DNA Binding Test, X-Ray Crystal Structure, Spectral Studies, TG-DTA, and Electrochemistry of [CoX2(dmdphphen)] (Dmdphphen Is 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline, X = Cl, and NCS) Complexes

Two new neutral mixed-ligand cobalt(II) complexes, [CoCl2(dmdphphen)] 1 and [Co(NCS)2(dmdphphen)]  2, where dmdphphen is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, were synthesized and characterized by an elemental analysis, UV-Vis, IR, TG/DTA, cyclic voltammetry CV, and single X-ray diffraction. Complex 2 crystallized as monoclinic with a space group P21/c. Co(II) ions are located in a distorted tetrahedral environment. TG/DTA result shows that these complexes are very stable and decomposed through one-step reaction. The two complexes exhibit a quasireversible one-electron response at −550 and 580 mV versus Cp2Fe/Cp2Fe+, which has been assigned to Co(I)/Co(II) and Co(II)/Co(III) couples. Absorption spectral studies reveal that such complexes exhibit hypochromicity during their interaction with CT-DNA.

The ability of the cobalt phenanthroline complexes to bind and to cleave DNA under physiological conditions is of current interest because of their potential applications in nucleic acids chemistry [11]. Also, these complexes are useful in footprinting studies [12][13][14][15][16][17]. The cleavage of DNA usually occurs through the heterocyclic bases, deoxyribose sugar moiety, or phosphodiester linkage [18][19][20]. For the mixedligand complexes to interact efficiently with DNA, the ligands need to be flat, have large surface area, and have a spatial geometry to interact with the base pairs in DNA [15][16][17][18][19][20][21][22][23].
By changing the ligands or the metal ions, it is possible to modify the interaction with nucleic acids [23][24][25][26].

Experimental Section
2.1. Materials and Instrumentation. 2,9-Dimethyl-4,7diphenyl-1,10-phenanthroline ligand, CoCl 2 ⋅6H 2 O, and Co(NCS) 2 were purchased from Acros Organics. Elemental analyses were carried out on an Elementar vario EL analyzer. The IR spectra for samples were recorded using PerkinElmer Spectrum 1000 FT-IR Spectrometer. The UV-Vis spectra were measured by using a TU-1901 double-beam UV-Vis spectrophotometer. TG/DTA spectra were measured by using a TGA-7 PerkinElmer thermogravimetric analyzer. The cyclic voltammograms for the complexes were measured in CH 3 CN and 0.1 M tetrabutylammonium hexafluorophosphate (TBAHF) using BAS 100 B/W electrochemical workstation (Bioanalytical Systems, West Lafayette, IN, USA) and controlled by a standard 80486 personal computer (BAS control program version 2.0). All electrochemical experiments were carried out at room temperature under argon with a three-electrode cell. Voltalab 80 potentiostat PGZ402 with Pt-disk electrode (Metrohm, = 0.0064 cm 2 ) was used as working electrode. Platinum wire (m 1 mm) spiral with diameter 7 mm was used as a counter electrode. Haber-Luggin double reference electrode was used as a reference one. All potentials in this paper are reported to an external Cp 2 Fe 0/+ standard [32].

General Procedure for the Preparation of the Desired Complexes.
A mixture of CoX 2 salt (2 mmol) in distilled ethanol (15 mL) and free ligand (2.1 mmol) in methanol (10 mL) is stirred for around 0.5 h at room temperature until the precipitation appeared which was filtered, washed with ethanol, and dried. Suitable crystals for X-ray diffraction analysis were growing up by slow diffusion of ethanol into a solution of the complex in CH 2 Cl 2 after two days (yield 88%).

Crystallography.
A suitable single-crystal complex 2 with dimensions of 0.23 × 0.22 × 0.21 mm was chosen for an X-ray diffraction measurement. X-ray intensity data were collected at 296 K on a Bruker CCD diffractometer equipped with Cu K radiation ( = 1.54178Å). Data were collected with the and scan method. The final unit cell parameters were based on all reflections. Data reduction of all the collected reflections and absorption correction were carried out using the APEX 2 [33] package. The structure was solved by direct methods using SHELXS [34]. The structure was then refined by a full-matrix least-squares method with anisotropic temperature factors for nonhydrogen atoms using SHELXL [34]. All the nonhydrogen atoms were revealed in the first Fourier map itself. After several cycles of refinement, the final difference Fourier map showed peaks of no chemical significance and the residual saturated to 0.0671. Details of data collection and refinement are given in Table 1. The geometrical calculations were carried out using the program PLATON [35]. The molecular and packing diagrams were generated using the software MERCURY [36].

