Actinides can be stabilised in various oxidation states. For uranium specifically these are +II, +III, +IV, +V and +VI, with the +V and +VI oxidation states existing as a linear dioxo species (uranyl ion). Of these, U(IV) and UO2(VI) are the most commonly studied, with very little known about UO2(V) as it readily disproportionates in solution to U(IV) and UO2(VI). The mechanism is thought to occur via an inner-sphere electron transfer that is facilitated by cation-cation interactions (CCIs), Lewis acid-base interactions connecting neighbouring uranyl units. Luminescence spectroscopy has already been used to probe the nuclearity of a species (oxidation state, coordination number etc.) and is well defined for UO2(VI) but not for UO2(V) and more studies need to be carried out. It is also proposed that luminescence spectroscopy may be used to distinguish between different CCIs to provide valuable insight on structures in the solution and solid states. One method of trying to stabilise UO2(V) is through the use of sterically bulky ligands, which disrupt and prevent the formation of cation-cation interactions (CCIs) between uranyl units that enable the disproportionation. The syntheses of sterically bulky silylated polypyridyl ligands based on 6,6âdimethyl-2,2-bipyridine and 2,9-dimethyl-1,10-phenanthroline have been studied previously in the group, but subsequent reactions with lanthanides and actinides resulted in the formation of assorted side products. As such, a number of modelling reactions using unsilylated 6,6âdimethyl-2,2-bipyridine and 2,9-dimethyl-1,10-phenanthroline with various lanthanides were carried out and are reported here. The use of N-oxide derivatives as an alternate neutral ligand and 5,5â-substituted bipyridines is also explored. Imidodiphosphinates (R2P(O)NHP(O)Râ2) are seen as complementary inorganic structures to Î²-diketonates that are already well known for forming luminescent lanthanide complexes. Those with bulky substituents in particular are very attractive for use in f-block chemistry due to the potential to sterically protect the metal centre. This, along with the fact that there are no O-H, C-H or N-H oscillators within the binding site that can lead to quenching of the emission, means they are ideal for luminescence applications utilising the photophysical properties inherent to lanthanide(III) ions. Here we introduce two unexplored bulky imidodiphosphinate ligands to f-element chemistry; tetraiso-propylimidodiphosphinate (TIPIP) and tetratert-butylimidodiphosphinate (TBIP). Importantly, these ligands, which contain no chromophores within the ligand framework, enable us to study pure metal emission without any competing ligand emission processes that currently hamper observations of actinide luminescence. We will present the synthesis, coordination chemistry and photophysical properties of TIPIP and TBIP with lanthanide(III) ions as well as preliminary results of the reactions of the two ligands with uranium.
|Date of Award
|1 Aug 2019
- The University of Manchester
|David Collison (Supervisor) & Louise Natrajan (Supervisor)