Synthesis and Characterization of Schiff Base CoII, NiII and CuII Complexes derived from 2-Hydroxy-1-naphthaldehyde and Ethylenediamine


Kadhiravansivasamy, S. Sivajiganesan, T. Periyathambi, V. Nandhakumar, M. Pugazhenthi

Department of Chemistry A.V.V.M Sri Pushpam College Poondi, Thanjavur, Tamilnadu, India

*Corresponding Author E-mail:



New CoII, NiII and CuII complex has been synthesized and characterized by elemental analysis, UV-Vis, FT-IR and thermal analysis Binding of this CoII, NiII and CuII complex with calf thymus DNA was investigated by UV-Visible absorption, fluorescence spectroscopy techniques. The intrinsic binding constants Kb of complex with CT-DNA obtained from UV-Vis absorption studies. Further, the in vitro cytotoxic effect of the complexes examined on cancerous cell line, such as human breast cancer cells (MCF-7).


KEYWORDS: CoII, NiII and CuII complex, DNA interaction, Electrochemical studies and cytotoxicity activity.



There has been enormous report directed towards the development of novel chemical compounds able to arrest or reverse the development of cancer1, 2. Biological activities of transition metal complexes derived from Schiff base ligands are one of the most exhaustively studied topic in coordination chemistry, due to their enhanced activities compared to non - Schiff base complexes3-7. Schiff base complexes show important physiological and pharmacological activities due to their favorable cell membrane permeability8-12. For example, amino acid Schiff base metal complexes have a wide variety of applications including biological, clinical13, analytical and industrial area in addition to their important role in catalysis and organic synthesis14, 15. Accordingly, we intend to report herein, the synthesis of Co(II) Ni(II) Cu(II) complex with 2-Hydroxy-1-naphthaldehyde and Ethylenediamine ligand and characterization of complex was carried out by elemental analysis, IR, UV-Vis studies. The binding properties of this complex to CT-DNA have been carried out using different physico-chemical methods and the binding modes are discussed.



FT-IR, UV-Vis spectroscopy and mass spectra analysis:

The IR spectra of complexes (7-9)(Fig.1 ) show a sharp band in the region of 1616-1652 cm˗1 due to the presence of ν(C=N) [16] formed as a result of condensation reaction of amine and aldehyde, which confirms the presence of Schiff’s base in complexes. All complexes have bands in the region of 3117–3193 cm˗1 and 2916–2980 cm˗1, which can be assigned to aromatic and aldehyde C–H stretching, respectively. Further evidence of coordination of ligands with the metal ions is revealed by the band at 469–527 cm˗1 assigned to the metal-oxygen(M–O) vibration in all complexes [17].


Fig. 1. UV-Vis spectra of complexes 7-9

The absorption spectral data were obtained experimentally for all the complexes in DMF solution (Fig. 2 (a-c)). In the UV region, complexes (7-9) show peaks near 230- 280 nm due to π → π* transition of Schiff’s base ligands. In complexes 7-9, broad/slightly intense peaks or a shoulder were observed in the region of 305–413 nm which could be assigned for ligand to metal charge-transfer transitions18. In the visible region, d-d transition of copper(II) complexes 7-9was observed between 500 and 700 nm. In the visible region, weaker absorptions bands in the region of 500-700 nm obtained for the Co(II) complexes are assigned to 1A1g1T1g and 1A1g1T2g transitions, consistent with Co in an octahedral environment [19,20]. In general, three transitions (2B1g2B2g, 2B1g2A1g, and2B1g2Eg) are expected for a square planar geometry of copper complexes21, however, due to extension of charge transfer transition to low-energy visible region, only one/two d-d transition obtained for complexes 7-922. The nickel(II) complexes 7-9have peak in the visible region near 470-640 nm, which can be assigned to1A1g1A2g, transition confirming square planar geometry of these nickel complexes23.


Fig. 2a. FT-IR spectra of complex 7


Fig. 2b. FT-IR spectra of complex 8


Fig. 2c. FT-IR spectra of complex 9

Electron Spray Ionization (ESI) mass spectrum (Fig. 3(a-c)) are consistent with the proposed complexes (7-9) structures.


