Catalytic, Theoretical, and Biological Investigations of Ternary Metal (II) Complexes Derived from L-Valine-Based Schiff Bases and Heterocyclic Bases

A new series of ternary metal complexes, including Co(II), Ni(II), Cu(II), and Zn(II), were synthesized and characterized by elemental analysis and diverse spectroscopic methods. The complexes were synthesized from respective metal salts with Schiff’s-base-containing amino acids, salicylaldehyde derivatives, and heterocyclic bases. The amino acids containing Schiff bases showed promising pharmacological properties upon complexation. Based on satisfactory elemental analyses and various spectroscopic techniques, these complexes revealed a distorted, square pyramidal geometry around metal ions. The molecular structures of the complexes were optimized by DFT calculations. Quantum calculations were performed with the density functional method for which the LACVP++ basis set was used to find the optimized molecular structure of the complexes. The metal complexes were subjected to an electrochemical investigation to determine the redox behavior and oxidation state of the metal ions. Furthermore, all complexes were utilized for catalytic assets of a multi-component Mannich reaction for the preparation of -amino carbonyl derivatives. The synthesized complexes were tested to determine their antibacterial activity against E. coli, K. pneumoniae, and S. aureus bacteria. To evaluate the cytotoxic effects of the Cu(II) complexes, lung cancer (A549), cervical cancer (HeLa), and breast cancer (MCF-7) cells compared to normal cells, cell lines such as human dermal fibroblasts (HDF) were used. Further, the docking study parameters were supported, for which it was observed that the metal complexes could be effective in anticancer applications.


Introduction
The discovery of cisplatin (cis-dichlorodiammine platinum(II)) and its subsequent use as a drug for the treatment of numerous human tumors sparked the development

In Vitro MTT Assay
According to the previous literature [17,18], the cytotoxic activity of compounds [Cu(L)(phen)] 1c and [Cu(L)(bpy)] 1g was assessed with respect to three tumor cell lines, including the A549, HeLa, and MCF-7 lines, as well as NHDF normal cells. The medium was switched to DMEM with 1% FBS after one day of growth in Dulbecco's Modified Eagle Medium with 10% FBS. After one day, 2,5,10,25,50, and 100 µL doses of the produced compounds diluted in DMSO were applied to the cells, which were then incubated. Each well was then filled with 10 µL of MTT (5 mg/mL) and maintained in this state for 3 h. Farmazone crystals were produced and dispersed in 100 µL of DMSO; then, the absorbance at 570 nm was calculated using an ELISA spectrometer. The proportion of viable cells was determined using the following formula [17,19,20]: Cell viability (%) = (A 570 nm of treated samples/A 570 nm of control samples) × 100

Molecular Modeling
Complexes were modeled molecularly by Maestro software coupled with the Schrodinger equation. The complex 1(a-h) 3D structures were first sketched in the maestro builder panel before being optimized in the Ligprep system. The complexes' tailored structures were suitable for docking with receptors. The receptor thymidylate synthase (PDB ID: 1HZW) was chosen from the protein data bank (http://www.rcsb.org, accessed on 22 October 2022). Utilizing the force field OPLS-2005, the receptor was also extensively tuned in the protein preparation wizard. The grid parameters were generated by supplying coordinates of −23.213, 43.011, and 21.67 Å and maintaining a diameter of 20 × 20 × 20 Å. Finally, docking programs were run in the maestro workspace for optimized complexes with receptor grids and binding interactions (3D and 2D).

Computational Analysis
Jaguar software, which is included in the Schrödinger suite 2017, was used to run the computational program employed in this study. Density functional theory (DFT) and the B3LYP/LACVP++ basis set were used to examine the optimal geometries and perform molecular orbital analyses (HOMO-LUMO) of the complexes.

Computational Analysis
Jaguar software, which is included in the Schrödinger suite 2017, was used to run the computational program employed in this study. Density functional theory (DFT) and the B3LYP/LACVP++ basis set were used to examine the optimal geometries and perform molecular orbital analyses (HOMO-LUMO) of the complexes.

