N, 6.30. Measured value: C, 59.21; H, 5.40; N, 6.03. IR (KBr, cm-1):v(Ar-O-C): 1285,v(C=N): 1606,vsym(COO): 1415,vas(COO): 1649,v(OH): 2918.

2. 3 Synthesis of the complex

The ligand H2L1(0.44 g, 1 mmol) was dissolved in 10 mL benzene. CoCl2·6H2O (0.24 g, 1 mmol) solution dissolved with absolute ethanol of 5 mL was mixed with standard 0.25M NaOH and pH = 9, heated to boiling, refluxed for 1 hour and filtered by heat. After volatilizing the filtrate at room temperature for one week, purple crystals were precipitated, filtered and dried to obtain cobalt complexes (yield 62%). Elemental analysis (%): C28H22CoN2O8.50(1), calculated value: C, 57.84; H, 3.81; N, 4.81. Measured value: C, 57.79; H, 3.69; N, 4. 77. IR (KBr, cm-1):v(Ar-O-C): 1269,v(C=N): 1576,vsym(COO): 1385,vas(COO): 1635,v(Co-O): 707,v(Co-N): 640.1H NMR(D2O, ppm):δ7.56 (d, H,J= 8.4, -N=CH), 7.21～6.75 (m, 3H, Ar-H), 4.85 (s, 2H, O-CH2), 3.92 (s, 4H, CH2-CH2), 3.73(s, 3H, O-CH3).

2. 4 Structure determination

The single crystal of complex 1 with suitable size and no crack was selected, and single-crystal X-ray diffraction data were collected at 293(2) K with a Bruker SMART APEX II CCD diffractometer equipped with a graphite-monochroma- tized MoKαradiation (λ= 0.71073 A) in the range of 3.06≤2θ≤49.42° for 1. Absorption corrections were applied using multi-scan technique and all the structures were solved by direct methods with SHELXS-97[10]and refined with SHELXL-97[11]by full-matrix least-squares techniques onF2. The finalR= 0.0541 andwR= 0.1565.

3 RESULTS AND DISCUSSION

3. 1 Synthesis conditions of the complex

The asymmetric mono-Schiff base metal complex 1 was obtained by mixing the ligand H2L1with cobalt chloride and adjusting the pH to 9. Two C=N of symmetric double Schiff base are partially hydrolyzed under alkaline condition and transformed into the asymmetric mono-Schiff base.

3. 2 Spectral properties

The infrared spectra of ligand H2L1are similar to those of complex 1. However, compared with ligand H2L1, the corresponding absorption peaks in the infrared spectra of the complexes have changed obviously, because the oxygen atoms on the carboxy group are bonded to metal ions, and the phenoxyl oxygen and nitrogen atoms form intramolecu- lar coordination bonds with metal ions, thus making the absorption peaks ofvas(COO),vsym(COO) andv(Ar-O-C) move to low wavenumber. Thevas(COO) absorption peak of ligand H2L1appears at 1649 cm-1,vsym(COO) absorption peak at 1415 cm-1andv(Ar-O-C) absorption peak at 1285 cm-1; complex 1vas(COO) absorption peak at 1635 cm-1,vsym(COO) absorption peak at 1385 cm-1,andv(Ar-O- C)absorption peak at 1269 cm-1. The movement is influenced by the intermolecular coordination or the bonding of oxygen atoms with metal ions[12,13].

In the 1H NMR spectra of ligand H2L1,δCOO-Happears at 10.30 ppm. After the complex is formed by the reaction with cobalt chloride, the peak disappears in the 1H NMR spectrum of complex 1, which is caused by the bonding of oxygen atoms on carboxy groups with cobalt ions. At the same time, theδ-N=CH,δO-CH2andδCH2-CH2values have changed: theδ-N=CHvalue of H2L1is 8.28, while 7.56 in complex 1; theδO-CH2value of H2L1is 4.82, and that of complex 1 is 4.85; TheδCH2-CH2value of H2L1is 3.89, and for 1 it is 3.92. The reason for these chemical shifts is that the shielding effect of hydrogen protons is changed due to the different solvents and the coordination of ligands with cobalt ions[14].

3. 3 Crystal structure description

The molecular structure of the asymmetric unit part of complex 1 with the coordination environment is shown in Fig. 1, and the one-dimensional chain diagram formed by hydrogen bonding interaction is shown in Fig. 2. The selected bond lengths and bond angles are shown in Table 1.

Table 1. Selected Bond Lengths (nm) and Bond Angles (o) for Complex 1

Symmetry transformations used to generate equivalent atoms: A: -x,y, -z+ 1/2

Fig. 1. Molecular structure of complex 1 (Hydrogen atoms and ethanol molecule were omitted. Symmetry transformations used to generate the equivalent atoms: A -x, y, -z + 1/2)

Fig. 2. One-dimensional chain diagram formed by hydrogen bonds of complex 1 (Dotted line represents hydrogen bond

The cobalt ion in complex 1 is six-coordinated by two nitrogen and four oxygen atoms. The configuration of complex 1 is more distorted than the octahedral structure, which can be approximately regarded as the distorted CoO4N2triangular prism configuration centered on Co(II). The dihedral angle between the planes formed (N(1), O(4), O(3)) and (N(1A), O(4A), O(3A)) is 5.4°, and that between planes (N(2), O(1), O(2)) and (N(2A), O(1A), O(2A)) is 6.7° (symmetry code for A: -x,y, -z+1/2). Because the two dihedral angles are approximately equal, it can be approximately considered that the planes are parallel and are the upper and lower sides of the two trihedral prisms.

