Physicochemical Properties and Theoretical Modeling of Actinide Complexes with a para-tert-Butylcalix[6]arene Bearing Phosphinoyl Pendants. Extraction Capability of the Calixarene toward f Elements

The coordination ability of the hexaphosphinoylated p-tert-butylcalix[6]arene $B_6bL^6$ toward actinides is established, as well as its good separation ability of the actinide ions $UO_2 ^{2+}$ and Th(IV) over trivalent rare earths such as La(III), Eu(III), and Y(III). Spectrophotometric titration of uranyl with $B_6bL^6$ in $CH_3CN$ yields $\log\beta_{11}= 7.1$ and $\log\beta_{12}= 12.5$ for the 1:1 and 1:2 ($UO_2 ^{2+}/B_6bL^6$) species, respectively. Actinide complexes with 1:1 and 1:2 (M/L) stoichiometries are isolated and characterized by elemental analysis, IR, and UV-vis. Compounds 1 and 3 fulfill their $CN = 8$ just with $B_6bL^6$, while compounds 2 and 4 require coordinated nitrates and/or water molecules. The luminescence spectra of the uranyl complexes and the parameters such as FWMH, vibronic spacing ($\upsilon_{sp}$), and the U-O bond length, as well as the luminescence lifetimes, permit the understanding of the coordination chemistry of these actinide calixarene complexes. Energy transfer from the $B_6bL^6$ ligand to the uranyl ion is demonstrated to be relevant in compound 1 with $Q_{abs} = 2.0\%$. The uranyl complex emission reveals a biexponential decay with $\tau_s$ from 210 to 220 μs and $\tau_L$ from 490 to 650 μs for compounds 1 and 3, respectively. The liquid-liquid extraction results demonstrate the good extraction capability of $B_6bL^6$ toward actinides but not for rare earths at room temperature. The extracted species keeps the 1(cation)/1(calixarene) ratio for the $UO_2 ^{2+}$, $Th^{4+}$, and $Eu^{3+}$ ions. A good capacity of $B_6bL^6$ toward $Th^{4+}$ ions using aqueous phase 2 containing even up to 0.3 M thorium nitrate and an organic phase of $2.47 \cross 10^{-4} M$ $B_6bL^6$ in chloroform is found. The spectroscopic properties of the isolated uranyl complexes and the extraction studies reveal a uranophilic nature of $B_6bL^6$. The molecular modeling results are in good agreement with the experimental findings.