Results and Discussion
The metal(II) complexes 1(a-h) were synthesized by a stoichiometric (1:1:1) template reaction of a methanolic solution containing ligand potassium (E)-2-((5-bromo-2-hydroxybenzylidene) amino)-3-methylbutanoate (HL), heterocyclic bases (1,10-phenanthroline or 2,2′-bipyridyl), and hydrated metal(II) salts (Scheme 1). Table 1 depicts the physical and elemental analyses of the metal(II) complexes. The complexes presented high yields and stability at ambient temperatures. The complexes were highly soluble in many solvents, such as methanol, acetonitrile, DMSO, and DMF, and were partially soluble in water.  As shown in Table 2, the FT-IR spectra of the mixed ligand complexes 1(a-h) were measured in the 450-4000 cm −1 wavelength range ( Figure S1 from Supplementary Materials). A broad peak was observed in the ligand 3300-3500 cm −1 region due to the -OH stretching vibration of the ligand (HL), which was not present in the complexes for which the coordination mode of the deprotonated hydroxyl group with metal ions was confirmed. The C=N-(azomethine) stretching frequency of the ligand showed a strong absorbance peak at 1643 cm −1 ; a shift toward the higher wavelength region (1610-1635 cm −1 ) of the metal(II) complexes revealed that the azomethine nitrogen had coordinated with metal ions ( Table 2) [21]. The bands at (1570-1590 cm −1 ) and (1350-1380 cm −1 ) were attributed to the asymmetric and symmetric vibrational frequencies, respectively, of the carboxyl groups present in the amino acid [22][23][24][25]. The absence of a band in the 1700-1750 cm −1 region suggested that the COO − group of L-valine was coordinated with the central metal ion and that the difference in the frequency value between the asymmetric and symmetric stretching vibrations of the complexes lay between 180 and 216 cm −1 , for which the latter is practically larger than that of a free carboxylate ion [26]. These findings supported the notion of a monodentate coordination of the carboxyl group of L-valine with metal ions. The spectra of all the complexes showed a strong band at 725-727 cm −1 , thus indicating that -CH was out of the plane in the center ring of 1,10-phenanthroline. The lower frequency regions of the metal(II) complexes around 540-565 cm −1 and 450-505 cm −1 , which are attributed to the (M-O) and (M-N) bands, respectively, confirmed the coordination of ligands with metal ions [27,28]. Hence, all the above facts are in good agreement with the complexes coordinated as a tridentate mode.

UV-Visible Spectral Analysis
The electronic spectra of metal(II) complex 1(a-h) were measured using methanol solution (Figure 1a-c) and are presented in Table 2. A series of intense bands were observed for the ligand (HL) at 264, 353, and 409 nm, which were assigned to π-π* and n-π* transitions involved in azomethine nitrogen and aromatic ring systems, and this transition showed a decrease in wavelength at 230-300 nm upon complexation [29]. A moderately intense band at around 330-390 nm corresponds to the ligand-to-metal charge transfer transition [30]. In the visible region, cobalt(II) complexes 1a and 1e showed two weak absorption bands at 494 and 634 nm as well as 463 and 619 nm, respectively, suggesting a distorted trigonal bipyramidal geometry around the metal ion [31]. Whereas the nickel(II) complexes 1b and 1f and copper(II) complexes 1c and 1g showed one weak broad absorption peak in the 620-650 nm region due to the distorted square pyramidal geometry around the metal(II)complexes [32]. Moreover, the zinc(II) complexes 1d and 1h did not present any bands in the visible region due to their diamagnetic behavior and the d 10 electronic configuration of the metal ion [33].

Mass Spectral Analysis
The ESI + mass spectra of complexes 1a and 1b ( Figure S2) were determined to have molecular peaks at m/z = 536. 50 and 536.20, which Figures S1 and S2). In the same way, other metal(II) complexes (1c-1h) exhibited molecular ion peaks at m/z = 541.78, 543. 28, 513.03, 512.89, 517.69, and 519.38, which Ni, Cu, and Zn). The resulting spectral data of the complexes showed the formation of the suggested molecular structure.

