Further Mechanistic Information on the Reaction between FeIII(edta) and Hydrogen Peroxide: Observation of a Second Reaction Step and Importance of pH

A detailed study of the effect of buffer, temperature, and pressure on the reaction of hydrogen peroxide with [FeIII(edta)H2O]- was performed using stopped-flow techniques. The reaction was found to consist of two steps and resulted in the formation of the already characterized high-spin Fe(III) side-on bound peroxo complex. The second step of the reaction was found to be independent of the hydrogen peroxide concentration. Formation of the purple peroxo complex is only observable above pH 7.5. Both reaction steps are affected by specific and general acid-catalysis. Five different buffer systems were used to clarify the role of general acid-catalysis in these reactions. Both reaction steps reveal an element of reversibility, which disappears on decreasing the acid concentration. The positive volumes of activation for both the forward and reverse reactions of the first step suggest a dissociative interchange substitution process for the reversible end-on binding of hydrogen peroxide to [FeIII(edta)H2O]-. Th...