X-ray Spectroscopic Probing and Tuning of Lanthanide Binding Properties

A deep understanding of the electronic structure and chemical bonding properties of the lanthanide (Ln)
elements remains a significant challenge [1-3]. Techniques such as core-to-core and valence band resonant
inelastic X-ray scattering (CC/VB-RIXS) and high-energy-resolution X-ray absorption near edge structure
(HR-XANES) at the Ln L₂,₃ absorption edges allow for probing both the occupied and unoccupied states of
the valence band in lanthanide elements with exceptional energy resolution. When combined with
quantum chemical calculations, these methods can yield detailed insights into chemical bonding [1–3].
This contribution highlights two examples that illustrate the application of high-resolution X-ray
spectroscopic tools:
(A) It will be demonstrated that the ionic bonding in the Sm(II) complex with cyclononatetraenyl (CNT)
ligands, [Sm(CNT)₂], can be modulated and converted into a more covalent interaction via photon-induced
promotion of Sm f electrons to d/s orbitals. This photoinduced change in bonding character can potentially
activate the otherwise chemically inert [Sm(CNT)₂], opening new avenues for designing lanthanide-based
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molecular materials. [2]
(B) The use of radiopharmaceuticals based on alpha-particle-emitting radionuclides has grown
considerably in recent years. Pre-clinical research and early clinical trials have demonstrated the promise
of targeted alpha therapy. However, several challenges remain, including the need for robust chelation of
both the parent radionuclides and their radioactive daughters. Our goal is to understand how bonding
characteristics influence the stability of such compounds.
To this end, we have developed spectroscopic methods based on Ln L₃-edge CC-RIXS and HR-XANES, which
we applied in conjunction with quantum chemical calculations to investigate the bonding behavior of
lanthanum (La)—a chemical homolog of actinium (Ac)—with various ligands relevant to
radiopharmaceutical design. Systems studied include La³⁺ in aqueous and buffered solutions, as well as
the complexes [La(DOTA)(H₂O)]¹⁻, [La(MACROPA)]¹⁺, and [La(PSMA-617)(H₂O)]. A combination of
spectroscopic and theoretical approaches was used to assess the covalency of the La–ligand bonds.
Taken together, these findings represent a first step toward the development of new spectroscopic tools
for probing electronic structure and bonding in such systems—tools that may ultimately support the
design of chelating ligands for targeted alpha therapy. [3]
References: [1] T. Vitova et al., Commun. Chem., 2022, 5 (1), 1-4; [2] T. Vitova et al., JACS, 2024, 146,
20577-20583; [3] H. Ramanantoanina et al., submitted