Advancing f-element-ligand multiple bonding scaffolds : lanthanide and actinide yldiide and aryloxide complexes
Authors:   , Gremillion Alexander J 
Published: September 2024

Organometallic actinide chemistry is still a field that is in its infancy. The birth of the area can be traced back to the isolation of the first "actinocene", Uranocene, in 1968. Given the complexity of the 5f orbital manifold, descriptions of actinide ligand bonding are to this day still being investigated at a fundamental level, with competing schools of thought asserting their perspective on bonding and electronic structure. This task is not made easier through comparisons with the lanthanides which reside one period above, utilizing the 4f manifold. The reason for this is attributed to the almost entirely ionic bonding modalities established by the lanthanide elements, in which the ionic radius drives the be havior of each element rather than any highly significant level of metal ligand orbital overlap. This divergence does prove most beneficial to the field at large, however. On a fundamental level, it provides a benchmark with which to compare the degree of covalency that any given actinide element may invoke. On a practical level, it provides a difference in behavior that can be exploited in situations where these elements have a co-occurrence, namely in the nuclear fuel cycle and reprocessing schema. This dissertation seeks to add to the conversation around this burgeoning field of study by contributing a discussion surrounding two highly literature relevant ligand sets: the carbene and the aryloxide. The first chapter discusses a continued effort to push forward our understanding of the phosphorano stabilized carbene complexes for which our group is now known. Chapter two then continues the discussion by altering the ligand framework to accommodate a new carbene scaffold altogether; the yldiide. In this regime, the yldiide ligand can be tailored and parameterized to alter electronic and steric properties, while maintaining a carbanionic center that is stabilized through negative hyperconjugation. Additionally, these ligands still feature phosphorous which aids in their spectroscopic characterization, given the high isotopic abundance and molar receptivity of 31 P NMR. Chapter three is the first of two aryloxide focused chapters and seeks to define a system in which Ce(IV) is easily accessible through a convenient and mild oxidant. Although the Ce(III/IV) couple is well studied, and Ce(IV) is known to be stable, the incidence of this work in an air free, nonaqueous environment is substantially lower than that of the existing aqueous literature. This change in the overall chemical environment not only leads to changes in redox potentials from their standard potentials but allows for transformations that would not normally be possible in an aqueous medium. Here, we report evidence of a rare example of a Ce(IV) fluoride as well as a pair of isostructural chloride, bromide, and iodide complexes that represent a system in which facile oxidation is achieved in a non aqueous medium and interrogated with cyclic voltammetry. Finally, chapter four focuses on the chemistry of neptunium (237 Np), an important element found in the waste produced by nuclear reactors, and the first of the transuranium elements. As this element is particularly less studied than its early actinide congeners, its chemistry is considerably less developed. In this study we report a system in which a homoleptic aryloxide complex is described and parameterized via small molecule activation through one electron oxidation. The study of Np(III) and Np(IV) in this way is only possible via air free techniques, and contributes to the overall understanding of low valent neptunium chemistry, that is currently not well described in comparison to the ubiquitous Np(V) actinyl system.