Biphasic Chemistry Utilising Ionic Liquidsa

LAUREATES: AWARDS AND HONORS SCS FALL MEETING 2004 66 CHIMIA 2005, 59, No. 3 Chimia 59 (2005) 66–71 © Schweizerische Chemische Gesellschaft ISSN 0009–4293 Biphasic Chemistry Utilising Ionic Liquidsa Paul J. Dyson* Werner Prize Winner 2004 Abstract: Biphasic and multiphasic processes have emerged as effective methods to reduce the amount of volatile organic solvents used in the manufacture of chemicals. In this article some of the research conducted in my laboratory in this domain utilizing ionic liquids is described. Other dimensions to our research interests in ionic liquids are also discussed including their conversion to porous nanomaterials. Keywords: Biphasic catalysis · Catalysis · Ionic liquids · Reaction mechanisms · Nanoparticles Introduction Montreal, Geneva and Kyoto Protocols set lytic applications, usually incorporating a targets for reducing and eventually phasing methyl group in combination with an alkyl Decreasing the use of volatile organic sol- out many solvents [3]. Considerable efforts chain. A wide range of cations now give rise vents represents one of the major challenges are being made to find alternative solvents to low melting salts and some examples are of the chemical and pharmaceutical indus- that will conform to future controls, and illustrated in Fig. 1. The nature of the anion tries [1][2]. While solvents are used to fa- in particular, there has been an explosion is also important; halides giving relatively cilitate reactions (allowing reactants to mix of interest in the use of non-volatile, ionic high melting points, with anions such as and supplying or dissipating heat), their use liquids as alternative media in which to – – – – – – BF4 , PF6 , AsF6 , SbF6 , NO3 , CH3CO2 , is inherently wasteful as the solvent is sub- conduct reactions. In our laboratory we are – – – CF3SO3 , (CF3SO2)2N , (CF3SO2)3C pro- sequently removed from the chemical prod- interested in fundamental aspects of ionic viding low melting salts. uct and then disposed of, often by incinera- liquids chemistry, including method de- Ionic liquids are finding applications in tion. Solvents tend to be highly volatile as velopment and the elucidation of reaction separation processes [8], in electrochemis- this property facilitates their removal from mechanisms in ionic liquids, and based on try [9], as electrolytes in solar cells [10], chemical products, but unfortunately their these basic data we attempt to develop new as lubricants [11], as matrices in MALDI volatility has led to major environmental catalysts and new ionic liquids that are su- mass spectrometry [12] and even as pro- and toxicological problems. Atmospheric perior to those in current use, and superior pellants in small satellites [13]. However, pollution resulting from the evaporation of to other catalyst–solvent regimes. the most active area of ionic liquid research solvents represents one of the major glo- involves their use as alternative solvents in bal environmental issues of our time, with which to conduct synthetic chemistry [14]. emissions of some categories of volatile or- Ionic Liquids – Composition, The general properties of ionic liquids that ganic solvents implicated in the depletion Uniqueness and Applications make them suited to synthesis and catalysis of the ozone layer and in the greenhouse include the following: effect. Ongoing legislation including the Ionic liquids, which until recently were – They have no (or negligible) vapour referred to as room-temperature molten salts, pressure and therefore do not evapo- are merely salts with low melting points, by rate. definition being below 100 °C [4]. The first – They have favourable thermal proper- ionic liquid, at least the first substance be- ties. ing recognised as such, was [EtNH3][NO3], – They dissolve many metal complexes, with a melting point of 12 °C, discovered in catalysts, organic compounds and gas- 1914 [5]. About thirty years later the next es. ionic liquids were discovered, being com- – They are immiscible with many organic *Correspondence: Prof. P.J. Dyson Swiss Federal Institute of Technology (EPFL) posed of alkylpyridinium cations with tetra solvents and water. Institute of Chemical Sciences and Engineering chloroaluminate(III) and related anions [6]. In particular, it is the first property, viz. EPFL-BCH-LCOM Today the most widely studied class of ionic the fact that they do not evaporate, which CH–1015 Lausanne liquids is based on 1,3-dialkylimidazolium Tel: +41 21 693 98 54 makes them particularly attractive, due to Fax: +41 21 693 98 85 cations and many of these liquids have very the problem associated with volatile sol- E-Mail: [email protected] low melting points, some approaching –100 vents mentioned in the introduction. How- °C [7]. It is these 1,3-dialkylimidazolium ever, what really makes ionic liquids such aThis article is based on the Werner Prize Lecture given at the Fall Meeting of the Swiss Chemical cation based ionic liquids that are most fre- fascinating solvents for synthetic chemists Society in Zürich in 