The ligand HL was synthesized by combining 3-acetyl coumarin in a 1:1 molar ratio with 2-methylbenzyl hydrazinecarbodithioate. The Schiff base transition metal complexes (1-4) were then synthesized by refluxing the ligand (HL) in ethanol with Co(OAc)2⋅4H2O, Ni(OAc)2⋅4H2O, Cu(OAc)2⋅4H2O and Zn(OAc)2.2H2O respectively (Scheme 1). FT-IR, UV-Vis, 1H NMR, EPR and mass spectroscopy were used to characterize all of the compounds. The analytical data are well aligned with their formulations’ proposed structure of the complexes.
3.1. FT-IR spectra
Table 1 shows the IR band assignments of the ligand and metal complexes. The IR spectrum of the ligand shows a band at 3162 cm−1 ascribed to the ν(N-H) stretching frequency  and a band around 948 cm-1 due to the ν(C=S) stretching frequency . The ʋ(C=O) vibration of ligand and complexes was observed in the 1684-1692 cm-1 range . At 1611 cm-1, the ʋ(C=N) vibrations for the azomethine group in the ligand are observed. These peaks shifted to lower frequencies in all of the complexes (1601-1609 cm−1), indicating the presence of azomethine nitrogen in the metal ion . In the case of complexes, the appearance of two distinct bands in the 1551-1570 cm-1 and 1358-1381 cm-1 ranges was assigned to ʋ(COO-)s and ʋ(COO-)as, respectively. The carboxylate bands show that the carboxylate oxygen (OC(O)CH3) is involved in the complex formation . The ligand exhibits a band at 1041 cm-1, which was assigned to ʋ(N-N), and the shift of hydrazinic ʋ(N-N) bands in the metal complexes (1003-1026 cm-1) may be due to complex formation . The appearance of the new bands in the 585-596 and 457-472 cm-1 regions due to the ʋ(M-O), ʋ(M-N) vibrations, respectively, further supports the bonding of the azomethine ligand to the metal ion .
3.2. Electronic absorption spectra.
All of the compounds in DMSO solution had visible absorption bands in their electronic spectra, which were recorded (Table 2 & Fig. 1). The ligand exhibits an absorption band around 230 nm, which can be due to the π→π* transition . On complexation, these bands shift to a higher wavelength region (240-260 nm), indicating that the metal ion coordinates with azomethine nitrogen (C=N) atoms. The ligand to metal charge transfer transitions (LMCT) of complexes are attributed to the absorption bands that formed in the 340-370 nm region . At 520 nm, the cobalt(II) complex has a d-d transition, indicating a square planar geometry [50, 51]. The d-d transition causes the absorption band at 560 nm, showing the square planar nickel(II) complex . At 530 nm, the copper(II) complex shows a d–d transition band, indicating square planar geometry . The Zn complex exhibits no absorption in the range above 500 nm, which is consistent with the d10 electronic configuration of the Zn(II) ion .
3.3. 1H NMR spectra
The 1H NMR spectrum (Fig. 2) of the ligand in CD3OD reveals a singlet at 1.58 ppm, which corresponds to methyl protons on a benzene ring. Methyl protons are assigned a singlet at 2.37 ppm. At 4.55 ppm, two methylene protons show as a singlet. Aromatic protons are responsible for the multiplet at 7.26-7.59 ppm. The amine proton is responsible for the singlet at 9.95 ppm.
The 1H NMR spectrum of the Zn(II) complex was recorded in DMSO. The resonance of methyl protons to a benzene ring was 2.27 ppm. Methyl protons of -C-CH3 unit appear at 2.53 ppm. Methylene protons cause a singlet at 3.83 ppm. Around 6.67-8.23 ppm, aromatic protons appear as a multiplet.
3.4. ESI mass spectral studies
The stoichiometric composition of the metal complexes was determined using the ESI mass spectra, which show moderate to high relative intensity molecular ion peaks. The molecular ion peaks of complexes 1 and 2 are observed at m/z 517 and 537.
3.5. Molar conductance data
The molar conductivity data of Schiff base metal complexes are recorded in DMSO at 25 °C (Table 2). Low molar conductance values of the mononuclear complexes, in the 12-19 ohm-1 cm-1 mol-1 range, reveal non-electrolytic nature .
