Cations imidazolium

Imid azolin-2-ones polymers, 1, 296 synthesis, 5, 466, 491 Imidazolium cations, 2-fluoro-nucleophilic substitution, 5, 413 Imidazolium chloride, 4-amino-l-methyl-2,3-diphenyl-... 

Up to the present the principal interest in heteroaromatic tautomeric systems has been the determination of the position of equilibrium, although methods for studying fast proton-transfer reactions (e.g., fluorescence spectroscopy and proton resonance ) are now becoming available, and more interest is being shown in reactions of this type (see, e.g., references 21 and 22 and the references therein). Thus, the reactions of the imidazolium cation and imidazole with hydroxyl and hydrogen ions, respectively, have recently been demonstrated to be diffusion controlled. ... 

A variation on this overall synthetic approach allows the formation of related TSIL ureas by initial conversion of l-(3-aminopropyl)imida2ole into an isocyanate, followed by treatment with an amine and allcylating agent. This approach has been used to append both amino acids and nucleic acids onto the imidazolium cation skeleton [14]. 

The viscosities of non-haloaluminate ionic liquids are also affected by the identity of the organic cation. For ionic liquids with the same anion, the trend is that larger allcyl substituents on the imidazolium cation give rise to more viscous fluids. For instance, the non-haloaluminate ionic liquids composed of substituted imidazolium cations and the bis-trifyl imide anion exhibit increasing viscosity from [EMIM], [EEIM], [EMM(5)IM], [BEIM], [BMIM], [PMMIM], to [EMMIM] (Table 3.2-1). Were the size of the cations the sole criteria, the [BEIM] and [BMIM] cations from this series would appear to be transposed and the [EMMIM] would be expected much earlier in the series. Given the limited data set, potential problems with impurities, and experimental differences between laboratories, we are unable to propose an explanation for the observed disparities. 

In the binary haloaluminate ionic liquids, an increase in the mole percent of the imidazolium salt decreases the density of the liquid (see Table 3.2-2). The bromo-aluminate ionic liquids are substantially denser than their chloroaluminate counterparts, being between 0.57 g cm and 0.83 g cm denser than the analogous chloroaluminate ionic liquids (see Table 3.2-2). Variation of the substituents on the imidazolium cation in the chloroaluminate ionic liquids has been shown to affect the density on the basis of the cation size [17]. 

In general, isotopic exchange is both expensive and difficult. In the case of many room-temperature ionic liquids, however, the manufacture of deuterated ionic liquids is relatively easily achievable. For example, the general synthesis of l-allcyl-3-methylimidazolium salts is shown in Scheme 4.1-1 [2]. This methodology allows maximum flexibility in the deuteration on the imidazolium cation that is, it can be either ring or side chain deuteration or both. 

Bowron et al. [11] have performed neutron diffraction experiments on 1,3-dimethylimidazolium chloride ([MMIM]C1) in order to model the imidazolium room-temperature ionic liquids. The total structure factors, E(Q), for five 1,3-dimethylimidazolium chloride melts - fully probated, fully deuterated, a 1 1 fully deuterated/fully probated mixture, ring deuterated only, and side chain deuterated only - were measured. Figure 4.1-4 shows the probability distribution of chloride around a central imidazolium cation as determined by modeling of the neutron data. 

Another ionic liquid, containing a nonyl-rather than a butyl-side chain, is shown in Figure 4.2-2. There is little difference between the basic structures of these two ion-pairs (Figures 4.2-1 and 4.2-2) with respect to the non-bonded interactions (hydrogen bonds) occurring between the F atoms on the anion and the C-H moieties on the imidazolium cation. 

Scheme 5.2-3 Formation of a Pd-carbene complex by deprotonation of the imidazolium cation.

Another means of in situ metal-carbene complex formation in an ionic liquid is the direct oxidative addition of the imidazolium cation to a metal center in a low oxidation state (see Scheme 5.2-2, route b)). Cavell and co-workers have observed oxidative addition on heating 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPli3)4 in refluxing THF [32]. The Pt-carbene complex formed can decompose by reductive elimination. Winterton et al. have also described the formation of a Pt-car-bene complex by oxidative addition of the [EMIM] cation to PtCl2 in a basic [EMIM]C1/A1C13 system (free CP ions present) under ethylene pressure [33]. The formation of a Pt-carbene complex by oxidative addition of the imidazolium cation is displayed in Scheme 5.2-4. 

Scheme 5.2-4 Formation of a Pt-carbene complex by oxidative addition of the imidazolium cation.

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. 

Water molecules or anions close to the active sites in the protease enzymes, mentioned above, may not be considered circumstantial, but may effectively contribute to the removal of the surplus proton from the imidazolium cation before the actual catalytic event. They could serve well to create the initial ion/neutral form of the Aspl02-His57 couple which is important for the initial step of the catalytic process in most discussions 11611 .13i. such a proton removal may be caused by the productive binding of a true substrate (or inhibitor) of the enzyme to the neighboring recognition clefts of the active site. 

BMI.PFg [HAuC14.3H20] Imidazolium cation Prismatic particles ... 

Interestingly, in some cases the IL itself can act as the reductive agent. Spherical metal silver NPs were prepared in a hydroxyl-functionalized IL (2) (entry 30, Table 1.1) [17]. In this case, the hydroxyl moiety of the IL plays a reductive role, being oxidized to the corresponding aldehyde. In a similar manner, for Au(III) precursors, the imidazolium cation itself can act as a reducing agent to yield prismatic particles in BMI.PF6 with a very broad size range of diameter 3-20 pm and thickness... 

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