Thermal stability profiling of Co(II), Mn(II), and Ni(II)-hydrazone based complexes: The role of kinetic and thermodynamics parameters

Abstract

The thermal stability of the ligands ( (E)–4- [(2 – (2, 4 – dinitrophenyl)hydrazono)methyl)phenol (L₁) and N- (4 – hydroxybenzaldehyde) – p - fluoroaniline (L₂) along their complexes of Co(II), Mn(II), and Ni(II) were profiled by thermo-gravimetric analysis (TGA), different thermal (kinetic and thermodynamics) parameters viz. Energy of activation (E_a), Entropy of activation (ΔS), Free energy of activation (ΔG), Enthalpy of activation (ΔH) and Frequency factor (A) were calculated using Freeman-Carroll’s and Horowitz-Metzger’s approximation methods. The thermal results of the ligands showed that they do not contain crystals of water while the complexes contain two molecules of water of hydration. The kinetic parameters revealed that, the decomposition reactions of the synthesized compounds followed first order reaction with the rate constant values ranging from 0.002-0.129 and the activation energies were 55.22, 76.59, 52.90, 52.88 and 75.23 kJmol^-1 for ( (E)–4- [(2 – (2, 4 – dinitrophenyl)hydrazono)methyl)phenol (L₁), N- (4 – hydroxybenzaldehyde) – p - fluoroaniline (L₂), Co(II), Mn(II) and Ni(II) complexes respectively which showed that, L₂ and Ni(II) complex requires extra energy to form activated complex as compared to L₁, Co(II) and Mn(II) complexes. On the other hand, the frequency factor acquired were 3.56, 5.06, 3.48, 3.50 and 4.97 min^-1 for L₁, L₂, Co(II), Mn(II) and Ni(II) complexes accordingly indicating that, more spaces existed in L₂ and Mn(II) complexes than L₁, Ni(II) and Co(II) complexes. Thermodynamics parameters results showed that, the Gibb’s free energy (ΔG) of the synthesized compounds were positive indicating that, the decomposition was non-spontaneous, The positive values of enthalpy (ΔH) showed that enthalpy is the driving force for the decomposition of the synthesized compounds and exothermic in nature. The negative values of entropy (ΔS) indicate the degree of disorder of the products formed by the dissociations of the bonds is lower than that of the initial reactants.