Document Type : Research Paper
Authors
Department of Chemistry, College of Science, University Al- Muthanna, Iraq
Abstract
Keywords
INTRODUCTION
Today, azo dyes form the majority of dye chemistry output, and their popularity may grow in the years to come. The synthesis of most azo dyes involves the diazotization of an aromatic primary amine, followed by its subsequent coupling to one or more electron-rich nucleophiles, commonly amino or hydroxyl groups. Azo dyes get their color from the chromophores and auxochromes that bind to their azo bonds. Due to their exceptional biological and physicochemical qualities and wide range of uses in fields as varied as analytical chemistry, pharmaceuticals, cosmetics, painting, and the dyeing industry, azo dyes have garnered considerable interest [1,2].
Extensive investigation has been dedicated to exploring the notable therapeutic properties exhibited by small-ring heterocycles that incorporate N and S atoms [3]. Thiazole and imidazole derivatives are among the most important heterocyclic compounds due to their biological and chemical potential as well as their presence in major classes of naturally occurring compounds (such as alkaloids, vitamins, hormones, and antibiotics). There has been an increase in the usage of imidazoles and thiazoles as organic synthesis intermediates, catalysts for asymmetric synthesis, and active ligands for transition metals and their complexes [4,5]. These chemicals affect DNA by changing its structure, activity, and stability. Due to the suppression of enzymes involved in DNA metabolism, including topoisomerases, several critical cellular processes are disrupted [6]. The biological activities exhibited by thiazole and imidazole are anti-inflammatory [7], antibacterial, antifungal [8], antitumor, anti-tubercular, anti-diabetic, antiviral, and anticancer [9].
More than 10 million people die every year from cancer, which is part of a group of disorders characterized by aberrant cell division and unchecked cell development. The challenge for synthetic chemists is to create anticancer medications that are both more selective and less toxic. In pharmacology, heterocyclic molecules typically play an important function. Thiazoles and imidazoles are just two of the many NN and NS-donating ligands and their metallic complexes that were discovered as anti-cancer agents in the past 35 years [10,11].
This research involved the synthesis of a new thiazolyl azo ligand (5-MeTADMBI) and the evaluation of its antibacterial activity against Gram (-ve, +ve) bacteria using a range of Co(III), Ni(II), and Co(II) complexes [12]. Finally, the chemicals were tested for their anti-cancer activity in a cytotoxicity experiment using Lung cancer (A549) cell lines [13].
MATERIALS AND METHODS
Instruments
Infrared compound spectra were registered using (KBr disc) in the range (400- 4000) cm-1 of a Shimadzu 8400S FT-IR spectroscopy. The 1H and 13C NMR spectra were run at 500 MHz employing dimethylsulfoxide (DMSO-d6) as the chosen solvent and tetramethylsilane (TMS) as the designated reference compound, we shall proceed with the utilization of a Brukere 500 Analyzer for our experimental endeavors. The mass spectra were recorded with the aid of the AB Sciex 3200 QTRAP device. The UV-Vis spectra were measured using a PerkinElmer Ultraviolet-Visible Spectrometer Lambda 35. Spectroscopy (C.H.N.) was performed using a Euro EA 1106 elemental analyzer. The BuchiSMP-20 was used to determine the ligand and complex melting points. The magnetic susceptibility of the complexes was tested for each sample using the Susceptibility Balance Magnetic Model M.S.B.-Auto. The X-ray diffraction (XRD) analysis was carried out on a Siemens D5000. The scanning electron microscope technique was used in the emitted field (FESEM TESCAN BRNO-MIRA3 LMU, which is of Czech-French origin). At room temperature (240 °C), ethanol solutions were tested using a 31A digital conductivity meter to determine their conductivity. Perkin-Elmer instruments were utilized to analyze the compounds’ thermal stability after synthesis.
Material
All of the substances in this experiment come from commercially available, high-quality sources of analytical reagents. Aldrich, Sigma (Germany), Merck (Germany), and J&K chemical (China) supplied the 2-amino-5- methyl thiazole, 5,6-dimethyl benzimidazole, DMSO (Dimethyl sulfoxide), pure ethanol, NaNO2, NaOH, HCl, CoCl2.6H2O, and NiCl2.6H2O.
