Microwave Synthesis, Characterization, Biological Activity of N- (p-Chlorophenyl)-N'-Benzoyl Thiourea and its Complexes

Ali MH¹, Zaghloul IE² and Khalid MW^3*
*Correspondence: Khalid MW, Researcher, Faculty of Science, Egypt, Tel: +201007068070, Email:

Keywords

Thiourea; Transition metals complexes; Microwave irradiation; Green chemistry; Biological activity; Antimicrobial activity; Antifungal activity

Introduction

Coordination compounds attracted great attention due to their structural variety, interesting physical & chemical properties and promising applications in many fields. Metal ions play an important role in the structure and function of many bio macromolecules and have important roles in the biological processes of metabolism as well as in pharmaceutical chemistry due to their chemical properties. Compounds bearing carbonyl and thio carbonyl groups are used as potential donor ligand for the preparation of complexes [1,2]. Among these, thiourea and its derivatives are versatile ligands that coordinate to form stable compounds. Thiourea (NH₂)₂C=S is a compound where the oxygen atom of urea compound replaced by a Sulphur atom. They are able to coordinate with metal either as neutral or mono-anion or dianion ligand [3,4]. Chemotherapy concerned in treatment of disease in which a chemical is specifically targeted for a microbial agent or a specific tissue. These thiourea ligands and their metal complexes were reported to act as antimicrobial, antibacterial, antifungal, antimalarial, anti-tuberculous and anticancer activities [7]. They also form a variety of complexes of different symmetries with various metal ions [7,8]. In view of the importance of thiourea and their derivatives it was worth interesting to synthesize Nsubstituted thiourea ligand and their complexes with transition metal elements because it was observed that this activity was enhanced by complexing with certain transition metal elements [3,9,10]. Complexes were synthesized using microwave-assisted irradiation. Microwave gives shorter reaction times, clean, high yields, and low cost [11].

Materials and Experimental Method

All purchased chemicals were of Annular AR grade and were obtained from Sigma Aldrich and all Metals salts were purchased from ADWIC. The Microwave-assisted synthesis was carried out in a domestic microwave energy output 900 W. Purity of Schiff base ligand and its complexes were detected by using thin-layer chromatography (TLC) technique. Melting points were recorded in open capillaries with Barnstead Thermolyne Mel-temp 1001D Electrothermal Melting Point. Elemental analysis was done on automatic analyzer CHNS Vario El III-Elementar, Germany. The FT-IR spectra samples were ground with (CsBr) powder. Then pressed into a disk and recorded on Shimadzu FTIR spectrometer. Mass spectra were determined by using Mass GC-2010 Shimdazu instrument. Metals content were determined by complexometric titration using xylenol orange (XO) as indicator and hexamine as a buffer (pH = 6). Electronic absorption spectra in DMF were measured using automated UV/Vis-NIR 3101 PC Shimadzu spectrophotometer ranged from 200-900 nm. ¹HNMR spectra for Schiff base ligand was recorded in 300MHz Varian-Oxford Mercury in DMSO-d₆ as solvent and the chemical shifts were recorded in ppm relative to TMS as an internal standard. Magnetic susceptibility of complexes was measured on powdered samples using the faraday method. Thermal analysis measurements (TGA) were carried out with Shimadzu thermal analyzer model 50 at Microanalytical. The ESR spectra of the powdered Cu^II complex recorded at room temperature by X-band EMX spectrometer (Bruker, Germany) using a standard rectangular cavity of ER 4102 with 100 KHz frequency.

Schiff base ligand and their metal complexes were screened for in-vitro antibacterial activity against two species of Gram-positive bacteria and two species of Gram-negative bacteria as well as two species of fungi. All of these were carried out in faculty of Science, Cairo University.

Chemicals

All consumed chemicals were from analytical grade and were used as received without further purifications. Chemicals used are Benzoyl chloride, ammonium thiocyanate, p-chloro analin, acetone, cobalt acetate, nickel acetate, copper acetate, zinc acetate and methanol.

