Mixed metal double salt ionic liquids comprised of [HN222]2[ZnCl4] and AlCl3 provide tunable Lewis acid catalysts related to the ionic environment

Abstract

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.