Hydrophobicity ionic liquids

Without special drying procedures and completely inert handling, water is omnipresent in ionic liquids. Even the apparently hydrophobic ionic liquid [BMIM][(CF3S02)2N] saturates with about 1.4 mass% of water [15], a significant molar amount. For more hydrophilic ionic liquids, water uptake from air can be much greater. Imidazolium halide salts in particular are laiown to be extremely hygroscopic, one of the reasons why it is so difficult to make completely proton-free chloroaluminate ionic liquids. 

In the same area, Moreau et al. have developed new hydrophobic ionic liquids containing chiral camphorsulfonamide units and showed that they could be used as ligands for the Ti-catalysed addition of ZnEt2 to benzaldehyde. The ionic... 

Thiolates, generated in situ by the action of ammonium tetra-thiomolybdate on alkyl halides, thiocyanates, and disulfides, undergo conjugate addition to a, (1-unsaturatcd esters, nitriles, and ketones in water under neutral conditions (Eq. 10. II).29 Conjugate addition of thiols was also carried out in a hydrophobic ionic liquid [bmim]PF6/water-solvent system (2 1) in the absence of any acid catalyst to afford the corresponding Michael adducts in high to quantitative yields with excellent 1,4-selectivity under mild and neutral conditions (Eq. 10.12). The use of ionic liquids helps to avoid the use of either acid or base catalysts... 

Hydrophobic ionic liquids, 26 850, 860-861 Hydrophobicity. See also Superhydrophobicity of silicone fluids, 22 578 of silicones, 22 603 solvent, 20 517 of surfaces, 22 111—112 Hydrophobic materials, encapsulation of, 11 545-546... 

According to their miscibility with water, ionic liquids are also frequently classified as hydrophilic or hydrophobic. The hydrophilic ionic liquids are typically salts composed of halide, acetate, nitrate, trifluoroacetate, and, in some cases, tetrafluoroborate anions, in particular their salts with [AMIM] having short alkyl chains, as these ionic liquids are totally miscible with water. The ionic liquids composed of PF and (CF3S02)2N with [AMIM] are immiscible with water in bulk, and are therefore referred to as hydrophobic ionic liquids. The ionic liquids consisting of BF4 and CFsSOi" ions with [AMIM] can be totally miscible or immiscible depending on the substituents on the cation, and they are therefore sometimes called tunable ionic liquids (22). A recent review covers relevant properties of some ionic liquids for catalysis (42,43). 

As a strong polar proton donor, water is strongly absorbed in most ionic liquids. In the extreme case, it is fully miscible with ionic liquids, such as [BMIM]Bp4 and [BMIMJCFsCOj. Even the hydrophobic ionic liquids can pick up a low concentration of water, resulting in a reduced absorption affinity for some organic compounds (27). The presence of water may, therefore, influence the performance of an ionic liquid in catalytic applications. 

The solubility of a Lewis acidic salt in an ionic liquid is dependent on the nature of both the salt and the ionic liquid. Hydrophobic ionic liquids have lower solubilities for such salts. For example, metal triflates such as Sc(TfO)3 readily dissolve... 

BMIM]BF4 was applied to a Suzuki reaction. The active catalyst was a trico-ordinated [Pd(PPh3)2(Ar)][X] complex that formed after oxidative addition of aryl halide to [Pd(0)(PPh3)4] 211). The hydrophobic ionic liquid does not compete with the unsaturated organic substrate for the electrophilic active metal center. 

In general, hydrophobic ionic liquids, [BMIM]Tf2N, [BMIMJPFe, and [OMIM]PFg, provide environments that are less disruptive of the structures of biocatalysts than molecular solvents of similar polarity, methanol or DMSO (220). 

Biphasic systems consisting of ionic liquids and supercritical CO2 showed dramatic enhancement in the operational stability of both free and immobilized Candida antarctica lipase B (CALB) in the catalyzed kinetic resolution of rac- -phenylethanol with vinyl propionate at 10 MPa and temperatures between 120 and 150°C (Scheme 30) 275). Hydrophobic ionic liquids, [EMIM]Tf2N or [BMIM]Tf2N, were shown to be essential for the stability of the enzyme in the biotransformation. Notwithstanding the extreme conditions, both the free and isolated enzymes were able specifically to catalyze the synthesis of (J )-l-phenylethyl propionate. The maximum enantiomeric excess needed for satisfactory product purity (ee >99.9%) was maintained. The (S)-l-phenylethanol reactant was not esterified. The authors suggested that the ionic liquids provide protection against enzyme denaturation by CO2 and heat. When the free enzyme was used, [EMIM]Tf2N appeared to be the best ionic liquid to protect the enzyme, which... 

The hydrophobic ionic liquid [BMIMJPFg has been consistently shown to provide the desired conformational flexibility to enzymes without drastically altering their catalytically active conformations. Therefore, this ionic liquid has been widely studied as a suitable medium for enzymatic transformations. 

