A new series of quinoline hydrazone derivatives and their metal complexes have been synthesized and their biological properties have been evaluated against
The tuberculosis also known as TB and “white plaque” is caused by infection with different species of bacteria including
Major factors associated with drug resistance development include nonadherence to therapy due to multiple factors such as high cost of drugs, long duration of therapy and combination of multiple drugs used in treatment regimens and drug related coinfection with HIV, adverse reactions, prior history of treatment, and treatment failures with antituberculosis (anti-TB) drugs [
The treatment of tuberculosis involves first-line drugs including Streptomycin, Isoniazid (INH), Rifampin (RMP), Ethambutol, and Pyrazinamide. The second-line treatments of tuberculosis involve the application of p-aminosalicylic acid, Ethionamide, Cycloserine, Azithromycin, Clarithromycin, and Fluoroquinolones. However, the main complication associated with TB therapy is the poor assent with the long duration of the treatment and mainly with the drugs used to treat MDR-TB that are increasing resistance, expensive, relatively ineffective, and having long duration of treatment. The Isoniazid is one of the commonly used and effective antitubercular drugs, but recently because of the appearance of MTB resistant strains many attempts have been made to explain the mechanism of interaction of this drug and the origin of drug resistance and research its novel/new drugs for the treatment of tuberculosis. Therefore it is still a challenge for the researchers to develop drugs of more efficiency, more effective with less toxicity to treat signs and symptoms of tuberculosis.
On the side the various pharmacological properties of quinoline and their derivatives attracted great attention in the last few decades because of their vast occurrence in natural products and drugs [
The chemicals and solvents used in this work were of analytical grade and purchased from Merck and Sigma-Aldrich Chemicals. The zinc chloride, copper chloride, and DMF (N,N-dimethylformamide) were purchased from SD Fine Chemicals.
6-Fluoro-2-hydroxyquinoline-3-carbaldehyde was synthesized by Vilsmeier-Haack reaction starting with 4-fluoroacetanilide as per reported method [
6-Fluoro-2-hydroxyquinoline-3-carbaldehyde (0.200 g, 0.0015 moles) was dissolved in 5 mL ethanol and compounds 1a–1e (0.0015 moles) were added. A drop of glacial acetic acid was added as a catalyst for the reaction. The reaction mixture was refluxed for half an hour. The reaction mixture was cooled in ice bath and precipitated product was filtered. The product was then dried in oven. Structures of synthesized hydrazone derivatives have been shown in Figure
Structures of hydrazones 2a–2e (see Figure
Entry | Hydrazide (1) | Hydrazone (2) | Yield (%) | Colour |
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a |
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84 | Dark yellow |
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b |
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87 | Yellow |
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c |
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76 | Faint yellow |
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d |
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90 | Pale yellow |
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e |
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84 | Pale yellow |
M.P.: 293–295°C; UV
M.P.: >300°C; UV
M.P.: >300°C; UV
M.P.: >300°C; UV
M.P.: >300°C; UV
The solution of metal salt [ZnCl2, CuCl2] dissolved in ethanol was added gradually to a stirred ethanolic solution of the Schiff base hydrazones [2a–2e], in the molar ratio 1 : 2. The reaction mixture was further stirred for 2–4 hr at 60°C. Then it was cooled in ice bath to ensure the complete precipitation of the formed complexes. The precipitated solid complexes were filtered and washed four times with water. Finally, the complexes were washed with diethyl ether and dried in vacuum desiccators over anhydrous CaCl2. Structures of synthesized complexes have been shown in Figure
Proposed structures of Zn(II) and Cu(II) complexes from hydrazones 2a–2e (see Figure
Entry | Metal complex | Yield (%) | Colour |
---|---|---|---|
3a | |
92 | Bright yellow |
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3b | |
90 | Dark yellow |
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3c | |
93 | Dark yellow |
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3d | |
88 | Shiny yellow |
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3e | |
92 | Dark yellow |
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3f | |
90 | Dark green |
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3g | |
87 | Green |
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3h |
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89 | Green |
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3i |
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86 | Green |
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3j |
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91 | Dark green |
All the synthesized hydrazone ligands 2a–2e and their Cu2+ and Zn2+ complexes 3a–3j are stable at room temperature and are nonhygroscopic in nature. The metal complexes are insoluble in H2O but are soluble in DMF. The spectral characterizations of synthesized compounds confirm the suggested structures of the hydrazones as well as their metal complexes. The elemental analysis, physical properties, and spectral data of the ligand and complexes are summarized below.
