Developing Ligand Systems for Selective Uranyl Binding and Methods to Investigate the Radiolytic Stability of Monoamides

Author

Brandon G. Wackerle, Clemson UniversityFollow

Date of Award

12-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Committee Chair/Advisor

Dr. Julia L. Brumaghim

Committee Member

Dr. Shanna L. Estes

Committee Member

Dr. Brian A. Powell

Committee Member

Dr. Daniel C. Whitehead

Committee Member

Dr. William T. Pennington

Abstract

Nuclear energy has started to make a resurgence due to increasing demand for clean energy and the push for reduced carbon emissions. To make nuclear power more attractive, methods for waste remediation and recycling of fissile material for new fuel need to be developed. Developing ligand systems capable of selectively binding uranium is crucial for developing systems that can extract uranium from seawater, which contains most of the world's uranium at very low concentrations. This would lessen nuclear energy’s environmental impact. The work in this dissertation describes two different approaches to help alleviate challenges associated with closing the nuclear energy fuel cycle. The first is examining the degradation of next-generation monoamide extractants. The second approach leverages secondary-sphere interactions to develop a selective uranium ligand for separations from complex matrices.

The first chapter reviews the recent literature discussing various uranyl bond lengths and angles in the solid state as a function of -yl oxo interactions. Uranium in the 6+ oxidation state adopts a linear trans dioxo (oxygen-uranium-oxygen) species in an aqueous environment. The Cambridge Structure Database was used to determine solid-state uranyl-oxo interactions with any type of atom (X) and bonding interaction. The bond angles and distances observed for these uranyl oxo–X interactions highlight structural differences and similarities across multiple structures. Analyzing the oxygen-uranium bond lengths and angles of uranyl offer the potential to guide future development of ligand scaffolds to chelate uranyl in various ligand environments, with an emphasis on incorporating secondary-sphere interactions into the ligand design amenable to -yl oxo interactions.

The second chapter discusses development of a high-throughput, low-cost, non-radioactive, radical assay method that was developed as a screening tool for studying the radiolytic degradation of monoamides, namely N,N-di(2-ethylhexyl)butyramide (DEHBA) and its isomer N,N-di(2-ethylhexyl)isobutyramide (DEHiBA), for nuclear waste remediation. Our non-radioactive radical assay results correlate to those from gamma radiolysis because many of the same decomposition products are observed between the two methods. Establishing this radical assay as a proof-of-concept screening tool for radiolytic stability will offer a more cost-effective method to develop next-generation monoamide extractants.

The third chapter further discusses the radiolytic stability of DEHBA and DEHiBA in biphasic, nitric acid-contacted conditions. These results are compared to gamma irradiations performed in organic only conditions to determine the effects of a nitric acid phase on the stability of DEHBA and DEHiBA and the change in degradation products observed. Additionally, a series of four smaller monoamides were studied using the radiolytic and non-radiolytic methods to extend this work to a broader set of monoamide and to determine the effects of N- and C-terminal branching on the overall monoamide stability and degradation products.

The fourth chapter discusses the design and synthesis of a bio-inspired β-peptoid ligand that mimics the bacterial siderophore enterobactin, a well-studied iron(III) chelator. Enterobactin is a triscatecholate ligand synthesized by bacteria to sequester iron(III) from the environment. Iron(III) is extremely insoluble at pH 7 (K_sp = 4 10^-38), but is required for bacterial growth. The stability constant for the iron(III)-enterobactin complex is (10⁴⁹) one of the highest for iron(III) metal complexes. Developing synthetic analogs of enterobactin has traditionally been difficult due to its cyclic trilactone backbone. Our modular and facile synthetic protocol develops a cyclic β-peptoid based scaffold as a proof-of-concept ligand capable of mimicking iron binding in enterobactin. Although no iron coordination studies were performed with the synthesized ligand, these enterobactin analogs may hinder bacterial growth by limiting useable concentrations of bioavailable iron.

The fifth chapter discusses development of next-generation organic ligands designed to selectively chelate the unique trans-dioxo (actinyl) species of early actinides (U, Np, Pu, and Am), a poorly explored ligand design challenge. To increase actinyl ion affinity, we synthesized a ligand that incorporates amine functional groups for outer-sphere interactions with one or both actinyl oxygens and is based on a modular, catechol-containing ligand design that allows for varying oxo-arm and linker lengths without overhauling the synthetic scheme. The seven ligands synthesized using these methods incorporated Lewis acidic and basic moieties to interact with the uranium metal center and the -yl oxos. One ligand was crystallized highlighting its preorganized structure where the catechol and amine groups are pointed in toward each other, conducive to metal binding. After synthesis, these ligands were coordinated to uranyl, and mass spectrometry, ¹H NMR, and Raman and infrared spectra of these complexes were analyzed to establish ligand binding modes and to determine interactions to the -yl oxo of uranyl. Mass spectrometry revealed these complexes form primarily ML^‑1 complexes and, to a lesser extent, dimeric M₂L₂^‑1 species. ¹H NMR showed that both of the catechol groups had successfully bound uranyl. Raman and infrared spectroscopies were used to determine shifting of the uranyl Raman active symmetric (ν₁) and infrared active antisymmetric (ν₃) vibrational modes when complexing the uranyl to the ligand. These results indicate that uranyl is equatorially bound and that there are likely secondary sphere hydrogen bonding interactions present in some of these complexes, which allows the ν₁ and ν₃ vibrational modes to be both Raman and infrared active. Experimental Raman and infrared results were compared to calculated spectra which determined that hydrogen bonding in these complexes can cause significant shifts in the vibrational modes, even when compared to similar ligands without hydrogen bonding abilities. Results show these ligands bind uranium through the catechol groups as well as binding a uranyl oxo atom through hydrogen bonding. Uranyl was used as a model system for actinyl binding due to its relevance to nuclear energy applications and its potential for separation from seawater.

Recommended Citation

Wackerle, Brandon G., "Developing Ligand Systems for Selective Uranyl Binding and Methods to Investigate the Radiolytic Stability of Monoamides" (2025). All Dissertations. 4111.
https://open.clemson.edu/all_dissertations/4111

Author ORCID Identifier

0000-0001-8950-6195