Ionic catalytic reactions

R. Sheldon, Catalytic reactions in ionic liquids, J Chem Soc, Chem Commun 2399-2407 2001. 

While certain TSILs have been developed to pull metals into the IL phase, others have been developed to keep metals in an IL phase. The use of metal complexes dissolved in IL for catalytic reactions has been one of the most fruitful areas of IL research to date. LLowever, these systems still have a tendency to leach dissolved catalyst into the co-solvents used to extract the product of the reaction from the ionic liquid. Consequently, Wasserscheid et al. have pioneered the use of TSILs based upon the dissolution into a conventional IL of metal complexes that incorporate charged phosphine ligands in their stmctures [16-18]. These metal complex ions become an integral part of the ionic medium, and remain there when the reaction products arising from their use are extracted into a co-solvent. Certain of the charged phosphine ions that form the basis of this chemistry (e.g., P(m-C6H4S03 Na )3) are commercially available, while others may be prepared by established phosphine synthetic procedures. 

Stoichiometric - or, more simply, non-catalytic - reactions are an important and rapidly expanding area of research in ionic liquids. This section deals with reactions that consume the ionic liquid (or molten salt) or use the ionic liquid as a solvent. 

The distribution of the products obtained from this reaction depends upon the reaction temperature (Figure 5.1-4) and differs from those of other poly(ethene) recycling reactions in that aromatics and alkenes are not formed in significant concentrations. Another significant difference is that this ionic liquid reaction occurs at temperatures as low as 90 °C, whereas conventional catalytic reactions require much higher temperatures, typically 300-1000 °C [100]. A patent filed for the Secretary of State for Defence (UK) has reported a similar cracking reaction for lower molecular weight hydrocarbons in chloroaluminate(III) ionic liquids [101]. An... 

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

With respect to the ionic liquid s cation the situation is quite different, since catalytic reactions with anionic transition metal complexes are not yet very common in ionic liquids. However, an imidazolium moiety as an ionic liquid cation can act as a ligand precursor for the dissolved transition metal. Its transformation into a lig-... 

Obviously, with the development of the first catalytic reactions in ionic liquids, the general research focus turned away from basic studies of metal complexes dissolved in ionic liquids. Today there is a clear lack of fundamental understanding of many catalytic processes in ionic liquids on a molecular level. Much more fundamental work is undoubtedly needed and should be encouraged in order to speed up the future development of transition metal catalysis in ionic liquids. 

Moreover, these experiments reveal some unique properties of the chlorostan-nate ionic liquids. In contrast to other known ionic liquids, the chlorostannate system combine a certain Lewis acidity with high compatibility to functional groups. The first resulted, in the hydroformylation of 1-octene, in the activation of (PPli3)2PtCl2 by a Lewis acid-base reaction with the acidic ionic liquid medium. The high compatibility to functional groups was demonstrated by the catalytic reaction in the presence of CO and hydroformylation products. 

The first application involving a catalytic reaction in an ionic liquid and a subsequent extraction step with SCCO2 was reported by Jessop et al. in 2001 [9]. These authors described two different asymmetric hydrogenation reactions using [Ru(OAc)2(tolBINAP)] as catalyst dissolved in the ionic liquid [BMIM][PFg]. In the asymmetric hydrogenation of tiglic acid (Scheme 5.4-1), the reaction was carried out in a [BMIM][PF6]/water biphasic mixture with excellent yield and selectivity. When the reaction was complete, the product was isolated by SCCO2 extraction without contamination either by catalyst or by ionic liquid. 

In comparison with catalytic reactions in compressed CO2 alone, many transition metal complexes are much more soluble in ionic liquids without the need for special ligands. Moreover, the ionic liquid catalyst phase provides the potential to activate and tune the organometallic catalyst. Furthermore, product separation from the catalyst is now possible without exposure of the catalyst to changes of temperature, pressure, or substrate concentration. 

