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Abstract
Hydrogen transfer chemistry is important due to its wide variety of applications in industrial processes and pharmaceutical development. There has been extensive research into catalyst design for reactions involving hydrogen transfer reactivity. Homogenous catalysts are attractive for studies due to the relative ease of their characterization. Metal-ligand cooperativity can allow first row transition metals to catalyze multi-electron and multi-proton processes, and, as such, has become an important tool in in transition metal catalyzed chemical synthesis and industrial transformations such as hydrogenation. For example, the use of redox-active ligands can allow first row transition metals that often facilitate 1 electron chemistry to facilitate 2 electron chemistry. Pendant protons on the ligand have been found to facilitate proton shuttling and engage in hydrogen bonding interactions that stabilize reactive intermediates. Recent work combining these two strategies into a single ligand scaffold has been found to be extremely effective at facilitating challenging multi-electron and multi-proton reactivity. In these studies, a 2,5-dihydrazonopyrrole (DHP) ligand scaffold was utilized in complexes with Ni and Fe. This ligand scaffold can store a full H2 equivalent in the ligand scaffold itself in addition to the redox capabilities of the metal center.
In Chapter 1, I discuss a DHP complex with Ni, where an H2 equivalent can be stored on the ligand periphery without metal-based redox changes and can be leveraged for catalytic hydrogenations. This complex is an unusual example where a synthetic system can mimic biology’s ability to mediate H2 transfer via secondary coordination sphere-based processes.
DHP ligands have been isolated in a variety of redox and protonation states when complexed to Ni, but the redox-state of this ligand scaffold is less obvious when complexed to metal centers with more accessible redox couples. In Chapter 2, I discuss the synthesis of a new series of Fe-DHP complexes with phenyl groups on the hydrazone arms in two distinct oxidation states. Detailed characterization supports that the redox-chemistry in this set is still primarily ligand based.
In Nature, enzymes carefully control the movement of protons and electrons via amino acids in the secondary sphere of the enzyme active site. This allows for precise reactivity using kinetically inert oxidants, such as O2. Harnessing metal-ligand cooperativity to control the secondary sphere of molecular catalysts mimics the strategies used in nature. Chapter 3 discusses the DHP ligand with tert-butyl groups complexed to Fe. This complex has a hydrogenated ligand which can donate two electrons and two protons to a substrate. In the presence of O2, this complex reduces O2 via a high spin Fe(III)-hydroperoxo intermediate which features a DHP• ligand radical. This intermediate is characterized by a variety of spectroscopic and computational techniques.
In Chapter 4, we discuss a family of bisneocuproine complexes of Fe2+ and Co2+ have been investigated for neocuproine redox noninnocence. A series of redox isomers of M(neocuproine)2n+ (where n = 2, 1, and 0 for Co and 2 and 0 for Fe) were synthesized and thoroughly characterized. All techniques were consistent with ligand-based reduction events to generate radical neocuproine complexes.