Zinc(II) Complexes Derived from Schiff Bases: Syntheses, Structures, and Biological Activity

Three new zinc(II) complexes, [Zn2I2(L)2] (1), [Zn(HL)2(NCS)2] (2), and [ZnIL3] (3), where L1 is the anionic form of 2-[(6-methylpyridin-2-ylimino)methyl]phenol (HL1), HL2 is the zwitterionic form of 2-(cyclopropyliminomethyl)-5-fluorophenol (HL2), and L3 is the anionic form of 5-bromo-2-[(3-morpholin-4-ylpropylimino)methyl] phenol (HL3), have been prepared and characterized by elemental analyses, IR, UV and NMR spectra, and single crystal X-ray crystallographic determination. Complex 1 is a dinuclear zinc complex, and complexes 2 and 3 are mononuclear zinc complexes. The Zn atoms in the complexes are in tetrahedral coordination. The effect of the complexes on the antimicrobial activity against Staphylococcus aureus, Escherichia coli, and Candida albicans were evaluated.


Introduction
The rapid increasing interest in the synthesis and structural studies of Schiff bases is due to their bioactivity and coordination properties. 1 Schiff bases are active against fungal, cancer, convulsant, oxidant and diuretic activities. 2 Metal complexes of Schiff bases have attracted considerable attention due to their versatile biological activity, such as antifungal, antibacterial and antitumor. 3 And, in general, the metal complexes have higher biological activities than the free Schiff bases. It has been shown that the Schiff base complexes derived from salicylaldehyde and its derivatives with primary amines, bearing the N 2 O, N 2 S, NO 2 or NSO donor sets, have potential antimicrobial activities. 4 Zinc is an important biological element, its complexes derived from Schiff bases have received particular attention due to their interesting antimicrobial potential. 5 Recent research indicated that the halide and pseudohalide groups can severely increase the antimicrobial activities. 6 Our research group has reported some metal complexes with effective antimi-crobial activities. 7 In pursuit of new and efficient antimicrobial agents, in the present work, three new zinc(II) complexes, [Zn 2 I 2 (L 1 ) 2 ] (1), [Zn(HL 2 ) 2 (NCS) 2 ] (2), and [ZnIL 3 ] (3), where L 1 is the anionic form of 2-[(6-methylpyridin-2-ylimino)methyl]phenol (HL 1 ), HL 2 is the zwitterionic form of 2-(cyclopropyliminomethyl)-5-fluorophenol (HL 2 ), and L 3 is the anionic form of 5-bromo-2-[(3-morpholin-4-ylpropylimino)methyl]phenol (HL 3 ) (Scheme 1), are reported. To our knowledge, only two complexes with HL 1 , 8 and no complexes with HL 2 and 2. Experimental

4. X-Ray Diffraction
Data were collected from selected crystals mounted on glass fibres. The data were collected with MoK α radiation (0.71073 Å) at 298(2) K with a Bruker SMART 1000 CCD area diffractometer. The data for the complexes were processed with SAINT 9 and corrected for absorption using SADABS. 10 Multi-scan absorption corrections were applied with ψ-scans. 11 The structures were solved by direct method using SHELXS-97 and refined by full-matrix least-squares techniques on F 2 using anisotropic displacement parameters. 12 The imino H atom in complex 2 was located form a difference Fourier map and refined with N-H distance of 0.90(1) Å. All other hydrogen atoms were placed at the calculated positions. Idealized H atoms were refined with isotropic displacement parameters set to 1.2 (1.5 for methyl groups) times the equivalent isotropic U values of the parent carbon atoms. The crystallographic data for the complexes are listed Table 1.

5. Antimicrobial Assay
Qualitative determination of antimicrobial activity was done using the disk diffusion method. 13 The antibac- terial activity was tested against B. subtilis, E. coli, P. fluorescence and S. aureus using MH medium (Mueller-Hinton medium). The MICs (minimum inhibitory concentrations) of the test compounds were determined by a colorimetric method using the dye MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide]. A stock solution of the synthesized compound (50 μg mL -1 ) in DMSO was prepared and quantities of the test compounds were incorporated in specified quantity of sterilized liquid MH medium. A specified quantity of the medium containing the compound was poured into micro-titration plates. A suspension of the microorganism was prepared to contain approximately 10 5 cfu mL -1 and applied to micro-titration plates with serially diluted compounds in DMSO to be tested and incubated at 37 °C for 24 h. After the MICs were visually determined on each of the micro-titration plates, 50 μL of PBS containing 2 mg of MTT per millilitre was added to each well. Incubation was continued at room temperature for 4-5 h. The content of each well was removed and 100 μL of isopropanol containing hydrochloric acid was added to extract the dye. After 12 h of incubation at room temperature, the optical density (OD) was measured with a micro-plate reader at 550 nm.

1. Chemistry
The Schiff bases were readily prepared by the reaction of equimolar quantities of aldehyde and amines in methanol, which were used directly for the preparation of the zinc complexes at ambient temperature. The zinc complexes are stable at room temperature in the solid state and soluble in common organic solvents, such as methanol, ethanol, chloroform, and acetonitrile. The results of the elemental analyses are in accord with the composition suggested for the complexes.

