Reactivity and Thermochemistry of First-Row Transition Metal Complexes with Stable Organic Radicals

Abstract

Reactions involving transition metals and organic free radicals are critically important in a variety of chemical and biological processes. Because of their prevalence, there is a fundamental interest in better understanding these types of reactions to fully realize their potential for new applications. The work presented in this dissertation describes the free radical reactivity and thermochemistry of several different transition metal systems with stable organic radicals. Chapter 1 provides an introduction to transition metal reactivity involving organic free radicals. Chapter 2 describes the catalytic disproportionation of a hydroxylamine by (TMP)FeIII-OH (TMP = meso-tetramesityl porphyrin) and some of the radical reactions that make up the catalytic cycle. Chapter 3 describes the preparation, structural characterization and thermochemistry of a previously unreported stable organic radical, tBu2NPArO* (2,6-di-tert-butyl-4-(4'-nitrophenyl)phenoxyl). Chapter 4 describes the preparation of several [TptBuCuII]+ (TptBu = hydro-tris(3-tert-butyl-pyrazolyl)borate) and [TptBuMeCuII]+ (TptBuMe = hydro-tris(3-tert-butyl-5-methyl-pyrazolyl)borate) alkoxide compelxes as models for potential intermediates in copper/radical alcohol oxidation catalysis. Treating these complexes with stable radicals such a tBu3ArO* (2,4,6-tri-tert-butyl-phenoxyl) did not result in alkoxide oxidation despite having a large driving force. From these studies, we conclude driving force is not a primary predictor for copper/radical alcohol oxidation. Chapter 5 discusses the coordination chemistry of [TptBuCuII]+ and [TptBuZnII]+ with 4-nitro-phenols. With the bulky 2,6-disubstituted 2,6-di-tert-butyl-4-nitro-phenoxide, coordination to either metal occurs through a nitronate resonance form. The 2,6-unsubstituted 4-nitro-phenol binds through the phenoxide resonance form. Chapter 6 highlights the large kinetic barrier for ketone reduction or oxidation by titanocene(III/IV) and the hydrogen atom donor/acceptor, tBu3ArO(-H). Finally, Chapter 7 describes the selective and stoichiometric reduction of aromatic and aliphatic nitro groups by photoreduced titanium dioxide nanoparticles in acidic aqueous solutions. From thermochemical analysis, it is likely that these reactions proceed through a rate determining H+/e- transfer.