Synthesis of 2-[2-(5-Methyle thiazolyle) azo]-5, 6-dimethyl benzimidazole (5-MeTADMBI)
Fig. 1 shows the steps taken to synthesize a new thiazolyl azo dye ligand, 2-[2-(5-Methyle thiazolyle) azo]-5, 6-dimethyl benzimidazole (5-MeTADMBI). According to the literature[14,15], we dissolved 2-amino-5-methylthiazole (1.2g, 0.001 moles) in concentrated hydrogen chloride (5 mL) and distilled water (30 mL) to make the diazonium salt. Within 30 minutes of cooling in an ice-water bath with constant stirring, (1g, 0.015 mol.) of NaNO2 dissolved in, (35 mL) of deionized water was applied. However, after stirring the reaction mixture in an ice bath (0-5 °C) for an hour, a solution of (diazonium salt) was slowly added to a solution of 5,6-dimethyl benzo imidazole (1.5g, 0.01 mole) dissolved in the mixture (25 mL Ethyl alcohol, 12 mL 10% NaOH). The precipitate was obtained after filtering and cooling. Several washes in distilled water, a recrystallization in Ethyl alcohol, and an overnight drying at 50 °C resulted in a pure precipitate.
General synthesis of metal complexes
To produce metal complexes, ligand (0.3g, 0.002 moles) was dissolved in ethanol solution (50 mL). In a stoichiometric ratio of 1:2 [M: L] for Co(III) and Cu(II) and 1:1 [M: L] for Ni(II), add (0.15g, 0.0001 moles) of metal chlorides while stirring and utilize the buffer solution (ammonium acetate). Over 30 minutes at (50-70oC) and left overnight, the reaction mixture was gradually lowered. Several methods were used to re-crystallize the solid metal complexes after washing them in a distilled water/ethanol solution and drying them Table 1 [16].
RESULTS AND DISCUSSION
Characterization of thiazolyl azo dye ligand and their complexes
Brown crystals of the novel (5-MeTADMBI) ligand were seen, while crystals of the synthesized metallic complexes appeared to be of varied colors. The compounds exhibit limited solubility in aqueous media while demonstrating appreciable solubility in organic solvents such as ethanol, DMSO-d6, or methanol [17,18]. All of the compounds are stable at ambient temperature. Thiazolyl azo ligand and chelate complex yields, melting temperatures, and elemental analyses are shown in Table 1.
Molar conductivity and stability constants measurements
Table 2 shows that all of the metal complexes are not an electrolyte in a 10-3 Molarity ethanol solution at ambient temperature, however, based on its conductivity, the Co(III)-complex is an electrolyte[19,20]. Furthermore, the determination of stability constants for all complexes entails the utilization of absorbance coefficients obtained from solutions comprising both ligand and metal ions at a fixed wavelength (λmax). In relation to the equations:
β = (1-α)/4 α3C2, [1:2] [M: L]
β = (1-α)/ α2C, [1:1] [M: L]
α = Am-As/Am
Where As and Am are the Absorbance of fully and partially produced metallic complexes at best concentrations, respectively [21,22].
UV-Vis Studies
In order to elucidate the stereochemistry or geometry of metal complexes, it is imperative to consider two key parameters: the maximum wavelength of electronic absorption (λmax, nm) and the magnetic moment (μeff B.M.) at room temperature. The UV-Vis spectra of a 10-4 M solution of the free ligand (5-MTADMBI) and its complexes in absolute ethanol at ambient temperature within the wavelength range of 200 to 1100 nm are depicted in both Fig. 2 and Table 3 [23].
The electronic spectrum of a free ligand (5-MTADMBI) showed four absorption peaks, the first at wavelength 458nm (21834.06)cm-1 belonging to an electron transition (n→π(* of an azo group -)N=N(- and the second at wavelength 398nm (25125.63)cm-1 belonging to the electron transition (n→π (*of an )C=N ( imine group in the thiazole ring, as well as, a third peak appeared at 326nm (30674.85 cm-1) and the fourth peak at 270nm (37037.04 cm-1) is related to the electronic transition (π→π (*of the )C=C( bond in the thiazole ring and the aromatic ring [24,25].
