Introduction
Hydrazones are compounds that have an azomethine group, such as CH=N–NH2 and are vital in applications of medicinal chemistry.1 Hydrazones are the condensation products of amines and carbonyl compounds. Hydrazone ligand and metal complexes are commonly used as analytical reagents, as well as for treatment of various diseases.2 In some chemical reactions, metal compounds of hydrazone are used as catalysts.3 A Schiff base ligand forms a coordinated complex with metal ions. This metal complex exhibits a reversible association of ions or atoms by weak coordinate covalent bond formation. Schiff bases are important due to their antimicrobial activity and are remarkable due to their stability and chelating properties.4 Schiff bases can be used for the production of novel drugs. Schiff base complexes with metal ions have interested chemists due to applications of imines for their antituberculosis, antibacterial, antifungal, antimalarial, and antiviral activity.5 Schiff bases and their metal complexes contain halogens that display antimicrobial activity.6
Herein, we report the synthesis and characterization of novel Schiff base hydrazone: (1-((3-(4-fluorophenyl)-1-isopropyl-1H-indol-2-yl)methylene) hydrazine) ligand. The ligand was prepared by condensing hydrazine hydrate and 3-(4-fluorophenyl)-1-isopropyl-1H-indole-2-carbaldehyde. The synthesized ligand and its metal complexes were screened for antimicrobial, anti-tubercular, and antimalarial activities.
Materials and methods
All metal salts, solvents, and chemicals purchased were analytical reagent grade and did not require further purification.
Synthesis of [1-((3-(4-fluorophenyl)-1-isopropyl-1H-indol-2-yl)methylene) hydrazine] ligand (L2)
A mixture of 1 mmol of 3-(4-fluorophenyl)-1-isopropyl-1H-indole-2-carbaldehyde (1) and 8 mmol of hydrazine hydrate (2) was refluxed in ethanol in the presence of 1-2 drops of concentrated sulfuric acid for 5 h. The reaction progress was monitored using thin layer chromatography (TLC) in ethyl acetate:n-hexane (1:4). Upon completion of the reaction, the reaction mixture was cooled and poured onto crushed ice. The resulting product (3) was filtered off, dried, and purified by recrystallization from ethanol (Figure 1).
Synthesis of metal complexes
An ethanolic solution of metal salt (chlorides or nitrates) was mixed with an ethanolic ligand solution in a 2:1 (mmol) ratio. A slightly basic pH of the resulting reaction mixture was maintained with the addition of dilute ammonia, and the contents were refluxed for 6 h and the reaction was monitored using TLC in ethyl acetate: n-hexane as the mobile phase (1:4). After completion of the reaction, products were cooled, filtered off, dried (Figure 2), and confirmed using UV and IR spectra (Table 1).
Table 1Physical and analytical data of the synthesized ligand and complexes
| Compound | Melting point (°C) | Color |
|---|
| Ligand (L2) | 119–120 | Yellow |
| ZnL2 | 258–260 | Yellowish Brown |
| CuL2 | 239–240 | Yellowish Brown |
| NiL2 | >300 | Yellow |
| CoL2 | 268–270 | Yellow |
| Mn L2 | >300 | Yellow |
| Hg L2 | 279–280 | Yellow |
| CdL2 | 263–265 | Yellow |
| SnL2 | >300 | Yellow |
| ZrL2 | 279–280 | Yellow |
| FeL2 | >300 | Yellow |
Characterization
[1-((3-(4-fluorophenyl)-1-isopropyl-1H-indol-2-yl) methylene) hydrazine] L2:1HNMR (DMSO-d6) δ ppm: 8.34 (s, 1H, CH, hydrazide) 6.95 (s, 2H, NH2) δ 6.1 (m, 1H, CH, methine) 2.03 (s, 3H) 2.06 (s, 3H, CH3) 1.57 (d, 3H) 1.62 (d, 3H CH3) 7.51 (d, 2H, Ph), 7.67 (d, 2H, Ph) MS: m/z 295; FTIR: cm−1 3,385 (NH), 1,600 (C=N), 3,053 (CH-Ar), 1,529 (C-C Ar), 2,972 (CH-Aliphatic)
IR Spectral analysis
IR spectral data ν cm−1 for C-H, M-N, C=N of ligand, and metal complexes are reported in Table 2. The IR frequency band due to the N-H bond in the free ligand was shifted to a lower value in the spectra of all synthesized complexes, showing the involvement of an N-H group in the complexes.
Table 2IR spectral interpretation of ligand and metal complexes
| Compound | ν cm−1 (C-H) | ν cm−1 (M-N) | ν cm−1 (C=N) | ν cm−1 (N-H) |
|---|
| Ligand | 3,053 | – | 1,600 | 3,385 |
| ZnL2 | 3,064 | 420 | 1,531 | 2,966 |
| CuL2 | 3,062 | 426 | 1,531 | 2,970 |
| NiL2 | 3,059 | 426 | 1,527 | 2,873 |
| CoL2 | 3,064 | 420 | 1,531 | 2,906 |
| Mn L2 | 3,062 | 426 | 1,531 | 2,968 |
| Hg L2 | 3,062 | 429 | 1,529 | 2,968 |
| CdL2 | 2,980 | 424 | 1,527 | 2,665 |
| SnL2 | 2,974 | 422 | 1,527 | 2,974 |
| ZrL2 | 3,064 | 567 | 1,531 | 2,964 |
| FeL2 | 3,053 | 516 | 1,531 | 2,978 |
UV Spectral analysis of metal complexes
The λmax values observed in the UV spectra of the synthesized metal complexes are summarized in Table 3. The UV spectra of the complexes were recorded in DMSO.
