Abstract
Some new coordination compounds of palladium(II) have been synthesized by the reaction of palladium(II) acetate with azomethines in a 1 : 2 molar ratio using acetonitrile as a reaction medium. Azomethines used in these studies have been prepared by the condensation of 2-acetyl fluorene and 4-acetyl biphenyl with glycine, alanine, valine, and leucine in methanol. An attempt has been made to probe their bonding and structures on the basis of elemental analyses and IR, 1H, and 13C NMR spectral studies. Pd(II) compounds have been found to be more active than their uncomplexed ligands as both of them were screened for antibacterial, antifungal, and insecticidal activities.
1. Introduction
Active and well-designed Schiff base ligands are considered as privileged ligands because they are easily prepared by the condensation between aldehyde or ketones and amines and able to stabilize different metals in various oxidation states. The chemistry of Schiff bases has occupied a place of considerable importance because of their well-established biological properties [1].
The Schiff base ligands are coordinated to the investigated metal ion through the azomethine nitrogen either alone or in combination with other electroactive site such as oxygen or sulphur. The N- and O-containing ligands and their complexes have become important due to their wide biological activities [2–5]. It is known that the existence of metal ions bonded to biological active material can enhance their activity [6]. Amino acids and their compounds with different metal ions play an important role in biology, pharmacy, and industry [7–12]. It has been reported that metal complexes of amino acid Schiff bases with transition metals possess anticarcinogenic [13], antimicrobial [14], and antitumor [15] activity.
Therefore, the present study was focused on the synthesis, spectral studies of Schiff bases containing, for instances, 2-acetylfluoreneglycine, 2-acetylfluorenealanine, 2-acetylfluorenevaline, 2-acetylfluoreneleucine, 4-acetylbiphenylglycine, 4-acetylbiphenylalanine, 4-acetylbiphenylvaline, and 4-acetylbiphenylleucine moiety and their complexes with Pd(II). The synthesized amino acid derived compounds (L1H–L8H) have been exposed to act as bidentate towards divalent metal atom solely through the azomethine nitrogen and carboxylate oxygen forming a stable five-membered chelate ring.
2. Experimental
2.1. Analytical Methods and Physical Measurements
All the chemicals used in this work were of AR grade and solvents were dried by a standard method. The reactions were carried out under strictly anhydrous conditions. Nitrogen was estimated by Kjeldahl’s method. IR spectra in the range 4000–250 cm−1 were recorded on a Nicolet Protége 460 FT-IR spectrometer as KBr pellets. A Jeol AL 300 MHz spectrometer was used to obtain the 1HNMR and 13C NMR spectra using DMSO-d6 as a solvent. The chemical shifts are reported in ppm and trimethylsilane (TMS) is used as a reference compound. Molecular weight determinations were carried out by the Rast camphor method 16.
2.2. Synthesis of Ligands
The azomethine were synthesized by the condensation of 2-acetylfluorene and 4-acetylbiphenyl with amino acids (glycine, alanine, valine, and leucine) in 1 : 1 molar ratio using methanol as a reaction medium. The solution was refluxed on a water bath from 5 to 7 h and then allowed to cool at room temperature. The crystalline solids were separated out and purified by recrystallization from the same solvent. The physical properties and analytical data are recorded in Table 1.
2.3. Synthesis of Complex
Azomethine of amino acids (0.416 g–0.642 g 2 mmol) was dissolved in 10 mL of dry acetonitrile in a round bottom flask. At the same time, palladium(II) acetate (0.225 g, 1 mmol) was dissolved separately in 10 mL of dry acetonitrile. Then, the metal solution was added dropwise into the flask containing the ligand solution. The contents were refluxed for about 5 hours. The solid derivative obtained was filtered, washed repeatedly with ethanol, and dried in vacuo. The purity of the compounds was checked by TLC using silica gel-G as an adsorbent. The physical properties and analysis of these complexes are listed in Table 2.
