1. INTRODUCTION:

Schiff bases play an important role in coordination chemistry and have found considerable applications in various fields of science. Nowadays, Nitrogen-Oxygen-Sulfur containing Schiff base ligands have fascinated an enormous interest of researchers due to having potential donor sites or atoms accessible as chelating agents for various metals to form Schiff base metal complexes.^1-4 The deprotonated phenolic O, imine N, and thiocarbonyl S of the thiosemicarbazone ligands permit them to function as tridentate ligands or they can exist as negatively charged tridentate ligands by losing one of their protons.^5,6The chemistry dealing with hydrazine derivatives, such as thiosemicarbazide and its hydrazones have a massive attraction due to their widespread synthetic and systematic applications and biological activity.^7,8

The thiosemicarbazone Schiff bases have been introduced with remarkable interest in many fields such as chemistry and biology due to the existence of their antifungal, antiviral, antibacterial, antimalarial, antitumor, anticancer and anti-HIV and antineoplastic activities.^9-14 The biochemical performance of thiosemicarbazide is identical to its correlated semicarbazide, yet, it is greater chemical flexibility of the thione group as related to that of a keto group and is responsible for more various activities of thiosemicarbazide.^15,16 In coordination chemistry, several reasons such as steric assembling on the azomethine bond, intermolecular hydrogen bonding, larger coligand interactions, solvent effect, etc. are responsible for complexation.¹⁷ The synergic effect of those metal complexes increases the biological activity and reduced toxicity.¹⁸ In this study we have Synthesized a Schiff base from thiosemicarbazide and salicylaldehyde, its complexation with V(IV), Ni(II), Cu(II), and Zn(II) and investigated for in vitro antibacterial activity against two gram-positive (Staphylococcus aureus, Bacillus subtilis) and two germ negative (Escherichia Coli, Shigella boydii) bacterial strains. In vitro, brine shrimp (Artemia salina) bioassay has also been carried out to study the cytotoxic activities of these compounds

2. MATERIALS AND METHODS:

2.1 Materials and Physical measurements:

All utilized chemicals were widely available on the market, had analytical-grade purity, and used without any further purification. FTIR spectra of all the synthesized compounds were collected as KBr pellet using an FTIR spectrophotometer (JASCO, FTIR/4100 Japan). The Lambda 25 spectrophotometer by PerkinElmer was used to obtain the UV-Visible spectra. Digital melting point equipment was used to regulate the melting points of synthesized compounds (Melting-Point Apparatus with Microscope). Molar conductance of the complexes (1.0 × 10^-3M in DMSO) was measured using a WPACM35 conductivity meter and a dip-cell with a platinum electrode. The magnetic susceptibility of the solid samples was measured with the help of Sherwood Scientific Susceptibility balance that followed the Gouy methods. The thermal stability of the complexes was obtained by TGA-DTA analysis using Perkin Elmer Simultaneous Thermal Analyzer, STA-8000. The Thin Layer Chromatography (TLC) was used to reaction progress and purity of ligands and their complexes.

2.2 Synthesis of Schiff base Ligand (L), (C₈H₉N₃OS):

In a two-neck round bottom flux, a hot methanolic solution (30 mL) of thiosemicarbazide (20 mmol, 1.824 g) and a solution of salicylaldehyde (20 mmol, 2.105 mL) in the same amount of solvent were mixed thoroughly. In this mixture, about 1 ml of acetic acid was poured (due to protonation in carbinolamine and produced a good leaving group for a faster reaction) White-colored precipitate (ppt) was observed after five minutes, then it refluxed for three hours. The completion of the reaction was monitored by TLC. The ppt was separated through filtration before being washed with hot ethanol and diethyl ether. Finally, the solid sample was dried and kept in a desiccator over anhydrous CaCl₂.

Color: White, yield: 81%, m.p.: (228-230 °C), FTIR spectral peak (n/cm^-1): 1617 (-HC=N-), 3436 (-NH₂), 3138 (-NH-), 1103 (N-N), UV-Vis. spectral peak (l_max/nm): 256, 313, 337.

