Abstract
A series of metal complexes of La(III) and Th(IV) have been synthesized with newly derived biologically active ligands. These ligands were synthesized by the condensation of 3-substituted-4-amino-5-hydrazino-1,2,4-triazole with 8-formyl-7-hydroxy- 4-methylcoumarin. The structure of the complexes has been proposed by elemental analyses, spectroscopic data i.e. i.r., 1H nmr, Uv-Vis, FAB-mass and thermal studies. The elemental analyses of the complexes conform to the stoichiometry of the type [La(L)·3H2O]·2H2O and [Th(L)(NO3)·2H2O]·2H2O where (L = LI-LIV). All the complexes are soluble in DMF and DMSO and are non-electrolytes in DMF and DMSO. All these ligands and their complexes have also been screened for their antibacterial (Escherichia coli, Staphylococcus aureus, Staphylococcus pyogenes and Pseudomonas aeruginosa) and antifungal activities (Aspergillus niger, Aspergillus flavus and cladosporium) by the MIC method. The brine shrimp bioassay was also carried out to study their invitro cytotoxic properties.
Introduction
Schiff base complexes with transition metals have played a prominent role in the development of coordination chemistry [Citation1,Citation2]. Synthesis of 1,2,4-triazole derivatives has attracted widespread attention due to their diverse biological activities. Triazole derivatives Citation3,Citation4,Citation5,Citation6 are known to possess antibacterial, fungicidal, hypotensive and hypothermic activities. In addition to 1,2,4-triazole derivatives coumarin derivatives display an additional significant biological Citation7,Citation8,Citation9,Citation10,Citation11 and pharmacological [Citation12,Citation13] properties. Many of these compounds possess antibacterial [Citation12], antifungal [Citation13] and insecticidal [Citation10] activities and the hydroxycoumarins are the typical phenolic compounds and therefore, act as potent metal chelators and free redical scavengers. They are powerful chain-breaking antioxidants [Citation14]. Metal complexes of 1,2,4-triazole derivatives have been extensively investigated and reported from our laboratory Citation15,Citation16,Citation17,Citation18.
Recently, a number of attempts have been made to obtain Co(II), Ni(II), Cu(II) and Zn(II) complexes with the Schiff bases derived from cinnamaldehyde and 4-amino-3-ethyl-5-mercapto-s-triazole and 4-amino-5-mercapto-3-n-propyl-s-triazole [Citation19] and cobalt(II), Nickel(II) and copper(II) complexes with Schiff base derived from furfuraldehyde and 3-substituted-4-amino-5-mercapto-s-triazole have been isolated and characterized by elemental analyses, magnetic, spectral (IR, Uv-Vis, EPR) and thermal studies [Citation20].
In continuation of our efforts on 1,2,4-triazole derivatives we have synthesized a new series of Schiff bases derived from 3-substituted-4-amino-5-hydrazino-1,2,4-triazole and 8-formyl-7-hydroxy-4-methylcoumarin. The survey of the literature reveals that, no work has been carried out on the synthesis of metal complexes with these Schiff bases. These ligands have donor sites with the ONNNO sequence and varied coordination abilities. Because of the importance of triazole and coumarin compounds in biological and medicinal fields and also the La(III) and Th(IV) complexes are well reported as antimicrobial and cytotoxic effect with coumarin derivatives Citation21,Citation22,Citation23. Thus, it attracted our attention and aroused our interest in elucidating the structure of La(III) and Th(IV) complexes with these bioactive Schiff bases (). The Schiff base metal complexes are characterized by spectral, thermal and molar conductance studies and these are evaluated for their antibacterial and antifungal properties against various pathogenic bacterial strains using the minimum inhibitory concentration (MIC) method.
Figure 1. Synthesized Schiff bases.
