Ionic liquids based on polynitrile anions: hydrophobicity, low proton affinity, and high radiolytic resistance...

Ionic liquids (IL) are being considered as replacements for molecular diluents in spent nuclear fuel reprocessing. This development is hampered by the dearth of constituent anions that combine high hydrophobicity, low metal cation and proton affinity, and radiation resistance. We demonstrate that polynitrile anions have the potential to meet these challenges. Unlike the great majority of organic anions, such polynitrile anions are resistant to oxidative fragmentation during radiolysis, yielding stable N- and C-centered radicals. Moreover, their radical dianions (generated by reduction of the anions) generally undergo protonation in preference to elimination of the cyanide. This is in contrast to fluorinated anions (another large class of anions with low proton affinity), for which radiation-induced release of fluoride is a common occurrence. The "weak spot" of the polynitrile anions appears to be their excited-state dissociation, but at least one of these anions, 1,1,2,3,3-pentacyanopropenide, is shown to resist fragmentation in room temperature radiolysis. We suggest beginning the exploration of ionic liquids based on such polynitrile anions.