The oxidative properties of a manganese(iv) hydroperoxide moiety and its relationships with the corresponding manganese(iv) oxo and hydroxo moieties

Abstract

Clear elucidation of the oxidative relationships of the active metal hydroperoxide moiety with its corresponding metal oxo and hydroxo intermediates would help the understanding of the different roles they may play in redox metalloenzymes and oxidation chemistry. Using an Mn(Me₂EBC)Cl₂ complex, it was found that, in t-butanol–water (4 : 1) with excess H₂O₂ at pH 1.5, the Mn^IV-OOH moiety may exist in the catalytic solution with a mass signal of m/z = 358.1, which provides a particular chance to investigate its oxidative properties. In catalytic oxidations, the Mn^IV-OOH moiety demonstrates a relatively poor activity in hydrogen abstraction from diphenyl methane and ethylbenzene with TOF of only 1.2 h⁻¹ and 1.1 h⁻¹ at 50 °C, whereas it can efficiently oxygenate diphenyl sulfide, methyl phenyl sulfide and benzyl phenyl sulfide with TOF of 13.8 h⁻¹, 15.4 h⁻¹ and 17.8 h⁻¹, respectively. In mechanistic studies using H₂¹⁸O and H₂¹⁸O₂, it was found that, in the Mn^IV-OOH moiety mediated hydrogen abstraction and sulfide oxygenations, the reaction proceeds by two parallel pathways: one by direct oxygen insertion/transfer, and the other by plausible electron transfer. Together with a good understanding of the corresponding manganese(IV) oxo and hydroxo intermediates, this work provides the first chance to compare the reactivity differences and similarities of the active metal oxo, hydroxo and hydroperoxide intermediates. The available evidence reveals that the Mn^IV-OOH moiety has a much more powerful oxidizing capability than the corresponding Mn^IVO and Mn^IV–OH functional groups in both hydrogen abstraction and oxygenation.