Alkyl Imidazolium-based Ionic Liquids

S Alkyl Imidazolium-based Ionic Liquids Isolated Imidazolium Cations 

Studies of the isolated imidazolium cation have been undertaken primarily to produce parameters for use in force field methods. Shah et al., have calculated the [BMIM]+ cation at the HF/6-31G(d) level, and determined CHdpG [57] charges [58]. De Andrade et al., have calculated the [EMIM]+ and [BMIM] cations at the HF/6-31G(d) level, and determined the RESP charges using AMBER [59]. They also compared the ab initio dipole moments and vibrational frequencies with those obtained using force field methods [60]. 

The vibrational spectrum of [BM1M]+ has been determined at the HF/6-31+G(d) level [62], and at the B3LYP and MP2 levels and examined for evidence of hydrogen bonding with a Cl anion [17]. 

Recently the [RMIM]X (X = Q, Br, 1 and R = methyl, ethyl, propyl, butyl) ionic liquids have been investigated in some detail. A crucial problem when studying loosely bound ion pairs is the large number of possible conformations. Two key structural features are dominant, they involve the position of the hahde around the imidazolium ring, Fig. 4.2-3(a), and rotation of the alkyl chain. Fig. 4.2-3(b). In both cases 

Interaction energies have been computed for a range of conformers, (interaction) = (ion-pair) - [ (cation) -h (anion)j. Chaumont et al, have computed gas-phase interaction energies using the AMBER [59] force field [44]. For [BMIM]C1 with the Cl hydrogen bonding to the C -H (with unspecified butyl orientation) or -363.59 and -353.13 kj mol have been obtained, respectively 

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Local structure formation in alkyl-imidazolium-based ionic liquids as revealed by linear and nonlinear Raman spectroscopy 07ACR1174. 

Iwata, K., Okajima, H., Saha, S. and Hamaguchi, H.-O., Local structure formation in alkyl-imidazolium-based ionic liquids as revealed by linear and nonlinear Raman spectroscopy, Acc. Chem. Res. 40, 1174-1181 (2007). 

Yanes, E.G. et al.. Capillary electrophoretic application of l-alkyl-3-methyl-imidazolium-based ionic liquids. Anal. Chem., 73, 3838, 2001. 

He, L., Zhang, W., Zhao, L., Liu, X., and Jiang, S., Effect of l-alkyl-3-methyl-imidazolium-based ionic liquids as the eluent on the separation of ephedrines by liquid chromatography, J. Chromatogr. A, 1007, 39-45, 2003. 

Blocking the C2 position with alkyl groups may afford C - H oxidative additions of the imidazolium salts yielding abnormal carbenes. This strategy was followed in the reaction a Pt(0) complex with C2-methylated imidazolium salts, which provided the oxidative addition of the C4,5 - H bond, as shown in Scheme 42 [158]. This behavior provides evidence that the substitution of imidazolium-based ionic liquids at the C2 may not be enough to prevent their involvement in reactions for which they are solvents. 

Fig. 3 Schematic representation of the transferability premise developed as a built-in characteristic of the CLAP force-field, a cl, c2, c3, and c4 stand for imidazolium- pyridinium-, phosphonium- and guanidinium-based cations, al to a5 stand for the hexafluorophosphate, chloride, nitrate, bis-(trifluoromethanesulfonil)imide, and triflate anions. The c3-a4 combination was selected in this case, b The possibility to grow an alkyl side chain in, for example, imidazolium-based ionic liquids in order to study an entire homologous series of ionic liquids is illustrated...

At 100 °C no product was ever observed. At 150 °C the imidazolium-based ionic liquids could be prepared. For ionic liquids bearing pyridinium cations a reaction temperature of 200 °C was required. The reactions could be scaled up to 50 g and no large excess of alkyl halide was needed. Deetlefs and Seddon studied the synthesis of pyrazolium, thiazolium, imidazolium, and pyridine-based ionic liquids [21]. Selected results are summarized in Table 7.1. The authors reported that the reactions are up to 72 times faster than when using conventional heating. They also found that if the microwave irradiation is prolonged, decomposition of the ionic liquid occurs. Imidazolium halide based ionic liquids could be prepared in 150,... 

Early quantum chemical calculations on ionic liquids were focused towards the haloaluminate, and related metal- (Au and Fe) containing melts, these are examined in the foUovying subsection. As the field has developed, this focus has shifted towards imidazolium-based ionic liquids because of their lower melting points and more fevorable physical properties. Imidazolium-based ionic liquids are discussed in the third subsection which examines imidazolium cations with small alkyl chains (methyl, ethyl and butyl). The ionic liquids which can be formed from imidazolium cations and small anions such as halides or [PFe]" are then discussed, mention is also made of calculations carried out on a few more diverse systems. The electronic structure of the imidazolium-based ionic liquids is the focus of the fourth and final subsection. 

