SYNTHESIS AND CHARACTERISATION OF BIO-INORGANIC TRANSITION METAL COMPLEXES DERIVED FROM NOVEL BIGINELLI ADDUCT COUPLED SCHIFF’S BASES

SYNTHESIS AND CHARACTERISATION OF BIO-INORGANIC TRANSITION METAL COMPLEXES DERIVED FROM NOVEL BIGINELLI ADDUCT COUPLED SCHIFF’S BASES

Rohini Chandran ¹ and S. Arul Antony ^{* 2}

Department of Chemistry ¹, Sriram Engineering College, Perumalpattu, Thiruvallur - 602024, Tamil Nadu, India.

PG and Research Department of Chemistry ², Presidency College, Chennai - 600005, Tamil Nadu, India.

ABSTRACT: Novel Schiff base comprising 4-aminoantipyrine with Biginelli adducts of salicylaldehyde has been designed and synthesized. Solid metal complexes of this Schiff base with Ni(II), Zn(II), Cd(II) and Hg(II) metal ions were synthesized and characterized by elemental analysis, magnetic susceptibility, molar conductance, ESI mass, IR, NMR, and UV-Vis spectral studies. From the data, it has been observed that the complexes had the composition of ML₂ type, diamagnetic nature and a tetrahedral geometry around the metal ion except for Ni(II) complex which has square planar geometry. The Ligand and its complexes have been screened for their in-vitro antifungal and in-vitro antibacterial activities against the fungi i.e., Candida albicans, Aspergillus flavus, Penicillium spp., Aspergillus niger and Trichophyton and the gram-negative bacteria Escherichia coli, Vibrio spp., Pseudomonas aeroginosa, Vibrio para haemolytic, Salmonella spp., Aeromonas spp., Klebsiella spp., Proteus spp. and gram-positive bacteria Bacillus spp and Staphylococcus aureus. The in-vitro antimicrobial activities of these metal complexes found to be stronger than the ligand.

Schiff base, 4-aminoantipyrine, Biginelli reaction, Metal complexes, Spectral studies, In-vitro antimicrobial activity

INTRODUCTION: Schiff base transition metal complexes possess excellent characteristics, structural similarities with natural biological substances, prepared by relatively simple preparatory procedures and the synthetic flexibility that enables the design of suitable structural properties ¹. Schiff base is derived by the condensation reaction of aldehyde or ketones or primary amine and these containing azomethine (RCH=N) group. Many Schiff base ligands have been synthesized from heterocyclic compounds ².

The heterocyclic ring contained sulphur, nitrogen, and oxygen import special biological activity ^3-8 to Schiff base and their metal complexes. The coordinating property of Schiff base with 4- aminoantipyrine has a variety of applications in the biological, clinical, analytical, and pharmacological areas ^9-10. Aminoantipyrine is very much used in medicine ^2, and it is believed that its amino derivative would equally be of much use in medicine possibly as intermediate in antipyretic and analgesic drugs ¹¹. A thorough search of the literature reveals that no work has been done on the transition metal complexes of Schiff base condensed by 4-aminoantipyrine with Biginelli reaction through salicylaldehyde which is a characteristic aroma component of buckwheat ¹² and has good antimicrobial activity against certain bacteria or fungi ¹³.

Moreover, Biginelli reactions (Multicomponent reactions) are of increasing importance in the organic and medicinal chemistry due to their high degree of atom economy, applications in the combinatorial chemistry and diversity-oriented synthesis ¹⁴.

In the present context, bidentate ligand from condensing 4-amionoantpyrine with Biginelli adduct of salicylaldehyde was synthesized. The synthesized compound and its chelation behavior with metal ions were studied through physical parameters and spectral analysis. The in-vitro antimicrobial properties of ligand and metal complexes were also investigated.

MATERIAL AND METHODS:

Chemicals and Reagents: All the chemicals and solvents were of analytical grade procured from Spectrochem Pvt Ltd, Mumbai, the solvent ethanol was dried by the standard procedures ¹⁵.

Reagents Required: Salicylaldehyde, Urea, Ethyl acetoacetate, acetic acid, 4-aminoantipyrine, Ethanol.