Figure 1: Synthesis pathway of thiosemicarbazone ligand

2.3 Synthesis of the Cu(II) complex (CuL.H₂O):

The synthesized thiosemicarbazone ligand (5 mmol 0.976 g) was dissolved in 30 mL methanol and a hot methanolic solution of dihydrate copper chloride (5 mmol, 0.858 g) was poured into a round bottom flask. Immediately light green color ppt was obtained. After refluxing for 3 hours, the ppt was filtered off, washed with hot ethanol and diethyl ether, and dried over anhydrous CaCl₂.

Color: Light green, yield: 71%, m.p.: (>300 °C), Molar conductivity (L/Ω^-1cm²mol^-1): 8, Magnetic moment (µ_eff/BM): 1.59, FTIR spectral peak (n/cm^-1): 1636 (-HC=N-), 3436 (-NH₂), 1103 (N-N), 604 (M-O), 531 (M-N), 443 (M-S); UV-Vis. spectral peak (l_max/nm): 282, 329, 357, 399.

2.4 Synthesis of the Ni(II) complex (NiL.3H₂O):

The same procedure was followed to carry out for all the complexes. In this case, tetrahydrate nickel acetate (5 mmol, 1.244 g) was used as the metal salt.

Color: Deep brown, yield: 78%, m.p.: (>300 °C), Molar conductivity (L/Ω^-1cm²mol^-1): 4, Magnetic moment (µ_eff/BM): 2.56, FTIR spectral peak (n/cm^-1): 1606 (-HC=N-), 3410 (-NH₂), 1124 (N-N), 658 (M-O), 561 (M-N), 457 (M-S); UV-Vis. spectral peak (l_max/nm): 264, 304, 376, 422.

2.5 Synthesis of the Zn(II) complex (ZnL.H₂O):

In this procedure, dihydrated zinc acetate (5 mmol, 1.095 g) was used as the metal salt.

Color: Off white, yield: 67%, m.p.: (>300 °C), Molar conductivity (L/Ω^-1cm²mol^-1): 3, Magnetic moment (µ_eff/BM): 0.48, FTIR spectral peak (n/cm^-1): 1626 (-HC=N-), 3397(-NH₂), 1111(N-N), 609(M-O), 563(M-N), 459 (M-S); UV-Vis. spectral peak (l_max/nm): 253, 319, 386.

2.6 Synthesis of the VO(IV) complex (VOL.2H₂O):

To synthesize VO(IV) complex, vanadyl sulfate (5 mmol, 0.820 g) was used as the metal salt.

Color: Light green, yield: 80%, m.p.: (>300 °C), Molar conductivity (L/Ω^-1cm²mol^-1): 6, Magnetic moment (µ_eff/BM): 1.76 Paramagnetic. FTIR spectral peak (n/cm^-1): 1605 (-HC=N-), 753 (C=S), 3431(-NH₂), 1105 (N-N), 657(M-O), 603 (M-N), 472(M-S); UV-Vis. spectral peak (l_max/nm): 263, 339, 381, 414.

3. RESULT AND DISCUSSION:

By condensing salicylaldehyde and thiosemicarbazide to form a white crystalline solid, the Schiff base has been produced. All complexes are soluble in DMSO and DMF, partially soluble in CHCl₃ but insoluble in water and other common organic solvents and intensely colored, powdered solids, which decompose above 300 °C. The low value of molar conductance (3-8 Ω^-1cm² mol^-1) indicated that all the complexes are non-electrolytes.¹⁹

3.1 IR Spectral Studies:

Figure 2: Tautomeric form of (a) thione and (b) thiol.

Figure 3: FTIR spectra of Schiff base ligand and its complexes.

