Synthesis and antimicrobial studies of structurally-related Schiff bases and their metal complexes

ABSTRACT

Schiff bases were obtained from the reaction between 2-aminopyridine and 4-ethoxybenzaldehyde and the reaction of 4-ethoxyaniline and 2-pyridinecarboxaldehyde using the reflux method in ethanol for 2 hours producing 88.2% and 83.5% yield respectively. The 2-aminopyridine-4-ethoxybenzaldehyde Schiff base was dark yellow crystals with a melting point of 95 – 97 °C, while the 4-ethoxyaniline-2-pyridinecarboxaldehyde compound was dark brown crystals with a melting point of 118 – 120 °C. The IR spectrum showed the absorption band at 1681cm^-1 (C=N, imino), ¹³C NMR spectrum showed chemical shift at 158.48 ppm (C=N, imino) while ¹H NMR spectrum gave a 1 hydrogen singlet peak at 9.71 ppm, (HC=N-) for the 2-aminopyridine-4-ethoxybenzaldehyde Schiff base. Conversely, the IR spectrum of 4-ethoxyaniline-2-pyridinecarboxaldehyde Schiff base presented an absorption band at 1625 cm^-1 for the imino (C=N) group, while the ¹³C NMR  and ¹H NMR showed peaks at chemical shift 158.68 ppm and 8.69 ppm for the imino carbon and imino proton respectively. Furthermore, the zinc and chromium complexes of the Schiff bases were synthesized using the same method but for 3 hours giving yields above 50%. The IR data for the metal complexes showed shifts in the absorption bands when compared to the ligands and with a specific interest in the imino group absorption, it showed that the chelation occurred at the imino nitrogen for all metal complex. In addition, only the chromium complex of 4-ethoxyaniline-2-pyridinecarboxaldehyde showed antifungal activity against Saccharomyces cerevisiae with a zone of inhibition of 15 mm and 11 mm. All other compounds were inactive against all the strains of pathogens tested.