Redox-switchable carboranes for uranium capture and release.

The uranyl ion (UO; U(VI) oxidation state) is the most common form of uranium found in terrestrial and aquatic environments and is a central component in nuclear fuel processing and waste remediation efforts. Uranyl capture from either seawater or nuclear waste has been well studied and typically relies on extremely strong chelating/binding affinities to UO using chelating polymers, porous inorganic or carbon-based materials, as well as homogeneous compounds. By contrast, the controlled release of uranyl after capture is less established and can be difficult, expensive or destructive to the initial material. Here we show how harnessing the redox-switchable chelating and donating properties of an ortho-substituted closo-carborane (1,2-(PhPO)-1,2-CBH) cluster molecule can lead to the controlled chemical or electrochemical capture and release of UO in monophasic (organic) or biphasic (organic/aqueous) model solvent systems. This is achieved by taking advantage of the increase in the ligand bite angle when the closo-carborane is reduced to the nido-carborane, resulting in C-C bond rupture and cage opening. The use of electrochemical methods for uranyl capture and release may complement existing sorbent and processing systems.