Thermal Analysis
The thermal behaviors of the metal complexes were determined under a nitrogen atmosphere, as shown in Figure 2. Upon comparison, it is clear that the weight loss of the metal(II) complexes correlates with different compositions at specific temperatures. The [Cu(L)(bpy)] 1g complex displays a weight decrement in three steps from 38 to 800 • C. The first one is related to the decrement in the outer lattice dehydration of a single H 2 O molecule at 30-120 • C. The second step temperature range of 120-270 • C is due to the loss of organic molecules of C 10 H 8 N 2 (2,2 -bipyridyl). A peak corresponding to a weight loss of 24.52% (calcd. 24.46%) at 420-510 • C was related to the ligand moiety in the metal complexes in the third step and the subsequent complex, thus leaving CuO as a residue. The same trend was traced in the TGA plots of the other metal complexes.

EPR Spectral Analysis
The EPR spectra of the copper(II) complexes [Cu(L)(phen)] 1c and [Cu(L)(bpy)] 1g were measured in polycrystalline conditions at 298 K. The hyperfine splitting patterns of the EPR spectra revealed that the discrete g || and g ⊥ values obtained correspond to axial spectral features ( Figure 3). The spectral data suggested that unpaired electrons of the copper(II) complex lay in the d x 2 -y 2 molecular orbital having a ground state(( 2 B 1g ) with g || > g ⊥ > 2.0023. The g || magnitudes elucidated the types of bonds (ionic or covalent) included in the observed metal-to-ligand coordination process. Usually, ionic bonds showed g || > 2.4 and covalent bonds indicated g || < 2.4. The g || values of complexes 1c and 1g were 2.184 and 2.186, thus confirming the covalent nature of the copper(II) complexes. The attained g values confirmed the square pyramidal geometry of the copper complexes reported in previous studies [34]. According to Hathway [35], the geometric Molecules 2023, 28, 2931 8 of 27 parameter 'G' indicates an exchange interaction between the copper-copper centers and is calculated as follows: G = (g || − 2)/(g ⊥ − 2) for axial spectra served for the ligand (HL) at 264, 353, and 409 nm, which were assigned to π-π* and nπ* transitions involved in azomethine nitrogen and aromatic ring systems, and this transition showed a decrease in wavelength at 230-300 nm upon complexation [29]. A moderately intense band at around 330-390 nm corresponds to the ligand-to-metal charge transfer transition [30]. In the visible region, cobalt(II) complexes 1a and 1e showed two weak absorption bands at 494 and 634 nm as well as 463 and 619 nm, respectively, suggesting a distorted trigonal bipyramidal geometry around the metal ion [31]. Whereas the nickel(II) complexes 1b and 1f and copper(II) complexes 1c and 1g showed one weak broad absorption peak in the 620-650 nm region due to the distorted square pyramidal geometry around the metal(II)complexes [32]. Moreover, the zinc(II) complexes 1d and 1h did not present any bands in the visible region due to their diamagnetic behavior and the d 10 electronic configuration of the metal ion [33].

Mass Spectral Analysis
The ESI + mass spectra of complexes 1a and 1b ( Figure S2) were determined to have molecular peaks at m/z = 536. 50  [Cu(L)(bpy)] 1g complex displays a weight decrement in three steps from 38 to 800 °C. The first one is related to the decrement in the outer lattice dehydration of a single H2O molecule at 30-120 °C. The second step temperature range of 120-270 °C is due to the loss of organic molecules of C10H8N2 (2,2′-bipyridyl). A peak corresponding to a weight loss of 24.52% (calcd. 24.46%) at 420-510 °C was related to the ligand moiety in the metal complexes in the third step and the subsequent complex, thus leaving CuO as a residue. The same trend was traced in the TGA plots of the other metal complexes.

