New cyclen derivative ligand for thorium(IV) separation by solvent extraction

A new N-containing ligand, 1,4,7,10-tetra-(4-nitrobenzyl)-1,4,7,10-tetraazacyclo-dodecane (L), was synthesized, and its structure was determined by H-1 NMR, high resolution mass spectrometry and X-ray diffraction. L crystallized in the monoclinic system (P2(1)/n space group; a = 7.7895(2) , b = 22.9592(5) , c = 9.9204(2) ; alpha = 90.00A degrees, beta = 105.481(3)A degrees, gamma = 90.00A degrees; Z = 2). Slope analysis and the continuous variation method demonstrated that 1:2 complexes between Th(IV) and L are formed; furthermore, the XPS analysis suggested that two oxygen atoms might be provided by two water molecules and that eight nitrogen atoms might be provided by two L molecules to form a ten-coordinate compound with Th(IV). The extraction equilibrium constant for the complex formation between Th(IV) and L was logK (ex) = 6.95 +/- A 0.15 (25 A degrees C), and the Gibbs free energy, Delta G (o) (25 A degrees C), of the 1:2 Th-L complex in dichloromethane was -39.56 kJ/mol. The L ligand in dichloromethane only slightly extracted Th(IV) from HNO3 solution at pH = 1-3; however, an extraction efficiency of E = 94.9 +/- A 0.3 % was observed at pH = 4.63. The selectivity of L for the Th(IV) cation over other cations (i.e., Cs(I), Sr(II), Y(III), La(III), Sm(III), Eu(III), U(VI), and Am-241(III)) was evaluated. Furthermore, the stripping experiments showed that the stripping agent (0.5 mol/L Na2CO3 + 0.1 mol/L EDTA) could provide an optimal condition for stripping thorium, and thorium recovery was up to 91.6 +/- A 0.1 %.