Lewis Acidic Ionic Liquids
Lewis Acidic Ionic Liquids 0
Lucy C. Brown 0
James M. Hogg 0
Małgorzata Swadz´ba-Kwas´ny 0
0 School of Chemistry and Chemical Engineering, The Queen's University of Belfast , David Keir Building, Stranmillis Road, Belfast BT9 5AG , UK
Until very recently, the term Lewis acidic ionic liquids (ILs) was nearly synonymous with halometallate ILs, with a strong focus on chloroaluminate(III) systems. The first part of this review covers the historical context in which these were developed, speciation of a range of halometallate ionic liquids, attempts to quantify their Lewis acidity, and selected recent applications: in industrial alkylation processes, in supported systems (SILPs/SCILLs) and in inorganic synthesis. In the last decade, interesting alternatives to halometallate ILs have emerged, which can be divided into two sub-sections: (1) liquid coordination complexes (LCCs), still based on halometallate species, but less expensive and more diverse than halometallate ionic liquids, and (2) ILs with main-group Lewis acidic cations. The two following sections cover these new liquid Lewis acids, also highlighting speciation studies, Lewis acidity measurements, and applications. This article is part of the Topical Collection ''Ionic Liquids II''; edited by Barbara Kirchner, Eva Perlt.
Lewis acidity; Halometallate ionic liquids complexes; Solvate ionic liquids; Borenium cations; Liquid coordination
1 Introduction
The development and applications of Lewis acids hold an important place in
chemical research. In industrial processes, heterogeneous Lewis acids are dominant,
from simple metal halides to metal oxides with Lewis acidic sites (alumina,
zirconia, titania) [1, 2]. Lewis acids, such as BF3 or AlCl3, are also combined with
Brønsted acids to yield Brønsted superacidic catalysts. In organic synthesis, a wide
variety of Lewis acids are used, both in stoichiometric and catalytic quantities, with
acidic metal centers varying from alkali metals (Li, Na), through to group 13
elements (AlIII, GaIII, InIII), to ZnII, SnII and SnIV, HfIV and lanthanides [3]. In
addition to simple halides, metal triflates and bistriflamides are also used, as well as
organometallic Lewis acids, with an increasing focus on water-stable Lewis acids
[4]. Elaborate chiral ligands are used for asymmetric reactions [5]. In main-group
chemistry, frustrated Lewis pairs, which are combinations of Lewis acids and bases
prevented from forming an adduct by steric hindrance [6], have recently opened up
the field of metal-free catalysis and small molecule activation [7]. In this context,
the archetypical Lewis acid is B(C6F5)3, though a plethora of charge-neutral and
cationic Lewis acids, based predominantly on boron, but also on phosphorus, silicon
and main-group metals, have been synthesized [
8–10
].
Against this backdrop, the chemistry of Lewis acidic ionic liquids appears
structurally monotonous, with a strong focus on halometallate Lewis acidic
anions—in particular chloroaluminate(III) ILs. These were the first group of ionic
liquids to capture the attention of chemists across disciplines, and remained the
center of research efforts until ca. 2000 [
11, 12
]. Afterwards, the spotlight has
shifted to ‘air- and water-stable’ systems [13], but research on Lewis acidic
halometallate ILs has been under continuous development, which was reviewed in
2014 by Estager et al. [
14
]. Recent years have been marked by: (1) the maturing of
chloroaluminate(III) ILs in catalysis, expressed in engineering advances and the
announcement of several full-scale industrial processes, (2) the development of new
ionic liquid-like systems, based on metal chloride eutectics (liquid coordination
complexes), and (3) the naissance of ILs with Lewis acidic cations, with hopefully a
lot of inspiring chemistry still to come. All three families of Lewis acidic ionic
liquids will be discussed, including the historical context, speciation studies,
quantification of acidity, and examples of applications. Abbreviations used
throughout the text are listed in Table 1.
2 Chlorometallate Ionic Liquids
2.1 Historical Context
The study of modern ionic liquids originated from high-temperature inorganic
molten salts (studied as heat transfer fluids and electrolytes), which lead to lower
melting organic salts, especially chloroaluminate(III) ILs (Fig. 1, left), based on
[AlCl4]- and [Al2Cl7]- anions [
15
]. In parallel, mechanistic studies on
AlCl3promoted Friedel–Crafts chemistry revealed [AlCl4]- and [Al2Cl7]- anions to
balance the charge of the intermediate, a protonated toluenium cation [
12, 16
]. This
naturally led to studies of Friedel–Crafts chemistry in the [C2mim]Cl–AlCl3 system
[17], which may be considered the starting point of Lewis acid catalysis in
chlorometallate ILs, with over 400 citations to date.
Boon’s work was preceded—by over a decade—by Parshall, who in 1972 used
organic molten salts, [N2222][SnCl3] and [N2222][GeCl3], as solvents for PtCl2 [
19
].
Th (...truncated)