Interestingly these complexes showed high activity without addition of alkyl aluminum compounds in the ionic liquid while they are almost inactive in toluene. These results are interpretable in terms of catalyst stabilization by the imidazolium-based ionic liquid. Reductive elimination of imidazolium is also possible as in toluene as in the ionic liquid but in the ionic liquid, a rapid reoxidation via addition of the solvent imidazolium cation seems possible and may prevent the formation of Ni deposits associated with catalyst deactivation. The carbene complex with R = n-Bu showed the highest activity with a dimer yield of 70.2% (TOF = 7020 h ). The preferred product of the nickel-catalyzed reaction is methylpentene. Additional phosphine ligand had no significant influence on the distribution of the products in this case. 

In most cases ionic liquids show remarkably high thermal stabihties of >200 °C. A decomposition pathway of the usually very stable imidazolium-based ionic liquid is the back alkylation of the anion (Scheme 9-10). 

The physical properties of ionic liquids play an important role on the efficiency of phase separations. In ionic liquid-based extractions, the viscosity of an ionic liquid affects the mass transfer efficiency. For imidazolium-based ionic liquids (Cnmim), the viscosity increases by increasing the alkyl chain length due to the increased van der Waals interactions, while the choice of the anion also affects the viscosity as it... 

The densities of ionic liquids are also affected by the choice of the cation and anion. For imidazolium ionic liquids the density decreases slightly as the alkyl chain of the cation increases in length. In addition, the absoption of water by ionic liquids by increasing nitric acid concentration can also cause a decrease in the density (Giridhar et al. 2004). Regarding imidazolium-based ionic liquids, by increasing the alkyl chain it was found that the surface tension decreased due to the orientation of hydrocarbon tails on the surface (Kilaru et al. 2007). The vapour pressure of an ionic liquid is usually unmeasurable at room temperature. Studies on the thermal stability of ionic liquids showed that the type of the associated anion has the primary effect on the thermal stability of the ionic liquids (Holbrey and Seddon... 

Figure 5 Separation of poly(phenols) using (A) 1-ethyl-3-methyl-imidazolium tetrafluoroborate, (B) 1-butyl-3-methylimidazolium tetrafluoroborate, and (C) 1-ethyl-3-methylimidazolium hexafluoro-phosphate. The electrolyte solution was a 150mmoll concentration of the ionic liquid in water. The fused-silica capillary column was 50cm x 50 m ID, operating voltage 16kV, and absorbance detection at the anode end at 240nm. (Reprinted with permission from Yanes EG, Gratz SR, Baldwin MJ, Robison SE, and Stalcup AM (2001) Capillary electrophoretic application of 1-alkyl-3-methyl-imidazolium-based ionic liquids. Analytical Chemistry 73 3838-3844 American Chemical Society.)...

Common UCST behaviour has also been observed for mixtures consisting of two ionic liquids with the same anion and a different cation.Less common lower eritical solution temperature (LCST) behaviour has been observed for binary ionic liquid mixtures with benzene,chloroalkanes " and polymers. Interestingly, the LCST behaviour of benzene with an imidazo-lium-based ionic liquid changed via hour-glass to UCST behaviour when the imidazolium alkyl chain length was increased.Moreover, imidazolium-based ionic liquids were found to form clathrate structures with benzene at low temperatures, whereas the solid-liquid equilibria (SLE) of other organics or water with ionic liquids are simple eutectic systems. 

Mandai, T., Masu, H., Imanari, M. and Nishikawa, K., Comparison between cycloalkyl-and n-alkyl-substituted imidazolium-based ionic liquids in physicochemical properties and reorientational dynamics, J. Phys. Chem. B 116 (7), 2059-2064 (2012). 

Ultrasonic irradiation was apphed to suspensions containing granular carbon and activated zero-valent iron, and eight different imidazolium-based ionic liquids. For example, about 90% of l-alkyl-3-methyIimidazolium bromide was degraded in about 110 minutes. Degradation produets were also identifred such as l-alkyl-3-methyl-2,4,5,-triox-... 

Stoppa A, Zech O, Kunz W, Buchner R (2010) The conductivity of imidazolium-based ionic liquids from (—35° to 195)C. A. Variation of cation s alkyl chain. J Chem Eng Data 55 1768-1773... 

This study describes the utilization of the alkyl substituted imidazolium based ionic liquid as a stmctuie directing agent of the MFI type zeolite stracture in water rich environment. The resulted zeolite like stmeture presents some similarities with the parent MFI stracture, but grows preferentially in 1 D leading to zeohte like nanofibers. 

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