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Synthesis, Characterization and in-vitro Antimicrobial activity Studies of Co (II), Ni (II) and Cu (II) Complexes with ONSO Donor Coumarin Schiff Bases

Der Pharma Chemica
Journal for Medicinal Chemistry, Pharmaceutical Chemistry, Pharmaceutical Sciences and Computational Chemistry

ISSN: 0975-413X

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Research - Der Pharma Chemica ( 2022) Volume 14, Issue 2

Synthesis, Characterization and in-vitro Antimicrobial activity Studies of Co (II), Ni (II) and Cu (II) Complexes with ONSO Donor Coumarin Schiff Bases

Shrishila N. Unki* and Shekappa D Lamani
 
Department of Chemistry, SB Arts and KCP Science College, Vijaypura-586103. Karnataka, India
 
*Corresponding Author:
Shrishila N. Unki, Department of Chemistry, SB Arts and KCP Science College, Vijaypura-586103. Karnataka, India, Email: [email protected]

Received: 27-Jan-2022, Manuscript No. dpc-22-52528; Accepted Date: Jan 29, 2022 ; Editor assigned: 29-Jan-2022, Pre QC No. dpc-22-52528; Reviewed: 15-Feb-2022, QC No. dpc-22-52528; Revised: 21-Feb-2022, Manuscript No. dpc-22-52528; Published: 27-Feb-2022

Abstract

A series of Co(II), Ni(II) and Cu(II) complexes have been synthesized with Schiff bases derived from 3-substituted-4-amino-5-mercapto-1,2,4-triazole and 8-acetyl-7-hydroxy-4-methylcoumarin. The chelation of the complexes has been proposed in the light of analytical, spectral (IR, UV–Vis, 1H NMR, ESR and FAB-mass), magnetic and thermal studies. The measured molar conductance values indicate that, the complexes are non-electrolytic in nature. The redox behavior of the complexes was investigated with electrochemical method by using cyclic voltammetry. The Schiff bases and their metal complexes have been screened for their in vitro antibacterial (Escherichia coli, Staphylococcus aureus, Bacillus subtilis and Salmonella typhi) and antifungal activities (Candida albicans, Cladosporium and Aspergillus niger) by MIC method.

Keywords

Coumarin; Antimicrobial; IR Spectra; Schiff bases; Spectroscopic studies; Transition metal complexes

Introduction

Chromenes, especially 2-oxo-2H-chromenes (coumarins), have been extensively studied due to their commercial applications in several fields. These compounds have outstanding optical properties, including an extended spectral range, high quantum yield, superior photostability and good solubility in common solvents. Typical feature of chromene derivatives is that photophysical and spectroscopic properties can be readily modified by introduction of substituents in the chromene ring, giving themselves more flexibility to fit to various applications [1-3]. Coumarins display a remarkable array of biochemical and pharmacological actions, some of which suggest that, certain members of this group of compounds may significantly affect the function of various mammalian cellular systems [4].

Schiff bases are important class of ligands in coordination chemistry and their complexing ability containing different donor atom is widely reported. The chemistry of transition metal complexes containing heterocyclic donor continues to be of interest on account of their biological importance. There is a growing interest in the studies on the metal complexes of Schiff bases derived from triazoles and its derivatives which are biologically important ligands [5]. Schiff base metal complexes have been widely studied because of their industrial and biological applications; several derivatives of these have been used as drugs. The triazole Schiff bases constitute one of the most important classes of O, N, and S donor atoms. Triazoles and their derivatives have been proved effective bacteriocides [6], pesticides [7], fungicides [8-9] and insecticides [10-11].

