Synthesis, characterization, biological activity of Schiff base metal complexes derived from 3-Acetylcoumarin and 2-methylbenzyl hydrazinecarbodithioate

doi:10.21203/rs.3.rs-1513188/v1

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Synthesis, characterization, biological activity of Schiff base metal complexes derived from 3-Acetylcoumarin and 2-methylbenzyl hydrazinecarbodithioate

https://doi.org/10.21203/rs.3.rs-1513188/v1

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New Schiff base metal complexes of Co(II), Ni(II), Cu(II) and Zn(II) with 3-Acetylcoumarin and 2-methylbenzyl hydrazinecarbodithioate {[Co(L)(OAc)] (1), [Ni(L)(OAc)] (2), [Cu(L)(OAc)] (3) and [Zn(L)(OAc)] (4)} were synthesized and characterized by elemental analysis, IR, UV-Vis, ¹H-NMR, molar conductance, ESR and mass spectrometry. Based on these data, it is clear that the Schiff base act as tridentate for metal complexes via the ionic sulphur of thiol and azomethine nitrogen atom, as well as the oxygen atom of the carbonyl group. Electronic spectral results were used to infer the geometry of the complexes, which was then validated using DFT calculations. The molar conductance discloses that the Schiff base complexes are non-electrolytic. The in vitro antibacterial activity suggest that the metal complexes are highly active against two bacteria (Streptococcus pneumonia and Escherichia coli). Furthermore, the in vitro antioxidant activity of all the complexes was determined by DPPH radical scavenging method.

3-Acetylcoumarin

Schiff base

DFT studies

non-electrolytic

antibacterial

antioxidant

In recent years, transition metal complexes of Schiff bases have long been used in pharmacological, biological, catalytic, analytical, and other domains due to their preparative accessibility and unique ligation characteristics [1-5]. Schiff bases are a momentous class of compounds in the medical and pharmaceutical fields. They have biological properties like antibacterial [6], antifungal [7], anticancer [8] and herbicidal [9]. Moreover, Schiff bases can be used to tune metal-centered electronic parameters, induce substrate chirality, and improve the solubility and stability of homogeneous or heterogeneous catalysts [10-13].

Coumarins are a significant class of heterocyclic compounds that include a variety of pharmacological agents. Many structurally new coumarin derivatives have been discovered to have significant cytotoxic activity in vitro and in vivo [14, 15]. Due to its potential to inhibit human immunodeficiency virus integrase, coumarin derivatives have recently been assessed to treat the human immunodeficiency virus [16, 17]. Coumarin nucleus and related derivatives [18, 19] exhibit bioactivities such as antibacterial [20], vasodilatory and antithrombotic [21], lipoxygenase and cyclooxygenase inhibition [22, 23], antimutagenic [24] and antitumorigenic [25]. Few coumarin derivatives enhance biological function by binding to metal ions. Some reviews describe achievements in various medical applications of complexes of coumarins [26, 27]. In specific, Coumarins with diverse substituents at position 3, have a broad range of biological effects [28] and are spectroscopically interesting [29]. Antimicrobial properties have been found for 3-acetylcoumarins, effective synthons for chloramphenicol coumarin analogs [30, 31] and 3-coumarinylthiazoles [32].

Schiff bases derived from S-alkyl/aryl ester of dithiocarbazic acid and their metal complexes have been intensively explored over recent years [33-35]. The bioactivities of dithiocarbazate derivatives varied dramatically in many cases, despite minor structural differences. Furthermore, because these ligands contain both soft sulphur and hard nitrogen donor atoms, they can coordinate with various types of metal ions, resulting in metal complexes with fascinating physicochemical properties and potentially useful bioactivity [36-38].

Keeping all of the above factors in mind, we report the synthesis of the ligand by using 2-methylbenzyl hydrazinecarbodithioate and 3-acetylcoumarin moieties. By complexation, the coordination of the above said azomethine ligand with transition (Co(II), Ni(II), Cu(II) and Zn(II)) metal ions resulted in Schiff base metal complexes with square planner geometry. We have explored their in vitro antibacterial activity against Streptococcus pneumonia and Escherichia coli by disc diffusion method. Moreover, the antioxidant activity of the complexes was established by DPPH radical. These results divulge that the complexes possess potential antimicrobial agents and antioxidants.

2.1 Materials and physical measurements

3-Acetylcoumarin was purchased from Sigma Aldrich. Sigma Aldrich provided the 3-acetylcoumarin. The solvents used were analytical grade (AR) and did not need to be purified further. Merck was the source for all of the metal salts. A Perkin Elmer Frontier FT-IR spectrophotometer was used to capture FT-IR spectra. A LECO ONH 836 analyzer was used to perform the elemental studies. A Perkin Elmer LAMBDA 950 spectrophotometer was used to produce UV-visible spectra of all the compounds. The BRUKER AMX 400 MHz spectrometer was used to record¹H NMR spectra. On a JEOL GCMATE II GC-MS, the complexes’ electrospray ionization mass spectra (ESI-MS) were recorded. A Jenway 4010 conductivity meter was used to measure molar conductivities. At room temperature, ESR spectra were obtained using a JEOL Model JES FA200 equipment operating at X-band frequency.

