Imidazohum-based ILs

Methylation at the 2-position of the imidazolium ring also results in an increase in the stability of imidazoHum-based ILs [13], This is probably best explained by the addition of an electron-donating substituent leading to a reduction in the electro-philicity of the a-carbons of the alkyl chains. 

Carter [9] and AlNashef [10] research works were focused on imidazohum-based ILs with similar structures (e.g. l-n-aIkyl-3-methylimidazolium), differing only in the length of the alkyl chain at the 1-position and the nature of the anion (Cl vs. PFg ), however, only the latest study showed reversibility of the 02/02 redox couple. Therefore, these contradictory results indicated that a deeper knowledge of the influences of the cations and anions of the imidazolium-based ILs on the oxygen reduction reaction was stiU needed. 

Konig et al. [274] investigated the long-term stability of two selected ILs over several months under process-hke conditions with a subsequent IC-MS analysis to identify the resulting decomposition products. As an example, samples of the imidazohum-based ILs l-ethyl-3-methylimidazolium chloride (EMIM Cl) and 1-ethyl-3-methyhmidazohum acetate (EMIM Ac) were thermally and catalytically stressed. The most obvious decomposition of a dialk)dimidazolium cation is the reverse reaction of the quaternization reaction. The typical products are imidazole derivatives and alkylated anions (reaction (1) in Figure 8.120). Further considerable decomposition products are cations with scrambled alkyl chains. These cations can be formed either due to a realkylation of a previously dealkylated... 

Imidazohum-based ILs were used as nonconventional media in alcohol dehydrogenase (ADH)-catalyzed reactions in enzymatic catalysis. When containing up to 50% of the IL, the overall conversion could be improved in some cases, while the stereoselectivity of the enzyme remained unaltered. 

In order to make ILs a more effective lubricant for steel/aluminum friction pair especially at higher loads to reduce corrosion and wear Al matrix, a series of phosphate ester functional group-containing ILs was designed and synthesized. Mu et al. [84-86] synthesized a new series of imidazohum cation-based ILs with 0,0-diethylphosphonyl groups on the alkyl side chain (Fig. 9.12), evaluated the tribological properties of the ILs, and discussed possible mechanisms of action. 

Welton and coworkers used NMR spectroscopy to measure hydrogen solubility in a wide variety of ILs, the majority of which are imidazohum-based cations paired with anions such as [PFe], [BF4]", and Tf2N] [16,59]. Berger et al. [59] reported on the conversion and enantioselectivity of a hydrogenation reaction, as well as kinetic resolution by catalysts immobilized in the ILs. For these reactions, the H2 concentration in the ionic phase, rather than pressure in the gas phase, was the important kinetic parameter because the reaction occurred in the IL. For the ILs investigated in both studies, Henry s constants ranged from 0.7-7.2 x 10 bar. 

If ILs are to be used in metal-catalyzed reactions, imidazoHum-based salts may be critical due to the possible formation and involvement of heterocyclic imidazo-lylidene carbenes [Eqs. (2)-(4)]. The direct formation of carbene-metal complexes from imidazolium ILs has already been demonstrated for palladium-catalyzed C-C reactions [40, 41]. Different pathways for the formation of metal carbenes from imidazolium salts are possible either by direct oxidative addition of imidazolium to the metal center in a low oxidative state [Eq. (2)] or by deprotonation of the imidazolium cation in presence of a base [Eq. (3)]. It is worth mentioning here that deprotonation can also occur on the 4-position of the imidazolium [Eq. (4)]. The in-situ formation of a metal carbene can have a beneficial effect on catalytic performances in stabilizing the metal-catalyst complex (it can avoid formation of palladium black, for example). However, given the remarkable stability of this imidazolylidene-metal bond with respect to dissociation, the formation of such a complex may also lead to deactivation of the catalyst This is probably what happens in the telomerization of butadiene with methanol catalyzed by palladium-phosphine complexes in [BMIMj-based ILs [42]. The substitution of the acidic hydrogen in the 2-position of the imidazolium by a methyl group or the use of pyridinium-based salts makes it possible to overcome this problem. Phosphonium-based ILs can also bring advantages in this case. 

In subsequent work, Blanchard et al. [5] carried out a systematic study on the phase behavior of CO2 with a number of imidazoHum, and pyridinium-based ILs. The solubility of CO2 in [BM IM][PFg] was found to increase with increasing pressure reaching 0.72 mole fraction CO2 at 40 °C and 9.3 MPa, as illustrated in Figure 1. Conversely, the solubility of the IL in CO2 was determined to be less than 5 10 mole fraction. Perhaps the best indication of solubility within SCCO2 is vapor pressure the more volatile a solute, the more soluble it will be. The absence of IL dissolution within the gas-rich phase therefore reflects the vanishingly small vapor pressure of these materials and the inability of CO2 to solvate charge-separated salts. 

Recently, interest in morpholinium-based ILs has been increased becanse of their attractive physicochemical and thermal properties and large electrochemical windows. In fact, they are less toxic than the commonly used imidazohum-, pyridin-ium-, or tetraalkylammonium-based ILs [137]. In particular, n.s based on the morpholinium cation are preferred because of their electrochemical stability [138]. However, little data on the (eco)toxicity of morpholinium-based ILs are available. The functionalized morpholinium compounds tested so far have shown low acute... 

A new approach in the use of organic solvents for preparation of SLMs is the apphcation of ionic liquids. ILs are compounds composed of two parts cationic organic moieties and an anionic, organic, or inorganic part. The cationic parts of most ionic liquids are organic based, including imidazohum,... 

The low solubility of most organometaUic catalysts in COj means that in ideal cases no modification catalyst is required, provided it is suffidenfly soluble in the IL. This was foimd to be the case for the hydrovinylation system which employed a nickel-based chiral catalyst first reported by Wilke and co-workers [23], In the case of the hydroformylation system, however, the activity was foimd to be highest when using [Rh(acac)(CO)2] combined with a sulfonated triphenylphosphine analogue that had an imidazoHum cation as its counterion. In practice it is likely that neutral catalysts may require charged ligands, while those which are themselves charged can be used unmodified. 

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