A Mononuclear Non-heme Manganese(III)-Aqua Complex as a New Active Oxidant in Hydrogen Atom Transfer Reactions

A mononuclear non-heme Mn(III)-aqua complex, [(dpaq)Mn^III(OH₂)]²⁺ (1, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate), is capable of conducting hydrogen atom transfer (HAT) reactions much more efficiently than the corresponding Mn(III)-hydroxo complex, [(dpaq)Mn^III(OH)]⁺ (2); the high reactivity of 1 results from the positive one-electron reduction potential of 1 ( E_red vs SCE = 1.03 V), compared to that of 2 ( E_red vs SCE = -0.1 V). The HAT mechanism of 1 varies between electron transfer followed by proton transfer and one-step concerted proton-coupled electron transfer, depending on the one-electron oxidation potentials of substrates. To the best of our knowledge, this is the first example showing that metal(III)-aqua complex can be an effective H-atom abstraction reagent.