Synthesis, characterisation and extraction behaviour of calix[4]arene-based phosphonic acidsElectronic supplementary information (ESI) available: Tables S1–S3 and Figs. S1 and S2. See http://www.rsc.org/suppdata/p2/b1/b105489a

Three new bis(phosphonic) acids were prepared by derivatization of p-tert-butylcalix[4]arene on the lower rim by –(CH2)xPO3H2 groups and were fully characterized by NMR spectroscopy; some of them were further investigated by X-ray analysis. The compounds obtained were tested as extractants for La3+, Eu3+, Yb3+ and Th4+ in 0.1–1 M HNO3. Structural details of the complex HLn(H2L1)2 where Ln = Y, La, Eu were elucidated by NMR techniques in solution. The extraction efficiencies decrease with increasing length of the CH2 spacer; the dependence is not monotonic, and an extraordinary selectivity was observed for Yb. The extraction efficiency of Ln3+ for all of the ligands studied increased significantly with decreasing ionic radius.