Synthesis, Characterisation and Antimicrobial Evaluation of a New Bis(semicarbazone) Ligand and Its Coordination Complexes

This study aimed to synthesise a new multidentate bis(semicarbazone) ligand, (2Z,2'E)-2,2'-(((propane-1,3-diylbis(oxy))bis(2,1-phenylene))bis(methanylylidene))-bis(hydrazine-1-carboxamide) (H₂L), and to investigate its paramagnetic coordination compounds with bivalent transition metals (M(II) = Mn, Fe, Co, Ni, and Cu). The focus was on characterising both the ligand and its coordination compounds through physicochemical properties and evaluating their antimicrobial activity. H₂L was synthesised via a two-step process: first, ligand L was prepared and subsequently reacted with semicarbazide in a 1:2 mole ratio. A range of monomeric paramagnetic coordination compounds was isolated by reacting H₂L with metal chlorides of manganese(II), iron(II), cobalt(II), nickel(II), and copper(II) in a 1:1 molar ratio. The structures of H₂L and its complexes were confirmed using various analytical techniques, including FT-IR, UV-Vis spectroscopy, mass spectrometry, NMR spectroscopy, elemental analysis, magnetic moment measurements, and molar conductance measurements. TGA and DTA analyses for H₂L and selected complexes were also conducted. The antimicrobial activity of H₂L and its coordination compounds was evaluated against a broad spectrum of bacterial (Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Proteus vulgaris) and fungal (Candida albicans) species. The coordination compounds exhibited enhanced antimicrobial activity compared to free H₂L, with nickel(II) and copper(II) complexes demonstrating the highest activity against bacterial strains, surpassing the efficacy of amoxicillin. Additionally, the cobalt(II) and copper(II) complexes showed significant inhibition against Candida albicans compared to the commercial antifungal metronidazole.