Modification of calix[4]arenes with CMPO-functions at the wide rim. Synthesis, solution behavior, and separation of actinides from lanthanides

Abstract

Two calix[4]arene tetraethers (Y = C₅H₁₁, C₁₄H₂₉) bearing on their wide rim four –N(Me)–CO–CH₂–P(O)Ph₂ residues were synthesized for the first time. Their ability to extract lanthanides and actinides from an acidic aqueous phase to organic phases (CH₂Cl₂, NPHE) was studied. In comparison to the corresponding –NH-analogs, they are less efficient extractants, the selectivity for the light over the heavy lanthanides is less pronounced, while there is still an interesting selectivity of Am³⁺ over Eu³⁺. Stability constants for selected lanthanide salts were determined also in homogenous phase (methanol, acetonitrile) but do not account for the different extraction results. The complexation of Gd³⁺ was also followed by relaxivity (NMRD) measurements, which suggest an even stronger aggregation for the N-methyl compound while the 1 : 1 complex is reached for a smaller ratio [L]/[Gd³⁺] compared to the NH analog. The formation of aggregates is also supported by dynamic light scattering measurements. A single crystal X-ray structure of the pentyl ether reveals a C₂-symmetrical pinched cone conformation for the free ligand.