Salicylaldehyde-based Schiff bases and their transition metal complexes: an overview on synthesis and biological activities

Due to their varied biological actions, salicylaldehyde-based Schiff bases and their transition metal complexes have received interest. The imine and hydroxyl groups of Schiff bases are essential in the mechanism of biological activities. Phenyl-derived metal complexes have increased antimicrobial activity owing to lipophilicity and delocalized p-electrons. Chelation lessens metal ion polarity and increases lipophilicity, allowing cell penetration and enzyme inhibition to cause higher antimicrobial and anticancer activity. A positive charge on the metal atom in salicylaldehyde-based Schiff-base metal complexes enhances the acidity of proton-containing Schiff-base metal complexes that promote stronger hydrogen bonding and increased anticancer activity. The t-butyl group substituents of the metal complexes improve hydrophobicity and anticancer action. Diorganotin(IV) and Ru(III) complexes have higher anticancer activity. Salicylaldehyde-based Schiff bases act as antioxidants by providing protons and electrons to remove free radicals that are improved by the electron-donating substituent. The complexes formation of the ligand develops antioxidant effectiveness, and nitro-substituted Cu(II) complexes, phenyl-containing diorganotin(IV) complexes, and Ru(III) complexes having d5 electronic configurations are more antioxidant. More investigation is needed to understand the biological activities of salicylaldehyde-based Schiff base and their transition metal complexes. This review summarizes synthetic methods, biological activities, and structure-activity relationships.