Two bidentate Schiff base ligands having nitrogen sulphur donor sequence were derived from the condensation of S-benzyldithiocarbazate (SBDTC) with 2-chloroacetophenone and 4-chloroacetophenone to give S-benzyl-
The past few decades have seen a growing interest in transition metal complexes of Schiff base ligands as there have been several studies done on complexes that have nitrogen-sulfur donor ligands [
SBDTC is interesting due to the fact that its derivatives have the potential to be modified in various ways by introducing several different substituents [
The extensive literature review that has been made revealed that there were no studies performed on the properties and biological activities of Schiff base ligands and complexes derived from SBDTC, possessing a benzene ring with different halogen isomers. Therefore, our studies were dedicated to form ligands by condensation of 2-chloroacetophenone with SBDTC (NS2) and the condensation of 4-chloroacetophenone with SBDTC (NS4). This was done in order to investigate the changes in properties and biological activities brought about by the position of the chlorine atom on the benzene ring.
The synthesized Schiff bases were chelated with Zn2+, Cu2+, Cd2+, and Ni2+ salts due to the reported significant antimicrobial activity brought about by transition metal ions [
Schematic diagram of the synthesis procedure for NS4 and NS2 ligands.
Proposed structure of the complexes.
100% hydrazine hydrate, carbon disulphide, potassium hydroxide, and 4-chloroacetophenone were obtained from Merck (Germany), while 80% hydrazine hydrate, cadmium acetate, and copper acetate were obtained from R&M Marketing (UK). Benzyl chloride and 2-chloroacetophenone were obtained from Acros (Belgium) while absolute ethanol, zinc acetate, and copper acetate were obtained from Friedemann schmidt.
The IR spectra were recorded by a Perkin Elmer FTIR spectrophotometer within the range (4000–400 cm−1) using KBr discs while the melting point has been measured using an Electrothermal IA9100 digital m.p. apparatus measuring within a range of (0°C–400°C).
The synthesis of SBDTC was carried out as previously reported [
The method used was similar to SBDTC synthesis using 100% N2H4; however, some modifications needed to be implemented where 13 g (0.41 moles) of 80% N2H4 was used and the time needed for stirring after the complete addition of PhCH2Cl was 2 hours.
The method used for synthesis was a modified form of the one reported in [
1.98 g (0.01 moles) of SBDTC was dissolved in 40 mL of absolute ethanol and then heated on a heating plate with constant stirring in order to ensure the complete dissolving of the SBDTC. Similarly, 1.30 mL (0.01 moles) of 4-chloroacetophenone was mixed with 40 mL of absolute ethanol and heated on a heating plate for 10 minutes, which was later followed by mixing both of the reactants followed by the addition of 2–4 drops of concentrated H2SO4. The mixture was kept on the heating plate for 5 more minutes and then cooled to 0°C in an ice bath until the Schiff base precipitated. The Schiff base precipitated was filtered via suction filtration and washed with cold ethanol and dried over silica gel.
Method used was similar to the one used for NS4 synthesis; however, 2-chloroacetophenone was mixed with 20 mL of absolute ethanol and at the final stage, crystals were produced by slow heating to evaporate.
The method used for synthesis was similar to the one reported in [
The crystals for analysis were prepared by the slow evaporation method where pale yellow crystals were formed. One of these crystals was selected later to be mounted on a SMART CCD diffractometer with reflection data measured at 20°C and the source of X-ray was graphite monochromated copper radiation that produced X-ray with a characteristic wavelength of 1.54180 Å. The detector was at a distance of 4 cm and a swing angle of −35°. The collected data were reduced using the program SAINT [
Crystallographic data and structure refinement details of NS4.
