Unlocking uranium’s chemistry

Materials World magazine
,
1 Apr 2008

Scientists at the University of Edinburgh, UK, have taken the most common form of uranium, uranyl dication (UO22+), found in the natural environment and nuclear waste, and converted the chemically un-reactive compound into a reactive molecule. The team believes this could improve understanding of nuclear materials and waste, and the ability to handle them.

‘Uranium is enormous, and its chemistry is under-explored and often very difficult to predict,’ says Dr Polly Arnold, Reader in Organic Chemistry at Edinburgh. ‘Its size makes it difficult to model accurately with computational methods. We have found a way to trick uranium into behaving like a lighter metal – no-one has been able to do this before.’

The uranyl ion oxygens in uranyl dication are normally almost totally un-reactive chemically, unlike their transition metal analogues. Senior Lecturer in Inorganic Chemistry Dr Jason Love and his team have obtained a selective covalent bond formation at one oxygen atom and ended up with singly reduced uranyl ion UO2+.

Arnold explains, ‘Love’s group have designed a rigid ligand – a molecular scaffold – that binds two normal metal cations in its loop, forming a mouth shape. This has been dubbed his “Pacman” ligand (see image. Oxygen atoms are red, the uranium atom is green and the organic group picked up is yellow and grey).

‘We realised that because the uranyl ion always remains rigorously linear, when it is bound into the loop by the uranium atom, the uranyl unit pierces Pacman, placing one oxygen [atom] inside Pacman’s mouth and leaving the other one sticking out like a horn. This results in an asymmetric uranyl environment.’

When two potassium cations are bound inside the mouth to the endo oxygen, the outer oxygen atom becomes sufficiently reactive to break carbon-silicon and nitrogen-silicon bonds in other substrates.

Arnold says, ‘Fundamentally, this new uranium-oxo bond should improve our understanding of the bonding in uranyl ions, and provide models of singly reduced or functionalised uranyl groups. The more we know about uranium, the better we [can] deal with our huge waste stockpile.’

Furthermore, the new molecule behaves similarly to the more reactive and radioactive plutonium, another by-product of nuclear power generation. This could broaden the potential use of uranium as a safer alternative to better understand plutonium compounds.