Probing the chemistry of actinide cation-cation complexes

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Chemistry

Abstract

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.

Publications

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Arnold P (2009) Pentavalent uranyl complexes in Coordination Chemistry Reviews

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Arnold PL (2011) Oxo group protonation and silylation of pentavalent uranyl Pacman complexes. in Angewandte Chemie (International ed. in English)

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Arnold PL (2011) Single-electron uranyl reduction by a rare-earth cation. in Angewandte Chemie (International ed. in English)

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Jones GM (2012) Controlled deprotection and reorganization of uranyl oxo groups in a binuclear macrocyclic environment. in Angewandte Chemie (International ed. in English)

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Jones GM (2013) Oxo-group-14-element bond formation in binuclear uranium(V) Pacman complexes. in Chemistry (Weinheim an der Bergstrasse, Germany)

 
Description fundamental understanding of f-block bonding.
new actinide oxo geometries and reactivities.Unprecedented oxo-group reactivity of the uranyl ion, and control of metal-ligand multiple bonds

Ten years ago, we started using Pacman-shaped N-donor macrocycles with collaborator Prof. Jason Love for binding the uranyl dication in a new desymmetrising mode. In teh textbooks, the uranyl ion is the most prevalent and stable form of uranium in the environment. It is always linear with two strong axial oxo groups, and it was assumed to have no important oxo-reactivity.We have spent the last decade disproving this using new molecular uranyl chemistry.



Uranyl oxo reactivity: The Arnold/Love collaboration reported the first covalent bond forming reaction at the oxo group of the uranyl ion in 2008, here to silicon 2.1 (Nature '08). We have gone on to develop oxo-bond chemistry to make a range of singly reduced U(V) uranyl complexes that were previously considered unisolable. The new UV uranyl complexes also make much better models for the fn transuranic actinyl cations NpO2n+ and PuO2n+ which are thousands of times more radioactive, but known to have greater oxo-group reactivity than UVI uranyl.
We have shown how to control the oxo group activation by Lewis acid coordination (Inorg. Chem. '15), enabling unprecedented thermal hydrocarbon C-H bond cleavage (Nature Chem. '10), a new reactivity for the f-block that mimics transition metal oxo catalysts, hinting at new vistas in catalysis. She demonstrated oxo-rearrangement from the ubiquitous ­trans-dioxo to the previously unseen cis-dioxo geometry (2.2, Nature Chem. '12). Containing the shortest U-U bond yet reported, air-stable 2.2 also contradicts conventional wisdom that adjacent f1centres like this will disproportionate in nuclear waste solutions. As a result, theoreticians and spectroscopists in EU and US National labs are now studying these new actinyl motifs to inform our understanding of transuranic actinyl ion migration and aggregation in the environment and nuclear waste separations.
Arnold has now shown selective oxo-group metallation by cations from across the periodic table, from the proton (Angew. Chem. '12), to neptunium 2.3 and plutonium (Angew. Chem. '16). In-depth synthetic/electronic/computational studies have in many cases identified unusual electronic properties such as single molecule magnetism (JACS '13).
Most recently, she has shown that these electron transfers between actinyl salts and the rare earth ions do not even require complicated ligand architectures, just clever use of donor solvents. This latest breakthrough reports the controlled one or two-electron reduction of one or both uranyl oxos to form linear oligomers whose length (and therefore magnetic properties) is again controlled by donor solvent choice 2.4 (Angew. Chem. '17).

Unusual and reactive multiple bonds; thorium and cerium: Again, using simple ligands, we developed a simple and general reaction to make metal nitrogen M=N double bonds, the first thorium complex containing two Th=NR imido ligands 2.5 (JACS '15). The surprising cis M(=N)2 geometry contrasts with uranium's linear structures, provide strong new evidence for one side of the long-running argument that thorium should behave more like a transition metal than an actinide, and suggesting it might participate in new hydrocarbon C-H bond activation chemistry. our established expertise in unusual f-block metal ligand multiple bonding has led many others in the international f-block chemistry community to seek our expertise; for example, we is now collaborating on reactivity studies of the first terminal cerium oxo Ce=O complex (Inorg. Chem. '16; manuscript submitted, '18).
Exploitation Route research
Sectors Aerospace, Defence and Marine,Energy,Environment,Other

 
Description ERC advanced grant
Amount € 2,500,000 (EUR)
Organisation European Research Council (ERC) 
Sector Public
Country Belgium
Start 10/2017 
End 09/2023
 
Description ITU 
Organisation Institute for Transuranium Elements (ITU)
Country Germany 
Sector Public 
PI Contribution synthesis and electronic structure studies on actinide compounds of relevance to understanding fundamental nuclear waste behaviour.
Collaborator Contribution transuranic handling facilities and magnetic modelling
Impact jobs for the researchers, new knowledge, conference and journal publications.
Start Year 2007