Acid Catalysis in the Oxidation of Substrates by Mononuclear Manganese(III)-Aqua Complexes.

Acids are known to enhance the reactivities of metal-oxygen intermediates, such as metal-oxo, -hydroperoxo, -peroxo, and -superoxo complexes, in biomimetic oxidation reactions. Although metal-aqua (and metal-hydroxo) complexes have been shown to be potent oxidants in oxidation reactions, acid effects on the reactivities of metal-aqua complexes have never been investigated previously. In this study, a mononuclear manganese(III)-aqua complex, [(dpaq)Mn(OH)] (; dpaq = 2-[bis(pyridin-2-ylmethyl)]amino--quinolin-8-ylacetamidate with an NO substituent at the 5 position), which is relatively stable in the presence of triflic acid (HOTf), is used in the investigation of acid-catalyzed oxidation reactions by metal-aqua complexes. As a result, we report a remarkable acid catalysis in the six-electron oxidation of anthracene by in the presence of HOTf; anthraquinone is formed as the product. In the HOTf-catalyzed six-electron oxidation of anthracene by , the rate constant increases linearly with an increase of the HOTf concentration. Combined with the observed one-electron oxidation product, anthracene (derivative) radical cation, and the substitution effect at the 5 position of the dpaq ligand in on the rate constants of the oxidation of anthracene, it is concluded that the oxidation of anthracene occurs via an acid-promoted electron transfer (APET) from anthracene to . The dependence of the rate constants of the APET from electron donors, including anthracene derivatives, to on the driving force of electron transfer is also shown to be well fitted by the Marcus equation of outer-sphere electron transfer. To the best of our knowledge, this is the first example showing acid catalysis in the oxidation of substrates by metal(III)-aqua complexes.