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 TiCl offers a way to react tetravalent metal ions in an IL. Reactions of TiCl with 1-butyl-3-methylimidazolium ([Cmim])-based ILs containing chloride or bromide lead to mixtures of highly metastable amorphous solids and small amounts of crystalline chlorotitanate salts including [Cmim][TiCl] and two polymorphs of [Cmim][TiCl] in a manner not well correlated with stoichiometry or anion identity. The reaction of TiCl with [Cmim][BF] yields crystals of the mixed fluoro-chloro complex [Cmim][TiFCl], 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 Ti and the unusual behavior of TiCl among metal halides as a nonpolar molecular compound.