Uranium-Ligand Multiple Bonds: From Molecules to Materials

Since the 2011 Fukushima disaster, a major priority for the nuclear sector has been to develop accident tolerant fuels (ATFs). A very promising ATF is uranium-nitride (UN). UN has a high thermal conductivity, enabling heat to be transferred efficiently so the fuel is meltdown-resistant. UN has a high fissile content, so more power can be generated than with existing oxide fuels for the same enrichment level. Mixed UN/PuN is a fuel option for Generation IV reactors breeding fissile material and producing less long-lived radioactive waste. So, UN is a safer, more environmentally friendly, and sustainable nuclear fuel. For similar reasons uranium-carbides are also attractive ATFs. However, preparing uranium-nitrides and -carbides by traditional routes presents challenges. An attractive approach is to use molecular uranium-nitride and -carbyne precursors and decompose them to binary nitrides and carbides. Sadly, for decades there were few molecular uranium-nitrides so a molecules-to-materials approach was not realistic. The situation for uranium-carbynes is worse; there are only two spectroscopic reports of uranium-carbynes at ~10 Kelvin. Recently, we prepared the first molecular uranium-nitride triple bonds (Science, 2012, 337, 717; Nature Chemistry, 2013, 5, 482). Metal-ligand multiple-bonding is fundamentally important in chemistry and we have made a number of contributions in this area (e.g. J. Am. Chem. Soc. 2014, 136, 5619; Angew. Chem. Int .Ed. 2014, 53, 4484) and preliminary results show that our molecular nitrides can be controllably decomposed to binary nitrides which opens up a molecules-to-materials approach. This Proposal aims to apply our recent coordination chemistry to the preparation of materials for energy in Grand Challenge and Priority Areas. We will develop a new range of uranium precursors to generate a platform to expand the range of nitrides. This exploits a blend of steric and electronic properties uniquely suited to stabilising uranium-ligand multiple bonds. Using these precursors we have identified four routes to maximise our chance of success to prepare high-value uranium-carbynes which have no precedent. With an expanded range of molecular uranium-nitrides and new uranium-carbynes we will build on preliminary results and investigate their decomposition to binary materials. The availability of new precursors leads to the possibility of exploring high pressure phase transitions to give new polymorphs. This is directly relevant to understanding fuels under extreme conditions in nuclear reactors and these metallic polymorphs are interesting to study as their itinerant vs localised 5f electron behaviour is magnetically fascinating and crucial to designing better ATFs. We will combine synthetic, structural, and materials studies with interdisciplinary magnetometric, computational, and spectroscopic studies with collaborators to give a comprehensive understanding of uranium-nitrogen and -carbon bonding, reactivity, and materials applications. A Fellowship will provide the best opportunity to oversee this complex programme of research, manage an intensive array of collaborations, and make the time to engage with the nuclear industry and translate academic advances on to the next level into industrially relevant applications. The researchers on this project will develop a range of skills in a recognised strategic skills shortage area. Our molecules provide unique opportunities to probe the nature and extent of covalency in uranium bonding; this issue is long-running, still hotly debated, and important because of the nuclear waste legacy in the UK. Spent nuclear fuel is ~96% uranium and the official Nuclear Decommissioning Authority figure for nuclear waste clean-up bill is 70 billion pounds. If we can better understand the chemistry of uranium this may in the future contribute to ameliorating the UK's nuclear waste legacy and provide new routes to ATFs to be developed with the Nuclear Industry.