Preparation, Characterization and In Vitro Biological Activities of New Diphenylsulphone Derived Schiff Base Ligands and Their Co(II) Complexes

The present work describes the chemical preparation of Schiff bases derived from 4,4′-diaminodiphenyl sulfone (L1–L5) and their Co(II) metal complexes. The evaluation of antimicrobial and anticancer activities against MCF-7 cell line and human lung cancer cell line A-549 was performed. The aforementioned synthesized compounds are characterized by spectroscopic techniques and elemental analysis confirms successful synthesis. The results from the above analytical techniques revealed that the complexes are in an octahedral geometry. The antimicrobial activity of the synthesized Schiff base ligands and their metal complexes under study was carried out by using the agar well diffusion method. The ligand and complex interactions for biological targets were predicted using molecular docking and high binding affinities. Further, the anticancer properties of the synthesized compounds are performed against the MCF-7 cell line and human lung cancer cell line A-549 using adriamycin as the standard drug.


Introduction
Coordination compounds play an important role in our daily lives, with applications ranging from biology to industry. Because of their high selectivity and target specificity in 2-Hydroxy-1-naphathaldehyde, 5-bromo-2-hydroxybenzaldehyde, 4,4 -diaminodiphe nylsulphone, 2-hydroxy-3-methoxy benzaldehyde, 5-chloro-2-hydroxybenzaldehyde, and 2hydroxy-benzaldehyde were of AR grade from Sigma Aldrich. CoCl 2 .6H 2 O was purchased from Merck Chemical Ltd. (Mumbai, India). All chemicals/reagents were used as received. Euro EA CHNS elemental analyzer is used to determine the elemental composition of synthesized compounds. The functional groups in the molecules were determined by FT-IR spectroscopy in the range of 400-4000 cm −1 in KBr disc using Perkin-Elmer 1200 FT-IR spectrometer. The thermal response of the investigated complexes were taken using the Shimadzu DT-50 thermal analyzer with a heating rate of 10 • C/min in an atmosphere saturated with nitrogen. Electronic absorption of the compounds was investigated using a UV-Visible spectrophotometer (Shimadzu, UV-1800). 1 H-NMR of the Schiff base ligands was recorded using Bruker Avance (400 MHz) 1 H-NMR spectrometer.

Synthesis of Co(II) Complexes (C1-C5)
An ethanolic solution of 0.1 M (4.56 g in 25 mL of ethanol) of the Schiff base ligand (L1) was added to a Co(II) chloride solution of 0.102 M (2.59 g in 10 mL of ethanol). The above reaction mixture was refluxed under continuous stirring for about 6 h. The progress of reaction was checked by TLC and spots were visualized under UV light. The precipitate obtained was then filtered, washed thoroughly with ethanol, and dried in desiccator over anhydrous CaCl2. Co(II) complexes with other Schiff bases (L2 (5.56 g), L3 (6.14 g), L4 (5.16 g) and L5 (5.24 g)) were synthesized using the above procedure. The synthesized complexes are characterized using physico-chemical techniques. The tentative structures of the prepared complexes are depicted in Figure 1.  (4.39), N 6.21 (6.14).
1,1 -(4,4 -Sulfonylbis(4,1-phenylene)bis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)dinapthalen-2-ol (L 2 ) An ethanolic solution of 0.1 M (4.56 g in 25 mL of ethanol) of the Schiff base ligand (L 1 ) was added to a Co(II) chloride solution of 0.102 M (2.59 g in 10 mL of ethanol). The above reaction mixture was refluxed under continuous stirring for about 6 h. The progress of reaction was checked by TLC and spots were visualized under UV light. The precipitate obtained was then filtered, washed thoroughly with ethanol, and dried in desiccator over anhydrous CaCl 2 . Co(II) complexes with other Schiff bases (L 2 (5.56 g), L 3 (6.14 g), L 4 (5.16 g) and L 5 (5.24 g)) were synthesized using the above procedure. The synthesized complexes are characterized using physico-chemical techniques. The tentative structures of the prepared complexes are depicted in Figure 1. An ethanolic solution of 0.1 M (4.56 g in 25 mL of ethanol) of the Schiff base ligand (L1) was added to a Co(II) chloride solution of 0.102 M (2.59 g in 10 mL of ethanol). The above reaction mixture was refluxed under continuous stirring for about 6 h. The progress of reaction was checked by TLC and spots were visualized under UV light. The precipitate obtained was then filtered, washed thoroughly with ethanol, and dried in desiccator over anhydrous CaCl2. Co(II) complexes with other Schiff bases (L2 (5.56 g), L3 (6.14 g), L4 (5.16 g) and L5 (5.24 g)) were synthesized using the above procedure. The synthesized complexes are characterized using physico-chemical techniques. The tentative structures of the prepared complexes are depicted in Figure 1