A survey of the literature reveals that, no work has been carried out on the synthesis of Co(II), Ni(II) and Cu(II) complexes with newly synthesized Schiff bases derived from 3-substituted-4-amino-5-mercapto-1,2,4-triazole and 8-acetyl-7-hydroxy-4-methylcoumarin. These Schiff bases have donor sites with the OONS sequence and varied coordination abilities. This nature of the Schiff bases (Figure-1) have attracted our attention and aroused our interest in elucidating the structure of metal complexes. The Schiff bases and their metal complexes were screened for their in-vitro antimicrobial activity.

derpharmachemica-Structure

Figure 1: Structure of Schiff bases: Thiol–Thione tautomerism

Experimental

Analysis and Physical Measurements

Carbon, hydrogen and nitrogen were estimated by using Elemental Analyzer Truspec (Leco Corporation USA). The IR spectra of the Schiff bases and their Co(II), Ni(II) and Cu(II) complexes were recorded on a HITACHI-270 IR spectrophotometer in the 4000-250 cm-1 region in KBr disc. The electronic spectra of the complexes were recorded in HPLC grade DMF and DMSO solvent on a VARIAN CARY 50-BIO UV-spectrophotometer in the region of 200-1100 nm. The 1H-NMR spectra of ligands were recorded in DMSO-d6 on a BRUKER 300 MHz spectrometer at room temperature using TMS as an internal reference. The fluorescence studies of Schiff bases and their metal complexes were recorded on HITACHI F- 7000 Fluorescence Spectrophotometer (Made in Japan). The solutions of 10-3 M concentration were prepared in HPLC grade DMF and DMSO solvents and the experiment was carried out at room temperature. The electrochemistry of the metal complexes was recorded on CHI1110Aelectrochemical analyzer (Made in U.S.A) in dimethyl formamide (DMF) containing 0.05 M n-Bu4NClO4 as the supporting electrolyte. The ESR spectrum was recorded on Varian-E-4X-band EPR spectrometer and the field set is 3000 G at modulation frequency of 100 K Hz under liquid nitrogen temperature using TCNE as ‘g’ marker. FAB-Mass spectra were recorded on a JEOL SX 102/DA-6000 mass spectrometer/data system using Argon/Xenon (6KV, 10Am) as the FAB gas. The accelerating voltage was 10 KV and the spectra were recorded at room temperature and mnitrobenzyl alcohol was used as the matrix. Thermogravimetric analyses data were measured from room temperature to 1000ºC at a heating rate of 10ºC/min. The data were obtained by using TA Instruments Water LLC, New castel, Delware. USA. Model; DCS Q 20, 2009. Molar conductivity measurements were recorded on ELICO-CM-82 T Conductivity Bridge with a cell having cell constant 0.51 and magnetic moment of the complexes was carried out by using faraday balance.

Synthesis

All chemicals and solvents used were of AR grade. All metal (II) salts were used as their chlorides. 3-substituted-4-amino-5-mercapto-1,2,4- triazoles were synthesized by reported methods [12].

Synthesis of 7-Hydroxy-4-Methylcoumarin [13]

A mixture of dry resorcinol (0.2 mol) and ethylacetoacetate (0.2 mol) is cooled to 0-5ºC and conc. sulphuric acid (25 mL) is added gradually with constant shaking. The reaction mixture is then kept in a refrigerator for 24 h. and poured into crushed ice with stirring. The separated solid is filtered washed with water and recrystallized from ethanol as cream colored needles. Yield: 80%; MP: 182ºC.

Synthesis of 7-Acetoxy-4-Methyl Coumarin [14]

A mixture of 7-hydroxy-4-methyl coumarin (0.16 mol) and freshly distilled acetic anhydride (0.56 mol) was refluxed for 1.5 hr under anhydrous conditions. While the solution was hot, it was poured to crushed ice and the separated product was filtered and washed with water. It was crystallized from methanol as colorless needles. Purity of the compound was established by a single spot in TLC and the melting point was in agreement with the literature value (150ºC); m.p 152-154ºC.

Synthesis of 8-Acetyl-7-Hydroxy-4-Methyl Coumarin [14]

A mixture of 7-acetoxy-4-methyl coumarin (0.01 mol) and anhydrous AlCl3 (0.03 mol) was heated under anhydrous conditions in an oil bath at 125oC and the temperature was raised during 2.5 hr period to 170ºC. To the reaction mixture, crushed ice and dilute HCl were added with stirring and the mixture was left for 2-3 hr in order to decompose the complex. The separated product was filtered, washed with water and recrystallized from ethanol. A single spot in TLC indicated the purity of the compound. m.p. 159-160ºC, yield 68%.

