Synthesis and evaluation of lipophilic BTBP ligands for An/Ln separations in nuclear waste treatment: the effect of alkyl substitution on extraction properties and implications for ligand design

Four new 6,6’-bis(1,2,4-triazin-3-yl)-2,2’-bipyridine (BTBP) ligands containing either additional alkyl groups on the pyridine rings, or 7-membered aliphatic rings attached to the triazine rings, have been synthesized, and the effects of additional alkyl substitution in the 4- and 4’-positions of the pyridine rings on their extraction properties with Ln(III) and An(III) cations in simulated nuclear waste solutions were studied. The speciation of ligand 13 with some trivalent lanthanide nitrates was elucidated by 1H NMR titrations and electrospray-ionization mass spectrometry. Whereas ligand 13 formed both 1:1 and 1:2 complexes with La(III) and Y(III), only 1:2 complexes were observed with Eu(III) and Ce(III).Quite unexpectedly, both alkyl-substituted ligands 12 and 13 showed lower solubilities in certain diluents than the non-substituted ligand CyMe4-BTBP. Compared to CyMe4-BTBP, alkyl-substitution was found to decrease the rates of metal ion extraction of the BTBPs in both 1-octanol and cyclohexanone. A highly efficient (DAm > 10) and selective (SFAm/Eu > 90) extraction was observed for BTBPs 12 and 13 in cyclohexanone, and for BTBP 13 in 1-octanol in the presence of a phase-transfer agent. The implications of these results for the design of improved extractants for radioactive waste treatment are discussed.