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.
basic_science · Level V
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- Record sourced from PubMed, PMID 38181087.
- Also identified by DOI 10.1126/sciadv.adj8765 and PMC identifier 10776001.
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Abstract
Numerous technologies-with catalytic, therapeutic, and diagnostic applications-would benefit from improved chelation strategies for heavy alkaline earth elements: Ra<sup>2+</sup>, Ba<sup>2+</sup>, and Sr<sup>2+</sup>. Unfortunately, chelating these metals is challenging because of their large size and weak polarizing power. We found 18-crown-6-tetracarboxylic acid (<b>H</b><sub><b>4</b></sub><b>COCO</b>) bound Ra<sup>2+</sup>, Ba<sup>2+</sup>, and Sr<sup>2+</sup> to form <b>M(H</b><sub><b><i>x</i></b></sub><b>COCO)</b><sup><b><i>x</i>-2</b></sup>. Upon isolating radioactive <sup>223</sup>Ra from its parent radionuclides (<sup>227</sup>Ac and <sup>227</sup>Th), <sup>223</sup>Ra<sup>2+</sup> reacted with the fully deprotonated <b>COCO</b><sup><b>4-</b></sup> chelator to generate <b>Ra(COCO)</b><sup><b>2-</b></sup><sub><b>(<i>aq</i>)</b></sub> (log <i>K</i><sub><b>Ra(COCO)2-</b></sub> = 5.97 ± 0.01), a rare example of a molecular radium complex. Comparative analyses with Sr<sup>2+</sup> and Ba<sup>2+</sup> congeners informed on what attributes engendered success in heavy alkaline earth complexation. Chelators with high negative charge [-4 for <b>Ra(COCO)</b><sup><b>2-</b></sup><sub><b>(<i>aq</i>)</b></sub>] and many donor atoms [≥11 in <b>Ra(COCO)</b><sup><b>2-</b></sup><sub><b>(<i>aq</i>)</b></sub>] provided a framework for stable complex formation. These conditions achieved steric saturation and overcame the weak polarization powers associated with these large dicationic metals.