Spectral, magnetic, thermal and electrochemical studies on phthaloyl bis(thiosemicarbazide) complexes

Abstract

The coordination behaviour of phthaloyl bis(thiosemicarbazide), H4PBT, towards VO2+, Co(II), Ni(II), Cu(II), Zn(II) and ions has been investigated. The elemental analyses, magnetic moments, spectra [UV-Vis, IR, ESR and MS] and thermal studies were used to characterize the isolated complexes. Cyclic voltammetry was used to study the electrochemical behaviour of the Cu(II) complex. The IR spectra showed that the ligand is deprotonated in the complexes and acts as a binegative SSNN donor. The f U–O (mdyn Å−1) and the length r U−O (Å) of the UO bond are calculated spectroscopically from the IR data. The loss of thiol hydrogen during the formation of complexes was confirmed from the pH-metric titrations of the ligand and its metal(II) complexes. The protonation constants of the ligand (pK1 = 9.45 and pK2 = 7.25) as well as the stability constants of its complexes were calculated. The intensity and position of the V=O band in the IR spectra reflect the geometry of the formed complex. Also, its colour (dark green) is consistent with the suggested stereochemistry. The binuclear VO2+ complex was isolated and confirmed by ESR spectra. The TG analyses for the VO2+, Co(II), Ni(II) and Zn(II) complexes suggest different decomposition steps. The Coats–Redfern and Horowitz–Metzger equations have been used to calculate the kinetic and thermodynamic parameters for the different thermal decomposition steps of some complexes. The redox properties, nature of the electroactive species and the stability of the Cu(II) complex toward oxidation were examined. The electrochemical data are discussed in terms of the kinetic parameters and the reaction mechanism.