Empirical formula | C16H15ClN2S2 |
Formula weight | 334.89 |
Temperature (K) | 150 |
Crystal class | Triclinic |
Space group |
|
Unit cell dimensions | |
|
6.2206(7) |
|
9.7222(10) |
|
13.5943(14) |
|
89.729(8) |
|
101.960(9) |
|
97.447(9) |
Volume (Å3) | 797.32(15) |
|
2 |
|
1.39 |
Radiation type | Cu K |
Wavelength (Å) | 1.5418 |
Crystal size (mm) | 0.06 × 0.08 × 0.18 |
|
3–72 |
Reflections measured/independent | 7883/2834 ( |
|
71.6937 |
Limiting indices |
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|
|
Refinement on |
|
|
0.066 |
|
0.178 |
Goodness of fit (S) | 0.97 |
Minimum and maximum residual electron density (eÅ−3) | −0.45 and 0.80 |
Reflections used | 2821 |
Number of parameters | 190 |
The method that has been used for the antibacterial assay was the disc diffusion assay reported in [
The quantitative antimicrobial assay has been carried out by determining the minimum inhibitory concentration (MIC, mg/mL) of the compounds which is defined as the lowest concentration that managed to inhibit visible microbial growth. The method carried out to measure the MIC was quite similar to the disc diffusion assay method used in the qualitative antimicrobial assay. However, the assay was not performed on all compounds as only those compounds that caused inhibition zones of diameters greater than or equal to 9 mm (i.e., moderate effect) were considered and only bacteria were considered for this assay.
The concentrations were prepared by two fold serial dilutions of the synthesized compounds dissolved in DMSO starting from 100 mg/mL to 6.25 mg/mL.
See Table
Physical properties and yield of the Schiff bases and their complexes.
Compound | Colour | Yield | Melting point (°C) |
---|---|---|---|
SBDTC | White | 7.67 g | 124 |
NS4 | Light yellow | 1.46 g | 138.5 |
NS2 | White | 0.61 g | 178 |
Zn(NS4)2 | Yellowish green | 0.65 g | 200 |
Cd(NS4)2 | Yellowish brown | 2.81 g | 189 |
Ni(NS4)2 | Dark green | 0.52 g | Decomposes at 309.5 |
Cu(NS4)2 | Dark brown | 0.52 g | 190 |
Zn(NS2)2 | White | 0.35 g | 159 |
Cd(NS2)2 | Greyish green | 0.59 g | Decomposes at 170 |
Ni(NS2)2 | Light green | 0.12 g | Decomposes at 160 |
Cu(NS2)2 | Dark brown | 0.10 g | Decomposes at 205 |
See Table
IR spectral data of SBDTC, free ligand, and their complexes.
Compound | Infrared absorption bands (frequency, cm−1) | |||||||
---|---|---|---|---|---|---|---|---|
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SBDTC | 3451 | 3250, 3172 | 950 | 1048 | — | — | — | 2 peaks at 710, 698 |
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NS4 | 3436 | — | 950 | 1051 | 1638 | — | — | (i) Aromatic C–H at 3166 |
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NS2 | 3447 | — | 972 | 1023 | 1590 | — | — | (i) Aromatic C–H at 3285 |
| ||||||||
Zn(NS4)2 | — | — | — | 1000 | 1400 | Obscured by other peaks | 3432 | Broad peak ~(500–900) |
| ||||||||
Cd(NS4)2 | — | — | — | 1086 | 1450 | 600 | 3430 | (i) Peak at 822 |
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Zn(NS2)2 | — | — | — | 1121 | 1578 | 750 | 3436 | (i) Peak at 697 |
| ||||||||
Cd(NS2)2 | — | — | — | 1120 | 1421 | Obscured by other peaks | 3448 | (i) Peak at 700 |
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Ni(NS4)2 | — | — | — | 1116 | 1400 | 500 | 3400 | (i) Peak at 827 |
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Cu(NS4)2 | — | — | — | 1117 | 1406 | 619 | 3420 | (i) Peak at 825 |
| ||||||||
Cu(NS2)2 | — | — | — | 1120 | 1406 | Obscured by other peaks | 3410 | Peak at 619 |
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Ni(NS2)2 | — | — | — | 1110 | 1421 | 510 | 3430 | Peak at 620 |
(M–N)*: this refers to the dative bond that is formed between the metal ion (M) and the nitrogen (N) atom.
The peaks at 3436 cm−1 and 3447 cm−1 found in NS4 and NS2 spectra, respectively, are attributed to the presence of a secondary amine group; however, unlike SBDTC, the peaks found at around 3166 cm−1 and 3285 cm−1 in NS4 and NS2 spectra cannot be assigned to the primary amine group. Instead, it can refer to the aromatic (C–H), and this shows that the primary amine no longer exists and the reaction of SBDTC with each of the ketones took place. The peaks at 1590 cm−1 and 1638 cm−1 found in the spectra of NS2 and NS4, respectively, can be assigned to (C=N) which confirms the formation of the Schiff base ligands. The peaks at around 1023 cm−1 and 1051 cm−1 can be assigned to (N–N) of NS2 and NS4, respectively. The fact that the Schiff base ligands have been derived from SBDTC might cause them to exhibit thione-thiol tautomerism as shown in Figure
Thione-thiol tautomerism.
