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Kinetic and Thermodynamic Descriptions of PCET Reactions by Manganese(IV) Oxo Complexes
Mayfield, Jaycee R
Mayfield, Jaycee R
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Abstract
Catalysts featuring a manganese center are responsible for the selective activation of strong C-H bonds in biological systems. Their capability of oxidizing C-H bonds with bond dissociation free energies (BDFEs) of up to 85 kcal mol-1 has motivated the study of biomimetic manganese-containing complexes. One class of synthetic compounds that has emerged in these studies features aminopyridyl ligands bound to a manganese center in a pentadentate fashion. Small structural changes to the ligand field are shown to have dramatic effects on the reactivity of these compounds. One of the early ligand scaffolds in these studies is the N4py ligand. In its reactive state, the manganese adduct featuring the neutral pentadentate ligand is characterized as [MnIV(O)(N4py)]2+. This thesis focuses on kinetic and thermodynamic descriptions of hydrogen-atom transfer (HAT) reactions performed by a library of MnIV-oxo complexes that feature derivatives of the N4py ligand. Studies of many manganese(IV) oxo species have been motivated by their analogies to FeIV-oxo species. There is a discrepancy in relative rates of HAT reactions performed by MnIV-oxo and FeIV-oxo complexes with the same ligands. For example, FeIV-oxo complexes featuring 2pyN2B and 2pyN2Q ligands show much higher reactivity when bound to 2pyN2B. However, our studies of MnIV-oxo complexes featuring the same ligands show very sluggish reactivity when bound to 2pyN2B. In order to understand this further, we have performed Eyring analysis for reactions of four different MnIV-oxo complexes bound to the N4py, DMMN4py, 2pyN2Q, and 2pyN2B ligands with the hydrocarbon substrates EtBn and DHA. To our surprise, the results show ΔG‡ values ordered correctly with respect to their relative rates of reaction, but the ΔH‡ and ∆S‡ show unexpected differences in magnitude that do not reflect their reaction rates and structural similarities. Further Arrhenius and Eyring analyses for reactions of [MnIV(O)(N4py)]2+ and [MnIV(O)(2pyN2Q)]2+ with d4-DHA suggest that hydrogen atom tunneling is complicating our analysis of the kinetic data. We then performed computational studies to determine if a thermodynamic description would be better suited to predicting reactivity trends of MnIV-oxo intermediates. The MnIV-oxo adducts we have studied previously, as well complexes featuring four other variations of the N4py ligand (N4pyMe2, N3pyQ, DMAN4py, and 2pyN2I), were examined. This approach looks at the concerted proton electron transfer (CPET) reaction as separate electron transfer and proton transfer steps to determine theoretical pKa and BDFE values for the resulting MnIII-OH species. Once these values were obtained, they were plotted against experimental log(k2) and E1/2 values to determine if this method is suitable for predicting relative reaction rates for MnIV-oxo adducts in HAT reactions. The appeal of this method lies in its lower computational cost than more complicated methods such as CASSCF. Additionally, it has already been shown to reproduce reactivity trends seen for OAT and HAT reactions by previously reported MnIV-oxo complexes.
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Date
2022-08-31
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University of Kansas
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Keywords
Chemistry, Inorganic chemistry, HAT, hydrogen atom transfer, Manganese, N4py, Oxo, PCET