catena-Poly[1-butyl-3-methyl-imidazolium [[dichlorido(methanol-κO)(propan-2-ol-κO)lanthanate(III)]-di-μ-chlorido]]

Crystallographic Insights into the Behavior of Highly Acidic Metal Cations in Ionic Liquids from Reactions of Titanium Tetrachloride with [1-Butyl-3-Methylimidazolium][X] Ionic Liquids (X = Chloride, Bromide, Tetrafluoroborate)

Highly charged metal ions are difficult to investigate in weakly coordinating ionic liquids (ILs) because of the insolubility of their solid forms, but the molecular liquid TiCl₄ offers a way to react tetravalent metal ions in an IL. Reactions of TiCl₄ with 1-butyl-3-methylimidazolium ([C₄mim]⁺)-based ILs containing chloride or bromide lead to mixtures of highly metastable amorphous solids and small amounts of crystalline chlorotitanate salts including [C₄mim]₂[TiCl₆] and two polymorphs of [C₄mim]₂[Ti₂Cl₁₀] in a manner not well correlated with stoichiometry or anion identity. The reaction of TiCl₄ with [C₄mim][BF₄] yields crystals of the mixed fluoro-chloro complex [C₄mim]₂[Ti₄F₆Cl₁₂], indicating spontaneous reaction of the IL ions to generate HF in situ. These unusual behaviors are explained in terms of the exceptionally high acidity of Ti⁴⁺ and the unusual behavior of TiCl₄ among metal halides as a nonpolar molecular compound.