Imidazolium stabilization

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

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. 

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

Generally, it is most likely that metal NPs are stabilized by the aggregates of the non-functionalized imidazolium ILs rather than by the isolated ions. In addition, the interaction between ILs and the metal NPs have been evidenced by X-ray photoelectron spectroscopy (XPS), small-angle X-ray scattering (SAXS), isotope labeling, and surface-enhanced Raman spectroscopy (SERS) techniques. 

The chelate effect also favors oxidative addition of the C2—H bonds of imidazo-lium salts because it provides stabilized complexes. The reaction of a pyridine-imidazolium salt with [lrCl(cod)]2 yields the oxidative addition product, even in the absence of a base (Scheme 3.9), thus confirming that the oxidative addition of an imidazolium salt should be considered as a vahd process for the preparation of NHC—M—H complexes [24]. 

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. 

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