Flexible coordination environments of lanthanide complexes grown from chloride-based ionic liquids

Hydrated lanthanide(III) chlorides, LnCl₃·xH₂O (Ln = La, Pr, Nd, Sm, Eu, Gd; x = 6–7) readily dissolve in the low melting ionic liquid 1-ethyl-3-methylimidazolium chloride ([C₂mim]Cl) in an open vessel at 110 °C, and upon cooling crystallize as the anhydrous [C₂mim]₃[LnCl₆]. The crystal structures exhibit a face-centered packing arrangement of the [LnCl₆]^3- anions, with the cations located as slip aligned pairs in the void spaces which participate in hydrogen-bonding to chlorides. A second crystalline form of the Gd³⁺ complex, GdCl₃(OH₂)₄·2([C₂mim]Cl), was isolated when the above reaction was conducted in a sealed system. For comparison, a third Gd³⁺ compound was grown from the ionic liquid 1-butyl-3-methylimidazolium chloride ([C₄mim]Cl) using the same unsealed conditions as above, and was found to be [C₄mim]₃[GdCl₆]. This compound exhibits a different packing arrangement to that observed for the [C₂mim]⁺ analogs. Based on these findings, ILs would appear to offer new crystallization process options based on their often high thermal stabilities and low to negligible vapor pressures.