Synthesis, spectroscopic characterization, density functional theory, molecular docking studies, antioxidant and antimicrobial potential of novel Schiff base macrocyclic complexes of bivalent manganese

The threat of antibiotic resistance to public health is becoming a major cause of mortality on a worldwide scale. It has stimulated the interest of researchers in the development of novel antibiotics. Therefore, a new series of Schiff base macrocyclic ligands (N(8)MacL(1)-N(8)MacL(3)) and their bivalent manganese complexes were synthesized and analyzed for biological activities. Characterization of the synthesized macrocyclic compounds was done by various spectroscopic techniques like elemental analysis, infrared (IR), proton nuclear magnetic resonance (NMR), C-13 NMR, ultraviolet-visible, mass, electron paramagnetic resonance (EPR), and powder X-ray diffraction (PXRD). Based on spectral studies, an octahedral environment has been proposed around the manganese metal center in the synthesized macrocyclic complexes. Density functional theory (DFT) calculations were used to evaluate the stability and the electronic properties of the synthesized macrocyclic ligands and their Mn (II) complexes. The structures of the synthesized compounds were optimized by using the def2-SVP basis set at B3LYP level. The synthesized compounds were evaluated against gram-positive bacterial strains (Staphylococcus aureus and Bacillus cereus), gram-negative bacterial strains (Escherichia coli and Xanthomonas campestris), and fungal strains (Candida albicans and Fusarium oxysporum) for their antimicrobial activity by agar well diffusion method. Among all of the newly synthesized compounds, the macrocyclic complex [Mn(N(8)MacL(3))Cl-2] exhibited high antimicrobial activity against all bacterial and fungal strains except S. aureus. The virtual screening of synthesized Schiff base ligand [L], macrocyclic ligands (N(8)MacL(1)-N(8)MacL(3)), and their Mn (II) complex [Mn(N(8)MacL(1))Cl-2-Mn(N(8)MacL(3))Cl-2] are performed against the receptors, which were used for in vitro antimicrobial testing, to identify the most plausible drug-ligand interactions. The scavenging activity of DPPH radicals was utilized to study the antioxidant activity of the synthesized compounds. The macrocyclic metal complex [Mn(N(8)MacL(3))Cl-2] exhibited excellent antioxidant activity.