Synthesis of Schiff Bases [I-IV]

The Schiff bases were synthesized by the condensation of 3-substituted-4-amino-5-mercapto-1,2,4-triazole (0.01 mol) and 8-Acetyl-7-Hydroxy-4- Methylcoumarin (0.01 mol), dissolved in 30 ml alcoholic medium containing few drops of concentrated HCl. The resulting mixture was refluxed for 3-4 h. The solid product separated on evaporation of the solvent was filtered, washed with alcohol and then finely recrystalized from EtOH.

Synthesis of Co (II), Ni (II) and Cu (II) Complexes

An alcoholic solution (30ml) of Schiff bases (I-IV) 1 mmol was refluxed with 1 mmol of CoCl2.6H2O/NiCl2.6H2O/CuCl2.2H2O in 30ml ethanol solution on steam bath for 1h. Then, to the reaction mixture 2 mmol of sodium acetate was added and refluxtion was continued for 3h. The separated complex was filtered, washed thoroughly with water, Ethanol, Ether and finally dried in vacuum over fused CaCl2

. Pharmacology

In vitro antibacterial and antifungal assay

The biological activities of Synthesized Schiff bases and their Co (II), Ni (II) and Cu(II) complexes have been studied for their antibacterial and antifungal activities by agar and potato dextrose agar diffusion [15-16] method respectively. The antibacterial and antifungal activities were done at 25, 50 and 100 μg/mL concentrations in DMSO solvent by using four bacteria (Escherichia coli, Staphilococcus aureus, Bascillus subtilis and Salmonella typhi) and three fungi (Aspergillus niger, Candida albicans and cladosporium) by the MIC method [32]. These bacterial strains were incubated for 24h at 37ºC and fungal strains were incubated for 48h at 37ºC. Standard antibacterial and antifungal drug (Gentamycine) and antifungal drug (Flucanazole) were used for comparison under similar conditions.

Result and Discussions

All the Co(II), Ni(II) and Cu(II) complexes were stable at room temperature, non-hygroscopic, insoluble in water and many common organic solvents but soluble in DMF and DMSO and infusible at high temperature. All the metal complexes are thought to be polymeric in nature. The elemental analysis results for the Co (II), Ni (II) and Cu (II) complexes agree with the calculated values showing that the complexes have 1:1 stoichiometry of the type ML.2H2O, where ‘L’ stands for a deprotonated ligand. The observed molar conductances of the complexes in DMF for 10-3 M solutions at room temperature were consistent with the non-electrolytic nature (Table 1).

Table 1: Elemental Analyses of Co (II), Ni (II) and Cu (II) Complexes along with Molar Conductance and Magnetic Moment Data

Comp.
No.
Empirical formula M% C% N% S% Molar conductance Ohm-1 cm-2 mole-1 Mag. Moments (µeff BM)
Obsd Calcd Obsd. Calcd Obsd. Calcd Obsd. Calcd
1 Co(C14H10N4O3S).2H2O 14.36 14.42 41.05 41.07 13.60 13.69 7.80 7.82 19.22 4.61
2 Co(C15H12N4O3S).2H2O 13.90 13.94 42.51 42.55 13.16 13.23 7.50 7.56 22.06 4.76
3 Co(C16H14N4O3S).2H2O 13.48 13.50 43.90 43.93 12.77 12.81 7.28 7.32 24.52 4.90
4 Co(C17H16N4O3S).2H2O 13.00 13.08 45.21 45.23 12.33 12.41 7.03 7.09 17.28 4.96
5 Ni(C14H10N4O3S).2H2O 14.11 14.21 41.15 41.17 13.70 13.72 7.79 7.84 20.01 3.11
6 Ni(C15H12N4O3S).2H2O 13.71 13.74 42.62 42.65 13.22 13.27 7.52 7.58 23.07 3.20
7 Ni(C16H14N4O3S).2H2O 13.26 13.30 44.00 44.03 12.81 12.84 7.26 7.33 25.85 3.09
8 NI(C17H16N4O3S).2H2O 12.82 12.88 45.31 45.33 12.40 12.44 7.10 7.11 18.03 3.29
9 Cu(C14H10N4O3S).2H2O 15.22 15.25 40.65 40.67 13.53 13.55 7.72 7.74 21.14 1.77
10 Cu(C15H12N4O3S).2H2O 14.70 14.45 42.13 42.15 13.10 13.11 7.45 7.49 23.43 1.76
11 Cu(C16H14N4O3S).2H2O 14.23 14.25 43.52 43.53 12.65 12.69 7.22 7.25 25.66 1.79
12 Cu(C17H16N4O3S).2H2O 13.82 13.84 44.80 44.83 12.01 12.03 7.01 7.03 18.46 1.73

