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 Ln³⁺ 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 , [Ln₂( L )₂(H₂O)]²⁺ ( 1–5 ), in solution, whereas uranyl formed a 1 : 1 M : L complex [UO₂( L )(MeOH)]_∞ ( 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: β_{UO /La} =121.0 and β_{UO /Yb} =70.0.