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 compounds 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 reactions that are characteristic of transition metal dioxide analogues which have chemical and catalytic uses in both biological and industrial environments. Due to relativistic effects, thorium, another component of nuclear waste, and a potential nuclear fuel of interest due to the lower proliferation risk, also does not have straightforward, predictable chemistry, and is a remarkably soft +4 metal ion. The behaviour of its molecular oxides is poorly understood, although tantalising glimpses of what might be possible come from gas phase studies that suggest oxo structures completely unlike the other actinyl ions. Uranium's man-made and highly radioactive neighbour neptunium forms linear O=Np=O dications like uranium, but due to the extra f-electron, shows much more oxygen atom reactivity. In nuclear waste, cation-cation complexes form with U, Np, and Pu when the oxo groups bind to another metal dioxo cation, making the behaviour of the mixtures harder to predict. However, by adding an electron to the uranyl ion, we and others have shown in recent years that the singly reduced uranyl can provide a more oxo-reactive, better model for the heavier actinyls. Since the route for precipitating uranium from groundwater involves an initial one-electron reduction to an aqueous-unstable intermediate, these stable U(V) uranyl complexes are potentially important models for understanding how uranium is precipitated.
Our work to uncover actinyl ion reactivity similar to that seen in transition metal oxo chemistry has focused on using a rigid organic ligand framework to expose one of the oxygen atoms. We have most recently reported a smaller, more constrained macrocycle that can bind one or two uranium or thorium cations, so far in the lower oxidation states. This also allowed us to look at covalency in the metal-ligand and metal-metal interactions.
We will use the control afforded by these two rigid ligands to make a series of actinide oxo complexes with new geometries. Some, including more chemically esoteric projects, are initially anticipated to be purely of academic interest, and an important part of researcher training. Some of the reactions will have more relevance to environmental and waste-related molecular processes, including proton, electron, and oxo group rearrangement, transfer, and abstraction. Results concerning the reactivity of these new complexes will help us better understand the more complex metal oxo systems found in nuclear wastes and the environment.
We will look at hydrocarbon C-H bond cleavage by the most reactive actinide oxo complexes, working on pure hydrocarbon substrates, but recognising the relevance to the destruction of organic pollutants induced by photolysis of uranyl.
Working at the EU Joint research centre for transuranic research at the ITU (Karlsruhe), we will also study the neptunium analogues of these complexes. The molecularity of these systems will also make the magnetism of mono- and bimetallic complexes easier to understand and model than solid-state compounds. The experts at the ITU will be able to identify whether the two metals communicate through a central oxo atom or even through ligand pi-systems. We will also provide samples to collaborators at the INE (institute of nuclear waste disposal), Karlsruhe and Los Alamos National Labs, USA, to obtain XAS data that allow the study of the valence orbitals, metal-metal distances/interactions (from the EXAFS) and covalency (from the ligand edge XAS).
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