IR Spectra

In order to study the binding mode of Schiff bases to metal ion in the complexes, IR spectrum of the Schiff bases were compared with the spectra of the metal complexes. The IR spectra of the Schiff bases (Table-2) display a broad band in the range 3109-3120 cm-1, which is ascribed to the stretching vibration of -NH of the pyrrolic nitrogen atom. Although Schiff bases in solution exists in two tautomeric conformations exhibiting thiol ↔ thione isomerism involving –N=C-SH and –NH=C-S groups in a thiol-thione equilibrium [17-18], the IR spectrum of this ligands provides evidence of the thione form in the solid state along with little of the thiol structure, where, in addition to the weak band observed at 2440-2482 cm-1, which is ascribed to the stretching vibration of SH [19-21]. The IR spectra of Schiff bases exhibited a broad band at 3251-3271 cm-1, strong band at 1714-1720, 1600-1610 and 1271-1296 cm-1 assigned to H-bonded -OH stretching, ν(C=O) lactonic carbonyl, ν(C=N) and phenolic ν(C-O) vibrations respectively. A medium band around 1055 cm-1 is characterized for ν (O-C-O).

Table 2: The Important Infrared Frequencies (in cm -1 ) Schiff Bases (I-IV)

Schiff bases
No.
Empirical Formula H-bonded –OH Stretching ν (NH) Lactonil
ν (C=O)
ν (C=N) ν (SH) ν (C=S) Phenolic
ν (C-O)
I C14H12N4O3S 3251 3120 1720 1600 2458 1107 1269
II C15H14N4O3S 3269 3118 1715 1608 2440 1040 1282
III C16H16N4O3S 3271 3113 1714 1610 2480 1075 1289
IV C17H18N4O3S 3258 3109 1719 1605 2482 1182 1296

In comparison with the spectra of the Schiff bases, all the metal complexes exhibit downward shift 8-20 cm-1 of ν(C=N) indicating the participation of azomethine nitrogen in the coordination to the metal ion (Table-3).

Table 3: Important Infrared Frequencies (in cm-1) of Metal Complexes

Complex
No.
ν(OH) ν(C=O) ν(C=N) Phenolic
ν(C-O)
ν(M-N) ν(M-O)
1 3432 1705 1592 1325 468 489
2 3440 1700 1600 1333 473 467
3 3429 1704 1599 1342 463 471
4 3412 1708 1590 1346 468 484
5 3422 1706 1595 1344 474 487
6 3435 1699 1589 1358 465 481
7 3436 1695 1593 1346 463 484
8 3408 1702 1602 1348 484 456
9 3425 1706 1587 1352 470 487
10 3408 1703 1590 1345 481 459
11 3417 1700 1600 1349 446 486
12 3432 1709 1590 1358 443 467

The absorption bands associated with the ν(OH) of the phenolic groups (observed at 3251-3271 cm-1 in the free ligands) disappeared in the IR spectra of the metal complexes, indicating that, the loss of phenolic proton on complexation and formation of metal-oxygen bonds. The high intensity band due to phenolic ν(C-O) appeared in the region at 1269-1296 cm-1 in the Schiff bases appeared as a medium to high intensity band in the 1325-1358 cm-1 region in the complexes and supports the suggestion that the ligands coordinate through their deprotonated form.

