Hydrogen bonding ionic liquids

Kohmoto S, Kara Y, Kishikawa K (2010) Hydrogen-bonded ionic liquid crystals pyridinyl-methylimidazolium as a versatile building block. Tetrahedron Lett 51 1508-1511... 

The application of ionic liquid to biomass separation recently started to attract a great deal of attention. Cellulose is poorly soluble in conventional solvents due to its many intermolecular hydrogen bonds. Ionic liquid was believed to be capable of disrupting such hydrogen bonds between different polysaccharide chains, thus decreasing the compactness of cellulose and... 

Keywords l-Ethyl-3-methyliinidazoliuin acetate Acid-base equilibria Basic ionic liquids Carbene Catalysis CO2 capture DFT calculations Hydrogen bonding Ionic liquids Mass spectroscopy N-Heterocyclic carbenes Organocatalysis Photoelectron spectroscopy Proton transfer Synthesis Vaporization mechanism... 

It should be mentioned that in addition to hydrogen bonding, ionic interactions [103, 104], charge-transfer interactions [105], host guest interaction [106], halogen bridges [107] also direct liquid crystalline molecular assemblies. Charge-transfer interactions have been used for the enhancement of columnar and smectic phases [108-111]. 

Salt in water Requires breaking strong bonds between ions, and disrupting hydrogen bonding in liquid water, but ions often become strongly hydrated as polar water molecules are attracted to the ions. Some, but not all, ionic solids are very soluble in water. 

The mesogen structures may be formed not only by covalent bonds, but also by non-covalent interactions, such as hydrogen bonds, ionic interactions, and metal coordination [71]. A recent example [72] of this concept comprised the self-assembly of complex salts into stable hierarchical aggregates with a dense core and a diffuse shell. These materials were made from diblock copolymers poly(acrylic acid)-block-poly(acrylamide) and the cationic surfactant dodecyltrimethylammonium. Due to non-covalent interactions the surfactant/polymer aggregates exhibited a liquid crystalline structure of cubic symmetry. 

Finally, none of the ionic liquids were found to be hydrogen bond acids [5], although this may well be a consequence of the salts selected, none of which had a cation that would be expected to act as a hydrogen bond donor. Earlier qualitative measurements on ionic liquid stationary phases of mono-, di-, and trialkylammo-nium salts suggest that hydrogen bond donation can be important where a potentially acidic proton is available [7-9]. More recent work, with [BMIM] salts, also indicates that these ionic liquids should be considered to be hydrogen bond donor solvents [10]. However, this has yet to be quantified. 

If the cation has been unchanged, its ability to act as a hydrogen-bond donor has been unchanged, so why is an effect seen at all I propose that there is competition between the anion and the Reichardt s dye solute for the proton. Thus, the values of the ionic liquids are controlled by the ability of the liquid to act as a hydrogen bond donor (cation effect) moderated by its hydrogen bond acceptor ability (anion effect). This may be described in terms of two competing equilibria. The cation can hydrogen bond to the anion [Equation (3.5-2)] ... 

It can easily be shown that the value of K" is inversely proportional to the value of K and that K is dependent on both the cation and the anion of the ionic liquid. Eience, it is entirely consistent with this model that the difference made by changing the anion should depend on the hydrogen bond acidity of the cation. 

Attempts have also been made to separate non-specific effects of the local electrical field from hydrogen-bonding effects for a small group of ionic liquids through the use of the k scale of dipolarity/polarizability, the a scale of hydrogen bond donor acidity, and the (i scale of hydrogen bond basicity (see Table 3.5-1) [13, 16]. 

At first glance the hydrogen bond basicity (3 is controlled solely by the anions, with basicity decreasing in the order Cl > [RS03] >[BF4] > [PF ] > [N03] > [SCN] . However, while the general trend is clear, this is not the order that one would have expected, and the cations are obviously playing a role. Again, this may be a consequence of competition for the basic site (anion) between the test solute and the acidic site (cation) of the ionic liquid. It is unfortunate that no study to date has used a common anion across all possible cations. 

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. 

A similar study performed by Welton and co-workers studied the rate and selec-tivities of the Diels-Alder reaction between cyclopentadiene and methyl acrylate in a number of neutral ionic liquids [44]. It was found that endo. exo ratios decreased slightly as the reaction proceeded, and were dependent on reagent concentration and ionic liquid type. Subsequently, they went on to demonstrate that the ionic liquids controlled the endo. exo ratios through a hydrogen bond (Lewis acid) interaction with the electron-withdrawing group of the dienophile. 

Many organic chemical transformations have been carried out in ionic liquids hydrogenation [4, 5], oxidation [6], epoxidation [7], and hydroformylation [8] reactions, for example. In addition to these processes, numerous synthetic routes involve a carbon-carbon (C-C) bond-forming step. As a result, many C-C bondforming procedures have been studied in ambient-temperature ionic liquids. Among those reported are the Friedel-Crafts acylation [9] and allcylation [10] reactions, allylation reactions [11, 12], the Diels-Alder reaction [13], the Heck reaction [14], and the Suzuld [15] and Trost-Tsuji coupling [16] reactions. 

The mp and bp increase in the order of nonpolar to polar to ionic compounds. Associated liquids, in which the molecules are held together by hydrogen bonds, show higher bp than nonassociated polar compounds of similar molecular w eights. 

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