INVESTIGATIONS INTO THE COORDINATION CHEMISTRY OF SUBSITUTED THIOBIURETS AND THEIR ANALOGUES WITH LANTHANIDES AND EARTH-ABUNDANT METALS
Rare earth elements (REE) are increasingly becoming important in modern society as those elements are widely added to novel inventions to enhance performance, such as lanthanum (La) being used to increase refraction and keep dispersion low in optical glasses or neodymium (Nd) being commercially used in permanent magnets, but our high demand for REE is not fully satisfied since it remains very difficult to separate REE from each other during hydrometallurgical processes even though the natural abundance of REE is rather high. The “rarity” of REE comes from their unique characteristics and also separation issues because of similar chemical and physical properties. Use of common extractants result in a low differentiation between different elements and low efficiency during separation is unavoidable. To result in effective separation (liquid-liquid, liquid-solid), different extractants has been designed and among those extractants, bifunctional (two donor atoms) extractants are well-developed such as diglycolamide, ethylenediaminetetraacetic acid (EDTA), beta-diketone, malonamide and succinamide. This thesis starts the idea of expanding the choice of bifunctional extractants and make exploratory attempts in the extraction or even in the separation of lanthanide elements. The initial hypothesis for this thesis was that by varying the degree of hardness of the donor atoms on a biuret-based ligand skeleton from relatively polarizable Sulfur atoms to relatively no-polarizable O donor atoms, we could control the preference of ligands to the rare earth elements. Biuret-based ligands were chosen due to their ease of synthesis and success within the Foley group with regards to selective leaching of gold by dithiobiuret-based ligands. The first part of the thesis explores methodologies by which substituted dithiobiurets can be selectively oxidized into their corresponding monothiobiuret or biuret derivatives with an easy control and good yield under mild conditions. Initial focus was on dithiobiurets synthesised from secondary amines, CS2, and a carbodiimide. General procedures were established resulting in the synthesis of several previously unreported monothiobiuret and biuret derivatives in high yields. Alternative synthetic strategies involving thiocarbomoyl isothiocynate derivatives as the starting materials were also explored however the resulting thiobiurets were found to be relatively unstable compared to those obtained via the CS2 route. After determining how to efficiently and selectively convert substituted dithiobiuret into their corresponding monothiobiuret or biuret derivatives, the second part of the thesis involves investigation towards the selective leaching of lanthanides. While preliminary results indicated a degree of coordination of select ligands to lanthanides, progress was complicated by insolubility issues and paramagnetism of the resulting compounds. Under the conditions evaluated, biuret-based ligands did not yield conclusive results for the selective extraction of lanthanides. Due to the inconclusive nature of the investigation involving the selective leaching of lanthanides. The research focus shifted to the exploring the coordination chemistry of dithiobiuret, monothiobiuret and biuret-based ligands with earth-abundant metals. While previous members of the Foley group have shown the dithiobiuret ligands have high selectivity towards gold over base metal, little research have been carried to study the efficiency of the ligands in coordination to base metals or the nature of the resulting complexes that might form. To better understand the chemistry of the dithiobiuret ligands and their oxidized analogues, their coordination chemistry was investigated with earth abundant metal including Fe, Ni, Cu and Zn.
Lanthanide, Dithiobiuret, Solvent extraction
Master of Science (M.Sc.)