The molecular structure, the Natural Bond orbital (NBO) and the Time Dependent-DFT of both isomers cis or γ-Cl and trans or δ-Cl of RuCl 2 (L) 2 , where L stands respectively for 2-phenylazopyridine (Azpy), 2,4-dimethyl-6-[phenylazo]pyridine (Dazpy), 2-[(3,5-dimethylphenyl)azopyridine] (Mazpy) and 2-pyridylazonaphtol (Nazpy) were calculated with DFT method at B3LYP/LANL2DZ level. The prediction of the frontier orbitals (Highest Occupied Molecular Orbital or HOMO and Lowest Unoccupied Molecular Orbital or LUMO) shows that the most active complexes suitable for electronic reactions are admitted to be the trans isomers. Moreover, δ-RuCl 2 (Azpy) 2 is discovered to react more actively as photo-sensitizer since its energy gap is the minimum. Besides, electronic structures of all complexes through NBO calculation indicate that Ru-N bonds are made of delocalization of occupancies from lone pair orbital of N atoms to the ruthenium. Moreover, Ru was assumed to have almost the same charge regardless the structure of the azopyridine ligands in the complex indicating that the ligands provide only a steric effect that is responsible for the ruthenium's selectivity. Concerning the transition state, NBO analysis also highlights that the transition LP(Ru)π*(N 1 -N 2 ) does correspond to t 2g π*(L). This transition is assumed to correspond to Metal to Ligand Charge Transfer (MLCT) that is responsible for the photo-sensitiveness of the metallic complex. Besides, TDDFT calculation of complexes showed that δ-RuCl 2 (Nazpy) 2 displays the largest band during the absorption. For that reason, it is admitted to be the best photosensitizer due to a large system of conjugation provided by Nazpy ligand.

A series of ruthenium azopyridine complexes have recently been investigated due to their potential cytotoxic activities against renal cancer (A498), lung cancer (H226), ovarian cancer (IGROV), breast cancer (MCF-7) and colon cancer (WIDR). Thus, in order to predict the cytotoxic potentials of these compounds, quantitative structure-activity relationship studies were carried out using the methods of quantum chemistry. Five Quantitative Structure Activity Relationship (QSAR) models were obtained from the determined quantum descriptors and the different activities. The models present the following Moreover, the charge of the ligand is the priority descriptor for the prediction of the cytotoxicity of the compounds studied. Furthermore, QSAR models developed are statistically significant and predictive, and could be used for the design and synthesis of new anti-cancer molecules.

Structure Activity-Relationships (SARs) of the five possible isomers of RuCl2(Azpy)2 were predicted thanks to DFT method. Azpy stands for 2-phenylazopyridine and the structure of the isomers α-RuCl2(Azpy)2, β-RuCl2(Azpy)2, γ-RuCl2(Azpy)2, δ-RuCl2(Azpy)2 and ε-RuCl2(Azpy)2 call respectively α-Cl, β-Cl, γ-Cl, δ-Cl and ε-Cl are defined according to chlorine atoms orientations. Hence, they are divided into two groups. In the first group comprising α-Cl, β-Cl and ε-Cl, both chlorine atoms are in cis position and Azpy ligands are intervertical. Whereas the two others isomers (γ-Cl and δ-Cl), they form the second group. Here, both chlorine are in trans position and Azpy are planar. The five synthesized isomers were investigated as potential antitumor agents. Then, regarding the DNA, its bases are stacked by pair. Therefore, complexes are assumed to insert and to stack on them through intercalative mode. So the electronic and geometric structures become more important to describe their SARs. Consequently, group 2 regarding γ-Cl and δ-Cl presents the best structure to allow intercalation between DNA base-pairs. Besides, the energy order of the lower unoccupied molecular orbital (LUMO) of the isomers is ELUMO(β-Cl) > ELUMO(α-Cl) > ELUMO(ε-Cl) > ELUMO(γ-Cl) > ELUMO(δ-Cl). The energy gap between LUMO and HOMO was also sorted as ∆(L-H)(β-Cl) > ∆(L-H)(α-Cl) > ∆(L-H)(ε-Cl) > ∆(L-H)(γ-Cl) > ∆(L-H)(δ-Cl). In addition, the total dipole moment was classified as μ(ε-Cl) > μ(β-Cl) > μ(α-Cl) > μ(γ-Cl) > μ(δ-Cl). Finally, net charge of the ligand Azpy was also classified as QL(δ-Cl) > QL(γ-Cl) > QL(ε-Cl) > QL(α-Cl) > QL(β-Cl). All those parameters show that δ-Cl isomer displays the highest activity as antitumor drug when intercalating between the DNA basepairs Cytosine-Guanine/Cytosine-Guanine (CG/CG).

