Some mixed-ligand complexes of Zn(II) and Ni(II) derived from the sodium salt of
The chemistry of transition metal complexes with mixed ligands is an area of current interest due to their structural diversity and usefulness. They have been reported to possess properties such as electrical conductivity [
Inflammation is recognized as a defensive response by the body’s vascular tissue to different external inflammatory stimuli, in which the body induces physiological adaptation to minimize tissue damage and to remove the extraneous variable [
Various synthesized zinc and nickel dithiocarbamate based compounds have been reported as potent antibacterial, antifungal, and anticancer drugs and also as imaging agents [
Nickel(II) chloride hexahydrate, zinc(II) sulphate heptahydrate, carbon disulfide, bovine serum albumin, FeSO4·7H2O, 2,2-diphenyl-1-picrylhydrazyl (DPPH), phosphate buffer, 1,10-phenanthroline, benzoic acid,
The percentages of C, H, N, and S were determined by Elementar, Vario EL Cube, set up for CHNS analysis (InnoVenton, NMMU). UV-Vis spectra were obtained on a Perkin Elmer Lambda 40 UV-Vis spectrophotometer. FTIR spectra were recorded in the range of 4000–400 cm−1 on a Bruker alpha-P FTIR spectrophotometer. Magnetic susceptibilities were measured on a Johnson Matthey magnetic susceptibility balance and diamagnetic corrections were calculated using Pascal’s constant. Conductivity measurements were conducted using MC-1, Mark V conductivity meter with a cell constant of 1.0.
Sodium
Sodium
NiMDBz: (Yield 0.35 g, 70%). Elemental analysis for [NiC15H13NS2O2]·H2O (380.12): C, 47.40; H, 3.99; N, 3.57; S, 16.72. Found: C, 47.39; H, 3.97; N, 3.68; S, 16.87. FTIR:
ZnMDBz: (Yield 0.42 g, 82%). Elemental analysis for [ZnC15H13NS2O2]·2H2O (404.81): C, 44.29; H, 4.53; N, 3.38; S, 15.80. Found: C, 44.50; H, 4.23; N, 3.46; S, 15.84. FTIR:
The ligand, NaED (0.5 g, 2.3 mmol), was dissolved in ethanol (10 mL) and mixed with a solution of benzoic acid (0.28 g, 2.3 mmol) in 5 mL of ethanol. The mixture was stirred for 5 min. To the resultant solution, 5 mL of ethanol solution of 2.3 mmol was added (ZnSO4·7H2O and NiCl2·6H2O) and stirred at room temperature for 3 h. The precipitate obtained was filtered and stored under vacuum.
NiEDBz: (Yield 0.32 g, 64%). Elemental analysis for [NiC16H15NS2O2]·2H2O (412.15): C, 46.63; H, 4.65; N, 3.40; S, 15.56. Found: C, 46.58; H, 4.56; N, 3.21; S, 15.50. FTIR:
ZnEDBz: (Yield: 0.42 g, 82%). Elemental analysis for [ZnC16H15NS2O2]·2H2O (418.84): C, 45.88; H, 4.57; N, 3.34; S, 15.31. Found: C, 45.78; H, 4.52; N, 3.30; S, 15.53. FTIR:
Quantum chemical calculations were carried out on the ligands (NaMD, NaED, and Bz) and their metal complexes (NiMDBz, NiEDBz, ZnMDBz, and ZnEDBz)
The 6-31+G(d,p) basis set was used for C, H, N, O, and S atoms, while the metal ions were described by the LANL2DZ relativistic pseudopotential. The LANL2DZ relativistic pseudopotential has been found to be reliable for quantum chemical studies on transition metal complexes [
Geometry, electronic, and thermodynamic parameters were obtained from the optimized geometries. The frontier molecular orbital energies, the energies of the highest occupied and the lowest unoccupied molecular orbitals (
According to Scheme
The assay was carried out on the ligand and its metal(II) complexes
A disc application technique was employed
Bovine serum albumin denaturation assay is used to assess the antidenaturation/anti-inflammatory effect of compounds. The study was done following the method of Mizushima and Kobayashi [
2,2-Diphenyl-1-picrylhydrazyl (DPPH) is a stable free radical that has been widely used as a tool to estimate the free-radical scavenging activity of antioxidants. The reduction capacity of the DPPH radical was determined by the decrease of absorbance induced by antioxidants, according to Brands-williams et al. [
The ferrous ion-chelating ability was determined by the standard colorimetric method [
Treatment of the respective metal salts (nickel(II) chloride hexahydrate and zinc(II) sulphate heptahydrate) with a mixture of benzoic acid and the corresponding dithiocarbamate ligands in ethanol (1 : 1 : 1 ratio) afforded different precipitates of the mixed-ligand complexes. The resulting complexes are colored solids, soluble in DMF and DMSO, and are partially soluble in common organic solvents. They are stable at room temperature, and their geometry was elucidated based on their elemental and spectral studies which were found to be in agreement with the proposed structure of the metal complexes.