Antimicrobial Studies
The antimicrobial activities of the synthesized Schiff base ligands and their cobalt metal complexes under study were carried out by using the agar well diffusion method. The Gram-positive pathogens Bacillus subtilis (ATCC 6633) and Staphylococcus aureus (ATCC6538) and Gram-negative pathogens Klebsiella pneumonia (ATCC 13883), Pseudomonas aeruginosa (ATCC9027), and Escherichia coli (ATCC 8739) were used in the biological potency evaluation. The antifungal activity of the compounds was tested against Aspergillus niger (ATCC 16404) and Candida albicans (ATCC 10231). Standard drugs used for the study were Ciprofloxacin and Nystatin for bacterial and fungal pathogens respectively [24].
The Muller Hinton agar plate surface was inoculated by the spread plate method with microbial inoculum over the entire agar surface [25]. Then, a hole with a diameter of 6-8 mm is punched aseptically with a sterile cork borer, and the volume of the desired antimicrobial compound dissolved in DMSO with desired concentration was introduced into the well. Then, agar plates were incubated under suitable conditions depending on bacterial/fungal species requirements. The antimicrobial agents diffuse in the agar medium and inhibit the growth of the microbial strain tested. The inhibition zones exhibit around the antimicrobial compound and are measured as the diameter of the zone of inhibition in millimeters (mm) [26,27].

Molecular Docking Studies
Molecular docking is one of the most essential tools used in drug discovery due to its ability to predict, the conformation of small-molecule ligands within the appropriate target binding site with a substantial degree of accuracy. To find out the possible mode of action of the synthesized Schiff bases (L1-L5) and Co(II) complexes (C1-C5), molecular docking calculations of cysteine protease human cathepsin ki.-e.CDK7 (Cyclin dependent kinase-7) PDB ID: 1au2 were carried out using AutoDock 4.2. Lamarckian genetic algorithm. Auto Grid was used to define the active site and the grid size was set to 46 × 54 × 56 points which covers all the active site residues [20]. The grid spacing of 0.375 Å was centered on the selected flexible residues, which are the active sites of the CDK-7. The step size of 2.0 Å for translation and 10° for rotation were selected. The maximum number of energy evaluations was set to 2,500,000. A total of 10 runs were performed, and for each run, a maximum number of 27,000 genetic algorithms (GA) generations were performed on a single population of 150 individuals. The best-docked conformation among 10 conformations was obtained with the lowest binding energy values [28]. Interaction between cysteine protease human cathepsin ki.-e.CDK7 (Cyclin-dependent kinase-7) PDB ID: 1au2 with various ligands under study was visualized using molecular visualization tools such as Chimera (Pettersen et al., 2004). In addition, by using LigPlot+ tool hydrogen bonding and hydrophobic interactions were predicted for the complex [29].

