Isolation of anhydrous tetrabutylphosphonium lanthanide hexa- and penta-nitrates from ionic liquids

Our attempts to use an ionic liquid strategy to force f-element coordination of soft donor adenine and 4,5-dicyanoimidazole using 1-pot reactions in water using hydrated lanthanide salts failed, instead resulting in the isolation of anhydrous lanthanide nitrate salts. Reactions of previously ground mixtures of 40 % aq. [P4444][OH] and adenine with Nd(NO3)3·6H2O or Sm(NO3)3·6H2O followed by hand grinding and heating to 90 °C for 10 h led to hot crystallization of anhydrous [P4444]2[Nd(NO3)5] and [P4444]2[Sm(NO3)5]. The same reaction with 4,5-dicyanoimidazole and Nd(NO3)3·6H2O but without heating (10 h, room temperature) to prevent ligand decomposition, led to crystallization of anhydrous [P4444]3[Nd(NO3)6]. [P4444]3[Nd(NO3)6] could also be isolated by conducting the first reaction without using adenine. The single crystal X-ray diffraction structures of the three new anhydrous salts were examined to provide clues to the observed behavior. The [P4444]+ cations allow charge balance while forming box-type encapsulations around the anionic counterpart which has been previously reported in similar structures. The fact that these anhydrous salts were prepared from water with hydrated starting materials and no precautions taken to keep the crystal growing process anhydrous, suggests synthetic strategies that can potentially be beneficial for more advanced separations procedures.