Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Cover Picture (Angew. Chem. Int. Ed. 1/2/1998)
The cover picture shows a section from the crystal structure of the neptunium complex ion [NpO2( crown-6)]+. This is the first structurally and spectroscopically characterized transuranium crown ether inclusion complex. The X-ray structure analysis reveals that the radioactive neptunium ion (yellow) is completely encapsulated in the equatorial plane of the crown-6 ligand; the oxygen atoms (red) of the macrocycle bind directly to the actinide metal center (yellow). This direct coordination, for which there are only a few examples with uranium as the central metal, is probably facilitated by the large radius of the NpO2+ ion. In comparison to the related UO+2 ion, the NpO+2 ion undergoes encapsulation relatively readily in aqueous solution. This holds promise for future application of crown ether ligands for the selective separation of actinide ions from radioactive process streams or solutions of nuclear waste. The picture was generated by J. Smith and B. L. Scott with the programs Biosym (Biosym Technologies) and Photoshop (Adobe); the axial oxygen ligands at the neptunium center are not shown. More details about the inclusion complex are reported by D. L. Clark et al. on pages 164–166.