Synthesis, Molecular Structure, and Antibacterial Efficacies of Transition Metal Azo-Oximates of Fe(II) and Mn(II), a Sustainable Solution to Multidrug Resistance in Bacteria.

Azo-oxime ligands, namely, Ar-HLPh, 1 (Ar = phenyl, pyridyl, and naphthyl) and Ar⁄-HLpy 2 (Ar⁄ = phenyl, p-tolyl) were used to synthesize three trinuclear mixed valence iron complexes of type [FeIII(FeII(Ar-LPh)3)2]ClO4 3+ClO4 - and four bis-chelates [MII(Ar⁄-Lpy)2] (M = Fe, 4 and M = Mn, 5). The SCXRD structures of [MnII(Ar⁄-Lpy)2] and [FeII(Ar⁄-Lpy)2] (Ar⁄ = p-tolyl) are consistent with the theoretically optimized structures. The N─N (azo) distances are longer in comparison to their nickel(II) congeners and the order of average N─N (azo) lengths of [NiII(Ar⁄-Lpy)2] < [FeII(Ar⁄-Lpy)2] < [MnII(Ar⁄-Lpy)2], reported earlier. This is ascribed to greater π-acceptance by azo function with Ni(II) → Fe(II) → Mn(II). The nature of the FMOs of [MnII(Ar⁄-Lpy)2] and [FeII(Ar⁄-Lpy)2] (Ar⁄ = p-tolyl) has been inspected, and their absorption spectra were supported by TDDFT analysis. The complexes were subjected to anti-bacterial screening against two Gram-positive, namely, Streptococcus mutans and Staphylococcus aureus and two Gram-negative strains, namely, Escherichia. coli, and Salmonella sp. and their mode of action have also been detected. IC50 values of the complexes for all four bacterial strains designates that Gram-negative bacterial strains were more susceptible than the Gram-positive strains and among the two Gram-negative strains, Salmonella sp. was more affected than Escherichia coli. All seven complexes have significant broad-spectrum antibacterial potential for MDR bacterial strains, and the mode of antibacterial action appears to progress via a versatile molecular mechanism, including all the biomolecular fronts, namely, proteins, lipids, and nucleic acids damage.