In the analysis, IR spectroscopy is much important for the comparison of spectral lines of metal complexes and free Schiff base. The tautomeric form of Schiff base was observed due to conversion of thiol protons. In the solid state, there was no peak obtained in the range between 2250 cm^-1to 2700 cm^-1 that was assigned to absence of thio-enol isomerization.²⁰ The sharp band at 3138 cm^-1 is ascribed to secondary amine of ligands. In the solution, the unstable monomeric form of thione groups converted to stable form of thiol isomer. The N-H bands of secondary amine disappeared in the complexes and the thiolate anions existed in solution due to deprotonation. ²¹ The strong stretching frequency band 1617 cm^-1 that was indicated the presence of azomethine (-HC=N-) linkage.²² During complexation, the azomethine band was shifted to lower frequencies for Ni(II) and VO(IV), and to higher for Zn(II), and Cu(II) complexes.²³ The emerging absorbed bands were 3443 cm^-1 due to the free -OH group in the Schiff base and the board peaks around 3300 cm^-1 to 3450 cm^-1were obtained due to presence of hydroxyl group of water molecule and primary amine of thiosemicarbazide moiety in the metal complexes.²⁴ The appeared 1111 cm^-1 stretching band was for the N-N linkage and the band was weaker for metal complexes. The new band at (604-658, 531-602, 443-472) cm^-1 for M-N, M-O, and M-S respectively assigned the coordination of the N, O, S donor atom with central metal ion in the complexes.²⁵

3.2 Electronic Spectra and magnetic moment:

The electronic spectral data are frequently helpful in the confirmation of outcomes from other structural assessment techniques. In The electronic spectra of the ligand the absorption maxima at 256 nm arise from π→π* (Conjugated π bond of aromatic moiety), 313 nm arise from n→π* for the azomethine family of thiosemicarbazone portion, and 337 nm arise from n→π* for the thioamide portion of free ligands moiety. At room temperature, the magnetic moment of Cu(II) complex is 1.59 BM data indicating that has a paramagnetic nature with one unpaired electron. The electronic spectra of the complex showed four bands at 282 nm for π→π*, 329 nm, and 357 nm for n→π*, and 399 nm for charge transfer transition subsequently.²⁶ The complex exhibited a tetrahedral geometry with sp³ hybridization. The magnetic moment value of Ni(II) complex is 2.56 BM corresponds to two unpaired electrons and the electronic spectrum of various bands at 264 nm for π→π*, 304 nm for n→π*, 376 nm for charge transfer transition, and 422 nm for weak d-d transition. The Ni(II) complex adopt octahedral geometry with sp³d² hybridization.²² The electronic spectra and magnetic moment value indicated that Zn(II) complex showed a diamagnetic nature. The spectrum of 253 nm for π→π*, 319nm for n→π*, and 386 nm for charge transfer transition. In those studies, d-d transition was absent and proposed a tetrahedral structure with sp³ hybridization. The magnetic moment value of VO(IV) complex is 1.76 BM corresponds to one unpaired electron and the electronic spectrum of various bands at 263nm for π→π*, 339 nm for n→π*, 381 nm for charge transfer transition, and 414 nm for weak d-d transition. The oxovanadium complex adopt octahedral geometry with sp³d² hybridization.²⁷

Figure 4: Electronic spectra of Schiff base ligand and its complexes.

3.3 Thermal analysis:

The thermal stability of Ni(II), Cu(II), Zn(II), and VO(IV) complexes was analyzed with a TGA-DTA curve obtained from a Simultaneous Thermal Analyzer, STA-8000 (Perkin Elmer) at a heating rate 15 °C min⁻¹ in N₂ atmosphere with a temperature range of 30–800 °C. The samples were performed in ceramic pans crucibles to obtain signal to the recorder which was exhibited by a computer interface. The data were plotted in the form of weight loss of sample vs. temperature for TGA and microvolts vs. temperature for DTA.²⁸ The proposed step-by-step thermal deprivation form of all the complexes with respect to temperature is shown in Table 1.

