Copper(II) complexes of aroylhydrazones: Preparation and structural characterization

Structures of solid copper complexes with aroylhydrazones obtained from nicotinic acid hydrazide and salicylaldehyde derivatives differing in substituents on the benzene ring were analysed by means of infrared (IR) and Raman spectroscopy. Studied aromatic hydrazones acted as neutral, monoanionic or dianionic tridentate ligands, depending on the tautomer form adopted during the complexation process. Various additional substituents on the salicylaldehyde originated ring were not involved in metal coordination, thus carbonyl/enolic oxygen, azomethine nitrogen and oxygen of the phenolic hydroxyl group were proposed as the coordination sites for the copper(II) ion. In all applied synthesis conditions Cu(II) complexes were obtained in Cu(II)/hydrazone molar ratio of 1/1, except for a hydrazone with nitro group, which did not form a complex with Cu(II) regardless of the synthetic conditions. Vibrational spectra indicated that hydrazone having only ortho-hydroxyl group with respect to the double CN bond as well as a hydrazone with an additional OH group in para-position on the benzene ring, formed monomeric copper(II) complexes including water as a ligand, while hydrazones with meta-substituents (Cl, OCH3) formed binuclear copper complexes, in which two copper(II) ions bridge hydrazone molecules via phenolic oxygen atom. Thermal properties of prepared copper(II) complexes were investigated by means of thermogravimetric analysis. [Display omitted] •The complexes of Cu(II) and different aroylhydrazones were prepared.•Whatever Cu:ligand ratio was used for synthesis, only 1:1 complexes were isolated.•Various tautomeric forms were involved in complex formation.•Hydrazones acted as neutral, mono- and dianionic tridentate ligands.