Functionalised ionic liquids

The use of functionalised ionic liquids in catalysis is still very much in its infancy. The presence of a functional group may serve any of the following purposes in order to improve the catalytic reaction (i) assist in the activation of the catalyst (ii) generate a novel catalytic species (iii) improve the stability of the catalyst (iv) optimise immobilisation and recyclability (v) facilitate product isolation and (vi) influence the selectivity of the reaction. In addition, the functionalised ionic liquid has to meet certain requirements to be of use in synthesis/catalysis. Most important is their inertness with respect to the reaction in question. Also, the right balance between catalyst stabilisation and activation has to be found and this might require careful tuning of the reaction conditions. 

Catalyst Stabilisation Coordination Polymers Catalyst Stabilisation 

Lubricants Surface Modification Lipophilic Ionic Liquids 

Combining functionalised cations with functionalised anions may lead to highly task-specific ionic liquids, but very little has been undertaken in this area in the catalysis domain. However, a number of so-called dual-functionalised ionic liquids have been reported1271 and it has been shown 

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In 2002 Mehnert and co-workers were the first to apply SILP-catalysis to Rh-catalysed hydroformylation [74], They described in detail the preparation of a surface modified silica gel with a covalently anchored ionic liquid fragment (Scheme 7.7). The complex N-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazole was reacted with 1-chlorobutane to give the complex l-butyl-3-(3-triethoxysilylpropyl)- 4,5-dihydroimidazolium chloride. The latter was further treated with either sodium tetrafluoroborate or sodium hexafluorophosphate in acetonitrile to introduce the desired anion. In the immobilisation step, pre-treated silica gel was refluxed with a chloroform solution of the functionalised ionic liquid to undergo a condensation reaction giving the modified support material. Treatment of the obtained monolayer of ionic liquid with additional ionic liquid resulted in a multiple layer of free ionic liquid on the support. 

Scheme 2.4 Preparation of carboxylic acid-functionalised ionic liquids via a zwitterionic intermediate...

Sulfonic acid functionalised ionic liquids may also be prepared via a zwitterionic intermediate from a Michael-type addition, as shown in Scheme 2.5.[84 85] In the first step 1-methylimidazole reacts with the sulfonic acid precusor 1,3-propane sultone to form a zwitterionic intermediate. Protonation with Bronsted acids affords ionic liquids with high purity that have proven to be highly efficient reaction media in, for example, esterification reactions.[86] Task-specific ionic liquids may also be prepared using semi-combinatorial methods.1871... 

Scheme 2.5 Preparation of sulfonic-acid functionalised ionic liquids...

Functionalised ionic liquids based on cations other than imidazolium have also been developed. For example, pyridinium cations functionalised with pentafluorosulfanyl[89] or alkyl-nitrile groups1901 have been prepared as cheaper alternatives to their imidazolium-based counterparts (see Figure 2.8). The latter have been evaluated in palladium catalysed C-C cross coupling reactions and improved catalyst retention and stability were observed in the nitrile-functionalised ionic liquid compared to the simple alkyl-analogue. Consequently, the nitrile-functionalised ionic liquid solution can be reused repeatedly without significant decrease in activity (see Chapter 6 for further information). 

An area in which functionalised ionic liquids are already playing an important role in catalysis is heterogenisation on solid supports. The general concept involves the immobilisation of imidazolium and other cationic fragments onto solid supports using appropriate functional groups attached to the cation. An ionic catalyst then resides within the ionic matrix and several examples of such supported ionic liquid phase catalysts are provided in the subsequent chapters of this book. The concept is illustrated in Figure... 

Figure 2.11 Surface modification using a functionalised ionic liquid...

The effect of nitrile-functionalised ionic liquids on the recycling potential was investigated in the coupling between iodobenzene and tributylphenyltin.[15] Several, closely related palladium-precursors, based on the ionic liquid cation [C3CNpy]+ and PdCP (Figure 6.5), were tested in the reaction, but the nature of the pre-catalyst did not have a significant influence on the rate. Independent from the ionic liquid used, rather low catalytic activity was observed and with 5 mol% catalyst, yields ranged between 43-65% after 12 hours at 80°C. 