The TGA-DTA curve of Ni(C₈H₉N₃OS)(H₂O)₃complex was shown in the figure 5(a). In the 1st step, the coordinated water molecule (3mol) was removed in the range between 222-304°C with a weight loss of 17.01% (calc. 17.64%).²⁹ The second step involves removing the aromatic moiety of C₇H₅ is 29.01% (calc. 29.08) in 304-393 °C. Thiosemicarbazide moiety of H₃N₃CS was decomposed at the range of temperature 394-600 °C (exp. 30.67%/calc. 28.10%). The temperature up to 600 °C, the straight line of the curve was obtained which is indicated as NiO/Ni residue (exp.19.75%/calc. 22.54%). The DTA data indicated graphically that consequently weight losses appeared by endothermic peaks at 39 °C, 301 °C, 515 °C and exothermic peaks at 395 °C, 505 °C.

The TGA-DTA curve of Cu(C₈H₉N₃OS)(H₂O)complex was shown in the figure 5(b). In the 1st step, the coordinated water molecule was removed in the range between 200-260 °C with a weight loss of 6.49% (calc. 6.50%). The second step involves removing the aromatic moiety of C₇H₅ is 29.05% (calc. 32.38%) in 269-396 °C. Thiosemicarbazide moiety of H₃N₃CS was decomposed at the range of temperature 399-701°C (exp. 32.87%/ calc. 31.81%). The temperature up to 700 °C, the straight line of the curve was obtained which is indicated as CuO/Cu residue (exp. 29.42%/calc. 31.81%). The DTA data indicated graphically that consequently, weight losses appeared by endothermic peaks at 265 °C and 554 °C and exothermic peaks at 338 °C and 486 °C.

The TGA-DTA curve of Zn(C₈H₉N₃OS)(H₂O)complex was shown in the figure 5(c). In the 1st step, the coordinated water molecule was removed in the range between 270-332 °C with a weight loss of 6.17% (calc. 6.51%). The second step involves removing the aromatic moiety of C₇H₅ is 32.02% (calc. 32.17%) in 332-451 °C. Thiosemicarbazide moiety of H₃N₃CS was decomposed at temperature 450-651°C (exp. 21.45%/calc. 20.54%). The temperature up to 690 °C, the straight line of the curve was obtained which is indicated as ZnO/Zn residue (exp. 28.93%/calc. 31.30%). The DTA data indicated graphically that consequently, weight losses appeared by endothermic peaks at 42 °C and 534 °C and exothermic peaks at 473 °C and 578 °C.

The TGA-DTA curve of VO(C₈H₉N₃OS)(H₂O)₂complexes was shown in the figure 5(d). In the 1st step, the coordinated water molecule (2 mol) was removed in the range between 93-212 °C with a weight loss of 11.83% (calc. 12.15%). The second step involves removing the aromatic moiety of C₇H₅ is 29.77% (calc. 30.0%) in 212-336 °C. Thiosemicarbazide moiety of H₃N₃CS was decomposed at temperature 336-510 °C (exp. 36.98%/ calc. 35.11%). The temperature up to 520 °C, the straight line of the curve was obtained which is indicated as VO residue (exp. 21.42%/calc. 22.61%). The DTA data indicated graphically that consequently weight losses appeared by endothermic peaks at 37 °C, 393 °C and 662 °C and exothermic peaks at 327 °C and 465 °C.

Figure 5: TGA and DTA curve of (a) Ni(II) complex (b) Cu(II) complex (c) Zn(II) Complex and (d) VO(II) complex.

Table 1: Thermal data of the metal complexes

Complexes	Steps	Decomposition temperature (°C)	Weight loss (%), (obs/cal)	DTA peak (°C)	Description
NiL(H₂O)₃	1^st	222-304	17.01/17.64	Endothermic: 395, 505 Exothermic: 39, 301, 471, and 515	3H₂O
	2^nd	305-393	29.01/29.08		C₇H₅
	3^rd	399-600	30.67/28.10		CN₃SH₂
	4^th	>600	19.75/22.54		NiO/Ni
CuL(H₂O)	1^st	200-260	6.49/6.50	Endothermic: 338, and 486 Exothermic: 265, and 554	H₂O
	2^nd	269-396	29.05/32.38		C₇H₅
	3^rd	399-701	32.87/31.81		CN₃SH₂
	4^th	>700	29.42/31.3		Cu/CuO
ZnL(H₂O)	1^st	270-332	6.17/6.51	Endothermic: 473, and 578 Exothermic: 42, and 534	H₂O
	2^nd	332-451	32.02/32.17		C₇H₅
	3^rd	450-685	21.45/20.54		CN₃SH₂
	4^th	>690	28.93/31.30		Zn/ZnO
VOL (H₂O)₂	1^st	93-212	11.83/12.15	Endothermic: 327, and 465 Exothermic: 37, and 393	2H₂O
	2^nd	212-336	29.77/30.0		C₇H₅
	3^rd	336-510	36.98/35.11		CN₃SH₂
	4^th	>520	21.42/22.61		VO

