Two novel Mg(II)-based and Zn(II)-based complexes: inhibiting growth of human liver cancer cells

Two new Mg(II)-based and Zn(II)-based coordination polymers, {[Mg3(BTB)(DMA)4](DMA)2}n (1, H3BTB=1,3,5-benzenetrisbenzoic acid, DMA=N,N-dimethylacetamide) and {(H2NMe2)2[Zn3(BTB)2(OH)(Im)](DMF)9(MeOH)7}n (2, Im=imidazole, DMF=N,N-dimethylformamide), have been successfully synthesized and structurally characterized under solvothermal conditions. 1 contains a linear [Mg3(COO)6] cluster that connected by the fully deprotonated BTB3- ligands to give a kgd-type 2D bilayer structure; 2 represents a microporous 3D pillar-layered system based on the binuclear Zn units and pillared Im ligands, which shows a (3,5)-connected hms topological net. In addition, in vitro anticancer activities of compounds 1 and 2 on 4 human liver cancer cells (HB611, HHCC, BEL-7405 and SMMC-7721) were determined.


Introduction
Cancer is presently responsible for about 25% of deaths in developed countries and for 15% of all deaths worldwide (1). It is therefore considered one of the foremost health problems, with about 1.45 million new cancer cases expected yearly (2,3). Antitumor chemotherapy is a very active field of research, and a large amount of information on the topic is generated every year (4,5). However, there is a clear need for new treatments, from the medicinal chemistry and drug design point of view (6).
In recent years, aromatic multi-carboxylate acid compounds have been widely used as versatile ligands involved in various metal chelation reactions to form transition metal complexes with interesting properties in material sciences and biological systems (7,8). These complexes can be easily synthesized from simple starting materials, where the metal ions, ligands, and coordination modes are the important factors for the self-assembly processes (9,10). 1,3,5-benzenetrisbenzoic acid is a versatile ligand, which acts as monodentate or bridging group with end-on or end-to-end coordination mode to form complexes with interesting structures.

Apparatus and materials
All starting materials and reagents used in this research were obtained commercially and used without further purification. To obtain infra-red (IR) spectra (400B4000 cm -1 ), we used a Brucker Equinox-55 spectrophotometer (Bruker, Germany). Element analyses (C, H, and N) were determined with a Vairo EL III analyzer (Elementar, Germany). Single crystal X-ray diffraction was carried out by an Oxford Xcalibur E diffractometer (Bruker Optics, Germany). A XT-4 micro melting apparatus (Ledon, China) was used to determine the melting points, and the thermometer remained uncorrected.

Crystal structure determination
Suitable single crystals of compounds 1 and 2 were carefully selected under optical microscope and glued on thin glass fibers. The intensity data of 1 and 2 was obtained on an Oxford Xcalibur E diffractometer. The empirical absorption corrections were applied to the data using the SADABS system. This structure was solved by direct method and refined by full-matrix least-squares method on F 2 using the SHELXS-97 program (11). All non-hydrogen atoms of 1 and 2 were refined anisotropically, and all the hydrogen atoms attached to carbon atoms were fixed at their ideal positions. Pertinent crystal data and structural refinement results for compounds 1 and 2 are summarized in Table 1.

Antitumor activity
Four human liver cancer cells (HB611, HHCC, BEL-7405 and SMMC-7721) were grown in a RPMI 1460 medium supplemented with 10% fetal calf serum, 100 mg/mL penicillin and 100 mg/mL streptomycin. Cells were incubated at the temperature of 37°C in a moist incubator with 95% air and 5% CO 2 . Cells at exponential growth were diluted to 5 Â 10 4 cells/mL with RPMI1640, and then  seeded on 96-well plates at a volume of 100 mL per well, and incubated for 24 h at 37°C in 5% CO 2 . After incubation of cells for up to 96 h, the medium was removed from each cell and 150 mL of MTT (0.5 mg/mL) solution, diluted 10-fold by RPMI 1460 was subsequently added. The IC 50 values were measured by depicting the ratio viability versus concentration on a logarithmic chart and obtaining the concentration where 50% of cells were inhibited. In order to get mean values, each experiment was conducted at least three times in the same way.

