Synthesis, Antioxidant Evaluation, and Electrochemical Behavior of Novel Schiff Base-Metal Complexes

Introduction: This study presents the synthesis and evaluation of five novel Schiff base– metal complexes containing Cu²⁺, Cr³⁺, Co²⁺, Fe³⁺, and Hg²⁺ ions coordinated to a bis(indole)-based Schiff base ligand. The aim was to investigate their antioxidant and electrochemical properties in a comparative framework. Methods: The complexes were synthesized from indole-3-carbaldehyde and trans-cyclohexane-1,2- diamine and characterized using FT-IR spectroscopy. Antioxidant activities were assessed by DPPH, ABTS, and CUPRAC assays. Electrochemical behavior was analyzed by cyclic voltammetry using a glassy carbon electrode in TBAP/DMSO electrolyte medium. Results: The Cu(II) complex exhibited the highest DPPH radical scavenging activity, while the Cr(III), Co(II), and the free Schiff base showed superior ABTS activity compared to the standard antioxidant BHA. The free ligand also demonstrated CUPRAC activity comparable to BHA. Cyclic voltammetry revealed that all metal complexes exhibited anodic shifts in oxidation peaks relative to the free ligand. Additionally, the Cu(II) and Co(II) complexes showed extra redox waves, indicating unique redox-active behavior. Discussion: These findings suggest that metal coordination alters the electron-donating capacity of the Schiff base ligand and enables tunable redox behavior. The observed differences across metal ions underscore their role in defining both antioxidant potential and electrochemical reactivity. Conclusion: The synthesized Schiff base-metal complexes possess significant antioxidant activity and diverse redox properties, particularly the Cu(II) and Co(II) derivatives. These characteristics make them promising candidates for applications in redox-sensitive medicinal and analytical technologies.