Figure 7: Pictorial representation of antibacterial activity

Figure 8: Photographic representation of zone of inhibition of (1) Schiff base, (2) Ni(ll) complex, (3) Cu(ll) complex, (4) Solvent (5) Zn(ll) complex (6) VO(lV) complex against Bacillus subtilis, Staphylococcus aureus, Escherichia coli and Shigella boydii, bacterial stains.

5. In Vitro Cytotoxicity Effect:

In vitro cytotoxic activity of all the synthesized ligands and their metal complexes were studied using the protocol of Meyer et al.³¹ Brine shrimp (Artemia salina) eggs were hatched in a shallow rectangular plastic dish and filled with artificial seawater, which was prepared with commercial salt mixture and double-distilled water. Data were analyzed by Finney computer program to determine the LD₅₀ values.³² For toxicity screening, for probable and convenient pharmacological activity, and for cheaper accessibility, artemia-based toxicity assays of metal complexes are a key solution.³³ Even at the lowest dose of 50 μg/mL, there was a mortality rate of about 10-25%; however, as concentration raised to 100 μg/mL, 150 μg/mL, 200 μg/mL, and 250 μg/mL, death increased up to about 85%. The toxic consequences and rate of mortality were represented as LD₅₀. According to Karbar’s methods, the LD₅₀values are 148.25 µg/mL for Schiff base, 138.25 µg/mL for Cu(II) complex, 158.75 µg/mL for Ni(II) complex, 161.75 µg/mL for Zn(II) complex, and 152.50 µg/mL for VO(lV) complex.

Figure 9: Graphical representation of the mortality rate (24 hour) of brine shrimp Artemia nauplii exposed to different concentrations of Schiff base and its complexes.

6. CONCLUSION:

In the work, transition metal complexes of Cu(II), Ni(II), Zn(II) and VO(IV) ion with tridentate nitrogen, Oxygen, and sulfur donor thiosemicarbazone Schiff base ligand were Synthesized. FTIR data revealed that metal is bound to NOS coordinating site. Electronic spectra and magnetic moment values designated the geometry of the complexes. The Cu(II) and Zn(II) complexes showed a tetrahedral geometry, on the other hand, Ni(II) and VO(IV) complexes adopt an octahedral geometry. All the complexes were thermally stable up to 220 °C. All the complexes exhibited greater antibacterial activity than Schiff base and Copper complexes showed more precise results among the complexes compared to kanamycin. Schiff base and its complexes adopted cytotoxic activity.

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Received on 22.11.2023 Modified on 11.12.2023

Asian J. Research Chem. 2024; 17(1):6-12.

DOI: 10.52711/0974-4150.2024.00002

Name of Bacteria	Gram staining	Diameter (mm) of zone of inhibition
Name of Bacteria	Gram staining	Ligand, 100 µg/mL	Ni(II) Complex, 100 µg/mL	Cu(II) Complex, 100 µg/mL	VO(IV) Complex, 100 µg/mL	Zn(II) Complex, 100 µg/mL	Kanamycin, 30 µg/disc
*Escherichia coli*	-	6	14	18	7	9	20
*Shigella boydii*	-	6	14.5	19	7	8.5	20
*Bacillus subtilis*	+	-	11	16.5	7	9	23
*Staphylococcus aureus*	+	6	12.5	18.5	8	9.5	19.5