Rare-earth-metal solvent extraction with calixarene phosphates

Several phosphate esters in which calixarenes act as either uni- or bi-dentate ligands towards P-V have been investigated as solvent-extraction agents for the rare-earth metals (La to Lu, excluding Pm, plus Y). Extraction of picrate salts into dichloromethane, ethyl acetate and tributyl phosphate-hexane (1:1) is considerably enhanced by some of these esters but in no case is any marked selectivity exhibited, although some features of the observed extraction patterns suggest ways in which the molecular design of such extractants may be improved. Crystal structure determinations on two phosphate esters in which the calixarene unit is bidentate indicate that the relatively poor metal-ion binding abilities of these ligands may be a consequence of a preferred orientation of the phosphoryl donor groups in which they diverge from the macrocyclic ring. A crystal structure determination on the 2:1 complex of tetrakis-O-(diethoxyphosphoryl)-p-tert-butylcalix[4]arene and lanthanum picrate (2,4,6-trinitrophenolate) shows that a convergent array of phosphoryl donor groups can form in such cases where the functional groups are flexible. This does not, however, appear to lead to a particular enhancement of stability, as the complex readily dissociates into a 1:1 species in solution.