The broad peaks at around 3400 cm−1 to 3450 cm−1 that were found in all of the spectra for the metal complexes are attributed to water coordination with the metal ion as the peak is very broad and cannot be attributed to the secondary amine. The peaks between 1400 cm−1 and 1580 cm−1 found in all the spectra of the metal complexes can be assigned to the imine (C=N) group and the negative shift in the absorption frequency of the imine (C=N) group in the complexes relative to their Schiff base ligands proves that the imine group participates in the complexation process. Moreover, most of the IR spectra of the complexes show peaks between 1000 cm−1 and 1120 cm−1 which can be assigned to the (N–N) group and the slight increase in the absorption frequency of the (N–N) group might be due to the decrease in repulsion between the two nitrogen atoms. This is due to the fact that the nitrogen atom of the imine group forms a coordinate bond with the metal ion; therefore, the pair of electrons used to form the bond are no longer in close proximity with the electrons of the adjacent nitrogen atom leading to a decrease in repulsion [
There is an absence of a peak with reasonable intensity at 950–970 cm−1 which indicates the absence of the (C=S) group and this proves that the Schiff base ligand forms a complex with the metal ion through its thiolate group and not through its thioketo sulfur. This would also mean that in solution, the ligand exists in its thiol form and this is proven further by the absence of the (N–H) group.
In the presence of KOH, the thiol proton undergoes deprotonation forming thiolate anions (Figure
Deprotonation of the thiol by KOH.
In an attempt to synthesize the metal complex without the presence of KOH, two crucial points were deduced from this unsuccessful attempt. This attempt confirmed the fact that the ligand was not in the thione form in solution as if it did, there would be no need for deprotonation by KOH and the attempt would have been successful. Secondly, it confirms the initial assumption that the ligand chelates with metal ion via an ionic bond between the negatively charged deprotonated thiol group and the metal ion. On the contrary, when KOH was added, the attempt was successful due to the deprotonation of the thiol group. Therefore, it can be deduced that the metal complex structure would be a bischelated bidentate complex as shown in Figure
Figure
Summary of selected bond lengths found in NS4.
Bond lengths (Å) | |||
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Cl1–C2 | 1.746(4) | C6–N7 | 1.285(5) |
C2–C21 | 1.381(6) | N7–N8 | 1.370(4) |
C3–H31 | 0.938 | N8–H81 | 0.870 |
C5–C20 | 1.387(6) | C9–S18 | 1.660(4) |
C9–S10 | 1.765(4) | N8–C9 | 1.343(5) |
S10–C11 | 1.825(4) |
Summary of the bond angles in NS4.
Bond angles (°) | |||
---|---|---|---|
Cl1–C2–C3 | 119.0(3)° | C6–N7–N8 | 119.1(3)° |
N7–C6–C19 | 124.1(3)° | N7–N8–H81 | 120.673° |
N7–N8–C9 | 118.7(3)° | N8–C9–S10 | 112.9(3)° |
N8–C9–S18 | 122.3(3)° | S10–C9–S18 | 124.8(2)° |
C9–S10–C11 | 102.76(17)° | S10–C11–C12 | 115.4(3)° |
ORTEP plot of NS4 showing 50% probability displacement ellipsoids in addition to the atomic numbering scheme.
The characteristic bond length is that of Cl1–C2 possessing a bond length of 1.75 Å. This is the first chlorine-carbon bond length to be reported in a dithiocarbazate derived compound at the time the experiment was carried out. The bond N8–C9 had a length of 1.34 Å while C6–N7 had a length of 1.29 Å which is shorter than that of N8–C9 due to the double bond character of the latter indicating that it is an imine functional group. The case is similar when comparing the bonds C9–S10 and C9–S18 with bond lengths of 1.77 Å and 1.66 Å, respectively, with the latter having a shorter bond length confirming its double bond character, thus indicating that the ligand existed in its thione form. The bond lengths of imine group C6–N7 and that of C9–S18 are similar to the ones found in the previous literature [
The structure geometry is considered to be planar except for the benzene ring derived from SBDTC which is out of plane and is nearly perpendicular to the plane with a torsion angle of 86.8°. The molecules seem to be stabilized by intramolecular and intermolecular H81–S18, H191–S18, and Cl1–H141 hydrogen bond interactions (Table
Hydrogen bond geometry (Å, °).
