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Synthesis and Antibacterial Study of Some Schiff Bases Complexes

Novel Schiff bases (E)-N-(4-chlorobenzylidene)-2-(2,4-dichlorophenyl) acetohydrazide and (E)-2-(2,4-dichlorophenyl)-N'-((1-methoxynaphthalen-2-yl)methylene) acetohydrazide were synthesized and further used for the synthesis of metal complexes. All these synthesized Schiff bases and metal complexes were characterized and screened for antimicrobial activity against bacteria Escherichia coli, Pseudomonas aeruginosa, and Bacillus subtilis.


Introduction
Schiff bases and their complexes are of high interest among the researchers because of their biological activity including anti-tumor, antibacterial, fungicidal, antidepressants, antiphlogogistic, nematocide, anti-carcinogenic and catalytic activity [1][2].
The microorganisms adsorb metal ions on their cell walls and through respiration process of cells these ions are disturbed and the process of protein synthesis is blocked which is the requirement for further growth of organisms. Gram-negative bacteria membrane is surrounded by an outer lipopolysaccharide membrane. Schiff base metal complexes combine with the lipophilic layer to enhance the membrane permeability of the Gram-negative bacteria. The lipid membrane surrounding the cell favours the passage of lipid soluble materials only; thus the lipophilicity is an important factor that controls the antimicrobial activity. Increase in lipophilicity enhances the penetration of Schiff base and its metal complexes into the lipid membranes and thus restricts growth of the organism [3].

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These Schiff base ligands were heated with different metals for the formation of corresponding complexes. Further the Schiff bases and complexes were screened for antimicrobial activity against bacteria Escherichia coli, Pseudomonas aeruginosa, and Bacillus subtilis.

General Procedure for the Synthesis of Schiff Base
In order to prepare Schiff base firstly esters of substituted phenylacetic acid were prepared, which were further reacted with hydrazine hydrate to obtain hydrazides from which Schiff base were prepared. a) General procedure for the synthesis of esters: To a magnetically stirred ice cold solution of carboxylic acid (20 mmol) in methanol (20 mL), a catalytic amount of concentrated H 2 SO 4 (2-3 drops) was added dropwise. The contents were gently warmed to room temperature and then refluxed for 2-3 h. After completion of the reaction as indicated by TLC (20% ethyl acetate: n-hexane); excess methanol was removed under reduced pressure on rotary evaporator. The reaction mixture was cooled to 0 °C, basified with saturated aqueous NaHCO 3 and finally extracted with dichloromethane (3x15 mL). The combined organic layer was washed with water, separated, dried over sodium sulphate and concentrated on rotary evaporator to afford the corresponding esters as oily liquids (Table-1). b) General procedure for the synthesis of hydrazides: A mixture of carboxylic ester (20 mmol) and hydrazine hydrate (100 mmol) was refluxed at 100 °C for 1h. Progress of the reaction was monitored by TLC (50 % ethyl acetate: n-hexane). After completion of reaction; the excess amount of hydrazine hydrate was evaporated under reduced pressure. The crude product was triturated with petroleum ether under ice-cold condition, washed several times with water and dried by toluene azeotrope to get the corresponding hydrazide as the crystalline white solid. c) General procedure for the synthesis of Schiff base: To an equimolar mixture of aldehyde (2 mmol) and hydrazide (2 mmol) in ethanol (5 mL), 2-3 drops of glacial acetic acid were added at room temperature and the content was refluxed till completion of reaction for appropriate time as specified in Table-

Spectral Analysis of Metal Complexes:
Characterization of all the prepared hydrazone-based Schiff base ligands complexes with metals Zn(II), Pd(II), Co(II), Cu(II), and Fe(II) were done.

Antibacterial Activity
Antibacterial activity of synthesized Schiff bases and their complexes has been screened against bacteria Escherichia coli, Bacillus subtilis & Salmonella aeruginosa. Selective media (HiMedia) were used for each type of bacteria.The antibacterial activity was evaluated using agar plate method. Agar plate's surface was inoculated by spreading a standardized inoculum of the test microorganism over entire surface. Then, a hole with a diameter of 6 mm is punched aseptically with a sterile cork borer or a tip, and a volume (20 µL) of the test compound solution is introduced into the well. The Petri dishes are incubated under suitable conditions. The test compounds diffuse into the agar and inhibits germination and growth of the test microorganism and then the diameters of inhibition growth zones are measured as a zone of inhibition [13,14]. Results of the ligands and complexes prepared were showed moderate to excellent activity as compared to standard Penicillin Table 3.

Conclusion
In conclusion we have synthesized novel Schiff bases and their metal complexes. These synthesized metal complexes were screened for antibacterial activity and found to exhibit good to moderate activity for complex A1, A2, A4 and A9. Complex A8 does not show any antibacterial activity while compound A6 showed very week antibacterial property. All other compounds were variable but have less than standard antibacterial activity.