1H NMR spectra were recorded on Varian-NMR-Mercury 300 MHz instrument. The DMSO-d6 was used as a solvent with TMS (tetramethylsilane) as an internal standard. The chemical shifts are expressed as
1H NMR signals for hydrazones (2a–2e) and their assignments.
Hydrazones | Amidic -NH |
Phenolic -OH |
Azomethine -CH |
---|---|---|---|
2a | 12.16 | 12.09 | 9.08 |
2b | 12.26 | 12.13 | 8.77 |
2c | 12.11 | 12.11 | 8.97 |
2d | 12.13 | 11.82 | 8.51 |
2e | 12.10 | 11.76 | 8.42 |
In the 13C spectra azomethine carbon atom appeared most downfield as reported in literature values; in the hydrazones (2a–2e) it was observed in the range of 140.97–146.95 ppm shown in Table
13C NMR for hydrazones (2a–2e) and their assignments.
Hydrazones | -C |
-C |
-C |
---|---|---|---|
2a | 163.01 | 161.63 | 144.63 |
2b | 164.80 | 162.52 | 140.97 |
2c | 165.31 | 160.40 | 141.29 |
2d | 168.50 | 162.75 | 143.72 |
2e | 168.40 | 161.96 | 146.95 |
The formation of imine is confirmed by the presence of intense molecular ion peak in the mass spectra of hydrazone derivatives (2a–2e). Spectral evaluation predicts the molecular weights of the desired hydrazone compounds.
The infrared spectra were recorded on FTIR-7600 Lambda Scientific Pty. Ltd. using KBr pellets. From the interpretation of IR spectra we get valuable information regarding the nature of functional group present in the hydrazone derivatives (2a–2e) and metal complexes (3a–3j). In the IR spectra of hydrazones the imine group (-HC=N-) and hydroxyl group show strong peak in the regions of 1625–1630 cm−1 and 3193–3538 cm−1, respectively. All metal complexes show broad peak in the region of 3200–3400 cm−1 due to coordinated water molecules.
In order to study the bonding mode of hydrazone ligand to the central metal atom the IR spectra of the free hydrazones were compared with the spectra of the complexes. The important IR bands and their assignments are listed in Tables
FTIR bands for hydrazones (2a–2e) and their assignments.
Compounds | Phenolic |
Amide |
Imine |
Phenolic |
|
---|---|---|---|---|---|
2a | 3208 | 1660 | 1625 | 1425 | 1294 |
2b | 3488 | 1650 | 1630 | 1427 | 1288 |
2c | 3444 | 1662 | 1628 | 1438 | 1234 |
2d | 3538 | 1656 | 1627 | 1425 | 1263 |
2e | 3193 | 1668 | 1625 | 1428 | 1232 |
FTIR bands for metal complexes (3a–3j) and their assignments.
