Advances in heavy alkaline earth chemistry provide insight into complexation of weakly polarizing Ra <sup>2+</sup> , Ba <sup>2+</sup> , and Sr <sup>2+</sup> cations
James C. Gilhula, Lei Xu, Frankie D. White, Sara L. Adelman, Kelly E. Aldrich, Enrique R. Batista, David Dan, Zachary R. Jones, Stosh A. Kozimor, Harris E. Mason, Rachel L. Meyer, Nikki A. Thiele, Ping Yang, Mingbin Yuan
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.