Fundamental An(iii)-Ln(III) separation chemistry of N-donor ligands

For three decades lipophilic N-donor extracting agents have been used to separate trivalent actinides from the chemically similar lanthanide ions in the context of advanced nuclear fuel cycles. 15 years ago, bis-triazinyl-pyridines (BTP) were shown to achieve this separation under conditions relevant to the reprocessing of nuclear fuels, i.e. from nitric acid solutions. BTP were later followed by the bis-triazinyl-bipyridines (BTBP). More recently, water soluble BT(B)P were used as complexing agents selective for actinides. These could be useful for simplified process schemes. Despite BT(B)P’s advantageous properties, current work in international projects is directed towards finding ligands with even better properties. This requires a deep understanding of how such ligands work on the molecular level. This question has been addressed in several studies, the outcome of which has recently been summarised. The following conclusions are drawn: Only N-­‐donor ligands forming strong complexes with actinides(III) show adequate performance in highly acidic solutions. Furthermore, high selectivity over lanthanides(III) seems to correlate with the ligands’ ability of forming complexes with exclusive coordination via N atoms. ... mehrHowever, several important questions remain unanswered: Why are BT(B)P the only compounds able to extract actinide nitrates from nitric acid with high selectivity, while most other compounds require a more lipohilic anion to form extractable complexes? Is selectivity an intrinsic property of the ligand, or is it induced by its ability of forming complexes with pure N-­‐donor coordination? Can we find even better ligands? Or are BT(B)P the holy grail of An(III)/Ln(III) separation? This presentation discusses the development of N-­‐donor ligands and their properties as selective extracting or complexing agents for actinides(III). Furthermore, it gives an overview of the more fundamental studies performed with BT(B)P (but also with some less efficient N-donor ligands). We show on the one hand how these studies have forwarded our understanding of N‐donor‐actinide interactions and on the other hand point out some of the open questions to be addressed in the future.