Instrumental Analysis: The molar conductance was measured in ELICO – CM 180 instrument using a 10^-3 M Solution of DMSO as the solvent at room temperature. The elemental analysis was performed at IIT, Madras, India using analyzer Perkin Elmer 2400 for C, H, and N.  The metal contents were determined by standard gravimetric method ¹⁶.

IR characteristic vibrations were recorded in JASCO FTIR/4100 spectrometer in the range of 4000-400 cm^-1 using KBr pellet. UV visible spectra were recorded in JASCO UV spectrometer in the wave length range of 200-800 nm. H¹NMR and C¹³ NMR were recorded in CDCl₃ on a JASCO FTNMR Spectrometer. ESI mass spectra were recorded in Mass QTOFmicro mass spectrometer using nitrogen (CE-8ev, CV-27ev).

Magnetic susceptibility measurements of the complexes in the solid-state were determined by the Faraday method at room temperature using Lake Shore Model 7410 VSM, Std Ni as calibrant. Diamagnetic corrections were applied in compliance with Pascal’s constant.

The in-vitro antimicrobial activities were carried out using disc and well diffusion method. All the spectral activities carried out at IIT, Madras, India.

Synthesis of Schiff Base Ligand: The synthesis of Schiff base ligand consisted of two steps described as follows and is shown in Scheme 1.

Step 1: Preparation of 4-(2-Hydrophenyl)-6-mehtyl-2-oxo-1, 2, 3, 4-tetrahydropyrimidine-5-carboxylate by Biginelli reaction: A mixture of urea (1.32g. 22mmol), salicylaldehyde (1.20ml, 10mmol), ethylaceto acetate (1.60ml, 12mmol), ethanol (25ml) and acetic acid (2ml) was heated under reflux for six hours until the completion of the reaction (60 ºC, mp-180, monitored by TLC).

The solvent was removed under reduced pressure; the yellow colored solid obtained was treated with 50ml of water, filtered, washed with water, dried and crystallized from ethanol ¹⁷.

SCHEME 1(a): SYNTHETIC ROUTE FOR BIGINELLI REACTION

Step 2: Preparation of 2-ethyl 2-(2,5-dihydro-2,3-dimethyl-5-oxo-1-phenyl-1H-pyrazol-4-ylimino)-1,2,3,4-tetrahydro-4-(2-hydroxyphenyl) -6 methyl pyrimidine-5-carboxylate: The Schiff base ligand was prepared by adding an ethanolic solution (50ml) of 4-aminoantipyrine (2.03g, 0.01 mol) to Biginelli product (2.76, 0.01 mol). The equimolar mixture was stirred and refluxed for 8-12 h until the completion of the reaction (60 ºC, mp-232, monitored by TLC). This was then filtered and kept aside for 24 h to facilitate crystallization. The crystal formed was recrystallized with hot ethanol and dried in a desiccator over CaCl₂ vacuum ². The yellow colored yield was measured.

SCHEME 1 (B): SYNTHESIS OF PROPOSED SCHIFF BASE LIGAND

Preparation of Metal Complexes: The complexes were prepared by the reaction of the ligand (9.23g, 0.02 mol) with the respective 0.01M metal (II) salts in ethanol medium (2.34g NiCl_2. 6H₂O, 1.36 g ZnCl₂, 2.88g CdCl_2, and 2.71g HgCl₂).

The various metal salts and ligand in 1:2 ratios were refluxed and washed several times with ethanol after which kept aside for 24 hrs to facilitate crystallization ². The resulting crystals were then dried. The proposed structure of the complex is as shown in Fig. 1.

FIG. 1: PROPOSED STRUCTURE OF METAL COMPLEX

Pharmacological Assays:

Antimicrobial Assays: The biological activities of the synthesized Schiff base ligand and its Ni (II), Zn(II), Cd(II), and Hg(II) complexes were screened for their antibacterial and antifungal activities by the disc diffusion ^18-19 and agar plate ^20-21 method respectively. The in-vitro antibacterial activities of the compounds were tested against eight gram-negative bacteria like Escherichia coli, Vibrio spp., Pseudomonas aeroginosa, Vibrio parahaemolytics, Salmonella spp., Aeromonas spp., Klebsiella spp., Proteus spp. and two gram-positive bacteria like Bacillus spp and Staphylococcus aureus.