2.2. Synthesis of Schiff base ligand (HL)

The ligand HL was prepared by the condensation of 3-acetylcoumarin (0.005 mol, 1 g) with 2-methylbenzyl hydrazinecarbodithioate (0.005 mol, 1.06 g) in ethanol under reflux for 6 h. Around this period, the orange colour precipitate was formed. Then the solution was allowed to room temperature and the solid obtained was filtered. Finally, the residue was washed with cold ethanol and air dried. Yield: 85%, Orange colour powder, m.p.: 178 °C. Anal. Calc. for C₂₀H₁₈N₂O₂: C, 62.80; H, 4.74; N, 7.32. Found: C, 62.74; H, 4.70; N, 7.30.

2.3. Synthesis of metal complexes

2.3.1. [Co(L)(OAc)] (1)

A drop of triethylamine was added to a hot ethanolic solution of HL (1 mmol, 382 mg), then the mixture was refluxed for 3 hours with an ethanolic solution of cobalt(II) acetate tetrahydrate (1 mmol, 177 mg). The resulting brown product was filtered, ethanol-washed and air dried. Chloroform was used to recrystallize the compound. Yield: 78 %, brown solid, m.p.: 238 °C. Anal. Calc. for C₂₂H₂₀CoN₂O₄S₂: C, 52.90; H, 4.04; N, 5.61. Found: C, 52.85; H, 4.0; N, 5.60. The nickel (1 mmol), copper (1 mmol), and zinc (1 mmol) complexes were synthesized using the Schiff base ligand HL (1 mmol), as described previously.

2.3.2. [Ni(L)(OAc)] (2)

Yield: 78 %, Reddish brown solid, m.p.: 242 °C. Anal. Calc. for C₂₂H₂₀NiN₂O₄S₂: C, 52.93; H, 4.04; N, 5.60. Found: C, 52.88; H, 4.0; N, 5.58.

2.3.3. [Cu(L)(OAc)] (3)

Yield: 79 %, Dark red solid, m.p.: 240 °C. Anal. Calc. for C₂₂H₂₀CuN₂O₄S₂: C, 52.42; H, 4.0; N, 5.56. Found: C, 52.37; H, 3.96; N, 5.54.

2.3.4. [Zn(L)(OAc)] (4)

Yield: 84 %, Light yellow solid, m.p.: 244 °C. Anal. Calc. for C₂₂H₂₀ZnN₂O₄S₂: C, 52.23; H, 3.98; N, 5.54. Found: C, 52.18; H, 3.95; N, 5.53.

2.4. Computational studies

An attempt was made to better understand the ligand and complex molecular structures through computational studies in the absence of crystal structures. The geometry optimization process is done by Gaussian 09W software. The ground state geometry optimization process was carried out using hybrid density functional B3LYP integrated with the LANL2DZ basis set in the gas phase. The ground state geometries of complexes and their frontier molecular orbitals of HOMO and LUMO are visualized with GaussView 5.0 program.

2.5. Biological studies

2.5.1. Antibacterial activity

The agar well diffusion method, which involves measuring the inhibited region (mm) with a ruler and is connected to one of the standard medications, was used to study the antibacterial activity of the free ligand, its complexes, and the solvent control dimethylsulfoxide (DMSO). Using the nutrient agar medium and ampicillin as standard medications, tests were performed on Streptococcus pneumonia as Gram-positive bacteria and Escherichia coli as Gram-negative bacteria. A well was created on the agar medium and inoculated with bacteria that had been grown overnight [39]. The test solution (20 g) was pipette-filled into each well and incubated at 35 °C for 48 hours for bacteria. Each assay was carried out twice, with the average values being recorded.

2.5.2. Antioxidant activity

2.5.2.1. DPPH radical scavenging activity

Choi [40-42] evaluated the DPPH radical scavenging activity of all the compounds. To a 0.004 percent (w/v) DPPH methanolic solution, equal volumes of different concentrations (0.2, 0.4, 0.6, 0.8 and 1.0 mg/1 mL DMSO) were added and incubated at 28 °C in the dark for 30 minutes. Using a UV-Vis spectrophotometer, the absorbance of the incubated solution was measured at 517 nm. As a standard antioxidant, ascorbic acid was used. The percentage of free radicals inhibited by DPPH (I percent) was calculated using the following equation:

I percent = [(A₀(control) - A₁ (sample)/A₀ (control)] x 100

Where A₀ (control) is the absorbance of DPPH solution in the absence of the test compound and A₁ (sample) is the absorbance of the test compound. Experiments were carried out twice, and the reproducibility results were noted.