The most notable change in the Schiff bases spectral features when coordinated to metal ion is the lower frequencies about 10-20 cm-1 in lactone ν(C=O), suggesting that, the metal is coordinated to the lactone oxygen [22]. This further supported by downward shift in ν (O-C-O) of the coumarin ring [23]. The presence of coordinated water was suggested by the very broad absorption band around 3408-3440 cm-1 in the IR spectra of complexes. Bands at the region 760-800 and 705-720 cm-1 may be attributed to rocking and wagging modes of coordinated water [24-26]. New bands at 500-400 cm-1 are tentatively assigned to ν(M-O), ν(M-N) and ν(M-S) (metal–ligand) stretching bands. On the basis of IR data, it is concluded that, all the metal ions are coordinated to the azomethine nitrogen, phenolic oxygen, sulphur atom and lactone oxygen.

Thus, the IR spectral data results provide strong evidences for the complexation of the tetradentate Schiff bases and also suggests that, the complexes are exist in the solid state as polymeric structure with bonding of metal likely to both the deprotonated phenolic oxygen and lactose carbonyl oxygen.

1H NMR Spectral Studies of Schiff Bases

The spectral data of 1H NMR of all the Schiff bases are given in Table-4. In the 1H NMR spectra of Schiff bases II exhibit the singlet at 11.97 ppm (s, 1H) is ascribed to NH and a sharp signal at 10.09 ppm (s, 1H) is attributed to OH protons respectively. As multiplet, the aromatic ring protons are observed in the range 6.9-7.3 ppm (m, 4H). In addition to these signals, a sharp singlet at 3.45 ppm (s, 1H) is due to SH protons. The singlet observed at 2.48 ppm (s, 3H) and 2.85 ppm (s, 3H) are attributed to the coumarin methyl protons and the triplet observed at 2.70(t, 3H) are due to triazole methyl protons (Table-4).

Table 4: The Important 1H NMR Data of Schiff Bases I-IV.

  Schiff base   1H NMR (d6-DMSO) (ppm)
I 12.09 (s, 1H, NH), 10.01 (s, 1H, OH), 6.7-7.2 (m, 4H, Ar-H), 3.42 (s, 1H, SH), 2.46 (s, 3H, CH3), 2.83 (s, 3H, CH3).
II 11.97 (s, 1H, NH), 10.09 (s, 1H, OH), 6.9-7.3 (m, 4H, Ar-H), 3.45 (s, 1H, SH), 2.48 (s, 3H, CH3), 2.85 (s, 3H, CH3), 2.70 (s, 3H, CH3 triazole).
III 12.02 (s, 1H, NH), 10.10 (s, 1H, OH), 6.5-7.1 (m, 4H, Ar-H), 3.43 (s, 1H, SH), 2.49 (s, 3H, CH3), 2.81 (s, 3H, CH3), 2.10 (q, 2H, CH2), 2.72 (s, 3H, CH3 triazole).
IV 12.96 (s, 1H, NH), 10.12 (s, 1H, OH), 6.6-7.0 (m, 4H, Ar-H), 3.41 (s, 1H, SH), 2.48 (s, 3H, CH3), 2.82 (s, 3H, CH3), 2.12 (t, 2H, CH2), 1.90 (m, 2H, CH2), 2.70 (t, 3H, CH3 triazole).

GC-Mass spectral studies of Schiff base

The GC-mass spectrum of the Schiff base I (Figure-2) shows a molecular ion (M+) peak at m/z 316 which is equivalent to its molecular weight, which confirms the proposed formula.

derpharmachemica-Spectrum

Figure 2: GC-Mass Spectrum of Schiff base-I

Electronic Absorption Spectral Studies

The electronic absorption spectra of the complexes in DMF were recorded at room. The cobalt(II) complexes exhibited two distinct absorption in the region 9880-9997 cm-1 and 17760-20660 cm-1 corresponding to 4T1g(F) → 4T2g(F) (ν1) and 4T1g (F) → 4T1g (P) (ν3) transitions respectively which suggests an high spin octahedral geometry around the cobalt(II) ion [27]. The ν2 band that involves a two-electron transition is not observed in spectra because of its proximity to strong ν3 transition.

