ABSTRACT
Among all of the options to improve the catalytic activity of transition metal catalysts, in the last decades, especially since the 1990s, ionic liquids (ILs) have become an exceptional media to test traditional and new catalysts.6-9 Those liquids may be dened as structures that are entirely composed of ions that melt below 100°C, typically displaying an organic cation associated with a relatively weak coordinating anion. The positive effect of those uids over a reaction is a consequence of their inherent ionic nature and due to their unique physicochemical properties. Indeed, the origin of this effect and how it operates is the subject of many controversies. The positive effect of ILs on catalytic rates10 has been well documented over the years. The so-called tunable properties of ILs provide these materials with almost unlimited possibilities, especially when considering the estimation of 106 possible combinations of known cations and anions to form an IL.11 Imidazolium-based ILs constitute an important, rich, and controversial class of ILs, but this has a prominent position among all types of ILs.12-15 Their physicochemical properties are the subject of major debates, but no one has questioned their importance and applicability. Their negligible vapor pressure, for instance, allows their use for the investigation of the size and shape of metal nanoparticles (NPs) by in situ X-ray photoelectron spectroscopy (XPS)16 or transmission electron microscopy (TEM)17 analyses. The supramolecular three-dimensional structural organization of imidazolium-based ILs has a direct impact on their physicochemical properties and helps to explain their inuence over several organic transformations.18 It is not rare to nd an example where the reaction yields, rates, and selectivities are improved when a reaction is carried out in ILs.19-21 ILs are capable of forming ion pairs with charged intermediates and of participating in polar transition states, therefore stabilizing them. Indeed, ILs are able to form large supramolecular aggregates, thus reducing activation barrier energies and facilitating the transformation. This specic property gives ILs the title “entropic drivers,” and the aforementioned description is the IL effect. This effect is a consequence of their inherent ionic nature associated with their supra molecular organization. The forces responsible for the high supramolecular organization and nanoorganization of inclusion compounds are the subject of intense debates.22 For some, H-bonds play a crucial role,13 whereas others consider the relevance of such bonds minor, attributing high importance to the Coulombic interactions.23 Another important property to be considered is their capacity to solubilize a great number of organic and inorganic compounds independently of their polarities. This unique property, especially observed in imidazolium-based ILs, is a consequence of their natural segregation in two main domains: polar and nonpolar.24 As a consequence, polar substrates are preferentially dissolved in
polar domains, and vice versa.25-29 This is of particular importance for the facilitation of close contact among two different reagents when dissolved in the same phase. For all of the desirable characteristics found in imidazolium ILs, this cation was later naturally incorporated as a charge tag in many different ligands (task-specic ionic liquids (TSILs)), which were expected to display similar physicochemical properties to those observed in pure ILs, as reviewed elsewhere.12,30-32 This elegant strategy enabled mass spectrometry mechanistic evaluations and a better support of novel catalysts in imidazolium ILs. Indeed, as will be discussed, this efcient strategy resulted, overall, in very impressive improvements on yields and selectivities for several reactions. Despite all of the controversies, improved yields, rates, and selectivities are observed, therefore justifying deeper investigation and investment in this eld of research. Considering that the number of research groups working in this eld has multiplied, we have selected works focusing on hydrogenation, cross-coupling, oxidation, multicomponent reactions, and biomass transformation performed in imidazolium-based ILs using transition metal catalysts.
