X-ray Structures, Redox Behavior, and Antibacterial Evaluation of Zn(ii), Ni (ii), Mn(iii), and Co(iii) Complexes with a Tridentate Ono Schiff Base Ligand

Four new metal complexes, [Zn(HL)2] (1), [Ni(HL)2] (2), [Mn3(L)2(OAc)4(OH2)] (3) and [Co(L)(HL)] (4) were synthesized using a tridentate ONO-type Schiff base ligand (H2L), derived from the condensation of 2-amino-2-methyl-1-propanol and 5-bromo salicylaldehyde. Single crystal X-ray diffraction revealed that the mononuclear Zn(II), Ni(II), and Co(III) complexes (1, 2 and 4) exhibit distorted octahedral geometries, whereas the mixed-valance Mn(III)/Mn(II) complex (3) crystallizes as a trinuclear specieswith acetate-bridged Mn centers. Electrochemical investigations in DMF demonestrated irreversible redox processes for complexes 2-4, consistent with the expected electrochemical characteristics of Ni(II), Mn(III), and Co(III) centers in distorted octahedral environments. The antibacterial activity of the complexes 1-4 was evaluated against Staphylococcus aureus, Bacillus subtili, Escherichia coli and Salmonella enterica using Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) assays. Among the series, complex 3 displayed the most antibacterial potency, with MIC values as low as 1.56 mu g/mL against B. subtilis and 3.12 mu g/mL against S. enterica. These findings demonstrate that the nature of the central metal ion and its coordination environment play a decisive role in tuning the redox behavior and antibacterial properties of Schiff base complexes, highlighting their potential for further functional and bioinorganic applications.