Mechanistic Studies on Peroxide Activation by a Water‐Soluble Iron( III )–Porphyrin: Implications for OO Bond Activation in Aqueous and Nonaqueous Solvents

The reactions of a water‐soluble iron( III )–porphyrin, [ meso ‐tetrakis(sulfonatomesityl)porphyrinato]iron( III ), [Fe III (tmps)] ( 1 ), with m ‐chloroperoxybenzoic acid ( m CPBA), iodosylbenzene (PhIO), and H 2 O 2 at different pH values in aqueous methanol solutions at −35 °C have been studied by using stopped‐flow UV/Vis spectroscopy. The nature of the porphyrin product resulting from the reactions with all three oxidants changed from the oxo–iron(IV)–porphyrin π‐cation radical [Fe IV (tmps .+ )(O)] ( 1 ++ ) at pH<5.5 to the oxo–iron( IV )–porphyrin [Fe IV (tmps)(O)] ( 1 + ) at pH>7.5, whereas a mixture of both species was formed in the intermediate pH range of 5.5–7.5. The observed reactivity pattern correlates with the E °′ versus pH profile reported for 1 , which reflects pH‐dependent changes in the relative positions of E °′   Fe   IV /Fe   IIIand E °′   P   ⋅+ /Pfor metal‐ and porphyrin‐centered oxidation, respectively. On this basis, the pH‐dependent redox equilibria involving 1 ++ and 1 + are suggested to determine the nature of the final products that result from the oxidation of 1 at a given pH. The conclusions reached are extended to water‐insoluble iron( III )–porphyrins on the basis of literature data concerning the electrochemical and catalytic properties of [Fe III (P)(X)] species in nonaqueous solvents. Implications for mechanistic studies on [Fe(P)]‐catalyzed oxidation reactions are briefly addressed.