Green synthesis, characterization and antimicrobial activities of selected Schiff bases

Abstract

Antimicrobial resistance is a serious global health challenge with alarming rates of emergence and spread of resistant microbes necessitating the development of novel antimicrobial agents. Schiff bases, also called imines or azomethines are synthesized from the condensation of aldehydes and amines. They have multiple properties including antibacterial and antifungal properties. Synthetic procedures of Schiff bases often involve the use of catalysts and solvents. The disposal of the latter, often leads to environmental pollution hence the need for the development of novel procedures that are applicable, environmentally friendly and economically realistic. Schiff bases were synthesized through grinding (for the solid aldehydes and amines) and stirring (for liquid aldehydes and amines) techniques without the use of catalysts and solvents in this work. This led to reduced reaction times, clean products, reduced possibility of pollution, no need for extra work up in the removal of solvents and catalysts and easy handling of the reagents. Six Schiff bases were synthesized. Schiff base S1, from benzaldehyde and 4-aminophenol, S2 from 4-aminophenol and 4-nitrobenzaldehyde, S3 from 4-aminophenol and salicylaldehyde, S4 from aniline and 4-nitrobenzaldehyde, S5 from aniline and salicylaldehyde and S6 from aniline and benzaldehyde. The compounds melted at constant temperatures demonstrating their purity. Characterization of the Schiff bases was done using FT-IR, UV-VIS and NMR (proton and carbon NMR) spectroscopy. Sharp IR peaks at 1628.95- 1617.38 cm-1 in the IR spectra of the compounds confirmed the formation of the imine bond, C=N. The disappearance of the carbonyl C=O peak at around 1600 cm-1 further confirmed the conversion of the aldehydes to imines. Free O-H broad bands were observed at 3447.91- 3339.29 cm-1. In compound S3, the O-H band was shallow and broad which was attributed to keto-enol tautomerism. In the UV-Vis spectra, bands at 261.40- 287.60 nm were observed corresponding to the n–π transitions of the azomethine C=N bond. NMR peaks at 7.26- 7.54 ppm for 1H NMR and 161.15- 158.34 ppm for 13C NMR further confirmed the presence of the imine protons and imine carbon atoms respectively. The disappearance of free NH2 peaks which usually occurs at 4.00- 6.00 ppm showed that the amines had been converted to imines. The synthesized compounds were subjected to antibacterial susceptibility tests against three Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli and Salmonella typhi), one Gram-positive bacteria (Staphylococcus aureus) and one fungi Candida albicans at concentrations of 500 ppm, 250 ppm and 125 ppm. The zones of inhibition developed from 7.16- 20.00 mm on all the organisms at all concentrations indicating that all the compounds were biologically active. The data was analyzed using One- Way Anova. Significant difference between the means indicated that compounds with nitro and hydroxyl substitution had greater activity against gram negative bacteria while the compound lacking the substitution had better activity against gram positive bacteria. The compounds also showed better antifungal activity against Candida albicans than the positive control. This indicated that the compounds had great potential for development of antibacterial and antifungal drugs.