Synthesis, spectroscopic characterizations, conductometric titration and investigation of potent antioxidant activities of gallic acid complexes with Ca (II), Cu (II), Zn(III), Cr(III) and Se (IV) metal ions

Gallic acid metal complexes were prepared and characterised using elemental analysis, infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, electronic spectroscopy, magnetic susceptibility, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The non-electrolytic nature of the gallic acid complexes was confirmed on the basis of their molar conductance values. Conductometric titrations for Cu+2 with gallic acid confirmed the formation of a binary Cu(II) gallic acid complex. Spectral data showed that gallic acid acted as a bidentate ligand with different metal ions through meta- and para-OH phenolic groups. The complexes had different geometries, including octahedral and square planar. The thermal properties of the complexes were explored using thermogravimetric and differential thermogravimetric techniques. The surface morphology of the gallic acid complexes was observed via scanning electron microscopy. Results showed that small particles can form agglomerates with different shapes. Transmission electron microscopy revealed that the gallic acid complexes have spherical black spots with particle sizes ranging from 4.90 nm to 93.87 nm. X-ray diffraction analysis showed that the crystallinity of gallic acid was not similar to its metal complexes, confirming that the complexes formed had well-defined crystalline structure. The antioxidant activities of gallic acid and its metal complexes were assessed. Results showed that the gallic acid complexes with Cu, Zn, Cr or Se showed considerable antioxidant activities. Further studies could evaluate the potency of these complexes to elevate the antioxidant defence system and enhance body functions against degenerative diseases, such as Alzheimer disease, and viral diseases, such as COVID-19.