Complex | Lattice water |
Amide |
Imine |
Phenolic |
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3a | 3399 | 1650 | 1631 | 1376 | 574 | 474 |
3b | 3372 | 1646 | 1614 | 1369 | 590 | 449 |
3c | 3392 | 1652 | 1619 | 1380 | 599 | 482 |
3d | 3369 | 1650 | 1631 | 1376 | 599 | 468 |
3e | 3392 | 1621 | 1589 | 1425 | 592 | 470 |
3f | 3436 | 1665 | 1616 | 1380 | 597 | 470 |
3g | 3357 | 1610 | 1558 | 1375 | 501 | 460 |
3h | 3432 | 1658 | 1616 | 1386 | 566 | 474 |
3i | 3426 | 1658 | 1617 | 1382 | 530 | 453 |
3j | 3368 | 1663 | 1616 | 1380 | 599 | 472 |
The phenolic -OH band appears at 3193–3588 cm−1 which disappears in IR spectra of the metal complexes; however new broad peak has been observed at 3200–3400 cm−1 due to coordinated water molecules which confirms the complexation of hydrazones with central metal atom through phenolic -OH. The IR spectra of all the metal complexes show prominent band at about 501–599 cm−1 due to
From the mathematical relation
Molar conductance (Λ
Complexes | Molar conductance Λ |
|
---|---|---|
Zn complexes | 3a | 3.6 |
3b | 4.9 | |
3c | 3.2 | |
3d | 2.9 | |
3e | 5.3 | |
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Cu complexes | 3f | 7.30 |
3g | 4.83 | |
3h | 6.42 | |
3i | 4.74 | |
3j | 3.32 |
The quantitative estimation of Cu(II) and Zn(II) has been done by complexometric titration with standard EDTA solution. In a titration an accurately known mass of metal complex is dissolved in an aqueous solution by chemical treatment such as acid-digestion of solid metal complex samples and diluted with high purity water to an accurately known volume. Then an accurately known volume of the aliquot is pipetted into a titration vessel and the analyte of interest is carefully titrated with a standardized EDTA solution to the endpoint of the titration [
Percentage content of Zn and Cu in metal complexes 3a–3j.
Zn(II) complexes | % Zn content observed (calculated) |
---|---|
3a | 9.21 (9.08) |
3b | 8.96 (9.08) |
3c | 8.63 (8.74) |
3d | 6.32 (6.44) |
3e | 7.80 (7.62) |
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Cu(II) complexes | % Cu content observed (calculated) |
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3f | 8.73 (8.85) |
3g | 8.93 (8.85) |
3h | 8.64 (8.52) |
3i | 6.31 (6.27) |
3j | 7.40 (7.42) |
Showing comparative % of Zn and Cu contents in metal complexes 3a–3j.
From the experimental study it is clear that practical observations are in good agreement with the theoretical values calculated for 1 : 2 ratio of metal : ligand stoichiometry. Regarding the above explanation of the results of various spectroscopic details, it may be concluded that the proposed geometry for the transition metal complexes with general formula ML2·2H2O is octahedral for Zn+2 and Cu+2 complexes. The probable structures are shown in Table
The Cresset software Forge is a molecular design and SAR (structure activity relationship) interpretation tool that generates and uses molecular alignments as a way to make meaningful comparisons across chemical series. The interaction between a ligand and a protein involves electrostatic fields and surface properties (e.g., hydrogen bonding and hydrophobic surfaces). Two molecules which both bind to a common active site tend to make similar interactions with the protein and hence have highly similar field properties.
Accordingly, using these properties to describe molecules is a powerful tool for the medicinal chemist as it concentrates on the aspects of the molecules that are important for biological activity. In Forge, molecules can be aligned by using the fields of the molecules, by using shape properties, or by using a common substructure. Using the fields gives a “protein’s view” of how the molecules would line up in the active site, generating ideas on how molecules with different structures could interact with the same protein. Using substructure or common shape properties shows how the fields around a single chemical series vary with activity and in many cases these can be automatically examined to give a 3D structure active relationship (SAR) with predictive power for new ideas for synthesis.
Molecules bear different types of field points. Larger field points represent stronger points of potential interaction. Throughout Cresset’s software the blue points are negative field points which like to interact with positives/H-bond donors on a protein, whereas red points are positive field points which like to interact with negatives/H-bond acceptors on a protein. Similarly the yellow points are van der Waals surface field points which describe possible surface/van der Waals interactions. It can be seen that ionic groups give rise to the strongest electrostatic fields. Hydrogen bonding groups also give strong electrostatic fields. Aromatic groups encode both electrostatic and hydrophobic fields. Aliphatic groups such as the methyl or cyclopentyl group give rise to hydrophobic and surface points but are essentially electrostatically neutral.
To generate these fields, we use XED (Extended Electron Distribution) molecular mechanics force field, which uses off-atom sites to more accurately describe the electron distribution in a molecule, as opposed to other force fields where charges are placed at the atomic nuclei only.