2. IR and Electronic Spectra
The IR spectra of the complexes were analyzed and compared with those of their free Schiff bases. The intense absorption bands at 1623, 1627 and 1633 cm -1 in the spectra of the Schiff bases HL 1 , HL 2 and HL 3 , respectively, can be assigned to the C=N stretching. In the complexes, these bands are shifted to 1615, 1653 and 1637 cm -1 upon complexation with the zinc atoms, which can be attributed to the coordination of the imine nitrogen to the metal centre. 14 The absorption of this band for complex 2 is located at higher wavelength than complexes 2 and 3, which is due to the protonation of the imino group. The typical absorption at 2073 cm -1 in the spectrum of complex 2 is assigned to the vibration of the NCS ligand. 15 UV-Vis spectra of the free Schiff bases and the complexes were recorded in HPLC grade acetonitrile solution. The spectra of the Schiff bases exhibit bands at 220-280 nm and 330-360 nm attributed to π→π* and n→π* transitions. In the spectra of the complexes the charge transfer bands at 220-280 nm remain intact, in agreement with the π→π* transitions of the Schiff base ligands. The remaining bands at 350-410 nm in the spectra of the complexes are assigned to the metal to ligand charge transfer (MLCT) transition. 16

NMR Spectra
The 1 H NMR spectra of the Schiff bases exhibit OH (phenolic) proton resonances at 11.78-12.15 ppm, imine proton resonances at 8.58-8.67 ppm and aromatic proton resonances in the range 6.43-7.68 ppm, respectively. On coordination, the signal due to OH proton disappears, indicating deprotonation of the phenolic OH and subsequent coordination of the phenoxide oxygen to the metal atoms. Involvement of the imine nitrogen in coordination has shifted the resonance signal of the imine proton by 0.7-0.9 ppm. The peaks observed in 13 C NMR spectra of the Schiff bases and the complexes are in expected range and values are given in experimental section. The carbonyl C and imine C atoms in the complexes have shifted compared to their ligands, confirming the coordination through carbonyl O and imine N atoms. 17

4. Crystal Structure Description of the Complexes
Complex 1 is a phenolato-bridged dinuclear zinc(II) compound ( Figure 1), with Zn···Zn separation of 3.096(2) Å. The crystal of the complex possesses a crystallographic inversion symmetry, with the inversion center located at the middle of the two zinc atoms. The Zn atom in the complex is coordinated by one pyridine N and two phenolate O atoms from two Schiff base ligands, and one I atom, generating tetrahedral geometry. The Schiff base acts as a bidentate ligand, and forms a six-membered chelate ring with the metal center through the phenolate O and imine N. The bond distances subtended at the metal atoms are comparable to those observed in similar zinc(II) complexes with Schiff bases. 18 In the crystal structure of the complex, molecules are stacked via π···π interactions (Table 3) along the b axis (Figure 2).
Complex 2 is a thiocyanate-coordinated mononuclear zinc(II) compound (Figure 3). The Zn atom in the complex is coordinated by two phenolate O atoms from two zwitterionic Schiff base ligands, and two thiocyanate N atoms, generating tetrahedral geometry. The Schiff base   acts as a monodentate ligand, with the phenol H atom transferred to the imino N group. The bond distances subtended at the metal atoms are comparable to those observed in similar zinc(II) complexes with Schiff bases and thiocyanate ligands. 19 In the crystal structure of the complex, molecules are stacked via π···π interactions (Table 3) including the pyridine ring N(2)-C(8)-C(12)-C(11)-C(10)-C(9) and the chelate ring Zn(1)-N(1)-C(8)-N(2), along the b axis (Figure 4).
Complex 3 is an iodide-coordinated mononuclear zinc(II) compound ( Figure 5). The Zn atom in the complex is coordinated by one phenolate O, one imino N, and one morpholine N atoms of the Schiff base ligand, and one I atom, generating tetrahedral geometry. The Schiff base acts as a tridentate ligand, with the morpholine ring adopts chair configuration. The bond distances subtended at the metal atoms are comparable to those observed in similar zinc(II) complexes with Schiff bases and thiocyanate ligands. 20 In the crystal structure of the complex, molecules are stacked via π···π interactions (Table 3) including the phenyl ring C(1)-C(2)-C(3)-C(4)-C(5)-C(6) and the chelate ring Zn(1)-O(1)-C(2)-C(1)-C(7)-N(1), along the b axis ( Figure 6).

5. Antimicrobial Activity
The results of the antimicrobial activity are summarized in Table 4. A comparative study of minimum inhibitory concentration (MIC) values of the Schiff base and the zinc complex indicated that the complex has more effective activity against Staphylococcus aureus, Escherichia coli, and Candida albicans than the free Schiff base. Generally, Xue et al.: Zinc(II) Complexes Derived from Schiff Bases: ... this is caused by the greater lipophilic nature of the complex than the ligand. Such increased activity of the metal chelates can be explained on the basis of chelating theory. 21 On chelating, the polarity of the metal atoms will be reduced to a greater extent due to the overlap of the ligand orbital and partial sharing of positive charge of the metal atoms with donor atoms. Further, it increases the delocalization of π-electrons over the whole chelate ring and enhances the lipophilicity of the complex. This increased lipophilicity enhances the penetration of the complex into lipid membrane and blocks the metal binding sites on enzymes of micro-organisms.

Conclusions
In summary, three new zinc(II) complexes with halide and pseudohalide ligands derived from Schiff bases have been prepared and characterized. The structures of the complexes are confirmed by single crystal X-ray crystallographic determination. The Zn atoms in the complexes are in tetrahedral coordination. The complexes have better activities on the bacteria Staphylococcus aureus and Escherichia coli than the control drug Tetracycline. Moreover, the complexes have stronger activities against Candida albicans than Tetracycline. Interestingly, complex 2 has the most activity against Staphylococcus aureus with MIC value of 2 μg/mL.