The ultraviolet bands for the Ni(II) and Zn(II) MDBz mixed-ligand complexes were observed around 46950 (
The visible spectra for the zinc complexes of MDBz and EDBz series showed no d-d transitions but metal-to-ligand charge transfer transition at 24940 cm−1 for ZnMDB and at 24960 cm−1 for ZnEDBz [
The magnetic susceptibility measurement was carried out for the metal complexes at room temperature, and the diamagnetic corrections were calculated using Pascal’s constant. The ZnEDBz and ZnMDBz complexes were found to be diamagnetic. This is consistent with Zn(II) complexes of d10 system which has all the 3d electrons paired [
The conductivity measurements were carried out in DMSO at a concentration of 10 mmol and recorded as Ω−1cm2mol−1. The complexes showed values between 5.16 and 19.9 Ω−1cm2mol−1. Values below 60 Ω−1cm2mol−1 are expected for nonelectrolytic complexes, while, for 1 : 1 electrolytes, a value between 60 and 90 Ω−1cm2mol−1 is expected [
The infrared spectra of the complexes were measured from 400 to 4000 cm−1. The stretching vibration bands due to the O-H group of water of crystallization molecules in the complexes appeared between 3478 and 3577 cm−1. The C=O stretching vibration bands of the coordinated carbonyl group of the benzoic acid were observed as sharp bands between 1556 and 1578 cm−1 [
The optimized structures of the studied ligands (MD, ED, and Bz) and their Ni(II) and Zn(II) complexes are displayed alongside their HOMO and LUMO graphics in Figures
Optimized structures and HOMO and LUMO graphics of MD, ED, and Bz at B3LYP/6-31+G(d,p).
Optimized structures and HOMO and LUMO surfaces of NiMDBz, NiEDBz, ZnMDBz, and ZnEDBz at B3LYP/6-31+G(d,p)/LANL2DZ.
Selected bond lengths, bond angles, and dihedral angles of the studied ligands and their metals complexes are presented in Table
Selected geometry parameters: bond lengths (
Geometry parameter | NiMDBz | NiEDBz | ZnMDBz | ZnEDBz | MD | ED | Bz |
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Ni-S1/Zn-S1 | 2.253 | 2.289 | 2.456 | 2.454 | — | — | |
Ni-S2/Zn-S2 | 2.256 | 2.291 | 2.469 | 2.466 | — | — | |
Average Ni-S/Zn-S | 2.254 | 2.290 | 2.462 | 2.460 | — | — | |
Ni-O1/Zn-O1 | 1.946 | 1.964 | 2.093 | 2.092 | — | — | |
Ni-O2/Zn-O2 | 1.952 | 1.969 | 2.093 | 2.094 | — | — | |
Average Ni-O/Zn-O | 1.949 | 1.966 | 2.093 | 2.093 | — | — | |
C1-C2 | 1.479 | 1.463 | 1.470 | 1.470 | — | — | 1.552 |
C2-O1 | 1.282 | 1.310 | 1.307 | 1.307 | — | — | 1.260 |
C2-O2 | 1.281 | 1.309 | 1.307 | 1.307 | — | — | 1.260 |
C3-S1 | 1.726 | 1.778 | 1.794 | 1.796 | 1.719 | 1.717 | — |
C3-S2 | 1.727 | 1.784 | 1.794 | 1.799 | 1.719 | 1.722 | — |
C3-N | 1.341 | 1.338 | 1.341 | 1.342 | 1.395 | 1.398 | — |
C4-N | 1.471 | 1.495 | 1.485 | 1.497 | 1.463 | 1.469 | — |
C5-N | 1.445 | 1.454 | 1.456 | 1.456 | 1.434 | 1.434 | — |
S1-(Ni/Zn)-S2 | 78.98 | 80.05 | 77.78 | 77.75 | — | — | — |
O1-(Ni/Zn)-O2 | 67.87 | 68.23 | 64.47 | 64.47 | — | — | — |
S1-C3-S2 | 112.31 | 111.50 | 119.00 | 118.36 | 125.08 | 124.39 | — |
O1-C2-O2 | 116.20 | 114.78 | 117.32 | 117.32 | — | — | 129.17 |
O1-(Ni/Zn)-S1-C3 |
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133.63 | — | — | — |
O1-(Ni/Zn)-S2-C3 | 178.11 |
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135.75 |
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O2-(Ni/Zn)-S1-C3 | 179.50 | 179.83 | 133.96 |
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— | — | — |
O2-(Ni/Zn)-S2-C3 |
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179.77 |
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135.52 | — | — | — |
Common bonds such as C3-S1, C3-S2, C3-N, and C4-N in MD and ED ligands differ only by ±0.002–0.003 Å, being generally longer in ED. This is due to the slightly better electron-donating ability of ethyl group than of methyl group, which increases the electron density around these bonds in ED as compared to MD. The C5-N bond in the ligands seems to be invariant with the change in alkyl chain. Bonds such as C1-C2, C2-O1, C2-O2, C3-S1, C3-S2, C3-N, C4-N, and C5-N in the ligands appear to be slightly shorter or longer upon complexation with metal ions. This is evidence of redistribution of electron density around these bonds after complexation. The bond angles S1-M-S2 and O1-M-O2 are not significantly affected by the change in alkyl substituent on the ligand from -CH3 (MD) to -CH2CH3 (ED). The dihedral angles around the central metal ions in the studied complexes clearly revealed the planar geometry of Ni(II) ions and distorted tetrahedral geometry of Zn(II) ions in their respective complexes.
