Synthesis and Evaluation of Lipophilic BTBP Ligands for An/Ln Separation 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, which contain either additional alkyl groups on the pyridine rings or seven‐membered aliphatic rings attached to the triazine rings, have been synthesized, and the effects of the 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 have been studied. The speciation of ligand 13 with some trivalent lanthanide nitrates was elucidated by 1 H NMR spectroscopic titrations and ESI‐MS. Although 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 unsubstituted ligand CyMe 4 ‐BTBP. Compared to CyMe 4 ‐BTBP, alkyl‐substitution was found to decrease the rates of metal‐ion extraction of the ligands in both 1‐octanol and cyclohexanone. A highly efficient ( D Am > 10) and selective (SF Am/Eu > 90) extraction was observed for 12 and 13 in cyclohexanone and for 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.