High-valent transition metal-oxo intermediates play essential roles as active oxidizing species in enzymatic and biomimetic catalytic systems. Extensive research has been conducted on a variety of transition metal catalysts being studied as models for the ubiquitous cytochrome P450 enzymes. In doing so, the production of enzyme-like oxidation catalysts and probing studies on the sophisticated oxygen atom transfer mechanism are taking place.
In this work, visible-light irradiation of highly-photo-labile corrole-manganese(IV) bromates and chlorates was studied in two corrole systems with differing electronic environments, i.e. 5,10,15-trisphenylcorrole (H3TPC) and 5,10,15- tris(pentafluorophenyl)corrole (H3TPFC). In both systems, homolytic cleavage of the O- Br and O-Cl bonds in the ligands was observed to result in one-electron photo-oxidation to afford the corrole-manganese(V)-oxo species as determined by their distinct UV-vis spectra. Kinetics of oxygen atom transfer (OAT) reactions by each photo-generated [MnV(Cor)O] species with various substrates were conducted in two solvents, CH3CN and CH2Cl2. It was found that [MnV(Cor)O] exhibits noteworthy solvent and ligand effects on its reactivity and spectroscopic behavior. In the more electron-withdrawing TPFC species in polar CH3CN solvent, MnV-oxo corrole returned to MnIII corrole following oxidation of substrate. However, the TPFC species in the less polar CH2Cl2 solvent, and in both solvents for the TPC system, MnIV product was formed instead of MnIII. An inverted reactivity pattern, i.e. TPC > TPFC, for the MnV-oxo corroles was observed. These spectral and kinetic results were rationalized by a multiple oxidation pathway model, where either a two-electron oxidation for oxygen atom transfer reactions takes place or a disproportionation reactive takes place forming the elusive manganese(VI)-oxo as the true oxidant. The preferred pathway is highly dependent on the nature of the corrole ligand and the solvent.
Furthermore, a variety of [MnIV(Cor)Cl] complexes were investigated as biomimetic catalysts for the selective catalytic oxidation of the organic sulfide with mild sacrificial oxidant PhI(OAc)2. It was found that catalytic activity was affected by the oxidation state and electron environment of the catalyst. It was also found that in the same TPC system, [MnIV(TPC)Cl] was more reactive than [MnIII(TPC)], presumably due to the MnIV-corrole having easier access to the active metal-oxo intermediates than MnIII-corrole. In the same oxidation state, catalytic sulfoxidation of thioanisole resulted in a slower reaction rate for corrole species with more electron withdrawing ligands. In addition to thioanisole, [MnIV(TPC)Cl] was tested for its reactivity under catalytic conditions for eight other substrates. In most cases, quantitative conversions and excellent selectivity for sulfoxide were achieved.
| Identifer | oai:union.ndltd.org:WKU/oai:digitalcommons.wku.edu:theses-4064 |
| Date | 01 July 2018 |
| Creators | Ranburger, Davis Ray |
| Publisher | TopSCHOLAR® |
| Source Sets | Western Kentucky University Theses |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Masters Theses & Specialist Projects |
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