Advances in heavy alkaline earth chemistry provide insight into complexation of weakly polarizing Ra2+, Ba2+, and Sr2+ cations | Science Advances

Numerous technologies—with catalytic, therapeutic, and diagnostic applications—would benefit from improved chelation strategies for heavy alkaline earth elements: Ra ²⁺ , Ba ²⁺ , and Sr ²⁺ . Unfortunately, chelating these metals is challenging because of their large size and weak polarizing power. We found 18-crown-6-tetracarboxylic acid ( H ₄ COCO ) bound Ra ²⁺ , Ba ²⁺ , and Sr ²⁺ to form M(H _x COCO) ^{x –2} . Upon isolating radioactive ²²³ Ra from its parent radionuclides ( ²²⁷ Ac and ²²⁷ Th), ²²³ Ra ²⁺ reacted with the fully deprotonated COCO ⁴⁻ chelator to generate Ra(COCO) ²⁻ _{( aq )} (log K _Ra(COCO)2− = 5.97 ± 0.01), a rare example of a molecular radium complex. Comparative analyses with Sr ²⁺ and Ba ²⁺ congeners informed on what attributes engendered success in heavy alkaline earth complexation. Chelators with high negative charge [−4 for Ra(COCO) ²⁻ _{( aq )} ] and many donor atoms [≥11 in Ra(COCO) ²⁻ _{( aq )} ] provided a framework for stable complex formation. These conditions achieved steric saturation and overcame the weak polarization powers associated with these large dicationic metals.