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Investigations into photocatalysis and electronic structure for transition metal and actinide complexes




Higgins, Robert F., author
Shores, Matthew P., advisor
Rappé, Anthony K., committee member
Neilson, James R., committee member
McNally, Andrew, committee member
Wu, Mingzhong, committee member

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Presented herein are investigations into the electronic structure of various metal complexes and how they effect reactivity. The first chapters are centered on how [Cr(Ph2phen)3]3+ reacts as a photooxidant. The latter part of this work concerns magnetic properties of various first row transition metal and actinide complexes. In Chapter 1, I provide a background on how understanding electronic structure of transition metal complexes has motivated later work in reactivity. This Chapter also includes a detailed background in photoredox catalysis and different electronic structures of Ru-, Ir- and Cr-containing photosensitizers. It ends with a lead-in to our initial hypotheses and motivations for using Cr as a paramagnetic, Earth-abundant congener to Ru photosensitizers in photoredox manifolds. Chapters 2-4 illustrate our mechanistic studies into transformations using Cr as a photooxidant to perform [4+2] cycloaddition reactions between (trans and cis)-anethole and dienes. Chapter 2 focuses on the interactions of oxygen (O2) in the reaction of trans-anethole and isoprene mediated by [Cr(Ph2phen)3]3+. We determined three separate, yet invaluable roles that oxygen performs in this reaction, which include: (1) protection of the catalyst through excited-state energy-transfer giving 1O2, (2) 1O2 oxidation of the reduced form of the catalyst, regenerating the ground state species and giving 2O2•- as well as (3) 2O2•- reduction of the radical cation of the [4+2] product, completing the catalytic cycle. In Chapter 3, I discuss the association that trans-anethole and similar dienophiles show with [Cr(Ph2phen)3]3+ and how this affects the overall reactivity. Interestingly, diamagnetic analogues do not show the same association. Finally, in Chapter 4, trans-anethole is replaced with cis-anethole to determine how the overall reactivity changes. These data are supported by reactivity, kinetic and quenching studies to probe the reactivity. Chapters 5-7 concern similar mechanistic details involving [Cr(Ph2phen)3]3+ in photocatlytic cycloaddition reactions, except that trans-anethole, which is electron-rich, is replaced by 4-methoxychlacone, which is electron-poor. Chapter 5 discusses the synthetic utility of this reaction manifold and initial mechanistic details of the transformation, which reveal an orthogonal mechanism which proceeds through energy transfer when compared to the reactivity of trans-anethole with [Cr(Ph2phen)3]3+. In Chapter 6, the observation of enhanced regioselectivity that is observed when [Cr(Ph2phen)3]3+ is used is investigated, specifically in comparison to all other Cr- and Ru-photooxidants attempted. This regioselectivity is manifested in the stabilization of a one-bond intermediate, as well as an association between 4-methoxychalcone and [Cr(Ph2phen)3]3+. To conclude this section, Chapter 7 focuses on the interesting solution-phase equilibria of 4-methoxychalcone and how the association of 4-methoxychlacone with itself and [Cr(Ph2phen)3]3+ impacts the overall reaction mechanism. Chapter 8 provides an interesting method of using ferrocenium as an inexpensive and abundant electron-transfer reagent in reactions similar to common photoredox reactions. This uncommon reaction pathway provides an interesting reactivity compared to traditional pericyclic reactions. The remaining Chapters (9-13) explore the magnetic properties and electronic structures of a variety of first-row and actinide complexes and clusters. Chapter 9 focuses on spin-state switching through oxidation chemistry of both iron and nitrogen atoms in organometallic complexes. The ground states of these complexes can be controllably tuned through sequential oxidation reactions. In Chapter 10, I present the synthesis and magnetic properties of mono- and bis-terpyridine Co(II) complexes. These Co complexes display a variety of coordination geometries which affect their dynamic magnetic properties. Chapter 11 focuses on the reactivity and magnetic properties of a family of U-acetylide species, where interesting redox chemistry is noted upon addition of redox-inactive crown ether molecules. In Chapter 12, I discuss the magnetic properties of 3 different families of uranium complexes measured in collaboration with Prof. Suzanne Bart's group at Purdue University. Finally, in Chapter 13, I give some broad conclusions about what was learned in the mechanistic studies of Cr-photocatalysis and possible interesting avenues for future work.


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