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 [43] and a band around 948 cm^-1 due to the ν(C=S) stretching frequency [44]. The ʋ(C=O) vibration of ligand and complexes was observed in the 1684-1692 cm^-1 range [45]. 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 [46]. In the case of complexes, the appearance of two distinct bands in the 1551-1570 cm^-1and 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 [47]. 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 [46]. The appearance of the new bands in the 585-596 and 457-472 cm^-1regions due to the ʋ(M-O), ʋ(M-N) vibrations, respectively, further supports the bonding of the azomethine ligand to the metal ion [48].

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 [49]. 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 [49]. 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 [52]. At 530 nm, the copper(II) complex shows a d–d transition band, indicating square planar geometry [53]. The Zn complex exhibits no absorption in the range above 500 nm, which is consistent with the d¹⁰electronic configuration of the Zn(II) ion [54].

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 [55].

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 [56].

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

3.8.2.1. 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 [61]. 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.

A series of mononuclear Schiff base cobalt(II), nickel(II), copper(II) and zinc(II) complexes involving tridentate ligand derived from 3-Acetylcoumarin and 2-methylbenzyl hydrazinecarbodithioate have been synthesized and characterized by various physico-chemical techniques. The elemental analysis and electronic spectra of the complexes designate square planar geometry. DFT calculations explore the geometry and the electronic charge transfer mechanism on the complexes. All the compounds were exposed to in-vitro antibacterial activity studies, which possess better biological properties and are comparable to the standard. Moreover, copper(II) complex 3 was obtained to be better antibacterial activity among all the test compounds. The compounds also showed better antioxidant activity in DPPH scavenging free radicals. DPPH scavenging activity results revealed that complexes are better antioxidants than the ligand.

Acknowledgements

We thank the Sophisticated Analytical Instrumentation Facility (SAIF), Indian Institute of Technology - Madras for Mass analyses and Bombay for EPR analyses. The author thanks SSN Trust for financial support under the Postdoc fellowship scheme.

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Table 1 IR spectral data of ligand and its metal complexes
Compound	ν(N-H) (cm^-1)	ν(C=S) (cm^-1)	ν(C=O) (cm^-1)	ν(C=N) (cm^-1)	ν(N-N) (cm^-1)	ν(COO)s, ν(COO)as (cm^-1)	ν(M-N) (cm^-1)	ν(M-O) (cm^-1)
Ligand (HL)	3162	948	1692	1611	1041	-	-	-
[Co(L)(OAc)] (1)	-	-	1689	1609	1003	1554, 1381	472	596
[Ni(L)(OAc)] (2)	-	-	1686	1601	1026	1551, 1368	468	591
[Cu(L)(OAc)] (3)	-	-	1684	1607	1025	1563, 1358	457	585
[Zn(L)(OAc)] (4)	-	-	1689	1603	1025	1570, 1360	460	587

Table 2 Electronic spectra, molar conductance values of the compounds
Compound^a	Absorption (nm)		d-d transition	Ʌ_m (ohm^-1 cm^-1 mol^-1)
Compound^a	π- π*	L → M	d-d transition	Ʌ_m (ohm^-1 cm^-1 mol^-1)
HL	230	-	-	-
1	240	370	520	15
2	230	360	560	16
3	250	340	530	12
4	260	380	-	19

^aCompound details are as listed in table 1

Table 3 Calculated and experimental bond length (Å) values of the complexes
Metal contacts	Calculated				Experimental (57, 58)
Metal contacts	1	2	3	4	Experimental (57, 58)
M–O	1.936	2.061	1.917	2.085	1.958
M–N	1.974	2.041	1.944	2.183	1.994
M–S	2.278	2.357	2.235	2.437	2.244
M–O (OC(O)CH₃)	1.845	1.873	1.833	2.146	-

Table 4 Electronegativity (χ), Ionization potential (IP), Electron affinity (EA), Hardness (η), Softness (S) and electrophilicity (ω) values of complexes (1, 2, 3, 4) in eV calculated by DFT/B3LYP/LANL2DZ basis set
Complexes^a	Electronegativity (χ)	Ionization potential (IP)	Electron affinity (EA)	Hardness (η)	Softness (S)	Electrophilicity (ω)
1	4.74	5.69	3.79	0.95	0.52	11.81
2	4.78	5.78	3.78	0.99	0.50	11.46
3	4.82	5.95	3.69	1.13	0.44	10.26
4	4.65	5.92	3.37	1.28	0.39	8.45

^aCompound details are as listed in table 1

Table 5 The zone of inhibition against the growth of bacteria by the well diffusion method (mm)

Samples (20 µg)		Bacterial Pathogens
		Streptococcus pneumonia		Escherichia coli
HL		11		05
1		21		17
2	18		16
3	23		19
4	20		17
Ampicillin (Antibiotic)	24		21
Control (DMSO)	-		-

^aCompound details are as listed in table 1

Scheme 1 is available in supplementary section.

Scheme01.png
Scheme 1. The reaction pathway for the synthesis of Schiff base and metal complexes

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Synthesis, characterization, biological activity of Schiff base metal complexes derived from 3-Acetylcoumarin and 2-methylbenzyl hydrazinecarbodithioate

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Abstract

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3. Results And Discussion

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