Material and methods
All the chemicals used were of reagent grade. 7-hydroxy-4-methyl-coumarin was obtained from Acros Chemical Company. 3-substituted-4-amino-5-mercapto-1,2,4-triazole was prepared as described in the literature [Citation24,Citation25]. The IR spectra of the Schiff bases and their La(III) and Th(IV) complexes were recorded on a HITACHI-270 IR spectrophotometer in the 4000-250 cm− 1 region in KBr disks. The electronic spectra of the complexes were recorded in DMF on a VARIAN CARY 50-BIO UV-spectrophotometer in the region of 200-1100 nm. The proton NMR spectra of ligands and its metal complexes were recorded in CDCl3 on a BRUKER 300 MHz spectrometer at room temperature using TMS as an internal reference. FAB mass spectra were recorded on a JEOL SX 102/DA-6000 mass spectrometer/data system using Argon/Xenon (6 KV, 10Am) as the FAB gas. The accelerating voltage was 10 KV and the spectra were recorded at room temperature m-nitrobenzyl alcohol was used as the matrix. The mass spectrometer was operated in the positive ion mode. Particle size was measured by Zetasizer technique, Model 3000HS, Malvern, U.K. Thermogravimetric analysis data were measured from room temperature to 1000°C at a heating rate of 10°C/min. The data were obtained by using a PERKIN-ELMER DIAMOND TG/DTG instrument. Molar conductivity measurements were recorded on a ELICO-CM-82 T conductivity bridge with a cell having cell constant 0.51 and magnetic moment was carried out on faraday balance.
Synthesis of 3-substituted-4-amino-5-hydrazino-1,2,4-triazole
A mixture of 3-substituted-4-amino-5-mercapto-1,2,4-triazole and N2H4. H2O in 1:1 molar proportions in EtOH was boiled under reflux for 4-5 h on a water bath. The reaction mixture was cooled at room temperature; within 1 h the compound separated from the clear solution. It was filtered, washed and recrystalized from EtOH.
Synthesis of Schiff bases (LI-LIV)
A mixture of 3-substituted-4-amino-5-hydrazino-1,2,4-triazole and 8-formyl-7-hydroxy-4-methylcoumarin [Citation26] in 1:1 molar proportion in an alcoholic medium containing few drops of concentrated HCl was refluxed for 3-4 h. The product separated is filtered, washed with alcohol and was recrystalized from EtOH.
Synthesis of La(III) complexes (1-4)
The lanthanum(III) nitrate (0.01 mol) in super-dry alcohol was treated with (0.01 mol) ligand in super-dry alcohol. The reaction mixture was boiled under reflux for 2-3 h. After cooling, the pH of the reaction mixture was adjusted to ca. pH 7 by adding dilute ammonia with constant stirring. The dark green precipitate of the complexes was filtered, washed thoroughly with dry alcohol and ether and finally dried over fused CaCl2 in vacuum.
Synthesis of Th(IV) complexes (5-8)
Thorium(IV) nitrate (0.01 mol) was boiled under reflux with the ligand (0.01 mol) in alcohol for 3 h. The complex was precipitated by adding distilled water containing liquor ammonia. The precipitated complex was filtered, washed thoroughly with distilled water, alcohol then with ether, and dried under reduced pressure over fused CaCl2.
In vitro antibacterial and antifungal assay
The synthesized Schiff bases and their corresponding La(III) and Th(IV) complexes were screened for their biological activity by using five bacteria, namely E. coli, S. aureus, S. pyogenes, Salmonella typhi and P. aeruginosa and three fungi namely A. niger, A. flavus and cladosporium by the reported method [Citation27,Citation28].
The bacteria were subcultured in agar medium. The petridishes were incubated for 24 h at 37°C. Standard antibacterial drug (gentamycine) was also screened under similar conditions for comparison. The fungi were subcultured in potato dextrose agar medium. Standard antifungal drug (fluconazole) was used for comparison. The petridishes were incubated for 48 h at 37°C. The wells were dug in the agar media using a sterile metallic borer. Activity was determined by measuring the diameter of the zone showing complete inhibition (mm). Growth inhibition was compared with the standard drugs. In order to clarify any effect of DMF on the biological screening, separate studies were carried out with solutions alone of DMF and they showed no activity against any microbial strains The stock solution (1 mg mL− 1) of the test chemical was prepared in DMF.
Minimum inhibitory concentration (MIC)
Some compounds showing promising antibacterial/antifungal activity were selected for minimum inhibitory concentration studies [Citation29,Citation30].
In vitro cytotoxicity
The synthesized Schiff base and its La(III) and Th(IV) complexes were screened for their cytotoxicity (brine shrimp bioassay) using protocol of Meyaer [Citation31]. Brine shrimp (Artemia salina leach) eggs were hatched in a shallow rectangular plastic dish (22 × 32 cm) filled with artificial seawater, which was prepared with a commercial salt mixture and double distilled water. An unequal partition was made in the plastic dish with the help of a perforated device. Approximately 50 mg of eggs were sprinkled into the large compartment, which was darkened while the minor compartment was open to ordinary light.
