title: Sterically Encumbering Tris(aryloxide)- and Cyclopentadienyl-Supported Uranium Complexes and Their Reactivity
Creators: name: Löffler, Sascha
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: The abundance of depleted uranium as a waste product of the enrichment process and the relatively high abundance of natural uranium makes it a considerable object of research. Any contribution of fundamental research in uranium (coordination) chemistry helps to understand the unique physicochemical properties and reactivity of the metal and its complexes. This lays the foundation for any beneficial use and application of the uranium complexes in the future. The present thesis consists of four chapters that correlate to four publications. Three chapters (3.2, 3.4, and 3.5) include uranium complexes stabilized by tris(aryloxide) ligands and their reactivity and interactions with the (small) molecules methylcyclohexane, methylene chloride, silver fluoride, water, nitrous oxide, and 9,10-diphenylanthracene-9,10-endoperoxide. Chapter 3.3 focusses on uranium complexes with cyclopentadienyl ligands and their molecular and electronic structures. In chapter 3.2, the intermolecular forces behind a uranium–alkane adduct formation are discussed. This study follows up on a publication in JACS 2003 that reports an alkane coordination to the uranium center of the complex [UIII((Ot-Bu,t-BuAr)3tacn)]. To gain insight into the forces behind the adduct formation, the adduct was resynthesized and subsequently analyzed and characterized by X-ray diffraction measurements at 6 K and a variety of spectroscopic methods. The experimental results served as a benchmark to adjust the theoretical methods and calculations that were used to describe the interactions between the uranium and the alkane. Chapter 3.3 includes bent and linear uranium pentabenzyl cyclopentadienyl complexes. It is shown that the electronic structure changes significantly when going from bent to linear. For this purpose, several bent and linear complexes were synthesized and thoroughly characterized by 1H NMR spectroscopy, CHN elemental analysis, UV/Vis/NIR spectroscopy, and SQUID magnetization measurements. The spectroscopic findings were supported and explained by theoretical studies. Chapter 3.4 deals with the activation of small molecules, such as methylene chloride, silver fluoride, water, and nitrous oxide. The primary focus is on the synthesis of a uranium(IV/V/VI) oxide and a uranium(IV/V) hydroxide series. All complexes are stabilized by the same cyclen-based tris(aryloxide) ligand. This allows for an unprecedented comparison of the oxide and hydroxide complexes, which aids drawing conclusions about the physicochemical properties of the uranium complexes. For this purpose, all compounds were extensively characterized by 1H NMR spectroscopy, CHN elemental analysis, UV/Vis/NIR spectroscopy, IR spectroscopy, cyclic voltammetry, and SQUID magnetization measurements. Chapter 3.5 describes a rare uranium-mediated peroxide activation of 9,10-diphenylanthracene-9,10-endoperoxide. The resulting uranium bis(alkoxide) complex was subsequently reduced to serve as a precursor for the formation of a uranyl complex via photochemical release of diphenyl anthracene. Theoretical calculations indicate that the synthesis of the uranyl complex proceeds through an elusive uranium cis-dioxo intermediate.