Antimicrobial Studies
The antimicrobial activities of the synthesized Schiff base ligands and their cobalt metal complexes under study were carried out by using the agar well diffusion method. The Gram-positive pathogens Bacillus subtilis (ATCC 6633) and Staphylococcus aureus (ATCC6538) and Gram-negative pathogens Klebsiella pneumonia (ATCC 13883), Pseudomonas aeruginosa (ATCC9027), and Escherichia coli (ATCC 8739) were used in the biological potency evaluation. The antifungal activity of the compounds was tested against Aspergillus niger (ATCC 16404) and Candida albicans (ATCC 10231). Standard drugs used for the study were Ciprofloxacin and Nystatin for bacterial and fungal pathogens respectively [24].
The Muller Hinton agar plate surface was inoculated by the spread plate method with microbial inoculum over the entire agar surface [25]. Then, a hole with a diameter of 6-8 mm is punched aseptically with a sterile cork borer, and the volume of the desired antimicrobial compound dissolved in DMSO with desired concentration was introduced into the well. Then, agar plates were incubated under suitable conditions depending on bacterial/fungal species requirements. The antimicrobial agents diffuse in the agar medium and inhibit the growth of the microbial strain tested. The inhibition zones exhibit around the antimicrobial compound and are measured as the diameter of the zone of inhibition in millimeters (mm) [26,27].

Molecular Docking Studies
Molecular docking is one of the most essential tools used in drug discovery due to its ability to predict, the conformation of small-molecule ligands within the appropriate target binding site with a substantial degree of accuracy. To find out the possible mode of action of the synthesized Schiff bases (L 1 -L 5 ) and Co(II) complexes (C 1 -C 5 ), molecular docking calculations of cysteine protease human cathepsin ki.-e.CDK7 (Cyclin dependent kinase-7) PDB ID: 1au2 were carried out using AutoDock 4.2. Lamarckian genetic algorithm. Auto Grid was used to define the active site and the grid size was set to 46 × 54 × 56 points which covers all the active site residues [20]. The grid spacing of 0.375 Å was centered on the selected flexible residues, which are the active sites of the CDK-7. The step size of 2.0 Å for translation and 10 • for rotation were selected. The maximum number of energy evaluations was set to 2,500,000. A total of 10 runs were performed, and for each run, a maximum number of 27,000 genetic algorithms (GA) generations were performed on a single population of 150 individuals. The best-docked conformation among 10 conformations was obtained with the lowest binding energy values [28]. Interaction between cysteine protease human cathepsin ki.-e.CDK7 (Cyclin-dependent kinase-7) PDB ID: 1au2 with various ligands under study was visualized using molecular visualization tools such as Chimera (Pettersen et al., 2004). In addition, by using LigPlot+ tool hydrogen bonding and hydrophobic interactions were predicted for the complex [29].

Anticancer Activities by SRB Assay
The cell lines were cultured and grown in an appropriate medium containing 10% fetal bovine serum (FBS) and 2 mM L-glutamine. In the presentation investigation of the screening experiment, 5000 cells/well were inoculated into 96 well microtiter plates in 100 µL. After cell inoculation, the microtiter plates were incubated at 37 • C, 5% CO 2 , 95% air, and 100% relative humidity for 24 h before the addition of experimental drugs. Experimental drugs were solubilized in a suitable solvent (100 mg/mL) and diluted to 1 mg/mL using double distilled water and frozen before use. During the time of drug addition, an aliquot of frozen concentrate (1 mg/mL) was thawed and diluted to 100, 200, 400, and 800 µg/mL with a complete medium containing the test sample. Aliquots of 10 µL of these different drug dilutions were added to the appropriate microtiter wells already containing 90 µL of the medium, resulting in the required final drug concentrations, i.e., 10, 20, 40, 80 µg/mL. After the addition of the test compound, the plates were incubated at standard conditions for 48 h, and the assay was terminated by the addition of cold TCA. Cells were fixed in situ by the gentle addition of 50 µL of cold 30% (w/v) TCA (final concentration, 10% TCA) and incubated for 60 min at 4 • C. The supernatant was discarded. The plates were washed five times with tap water and air dried. Sulforhodamine B (SRB) solution (50 µL) at 0.4% (w/v) in 1% acetic acid was added to each of the wells, and plates were incubated for 20 min at room temperature. After staining, the unbound dye was recovered, and the residual dye was removed by washing five times with 1% acetic acid. The plates were air dried. The bound stain was subsequently eluted with 10 mM trizma base, and the absorbance was read on a plate reader at a wavelength of 540 nm with 690 nm reference wavelength. Percent growth was calculated on a plate-by-plate basis for test wells relative to control wells. Percent growth was expressed as the ratio of the average absorbance of the test well to the average absorbance of the control wells × 100. This has been shown in the form of the equation below: Percentage growth = Average absorbance of the cell test Average absorbance of the control well × 100

