Binding and Extraction of Pertechnetate and Perrhenate by Azacages

The design and synthesis of anion receptors of technical and biochemical significance is receiving more and more attention.^1-8 Currently, effective binding and selective phase transfer of the oxoanions pertechnetate and perrhenate is of considerable interest from different point of view. Due to its long half-life and environmental mobility, the radioactive pertechnetate is one of the most hazardous contaminants. In this context, effective and selective separation processes are of utmost importance.^9-12 On the other hand, there are some emerging possibilities for the application of the radiochemically active oxoanions pertechnetate and perrhenate in nuclear medicine.^13,14 The most commonly used isotope in diagnostic nuclear medicine ^99mTc is readily available from a ⁹⁹Mo/^99mTc generator system.^15-19 Likewise, the b-emitting ¹⁸⁸Re – discussed as one of the most interesting radionuclides for specific therapeutic applications – is conveniently produced by a ¹⁸⁸W/¹⁸⁸Re generator.^20-22 In both cases the radionuclides are available as oxoanions in isotonic solution, and it appears highly desirable to directly complex ^99mTcO₄ ^- and ¹⁸⁸ReO₄ ^- as they exist in the generator eluate itself. But, the binding of such large, lowly charged anions is a difficult venture. The enthalpic contribution for complexation is rather small. Hence, host compounds being capable to encapsulate these oxoanions are of great interest.
For both directions discussed above, some different requirements have to be fulfilled for the design of ligands. The essential properties of ligands serving as extractants and imaging/therapeutic agents are summarized in Table 1.