Chapter 22
Acids and Bases in Ionic Liquids Douglas R. MacFarlane and Stewart A. Forsyth
Downloaded by CORNELL UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch022
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
Salts that are liquid at ambient temperatures are of interest in an enormous range of applications from 'green' synthesis of chemicals, to electrolytes in devices such as artificial muscles and electrochromic windows, to media for biochemical and biological processes. All of these applications require the ionic liquid (IL) to take on the role as solvent for a variety of dissolved species, yet an understanding of the range of behaviour of these new liquids as solvents is only beginning to emerge. The commonplace assumption that they are inert may not be correct in all cases. Drawing on a range of ionic liquids we present an investigation of their solvent properties as Lewis acids and bases using a number of probe acids to determine the relative state of dissociation of the acids in the IL. This allows us to classify ionic liquids into a number of distinct classes, each expressing distinctly and importantly different behaviours. We demonstrate that the Lewis base class may exhibit a general base catalysis phenomenon which may be of importance in synthetic use of ionic liquids. The role of low levels of water contamination is also discussed. It is clear that there is no single "ionic liquid" behaviour; rather there are a range of properties possible across the ever growing range of ionic liquids discovered and yet to be discovered.
264
© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
265 Ionic liquids have been known for quite some time, some of the first reports dating back to 1914. They became of interest to the electrochemist as a result of their high ionic conductivity which made them interesting as solvents for electrochemically active species. A range of familiar and important compounds including various acids and bases are quite soluble in ionic liquids and this has stimulated interest in their use as reaction media for a range of important chemical and biochemical processes. More recently they have been recognized as superb media for "green" chemical synthesis by virtue of their virtually non-existent vapour pressure. This lack of volatility is a general feature of ionic liquids that do not contain an active proton. Other investigations have recognized their special properties as electrolytes for electrochemical devices, and as solvents for in-vitro enzymatic and biochemical processes. 1
2
3
4
5
Downloaded by CORNELL UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch022
6
The majority of ionic liquids that are commonly investigated consist of an organic cation based on a quaternized nitrogen such as those shown in Scheme 1. Other cations of similar structures to those in Scheme 1, but having a N-H type group (R=H) are also known, but the components of these tend to be volatile to some extent.
*
/^A /
N
^
300°C and their electrochemical stability is usually in excess of +2 V (vs Ag/Ag ) with respect to oxidation and -2V with respect to reduction. " For these reasons the ionic liquids are often described 6
7
9
8
10
n
+
12
14
In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
266 BF "
PF ~
N0
[4]
[5]
[6]
6
4
Ο
Ο
II
F C—S—N—S—CF 3
3
Il
II
ο
ο
3
Downloaded by CORNELL UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch022
[9] Scheme 2: Weakly basic anions; tetrafluoroborate [4], hexafluorophosphate [5], nitrate [6], mesylate [7], thiocyanate [8], bis(trifluoromethane)sulfonimide [9], tricyanomethide [10] and tosylate [11].
as being inert and hence of exceptional interest as potential solvents. Nonetheless many aspects of their ability to dissolve and interact, or not, with solutes is, as yet, far from understood. More recently ionic liquids have been discovered involving cations bearing other functional groups, eg ether functionality [13], and anions of varying properties, for example the dicyanamide anion [14] (scheme 3). These ionic liquids express a variety of properties that are not easily accommodated in the description of the Class I ionic liquids given above and serve to illustrate the fact that there is a rich diversity of solvent properties available within the broad family of ionic liquids. A number of recent reports have described the preparation and properties of families of ionic liquids involving anions that would not normally be described as "very weak bases". These include the acetate[15], and dieyanamide[14] anions (Scheme 3). While expected to be stronger bases than the anions in Scheme 2, these salts would nonetheless only produce slightly basic solutions in water. This difference in basicity can be expected to produce different solvent properties in the ionic liquids of these anions. 15
16
7
16
Ο
[13]
-O"
H,C-
N-
[14]
[15]
Scheme 3: Ether functionality in cation, l-methyl-3-ethylmethoxyimidazolium [13] and basic anions dicyanamide [14] and acetate [15].
In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
267
Downloaded by CORNELL UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch022
One of the important and very revealing properties of any solvent, and one that is very familiar to all of the fields in which water is a key solvent, is the way that the solvent responds to the presence of an acidic or basic solute. The state of a traditional acid or base compound, such as acetic acid or ammonia, in an ionic liquid solvent is not immediately clear and the question challenges traditional understanding of acid/base behaviour. In this paper we describe a number of experiments designed to elucidate this important behaviour in a broad range of ionic liquids. It appears that some ionic liquids can turn a strong acid into a weaker acid, while other ionic liquids turn weak into strong. These results suggest a classification of ionic liquids with respect to their acid/base properties as discussed further below.
