Many fundamental processes of life depend on the chemical energy stored in the O-O bond of dioxygen (O2), are catalyzed by manganese (Mn), Copper (Cu), or iron (Fe), and involve M-superoxo, peroxo, and/or oxo (M= Mn, Fe) intermediates. Most of the O2on our planet is derived from Mn-catalyzed H2O oxidation. Mechanisms for O-O bond cleavage have been shown with Fe and Cu to involve either protonation of a M-OOH at the distal oxygen, or the addition of a second metal ion to the distal oxygen to form a bridging peroxo. In contrast, mechanistic details regarding Mn-promoted O-O bond cleavage remain relatively unexplored. This talk will focus on a collaborative project with Professor Rybak-Akimova involving the dioxygen chemistry of a series of structurally related Mn complexes, containing readily derivatized N–heterocycles amines (NAr= 6-H-pyridine, 6-Me-pyridine, and quinoline). The steric and electronic properties of the complexes were tuned by varying NArsubstituents. We will show that the barrier to selected steps of O2binding and release, peroxo O-O bond cleavage, and H-atom abstraction, is highly dependent on the NArsubstituents. Incorporation of a more electron rich, less sterically encumbering 6-OMe substituent decreases the barrier to O2release, relative to O2binding, facilitates an accumulation of a Mn(III)-superoxo intermediate, allows us to monitor the kinetics of the O-O bond cleaving step. High-valent intermediates following the peroxo are shown to abstract a H-atom from TEMPOH (kH/kD= 2.2) or 1,4-cyclohexadiene

Tuning the reactivity and relative stability of manganese dioxygen intermediates via systematic ligand modification

2019 Spring National Meeting

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