There is a wide variety of solid electrolytes and, depending on their composition, these anionic, cationic or mixed conducting materials exhibit substantial ionic conductivity at temperatures between 25 and 1000°C. Within this very broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used to induce the NEMCA effect and thus activate heterogeneous catalytic reactions. As will become apparent throughout this book they behave, under the influence of the applied potential, as active catalyst supports by becoming reversible in situ promoter donors or poison acceptors for the catalytically active metal surface. 

Conclusion when using ionic conductors where the conducting, i.e. backspillover ion participates in the catalytic reaction under study (e.g. O2 ions in the case of catalytic oxidations) then both galvanostatic and potentiostatic operation lead to a steady-state and allow one to obtain steady-state r vs Uwr plots. 

S. Bebelis, M. Makri, A. Buekenhoudt, J. Luyten, S. Brosda, P. Petrolekas, C. Pliangos, and C.G. Vayenas, Electrochemical activation of catalytic reactions using anionic, cationic and mixed conductors, Solid State Ionics 129, 33-46 (2000). 

R.M. Lambert, M. Tikhov, A. Palermo, I.V. Yentekakis, and C.G. Vayenas, Electrochemical Promotion of Environmentally Important Catalytic Reactions, Ionics 1, 366-376 (1995). 

Room temperature ionic liquids are air stable, non-flammable, nonexplosive, immiscible with many Diels-Alder components and adducts, do not evaporate easily and act as support for the catalyst. They are useful solvents, especially for moisture and oxygen-sensitive reactants and products. In addition they are easy to handle, can be used in a large thermal range (typically —40 °C to 200 °C) and can be recovered and reused. This last point is particularly important when ionic liquids are used for catalytic reactions. The reactions are carried out under biphasic conditions and the products can be isolated by decanting the organic layer. 

Sheldon, R.A. (2001) Catalytic Reactions in Ionic Liquids. Chemical Communications, 23, 2399-2407. 

Nowadays, a number of commercial suppliers [20] offer ionic liquids, some of them in larger quantities, [21] and the quality of commercial ionic liquid samples has clearly improved in recent years. The fact that small amounts of impurities significantly influence the properties of the ionic liquid and especially its usefulness for catalytic reactions [22] makes the quality of an ionic liquid an important consideration [23]. Without any doubt the improved commercial availability of ionic liquids is a key factor for the strongly increasing interest in this new class of liquid materials. 

It is generally observed that the rate of reaction can be altered by the presence of non-reacting or inert ionic species in the solution. This effect is especially great for reactions between ions, where rate of reaction is effected even at low concentrations. The influence of a charged species on the rate of reaction is known as salt effect. The effects are classified as primary and secondary salt effects. The primary salt effect is the influence of electrolyte concentration on the activity coefficient and rate of reaction, whereas the secondary salt effect is the actual change in the concentration of the reacting ions resulting from the addition of electrolytes. Both effects are important in the study of ionic reactions in solutions. The primary salt effect is involved in non-catalytic reactions and has been considered here. The deviation from ideal behaviour can be expressed in terms of Bronsted-Bjerrum equation. 

AH of these experimental observations point to a remarkable hydrogen-bonding chemistry among the components of the catalytic reaction. In addition, MP2 calculations performed on model compounds allow the formulation of an ionic, outer sphere, bifunctional hydrogenation mechanism, as shown in Scheme 2.32. 

The so-called second generation ionic hquids were prepared from organic cations and AlCl anions [170]. Since AICI3 was present in these liquids, they were used as catalysts in Lewis acid catalyzed reactions. Also many of the third generation ionic liquids have been used as solvents for catalytic reactions [171-174], However, it is also known that third generation ionic liquids are capable of catalyzing reactions, either in substoichiometric amounts or as reaction medium. This will be discussed in this section. 

Although the number of enantiopure ionic liquids as successful asymmetric catalytic reaction media is still very limited, the research field has attracted considerable attention. Due to the large number of possible applications in combination with the advantages of easy recoverability, the further development of the field is very important. However, it shall be mentioned here that some reported examples of catalytic activities of ionic liquids have to be investigated in more detail. In particular, ionic liquids incorporating [BF ] and [PF ] have to be very pure and normally should not be used with water for a prolonged time, since the anions could decompose and release HF, which could be itself the cause of the observed activity [164]. 

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