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Phosphinoylated calixarenes feature high coordination ability toward f elements and a great potentiality toward actinide/rare earth separation. Here, we report three characteristic properties of a tetra-phosphinoylated p-tert-butylcalix[4]arene, B(4)bL(4) functionalized with phosphinoyl pendant arms: (i) its coordination ability toward Th(IV) complexation in organic medium, (ii) its ability to separate thorium from yttrium, lanthanum, and europium in three different organic media, and (iii) the X-ray crystal structure of the La complex. Thorium(IV) forms 1:1 and 1:2 (M:L) complexes with B(4)bL(4): Th(NO3)(4)(B(4)bL(4))(n)center dot xH(2)O (n = 1, x = 1, 1; n = 2, x = 4, 2). Spectroscopic data point to the inner coordination sphere of 1 and 2 containing nitrate ions and water molecules. Molecular modeling of 1 yielded an 8-coordinate species and its coordination polyhedron can be described as a distorted square antiprism while that for 2, a 9-coordinate species, as a distorted tricapped trigonal prism. The extraction study of tetravalent thorium and trivalent rare-earth (Y, La, Eu) ions from acidic nitrate media by B(4)bL(4) in chloroform shows thorium being much more extracted than the rare earths, with selectivity close to 100%. The extraction behavior can be easily modulated by changing the initial conditions (pH, nitrate concentration). The X-ray structure of [LaB(4)bL(4)(H2O)(5)] CH3CN center dot(ClO4)(3) points to the La-III ion lying on a C-4 axis and being 9-coordinated by the four O(P) atoms and five O atoms from water molecules. It is located in the middle of the void formed by the four O-CH2-PO(Me)(2) pendant arms. (C) 2011 Elsevier B.V. All rights reserved.