Ring transformation and complex formation of 3-acetyl-4,5-dihydro-1,2,4-triazole oximes

Oximic 1,2,4-triazole ligands 2a–e were prepared from the reaction of 3-acetyl-4,5-dihydro-1H-1,2,4-triazoles 1a–e with hydroxylamine hydrochloride at room temperature. At higher temperatures, the reaction afforded, however, the novel ring transformation product 4-amino-2-(4-chlorophenyl)-5-methyl-2H-1,2,3,6-oxatriazine 3. The reaction of the ligands 2a–e with nickel (II) and palladium (II) acetates in ethanol at room temperature yielded the respective square planar complexes 5a–e, 6a,e. X-ray structure determination of one of these complexes (5a) revealed that metallation led to unexpected ring transformation of the triazole ligand. It is probable that such ring transformation generated the imidazole-N-oxide intermediate 4a which coordinated to Ni(II) ion, and the 4N-donor set comprises both imidazole nitrogen and arylhydrazone nitrogen. The whole process is associated with loss of one hydrogen molecule and formation of one new π-bond. The new compounds were characterized by elemental analysis, IR, 1H NMR, 13C NMR and HRMS.