THREE Co(II), Ni(II) AND Cu(II) SCHIFF BASE COMPLEXES INCORPORATING

. New 2-[(4-[(4-methylphenyl)sulfonothioyl)methylene]aminobenzoic acid ligand-Co(II), Ni(II)

antifungal, antiviral, antimalarial, antitubercular, anti-inflammatory, and antioxidant activities that make them potential candidates for drug design and development [7].They can act as corrosion inhibitors for various metals and alloys in different acidic and alkaline media.They can serve as analytical reagents for the spectrophotometric determination of various metal ions, such as copper, iron, nickel, cobalt, zinc, and manganese.Therefore, Schiff-bases derived from anthranilic acid and their metal complexes are important and useful compounds that have a wide range of applications in various fields of science and technology.
On the other hand, Sulfonate compounds are organic molecules that contain the functional group R-S(=O)2-O-, where R is an organic group [8,9].They can be classified into sulfonate salts and sulfonate esters, depending on the nature of the group attached to the oxygen atom.Sulfonate compounds have a wide range of applications in various fields of chemistry, biology, and medicine, due to their stability, solubility, and reactivity [10,11].One of the important applications of sulfonate compounds is their ability to act as ligands for metal ions, forming sulfonate metal complexes [12,13].These complexes have diverse structures, properties, and functions, depending on the type of metal ion, the number and geometry of the ligands, and the presence of other coordinating groups.Sulfonate metal complexes have been extensively studied for their potential uses in catalysis, luminescence, sensing, optoelectronics, drug design, and corrosion inhibition.
The aim of this research article is to explore the coordination chemistry of novel Schiff-base ligands derived from 4-tosyl chloride and anthranilic acid, which have not been reported in the literature before.We synthesized and characterized a series of new Co(II), Ni(II), and Cu(II) complexes with these ligands, and investigated their antibacterial and antifungal activities against various microorganisms.We also performed density functional theory (DFT) calculations and molecular docking studies to understand the structural and electronic properties of the ligands and complexes, as well as their possible binding modes with microbial enzymes.This study provides new insights into the synthesis, structure, and biological applications of Schiff-base metal complexes derived from anthranilic acid.

EXPERIMENTAL Materials
All of the chemicals utilized in the study, such as CoCl2, NiCl2, and CuCl2, were of reagent quality and were used straight from the chemical companies from whence they were bought.Every one of the solvents was of spectroscopic quality.DMF, DMSO, and acetonitrile were among the solvents used.

Synthesis of the Schiff-bases ligand (HL)
After 10 minutes of stirring a mixture of 4-formylphenyl-4-methylbenzenesulfonate (1 mmol, 2.76 g) and anthranilic acid (1 mmol, 1.37 g), the corresponding Schiff base was produced.After the solid precipitate formed, it was filtered, washed with ethanol solutions of decreasing concentration, air-dried, and crystallized.

Synthesis of the metal complexes
The metal complexes were prepared by dissolving the ligand (1.5 mmol) in hot ethanol (20 mL) and then adding 1.5 mmol each of CoCl2, NiCl2, and CuCl2 in water (20 mL).The process was executed at a comfortable temperature of about 80 degrees Celsius.The solution was refluxed in a water bath with continual magnetic stirring and a nitrogen atmosphere for eight hours.The last step in this process was to dry the product after it had been filtered and washed many times with ethanol.
Preparation of Schiff-base ligand and its metal complexes.

Characterization
The compositions of the complexes were determined using elemental analysis (for C, H, and N content), molar conductance (10 -3 mol/L), thermal decomposition (TGA), magnetic measurements, infrared (IR) spectroscopy, and electronic spectra (10 -3 mol/L).Elemental analysis was performed at the University of Cairo's Central Laboratory using a Perkin-Elmer 2408 analyzer.Near infrared spectra of KBr pellets were obtained using a Shimadzu DR-8001 spectrometer and an infrared spectrometer. 1H-NMR spectra of DMSO-d6 were obtained using a Bruker DRX 400 MHz instrument with TMS as a reference.UV-Vis spectra were obtained using a Jenway UV-Vis spectrophotometer.Thermal decomposition (TGA) analysis was performed on the required compounds using a Shimadzu type 60 H analyzer.The molar conductance of a DMF solution (10 -3 mol/L) was measured using a JENWAY model 4320 electronic conductivity meter.The molar magnetic susceptibility of powdered materials was determined using a Bartington Susceptibility machine (model 4320).The stoichiometry of the compounds was determined using a continuous-variation spectrophotometric method.