Fig. 3a. Mass spectra of complex 4


Fig.3b.ESI mass spectrum of complex 5



Fig.3c.ESI mass spectrum of complex 6


The complex 5shows the peak at m/z 378 which is assignable to [M+]. The loss of -NSC molecule leads to formation of peak at m/z 320 [M-NSC]. The complex 6 shows the peak at m/z 382 which is assignable to [M+]. The loss of -NSC ion leads of peak at m/z 324 due to formation of [M+NSC)]. Few other intense peaks are also obtained for complexes (4-6).


Electrochemical Studies:

The electrochemical behavior of the complexes (1-3) (10-3 M) have been studied using cyclic voltammetry in the potential range of 0 to -1.2 V in the DMF solution containing 10-1M tetra(n-butyl) ammoniumperchlorate and scan rate 50 mVs-1. The voltammograms of the complexes 7-9 were displayed in Fig. 4. The cyclic voltammograms of all the complexes 7-9 have almost the same shape, and exhibit one irreversible redox couple at -0.93, -0.96 and -0.91 for complexes 7-9, respectively.


Fig. 4. Cyclic voltammograms of complexes 7-9



Absorption studies:

Electronic absorption spectroscopic technique was used to investigate the binding of DNA with metal complexes. To achieve this, the absorption spectra of complexes in the absence and presence of calf thymus DNA (CT-DNA) at different concentrations were measured. Transition metal complexes can bind to DNA via both covalent (via replacement of a labile ligand of the complex by a nitrogen base of DNA) and/or noncovalent (intercalation, electrostatic or groove binding) interactions24. In the literature, metal complexes have been found to bind to DNA via the interaction mode25. The UV spectra have been recorded for a constant CT DNA concentration in different [compound]/[DNA] mixing ratios (r). The absorption spectra of the complexes in the absence and presence of calf thymus DNA are shown in Fig. 5. With increasing CT-DNA concentration, for complex 7, the hyperchromism in the band at 266 nm reaches as high as 44% with a blue shift of 5 nm at a ratio of [DNA]/[Complex] of 7. For complexes8 (276 nm, blue shift3 nm)and complex 9 (276 nm, blue shift3 nm), under the same experimental conditions. The results derived from the UV titration experiments suggest that all the complexes can bind to CT DNA11-17 although the exact mode of binding cannot be merely proposed by UV spectroscopic titration studies. The existence of hyperchromism could be considered as evidence that the binding of the complexes involving intercalation between the base pairs of CT DNA cannot be ruled out26-27.


Fig. 5.Absorption spectra of complexes (7-9) in 5 mM Tris-HCl/ 50 mM NaCl buffer upon addition of DNA. Arrow shows the absorbance changing upon increase of DNA concentration. The inner plot of [DNA/(εa-εf) vs [DNA] for the titration of DNA with complexes.

In order to further evaluate the binding strength of the compounds, the intrinsic binding constants (Kb) of them with CT-DNA were determined from the following equation [28]:


Where, [DNA] is the concentration of DNA in base pairs and the apparent absorption coefficient, εa, corresponds to Aobs/[compound]. εrefers to the extinction coefficient of the free compound and εb is the extinction coefficient of the compound when fully bound to DNA. The plot of [DNA]/(εa - εf) vs [DNA] gave a slope and intercept which are equal to 1/(εb - εf) and 1/Kbb - εf), respectively; Kb is the ratio of the slope to the intercept (Fig. 6.).


Fig.6. The plot of [DNA/(εa-εf) vs [DNA] for the titration of DNA with complexes.


The λmax, hypochromism, blue shift and binding constant values of all the complexes with CT-DNA indicate that there was a finite interaction between these complexes and CT-DNA. The observed Kb values have also revealed that the complexes bind to DNA via an intercalative mode18. The observed Kb value of complexes 1-3, 2.65 X 10-4, 2.91 X 10-4and 3.51 X 10-4M, respectively. This is expected as the incorporation of substituent groups on aromatic ligand would hinder the insertion of the aromatic ring in between the DNA base pairs29. Thus, in general a planar extension of the intercalating ligand would increase the strength of interaction of the complexes with DNA Thus, the numbers of aromatic rings in the co-ligand and substituent groups on aromatic ring dictate the DNA binding affinity and binding structure of the mixed ligand complexes. Though it has been found that the complexes can bind to DNA groove from the electronic absorption studies, the binding mode needed to be through some more experiments.


