Synthesis, thermal behaviour and evaluation of the antioxidant capacity of ZnII complexes with polydentate Schiff bases

The aim of the study presented in this work was to obtain the new Zn-II complexes with different N2O2-, N3O4-, N2O5 and N2O3- donors Schiff bases (H2L1 (C29H26N2O2), H2L2 (C29H26N2O2), H4L3 (C30H29N3O4), H5L4 center dot H2O (C29H28N2O6), H3L5 center dot H2O (C19H24N2O4)) and characterized them by various physicochemical methods, such as: elemental analysis, spectroscopic (FTIR) and thermal analysis (TG/DSC, TG-FTIR). The synthesized homonuclear complexes were obtained as polycrystalline solids, stable at ambient temperature with the following formulas: [Zn-3(L1)(2)(CH3COO)(2)] (1), [ZnL2]center dot H2O (2), [ZnH2L3]center dot 0.5H(2)O (3), [Zn2L4(CH3COO)] (4), [Zn2L5(CH3COO)]center dot 1.5H(2)O (5). The results of FTIR spectroscopy indicate that azomethine nitrogen and phenoxide oxygen atoms participate in the coordination of zinc ions, and in some cases, the coordination sphere is supplemented with amino nitrogen atom or oxygen atoms from an acetate ion, a deprotonated hydroxyl group or a water molecule. TG/DSC and TG-FTIR analysis indicate that the thermal decomposition of the ZnII complexes is a multi-stage process depending on the applied Schiff base ligands and the used atmosphere (air or nitrogen). After heating, the complexes containing water molecules undergo a dehydration process, then an organic part is gradually defragmented and combusted/pyrolysed. The pyrolysis processes of the complexes are mainly associated with the release of water, carbon dioxide, ammonia, phenol, methane, carbon monoxide, and acetic acid (for 1, 4 and 5). Additionally, the antioxidant activities of the complexes in reducing power, FRAP (ferric reducing antioxidant power) were investigated. As a standard antioxidant compound, the ascorbic acid was used and the results were expressed as ascorbic equivalent (mu mol of ascorbic acid equivalents (AAE) per L). Schiff bases exhibit good antioxidant activity, which is enhanced or comparable when complexed with zinc ions. The exception is complex 3, which has a significantly lower reducing capacity than the corresponding free ligand (H4L3).