2.4. Biological Activity
2.4.1. Antibacterial Activity
All the synthesized ligands and their corresponding palladium(II) complexes were screened in vitro for their antibacterial activity against two Gram-negative (Escherichia coli and Proteus mirabilis) and two Gram-positive (Bacillus thuringiensis and Staphylococcus aureus) bacterial strains using a paper disc plate method [16, 17]. The nutrient agar medium (peptone, beef extract, NaCl and agar-agar) and 5 mm diameter paper discs of Whatman filter paper no. 1 were used. The compounds under investigation were dissolved in methanol to give concentrations of 500 and 1,000 ppm. The filter paper discs were soaked in these solutions, dried, and then placed in petri plates previously seeded with the test organisms. The plates were incubated for 24 h at 28 ± 2°C and the inhibition zone around each disc was measured. The antibacterial activity displayed by various compounds is shown in Table 3.
2.4.2. Antifungal Activity
The antifungal activity was evaluated against Aspergillus flavus, Fusarium oxysporum, Aspergillus niger, and Rhizopus phaseoliby the agar plate technique. Solutions of the compounds in different concentrations in DMF were then mixed with the medium. The linear growth [18] of the fungus was recorded by measuring the diameter of colony after 96 h, and the percentage inhibition was calculated as , where and are the diameters of the fungus colony in the control and test plates, respectively (Table 4).
2.4.3. Insecticidal Activity
H. armigera has a long history of insecticide resistance to DDT, pyrethroids, carbonates, organophosphates, and endosulfan. But against endosulfan, it shows less resistance [19]. Hence, the present study is a humble effort in the direction of accomplishing to make new insecticides, which shows less resistance to Helicoverpa armigera like endosulfan.
Four synthesized novel azomethine derivatives of amino acids and their complexes have been screened for their insecticidal activity against Helicoverpa armigera, and results are presented in Table 5. The study has been conducted on the second and third instar larval stages of the said insect. Two concentrations, namely, 0.05% and 0.075%, of the test compounds were selected along with the standard check, the endosulfan 35, together with an untreated control. The mortality counts of the insect pests were recorded daily up to ten days.
3. Result and Conclusion
Bimolar reaction of palladium(II) acetate with the aforementioned ligand of amino acids in 1 : 2 molar ratio in the presence of acetonitrile can be represented as follows:
The acetic acid formed in these reactions remains soluble in the reaction mixture and solid complexes could be separated by filtration. The newly synthesized complexes are brightly colored solid and soluble in DMF and DMSO. The molecular weight determined by the Rast Camphor method showed them to be monomer. The molar conductance measurement in DMF at room temperature shows the value in the range 10–15 Ω−1 cm2 mol−1 indicating nonelectrolyte natures of the complexes.
3.1. Spectroscopic Characterization
3.1.1. Significance IR Bands of Starting Material, Ligand, and Complex
The IR spectrum of a starting material amino acids shows two bands at 3374 cm−1 and 3308 cm−1 which are characteristics of the NH2 stretching mode while the carbonyl group shows a strong absorption band at 1635 cm−1 which is assigned to the >C=O stretching band. From IR spectrum of Schiff base ligand, new strong and sharp absorption band can be observed at 1618 cm−1 which is assigned for azomethine group >C=N [20] stretching mode. Besides, the absorption bands for NH2 stretching mode and C=O stretching mode have totally disappeared confirming the formation of azomethine compound.
The IR spectrum of palladium(II) complex shows that the absorption band for >C=N stretching mode has been shifted from 1618 cm−1 in free ligand to the strong and sharp absorption band at 1595 cm−1 in the complex indicating that the nitrogen atom is involved in bond formation with the metal ion. Besides, the broad O–H band at 3428 cm−1 in the ligand has been disappeared in the complex, suggesting the possible deprotonation on complexation and the formation of Pd–O bond. The appearance of new and strong medium intensity bands in the spectra of complexes in the region 360–365 cm−1 and may be attributed to (Pd–N) [21] 400–410 cm−1 due to (Pd–O) [22], respectively.