D–H |
D–H | H |
D |
D–H |
---|---|---|---|---|
C14–H141 |
0.94 | 2.88 | 3.813(5) | 173 |
N8–H81 |
0.87 | 2.67 | 3.526(3) | 166 |
C19–H191 |
0.98 | 2.80 | 3.440(4) | 124 |
C11–H112 |
0.98 | 2.65 | 3.145(4) | 112 |
C4–H41 |
0.94 | 2.41 | 2.742(5) | 100 |
Symmetry codes: i
(a) Packing of molecules per unit cell viewed along the
Table
(a) Qualitative antimicrobial assay mean results* (diameter of inhibition zones in mm). (b) Qualitative antimicrobial assay mean results* (diameter of inhibition zones in mm).
Compound | Gram-positive bacteria | Gram-negative bacteria | Fungus | ||||
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NS2 | 17.60 | 21.63 | 19.01 | 15.62 | 13.88 | 19.60 | — |
Zn(NS2)2 | 16.23 | 18.12 | 18.34 | 8.60 | 13.63 | 16.99 | 23.15 |
Cd(NS2)2 | 8.90 | 8.16 | 25.76 | 7.43 | 15.27 | 8.33 | 29.98 |
Ni(NS2)2 | 7.40 | 8.51 | 13.53 | — | 8.88 | — | 17.17 |
Cu(NS2)2 | 7.09 | — | — | — | 7.45 | — | — |
NS4 | — | 6.53 | — | — | — | 11.50 | — |
Zn(NS4)2 | 9.19 | 8.17 | 7.40 | — | — | 9.69 | — |
Cd(NS4)2 | 7.77 | 6.61 | 30.05 | — | — | 6.81 | — |
Ni(NS4)2 | 10.87 | 10.22 | 9.13 | — | — | 12.99 | — |
Cu(NS4)2 | 10.33 | 6.64 | 8.21 | — | — | 9.99 | — |
Controls and reference compounds | Gram-positive bacteria | Gram-negative bacteria | Fungus | ||||
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DMSO | — | — | — | — | — | — | — |
Streptomycin | 14.08 | 15.67 | 12.20 | 14.44 | 13.54 | Not available | |
Neomycin | 18.14 | 18.52 | 18.57 | 16.58 | 21.87 | 18.91 | |
Chloramphenicol | 25.86 | 21.70 | 25.87 | — | 15.01 | 19.80 | |
SBDTC | 19.07 | 22.73 | 13.33 | 15.70 | 14.90 | 21.19 | 18.57 |
Amphotericin B | 19.17 |
NS2 has inhibited the growth of all types of bacteria strongly with the strongest inhibition observed against
NS4 seems to possess a very low antimicrobial activity, and the case was similar to Zn(NS4)2. Cd(NS4)2 showed lack of antifungal activity and a very low and insignificant effect on Gram-negative bacteria and a slightly higher effect has been seen with Gram-positive bacteria although these effects were insignificant. However, there was a very high effect exerted on
It can be deduced from the results that the activity of the NS4 complexes is generally higher than that of the Schiff base ligand which might be explained via Overtone’s concept and chelation theory [
The results show that Gram-positive bacteria were inhibited more strongly than Gram-negative bacteria and that can be explained by considering the structural features of both bacterial types. Gram-negative bacteria possess an extra outer layer on top of the peptidoglycan and this has been found to be highly impermeable. Moreover, Gram-positive bacteria have polysaccharides in their cell wall called teichoic acids, which are negatively charged and have facilitated the passage of the positive metal ions. The position of chlorine in the benzene ring is crucial as it affected the activity of the ligand where it showed that NS2 has had a much higher activity than that of NS4. This might suggest that the binding pocket of the target in the bacteria favours the orthoposition of the chlorine. Despite the fact that NS2 and NS4 derivatives acted with variable strengths on the microbes tested, it has been found that the strongest inhibition level observed was by the Cd complexes of both ligands against
The activities of the synthesized compounds were compared with those of the reference compounds, by forming a ratio of the mean inhibition diameter of the synthesized compound to that of the reference compound (Table
Ratio of mean inhibition zone diameter of synthesized compounds to that of reference compounds.