Theoretical study of a series of isomeric α-, β-, γ-, δ-, ε-RuCl 2 L 2 (L= azpy, tazpy, 4mazpy, 5mazpy) complexes is carried out using the density functional theory (DFT) method at B3LYP/LanL2DZ level. The effects of the ligand on the electronic structures and related properties, e.g. the components and the energies of some frontier molecular orbital, the net charge populations of some main atoms of the complexes, the effect of substituent methyl as well as the Structure Activity-Relationships (SARs) of the complexes were investigated. The results show that the sterical differences between isomeric structures of these complexes have serious influence on their electronic structures and related properties. First and foremost, the geometric configuration of δ-Cl and γ-Cl isomers must be advantageous to the conjugative ligand to intercalate between DNA-base-pairs in comparison with α-Cl, β-Cl and ε-Cl complexes. Secondly, the energy order of the lowest unoccupied molecular orbital (LUMO) of the isomers is E LUMO (δ-Cl) < E LUMO (γ-Cl) < E LUMO (ε-Cl) < E LUMO (α-Cl) < E LUMO (β-Cl). And their HOMO-LUMO gap energy is classified as ΔE(δ-Cl) < ΔE(γ-Cl) < ΔE(ε-Cl)< ΔE(α-Cl)< ΔE(β-Cl). Thirdly, the dipole moments (µ) of the isomers, expressing the hydrophobic parameters of the molecules, was also classified as μ(ε-Cl) > μ(β-Cl) > μ(α-Cl) > μ(γ-Cl) > μ(δ-Cl). Finally, the net charge of the ligands azopyridine that defines the aptitude for the ligand to accept the electron from DNA, are classified as. These electronic and geometric structural characteristics can be used to explain the trend in the anticancer-activities (A) of isomeric α-, β-, γ-RuCl 2 L 2 (L= azpy, tazpy, 4mazpy) or to predict the order of activity of the five δ-Cl, γ-Cl, α-Cl, β-Cl and ε-Cl isomers of the three complexes RuCl 2 (azpy) 2 , RuCl 2 (tazpy) 2 and RuCl 2 (4mazpy) 2 . They are also suitable to predict the activity of five non synthesized isomers of RuCl 2 (5mazpy) 2 since the three azopyridine ligands tazpy, 4mazpy and 5mazpy display the same number of electrons.

The theoretical study of α-, β-, γ-, δ-, ε- MCl2(Azpy)2 isomers with (M = Fe, Os and Ru) complexes is carried out using Density Functional Theory (DFT) at the B3LYP / LANL2DZ level. This study is focused not only on the effect of metals over geometric, electronic and reactivity parameters, but also on their anti-cancer effect. Its results that the geometric parameters undergo small modifications. These modifications evolve from iron to osmium through ruthenium complexes. Thus, the lengths of the bonds M-X (with X = Cl, N2, Npy) follow the following order Fe-X <Ru-X <Os-X. However, regarding their angular variation that undergoes deformation through the octahedron shape, it could be related to Jahn Teller effect. Also, the substitution of Ru by Os would increase the reactivity of these complexes. Among the isomers studied, the ε-Fe, δ-Ru and δ-Os complexes are likely to bind easily to the DNA. The values of the dipole moments are arranged in the following order: μ (ε-M)> μ (β-M)> μ (α-M)> μ (γ-M)> μ (δ-M) within these azopyridine complexes. Finally, we notice that the substitution of Ru by Os improves the cytotoxicity and the fluorescence of these complexes. The δ-Os isomer has the best cytotoxic and photosensitive characteristics of these azopyridine complexes and would be the ideal isomer for the diagnosis and treatment of cancers.