Using Crystal Structures of Ionic Compounds to Explore Complexation and Extraction of Rare Earth Elements in Ionic Liquids

Abstract

Despite only being able to directly measure the solid state, single crystal X-ray diffraction is of use in understanding how the fully ionic environment of an ionic liquid (IL) affects complexation of rare earth element ions. Here we examine crystal structures of ionic lanthanide complexes as case studies in this context. The complexation of a dithiophosphate IL is compared to non-IL systems, where the crystal structure illustrates the formation of hydrophobic domains despite the presence of highly charged ions. The crystal structure of a lanthanum complex with 1,2-di(4-pyridyl)ethylene illustrates how a large, neutral ligand in a fully ionic system interrupts the ionic packing, leading to the inclusion of noncoordinating ligands in the outer coordination sphere. The crystal structure of a salt composed of discrete terbium and thorium complex ions illustrates how in two metal ions in a fully ionic environment need not be bridged directly with a ligand but can be linked entirely through noncovalent interactions. Because ILs are fully ionic systems, these effects can be interpreted in the context of rare earth element behavior in ILs.