After two days, nauplii were collected by a pipette from the lighted side. A sample of the test compound was prepared by dissolving 20 mg of each compound in 2 mL of DMF. From this stock solutions 100, 50 and 10 μg/mL were transferred to 9 vials (three for each dilutions were used for each test sample and LD50 is the mean of three values) and one vial was kept as control having 2 mL of DMF only. The solvent was allowed to evaporate overnight. After two days, when shrimp larvae were ready, 1 mL of seawater and 10 shrimps were added to each vial (30 shrimps/dilution) and the volume was adjusted with seawater to 5 mL per vial. After 24 h the number of survivors was counted. Data were analyzed by a finney computer program to determine the LD50 values [Citation32].
Results and discussion
All the La(III) and Th(IV) complexes are colored, stable in air and non-hygroscopic solids. They are soluble in DMF and DMSO. The elemental analyses show that, the La(III) and Th(IV) complexes have 1:1 stoicheometry. The molar conductance values at the 10− 3M concentration are too low to account for any dissociation of the complex in DMF. Hence, the La(III) and Th(IV) complexes may be regarded non-electrolytes. In order to establish whether the water molecules present in the complexes coordinated to the metal ion, weighed complexes were dried in over P2O5 in a vacuum for ca 1 h. and then weighed again. No loss in weight was observed. These observations suggest the water molecules in the complexes are coordinated to the metal ion. The metal contents were estimated gravimetrically by the standard method [Citation33]. Carbon, hydrogen, nitrogen and sulphur were estimated by using C, H, N and S analyzer. The results of elemental analyses and molar conductance values are listed in .
Table I. Elemental analysis and molar conductance data of Schiff bases (I-IV) and their La(III) and Th(IV) complexes (1-8).
IR spectra
The important IR frequencies of the Schiff bases and their La(III) and Th(IV) complexes are listed in the and respectively.
Table II. Important infrared frequencies (cm-1) of 3-substituted-4-amino (8-formyl-7-hydroxy-4-methylcoumarin)-5-hydrazino-1,2,4-triazole Schiff bases.
Table III. Important infrared frequencies (cm-1) of La(III) and Th(IV) complexes (1-8).
In the IR spectra of Schiff bases a medium band appeared in the region 3130-3058 cm− 1 due to ν(NH). A medium of high intensity band ca 1630-1615 cm− 1 was assigned to ν(C = N) so confirming coumarin residues.
In addition to the above, a broad weak band with fine structure in the region 2758-2750 cm− 1 and that can be attributed to the intramolecular H-bonded-OH. Medium intensity bands in the 1610-1590 cm− 1 region are regarded as a combination of C = N of triazole ring and aromatic C = C stretching vibrations. A high intensity band is present in the 1295-1279 cm− 1 region with an additional band in the region ca 1600-1590 cm− 1 was assigned to the phenolic ν(C-O) vibrations. The band located at 1715-1700 cm− 1 in all these Schiff bases attributed to ν(C = O) [Citation34].
In the complexes we observed the following changes.
The disappearance of medium band appeared in the region 3130-3058 cm− 1 of the Schiff bases due to ν(NH) in the La(III) and TH(IV) complexes, supports the formation of M-N bands via deprotonation. So the H atoms of –NH group have been replaced by the metal ion. The characteristic band due to ν(C = N) appears in the region ca 1611-1603 cm− 1 for these La(II) and Th(IV) complexes. The low frequency shift of ν(C = N) band suggesting that, the C = N group is coordinated to the metal ion through nitrogen and this has resulted in lowering of the bond order of carbon to nitrogen link. The high intensity band due to phenolic C-O appeared in the region 1295-1285 cm− 1 in the Schiff bases appeared as a medium to high intensity band in the 1380-1392 cm− 1 region in the complexes. These observations support the formation of M-O bands via deprotonation. So the H atoms of –OH groups have been replaced by the metal ion [Citation35].
All these complexes exhibit a broad through band in the region 3422-3442 cm− 1 indicates the presence of lattice or coordinated water molecule [Citation36] and the additional two weaker bands in the region 800-750 and 720-700 cm− 1 due to ν(-OH) rocking and wagging mode of vibrations, confirms the presence of coordinated water in all La(III) and Th(IV) complexes[Citation37]. The medium intensity band in the region ca 555-537 cm− 1 is characteristic of ν(M-O) vibration of the phenolic oxygen and the coordinated water molecules [Citation38] and a band at ca 492-480 cm− 1 is due to ν(M-N) modes [Citation39].
