Rare-earth iodides in ionic liquids: Crystal structures of [bmpyr]4[LnI6][Tf2N] (Ln = La, Er)

Deliberately designed ionic liquids can be excellent solvents for organic reactions with lanthanide compounds, e.g. Lewis catalysis with trivalent lanthanides. Little is known about the solvation and complexation of these Lewis-acid catalysts in these—still uncommon—solvents, although the knowledge of these processes is a prerequisite for a basic understanding of reaction mechanisms and catalytic cycles. Therefore, we have investigated the chemical behaviour of rare-earth metal iodides in the ionic liquid [bmpyr][Tf2N] (bmpyr = 1,1-n-butyl-methylpyrrolidinium; Tf2N = bis(trifluoromethanesulfonyl)-amide). Compounds of the general composition [bmpyr]4[LnI6][Tf2N] could be crystallized from solutions of LnI3 (Ln = La, Er), in [bmpyr][Tf2N]. Single-crystal X-ray diffraction data show that the trivalent rare-earth cations are octahedrally coordinated by six iodide anions. Eight cations of the ionic liquid are located tangentially above each of the triangular faces of the [LnI6] octahedron. According to the size of the trivalent cation, the crystal structure adjusts itself by tilting of the [LnI6] octahedra to accommodate one anion of the ionic liquid, bis(trifluoromethanesulfonyl)-amide, which completes the crystal structure of the composition [bmpyr]4[LnI6][Tf2N].