Solid State Synthesis, Spectroscopic, X-ray and Antibacterial Studies of Zn(II) and Fe(III) Complexes of Pyridine-3-Carboxamide and 2-Hydroxybenzoic Acid

Mixed-ligand metal complexes are being prepared under solvent-free conditions and this has formed a vital ongoing research focus. One of the purposes is to overcome antimicrobial resistance while offering synthetic alternative to overcome environmental issues left behind by conventional use of solvents. In this work, zinc (II) and iron (III) complexes of pyridine-3-carboxamide (Pnc) containing 2-hydroxybenzoic acid (Hba) were synthesized as potential antibacterial agents through solvent-free process. The complexes were characterized using magnetic moment determination, spectroscopic and X-ray powder diffraction techniques. Analytical, magnetic, and spectral results suggested that the compounds have molecular formulae [Zn(C₆H₆N₂O)₂(C₇H₅O₃)₂] and [Fe(C₆H₆N₂O)₂(C₇H₅O₃)₂(H₂O)₂] with octahedral d¹⁰ and d⁵ configurations respectively. Pnc behaved as a bidentate ligand in the Zn(II) complex (binding through the two nitrogen atoms) and as a monodentate ligand toward Fe(III) ions while Hba acted as a monodentate ligand. NMR spectrum of the Zn(II) complex confirmed deprotonation of carboxyl group in Hba suggesting coordination of the metal via oxygen atom. Sharp X-ray diffraction peaks revealed highly crystalline complex compared to those of the ligands. Zeta potential and particle size measurements indicated that Fe^III complex could have higher uptake in a biological system whereas the Zn^II complex could penetrate better. Antibacterial sensitivity tests showed that metal coordination improved activities of the two parent ligands against Staphylococcus aureus, Escherichia coli, Salmonella typhi and Bacillus subtilis. The synthetic procedure presented for these potential antibacterial agents was fast, simple to follow with good yields, and environmentally safe without the need of toxic solvents or external heating. Graphical Abstract: (Figure presented.)