The presence of small amounts of water was found to be essential even for hydrophobic ionic liquids (284). When a-chymotrypsin (in the form of salt-free lyophilized powder) was applied for the transesterification of Ai-acetyl-L-phenyl-alanine ethyl ester with 1-propanol in the dry ionic liquids [BMIM]PFg and [OMIMJPFg, little enzymatic activity was observed. The maximum activity was observed when 0.5 vol% water was added to the ionic liquids. Supercritical CO2 was also sufficient to activate the enzyme in dry ionic liquids. The addition of water to the supercritical C02-ionic liquid system further enhanced the enzymatic activity. 

Visser, A.E., Holbrey, J.D., Rogers, R.D., Hydrophobic ionic liquids incorporating N-alkylisoquinolinium cations and their utilization in liquid-liquid separations, Chem. Commun., 2484-2485,2001. 

Ionic liquids are a class of solvents and they are the subject of keen research interest in chemistry (Freemantle, 1998). Hydrophobic ionic liquids with low melting points (from -30°C to ambient temperature) have been synthesized and investigated, based on 1,3-dialkyl imidazolium cations and hydrophobic anions. Other imidazolium molten salts with hydrophilic anions and thus water-soluble are also of interest. NMR and elemental analysis have characterized the molten salts. Their density, melting point, viscosity, conductivity, refractive index, electrochemical window, thermal stability, and miscibility with water and organic solvents were determined. The influence of the alkyl substituents in 1,2, 3, and 4(5)-positions on the imidazolium cation on these properties has been scrutinized. Viscosities as low as 35 cP (for l-ethyl-3-methylimi-dazolium bis((trifluoromethyl)sulfonyl)amide (bis(triflyl)amide) and trifluoroacetate) and conductivities as high as 9.6 mS/cm were obtained. Photophysical probe studies were carried out to establish more precisely the solvent properties of l-ethyl-3-methyl-imidazolium bis((trifluoromethyl)sulfonyl)amide. The hydrophobic molten salts are promising solvents for electrochemical, photovoltaic, and synthetic applications (Bon-hote et al., 1996). 

Alternatively, the enzyme can be modified such that it dissolves in a hydrophobic ionic liquid with retention of activity. This approach was demonstrated with cyt c, which, when covalently modified with polyethylene glycol (PEG), dissolved in [EMIm][ Tf2N] with retention of activity. The best results were obtained when the molecular weight of the polymer chain was >2000 [88]. Similarly, a copolymer of PEG and maleic anhydride solubilized subtilisin in [EMIm][NTf2] and a range of similar ionic liquids with good retention of activity and operational stability [89, 90]. 

We conclude that hydrophobic ionic liquids are promising and attractive replacements for molecular solvents in whole-cell biotransformations. 

They are immiscible with a number of organic solvents and provide a non-aqueous, polar alternative for two-phase systems. Hydrophobic ionic liquids can also be used as immiscible polar phases with water. 

Recently [111], the ET was probed at the interface between water and a hydrophobic ionic liquid (EL), l-octyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide. Ferrocene was dissolved in an ionic liquid, and ferrocyanide—in the aqueous phase. The tip was immersed in the aqueous phase. Ferricyanide, electrogenerated at the tip, diffused toward the interface where it was reduced by ferrocene... 

Appetecchi, G., Scaccia, S., Tizzani, C., Alessandrini, F., and Passerini, S. (2006) Synthesis of Hydrophobic Ionic Liquids for Electrochemical Applications./. Electrochem. Soc., 153 (9), A1685-A1691. 

Carbonyl compounds as well as their 0,0-acetals undergo thioacetalisation and ram-thioacetalisation in ionic liquids with scandium(III) triflate as catalyst under facile reaction conditions.1681 Both hydrophilic and hydrophobic ionic liquids give good results and the catalyst could be recycled at least three times without change in activity after extraction of the product with diethyl ether. Higher reaction rates were observed relative to the reaction in dichloromethane. The procedure is highly chemoselective in that in the presence of both an aldehyde and a ketone only the aldehyde reacts to form the corresponding thioacetal. 

Zhou Z B, Matsumoto H, Tatsumi K. Low-melting, low-viscous, hydrophobic ionic liquids l-alkyl(alkyl ether)-3-methyhmidazolium perfluoroalkyltrifluoroborate. Chem. Eur. J. 2004. 10, 6581-6591. 

Zhou Z B, Takeda M, Ue M. New hydrophobic ionic liquids based on perfluoroalkyltrifluoroborate anions. J. Fluorine Chem. 2004. 125, 471-476. 

Mikkola J P, Virtanen P, Sjohohn R. Aliquat 336—a versatile and affordable cation source for an entirely new family of hydrophobic ionic liquids. Green Chem. 2006. 8, 250-255. 

Finally, it has to be noted that hydrophobic ionic liquids can take up significant amounts of water, even though they will not mix with water. As a result, the absence, presence, and amount of water in an IL may not be neglected. While in some cases it is sufficient to determine the fraction of water in the IL, other cases are more critical. For example, water has been shown to reduce dramatically the emission yield from lanthanides dissolved in ILs (see also the respective chapter by Mudring, in this volume) [142, 148, 150-153],... 

Synthesis of functionalized hydrophobic ionic liquids bearing the 2-hydroxyben-zylamine substructure, and their application in partitioning of metal ions from water have been described (Scheme 15) [94],... 

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