Browsing by Subject "Nonheme"

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    The Chemistry of S = 2 Nonheme Oxoiron(IV) Complexes
    (2016-05) Puri, Mayank
    Nonheme oxoiron(IV) intermediates are proposed to be involved in a number of important biological oxidation reactions, where all characterized examples thus far have a S = 2 ground spin state. In contrast, the majority of synthetic nonheme oxoiron(IV) complexes have a S = 1 ground spin state. In order to gain a deeper understanding of these important biological species, it is critical to expand the number of synthetic S = 2 nonheme oxoiron(IV) complexes and to study their reactivity with organic substrates. This thesis explores a synthetic strategy to obtain S = 2 nonheme oxoiron(IV) complexes by utilizing weak-field equatorial quinoline donors, in contrast to the relatively strong-field pyridine donors that are often used. The resulting S = 2 nonheme oxoiron(IV) complexes demonstrate spectroscopic signatures similar to those of the enzymatic oxoiron(IV) intermediates and reproduce the reactivity observed by these nonheme iron enzymes. Thus, this research has given rise to the first electronic and functional models of the nonheme oxoiron(IV) intermediates found in the enzymes TauD, CytC3 and SyrB2. In addition, the reactivity of known S = 1 nonheme oxoiron(IV) complexes were explored in the context of oxygen-atom exchange with H2O, which is an important reaction in tracking metal-oxo intermediates proposed to be involved in catalytic substrate oxidation reactions. Finally, a six-coordinate S = 1 nonheme imidoiron(IV) complex supported by a tetradentate ligand was synthesized and compared to its isoelectronic S = 1 nonheme oxoiron(IV) analogue.
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    High-valent Iron Intermediates in Nonheme Iron Catalytic Systems Designed for Hydrocarbon Oxidations
    (2019-05) Kal, Subhasree
    Inspired by nonheme iron enzymes, synthetic chemists have developed iron complexes to catalyze hydrocarbon oxidation reactions. High-valent iron intermediates have been proposed to be the oxidant for both enzymes and synthetic catalysts. For future development of catalysts, it is critical to discover and understand pathways for forming high-valent iron oxidants that can perform difficult oxidative transformations such as alkane and aromatic hydroxylation. Additionally, understanding the pathways to generate iron-based oxidants in model synthetic systems can help in elucidating mechanisms of the enzymes. This thesis describes a new pathway to form reactive high-valent FeV oxidants by utilizing strong Lewis and Brϕnsted acids. The acids facilitate heterolytic cleavage of the O–O bond in FeIII–OOH intermediates generated from the reaction of nonheme FeII complexes and H2O2. This pathway converts an inefficient catalyst for cyclohexane hydroxylation into an efficient catalytic system, forming an FeV oxidant in the catalytic cycle that hydroxylates cyclohexane within seconds at -40 °C. This new oxidant can also perform benzene hydroxylation equally efficiently. FeIII(OTf)3 is one of the Lewis acids that does this chemistry, giving rise to the first synthetic example where a mononuclear FeIII–OOH intermediate is activated by a second iron(III) ion to form an FeV oxidant. This work introduces the idea that the second iron in diiron nonheme enzymes can also act as a Lewis acid to activate O2 and form high-valent iron oxidants like Q in sMMO, which oxidizes methane to methanol. In addition, this thesis explores the importance of ligand topology around the iron center by comparing the effect of Lewis acid on the reactivity of three different catalytic systems. The effect of ligand topology was also investigated in the case of FeV intermediates that were generated stoichiometrically via one-electron oxidation of two topological isomers of an FeIV compound. The properties of the isomeric FeV intermediates, and the effect of Lewis acid in each case were explored.
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    Mono- and Dinuclear Nonheme Iron Model Complexes: O-O Bond Activation, Structural Characterization and Reactivity Study
    (2015-05) Rohde, Gregory
    The structures and reactivities of mono- and dinuclear nonheme iron model complexes were investigated. In Chapters 2 and 3, O-O bond activation of H2O2 by the dinuclear complexes [(FeIII2(μ-O)(μ-OH)L2]3+ (1A) and [(FeIII2(μ-OH)2L2]4+ (2A), L = tris(3,5-di-methyl-4-methoxypyridyl-2-methyl)amine, to form the high-valent [(FeIV2(μ-O)(OH)(O)L2]3+ (3A) and [(FeIV2(μ-O)2L2]4+ (4A) was studied. H2O2 and H2O competed for binding to the Fe centers of 1A and 2A, and [H2O2] was rate limiting under the concentrations studied. The presence of base increased the H2O2 activation rate for 2A, but not for 1A. The H2O2 activation rates by 1A and 2A were comparable to that of the mononuclear nonheme iron complex [FeII(TMC)]2+ (TMC = tetramethylcyclam) (J. Am. Chem. Soc. 2010, 2134-2135) after accounting for water inhibition. A crystal structure of [(FeIV2(μ-O)2L2]4+ (4A), or diamond core, was solved and described the Fe2O2 core in more detail than the original EXAFS structural assignment. In addition, structures of other complexes with Fe2O2L2 cores in different oxidation and protonation states were also studied and compared to the Fe2O2 cores of the high-valent enzymes intermediates RNR-X and sMMO-Q. In Chapter 4, iron complexes supported by the TMC ligand were studied by X-ray crystallography. A second isomer of the [FeIV(O)(TMC)]2+ complex was found, and the mechanism of conversion to the original isomer was explored. Additionally, the crystal structure of (TMC)FeIII(μ-O)Sc(NCCH3)(OTf)4 complex was obtained and used to reassign the Fe oxidation state of the originally reported (TMC)FeIV(μ-O)Sc(OH)(OTf)4 complex.(Nat. Chem. 2010, 756-759) In Chapter 5, the hydrogen atom transfer (HAT) rates of a series of S = 2 mononuclear nonheme iron complexes, [FeIV(O)TMG2dien(X)]2+,+ (X = CH3CN, Cl-, Br-, N3-, CH3CO2- and CF3CO2-; TMG2dien = 1,1-bis{2-[N2-(1,1,3,3- tetramethylguanidino)]ethyl}amine), were reported. Substitution of CH3CN with carboxylate and halide anions cis to the oxo ligand increased the HAT oxidation rate by as much as 15 times. A series of S = 1 nonheme iron complexes, [FeIV(O)TPA(Y)]2+,+ (Y = CH3CN, Cl-, CH3CO2- and CF3CO2-; TPA = tris(pyridyl-2-methyl)amine), was also investigated to explore what effect spin state has on reactivity. The HAT rates were similar for the [FeIV(O)TPA(Y)]2+,+ series, while OAT rates were much faster for the [FeIV(O)TPA(CH3CN)]2+ species.