Kinetico-mechanistic Study on the Oxidation of Biologically Active Iron(II) Bis(thiosemicarbazone) Complexes by Air. Importance of NH···O2 Interactions As Established by Activation Volumes.
Air oxidation of methanolic solutions of biologically active tridentate pyridyl thiosemicarbazone (TSC) complexes of the general formula [FeII(TSC)2] has been studied at varying dioxygen concentrations, temperatures, and pressures. The data collected indicate that the activation entropy of the reaction increases linearly with the redox potential of the complexes in a more definite way than the activation enthalpy. However, a very distinct behavior is observed for the values of the activation volumes, which do not follow the expected entropy-volume parallel trend for all of the systems studied. The involvement of important interactions between the terminal NH groups of the coordinated TSC ligand and molecular dioxygen has been found to be significant by measurements carried out at varying hydrostatic pressures. Kinetic experiments run on analogous N-deuterated complexes confirm the importance of these noncovalent interactions, which are weaker for the less acidic ND groups. These interactions show the existence of an ordering/expansion process upon going from the reactants to the transition state, whenever an interaction between the polar terminal amino groups and dioxygen can be established.