Tetra-Substituted p-Tert-Butylcalix[4]Arene with Phosphoryl and Salicylamide Functional Groups: Synthesis, Complexation and Selective Extraction of f-Element Cations

A new series of lanthanide (1-5) and uranyl (6) complexes with a tetra-substituted bifunctional calixarene ligand H2L is described. The coordination environment for the Ln(3+) and UO22+ ions is provided by phosphoryl and salicylamide functional groups appended to the lower rim of the p-tert-butylcalix[4]arene scaffold. Ligand interactions with lanthanide cations (light: La3+, Pr3+; intermediate: Eu3+ and Gd3+; and heavy: Yb3+), as well as the uranyl cation (UO22+) is examined in the solution and solid state, respectively with spectrophotometric titration and single crystal X-ray diffractometry. The ligand is fully deprotonated in the complexation of trivalent lanthanide ions forming di-cationic complexes 2 : 2 M : L, [Ln(2)(L)(2)(H2O)](2+) (1-5), in solution, whereas uranyl formed a 1 : 1 M : L complex [UO2(L)(MeOH)](infinity) (6) that demonstrated very limited solubility in 12 organic solvents. Solvent extraction behaviour is examined for cation selectivity and extraction efficiency. H2L was found to be an effective extracting agent for UO22+ over La3+ and Yb3+ cations. The separation factors at pH 6.0 are: beta(UO)22+ (/La)3+ =121.0 and beta(UO)22+ (/Yb)3+ =70.0.