In this complex, the Co(II) coordinates with N atom on the imino group, O atom on the phenoxyl group and O atom on the deprotonic group. The bond angle of CoO4N2is between 76.26° and 160.22°, the bond length of Co-O falls in the 0.1993～0.2225 nm range, and that of Co-N is between 0.2055 and 0.2073 nm. The Co-O (O atom of the phenoxyl group) bond has the longest length (0.2220 and 0.2225 nm), Co-N is the middle, while Co-O (O atom on proton decarboxyl group) is the shortest (0.1993 and 0.1997 nm). The unequal bond length of Co-O proves that Co(II) coordinates with O atom on the phenoxyl group and bonds with O atom on the deprotonic group. The bond lengths of Co-N and Co-O are close to those reported[15,16]. Interestingly, there are C-H···O hydrogen bonds in complex 1. The hydrogen bonds of C(9)-H(9A)···O(8) and C(21)-H(21B)···O(4) (H(9A)···O(8) = 0.257 nm, C(9)···O(8) = 0.3385 nm and C(9)-H(9A)···O(8) = 142°; H(21B)···O(4) = 0.255 nm, C(21)···O(4) = 0.3370 nm and C(21)-H(21B)···O(4) = 143°) extend the structure of 1 into an infinite one-dimensional chain structure, with the distance between two adjacent Co ions to be 0.6750 nm.

3. 4 Oxygen carrying performance

We install the instrument as shown in Scheme 2. The mixture of 2 mmol ligand dissolved in ethanol was put into a dry round bottom flask, and then 2 mmol cobalt chloride was dissolved in a liquid separation funnel with 10 mL ethanol. The piston was opened to connect the round bottom flask, the measuring pipe and oxygen cylinder, so that the oxygen can be filled with this flask and the measuring pipe, and the oxygen inlet will be closed so that the measuring pipe can be connected with the flask. The leveling regulator was moved to the right side of the measuring pipe to keep the liquid level of the two at the same level, and then electric agitator was opened to slowly drop the ethanol solution of cobalt chloride into the round bottom flask. After that, we opened the piston and moved the leveling regulator to the right side of the measuring pipe in order to keep the liquid level of the two pipes at the same level. At last, the position of the liquid level was recorded in the measuring pipe and the liquid level in the trachea was recorded every 5 min. The change of the reaction was observed until the oxygen absorption reaction of the complex is complete. In this experiment, the oxygen absorption of Schiff base cobalt complex was measured directly after the preparation of Schiff base cobalt complex. The amount of oxygen was calculated according to the ideal gas formula. The experimental results show that the amount of oxygen is between 1 and 2 mmol. Schiff base cobalt complex can exist in two different forms, the active type which can absorb oxygen rapidly at room temperature and the inactivated one which is stable at room temperature and does not absorb oxygen[7]. The active type is a dimer, in which one Schiff base cobalt complex binds to the Co atom in another Schiff base cobalt complex molecule, and the inactive type is also a dimer with the Co atom in a Schiff base cobalt complex molecule binding to the O atom in another same molecule. The oxygen absorption curve of complex 1 on drug sample is shown in Fig. 3. Through experiment, complex 1 proves to be active and the molar ratio of 1 to O2is between 1:1 and 2:1.

Scheme 2. Test device for oxygen-carrying amount of the complex material

Fig. 3. Oxygen absorption curve of complex 1 on the drug sample

3. 5 Antibacterial activity

On the basis of diffusion method, the antibacterial activity of complex 1 againstEscherichia coli(G) andBacillus subtilis(M) was tested, and the diameter inhibition was described by ring size. The greater the inhibition zone, the better the inhibitory effect. The strains of G and M to be tested are pre-activated for two generations, and then are inoculated into a liquid culture medium and cultured for 6 to 8 h under the condition of 37 oC, so that the concentration of the bacterial liquid is 106/mL, and the bacterial liquid is used as a test bacterial liquid. A 50 mL small triangular bottle was sterilized and then 10 mL ethanol solution was added. At the same time, the sterilized sample was put into solution at the concentration of 1000 μg/mL. A sterile pipette was used to suck 0.2 mL of the bacterial liquid, and the solution was added to the plate and the coating was uniform. The sterilized samples were simultaneously taken out of the sterile forceps on the coated plates which were then placed in 37 °C for 18～24 h. In experiment, ligand H2L1-M has no antibacterial ring. H2L1-G has a bacteriostatic ring with a diameter of 2 mm, while Co(II)-G is 8.1 mm in diameter. Co(II)-M has no antibacterial ring. The data show that there is no antibacterial display ring around sample M, so complex 1 does not show antibacterial activity against M. Compared with the Schiff-base ligand, the Co(II) complex can obviously improve the antibacterial action against G. Thus we can say that complex 1 has stronger antibacterial activity than ligand H2L1against the same bacteria G.

3. 6 Photoluminescent properties

Luminescent complexes are currently of great interest because of their various applications in photochemistry and photophysics. At room temperature, the fluorescence excitation wavelength and emission wavelength of complex 1 were searched by multispectral scanning in methanol solution with incident and emission slit width of 5 nm in 10-3mol/ L methanol solution. The fluorescence spectra of the complex were determined, as shown in Fig. 4. When the excitation wavelength is 325 nm, the maximum emission wavelength of 1 is 425 nm in the blue spectral range. The emission band for the title complex is probably attributable to theπ*-n transitions. Furthermore, the absolute emission quantum yield estimated for complex 1 is 3.8%, and the result suggests 1 has good performance of light emitting. However, the effect of microenvironment between ligand and complex on luminous properties needs to be further studied.