Polyoxometalate-Assisted Crystallization: A General Strategy Enabling Structural Characterization of Molecular Radium...

A fundamental understanding of radium (Ra) coordination chemistry has been hindered by the scarcity and radiological hazards of ²²⁶Ra, leaving the structural characterization of molecular Ra complexes almost entirely unexplored. Here, we introduce a polyoxometalate (POM)-assisted crystallization strategy that enables single-crystal X-ray diffraction analysis of Ra-chelator complexes from microgram-scale samples. Employing the plenary Keggin anion [SiW₁₂O₄₀]^4-, we isolated and structurally characterized Ra²⁺ complexes of 18-crown-6 and the bis-picolinate macrocycle macropa, together with their Sr²⁺ and Ba²⁺ analogues. The resulting structures reveal systematic, size-dependent trends in coordination number and metal-donor distances across the alkaline earth series and provide the first direct experimental measurements of Ra-N and Ra-O_COO bond distances. Together, these results establish POM-assisted crystallization as a robust approach for obtaining solid-state structural information on Ra²⁺ complexes of organic chelators, opening new opportunities to advance the coordination chemistry needed to fully realize radium's potential in isotope production and targeted radiotherapy. More broadly, this approach expands the experimental toolkit available for studying scarce, highly radioactive elements accessible only in microgram quantities.