Uranyl to Uranium(IV) Conversion through Manipulation of Axial and Equatorial Ligands

The controlled manipulation of the axial oxo and equatorial halide ligands in the uranyl dipyrrin complex, UO 2 Cl(L), allows the uranyl reduction potential to be shifted by 1.53 V into the range accessible to naturally occurring reductants that are present during uranium remediation and storage processes. Abstraction of the equatorial halide ligand to form the uranyl cation causes a 780 mV positive shift in the U V /U IV reduction potential. Borane functionalization of the axial oxo groups causes the spontaneous homolysis of the equatorial U–Cl bond and a further 750 mV shift of this potential. The combined effect of chloride loss and borane coordination to the oxo groups allows reduction of U VI to U IV by H 2 or other very mild reductants such as Cp* 2 Fe. The reduction with H 2 is accompanied by a B–C bond cleavage process in the oxo-coordinated borane.