Insights into O2 Reductive Activation by Copper-Containing Synthetic Model Complexes
Embargo until
2022-12-01
Date
2018-10-24
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Johns Hopkins University
Abstract
The reductive activation of dioxygen, which is often coupled to other processes such as substrate hydroxylation or oxidation, is of fundamental importance in biological and synthetic systems. While the stepwise reduction of dioxygen to water is well-known in aqueous systems, gaining information on the thermodynamic parameters of metal-bound reduced-O2 intermediates is of great interest, especially for their use as catalysts in the cathodic half of fuel cells. The formation and reactivity of (di)copper-dioxygen adducts has been well documented in the literature, yet there is still much to learn about factors that dictate the identity of the formed adduct upon reaction of copper(I) with dioxygen. This dissertation explores the effect of secondary coordination sphere moieties (hydrogen bonding and sterics) on the (di)copper-dioxygen formed. Thermodynamics parameters of reduced-O2 species bound to a dicopper(II) complex are also investigated.
Chapter 1 provides a detailed overview of reductive dioxygen activation by synthetic and enzymatic copper-containing systems. The known (di)copper-dioxygen adducts that have been synthesized and characterized in synthetic systems, as well as their importance as intermediates in the catalytic reduction of dioxygen by synthetic copper complexes, are discussed. The significance of cupric superoxide complexes in copper monooxygenases and the synthetic strategies utilized to stabilize these species in organic solvent at cryogenic temperatures is also explored.
In chapter 2, the influence of secondary coordination hydrogen bonding (H-bonding) interactions on the reactivity of synthetic copper(I) complexes with dioxygen and the reactivity of H-bonded cupric superoxide species is reported. In fact, increasing the number or strength of the H-bonding moieties greatly enhances the electrophilic reactivity of the cupric superoxide complexes.
Chapter 3 discusses the CuI/O2 reactivity of a copper complex bearing a novel tetradentate, tripodal ligand containing an appended methoxy moiety. Unprecedented copper(I)-dioxygen reactivity was observed, with this being the first report of the direct conversion of a cupric superoxide species to a dicopper(III) bis--oxo complex.
In chapter 4, the interconversion of reduced O2 species (superoxide, peroxide, and hydroperoxide) while bond to a dicopper(II) complex is investigated. Through chemical titrations and, finally, using the Bordwell relationship, the O–H bond dissociation energy of the dicopper(II) hydroperoxide complex was determined.
Finally, chapter 5 details final conclusions and future outlooks on the research presented herein, including possible new H-bonding interactions to explore and suggestions for varying the O–H bond dissociation energy of dicopper(II) hydroperoxide complexes by varying the dicopper-bound ligand.
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Keywords
copper, dioxygen activation, cupric superoxide, enzymes, bioinorganic chemistry, dicopper(III) bis-mu-oxo, monooxygenase mechanisms, monooxygenase