The Chemistry of the Uranium-Nitride Triple Bond

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.

The 2009 EPSRC International Review of Chemistry stated: "The resurgence in radiochemistry is a potentially important development given the rising interest worldwide in nuclear power. There are obvious needs for better understanding of the chemistry related to the radioisotopes and for educating a new generation of personnel trained to deal with these materials. Moreover, this presents an opportunity to explore the fundamental chemistry of compounds rarely accessed in academic laboratories".
We therefore posit that this project will have significant impact, as evidenced by publication of our preliminary results. Furthermore, the proposed research will address many Grand Challenge issues identified by the EPSRC and will deliver the following tangible outcomes and inventions:
1. Delivery of a better understanding of actinide science;
2. Materials synthesis and small molecule activation and conversion into value-added products;
3. Fundamental scientific knowledge and understanding of academic and technological importance;
4. Maximise knowledge-exchange, -transfer, and -impact in exploitation, and commercialisation;
5. An early career researcher available for the UK economy, coupled to significant outreach.

In order to maximise the impact of our work, we will communicate and engage with the private and public sectors, academia, and the public through a series of activities with well-defined milestones and timelines.

Outreach: The PI has established a strong record of science communication through university open days, school visits, the Royal Society MP/Scientist Pairing Scheme, the 2012 Royal Society Summer Science Exhibition, the Sunday Times Magazine, and internationally acclaimed Periodic Videos and EPSRC funded MolVids. We will build on our approach by constructing a new website, producing new videos on uranium-nitride and the Haber-Bosch process, and use our 2012 public lecture experience to culminate with a Royal Society Summer Science Exhibition stand in Year 3.

Industry and Commerce: The Business Partnership Unit, which is uniquely embedded in chemistry at Nottingham, maintains an excellent awareness of industrial needs and will manage potential exploitation of research outputs. Progress will be regularly monitored and contact with end-users will be made as exploitable outputs are identified. As demonstrated by the letters of support for this Proposal the team already have established contacts with the nuclear industry and will develop them as appropriate. Through departmental advisory boards we have access to representatives from industry. Knowledge exchange will be exploited using standard protocols as collaborative opportunities are identified. The BPU has extensive experience of negotiating contracts covering collaboration, confidentiality, material transfer, and licenses and all necessary arrangements are in place with the project partners.

EU: The impact of this work will be maximised through the PI's COST Action at three levels: (i) annual talks at the Action meetings which are attended by EU f-element researchers at all levels; (ii) annual presentation of research to the COST Chemistry Domain Committee (DC) who regularly liaise with the EU Commission; (iii) participation in science fairs run by the COST Office in concert with the COST Committee of Senior Officials (CSO) which is attended by the EU Commission.

Training: The PDRA working on this research will gain a comprehensive background in uranium handling (a rare skill), synthesis, spectroscopy, magnetism, and computation. They will receive external RPS and UoN radiochemical training and attend conferences and exhibitions in addition to acquiring a host of transferable skills. This will enhance their career, make them very attractive, valuable candidates for scientific careers, and help restructure the UK science base.