The in-vitro antifungal activities were carried out against Candida albicans, Aspergillus flavus, Penicillium spp., Aspergillus niger, and Tricho -phyton fungi ^22-23. The stock solutions of the test chemicals (1 mg mL⁻¹) were prepared by dissolving 10 mg of each test compound in 10 mL of distilled DMSO solvent. The different concentrations of the test compounds (100, 75, 50, and 25 μg mL⁻¹) were prepared by diluting the stock solution with the required amount of freshly distilled DMSO. Further, the controlled experiments were carried out by using freshly distilled DMSO solvent alone.

Antibacterial and Antifungal Screening: Muller-Hinton agar media was used for antibacterial studies.

The pure dehydrated Muller-Hilton agar (38 g) was dissolved in 1000 mL distilled water. The pure cultures of the bacterial strains E. coli, Vibrio spp., Staphylococcus aureus, Pseudomonas aeroginosa, Bacillus spp., Vibrio parahaemolytics, Salmonella spp., Aeromonas spp., Klebsiella spp., Proteus spp. were subcultured by inoculating in the nutrient broth, and they were incubated at 37 °C for about 18 h. The agar plates were prepared by using Muller-Hinton agar media, and wells were dug with the help of 6 mm sterile metallic cork borer.

Each plate was inoculated with 18-h-old bacterial culture (100 μL) using a micropipette and spread uniformly using a bent glass rod on each plate. The drug gentamycin was used as standard. Different concentration of the test compounds was incorporated into the wells using micropipette, and the plates were kept for incubation at 37 °C for 24 h. Soon after the completion of the incubation period, the diameter of the inhibition zone generated by each test compound against bacterial growth was measured using the antibiogram zone measuring scale. A similar procedure was followed for antifungal screening using Potato dextrose agar (PDA) media and Fluconazole drug as standard.

RESULTS AND DISCUSSION: All the complexes were characteristically colored, crystalline solid, non-hygroscopic, and air-stable at room temperature and could be stored for months without any appreciable changes. These were soluble in common organic solvents like Ethanol, Chloroform, DMSO, DMF, etc. but insoluble in water and decomposed on heating at a high temperature around equal or greater than 250 ºC.

Molar Conductance: The conductance measurements of the complexes were carried out in DMSO at room temperature and listed along with other parameters in Table 1. The obtained values were taken as good evidence for the existence of a non-electrolyte nature ^22-23 of the complexes, and the values were too low to account for any dissociation.

Elemental analysis or Microanalysis: Analytical data of the compound together with their physical properties are consistent with the proposed molecular formula as given in Table 1. The data found are in good agreement with the theoretical calculation. The results showed that all the metal chelates have 1:2 (M: L) stoichiometry. Sharp melting point and consistency of both experimental and calculated percentage of C, H, N, and M show the purity of the Schiff base complexes. Further, the purity is further supported by TLC technique.

TABLE 1: PHYSICAL PROPERTIES AND ELEMENTAL ANALYSIS OF PROPOSED LIGAND AND ITS COMPLEXES

*L – Schiff Base Ligand, *Molecular Weight- g/mol, *Molar Conductance (ohm^-1 Cm² Mol^-1)

ESI Mass Spectra: ESI mass spectra provide a vital clue for elucidating the structure of complexes, and the mass spectrum of the ligand is presented in Fig. 2.

It shows a molecular ion peak (M⁺) m/z at 461.24 which is equivalent to its molecular weight and the isotopic peak at (M⁺+1) at m/z = 462.24 due to  C¹³ and N¹⁵ isotopes. The base peak at m/z = 187 was due to 4-Aminoantipyrine (C₁₁ H₁₁ N₂O) ⁺ ion.

Another intense peak at m/z= 274 is due to (C₁₄H₁₆N₃O₃) ⁺ ion. The different competitive fragmentation pathways of ligand give the peaks at different mass numbers at 257, 185, 173, 171, 147, 132, 82, 77 and 56 due to (C₁₄H₁₅N₃O₂)^+, (C₁₁H₁₁N₃)⁺, (C₁₀H₉N₂O)⁺, (C₁₀H₉N₃)⁺, (C₈H₉N₃)⁺, (C₈H₈N₂)⁺, (C₃H₂N₂O)⁺, (C₆H₅)⁺, (C₂H₄N₂)⁺ ions respectively. The intensity of these peaks reflects the stability and abundance of the ions ^24-25.

