Potentiometric, equilibrium studies and thermodynamics of novel thiosemicarbazones and their bivalent transition metal(II) complexes

Abstract

Protonation constants of novel thiosemicarbazones including 2-(1-(2-phenyl-hydrazono)-propan-2-ylidene) hydrazine-carbothioamide (TPHP) and N-methyl-2-(1-(2-phenyl-hydrazono)-propan-2-ylidene)hydrazinecarbo-thioamide (MTPHP) ligands and their corresponding metal-ligand formation constants with Cu(II), Ni(II), Mn(II) and Co(II) ions were determined at 15°C, 25°C and 35°C in 50% DMSO solution at I=0.1mol·dm−3 NaNO3. The stability order of complexes with reference to the metal ions has been followed this order Cu(II)>Ni(II)>Co(II)>Mn(II) in concord with the Irving-Williams stability order. Also, chemical equilibrium studies indicated that substitution of hydrogen in thiosemicarbazone (TPHP) by methyl group causes lowering of acidity as a result of the electron-releasing effect of the methyl group in the substituted amino group of the thiosemicarbazide moiety. The speciation of different species in solution has been evaluated as a function of pH. Additionally, the effect of temperature on protonation of thiosemicarbazone ligands and formation of their M(II)-thiosemicarbazone complexes was investigated. The thermodynamic parameters (ΔH, ΔS and ΔG) were calculated and discussed. It was found that both log K1 and –ΔH1, for M(II)-thiosemicarbazone complexes are somewhat larger than log10 K2 and –ΔH2, indicating a change in the dentate character of these ligands from tridentate (SNN-donors) in 1:1 chelates to bidentate (SN-donors) in 1:2; M:L chelates. Also, the lower values of log10 K2 and –ΔH2 than log K1 and –ΔH1 may be attributed to steric hindrance produced by the entrance of a second molecule.