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The effect of the presence of an additional ester or amide carboxy group in the 6position of the pyridine nuclei of calixarenebased picolinamide ligands on the extraction and complexation properties of lanthanideIII and actinideIII metal ions was studied. For this purpose, six new ligands 1–6were synthesized; their conformational properties were studied both in solution and in the solid state, and their binding properties towards lanthanide LnIII and actinide AnIII metal ions were determined under extracting conditions simulating those present in radioactive waste. In the presence of BrCosan as synergizer, a rather high efficiency in the extraction of trivalent metal ions by these ligands was observed even at [HNO3] > 2–3 M. Complexation of LnIIImetal ions was also studied under homogeneous conditions methanol solution, both in chloride and nitrate media, by using spectrophotometry and calorimetry. A comparison with the data obtained with ligands L1–L3, lacking the additional binding sites in the 6position of the pyridine nuclei, is reported and the effects of the structure of the ligands on the stoichiometry of the complexes, coordination of the metal ions, basicity of the pyridine binding groups and efficiency and selectivity in the extractions are also discussed.

The cover picture showsthe complexation of actinide metal ions by picolinamidebased calix[6]arenes. These ligands possess remarkable efficiency in the extraction of trivalent metal ions even at very high HNO3concentration and fair selectivity for actinide ions over lanthanide ions, which is useful in the recycling of the spent nuclear fuel. The Partitioning and Transmutation P&T strategy would allow the longlived minor actinides Np, Am, Cm in fast nuclear reactors to be separated and transmutated and would also allow the radiotoxicity in the ultimate waste to be minimized, making nuclear power plants more efficient and sustainable. Details are discussed in the article by M. Mariani, A. Casnati et al. on p. 2675 ff.