3.6. EPR spectra
X-band ESR spectrum (Fig. 3) of the [Cu(L)(OAc)] complex 3 was recorded in a solid state at ambient temperature. The spectrum of the complex consists of at g value 2.18. The observed spectrum support the square planar environment .
3.7. DFT calculations
3.7.1. Geometry optimization
The ground state geometry of molecules is shown in Fig. 4. The metal coordination bonds are listed in Table 3. We observed M-O bond length values in the limit of 2.085-1.917 Å, which corresponds to the available experimental values [57, 58]. The calculated bond length values of M-N are in the range of 2.183-1.944 Å, which are consistent with reported values [57, 58]. Moreover, the observed bond length of M-S in the range of 2.437-2.235 Å, which agrees with reported values [57, 58].
Further, M-O calculated bond length values are in the range of 2.146-1.833 Å, and the values correspond to the literature values [57, 58]. Based on the calculated values, it can be seen that bond lengths are reasonably consistent with experimental data. The computed bond length values are slightly higher than the available experimental bond length due to the lack of intermolecular interactions in gas phase geometry.
3.7.2. Frontier molecular orbital analysis
In frontier molecular orbital analysis (FMO), the highest occupied molecular orbital (HOMO) represents the capability to donate electrons, whereas as lowest unoccupied molecular orbital (LUMO) represents the ability to accept electrons. Fig. 5 shows the FMO of Co(II), Ni(II), Cu(II) and Zn(II) complexes. The HOMO is primarily localized over the moiety in Co(II), Ni(II), Cu(II) complexes. The LUMO is localized over the moiety for the Zn(II) complex. However, the localization of LUMO on the four molecules placed over the moiety. The localization states disclose that the charge transfer of Co, Cu and Ni is the same, and the Zn derivative charge transfer mechanism is different from others. The energy gap of complexes 1, 2, 3 and 4 are 1.91 eV, 1.99 eV, 2.26 eV and 2.55 eV, respectively. Both the HOMO and LUMO orbitals play a significant role in chemical stability molecules. The reactivity of molecule, chemical stability, chemical hardness, chemical potential, ionization potential and electron affinity of molecules are calculated from HOMO and LUMO values using Koopmann’s theorem and are listed in Table 4. Molecules with a small energy gap have high polarization, less kinetic stability, and more chemical reactivity. The Co(II) and Ni(II) complexes have higher chemical reactivity and lower molecular kinetic stability. Thus, Cu(II) and Zn(II) complexes exhibit lower chemical reactivity and higher molecular kinetic stability. Moreover, complexes 1 and 3 are more polarizable than 2 and 4.
3.8. Biological study
3.8.1. Antibacterial activity
The antibacterial efficiencies of the Schiff base, its complexes and solvent control have been tested against human pathogens (Streptococcus pneumonia and Escherichia coli). The results are displayed in Table 5. The well diffusion technique was utilized to assess the antibacterial activity of the synthesized compounds. According to the zone of inhibition values obtained, the ligand shows moderate activity against Streptococcus pnenumonia and Escherichia coli. Copper(II) complex 3 shows excellent activity, Cobalt(II) complex 1 exhibits good activity, Zinc(II) complex 4 shows significant activity and Nickel(II) complex 2 displays moderate activity against Streptococcus pneumonia and Escherichia coli. The complexes moderate to excellent activities compared to the ligand against human pathogenic microorganisms.
The higher activity of metal complexes compared to the ligand is due to structural changes caused by coordination and chelating, making metal complexes more effective and potent bacteriostatic agents, preventing microbe development [59, 60].
3.8.2. Antioxidant activity
18.104.22.168. DPPH radical scavenging activity
The antioxidant performance of Schiff base and its metal complexes was probed by DPPH radical scavenging activity (Fig. 6). The deep purple colour of the DPPH radical exhibits an absorption band at 517 nm. It reacts directly with electron and hydrogen donors to form the more stable and light yellow DPPH or DPPH2 molecules . The change of DPPH colour and a decrease in absorbance is monitored by UV-vis spectrophotometry and it is used to describe the antioxidant efficiency of the synthesized compounds. The ligand shows moderate activity and Copper and zinc complexes show good antioxidant activity while cobalt and nickel complexes also reveal significant activity. All the Schiff base metal complexes indicate better radical scavenging activity, which the presence of electron-donating groups may cause. So, the array of antioxidant activity of metal complexes is as Ascorbic acid >Cu > Zn > Co > Ni > HL.