Mixed metal double salt ionic liquids comprised of [HN222]2[ZnCl4] and AlCl3 provide tunable Lewis acid catalysts related to the ionic environment†
Two solids of differing Lewis acidities, triethylammonium tetrachlorozincate ([HN222]2[ZnCl4]) and AlCl3, have been combined across 0.1 to 0.9 mole fractions (with respect to [HN222]2[ZnCl4]), and homogeneous solid double salts or mixed metal [HN222]2x[(1 − x)AlCl3 + xZnCl4] double salt ionic liquids (DSILs) were obtained at x = 0.33, 0.4, and 0.5 with varying Lewis acidities. The characterization of the prepared DSILs (melting point, color, homogeneity, 1H, 13C, and 27Al nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (IR), single crystal X-ray diffraction (SCXRD), and matrix-assisted laser-desorption ionization time-of-flight (MALDI-TOF) mass spectrometry) suggests that [HN222]2[ZnCl4] transfers Cl− from [ZnCl4]2− to the stronger Lewis acid AlCl3, forming intermediate acidic DSILs with [AlCl4]− and [Al2Cl7]− anions and mixed anionic Zn species. Qualitative Lewis acidity measurements using acetonitrile as an IR-active probe showed that the acidity of the DSILs decreased as the amount of [ZnCl4]2− increased. In a Beckman rearrangement reaction of acetophenone oxime, significant catalytic activity was observed for the DSIL at x = 0.33, where the activity of the DSIL was found to be even higher than [HN222][Al2Cl7], AlCl3, [HN222]2[ZnCl4], or ZnCl2 despite its lower Lewis acidity, apparently due to the synergistic effect of AlCl3 and [ZnCl4]2− as Cl− donors for the formation of catalytically active species. The findings illustrate a DSIL-based approach for modifying the catalytic activity of a known complex without changing its inner coordination sphere.