Competitive study with ethidium bromide:

Ethidium bromide (EB) is a typical indicator of intercalation since it can form soluble complexes with nucleic acids resulting in the emission of intense fluorescence due to the intercalation of the planar ring between adjacent base pairs on the double helix of CT DNA. The changes observed in the spectra of EB on its binding to CT DNA are often used for the interaction study between DNA and metal complexes30,31. Complexes 1-3 show no fluorescence at room temperature in solution or in the presence of CT DNA, and their binding to DNA cannot be directly predicted through the emission spectra. Hence competitive EB binding studies may be undertaken in order to examine the binding of each compound with DNA since the fluorescence intensity is highly enhanced upon addition of CT DNA, due to its strong intercalation with DNA base pairs. Addition of a second molecule, which may bind to DNA more strongly than EB, results in a decrease the DNA-induced EB emission due to the replacement of EB, and/or electron transfer32,33. The emission spectra of EB bound to CT DNA in the absence and presence of each compound have been recorded for [EB] = 20 µM, [DNA] = 4 µM for increasing amounts of each compound. The addition of each complex 1-3 at diverse r values (Fig. 7) results in a significant decrease of the intensity of the emission band of the DNA-EB system at 600 nm (up to 24% of the initial EB-DNA fluorescence intensity for complex 1, 34% for 2and 55% for complex 3 (Fig. 5.8.) indicating the competition of the complexes with EB in binding to DNA. The observed significant quenching of DNA-EB fluorescence for complexes 1-3 suggests that they displace EB from the DNA-EB complex and they can probably interact with CT DNA by the intercalative mode34-36.


Fig. 7.Fluorescence emission spectra of the EB-DNA in presence of complexes (7-9) in 5 mM Tris HCl/ 50 mM NaCl buffer (pH 7.2).


Fig. 8. Plot of EB relative fluorescence intensity at λem = 598 nm (I/I0 (%)) vs r (r = [compound]/[DNA]) for complexes (7-9) in buffer solution (5 mM HCl/ 50 mM NaCl at pH 7.2).


Fig. 9.the Stern-Volmer plot illustrating the quenching of EB bound to DNA by complexes (7-9).


The Stern-Volmer constant Ksv may be used to evaluate the quenching efficiency for each compound according to the equation:


where I0 and I are the emission intensities in the absence and the presence of the quencher, respectively, [Q] is the concentration of the quencher (complexes 1-3) and Ksv is obtained by the slope of the diagram I0/I vs [Q]. The Stern-Volmer plots of DNA-EB for the compounds (Fig.9.) illustrate that the quenching of EB bound to DNA by the compounds is in good agreement with the linear Stern-Volmer equation, which proves that the replacement of EB bound to DNA by each compound results in a decrease in the fluorescence intensity. The high Ksv (2.56 X 10-5 for complex 1, 2.81 X 10-5 for complex 2and 3,31 X 10-5M for complex 3) values of the compounds show that they can bind tightly to the DNA and are higher than those found for the metal(II) complexes.



Binding of the complexes to serum albumin:

The study of the interaction of drugs and their compounds with blood plasma proteins and especially with serum albumin, which is the most abundant protein in plasma and is involved in the transport of metal ions and metal complexes with drugs through the blood stream, is of increasing interest. Binding to these proteins may lead to loss or enhancement of the biological properties of the original drug, or provide paths for drug transportation37. Bovine serum albumin (BSA) is the most extensively studied serum albumin, due to its structural homology with human serum albumin (HSA). HSA (one Trp-214) and BSA (containing two tryptophan's, Trp-134 and Trp-212) solutions exhibit a strong fluorescence emission with a peak at 351 nm and 343 nm, respectively, due to the tryptophan residues, when excited at 295 nm38. The interaction of complexes 1-3 with serum albumins has been studied from tryptophan emission-quenching experiments. The changes in the emission spectra of tryptophan in BSA are primarily due to change in protein conformation, subunit association, substrate binding or denaturation complexes 1-3 exhibited a maximum emission at 354 nm under the same experimental conditionsm (Fig. 10) and the SA fluorescence spectra have been corrected before the experimental data processing39.


Fig. 10.Fluorescence spectra of BSA in presence of various concentration of complexes (7-9)


Fig. 11. Plot of % EB relative fluorescence intensity at λem = 353 nm (I0/I (%)) vs r (r = [complex]/[BSA]) for complexes (7-9) in buffer solution (150 mM NaCl and 15 mM trisodium citrate at pH 7.0)


Fig. 12. Fluorescence spectra of BSA in emission intensity I0/I vs [Q]. [Q = complexes (7-9)


Fig. 13. Determination of the complex-BSA binding constant and the number of binding of BSA.