Results and Discussion
The elemental analysis data of synthesized Schiff bases and their complexes were found to be consistent with the expected result. The analytical and physical data of all the synthesized Schiff bases (L 1 to L 5 ) and their Co(II) complexes (C 1 to C 5 ) are given in Table 1. Theoretical and experimentally observed values of elemental analysis of compounds are in good agreement with the molecular formula.

Electronic Spectral Studies amd Magnetic Moment Studies
Electronic absorption spectra of Co(II) Schiff base complexes were recorded at room temperature in DMSO solution and the results are shown in Table 2 and Figure 2 respectively. The electronic spectra of C 1 , C 2 , and C 3 showed three absorption bands. The first two were in the range 248-290 nm, which are due to π → π* transition of the aromatic ring and the third one was in the range 345-399 nm, which was assigned to n→ π* transition of azomethine group (C=N). The UV-Visible spectrum of C 4 and C 5 showed two absorption bands ( Figure S24). First, one was at 255 and 245 nm, respectively, which was attributed to π → π* transition of the aromatic ring, and the second one at around 310 and 342 nm, which was assigned to n → π* transition of azomethine group (C=N) of Schiff base. The magnetic moment values of Co(II) complexes were observed in the range of 4.37-5.08 B.M. (Table 2) because of high-spin magnetic moments, corresponding to the unpaired electrons. It appears from the magnetic moment data of Co(II) complexes that they are paramagnetic in nature, and hence are of six-coordinate complexes [30]. 248 π-π* C 1 289 π-π* 4.47 345 n-π* 250 π-π* C 2 283 π-π* 4.37 399 n-π* 277 π-π* C 3 290 π-π* 5.08 375 n-π* C 4 255 310 π-π* n-π* 4.86  248 π-π* C1 289 π-π* 4.47

FT-IR Spectral Studies
The formation of Co(II) complexes were confirmed as important shifts in azomethine group (C=N) and phenolic -OH bands by comparing the infrared spectroscopic data of metal complexes and their respective ligands. The IR spectral data of metal complexes are presented in Table 3. The stretching vibrational band observed around 1627-1614 cm −1 is a characteristic of the azomethine (C=N) nitrogen atom present in the free Schiff base ligand. Due to metal coordination, the expected typical imine band in the range 1620-1596 cm −1 was observed in the Co(II) complexes. In addition, the -OH stretching and bending vibrational frequencies of the substituted salicylaldehyde appeared in the region 3459-3427 cm −1 . The disappearance of these two peaks in the spectra of all the Co(II) complexes indicates that the coordination takes place via the enolic -OH group. Furthermore, the presence of a broad band at around 3435-3367 cm −1 in the Co(II) complexes suggests the presence of coordinate water molecules to the central metal ion [31][32][33]. Additional evidence for the coordination of the azomethine nitrogen is the presence of ν(M-N) bands in the frequency range of 562-544cm −1 and ν(M-O) bands in the frequency range of 517-505 cm −1 [34][35][36]. It is noteworthy that the unchanged band position of sulphonyl groups suggests that sulphone is not taking part in the coordination (Figures S1-S9) [37,38].

