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https://hdl.handle.net/2142/19923
Description
Title
Photochemistry of metalloporphyrins
Author(s)
Watson, Randall Alan
Issue Date
1991
Doctoral Committee Chair(s)
Kenneth S. Suslick
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Inorganic
Chemistry, Physical
Language
eng
Abstract
Metalloporphyrins serve many functions in biological systems. Their important roles in the reduction of oxoanions such as nitrite and sulfite in bacteria and in the oxidation of organic substrates by cytochrome P450 has prompted our current investigations into the photochemistry of several metalloporphyrin oxoanion complexes. The use of photochemistry with model systems in the laboratory opens new avenues for the direct production of proposed active species in the biological systems.
Irradiation of Mn(TPP)(NO$\sb3$) and Mn(TPP)(NO$\sb2$) (where TPP = 5,10,15,20-tetraphenylporphyrinate(-2)) produces the high valent metal-oxo species O = Mn$\sp{\rm IV}$(TPP) quantitatively, with quantum yields of 1.58 $\times$ 10$\sp{-4}$ and 5.30 $\times$ 10$\sp{-4}$, respectively. This metal-oxo species is capable of oxidizing substrates, as demonstrated in reactions with styrene or triphenylphosphine. Mn(TPP)(NO$\sb2$) is formed as an intermediate in the complete photolysis of Mn(TPP)(NO$\sb3$). Similarly, the photochemistry of Fe(TPP)(NO$\sb3$) produces substrate oxidation, including C-H hydroxylation, which suggests the photochemical formation of O = Fe$\sp{\rm IV}$(TPP$\sp{\cdot +}$) as the active oxidant. Remarkably, all three oxygen atoms of the initially bound NO$\sb3\sp-$ can be used for substrate oxidation.
Irradiation of (Mn(TPP)) $\sb2$(SO$\sb4$) and Mn(TPP)(OSO$\sb3$H) produces Mn$\sp{\rm II}$(TPP) quantitatively, with quantum yields of 7.1 $\times$ 10$\sp{-4}$ and 9.8 $\times$ 10$\sp{-4}$, respectively. In contrast to the nitrate and nitrite complexes, metal-oxo species are not formed and oxidation of hydrocarbons does not occur.
The photochemistry of a number of metalloporphyrin oxoanion complexes has also been examined by matrix isolation techniques using both frozen solvent glasses and polymer films. Results suggest that the photoreduction of porphyrin metal centers is the primary photoreaction and that apparent differences in solution photochemistry actually result from secondary thermal reactions. A 3: 1 mixture of 2,2-dimethylbutane: t-butylbenzene was shown to work well as a non-coordinating, frozen solvent glass with little cracking below 70K. Additionally it was found that the photochemistry of metalloporphyrins could easily be monitored in polymer films by the co-evaporation of toluene solutions of either polymethylmethacrylate or poly-$\alpha$-methylstyrene on a sapphire window. The polymer films have the added advantage of a greatly increased temperature range that can provide isolation even at room temperature.
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