Catecholase-mimetic activity and antioxidant properties of in situ Cu(II) complexes of histamine-derived ligands explored by spectroscopic studies

Three new histamine-derived Schiff base ligands were synthesized and studied: 4-[(E)-{[2-(1H-imidazol-4-yl)ethyl]imino}methyl]-N,N-dimethylaniline (L1), 2-ethoxy-4-[(E)-{[2-(1H-imidazol-4-yl)ethyl]imino}methyl]phenol (L2), and 1-[(E)-{[2-(1H-imidazol-4-yl)ethyl]imino}methyl]-5,8-dihydronaphthalen-2-ol (L3). The ligands were fully characterized by elemental analysis, mass spectrometry, 1H and 1³C NMR spectroscopy, infrared (IR), and UV–visible spectroscopic techniques, confirming their structures and functional groups. The catalytic properties of the in situ Cu(II) complexes were evaluated in the oxidation of catechol, and the kinetic parameters were determined from the Michaelis–Menten plots. The results showed that the structural features of the ligands strongly influenced the catalytic efficiency, as reflected by the variations in Vmax, Km, and kcat values. Furthermore, the antioxidant activity was assessed using the DPPH radical scavenging method. Ligand L1 demonstrated the strongest activity (IC50 = 0.326 µg/L), even higher than ascorbic acid (IC50 = 0.457 µg/L), while L2 and L3 exhibited moderate but significant antioxidant potential. Overall, these findings highlight the dual catalytic and antioxidant properties of histamine-based Schiff bases, supporting their potential applications in coordination chemistry, bioinorganic catalysis, and antioxidant research.