The Spectrum of (NiL₂) Cl₂, (ZnL₂) Cl₂, (CdL₂) Cl₂ and (HgL₂) Cl₂ show a molecular ion peak (M⁺) at m/z 1052.29, 1059.28, 1106.35 and 1194.32 respectively that is corresponding to their molecular weight.  All these fragments lead to the formation of the species (ML₂)⁺ which further undergo demetallation to yield the species (L)⁺ which gives the fragment ion peak at m/z 461.

Thus, the data obtained from mass spectra confirm the stoichiometry of metal (1:2) ratio and in good agreement with the micro-analytical data.

FIG. 2: MASS SPECTRUM OF SCHIFF BASE LIGAND

IR Spectrum: The IR spectra of complexes are compared with those of the free ligand to determine the involvement of co-ordination sites in chelation Table 2 and Fig. 3.

The IR spectrum of the Schiff base exhibited characteristic band due to ⱱ_(C=N) & ⱱ_(C=O) at 1620 cm^-1 and 1684 cm^-1 respectively ²⁶. Also, the strong band at 3372cm^-1 and a characteristic intensity band at 3355cm^-1were assigned to ⱱ_(N-H) and ⱱ_{(phenolic - OH)} group respectively ²⁶. In comparison with the spectra of the Schiff base, all the metal complexes exhibit the band of ⱱ_(C=N) in the region of 1618-1602cm^-1 showing the shift of band to lower wave numbers indicates that the azomethine nitrogen coordinated to the metal ion ²⁷. The band of ⱱ_(C=O) in the region of 1680-1658 cm^-1 in the metal complexes showing the shift of band to lower wave numbers confirmed that the carbonyl oxygen is coordinated to the metal ion ²⁷.

The unaltered position of a band due to ⱱ_(N-H) and ⱱ_{(phenolic OH)} in all-metal complexes indicate that these groups are not involved in co-ordination.  The new bands in the region of 410-458 cm^-1 and 498-552 cm^-1 in the spectra of the complexes are assigned to stretching frequencies of M-O and M-N bonds respectively ²⁸.

Thus, the IR spectral results provided strong evidence for the complexation of Schiff base with metal ions through bidentate mode by azomethine and carboxyl oxygen groups.

TABLE 2: IR SPECTRAL DATA FOR THE PROPOSED LIGAND AND ITS COMPLEXES

*L – Schiff Base Ligand

FIG. 3: IR SPECTRA OF LIGAND AND ZINC METAL COMPLEX

NMR Spectra:

H¹ NMR Spectra: The H¹NMR spectra of the Schiff base and its metal complexes were recorded at room temperature in CDCl₃ Fig. 4.

FIG. 4: H¹ NMR SPECTRUM OF SCHIFF BASE LIGAND

The ligand shows the following signals: C₆H₅ as multiplet at 6.10-7.50 ppm (10H); N-CH₃ as single at 2.42 ppm (3H); -NH as singlet at 2.34 ppm (2H), H₃C-C as singlet at 1.85ppm (6H); H₃C-CH₂as triplet at 1.32ppm (3H) and H₂C-CH₃ as quartet at 4.21-4.25 ppm (2H) and CH as singlet at 4.58 ppm ²⁹.

The H¹-NMR of metal complexes compared with Schiff base ligand infers that there is no appreciable change in the signals of the complex as no hydrogen involves in chelation process.

C¹³ NMR Spectra: C¹³ NMR spectra of Schiff base ligand and metal complexes are summarised in Table 3 and given in Fig. 5.

In the ligand, the C=O carbon resonated at 160 ppm, but in the complexes, C=O band shifted down the field due to complexation ³⁰. The azomethine carbon C=N appears at 163ppm in ligand while in the complexes, C=N band shifted downfield due to chelation ³¹. There is a prominent peak that appeared in all spectra at 77ppm; this resulted from the solvent CDCl₃used in the analysis. There are a total number of 22 peaks which confirmed the structure of the ligand. There is also a concentration of peaks between 104-150ppm which indicate sp² hybridized carbons and they consist of azomethine carbons, C=C carbons, benzylic carbons and aromatic carbons ²⁹. Thus, NMR information further supports the apprehension of IR results.

