Advances in heavy alkaline earth chemistry provide insight into complexation of weakly polarizing Ra²⁺, Ba²⁺, and...

Numerous technologies-with catalytic, therapeutic, and diagnostic applications-would benefit from improved chelation strategies for heavy alkaline earth elements: Ra²⁺, Ba²⁺, and Sr²⁺. Unfortunately, chelating these metals is challenging because of their large size and weak polarizing power. We found 18-crown-6-tetracarboxylic acid (HCOCO) bound Ra²⁺, Ba²⁺, and Sr²⁺ to form M(HCOCO). Upon isolating radioactive ²²³Ra from its parent radionuclides (²²⁷Ac and ²²⁷Th), ²²³Ra²⁺ reacted with the fully deprotonated COCO chelator to generate Ra(COCO) (log K = 5.97 ± 0.01), a rare example of a molecular radium complex. Comparative analyses with Sr²⁺ and Ba²⁺ congeners informed on what attributes engendered success in heavy alkaline earth complexation. Chelators with high negative charge [-4 for Ra(COCO)] and many donor atoms [≥11 in Ra(COCO)] provided a framework for stable complex formation. These conditions achieved steric saturation and overcame the weak polarization powers associated with these large dicationic metals.