This thesis describes the preparation, detailed characterization, and reactivity of iron and cobalt complexes designed to understand molecular redox transformations. Despite their essential roles in the redox chemistry of small molecules (for example, O2, CO2, N2), the mechanisms by which metal chalcogenide species orchestrate important reactions remain poorly understood.
Chapter 1 examines the relative kinetics of C-H activation by heterobimetallic and homobimetallic oxo complexes. Inspired by natural systems like methane monooxygenase that use two metal centers to cooperatively bind and activate O2, we describe a structural, spectroscopic, and computational analysis to characterize bimetallic Fe/Co and Co/Co complexes that differ in a single metal center. We interrogate mechanisms of C-H activation to quantify the influence of the metal on C-H activation, and our results demonstrate that exchange of a single cobalt atom with iron to generate a Fe/Co heterobimetallic complex results in much faster C–H bond activation. These findings corroborate previous proposals of heterobimetallic oxo species in redox reactions, and include the first structurally characterized heterobimetallic oxo complex of any two transition metals.
Chapter 2 interrogates CO2 reduction by a diketiminate supported cobalt(I) complex. We report a reaction that converts CO2 to fully characterized CO and carbonate products, and conduct kinetics experiments to determine the rate law of this reaction. Computations conducted by collaborators supplement the experiment work, and suggest the intermediacy of a dicobalt oxo. Through independent synthesis of this proposed intermediate, we demonstrate the kinetic competency of the dicobalt oxo in the reduction of CO2 by the cobalt(I) complex. This chapter reveals a well-characterized mechanistic picture for cobalt mediated CO2 reduction.
Chapter 3 describes synthetic [4Fe-3S] iron-sulfur clusters with unusual structural, electronic, and reactivity properties. This chapter evaluates one hypothesis for enabling reactivity in iron-sulfur clusters: that three-coordinate Fe sulfide, possibly formed by rupture of weak Fe-S bonds at tetrahedral sites, serves as a binding site in clusters like nitrogenase. We describe the synthesis of new [4Fe-3S] clusters that feature the first example of an iron site supported only by three sulfide ligands. Detailed spectroscopic characterization and computational analysis is discussed, which reveals an unusual electronic structure in [4Fe-3S] clusters. Biomimetic reactivity of [4Fe-3S] demonstrates that three-coordinate iron in an all-sulfide coordination sphere is a viable precursor to substrate binding by iron-sulfur clusters.
Chapter 4 reports the synthesis of a [2Fe-1S] cluster with a reactivity pattern that leads to various other iron-sulfur clusters. We describe the stabilization of the otherwise highly reactive [2Fe-1S] complex using phosphine ligands to protect low-coordinate iron-sulfide, which enables its thorough characterization. Through phosphine removal, a highly reactive species is formed that can be elaborated into higher nuclearity iron-sulfur clusters, including the [4Fe-3S] cluster described in Chapter 3 in addition to a partially characterized [10Fe-8S] cluster with a previously unobserved geometry.
Chapter 5 offers a perspective on the possible implications of the [4Fe-3S] cluster family for the mechanism of nitrogenase and for the electronic structure of iron-sulfur clusters more generally. We describe connections between one [4Fe-3S] cluster and a recently reported crystal structure of a nitrogenase cofactor trapped during catalytic turnover. The electronic structure of [4Fe-3S] is placed in the context of known mixed-valent iron-sulfur clusters. Chapter 5 closes with a discussion of the opportunities afforded by [4Fe-3S] and other synthetic clusters for understanding electronic structure and reactivity in biological iron-sulfur cofactors.
|Advisor:||Holland, Patrick L.|
|Commitee:||Mayer, James M., Hazari, Nilay|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 81/3(E), Dissertation Abstracts International|
|Subjects:||Chemistry, Inorganic chemistry|
|Keywords:||Hioinorganic chemistry, C-h activation, Iron-sulfur clusters, Metal oxo, Nitrogenase, Synthetic modeling|
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