This is the first comprehensive review of the biological activity of hydrazone-transition metal complexes. Hydrazone complexes gained much attention because of their antifugial, antibacterial anticonvulsant, and analgesic, antiinflammatory, antimalarial, antimicrobial, antituberculosis, anticancer, and antiviral activities. Additionally, some of the hydrazone complexes were used in treatment of iron overload diseases. One application, which reflects the importance of hydrazone complexes, is their use in detection and determination of metals and some organic constituents in pharmaceutical formulations. This review will provide an overview of the biological, analytical and catalytic applications of this category of complexes.

A series of biologically active 3-quinoline carboxylic acid hydrazide-hydrazones has been synthesized from 3-quinoline carboxylic acid hydrazide and a variety of aldehydes, with moderate to good yields. The chemical structures of the new products were confirmed by elemental analysis, IR, and 1H NMR, 13C NMR spectral data. The structural and frontier molecular orbital (FMO) properties and the density functional theory (DFT) calculations were conducted for the new compounds. The new hydrazide-hydrazones exhibited low to moderate antibacterial activity against Staphylococcus aureus and Esherichia coli in comparison with gentamycin. Among the tested compounds, compounds 9 and 13 were found to be the most active. Phthalimide derivative 2 of 3-quioline carboxylic acid hydrazide showed remarkable antibacterial activity.

Schiff bases have played significant roles in the development of inorganic or coordination chemistry. Three Schiff base (NB, CB and HB) ligands, prepared for the reaction of 2-amino-6-methoxy-benzothiazole with 2-Nitrobenzaldehyde, 2-chlorobenzaldehyde and 2,4-Dihydroxybenzaldehyed, respectively, were investigated for their transition metal complexes, which were prepared by reacting the ligand (2:1 molar ratio) with Co(II), Ni(II), Cu(II), Cd(II), Cr(III) and Fe(III) chlorides. The nature of the interaction between the metal ions and ligands (L) was studied with the aid of magnetic susceptibility, elemental analysis, FTIR and ¹H-NMR spectroscopy. Based on the magnetic superstability and elemental analysis results, octahedral structures of the complexes, such as [ML₂Cl₂] or [ML₂Cl(OH)], were proposed for Cu(II), Cd(II), Co(II) and Ni(II) in which the ligand (L:NB, CB or HB) is bidentate through the azomethine and benzothiazole nitrogen. For Cr(III) and Fe(III) complexes, octahedral ML₂Cl(OH)₂ or ML₂(OH)₃ structures were proposed, where one ligand is monodentate and the other is bidentate. The azomethine ν(-HC=N-) and ¹H-NMR peaks of NB and CB were shifted to a higher frequency and downfield, respectively, upon complexation with metal ions. The bonding of OH groups of HB to Co(II), Cu(II) and Ni(II) enables π-backdonation from these metals to the azomethine of Schiff bases and the consequent shift of ν(-HC=N-) to a lower frequency and changes in the intensity of the ¹H-NMR peak of OH. On the other hand, this backdonation was not evidenced in the FTIR of HB complexes with high-charge Cr(III) and Fe(III) ions.