Addition of complexes to BSA results in relatively moderate fluorescence quenching (up to 30% of the initial fluorescence intensity of BSA for complex 1, 46% for 2and 54% for complex 3 as calculated after the correction of the initial fluorescence spectra) (Fig. 11), due to possible changes in protein secondary structure of BSA indicating the binding of the compounds to BSA40. The Stern-Volmer and Scat chard graphs may be used in order to study the interaction of a quencher with serum albumins. According to Sterne Volmer quenching equation9,16:


Where I0 = the initial tryptophan fluorescence intensity of SA, I = the tryptophan fluorescence intensity of SA after the addition of the quencher, kq = the quenching rate constants of SA, Ksv = the Stern-Volmer constant, t0 = the average lifetime of SA without the quencher, [Q] = the concentration of the quencher respectively,  and, taking as fluorescence lifetime (t0) of tryptophan in SA at around 10-8 s, the Stern-Volmer  quenching constant (Ksv, M-1) can be obtained by the slope of the diagram I0 /I vs [Q] (Fig. 12), and subsequently the approximate quenching constant (kq, M-1 s-1) may be calculated.


The calculated values of Ksv and kq for the interaction of the compounds with BSA are given in    Table 1 and indicate a good BSA binding propensity of the complexes exhibiting the highest BSA quenching ability. The kq values are higher than those characterizing diverse kinds of quenchers, pointing towards the existence of a static quenching mechanism41,42. Using the Scat chard equation43:


where n is the number of binding sites per albumin and K is the association binding constant, K (M-1), may be calculated from the slope in plots (DI0/I)/[Q] vs (DI/I0) (Fig. 13) and n is given by the ratio of the y intercept to the slope. It is obvious (Table 5) that the coordination of Co(III) complexes results in a decreased K value for BSA with complex 1 exhibiting the highest K value among the other complexes. The Stern-Volmer equation applied for the interaction with BSA in Fig. 12 shows that the curves have fine linear relationships (r = 0.9798-0.9902). The calculated values of KSV and kq are given in Table 5 and indicate their good BSA binding propensity with complex 1 exhibiting the highest BSA quenching ability. From the Scat chard graph (Fig. 12), the associated binding constant to BSA of each compound has been calculated (Table 5). The n values of complexes (1 & 2) are given in Table 5. Additionally, complexes exhibited higher binding affinity for BSA than other complexes, which occurs in a similar way to that observed in the DNA binding studies.


Table5. The BSA binding constant and parameters (Ksv, kq, K, n and r) derived for complexes (1 & 2). 0.9798-0.9902








2.0 × 105

2.0 ×1013





2.6 × 105

2.6× 1013





2.9 × 105

2.9× 1013





Anticancer activity studies:

MTT assay:

Since all the present metal(II) complexes have the ability to strongly bind and cleave DNA in the absence of a reductant, and since DNA cleavage is considered44-48 as essential for a drug to act as an anticancer agent, the cytotoxicity of the complexes against both human cervical cancer cell lines (HeLa) were investigated in aqueous buffer solution in under identical conditions by using MTT assay. All the complexes are found to be very active against cancer cells and their IC50 values obtained by plotting the cell viability against concentrations of the complexes reveal that all the complexes exhibit cytotoxicity (Fig. 14). Further, as revealed by the observed IC50 values (7.36 µM for complex 1,4.16 µM for complex 2and 3.21 µM for complex), the potency of the complexes to kill the cancer cells, revealing that it varies with the mode and extent of interaction of the complexes with DNA. Thus, the highest cytotoxicity exhibited by the complexes is consistent with the stronger binding of the complex through deeper insertion of the co-ligand in between the base pairs of DNA and its higher ability to cleave DNA in the absence of a reductant is responsible for its potency to induce cell death through different modes.



Fig. 14. Cytotoxic effect of complexes (1-3) against MCF-7 at different concentration. Cell viability decreased with increasing concentration of complex 1.



Thank the A.V.V.M Sri Pushpam College Poondi  Thanjavur – Di Tamilnadu,.



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Received on 25.11.2016         Modified on 02.01.2017

Accepted on 27.01.2017         © AJRC All right reserved

Asian J. Research Chem. 2017; 10(2):106-114.

DOI:  10.5958/0974-4150.2017.00016.5