EPR Spectral Analysis
The EPR spectra of the copper(II) complexes [Cu(L)(phen)] 1c and [Cu(L)(bpy)] 1g were measured in polycrystalline conditions at 298 K. The hyperfine splitting patterns of the EPR spectra revealed that the discrete g||and g⊥ values obtained correspond to axial spectral features ( Figure 3). The spectral data suggested that unpaired electrons of the copper(II) complex lay in the dx 2 -y 2 molecular orbital having a ground state(( 2 B1g) with g|| > g⊥ > 2.0023. The g|| magnitudes elucidated the types of bonds (ionic or covalent) included in the observed metal-to-ligand coordination process. Usually, ionic bonds showed g||> 2.4 and covalent bonds indicated g|| < 2.4. The g||values of complexes 1c and 1g were 2.184 and 2.186, thus confirming the covalent nature of the copper(II) complexes. The attained g values confirmed the square pyramidal geometry of the copper complexes reported in previous studies [34]. According to Hathway [35], the geometric parameter 'G' indicates an exchange interaction between the copper-copper centers and is calculated as follows: G = (g|| − 2)/(g⊥ − 2) for axial spectra The G values calculated for complexes 1c and 1g were 2.61 and 3.064, respectively. As these G values were found to be around 3.0, the presence of a dx 2 -y 2 ground state of a square pyramidal geometry and a substantial exchange interaction in the polycrystalline state were confirmed [36,37]. 3.2.6. XRD Analysis The purity, crystallite size, and crystallinity of the synthesized complexes were analyzed by Powder XRD [38][39][40][41]. The diffractograms of complexes 1a, 1b, 1f, and 1h are shown in Figure 4a,b. In the region 5-50° (2θ), complexes 1(a-c), and 1g showed sharp The G values calculated for complexes 1c and 1g were 2.61 and 3.064, respectively. As these G values were found to be around 3.0, the presence of a d x 2 -y 2 ground state of a square pyramidal geometry and a substantial exchange interaction in the polycrystalline state were confirmed [36,37].

XRD Analysis
The purity, crystallite size, and crystallinity of the synthesized complexes were analyzed by Powder XRD [38][39][40][41]. The diffractograms of complexes 1a, 1b, 1f, and 1h are shown in Figure 4a,b. In the region 5-50 • (2θ), complexes 1(a-c), and 1g showed sharp diffraction patterns, suggesting that the complexes exist in a crystalline structure. While complexes 1(d-f), and 1h showed very few diffraction peaks, they indicated that the complexes were between crystalline and amorphous phases [42]. The crystallite sizes of the samples were evaluated using Scherrer's equation [43,44]. From the calculated data, the average crystallite size of complexes is in the range of 50-60 nm.

Electrochemical Studies
The redox potential for metal(II) complexes 1(a-h) was calculated via cyclic voltammetry in DMF containing 0.1 M of tetra(n-butyl) ammonium perchlorate and at a scan rate of 100 mVs −1 , for which the results are listed in Table 3. The absence of a cyclic voltammogram for complexes 1d and 1h was due to the electrochemically inactive nature of the zinc(II) ion. The anodic and cathodic peak current ratios (Ipc/Ipa) were found to be less

Electrochemical Studies
The redox potential for metal(II) complexes 1(a-h) was calculated via cyclic voltammetry in DMF containing 0.1 M of tetra(n-butyl) ammonium perchlorate and at a scan rate of 100 mVs −1 , for which the results are listed in Table 3. The absence of a cyclic voltammogram for complexes 1d and 1h was due to the electrochemically inactive nature of the zinc(II) ion. The anodic and cathodic peak current ratios (Ip c /Ip a ) were found to be less than unity, thereby suggesting that the metal(II) complexes were irreversible during the redox process.

