Complex‐Formation Ability of Salicylaldehyde Thiosemicarbazone towards Zn^II, Cu^II, Fe^II, Fe^III and Ga^III Ions

Abstract

Abstract The stoichiometry and stability of copper(II), zinc(II), iron(II)/(III) and gallium(III) complexes of salicylaldehyde thiosemicarbazone (STSC, H 2 L ) have been determined by pH potentiometry, UV/Vis spectrophotometry, and 1 H NMR and EPR spectroscopy in aqueous solution (with 30 % DMSO), together with the characterization of the proton dissociation processes. Mono‐ and bis‐ligand complexes in different protonation states were identified for Fe II , Fe III and Ga III , whereas Cu II and Zn II ions only form complexes with a 1:1 metal/ligand ratio. The coordination mode in the complex [Zn 2 (H L )( L )(OAc)EtOH] was confirmed by X‐ray diffraction. The metal‐binding ability of STSC at physiological pH is in the following order: Ga III < Zn II < Fe II < Fe III < Cu II . Ga III –STSC complexes show unambiguously higher stability, whereas Fe II –STSC species show significantly lower stability relative to the corresponding α( N )‐pyridyl thiosemicarbazones like 2‐formylpyridine thiosemicarbazone or Triapine. Furthermore, the fluorescence properties of the ligand were investigated in aqueous solution, and their changes caused by complexation with Ga III and Zn II were studied.