Exploring the Reactivity of Bio-inspired Manganese Complexes with Dioxygen and its Derivatives

Abstract

The reactivity of manganese centers with dioxygen and its derivatives is well known in nature, but the exact nature of the intermediates involved is often unclear. Synthetic manganese complexes have been developed to serve as models of some of these enzymatic systems and aid in our understanding of what factors control these important pathways. The work presented herein focuses on the reactivity of Mn complexes supported by pentacoordinate amide-containing ligands with O2, alkylhydrogenperoxide, and H2O2. These studies have resulted in the formation and characterization of many new intermediates, adding to the general knowledge base regarding this kind of reactivity. The dioxygen activation pathways of mononuclear MnII complexes supported by pentacoordinate amide-containing ligands, [MnII(dpaq)](OTf) and the sterically modified [MnII(dpaq2Me)](OTf), are investigated. Through titration experiments and isotopic-labeling studies, a change of mechanism is observed as a result of the steric modification of [MnII(dpaq2Me)](OTf). This highlights the use of steric effects as a means to control which intermediates form along dioxygen activation pathways. The formation and properties of two new MnIII-alkylperoxo complexes, [MnIII(OOtBu)(dpaq)]+ and [MnIII(OOtBu)(dpaq2Me)]+, are described. These complexes were generated by reacting the corresponding MnII precursors with a large excess of tBuOOH at -15 °C in MeCN. These unstable MnIII-alkylperoxo complexes were spectroscopically characterized, and complementary density functional theory (DFT) calculations were also performed to gain insight into their bonding and structural properties. Additionally, the development of a new derivative of the dpaq ligand with steric functionalization is reported herein. The MnII and MnIII-hydroxo complexes of this ligand have been both spectroscopically and structurally characterized, revealing elongated equatorial Mn-N bonds as a result of the steric functionalization. These new complexes react with tBuOOH at room temperature to generate the first room temperature-stable MnIII-alkylperoxo species. Based on the ability to form MnIII-alkylperoxo species from the reaction between MnIII-hydroxo species and alkylhydrogenperoxides, the reactivity of MnIII-hydroxo complexes with H2O2 has been investigated as a possible means to form rare MnIII-hydroperoxo species.