Zn Complex with Homovanillic Acid: Theoretical (B3LYP/6-311++G(d,p)), Structural (FT-IR, NMR), Thermal (TG, DTG, and DSC) and Biological (Antioxidant and Antimicrobial) Characteristics

In this study, the structure of the synthesized Zn(II) complex with homovanillic acid (HVA) was investigated using the FT-IR, UV/Vis, and NMR spectroscopic methods, as well as elemental and thermal (TG, DTG, and DSC) analysis. The stoichiometric molar ratio of metal:ligand for the solid form of the complex was established as 1:2, with coordination through the carboxylate group and aromatic ring substituents. The theoretical structural and electronic parameters were calculated by the use of the B3LYP/6-311++G(d,p) method. Antioxidant properties were examined using spectroscopic tests: DPPH (1,1-diphenyl-2-picrylhydrazyl radical), FRAP (ferric reducing antioxidant activity), and ABTS (2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (diammonium salt radical cation). The Zn(II) complex with HVA showed similar or lower antioxidant properties compared to the ligand, depending on the antioxidant assay. The antimicrobial activity of acid and its complex with Zn(II) against <i>Escherichia coli</i>, <i>Bacillus subtilis</i>, and <i>Candida albicans</i> were also investigated by evaluation of the minimum inhibitory concentration (MIC). The Zn(II) complex shows higher antibacterial and antifungal activity compared to HVA.