For SAR study we have selected ciprofloxacin as reference compound as it shows strong antitubercular activity against
Showing different electrostatic regions of products 2a–2e.
Showing point charges of products 2a–2e.
Hydrophobic regions for products 2a–2e.
The antitubercular activity of the hydrazone ligands and their metal complexes was tested against
The antibacterial activity of hydrazones and their metal complexes was tested against
To minimize the evaporation of medium in the test wells during incubation, 200
When we compare MIC values of hydrazone and its complexes, it indicates that metal complexes exhibit higher antimicrobial activity than the free hydrazone ligands and the same is represented from the results given in Table
Showing comparative antituberculosis screening results by MIC method.
Test samples | Sample concentration in |
|
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Hydrazones | 2a | 50 |
2b | 25 | |
2c | 50 | |
2d | 50 | |
2e | 25 | |
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Zn complexes | 3a | 25 |
3b | 25 | |
3c | 12.5 | |
3d | 25 | |
3e | 12.5 | |
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Cu complexes | 3f | 6.25 |
3g | 12.5 | |
3h | 6.25 | |
3i | 12.5 | |
3j | 25 |
There was moderate to good antibacterial activity observed against
Schematic diagram of mycobacterium cell wall.
Metal complexes showed greater activity as compared with hydrazone ligand. This may be probably because of the greater lipophilic character of the complexes. This increased activity of the metal complexes can be explained on the basis of coordination theory [
However, in case of test samples 3f and 3h complexes showed good activity up to MIC value of 6.5
The UV-Visible absorption spectra were recorded with UV spectrophotometer model Shimadzu UV-1800. The path length of the measurements was 1 cm. The fluorescence study was done on a Spectrofluorophotometer model Shimadzu RF-5301pc having 1 cm path length. The concentration 200 ppm of ligand and metal complexes which was in DMF (N,N-dimethylformamide) was prepared for study. Figures
The absorption and emission wavelength with intensity.
Compound | Absorption |
Emission |
---|---|---|
2a | 383 (1.73) | 456 (200.11) |
2b | 388 (2.37) | 452 (222.03) |
2c | 385 (1.42) | 459 (203.63) |
2d | 382 (2.78) | 458 (110.40) |
2e | 381 (1.65) | 451 (400.14) |
3a | 405 (1.49) | 479 (791.91) |
3b | 408 (0.63) | 485 (654.31) |
3c | 405 (2.75) | 485 (766.40) |
3d | 391 (1.50) | 466 (453.99) |
3e | 392 (1.69) | 464 (409.44) |
3f | 401 (0.99) | 473 (138.56) |
3g | 402 (0.69) | 464 (284.08) |
3h | 394 (1.07) | 460 (359.55) |
3i | 399 (0.34) | 466 (446.55) |
3j | 390 (1.87) | 456 (704.06) |
Electronic spectra of hydrazones (2a–2e).
Electronic spectra of metal complexes (3a–3j).
Fluorescence spectra of hydrazones (2a–2e).
Fluorescence spectra of metal complexes (3a–3j).
All the ligands in UV-Visible spectra exhibit bands around 381–388 nm. The broad intense band around 280 nm in the ligands can be assigned to intraligand
The emission spectra of compounds 2a–2e showed emission band in the range of 451–459 nm and compounds 3a–3j showed emission band in the range of 456–485 nm. Compounds 3a, 3b, 3c, and 3j showed higher emission intensity. Complex formation of hydrazones induces marked hyperchromic and bathochromic shifts. The Zn(II) complexes showed intense fluorescent properties as compared to Cu(II) complexes and their parent ligands.
To assess the antimycobacterial activity potential of this class of compounds, some of the compounds synthesized were checked against
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank Principal, Head of Department of Chemistry, Government of Maharashtra, Ismail Yusuf Arts, Science and Commerce College, for providing research and library facilities. The authors also thank Dr. Kishore Bhat of Governmental Dental College, Belgaum, for facilitating anti-TB assays and providing the procedure for the same.