Unusually Kinetically Inert Monocationic Neptunyl Complex with a Fluorescein-Modified 1,10-Phenanthroline-2,9-dicarboxylate Ligand: Specific Separation and Detection in Gel Electrophoresis

We found a singly charged Np(V)O2+ complex with unprecedented kinetic inertness in aqueous solution, one million times slower than the widely accepted fast kinetics of neptunyl complexes. An inert NpO2+ complex with a fluorescent 1,10-phenanthroline-2,9-dicarboxylate derivative was found by kinetic selection using polyacrylamide gel electrophoresis (PAGE) from a small chemical library. Autoreduction from Np(VI)O22+ to Np(V)O2+ via complexation was observed. A remarkably small spontaneous dissociation rate constant of 8 × 10-6 s-1 (half-life of 23 h) was determined using PAGE. Selective detection of Np(V)O2+ was achieved in PAGE with a detection limit of 68 pmol dm-3 (17 fg). This system was successfully applied to simulated radioactive waste samples. Our finding that electron-rich NpO2+ forms a uniquely inert complex with no strong electrostatic interaction reveals a new aspect of actinide chemistry for developing a novel separation system of real radioactive material samples.