Solid structure of imidazolium salts 6


Although most used and preferred ionic liquids are those that are liquid at room temperature, detailed structural features are easily accessible by looking at those that crystallize and that have their crystal structure determined (indeed, the investigation of structural properties in the liquid state requires considerable experimental effort, as for example the X-Ray spectroscopy, and little information can be gained if heavy metals are missing).
A simple comparison of these data can yield important information about the structural features of the same material in the liquid phase.

Two-dimensional simplified solid-state model of the polymeric
supramolecular structure of 1,3-dialkyl imidazolium ionic
liquids showing the hydrogen bonds between the imidazolium
cation(C) and the anions (A) (one cation is surrounded by three
anions and vice-versa). An over view in the X-ray studies reported in the last years on the structure of 1,3-dialkylimidazoilum salts reveals a typical trend: they form in the solid state an extended network of cations and anions connected together by hydrogen bonds. The monomeric unit is always constituted of one imidazolium cation surrounded by at least three anions and in turn each anion is surrounded by at least three imidazolium cations (Figure 2). Although the number of anions that surround the cation (and vice-versa) can change depending upon the anion size and type of the N-alkyl imidazolium substituents, this structural pattern is a general trend in imidazolium salts (for examples see Table 2).

The strongest hydrogen bond always involves the most acidic H2 of the imidazolium cation (pKa=23.0 for the 1,3-dimethyl imidazolium cation) followed by the other two hydrogens (H4 and H5) of the imidazolium nucleus and/ or the hydrogens of the N-alkyl radicals (H6, H7 and H8, Table 2). These bonds possess properties of weak to moderate hydrogen bonds – they are mostly electrostatic in nature - (H……X bond lengths > 2.2Å; C-H……X bond angles between 100°-180°).

Note that even in the case B(Ar)4 anions the network is formed through relatively strong C-H…π hydrogen bonds. In the case of the octahedral PF6 anion the equatorial F atoms participate preferentially in hydrogen bonding network and in the case of the tetrahedral BF4 anion only three of the F atoms are usually involved in the linkage.

A schematic view of the arrangements of imidazoliumThe three dimensional arrangement of the imidazolium ionic liquids are generally formed trough chains of the imidazolium rings (Figure 3). In some cases there are typical π-π stacking interactions among the imidazolium rings and in the case of 1-alkyl-3-methylimidazolium salts a relatively weak C-H…π interaction via the methyl group and the imidazolium ring-π system can also be found. This molecular arrangement can generate channels in which the spherical anions are accommodated as chains. This structural pattern depends on the anion geometry, and the internal arrangements along the imidazolium columns vary with the type of the N-alkyl substituents. Of note that other effects than π-π stacking, such as the entropy effect and electrostatic interactions may not favorable the formation of structures of the type shown in Figure 3. Therefore, it can be proposed that the best representation for the imidazolium salts in the solid phase is: [(DAI)x(X)x-n]n+ [(DAI)x-n(X)x)]n-, where DAI is the 1,3-dialkylimidazolium cation and X is the anion.

Liquid crystalline properties

Long-chain IL salts have attracted some interest due to their liquid crystalline (LC) properties. The origin for these can be found in the formation of domains "Coulombic layers" where the ionic head-groups interact with the counterions, and "van der Waals" layers built from (anti)parallel stacking of the alkyl chains.
Hexafluorophosphate salts with cations up to C20MIM have been investigated by differential thermal analysis and show one or more LC transitions. Melting to isotropic liquids occur at rather high temperatures (> 100°C). The phase behaviour of long chain-length imidazolium and pyridinium chlorides, tetrachlorocobaltates and tetrachloronickelates showing LC properties has also been reported. Structure IntroductionBack to Structure Introduction


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