FORTRESS: F block cOvalency and Reactivity defined by sTructural compRESSibility

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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.