DFT studies
Theoretical simulations have been used in the process of verifying the structure of newly synthesized substances.Gaussian 09w [14] was utilized to do computational research.Density functional theory (DFT) was conducted using B3LYP method [15,16] using 6-311G (d,p) [17,18] basis set for H, C, N, S and O-atoms and LANL2DZ basis set [19] for metal-atom (Co/Ni/Cu).

In vitro antibacterial and antifungal activity screening
Using the disc diffusion method, we investigated the antibacterial and antifungal characteristics of the free ligand and its metal complexes [20,21].Some of the bacteria and fungi were B. cereus (+ve), S. aureus (+ve), P. aeruginosa (-ve), and E. coli (-ve).In addition to that, A. flavus, T. rubrum and C. albicans fungi were used.DMSO (diamethylene sulfoxide) was utilized to dissolve the chemicals for solution preparation.
Assaying the free ligand and its complexes for in vitro antibacterial and antifungal activity, we employed the disc diffusion method.This experiment was performed on a petri dish.The study's major focus was on eliminating potentially harmful bacteria and fungi.Petri plates were prepared with nutritional agar, and the organisms of interest were seeded into them.Discs 5 mm in diameter and 100 ppm in concentration of the ligand and the metal complexes were distributed uniformly throughout the culture plates.The dishes were then baked for another 24 hours at 37 degrees Celsius.To ascertain whether or not the medicine is antibacterial, inhibition zone (IZ) diameter was.
Chloramphenicol, a popular conventional antibiotic, was shown to be antibacterial and antifungal when used at the same doses and in the same solvent.The activity indices were converted to percentages using the following method so that the antibacterial and antifungal efficacies of the compounds could be compared to a standard.To calculate the percentage of activity (%) between the test material and the reference material, % = (IZtest/IZRefrence) x 100 [22,23].Where; IZtest is inhibition zone of the test Material, and IZRefrence is inhibition zone of the Reference Material.

Molecular docking studies
The binding mechanism and ligand-DHPS (dihydropteroate synthase) interactions were theorized by molecular docking experiments.Essential for prokaryotes is the existence of this enzyme, which allows them to produce folic acid.Since practically all bacterial species need folic acid for nucleic acid synthesis [24,25], it makes for a suitable antibiotic target.Crystal structures of DHPS from E. coli and S. aureus were utilized as receptors in molecular docking simulations [26,27], after accessing the RCSB protein data repository.The corresponding PDB identifiers are 5JQ9 and 6CLV.

Structural characterization of the HL-ligand
A high yield of the HL ligand was achieved during synthesis (Table 1).Scheme 1 details the steps required to synthesize HL.
FTIR, 1 H-NMR, 13 C-NMR, and chemical investigations all corroborated the ligand's structure.In the IR spectra, the (-CH=N) band is located at 1670 cm -1 , while the OH group stretching vibration is located at 3322 cm -1 .The frequencies of 1581 cm -1 for ʋaas(COO -) and 1424 cm -1 for ʋs(COO -) were also seen.The 1 H-NMR (DMSO-d6) spectrum, Figure 1, revealed the presence of a carboxylic OH group at 12.84 ppm, aromatic proton signals in the range 8.62-78.20 ppm, and the methyl group at 2.44 ppm.Furthermore; the 13 C-NMR spectra (Figure 1) displays the C-OH and CH3 at 192.34 and 21.64 ppm, respectively.Furthermore, the DEPT 13 C-DEPT-135 confirm the proposed structure, Figure 1.

