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


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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 (H4COCO) bound Ra2+, Ba2+, and Sr2+ to form M(HxCOCO)(x-2). Upon isolating radioactive Ra-223 from its parent radionuclides (Ac-227 and Th-227), Ra-223(2+) reacted with the fully deprotonated COCO4- chelator to generate Ra(COCO)(2-)((aq)) (log KRa(COCO)2- = 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)(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.
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