Angew. Chem. Int. Ed. (2012)

Technetium-99 is a long-lived radioactive isotope with a half-life of more than 200,000 years and, as the pertechnetate ion (TcO4), is a notable component of spent nuclear fuel. The high solubility of this oxoanion in water means that it can spread quite widely in the environment. The development of molecules that can selectively trap this hazardous species could lead to materials that extract it from nuclear-waste streams. Owing to its radioactivity, much academic research on the design of receptors for TcO4 has used the structurally similar perrhenate ion (ReO4) as an analogue. Those studies that have focused on TcO4 itself have not typically been done in water.

Credit: © 2012 WILEY

Now, a team of researchers based in Italy and Switzerland, led by Valeria Amendola at the University of Pavia, have shown that a poly-protonated azacryptand (pictured) has a high binding affinity for 99TcO4 in aqueous solution. This cage-like compound had previously been shown to bind ReO4 in water and so was an ideal candidate as a receptor for the analogous pertechnetate anion. Isothermal titration calorimetry experiments revealed that the receptor binds 99TcO4 with a slightly greater affinity than ReO4 and exhibits the strongest binding of 99TcO4 in aqueous solution for any receptor reported so far. The researchers suggest that the lower hydration energy of the pertechnetate anion — which makes it easier to desolvate than its rhenium analogue — explains its increased affinity for binding with the azacryptand.

The interaction between the 99TcO4 ion and the azacryptand host was also studied using NMR spectroscopy, looking at both 1H and 99Tc spectra. In the 99Tc experiments, the linewidth of the 99Tc signal was observed to broaden as more of the receptor was added, up to a limiting value where all of the 99TcO4 ion is bound to the azacryptand. Finally, further proof that the 99TcO4 ion binds inside the cavity of the azacryptand receptor was provided by single-crystal X-ray diffraction data, which showed the anion sitting just off-centre forming hydrogen bonds to the protonated amine groups — directly, as well as through water bridges.