Kinetic investigations of the formation of iron(IV) oxido complexes

Oxido iron(IV) complexes are key intermediates in oxygenation reactions in biological systems. They can form during oxidation experiments with various oxidants such as oxygen, hydrogen peroxide, organoiodine compounds, or ozone. However, kinetic studies for these reactions are rare because the intermediates are usually labile. Here the results of a detailed investigation on the oxidation of iron(II) complexes with macrocyclic (e.g. TMC = tetramethylcyclam = 1,4,8,11-tetramethyl-1,4,8,11-tetraaza-cyclotetradecane) ligands are reported that provide a better understanding of the mechanisms of these reactions. For formation of the quite stable oxido iron complex with an amide derivative of TMC, activation parameters with Delta H-double dagger = 28 +/- 4 kJ center dot mol(-1) and Delta S-double dagger = -144 +/- 15 J center dot mol(-1)center dot K-1 could be obtained that allowed postulation of an associative mechanism.