Publication Date

Fall 2017

Advisor(s) - Committee Chair

Rui Zhang (Director), Stuart Burris and Kevin Williams

Degree Program

Department of Chemistry

Degree Type

Master of Science

Abstract

High-valent transition metal-oxo intermediates are of fundamental importance because of their central role as active oxidizing species in enzymatic and synthetic catalytic oxidations. Many transition metal catalysts have been extensively studied as models of the ubiquitous cytochrome P450 enzymes to probe the sophisticated oxygen atom transfer mechanism as well as to invent enzyme-like oxidation catalysts.

In this work, photolysis of highly photo-labile corrole-manganese(IV) bromates or nitrites by visible light was studied in two corrole systems with different electronic environments. The corrole systems under studied include 5,10,15- tris(pentafluorophenyl)corrole (TPFC) and 5,10,15-triphenylcorrole (TPC). As observed in both systems, homolytic cleavage of O-Br or O-N bonds in the ligands resulted in one-electron photo-oxidation to generate corrrole-manganese(V)-oxo species, as determined by their distinct UV-vis spectra and kinetic behaviors. The kinetic of oxygen atom transfer (OAT) reactions with various substrates by these photogenerated [MnV(Cor)(O)] were studied in CH3CN and CH2Cl2 solutions. [MnV(Cor)(O)] exhibits remarkable solvent and ligand effects on its reactivity and spectral behaviors. In the more electron-deficient TPFC system and in the polar solvent CH3CN, MnV-oxo corrole returned MnIII corrole in the end of oxidation reaction. However, in the less polar solvent CH2Cl2 or in the less electron-deficient TPC systems, MnIV product was formed instead of MnIII. With the same substrates and in the same solvent, the order of reactivity of MnV-oxo corrole was inverted as TPC > TPFC. The spectra and kinetic results are rationalized by a multiple oxidation model, where the electron-demand MnVoxo species may serve as direct two-electron oxidation for oxygen atom transfer reactions; and less electron-demand systems undergo a disproportionation reaction to form a putative manganese(VI)-oxo corrole as the true oxidant. The choice of pathways is strongly dependent on the nature of the solvent and corrole ligand.

Furthermore, porphyrin-manganese(V)-oxo were produced in organic solvents by visible light irradiation of the corresponding porphyrin-manganese(III) nitrite complexes. The porphyrin systems studied were 5,10,15,20-tetrakispentafluoro phenylporphyrin (TPFPP), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin (TDFPP) and 5,10,15,20-tetrakis(2,6-dichlorophenyl)-porphyrin (TDCPP). Heterolytic cleavage of O-N bond of nitrite ligand results in two-electrons photo-oxidation. Under visible light irradiation, MnV-oxo porphyrins quickly returned to MnIII product. However, in absence of light, MnIV-oxo species were formed, as determined by their distinct UV-vis spectra, which permitted direct kinetic studies. The apparent rate constants for reaction of [MnIV(Por)(O)] species show inverted reactivity order with (TPFPP) < (TDFPP) < (TDCPP) in reactions with ethylbenzene. A model for oxidation under catalytic condition was presented.

Disciplines

Chemistry | Inorganic Chemistry

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