National Renewable Energy Laboratory,
Keywords: computational study, lanthanide, photochemistry, photophysics, oxygen atom transfer, critical materials
Summary:Rare earth (RE) elements are in high demand as an essential component to many advanced materials and as such, have been designated critical materials. However, the separation of REs is still a very challenging task due to the similarities among different elements in their physical and chemical properties. At an industrial scale, separations are currently based on ionic radii through liquid-liquid extraction, which is time, energy, and resource consuming with a considerable negative environmental impact. The unique and discontinuous manifold of energy states associated with the f-electron configurations of the RE ions offer an opportunity to overcome some of these limitations by incorporating light into the separation process. Manifesting chemical differences in individual RE element complexes is challenging due to the similar sizes of the tripositive cations and the core-like 4f-shell. We disclose a new strategy for differentiating between similarly sized Dy3+ and Y3+ ions through a tailored photochemical reaction of their isostructural complexes in which the f-electron states of Dy3+ act as an energy sink. Complexes RE(hfac)3(NMMO)2 (RE = Dy (2-Dy), Y (2-Y), hfac = hexafluoroacetylacetonate, NMMO = 4-methyl-morpholine-N-oxide) showed variable rates of oxygen-atom transfer (OAT) to triphenylphosphine under ultraviolet (UV) irradiation. The complex nature of this project required a joint experiment-computational collaboration to investigate the chemical differences between the Y and Dy complexes. The aim would be to fully understand the underlying mechanisms and optimize it to separate other RE elements. Ultrafast transient absorption spectroscopy (TAS) identified the excited state(s) responsible for the photochemical OAT reaction and allowed us to propose an explanation for the rate difference between 2-Y and 2-Dy but also to come up with a possible mechanic involved in the significant differences in OAT reactivity between Y and Dy. Computational calculations of the ground state showed a difference in the nature of the bond between both RE and NMMO but the thermodynamic results allude to the requirement for photoexcitation. Furthermore, the excited state studies point to the observed bond weakening and suggest that an intersystem crossing can take place between the S1 and T1 energy surfaces, consistent with the observation of triplet contributions in the TAS studies of Y complex. Both experiments and computational results indicate that the rate difference can be attributed to the distinct lifetimes of the different 2-RE* species. The different lifetimes can be rationalized by the presence or lack of an available metal-based excited state manifold that quenches the ligand excited states. For 2-Y*, the long-lived NMMO-localized excited state can react with PPh3 to form O=PPh3 and finally 4-Y. In contrast, a significant portion of the ligand-centered excited states on 2-Dy* are deactivated via energy transfer to the metal center within 1 ns, meaning the likelihood of reactive 2 Dy* encountering PPh3 within the excited-state lifetime is small. A complete study of the potential energy surface is therefore needed to better understand and optimize the process and apply it to future separation of other RE.