Two New Zinc(II) Complexes with Symmetric and Asymmetric 2,6-Diacetylpyridine–Aminoguanidine Schiff Bases: Synthesis, Crystal Structures, and Comparative Study

Novel compounds based on symmetric and asymmetric 2,6-diacetylpyridine (DAP) Schiff bases were synthesized. A new salt of the symmetric Schiff base 2,6-diacetylpyridine-bis(aminoguanidine), L1∙3HCl∙2H2O, and its zinc(II) complex, [Zn(L1)(SCN)2] (1), were prepared. By template synthesis, the zinc(II) complex with an asymmetric DAP derivative containing aminoguanidine and thiosemicarbazide moieties (L2), [Zn(L2–H)Cl] (2), was obtained. All compounds were characterized by elemental analysis, FTIR spectroscopy, molar conductivity, thermogravimetric analysis, antioxidant activity measurements, and, for the complexes, SC-XRD. In 1, the first complex containing the Schiff base of DAP and aminoguanidine, the chelating ligand coordinates as a pentadentate N5 donor through the pyridine, two azomethine, and two guanidine imino nitrogen atoms. Two SCN– are coordinated through sulfur atoms, an uncommon coordination mode with zinc(II). In 2, the ligand acts as an N3S tetradentate donor involving one azomethine, one hydrazine, the pyridine nitrogen, and thioamide sulfur atom. The chloride anion completes the coordination sphere. Thermal analysis demonstrated stepwise desolvation of the hydrated ligand, whereas the solvent-free complexes underwent continuous thermal decomposition with characteristic gaseous products. The ligand displayed considerable antioxidant activity, which was markedly diminished upon coordination.