Table 3λmax value of the synthesized metal complexes
| Compound | Wavelength (λmax) |
|---|
| ZnL2 | 256.50 |
| CuL2 | 205 |
| NiL2 | 206.50 |
| CoL2 | 205.50 |
| Mn L2 | 205.00 |
| Hg L2 | 204 |
| CdL2 | 203.4 |
| SnL2 | 229 |
| ZrL2 | 205 |
| FeL2 | 204.5 |
Biological activity
Antimicrobial study
The metal complexes were screened against four bacteria (S. Pyogenus MTCC 442, E. Coli MTCC 443, P. Aeruginosa MTCC 1688, and S. Aureus MTCC 96) and three fungal species (C. Albicans MTCC 227, A. Niger MTCC 282, and A. Clavatus MTCC 1323).
Antimicrobial activity was determined using the Broth dilution method.7 Mueller–Hinton agar nutrient medium was used. The Hinton Broth Method was used to grow microbes and dilute the microbe compound suspension for the test.
Solutions of synthesized compounds were made in DMSO solvent (control). The sample tubes were also incubated at 37°C overnight. The minimal inhibition concentration (MIC) for the control test microbes was recorded to study the antimicrobial potential of the synthesized compounds. The MIC values for the synthesized metal complexes compared with ampicillin, chloramphenicol, nystatin, and greseofulvin are summarized in Table 4.
Table 4Antimicrobial results of metal complexes
| Compound | MIC
|
|---|
Antibacterial Activity
| Antifungal Activity
|
|---|
| S.PYOGENUS | S.AUREUS | E.COLI | P.AERUGINOSA | A.NIGER | A.CLAVATUS | C.ALBICANS |
|---|
| ZnL2 | 500 | 500 | 250 | 500 | >1,000 | >1,000 | 500 |
| CuL2 | 100 | 250 | 100 | 100 | 1,000 | 1,000 | 1,000 |
| NiL2 | 500 | 50 | 50 | 250 | >1,000 | >1,000 | 500 |
| CoL2 | 100 | 250 | 125 | 62.5 | 500 | 500 | 500 |
| Mn L2 | 500 | 250 | 100 | 100 | 1,000 | 1,000 | 250 |
| Hg L2 | 500 | 500 | 250 | 250 | >1,000 | >1,000 | 500 |
| CdL2 | 250 | 200 | 100 | 250 | 500 | 1,000 | 250 |
| SnL2 | 500 | 250 | 500 | 500 | 1,000 | 1,000 | 500 |
| ZrL2 | 250 | 12.5 | 250 | 62.5 | >1,000 | >1,000 | 250 |
| FeL2 | 500 | 500 | 100 | 250 | 500 | 1,000 | 1,000 |
| Ampicillin | 100 | 250 | 100 | – | – | – | – |
| Chloramphenicol | 50 | 50 | 50 | 50 | – | – | – |
| Nystatin | – | – | – | – | 100 | 100 | 100 |
| Greseofulvin | – | – | – | – | 100 | 100 | 500 |
Antituberculosis activity
In vitro bacterial susceptibility tests were performed in bottles to determine antitubercular activity. Mycobacterium Tuberculosis (H37Rv strain) cultures were studied against the synthesized complexes.8
MIC values were determined for the antituberculosis activity. L.J inoculum nutrient medium (1 mg/mL) was used to grow the microorganisms. DMSO solvent was used to achieve the required concentration of test compounds. For primary and secondary screening, serial dilutions were prepared.
The MIC value was recorded as the highest dilution showing a minimum of 99% inhibition. MIC values of the synthesized compounds were recorded and compared with rifampicin and isoniazid as shown in Table 5.
Table 5Anti-tubercular and antimalarial activity of metal complexes
| Compound | Anti-tubercular activity against H37Rv (MIC µg/mL) | Anti-malarial Activity (MEAN IC50 values) |
|---|
| ZnL2 | 125 | 2.05 µg/mL |
| CuL2 | 500 | 1.46 µg/mL |
| NiL2 | 250 | 2.35 µg/mL |
| CoL2 | 250 | 2.42 µg/mL |
| Mn L2 | 62.5 | 3.10 µg/mL |
| Hg L2 | 250 | 2.61 µg/mL |
| CdL2 | 125 | 1.68 µg/mL |
| SnL2 | 250 | 1.87 µg/mL |
| ZrL2 | 250 | 3.82 µg/mL |
| FeL2 | 500 | 2.25 µg/mL |
| Standard | Isoniazid 0.20 µg/mL, 99% inhibition | Chloroquine IC50–0.020 µg/mL |
| Standard | Rifampicin 40 µg/mL, 99% inhibition | Quinine IC50–0.268 µg/mL |
Antimalarial activity
The compounds were studied for antimalarial activity using the Rieckmann K.H. and co-worker’s method.9 An in vitro assay was used to evaluate antimalarial activity against Plasmodium falciparum; compound solutions were executed in 96 well microtiter plates.10 Culture medium RPMI 1640 was used to grow the P. Falciparum strain. Test compounds were diluted using DMSO and further dilutions were made with culture medium. Results of the antimalarial activity of the metal complexes are summarized in Table 5.
