Capture and Characterization of High-Valent Uranium on Carbon Electrodes with a Hybrid Molecular Platform

Abstract

Abstract

Abstract Studies of molecular actinide species on conducting surfaces could enable new approaches to understanding bonding, electronic properties, and reactivity, but no strategies have been described to date that afford on-surface control over the composition, structure, redox, or stability of molecular actinide species. Here, a hybrid molecular strategy for uranium capture on surfaces in the form of the prototypical uranyl dication (UO22+) is described. This strategy relies on a solution-processable organic ligand for uranyl that can be selectively immobilized, enabling electrochemical and spectroscopic studies of well-defined surface-bound uranyl species. Flexible, noncovalent interactions enabled by pyrene groups installed on the periphery of an organic chelating group have been used to assemble quasi-monolayer coverages of the desired species on graphite electrodes. Thermodynamic adsorption isotherm measurements, along with computational structural predictions, demonstrate that uranium is held tightly to the graphite surface in a manner that is enhanced by the presence of uranium in the ligand. In accord with these findings, Laviron analysis shows that the uranium redox on this platform is characterized by fast heterogeneous electron-transfer kinetics (k0 = 5 s–1). X-ray photoelectron spectroscopy (XPS) reveals rather intense U 4f satellite features for the on-surface molecular species, an effect originating in covalency between U and the hybrid ligand but not previously explored for molecular coordination complexes. Extended cathodic polarization can reversibly release uranium from the electrode, showcasing the potential of noncovalent immobilization strategies for studies of fundamental and applied actinide chemistry.