Reduction Process
The cyclic voltammograms of the Co(II), Ni(II), and Cu(II) metals were studied at the cathodic region from 0 to -1.4 V (Figure 5a). The voltammogram peaks of cobalt(II) complex 1a, nickel(II) complex 1b, and copper(II) complex 1c showed irreversible oneelectron transfer reduction at −0.71 V, −0.71 V, and −0.54 V. Likewise, the absence of counter-reduction patterns in the reverse scan corroborated the irreversible character of the metal in complexes 1(e-g) [45,46]. These reduction potential peaks were observed at −0.66 V, −0.58 V, and −0.58 V, respectively. It was suggested that the following one-electron reduction method should be used: M II (L)(phen or bpy) e − → M I (L)(phen or bpy)

Oxidation Process
The cyclic voltammograms of the Co(II), Ni(II), and Cu(II) metals were studied at the cathodic region from 0 to -1.4 V (Figure 5b). The cyclic voltammograms for cobalt(II) complex 1a and nickel(II) complex 1b revealed irreversible transfer oxidation at +1.19 V and +1.12 V, respectively. Likewise, the cobalt(II) complex 1e and nickel(II) complex 1f showed oxidation potential at +1.30 and +1.17, respectively; finally, the absence of a counteroxidation peak in the reverse pattern verified the irreversible nature of the metallic ions [47]. The oxidation step at the anodic region is defined as follows:

Catalytic Activity
Subsequently, all metal(II) complexes utilized for multi-component reactions in a single pot were examined at various concentrations with or without a solvent (Scheme 2). To improve the catalytic performance of Schiff base metal(II) complex 1c, substituted β-amino carbonyl derivatives were synthesized in one pot repeatedly at different concentrations, including 0.0384 mmol, 0.0769 mmol, 0.1154 mmol, and 0.1539 mmol. While floating the amount of the catalyst from 0.1154 mmol to 0.1539 mmol, the yield rate and reaction time remained unaffected.

Catalytic Activity
Subsequently, all metal(II) complexes utilized for multi-component reactions gle pot were examined at various concentrations with or without a solvent (Schem Various derivatives of substituted β-amino carbonyl compounds were synthesized using conventional and solid-phase techniques, for which reactions were continued at multiple varying concentrations. These results are shown in Tables 4 and 5. In the conventional technique, the zinc enolate is produced by linkage, and the catalyst then deprotonates the alkynyl ketone. However, the reaction time and percentage of yield are notable for the pricey Zn-Pro Phenol complex catalyst. β-Amino carbonyl groups were synthesized with less than 0.5 mmol of a nanocomposite catalyst, for which more than 10 h were required to complete the investigation. Furthermore, 60 mg of metal (II) salt was used for 5 min under the silica-supported method.
The synthesized metal (II) complexes were utilized as a model catalyst for the acceleration of the Mannich reaction. Under the same reaction conditions, the catalyst was used up to four times in the synthesis of substituted β-amino carbonyl compounds. The final product was the same as when it was first used, even after the fourth recycling phase, as shown in Table 6. Table 4. Synthesis of β-Amino carbonyl derivatives assisted by various concentrations of metal complex 1(a-d).

S.No
Catalyst Derivative  Various derivatives of substituted β-amino carbonyl compounds were synthesized using conventional and solid-phase techniques, for which reactions were continued at multiple varying concentrations. These results are shown in Tables 4 and 5. In the conventional technique, the zinc enolate is produced by linkage, and the catalyst then deprotonates the alkynyl ketone. However, the reaction time and percentage of yield are notable for the pricey Zn-Pro Phenol complex catalyst. β-Amino carbonyl groups were synthesized with less than 0.5 mmol of a nanocomposite catalyst, for which more than 10 h were required to complete the investigation. Furthermore, 60 mg of metal (II) salt was used for 5 min under the silica-supported method.  The synthesized metal (II) complexes were utilized as a model catalyst for the acceleration of the Mannich reaction. Under the same reaction conditions, the catalyst was used up to four times in the synthesis of substituted β-amino carbonyl compounds. The final product was the same as when it was first used, even after the fourth recycling phase, as shown in Table 6.

