HIGHLY REACTIVE METAL PORPHYRAZINES

Abstract

Nature has evolved a variety of different metalloenzymes for the selective oxygenation of C–H bonds. Cytochrome P450s, for example, are heme proteins that use an oxoiron(IV)-porphyrin π-radical cation to break strong C–H bonds. Lipoxygenases, on the other hand, utilize hydroxo iron(III) or manganese(III) intermediates to activate allylic C–H bonds. These mid-valent hydroxo metal(III) complexes can selectively activate C–H bonds in the presence of oxidatively labile functionalities. However, only a limited number of lipoxygenase model compounds have been reported, and they are quite unreactive compared with enzymatic systems. In this dissertation, several highly reactive mid-valent hydroxo-metal porphyrazines are presented.

Chapters 2 and 3 concentrate on a highly reactive hydroxo iron(III) porphyrazine complex, (PyPz)FeIII–OH (PyPz = tetramethyl-2,3-pyridino porphyrazine). Fe(II) and Fe(III) porphyrazines were synthesized and characterized. Electrochemical analyses revealed a remarkably high, pH-dependent FeIII–OH/FeII–OH2 reduction potential. The O–H bond dissociation energy of the (PyPz)FeII–OH2 complex is estimated to be 84 kcal/mol. (PyPz)FeIII–OH reacts rapidly with hydrocarbon substrates via a hydrogen atom transfer (HAT) mechanism, with second-order rate constants 5-6 orders of magnitude faster than other reported hydroxo iron(III) complexes. Kinetic isotope effects and unusually fast kinetics were observed when studying the oxidation of phenols by (PyPz)FeIII–OH. A mechanism of “partial electron transfer followed by proton transfer” is proposed accordingly.

In chapter 4, I report the first example of a fully characterized mononuclear CoIII–OH complex which can abstract benzylic C–H bonds as strong as 82 kcal/mol. This Co(III) porphyrazine was also determined to react with hydrocarbon substrates through a HAT mechanism. Oxidation of TEMPO and ferrocene by (PyPz)CoIII–OH follows Marcus theory with a moderate reorganization energy. Phenols react with Co(III) porphyrazine via “partial electron transfer followed by proton transfer” mechanism.

In the final chapter, I extend my research to manganese porphyrazine. (PyPz)Mn has more complicated oxidation states and redox properties. It can catalytically generate chlorine dioxide from chlorite at extremely fast rates with relatively high yields. The oxidized form of (PyPz)Mn abstracts C–H bonds with the fastest rates and smallest activation enthalpies among the three reported porphyrazines.