Two novel Co(II) complexes with two different Schiff bases: inhibiting growth of human skin cancer cells

Using two flexible Schiff bases, H2L1 and H2L2, two new cobalt II (Co(II))-coordination compounds, namely, Py3CoL1 (1) and Py3CoL2 (2) (Py=pyridine, L1=3,5-ClC6H2(O)C=NC6H3(O)-4-NO2, L2=3,5-BrC6H2(O)C=NC6H3(O)-4-NO2) have been synthesized under solvothermal conditions. Single crystal X-ray structural analysis revealed that compounds 1 and 2 are both six-coordinate in a distorted octahedral geometry, and the 1D chain structure was formed by the π…π and C-H…O interactions or C-H…Cl interaction. The in vitro antitumor activities of 1, 2 and their corresponding organic ligands Py, L1, and L2 were studied and evaluated, in which three human skin cancer cell lines (A-431, HT-144 and SK-MEL-30) were used in the screening tests.


Introduction
Cancer is a proliferation disorder disease with apoptosis obstacles (1,2). It strikes more than one-third of the world's population and causes over 20% of all deaths (3). Standard cancer treatment protocols include surgery, radiotherapy and chemotherapy (4). Unfortunately, chemotherapy is not effective in treating cancers associated with innate resistance to apoptosis and/or acquired resistance to drugs during treatment. Discovery of novel effective anticancer medicines is therefore of great importance (5).
Cobalt complexes with Schiff bases have received considerable attention in the fields of coordination chemistry and biological chemistry (6). Cobalt functions as the active site of hydrolytic enzymes, such as carboxypeptidase and carbonic anhydrase where it is in a hard-donor coordination environment of nitrogen and oxygen (7). Cobalt has been recognized as an important cofactor in biological molecules, either as a structural template in protein folding or as a Lewis acid catalyst that can readily adopt four-, five-, or six-coordination (7,8). The cobalt (Co) complexes with the Schiff bases derived from salicylaldehyde and its analogues have been extensively studied (9). In this work, two new Co(II) complexes, Py 3 CoL 1 (1) and Py 3 CoL 2 (2) (Figure 1)

Apparatus and materials
All starting materials and reagents used in this research were obtained commercially and used without further purification. Element analyses (C, H, and N) were determined with an elemental Vario EL III analyzer (Elementar, Germany). Single-crystal X-ray diffraction data for compounds 1 and 2 were recorded on Mercury CCD diffractometer (Bruker Optics, Germany). The melting points were taken on a XT-4 micro melting apparatus (Ledon, China), and the thermometer was uncorrected. Three human skin cancer cell lines (A-431, HT-144 and SK-MEL-30) were purchased from Sigma-Aldrich (USA).

Synthesis and characterization of compounds 1 and 2
A mixture of CoCl 2 (1.0 mmol, 0.130 g), and 3,5dichlorosalicylaldehyde-2-amino-4-nitrophenol Shiff base (H 2 L 1 , 1.0 mmol, 0.327 g) were mixed in 25 mL methanol. After heating and dissolving in air, 10 mL methanol solution of CoCl 2 (50 mM) was added to the reaction flask. Most of the solvent was removed by rotary evaporators after 2 h of reaction. Then, pyridine was dropped to dissolve, and the solution continued to reflux for 2 h. The solution was cooled down to room temperature and filtered and the brown crystals of solution 1 were obtained. Analytical characteristics found for compound 1 (C 28 H 21 Cl 2 CoN 5 O 4 ) were: C, 54.18; H, 3.39; N, 11.30%. Calculate: C, 54.13; H, 3.41; N, 11.27%.

Crystal structure determination
Structural measurement was performed on the computer-controlled Mercury CCD diffractometer with graphite-monochromated Mo-Ka radiation (l=0.71073 Å) at T=293 (2) K. Absorption correction was made using the SADABS (Bruker AXS Inc., USA) program. The structure was solved using the direct method and refined by fullmatrix least-squares methods on F 2 using the SHELXS-97 program package (10). Crystallographic data and structural refinements for compounds 1 and 2 are summarized in Table 1. CCDC numbers for compounds 1 and 2 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam. ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033; E-mail: E-mail: deposit@ccdc.cam.ac.uk)

Antitumor activity
Stock solutions of 1, 2 and their corresponding organic ligands Py, L 1 and L 2 were prepared in DMSO and kept at -20°C. Appropriate dilutions of the compounds were freshly prepared just prior to the assays. Final concentrations of DMSO did not interfere with the cell growth.

Antitumor activity
The tumor cell growth inhibition activities of 1, 2 and their corresponding organic ligands Py, L 1 and L 2 were assessed in vitro on 3 human skin cancer cell lines (A-431, HT-144, and SK-MEL-30) after continuous exposure for 48 h. The results were compared to the antiproliferative effects of the reference control doxorubicin. All compounds were dissolved in DMSO at 1 mg/mL immediately before use and diluted just before addition to the cell culture.
Data are reported as means±SE of 3 independent experiments performed in duplicate ( Table 2). The antiproliferative activity of the test compounds against each of the title tumor cell lines may be arranged in a descending order according to the measured concentration required to inhibit tumor cell proliferation by 50% (IC 50 m/M). From the results, we can see that compounds 1 and 2 showed significant growth inhibition activity on the 3 tumor cell lines (IC 50 =11.3B19.8 m/M), compared to their corresponding organic ligands Py, L 1 and L 2 (IC 50 =90.8B120.5 m/M).
In conclusion, we successfully obtained two new Co(II)-coordination compounds (1 and 2) by employing two different flexible Schiff base ligands. From the biological activity investigation, we observed that the antitumor activity of compounds 1 and 2 advanced greatly when organic ligands pyridine and Schiff bases were in coordination with Co ion. However, additional studies are needed to define the mechanism underlying this antitumor activity and evaluate the drug efficacy in vivo.