The Royal Society of Edinburgh Lord Kelvin Prize Lecture Professor Polly

The Royal Society of Edinburgh Lord Kelvin Prize Lecture Professor Polly L Arnold FRSC FRSE, Crum Brown Chair of Chemistry, University of Edinburgh Putting the ‘f’ in Chemistry: Molecular Exploration through the Footnotes of the Periodic Table and our Nuclear Waste Legacy Monday 18 September 2017 Report by Kevin Parker Professor Polly Arnold’s curiosity-driven research into the chemistry of hitherto obscure areas of the periodic table could help us improve the clean-up of radioactive waste and lead to other industrial opportunities. Down at the bottom of the periodic table (a little bit like the Channel Islands on a map of the UK) are the f block elements, the lanthanides and actinides. Long thought of as laboratory curiosities, these elements suddenly became significant in the latter part of the 20th Century. The 14 lanthanides (aka ‘rare earth elements’) have interesting optical properties in glasses and touch screens, and make light, strong magnets important in wind power generators. More sinisterly, the actinides, especially thorium, uranium and plutonium, have been the key materials used in both peaceful nuclear power and atomic weapon construction. Most of us will recall from school that chemical behaviour is driven by the accessibility and energy of the outer electrons around the nucleus. Sodium has a single outer electron, readily lost; while chlorine, with seven outer electrons, readily accepts an electron to form a stable outer shell of eight. Moving along the lanthanide series, from lanthanum to lutetium, electrons are added to an inner (4f) shell, not changing the chemistry very much from one element to the next. Meanwhile, the outermost shell with, three ‘valence’ electrons, gives the lanthanides a chemistry similar to that of aluminium and very similar to each other. The separation of the individual rare earths by fractional crystallisation was a triumph of 19th-Century ‘wet chemistry’. The actinides are similar to the lanthanides in that electrons are added to a 5f shell inside the valence electrons. The result is that actinides tend to be similar to one another, and to the lanthanides. However, Professor Arnold’s group theorised that actinide chemistry might just be a little more interesting than ‘pseudo-aluminium’. Uranium for example, has a 6+ (VI) oxidation state, whereas the most common state of the lanthanides is 3+ (III). The 5f electrons are closer to the outside of the atom than the 4f lanthanide electrons, and might interact and hybridise with the outer electrons in the 6s and 6p shells. Because of this, its relative ease of handling, and its environmental importance, uranium has been one of the main areas of Professor’s Arnold’s study. Much new research has been carried out on these elements, driven by two factors. The first is a need for better processing, as their industrial importance continues to grow. Their chemical similarity can lead to cross-contamination – for example, lanthanide mining often produces waste thorium and uranium. Mining lanthanides has virtually ceased everywhere outside China, due to the complexity and expense 1 of waste clean-up. On the other hand, uranium and plutonium (actinide) nuclear fuels are contaminated by lanthanide fission products (which absorb neutrons) during their operating life. Lanthanides are difficult to extract from actinides, and are very effective ‘poisons’ of spent nuclear fuel. Radioactive nuclear waste contains a hitherto intractable mixture of actinides, lanthanides and other radioactive fission products, currently vitrified in glass, but dangerous for 105–106 years. This could be reduced to 1000s of years by extracting the most dangerous metals and passivating them by methods such as neutron bombardment. All this has led to a great deal of research into ways of separating lanthanide from actinide elements. There are organic materials that selectively bind (‘co-ordinate’ in chemical phraseology) to either actinides or lanthanides, which could facilitate this desirable separation. However, the science is a little empirical/ad hoc at present. There is a need for a deeper understanding of the co-ordination chemistry of both lanthanide and actinides. The other factor is that their chemistry has proven to be much more interesting than anticipated. In 1983, a paper by Watson et al. showed the first example of a compound that could bind methane, and ‘labilise’ the C-H bond. Methane complexes are extremely rare and transient, but are a first step to reactions of exceptional industrial importance, such as the liquefaction of natural gas reserves. The f block elements would be “great for catalysis” if their chemistry could be “tamed” – their steady variation in size means that catalysts could be ‘tuned' to give very specific reactions, and their low toxicity (around that of common table salt) means that they would be safe in industrial applications. Professor Arnold’s insights into uranium chemistry has led her group to investigate novel uranium co-ordination chemistry. The aim is to clarify fundamental