Browsing by Subject "Transition metals"

Now showing 1 - 3 of 3
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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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    Design and synthesis of new ligand saffolds and transition metal complexes for small molecule activation
    (2013-08) Miller, Deanna Lynn
    The objective of this thesis is to explore the synthesis of new ligand scaffolds designed to support late first row transition metals and to study their potential for small molecule activation. Many researchers have utilized sterically encumbered ligands to protect reactive metal centers. In the second chapter, the design and synthesis of novel cage ligands featuring a hydrophobic cavity to provide protection to reactive metal centers will be explored. Ideally, the cavity will effectively prevent bimolecular decomposition reactions that often plague small molecule activation. The synthesis and characterization of two cage ligands, a trianionic tri(amido)amine and a neutral tri(amino)amine variant, will be presented. Additionally, the preparation and characterization of the zinc(II) complex of the tri(amido)amine cage ligand will be discussed and its uptake of small molecules explored. In addition to employing protection of reactive metal centers in ligand design, a bio-inspired ligand design will be explored. The biological cofactor dihydronicotinamide adenine dinucleotide (NADH) can efficiently reduce and oxidize substrates through the transfer of a hydride (or a proton and two electrons). The third chapter explores the design and synthesis of a NADH-type ligand scaffold. The systems presented herein have three NADH-like moieties built into the ligand, designed to reduce substrates via either hydride transfer or proton coupled electron transfer. Having three NADH moieties allows for the possibility of multi-electron redox chemistry. The synthesis and characterization of zinc(II) and cobalt(II) complexes will be discussed. Additionally, the reaction of both the NADH ligand and the zinc(II) complex with known hydride acceptors will be explored. Finally, in chapter four, the synthesis and characterization of a family of diiron and iron cobalt bimetallic complexes with a third type of ligand design will be presented. Previous work in the Connie Lu group has shown reactivity toward small molecules using heterobimetallic complexes supported by a tripodal phosphine amide ligand scaffold. Herein, the same ligand scaffold is applied to late transition metals to explore their synthesis, reactivity toward small molecules, and electronic and magnetic properties to allow for a better understanding of metal-metal bonding interactions.
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    Exploring Small Molecule Reactivity with Low-Valent Nickel and Cobalt Complexes Supported by Lewis Acidic Metalloligands
    (2019-05) Vollmer, Matthew
    This thesis details/describes the synthesis and characterization of heterobimetallic complexes that utilize a bifunctional amido-phosphine ligand to stabilize bonds between a Lewis acid (aluminum, gallium, or indium) and a late transition metal center (cobalt, nickel, or copper). Ultimately, the ability to tune the electronic properties of the transition metal center is utilized to design new catalysts for small molecule conversions. General characterization of these species includes single crystal x-ray diffraction, electronic structure calculations, nuclear magnetic resonance studies, cyclic voltammetry, and electron paramagnetic resonance spectroscopy. In the first chapter, a Lewis acid-cobalt interaction is utilized to stabilize sub-valent cobalt species that bind dihydrogen in a side on fashion with moderate activation. In the second chapter, these cobalt dihydrogen complexes are studied by high-pressure NMR spectroscopy and studies reveal that they catalyze the room temperature hydrogenation of carbon dioxide. These catalysts operate via a unique mechanism and produce formate with high turnover frequencies and numbers in the presence of organic bases. The third chapter focuses on the electronic structure of related nickel species, specifically towards understanding the role of 4p orbitals in the nickel-metal bond. The last chapter details thermodynamic parameters and small molecules reactivity of a unique class of metal hydride complexes supported by direct Lewis acid- metal interactions.