Geometry Optimization
With the help of the Jaguar 8.8 software, which is based on DFT with the B3LYP and LACVP++ basis sets, the optimal geometry for the metal(II) complexes was obtained. The chosen bond angles and bond lengths are shown in Tables 7 and 8, and the molecular structures of the metal(II) complexes are illustrated in Figures 6 and 7. It was discovered that the computed values of bond angles and bond lengths were more suitable for forecasting the geometry of the metal(II) complexes. The geometric parameter 'τ' determined the molecular structure of the complex for the pentacoordinate system, and it was computed using the following formula [48].

Molecular Orbital Analysis
The molecular orbitals for the synthesized metal(II) complexes were theoretically analyzed by frontier molecular orbital theory. This theory states that the interaction between the frontier orbitals (HOMO and LUMO) of the reacting molecules generate an electric change. The energies of the primary orbitals were used to determine whether a compound would be stable or chemically reactive at the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). A tendency to donate an electron corresponds to the HOMO, while a tendency to withdraw an electron corresponds to the LUMO [49]. In many molecular systems, the energy difference (∆E) between the HOMO and LUMO is thought to be a crucial stability index for highlighting structural and conformational barriers. Charge transfer between the HOMO and LUMO happens when ∆E is low. The increased activity of the molecule is influenced by the lower ∆E value.
In this study, the significant quantum parameters, such as the HOMO-LUMO energy gap (∆E), chemical potentials (Pi), absolute electronegativities (χ), absolute softness ((σ), absolute hardness (η), global electrophilicity (ω), global softness (S), and additional electronic charge (∆ Nmax ), were calculated in accordance with Koopman's hypothesis using the following equations [50]: The obtained parameters for the complexes are given in Table 9. The ∆E values of complex 1(a-h) were found to be 3.07, 2.34, 2.38, 2.21, 3.10, 2.37, 2.34, and 2.25 eV, respectively. Particularly, the zinc(II) complexes 1d and 1h have the lowest energy band gap, suggesting higher activity and lesser stability. The lowest energy gap value of the zinc(II) complex 1d demonstrated an undemanding electronic transition between the HOMO and LUMO, which may be reason for the higher bioactivity observed.

Molecular Orbital Analysis
The molecular orbitals for the synthesized metal(II) complexes were theoretically analyzed by frontier molecular orbital theory. This theory states that the interaction between the frontier orbitals (HOMO and LUMO) of the reacting molecules generate an electric change. The energies of the primary orbitals were used to determine whether a compound would be stable or chemically reactive at the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). A tendency to donate an (i) (ii) (iii) (iv) Figure 7. Frontier molecular orbitals of investigated complexes, namely, (i) cobalt(II) complex 1e, (ii) nickel(II) complex 1f, (ii) copper(II) complex 1g, and (iv) zinc(II) complex 1h, using B3LYP/LACVP ++ basis set.