The Ni(II) complex (6), exhibited three bands at 10215, 15620 and 26112 cm-1 attributed to the 3A2g3T2g (ν1); 3A2g 3T1g (F) (ν2) and 3A2g 3T1g (P) (ν3) transitions respectively, which indicate octahedral geometry around Ni(II) ion. The value of ν2/ ν1 is found to be around 1.529 and the μeff value is around 3.179 which is within the range of 2.8-3.5 BM, suggesting the octahedral environment. The values of the nephelauxetic parameters, β, indicate low covalent character of the metal-ligand σ bonds [28]. Hence the ligand field parameters correlate the electronic spectral and magnetic properties. The ligand field parameters calculated for the Co (II) complexes are given in Table-5.

Table 5: Ligand Field Parameters of Ni (II) Complexes

Complex
No.
Transitions (cm-1) ν2­ Cald.
cm-1
  Dq cm-1   B1 cm-1 % Distortion   ν1/ ν2   LSFE µeff
Cald. BM
  β   βº %
ν1 ν2 ν3
5
6
7
8
10239
10215
10231
10226
15615
15620
15606
15631
26108
26112
26123
26110
16245.00
16225.78
16242.80
16234.79
1023.9
1021.5
1023.1
1022.6
775.76
779.51
778.18
777.78
3.881
3.733
3.921
3.719
1.525
1.529
1.525
1.529
35.105
35.023
35.078
35.061
3.178
3.179
3.179
3.179
0.735
0.738
0.737
0.737
26.537
26.182
26.308
26.346

The electronic spectra of Cu (II) complexes showed low intensity broad band around 14380-14423 cm-1 is assignable to 2T2g ← 2Eg transition. Another high intensity band at 25477-25513 cm-1 is due to symmetry forbidden ligand → metal charge transfer. On the basis of electronic spectra distorted octahedral geometry around Cu (II) ion is suggested [29]. On the basis of electronic spectra, distorted octahedral geometry around Cu(II) ion is suggested.

Magnetic Studies

The magnetic moments obtained at room temperature are listed in Table-1. The magnetic measurement for Co (II) complexes showed magnetic moment value 4.6-5.0 which is well within the range of 4.3-5.2 BM and Ni(II) complexes showed the magnetic moment value 3.0-3.3 within the range of 2.8-3.5 BM suggesting consistency with their octahedral environment [30, 31]. The Cu (II) complexes showed magnetic moment 1.74-1.79 BM, is slightly higher than the spin-only value 1.73 BM expected for one unpaired electron, which offers possibility of an octahedral geometry [32].

ESR Studies

The ESR spectral studies of Cu (II) complex provide information of the metal ion environment. The ESR spectrum of the Cu (II) complex was recorded in DMSO at room temperature (RT) and depicted in Figure-3. The Cu (II) complex (10) exhibits the g║ value of 2.0513 and g┴ value of 2.0248. These values g║>g┴ indicate that, the unpaired electron lies predominantly in the dx2 –y2 orbital [33]. The trend g║>g┴>2.0023 observed for the complexes indicate that, the unpaired electron is localized in the dx2-y2 orbital of the Cu(II) ion and are characteristic of the axial symmetry. Thus, the results suggested that, the Cu (II) complex (10) possess distorted octahedral geometry.

derpharmachemica-ESR

Figure 3: ESR Spectrum of Cu (II) Complex (10)

Thermal Studies

The thermal behavior of Co (II), Ni(II) and Cu(II) complexes has been studied as a function of temperature. The thermal behavior of all the complexes is almost same. Hence, only the representative TG/DTG of Co (II) (2), Ni (II) (6) and Cu (II) (10) complexes have been discussed here. Their stepwise thermal degradation data are given in Table-6.

Table 6: Thermogravimetric data of Co (II), Ni (II) and Cu (II) complexes

Empirical Formula Decomposition
temperature
%Weight   loss Metal Oxide % Inference
oC Obsd. Calcd. Obsd. Calcd.
Co(C14H10N4O3S).2H2O 190-230
305-330
350-410
8.51
30.26
45.75
8.60
30.36
45.79
17.73 17.80 Loss of coordinated water molecules
Loss of 1,2,4-triazole moieties
Loss of acetyl coumarin moieties
Ni(C14H10N4O3S).2H2O 195-236
325-335
400–430
8.53
30.08
47.86
8.72
30.19
47.95
17.53 17.59 Loss of coordinated water molecules
Loss of 1,2,4-triazole moieties
Loss of acetyl coumarin moieties
Cu(C14H10N4O3S).2H2O 200-235
290-330
410–450
8.43
29.74
47.30
8.46
29.71
47.35
17.79 17.80 Loss of coordinated water molecules
Loss of 1,2,4-triazole moieties
Loss of acetyl coumarin moieties

