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Issue 23, 2018

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

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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.

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

Supplementary files

Article information


Submitted
13 Mar 2018
Accepted
10 May 2018
First published
11 May 2018

Dalton Trans., 2018,47, 7795-7803
Article type
Paper

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

R. Kore, Steven. P. Kelley, P. Aduri and R. D. Rogers, Dalton Trans., 2018, 47, 7795 DOI: 10.1039/C8DT00976G

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