Structural characterization of the metal-ligand complexes
The complexes are almost completely soluble in DMF and DMSO at room temperature.Table 1 provides the analytical data and physicochemical properties for each molecule produced.Several techniques were used, including elemental analysis, Fourier transform infrared spectroscopy, molar conductivity, thermal analysis, magnetic susceptibility, and mass spectrometry, to provide a comprehensive account of the structure of HL ligand and its complexes.This outcome was the combined consequence of all of these approaches.Spectroscopic and analytical data was used to propose possible structures for the newly synthesized metal complexes (Scheme 1).Nonetheless, spectroscopic and analytical results corroborated the hypothesized frameworks.In addition, the data obtained from the elemental studies (C, H, and N) matched the expected structures.

Electronic spectra measurements
The following Table 1 displays the electronic absorption bands of the ligand and its metal complexes in DMF solutions.Figure 2 shows both the HL-ligand spectrum and the spectra of its metal complexes.
The bands seen at 315 and 370 nm for the unbound HL-ligand are the result of the π→π* and n→π* transitions.As a result of the fact that the electronic spectra of the Co(II) complex revealed two bands at 475 and 525 nm, an octahedral geometry was found to be the most likely explanation.The transitions 4 T1g→ 4 A2g(F) and 4 T1g→ 4 T2g (F), respectively, were able to be assigned to these bands [28], Table 1 Two distinct bands could be seen in the electronic spectra of the Ni(II) complex, one at 505 nm and the other at 640 nm.These bands were able to be assigned to the 3 A2g(F)→ 3 T2g(p) transition and 3 A2g(F)→ 3 T1g(F) transition, respectively, both of which suggested a tetrahedral geometry around the Ni(II) centre [29], Table 1.
The d-d transition and the 2 B1g → 2 A1gbands at 495 and 625 nm, respectively, in the electronic spectra of the Cu(II) complex may be explained by either a transition or a phonon resonant absorption.These transitions point to the existence of an octahedral geometry in the vicinity of the Cu(II) center, as shown in Table 1.

Complex stoichiometry
Figure 2 shows the results of a spectrophotometric Job's technique of continuous variation [30] experiment designed to calculate the metal-to-ligand molar ratio of the metal complexes.As expected from Job's law, maximum absorbance was seen at concentrations 0.65 times that of the ligand.Complexes of Co(II), Ni(II), and Cu(II) need a metal-to-ligand molar ratio of 1:2.(M:L).

FT-IR spectra
The distinctive vibration bands of the ligand and its metal complexes may be seen by studying the FT-IR spectra of the resulting compounds (in the range of 400-4000 cm -1 ).Complex formation may affect bond strength, and IR spectroscopy can be used to probe atomic vibration at a certain stretching frequency.The IR spectrum line follows the motion of vibrational bands (Table 1).The HL ligand spectrum showed a band around 3322 cm -1 , which is indicative of the stretching vibration of the -OH group.In the HL ligand, the absorption was prominent at 1670 cm -1 , where the (CH=N) was shown to be responsible.Additionally, the υ(-COO)asymetric frequency in the area of 1581 cm -1 and υ(-COO)symmetric frequency in the range of 1424 cm -1 are seen in the spectra of HL-ligand, Table 1.
For metal ion coordination in metal complexes, the nitrogen atom of the azomethine group shifts the (-CH=N) band to lower frequencies (1589-1615 cm -1 ) [31,32].Metallic complexes showed elongation bands at 1605-1614 cm -1 for υ(-COO)asymetric and at 1480-1490 cm -1 for υ(-COO)symetric.Therefore, the oxygen atom in the carboxyl group must play a function in metal ion coordination.Using the azomethine group's nitrogen atom and the deprotonated carboxyloxygen, the HL-ligand forms a stable 6-membered chelate ring with the M(II) ions [33,34].In the spectra of metal-ligand complexes, there are two additional, weak bands seen at 490-494 cm -1 and 577-580 cm -1 [35], Table 1.The IR spectra of all the complexes matched with the elemental analyses, showing that the bandwidth at 3475-3481 cm -1 is occupied by the (OH) stretching vibration of water molecules [36].

