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 H(2)L is described. The coordination environment for the Ln(3+) and UO(2) (2+) 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: La(3+), Pr(3+); intermediate: Eu(3+) and Gd(3+); and heavy: Yb(3+)), as well as the uranyl cation (UO(2) (2+)) 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)(H(2)O)](2+) (1–5), in solution, whereas uranyl formed a 1 : 1 M : L complex [UO(2)(L)(MeOH)](∞) (6) that demonstrated very limited solubility in 12 organic solvents. Solvent extraction behaviour is examined for cation selectivity and extraction efficiency. H(2)L was found to be an effective extracting agent for UO(2) (2+) over La(3+) and Yb(3+) cations. The separation factors at pH 6.0 are: β(UO) [Formula: see text] (/La) [Formula: see text] =121.0 and β(UO) [Formula: see text] (/Yb) [Formula: see text] =70.0.