The TG and DTG curves of Ni (C14H10N4O3S).2H2O are shown in Figure-4. The DTG curve of this complex shows three stages of decomposition within the temperature range (180-650ºC). The first step of decomposition within the temperature range (180-250ºC) corresponds to the loss of coordinated water molecule with mass loss of 8.53% (calcd. 8.72%). The second step (250-350ºC) corresponds to the loss of 1,2,4-triazole moieties with mass loss of 30.08% (calcd. 30.19%). The third step (400-560ºC) corresponds to the loss of acetylcoumarin (mass loss 47.86%; calcd. 47.95%). Finally the metal complexes decompose gradually with the formation of metal oxide above 560ºC (mass loss 17.53%).

Thus, the TG and DTG provide the useful information about the coordination of water molecules to the metal ion and the stability of the complexes.

Electrochemistry

Electrochemical properties of the complexes were studied on a CHI1110A-Electrochemical analyzer in N,N-dimethyl formamide (DMF) containing 0.05 M n-Bu4NClO4 as the supporting electrolyte. A cyclic voltammogram of Cu (C14H10N4O3S).2H2O radical displays a reduction peak at Epc= 0.447V with a corresponding oxidation peak (Cu (I) radical) at Epa= 0.1. The peak separation of this couple (ΔEp) is 0.347V at 0.05V and increases with scan rate. The most significant feature of the Cu (II) complex is the Cu(II)/Cu(I) couple. The difference between forward and backward peak potentials can provide a rough evaluation of the degree of the reversibility of one electron transfer reaction. The analyses of cyclic voltametric responses with the scan rate varying 50 to 250 mV/s gives the evidence for quasi-reversible one electron oxidation state. The ratio of cathodic to anodic peak height was less than one. However, the peak current increases with the increase of the square root of the scan rates. This establishes the electrode process as diffusion controlled [33,34] (Figure 5,6).

derpharmachemica-Thermogravimetric

Figure 4: Thermogravimetric (TG/DTG) curves of Ni(C14H10N4O3S).2H2O complex

derpharmachemica-voltammogram

Figure 5: Cyclic voltammogram of Cu(C14H10N4O3S).2H2O

Pharmacology Results

The Schiff bases and their metal complexes were evaluated for antimicrobial activity. The obtained results are systematized in Table 7 & 8. The antibacterial studies inferred that, Both the Schiff bases and their Co(II), Ni(II) and Cu(II) complexes showed high antibacterial and antifungal activity against all the bacterial strains. All the metal complexes possess higher antifungal activity than the Schiff bases. This higher activity of the metal complexes compared to Schiff bases is may be due to the change in structure due to coordination, which make the metal complexes act as more powerful and potent bactereostatic agents, thus inhibiting the growth of the microorganisms.

The minimum inhibitory concentration (MIC) of some selected compounds, which showed significant activity against selected bacterial and fungi strains were studied. The results indicated that these compounds were most active in inhibiting the growth of the tested organisms at 10 mgmL-1 (Table 9).

Table 7: Anti-bacterial and Anti-fungal results of Schiff bases

Compd. Conc
.(μg ml-1)
Antibacterial activity (Zone of inhibition in %) Antifungal activity (Zone of inhibition in %)
E. coli S. aureus B. subtilis S. typhi C. albicans Cladosporium A Niger
I 25
50
100
37
47
55
-
20
31
39
41
53
29
49
58
52
58
67
-
33
44
52
67
69
II 25
50
100
38
52
59
-
24
30
37
47
55
31
47
52
53
58
66
-
31
42
57
62
69
III 25
50
100
33
58
60
-
23
33
34
57
59
33
46
59
55
65
68
-
39
41
55
66
68
IV 25
50
100
37
44
58
-
22
31
33
46
57
31
43
58
53
61
63
-
37
47
66
67
67
Gentamycine 25
50
100
81
85
86
62
73
75
77
79
81
73
78
80
- - -
Flucanazole 25
50
100
- - - - 85
93
95
78
83
88
88
89
89

