| dc.description.abstract | Many biochemical reactions critical for life require catalysis by manganese-containing enzymes that react with dioxygen or its reduced forms to generate manganese-oxo, peroxo, or hydroxo intermediates. These reactions are prevalent throughout nature and include the synthesis of DNA by ribonucleotide reductases, free radical detoxification by superoxide dismutases, and metabolic pathways catalyzed by cytochrome P450 enzymes. Peroxomanganese(III) intermediates are proposed in many of the catalytic cycles that are governed by manganese-containing enzymes, and are often precursors to high-valent active oxidant species that are capable of substrate oxidation. While there have been successful synthetic models of the active sites of many enzymes, the factors that direct the reactivity of these peroxomanganese(III) intermediates are not well defined. In particular, the electronic and geometric effects of the supporting ligand system on the reactivity of peroxomanganese(III) species are poorly understood. To characterize the geometric structures and inherent reactivity of these intermediates, low-temperature spectroscopic techniques have been utilized, including electronic absorption, electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), and X-ray absorption spectroscopy (XAS). In addition, these complexes and their reactivity have been computationally explored through density functional theory (DFT), time-dependent DFT (TD-DFT), and multi-reference ab initio computations. A MnIII-peroxo species supported by the neutral, cross-clamped Me2EBC ligand was prepared and spectroscopically and computationally characterized. The reactivity of the [MnIII(O2)(Me2EBC)]+ species with redox-active MnII was explored and found to display unique reactivity compared to other MnIII-peroxo species by generating mononuclear high-valent products and avoiding dinuclear product formation. This reactivity is reminiscent of the catalytic cycle observed in enzymatic systems and is the one of the few examples of activation of a MnIII-peroxo species to mononuclear, high-valent intermediates. Additional reactivity studies of [MnIII(O2)(Me2EBC)]+ were explored, including activation by the addition of protons from Brønsted acids, hydrogen atom addition from electrophilic substrates, and modulation of the electron density of the MnIII-peroxo moiety through Lewis acid addition. A trispyrazolylborate scorpionate-type ligand (TpPh2) was used to support a stable MnIII-peroxo species that was characterized by spectroscopic and computational methods. The [MnIII(O2)(TpPh2)(THF)]+ complex displays a blue-shift in the lowest energy transition compared to other MnIII-peroxo species, and this shift was determined to be a result of an axial bond elongation that stabilized the donor MO in this transition. Perturbations in the electronic structure of this MnIII-peroxo species and two isomers of the related [MnIII(O2)(TpiPr2)(pziPr2H)] species were explored through TD-DFT and multireference ab initio computations. Controlling O—O bond reactivity is a critical step in the modulation of the activation of MnIII-peroxo species, but there are few successful studies in this area. A MnIII-peroxo species supported by a pentadentate, N4O- ligand was electrochemically formed and displayed reductive activation that was controlled by the strength acid added to the reaction. In the presence of a strong acid, Mn—O bond cleavage with generation of H2O2 occurred; but in the presence of a weak acid, O—O bond cleavage was observed. The mechanism of O—O bond activation in MnIII-peroxo species supported by the N4O-, Me2EBC, and TMC ligands was explored by DFT calculations, and the role of the ligand in in the reductive activation process was evaluated. These calculations indicate that a stable supporting framework with a moderate amount of steric bulk is optimal for the reductive activation of MnIII-peroxo species. In addition to characterization and reactivity of MnIII-peroxo species, this work also contributes to the investigation of MnIII complexes with unusual spin states. While most MnIII complex are high-spin (S=2), several low-spin (S=1) MnIII species supported by trispyrazolylborate scorpionate-type ligands (Tp, Tp*) and a related N-heterocyclic carbene ligand were evaluated spectroscopically and computationally. The effect of the ligand in stabilizing these unusual spin states was examined through variable-temperature variable-field MCD spectroscopy and multireference ab initio calculations. While there are few methods available to treat the potentially orbitally-degenerate ground state of these low-spin MnIII complexes, NEVPT2/CASSCF computations were used to successfully reproduce the experimental zero field splitting parameters of these complexes. The results from these studies determined that a shift in the lowest lying triplet state is a result of strong σ-donation from the ligand. In many enzymatic catalytic cycles, metal-peroxo intermediates are precursors to the active metal-oxo species, and these metal-oxo species display high specificity for product distribution in substrate oxidation. While there are synthetic MnIV-oxo model species that display substrate reactivity, it is an ongoing investigation to replicate the controlled specificity present in enzyme systems. Two model complexes, [MnIV(OH)2(Me2EBC)]2+ and [MnIV(O)(OH)(Me2EBC)]+ differ only by one proton and are able to the direct the reaction with DHA to desaturated and hydroxylated products, respectively. To determine the fundamental parameters directing this reactivity, DFT calculations of hydrogen atom transfer, as well as the product formation steps, were performed for both MnIV species with DHA. | |