3.2. 1H NMR Spectra
The coordination of the metal to nitrogen and oxygen atoms is further supported by comparison of the 1H NMR spectral data of the ligand and its complexes. In the proton magnetic resonance spectra of the ligands, a sharp signal at δ1.80 ppm is observed due to –C(CH3)=N–. This moves downfield (δ1.98 ppm) in the complexes in comparison to its original position in the ligands due to coordination of azomethine nitrogen to the metal atom [23]. The ligands show the OH proton signal at δ11.10 ppm. However, in the complexes, these signals disappear showing the chelation through the carboxylic group. The ligand shows a complex multiplet signal in the region at δ7.28–8.39 ppm for the aromatic protons and it remain almost at the same position in spectra of the metal complexes. The results are given in Table 6.
3.3. 13C NMR Spectra
The 13C NMR spectral data for ligands and its corresponding Pd complexes in dry DMSO have been recorded in Table 7 and Scheme 1. The 13C NMR spectrum showed the displacement of azomethine carbon (>C=N–) from δ175.13 in the noncoordinated ligand to the downfield δ219.15 in the complex due to the coordination of azomethine nitrogen atom to the palladium metal. Therefore, a four-coordinate square planar geometry may be proposed for the resulting Pd(II) complexes.
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Thus, on the basis of the above discussion, it is clear that the ligand, by coordinating to Pd atom through the azomethine nitrogen, behaves as a bidentate ligand.
3.4. Antimicrobial Results
Antimicrobial tests of the ligands and their complexes on two Gram-negative (Escherichia coli and Proteus mirabilis) and two Gram-positive (Bacillus thuringiensis and Staphylococcus aureus) bacterial and selected fungi Aspergillus flavus, Fusarium oxysporum, Aspergillus niger and Rhizopus phaseoli were carried out. These results clearly indicate that the metal complexes are more active than the starting material and this is in accordance with the fact that chelation increases the activity. The chelation reduces the polarity [24, 25] of the metal ion, mainly, because of the partial sharing of its positive charge with the donor groups and possibly the -electron delocalization within the whole chelate ring [26–28]. This process of chelation thus increases the lipophilic nature of the central metal atom, which in turn favours its permeation through the lipid layer of the membrane [29–31].
3.5. Antiinsecticidal Result
To study the structure-activity relationship here we, have selected four azomethine amino acids and their metal complexes’ derivatives where the aromatic ring was the same; that is, fluorene ring. The carbonyl moiety was also the same; the only variation was in the alkyl substituted group. This study has suggested that an increase in the bulkiness of alkyl substituent and presence of the C=N moiety enhanced the bioactivity as evidenced from the experimental details of the study (Table 5, Figure 1). Compound 3 and compound 4 at concentrations 0.05% and 0.075% showed the best insecticidal activity, which was found to be superior to that of a standard insecticide endosulfan. Thus, the observed enhancement of activity of those complexes that were found to be more active than ligand must be due to a combination effect associate with the derivatization and complexation of the ligand and presence of the side group of the amino acid.
4. Conclusion
All the synthesized compounds show higher activity than the ligands but slightly lower than the standard drug. The compounds showed good antibacterial activity against S. aureus and B. thuringiensis than E. coli and P. mirabilis. The ligands and their complexes exhibit more significant effect on R. phaseoli and A. niger species than on F. oxysporum and A. flavus. The compounds showed toxicity at 200 ppm against all species of fungi. The activity decreased on dilution.
Acknowledgments
The authors are thankful to the Head of the Department of Chemistry, University of Rajasthan, Jaipur, for providing laboratory facilities and constant encouragement. M. Gupta and S. Sihag are thankful to CSIR, New Delhi, for providing financial assistance.