Some electronic and thermodynamic parameters were calculated for the metal complexes and the results are presented in Table
Some electronic and thermodynamic parameters of NiMDBz, NiEDBz, ZnMDBz, and ZnEDBz
Parameters | NiMDBz | NiEDBz | ZnMDBz | ZnEDBz |
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3.81 | 3.66 | 4.98 | 4.97 |
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6.14 | 6.04 | 6.39 | 6.37 |
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2.33 | 2.33 | 1.41 | 1.40 |
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4.24 | 4.18 | 3.90 | 3.88 |
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1.91 | 1.86 | 2.50 | 2.48 |
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4.14 | 4.31 | 3.66 | 3.73 |
BE (kcal/mol) |
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498.58 |
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394.52 |
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The growth of inhibition zones after incubation is presented in Table
A summary of the antimicrobial activities of the mixed-ligand complexes.
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Streptomycin |
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R | R |
Fluconazole | — | — | — | — | — | — |
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DMSO | R | R | R | R | R | R | R | R |
Values represent the average of three replications. Streptomycin and fluconazole were used as standard for the antibacterial and antifungal evaluation, respectively. “R” denotes “resistant.”
Histogram presentation of the antimicrobial properties of the mixed-ligand complexes.
Finally, the antimicrobial activity of the mixed-ligand complexes may be attributed to the fact that dithiocarbamates and benzoates are known to possess antimicrobial property; hence, their structural combination gave rise to heteroleptic metal complexes with improved antimicrobial character. Also, the presence of metal ions in the compound increased toxicity of the ligands towards the microbes [
The variation in the effectiveness of different compounds against different organisms depends either on the impermeability of the cells of the microbes or on differences in ribosome of microbial cells [
The results of denaturation studies presented in Table
Antidenaturation (anti-inflammatory activities) results.
Samples | Concentration ( |
% inhibition | Mean ± SD |
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Control | — | — |
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Diclofenac sodium | 100 | 83.39 |
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200 | 73.75 |
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500 | 67.44 |
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ZnEDBz | 100 | 67.59 |
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200 | 60.13 |
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500 | 59.80 |
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NiEDBz | 100 | 72.43 |
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200 | 68.11 |
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300 | 62.21 |
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ZnMDBz | 100 | 43.19 |
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200 | 41.86 |
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500 | 39.20 |
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NiMDBz | 100 | 57.81 |
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200 | 54.15 |
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500 | 49.37 |
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NiEDBz was found to have the highest percentage inhibition of 72.43% of all the test compounds while ZnMDBz showed the lowest inhibition property of 39.20%. However, the EDBz complexes had the highest anti-inflammatory property but not greater than that of diclofenac sodium that was used as the control drug.
DPPH is a stable free radical containing an odd electron in its structure and usually used for the detection of the radical scavenging activity in chemical analysis [
Graph representing DPPH radical scavenging ability of the compounds.
Metal chelation is an example of a complexation reaction. 1,10-Phenanthroline was used as complex forming agent of Fe(II) and forms a red coloured Fe(II)-1,10 Phen complex with maximum absorbance at 546 nm. Hence, in the presence of a reducing agent, the complex formation is hampered resulting in the decrease in the colour of the complex and a decrease in the absorbance. Measurement of absorbance therefore allows estimation of the metal chelating activity of the coexisting chelator. The prooxidant metal chelation is one of the most important mechanisms of secondary antioxidants’ action. Chelation of metals by certain compounds decreases their prooxidant effect by reducing their redox potentials and stabilizing the oxidized form of the metal [
Graph representing % ferrous chelating ability of the compounds.
Some heteroleptic metal complexes of Zn(II) and Ni(II) have been synthesized and characterized. The complexes are four coordinate geometries by bonding to the sulphur atoms of the dithiocarbamate and oxygen atoms of benzoic acid. The electronic spectral and magnetic moment data is in favour of square planar geometry for Ni(II) complexes and tetrahedral geometry for Zn(II) complexes. The covalent nature of the complexes was determined by their conductance measurement. The synthesized compounds showed antibacterial/antifungal properties. In comparison, the
The authors declare that they have no competing interests.