Comprehensive structural and antioxidant investigations of new Cu(Ⅱ) and Ni(Ⅱ) complexes based on an oxadiazole type ligand: syntheses, X-ray, and Hirshfeld analysis

Two new complexes incorporating the oxadiazole structural motif were synthesized and characterized using spectroscopic methods, including FTIR, and single crystal X-ray diffraction. The complexes [Cu(L)(ClO₄)₂(H₂O)]; 1 and [Ni(L)(H₂O)₃](NO₃)₂; 2 are mononuclear and hexa-coordinated, with one ligand molecule as a neutral tridentate NNN-chelate. The coordination sphere of complex 1 is completed by two perchlorate anions and one water molecule, while that of complex 2 is completed by three water molecules. The coordination geometry around metal ions is distorted octahedral for both complexes. Hirshfeld analysis was employed to explore the supramolecular features for both complexes where O···H interactions are the most significant. The synthesized complexes were assessed for their in vitro antioxidant activities using DPPH, nitric oxide and superoxide anion scavenging. The results demonstrated that the synthesized Cu(II) and Ni(II) complexes exhibited outstanding antioxidant activities. Complexes 1 and 2 (with IC₅₀ values of 53.24 ± 1.21 and 101.09 ± 2.21µM, respectively) exhibited notable nitric oxide radical inhibition, being 3.49- and 1.84- fold more potent than ascorbic acid. Also, they displayed 2.65 - to 2.31-fold higher superoxide anion scavenging efficacy than ascorbic acid, with IC₅₀ values of 33.47 ± 0.50 and 38.40 ± 1.89 µM, respectively.