Three absorption peaks were observed in the UV spectrum of the Co(III)-Complex at 629nm (15898.25 cm-1), 319nm (31347.96 cm-1), and 266nm (37593.98 cm-1) due to 1A1g→1T2g (F)(υ1), 1A1g→1T1g (F)(υ2), and 1A1g →1T1g (p)(υ3), and other peaks returning to the transitions of the intra ligand. Transitions with an octahedral geometry have been measured to have a magnitude of 10Dq/B (34.5) using the ratio υ2/υ1 (1.73). For the octahedral structure, we can use the tanaba-Sugano diagram for the d6 structure, the magnitude of B’ (565) as well as a location of υ1(10Dq) (12447.36) cm-1 have been examined. The value of β (0.514) denotes some covalent character. According to the conductivity test done with ethanol as the solvent, the Co(III)-Complex is an electrolyte[26][27].
The electronic spectrum of the Ni(II)-complex gave four absorption peaks, three of them at 505 nm (19801.98 cm-1), 325 nm (30769.23 cm-1), and 269 nm (37174.72 cm-1) due to the electron transitions 1A1g→1A2g(υ1) and 1A1g→1B1g(υ2) and 1A1g→1Eg(υ3) respectively, while the other transition at 420 nm (23809.52) cm-1 is related to the intra ligand, respectively, of square planar geometry [28].
The spectrum of the Cu(II)-complex showed a wide absorption peak of moderate intensity at 637nm (15698.59 cm-1), and the reason for the width of the peak indicates the occurrence of three electronic transitions of similar energy, which are 2B1g→2A1g (dx2-y2→dz2)(υ1) and 2B1g→2B2g (dx2-y2→dyz)( υ2) and 2B1g→2B2g(charge transfer)(υ3) . The reason for the width of the peak in the octahedral field being divided into three peaks is due to the distortion that occurs for this type of complex due to the Jean Teller effect. As for the other absorption peaks at 427nm (23419.20 cm-1) and 292nm (34246.58 cm-1), they are due to intra-ligand transitions. The non-ionic complex conclusion from conductivity measurements [29,30].
C.H.N.S Elemental Analysis
A thorough elemental analysis was conducted employing a CHNS elemental analyzer to elucidate the intricate metallic chemical composition. The elemental analysis yields valuable insights regarding the relative abundances of carbon, hydrogen, nitrogen, and sulfur (CHNS) elements within the material. The precise results of the elemental analysis are meticulously displayed in Table 4.
Upon comparing the experimental and theoretical values, a remarkable degree of concordance was observed, indicating a high level of agreement between the two sets of data. The obtained experimental data provide compelling support for the validity of the selected molar ratios between the metal and ligand species [M: L], thereby bolstering the robustness of the proposed chemical formulas for the synthesized metal complexes [31,32]
1H-NMR studies
The investigation involved the analysis of the (5-MTADMBI) ligand and Co(III) complex’s 1H-NMR spectrum at ambient temperature. A (Bruker 500 MHz) spectrometer was employed for this purpose, with DMSO-d6 serving as the solvent. The reference sample utilized was TMS [33]. The presence of a resonance peak at a chemical shift of (11.73) ppm can be attributed to the protons associated with the (NH) imidazole moiety within the free ligand, specifically 5-MTADMBI. Additionally, the occurrence of a signal at a chemical shift of δ = 7.95 ppm can be attributed to the proton within the thiazole ring. The aromatic protons in the benzimidazole ring of the (5-MTADMBI) compound exhibit a signal at a chemical shift range of δ= (6.90 - 7.24) ppm. On the other hand, the protons in the methyl groups (C-(CH3)) of the thiazole and benzimidazole rings display signals at chemical shifts of δ = (2.37 and 2.19) ppm, respectively. The proton resonance is observed at a chemical shift of δ = (2.39-2.41) ppm, indicating a singlet signal in the presence of the solvent. This information is referenced from source [34]. The 1H-NMR spectrum of the thiazolyl azo ligand (5-MTADMBI) is depicted in Fig. 3.
In the 1H- NMR spectrum of the Co(III)-complex, the methyl group (C-(CH3)) on the thiazole ring generates a signal at δ = (2.01- 2.06) ppm, while the (C-(CH3)2) on the benzimidazole ring provides a signal at δ = (1.75- 1.93) ppm. The NH group produces a signal at δ = 11.23 ppm (imidazole proton), while the proton in the thiazole ring generates a signal at δ = (7.06 - 7.31) ppm, and the aromatic protons in the benzimidazole ring provide a signal at = (5.20 - 6.63). In the case of the solvent proton shown at = (2.39 - 3.85) ppm [35]. Fig. 4 shows the 1H-NMR spectrum of the Co(III)-complex.
