Actinide Polyoxo Chemistry

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 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).

Planned Impact

UK-plc:
The ability to handle actinides is still rare, and a known skills shortage (NDA data), both in the UK and worldwide. In the short term, UK-plc will benefit from scientists trained to work with nuclear materials and who are skilled in developing and carrying out a complex scientific project that runs across different laboratories and countries.

UK nuclear research labs:
The transuranic ions are more radioactive and much harder to study experimentally, and more difficult to model computationally. Better model systems, and experimental proof of their validity, will save money and time if transuranic manipulations can be minimised.

UK and worldwide actinide and rare earth resources and environmental protection:
The demand for heavy rare earths (HREEs) such as Nd for modern technology applications releases large amounts of unwanted Th and U from the mineral separations, so a better understanding of their oxo chemistry in unconventional environments (e.g. extremes of pH, high carbonate concentrations) will improve our grasp of pathways for proton and electron transfer in environmental migration and precipitation processes, and in the cluster formation that hampers nuclear waste separations. The UK already has a UK waste legacy with an estimated clean-up bill of £70 billion (NDA data) and so better understanding and manipulation of wastes can only reduce this cost. Conversely, the security of supply of U (and Th) for civil nuclear reactors could be ensured by extraction of the abundant ions from seawater or even from deposits in Orkney. These will not be possible without improvements in separations technologies that require better bonding models.

SMEs:
If new catalyses or materials are discovered that are quickly exploitable, new technology SMEs and spin-outs will benefit. Millions of tons of basic hydrocarbons from fossil fuels and biomass are consumed by the chemical industry each year; the contribution of the UK's chemical and related sectors is equivalent to 21 % of its GDP and supports over 6 million jobs. The growing worldwide market (estimated at 6+%/yr) presents an opportunity for new catalysed transformations of hydrocarbons to give the UK a major competitive advantage. Homolytic C-H scission by heterogeneous early-to-middle transition metal oxo materials, and solid-state uranium oxide-derived materials have been used abroad in industrial oxidation processes. Even if uranium or thorium catalysts are not acceptable to the public in the next couple of decades, significant success in catalysis arising from these compounds may prompt others to redesign d-block metal oxo catalysts to mimic the activities we find, perhaps encouraging exploration of the early d-block where people have conventionally assumed that all complexes are inert. A better understanding of electron interactions, for example, the pathways that lead to slow magnetic relaxation in actinides and lanthanides will open up the whole periodic table for use in component design and manufacture in nano-electronics, information storage, and computing at the scale anticipated for future miniaturisation demands of technology.

Policymakers:
The bodies higher up in the European Commission who fund the ITU, and the UK politicians and decision makers who we come into contact with at 'Science in the Parliament', one-to-one briefing meetings, and related events will see potentially useful new technologies coming out of actinide research and thus will be persuaded of the importance of supporting fundamental research.

Public:
UK HEIs, pupils and the public will learn about the fundamental issues associated with the safe and clean long-term operation and decommissioning of low-carbon nuclear builds, the behaviour of the heavy ions, and the potential for chemistry that can be harnessed. PLA's Women in Science work will be threaded into all aspects of dissemination, adding value by encouraging 50 % of the STEM-trained workforce to continue in science.

Publications

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Arnold PL (2020) Dicerium letterbox-shaped tetraphenolates: f-block complexes designed for two-electron chemistry. in Dalton transactions (Cambridge, England : 2003)

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Arnold PL (2019) Applications of boroxide ligands in supporting small molecule activation by U(iii) and U(iv) complexes. in Dalton transactions (Cambridge, England : 2003)

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Arnold PL (2017) Organometallic Neptunium Chemistry. in Chemical reviews

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Bell NL (2018) Uranyl to Uranium(IV) Conversion through Manipulation of Axial and Equatorial Ligands. in Journal of the American Chemical Society

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Bell NL (2015) Thorium Mono- and Bis(imido) Complexes Made by Reprotonation of cyclo-Metalated Amides. in Journal of the American Chemical Society

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Cowie BE (2018) Double uranium oxo cations derived from uranyl by borane or silane reduction. in Chemical communications (Cambridge, England)

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Dutkiewicz MS (2016) Organometallic neptunium(III) complexes. in Nature chemistry

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Hlina JA (2017) Uranium rhodium bonding in heterometallic complexes. in Dalton transactions (Cambridge, England : 2003)

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Hlina JA (2016) Metal-Metal Bonding in Uranium-Group 10 Complexes. in Journal of the American Chemical Society

 
Description To date, new evidence for the ability of main group heavy metals to interact with, and stabilise unusual actinide oxo groups has been found.
we have also further reported on the synthesis and reactivity of the first thorium compounds with two multiple bonds to nitrogen atoms. 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. 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).
We have 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 (Angew. Chem. '17).

Again, using simple ligands, Arnold 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.
Exploitation Route This is of relevance to the fundamental understanding of structure and bonding in the f-block, which is important for dealing with our nuclear waste legacy.
Sectors Aerospace

Defence and Marine

Energy

Environment

Other

 
Description Energy Frontier Research Center 
Organisation Los Alamos National Laboratory
Country United States 
Sector Public 
PI Contribution Results from this grant have led to an invitation to join a new, large collaborative team exploring actinide chemistry in extreme environments.
Collaborator Contribution knowhow on the real-world behaviour of nuclear materials under duress, and following events such as the Fukushima Daiichi reactor accident.
Impact Research Center proposal.
Start Year 2020