A second source of amino functionalised imidazolium salts is the quest for functionalised ionic liquids [146,147]. Wan et al. used a piperidine functionaUsed imidazolium salt as a base in a palladium catalysed Heck reaction in an ionic liquid medium. The system was designed such that the phosphane Ugand, the base and the solvent were all imidazolium based ionic Uquids [148] (see Figure 3.51). Given the fact that imidazolium based ionic liquids can act as carbene precursors for coordination to transition metals [149] that coordinate better than phosphane Ugands (with cationic substituents in the present case), it is far from certain which is acmally the ligand in that carefully designed system. 

Figure 3.51 A multi-functionalised ionic liquid composition (MFILC) containing an amino functionalised imidiizolium salt for the Heck reaction of aryl iodides and bromides.

Yang et al. used a similar protocol (an ether functionality supported on a primary alkyl halide carrier) to introduce an acetal on either side of the imidazole ring generating an ether functionalised ionic liquid (IL) imidazolium salt [183] (see Rguie 3.58). The anion could be varied without loss of the IL property (melting point below 1(X) °C) [184]. Synthesis of the transition metal carbene complexes (palladium) was done by carbene transfer ftom the corresponding silver(I) complexes or by reaction with the metal acetate (nickel) [162] (see Figure 3.64). 

MFILC multi-functionalised ionic liquid composition... 

Zhao D, Fei Z, Ohlin C A, et al. Dual-functionalised ionic liquids Synthesis and characterisation of imidazolium salts with a nitrile-functionalised anion. Chem. Commun. 2004. 2500-2501. 

Holbrey JD, Rogers RD (2003) Ionic liquids in synthesis. WILEY-VCH, Germany Fei Z, Geldbach TJ, Zhao D, Dyson PJ (2006) From dysfunction to bis-function on the design and applications of functionalised ionic liquids. Chem Eur J 12 2122-2130 Wu J, Zhang J, Zhang H, He J, Ren Q, Guo M (2004) Homogenous acetylation of cellulose in a new ionic liquid. Biomacromolecules 5(2) 266-268... 

Some recyclable acyclic SOsH-functionalised ionic liquids have been nsed as catalysts for the synthesis of chalcones via Claisen-Schmidt condensation [198]. The chalcones could simply be separated from the catalyst by decantation. After removing water from the reaction mixture, the catalysts could be recycled and rensed several times without a noticeable decrease in the catalytic activity. Recently, 2,4,5-trimetohoxy... 

New functionalised ionic liquids from Michael-type reactions —a chance for combinatorial ionic liquid development, Chem. Comm., vol. 39, n°16, pp.2038-2039, (August 2003), ISSN 1359-7345... 

Olivier JH, Camerel F, Ziessel R (2011) Lanthanide ion extraction by trifluoromethyl-L3-diketonate-functionalised ionic liquids adsorbed on silica. Chem Eur J 17(33) 9113-9122... 

Galan-Sanchez, M (2008) Functionalised ionic liquids, absorption solvents for CO2 and olefin separation. Ph. D. thesis... 

Fei Z, Geldbach TJ, Zhao D, Dyson PJ (2006) From dysfunction to bis-function on the design and applications of functionalised ionic liquids. Chem Eur J 12(8) 2122-2130... 

Wasserscheid, P. DriePen-Holscher, B. van Hal, R. Steffens, H. C. Zimmermann, J. (2003). New, functionalised ionic liquids from Michael-typ>e reactions -a chance for combinatorial ionic liquid development. Chem. Commun., 2038-2039 Wilkes, J. S. Zaworotko, M. J. (1992). Air and water stable l-ethyl-3-methylimidazolium based ionic liquids. /, Chem. Soc., Chem. Commun., 965-966... 

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