The Chemistry of the Uranium-Nitride Triple Bond

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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.