Halogenated chemicals represent a large and toxic class of environmental pollutants. Although regulation of certain halogenated organics resulted in decreasing production of these chemicals in the United States since the 1970s, others were yet unknown when the regulations were written and production is increasing. In many cases, halogenated organics are persistent in the environment, bioaccumulative, and bioactive, causing toxicological concerns. As such, environmental scientists have studied the processes by which these chemicals can be broken down, and the products that form in these breakdown reactions. In some cases, the toxic effects associated with halogenated organic pollutants can be ameliorated by complete dehalogenation, while incomplete dehalogenation or other transformations can result in the production of harmful compounds. The mechanisms of these transformations are in most cases not yet well understood, but a fundamental understanding of these reactions helps in the development of effective remediation strategies, and informs the fundamental chemistry inherent to these reactions.
Concern about halogenated environmental pollutants has led to investigations of a number of means of dehalogenation including biological attenuation. Microbially mediated dehalogenation represents a major transformation pathway for halogenated pollutants in the environment. Metal-containing cofactors have been implicated in these processes including cobalamin (vitamin B12), factor F430, and hemitin. These cofactors are responsible for the reductive dehalogenation of environmental pollutants. These reactions can proceed by a various intermediates, but one of particular interest is the formation of radicals. Radicals have at least one unpaired electron, and as such are highly reactive and transient intermediates. These features can make them difficult to study but their powerful reactivity underscores their importance in environmental transformations.
Radical intermediates are often proposed but rarely fully understood in a range of environmental systems. In this thesis, the role of radicals in dehalogenation reactions is explored with particular attention to cobalamin-mediated and nickel-mediated reactions. The mechanism of cobalamin-mediated dechlorination has been studied extensively and evidence for both outer-sphere (radical based) and inner sphere (nonradical based) mechanisms has been presented. In this thesis the literature concerning cobalamin-mediated dehalogenation is reviewed in detail (Chapter 1) and a mechanistic study on the role of radicals in cobalamin-mediated dechlorination of chloroethylenes reconciles previously seemingly contradictory data (Chapter 2).
Similarly, both radical and nonradical pathways have been invoked in nickel-mediated dehalogenation of a variety of substrates. Nickel-mediated dehalogenation has not been studied as extensively as cobalt-mediated reactions and the understanding is complicated by the fundamental chemistry of nickel complexes. In order to better understand the chemistry of reduced nickel complexes, particularly their reaction with halogenated organics, a series of nickel complexes was synthesized and characterized (Chapter 3). The relationship between reduced transition metal complexes and their ligands is inextricably linked to whether and how radical intermediates are formed in these systems. The reactivity of two reduced nickel complexes precursors show that these complexes are highly sensitive to slight changes in ligand structure (Chapter 4).
University of Minnesota Ph.D. dissertation. Novemver 2009. Major: Chemistry. Major: Kristopher McNeill. 1 computer file (PDF); x, 94 pages. Ill. (some col.).
Kliegman, Sarah Isabella Morningstar ..
Radical involvement in cobalt- and nickel-mediated dehalogenation reactions..
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