13C-NMR studies
It is possible to determine the type and number of carbon atoms in a molecule by employing 13C-NMR, which is one form of nuclear magnetic resonance technology. Table 5 and Figs. 5 and 6 display the results of 13C-NMR (76 MHz, DMSO-d6) spectra performed on the investigated (5-MTADMBI); these spectra revealed numerous signals at the chemical shifts 13C-NMR= (179.27, 161.76, 156.01, 152.66, 150.57, 143.45, 125.52, 123.54, 55.44, 26.65, 16.06, 14.71, 14.42, and 39.39 ppm) to the carbon atoms at the locations (C2, C12, C16, C4, C15, C5, C17, C14, C9, C11, C18, C19, C6, and C solvent) respectively. Signals at δ= (170.80, 161.79, 156.06, 150.49, 149.93, 140.37, 125.55, 117.79, 40.41, 26.66, 16.07, 14.43, 11.65, and 39.39 ppm) were assigned by chemical shift to carbon atoms in the identical thiazolyl azo ligand positions in the Co-complex spectrum [36,37].
FT-IR spectra studies
Upon conducting a comparative analysis between the FTIR spectra of the ligand and its corresponding metal complexes, notable observations were made. Specifically, certain bands exhibited shifts in their positions, while novel bands emerged, alongside the persistence of unchanged bands. This observation provides evidence for the formation of a chemical complex. The FTIR spectroscopic data pertaining to the (5-MeTADMBI) ligand, along with its corresponding metal complexes, have been comprehensively presented in Table 6 and visually depicted in Fig. 7. Through the utilization of potassium bromide (KBr), the vibrational frequencies of the predominant molecular entities were observed within the spectral region spanning from 400 to 4000 cm-1 [38].
The observed strong absorption broad bands in the region of (3095, 3124, 3203, and 3134) cm-1 in all the ligand (5-MTADMBI) and its complexes with Co(III), Ni(II), Cu(II) can be ascribed to the stretching vibrations of the (NH) groups. The suggestion has been made that the spectra of most Co(III), Ni(II), and Cu(II) complexes exhibit the presence of (OH)aqua. This inference is based on the observation of significantly broad absorption bands at (3212, 3363, and 3335) cm-1, as well as the notable absence of this band in the spectra of the unbound ligand [39,40].
The spectra of the (5-MeTADMBI) ligand and its complexes exhibited a medium-broad band in the range of (3030, 2970, 2974, and 3080) cm-1, which can be attributed to the (C-H) aromatic vibrations. Additionally, the bands observed at (2962, 2939, 2939, and 2978) cm-1 can be assigned to the (C-H) aliphatic vibrations, respectively [41]. In the spectra of the free ligand and its metal complexes, we observed supplementary bands at wavenumbers 1477, 1498, 1502, and 1498 cm-1. These bands can be attributed to the stretching modes (N=N) of the azo group. Additionally, the band at 1708 cm-1 in the spectrum of (5-MeTADMBI) corresponds to the C=N stretching mode of the azomethine group in the benzimidazole ring, providing evidence for the existence of the ligand. The observed spectral band corresponding to the complexes exhibited an absence in comparison to the spectral band observed for the ligand. The FTIR spectra of the metal complex manifest novel bands that can be attributed to the vibrational mode of the metal-nitrogen azo (ν(M-Nazo)), as documented in Table 6. The FT-IR spectra obtained for the metal complexes indicate that (5-MeTADMBI) exhibits the capability to function as a bidentate ligand, specifically binding to nitrogen atoms from both the (N=N) and (C=N) positions of benzimidazole. This binding arrangement results in the formation of a hexagonal structure [42,43].
X-Ray Diffraction (XRD) studies
X-ray diffraction was used to investigate the crystal structures of the ligand (5-MeTADMBI) and its complexes in their solid states. The analysis was conducted within the angular range of (10-80°) 2θ to elucidate structural properties, including the crystalline structure and crystal size Fig. 8. Micro strains and dislocation density were determined as well to identify the degree of purity and crystalline structure defects during the conversion of the studied ligand into metal complexes [44].
