Structural diversity in copper(II) complexes of bis(thiosemicarbazone) and bis(semicarbazone) ligands

Abstract

Two symmetrical bis(carbazone) ligands, H₄L¹ and H₄L², were prepared by condensation of 4,6-diacetylresorcinol with thiosemicarbazide and semicarbazide, respectively. Their structures were elucidated by elemental analyses and IR, electronic and ¹H NMR spectroscopy. Both ligands are tetrabasic and contain two sets of SNO (H₄L¹) or ONO (H₄L²) coordinating sites. Binuclear, trinuclear and dimeric Cu^II complexes and also adducts with organic bases were prepared. Reactions of H₄L¹ with several Cu^II salts, including Cl⁻, AcO⁻,

,

and

, in addition to Cu^I as CuI, afforded different binuclear complexes depending on the salt and the working conditions, except for CuCl₂, where a trinuclear complex with a unique mode of bonding was obtained. These complexes reflect the strong coordinating power of Cl⁻,

and AcO⁻ compared to

. The mode of bonding and the basicity of the H₄L¹ ligand are also influenced by the counteranion, the pH of the reaction medium, the working conditions and the oxidation state of copper. The reactivity of the binuclear Cu^II complexes of both H₄L¹ and H₄L² towards 1,10-phenanthroline (Phen), 2,2′-bipyridyl (Bpy), N,N,N′,N′-tetramethylethylenediamine (Tmen) and 8-hydroxyquinoline (Oxine; Ox) were investigated. Adducts with organic bases were obtained in the mole ratio 2 : 1 : 1 for H₄L¹ and 2 : 1 : 2 for H₄L² [Cu^II : ligand : base (base = Phen, Bpy or Ox)], which reflects the difference in size of S and O. With Tmen, dimeric complexes were obtained containing no Tmen molecules instead of the expected adducts for both H₄L¹ and H₄L². Evidently the presence of Tmen in the reaction mixture enhances the dimerization process. Characterization and structure elucidation of the complexes was achieved by elemental and thermal analyses, electronic, IR, mass and ESR spectroscopy, as well as conductance and magnetic susceptibility measurements. Finally, the antifungal and antibacterial activities of H₄L¹ and its metal complexes were investigated.

Keywords:

• Binuclear
• trinuclear and dimeric CuII complexes
• Bis(thiosemicarbazone)
• Bis(semicarbazone)