Uranyl complexes formed with a para-t-butylcalix[4]arene bearing phosphinoyl pendant arms on the lower rim. Solid and solution studies

Abstract

The current interest in functionalized calixarenes with phosphorylated pendant arms resides in their coordination ability towards f elements and capability towards actinide/rare earth separation. Uranyl cation forms 1:1 and 1:2 (M:L) complexes with a tetra-phosphinoylated p-tert-butylcalix[4]arene, B₄bL⁴: UO₂(NO₃)₂(B₄bL⁴)_n· xH₂O (n = 1, x = 2, 1; n = 2, x = 6, 2). Spectroscopic data point to the inner coordination sphere of 1 containing one monodentate nitrate anion, one water molecule and the four phosphinoylated arms bound to UO₂²⁺ while in 2, uranyl is only coordinated to calixarene ligands. In both cases the U(VI) ion is 8-coordinate. Uranyl complexes display enhanced metal-centred luminescence due to energy transfer from the calixarene ligands; the luminescence decays are bi-exponential with associated lifetimes in the ranges 220 μs <τ_s <250 μs and 630 μs <τ_L < 640 μs, pointing to the presence of two species with differently coordinated calixarene, as substantiated by a XPS study of U(4f_5/2,7/2), O(1s) and P(2p) levels on solid state samples. The extraction study of UO₂²⁺ cation and trivalent rare-earth (Y, La, Eu) ions from acidic nitrate media by B₄bL⁴ in chloroform shows the uranyl cation being much more extracted than rare earths.