1 H-NMR Spectral Studies
The 1 H-NMR spectra of all the synthesized Schiff base ligands were recorded in DMSO solvent and expressed in ppm. The 1 H-NMR spectra of the synthesized compounds exhibit signals due to aromatic protons as a multiplet at 6.49-8.23 ppm. In the 1 H-NMR spectrum of the Schiff base ligands, a singlet observed downfield around 12.56-12.92 ppm, integrating for one proton, is assigned to -OH [39]. Similarly, the azomethine proton (attached to the carbon close to the nitrogen atom) appears around 8.68-8.80 ppm as a singlet signal [40]. Representative proton 1 H-NMR spectra of L 1 , L 2 , L 3 , and L 5 are shown in Figures S10-S13. The proton and carbon NMR of complexes are depicted in Figures S25-S30.

Thermogravimetric Analysis
The loss of water molecules was observed below 150 • C, which is due to the presence of lattice water molecules [41,42]. This further tells us that the coordinated water molecules occupy some position in the coordination sphere of the central metal ion. These water molecules are more strongly bonded to the metal ions thus eliminated at the higher temperatures. A further rise in the temperature, leading to the loss of mass in a slow gradual manner, was observed, which may be caused due to decomposition of metal complexes by the fragmentation and thermal degradation of organic parts and at the end, the metal oxide was formed as residue [43]. The thermal data are provided in Table 4 and representative thermal graphs of Co(II) complex (C 1 ) is shown in Figure 3.

1 H-NMR Spectral Studies
The 1 H-NMR spectra of all the synthesized Schiff base ligands were recorded in DMSO solvent and expressed in ppm. The 1 H-NMR spectra of the synthesized compounds exhibit signals due to aromatic protons as a multiplet at 6.49-8.23 ppm. In the 1 H-NMR spectrum of the Schiff base ligands, a singlet observed downfield around 12.56-12.92 ppm, integrating for one proton, is assigned to -OH [39]. Similarly, the azomethine proton (attached to the carbon close to the nitrogen atom) appears around 8.68-8.80 ppm as a singlet signal [40]. Representative proton 1 H-NMR spectra of L1, L2, L3, and L5 are shown in Figures S10-S13. The proton and carbon NMR of complexes are depicted in Figures S25-S30.

Thermogravimetric Analysis
The loss of water molecules was observed below 150 °C, which is due to the presence of lattice water molecules [41,42]. This further tells us that the coordinated water molecules occupy some position in the coordination sphere of the central metal ion. These water molecules are more strongly bonded to the metal ions thus eliminated at the higher temperatures. A further rise in the temperature, leading to the loss of mass in a slow gradual manner, was observed, which may be caused due to decomposition of metal complexes by the fragmentation and thermal degradation of organic parts and at the end, the metal oxide was formed as residue [43]. The thermal data are provided in Table 4 and representative thermal graphs of Co(II) complex (C1) is shown in Figure 3.

Mass Spectral Studies
The mass spectra of all the Schiff base ligands exhibit parent ion peaks, due to their respective molecular ion (M + ), corresponding to the molecular weight and confirming their molecular composition. The proposed molecular formula of these compounds was confirmed by comparing their molecular formula weights with the m/z values. The mass spectra of the Schiff base ligands are depicted in Figures S14-S18. In addition, the fragmentation pattern of L 1 is depicted in Figure S23.