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NS2 | |||||||
Streptomycin | 1.25 | 1.38 | 1.56 | 1.08 | 1.03 | Not available | |
Neomycin | 0.97 | 1.17 | 1.02 | 0.94 | 0.63 | 1.04 | |
Chloramphenicol | 0.68 | 1.00 | 0.73 | Control has no effect | 0.92 | 0.99 | |
SBDTC | 0.92 | 0.95 | 1.43 | 0.99 | 0.93 | 0.92 | 0.00 |
Amphotericin B | 0.00 | ||||||
Zn(NS2)2 | |||||||
Streptomycin | 1.15 | 1.16 | 1.50 | 0.60 | 1.00 | Not available | |
Neomycin | 0.89 | 0.98 | 0.99 | 0.52 | 0.62 | 0.90 | |
Chloramphenicol | 0.63 | 0.84 | 0.71 | Control has no effect | 0.91 | 0.86 | |
SBDTC | 0.85 | 0.80 | 1.38 | 0.55 | 0.91 | 0.80 | 1.25 |
Amphotericin B | 1.21 | ||||||
Cd(NS2)2 | |||||||
Streptomycin | 0.63 | 0.52 | 2.11 | 0.51 | 1.13 | Not available | |
Neomycin | 0.49 | 0.44 | 1.39 | 0.45 | 0.70 | 0.44 | |
Chloramphenicol | 0.34 | 0.38 | 1.00 | Control has no effect | 1.02 | 0.42 | |
SBDTC | 0.47 | 0.36 | 1.93 | 0.47 | 1.02 | 0.39 | 1.61 |
Amphotericin B | 1.56 | ||||||
Ni(NS2)2 | |||||||
Streptomycin | 0.53 | 0.54 | 1.11 | 0.00 | 0.66 | Not available | |
Neomycin | 0.41 | 0.46 | 0.73 | 0.00 | 0.41 | 0.00 | |
Chloramphenicol | 0.29 | 0.39 | 0.52 | Control has no effect | 0.59 | 0.00 | |
SBDTC | 0.39 | 0.37 | 1.02 | 0.00 | 0.60 | 0.00 | 0.92 |
Amphotericin B | 0.90 | ||||||
Cu(NS2)2 | |||||||
Streptomycin | 0.50 | 0.00 | 0.00 | 0.00 | 0.55 | Not available | |
Neomycin | 0.39 | 0.00 | 0.00 | 0.00 | 0.34 | 0.00 | |
Chloramphenicol | 0.27 | 0.00 | 0.00 | Control has no effect | 0.50 | 0.00 | |
SBDTC | 0.37 | 0.00 | 0.00 | 0.00 | 0.50 | 0.00 | 0.00 |
Amphotericin B | 0.00 | ||||||
NS4 | |||||||
Streptomycin | 0.00 | 0.42 | 0.00 | 0.00 | 0.00 | Not available | |
Neomycin | 0.00 | 0.35 | 0.00 | 0.00 | 0.00 | 0.61 | |
Chloramphenicol | 0.00 | 0.30 | 0.00 | Control has no effect | 0.00 | 0.58 | |
SBDTC | 0.00 | 0.29 | 0.00 | 0.00 | 0.00 | 0.54 | 0.00 |
Amphotericin B | 0.00 | ||||||
Zn(NS4)2 | |||||||
Streptomycin | 0.65 | 0.52 | 0.61 | 0.00 | 0.00 | Not available | |
Neomycin | 0.51 | 0.44 | 0.40 | 0.00 | 0.00 | 0.51 | |
Chloramphenicol | 0.36 | 0.38 | 0.29 | Control has no effect | 0.00 | 0.49 | |
SBDTC | 0.48 | 0.36 | 0.56 | 0.00 | 0.00 | 0.46 | 0.00 |
Amphotericin B | 0.00 | ||||||
Cd(NS4)2 | |||||||
Streptomycin | 0.55 | 0.42 | 2.46 | 0.00 | 0.00 | Not available | |
Neomycin | 0.43 | 0.36 | 1.62 | 0.00 | 0.00 | 0.36 | |
Chloramphenicol | 0.30 | 0.30 | 1.16 | Control has no effect | 0.00 | 0.34 | |
SBDTC | 0.41 | 0.29 | 2.25 | 0.00 | 0.00 | 0.32 | 0.00 |
Amphotericin B | 0.00 | ||||||
Ni(NS4)2 | |||||||
Streptomycin | 0.77 | 0.65 | 0.75 | 0.00 | 0.00 | Not available | |
Neomycin | 0.60 | 0.55 | 0.49 | 0.00 | 0.00 | 0.69 | |
Chloramphenicol | 0.42 | 0.47 | 0.35 | Control has no effect | 0.00 | 0.66 | |
SBDTC | 0.57 | 0.45 | 0.68 | 0.00 | 0.00 | 0.61 | 0.00 |
Amphotericin B | 0.00 | ||||||
Cu(NS4)2 | |||||||
Streptomycin | 0.73 | 0.42 | 0.67 | 0.00 | 0.00 | Not available | |
Neomycin | 0.57 | 0.36 | 0.44 | 0.00 | 0.00 | 0.53 | |
Chloramphenicol | 0.40 | 0.31 | 0.32 | Control has no effect | 0.00 | 0.50 | |