Synthesis of N- (p-chlorophenyl)-N'-Benzoyl thiourea (PCBT)

0.1 m of ammonium thiocyanate (7.6 gm) dissolved in 50 ml of acetone then added drop by drop to 0.1 m of benzoyl chloride (14.06 gm) (11.62 ml) taken in 3 neck flask with continuous stirring. The mixture is refluxed for 1 hour with continuous stirring. After 45 minutes white ppt (ammonium chloride) appeared and then disappeared at 1 hr. The mixture left in room temp until the precipitate appears again completely. Filtration done and precipitate washed by acetone to get all the filtrate (Benzoyl thiocyanate). The filtrate added drop by drop in a 3-neck flask contains 0.1 m (12.75 gm) of para chloro analin dissolved in 25 ml acetone with continuous stirring. The mixture refluxed for 2 hrs with continuous stirring. The mixture transferred to a baker and covered for two days for complete precipitation. Then the Precipitate was filtrated and washed by ethanol and acetone. Purity of ligand achieved by recrystallization using ethanol and dried over anhydrous CaCl₂ in a desiccator to give the pure ligand (PCBT).

Synthesis of metal complexes

The prepared ligand and the acetate salts of the metal Co (CH₃COO)₂.4H₂O, Ni (CH₃COO)₂.4H₂O, Cu (CH₃COO)₂.H₂O and Zn (CH₃COO)₂.2H₂O were mixed in (1:1) ratio. The reaction mixtures were then irradiated by the microwave oven by using drops of methanol as a solvent. The reaction was completed in a short time (3-5 min) with higher yields. The resulting product washed by hot methanol and ether and finally dried under reduced pressure over anhydrous CaCl₂ in a desiccator. The progress of the reaction and purity of the product was monitored by TLC using silica gel (yield: 78-83 %). The synthetic route of the prepared compounds is illustrated in Scheme 1.

Biological Activity

Antibacterial and anti-fungal effect

Some chelates exhibited a moderate inhibitory activity of complexes than that of the corresponding free ligands.

The free ligand (PCBT) and its metal complex Co^II in addition to the standard drugs were screened separately for their antibacterial activity against Staphylococcus aureus (ATCC:6538), streptococcus mutans (ATCC: 25175) (Gram-positive bacteria), Escherichia Coli (ATCC:9637) and Klebsiella Pneumonia (ATCC:10031) (Gram-negative bacteria) and antifungal activity against Aspergillus Nigar (ATCC:32856) and Candida albicans (ATCC:6538)fungi.

The antimicrobial activity against the growth of various microorganisms were determined by measuring the inhibition zone in millimeters around the well, also the activity index data was calculated [22].

The result is recorded in Table 5. As we can observe from the results metal complexes act as more powerful bactericides and fungicides agents and they may serve as a vehicle for activation of ligand where the metal ions being more hypersensitive against the microbial cells.

Table 5. Antibacterial and antifungal assay of ligand and its ZnII complex.

This behaviour of the metal complexes may be a result to the modification in structure upon coordination and formation of metal organic framework and can be explained on the basis of the overtone concept and chelation theory.

In general, the easy penetration of the metal complexes into lipid membranes, disturbance of the respiration process of the cell and blocking the synthesis of proteins are restrict further growth of the organism and lead to enhance of activity of metal complexes compared with the organic ligand [23-31]. We did different fungi and ligand interactions with proper docking of the compound.

Conclusion

Complexes were synthesized successfully and fully characterized by chemical and spectroscopic methods. Then biological activity was studied and compared between the ligand itself and the Zn^II complex. The study showed that the prepared compounds have an appreciable activity and can consider as an effective inhibitor towards the different microbial strains. Generally such activity enhanced upon complexation where metal complexes show better activity than their parent ligand.

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