Synthetic, structural and kinetic studies on the binding of cyclohexane-1,2-bis(4-methyl-3-thiosemicarbazone) to divalent metal ions (Co, Ni, Cu, Zn or Cd)

Abstract

The reactions of cyclohexane-1,2-bis(4-methyl-3-thiosemicarbazone) (CHMTSC) with MCl₂ (M = Co, Ni, Cu or Zn) and Cd(NO₃)₂ have been shown to produce complexes in which the thiosemicarbazone has been doubly deprotonated {[M(CHMTSC − 2H⁺)] (M = Co, Ni or Ni)}, analogous to those reported earlier with other Schiff base thiosemicarbazones. However, with ZnCl₂ and Cd(NO₃)₂, the complexes isolated are [ZnCl(CHMTSC)]Cl and [Cd(NO₃)(CHMTSC)]NO₃, containing the protonated forms of the ligand, which have been characterised by X-ray crystallography, as has free CHMTSC. The kinetics of the reactions between CHMTSC and all the various metal salts have been determined by stopped-flow spectrophotometry. In all cases, the reactions are complete on the seconds timescale. The reactions exhibit a first-order dependence on the concentration of metal salt and a first-order dependence on the concentration of CHMTSC. The thermodynamic and kinetic factors influencing the protonation state of the coordinated thiosemicarbazone are discussed.