The ionic liquid as innocent solvent

Ionic liquids with wealdy coordinating, inert anions (such as [(CF3S02)2N] , [BFJ , or [PFg] under anhydrous conditions) and inert cations (cations that do not coordinate to the catalyst themselves, nor form species that coordinate to the catalyst under the reaction conditions used) can be looked on as innocent solvents in transition metal catalysis. In these cases, the role of the ionic liquid is solely to provide a more or less polar, more or less weakly coordinating medium for the transition metal catalyst, but which additionally offers special solubility for feedstock and products. 

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For Rh-catalysed hydroformylation the role of the ionic liquid as an innocent solvent is by far the most important. To our knowledge, none of the published research in this area claims special chemistry. The selectivity found with the different Rh-ligand complexes corresponds in most cases to the values obtained in traditional organic solvent or water (with the surprisingly low selectivity of TPPTS ligands in ionic liquids being a remarkable exception). Overall activities were found to be very comparable if mass transfer effects between the gas phase and the two immiscible liquid phases were overcome by proper stirring. 

Depending on the coordinative properties of the anion and on the degree of the cation s reactivity, the ionic liquid can be regarded as an innocent solvent, as a ligand (or ligand precursor), as a co-catalyst, or as the catalyst itself... 

Is there a "universal ionic liquid at the present state of development The answer is clearly no. Many of the ionic liquids commonly in use have very different physical and chemical properties (see Chapter 3) and it is absolutely impossible that one type of ionic liquid could be used for all synthetic applications described in Chapters 5-8. In view of the different possible roles of the ionic liquid in a given synthetic application (e.g., as catalyst, co-catalyst, or innocent solvent) this point is quite obvious. However, some properties, such as nonvolatility, are universal for all ionic liquids. So the answer becomes, if the property that you want is common to all ionic liquids, then any one will do. If not, you will require the ionic liquid that meets your needs. 

Due to the good solubility of organometallic compounds, ionic liquids have been used as reaction media, replacing traditional molecular solvents, or as the catalyst-supporting phase in a biphasic system. Influences of the ionic liquid on the reaction rate and selectivity can mostly be explained by the reactivity of the anion, which can be noncoordinating or coordinating as well as Lewis-acidic, Lewis-basic or neutral. The cation, in contrast, is considered to be essentially noncoordinating and innocent. 

In addition, an exploration of other aluminium-centred anions, in which interaction with cations can be more subdy varied, has produced a range of low-melting salts and interesting structural types. Part of this is stimulated by an interest in minimising conplexation of metal (and other coordinatively unsaturated) centres by the anionic conponent of ionic liquids used as reaction solvents. The terms innocent and weakly coordinating have been used to describe such minimally interacting species, and the topic has been reviewed [649]. 

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