Structure-DFT-Bioactivity Correlation in Mixed-Ligand Fe(III), Mn(II), and V(IV)O Complexes with Bidentate NO Donors 1H-benzimidazole-2-carboxylic acid and Norfloxacin Ligands

Abstract

Three new mixed-ligand transition metal complexes of iron(III), manganese(II), and oxidovanadium (IV) were synthesized using 1H-benzimidazole-2-carboxylic acid and 1-ethyl-6-fluoro-4-oxo-7-(piperazin-1-yl)-1,4-dihydroquinoline-3-carboxylic acid (Norfloxacin) as coordinating ligands. The complexes were characterized by elemental analysis, infrared and electronic spectroscopy, mass spectrometry, molar conductance, magnetic measurements, and thermal analysis. Spectroscopic results indicate bidentate coordination of both ligands through nitrogen and oxygen donor atoms, leading to octahedral geometries for the iron(III) and manganese(II) complexes and a square-pyramidal geometry for the oxidovanadium (IV) complex. Molar conductance data supports the non-electrolytic nature of the complexes, while magnetic measurements are consistent with the proposed coordination environments. Thermal studies confirmed the presence or absence of coordinated water molecules in agreement with the suggested formulations. Density functional theory calculations revealed changes in frontier molecular orbital energies and global reactivity descriptors upon coordination, indicating modified electronic properties relative to the free ligands. In vitro antimicrobial studies showed that the metal complexes exhibited higher antibacterial and antifungal activities than the uncoordinated ligands, with inhibition zones comparable to a reference antibacterial drug. Anti-inflammatory evaluation demonstrated notable inhibitory effects for the oxidovanadium (IV) and manganese (II) complexes. Molecular docking studies suggested favorable interactions of the iron(III) and oxidovanadium (IV) complexes with DNA gyrase B, providing a possible molecular basis for the observed antibacterial trends. Overall, the results demonstrate that metal coordination significantly influences the structural, electronic, and biological properties of the ligand system.