1,3-dialkylimidazolium ILs

Preparation and Some Physico-Chemical Properties of 1,3-Dialkylimidazolium ILs... 

Multinuclear NMR spectroscopy experiments of various 1,3-dialkylimidazolium ILs dissolved in organic solvents have also pointed to the formation of floating aggregates through hydrogen bonds [81-84]. In particular, it has been demonstrated by heteronuclear NMR experiments on [C4CjIm]BF4 that contact ion pairs exist in the presence of small amounts of water and even in dimethyl sulfoxide (DMSO) solution [85]. 

All 1,3-dialkylimidazolium ILs reported to date are hygroscopic, and their miscibility with water is largely controlled by the nature of the anion. While those salts containing the nitrate, chloride and perchlorate anions are usually miscible with water in all proportions, those associated with hexafluorophosphate and bis(tri-fluoromethane) sulfonylamidate anions are almost completely immiscible with water [93]. Interestingly, the miscibility with water of those containing the tetraf lu-oroborate anion is temperature dependent (Fig. 3.5-7) [36]. It is also known that an increase of the N-alkyl chain lengths increases the hydrophobicity for a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ILs [94] The miscibility of water with ILs can be increased by the addition of short-chain alcohols [95] or diminished by the addihon of salts (salting-out effect) [96]. 

Inasmuch as most of the 1,3-dialkylimidazolium ILs have extremely low vapor pressure and relatively high viscosity at room temperature, in situ TEM observations in dispersed ILs can be carried out. Indeed, this method was applied for size and shape in situ analysis of various transition-metal NPs dispersed in the ILs (see Table 6.1). The size and shape determined by this in situ technique are the same as those obtained with NPs mixed with an epoxy resin distributed between two silicon wafer pieces and dried at 50 °C. In these cases the samples were pre-thinned mechanically to a thickness of about 20 nm and then ion milled to an electron transparency using 3kV Ar ion beams. 

In this review, the use of N-heterocyclic carbene complexes derived from imidazoHum salts as a part of the different strategies involving ionic liquids has not been considered. Nevertheless, it can be important to take into account this reactivity, especially when 1,3-dialkylimidazolium ILs are involved. For more details about this aspect and applications in hydroformylation, one can refer to Herrmanrfs recent review [55] and the later publication of Poyatos et al. in 2003 [56]. 

Alternatively, halide-free 1,3-dialkylimidazolium ILs can be prepared from the five-component reaction (glyoxal, formaldehyde, two different amines and acids. Scheme 2), and those containing alkyl sulfate or trifluoromethane sulfonate anions by simple alkylation of 1-alkylimidazole with the corresponding dialkylsulfate or alkyltrifluoro-methane sulfonate ester, respectively. " ... 

ILs are usually entirely innocent and non-coordinating solvents, but in some cases innocuous behavior is not always observed, in particular those based on the imidazolium cation. Although most of the 1,3-dialkylimidazolium ILs are stable toward organic and inorganic substances, under determined reaction conditions, both the cation and anion... 

The Heck reaction performed in ILs has various advantages compared with all previously described molecular solvents, such as catalytic efficiency for the vinylation of chloroarenes, improved thermal catalyst stability and lifetime during the reaction. In the reactions performed in 1,3-dialkylimidazolium ILs, jV-heterocyclic carbene complexes of palladium can be formed in situ. These palladium-carbene complexes are formed from the deprotonation of the imidazolium cation in the presence of the catalyst precursor. ... 

As observed in Heck reactions, in the reactions performed in 1,3-dialkylimidazolium ILs, the formation of carbenes that may be involved in the stabilization of catalytically active species is not uncommon. The in situ formation of a mixed phosphine-imidazolylidene palladium complex, in [G4GiIm]BF4, has been observed under conditions employed in many palladium-catalyzed coupling reactions (Scheme 40). ... 

It seemed interesting to use the ultrasonic chemical oxidative degradation method proposed for 1,3-dialkylimidazolium ILs with different anions (chloride, bromide,... 

Imidazolium-based ionic liquids (ILs) have been used extensively as media for the formation and stabilization of transition-metal nanoparticles [14—17]. These 1,3-dialkylimidazolium salts (Figure 15.3) possess very interesting properhes they have a very low vapor pressure, they are nonflammable, have high thermal and electrochemical stabilities, and display different solubilities in organic solvents [18-20]. 