SBDTC | 0.54 | 0.29 | 0.62 | 0.00 | 0.00 | 0.47 | 0.00 |
Amphotericin B | 0.00 |
It can be deduced from (Table
Zn(NS2)2 showed generally similar strength to the positive controls against all bacteria and the fungus except
Zn(NS4)2, Cd(NS4)2, Ni(NS4)2, and Cu(NS4)2 complexes showed generally an effect that is half the strength of the positive controls or slightly higher against
The effect of chloroacetophenone analogues on microbial activity can be analysed via comparing the results obtained from this experiment with the previous literature where a different ketone has been reacted with SBDTC. In order to attribute any differences in results to the structure that has been derived from the ketone. The parameter used for the comparison is the inhibition zone diameter.
The compound synthesized by Tarafder et al. [
The compound synthesized by Hossain et al. [
The compound synthesized by Tarafder et al. [
The activity of the bacteria has been assessed quantitatively via measuring the minimum inhibitory concentration (MIC). Some of the MIC of the controls used was obtained from the previous literature [
Shows the minimum inhibitory concentration (MIC) of the compounds.
Compound | MIC (mg/mL) | |||||
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NS2 | <6.25 | <6.25 | 25 | <6.25 | 50 | <6.25 |
Zn(NS2)2 | <6.25 | <6.25 | 12.50 | <6.25 | <6.25 | <6.25 |
Cd(NS2)2 | <6.25 | — | 100 | — | <6.25 | — |
Ni(NS2)2 | — | 100 | 100 | — | — | — |
Cu(NS2)2 | — | — | — | — | — | — |
NS4 | — | — | — | — | — | 12.50 |
Zn(NS4)2 | — | — | — | — | — | 50 |
Cd(NS4)2 | — | — | <6.25 | — | — | — |
Ni(NS4)2 | 25 | — | — | — | — | 50 |
Cu(NS4)2 | <6.25 | — | — | — | — | <6.25 |
SBDTC | <6.25 | <6.25 | <6.25 | <6.25 | 0.78* | <6.25 |
Streptomycin | 0.001563* | 0.000391* | >0.10* | 0.0008* | 0.004* | 0.004* |
Neomycin | N/A | 1.10* | 0.0039* | 0.0064* | N/A | 0.032* |
Chloramphenicol | 0.005* | 0.01* | 0.05* | 0.004* | 0.02* | 0.002* |
— refers to the absence of an MIC value for a certain compound as it was not investigated for its MIC due to its low activity. *refers to an MIC value that was obtained from the previous literature or experiments. N/A: (not available).
Ni(NS4)2 was the weakest compound relative to the antibiotics with MIC values equal to or greater than 25 mg/mL. NS2 and Zn(NS2)2 had relatively the lowest MIC values of <6.25 mg/mL for
Characterization of the synthesized compounds has revealed that the complexes are bidentate having nitrogen and sulfur donor atoms, while the X-ray analysis of NS4 revealed a planar structure with an out-of-plane benzyl ring. Qualitative antimicrobial assay confirmed the potential of NS2 and its complexes as strong antimicrobial agents and the generally higher biological activity possessed by NS2 when compared to NS4. It has been noted that the Cd complexes of NS2 and NS4 acted strongly specifically on
The authors would like to extend their appreciation to the Ministry of Higher Education Malaysia (MOHE) under FRGS (F0010.54.02) for providing the grant for this study.