Synthesis, Characterization, and Antibacterial Activity of Cu(II), Ni(II), and Zn(II)Complexes of 14-Membered MacrocyclicTetraaza Ligand

The macrocyclic 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-7,14-dienium bromide salt (LBr) was synthesised for use in complexation reactions with various metal salts of copper, nickel, and zinc. Twelve complexes were synthesised; five of them were successfully obtained in a crystal form that was suitable for X-ray crystallographic analysis. All the complexes were analysed by micro elemental analysis, Fourier Transform infrared spectroscopy, ultraviolet-visible spectroscopy, and magnetic susceptibility. The metals formed coordination bonds to the four aza nitrogen atoms, during the process of which the azomethine nitrogen atoms were deprotonated. The cationic charge in each complex was balanced with a bromide anion from the macrocyclic ligand. Deprotonation of the azomethine nitrogen atoms led to the formation of an acid/base complex with the metal salts. Preliminary studies for biological activity, based on the minimum inhibitory concentration (MIC) method, indicated that the macrocyclic complexes showed greater antibacterial activity than the bromide macrocyclic ligand, which showed lower MIC values with S. aureus(6.3 mg/mL) and E. aerogenes and B. Subtilis (3.1 mg/mL).