Kinetics and Mechanism of the Thermal Decomposition of o-Vanillin-L-Histidine Complexes of Transition Metal Ions

摘要

o-Vanillin-L-histidine forms 1:1 complexes with the transition metals like manganese,cobalt,nickel,copper and zinc.All the complexes are found to be non-electrolytes in distilled water and octahedral structures are assigned to them based on electronic.ESR and room-temperature magnetic measurements.IR studies reveal that the o-vanillin-L-histidine can act as a tridentate ligands towards the transition metals.The thermal decomposition of Mn(II),Co(II),Ni(II),Cu(II) and Zn(II) chelates of o-vanillin-L-histidine was studied by thermogravimetry.Thermal studies indicate that the decomposition takes place in two stages.The kinetic parameters,namely activation energy E,pre-exponential factor A and entropy of activation DELTAS were also calculated from the thermogravimetric curves using mechanistic and non-mechanistic (Coast-Redfern) equations.The mechanism of the decomposition has been established from thermogravimetric data.On the basis of our findings the relative thermal stabilities of the chelates can be given as [CoL(H_2O)_3]approx[CuL(H_2O)_3]<[ZnL(H_2O)_3<[MnL(H_2O)_3]<[NiL(H_2O)_3].