Synthesis, characterization, and DFT calculations of Mn(II)–amino acid complexes for their antioxidant and antimicrobial activities

Abstract Mixed-ligand manganese (II) (Mn(II)) complexes with the general formula [Mn(INAP) (L)(H 2 O) 2 ] (L = Histidine, Phenylalanine, or Tryptophan) were synthesized in a 1:1:1 M ratio, where INAP denotes isonitrosoacetophenone and L represents an amino acid ligand such as l -histidine, l -phenylalanine, or l -tryptophan. The complexes were characterized using elemental analysis, molar conductance measurements, and spectroscopic techniques including UV–Vis, infrared, and electron spin resonance spectroscopy. The molar conductance values indicate that the complexes are non-electrolytic in nature. FTIR spectral data suggest that both the amino acid and isonitrosoacetophenone ligands coordinate to the Mn(II) center in a bidentate manner via nitrogen and oxygen donor atoms. Electronic absorption spectra and magnetic susceptibility measurements support a distorted octahedral geometry around the central metal ion. Density Functional Theory calculations were performed to obtain optimized geometries, structural parameters, vibrational frequencies, and quantum chemical descriptors of the synthesized complexes. Cyclic voltammetry was employed to study their electrochemical behavior, revealing that the redox potentials are influenced by the chelating nature of the ligands and the electron-donating effects of their substituents. The antioxidant activities of the complexes, which are closely associated with their redox behavior, were evaluated using the DPPH radical scavenging assay. Furthermore, the antimicrobial potential of the complexes was assessed against a range of pathogenic microorganisms.