Stabilization imidazolium ionic liquid

Using electrospray ionization mass spectrometry in both positive and negative ion modes, the on-line scanning of the Morita-Baylis-Hillman reaction in the presence of imidazolium ionic liquids has been investigated. The interception of several supramolecular species indicated that ionic liquids co-catalyse the reactions by activating the aldehyde toward nucleophilic enolate attack and by stabilizing the zwitterionic species that act as the main intermediates.175... 

Stabilization of the Metal Nanoparticles in Imidazolium Ionic Liquids... 

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... 

Fonseca GS, LFmpierre AP, Fichtner PE (2003) The use of imidazolium ionic liquids for the formation and stabilization of Ir° and Rh nanoparticles efficient catalysts for the hydrogenation of arenes. Chemistry 9 3263-3269... 

Thus, the goal of this chapter is to compile the main findings in the reduction of oxygen from a fundamental point of view, and its reaction mechanisms within ILs. The chapter will be divided according to families of ionic liquids depicted in Scheme 18.1 (e.g. imidazolium, pyrrolidinium, quaternary ammonium and phos-phonium) since differences in the physical properties, structure and stability of ionic liquids will affect the stability of the electrogenerated species of oxygen. 

Imidazolium ionic liquids are known to be a remarkable solvent for CO2 solubility ref It has been found that anion play the important role in determining the gas solubility in ionic liquids. It has been reported that the bistriflamide anion has the greatest affinity for CO2 experimentally compared to other anions, such as the halides, triflate ([OTf] ), hexafluorophosphate ([PFe] ), tetrafluoroborate ([BF4] ), dicyanamide ([N(CN)2] ) and ethanoate ([MeC02] ) Although fluorination effect are shown to have greatly enhanced the solubility of CO2, these class of ionic liquids are very expensive and are not environmental friendly, due its high stability and low reactivity... 

Dash, P. Scott, R. W. J. (2009). 1-MethyIimidazoIe stabilization of gold nanopartides in imidazolium ionic liquids. Chemical Communication, No.7, pp. 812-814, ISSN 1359-7345... 

To decrease the induction period, DMCs have been combined with quaternary ammonium salts, montmorillonite clays modified with quaternary ammonium salts, or imidazolium ionic liquids [10,12,13]. Such additives were found to decrease the induction period by acting as nucleophiles in the catalytic cycles. As such, these compounds (1) facilitate the formation and activation of the zinc-monomer bond and (2) make the zinc-monomer bonds more active for polymerization, and/or they stabilize the polymerization centers and prevent their decomposition [10,13]. 

In the author s group, much lower-melting benzenesulfonate, tosylate, or octyl-sulfate ionic liquids have recently been obtained in combination with imidazolium ions. These systems have been successfully applied as catalyst media for the biphasic, Rh-catalyzed hydroformylation of 1-octene [14]. The catalyst activities obtained with these systems were in all cases equal to or even higher than those found with the commonly used [BMIM][PF6]. Taking into account the much lower costs of the ionic medium, the better hydrolysis stability, and the wider disposal options relating to, for example, an octylsulfate ionic liquid in comparison to [BMIM][PF6], there is no real reason to center future hydroformylation research around hexafluorophosphate ionic liquids. 

As well as phosphorus ligands, heterocyclic carbenes ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to be a simple inert component of the solvent system. However, the proton on the carbon atom at position 2 in the imidazolium is acidic and this carbon atom can be depro-tonated by, for example, basic ligands of the metal complex, to form carbenes (Scheme 5.3-2). 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

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]. 

Application may be found for ionic liquids of resonance-stabilized methanides which can easily be prepared from the alkali salts. The first synthetic step includes the formation of the nearly insoluble silver salts in water (AgX, X = methanide). Since the silver salts dissolve in 2N aqueous NH3, adding ethyl(methyl)imidazolium bromide (EMHBr") results in the formation of a water-soluble ethyl(methyl)imidazolium methanide (EMHX") which can be separated from the AgBr precipitate by filtration. ... 

TABLE 11. Properties of methanide based ionic liquids [RMI+X ] (R organic group, M = methyl, I = imidazolium X = resonance-stabilized nitrosomethanide)... 

Stabilized by bulky organic cations such as EMI+ or BMI+ [BMI+ = butyl(methyl) imidazolium]. Nitrosomethanide-based ionic liquids are very hygroscopic and immediately absorb water when exposed to air. The experimentally determined melting points vary between —4°C and 35 °C (Figure 40, Table 11), with the BMI+ salts always possessing the lower melting point. ... 

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. 

The cations in ionic liquids are generally bulky monovalent organics. The typical cations of ionic liquids, not including the familiar alkylammonium and alkylphosphonium ions, are shown in Fig. 2. It is primarily the cations, which account for the low melting points of ionic liquids. The dialkylimidazolium ions, such as 1-butyl-3-methyl imidazolium [BMIM], have been widely investigated because low-melting ionic liquids can be made readily from such cations and because of their thermal and chemical stability. 

---