Magnetic moment measurements
The change in magnetization orientation caused by an external magnetic field is one way to approximate a material's magnetic susceptibility.Paramagnetic materials are those with unpaired electrons and positive susceptibilities, making them magnetically attractive.They also feature an extra uncoupled electron compared to typical atoms.Coordination chemists may learn about the three-dimensional structure of metal complexes by examining their magnetic susceptibility.
The d 8 metal ion in the tetrahedral structure of the paramagnetic Ni(II) complex has an effective magnetic moment of 3.07 B.M., according to calculations.
In terms of magnetic susceptibility, complexes of cobalt(II) and copper(II) are both paramagnetic (4.15 and 1.74 B.M., respectively).The existence of three unpaired electrons is consistent with the reported value for the magnetic moment of the cobalt complex, which falls within the range typically seen for high-spin octahedral Co(II) complexes.The effective magnetic moment of 1.74 B.M. makes sense when considering the octahedral d 9 -system implied by the Cu(II) complex.

Microanalyses and molar conductance measurements
In order to measure the molar conductance at room temperature, freshly prepared DMF solutions of the complexes were diluted to a concentration of 1.0 x 10 -3 M and tested.Results showing molar conductance values for the complexes in the range of 9.07-9.99S.cm 2 mol −1 provide further proof that the complexes do not possess an electrolytic feature.

Thermal studies
Differential thermal analysis and thermogravimetry (DTA/TG) were performed on metal complexes from ambient temperature to 800 o C. As was said in relation to the proposed formula for the compounds, mass loss occurs at each stage of the breakdown process.After the coordination water in the complexes degraded, the rest of the molecule followed suit.Figure 2 provide the DTA and TG properties of the relevant compounds, respectively.
Separate thermal breakdown processes occurred twice in the Co(II) complex, [C42H40CoN2O14S2].The initial breakdown takes place between 30 -420 o C, and it results in the loss of 2H2Ocoord + 2H2Ohyd + 2C7H7SO3 (calculated mass loss = 45.00%,experimental mass loss = 45.12%).The second phase, the breakdown of the remaining ligand fragments, is expected to account for a total mass loss of 46.69% at temperatures between 420 -800 o C (estimated mass loss = 46.85%),leaving behind just CoO as a metallic residue.
The breakdown of the Ni(II) complex [C42H36N2NiO12S2] consists of two stages.At temperatures between 30 -415 o C, the mass of 2H2Ocoord + 2C7H7SO3 decreases by 42.48% (calculated mass loss = 42.76%).Second, between 415 -800 o C, the remaining ligand fragments dissolved, resulting in a total mass loss of 48.98% (calculated mass loss = 48.76%)and a metallic residue of NiO.
It was also demonstrated that the Cu(II) complex (with the formula [C42H42CuN2O15S2]) breaks down in two stages.The first stage included the loss of 3H2Ocoord + 2H2Ohyd + 2C7H7SO3 (estimated mass loss = 43.93%)at temperatures between 30 and 430 o C. CuO was determined to be the metallic remnant after the second phase, which occurred between 430 -800 o C and resulted in an estimated total mass loss of 47.75% (mass loss = 47.64%).
The complexes' initial mass loss might have been caused by the hydration water and/or organic group.The presence of water molecules in the complexes has been confirmed by elemental analysis.

Mass spectra
The mass spectrum is widely acknowledged as an important tool for describing structural features.The mass spectra of the given metal complexes were used to examine and compare the stoichiometric compositions of the compounds.Figure 2

DFT studies
Molecular structures of the HL ligand and its metal complexes that have been optimized through the use of the DFT method are depicted in Figure 3. HOMO and LUMO, Figure 4, also called frontier molecular orbitals, refer to the highest occupied and lowest unoccupied orbitals, respectively, in a given molecule (FMO).The kinetic stability and chemical reactivity of a molecule may be linked to its orbital energy gap (ΔE) [37,38].The border orbitals of highly polarizable, kinetically unstable molecules are packed closely together.The energy gap (ΔE) between the ground (HOMO) state and the most excited (LUMO) state of a ligand is often used as a proxy for the ligand's electronic structure.

