Enhanced acidity and activity of aluminum/gallium-based ionic liquids resulting from dynamic anionic speciation

To explore if the catalytic activity of single-metal species in halometallate catalysts can be enhanced with mixed-metal species, aluminum and gallium were simultaneously introduced to prepare a mixed-metal ionic liquid (IL) through a one-pot reaction of [HN₂₂₂]Cl, AlCl₃, and GaCl₃ in various amounts. The Lewis acidity in Friedel-Crafts acylation and alkylation reactions of these IL compositions were evaluated and all mixed-metal [HN₂₂₂][xAlCl₃ + (2-x)GaCl₃]Cl ILs exhibited higher catalytic activity than AlCl₃, GaCl₃, [HN₂₂₂][Al₂Cl₇], or [HN₂₂₂][Ga₂Cl₇]. The broadening in ²⁷Al nuclear magnetic resonance (NMR) of these ILs indicates the presence of dynamic anionic species, whereas matrix-assisted laser-desorption ionization time-of-flight (MALDI-TOF) and acetonitrile-probed Fourier transform infrared (FT-IR) analyses correlates the structure-activity relationship, for instance, identifying the presence of mixed-metal-containing [Cl₂GaOClAlClOGaCl₂]^- species in the better-performing ILs.