Enhanced Selectivity for Actinides over Lanthanides with CMPO Ligands Secured to a C3-Symmetric Triphenoxymethane Platform

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Abstract
A ligand system containing three preorganized carbamoylmethylphosphine oxide (CMPO) moieties anchored onto a rigid C3-symmetric triphenoxymethane platform has been developed for facile metal complexation and subsequent extraction from aqueous acidic nuclear waste streams. Intended to mimic the 3:1 CMPO−actinide stoichiometry of the extracted species in the TRUEX nuclear waste treatment process, the CMPO arms on this ligand are oriented such that all three CMPO moieties can cooperatively bind a metal ion. Extractions of simulated nuclear waste streams (10-4 M metal in 1 M nitric acid) with solutions of this ligand in methylene chloride (10-3 M) reveal a high affinity for the actinide thorium and a very low, but constant, affinity for the lanthanides across the series. Thorium and five lanthanide (lanthanum, cerium, neodymium, europium, and ytterbium) nitrate complexes of this ligand have been synthesized and fully characterized by X-ray crystallography, 1H and 31P NMR spectra, and FT-ICR-MS to elucidate the mechanism of this unique actinide selectivity. All six oxygen donors from the three CMPO arms of the ligand and one or two nitrate counterions coordinate these metals to afford 2+ cationic complexes in every case. Because of the large size of the ligand, both the thorium and lanthanide complexes present similarly charged and sized surfaces to the extraction solvents, but the thorium complex is extracted quantitatively over the lanthanide complexes. A possible rationale for this extraction behavior difference is presented and further illustrated by the extraction properties of this ligand system for the alkali metals (lithium, sodium, potassium, rubidium, and cesium) as picrate salts and by the solid- and solution-state structures of its lithium picrate complex.