Generation & Kinetic Studies of Porphyrin Chromium-Oxo Intermediates

Authors

Iyanu Olumide Ojo

Publication Date

5-2022

Advisor(s) - Committee Chair

Rui Zhang (Director), Bangbo Yan, Eric Conte

Degree Program

Department of Chemistry

Degree Type

Master of Science

Abstract

Recent emerging applications of chromium porphyrin complexes are generating increasing interest in oxidation chemistry and catalysis. Its recent application as metal-organic frames in spintronic devices supports the rich electronic system of chromium metal. Through the past decades, first-row transition metalloporphyrins have served as biomimetic oxidation catalysts and models of cytochrome P450 enzymes. However, the oxygen atom transfer mechanism of chromium porphyrin complexes, most especially in sulfide oxidation reactions, has been scarcely investigated.

In this study, high-valent chromium-oxo porphyrin species, namely Cr^IV-oxo and Cr^V-oxo species, with electron-donating tetramesitylporphyrin(TMP) ligand were successfully generated and characterized with UV-vis, ESI-MS, and NMR spectroscopies. [Cr^IV(TMP)O] species was chemically generated by oxidation of [Cr^III(TMP)Cl] with a mild oxygen source PhI(OAc)₂ or photochemically by visible light irradiation of the photo-labile [Cr^III(TMP)ClO₃]. Oxidation of [Cr^III(TMP)Cl] with m-CPBA as the sacrificial oxygen source gave the higher valent [Cr^V(TMP)O]Cl species. In addition, [Cr^V(TMP)O]ClO₄ was obtained by one-electron oxidation of [Cr^V(TMP)O] by AgClO₄. Subsequent kinetic studies of these formed oxo-species in sulfide oxidation reactions and single-electron reduction with triethylamine showed that both photochemically and chemically generated [Cr^IV(TMP)O] have similar reactivity. The representative second-order rate constants of oxidation of thioanisole are (3 ± 0.09) × 10^-3 M^-1s ^-1 for [Cr^IV(TMP)O] and (26 ± 2 ) × 10^-3 M^-1s ^-1 for [Cr^V(TMP)O]ClO₄. By comparison, [Cr^V(TMP)O]ClO₄ exhibits approximately one order of magnitude higher reactivity than iv [Cr^IV(TMP)O] with the same substrate. Furthermore, [Cr^V(TMP)O]Cl with (99.8 ± 1.9 ) × 10^-3 M^-1s ^-1 showed higher reactivity than that of [Cr^V(TMP)O]ClO₄ with thioanisole, suggesting that the axial ligand also plays an important role in the reactivity of these oxo-intermediates.

Disciplines

Biological and Chemical Physics | Physical Sciences and Mathematics | Physics

Recommended Citation

Ojo, Iyanu Olumide, "Generation & Kinetic Studies of Porphyrin Chromium-Oxo Intermediates" (2022). Masters Theses & Specialist Projects. Paper 3563.
https://digitalcommons.wku.edu/theses/3563