Browsing by Subject "Inorganic"

Now showing 1 - 4 of 4
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    Bimetallic Cooperativity of Nickel and Cobalt–Group 13 Pincer Complexes via Polarized Metal–Metal Bonds
    (2022-06) Graziano, Brendan
    The underpinnings of modern industrial chemistry necessitate the use of transition metals to catalyze a wide variety of challenging chemical transformations. Precious metals, such as platinum, palladium, rhodium, and iridium are an essential component to many of these processes and have dominated due to the high catalytic activity and predictable two-electron redox. In the area of homogenous catalysis, precious metals have been an indispensable tool in many industrial processes. The scarcity of these metals and their high environmental impact have prompted a more recent focus on earth-abundant 3d metals. Earth-abundant transition metals are a critical component in many difficult biological transformation and offer untapped potential for homogenous catalysis. However, unlike precious metals, base metal catalysis tends to suffer from unproductive one-electron pathways which often hinder performance.Metal-ligand cooperativity has emerged as a premier strategy to perform catalysis with base metals. Cooperative ligand supports promote two-electron chemistry with base metal by working in concert during catalysis. Main group metalloligand scaffolds have shown promise in enhancing and turning on new types of reactivity at transition metals. Among these, Lewis acidic group 13 metals have garnered much attention due to their ability to act as strong σ-acceptors and potential for ambiphilic character. In this regard, rational ligand design was used to synthesize group 13 pincer-type ligands aimed towards cooperative bond activations involving first-row transition metals. Two new bimetallic systems have been developed with an open ligand framework that allows direct access to the M−group 13 interaction and exhibit a diverse array of cooperative σ-bond activations. Specifically, the group 13 support embedded in the pincer framework functions to bind substrate, stabilize low-valent metal states, and stabilize intermediates during reactivity. This strategy enabled two-site C–H, C–X, and H–H cleavage in which both metal ions participate. A key finding of this study is the unique bond polarization towards the transition metal which is induced by the electropositive group 13 element. A Ni–alane moiety is isolated which undergoes a reversible transformation between a Z and X-type ligand via aryl group transfer between the two metals. Mechanistic investigations identify that the Ni−Al bond is formed or broken as necessary during metal-ligand cooperativity. Additionally, by systematically varying the ligand frameworks and group 13 element, structure property relationships are found, highlighting important principles in the design of pincer-type group 13 metalloligands. A pair of isostructural Co–Al and Co–Ga pincer complexes are isolated which feature an X-type aluminyl or gallyl ligands. The isostructural nature of this pair allows for direct comparison of the effects of the supporting metal for reactivity. Further investigation of the duo reveals disparate reactivity with a much more active Al ion compared to Ga. Collectively, experimental and theoretical results show that the Lewis acidic metalloligand is key to the observed reactivity
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    Catalyst Development For Selective Ti-Catalyzed Synthesis Of Multi-Substituted Pyrroles
    (2020-05) See, Xin Yi
    This thesis broadly covers our catalyst development efforts for selective Ti(II)/Ti(IV) redox catalysis towards the synthesis of multi-substituted pyrroles. Chapter 1 provides a literature background on the multitude of ways to access low-valent early transition metals and its application towards group transfer catalysis. Chapter 2 describes the synthesis of pyrroles from the [2+2+1] coupling of alkynes and azides mediated by simple Ti-halide catalysts. Chapter 3 covers the generality of accessing Ti(II) intermediates, in the absence of strong metal reductants, via the coupling of Ti(IV) imido precatalysts and alkynes. Lastly, Chapter 4 details utilizing iterative supervised principal component analysis as a strategy to aid in rational catalyst design in the Ti-catalyzed regioselective synthesis of pyrroles.
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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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    Responsive, multimodal imaging agents for MRI: advancing the detection of metals and oxidized species implicated in neurodegenerative disorders
    (2012-08) Smolensky, Eric Dominick
    The successful development of responsive, multimodal imaging agents required a bottom-up approach starting with the nature of the iron oxide nanoparticles. Thee relaxivity of the nanoparticles was found to be dependent on the total anisotropy of the particles themselves, which is in turn a function of the size, shape, composition, surface coating, and interparticle distance of the nanoparticles. Responsive, monomodal imaging agents designed to respond to Cu(I) via click chemistry were found to produces significant changes in transverse relaxivity, corresponding to regime changes upon nanoparticle aggregation. These changes agreed well with theoretical modeling and laid the foundation for the subsequent design of multimodal imaging agents.The first multimodal imaging probe, MION@polymer@Ln was designed to maximize relaxivity using a MION@PEG based system. The probe was found to have high relaxivities and exhibited traditional time delayed lanthanide luminescence. The second probe, a core-shell MION@organic@Au multimodal imaging probe was also designed. It was found that the organic intermediate layer maintain the relaxivity of the core nanoparticles, while the gold shell exhibited significant plasmonic absorbance, enabling the probes to function as multimodal imaging probes.Finally, responsive multimodal imaging probes using the previously designed multimodal imaging probes as templates were designed. By using Cu(I) induced aggregation of AuNP and MIONs, the aggregation of the probes was monitored via attenuation of the SPR absorbance and increases in relaxivity. Additional probes using MION@PEG based designs allowed for small molecule (dsDNA) detection as monitored by luminescence quenching and changes in relaxivity.