Reactivity of Bio-Inspired Manganese-Oxygen Intermediates

Abstract

Due to their ability to cycle between many stable oxidation states, Mn centers are employed by biological systems to react with oxygen and its reduced derivatives to mediate important chemical reactions. While these enzymes feature varied active-site structures, they utilize a common set of Mn-oxygen intermediates, which include oxygen ligands in the superoxo, peroxo, oxo, and hydroxo states. Taking inspiration from biology, Mn-based catalysts have been employed to perform C–H bond activation and olefin epoxidation using H2O2 as a cheap and environmentally-friendly oxidant. High-valent Mn-oxo centers are proposed to mediate difficult oxidation reactions for these catalysts. This dissertation focuses on the use of neutral, nitrogen-based ligands to support bio-inspired MnIII-peroxo and MnIV-oxo adducts. These species are characterized with spectroscopic and computational methods, and kinetic studies are utilized to investigate reactivity with a variety of substrates. A novel mononuclear MnII complex, [MnII(L7BQ)(OTf)2] was shown to react with H2O2 in the presence of base to access a new MnIII-peroxo species, [MnIII(O2)(L7BQ)]+. This intermediate was characterized by spectroscopic techniques, including electronic absorption and Mn K-edge X-ray absorption (XAS) methods. Aldehyde deformylation by [MnIII(O2)(L7BQ)]+ was only observed in the presence of acid. Aldehyde substrates often contain acid impurities that can protonate the MnIII-peroxo center prior to deformylation, making it difficult to assign the reactive species. MnIV-oxo species supported by derivatives of the N4py scaffold have allowed for direct investigation of structure-reactivity relationships. The introduction of benzimidazole moieties gave [MnIV(O)(2pyN2B)]2+, which was characterized by electronic absorption, XAS, and electron paramagnetic resonance methods. Kinetic studies of this MnIV-oxo complex and others in the N4py series has allowed access to a variety of equatorial field strengths. While the reactivity of MnIV-oxo centers in hydrogen atom transfer (HAT) has been studied extensively, much less is known about their oxygen atom transfer (OAT) reactivity. Recent calculations on the sulfoxidation of thioanisole by [MnIV(O)(N4py)]2+ suggest multi-state reactivity, but there is minimal experimental evidence to evaluate this theory. Kinetic studies for the oxidation of thioanisole and its derivatives offers insights into the reaction mechanism, and how that relates to the computationally predicted multi-state reactivity pathway.