Probing the chemistry of actinide cation-cation complexes

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Uranium, the heaviest naturally occurring element, is the main component of nuclear waste. In air, and in the environment, it forms dioxide salts called uranyl compounds, which are all based around a doubly charged, linear O=U=O group. These are very soluble, and are problematic environmental groundwater contaminants. The U=O bonds are also extraordinarily chemically robust and show little propensity to participate in the myriad of oxo-group and redox reactions that are characteristic of transition metal dioxide analogues which have chemical and catalytic uses in both biological and industrial environments. Uranium's man-made and highly radioactive neighbour neptunium also forms linear O=Np=O dications, but due to the extra f-electron, shows much more oxygen atom reactivity. In nuclear waste, cation-cation complexes form when the oxo groups bind to another metal dioxo cation, making the behaviour of the mixtures harder to predict, and suggesting that the less reactive uranyl dication is not a good model for the harder-to-handle neptunium and plutonium analogues.We recently showed that we can 'trick' the uranyl dication into reacting at just one oxygen atom to form a strong O-Si covalent bond, in a manner similar to that seen in transition metal oxo chemistry. We have been able to bring out this unprecedented behaviour in uranium chemistry by binding the uranyl dication within a rigid organic ligand framework which exposes just one of the two oxygen atoms to reactions while the other remains inaccessible within the cavity of the ligand framework. This makes the uranyl ion behave more like the neptunyl ion. Working at the EU Joint research centre for transuranic research at the ITU, we will place the neptunyl ion in the same organic framework, since it should more readily form O-Si bonds. This controlled reaction would be a new type of reactivity for the neptunyl ion. We will use the control afforded by the rigid ligand to make the first series of same- and mixed-metal bimetallics in which the two metals communicate through a central oxo atom. Again, work at the ITU will allow us to make molecular, and thus easy to study, uranium-neptunium systems. These new cation-cation complexes will help us better understand the more complex metal oxo systems found in nuclear wastes, so we will collaborate with Los Alamos National Labs to obtain XAS data to determine model metal-metal distance (from the EXAFS) and covalency (from the ligand edge XAS) information.The magnetism of these, and lower-oxidation state systems will be studied in collaboration with experts at the ITU.As part of the researcher training, more chemically esoteric projects will also be studied, for example, we will use the rigid ligand to try to trap the first example of a bent uranyl dication, and the also first U=C double bond. The current estimated bill for cleaning up all of our nuclear waste is 70 billion (official Nuclear Decommissioning Authority figure), and approximately 96% of used fuel is recyclable uranium. If we can understand better the chemistry of this ubiquitous uranium dioxo dication, how it relates to its more radioactive neighbour elements, and how it is precipitated from the environment, we might be able to help reduce the UK's nuclear waste legacy.