Comparative Solution Equilibrium Study of the Interactions of Copper(II), Iron(II) and Zinc(II) with Triapine (3‐Aminopyridine‐2‐carbaldehyde Thiosemicarbazone) and Related Ligands

The interactions of Cu II , Zn II and Fe II with Triapine (3‐aminopyridine‐2‐carbaldehyde thiosemicarbazone), which is currently undergoing phase II clinical trials as a chemotherapeutic antitumour agent, were investigated in a water/DMSO mixture. The proton‐dissociation constants of the ligands, the stability constants and the coordination modes of the metal complexes formed were determined by pH‐potentiometric, UV/Vis spectrophotometric, EPR, 1 H NMR spectroscopic and ESI‐MS methods. Two N‐terminally dimethylated derivatives of Triapine were also studied. Mono‐ and bis‐ligand complexes in different protonation states were identified. Furthermore, the formation of the dinuclear species [Cu 2 L 3 ] + was confirmed for all ligands by EPR spectroscopy and ESI‐MS measurements. The results showed that the N‐terminally dimethylated ligands are much more potent chelators than Triapine for the divalent metal ions studied. All three ligands formed the least stable complexes with Zn II , whereas the Fe II complexes were somewhat more stable than the corresponding Cu II species.