Low-viscosity polyalphaolefins were synthesised using AlCl3-based liquid coordination complexes (LCCs), in a sustainable and economically competitive process.

The first examples of ionic liquids based on borenium cations, [BCl2L]+, are reported. These compounds form highly Lewis acidic liquids under solvent‐free conditions. Their acidity was quantified by determining the Gutmann acceptor number (AN). Extremely high ANs were recorded (up to AN=182, δ31P=120 ppm), demonstrating that these borenium ionic liquids are the strongest Lewis superacids reported to date, with the acidity enhanced by the ionic liquid environment.

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.

Coordination complexes of Lewis acidic metal chlorides AlCl, GaCl, InCl, SbCl, SnCl, SnCl, ZnCl and TiCl with trioctylphosphine (P) and trioctylphosphine oxide (PO) were synthesised. All compounds formed liquid coordination complexes (LCCs) at ambient temperature, although decomposition via a redox mechanism was detected in some cases. The Lewis acidity of the metal chlorides (measured in 1,2-dichloroethane solutions) and the LCCs (measured neat) was quantified by using the Gutmann acceptor number (AN) approach. In general, LCCs were equally or more Lewis acidic than the corresponding metal chlorides. The AN values were compared with the catalytic activity of selected LCCs in a model Diels-Alder reaction. Insight into speciation of LCCs was gained using multinuclear NMR spectroscopy, revealing that most LCCs comprised charge-neutral complexes rather than ionic ones. The relationship between the speciation, Lewis acidity (AN scale) and catalytic activity is discussed in detail. This approach reveals several new, promising catalytic systems, such as PO-InCl, with Lewis acidity enhanced compared to chloroindate ionic liquids, and PO-TiCl, with hydrolytic stability enhanced with respect to neat TiCl.