The utilization of Bragg’s law [n λ = 2dsin Ѳ] facilitated the determination of the d-spacing, denoting the separation between the crystalline planes. In this context, n assumes integer values (1, 2, 3, 4), λ corresponds to the wavelength of X-ray Cu-kα (1.5405), and Ѳ signifies the deflection angle [45]. The Debye-Scherer equation, a fundamental tool in crystallography, allows for the determination of the average crystal size (D) through the utilization of various parameters. In this equation, D is calculated by dividing the product of the X-ray wavelength (λ) and the constant k (known as Blanks constant, with a value of 0.940) by the cosine of the deflection angle (Ѳ) and the FWHM of a peak observed during experimentation [46]. The ligand and metallic complexes being investigated exhibit a granular size below 100 nm, thereby placing them within the nanoscale regime. Simultaneously, these findings substantiate our previous data obtained through FESEM. Table 7 presents the pertinent data regarding the crystalline size, d-spacing, and diffraction angles pertaining to the synthesized ligand and complexes.
FESEM studies
Using a technique called field emission scanning electron microscopy (FE-SEM), the surface morphology, shape, and aggregation of the particles, as well as their location, of ligand (5-MeTADMBI) and metal complexes are studied. Before imaging, a 20 Kv acceleration voltage, 200 nm cross-sectional distance, and 10 KX magnification force were used with a field emission scanning electron microscope. Fig. 9 displays FESEM images of the thiazolyl azo ligand (LH) and its metal complexes, for which the particle sizes and specific properties were calculated using the (image J) program [47].
The image of the FE-SEM analysis of the (5-MeTADMBI) ligand shows that it has a shape with irregular crystalline particles with a heterogeneous surface and an average particle size of (77.42 nm). As for the image of the FESEM analysis of the Co(III)-complex, it appeared in the form of spherical aggregates with an average particle size of (73.63nm). While the image of the FESEM analysis of the Ni(II)-complex showed it in the form of particles, it had a smooth surface with an average particle size of (85.51nm). While the image of the FESEM analysis of the Cu(II)-complex appeared in the form of porous cheese slices and had an average particle size of (66.88nm) [48].
TGA-DTG analysis
The azo dye ligand and its metal complexes underwent thermal treatment in a controlled nitrogen environment, with the temperature ranging from 40 to 900 oC at a heating rate of 20 oC per minute. An experimental investigation utilizing thermogravimetric analysis (TGA) was undertaken with the primary aim of elucidating the thermolytic degradation pathways of the compounds under investigation. Fig. 10 presents the TGA outcomes for the ligand and metal complexes. The TGA data unambiguously demonstrated the occurrence of ligand and complex decomposition in singular, binary, or ternary steps [49].
The thermogravimetric curve for (5-MeTADMBI) one characterization step of decomposition, was seen at temperatures between (40.17 and 410.98°C), and is related to the evolution of moisture and CO2 gas with a loss of mass of 76.095%. There are two successive thermal decomposition stages in the Co(III), Ni(II), and Cu(II), complexes. The initial stage takes place between (39.99- 282.75°C), (40.24 -242.29°C), and (40.36 - 175.17°C) respectively, and it is estimated that (13.536, 16.117, and 5.967 %) of the mass is lost during this stage. A mass loss of about (33.213, 39.381, and 42.263 %) is anticipated to occur during the second stage, which takes place between (258.28- 499.16°C), (242.29 -500.66°C), and (175.17°C -496.19°C) [50].
Mass Spectral Studies
The determination of the molecular weight of the synthesized azo dye is accomplished through the utilization of mass spectrometry, a powerful analytical technique that yields valuable insights into the abundance of ions at various m/z values. Mass spectrometry is a powerful analytical technique that enables the direct identification of molecules by evaluating their mass-to-charge ratio [51].
The thiazolyl azo ligand’s mass spectrum revealed a group of peaks, and the compound’s predicted mass fractions are illustrated in Figs. 11 and 12. The molecular weight of this compound is 271.34, and the primary peak in the spectral line appeared at (m/z+= 270.1), which is consistent with the chemical formula [C13H13N5S] Fig. 11.
While the Ni(II)-complex mass spectrum showed a group of peaks and the proposed mass fractions of the Ni(II)-complex was shown in Fig. 12, respectively, the spectrum showed a main peak at (m/z+= 421.1) that corresponds to the chemical formula [C13H15N5SOCl2Ni] and belongs to the molecular weight of the compound (418.962) [52].