Antibacterial and Antifungal Activities
The antibacterial activity of synthesized Co(II) complexes was evaluated using different pathogens, including the Gram-positive Bacillus Subtilis and Staphylococcus aureus, Gram-negative Klebsiella Species, E. coli, and Pseudomonas aeruginosac, and fungal pathogens Aspergillus niger and Candida albicans. Antibacterial activity against DMSO and standard drugs Ciprofloxacin and Nystatin were also carried out. Bacterial and fungal pathogens showed the different zone of inhibitions against cobalt complexes (Figures S17-S20).
Considering the case of Bacillus Subtilis, for synthetic complex compounds (C 1 -C 5 ) and DMSO, there was no zone of inhibition observed. The zone of inhibition for the standard drug Ciprofloxacin was 32 mm. However, in the case of Staphylococcus aureus, for synthetic compounds, C 1 , C 3 , C 4 , and C 5 , the zone of inhibition was 16,16,27, and 20, respectively. There was no zone of inhibition in the case of C 2 and DMSO. The zone of inhibition for the standard drug Ciprofloxacin is 30 mm. Similarly, for Klebsiella pneumonia, for synthetic compounds, C 3 and C 4 , the zone of inhibition was 8 and 16 mm, respectively. There was no zone of inhibition in the case of C 1 , C 2 , C 5 , and DMSO. The zone of inhibition for the standard drug Ciprofloxacin is 28 mm. Similarly, with E. coli, for synthetic compounds, C 2 , C 4 , and C 5 , the zone of inhibition was 15, 36, and 30 mm, respectively. There was no zone of inhibition in the case of C 1 , C 3 , and DMSO. The zone of inhibition for the standard drug Ciprofloxacin was 30 mm. In the case of Pseudomonas aeroginosa, for synthetic compounds, C 4 , and C 5 the zone of inhibition was 16 and 12 mm, respectively. There was no zone of inhibition in the case of C 1 , C 2 , and DMSO. The zone of inhibition for the standard drug Ciprofloxacin was 28 mm (Figures S19-S22).
It is interesting to note that all the Schiff bases and their Co(II) complexes showed antibacterial activity against Gram-positive and Gram-negative bacteria (Tables 5 and 6). This indicates the broad-spectrum ability of these compounds against the different pathogens. Ciprofloxacin as a standard drug and DMSO as a control was used for all bacterial species.
The Schiff base L 4 showed the highest antibacterial activity when compared with the other ligands (Figure 4). These compounds are not only active against bacteria, but also exhibit antifungal activity (Figures S19 and S20). Further, L 4 exhibited strong antifungal activity against Aspergillus niger (ATCC 16404). It is noteworthy that antifungal activities were higher than the standard antifungal drug Nystatin.
On the other hand, it was observed from these results that the Co(II) complexes were less effective against both Gram-positive and Gram-negative. Moreover, only a few complexes show antifungal activity. There was no zone of inhibition shown by any cobalt complex against Bacillus subtilis. Out of five cobalt metal complexes, only C 4 showed antibacterial activity against both Gram-positive, Gram-negative, and fungal pathogens ( Figure 5). The Schiff base L4 showed the highest antibacterial activity when compared with other ligands (Figure 4). These compounds are not only active against bacteria, but exhibit antifungal activity (Figures S19 and S20). Further, L4 exhibited strong antifun activity against Aspergillus niger (ATCC 16404). It is noteworthy that antifungal activi were higher than the standard antifungal drug Nystatin. On the other hand, it was observed from these results that the Co(II) complexes w less effective against both Gram-positive and Gram-negative. Moreover, only a few c plexes show antifungal activity. There was no zone of inhibition shown by any co complex against Bacillus subtilis. Out of five cobalt metal complexes, only C4 showed a bacterial activity against both Gram-positive, Gram-negative, and fungal pathogens (
Compound L2 has the highest binding affinity followed by L1, L3, and L4, whereas compound L5 has the lowest affinity among all the docked ligands, which is 285.63 K. Cal/Mol. Further, the L2 molecule showed interaction with amino acids, e.g., ARG188, TYR190, and PHE156, as well as hydrophobic and other interactions with LUE158, LEU134, ARG136, ASP137, THR175, ARG176, ARG179, and PHE162. The highest binding affinity of L2 than the other Schiff bases is mainly due to the involvement of ARG188 in the hydrogen bonding interaction and TYR190, LEU134, ARG136, ASP137, LEU158, THR175, ARG176, ARG179, and PHE162 in hydrophobic and other interactions with amino acid.
Compound L 2 has the highest binding affinity followed by L 1 , L 3, and L 4 , whereas compound L 5 has the lowest affinity among all the docked ligands, which is 285.63 K. Cal/Mol. Further, the L 2 molecule showed interaction with amino acids, e.g., ARG188, TYR190, and PHE156, as well as hydrophobic and other interactions with LUE158, LEU134, ARG136, ASP137, THR175, ARG176, ARG179, and PHE162. The highest binding affinity of L 2 than the other Schiff bases is mainly due to the involvement of ARG188 in the hydrogen bonding interaction and TYR190, LEU134, ARG136, ASP137, LEU158, THR175, ARG176, ARG179, and PHE162 in hydrophobic and other interactions with amino acid.