DNA Binding and Cleavage Experiments.
Absorbance measurement was performed to clarify the binding affinity of cobalt(II) complexes by emissive titration at room temperature. The complexes were dissolved in mixed solvent of Tris-HCl buffer (5 mM Tris-HCl/50 mM NaCl buffer for pH = 7.2) for all the experiments and stored at 4 ∘ C for further use and used within 2 days. Tris-HCl buffer was subtracted through baseline correction. The absorption experiments were performed by keeping the concentration of cobalt(II) complexes constant (1.5 × 10 −4 mol/L) and increasing the concentration of DNA gradually (1.0 × 10 −4 -1 × 10 −3 mol/L).

Synthesis of the Desired Complexes. The mononuclear
CoCl 2 (dmdphphen) complex 1 and Co(NCS) 2 (dmdphphen) complex 2 were isolated in a good yield without side product as seen in Scheme 1.
The structures of the desired complexes were confirmed by using elemental analysis, IR, UV-Vis, TG/DTA, and Xray single-crystal measurement for complex 2.    moiety as indicated by the dihedral angle values of 43.2(4) ∘ and 47.2(4) ∘ , respectively. All coordination distances and bond angles are similar to those found in similar compounds [37]. No classic hydrogen bonds were observed. In the crystal structure there is a -stacking interaction between adjacent dmdphphen and distances 3.7109(17)Å and 3.8070(17)Å, which may account for stabilizing the crystal structure ( Figure 2). The packing of the molecules when viewed down along the axis indicates that the molecules are interlinked by weak hydrogen bonds to form one dimensional chain.

IR Spectrum.
The IR spectrum of complex 1 (Figure 3(b)) showed four characteristic absorptions peaks in the range of 3060, 2950, 550, and 350 cm −1 [7][8][9][10] which was assigned to H-Ph, H-CH 3 , Co-N, and Co-Cl stretching vibrations, respectively. New band at 2150 cm −1 which was assigned to NCS vibrations was observed in IR spectrum of complex 2 (Figure 3(c)). The H-Ph, H of CH 3 in dmdphphen bands appeared in their expected areas (Figure 3(a)).

Thermal Decomposition Analysis of Complexes 1.
The thermal analysis of complex 1 ( Figure 5) was investigated in the range of 0-600 ∘ C and heating rate of 10 ∘ C/min. Figure 5 shows that there is no uncoordinated or coordinated water in the range of 0-150 ∘ C and 150-180 ∘ C, respectively. Also, it shows that there are no decomposition intermediate steps of the coordinated chloride and dmdphphen ligands; both inorganic and organic ligands were destructured away from the Co metal with one-step broad decomposition in 200-330 ∘ C with weight loss ∼81% and an exothermic DTA signal at ∼315 ∘ C; the final residue was confirmed by IR to be CoO.
3.6. Electrochemistry. The electron-transfer behavior of the complexes in acetonitrile solution was examined by cyclic voltammetry. As a representative example, the cyclic voltammogram for complex 2 is shown in Figure 6. Complex 2 exhibited two single electron reversible oxidative responses at −550 and 580 mV versus Cp 2 Fe/Cp 2 Fe + , which has been assigned to Co(I)/Co(II) and Co(II)/Co(III) couples, respectively. The dmdphphen ligand is electroinactive over the studied range of +1.5 to −1.5 V. Both complexes exhibit the similar behavior during the cyclic voltammetry experiments.

DNA-Complex 1 Binding
Test. The affinity of Co(II) complexes for double-stranded CT-DNA was explored using UV-Vis titrations in deionised water. The results of representative titrations are shown in Figure 7. Complex 1 showed good DNA binding affinity. Complex 1 has three characteristic absorption peaks at 360 nm, 572 nm, and 655 nm, respectively. There is a decrease in an intensity for all peaks for complex 1 by adding several concentrations of DNA. This suggests that the cobalt complex might be bind to DNA by an intercalative mode [38]. However, by comparing the small shift for complex 1 with 7 nm red-shift values for Os(phen) 2 (dppz) 2+ [39] and 9 nm for [Co(phen) 2 (pdtp)] 3+ [40], this demonstrates that the intercalative strength of such complexes into DNA is not very strong.

Conclusions
Tetrahedral cobalt(II) complexes [CoCl 2 (dmdphphen)] 1 and [Co(NCS) 2 (dmdphphen)] 2 were made available in good yield. Complex 2 was solved by XRD as monoclinic with a space group P2 1 /c. Co(II) ions are located in a distorted tetrahedral environment. TG/DTA result shows that these complexes are very stable and decomposed through one-step   reaction; the complexes exhibit a quasireversible one-electron response at ∼ −550 mV assigned to Co(I)/Co(II) and ∼580 mV assigned to Co(II)/Co(III) versus Cp 2 Fe/Cp 2 Fe + . Absorption spectral studies reveal that such complexes exhibit good DNA binding.

Additional Material
Crystallographic