1,3-dialkylimidazolium cation

This chapter will concentrate on the preparation of ionic liquids based on 1,3-dialkylimidazolium cations, as these have dominated the area over the last twenty... 

The physical properties of ionic liquids have been extensively studied and some trends are beginning to emerge. In particular, ionic liquids based on 1,3-dialkylimidazolium cations have been investigated in detail, partly due the their wide use as solvents to conduct synthesis and catalysis. The attraction of the imidazolium cation in synthetic applications is because the two substituent groups can be varied to modify the properties of the solvent. For example, Table 4.1... 

The D/H exchange occurred mainly after complete consumption of the alkene, and no D-incorporated alkane was detected, which indicated that the coordinated NHC was easily displaced by the alkene and that these carbenes were less strongly bounded to the metal surface than was seen with mononuclear metal compounds [28]. These results strongly suggested that the imidazolium cations reacted with the nanoparticle surface preferentially as aggregates of the type [(DAI) (X) ] [(DAI) e (X)J" (where DAI is the 1,3-dialkylimidazolium cation and X the anion), rather than as isolated imidazolium cations. 

Heretofore, ionic liquids incorporating the 1,3-dialkylimidazolium cation have been preferred as they interact weakly with the anions and are more thermally stable than the quaternary ammonium cations. Recently, the physical properties of 1,2,3,4-tetraalkylimidazolium- and 1,3-dialkylimidazolium-containing ionic liquids in combination with various hydrophobic and hydrophilic anions have been systematically investigated (36,41). The melting point, thermal stability, density, viscosity, and other physical properties have been correlated with alkyl chain length of the imidazolium cation and the nature of the anion. The anion mainly determines water miscibility and has the most dramatic effect on the properties. An increase in the alkyl chain length of the cations from butyl to octyl, for example, increases the hydrophobicity and viscosity of the ionic liquid, whereas densities and surface tension values decrease, as expected. 

Significant developments were achieved with the discovery in the 1970s and 1980s of varied room-temperature ionic liquids.41,42 These were organoaluminate ionic liquids, typically a mixture of quaternary ammonium salts with aluminum chloride. A major breakthrough came in 1992 by the discovery of air- and moisture-stable ionic liquids.43 1,3-Dialkylimidazolium cations (1), specifically,... 

Stereochemical and kinetic studies have confirmed the enhancement of the hydrogen bond ability of the imidazolium cation on going from l,2-dimethyl-3-alkylimidazolium salts to 1,3-dialkylimidazolium cations. However, deprotonation of the site between nitrogens is not particularly simple, it requires strong bases and depends on the ionic liquid counter-anion (Scheme 4.1). ... 

Therefore, a remarkable reactivity of the C2-H group versus reducing agents, in addition to the one versus bases, must be kept in mind 1,3-dialkylimidazolium cations may be reduced to N-heterocyclic carbenes and dihydrogen. The reduction can be performed by chanical as well as by electrochanical (i.e. by the innocent... 

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). 

The preparation of these salts is usually performed by a simple N-alkylation of 1-methylimidazole generating a l-alkyl-3-methylimidazolium cation followed by anion metathesis [27]. The synthesis can also start with imidazole, which is consecutively alkylated with alkyl hahdes (Scheme 3.5-9). The 1,3-dialkylimidazolium cations will be abbreviated throughout this section as [C Cylm], where Im stands for imidazolium and x and y are the number of the carbons of the alkyl chains. 

As can be easily observed from the data presented in Table 3.5-3 the I Ls based on 1,3-dialkylimidazolium cations possess relatively low viscosity (as low as 26 mPa s) and high density (usually between 1.2 and 1.5 g cm ). They are liquid over a large range of temperatures (down to -80 C) have a high thermal stability (starting to decompose at temperatures over 400°C) and display a large electrochemical window (up to 7 V). 

Fig. 3.5-6 A three-dimensional simplified schematic view of the arrangements of 1,3-dialkylimidazolium cations showing the channels in which the spherical anions are...

This structural pattern is even maintained in the gas phase and evidence for the clustering - supramolecules formed through association of the 1,3-dialkylimidazolium cations with the anions - has been obtained by mass spectrometry expert-... 

This chapter will concentrate on the preparation of ionic liquids based on 1,3-dialkylimidazolium cations, as these have dominated the area over the last twenty years. The techniques discussed in this chapter are, however, generally applicable to the other classes of cations indicated in Fig. 2.1-1. The original decision by Wilkes et al. to prepare l-alkyl-3-methylimidazolium ([RMIM]+) salts was prompted by the requirement for a cation that had more negative reduction potential than Al(iii) [6]. The discovery that the imidazolium-based salts also generally displayed lower melting points than the 1-alkylpyridinium salts used prior to this cemented their position as the cations of choice since this time. Indeed, the method reported by... 

Fig. 5.M Ability ofthe 1,3-dialkylimidazolium cation to hydrogen bond methyl acrylate in the course of its reaction with cyclopentadiene.

Scheme 5.3-2 Different routes for an in situ ligand formation from the ionic liquid s 1,3-dialkylimidazolium cation.

Finally, a third way of ligand formation from a 1,3-dialkylimidazolium cation has been described by Dupont and coworkers [57]. They investigated the hy-drodimerization/telomerization of 1,3-butadiene with paUadium(ii) compounds in [BMIM][Bp4] and described the activation of the catalyst precursor complex [BMIM]2[PdCU] via a palladium(IV) compound, which is formed by oxidative addition of the imidazolium with cleavage of the C-N bond of the [BMIM]+ ion leading to dichloro-bis(methylimidazole)-palladium (Scheme 5.3-5). However, this reaction was only observed in the presence of water. 

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... 

It is crucial to find an appropriate combination of a catalyst and an ionic liquid which has to be harmonized with the hydrophilicity/hydrophobicity of the product. For example, hydrosilylation reactions in the presence of ionic liquids based rat 1,3-dialkylimidazolium cations did not give the desired polyethersiloxanes, as the 2-H position of the cation was too reactive. The authors assumed that formation of NHC (Af-heterocyclic carbenes) which coordinates to the Pt center is responsible of the catalyst deactivation. 

They have discovered that the use of 1,3-dialkylimidazolium cations led the reactions less efficient than when conventional solvents were used (it may be due to carbene formation [59]). In comparison, tetraalkylphosphonium ionic liquids led the reaction much more efficiently than organic solvents, or with neat reagents. Based on these observations, they have developed the following reaction and commercialized it (Eq. 20.5). 

In comparison with the oftentimes preferred imidazolium cations, the choice of a quartemary ammonium as cation is advisable for some applications the use of ILs with 1,3-dialkylimidazolium cation in high-energy electrochemical devices, for example, 4 V lithium batteries, is not possible because of the electrochemical instability of the cation. ILs containing quaternary ammonium ions are more resistant against reduction and oxidation, and the ILs with electrochemically stable anions display much wider electrochanical windows (Zhou et al., 2005). 

This study, and a H NMR study conducted in parallel on single ionic liquids with ethanol as cellulose model, clearly demonstrated that as halide anions exhibit strong and preferential interactions with the H2 of the 1,3-dialkylimidazolium cation rather than with the cellulose (or ethanol) hydroxyl group, the cellulose-dissolving anion... 

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