Anti-microbial activity studies
Antibacterial and antifungal activity
The anti- microbial assay was screened for (5-MTADMBI) ligand and their metal complexes in vitro against bacteria gram-positive, gram-negative (Staphylococcus aureus, Escherichia coli and Klebsiella) and fungi (Candida), using the method of diffusion on Mueller-Hinton agar medium. There was control of the growth-inhibiting zone surrounding the disc. The compounds were dissolved in DMSO to prepare the stock solution. The Petri dishes were subjected to incubation at a temperature of 37 °C for a duration of 24 hours, during which the majority of the conducted assays indicate that the complexes exhibit enhanced biological activity in comparison to analogous azo ligands [53,54]. The results are observed in Table 8 and Fig. 13.
Cytotoxicity Assays (MTT assay)
The MTT cell viability assay was employed to quantitatively assess the cytotoxic effects of the azo dye ligand and several metal complexes. The MTT test is commonly used to determine cell viability and calculate the IC50 (half maximal inhibitory concentration) of agents. The MTT assay is a rapid and easy in vitro method for assessing cell viability by measuring the amount of yellow tetrazolium salt reduced to purple formazan [55].
The cytotoxicity of (5-MTADMBI) was assessed by employing the MTT assay within 96-well plates. A confluent monolayer was attained by inoculating cell lines at a density of 1×104 cells per well and allowing them to adhere within a controlled environment of 5% CO2 incubator for a duration of 24 hours. The experimental setup involved the utilization of DMSO at a concentration of 0.5% (v/v) as the vehicle control for the cultured cells. The negative control, on the other hand, consisted of untreated cells.
The cells were subsequently exposed to several concentrations of (5-MTADMBI) for 48 hours (200, 100, 50, 25, and 12.5 μg/ml). The medium was then discarded, and a 5 mg/ml MTT solution was added to each well, where it remained for another 2 to 4 hours at 37 degrees Celsius. Following the successful formation of formazan, DMSO was introduced into the system. Subsequently, the absorbance at a wavelength of 595 nm was meticulously quantified utilizing the cutting-edge technology of an ELISA microplate reader, specifically the xMark Microplate Reader [55].
At 200 (μg.ml −1) of the thiazolyl azo ligand, lung cancer (A549) was inhibited at a rate of 60.10%, whereas normal cells (HdFn) were inhibited at a rate of 27.36%. In contrast, the Cu(II)-complex inhibited A549 cells at a rate of 46.528 % at a concentration of 200 (μg.ml −1) and HdFn cells at a rate of 28.05 % at the same dose.
The experimental findings presented in Tables 9 and 10 elucidate the impact of the thiazolyl azo ligand and its Cu(II)-complex on pulmonary cells, while concurrently providing a comparative analysis with the normal cell line at equivalent concentrations. The IC50 values, representing the concentration at which 50% inhibition occurs, were determined for lung cancer cells and yielded values of 68.51 and 64.20 (μg.ml-1), respectively. The observed selective cytotoxicity of the ligand and Cu(II)-complex was demonstrated against normal cell lines, with IC50 values of 76.85 and 83.4 (μg.ml-1), respectively. Figs. 14 and 15 depict the cell viability percentages of the (5-MTADMBI) ligand and its corresponding Cu(II)-complex, respectively.
CONCLUSION
A predictable diazonium coupling process at 0-5 oC has been used to synthesize a biologically active azo dye ligand, including 5-MeTADMBI Co(III), Cu(II), and Ni(II) were used for metallization with the synthesized azo dye ligand in 1:2 or 1:1 stoichiometric ratio as ligand: metal with the present base. Molar conductivity, elemental analysis, TGA/DTA, Mass, XRD, FE-SEM, FTIR, and nuclear magnetic resonance (NMR) all contributed to the establishment of the structure. The octahedral geometry of the Co(III) and Cu(II) complexes’, but square planar for Ni(II) complex, are supported by physicochemical and spectroscopic data. Biological assays showed that both the ligand and its complexes were effective in inhibiting pathogenic bacteria. In addition, each chemical compound gives effective inhibition against the A549 cell line, especially [Cu(L)2Cl2].H2O is considerably more efficient, with IC50 values of 64.20.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.
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