Rare earth elements (Ln) are in high demand due to their crucial role in various advanced technologies, including energy systems, communications, electronics, and manufacturing. Understanding their coordination behavior across the lanthanide series is necessary for improving separation and recycling processes. They are also important surrogates for understanding actinide separations, important for nuclear fuel production and reprocessing. Herein, we investigate the synergistic interaction between polyoxometalates (POMs) and macrocycles for selective separation of Ln at the microscale. POMs (e.g., Keggin H₃PM₁₂O₄₀, where M = W or Mo) are stable under harsh reaction conditions, and their high molecular weight is advantageous for precipitation-based separations. Meanwhile, macrocycles (e.g., 4’-carboxybenzo-15-crown-5 (4CB15C5)) exhibit size-specific binding, making them strong chelating agents and ideal candidates for separation studies. We performed a one-pot synthesis for each Ln, followed by crystallization of several complexes. Single crystal X-ray analysis showed early Ln’s (i.e., from La-Nd) complexed with the POM to form Ln mono-substituted Keggin ions, which co-crystallized with the protonated 4CB15C5 macrocycle to form polyoxoanions formulated as [Ln(PM₁₁O₃₉)(C₁₅H₂₀O₇)]^4-. Upon extension, these complexes form a double Keggin-type centrosymmetric dimer, [{Ln(PM₁₁O₃₉)(C₁₅H₂₀O₇)}₂]^8-. For the mid-to-late Ln (Sm to Yb), we isolated a series of supramolecular complexes in which the Ln³⁺ acts as a charge-balancing species without direct coordination to either ligand-producing structures with the general formula Ln[(C₁₅H₁₉O₇)_x(PM₁₂O₄₀)], where x = 0-2. In contrast, with later Ln’s such as Lu, Ln complexation occurs solely with the macrocycle (4CB15C5), and the resulting Ln-macrocycle complex co-crystallizes with the Keggin POM [(Lu-C₁₅H₁₉O₇)₆](PMo₁₂O₄₀) rather than forming a substituted double-Keggin framework. Across the series, we observed a trend in which early Ln (La–Nd) preferentially complex with POMs, while the later lanthanides (e.g., Lu) favor complexation with the macrocycle. This supports our initial hypothesis regarding size-dependent ligand preferences. We examined cluster formation in solution using Small-Angle X-ray Scattering, Raman Spectroscopy, and ³¹P, ¹H NMR techniques.

Doctor Stephen

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