Finally, X-ray diffraction has been used to examine the liquid structure of binary ionic liquids of 1,3-dialkylimidazolium fluoride with HF [173,174], The solid state and liquid structures are closely related as both contain [HF2] anions. In contrast, Shodai et al. reported that the structure of liquid [(CH3)4N]F HF (n = 3-5) has a range of anion structures of the form [(HF)jcFp (x = 1-3). In this case, structures with x = 4 or 5 were not found in the liquid phase although similar compositions have been found in the solid state [175], These studies show that even elements that are rarely studied in the IL context like P (other than in PF ( or phosphonium cations) or F could provide access to interesting and potentially useful ILs. 

The work from Sheldon s group [10] was the first to present the use of ionic liquids in the enzymatic synthesis of esters. Since then, there have been many reports on biosynthesis of esters in ionic liquids. De los Rios et al. [64,65] synthesised a wide range of aliphatic organic esters, commonly used in the perfumery, flavour and pharmaceutical industries, by transesteriflcation from vinyl esters and alcohols catalysed by free CaLB in different 1,3-dialkylimidazolium-based ILs (Fig. 7.2). They analysed the effects of the alkyl chain lengths of the acyl donor and the alcohol. The optimum (C6 for acyl donor and C4 for alcohol) chain lengths were found because the activity decreased with further increase in alkyl chain length. The authors attributed the enzyme behaviour to a substrate modulation mainly due to the different affinity of the lipase towards the different substrates and steric hindrance and denaturalisation by small alcohol molecules. 

With respect to ILs as cosolvents, the interest was centred on two types of ILs aUcylammonium and 1,3-dialkylimidazolium-based ionic liquids. For the first type, we selected ethylammonium nitrate, considering that it is a protic ionic liquid (PIL) and that it can act as a potential acid catalyst. With respect to the second type of ILs, we selected those based on l-methyl-3-n-butylimidazolium cation, characterised by a slighter HBD acidity than that of the alkylammonium type (Figs. 13.2, 13.3). These ILs exhibit a wide spectrum of physicochemical properties. It was demonstrated that their water content, density, viscosity, surface tension, melting point and thermal stability are affected by the length of the alkyl chain and the nature of the anion. Several anions were incorporated in this class of ILs. 

Fig. 3.5-4 Basic skeleton of the nitrogen-containing cations that can generate ILs N-alkylpyridinium 1,3-dialkylimidazolium 1,1 -...

Not surprisingly, these materials are very popular and enjoy a plethora of applications in various domains of the physical sciences, and an impressive number of spedahzed reviews and books has appeared dealing with their synthesis, physicochemical properties and appHcations in synthesis, catalysis and separation processes [12-26]. This section does not intend to be comprehensive on the vast area of synthesis and appHcations of ILs rather it will attempt to provide a critical update of the basic principles and latest developments on the structure and properties of ILs (mainly those based on the 1,3-dialkylimidazolium cation), and their... 

As already pointed out, of the various known ILs, those derived from the combination of the 1,3-dialkylimidazolium cation with various anions are the most popular and investigated class (Scheme 3.5-9). This is most probably due to their facility of synthesis stability and the possibility of fine-tuning their physico-chemical properties by the simple choice of the N-alkyl substituents and/or anions (Table 3.5-3). 

In some cases the anions of imidazolium ILs can easily undergo hydrolysis, particularly those containing AICI4 and PFg anions. In the case of the hexafluoro-phosphate anion, phosphate and HF are formed and 1,3-dialkylimidazolium phosphates and fluoride have been isolated during reactions [50] or purification proce-... 

Concerning cations, the most popular ILs contain a 1,3-dialkylimidazolium cation. However, liquids are also described in the literature in which a functional feature, such as (CH2hOR [20], CH2CH2SR [28], CH2CH2CN [29], CH2CH2C(0)CH3, or (CH2)3NHC0NHR, is incorporated instead of the alkyl chain on the nitrogen atom... 

Scheme 2 One-pot five-component synthesis of 1,3-dialkylimidazolium tetrafluoroborate ILs.

Keglevich G, Baan Z, Hermecz I, Novak T, Odinets IL (2007) The phosphorus aspects of green chemistry the use of quaternary phosphonium salts and 1,3 dialkylimidazolium hexa-fluorophosphates in organic synthesis. Curr Org Chem 11 107-126... 

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