Uranium-Ligand Multiple Bonds: From Molecules to Materials

自2011年福岛灾难以来，核能部门的一个主要优先事项是开发事故耐受燃料（atf）。一种非常有前途的ATF是氮化铀（UN）。UN具有高导热性，使热量能够有效地传递，因此燃料具有抗熔毁性。UN具有高裂变成分，因此在相同的浓缩水平下，可以比现有的氧化物燃料产生更多的能量。混合UN/PuN是第四代反应堆的一种燃料选择，用于繁殖可裂变材料，产生寿命较短的放射性废物。因此，UN是一种更安全、更环保、更可持续的核燃料。由于类似的原因，碳化铀也是有吸引力的atf。然而，用传统方法制备氮化铀和碳化物存在挑战。一种有吸引力的方法是使用分子氮化铀和碳炔前体，并将它们分解成二元氮化物和碳化物。遗憾的是，几十年来，氮化铀分子很少，所以分子到材料的方法是不现实的。铀-碳的情况更糟；在~10开尔文的温度下，只有两个铀-碳的光谱报告。最近，我们制备了第一个氮化铀分子三键（Science, 2012, 337, 717; Nature Chemistry, 2013, 5,482）。金属-配体多键在化学中是非常重要的，我们在这一领域做出了许多贡献。化学。社会科学学报，2014,36,5619；Angew。化学。Int。。2014, 53,4484)，初步结果表明，我们的分子氮化物可以可控地分解为二元氮化物，开辟了分子到材料的途径。本提案旨在将我们最近的配位化学应用于大挑战和优先领域的能源材料制备。我们将开发一系列新的铀前体，以建立一个扩大氮化物范围的平台。这种方法利用了一种空间和电子性质的混合，特别适合于稳定铀配体的多重键。利用这些前体，我们已经确定了四条途径，以最大限度地提高我们成功制备高价值铀-碳炔的机会，这是前所未有的。随着氮化铀分子和新型铀碳化合物范围的扩大，我们将在初步结果的基础上研究它们分解成二元材料的过程。新前体的可用性导致探索高压相变以获得新多晶的可能性。这与理解核反应堆极端条件下的燃料直接相关，这些金属多晶体的研究很有趣，因为它们的流动与局部5f电子行为在磁性上很吸引人，对设计更好的atf至关重要。我们将与合作者将合成、结构和材料研究与跨学科磁强计、计算和光谱研究相结合，以全面了解铀氮和碳键、反应性和材料应用。奖学金将提供最佳机会，监督这一复杂的研究项目，管理密集的合作，并抽出时间与核工业接触，将学术进展转化为与工业相关的应用。该项目的研究人员将在公认的战略技能短缺领域开发一系列技能。我们的分子为探索铀键共价的性质和程度提供了独特的机会；这个问题长期存在，仍在激烈辩论，而且由于英国的核废料遗留问题而很重要。用过的核燃料约96%是铀，官方核退役管理局的数字显示，核废料清理费用为700亿英镑。如果我们能更好地了解铀的化学性质，这可能在未来有助于改善英国的核废料遗产，并为与核工业一起开发atf提供新的途径。

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.