Advances in heavy alkaline earth chemistry provide insight into complexation of weakly polarizing Ra ²⁺ , Ba ²⁺ , and Sr ²⁺ cations

Abstract

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.