structure bonding relationships, understand separations and improve on Watson’s original methane activation discovery. In solution, uranium (VI) does not exist as a naked/hydrated cation, but reacts with the water molecules themselves to form the O≡U≡O++ cation, a highly stable, uniquely linear moiety termed the uranyl group. Due to its symmetry and strong U–O triple bonds, this cation was thought not to participate in any complex chemistry with organic ligands, unlike, for example, the ‘bent’ O=Mo=O moiety in molybdenum (an extremely important industrial oxidation catalyst). However, in collaboration with Professor Jason Love, also at Edinburgh, Professor Arnold’s group has utilised a novel polydentate cyclic amine ligand – which, because of the way it envelops the uranyl group, has been termed the ‘Pacman ligand’. Not only does the Pacman ligand bind strongly to the uranyl group, it binds the oxygen atoms asymmetrically, opening the way for differential chemical attack on the uranyl group. The ‘upper’ (exo) oxygen is exposed above the cyclic Pacman ring, while the ‘lower’ (endo) oxygen has been ‘devoured’ inside the ring. The first unusual product of this chemistry was a reduced and silylated uranyl complex, where the exo uranyl oxygen has a strong bond to a silyl group and the endo oxygen has a dative bond to a zinc or iron halide held in the Pacman ligand. This is the first ever example of a covalent bond to a uranyl oxygen atom. In addition, the compound is paramagnetic and is a rare example of a stable uranium 5+ (V) complex. If the zinc halide is replaced by an alkali metal, particularly lithium, a series of compounds is produced where not only the endo oxygen is co-ordinated to a lithium inside the Pacman ring, but the exo oxygen also has a dative bond to a lithium cation. Carrying out the reaction in a solvent with weak C–H bonds (such as 9,10 dihydroanthracene) results in a very clean quantitative reaction to a metallated uranium (V) complex, as a result of hydrocarbon C–H bond cleavage. The 2 interactions of these uranyl (V) complexes could be relevant in ‘real life’ interactions between uranyl and iron and/or microbes – microbes that process iron also are known to process uranyl in anaerobic or reducing conditions. Adding excess uranyl salts to the original uranyl Pacman complex produces another unexpected result. An unusual compound forms in moderate (30%) yield where two uranyl moieties are forced into the Pacman ring. These interact with each other, forming a previously unknown U2O4 geometry. Another 30% of the product yield is dark and rather insoluble, but led the group to the discovery that simple, room- temperature reactions can reduce, transform and rearrange these traditionally inert uranyl ions. This newly recognised chemistry could well have happened and gone unnoticed in real-life waste separations. At this point, Professor Arnold showed a photo of a white board completely covered with different possible reactions of the uranium/pacman/silylation system, with over 20 different reaction pathways… Changing the metal ion bound to the endo oxygen neatly changes the chemistry of the exo oxygen – magnesium causes metallation at the exo position, while zinc produces the silylated U(V) complex mentioned above, in good yields at low temperatures. It is possible to put a lanthanide(III) into the end position and the resulting 4f-5f complex shows unusual magnetic properties, with each molecule essentially a tiny, single-molecule magnet. This work has given easier access to stable U(V) chemistry, insight into ‘real life’ uranium chemistry, and possible routes towards novel catalysts – perhaps including the functionalisation of the C–H bond. Professor Arnold’s group has also investigated similar reactions of the transuranium (TRU) elements neptunium and plutonium, highly radioactive components in nuclear waste. TRU research is much more difficult than uranium research, largely because these TRUs are many thousands of times more radioactive. Work has been carried out in specialist joint EU facilities in France and Germany (involving negative pressure glove boxes). Most intriguingly, in anaerobic conditions, uranyl Pacman complexes interact with cyclopentadienyl complexes of uranium and neptunium. These are the first isolated molecules that combine uranyl with all its actinide neighbours. A surprising finding was the extent to which uranyl is spontaneously reduced by the other actinides. This may give insight into the ‘real life’ clustering processes that hamper actinide waste solution separations. Sadly, this work, very dependent on co-operation with the EU facilities, is endangered post-2019 by Brexit and the UK leaving Euratom. The final part of Professor Arnold’s talk discussed work on reacting lanthanide chlorides with simple uranyl compounds without the Pacman ligand, but carried out in polar co-ordinating solvents such as pyridine or acetonitrile. Here, we see an array of ‘sandwich’ structures featuring UO2–Ln–UO2 links, Ln–UO2–Ln, and reduced U(V)O2 –Ln-U(V)O2.

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