TABLE 3: C¹³ NMR SPECTRAL DATA FOR THE PROPOSED LIGAND AND ITS COMPLEXES

* L – Schiff Base Ligand

FIG. 5: C¹³ NMR SPECTRA OF LIGAND AND CADMIUM METAL COMPLEX

Electronic Spectra and Magnetic Moment: The electronic spectra and magnetic measurements are performed to obtain information regarding the geometry of the complexes.

The nature of the ligand field around the metal ion has been deduced from the electronic spectra Table 4 and Fig. 6. The ligand spectral data displays two bands at 32258 cm^-1 and 25445 cm^-1 due to л →л* (C=O in pyrazole) and n→л*( C=N ) transitions ³².

The Ni (II) complex showed two absorption bands at 16501 and 21459 cm^-1 due to ¹A_1g→¹A_2g and ¹A_1g→¹B_1g d-d transition respectively supporting the square planar structure of Nickel complex ³³. Besides this, diamagnetic nature ^34-35 as well as the absence of absorption band at 16000 cm^-1 for ³T₁ (F) →³T₁ (P) transition, which again confirmed the square planar geometry ³⁶. Zn (II), Cd (II) and Hg (II) complexes are diamagnetic evidenced by the absence of characteristic d-d transition except for INCT bands as expected for d¹⁰ systems in the visible region. The electronic spectral studies also confirm the tetrahedral geometry to these complexes ³⁷.

Structural Interpretation: The structure of the complexes of the Schiff base derived from condensing 4-aminoantipyrine by Biginelli adduct of salicylaldehyde with Ni(II), Zn(II), Cd(II) and Hg(II) are confirmed by the elemental analysis, IR, UV-Vis, NMR, ESI mass spectra, molar conductance and magnetic data. From the IR spectra it is concluded that the ligand behaves as a bidentate ligand in all the complexes through the azomethine ‘N’ and pyrazole‘O’ and further this results are supported by NMR data.

The mass spectra data are correlated with elemental analyses and from the molar conductance data; it is found that all the complexes are non-electrolytes.

On the basis of the above observations and from the diamagnetic nature, UV-Vis data, the structure for the complexes could be proposed as follows: a square planar geometry for Ni (II) and a tetrahedral geometry for Zn (II), Cd (II) and Hg (II) complexes Fig. 7.

TABLE 4: UV-VIS SPECTRAL DATA FOR THE PROPOSED LIGAND AND ITS COMPLEXES

* L – Schiff Base Ligand; *INCT- Intraligand Charge Transfer Band

FIG. 6: ELECTRONIC SPECTRA OF LIGAND AND NICKEL METAL COMPLEX

FIG. 7: OPTIMISED STRUCTURAL GEOMETRY OF METAL COMPLEXES

*Red – Metal; *White – ‘N’ and ‘O’ of ligand

Pharmacological Results:

In-vitro Antimicrobial Activity: The in-vitro antimicrobial activity of all the synthesized complexes are screened against eight gram-negative bacteria like Escherichia coli, Vibrio spp., Pseudomonas aeroginosa, Vibrio parahaemolytics, Salmonella spp., Aeromonas spp., Klebsiella spp., Proteus spp. and two gram-positive bacteria like Bacillus spp and Staphylococcus aureus and fungi like Candida albicans, Aspergillus flavus, Penicillium spp., Aspergillus niger and Trichophyton fungi fungal strains by minimum inhibitory concentration (MIC) method. The minimum inhibitory concentration (MIC) profiles of all the complexes against bacteria and fungi are summarized in Table 5 and 6.

The MIC values indicate that all the complexes exhibited promising results than the ligand against the mentioned microorganisms, and this activity is enhanced on coordination with the metal ions. This enhancement in the activity may be rationalized on the basis that ligands mainly possess C=N bond. The enhanced activity of the complexes over the ligand can be explained based on chelation theory ^38-39. It is observed that, in a complex, the positive charge of the metal is partially shared with the donor atoms present in the ligand, and there may be π-electron delocalization over the whole chelating ⁴⁰. This increases the lipophilic character of the metal chelate and favors its permeation through the lipid layer of the bacterial membranes.

