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

Abstract

0-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 ligand towards
the transition metals. The thermal decomposition of Mn(ll), Co(ll).
Ni(ll), Cu(ll) and Zn(ll) 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
λS were also calculated from the thermogravimetric curves
using mechanistic and non-mechanistic (Coats-Redfern) equations.
The inechanism of the  decomposition has been established from
thermogravimetric data. On the basis of nur findings the relative
thermal stabilities of the chelates can be given as
[CoL(H₂O)₃] ≈ [CuL(H₂O)₃] < [ZnL(H2O)3] ] < [MnL(H₂O)₃]
< [NiL(H₂O)₃]·