Three Zn(II) complexes with a sexidentate N 2 O 4 -donor bis-Schiff base ligand: Synthesis, characterization, DFT studies, in vitro antimicrobial evaluation and molecular docking studies

Abstract Three Zn(II) complexes with a sexidentate N2O4-donor bis-Schiff base ligand, namely Zn(C36H34N2O8)·2CH3OH (1), Zn(C28H34N2O8S2)·CH3OH (2) and Zn(C40H36N4O8)·3CH3OH (3), (L1, C36H34N2O8 = 1,2-bis(2-methoxy-6-formylphenoxy)ethane- l -phenylalanine; L2, C28H34N2O8S2 = 1,2-bis(2-methoxy-6-formylphenoxy)ethane- l -methionine; L3, C40H36N4O8 = 1,2-bis(2-methoxy-6-formylphenoxy)ethane- l -tryptophan) were synthesized and fully characterized by physico-chemical and spectroscopic methods. The X-ray crystallography shows that the metal atoms of three complexes are all six-coordinate with two nitrogen atoms from C N groups, two oxygen atoms from ether groups and two carboxylic oxygen atoms in the mono-ligand, forming a distorted octahedral geometry. Theoretical studies of the three complexes were carried out by density functional theory (DFT) Becke’s three-parameter hybrid (B3LYP) method employing the 6–31G basis set. Moreover, the antimicrobial activities of the complexes were evaluated against Escherichia coli and Staphylococcus aureus by the agar-well diffusion method. The experiment showed that complex 2 exhibited the highest antimicrobial activity. At the same time, molecular docking was investigated to determine the molecular interaction of the complexes with microbial synthase. The docking simulation exhibited that complex 2 was well embedded into the active pocket of the enzyme and showed a more stabilized structure than complex 1 and 3. Their structure-activity relationships were further discussed.