Co(II)-complex
Ni(II)-complex Cu(II)-complex Figure 3. 3D geometries of the free ligand and its metal complexes.
Nevertheless, its impact on quantum chemistry remains significant [39].Herein, we used density functional theory (DFT) with the B3LYP/6-311G(d, p) and LanL2dz basis sets to construct a three-dimensional model of the HOMO and LUMO states of the HL ligand and its metal complexes [40].Table 2 displays the HOMO-LUMO gap energy determined for the HL ligand and its metal complexes (4.11, 1.96, 2.57, and 1.44).The structural and conformational constraints of any sort of molecular system may be predicted by including ΔE, a basic stability property, into theoretical models.To roughly assess a molecule's reactivity, the energy gap may be employed (ΔE).The equations in the scientific literature [41] that may be used to determine a substance's chemical hardness (η), global softness (S), global electrophilicity (ω), and absolute electronegativity (χ).The resultant quantum chemical parameter is listed in Table 2.
The capacity to classify substances as hard or soft is crucial for grading them, characterizing molecular reactivity using the HSAB rule (hard-soft-acid-base rule).This means that weak acids are drawn to weak bases, and strong acids to strong bases, when it comes to forming chemical bonds.Natural sources of strong acids and bases have been discovered as well.Examples of soft biological molecules include proteins, DNA, and other cellular building blocks.That's because soft molecules are more likely to interact with biological molecules than hard ones.As a result, softness promotes biological activity whereas hardness suppresses it [42].Based on the available data, the reactivity ranking will be copper(II) complexes being the most stable, followed by cobalt(II), nickel(II), and the free ligand rank the complexes > cobalt(II) > nickel(II) >> the free ligand.The Table 2 clearly shows that when compared to the Co(II) complex, the Ni(II) complex, and the ligand, the Cu(II) complex is the superior option.Table 2. Calculated chemical parameters.
The sizes, shapes, and electrostatic potential values of molecules, as well as the distribution of their charges, are all shown in MEP diagrams.The picture makes it easy to see exactly where in the molecule nucleophilic and electrophilic processes take place.Red indicates an overabundance of electrons, whereas blue indicates an electrically weaker region of the molecule in MEP maps.MEP surface diagrams for the HL-ligand and its metal complexes were computed at the DFT/B3LYP/6-311G (d, p) and LanL2dz levels, Figure 4.
Studies of the MEP surfaces of the compounds that bore the names revealed that the oxygen and nitrogen atoms inside their structures had a negative charge density.The MEP had sway over this choice.The high charge density may have its origins in the high electronegativity of oxygen and nitrogen.According to MEP diagrams, this is where molecules are most likely to undergo a nucleophilic reaction.However, hydrogen atoms tend to occupy these electron-poor blue regions in isolated compounds.The surrounding oxygen and nitrogen atoms were far more electronegative than the atom itself, causing this effect.This happened because of this.Because electron-poor regions tend to cluster close to their targets, electrophilic reactions favor those regions where molecules come into contact.

In vitro antibacterial and antifungal activity screening
Studies of the HL ligand and its metal complexes against gram-positive (S. aureus (+ve) and B. cereus (+ve)) and gram-negative (P.aeruginosa (-ve) and E. coli (-ve)) bacteria and yeast (A.flavus, T. rubrum, C. albicans) were conducted using the disc diffusion method in DMSO solvent, Table 3. Inhibitory values against these bacteria and fungi as described in the literature are shown in Table 3.The antibiotic drug chloramphenicol was used in a positive control screening for fungi and bacteria.We used both the free ligand and the metal complexes of the ligand as a basis for our comparisons.Confirming their inhibitory capability is the finding that the generated complexes exhibit higher antibacterial/antifungal activity than the free ligand against all species (Table 3).In place of the common antibiotic chloramphenicol, these metal complexes function admirably.It was found that the complex of Cu(II) was very efficient against even the most resistant bacteria and molds.In addition, the Co(II) complex had the second largest inhibition gradients.The activity of the complex is higher than that of the free ligand.Perhaps the concept of chelation might shed some light on this observation.It was found that the polarity of metal complexes was greatly diminished as a result of the interaction between ligand ions and metal ions.In order to share electrons with ligands, metal ions must first donate some of their own positive charge, and their orbitals may overlap with those of the ligands [43].To do this, the lipophilicity of metal complexes is increased by promoting the delocalization of pi electrons inside the chelate ring.Metal complexes may penetrate deeper through the lipid membrane and reduce bacterial populations due to their increased lipophilicity.It's possible that these metal complexes impede cellular respiration, preventing the production of "proteins" that would otherwise limit the organism's capacity for future development.
To further assess the in vitro antimicrobial and antifungal activity of each chemical, we determined the minimum inhibitory concentration (MIC) of each compound against our test bacteria and fungi.The MIC of an antimicrobial or antifungal agent is the lowest concentration at which the growth of microorganisms is visibly inhibited.In Table 3, we present the compounds investigated, the bacteria and fungi used as test subjects, and the MICs for each component.Based on the data in Table 3, the activity index (%) of the aforementioned compounds ranged from 40 to 50% for free ligand and increased to 70 to 90% for metal complexes.