In the infrared spectra of Th(IV) complexes, the two bands associated with asymmetric stretch appear around 1440 and 1270 cm− 1, the magnitude of separation between these two is ca 170 cm− 1, typical of bidentate bonding of nitrates. The band located at 1715-1700 cm− 1 due to ν(C = O) in the Schiff bases are unaffected in the complexes indicating that the lactone Oxygens are not involved in the coordination.
1H NMR spectra
The 1H NMR spectra of Schiff bases exhibit signals at 13.58, 10.21, 8.62 and 7.2-7.5 ppm due to –NH, phenolic OH, -CH = N and aromatic protons respectively. The signals around 2.84 ppm are due to methyl protons.
In case of the La(III) and Th(IV) complexes we observed the following changes.
The signal due to azomethine proton of Schiff bases (ca 8.62 ppm) shows a downfield shift ca 9.21 ppm [Citation40] in the spectra of complexes indicating the coordination of the azomethine group to the metal ion through metal ion. This downfield shift is due to deshielding of the = CH proton. All these observations supported the IR inferences. The resonance due to phenolic –OH ca 10.21 ppm disappears in the spectrum of complexes; this confirms that, the hydroxyl group reacted with metal ion via deprotonation. The resonance due to –NH ca 13.58 ppm disappears in the spectrum of complexes, indicating the coordination of the amine group to the metal ion. The signal due to water protons in the complexes appears ca 5.45 ppm. All these observations support the structure suggested by IR.
The signal due to a multiplet ca 7.-7.8 ppm due to aromatic protons and signals around 2.84 ppm due to methyl protons are unaffected in the case of metal complexes.
13C NMR spectrum
The 13C NMR spectrum of Schiff base (II) exhibits the signals at 117.4, 119.2, 120.4, 128.1, 135.0 and 149.2 ppm are corresponding to aromatic carbons. The resonance at 11.6 ppm is due to aliphatic (-CH3) carbon and the azomethine (-HC = N) carbon shows peak at 159.2 ppm.
Electronic absorption spectra
The electronic absorption spectra of the complexes were recorded for freshly prepared solutions in DMF, DMSO, THF and MeCN at room temperature. These complexes exhibited a broad band ca 31000 cm− 1 which was assigned to L → M charge transfer transition.
Magnetic studies
As expected all of these complexes are diamagnetic
FAB mass spectrum of Schiff base (II) and its La(III)(2) and Th(IV) complex (6)
The FAB spectrum of Schiff base (II) showed a molecular ion peak at m/z 500 which is equivalent to its molecular weight. The fragments in the spectrum leading to the formation of the species [C25H20N6O6]+. The FAB mass spectrum of La(III) (2) (Figure 1S) complex shows a molecular ion peak M+ at m/z 726 which is equivalent to its molecular weight of the La(III) (2) complex. This molecular ion undergoes fragmentation with the loss of three water molecules, gave a fragment ion [La(LII)]·2H2O at m/z 672. This molecular ion gave the fragment ion [La(C22H14N3O6)·3H2O] at m/z 610 by the loss of triazole (C3H3N3) and two lattice held water molecules. Finally it undergoes dematallation to form the species [LII + H]+ gave a fragment ion at m/z 498. All these fragmentation patterns are well observed in the FAB mass spectrum.
In case of the Th(IV) (6) (Figure 2S) complexes, the spectrum shows a molecular ion peak M+ at m/z 863, which is equivalent to its molecular weight [Th(LII)(NO3)·2H2O]·2H2O. This species by the loss of two water molecules gave a fragment ion [Th(LII)(NO3)]·2H2O at m/z 827. This fragment ion lost the one nitrate molecules and gave a fragment ion [Th(LII)]·2H2O at m/z 765. Finally, it undergoes dematallation to form the species [LII + H]+ gave a fragment ion at m/z 498.
