The Chemistry of the Uranium-Nitride Triple Bond

金属-配体多键代表了化学的基本方面，并支持化学结构、成键、反应和催化。事实上，过渡金属-碳多键是2005年诺贝尔化学奖的基础，过渡金属-氮三键是生物过程(固氮酶)和氨合成的重要中间体。对于最重的天然元素铀来说，与氧的双键，如普遍存在的线性铀酰正离子，和氮是众所周知的，铀-碳双键的领域正在蓬勃发展。几十年来，被称为氮化铀的分子铀-氮三键一直是合成鳗系元素化学的终极目标；然而，它避开了所有制备它的尝试。最近，我们取得了里程碑式的进展，制备了第一个分子铀-氮化物三键的例子(《科学》，2012,337,717)。我们的突破性方法利用了一种非常大的配体，它在铀中产生一个口袋，在其中安装氮化物，再加上在合成过程中使用钠离子稳定，然后温和地移除钠以提供末端氮化物的连接。这个项目的目的是利用我们的进展来开发这一令人兴奋的领域，以便我们可以绘制出铀-氮化物三键的内在结构和反应性。我们将用我们成熟的方法来扩大铀-氮化物三键的范围，以生成一系列化合物，以便进行有意义的比较。令人惊讶的是，1909年哈伯-博世氨合成专利明确提到铀是最好的催化剂，其中涉及氮化物。因此，我们寻求评估氮化铀在氨合成中的作用，以回答有关铀作用的长期问题。此外，我们将评估铀氮化物在原子效率高的N原子转移反应中的潜力，该反应可以直接被15N同位素标记。我们将建立铀-氮化物键的内在反应性特征，并将检验这样的假设，即我们的氮化物代表了迄今为止无法获得的长目标、高价值的铀-碳三重键和无杂原子双键的入口点，这些双键是没有先例的。我们还寻求将这一化学扩展到更重的类似物，其中氮化物氮被磷或砷原子取代，这将提供一个机会来比较化学基团内的趋势。我们将把合成和结构研究与跨学科的磁测量、计算和光谱研究(曼彻斯特大学的EPSRC EPR National Service、斯图加特大学的FAR-IR和堪培拉大学的XANES)相结合，以全面了解铀-氮键。我们的铀-氮化物连接提供了一个独特的机会来探索铀-配体键合中共价的性质和程度。铀化学结合中的共价性问题由来已久，仍在激烈辩论，而且很重要，因为英国已经拥有核废料遗产。乏核燃料中约96%是铀，核退役管理局用于核废料清理的官方数据为700亿磅。如果我们能够更好地了解铀的化学，这个更高层次的知识平台未来可能会为改善英国的核废料遗产做出贡献。

Metal-ligand multiple-bonds represent fundamental aspects of chemistry and underpin chemical structure, bonding, reactivity, and catalysis. Indeed, transition metal-carbon multiple bonds are the basis for the 2005 Nobel Chemistry Prize and transition metal-nitrogen triple bonds are well established and important intermediates in biological processes (nitrogenases) and ammonia synthesis. For uranium, the heaviest naturally occurring element, double bonds to oxygen, exemplified by the ubiquitous linear uranyl dication, and nitrogen are well known, and the area of uranium-carbon double bonds is burgeoning. A molecular uranium-nitrogen triple bond, known as a uranium nitride, was for decades the ultimate target in synthetic actinide chemistry; however it eluded all attempts to prepare it. Very recently, we made a landmark advance and prepared the first example of a molecular uranium-nitride triple bond (Science, 2012, 337, 717). Our breakthrough method utilises a very bulky ligand which generates a pocket at uranium in which to install the nitride, coupled to stabilisation during synthesis using a sodium cation, followed by gentle removal of the sodium to furnish the terminal nitride linkage. This project aims to exploit our advance in order to develop this exciting area so that we may map out the intrinsic structure and reactivity of the uranium-nitride triple bond. We will expand the range of uranium-nitride triple bonds with our proven method to generate a family of compounds so that meaningful comparisons can be made. Surprisingly, the 1909 Haber-Bosch patent for ammonia synthesis, where nitrides are implicated, clearly references uranium as the best catalyst. We therefore seek to assess the role of uranium-nitrides in ammonia synthesis to answer long-standing questions regarding the role of uranium. Furthermore, we will assess the potential of uranium-nitrides in atom-efficient N-atom transfer reactions which may straightforwardly be 15N-isotopically labelled. We will establish the intrinsic reactivity character of the uranium-nitride linkage and will test the hypothesis that our nitrides represent a hitherto unavailable entry point to long-targeted, high value uranium-carbon triple and heteroatom-free double bonds that have no precedent. We also seek to extend this chemistry to heavier analogues where the nitride nitrogen is replaced by a phosphorus or arsenic atom which will afford an opportunity to compare trends within a chemical group. We will combine synthetic and structural studies with interdisciplinary magnetometric, computational, and spectroscopic studies (EPSRC EPR National Service at Manchester University, far-IR at Stuttgart University, and XANES at Canberra University) to give a comprehensive understanding of uranium-nitrogen bonding. Our uranium-nitride linkage provides a unique opportunity to probe the nature and extent of covalency in uranium-ligand bonding. The issue of covalency in uranium chemical bonding is long-running, still hotly debated, and important because of the nuclear waste legacy which the UK already has. 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 higher platform of knowledge may in the future contribute to ameliorating the UK's nuclear waste legacy.