Advances in heavy alkaline earth chemistry provide insight into complexation of weakly polarizing Ra, Ba, and Sr cations.

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