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Contenido archivado el 2024-06-18

UNCLE: Uranium in Non-Conventional Ligand Environments

Final Report Summary - UNCLE (UNCLE: Uranium in Non-Conventional Ligand Environments)

From a historical perspective, the chemistry of uranium can be considered to lag behind the rest of the periodic table because the rarity and perceived handling problems have restricted or discouraged its use. This is particularly surprising when the fact that uranium plays a central role in nuclear power is appreciated. There are enduring questions regarding how uranium undertakes chemical bonding to other elements and what effect this has on reactivity. The extent of covalency in uranium bonding, that is sharing of electrons, is poorly understood, but could be key to resolving nuclear waste problems since only a small fraction of the volume of nuclear waste is actually radioactive, but separating it is difficult.

We have prepared a number of uranium complexes stabilized by ligands, that is large organic groups which bind to uranium through donor atoms, which are designed to occupy most of the coordination sphere around the uranium centres. This leaves a pocket at which to bind only one group and prevent decomposition. We have developed new methods and generated a fifteen-fold increase in the number of uranium-metal bonds (Angew. Chem. Int. Ed. 2011, 50, 10388-10392; Nat. Commun. 2013, 4, 2323, doi:10.1038/ncomms3323; Angew. Chem. Int. Ed. 2013, 52, 13334-13337).

Whilst studying the reactivity of these complexes we have discovered a way to couple two carbon monoxide molecules together and to then release this fragment and convert it into a more complicated organic molecule (Proc. Nat. Acad. Sci. USA 2012, 109, 9265-9270). This is the first time this has been achieved, is recyclable, and is important because there is intense interest in generating organic molecules from sources other than crude oil.

As part of this project we have discovered uranium molecules that exhibit a property known as single molecule magnetism (SMM). SMMs are of great interest because they promise applications in ultra-high-density data storage and quantum computing. Whilst uranium is unlikely to find widespread appeal in these areas, the lessons learnt here (Nature Chem. 2011, 3, 454-460; Angew. Chem. Int. Ed. 2013, 52, 4921-4924; Angew. Chem. Int. Ed. 2013, 52, 3430-3433) might be applied to more technologically amenable metals.
Some of our precursors contain uranium-carbon double bonds and are known as carbenes. For over 30 years these carbenes were limited to compounds where uranium was in oxidation state IV. Now we have accessed uranium-carbenes incorporating uranium(V) and (VI). This has given unprecedented insight into the nature of uranium-ligand bonding and uranyl(V) clusters of relevance to environmental speciation and mobility in waste water ponds (J. Am. Chem. Soc. 2012, 134, 10047-10054; Angew. Chem. Int. Ed. 2011, 50, 2383-2386; Angew. Chem. Int. Ed. 2014, 53, 6696-6700; Angew. Chem. Int. Ed. 2014, 53, 9356-9359).

Uranium nitrides are of major interest to the actinide community because uranium-nitride has better thermal conductivity, higher density and melting points than uranium-oxide used in nuclear fuel rods, which would give safer nuclear reactors. Unfortunately, current routes to uranium-nitride usually require high temperatures and thus introduce impurities that are difficult to remove. A molecular nitride has been proposed as a lower temperature synthesis start point. The nature of the uranium-nitride triple bond is of significant interest as a benchmark of uranium-nitrogen bonding. Thus, this had been referred to as the ultimate synthetic target in actinide chemistry for half a century. Using a precursor molecule generated from this project we targeted and successfully made the first example of a uranium-nitride triple bond (Science 2012, 337, 717-720). This has formed the start point of a whole new area of actinide chemistry for the PI (Nat. Chem. 2013, 5, 482-488; J. Am. Chem. Soc. 2014, 136, 5619-5622; Angew. Chem. Int. Ed. 2014, 53, 4484-4488; Chem. Sci. 2014, 5, 2489-2497; Angew. Chem. Int. Ed. 2014, 53, 10412-10415).