Acids in Ionic Liquids In traditional solvents, acids and bases are categorized in terms of Bronsted-Lowry acidity which relates to the ability of the acid HA to donate a proton to a molecule of the solvent, S. HA + S
«
*
A" + H S
+
The equilibrium constant for this reaction, pKa(HA) , is a measure of the acidity of the acid, but it clearly has a dependence on the nature of the solvent S. The more general Lewis definition of acidity and basicity involves simply the ability of the molecule to accept or donate electrons. In this case the proton, notionally free of A', is the Lewis acid (electron acceptor) and A" is the Lewis Base (electron donor). The role of the solvent in the Lewis perspective can be to act as the predominant base and thereby to form HS (eg H 0 in water). Water is amphoteric; it is a very weak acid (forming OH") and is also a weak base (forming H 0 , which is a relatively strong acid). The experiments described below show that some ionic liquids can act as both Lewis and Bwnsted-Lowry bases. s
+
+
3
+
3
The exact values of acid dissociation constants, pKa, in ionic liquids are unknown at present and are not easily measurable by direct means. In the case of an ionic liquid involving the extreme of a very weakly basic anion such as C H S 0 \ some progress towards understanding the relative magnitude of pKa values can be achieved by asking the question: Is the ionic liquid anion less basic than water? Given that the anion is intentionally chosen at the extreme of the very weakly basic anions, the answer to this question is certainly yes. The corresponding conjugate acid is therefore much more acidic than the hydronium ion. Since the anion is a weaker Lewis base than water, one might immediately expect that an acid dissolved in such an ionic liquid would exhibit a lower degree of dissociation than in water. Thus the Bronsted acidity of a solute acid is expected to be lower in such ionic liquids than in water. The anions in Scheme 2 typically are conjugate bases of acids having pKa's ranging from similar to H 0 to very much larger than H 0 . 3
+
3
3
+
3
In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
268
Downloaded by CORNELL UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch022
On the other hand, the anions in Scheme 3 are typically more basic than water and therefore can be expected to act as stronger proton acceptors than water. Acid dissolved in such an ionic liquid may then exhibit a higher degree of dissociation than in water. Thus the Bronsted acidity of a solute acid may be higher in such ionic liquids than in water. The extent to which this is the case is a matter of the relative acceptor strength of the ionic liquid anion and the conjugate base anion of the solute acid. This understanding is confirmed by the data in Figure 1, which shows the visible spectrum of a weak organic acid, m-cresol purple [16] (Scheme 4), in water and in a number of ionic liquids in which it is soluble. The ionic liquids studied in this work include emlm acetate, emlm nitrate, emlm tricyanomethide, emlm dicyanamide, emlm TFSI, bmim hexafluorophosphate and bmim tetrafluoroborate. These were prepared, purified and characterized as described previously . Samples were dried under vacuum before use. Metacresol purple is chosen as a typical probe indicator acid here because of its strong degree of colouration in both acid and base forms, the acid form being a straw yellow colour and the base form a deep purple. This difference is dramatically observed in the visible spectra of the acid ^max = 440nm) and base forms (kmax = 600nm) (Figure 1). Such spectra allow an estimation of the degree of dissociation, a, 5,714,16
Scheme 4: Acid/Base indicators; metacresol purple [16], bromocresol purple [17] and alizarin red S [18].
In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
Downloaded by CORNELL UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch022
269
,
,
400
,
,
,
,
500
600
,
,
700
,
,
,
6-
800
Wavenumber (nm) Figure 1: Visible spectroscopy of a weak organic acid, m-cresol purple [16], in various pure ionic liquids and aqueous solutions. Concentration - 1.0 wt% in all cases. Not shown: [bmIm][PF ], [emIm][TFSI] and [bmIm][BF ] also produce spectra equivalent to acidic water. #
6
4
of the acid. In pure water this acid at the concentration involved is expected to be only approximately 0.02% dissociated into the base form at room temperature. Thus the base form is almost completely absentfromthe spectra of the acid in pure water. Equally the base form is not observable in the solutions of the acid in the ionic liquids shown, with the exception of the acetate ionic liquid where a slight lightening in the colour of the solution is observed and an increase in the absorption in the region of the base form absorption maximum (~ 600nm) is seen. Notably, in the dca, tcm, nitrate, PF , TFSI and B F ionic liquids the acid appears to be as little dissociated as it is in acidic water. However the comparison between neutral water and the acetate ionic liquid shows that the acid is more dissociated in the latter, as predicted on the basis of the basicity of this anion. Thus we estimate that the degree of dissociation of this acid is very small (
