Exploration of Iron(III) Complexes with Bidentate N, O-Donor Schiff Base Ligands through Synthesis, Characterization, DFT, and Antibacterial Studies

Abstract

Iron(III) complexes [Fe(SBn)2(H2O)2]NO3 (1, 2) (n=1 or 2) were synthesized via a reaction of Fe(NO3)3.9H2O with N, O-donor Schiff-base ligands (HSBn) in a ratio of 1:2 in terms of moles. Schiff-base ligands (HSBn) were synthesized when a solution of salicylaldehyde in methanol underwent reflux with aniline (HSB1) or 4-chloroaniline (HSB2) in methanol respectively. These aquated iron(III) complexes, 1 & 2 were further reacted with nitrogen-containing molecules as neutral donors (L) (such as pyridine, imidazole, and benzimidazole) in a ratio of 1 part to 2 parts by moles, yielding mixed-ligand complexes [Fe(SBn)2(L)2]NO3(3-8). The synthesized complexes were characterized by elemental analyses, mass spectrometry, and spectroscopic studies (electronic, infrared & NMR spectra), as well as measurements of magnetic susceptibility and conductance. Infrared spectra revealed that Schiff bases coordinated with iron(III) centers in bidentate and chelating mode, primarily through phenolic –O and azomethine–N groups. Electronic spectra confirmed the existence of octahedral iron(III) in a high-spin state within these complexes, consistent with assessments of magnetic susceptibility. DFT-based parameters were determined for ligands HSB1, & HSB2, and complexes 1, 2, & 8 using Gaussian 09 at the B3LYP level. Structural analysis showed slightly distorted octahedral geometry with high spin (d5) and sextet multiplicity in all complexes. The global hardness exhibited an increasing trend: HSB2 < 8 < 2 < 1 < HSB1, while chemical reactivity showed the opposite trend: HSB2 > 8 > 2 > 1 > HSB1. The antibacterial activity of ligands, HSB1 and HSB2, along with complexes 1 and 2, was evaluated against various bacterial strains. Schiff bases, HSB1 and HSB2 inhibited several strains, with complexes 1 and 2 demonstrating improved efficacy, possibly due to altered chemical properties from the complex formation. Despite lower potency than Ciprofloxacin, they offer therapeutic alternatives.