The MIC values and the results of antimalarial activity were compared with chloroquine and quinine.11
Results and discussion
A ligand (1-((3-(4-fluorophenyl)-1-isopropyl-1H-indol-2-yl) methylene) hydrazine) was synthesized from 3-(4-fluorophenyl)-1-isopropyl-1H-indole-2-carbaldehyde and hydrazine hydrate and used for the preparation of metal complexes which were characterized using spectroscopic methods and further studied for antimicrobial, antituberculosis, and antimalarial properties. The metal complexes of Zn(II), Cu(II), Ni(II), Co(II), Mn(II), Hg(II), Sn(II), Cd(II), Zr(II), and Fe(II) resulted in a ligand : metal ratio of 2:1.
The band at 1,600 cm−1 in the IR spectrum can be attributed to the stretching of the C=N group.12 In cases of metal complexes, the spectral band that appeared at 420 cm−1 to 516 cm−1 is attributed to the presence of M-N bonds.13 The IR band at 2,974 cm−1 to 3,064 cm−1 corresponds to the C-H stretching frequency. The ligand behaves as bidentate, coordinating with the metal ion through two nitrogen atoms present in the structure of the ligand. The λmax values for metal complexes in the UV spectra were found in the range of 203 nm to 256 nm.14 the Zn(II) complex showed a λmax value at higher absorption.
The antimicrobial screening of metal complexes showed that Cu(II) and Co(II) were remarkably active against S. Pyogenus MTCC 442. The Cd(II) and Zr(II) complexes were active against S. Aureus MTCC 96. The Cu(II), Co(II), Mn(II), Cd(II), and Fe(II) complexes showed good activity against E. Coli MTCC 443, while Cu(II), Mn(II), and Zr(II) showed excellent activity against P. Aeruginosa MTCC 1688 compared to the standard drugs. Co(II) and Cd(II) were found to be active against A. Niger MTCC 282. Co(II) was found to be active against A. Clavatus MTCC 1323. Zn(II), Ni(II), Co(II), Mn(II), Hg(II), Cd(II), Sn(II), and Zr(II) showed good to excellent activity against the C. Albicans MTCC 227 fungus compared to standard drugs.
Mn(II) exhibited excellent antituberculosis activity against MTB (H37Rv strain). Zn(II) and Cd(II) were active against MTB compared to the standard drugs (rifampicin and isoniazid).
Cu(II) and Cd(II) metal complexes exhibited promising antimalarial activity while Zn(II), Co(II), Sn(II), Ni(II), Hg(II), and Fe(II) were active against malaria.
Future directions
Coordination chemistry has remained a useful field in search of bioactive agents. In the present work, we reported the synthesis and characterization of metal complexes of bidentate (1-((3-(4-fluorophenyl)-1-isopropyl-1H-indol-2-yl)methylene) hydrazine) Schiff base ligands and demonstrated that these complexes had antitubercular, antimicrobial, and antimalarial properties. Future studies will focus on identifying new similar Schiff base ligands and their metal complexes as potential entities for searching bioactive metal complexes.
Conclusions
In conclusion, the present work reports the synthesis, characterization, and antimicrobial activity of a series of metal complexes of bidentate (1-((3-(4-fluorophenyl)-1-isopropyl-1H-indol-2-yl)methylene) hydrazine) Schiff base ligands. The antitubercular, antimicrobial, and antimalarial activity of the synthesized metal complexes revealed good biological antimicrobial potential of Cu(II), Co(II), Mn(II), and Cd(II) complexes and the Mn(II) was remarkably active against MTB. The Cu(II) and Cd(II) displayed excellent activity against malaria compared to standard drugs, thus making the (1-((3-(4-fluorophenyl)-1-isopropyl-1H-indol-2-yl)methylene) hydrazine) Schiff base ligands useful entities in coordination chemistry.
Abbreviations
- DMSO:
dimethyl sulfoxide
- IR:
infra-red
- MIC:
minimal inhibition concentration
- NMR:
nuclear magnetic resonance
- TLC:
thin layer chromatography
- UV:
ultraviolet
Declarations
Acknowledgement
The authors thank Principal, Deogiri College, Aurangabad 431005 (MS), India for providing laboratory facilities.
Data sharing statement
No additional data are available.
Funding
This research received no external funding.
Conflict of interest
The authors declare no conflict of interest.
Authors’ contributions
Contributed to study concept and design (NRJ), acquisition of the data (SGM), assay performance and data analysis (VAG), drafting of the manuscript (RDS, GTP), critical revision of the manuscript (SRB), supervision (RPP).