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)­MnIII(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)­MnIII­(OH)]+ (2); the high reactivity
of 1 results from the positive one-electron reduction
potential of 1 (Ered vs SCE
= 1.03 V), compared to that of 2 (Ered 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.