The heterocyclic Schiff bases with different functional groups have greater tendency to interact with nucleoside bases even after complexation with metal ion or with the essential metal ions present in the biosystem can act as promising bactericides because they always tend to interact with enzymatic functional groups, to achieve higher coordination numbers ⁴¹. There are also other factors which increase the activity, namely, solubility, conductivity, and bond length between the metal and the ligand.

TABLE 5: IN-VITRO ANTIBACTERIAL ACTIVITY OF PROPOSED SCHIFF BASE LIGAND AND ITS COMPLEXES

*Ba. spp - Bacillus spp *Stap. aurs - Staphylococcus aureus*E. coli - Escherichia coli, *Vib. Spp -Vibrio spp. *Pseu. aero - Pseudomonas aeroginosa, *Vib. para - Vibrio parahaemolytics, * Sal. Spp - Salmonella spp.,* Aero. Spp - Aeromonas spp.,*Kleb. Spp - Klebsiella spp.* Proteus Spp - Proteus spp.

*C = 5 mg/L. Inhibition zone diameter mm (% inhibition): +, 6-10 (27-45 %); ++, 10-14 (45-64 %); +++, 14-18 (64-82 %); ++++, 18-22 (82-100 %). Percent inhibition values are relative to the inhibition zone (22 mm) of the most active compound with 100 % inhibition.

TABLE 6: IN-VITRO ANTIFUNGAL ACTIVITY OF PROPOSED SCHIFF BASE LIGAND AND ITS COMPLEXES

*Cand.  Albi - Candida albicans, *Asper. Flav - Aspergillusflavus, *Pen. Spp -Pencillium spp.,*Asper. Nig –Aspergillus niger *Trichophyton - Trichophyton.

*C = 5 mg/L. Inhibition zone diameter mm (% inhibition): +, 6-10 (27-45 %); ++, 10-14 (45-64 %); +++, 14-18 (64-82 %); ++++, 18-22 (82-100 %). Percent inhibition values are relative to the inhibition zone (22 mm) of the most active compound with 100 % inhibition.

Antibacterial Activity: It has been observed that the antibacterial activity of the complexes followed the order: Ni (II) >Zn (II)>Cd (II) ~Hg (II)). The antibacterial activity of metal complexes is graphically represented in Fig. 8. The data reveal that E. coli is inhibited to a greater degree by almost all complexes and so they could be reasonably used for the treatment of some common diseases caused by E. coli Fig. 9.

Antifungal Activity: It has been observed that the antifungal activity of the complexes follows the order: Ni(II) >Zn (II)>Cd (II) ~Hg (II). The antifungal activity of metal complexes is graphically summarized in Fig. 10. The data showed that all metal complexes have great MIC towards Candida albicans and concluding that the complexes prepared with the new Schiff base could reasonably be used for the treatment of skin diseases caused by Candida albicans Fig. 11.

CONCLUSION: Ni (II), Zn (II), Cd (II) and Hg (II) complexes of the Schiff base derived from condensing 4-aminoantipyrine with Biginelli adduct of salicylaldehyde were prepared and characterized.  The study revealed that;

1. All the complexes are non-electrolytes.
2. The Schiff base behaves as a bidentate ligand and is co-ordinated to the central metal ion through azomethine ‘N’ and pyrazole ‘O’.
3. Ni (II) complex has square planar geometry while Zn (II), Cd (II) and Hg (II) complexes have tetrahedral geometry.
4. The in-vitro antimicrobial activity of all the complexes are higher than that of the free Schiff base ligand and follows order: Ni (II)>Zn(II)>Cd (II)>Hg (II).
5. This means that metal chelation significantly enhances the antimicrobial behavior of organic ligand.

ACKNOWLEDGEMENT: The author expresses their sincere thanks to The Principal, HOD and faculty members of Department of Chemistry, Sriram Engineering College, Thiruvallur and The Principal, HOD, PG and Research Department of Chemistry, Presidency College, Chennai for providing the research facilities.

CONFLICT OF INTEREST: Nil

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    How to cite this article:

    Chandran R and Antony SA: Synthesis and characterisation of bio-inorganic transition metal complexes derived from novel biginelli adduct coupled schiff’s bases. Int J Pharm Sci & Res 2014; 5(10): 4339-50. doi: 10.13040/IJPSR.0975-8232.5(10).4339-50.

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