FORTRESS: F block cOvalency and Reactivity defined by sTructural compRESSibility

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

Abstract

Civil nuclear waste contains a smorgasbord of metal ions from the elements in the f-block of the periodic table (the 4f and 5f), some of which render it extremely radioactive, and costly to handle safely. Clean-up of the UK's waste carries an estimated £73 bn price tag. Most f-block ions in the waste's complex mixture are physically very similar, but subtle differences in their electron density distribution should enable the most dangerous ions to be separated and passified, reducing waste storage times from millions to thousands of years. The state of the art for differentiation of the 4f (lanthanide) from the 5f (transuranic actinide) ions is the remarkable 4000-fold selectivity achieved by certain ion-selective organic molecules in solution extraction processes. However, solid-state structures of the different metal adducts show minimal differences in metal-ligand bond distances, and the current computer models of the 4f/5f metal-ligand bonding cannot explain the selectivity. New, conceptually pleasing proposals of multiple layers of weak solvation interactions are now suggested as the reason for differentiation but, as yet, neither solution nor solid state experiments have produced conclusive evidence/understanding of the role of these interactions.
The metal-ligand bonding in 5f ions is described as 'softer' or more covalent, which to the chemist tasked with designing the ion-selective organic molecules suggests a 'smearing out' of the electron density. However, the physicists' definition of covalency centres on the matching of the energy of the orbitals which contain the important electrons, and there is much debate and confusion as to what covalency actually means in this part of the periodic table. Soft metal ions also form more weak interactions with solvents. Metal hydrocarbon weak interactions are called 'agostic' and are key to the stereochemical control of polymerisation of propene by zirconium catalysts used by industry to make 30 million tons of polypropylene pa. Such weak interactions are also studied in the lab as the precursor to the cleavage of a single C-H bond of a hydrocarbon across a metal cation, the first step in the highly desirable atom-economical catalytic functionalisation of alkanes.
Both the strong metal-ligand bonds, and weak metal-CH interactions can been measured using X-ray diffraction on crystals of the compound. At Edinburgh we can both compress the crystals as their structures are measured, and grow the crystals from solution at pressure, which increases solvation.
Small organics and actinide alloys have already been studied at high pressure, with interesting results. Here, we will explore for the first time the effect of high pressures on the compressibility of metal-ligand bonds and on weak interactions with solvent/ligand peripheral groups in isostructural 4f- and 5f-block molecules. In Edinburgh we will focus on uranium, and at the EU centre for transuranics in Germany we will also study heavier members of the 5f series, e.g. neptunium and plutonium. We will combine diffraction, spectroscopy, and computational (at Manchester) analyses to measure and interpret the differences between the 4f- and 5f-ions.
We will focus on the compressibility of the strong, 'covalent' bond. Solids show different compressibility depending on the extent of covalency. We will combine pressure experiments on 4f and 5f complexes with theory to produce a completely new and conceptually simple yet rigorous definition of the much-debated degree of covalency in the f-block metal-ligand bond.
We will also focus on weak, agostic interactions, and grow crystals at high pressure (i.e. high solvation levels) to accentuate the 4f/5f solvation differences and propose new answers to explain the selectivity of nuclear waste extractant molecules.
Finally, we will use pressure to initiate in-crystal reactions where weak interactions can be set up then broken by the metal, targeting some very unusual new molecules.

Planned Impact

The PDRAs
The proposed research combines precision synthetic chemistry, high level computational work, and technically demanding structural studies. In the short term, UK-plc will benefit from scientists who are skilled in devising and delivering a complex scientific project that runs across different labs, countries, and central facilities. The ability to handle actinides is still rare, and there is an on-going skills shortage (source NDA) of trained actinide scientists both in the UK and worldwide.

Government nuclear and environmental agencies
Better models for the more radioactive 5f ions, and experimental proof of their validity, will minimise transuranic manipulations saving time and money. Better bonding models will enable extraction processes that meet the demands for heavy rare earths (HREEs) such as Nd for sustainable technologies. Improvements in separations will also avoid the unwanted co-extraction of Th from HREE mineral deposits, rendering many more financially viable. In the context of energy, security of uranium supply could be ensured by extraction of the abundant ions from seawater; the outputs of the proposed research will inform such processes.