CDK-7-C1
3.8. Effect of Ligands (L 1 -L 5 ) and Their Complexes (C 1 -C 5 ) on Antiproliferative Activity All the synthesized compounds were screened for their anticancer activity against human breast cancer cell line MCF-7 and human lung cancer cell line A-549. The growth curves of human breast cancer cell line MCF-7 and human lung cancer cell line A-549 of Schiff bases (L 1 -L 5 ) and their Co(II) complexes (C 1 -C 5 ) are represented in Figure 7. The anticancer activity was measured in vitro for the newly synthesized compounds against breast cancer cell line MCF-7and human lung cancer cell line A-549 using the Sulforhodamine B stain (SRB) assay method [43]. The average values for % control growth for the cell line MCF-7 and human lung cancer cell line A-549 are listed in Tables 9 and 10. Sulforhodamine B stain (SRB) assay method [43]. The average values for % control growth for the cell line MCF-7 and human lung cancer cell line A-549 are listed in Tables 9 and 10. The anticancer activity results based on average values for % control growth revealed that ligands and their Co(II*) complexes are active at 20 and 40 µg/mL concentration. It is interesting to note that the complexes are more potent as compared to their respective ligand, except in the case of L3 and its complex. Moreover, it is noteworthy that the Schiff base ligands and Co(II) complexes are more active as compared to the standard drug adriamycin at all concentrations.

Conclusions
To sum up, the synthesized Schiff bases act as a tetradentate ligand and coordinated with the Co(II) ion through imine nitrogen and phenolic oxygen atoms. The binding of ligand to a metal ion was confirmed by elemental analysis, spectral studies (UV-Visible and FT-IR), and TGA measurements. The Co(II) complexes were found to exhibit octahedral geometry. All the Schiff bases (L1-L5) and their Co(II) complexes demonstrated moderate to good antimicrobial activity against the tested microbial species. The anticancer studies of the ligands and their Co(II)complexes showed significant activities against MCF-7 and human lung cancer cell line A-549 cancer cells. Particularly, the ligand L3 showed potential  The anticancer activity results based on average values for % control growth revealed that ligands and their Co(II) complexes are active at 20 and 40 µg/mL concentration. It is interesting to note that the complexes are more potent as compared to their respective ligand, except in the case of L 3 and its complex. Moreover, it is noteworthy that the Schiff base ligands and Co(II) complexes are more active as compared to the standard drug adriamycin at all concentrations.

Conclusions
To sum up, the synthesized Schiff bases act as a tetradentate ligand and coordinated with the Co(II) ion through imine nitrogen and phenolic oxygen atoms. The binding of ligand to a metal ion was confirmed by elemental analysis, spectral studies (UV-Visible and FT-IR), and TGA measurements. The Co(II) complexes were found to exhibit octahedral geometry. All the Schiff bases (L 1 -L 5 ) and their Co(II) complexes demonstrated moderate to good antimicrobial activity against the tested microbial species. The anticancer studies of the ligands and their Co(II)complexes showed significant activities against MCF-7 and human lung cancer cell line A-549 cancer cells. Particularly, the ligand L 3 showed potential activity compared with the other tested compounds, which in turn was compared with the standard drug, adriamycin. Further, the molecular docking results revealed that ARG188, TYR190, and ARG136 residues of cysteine protease human cathepsin are involved in hydrogen bonding interactions.