The Chemistry of the Uranium-Nitride Triple Bond

金属配体多键代表了化学的基本方面，是化学结构、键、反应性和催化的基础。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（氮酶）和氨合成中的重要中间体。对于自然界中最重的元素铀来说，与氧的双键（例如普遍存在的线性铀酰化）和氮的双键是众所周知的，而铀-碳双键的领域正在蓬勃发展。一个被称为氮化铀的分子铀氮三键，几十年来一直是合成锕系化学的终极目标；然而，它躲过了所有准备它的尝试。最近，我们取得了里程碑式的进展，制备了第一个铀氮分子三键的例子（Science, 2012, 337, 717）。我们的突破性方法利用了一个非常大的配体，在铀上产生一个口袋来安装氮化物，在合成过程中使用钠离子进行稳定，然后轻轻地去除钠来提供末端氮化物连接。这个项目的目的是利用我们的进步来发展这个令人兴奋的领域，这样我们就可以绘制出铀氮化三键的内在结构和反应性。我们将用我们已证实的方法扩大铀氮三键的范围，以产生一系列化合物，以便进行有意义的比较。令人惊讶的是，1909年哈伯-博世（Haber-Bosch）关于氨合成的专利，其中涉及氮化物，明确指出铀是最好的催化剂。因此，我们试图评估氮化铀在氨合成中的作用，以回答关于铀的作用的长期问题。此外，我们将评估氮化铀在原子效率的n原子转移反应中的潜力，这些反应可以直接用15n同位素标记。我们将建立铀-氮化键的固有反应性特征，并将测试我们的氮化物代表迄今为止无法获得的长期靶向，高价值的铀-碳三键和无杂原子双键的切入点，这是前所未有的。我们还试图将这种化学扩展到更重的类似物，其中氮化氮被磷或砷原子取代，这将提供一个比较化学组内趋势的机会。我们将把合成和结构研究与跨学科的磁学、计算和光谱研究（曼彻斯特大学的EPSRC EPR国家服务，斯图加特大学的远红外和堪培拉大学的XANES）结合起来，以全面了解铀-氮键。我们的铀-氮化键提供了一个独特的机会来探索铀-配体键的共价性质和程度。铀化学键的共价问题长期存在，仍在激烈辩论，由于英国已经拥有核废料的遗产，这个问题很重要。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，这个更高的知识平台将来可能有助于改善英国的核废料遗产。

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.