The Characterization and Analysis of Lanthanide-Ligand Complexes in Aqueous Solution
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Authors
Licup, Gerra L.
Issue Date
2024
Type
Thesis
Language
Keywords
Alternative Title
Abstract
The rare earth elements (REEs) primarily consist of the lanthanides; they occur naturally in ores around the world and have a wide range of important technological applications. REEs are key components of electronics from semiconductors to permanent magnets, and they are involved in multiple green energy implementations like windmills and electric vehicles. REEs present an array of fluorescent and luminescent properties, and they are also used in medical imaging. Lanthanides are difficult to separate from each other due to their similar chemical and physical properties, which leads to separation processes with high environmental impacts. Solvent extraction using ligands is a primary method for isolating lanthanides through chelation, by forming lanthanide-ligand complexes soluble in organic solvents. After solvent extraction, REEs are stripped from the ligands with acid, which changes the solution pH and the protonation state of the complex. Most ligands used in solvent extraction have poor selectivity for particular lanthanides. Lanthanide-ligand complexes are present in many applications of REEs, e.g., magnetic resonance imaging contrast agents, chelation therapy, and lanthanide separations. The binding between a lanthanide and ligand, which can be quantified by a stability constant, depends on the structure of the lanthanide-ligand complex in solution and the structure of the ligand. Resolving the structures of lanthanide-ligand complexes is challenging due to the lability of the ions and to the large coordination spheres that can accommodate many binding partners; however, structures can be resolved using computation to model the geometries and chemical make-up in solution. From resolved structures, thermodynamic and electrostatic properties can be predicted. The first chapter of this work is an introduction that provides background information and highlights the relevance of the work, while the second chapter explains the computational approach implemented in later chapters. The third chapter of this work describes how classical molecular dynamics simulations and ab initio molecular dynamics simulations are used to determine the structure of Eu 3+ complexed with ethylenediaminetetraacetic acid (EDTA) in all protonation states in aqueous solution. Simulations show agreement with experimental structures and validate the predicted structures and the computational approach. The fourth chapter of this work includes studies on the structure, binding, and electron density of the Nd 3+ and Dy 3+ complexes with a macrocyclic chelator (macropa) and a glycine-functionalized macropa. There is quantitative agreement between relative free energies of binding from experiment and relative binding energies from computation. Electrostatic analyses based on the electronic structure of the optimized geometries explain the binding behavior and trends in stability constants. The fifth chapter of this work details studies on the binding of the Gd 3+ ion to another cyclic ligand (DOTA) and its variations. With the Gd-DOTA complex as reference, computational relative binding energies were compared to experimental relative free energies of binding, and there is good agreement between computation and experiment with all the DOTA-based ligands. Electrostatic analyses were also done, and the electron density of the Gd 3+ ion in the Gd-ligand complexes shows a direct relationship to the electrostatic energies of the coordination bonds in the Gd-ligand complex, where a greater electron density in the ion results in more stable complexes.