Activation of a transition metal catalyst in ionic liquids

3 Activation of a transition metal catalyst in ionic liquids 

Apart from the activation of a biphasic reaction by extraction of catalyst poisons as described above, an ionic liquid solvent can activate homogeneously dissolved transition metal complexes by chemical interaction. 

In the following section, the nature of the chemical interactions between an ionic liquid and a transition metal catalyst is systematically developed according to the role of the ionic liquid in the different systems. 

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Unfortunately, most attempts to characterize catalytically active transition metal complexes in ionic liquids have been based on product analysis. There is nothing wrong with the argument that a catalyst is more active because it produces more of the product. However, this is not the type of esqjlanation that can help to develop a more general understanding of what happens to a transition metal complex under catalytic conditions in a certain ionic liquid. 

Many transition metal-catalyzed reactions have already been studied in ionic liquids. In several cases, significant differences in activity and selectivity from their counterparts in conventional organic media have been observed (see Section 5.2.4). However, almost all attempts so far to explain the special reactivity of catalysts in ionic liquids have been based on product analysis. Even if it is correct to argue that a catalyst is more active because it produces more product, this is not the type of explanation that can help in the development of a more general understanding of what happens to a transition metal complex under catalytic conditions in a certain ionic liquid. Clearly, much more spectroscopic and analytical work is needed to provide better understanding of the nature of an active catalytic species in ionic liquids and to explain some of the observed ionic liquid effects on a rational, molecular level. 

This type of co-catalytic influence is well loiown in heterogeneous catalysis, in which for some reactions an acidic support will activate a metal catalyst more efficiently than a neutral support. In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transition metal catalysts dissolved in it. 

In general, the incorporation of an active transition metal catalyst into the anion of an ionic liquid appears to be an attractive concept for applications in which a high catalyst concentration is needed. 

In many other reactions where ionic or polar transition metal catalysts are used it has been demonstrated that the use of polar and weekly coordinating ionic liquids can result in a clear enhancement of catalytic activity [32a]. 

Even if the ionic liquid is not directly involved in creating the active catalytic species, a co-catalytic interaction between the ionic liquid solvent and the dissolved transition metal complex often takes place and can result in significant catalyst activation. When a catalyst complex is, for example, dissolved in a slightly acidic ionic liquid some electron-rich parts of the complex (e.g., lone pairs of electrons in the ligand) may interact with the solvent, providing increased activity to the resulting catalytic centre. Acidic ionic liquids can be considered as liquid acid supports for transition metal catalysts dissolved therein. 

Ionic liquids in which the anion is composed of an active transition metal catalyst providing essentially a liquid catalyst may be prepared, although it is too early to comment whether these will offer any advantages over catalysts dissolved in ionic liquids [23],... 

Catalysts other than homogeneous (molecular) compounds such as nanoparticles have been used in ionic liquids. For example, iridium nanoparticles prepared from the reduction of [IrCl(cod)2] (cod = cyclooctadiene) with H2 in [bmim][PF6] catalyses the hydrogenation of a number of alkenes under bipha-sic conditions [27], The catalytic activity of these nanoparticles is significantly more effective than many molecular transition metal catalysts operating under similar conditions. 

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