Synthesis, Characterization and Antimicrobial Activities of Homoleptic and Heteroleptic Ni²⁺, Cu²⁺, and Zn²⁺ Complexes

The emergence of multidrug-resistant bacteria and fungi has become a major challenge in the pharmaceutical industry. Therefore, there is urgent need for new synthetic antimicrobial agents. Schiff bases and their metal complexes are more relevant in the biological and medical domains. Their activity profile is significantly influenced by the kind of Schiff base and the metal that is included into the complex. In this paper, the synthesis, characterization and antimicrobial examination of the homoleptic and heteroleptic Ni²⁺, Cu²⁺, and Zn²⁺ complexes of N-(2-hydroxy-1-naphthaldehyde)-2-amino-6-nitrobenzothiazole and 2,2’-bipyridine chelators is portrayed. The homoleptic complexes were prepared at ligands to metal ratio of 2:1 and the heteroleptic complexes were prepared at 1:1:1 ratio of Schiff base, 2,2’-bipyridine and metal. The metal salts used include nickel (II) acetatetetrahydrate, copper (II) acetatedihydrate and zinc (II) acetatedihydrate while CH3CH2OH served as solvent and triethylamine served as buffer. There were good yields when the compounds were isolated after preparation, various colors, high melting/decomposition temperature (206 ≥ 360 oC) were observed. The compounds were soluble in DMSO and DMF but insoluble in water. The spectroscopic analysis confirmed various coordinations. Imine coordination was between 1616 – 1642 cm-1, whereas typical M-N coordination was seen between (647 - 513 cm-1) and M-O (580 - 457 cm-1). Comparing the biological activities of the homoleptic Cu2+ and Zn2+ complexes, to those of the heteroleptic complexes of the same metal systems, the heteroleptic were observed to be better inhibitors. While the Cu2+ and Zn2+ recorded zero activity against Proteus mirabilis, Cu2+ and Zn2+ heteroleptic typed complexes recorded 5.5 mm inhibitory value against the bacterial. Antimicrobial screening was carried out against six bacteria (e.g., S. aureus, B. cerus, P. aeruginosa, P. mirabilis, S. typhi and K. pneumoniae) and three fungi (e.g.; The strain of A. niger, A. flaus and R. stalonfer) with inhibition zone of 1.0 - 14.0 mm (bacteria) and 1.0 - 25.0 mm (fungi). Interestingly, the activities recorded for both homoleptic and heteroleptic complexes were impressive, which presents them as good antibacterial and antifungal drugs. However, the heteroleptic complexes had most of the highest inhibition zones showing them as better antimicrobial agents. The presented results indicate that mixed-ligand coordination increases the biological activity and therefore heteroleptic metal(II) complexes are promising compounds to be used to develop new antibacterial and antifungal drugs.