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

Surface spectroscopy and imaging techniques represent advanced tools for the study of bonding and electronic properties. Such techniques could be especially revealing for actinide species, but actinide-containing systems are not available that afford control over on-surface composition, structure, electrochemistry, stability, and other key properties. Here, a hybrid molecular platform for uranyl (UO22+) capture on surfaces is described, where a soluble organic ligand for uranyl is used that can be selectively immobilized on graphitic surfaces/electrodes. This hybrid platform enables surface imaging, electrochemical redox, and spectroscopic studies of the corresponding 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. A related control compound was prepared for homogeneous, diffusional studies. Scanning tunneling microscopy (STM), never before applied to a molecular actinide complex, and complementary computational structural predictions show that uranium is held close to the graphite surface in a variety of ligand-driven conformations, in line with the finding of quite fast heterogeneous electron-transfer kinetics (k0 = 5 s–1) for uranium redox on this platform. X-ray photoelectron spectroscopy (XPS) reveals rather intense U 4f satellite features for the molecularly defined species on surfaces, 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, which further showcases the potential of noncovalent strategies for immobilization of species on electrodes for studies of fundamental and applied actinide chemistry.