Synthesis, Spectroscopic, Calculated and Thermal Study of Copper and Cobalt Complexes with Dacarbazine

Dacarbazine, DAC, 5-(3,3-dimethyltriazeno)imidazol-4-carboxamideis is an imidazolecarboxamide derivative, that is structurally related to purines. DAC belongs to the triazene compounds, which are a group of alkylating agents with antitumour and mutagenic properties. DAC is a non-cell cycle specific drug, active on all phases of cellular cycle. In the frame of this work the 3d-metal complexes (cobalt and copper) with dacarbazine were synthesized. Their spectroscopic properties by the use of FT-IR, FT-Raman and 1HNMR were studied. The structures of dacarbazine and its complexes with copper(II) and cobalt(II) were calculated using DFT methods. The effect of metals on the electronic charge distribution of dacarbazine was discussed on the basis of calculated NBO atomic charges. The reactivity of metal complexes in relation to ligand alone was estimated on the basis of calculated energy of HOMO and LUMO orbitals. The aromaticity of imidazole ring in dacarbazine and the complexes was compared (on the basis of calculated geometric indices of aromaticity). Thermal stability of the investigated 3d-metal complexes with dacarbazine and the products of their thermal decomposition were analyzed.

The greatest obstacle in the treatment of patients with advanced stage of melanoma and other cancers is the unique tumor resistance, both primary and secondary, to all known compounds including dacarbazine. The complexation of metal ions with anticancer drugs creates the possibility of an increase in the bioavailability and activity of drugs [12,13]. Metal complexes play a very important role in modern cancer therapy. The complexation of metal ions with biologically active ligands, can change their therapeutic activity. Metal ions affect the distribution of the electronic system of ligands what in turn changes the reactivity and biological activiry of the ligands [14,15]. Complexes with some of the transition metals, e.g. Au, Ag, Co, Cu, Ni, Fe Pt(II) and (IV), Pd(II) and Ru(III) show high anticancer activity [16]. intercalation, comparable to DAC alone [19].
Our studies showed that there is a dependency between the electronic charge distribution in metal complex and salt molecules and the location of these metals in the periodic table [20]. Alkali metal cations and some heavy metal cations [Ag(I), Pb(II), Hg(I), Hg(II)] perturb the electronic system of ligands (benzoic, salicylic, pyridinecarboxylic acids and others), whereas 3d and 4f metal cations stabilize it [21][22][23][24][25][26]. This conclusion allows to foresee the changes in the electronic structure under the influence of metal cations and estimation of the physicochemical and biologically properties of complexes. of metal chloride (0,1 mol) (CuCl2, CoCl2). Then the mixture was shaken for 2 h in the water bath in room temperature. Next 50 ml of diethyl ether was added and it was left for 2 h. The precipitate was washed three times with 50 ml of diethyl ether. The obtained complexes were dried in vacuum.

Methods
The FT-IR spectra were recorded with an Alfa (Bruker) spectrometer within the range of 400-4000 cm -1 . Samples in the solid state were measured in KBr matrix pellets and ATR technique. FT-Raman spectra of solid samples were recorded in the range of 100-4000 cm -1 with a MultiRam (Bruker) spectrometer. The resolution of the spectrometer was 1 cm -1 . index (harmonic oscillator model of aromaticity) differs from all other geometry-based ones by assuming another reference bond length. In this model, instead of the mean bond length a concept of the optimal bond length is applied [28]: Within the confines of the HOMA model, it is possible to obtain two components which describe different contribution to decrease in aromaticity, i.e. (a) due to bond elongation (the EN component), and (b) due to bond length alternation (the GEO component). The value of HOMA index is equal 1 for the entire aromatic system; HOMA = 0 when structure is non-aromatic and HOMA < 0 for anti-aromatic ring.
where: nav -average binding order, n -bond order based on bond length: n = (a / R) -b, a and b -parameters depending on the type of atoms in the bond.
NBO analysis was performed for the optimized structures to determine the electronic charge distribution [30]. Calculations were made using the B3LYP/6-311++G(d, p) method.
Thermal analyses of the prepared complexes were performed by the thermogravimetric (TG) methods using the Perkin Elmer analyser. The products of dehydration and decomposition processes were determined from the TG curves. The TG measurement has been performed in temperature range 50 o C-890 o C in air atmosphere.
The crystal structure of the DNA dodecamer was obtained from Protein Database AutoGrid4.0 was used to compute grid maps using a grid box. The molecular docking was carried out by setting the grid box size to cover the predicted binding sites, using 52, 56, 118 x, y, z points with a grid spacing of 0,375 Å. The grid center was set to 14.72, 21.006, 8.801 x,y and z dimensions respectively. The Lamarckian genetic algorithm (LGA) was selected to generate the best ligand conformers. We performed successfully 1000 docking runs with AutoDock4.0 for each ligands (DAC1, DAC2) and Co(II)-DAC, Cu(II)-DAC complexes. As standard values for hydrogen bond formation assumed distance 2,9 Å and cutoff angle 60 degree (120 to 180 degrees). To visualize results BIOVIA Discovery Studio and USCF Chimera 1.10.2were used [33,34].

2.
Results and discussion

IR and Raman spectra
The IR and Raman spectra of dacarbazine and IR spectra of cobalt (II) and copper (II) complex of dacarbazine were shown in Fig. 2.  αC-NH2 * Abbreviation: band intensity s -strong, m -medium, w -weak, vw -very weak, type of vibrations: ν -stretching, ρ-bending out-of-plane β -bending in-plane, 3 -deformation in-plane, Δ-deformation out-of-plane, ringdef-deformation of ring, s-symmetric, as-asymmetric The band assigned to the deformations of the triazene group αNNN was located at 8 630 cm -1 in the spectra of ligand, and then was shifted toward higher wavenumbers in the 9 spectra of complexes (649 cm -1 -Cu complex and 646 cm -1 -Co complex). The symmetric 10 stretching vibrations of the methyl group νsCH3, which were present in the spectra of 11 ligand at 3147, 2946, 2753 and 2612 cm -1 , disappeared in the spectra of metal complexes. 12 Whereas band of asymmetric stretching vibrations of the methyl group νsCH3 were 13 significantly shifted in the spectra of complexes comparing with the spectra of ligand, i.e. 14 2905 cm -1 -in the IR spectra of dacarbazine and 2925 cm -1 and 2923 cm -1 in the IR spectra 15 of Co and Cu complexes, respectively. The band of the deforming in-plane vibrations of 16 the methyl group of dacarbazine, disappeared or decreased/ increased in their 17 wavenumbers in the spectra of Cu and Co complexes (  values. According to the calculated geometric aromaticity indices, the imidazole ring of 86 the DAC1 structure is more stable than the DAC2 structure (Table 3) (Table 3)  The energy values of the HOMO and LUMO orbitals for dacarbazine as well as 143 copper and cobalt complexes were calculated. The shapes of the orbitals are shown in 144 Figures 5 and 6, while in Table 5  quantifies the global electrophilic force of the molecule [42]. The electrophilicity index for 164 the copper complex is much higher than that of the free ligand (four times higher), while 165 for the cobalt complex it is twice as high as obtained for the ligand.