2004. quently encountered in synthetic and cata- is that there is no limit to the number of LAUREATES: AWARDS AND HONORS SCS FALL MEETING 2004 67 CHIMIA 2005, 59, No. 3 reactions conducted in ionic liquids, which in situ NMR spectroscopy and electrospray will ultimately lead to improved processes. ionisation mass spectrometry, it has been The reason for the lack of mechanistic possible to show that the active complex data is due, at least in part, to the new set in the catalytic hydrogenation of styrene is of problems associated with non-volatile a dihydrogen species with the dihydrogen solvents, which are largely alien to most undergoing heterolytic cleavage. The arene chemists. However, some simple solutions ligand may also undergo exchange, but it have been devised, for example, an elegant is not a prerequisite for catalysis, and the approach using entirely labelled substrates, arene does not undergo hydrogenation (al- as opposed to preparing labelled ionic liq- though at high pH arene hydrogenation is uids allow reactions in ionic liquids to be observed, but the active catalyst has been monitored in situ by NMR spectroscopy identified as nanoparticulate) [22]. Based on [17], and various mass spectrometric meth- our experiments the catalytic cycle shown ods can be used to analyse catalysts present in Scheme 1 was devised, and from bite an- in extremely low concentrations directly in gle effects arising from the use of different ionic liquids [18]. Clearly, there is consid- bisphosphine ligands, the rate determining erable interest in showing why a reaction step has been identified as dissociation of may be superior (or less effective) in an the chloride ligand. ionic liquid, in order to remove the guess- Subsequently, using NMR methods, the work from the field. solvation enthalpy of chloride in 1,3-di- It has been shown that some catalysed alkylimidazolium ionic liquids was quanti- reactions that operate in water are less ef- fied and found to be eight-fold lower than fective in ionic liquids, or do not even pro- in water [23]. Thus, catalysis does not take ceed in an ionic liquid [19], while other pa- place in these ionic liquid as loss of a chlo- pers indicate that addition of water to ionic ride ligand, a prerequisite for the ruthenium liquids improves activity [20]. Recently, we compound to enter the catalytic cycle, is dis- have proposed an explanation for these ob- favoured. Catalytic activity may be restored servations derived from the detailed mecha- by exchange of chloride by another ligand Fig. 1. Examples of cations used to provide low nistic analysis of a series of ruthenium(II) providing the impetus for future catalyst melting salts (ionic liquids) hydrogenation catalysts [21]. Based on an design, or by the addition of water, which investigation of catalysts of general formula solvates the chloride and explains why the [Ru(η6-p-cymene)(η2-P-P)Cl]+ (where P-P presence of water facilitates certain reac- is a bisphosphine) in aqueous solution, using tions (see above). In fact, ionic liquids can different liquids that can be made, and as our fundamental understanding of them in- creases, it should be possible to design specific ionic liquids for specific reactions and processes. A relatively detailed list of the physical properties of ionic liquids relevant to their applications in synthesis and catalysis may be found elsewhere [15]. Catalysed Reactions in Ionic Liquids A large and diverse range of chemical reactions, in particular, those catalysed by metal compounds, have been conducted in ionic liquids, and several useful features have been documented. It would appear that the non-nucleophilic environment pro- vided by many ionic liquids is conducive to stabilising the lifetime of sensitive ho- mogeneous catalysts, leading to increased yields and enantioselectivities. It has also been found that ionic liquid or ionic liquid– catalyst systems can be recycled and reused a large number of times. Products can often be extracted from ionic liquids directly, although a co-solvent may be necessary, and the amount of ionic liquid and catalyst present in the product is usually low (a particularly important parameter in the manufacture of pharmaceuticals [16]). Despite these significant benefits, very few studies Scheme 1. Proposed mechanism of styrene hydrogenation using [Ru(η6-p- have addressed in detail the mechanisms of cymene)(η2-P-P)Cl]+; P-P is a bisphosphine ligand. LAUREATES: AWARDS AND HONORS SCS FALL MEETING 2004 68 CHIMIA 2005, 59, No. 3 be used in combination with water for the approach involves derivatising the com- group is protonated, the catalyst becomes hydrogenation of water soluble substrates pound with an imidazolium group at a pe- more water soluble [36]. Such an effect has which not only eliminates organic solvents, ripheral site, which provides the necessary even been exploited in a biological context but also provides excellent catalyst recy- solubility properties, but does not interfere involving compounds related to those de- cling [24].

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