Results and Discussion
Molecular structure As shown in Figure 2A, the asymmetric unit of 1 contains two crystallographically independent Mg(II) with 1/2 (Mg1) and 1 (Mg2) occupancies, one BTB 3ligand, two coordinated DMA and one lattice DMA molecule. Both Mg1 and Mg2 reveal similar six-coordinated octahedral geometries. Mg1 is located at an inversion center of (-1/2, 0, 1/2) and surrounded by six carboxylate O atoms from six different BTB ligands with the Mg-O lengths ranging from 2.021(2) to 2.037(2) Å. Mg2 is coordinated by four carboxylate O atoms from three BTB 3- 6 ] subunit is achieved. These trimeric clusters are further extended by BTB 3ligands along the ab plane to give a 2D bilayer pattern ( Figure 2C). The Mg2?Mg2 distance in the linearly arranged Mg 3 array is about 7.199 (2) Å, which is long enough to accommodate the bilayer network. These 2D bilayers adopt a parallel stacking arrangement to afford a 3D supramolecular architecture without classic H-bonding or pi-pi interaction. Topologically, the Mg 3 SBUs and BTB ligands can be regarded as 6-and 3-connected nodes, respectively, and afford a binodal (3,6)-connected kgd network with the point symbol of {4 3 } 2 {4 6 Á 6 6 Á 8 3 } ( Figure 2D).
The structural analysis results indicate that compound 2 crystallizes in the monoclinic space group P2 1 /c, and shows a 2-fold interpenetrated network. The asymmetric unit of 2 consists of six Zn(II) ions, three BTB 3 -ligands, five deprotonated Im ligands, two coordinated DMF molecules, and two lattice disordered H 2 NMe 2 cations. The six Zn(II) ions exhibit two different coordinating modes ( Figure 3A): Zn1, Zn2, Zn4 and Zn5 atoms are four-coordinated by three O atoms from three different carboxylic acid groups on the BTB 3ligands and one N atom from the deprotonated Im ligand, forming a distorted tetrahedral coordination mode; atoms Zn3 and Zn6 are five-connected by one(two) N atom(s) from the Im ligand(s) and three(two) O atoms from the carboxylic acid groups, resulting in a pyrometric geometry. Zn1-Zn6, Zn2-Zn3 and Zn4-Zn5 atoms are linked by the carboxylic groups to afford the binuclear Zn secondary building units (SBUs), which are joined by the Im pillars along the c axis to give rise to the 1D SBU chains ( Figure 3B). In addition, the BTB 3ligand links with the binuclear Zn SBUs along bc plane to give the 6 3 layer, and the Im ligands act as pillars between 2D sheets to form the three-dimensional (3D) framework. Due to its large solvent-accessible volume, such a framework might be large enough to accommodate another identical one to be interpenetrated, thus forming a doubly interpenetrated 3D framework with one-dimensional (1D) channel ( Figure 3C). Notably, the 1D channels are decorated with Im groups, which might be favorable for the formation of a H-bond interaction. The calculated void space per unit cell for guest-free framework is 40% as  From the topological point of view, the 3D structure of 2 can be rationalized as a 2-fold interpenetrated hmstype (3,5)-connected network by considering the BTB 3ligand, binuclear Zn unit and the Im pillar as a 3, 5 and 2-connected node. ( Figure 3D).

Anticancer activity
The cytotoxicity of the title compounds 1 and 2, reference drug carboplatin, organic ligands H 3 BTB and Im against HB611, HHCC, BEL-7405 and SMMC-7721 cell lines were evaluated by MTT assay, and the IC 50 values derived from the experimental data are shown in Table 2. The two organic ligands were ineffective against all cell lines (IC 50 4100 mM). At this concentration, H 3 BTB and Im would exert high cytotoxicity against normal cells, thus we conclude that it did not exert inactivation towards these cell lines.
However, after the cancer cells were incubated in the presence of compounds 1 and 2 for 72 h, the IC 50 values for the compound ranged from 20 to 35 mM, which is similar or even lower than that of carboplatin (25-40 mM), indicating that the title compounds 1 and 2 exhibited anticancer activity against these cell lines in different degrees.
According to the above-mentioned data, it can be concluded that compared with organic ligands H 3 BTB and Im, the anticancer activity of compounds 1 and 2 was more effective. However, additional studies are needed to define the mechanism underlying the antitumor activity of these compounds and evaluate their efficacy in vivo.