Thermogravimetric analyses
TG and DTG studies were carried out for La(III) (2) and Th(IV) (6) complexes. These complexes decompose gradually with the formation of respective metal oxide above 600°C shown in Figures 3S and 4S. The nature of proposed chemical change with the temperature range and the percentage of metal oxide obtained are given in the Table IS. The thermograms of these complexes show initial weight loss in the temperature range 75-98°C is evidence for the presence of water molecules. The loss of water molecules in this temperature range indicates that they are present as lattice-held water [Citation41]. The percentage weight loss in this temperature range indicates that there are two to three water molecules each in all these complexes as lattice water. The second stage loss of water molecules in the range of 225-250°C in theses complexes correspond to the loss of water molecules in coordinated sphere. The third stage of decomposition of all theses complexes in the range of 475-520°C is corresponds to the loss of organic moiety and the final weight corresponded to that of the metal oxide.
Kinetic study
The Freeman and Corroll procedure [Citation42] was used to evaluate the kinetic parameters such as order of reaction and energy of activation from a single experimental curve from the plot of 
. The determined order of reaction and energy of activation are listed in Table IIS.
Antimicrobial results
The microbial results are systematized in and and shown in and . In the case of bacteriological studies it was observed that, the ligand shows moderate active toward E. coli, P. aeruginosa and S. aureus and low active towards S. pyogenes. Among the La(III) and Th(IV) complexes La(III) shows high activity towards E. coli, and P. aeruginosa. La(III) complexes show high active.
Table IV. Antibacterial and antifungal activities of Schiff bases (I-IV).
Table V. Antibacterial and antifungal activities of La(III) and Th(IV) complexes (1-8) and standards.
Figure 2. Antibacterial activity of Schiff base and its La(III) and Th(IV) complexes (100 μg/mL).
Figure 3. Antifungal study of Schiff base and its La(III) and Th(IV) complexes(100 μg/mL).
In case of antifungal activity it was observed that, the Schiff base shows high activity against A. flavus, A. niger and less active towards Cladosporium. In the case of La(III) complex, it shows high activity against A. niger and Cladosporium and moderate active towards A. flavus, in case of A. flavus we got interesting result that it shows high activity.
It has been suggested that the ligands with nitrogen and oxygen donor systems inhibit enzyme activity, since the enzymes which require these groups of their activity appear to be especially more susceptible to deactivation by metal ions on coordination. Moreover, coordination reduces the polarity [Citation31] of the metal ion mainly because of the partial sharing of its positive charge with the donor groups [Citation43] with in the chelate ring system formed during coordination. This process, in turn, increases the lipophilic nature of the central metal atom, which favors its permeation more efficiently through the lipid layer of the micro-organism [Citation44] thus destroying them more aggressively. The minimum inhibitory concentration μg/mL− 1 was shown by compounds ranging from 10-100 μg/mL− 1 against all the microbial strains are given in .
Table VI. Minimum inhibitory concentration (μg/mL−1) for La(III) and Th(IV) complexes.
Cytotoxic bioassay
It is evident from the data recorded in that, only the complexes 1, 4, and 7 displayed cytotoxic activity as LD50 at c. 1 × 10− 3 M against Artemia salina, while the Schiff bases and the remaining complexes were almost inactive for this assay.
Table VII. Brine shrimp bioassay data for the ligands (LI-LIV) and their metal (II) complexes 1-8.
Conclusion
The synthesized Schiff bases (I–IV) act as pentadentate ligands through the coordination of azomethine nitrogens, NH and phenolic oxygen atoms to the metal ion. The bonding of ligands to metal ion was confirmed by the analytical, i.r., 1H nmr, Uv-Vis, FAB mass and thermal studies.
All Schiff bases were found potentially active towards all microbial strains and the La(III) and Th(IV) complexes show even more promising results than the Schiff bases.
All compounds are soluble in DMSO and DMF solvents. All these observations put together lead us to propose the structures shown in and in which the complexes have the stoichiometry of the type [La(L)·3H2O]·2H2O and [Th(L)(NO3)·2H2O]·2H2O where (L = LI-LIV).
FAB-mass spectrum of La(III) Complex.
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FAB-mass spectrum of La(III) Complex.
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FAB-mass spectrum of La(III) Complex.
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FAB-mass spectrum of La(III) Complex.
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FAB-mass spectrum of La(III) Complex.
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Acknowledgements
The authors are grateful to the Chairman, Department of Chemistry, Karnatak University, Dharwad for the facilities. One of the authors (GBB) grateful to Karnatak University, Dharwad for grant of University Research Studentship.
Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
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