Published in the leading journal Nature Chemistry, recent work by David King, a PhD student in the research group led by Dr Steve Liddle, has shown that uranium can engage in chemical bonding that is surprisingly covalent. Remarkably, the findings have shown that uranium can behave in much the same way as its counterparts in the well-known transitional metals such as chromium, molybdenum and tungsten.
This latest discovery builds on previous work from the group which was published in Science last year (Science, 2012, 337, 717). For many years the isolation of a uranium-nitrogen triple bond (known as a nitride) was the top target in synthetic actinide chemistry because the paucity of data was in stark contrast to transition metal analogues which are well known.
Originally, the team prepared a uranium(V)-nitride by using a very ‘bulky’ nitrogen ligand (an organic molecule bonded to a metal) to wrap around the uranium centre and to create a protective pocket in which the nitride nitrogen can sit. The nitride was stabilised during the synthesis by the presence of a weakly bound sodium cation which blocked the nitride from reacting with any other elements. In the final stage, the sodium was gently teased away, removing it from the structure and leaving the final, stable uranium-nitride triple bond.
Now, the team have succeeded in oxidising the uranium(V)-nitride to produce a uranium(VI)-nitride. This was achieved using the simple oxidant iodine, which is surprisingly straightforward as a range of other oxidation strategies fail. The team also found the uranium(VI)-nitride to be sensitive to photochemical decomposition which likely explains why previous attempts to prepare such compounds failed.
With a series of compounds in hand it has been possible to make meaningful comparisons to analogous group 6 transition metal-nitrides and the data suggest the surprising conclusion that uranium-nitrides exhibit levels of covalency that are at least as high as those of chromium, molybdenum, and tungsten. This is particularly surprising because the bonding for group 6 would be expected to be substantially more covalent than uranium. Another surprise result is the apparent dominance of f-orbitals in the bonding with very little d-orbital involvement which may pave the way for new modes of chemical reactivity compared to transition metal analogues which only employ d-orbitals.
Dr Liddle said: “We are absolutely delighted that we now have two types of uranium-nitride with uranium in the (V) and (VI) oxidation states because now we can compare how they react. d-Block metal-nitride reactivity is often classified as nucleophilic or electrophilic, with the former for early-metals and the latter for late-metals, but preliminary results suggest that uranium-nitrides are ambiphilic which shows we have much to learn about uranium-ligand multiple bonding. We anticipate that our uranium-nitrides may play a key role in benchmarking our understanding of uranium electronic structure.”
The research has been funded and supported by the Royal Society, European Research Council, the EPSRC, and the UK National Nuclear Laboratory.
Citation: D. M. King, F. Tuna, E. J. L. McInnes, J. McMaster, W. Lewis, A. J. Blake, S. T. Liddle, Nat. Chem., 2013, doi:10.1038/nchem.1642.
Posted on Tuesday 7th May 2013