Molecular Docking Studies
Validation of the Active Site of Thymidylate Synthase Thymidylate synthase (TS) plays a decisive role in the biosynthesis of DNA precursors. An increased level of thymidylate synthase activity has been observed in colorectal, breast, cervical, kidney, and lung tumors. Many researchers have encountered difficulties when attempting to find a better way to regulate thymidylate synthase action for both the development of therapeutic strategies and tumor prevention. Hence, the prepared complexes were assessed with respect to the binding affinity of the thymidylate synthase receptor to understand the interactive behavior of the studied complexes with the TS receptor and thus develop a strategy for their optimization. By analyzing the metal(II) complexes 1(a-h) applied for docking with thymidylate synthase (PDB ID: 1HZW) and their binding affinity values, it was revealed that the complexes were more active on that site. The calculated docking scores and active sites of TS with various modes of interaction are shown in Table 10 ( Figures S3 and S4 from Supplementary Materials). The docking study's results revealed that all the complexes were located within the hydrophobic site of the TS receptor. The docking scores for the metal(II) complexes with a TS receptor were found to be −5.70, −5.615, −5.791, −5.367, −5.49, −4.97, −5.026, and −5.223 kcal mol −1 . In addition, it was found that complex 1c had the highest docking score due to binding interaction with the TS receptor via π-π stacking, hydrogen bonding, and hydrophobic interactions, as shown in Figure 8. This complex showed one hydrogen bond (distance of 2.33 Å) interaction between an amino hydrogen of residue ASN 226 with the carboxylate oxygen of the complex. Further, complex 1c showed π-π stacking interaction between the residue PHE 225 and a phenolate ring with an interaction distance of 4.01 Å. Furthermore, the complex showed an abundance of hydrophobic interactions with different amino acid residues, such as VAL 79, PHE 80, TRP 90, PHE 91, LEU 101, VAL 106, ILE 108, TRP 109, ALA 111,TYR 135,LEU 192,PRO 194,CYS 195,PRO 194,LEU 221,VAL 223,PRO 224,PHE 225, and TYR 258. The prepared metal(II) complexes showed better activity compared to that presented in our previously published article [51]. The highest docking score value of complex 1c was obtained with thymidylate synthase, which encouraged us to study the cytotoxicity of the metal complexes experimentally. In vitro antibacterial activity for metal(II) complex 1(a-h) was checked against two Gram-negative bacteria (Escherichia coli and Klebsiella pneumonia) and one Gram-positive bacterium (Staphylococcus aureus) in different concentration ranges from 5-20 µL/mL using the agar diffusion method [21,52,53]. The gradient changes were illustrated as graphs (Figure 9a-c), and their data are shown in Table 11. From the data, it is evident that the metal complexes showed better activity upon an increase in concentration. In vitro antibacterial activity for metal(II) complex 1(a-h) was checked against two Gram-negative bacteria (Escherichia coli and Klebsiella pneumonia) and one Gram-positive    cytotoxic morphologies of condensed nuclei, cell shrinkage, membrane blebbing, apoptotic bodies, bubbling, and echinoid spikes (Figure 10b). The toxicity results also suggested that complex 1c is less toxic than complex 1g with respect to the NHDF cell line, for which the following order was observed: MCF-7 > A549 > HeLa. The extended aromatic conjugation of the phenanthroline moiety and strong hydrophobic interaction of complex 1c showed higher potential anticancer activities compared to the bipyridyl coligand.

Conclusions
In this work, Schiff base ligands (HL) and heterocyclic bases (1,10-phenanthroline or 2,2 -bipyridyl) were used to synthesize mixed-ligand metal (II) complexes 1(a-h). Based on the spectral results, it was determined that the Co(II), Ni(II), Cu(II), and Zn(II) complexes assumed the geometry of a distorted square pyramidal shape. With regard to the electrochemical behavior of the complexes, it was observed that one electron was irreversibly transferred. The results regarding catalytic activity showed that the muffle furnace approach reached the target molecule much faster than the conventional method. Metal(II) complex 1c revealed significant antibacterial activity against Escherichia coli and Staphylococcus aureus at a 20 µg/mL concentration applied using the agar diffusion method. Similarly, the in vitro cytotoxicity of complex [Cu(L)(phen)] 1c showed promising anticancer activity against MCF-7 (17.13 ± 0.74), A549 (25.95 ± 1.82), and HeLa (26.26 ± 1.06) cancer cell lines, whose IC 50 values were very close in terms of activity to that of cis-platin. The frontier orbital (HOMO-LUMO) analyses revealed that the complex [Zn(L)(phen)] 1d exhibited a lower band gap (2.21 eV) than the other complexes. The smaller energy gap led to an easing of the transition occurring between energy levels and precipitated the higher biological activity of the metal(II) complexes. Finally, the docking score of complex 1c (−5.791 kcal mol −1 ) showed better binding interactions with the thymidylate synthase receptor, viz., several hydrophobic, stacking, and hydrogen bonding interactions. Further investigation and developments in the area of Schiff base complexes of transition metal ions would be highly beneficial for industries and drug-related research.