Table 8: Anti-bacterial results of Co(II) , Ni(II) and Cu(II) complexes (1-12)

Compd Conc.
(μg ml-1)
Antibacterial activity (Zone of inhibition in %) Antifungal activity (Zone of inhibition in %)
E. coli S. aureus B. subtilis S. typhi C. albicans Cladosporium A Niger
1 25
50
100
58
67
77
44
63
65
47
50
61
58
66
72
57
61
69
66
66
69
62
64
74
2 25
50
100
62
69
80
41
62
67
52
57
60
63
67
74
53
66
70
50
52
57
69
71
71
3 25
50
100
72
77
81
43
66
69
63
69
72
62
68
70
71
78
82
77
78
80
65
67
71
4 25
50
100
68
78
82
40
62
65
64
67
74
69
72
78
47
62
68
67
71
78
67
69
71
5 25
50
100
64
71
89
44
64
68
35
46
51
45
56
59
34
45
67
-
35
51
57
69
75
6 25
50
100
67
71
80
46
61
68
66
68
71
66
69
71
66
71
78
58
60
65
60
73
79
7 25
50
100
71
80
82
39
68
65
64
71
75
46
52
53
63
71
76
66
71
78
55
69
71
8 25
50
100
74
70
79
45
62
69
35
44
59
44
52
59
59
66
69
66
62
67
57
68
74
9 25
50
100
68
77
79
41
63
69
39
48
57
-
36
52
55
67
78
45
55
58
69
73
78
10 25
50
100
64
66
73
47
60
65
65
69
71
66
71
77
71
77
79
47
59
61
60
69
71
11 25
50
100
72
77
81
47
63
71
61
65
69
62
66
68
68
69
73
48
58
51
56
69
71
12 25
50
100
67
71
75
49
65
70
65
68
73
47
52
55
45
56
68
35
41
57
65
76
79
Gentamycine 25
50
100
81
85
86
62
73
75
77
79
81
73
78
80
- - -
Flucanazole 25
50
100
- - - - 85
93
95
78
83
88
88
89
89

Table 9: Results of Minimum inhibitory concentration (μg/ml)

Compound E. coli S. aureus B. subtilis S. typhi C. albicans Cladosporium A Niger
I 15 - 10 25 10 10 -
II 25 10 10 - - 25 25
III 10 - 10 - 25 - -
IV 10 10 25 25 25 15 25
1 25 10 10 25 - 25 -
2 25 10 - 10 15 25 -
3 10 - 25 10 10 - 25
4 10 10 10 25 10 10 10
5 10 15 10 25 - 25 25
6 - 25 25 25 - - 15
7 25 - - 10 25 - -
8 - - - 15 25 25 25
9 10 15 10 25 - 25 25
10 15 10 10 25 - - -
11 10 10 10 25 - 10 25
12 25 10 25 10 25 10 25

Conclusions

The Co(II), Ni(II) and Cu(II) complexes of Schiff bases with 3-substituted-4-amino-5-mercapto-1,2,4-triazole and 8-acetyl-7-hydroxy-4-methylcoumarin were prepared and characterized using different analytical techniques. The synthesized Schiff bases act as tetradentate Schiff bases. The metals are coordinated to azomethine nitrogen, lactonyl oxygen, Phenolic oxygen and sulphur atom. Suggest that, the complexes were polymeric in nature. The electronic spectral data and magnetic measurements suggest that Co (II) and Ni(II) complexes are Octahedral, while Cu(II) complex has distorted octahedral geometry. ESR spectrum of Cu (II) complex also reveals that it is distorted. TG/DTG analysis indicates the presence of coordinated water molecule in the complexes. The antimicrobial studies reveal that the complexes show higher activity than the Schiff bases.

Acknowledgement

The authors are grateful to the Research Center in Chemistry, Affiliated to Rani Channamma University, Belagavi, Principal, Department of Chemistry, SB Arts and KCP Science College and Management BLDE Associon’s, Vijaypura-586103 for the facilities.

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