Molecular docking studies
Table 4. Molecular docking findings of the prepared materials.
The synthesized compounds were tested for efficacy against bacteria by docking with the target protein [44,45].The purpose of the experiment was to see whether the inhibitors' binding attractiveness correlated with their in vitro antibacterial activity.[46].This means that we may be able to focus on the molecules that are most suited to form a bind with our target.Molecules participating in an interaction are represented as pieces of a jigsaw puzzle to facilitate a holistic, three-dimensional understanding of events like the active site binding of a target receptor and a substrate.
These synthetic materials serve as an analogue substrate for the protein receptor seen in two crystal structures of DHPS bound to pterin-sulfonamide conjugates [47] (PDB ID 5JQ9 and 6CLV, respectively, from E. coli and S. aureus).The docking findings for the compounds under study are summarized in Table 4, and the proper orientation of the molecules inside the binding pocket is shown in Figure 5.According to docking experiments, the estimated binding energies of the compounds with E. coli DHPS (5JQ9) were somewhat higher than those with S. aureus DHPS (6CLV).Evidence like this point refers to DHPS inhibition as the probable culprit in E. coli.The binding process is spontaneous, and the compounds may be employed as a therapy, however, all of the compounds examined exhibited negative projected binding energies (Table 4).
As demonstrated in Figure 5 and Table 4, docking to the active pocket of the target receptor necessitates a high number of hydrogen bonds, hydrophobic interactions, and negative docking scores (S).Activation of the receptor is shown as a result of the docked substrates interacting with the active site.Docking energies (also known as binding energies) are expected to be larger for metal complexes than for their parent ligands, according to computational modeling.Metal complexes are more responsive than their parent ligands, as has been shown by several biological studies.Due to these modifications to the parent ligands, potent new antibacterial agents have been identified.After trying other receptors, Cu(II) complex emerged as the clear victor.

CONCLUSION
New 2-[(4-[(4-methylphenyl)sulfonothioyl)methylene]aminobenzoic acid ligand-Co(II), Ni(II), and Cu(II) complexes are synthesized and characterized.Quantum chemical calculations, based on DFT/B3LYP with 6-311G (d,p) and LANL2DZ, revealed a tetrahedral geometry for the Co(II) complex and octahedral geometry for the Ni(II) and Cu(II) complexes.The HOMO-LUMO gap tests indicated that the Cu(II) complex was the most reactive and least stable of the newly synthesized compounds.The newly synthesized compounds were also examined in vitro for their antibacterial and antifungal properties.Molecular docking was used to learn more about how the synthesized chemicals bound to the 5JQ9 and 6CLV proteins.Molecular docking simulations' results corroborated those of in vitro antibacterial efficacy tests.Binding energy was the lowest for the Cu(II) complex (-8.46 and -8.38 kcal/mol), out of all the compounds tested.In light of these findings, the Cu(II) complex merits more investigation as a potential antibacterial and antifungal agent.

Table 3 .
The anti-bacterial and ati-fungal activity of the titled compounds in terms of inhibition zone (IZ, mm), and activity index (%).