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Spectroscopic and Computational Investigations of Biologically Inspired Manganese-Hydroxo and -Peroxo Complexes
Grotemeyer, Elizabeth N
Grotemeyer, Elizabeth N
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Abstract
Manganese(III)-oxygen species have been proposed as key intermediates in the catalytic cycles of both manganese enzymes and synthetic catalysts. For example, the proposed reaction mechanism for the manganese lipoxygenase enzyme (MnLOX) includes both MnIII-hydroxo and end-on MnIII-alkylperoxo intermediates. Synthetic catalysts are proposed to access highly reactive MnV-oxo species via the decay of an end-on MnIII-hydroperoxo species in the presence of carboxylic acids. While commonly proposed, many of these Mn-peroxo intermediates have yet to be observed, particularly in catalytic systems. The development of model complexes has therefore been an invaluable tool in understanding biological and catalytic mechanisms, however our understanding of MnIII-oxygen intermediates, and of end-on MnIII-peroxo complexes in particular, remains quite limited. This dissertation describes the development of MnIII-peroxo intermediates bearing the pentadentate anionic dpaq ligand and its derivatives generated from the reaction of MnIII-hydroxo complexes with hydrogen peroxide. In particular, this work describes the first report of a MnIII-hydroperoxo complex generated from the direct reaction between a Mn¬III-hydroxo complex and hydrogen peroxide, a key step in the proposed mechanisms of manganese catalysts. The influence of both ligand perturbations and the Lewis acid salt Al(OTf)3 on the structure and reactivity of the MnIII-hydroxo complex [MnIII(OH)(dpaq)]+ are investigated. The addition of a nitro group to the 5-position of the quinonlyl moiety of the dpaq ligand (dpaq5NO2) results in favorable changes in the MnIII/II potential and an increase the hydrogen atom transfer (HAT) reactivity of the corresponding MnIII-hydroxo complex with TEMPOH. The reactions of [MnIII(OH)(dpaq)]+ with Al(OTf)3 in MeCN in the presence of added water leads to the formation of new intermediates that show enhanced reactivity compared to [MnIII(OH)(dpaq)]+ and [MnIII(OH)(dpaq5NO2)]+. The spectroscopic and kinetic data for [MnIII(OH)(dpaq)]+ in the presence of Al(OTf)3 and water are essentially identical to those of the MnIII-aqua complex [MnIII(OH2)(dpaq)]2+, leading us to conclude that the addition of the Lewis acid increases Brønsted acidity of the solution, promoting protonation of the MnIII-hydroxo unit. The reaction of mononuclear MnIII-hydroxo complexes with H2O2 is also explored. We report the formation and characterization of a series of interconvertible products formed by the addition of hydrogen peroxide to solutions of [MnIII(OH)(dpaq2Me)]+ under varying reaction conditions. These formation of these products, identified as [MnIIIMnIV(µ-O)2(dpaq2Me)2]+, [MnIII(OOH)(dpaq2Me)]+, and [MnIII(O2)(dpaq2Me)] by spectroscopic measurements and electronic structure calculations, was controlled by the solution acidity and the amount of hydrogen peroxide present in solution. The interconversion of these peroxomanganese complexes by thermal decay or by the addition of acid or base is investigated along with the reactivity of these intermediates with substrates. Reactivity studies with this MnIII-hydroperoxo complex were complicated by the requirement of excess hydrogen peroxide and the low stability of this intermediate, even at low temperatures. Inspired by the improved stability afforded to MnIII-alklperoxo complexes by the 6Medpaq ligand, the reactivity of this complex with hydrogen peroxide in the presence of acid was explored. We report the formation and characterization of the MnIII-hydroperoxo complex [MnIII(OOH)(6Medpaq)]+ and its reactivity with OAT and HAT substrates. DFT calculations were performed to support the assignments of a species formed in the reaction of electrochemically-generated superoxide with the [MnII(dpaq)]+ complex as a mononuclear [MnIII(O2)(dpaq)] complex. Further analysis of the electronic structure of the [MnIII(O2)(dpaq)] and [MnII(O2)(dpaq)]- complexes revealed that the reduction occurs at the ligand rather than the manganese center of the complex.
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Date
2022-08-31
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University of Kansas
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Keywords
Inorganic chemistry, Density functional Theory, Manganese-hydroxo, Manganese-peroxo, Spectroscopy