SMEs and technology industries
Though extreme conditions research is most actively pursued in academia, our aim is to demonstrate its power in defining bonding in complex systems and hence to indicate to materials science-using industries that they may find direct applications from pressure-induced molecular modification (e.g. sensors). The engineering skills for extreme conditions studies are also of value to industries such as aerospace, renewable energy and advanced machining. Pressure is a more powerful thermodynamic variable than temperature and it is already used in materials synthesis and the food processing industry. New applications in fine chemicals production and catalysis are realizable in the 10-15 year time-scale. This growing chemical science space is beginning to be served by private sector manufacturers of research instrumentation and software. For example, we are working with the Cambridge Crystallographic Data Centre on methods, inspired by our work in high pressure, for the calculation of intermolecular interaction energies and visualisation of voids.
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 miniature scales anticipated from future technology demands.
If new C-H activation reactions were to be made catalytic in the long-term, chemical companies will benefit. Millions of tons of basic hydrocarbons from fossil fuels and biomass are consumed by the chemical industry pa. The growing worldwide market (estimated at 6+%/yr) is an opportunity for new catalysed hydrocarbon transformations to give the UK a major competitive advantage.

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 through advisory board work with the RSE and at 'Science in the Parliament' and related events will see potentially useful emergent new technologies and will be persuaded of the importance of supporting fundamental research.

The public
UK HEIs and the public will learn about the generation of very high pressures, the effect these have on materials, and about issues associated with handling critical HREE elements and nuclear materials; areas which spark the imagination of the public. We use and contribute to the PE activities in our universities, presenting complex material to the public in simple ways to achieve greatest impact. PLA's Women in Science work will be threaded into all aspects of dissemination, to encourage all the members of our STEM-trained workforce to pursue science careers.

Publications

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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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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 Polly (2019) Organometallic neptunium chemistry and the importance of spontaneous reduction reactions in ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY

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Arnold Polly (2019) Many-electron reductive activation of small molecules with dinuclear 4f-and 5f-organometallics in ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY

 
Description We have shown that the early actinide tetrakis(aryloxides) complexes of Th, U, and Np undergo a sudden shortening of the strong M-O bonds at high pressure. Quantum mechanical calculations show that these bonds enter a more covalently-bonded regime at the transition through a greater interaction of the ligands with the 6d and 5f orbitals of the actinide. The nature of metal-ligand bonding in actinide complexes is still a controversial area, and this work provides a new way to think about the spatial implications of covalent contributions to bonding. The 5f-orbitals in the early and middle actinides cannot be considered as core-like. Small but significant covalency involving the 5f orbitals is regularly invoked to explain the reactivity, structure and spectroscopic properties of these elements, and has been probed with a variety of experimental and theoretical techniques. Further, some of the experiments that set out to define covalency in actinide metal-ligand bonding provide answers that show the energy degeneracy of the orbitals, which will have a different effect than a spatial overlap of orbitals - a description with which chemists are generally more comfortable. Finally, the neptunium (IV) complex Np(OAr)4 is also a new compound, and a rare example of a Np(IV) coordination complex.
Exploitation Route Understanding the differences in electronic structure and orbital covalency is fundamentally important for the segregation and safe manipulation of the f-block ions in technology and energy applications.
Sectors Chemicals,Energy,Environment

 
Description European Commission, Joint Research Centre, P.O. Box 2340, 76125, Karlsruhe, Germany 
Organisation European Commission
Department Joint Research Centre (JRC)
Country European Union (EU) 
Sector Public 
PI Contribution Synthesis of actinide complexes; crystallography; analysis of crystallographic data.
Collaborator Contribution Expertise in handling highly radioactive materials containing actinide elements (neptunium)
Impact A paper describing our work is in review.
Start Year 2018
 
Description BBC R4 Brainwaves 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact media coverage of both research and Women in STEM work,
Year(s) Of Engagement Activity 2018
URL https://www.bbc.co.uk/programmes/b09trwf0
 
Description Chemistry World feature article 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Article in Chemistry World focussing on women in STEM research
Year(s) Of Engagement Activity 2020
 
Description Infinite Monkey Cage 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Media coverage of both research and Women in STEM work.
Year(s) Of Engagement Activity 2020
URL https://www.bbc.co.uk/programmes/m000937h