Investigating the coordination chemistry and oxidation state stability of actinyl and actinide ions and consequent effects on their emission profiles

Nuclear power currently plays a significant role in today’s balanced energy portfolio. However, with this expansion, comes the need for improved methods to characterise and manage radioactive wastes arising from fission activities throughout the fuel cycle, and the need for fundamental research into all aspects of the nuclear fuel cycle. Whilst the chemistry of actinides is enjoying a renaissance, in particular the study of unusual oxidation states which were previously unobtainable, many fundamental properties of the actinides remain unknown. This thesis explores the coordination chemistry and emission spectroscopy of a selection of actinides (U to Cm) with a view to accessing the unstable f 1 oxidation states of uranium and neptunium. In particular, the role of cation-cation interactions (CCIs) on the stability, redox and spectroscopic properties has been addressed. Work on the luminescence of uranyl(VI) complexes has led to an increased understanding of the non-aqueous solution behaviour of the actinyl ion, in particular by fingerprinting/identifying complexes bearing cation-cation interactions. The uranyl(VI) LMCT emission profiles are observed to significantly red shift when CCIs are present, accompanied by a drastic reduction in the radiative lifetime. These observations have been supported by the increasingly effective diffusion-ordered nuclear magnetic resonance technique. This leads to the conclusion that uranyl(VI) cation-cation interactions unsupported by bridging ligands are unstable in solution. Attempts to isolate stable uranyl(V) complexes of tetraphenylimidodiphosphinate (TPIP) and the fluorinated acacs have proved difficult. The TPIP ligand has shown an unprecedented preference to stabilise NpO2(VI). From these observations, it has been possible to conclude that TPIP has a strong preference for the actinyl(VI) oxidation states. A preliminary study on the minor actinides Am and Cm in the +III oxidation state has taken place and allowed preliminary comparisons between a novel series of bis-trialkylsilyl bipyridyl and phenanthroline ligands, which have been designed in an attempt to study the effect of steric bulk in supporting unusual oxidation states of the actinides. An initial study of these ligands with AnO2(V) and AnO2(VI) (An = U and Np) has also taken place.